ISSN:0430-0696    

DOI:10.1039/SF96903FX001

出版商:RSC    

年代:1969

数据来源: RSC

摘要:

General Introduction BY A. D. BUCKINGHAM Dept . of Theoretical Chemistry University of Cambridge Received 1 1 iIz Fehrrrury 1970 Most magneto optical effects have their root in the Zeeman splitting of energy levels. The symmetry properties of the magnetic interaction are contrasted with those of an electric interaction. In molecular spectroscopy measurement of the Zeeman splittings yields the magnetic moments of molecules in both ground and excited states. When lines are broad the new technique of magnetic circular dichroism is particularly useful for assigning spectra and determining magnetic moments. The Cotton-Mouton effect in absorption bands is briefly considered. It is appropriate that an International Symposium on magneto optical effects should have been arranged by our Society for Michael Faraday was indeed very active in this field.In 1846 he announced the discovery of the effect that now bears his name,l and which forms a central part of the business of this Symposium. The other great name associated with magneto optics is Zeeman who discovered his effect just half a century after Faraday found his. The Netherlands Physical Society celebrated the centenary of Zeeman’s birth with a Conference in Amsterdam in 1965,2 and in some respects that meeting was the forerunner of the present Symposium. In this introductory talk I shall not attempt to give a complete survey of our subject but shall deal with the background in an attempt to set the stage for the proceedings to follow. Early work in magneto optics is described in three well-known books by Zeeman,3 Wood and S c h u t ~ .~ The basic actions of a magnetic field on matter are to align the electronic and nuclear angular momenta and to induce orbital motion. These interactions affect the macroscopic equations of electromagnetism and also make the important phenomena of electron and nuclear magnetic resonance possible. These basic properties of magnetic interactions are expressed in the hamiltonian describing a molecule in a uniform magnetic induction Bo : where Po is the hamiltonian of the free molecule m is the electronic magnetic moment operator, pus = eA/2rne is the Bohr magneton L and S are the total electronic orbital and spin angular momentum operators in units of ti m(N) = y(N)tiI(N) is the magnetic moment of nucleus N and x ( ~ ) and dN)(d) the diamagnetism and diamagnetic nuclear shield-ing operators : m = -pus(L+2S) (2) p o is the permeability of a vacuum and in S.I.it has the value 47z x lo-’ J s2 C-2 m-I. 8 GENERAL INTRODUCTION The significance of the diamagnetic term was appreciated long before the advent of quantum mechanics through considerations of the Larmor precession of an electron with an angular velocity eBo/2rn about the axis of the magnetic field (the z-axis). This leads to an induced magnetic moment - (e2B0/4rn,)~($ +z) whose direction is opposite to that of the field; the bars indicate an average over the electron cloud and the sum extends over all the electrons. The hamiltonian (1) is sufficient for most purposes. However when considering the effects of spin-orbit coupling it may be necessary to include small additional term^."^ The magnetic moment operator (2) commutes with the parity operator 9 (which changes the sign of all spatial coordinates) and has even parity.Since Y2 = 1 the eigenvalues of 9 are + 1 and - 1 corresponding to even and odd parity respectively. 9 commutes with So (and the total angular momentum operator J) so the eigen-functions $,JM of Z0 (where J and M determine the magnitude and z-component of the total angular momentum and n defines the other constants of the motion of the molecule) are either of even or odd parity and associated with magnetic moments ($,JM I m I $nJM). In this respect the magnetic moment operator differs funda-mentally from the electric dipole moment p = CeirI whose parity is odd.A molecule can only possess a permanent electric dipole moment (and hence show a first-order Stark effect) if $n is degenerate; thus the H-atom spectrum exhibits large Stark splittings because the excited stationary states are of mixed parity (e.g. (l/J2) [t,bZs+- t,kzp,]). A magnetic field splits the (2J+ 1)-fold degeneracy associated with each value of the total angular momentum J and this fact is of great importance. If rn is 1 Bohr magneton (0.9274 x J T-I = 0.9274 x erg gauss-l) then in a magnetic flux density of 1T (i.e. lo4 gauss) the Zeeman energy J = 0.5 cm-l i.e. of an order of magnitude that is suitable both for high-resolution optical spectroscopy and for paramagnetic resonance. The fundamental difference between the perturbations due to external electric and magnetic fields can be illustrated by considering a linear molecule in a lZ vibronic state e.g.OCS. As is well known its microwave spectrum shows only second-order Stark splittings but it exhibits first- and second-order Zeeman effects.IO The magnetic moment in this case is generated by the rotational angular momentum and is proportional to J ; however the constant of proportionality the “ rotational g factor ” x pB is normally - of its value for atoms and molecules with electronic angular momentum. The second-order Stark effect-which depends on the electric dipole moment induced by the external field-arises because parity is not conserved in the presence of the external field (2 = Z 0 - p E does not commute with 9 if the external field E is unaffected by the inversion).Considerations of time-reversal symmetry and spin-operators 11-1 require that molecules with half-integral J must be at least doubly degenerate in the absence of an external magnetic field (Kramers’ theorem) ; furthermore the application of a magnetic field to a system of half-integral J causes a first-order Zeeman splitting of the pairs of states having angular momentum components +M. Where spectral lines are sufficiently sharp direct observation of the Zeeman splittings or shifts provides the most satisfactory way of extracting information about the magnetic moments-and hence about the electronic structure-of molecules in both ground and excited states. In astrophysics the roles are reversed and Zeeman splittings of the spectra of atoms are used to determine magnetic fields on stars-for example flux densities up to 3700 gauss have been detected on the sun during intense sunspot activity.14 Many of the papers presented to the Zeeman Centenary Conference describe Zeeman splittings as do those by Hochstrasser and Lin and 1 A .D . BUCKINGHAM 9 Marzzacco and McClure to be given at this Symposium. Even when the lines are broad large magnetic moments can sometimes be detected as in the work of Malley, Feher and Mauzerall l 5 on porphyrins in solution at lo5 gauss at room temperature and at 77 K. However the differential approach through magnetic circular dichroism is generally superior in these conditions. The modern techniques of atomic and molecular beam resonance spectroscopy optical pumping level crossing experi-ments,18 and various quantum mechanical interference effects l 9 are also relevant and of importance in very accurate measurements of small Zeeman splittings and hyperfine interactions.Paramagnetic resonance transitions have been detected in phosphorescent molecules by sweeping the magnetic field and observing changes in the intensity of the phosphorescence 20-22 ; information is obtained about the mode of entry into and decay from the meta-stable state. This work is represented at this meeting by Sharnoff's paper. As in optical pumping this type of experiment can also yield lifetimes of excited molecules in triplet states. A magnetic field can influence triplet-state lifetimes through the triplet-triplet annihilation probability in solids 2 3 and solutions 2 4 ; the effect is not very large and is most marked in single crystals.Magnetophotoconductive effects have been observed in copper phthalo-cyanine and attributed to the influence of the magnetic field on the charge-carrier generation rate.25 When the spectral lines are broad or when there are difficulties in assigning spectra, the methods of magnetic optical activity are particularly useful. Eight of the thirteen papers to be presented to this meeting are concerned with this particular " magneto optical effect "-the Faraday effect in absorption bands. The method of magnetic circular dichroism is particularly useful as it is absent in transparent regions the solvent does not contribute significantly and it is a direct measure of the difference in absorption of right- and left-circularly-polarized light.The technique is particularly important in assigning the spectra of transition metal complexes and in determining their magnetic moments in excited states ; it is also helpful in organic and biochemistry. There is a very useful recent review of the subject by Schatz and McCaffery.26 Magnetic optical activity is determined by the linear effect of a magnetic induction Bo on the frequency dependent electric polarizability a I aaS = + a$iBoy + + C ~ $ ~ ~ B ~ B ~ + . . . . (4) i.e. by the tensor a('). If the molecular tumbling motion is random g m a y be replaced by its isotropic part a(')eafir where a(l) = I gCXa/jy+fiy ( l ) and cap = 1 or -1 if aPy is an even or odd permutation of xyz and is zero otherwise.Then the permittivity can be written E = id 0 where i~' = E, = - E,, when Bo is in the z-direction. (:iet ; :) Since every molecule regardless of its symmetry has an a so too does it have an a(1), since Bo is an axial vector ; however distortion of a in an isotropic medium by a polar vector of any strength (e.g. an electric field) could not yield optical rotation about the field (unless the molecules are different in right-handed (x y z) and left-handed ( - x -y - z ) frames as in an optically active medium). and for a molecule in a non-degenerate state described by the ket I n ) may be 4;) = Z 2 h - ' [ f ( w ~ j J - i g ( ~ ~ j n ) I C ~ j n Re {<. I Pa I j ) < j I PS I .>I-The complex polarizability a is defined by the equation pa = 28 J iw Im I<.I I j ) < j I PS I .>I] ( 5 ) where A g have dimensions of o r 2 is the angular frequency of the electri 10 G EN ER A L I N TROD U CT I ON field E wjn = (EJ-En)/h and g and fare absorption and dispersion line shape func-tions of the form illustrated in fig. 1. FIG. 1.-Typical absorption (a) and dispersion (6) line shapes near an absorption frequency a j r l . Right (E+) and left (E-) circularly-polarized light of angular frequency co travelling in the uniaxial z-direction can be represented by the equation where i j k are unit vectors along the x y z axes ; E,t = n* - ik+ is the complex refractive index so right-circularly polarized light travels with a velocity c/n+ in the z-direction with an intensity Z = I. exp (- 2cok+z/c). The actual electric field is the real part of eqn (6) i.e., Re (E*) = exp (-ok,tz/c)[i cos w(t-n*z/c)+j sin o(t-n*z/c)] (7) and the beam associated with E k is comprised of photons whose component of angular momentum in the direction of propagation is T 1.On absorption it induces transitions with AM = + 1 which for a transition to an electronically degenerate E* = E(O) exp [ico(t- n^+z/c)] (i& ij) (6) -pI: M = - l (n+) (n-) M - 0 FIG. 2.-The origin of (a) magnetic circular dichroism and (6) magnetic optical rotation. The A-terms are shown for a non-degenerate ground state and a triply-degenerate excited state A . D . RUCKINCHAM 1 1 excited state from a non-degenerate ground state? are displaced to lower/higher frequency by interaction with Bo (see fig. 2).The difference between n+ and n- leads to optical rotation through an angle 6 = (toz/2c)(iz -n__) (8) about the direction of propagation. The sign of this expression is opposite to that commonly adopted but it agrees with the natural choice that a positive rotation of the electric vector and the direction of propagation form a right-hand screw ; it also fits naturally into the established definitions of the Cotton-Mouton and Kerr constants (see eqn (14)). This sign convention is a point we might discuss at this Symposium. Similarly the difference between k+ and k- leads to an ellipticity 8 = (012/2C)(k.# -k) (9) (10) and the two equations (8) and (9) can be combined in the coniplex form A a) = 4 - io = (cloz/2c)(ti+ -fi-). The importance of studying 8 or # in the vicinity of the absorption frequency ujn is that the contribution of the excited state l j ) can be isolated.Thus the awkward summation in eqn (5) is obviated. If the magnetic moment differs in states 1 p t ) and I j ) the Zeeman shift of the absorption frequencies from cojn leads to opposite displacements of the resonance frequencies for E+ and E- as illustrated in fig. 2. The result is the line shape in magnetic circular dichroism or in magnetic optical rotation that is characteristic of electronic degeneracy in the ground or excited state (called A-terms by Serber 29). Even when neither state possesses a magnetic moment, circular dichroism or rotation may arise from small differences in the intensities of the interaction with E+ and E- ; this is due to the effects of the perturbation -m Bo on the eigenfunctions I n) and I j ) in eqn (5) and is illustrated in fig.3. The familiar line shapes are associated with Serber’s B-terms which are presumably responsible for the magnetic circular dichroism detected in organic carbonyl compounds and reported in the paper by Barth Bunnenberg Djerassi Elder and Records. 0 4 FIG. 3.-(a) Magnetic circular dichroism and (b) magnetic optical activity when the states are non-degenerate (B-terms) . In the Cotton-Mouton effect there is a phase difference 8 = ~ n z - n x x ) ~ Y l c (1 1) induced in light travelling in the y-direction and initially linearly polarized in th 12 GENERAL INTRODUCTION z- and x-directions (Bo is again in the z-direction). Near absorption bands ^n = n - ik and A c 6 = ( 6 z z - G x x ) ~ y / ~ = 2nyCB& (12) * where C = (Zzz -^n,,)r;o(2n~B~)-~ is the Cotton-Mouton constant.30 The difference in the absorption of E and Ex leads to an optical rotation 4 given by tan 24 = sinh [(L - ~xx)~y/cl/cos [(nz - n,x>~Y/cl (1 3) which reduces to 4 = (k,,-kxx)~Y/2c for small 4.Thus the Cotton-Mouton effect in absorption bands is probably best studied through measurement of optical rotation. The effect is quadratic in the magnetic induction Bo and can therefore be related to a(2) in eqn (4). While magnetic circular dichroism arises from a difference in absorptance of photons having angular momentum m of - 1 and + 1 in the direction of Bo Cotton-Mouton dichroism results from a difference in absorptance of photons polarized parallel (m = 0) and perpen-dicular (m = 1) to Bo as illustrated in fig.4. This interesting new magneto-optical effect is analogous to the electro-optic Kerr effect,31* 32 and has recently been studied by Boccara Ferre Bria Billardon and B a d ~ z . ~ ~ If either the ground or excited state has a magnetic moment its peak intensity is in general proportional to the inverse cube of the line-width compared to an inverse square in magnetic circular dichroism. However because of its proportionality to B,2 the full description of the Cotton-Mouton effect is considerably more complicated than that of the Faraday effect. FIG. 4.-The origin of Cotton-Mouton dichroism and its frequency dependence for a non-degenerate ground state and a triply degenerate excited state as in fig.2. In transparent regions the Cotton-Mouton constant is real and related to the product of the anisotropy in the molecular electric and magnetic polarizabilities and to a small temperature-independent hyperpolarizability (like d2) in eqn (4)) describ-ing the quadratic effect of Bo on a. Measurements have been made on a number of gases using mercury 546.1 nm radiation,34 and recently with a He+Ne laser at 632.8 nm.35 The results lead to accurate values for the anisotropies of molecular magnetic susceptibilities. M. Faraday Phil. Mag. 1846 28[3] 294 ; Phil. Trans. Roy. Soc. 1846 1. Physica 1967 33,l-293. P. Zeeman Researches in Magneto-Optics (Macmillan London 191 3) A . D. BUCKINGHAM 13 R. W. Wood Physical Optics 3rd ed. (Macmillan London 1934).W. Schutz Magnetooptik vol. 16 of Handbuch der Experimentalphysik (Akademische Ver-lagsgesellschaft Leipzig 1936). J. Larmor Aether and Matter (Cambridge University Press 1900) p. 341. A. J. Stone Proc. Roy. SOC. A 1963,271,424. A. Abragam and J. H. Van Vleck Phys. Rev. 1953 92 1448. L. L. Lohr J. Chem. Phys. 1966,45,1362. l o W. H. Flygare W. Huttner R. L. Shoemaker and P. D. Foster J. Chenz. Phys. 1969,50 1714. '' E. P. Wigner Group Theory and its Application to the Quantiim Mechanics of Atomic Spectra, l 2 J. S. Griffith The Theory of Transition-Metal Ions (Cambridge University Press 1961) p. 205. l 3 A. S. Davydov Quantum Mechanics (Pergamon Press Oxford 1965) p.431. l4 H. W. Babcock Physica 1967,33,102. l 5 M. Malley G. Feher and D. Mauzerall J. Mol.Spectr. 1968 26 320. l 6 P. Kusch and V. W. Hughes Handbuch der Physik vol. 37 (Springer-Verlag Berlin 1959) 1 . l 7 A. Kastler J. Phys. Radium. 1950 11 255 ; R. A. Bernheim Optical Pumping (Benjamin, '' T. G. Eck Physica 1967,33 157 '' G. W. Series J. Phys. By 1970 3,84. 2o M. Sharnoff J. Chem. Phys. 1967,46,3263. 21 A. L. Kwiram Chem. Phys. Letters 1967 1,272. 22 J. Schmidt I. A. M. Hesselmann M. S. de Groot and J. H. van der Waals Chem. Phys. Letters, 23 R. C. Johnson R. E. Merrifield P. Avakian and R. B. Flippen Phys. Rev. Letters 1967,19,285. 24 L. R. Faulkner and A. J. Bard J. Amer. Chem. Soc. 1969,91,6495. 2 5 S . E. Harrison J. Chem. Phys. 1969,51,465. 26 P. N. Schatz and A. J. McCaffery Quart. Rev. 1969 23,552. (Errata in 1970 24 329.) 27 M. Born and K. Huang Dynarnical Theory of Crystal Lattices (Oxford University Press 1954), p.189. 28 A. D. Buckingham and P. J. Stephens Ann. Rev. Phys. Chem. 1966,17,399 ; A. D. Buckingham, Adv. Chem. Phys. 1967,12,107. 29 R. Serber Phys. Rev. 1932,41,489. 30 J . W. Beams Revs. Mod. Phys. 1932 4,132. 31 A. D. Buckingham Proc. Roy. SOC. A 1962,267,271. 32 M. P. Bogaard A. D. Buckingham and B. J. Orr Mol. Phys. 1967,13,533. 33 A. C. Boccara J. Ferre B. Briat M. Billardon and J. P. Badoz. J. Chem. Phys. 1969,50,2716. 34 A. 11. Buckingham W. H. Prichard and D. H. Whiffen Trans. Faraday Soc. 1967,63,1057. 35 M. G. Corfield Ph.D. Thesis (University of Bristol 1969). (Academic Press New York 1959) chap. 26. New York 1965). 1967 1,434

ISSN:0430-0696    

DOI:10.1039/SF9690300007

出版商:RSC    

年代:1969

数据来源: RSC

摘要:

Magnetic Circular Dichroism Studies of Ions in Solutions and Crystals BY P. N. SCHATZ R. B. SHIFLETT J. A. SPENCER A. J. MCCAFFERY' S. B. PIEPHO, J. R. DICKINSON AND T. E. LESTER~ Department of Chemistry University of Virginia Charlottesville, Virginia 22901 U.S.A. Received 2nd September 1969 Magnetic circular dichroism (MCD) studies are reported on several metal d1OsZ systems both in solutions at room temperature and as dopants in crystals at room temperature and liquid helium temperature. Magnetic moments of the excited T1,(3P1) state obtained from the data are con-siderably lower than theoretically expected for octahedral complexes and suggest substantial quenching of angular momentum probably at least in part through the participation of the metal p orbitals in covalent bonding.The TlU(lPl) region clearly indicates distortion from high symmetry in most cases with apparent complete quenching of the angular momentum in the excited state. In addition, high resolution MCD data are presented on CsaZrC16 Os4+ at liquid helium temperature. A vast amount of fine structure is revealed often exceeding in clarity the previously studied absorption spectrum. A preliminary analysis of the results is discussed. The utility of magnetic circular dichroism (MCD) spectroscopy in clarifying spectroscopic assignments characterizing the symmetry of transitions and extracting magnetic moments of excited states has now been widely demon~trated.~ The technique involves the measurement as a function of frequency of the difference induced in the absorption coefficients for left and right circularly polarized light by a longitudinal magnetic field.If the energy levels involved in the transition possess degeneracy they will generally be split by the magnetic field and the selection rules involving the various Zeeman sublevels will differ for left and right circularly polarized light. It is often convenient to distinguish three different effects which give rise in turn to A C and B terms. The first arises if the ground and/or excited state involved in the transition undergoes a Zeeman splitting (and hence possesses degeneracy). The absorption of left and right circularly polarized light will then occur at slightly different energies and a sigmoid dispersion curve ( A term) will result which has its zero at the zero-field absorption ma~imum.~ The C term arises if thermally accessible levels undergo a Zeeman splitting so that the absorption of left and right circularly polarized light occurs from levels with differing populations.This gives rise to a term peaking at the absorption maximum and varying inversely with absolute tem-perat~re.~ Finally the B term in general is always present and arises from field induced mixing of the unperturbed states of the system. It is temperature independent and has the same dispersion form as the C term.3 All of the systems we shall discuss have nondegenerate ground states and hence only A and B terms can occur. In this paper we present some recent applications of the MCD technique to in-organic ions of high symmetry both in solutions and crystals.We start with a room temperature solution study of a number of dlos2 systems which involve broad absorp-tion bands ; we then consider some of these same systems as dopants in alkali halide 1 SCHATZ SHIFLETT SPENCER MCCAFFERY PIEPHO DICKINSON LESTER 15 crystals at low temperature. Finally we discuss a high resolution study of the d4 system Cs,ZrCl Os4+ at liquid helium temperature where extensive fine structure is resolved in both the absorption and MCD spectra. These latter data have been obtained recently and we restrict ourselves at this time to only a few general remarks. dlos2 SYSTEMS The ultra-violet absorption spectra of dl0s2 metal complexes generally show two fairly strong absorption bands interpreted as arising from transitions of the type s2+sp.In the atomic case the configuration sp gives rise to the states 3P2 3P1 3P0, and lP1 and the only allowed transition from the ground state is lS0-+lP1. How-ever lP1 and 3P1 mix under spin-orbit coupling and the lower frequency less intense band is assigned as 1S0+3P1 (made allowed through the admixture of lP1) and the higher frequency band is assigned as lSO-+ lP1. The spectra of these systems has been known for a long time and a large number of studies have been made both in solution and in the solid state. Jarrgensen et aZ.4-6 have given discussions of the spectra with additional literature references and good summaries of the solid state work have been given by McClure and Honma.8 T2u -NO ATOM . s - 0 s-0 0; COUP LING COUPLING FIG. 1 .-Energy levels arising from the excitation s2 +sp for a metal atom in spherical and octahedral symmetry.< is the p-electron spin-orbit coupling constant and the star indicates double group. Throughout this paper term symbols without a left superscript designate double group states. Our MCD study was undertaken to measure excited state magnetic moments and to see if we could throw any additional light on the nature and symmetry of the species in solution and in the crystal. Since the transitions are presumed to be to degenerate excited states A terms are expected. Fig. 1 shows a schematic energy level diagram for the central atom in spherical and octahedral environments. The MCD of [Bu2NH2I3BiBr6 in CH3CN shown in fig. 2 is typical of the solution MCD of all of these systems except Sn(I1).Band 1 is certainly the Alg-+T1,,(3P1) (‘* triplet ”) transition and in all cases we have studied (Table 1) shows a distinct, positive A term (though in a few cases there is clear evidence of some splitting of thi 16 MAGNETIC CIRCULAR DICHROISM STUDIES OF IONS band). There is a small negative MCD at about 31,000 cm-1 which does not corres-pond to any obvious absorption band in solution but corresponds to a clear shoulder in crystalline KBr Bi3+. This is likely due to the vibronically allowed 1S0-+3P2 I -so,ooo -40,000 -30,000 -20,000 - I 0,000 - 0 30,000 40,000 50,000 frequency (cm- l) FIG. 2.-Absorptionand MCD spectrum of [Bu2NH213BiBr6 in CH3CN. [O] M is the molar ellipticity (defined as in natural optical activity in deg. dl dm-' mol-') per gauss in the direction of the light beam.E is the molar extinction coefficient. The numbering of the bands is indicated. TABLE 1 Experimental magnetic moment (AID) in Bohr magnetons for the T1,(3P1) excited state of some dlos2 complexes. A-values determined by the method of l o D-values obtained by numerical integration except as noted. T = 300 K except as noted. metal Sn(1I) Sb(II1) SbClg- SbBrz- TKI) solvent HC1 KCl(s) HC1 HBr CH3CN CH3CN HCl HBr KBr(s) KI(s)e A/Da > O b >Ob >Ob 0.60 >Ob 0.21d 0.86 0.76 0.97 0.84d metal Pb(I1) BI(II1) Bi& BiBrz- * solvent HCl HBr KI(s) KI(s)f HCl HBr KBr(s) CH3CN CH3CN A/Da 0.79 0.91 0.44 0.63d 0.95 0.97 0.68 0.76 0.74 {(c/v)dv; bD could not be reliably measured; a D = 9.1834~ =run as an alkyl ammonium salt; d D obtained by gaussian fit; e 1 2 K ; f16K.transition. Bands 2 and 3 are almost certainly associated with the Al,-+Tl,(lPl) (" singlet ") transition and the much more intense band 4 probably corresponds to a charge-transfer or intra-ligand type transition. In the 'P1 region the MCD first shows a positive peak ( N 36 000 cin-I) which possibly corresponds to a shoulde SCHATZ SHIFLETT SPENCER MCCAFFERY PIEPHO DICKINSON LESTER 17 in absorption and it then shows a negative peak (-40 000 cm-l) which is quickly overlapped by what appears to be the first half of a large positive A term associated with band 4. In contrast to all the other ions studied in solution Sn(1I) in con-centrated HCl (fig. 3) shows an apparently simple MCD throughout the triplet-singlet region.There are distinct positive A terms corresponding to Alg+T1u(3PJ N 36 000 cm-,) and Alg-+Tlu(lPl) (-46 000 cm-l) and the characteristic negative MCD peak in the region of the lS-+ 3P2 shoulder ( - 38 000 cm-l). I I I I I 30,000 4 0,000 50,000 frequency (cm-') FIG. 3.-Absorption and MCD spectrum of Sn2+ in concentrated HCI; symbols and units are as in fig. 2. If we assume initially that we are dealing with octahedral MX6 ions it is not difficult to derive for the A,,+T, transition a general group theoretical expression for the ratio of Faraday A parameter to dipole strength (AID) which in this case is simply the magnetic moment of the excited state. If one assumes that the transition is simply s2-+sp (i.e. that the tFu functions are pure metal p functions) the result is (1) where /3 is the Bohr magneton I c1 l2 and I c2 l2 are respectively the relative weights of lP and 3P1 in the T, excited state (I c c2 l2 = l) and p and p/2 in the parentheses are the spin and orbital contributions respectively of 3P1.If we con-sider the triplet region first (roughly I c2 l2 - 1 I c1 l2 -O) we would predict an A/D value approaching 1.5 P. Inspection of table 1 shows in fact that the experimental AID values are always less than one often considerably so. Quenching of the orbital angular momentum due to mixing of the metal p functions with ligand orbitals (covalency) does not seem able to account fully for the observed reduction in AID, certainly not for Sb(III) since even complete quenching (AID = I c2 I2p) gives an AID = I c1 12P+ I c2 12W+8/2) 18 MAGNETIC CIRCULAR DICHROISM STUDIES OF IONS AID value of around one Bohr magneton due to the spin angular momentum of the triplet state.MCD studies in the triplet region by Yoshikawa and Mabuchi l2 (In+), Onaka et aZ.13 (TI+ and Pb2+) and Topa et a1.I4 (Ag-) also indicate substantial reductions of the magnetic moment from the limiting value of 1.5 p. The singlet region is much more complex and only with Sn2+/HC1 is a simple result obtained. There a clear A term is observed and AID is found to be -0.33 p. Since I cl l2 - I and I c2 l2 -0 a reduction in orbital angular momentum by about a factor of 3 is indicated. This could be due to covalency effects lowering of symmetry or a combination of both. In none of the other systems studied in solution was there any clear indication of an A term in the singlet region.frequency (cm- l) FIG. 4.-ROOm temperature absorption and MCD spectrum of Sn2+ doped into crystalline KCl; symbols and units are as in fig. 2. The concentration of dopant is not known but a nominal value is obtained by comparing the crystal absorption spectrum at room temperature with that of a cor-responding solution spectrum of known concentration. In order to define these systems more clearly and to permit low temperature work, we have extended our studies to alkali halide crystals containing the d10s2 ions as dopants. Fig. 4-6 show some typical results. We note from table 1 that the AID values for the triplet region in the crystals agree reasonably well with the corres-ponding ions in solution suggesting that no drastic lowering of the symmetry occurs in solution.This is of interest because there is not complete agreement that MX6 is always the limiting species in solution though we have always chosen our conditions to favour this. On the other hand it is generally assumed that the metal ion is octahedrally coordinated in the alkali halide host crystal. KC1 Sn2+ (fig. 4) shows the typical complex singlet region. There are three distinct bands (starting with the shoulde SCHATZ SHIFLETT SPENCER MCCAFFERY PIEPHO DICKINSON LESTER 19 35.000 40,000 45,000 frequency (cm-') FIG. 5.-12 K absorption and MCD spectrum of T1+ doped into crystalline KI; symbols and units are as in fig. 2. Dopant concentration determined as in fig. 4 caption.30 2 20 SL 10 0 -10 -20 -30 W 60000 40000 20000 0 2 5. 30.630 35,000 4 0 b O frequency (cm-') FIG. 6.-.16 K (solid line) and room temperature (dashed line) absorption and MCD spectrum of PbZ+ doped into crystalline KI; symbols and units are as in fig. 2. Dopant concentration determined as in fig. 4 caption 20 MAGNETIC CIRCULAR DICHROISM STUDIES OF IONS at -41 700 cm-l) and a correspondingly complex MCD. A great reduction in symmetry of the ion has occurred possibly through a Jahn-Teller effect in the excited state. Unfortunately we have not yet succeeded in making MCD measurements on this system at low temperature. The TI(I)/KI MCD spectrum (fig. 5) in the singlet region appears relatively more simple with no indication of any A term character though there may be significant overlap from the so-called 2240A band which is thought to be due to an intra-ligand (iodide) transition.’ The Pb(II)/KI spectrum (fig.6) in the singlet region is especially complex particularly at low temperature. We have as yet made no attempt at a detailed analysis of these crystal results. In any case with the possible exception of Sn2+/HC1 it seems clear for all of the systems studied both in solution and crystal that there is substantial distortion from octahedral symmetry in the TJP1) excited state apparently with complete quenching of the orbital angular moment urn. I I -__ 1- 1 . I I I I I I I I I I I I I I I I I I I I I I ;,’ I,,,’ I frequency (cm-’) FIG.7.-The liquid helium (solid line) and room temperature (dashed line) absorption and MCD spectrum of crystalline Cs2ZrCls Os4+ over the region 19-24 000 cm-l; symbols and units are as in fig. 2. It has been suggested that some of the ions studied here may be tetrahedrally coordinated. Our MCD studies do not distinguish in any simple way between octa-hedral and tetrahedral species since one finds that eqn. (1) applies in both cases. CszZrCls Os4+ High resolution MCD measurements on crystals at low temperatures has great potential because of the possibility of studying individual vibronic lines rather than the broad bands generally present in solution spectra at room temperature. We have recently succeeded in interfacing a Spex 1400-11 2 m double monochromator with our Durrum-Jasco CD spectrophotometer.One of the first systems we studied was Cs2ZrCl Os4+ at liquid helium temperature and we were able to obtain excellen SCHATZ SHIFLETT SPENCER MCCAFFERY PIEPHO DICKINSON LESTER 21 quality high resolution MCD data. Most of our results all from one small dilute crystal are summarized in fig. 7-1 1 the absorption spectra having been recorded on a Cary 14. f I I ! I I -13000 - ROO0 - 6000 -;OW -2WO 24,000 2 5,000 2 6,000 2 7,000 frequency (cm-') FIG. 8.-The liquid helium (solid line) and room temperature (dashed line) absorption and MCD spectrum of crystalline Cs2ZrC16 Os4f over the region 24-27 000 cm-' ; symbols and units are as in fig. 2. The labelled points are discussed in the text. In regions containing sharp features we were able to make MCD measurements at spectral slit widths in the approximate range 2-6cm-l.Except in the 19000-22 000 crn-I region where the MCD is very small we had an excellent signal-to-noise ratio and were able to obtain clear reproducible data. (We have obtained a nominal value for the product of concentration and path length (cl) in the crystal by setting ( N 26 000 cm-') for the crystal at room temperature equal to 9 600 the value observed 24 in [Bu4V20sCl6/C2H4Cl2 at room temperature. In quantitative con-siderations ratios are always considered which are independent of cl.) The absorption spectrum of Os4+ in single cubic crystals Of K2PtCl6 and Cs2ZrC16 at 4.2 K have been studied in detail by Dorain Patterson and Jordan (hereafter abbreviated DPJ).They found that many of the transitions are characterized by series of very narrow absorption lines. All the bands below 35 OOO cm-l were interpreted as d-+d transitions on the basis of a crystal field calculation. Previously, those transitions occurring in solution between 23 900 and 30 000 cm-I had been assigned to charge transfer transitions since their extinction coefficients in solution range up to 9 000. Intensity calculations were made for the vibronically allowed d+d transitions and it was determined that the more intense transitions would be those from the A1J3T1,) ground state to the Tlg or T2g excited states with one quantum of the vq(tlu) v3(tlu) v7(tlu) or Vg(t2u) odd vibrational modes excited. Experimentally 22 MAGNETIC CIRCULAR DICHROISM STUDIES OF IONS vibronic transitions of this sort are found to occur as progressions in the totally symmetric vibration superimposed on one quantum of an odd vibration.The authors found that an excellent fit of experimental energies with those from their crystal field calculation resulted if it was asszrrned that transitions to T2 +~4(t,,) +n v,(a,,) excited states were much the most intense transitions. l 2 frequency (cm-') FIG. 9.-The liquid helium (solid line) and room temperature (dashed line) absorption and MCD spectrum of crystalline CszZrCls Os4+ over the region 27-28 500 cm-' ; symbols and units are as in fig. 2. Our absorption spectra agree well with DPJ and though the crystal is rather dilute so that we do not observe many of the weaker lines observed by them we can clearly make some preliminary tests of their detailed assignments.Os4+ has a 5d4 configuration. In a strong cubic electrostatic field the ground state is the 3T1g state of the tiQ configuration. But since the spin-orbit interaction is large (lsd-2 000-3 000 cm-l) the 3T1g state will be split into states of Alg TI, Es and T2g symmetry with the A state lying lowest by several thousand cm-l. Thus the MCD should consist of A and B terms only. When the ground state is nondegenerate the experimental parameter AID can be calculated as the product of a group theoretic factor and the magnetic moment of the excited state. If the transition is vibronically allowed and standard vibronic theory1** l9 is used AID will depend on the symmetry of the excited state (Tlg or T,,) the symmetry of the odd vibration (tl or t2,) and the sign and magnitude of the excited state magnetic moment.For example for an A1g-+T2g transition made allowed by vibronic coupling to a v4(tlu) v3(tlU) or v7(tlu) vibration (2) AID = *(T2,1 I IL? I T2,l) SCHATZ SHIFLETT SPENCER MCCAFFERY PICPIIO DICKINSON LESTER 23 10 2: 22 -2 c -2 -6 6 030 450C 3 On@ 1mo C frequency (cm-l) FIG. 10.-The liquid helium (solid line) and room temperatwe (dashed line) absorption and MCD spectrum of crystalline CsrZrC16 Os4+ over the region 28 500-32 000 cm-l ; symbols and units are as in fig. 2. The labelled points are discussed in the text. I I I I I I I I I frequency (cm-') FIG. 11. The liquid helium (solid line) and room temperature (dashed line) absorption and MCD spectrum of crystalline CszZrC16 Os4+ over the region 32-34 OOO cm-l; symbols and units are as in fig.2 24 MAGNETIC CIRCULAR DICHROISM STUDIES OF IONS where p = - p(Lz + 2Sz) while with a v,(t2,) vibration AID = -3(Tzg1 I p:B I T*gl). (3) The notation and symbols are those of Griffith.zO As an initial step in interpreting our MCD data we made a complete crystal field calculation using the DPJ parameters and with the eigen-vectors obtained calculated the entire magnetic moment matrix. All matrix elements for the spin-orbit and crystal field interactions were machine computed in the weak field basis using input from the d4 matrices of Nielson and Koster.21 The magnetic moment matrix was also calculated by computer using standard formulasz2 and orbital reduction factors of one.Agreement with the eigenvalues published 1 5 3 l7 was to within 4 1 cm-l in all cases and the g factors published15 were reproduced with a maximum deviation Using our magnetic moments we are thus able to calculate the AID value pre-dicted by the DPJ assignment for each line. Our MCD data indicate a number of difficulties with their assignments. Even with our relatively dilute crystal there are two spectral regions in which we clearly observe an MCD though the corresponding absorption spectrum is almost undetectable. The first such region (19-22 000 cm-l) shows only some broad weak MCD but the second (23-23 900 cm-l) exhibits a totally symmetric progression in at least six distinct features. We hope to be able to identify the absorption bands to which this MCD corresponds when we run more concentrated crystals.It is possible that these bands correspond to transitions between states primarily of different spin multiplicity. The MCD of such transitions is frequently an order of magnitude more intense than the optical spectrum of the corresponding absorption^.^ MCD in this spectral region might indicate that the 5Eg state lies considerably higher in energy than predicted by DPJ who assign none of their transitions to the 23 000 cm-' region. With the dilute crystal used for our studies no MCD or absorption spectra were observed for the band at 17 000 cm-l. We hope to examine the spectral region below 25 000 cm-l with a more concentrated crystal. The first intense absorption band (26-27 000 cm-l) shows a clear progression of negative A terms in MCD.Using the DPJ parameters and assignment (Alg-+TZg (3Tzg)) the observed sign of the A terms is obtained only if V g ( t 2 ) is chosen as the intensity-producing vibration. This clearly contradicts the DPJ assumption that the most intense vibronic lines arise from transitions to T2g + ~ 4 ( t 1 ) +nvl(alg) excited states. The second intense band (28 500-32 000 cm-l) assigned by DPJ as Alg+T2g(3A2g) seems from the MCD to consist of two different transitions (fig. lo) although in the absorption spectrum the dominant peaks appear to be members of progressions in the totally symmetric vibration which proceed through the entire band. The overall shape of the band is however asymmetrical. The two transitions might correspond to the peak and shoulder observed at 29 200 and 29 900 cm-' respectively in the solu-tion spectrum of (NH4)20sC16.z4 The MCD in fig.10 shows a totally symmetric progression in a complex unit containing five distinct features for what appears to be the first transition of the band. Lines A and B give negative B terms lines C and C' negative A terms while line D gives a positive B term. These features are repeated several times but at line L seem to disappear. The spectrum of the second transition of this absorption band is less clear but the MCD appears to be dominated by a positive A term together with a positive B term for each of the lines P R S and T. The DPJ assignments are consistent with our MCD for lines C G J and L but we do not observe any angular momentum for lines A E and I lines B and F or lines D, H and K which by the DPJ assignment can show A terms.of 50.01 p SCHATZ SHIFLETT SPENCER MCCAFFERY PIEPHO DICKINSON LESTER contradicts. For example they have assigned lines G and K (fig. 8) as 25 There are also other details in the exhaustive DPJ assignments which our MCD while our MCD gives a positive A term for each of these lines and thus demonstrates that the excited state must be degenerate. Certainly DPJ's most interesting assertion is that all transitions in the Os4+ spectrum below 35 000 cm-1 are ligand field (d-+d) transitions. We have measured the dipole strength of the two intense absorption bands-at room temperature by a gaussian fit (which was quite convincing) and at liquid helium temperature by numeri-cal integration.The room temperature dipole strength of the first band (-25-27 000 cm-I) is about twice the liquid helium temperature value while for the second (-28 500-32 000 cm-l) it is about three times that at liquid helium temperature. Simple theory1 * predicts a temperature dependence for oscillator strengths of approxi-mately this magnitude for vibronically allowed transitions and thus the intensity ratios support the DPJ contention. It is also possible that these transitions are partly or entirely parity forbidden charge-transfer bands. Such transitions would be expected to have greater oscillator strengths than d-+d transitions. It is clear from this preliminary discussion that the high resolution MCD spectrum contributes a great deal of additional information regarding the interpretation of the absorption spectrum.It is relatively easy to check suggested assignments and in spectra of the complexity observed for Cs,ZrCl Os4+ it is not surprising that contra-dictions are found in assignments based on the absorption spectrum alone. A more challenging task is to propose detailed assignments which fit both sets of data. In a forthcoming paper we argue that much of our observed spectrum arises from charge-transfer transitions contrary to the DPJ interpretation. We are greatly indebted to Prof. Paul B. Dorain for supplying the Cs,ZrCI Os4+ crystal used in this work. We thank Mr. R. L. Mowery for much help with the data processing. One of us (S. B. P.) acknowledges support under an N.D.E.A.fellowship. This work was supported by a grant from the National Science Foundation. permanent address Department of Chemistry University of Sussex Brighton Sussex. permanent address British Petroleum Co. Ltd. Sunbury Research Laboratories Sunbury, Surrey. For a review of the theory and recent experimental work in this field see P. N. Schatz and A. J. McCaffery Quart. Rev. 1969 552; A. J. McCafTery P. N. Schatz and T. E. Lester J. Chem. Phys. 1969 50 379 and references therein. C. K. Jerrgensen Absorption Spectra and Chemical Bonding in Complexes (Pergamon Press, Oxford 1962); chap. 10. C. K. Jlzrrgensen Halogen and the Noble Gas Complexes in Halogen Chemistry Vol. 1 ed. V. Gutmann (Academic Press New York 1967). R. A. Walton R. W. Matthews and C .K. Jerrgensen Inorg. Chim. Acta 1967 1 355. York 1959) p. 162. C. H. Henry S. E. Schnatterly and C. P. Slichter Phys. Rev. 1965 137 A583. ' D. S. McClure Electronic Spetrca of A/iolecules and Ions in Crystals (Academic Press New * A. Honma Sci. Light 1967 16 229. l o P. J. Stephens Chem. Phys. Letters 1968 2 241. I ' See for example Schatz et al. J. Chem. Phys. 1966 45 722. l 2 A. Yoshikawa and T. Mabuchi J. Phys. SOC. Japan 1968,24 1405. l3 R. Onaka T. Mabuchi and A. Yoshikawa J. Phys. SOC. Japan 1967,23 1036. l4 V. Topa L. Taurel J. C. Rivoal and B. Briat Phys. Stat. Sol. 1969 33 K17. l 5 P. B. Dorain H. H. Patterson and P. C. Jordan J. Chem. Phys. 1968 49 3845. l6 C. K. Jerrgensen Disc. Faraday Soc. 1958 26 175; C. K. Jnrrgensen Mol. Phys. 1959,2,309 ; C. K. Jrargensen 2. Naturforsc. 1967 22 945; R. B. Johannesen and G. A. Candela Inorg. Chem. 1963 2 67 26 MAGNETIC CTRCULAR DTCMROTSM STUDIES OF IONS P. C. Jordan H. H. Patterson and P. B. Dorain J. Chem. Phys. 1968,49 3858. C. J. Ballhausen Introduction to Ligand Field Theory (McGraw-Hill Book Co. Inc. New York 1962) chap. 8. l9 P. J. Stephens J. Chem. Phys. 1966,44,4060. *O J. S. Griffith The Theory of Transition-Metal Ions (Cambridge University Press CAmbridge, England 136J) table A16. C. W. Nielson and G. W. Koster Spectroscopic Coefficients for thepn dn and f n Configurations (M. I. T. Press Cambridge Mass. 1963). 22 ref. (20) chap. 5. 23 A. J. McCaffery P. J. Stephens and P. N. Schatz Inorg. Chem. 1967 6 1614. 24 G. N. Henning Dim (University of Virginia June 1968) p. 84-86

ISSN:0430-0696    

DOI:10.1039/SF9690300014

出版商:RSC    

年代:1969

数据来源: RSC

摘要:

Measurement and Interpretation of Magnetic Circular Dichroism and Magnetic Linear Dichroism Spectra BY J. BADOZ M. BILLARDON A. C. BOCCARA AND B . BRIAT Laboratoire d'Optique Physique,* E.P.C.T. 10 r w Vauquelin Paris V" Received 9th October 1969 This paper deals with three problems of ;t different nature. We first discus the conditions required to obtain reliable and usable magneto-optical and absorption data at any temperature down to that of liquid helium; this discussion includes the description of our cryostat and absorption device as well as a critical study of the optimization of the signal-to-noise ratio of the instruments. Secondly a new fitting procedure is proposed for the analysis of absorption and magnetic circular dichroism curves and the extraction of useful parameters.Finally the significance of these para-meters is examined for anisotropic centres isolated in an isotropic matrix ; the results thus obtained are illustrated by means of magneto-optical experiments on CaF2 +Nd3+ crystals. Magneto-optical studies have largely benefited from instrumental progress in the measurement of natural optical activity. Most results have been obtained through the measurement of magnetic circular dichroism (MCD) and to a less extent magneto-optical rotatory dispersion (MORD). More recently magnetic linear dichroism studies (MLD Voigt or Cotton-Mouton effect) have been demonstrated to be of interest for the investigation of some particular spectroscopic problems. All these techniques lead to the experimental determination of parameters which in one way or another are related to molecular data (mainly magnetic moments) to be checked against theoretical predictions.In this paper we make comments on three different topics. (i) The experimentalist has two major aims when performing MCD and MLD work. First he requires measurable or even eventually large signals ; this problem can be dealt with by considering the signal-to-noise ratio as a parameter to be optim-ized. Secondly he often needs well-resolved magneto-optical and absorption data at many temperatures down to that of liquid helium ; we here investigate the difficulties encountered with particular emphasis on the study of very small (1 mm2 for example) crystals at low temperatures. (ii) A new fitting procedure is proposed for the analysis of absorption and MCD curves without any assumption about the number of actual components within a given band.(iii) Finally the significance of the extracted para-meters is discussed in the particular case of anisotropic centres isolated in an isotropic matrix. Two pertinent examples are chosen among recent MCD and MLD experi-ment s. INSTRUMENTATION GENERAL CONSIDERATION Circular dichroism measurements have now become well established.2* We use a photoelastic modulator in our instrument rather than a Pockels cell. This type of a modulator requires a very low driving voltage and possesses a wide aperture thus * Equip de recherche no. 5 du C.N.R.S. 2 optimizing the signal-to-noise ratio. Secondly the use of a null-method increases the stability of our instrument thus allowing dichroic optical densities AD- to be detected using a 6 A spectral width and a time constant of a few Throughout this paper MCD is referred to as the difference in optical densities D+ - D- for B+ and cr- components.Linear dichroism (e.g. stress- or magnetically-induced) is defined as the difference D,- D in optical densities for two mutually perpendicular linear polarizations (see fig. 1). Its measurement can be performed by 5 6' 6' FIG. 1 .-Schematic diagram showing circular and linear polarization. c c coupling a circular dichrometer with a quarter-wave plate (e.g. a Fresnel rhomb 6). The two circularly-polarized components are thus alternatively transformed into x and y linearly polarized components. The compensating system still works properly and performances for MCD and linear dichroism (ADID ratio) are the same.Magnetic linear dichroism is now induced by orienting the magnetic field perpendicular to the direction of propagation of the light beam (along one of the x or y axes). We now comment on two problems which the instrumentalist has to deal with, viz. to optimize the signal to noise (s/n) ratio and to obtain good resolution. OPTIMIZATION OF s / n The signal-to-noise ratio of a dichrometer is proportional to the square root (40)* of the luminous flux going through it. 4o is dependent upon three factors the effective brightness of the system comprising the light source and some suitable optical device ; the light gathering power 5 of the apparatus ; and the spectral width of the monochromator.In this section emphasis is on the second factor only the last being discussed later. The light-gathering power is often limited by the size of the sample S and that of the polarizer P placed next to the modulator (see fig. 2a). If no lens is used between P and S { is (1) 5 = ApAs/L2 = A,Q, D M M 1 -4 L - 1 , FIG. 2.-Illustration for calculation of the light gathering power (see text for description) J . BADOZ M . BILLARDON A . C . BOCCARA AND B . BRIAT 29 where A and As are the cross section area of P and S respectively and L is the distance between them. The solid angle Cl should be as large as possible and this requirement emphasizes the advantage of our photoelastic modulator with respect to the electro-optic one. Now in work on crystals very often their size is quite small (e.g.1 mm2, and in such cases we use a lens of section AL in front of the sample. 5 then becomes (2) (fig. 2b) c' = ApAL/L2 = ALAs/12. Since I is chosen to be much smaller than L the lens improves considerably the s/n ratio. However much care should be taken in the experimental work since a poor quality lens or improper orientation of the lens might result in drastic changes in the MCD curve. In practice in all cases a zero line is obtained in the absence of the magnetic field. We sometimes use an alternative technique for the study of micro-crystalline samples. The powder is immersed in an inactive liquid of which the refractive index is very accurately adjusted. This method has been checked by comparing the MCD and absorption spectra of a massive sample (Nd3+ doped glass) and of the same sample broken into very small pieces which were then immersed.The same results were obtained although the noise level was slightly higher in the latter experiment. It follows from eqn (1) that L has to be reduced as much as possible in order to increase the light gathering power. Magnetic field generators should thus be as small as possible and hence the use of an electromagnet is discarded. Superconducting magnets have now become available and these induce a high magnetic field (at least 50 kG) over a reasonably short path length. (Such equipment should undoubtedly be used when very small signals are to be detected or when a small amount of material is available (bio-compounds or organics).The same problem arises when the sample is rather insoluble or contains only few impurity centres (crystals) and must thus be studied using a longer path length in order to increase the signal. 3 1 i\ CsCl - Pb2+ 2 I FIG. 3.-Absorption (- - - -) and MCD (---) of Pb2+ in CsCl. Such an example is shown in fig. 3 for Pb2++ CsCI. The impurity did not enter the matrix very well and we obtained small optical densities even over a 2 cm path. Here it proved necessary to use a high field (50 kG) provided by means of an Oxford instrument superconducting magnet. However we have made important progres 30 MCD AN11 MLD SPECTRA in the design of permanent magnets. In our laboratory room temperature routine work is conducted with a 8 cm long magnet which provides fields up to 8 kG in a 5 mm air gap.CRYOGENIC AND ABSORPTION DEVICES MCD work particularly on inorganic compounds must include low temperature measurements whenever possible and MCD signals are generally increased when the temperature is decreased due to either the paramagnetism of the sample (C terms 9), or a narrowing of the bands and the observation of A terms ( e g rare earths). Actually we have found that magnetic fields of a few hundreds or of a few thousands gauss were sufficient to conduct most of our experiments. Further the orientation of crystals in the light beam proved to be very critical in many instances. Thus rather than use a classical optical cryostat or the low-temperature aperture in our super-conducting magnet we designed our own cryostat with a number of major aims a short path length in order to be placed and oriented between the pole pieces of a permanent magnet ; the possibility of quickly cooling the sample down to any temp-erature without perturbing the light beam ; low cost and ease of use.Our cryostat s He FIG. 4.-Schematic diagram of the cryostat used in our experiments is shown in fig. 4. Cold nitrogen or helium gas is pumped up through I and evacuated through E by means of a vacuum pump. A thermocouple t allows the temperature of the sample to be measured. This temperature can be varied easily by adjusting the flow of gas. A double jacket is used to reduce the thermal loss. Silica windows W allow the light to pass through the sample ; they are sealed on copper sheets and do not contribute any stray birefringence.This device is only 2 cm length alon J . BADOZ M . BILLARUON A . C . BOCCARA A N D B . BRIAT 31 the light path and can be easily oriented between the pole pieces of the permanent magnet. The sample itself is further oriented with respect to the light beam by means of its holder S. The measured temperature is the actual one since the sample is placed in the cryogenic fluid. The operation is very easy since one has merely to plunge the T cylinder into the dewar and then pump. After 5-10 min temperatures down to 5 K can be obtained. Moreover any chosen temperature can be stabilized within kO.1 K by means of a flow-valve monitored by the thermometer. For low-temperature work we thought it was important to run MCD and absorption spectra under the very same experimental conditions such as position of the sample temp-erature and spectral width.The best way to achieve this is to record both spectra simultaneously on the same instrument. The device shown in fig. 5 has thus been FIG. 5.-Schematic diagram of our absorption attachment. added to our dichrometer. Two semi-reflecting plates L and L are used before and after the sample in order to direct part of the light on to two photomultipliers. Their readings are then equilibrated by means of a servo-mechanism monitoring a wedge W linear in optical density. The displacement of the wedge is thus recorded as a function of the wavelength. We have not yet considered the role of the slit width of the monochromator and t iiiis the resolution of the instrument.Actually except for lanthanides actinides (i.e. UClg- or UOZ-) and certain spin forbidden lines (ruby) the absorption com-ponents of most compounds are reasonably wide at room temperature and MCD and absorption spectra can be run on any commercial instrument. Low-temperature work on the other hand usually requires a much increased resolution. With a given ~iionochromator one has to reduce the slit width in order to gain resolution thus alter-ing the signal-to-noise-ratio. Using a very luminous grating monochromator (1 200 groves/mm (102 x 102) we have succeeded in detecting small signals optical density unit) with a 1 A spectral width. This resolution is insufficient in some cases and further improvement would be much appreciated. Even so reasonably accurate parameters can be extracted by means of a deconvolution of the experimental curves.ABSORPTION A N D MCD CURVES ANALYSIS ?‘HE PROBLEM We have now dealt with a number of devices which contribute to obtaining accur-ate and reliable experimental data. Unfortunately these data cannot as a general rule be used as such since significant parameters have first to be extracted from the curves before any detailed assignment can be attempted. We now comment on fitting procedures; it will be assumed throughout that the resolution of the apparatus is adequate but that absorption and MCD components are not fully resolved in the spectra due to the broadening of lines via any mechanism 32 MCD AND MLD SPECTRA An absorption curve is then considered as the sum of many unresolved compon-ents and one can write for the optical density* where Dm andf,(v) are the maximum optical density and shape function for the isolated ith component.Similarly one has (4) for the magnetic dichroic optical density (standardized to 1 G). As has been shown recently,l a description of an MLD curve requires generally a linear combination of the absorption curve and its first and second derivatives. The treatment given for both MCD and MLD assumes the Zeeman separations to be much smaller than both the line-width and kT as well as a rigid shift of the Zeeman lines. One way of fitting the curves is to assume a reasonable number of components as well as some standard (gaussian or lorentzian) line shape. A computer programme then allows the Dml ai bl and ci parameters to be determined.In simple cases, mere consideration of the shape of the curves and magnitude of the peaks can provide a reasonably accurate answer to be checked against rather crude theoretical calcula-tions. It has recently been argued lo that the method of moments can be profitably used in many instances. This is true as long as the actual number of components does not need to be known; on the other hand however it does not require any assumption about the rigid or non-rigid shift of the individual Zeeman lines. which does not require any assumption about the number of components and which gives good results when the accuracy of the experimental data is sufficient. We have developed a different graphical method ANALYSIS OF ABSORPTION CURVES For simplicity only Gaussian components are considered although the method can be generalized to say Lorentzian components.A Gaussian curve can be repre-sented by Y = Di = Dmi exp { -[(v-vr)/6,12>, where vi and ai are the central frequency and half-width at l/e of the maximum of the ith line. Now it is easily shown that a plot of Y’ dDJdv 2 - -- { - -+v-vi) Y Di ai as a function of the frequency is a straight line ; the abscissa for Y’/ Y = 0 gives vi and the slope provides the band width. The method applies as well when curves are recorded in terms of wave length. The procedure is only slightly more complicated when the absorption curve contains many unresolved components. A plot of Y’/ Y = f(1) shows those fre-quency ranges where one of the ith lines gives a predominant contribution.This is illustrated for a theoretical example in fig. 6a where Do is the computed sum of two gaussian components of different intensities that are widely separated. Y’/ Y = f(A) is linear on the left-hand side of the figure and R1 = 3 OOOA and 6 = 160A are * D is used here for absorbance (A) since this last parameter (A) is widely used in the description of MCD data J . BADOZ M . BILLARDON A . C . BOCCARA A N D B . BRIAT 33 extracted for the major component which turns out to be identical to D1. The difference Do- D1 gives a curve 0; which is then analyzed in the same way. Only one component is obtained and its characteristic parameters are the same as those of 0 2 -I- a A FIG. 6.-Analysis of absorption nd MCD curves (see text for comments).We applied this method to a number of practical cases.ll* l2 We thus showed that the first excited state of d10s2 ions (3P1 in cubic symmetry) to have its degeneracy completely lifted and to give rise to three b terms. We also obtained interesting results for tetrahedral complexes of the 3d transitions metals. An example is illus-trated below. ANALYSIS OF MCD CURVES The procedure for MCD curves can be derived from eqn (4). We assume first a single absorption band and obtain Thus a plot of [AD]JDi against v (or A) is again a straight line. The ordinate for v = i t i gives (b,+c,/kT) and the slope provides the a parameter. This is illustrated in fig. 66. When absorption and MCD curves are both complex the absorption curve analysis is first carried out and the data thus obtained allow accurate MCD parameters to be extracted from the analysis of the experimental curve.The example of the CuC1:- ion is worked out here to illustrate (fig. 7-8) this procedure. The experimental absorption curve is shown in fig. 7a. Its analysis over the 395-415 nm and 276-295 nm regions shows that there are two gaussian curves located at 410 nm (I) and 294 nm (11) respectively (fig. 7b). These components are in turn subtracted from the experimental curve and one finds two residues R I and R 11. The analysis of R I does not show any linear relationship between Y'/ Y and v whatever the assump-tion on the shape of the component is ; this might indicate that a number of forbidden lines are indeed present under the long red tail of R I.On the other hand the analysis of R I1 clearly demonstrates the presence of two gaussian components at 347 and 318 nm respectively. With this information the MCD curve (fig. 8a) analysis can be S3-34 a 300 400 50 0 b FIG. 7.-Analysis of the absorption curve of CuC1:- (a) experimental curve ; (b) resolved components. FIG. 8 .-Analysis 2 0 - -2 Q s 2 2 0 - 2 -4 of the MCD curve of CuCIi-; (a) experimental curve ; (b) resolved a components J . BADOZ M . BILLARDON A . C . BOCCARA A N D B . BRlAT 35 carried out easily. The results obtained are shown in fig. 8b. Bands I and I1 both give a and (b + c/kT) terms while band I1 only shows a (b + c/kT) term and band IV does not contribute appreciably. The parameters obtained are given e1sewhere.l SIGNIFICANCE OF THE PARAMETERS ISOTROPIC OR ORIENTED MOLECULES We have not commented so far on the exact significance of the parameters a b and c obtained through the analysis of MCD curves.For an isotropic or oriented molecule in a longitudinal magnetic field H along the +z axis it is easy to relate a b and c to the &’ 98 %? and 9 parameters * proposed by Buckingham and Stephens.’ For any line-shape function one has where (5) Here Yg and Ye stand for the spectroscopic splitting factors of the ground and excited states respectively and /? is the Bohr magneton. If the ground state is doubly degener-ate one has for example I Yg ] = 2 I { + ] Lz+2Sz 1 +)I if ] +) is one of the doublet’s sublevels. Both procedures lead to the same physical quantity viz. the diagonal and off-diagonal matrix elements of the magnetic dipole moment operator.However, there are some differences in their use. On the one hand d/9 and %/9 can theo-retically be expressed from group theoretical arguments in terms of reduced matrix elements of the magnetic dipole moment operator. These in turn can be calculated in particular situations once approximate zero magnetic field wave functions have been evaluated. This mathematical approach is specially fruitful if associated with Griffith‘s irreducible tensor method l4 for the evaluation of matrix elements. It has now been widely used for fully allowed charge transfer bands of octahedral and tetra-hedral l2 complexes of the transition metals. On the other hand eqn (4) and ( 5 ) are derived assuming a separation =Yg/IH and 9’$H between the extreme components of the ground and excited states respectively, and expressing the difference in absorption between components for + and - circu-larly polarized lights.For an absorption transition atO one has for example,? AD-N+f(v,v+) I ( a I m+ I 0) I -N-f(v,v-) I ( a I m- I 0) 1 where m+ = nix. im is the electric dipole moment operator for circularly polarized light ; f(v,v+) andf(v,v-) are the shape functions for the individual lines ; N+ and N-the respective populations of the ground-state sublevels from which the o+ and o-lines start. This practice provides a more phenomenological approach to some MCD prob-lems and avoids theoretical complications when they are unnecessary. It is simple when one considers singlet or doublet states but requires more care when dealing with triplet or quartet states.The method has been widely used for rare-earths lS when * Cursive letters are used here in agreement with ref. (10) in order to differentiate between the optical density D and the dipole strength 9. t This expression leads to the now widely accepted sign convention for [AD] namely that say, [.AD] (or the molar ellipticity [el,) is positive within the 420 nm absorption band of potassium fcrricyanide and the Verdet constant of water is negative within the visible spectral range 36 MCD A N D MLD SPECTRA only singlet and doublet states are encountered. The procedure can be summarized as follows. Eqn (4) and ( 5 ) are general providing a sign convention is used 9 is positive when the a+ line starts from the lowest sub-level of the ground state and Ye is positive if the same polarization is allowed to the highest sub-level of the excited state.The experiment thus provides the parameters together with their signs. Using this sign convention it is then possible to derive the levels and the polarizations of the transitions between the various sub-levels of the ground and excited states. The selection rules are known from the literature and it is thus easy to assign to each sub-level its representation in the proper group to which the molecule or ion under con-sideration belongs. ANISOTROPIC CENTRES So far our remarks have been relevant to isotropic or properly oriented systems. Examples of this kind are found with octahedral or tetrahedral inorganic ions in solution or in crystals and divalent rare-earth ions in fluorite type crystals (iso-tropic t) or in tripositive rare-earth ions in oriented uniaxial calcium tungstate.A case frequently encountered however is that of an anisotropic centre in an isotropic (or eventually anisotropic) matrix. This is the rule when the impurity ion replaces ions with different valences such as tripositive rare-earth ions in fluorite. In that case the local symmetry of the centre is determined first by its structure i.e. by the positions of the nearest ions i.e. among those ions which compensate the excess charge (e.g. 02- F- in CaF,). In cubic crystals it is now agreed that impurity centres are statistically uniformly oriented along one or another of the equivalent directions which are symmetry axes of the lattice (3C4 4C3 or 6C2) so that the crystal preserves its optical isotropy.The microscopic anisotropy of the individual centres is hidden due to the ensemble averaging. In conclusion we comment on the suitability of magneto-optics for detecting hidden anisotropic centres. Emphasis will be on the problem of an anisotropic centre in an isotropic matrix although we have also developed some methods for anisotropic centres randomly oriented in a glassy matrix. Two examples will be chosen among recent MCD and MLD experiments. First we consider the absorp-tion and MCD of a tetragonal centre in a cubic matrix. The question arises whether the absorption and MCD spectra will be changed when the common direction of the magnetic field and Poynting vector are along e.g.a C or C3 axis of the cube. We further assume that only a polarized transitions are allowed and that the perpendicular splitting factors of the various states is zero. We first suppose the light wave to be propagated along a C4 axis ; it will encounter three distinct successive centres oriented along the three four-fold axes. Only one centre of three will have its axis properly oriented with regards to H and thus give rise to MCD ; on the other hand however, absorption corresponds to that of two centres having their axes in a plane perpendicu-lar to the electric field vector. The [AD]/& ratio will thus be 3 that which it would have if the three centres were all oriented with their axis parallel to [IOO]. Suppose now the light propagates along a three-fold axis and H is also parallel to [ 1 I I].Fol-lowing Merle d'Aubigne and Duval,16 a+ light propagating along [11 I] is split up as aa +pa- +p along the three four-fold axes. It can then be shown that a absorption for each centre is a2 + p 2 = 3 of that which it would be if the centres were all aligned along the axis of propagation (the " uniaxial " case). On the other hand the MCD is reduced by 3. Altogether considering the three centres the [AD]/Dm ratio is again 3 of that which would be in the uniaxial case. t except for I's quadruplets J . BADOZ M . B I L L A R D O N A . C . BOCCARA AND B . B R I A T 37 The above arguments thus lead 11s to the following important conclusion u tetragonal centre in a cubic matrix behaves as if it were isotropic when absorption or MCD experiments are performed ; absorption and MCD are merely 3 and 3 respec-tively of that which they would be in the uniaxial case.Our conclusion is essentially unchanged (i.e. same reduction factors) when one considers the MCD of a trigonal centre or the MORD of either tetragonal or trigonal centres. We confirmed the above ideas experimentally for the Nd3+ ion in CaF,. We had two crystals each containing mainly tetragonal (pink) and trigonal (blue) centres. H was oriented along [loo] and then [l 1 I] for each specimen. Within 5 % (experi-mental errors) we found no difference among the results for absorption and MCD or MORD for the two types of centres when the field was oriented along one of the particular axes. These results are in some respects disappointing since it would have been interesting to be able to differentiate between different types of anisotropic centres through magneto-optical experiments.These results are also in serious disagreement with previous predictions l7 for anisotropic centres. Actually stress-induced linear dichroism measurements or Cotton-Mouton experiments are likely to provide a more valuable approach to the problem.l* In conclusion we report on this latter point in connection with some recent experiments performed in our laboratory. It will be shown that the temperature-independent MLD is anisotropic for impurity centres possessing tetragonal or trigonal local symmetry. We assume throughout a transition between two doublets and lines allowed with cr light only.Now the spin Hamiltonian for each doublet can be written as t*-;) where 911 and 9,- are the parallel and perpendicular spectroscopic splitting factors res-pectively ; (b and 8 are the polar angles of the direction of H in a centre-fixed axes system. Fig. 9 shows the eigenvalues and eigenfunctions of the Hamiltonian. The FIG. 9.-Eigenvalues and eigenfunctions of the hamiltonian used in the calculation of the magnetic linear dichroism 38 MCD AND MLD SPECTRA latter are expressed as linear combinations of the basis functions 1 g,pT) and I Z,p*) diagonal in p (magnetic dipole moment operator). We further assume that the transitions [ g,p,)-+ [ e,p*) are Q+ polarized when H is directed along the axis of the centre. When the direction of the magnetic field is different each of the four transitions I g,-t-)+ I e,i-) is both allowed with Q+ and Q- polarization.We call FIG. 10.4rientation of the Poynting vector (112) and magnetic field (IlX) in a space fixed axes system. XYZ the space-fixed axes (see fig. lo) propagate light along 2 and apply the mag-netic field along X. The calculation of MLD is carried out by evaluating I (e I nTX I g) I 2 - I ( e [ my I g) [ for each individual transition in terms of the matrix elements of m and my in the centre-fixed axes system. The calculation is tedious but not difficult and one finally finds 2YIeYIg sin2 2@ cos2 o- Yil,Yil sin4 w cos2 cos 2@)82~2 + cos 24D jP2H2. 1 1 [cOs:-l 2(1,2 + 12,)(cos2 o - 1) cos 2 0 + This result can now be applied to the trigonal centres oriented along the four three-fold axes of a cubic matrix.Let us assume first that the light propagates along [loo]. Then cos w = 1 / J3 and the MLD is obtained by summing up the contributions of the individual centres = @; (D2 = CD+n/2; m3 = @+n; (D4 = @+371/2). One finally gets AD - q(Yle91g( 1 + cos 0) cos 2@ + 4YL,YLg cos o sin2 2@ - 8 dv We were able to check this expression experimentally by measuring the MLD through the 584 nm line of a blue CaF + Nd3+ crystal (mainly trigonal centres). The light was propagating along one C axis and we recorded a signal proportional to the absorption curve second derivative for a given position of the crystal. The crystal was then rotated around C4 through successive angles CD and fig. 11 was obtained where the amplitude is plotted against @.It is clear from the figure that the ordinate varies as expected from eqn (6). If we now assume that the light is propagating along [ 1 1 11 the MLD is easily shown to remain constant when @ varies ; this again has been checked experimentally within instrumental errors. The above calculation and experiment J . B A D O Z M. B I L L A R D O N A . C . ROCCARA A N D B . B R I A T 39 demonstrate that one must be careful in performing MLD experiments and attempting to explain the data since orientational problems are of prime importance here. Further a similar derivation can be made for tetragonal centres in a cubic matrix when the light is propagated along [ 1001 or [ 1 1 13 ; essentially similar conclusions are reached. i.e. AD varies according to cos2 2 0 in the former case and remains invariant in the latter.An unambiguous-identification of trigonal or tetragonal centres from a) (deg.1 FIG. 11 .-Magnetic linear dichroism of trigonal Nd3+ centres in CaF (see text for comments). just two MLD experiments within one absorption band is thus again unexpected (as from MCD work) when all 9’ factors are non-zero. It can be expected however, that more useful conclusions are likely to be obtained for paramagnetic centres with an even number of electrons where Y1 splitting factors are zero and the term pro-portional to sin2 2@ in eqn (6) vanishes. Experimental investigations along these lines are under way in our laboratory. We thank Prof. Chapelle (Fac. des Sciences Orsay) and Dr. Toledano (CNET, Issy-les-Moulineaux) for kindly providing the crystals used in our experiments.The assistance of Mrs. Lenain (D6partement de Recherches Physiques Facult6 des Sciences Paris) for polishing and orienting the crystals was greatly appreciated. Some of the results described here would not have been obtained without the skilful help of MM. Ferre and Rivoal. la A. C. Boccara J. Ferre B. Briar M. Billardon and J. P. Badoz J. Chem. Phys. 1969,50,2716. ‘ b A. C. Boccara Compt. rend. 1969 268 62. M. Grosjean and M. Legrand Compt. rend. 1960,251,2150. M. Billardon and J. Badoz Compt. rend. 1966,263,139. M. Billardon and J. Badoz Compt. rend. 1966 262 1672. M. Billardon J. C. Rivoal and J. Badoz Rev. Phys. Appl. 1969,4,353. Such a CD attachment is manufactured by FICA Le Mesnil Daint Denis France. G. Barth E. Bunnenberg and C. Djerassi Chem. Comm. 1969 1246. * A. C. Boccara P. Roubeau and J. Badoz Compt. rend. 1967,265,513. P. J. Stephens W. Suetaka and P. N. Schatz J. Chem. Phys. 1966 44,4592. lo P. J. Stephens Chem. Phys. Letters 1968 2,241 l 1 M. Billardon F. Sicart and J. Badoz J. Phys. 1970 31 219. l 2 (a) B. Briat J. C. Rivoal and R. H. Petit J. Chim. Phys. 1970 67,462. B. D. Bird B. Briat, l 3 A. D. Buckingham and P. J. Stephens Ann. Rev. Phys. Chem. 1966. 17 399. l4 J. S. Griffith The Irreducible Tensor Method for Molecular Symmetry Groups (Prentice-Hall, l5 see e.g., P. Day and J. C. Rivoal this Discussion. Inc. Englewood Cliffs N. J. 1962). (a) A. C. Boccara and B. Briat J. Phys. 1969 30,445. (b) L. A. Alekseyeva N. V. Starostin and P. P. Feofilov Opt. Spectr. 1967 23 140. L. A. Alekseyeva and P. P. Feofilov Sou. Phys. Solid State 1968 10 1397. J. Duran Compt. rend. 1969 269 540. l6 Y. Merle d’Aubigne et P. Duval J. Phys. 1968 29 896

ISSN:0430-0696    

DOI:10.1039/SF9690300027

出版商:RSC    

年代:1969

数据来源: RSC

摘要:

Magnetic Circular Dichroism of Se;' and Te;' BY P. J. STEPHENS* Dept. of Chemistry University of Southern California Los Angeles, California U.S.A. Received 8th September 1969 Magnetic circular dichroism is used to investigate the species formed in sulphuric acid solutions of selenium and tellurium. The data strongly support the identification of absorption bands in these solutions with square-planar Sei+ and Te:+. These bands are shown to be in-plane polarized, consistent with assignment to x +x transitions. We present an application of magnetic circular dichroism (MCD) to the charac-terization of new chemical species. Magnetic optical activity has not previously been employed for this purpose unless we include its use in the identification of colour centres.l'* This can be attributed both to the lack of adequately sensitive instrumentation and to the nature (and complexity) of the phenomenon.While it is unlikely that magnetic optical activity will become widely used in detective chemistry even to the extent of visible-u.-v. spectroscopy we wish to demonstrate that in special cases it can be of considerable value. Gillespie and co-workers have shown that solutions of selenium and tellurium in oxidizing solvents such as oleum and HS03F + S206F2 contain a variety of polymeric cations. The species Sei+ Sei+ and Tei+ have been identified from cryoscopic, conductometric and magnetic susceptibility meas~rements.~* O Solid salts of Sez+ have been obtained l 1 and X-ray analysis of Se4(HS20,)2 has shown this ion to be square-p1anar.l2 Raman and i.-r.studies are consistent with this structure l3 and the Raman spectra of solutions containing Te:+ have been interpreted in terms of an analogous geometry.1° Visible-u.-v. spectra have been reported for Sei+, Sei+ Tei+ and for solutions obtained on oxidation of Tei+.9* lo Both Sei+ and Tez+ show one low-lying reasonably intense band. That in Set+ has been attri-buted l2 to a transition between n levels deriving from Sep,-orbitals. We have studied the MCD of selenium and tellurium in sulphuric acid solutions, with a view to further characterization of the species obtained. The data provide strong support for square-planar Seq+ and Te:+. Historical precedence for our experiments can actually be claimed by Bizette and S ~ h k r e r ~ ~ who in 1937 studied the magnetic optical rotation of solutions of tellurium in concentrated sulphuric acid in the region of the visible absorption band.However despite a field of 50 kG their data were not good and no firm conclusions were drawn. EXPERIMENTS Selenium dissolves in 30 % fuming HzS04 to give a green solution. On standing or on Further oxidation causes all addition of oxidizing agents the solution becomes yellow. * Alfred P. Sloan Research Fellow. 4 P . J . STEPHENS 41 $ I 1 I I L I 250 350 450 5 50 650 7 50 1 (nm) ‘1.0 A (nm) FIG. 1 .+u) Absorption spectrum and (b) MCD of Se/30 % fuming sulphuric acid (- 3.212 g/l. ; -- 1.036 g/l.). colour to disappear. The absorption spectra and MCD of fresh green solutions and a yellow solution obtained using (NH4)2S208 as oxidizing agent are shown in fig.1 and 2. Tellurium dissolves slowly in concentrated H2SO4 and rapidly in 30 % fuming H2SO4, giving purplish-red and wine-red solutions respectively. On addition of oxidizing agents the solutions turn orange yellow and then colourless. The absorption spectra and MCD of a solution in concentrated H2S04 and solutions in 30 % fuming HzSO4 with and without added (NH4)2S208 are shown in fig. 3 and 4. MCD was measured in Prof. C. Djerassi’s laboratory at Stanford University with the assistance of Dr. E. Bunnenberg and Nrs. R. Records on a Durrum-Jasco ORD/CD/UV-42 MCD O F Se;+ AND Tei+-n -0.5 1 - 1.0 \ I V b) FIG. 2 . 4 ~ 2 ) Absorption spectrum and (b) MCD of Se (NH4)2S208/30 % fuming sulphuric acid (0.980 g/l. and 14.880 g/l.respectively). equipped with a Lockheed superconducting magnet. In all cases the field was 49.5 kG. Absorption spectra were taken on a Cary 14. The noise in all spectra was negligible and is not indicated in the figures. We report absorption data in terms of optical density O.D., and MCD as A(0.D.) = (O.D.)L-(O.D.)R the magnetic field being in the same direction as the light beam. The optical path-length in all experiments was 1 mm. All solutions used were freshly prepared excepting the yellow selenium and concentrated H2S04+ tellurium solutions which were 12 h and 23 days old respectively. Note added in proof The MCD data in fig. 26 and the right-hand part of fig. 36 are in error and should be multiplied by 1.26 P . J. STEPHENS 43 r' n e .- g -1.0 c.l 2 2 % I I I I I 2 50 350 450 550 650 A .- 1.0 A (nm) FIG.3.-(a) Absorption spectrum and (b) MCD of Te+ conc. sulphuric acid (concentration unknown but < 3.8 g/l.). DISCUSSION The spectrum of the yellow selenium solution (bands at 410 and 320nm) is essentially identical to that given by Barr et aL9 for a 4 1 Se S206F2 solution in HS03F and attributed to Se:+. A lower limit for E,, of the 410 nm band obtained assuming all the selenium present as Sez+ is 5900. This compares with 7400 given by Barr et al. for H2S207 solutions. The green selenium solutions contain the yellow species (and this cannot be eliminated by reducing the SO concentration). However if its contribution is subtracted the spectrum remaining is the same (bands at 685 470 and 295 nm) up to -280 nm as Barr et al.for a 10.25 1 Se S206F2 solution in HS03F,9 which was ascribed to Seg+. At shorter wavelengths than used by Barr et al. we observe an additional band (-250 n d . Since neither the wor 44 of Barr et aL9 nor OUT studies found any evidence for other species this band can also be attributed to Sei+. All tellurium solutions show bands around 510 420 360 300 and 250nm, of relative intensities varying with age and with SO3 and (NHJ2S208 concentration. The spectra of solutions of Te and 2 1 Te S206F2 in HS03F given by Barr et a2.1° show bands at 510 and 430nm and 520 420 360 300 and 250 nm respectively. MCD OF Se$+ AND Teil-(m) FIG. 4.-(a) Absorption spectra and (b) MCD of Te/30 % fuming sulphuric acid (- 2.244 g/1; --.0.304 g/l.) and Te (NH4)2S20~ + 30 % fuming sulphuric acid (- - - 2.244 g Te/l.). The former is attributed to Tei+ and the latter to a more highly oxidized species. Our spectra show no additional bands and appear interpretable as a superposition of these two spectra in varying proportions. However our data do not rule out the presence of more than two species ; in particular no isosbestic points were observed. From data on concentrated H2S04 solutions we find a lower limit for E,, of the 510 nm band of 5000 assuming all tellurium present as Tei+ (Barr et a2.1° give 2150). MCD is observed for all bands of the selenium solutions. Indeed for the green solutions MCD indicates the bands more clearly than the absorption spectrum an P . J . STEPHENS 45 shows there to be a band at 580 nni which is not resolved in the spectrum.The most noticeable feature of the MCD is the large effect in the 410 nm band changing sign through the band; this will be shown below to be strong support for assignment of this band to square-planar Se:+. The MCD of the tellurium + concentrated H2S04 solution contains bands at 460 400 360 and 300 nm which are not clearly resolved in the absorption spectrum. The data for the fuming H2S04 solutions are more complicated and require extension to a wider range of solutions for their correlation with the absorption spectra to be unambiguously established. The distinctive feature of the MCD data is the behaviour of the 510nm band which is identical in form to that of the 410nm Se:+ band. This correspondence leads to the conclusion that these bands arise from similar transitions and species.The MCD thus provides strong support for the identification of the 510 nm tellurium band with square-planar Te:+ as assigned by Barr et a1.I' Brown et aZ.12 have proposed that the highest occupied and the lowest unoccupied orbitals of square-planar D4h Sei+ are 71-levels arising from the Se 4p orbitals as shown in fig. 5. On this basis in the absence of spin-orbit coupling the ground state is 'Alg (in agreement with the observed diamagnetism 9* 1 5 ) and the lowest eg-+bzu excitation leads to 'E and 3E excited states. Brown et al. assign the 410 nm Se:+ band to this transition. Since the Se 4p spin-orbit coupling constant is 2000-3000 cin-l excited-state spin-orbit interactions would at first sight be expected to be visible.However spin-orbit interactions within n-electron states of aromatic hydrocarbons are very small due to the form of the n-orbitals and the spin-orbit operator.16 Energy - eg aeu FIG. 5.-x-orbitals of Se:+. Crosses indicate electrons in ground state of Se:+. Identical arguments hold for Se:+ and lead to the expectation that the 3E splitting and 1E,-3E mixing are also small. In this case since other off-diagonal spin-orbit interactions can be ignored only the lAlS-+lE, transition is strongly allowed and only one absorption peak is expected (excluding Jahn-Teller effects). The observed intensity is consistent with the assignment to an electric-dipole-allowed transition. This picture does not account for the weak 320 nm band.If it is due to the same species an additional orbital close to either the e,(n) or b,,(n) orbitals, must be present. However this orbital does not give rise to strong absorption below 40,000 cm-' and without a better theoretical calculation we shall not speculate further on its nature. The important feature of the MCD of the 410 nm band is its large first moment about the mean absorption frequency. General expressions for the zeroth and first moments of the absorption and MCD of a transition are given elsewhere and here we quote only those results relevant to the present study. For a dilute liquid solution of diamagnetic molecular species the zeroth moment of the absorption band and the zeroth and first moments of the MCD of an allowed transition A+J are given b 46 MCD OF Sea-1- AND Teit O.D.a dv = -9O(A+J), (O.D.) = J;- 2 (A(O.D.)> = -dv = -a9Y0(A-+J)H IA( O. DJ (v- v O ) dv - a I1 = - {d 1 ( A + J ) + [S 1 ( A + J ) - h Y O&?O( A+ J ) ] ) H . A11 integrals are over the band. a is a constant depending on the concentration of the species the optical path-length the " effective-field '' correction and fundamental constants. v" is the frequency about which the first moment is taken. H is the magnetic field strength. go and d1 are given by g O ( A - 4 = c I ( A I m I Jn> I * 1 where m and p are the electronic electric and magnetic dipole moment operators, Jn runs over the excited electronic states contributing to the band and the wave-functions are appropriate to the free molecule at a single chosen geometry.go and &ll are parameters depending on off-diagonal matrix elements of p between A and J and other molecular states. Their detailed form will not be needed here. Of the assumptions involved in eqn (1) and (2) we note only that the effects of the solvent are taken into account to first order-i.e. the molecule-solvent interaction is included explicitly but assumed to be weak-and Jahn-Teller complications are fully catered for. The states J1 need not be exactly degenerate at any molecular geometry. The matrix (JA I p I J1.) is zero when there is just one J state. Hence, dl is non-zero only when the J manifold contains more than one absorbing state. We now assume that the 410 nm band is due entirely to a single species. Values for the moments defined in eqn (1) are given in table 1.The first moment is calculated withv" as the mean absorption frequency making J(O.D./v) (v - v")dv = 0. Corrections for the overlap with the 320 nm transition have been included. TABLE 1 Se a Te 0.25 0.28 p+v -0.48 10-4 -0.32 10-4 V" 24 080 19 290 cm-~ ~ ) ( v - v " ) d v -0.38 - 0.33 cm-1 r c 3000 2700 cm-I I 0.66 0.50 cz data for the 410 nm band ; b data for the 510 nni band ; C width at half-height. An outside limit on the contribution of the 99 terms to the MCD first moment can be placed at BoT where r is the band width and this is iikely to be a considerable over-estimate. From table 1 it can then be seen that the major contribution must come from d, P. J. S l E P H E N S 47 To calculate dl we write where all wave-functions refer to the D4, ground state equilibrium geometry; we ignore off-diagonal spin-orbit mixing other than of IE and 3Eu and the (A 1 SM,a) matrix diagonalizes the spin-orbit interaction within the E, 3Eu manifold.But dl is invariant to a unitary transformation on I Ja). Hence the I lEua) I 3EUMsa) basis can be used instead of I JJ to evaluate dl and it is unnecessary to find the (A I SM,a} matrix. Now (lAl 1 m I 3E,Mscx) = 0 (all M,,a). Only the lE, functions thus contribute to dl-i.e. d is unaffected by the presence of the triplet states. With (lAlg I in I ' E J ) = ('Al I my I ' ~ y ) = in, (4) (lE*,x I P I l E Y ) = -BM('Eux I LZ I l&Y) = - iZ&f, where Ex and Ey states transform like x and y and /IM is the Bohr magneton we then obtain dl = m21pM go = 2m2 dlpQ = 31PM.(5) If the a terms of the MCD first moment are neglected eqn (1) and (5) permit I to be evaluated with the result given in table 1. This calculation does not require any knowledge of the concentration optical path-length " effective-field " correction or the value of m. In particular it does not require the assumption that all selenium is present as Se:+. Such calculations have been carried out for several types of system.18-22 For 7c-electron states of planar molecules expressed in terms of p z atomic orbitals matrix elements of L, reduce to two-centre integrals and all one-centre terms vanish.l8. The two-centre integrals are sensitive to the form of the radial function of the orbitals. In view of the crudity of the n-electron model and the uncertainty in the quantitative form of orbitals occurring therein it does not appear worthwhile to consider calcula-tion of I in more detail.From the previous experience the magnitude obtained from the data is very reasonable. We can now see to what extent the MCD actually provides proof of the Sei+ structure and the absorption band assignment. Recognizing the large separation of the ground and excited states from other states (neglecting the 320 nm band excited levels) their spin-orbit mixing with other states can always be ignored to a good approximation. Then the first moment can only be explained if in the zero-spin-orbit-coupling limit the excited-state manifold contains more than one singlet state. This can arise either from true or accidental degeneracy.The latter is reasonably uncommon. Excluding this it then follows that the species has sufficient symmetry to allow electronic degeneracy. This limits the allowed molecular point groups. Accepting the molecular formula Se, such groups as D4h (square-planar) Td (tetra-hedral) and Dmh (linear) are possible. The existence of the first moment does not in itself allow a choice between them. In summary then as long as accidental degeneracy can be excluded the MCD is strong evidence for a highly symmetrical I can be calculated a priuri given explicit lEU wave-functions 48 Se;-' geometry and is consistent with a square-planar structure. Accepting the latter and a IA, ground state the symmetry of the excited singlet state of the 410 nm band must be E, since this is the only degenerate u representation in D4,,.The transition is therefore in-plane polarized. This is strongly suggestive of a 71-71 type of transition though not conclusive proof. The absorption spectrum and MCD of the 510 nm Te:+ band are essentially identical to those for the Se2f 410 nm band and we interpret them analogously. Spin-orbit coupling is larger in Te but spin-orbit energies should still be much smaller than the separations of other states from the ground and excited states of the band. Values for moments and parameters derived therefrom are given in table 1. They are close to those obtained for Sea+. We therefore draw the same conclusions as for Sei+. The observation of degeneracy in both Sei+ and Teif strengthens the assumption that it is caused by molecular symmetry and is not accidental.The data for both species are thus together strong support for their proposed square-planar geometries. CONCLUSION The existence and structure of Sei+ appear to be reasonably established by the work of Gillespie et al. Our studies put this latter species on a firmer basis. We have added little to the existing knowledge of Seg+ or the oxidized tellurium species since the MCD of the former is highly unsingular and that of the latter is both hard to disentangle from the Tei+ contribution and rather complex. Further work on these systems is required. However Tei+ is not so well characterized as yet. I am deeply grateful to Dr. Bunnenberg and Mrs. Records for their generous assistance in the experimental part of this work and to Prof.Djerassi for his hospitality. I also owe thanks to Dr. David Parker for help in the initial stages and to Prof. Anton Burg for the use of his dry-box. The work was supported in part by grants from the National Institutes of Health and the Alfred P. Sloan Foundation. Y . M. d'AubignC and J. Gareyte Compt. rend. 1965 261,689. F. C. Brown B. C. Cavenett and W. Hayes Phys. Letters 1965 19 167. J. C. Kemp W. M. Ziniker and J. A. Glaze Phys. Letters 1966 22,37. J. C. Kemp W. M. Ziniker and J. A. Glaze Proc. Brit. Ceramic SOC. 1967 109. 6. Gehrer and H. Langer Phys. Letters A 1968 26,232. H. Paus and F. Luty Phys. Rev. Letters 1968 20,57. ' J. A. Glaze and J. C. Kemp Phys. Rev. 1969,178,1502 1507. * B. C. Cavenett W. Hayes I. C.Hunter and A. M. Stoneham Proc. Roy. SOC. A 1969,309 53. J. Barr R. J. Gillespie R. Kapoor and K. C. Malhotra Can. J. Chem. 1968 46 149. l o J. Barr R. J. Gillespie R. Kapoor and G. P. Pez J. Amer. Chem. SOC. 1968 90,6855. l 1 J. Barr D. B. Crump R. J. Gillespie R. Kapoor and P. K. Ummat Can. J. Chem. 1968 46, l 2 I. D. Brown D. B. Crump R. J. GiIlespie and D. P. Santry Chem. Comnz. 1968 853. l3 R. J. Gillespie and G. P. Pez Inorg. Chem. 1969 8 1229. l4 H. Bizette and M. SchCrer Compt. rend. 1937 204 1931. l 5 We have also looked at the e.s.r. of all solutions studied here and found no evidence for radicals. l 6 D. S. McClure J. Chem. Phys. 1952,20,682. l7 P. J. Stephens J. Chem. Phys. 1970,52,3489. A preliminary account of this work appeared l 8 P. J. Stephens W. Suetaka and P. N. Schatz J. Chem. Phys. 1966,44,4592. l 8 P. N. Schatz A. J. McCaffery W. Suetaka G. N. Henning A. B. Ritchie and P. J. Stephens, ** P. N. Schatz P. J. Stephens G. N. Henning and A. J. McCaffery Inorg. Chem. 1968,7 1246. 21 P. J. Stephens P. N. Schatz A. B. Ritchie and A. J. McCaffery J. Chem. Phys. 1968 48 132. 22 P. J. Stephens A. J. McCaffery and P. N. Schatz Inorg. Chem. 1968,7 1923. 3607. in Chem. Phys. Letters 1968 2,241. J. Chem. Phys. 1966 45,722

ISSN:0430-0696    

DOI:10.1039/SF9690300040

出版商:RSC    

年代:1969

数据来源: RSC

摘要:

Magnetic Circular Dichroism Studies Part 10.-Investigations of Some Carbonyl Compounds BY GUNTER BARTH EDWARD BUNNENBERG CARL DJERASSI DANNY ELDER AND RUTH RECORDS Dept. of Chemistry Stanford University Stanford California 94305 Received 1st October 1969 The magnetic circular dichroism (MCD) spectra observed for the n-x* transition of fifty-three aldehydes and ketones are reported. The results show that with appropriate instrumentation MCD spectra can be obtained for all carbonyl compounds and that this technique can be very sensitive to the structural environment of the carbonyl group. In the past decade a resurgence of interest in the magnetic optical activity of organic compounds has become evident. During this period most investigators have directed their attention to the spectroscopic information which is available through Faraday effect studies.We have been motivated in part by the possibility that this technique may provide in addition information about the stereochemistry of organic compounds. Because of the successes obtained by us in the 1950's with naturally optically-active compounds containing the carbonyl chromophore in optical rotatory dispersion (ORD) and circular dichroism (CD) it is natural that investigations involving magnetic optical rotatory dispersion (MORD) and magnetic circular dichroism (MCD) should have turned in this direction as well. The first measurement of the MORD spectrum through the singlet n - IT* transition of a ketone (2-butanone) was made by Cotton and Servant in 1942. In recent years MORD effects have been reported in the 290nm region for cyclohexanone and camphor as well as for several acyclic ketone^.^'^ Recently Winkler reported MORD spectra for acetophenone and benzophenone and commented on some of the sources of error involved in earlier MORD measurements.Although Faraday effect studies of absorbing compounds are more conveniently made using the technique of MCD the lack of sufficiently sensitive instruments notwithstanding the use of superconducting magnets has precluded a general investigation of the MCD spectra of the carbonyl chromophore. In particular McCaffery and co-workers reported that while MCD Cotton effects could be observed for cyclobutanone 2-bromocyclo-butanone and acetophenone the effects observed for a number of cyclic and acyclic ketones were too small to be reliably measured.In the present communication we report the results of a preliminary investigation of some carbonyl compounds which show that the use of a superconducting magnet in conjunction with a circular dichroism instrument of increased sensitivity has consistently provided satisfactory MCD spectra for all carbonyl compounds thus far investigated. Furthermore the variety of the shapes and amplitudes of the MCD bands observed in the n-n* absorption band suggests that MCD may be more sensitive to the symmetry (i.e. structural environment) around the carbonyl group than had previously been supposed. 4 50 MCD OF CARBONYL COMPOUNDS EXPERIMENTAL MCD measurements were made using a Japan Spectroscopic Company Spectropolari-meter-circular dichrometer (Durrum-JASCO model ORD/UV/CD-5) which was modified to accept a superconducting magnet built by Lockheed Palo Alto Research Laboratories (model OSCM-103).A modification of this instrument permitted the use of sensitivities of 2 x AA/cm. All measurements were made using a magnetic field strength of 49 500 G . The temperature of the sample cell was 20°C. MCD data are reported as molar ellipticities per unit magnetic field ([e]M) using the sign convention previously adopted.lOgll On the sensitivity scale used for most of our measurements the peak-to-peak signal-to-noise ratios encountered are largely dependent on the stability characteristics of the particular lamp (OSRAM XBO 450 W/P) in use and on its age. Typical signal-to-noise ratios are indicated on the figures by vertical bars.Measurements can be made using either neat liquids in 0.1 mm path length cells (cyclohexanone 21 table 1) or solutions. For solution measure-ments concentrations and path lengths (usually 1 mm) were chosen such that the optical densities were about 2. The selection of compounds for this study was governed by several considerations. First, the observation l2 of two oppositely signed MCD bands in the region of the n-n* transition of cyclohexanone (21) whereas only a negative band was observed for cyclobutanone (19) prompted a survey of the MCD spectra of the carbonyl chromophore in a wide variety of stereochemical environments. Secondly it was necessary to use only samples of the highest purity since the MCD bands arising from very small amounts of impurities such as the one associated with the 225 nm absorption band of a,P-unsaturated ketones frequently resulted in severe overlap with the MCD band(s) associated with the n-n* transition of the saturated compound.MCD data are reported only for those samples for which this overlap was not severe. Marginal cases are noted in table 1. Each sample whether obtained commercially or from the coIIection of other investigators was checked for homogeneity by analytical gas chromatography. When necessary and feasible mixtures of isomers (e.g. cis- and trans-2-decalone) were separated by preparative gas chromatography. In these cases isomer identity was ascertained either by n.m.r. or by preparation of suitable derivatives. Finally, for some samples the shape of the MCD curve observed was unexpected.In these cases a structurally similar homologue was obtained for comparison. Spectral data for vibrational components are entered in table 1 only for those samples for which such fine structure was clearly evident. For these samples the values for A,, and [O]M are listed separately in addition to the values obtained by averaging the values of the separate components. RESULTS It is frequently difficult to determine unambiguously whether a particular com-pound shows more than one MCD band in the region of the n-n* carbonyl transition. In fig. 1 for example there are at least two oppositely signed components in the MCD spectrum of 3-methyl-2-butanone (6) whereas 2-butanone (2) and 3-methyl-2-hexanone (10) both show only a singly signed effect.The relatively close correspondence between the MCD and absorption maxima of (2) as well as the long wavelength tail in the MCD spectrum suggests the presence of a negative MCD band at about 314nm. For (10) the symmetry of the MCD band about its maximum suggests the presence of only one band whereas the relatively large separation between the MCD and absorption maxima suggests the presence of a positive and weaker component in the 310-320 nm region. In particular instances a second component can be observed in methanol but not in cyclohexane (see (34) in table 1). In the sequel, unless specifically noted reference is made to data observed in cyclohexane solution. The n-alkanones 2-propanone ( I ) 2-butanone (2) 2-pentanone (3) 3-heptanone (4) and 4-heptanone (5) show negative magnetic Cotton effects centred on the blue side of the absorption band maxima and as the environment of the carbonyl grou G .BARTH E . BUNNENBERG c. DJERASSI D . ELDER K . RECORDS 51 TABLE 1 .-MCD AND ABSORPTION SPECTRAL DATA OF CARBONYL COMPOUNDS compound ACYCLIC KETONES 1. 2-propanone 2. 2-butanone 3. 2-pentanone 4. 3-heptanone 5. 4-heptanone 6. 3-methyl-2-butanone 7. cyclopropyl methyl ketone 8. 2-methyl-3-pent anone 9. 3 -methyl-2-pen tanone 10. 3-methyl-2-hexanone 1 1. 3,3-dimethyl-2-butanone 1 2. 2,4-dimethyl-3-pentanone 13. dicyclopropyl ketone 14. 2,2,4-trimethyl-3-pentanone 1 5. 2,2,4,4-tetramethyl-3-pent an one 16. 1,1,1 -trichloro-2-propanone ALDEHYDES 17. 2-methylbutanal solvent a absorption b9c MCD =sd notes & Amax ([el x CH CH M CH M CH M CH CH CH M CH M CH M CH M CH M CH M CH M CH M CH CH M CH 18.2,2-dimethylbutanal CH b a x 278 279 272 280 276 280 277 285 284 277 269 284 280 286 282 285 280 287 282 290 285 272 265 290 288 297 290 285 295 295 14 275( - 1 1) 16 275( - 1 1) 17 263( - 9) 18 274( - 10) 19 270( - 8) 21 265( - 8) 23 267( - 5) 22 275( - 9) 20 262(-4) 302(+2) e 20 260(-10) 295( + 1 1) 19 255(-9) 288(+ 11) 23 265(-4) 300(+ 2) 27 265(-4) 297( + 2) 272( - 6) 22 26 265(-5) 300(+ 1) 24 275( - 6) 29 265( - 6) 21 259(-2) 297(+ 10) 24 250(-5) 292( + 10) 30 275(-5) 320(+ 1) 30 280( - 9) 25 265(-9) 300( + 4) 35 260(-14) 290( + 7) 300(+6) 296(+ 7) 15 22 19 305( + 18) 55 247(+17) 310(- 18) 49 270(+11) 307( - 18) 24 302( - 22) 16 272(+1) 302(- 5 52 MCI> OF CARBONYL COMPOUNDS compound CYCLIC KETONES-RING SIZE 19.cyclobutanone 20. cyclopentanone 21. cyclohexanone 22. cyclohept anone 23. cyclooctanone 24. cyclononanone 25. cyclodecanone 26. cycl ododecanone CY CLOPENTANONES 27. 2-chlorocyclopentanone 28. 3-met hylcyclopen tanone 29. trans-2,3-dimethylcyclo-pentanone 30. cis- 3,4- dime t h ylcycl o-pentanone 31. trans-3,4-dimethylcyclo-pent anone 32. 2,2,4,4-tetramethylcyclo-pentanone CY CLOHEXANONES 33. 2-methylcyclohexanone solvent a absorption b9c MCL) ci* CH M CH M CH neat M CH CH M CH M CH M CH M CH M CH M CH CH M CH M CH M CH M 4nax 282 280 297 286 289 288 28 1 291 29 1 283 293 285 288 284 287 283 308 302 296 287 296 297 287 297 287 299 294 288 284 6 1 max 18 17 14 18 15 275(-4) 16 276(-4) 15 270(-6) 18 269(-3) 15 17 14 17 17 17 17 267(-3) 24 265(-4) 36 31 19 21 22 21 25 20 21 22 270(-14) 27 274(-5) 18 275(-3) 19 273(-5) 290( - 30) .f 288( - 30) 287(- 17) 9 285( - 16) 31 1(+2.3) h 312(+2) 308(+ 1.5) 310(+ 2) 295( - 8) i 286(- 8) 289( - 5 ) 275(- 4) 285(- 5 ) 275( - 6) 302( + 3) 301(+ 5 ) 307(- 56) 300( - 62) 291(-26) 285( - 23) 288(- 14) 286( - 20) 285(- 19) 29 1 ( - 2.8) 286(- 31) 305( + 9) 304( + 1) 314(+ 3) 305(+ 3) k I m I G .UARTH E . BUNNENBERG C . DJERASSI D . ELDER R . RECORDS 53 compound solvent a a,bsorption brc MCD c9d notes E Amax ([el x 3 4. 3 -met hylcyclohexanone 35. 4-methylcyclohexanone 36. 4-ethylcyclohexanone 3 7. 2-t -but ylcyclohexanone 3 8. 3 - t - but ylc y clo hexan one 39. 4-t-butylcyclohexanone 40. 2-chlorocylohexanone 4 1 . 2,4-di-t-butyl-5,5-dimethyl-cyclohexanone 42. 2,2,6,6-tetramethylcyclo-hexanone 43. 2,2,6,6-tetraethylcyclo-hexanone 44. 3 3,5-trimethylcyclo-hexanone 45. 3,3,5,5-tetramethylcyclo-hexanone BRIDGED CYCLANONES 46. trans- 1 -decalone 47. cis-9-methyl-1-decalone 48. cis-2-decalone 49. trans-2-decalone 50.bicyclo[2,2,l]heptan-2-one 5 1. endo-3-chloro-bicyclo[2,2,1]-hep t an-2-one 52. adamantan-2-one CH M CH M CH M CH CH CH M CH CH CH CH CH CH CH CH CH CH CH CH CH M 288 278 289 281 290 283 295 288 290 282 304 300 303 310 291 296 291 295 290 288 294 304 292 284 53. dimethyl adamantan-2-one-175-dicarboxylate ACN 290 17 26 20 13 16 15 20 17 19 17 33 280( - 6) 275(- 8) 313(+ 1) 275( - 10) 315(+ 1) 270( - 8) 310(+ 1) 272( - 3) 3 1 1 (+ 2) 265( - 4) 305( + 2) 262( - 3) 0 P 270( - 1) 304(+4) q 265( - 3) 290( - 1 1) 302( + 3) 309(- 41) 28 300( + 3) 22 306( + 45) 25 31 5( + 27) 16 273(-4) 315(+ 1) 17 278( - 7) 22 260(-4) 304(+4) r 19 26q-7) 306(+ 13) s 16 285(-6) 325( + 1) 17 280(-9) 318(+ 1) 23 278( - 4) 43 300( - 55) 16 295(+ 9) 23 295( + 9) 18 297(+ 10) 54 MCD OF CARBONYL COMPOUNDS a.Spectrograde solvents CH = cyclohexane M methanol ACN =z acetonitrile. 6. Amax (nm) Emax (1000 cmz mol-'). c. Fine structure is given in separate footnotes. d. Amax (nm) [e]M (deg. cm2 mol-' G-') ; [e]M = 3300 [AE]M ; signs are in accordance with the convention established in ref. (10) and (1 1). Double entries are made for compounds exhibiting two MCD bands in the region of the carbonyl n-x* transition. e. Insufficient sample available for measurement in methanol. f Structure observed for cyclobutanone in CH : MCD 302sh(-O.00024); 290(-0.00030); 281(-0.00028); 273 sh(-O.00022) Absorption 301 sh (13) ; 289 (18) ; 280 (19) ; 270 sh (16) 9.Structure observed for cyclopentanone in cyclohexane : MCD 322 sh (-O.OO0 45) ; 311 (-O.OO0 13); 299 (-0.OOO 18) ; 289 (-O.OO0 19) ; 280 (-0.000 18); 267 sh (-O.OO0 12) Absorption; 324(6); 311 (13); 300(15.5); 289(14); 280(12); 269sh(9) h. Structure observed for cyclohexanone in cyclohexane-band at 275 was not structured : MCD 321 (+O.OOO 22) ; 312 (+0.000 03) ; 300 (+O.OOO 02) Structure observed for cyclohexanone neat-band at 276 was not structured : i. Structure observed for cyclooctanone in cyclohexane : Absorption no structure discernible MCD 321 (+O.OOO 019) ; 309 (+O.OOO 025) ; 300 (+O.O00 019) MCD 310 (-0.000 06) ; 301 (-O.OO0 09) ; 290 (-0.000 093) Absorption 310 (9) ; 300 (13) ; 288 (15) j .Structure observed for cyclodecanone in cyclohexane : MCD 315 (-0.000 04) ; 280 (-0.000 054) ; 270 sh (-0.000 046) 305 (-0.OOO 045) ; 297 (-0.000 05) ; 289 (-0.000 05); Absorption no structure discernible k. Structure observed for 3-methylcyclopentanone in cyclohexane : MCD 322 sh (-O.OO0 06); 310 (-0.000 15) ; 299 (-0.000 21) ; 288 (-O.OO0 23) ; 280 (-O.OO0 18); 270 (-0.000 14) Absorption 322(7.6); 311 (17); 299(21); 288(19); 280(15.5); 269sh(13) 1. Structure observed for trans-2,3-dimethyl cyclopentanone in cyclohexane : MCD ; 318 sh (-O.OO0 05) ; 309 (-O.OO0 10) ; 298 (-0.000 14); 290 (-O.OOO 15); 280 (-0.000 14) Absorption 319 sh (9) ; 308 (19) ; 297 (22) ; 288 (20) ; 280 (16) m. Structure observed for cis-3,4-dimethylcyclopentanone in cyclohexane : MCD 325 (-0.000 04) ; 312 (-0.000 12) ; 278 (-0.OOO 20); 271 sh (-O.OO0 18) 297 (-O.OO0 19) ; 288 (-0.000 20); Absorption 323 (9) ; 31 1 (18) ; 299 (23) ; 289 (22) ; 279 sh (19) ; 271 sh (18) n.Structure observed for trans-3,4-dimethylcyclopentanone in cyclohexane : MCD 326 (-0.O00 10) ; 312 (-0.000 21); 302 (-0.000 28) ; 292 (-0.000 31) ; 280 sh (-0.000 25); 273 sh (-O.OO0 20) Absorption 322 (8) ; 309 (17) ; 298 (21) ; 289 (20) ; 280 sh (16) 0. Position and intensity of negative band uncertain due to overlap. p . Structure observed for 3-t-butylcyclohexanone in cyclohexane : MCD 317 (-0.0004) ; 305 (-O.OO0 08) ; 290 (-O.OO0 11) ; 270 sh (-0.OOO 08) W 305 (13) ; 290 (broad) (1 1) q. Structure observed for 4-t-butylcyclohexanone in cyclohexane : MCD 322 (+O.OOO 024) ; 304 (+0.000 039) Absorption no structure r.Structure observed for trans-l-decalone in cyclohexone : MCD 320 (+O.OOO 02) ; 310 (+O.OOO 04) ; 300 (+O.OO0 05) ; 292 (t-0.000 02) Absorption 315 sh (10) ; 305 (18); 295 (22) ; 287 (21) s. Structure observed for cis-9-methyl-l-decalone in cyclohexanone : MCD 324sh(+0.00008); 313 sh(+0.000 12); 308 (+O.OOO 15); 300(0.000 13); 292 (+ 0.000 08) Absorption 310 sh (14) ; 300 (20) ; 290 (19) t. Structure observed for adamantanone in cyclohexone : MCD 320 sh (+0.000 03) ; 305 sh (+O.OOO 07) ; 295 (+O.OOO 09) ; 284 sh (+O.OOO 06) ; 260 sh (+O.OOO 01) Absorption 315 sh (8) ; 305 sh (14) ; 294 (16) ; 288 (16) ; 270 sh (15 G . BARTM E. BUNNENBERG C . DJERASSI D . ELDER R . RECORDS 55 becomes less symmetrical the separation between the two maxima increases.When the environment of the carbonyl group is made even less symmetrical by replacement of an a-hydrogen by a methyl group as for 3-methyl-2-butanone (6) (fig. l) cyclopropyl methyl ketone (7) and 2-methyl-3-pentanone (S) one observes two components of opposite sign. Although the positive band was observed for 3-methyl-2-pentanone (9) in methanol the presence of a positive component in the spectrum of 3-methyl-2-hexanone (IU) fig. 1 can only be assumed. Two oppositely signed bands are also observed when the a and a'-carbons are symmetrically substituted as in 2,4-dimethyl-3-pentanone (12) and dicyclopropyl ketone (13). When three a-hydrogens are replaced by methyl groups as in 3,3-dimethyl-2-butanone ( I I ) the positive long wavelength component becomes the larger one.Continued methyl substitution on the a'-carbon results in a further increase in the intensity of the positive component, a single positive effect being observed for 2,2,4-trimethyl-3-pentanone (14) and 2,2,4,4-tetramethyl-3-pentanone (15). A (nm) FIG. 1.-Absorption and MCD spectra of 2-butanone (2) (- - -) 3-methyl-2-butanone (6) (-) and 3-methyl-2-hexanone (10) (. . .) in cyclohexane. The perturbation in the carbonyl chromophore engendered by halogens (chlorine) in the alpha position is particularly pronounced. In 2-chlorocyclopentanone (27) and endo-3-chloro-bicycl0[2,2,l]heptan-Zone (51) the intensity of the MCD band is enhanced with respect to the parent ketones (20) and (50).For 2-chloro-cyclo-hexanone (40) a single strong negative effect is observed whereas oppositely signed effects were found for cyclohexanone (21). In 1 1 1-trichloro-2-propanone (16) the signs of the two bands are inverted and intensified with respect to the trimethyl-analog (11). A similar situation is evident in the MCD spectra of the two aldehyde 56 MCD OF CARBONYL COMPOUNDS investigated. Two oppositely signed components (long wavelength negative) are observed for 2,2-dimethylbutana1(18),l table 1 whereas 2-methylbutanal (17) shows only a strong negative effect. In the unsubstituted cyclanone series the multiplicity as well as the magnitude of the bands observed depends on the ring size. The largest effect was observed for cyclobutanone (19),7 table 1 where the single but structured negative band is centred on the red side of the absorption band maximum.The effect observed for cyclopentanone (20) is negative weaker and centred on the blue side of the absorption band maximum. Examination of the data given in table 1 (footnote (9)) shows the close correspondence of the vibrational band in the MCD and absorption spectra. Cyclohexanone (21),12 table 1 shows two oppositely signed MCD bands centred about the absorption maximum. The three well-resolved vibrational components in the positive long wavelength band of (21) are generally absent in the substituted cyclohexanones. In the medium and large cyclanones investigated cycloheptanone (22) and cyclododecanone (26) exhibit two bands of opposite sign whereas cyclo-octanone (23) cyclononanone (24) and cyclodecanone (25) show only negative effects.20 16 12 I E 8 4 l a FIG. 2.-Absorption and MCD spectra of 2-methylcyclohexanone (33) (- - -) 3-methylcyclohexanone (34) (-) and 4-ethylcyclhexanone (36) (. . .) in cyclohexane. The importance of substitwents alpha to the carbonyl group is also apparent in the substituted cyclopentanone series since two oppositely signed components were evident only in the MCD spectrum of 2,2,4,4-tetramethylcyclopentanone (32), whereas only a singly signed effect was observed for 2-chlorocyclopentanone (27) and trans-2,3-dimethylcyclopentanone (29). Methyl groups at positions 3 and 4 as i G . BARTH E . BUNNENBERG C. DJERASSI D . ELDER R . RECORDS 57 3-methylcyclopentanone (281 cis-3,4-dimethylcyclope1itanoiie (30) and trans-3,4-dimethylcyclopentanone (31) apparently cause an enhancement of the effect relative to cyclopentanone (20).The MCD spectra of substituted cyclohexanones are particularly sensitive to the nature and position of the alkyl substituent(s). Except for the absence of structure in the positive long wavelength band the MCD spectra (fig. 2) of 2-methylcyclo-hexanone (33) and 4-ethylcyclohexanone (36) closely resemble that observed for cyclohexanone (21). 3-Methylcyclohexanone ( 3 4 fig. 2 however shows a very weak positive component only in methanol. 2-t-Butylcyclohexanone (37) shows a poorly-defined negative effect suggesting that the two components are much more closely spaced. The size of the alkyl substituent is also important since in table 1 the magni-tude of the positive component increases in the order 4-methylcyclohexanone (35), 4-ethylcyclohexanone (36) 4-t-butylcyclohexanone (39).The effect of increasing the number of alkyl substituents adjacent to the carbonyl group in the cyclohexanone series parallels that found in the acyclic series. Although both 2,4-di-t-butyl-5,5-dime thylcy clo hex an one (41) and 2,2,6,6- tetraethy Icy clohexanone (43) show only a single positive effect the one observed for 2,2,6,6-tetramethylcyclohexanone (42), fig. 3 is the largest yet encountered. By way of contrasting the effects produced by several alkyl groups in the alpha positions the MCD spectra of 3,3,5-trimethyl-cyclohexanone (44) and 3,3,5,5-tetramethylcyclohexanone (45) are also shown in fig.3. A (nm) FIG. 3.-Absorption and MCD spectra of 2,2,6,6-tetramethylcyclohexanone (42) (. . .) 3,3,5-trimethylcyclohexanone (44) (- - -) and 3,3,5,5-tetramethylcyclohexanone (45) (-) in cyclohexane. Similar features are observed in the MCD spectra of several decalones ((46) (47), In particular the enhancement of the positive component produced (48) and (49)) 58 MCD OF CARBONYL COMPOUNDS by more than one alkyl group alpha to the carbonyl group is evident when the data for trans-1-decalone (46) and cis-9-methyl-1-decalone (47) are compared. The enhancement observed for a-haloketones (e.g. (51)) relative to the parent cyclanone, bicyclo[2,2,l]heptan-2-one (50) has already been noted. In the preceding discussion we have pointed out that in the acyclic and cyclic alkanone series the observation of a positive long wavelength MCD band could be correlated with the perturbing influence of alkyl groups on the local environment of the carbonyl group and that the intensity of the effect could be correlated with the substitutional symmetry in the a and a' positions.Adamantanone (52) however is the only sample investigated which can be rigorously considered as having CZv symmetry. In fig. 4 (52) shows only a positive but structured effect whereas 2,4-dimethyl-3-pentanone (12) exhibited (in cyclohexane) two oppositely signed effects. The MCD spectrum of dimethyl adamantan-2-one 1,5-dicarboxylate (53) is also displayed in fig. 4 for comparison. .I I I L- 1 (nm) FIG. 4.-Adsorption and MCD spectra of adamantan-2-one (52) (-) in cyclohexane and dimethyl adamantan-2-one 1,5-dicarboxylate (53) in acetonitrile.DISCUSSION These results show that the MCD spectra of carbonyl compounds in the region of the n-n* transition are more complex than had been supposed. Furthermore the signal-to-noise figures as well as the structure correlation noted suggests strongly that these features are real and cannot be associated with instrument artifacts. We now consider briefly some explanations which could rationalize the observed spectra G . BARTH E. B U N N E N B E R G C. DJERASSI D. ELDER R . RECORES 59 The possibility that the S-shaped curves which are observed in many samples can be classified as A terms l3 can be excluded since even in the most symmetrical com-pound investigated adamantanone (52) the symmetry is only C2,.A second possibility is that the MCD effects observed are associated with an accidental electronic degeneracy in the 290 nm absorption band such as the one observed in the 260nm band of the adenine ~ h r o m o p h o r e . ~ ~ ~ ~ ~ Excluding the occurrence of a " mystery band " the most likely possibility is the S - T(n - n*) transition and then only if the S(n-n*)-T(n-n*) interval for carbonyl compounds is in general much smaller than that (2996 cm-l) found for formaldehyde. The available data suggest, however that this is the case only for conjugated systems.16 Although we have recently been able to observe l7 an extremely weak S-shaped effect for benzophenone (in cyclohexane) at about 542 nm similar attempts with 2-propanone and cyclo-hexanone were not successful.A third and to us the most reasonable rationalization, is that for the locally electric dipole forbidden n-n* transition of aldehydes and ketones MCD is particularly sensitive to the ratio of the formally symmetry-allowed part of the transition to that part which is locally forbidden (but which is observable due to a non-totally symmetric vibration as in formaldehyde).lg The n-n* system of benzaldehyde 2o and other carbonyl compounds have been considered in this manner as well as the doubly-signed CD Cotton effects found in particular optically active ketones such as norcamphor.22 One can consider three situations with regard to the effective local symmetry of the carbonyl group in particular compounds. First for samples of Czu symmetry, such as adamantanone (52) the transition is symmetry-forbidden but vibrationally allowed.The polarization is constant throughout the band and one observes an MCD envelope of a single (positive) sign. If 2-propanone ( I ) is regarded as being effectively of Cz symmetry then it is necessary to invoke a different vibration than the one for adamantanone in order to rationalize the observed negative sign. An alternative explanation for the difference in signs observed in the two cases is that the effective local symmetry of the carbonyl group in 2-propanone is lower than C2u. Secondly in compounds of lower symmetry the polarization of the symmetry-allowed part cannot be the same as that of the symmetry-forbidden but vibrationally-allowed part.21 The relative intensities of the two bands of opposite sign is a reflection of the contribution of the two parts.Thirdly the symmetry obtaining in particular cases should be considered. For example although 2,2,4,4-tetramethyl-3-pentanone (15) shows only a singly-signed effect steric interactions necessarily pre-clude the attainment of even approximate C, symmetry. In conclusion the results presented illustrate the range of features that may be expected in the MCD spectra of saturated carbonyl compounds. It is however, difficult to assess the level at which stereochemical information appears. Although the suggestion of an underlying pattern can be imagined in a particular series experi-mental confirmation of a sector rule 23 must await investigation of particular and conformationally well-defined systems.In flexible systems low-temperature measurements are definitely of value and are contemplated in our laboratories. We express our gratitude to Professors A. Moscowitz (University of Minnesota), P. J. Stephens (University of Southern California) and 0. E. Weigang (Tulane Uni-versity) and to Dr. B. Briat (Laboratory of Physical Optics EPCI Paris) for stimu-lating discussions. Financial assistance was provided by the National Science Foundation (Grant no. GP7432) and by the National Institutes of Health (Grant no. GM 12173 and AM 12758). G.B. was the recipient of a Eugene Wigner Sti-pendium 60 MCD OF CARRONYL COMPOUNDS A. Cotton and R. Servant Compt. rend. 1942 214 513. F. H. Garner C. W. Nutt and A. Labbauf J . Inst. Petr. 1955,41,329.B. Briat M. Billardon and J. Badoz Compt. rend. 1963 256 3440. B. Briat Compt. rend. 1964 258,2788. V. E. Shashoua J . Amer. Chem. SOC. 1964 86,2109. J. Winkler Chem. Comm. 1968 648. A. W. Norvelle and P. J. Stephens Chem. Comm. 1966 520. S. R. Hawkins and J. H. Harshman Rev. Sci. Instr. 1967 38 50. Sproul Scientific Instruments Boulder Creek California through Durrum Instruments Corp., Palo Alto California. lo B. Briat D. A. Schooley R. Records E. Bunnenberg and C. Djerassi J . Amer. Chem. SOC., 1967,89,6170. l1 P. N. Schatz A. J. McCaffery W. Suetaka G. N. Henning A. B. Ritchie and P. J. Stephens, J . Chem. Phys. 1966,45,722. l2 G. Barth E. Bunnenberg and C. Djerassi Chem. Comm. 1969 1246. l3 A. D. Buckingham and P. J. Stephens Ann. Rev. Phys. Chem. 1966,17,399. l4 W. Voelter R. Records E. Bunnenberg and C. Djerassi J. Amer. Chem. SOC. 1968 90 6163. l5 W. Voelter G. Barth R. Records E. Bunnenberg and C. Djerassi J . Amer. Chem. SOC. 1969, l6 S. P. McGlynn T. Azumi and M. Kinoshita Molecular Spectroscopy of the Tr@let State l7 unpublished work with B. Briat. l9 J. A. Pople and J. W. Sidman J. Chem. Phys. 1957,27,1270. 2o J. M. Hollas E. Gregorek and L. Goodman J. Chem. Phys. 1968,49,1745. 21 L. Goodman and J. M. Hollas PhysicaZ Chemistry An Advanced Treatise ed. H. Eyring, 22 0. E. Weigang J. Chem. Phys. 1965,43 3609. 23 0. E. Weigang private communication. ’ A. J. McCaffery G. N. Henning P. N. Schatz A. B. Ritchie H. P. Perzanowski 0. R. Rodig, 91 6165. (Prentice-Hall Inc. Englewood Cliffs New Jersey 1969) p. 85. J. M. Hollas Spectrochim. Acta 1964 20 1563. D. Henderson and W. Jost (Academic Press New York 1969) vol. 111 chap. 7

ISSN:0430-0696    

DOI:10.1039/SF9690300049

出版商:RSC    

年代:1969

数据来源: RSC

摘要:

Magneto Optical Rotation Spectroscopy and its Application to Studies of Biological Molecules BY VICTOR E. SHASHOUA Dept. of Biology Massachusetts Institute of Technology Cambridge, Massachusetts Received 3rd November 1969 The general features and method of measurement of magneto opical rotation (MOR) spectra are described. A number of structure-property relationships are derived from studies of the MOR spectra of haemoglobin cytochrome c and chlorophyll a and b. The results suggest that in special cases conformational changes can came indirect interactions with the chromophoric groups of these biological molecules to give changes in their MOR spectra. Magneto optical rotation (MOR) spectroscopy is based on Faraday’s discovery in 1846 that any molecule will rotate the plane of polarized light when a magnetic field is applied parallel to the light beam.This phenomenon has been the subject of many investigations at the sodium D-line frequency.2 Cotton and Scherer were first to carry out dispersion measurements at the absorption region of the visible spectrum of cobaltous chloride solutions. This observation suggested the possibility that the Faraday effect might be useful for extending the optical rotatory dispersion (ORD) technique with its limitation to naturally optically active compounds to the study of the stereochemistry of non-optically active molecules. Ill IV v l + - MOR MCD + ABS FIG. 1.-General types of MOR and MCD spectra. The initital work was carried out with magnetic fields of up to 18 000 G.5 The results showed that MOR spectra could be classified into five types.Fig. 1 illustrates the dispersion features of the magnetic rotation for the five types of MOR spectra observed at the absorption band regions of the molecules. In addition the figure depicts the corresponding magnetic circular dichroism (MCD) curves derived from MCD measurements on CoC1 and FeCl by Schooley Bunnenberg and DjerassL6 Types I and I1 MOR spectra have positive and negative sigmoid curves respectively, with inflection points coincident with the absorption maxima. Types I11 and IV 6 62 M O R SPECTROSCOPY -B I 0 LO G I CA L MOLECULES MOR spectra have negative and positive magnetic rotation peaks at the position of the absorption band maxima. Type V shows no anomalous dispersion features and may be positive or negative depending upon the sign and magnitude of the magnetic rotation for neighbouring absorption bands.The apparent similarity of the MOR spectral types to those obtained in ORD measurements however does not lead to any simple correlation of the sign and shape of the MOR spectrum with stereo-chemical features of molecules. Experimental investigations in a number of labora-tories 8-11 and the theoretical studies of Buckinghani and Stephens suggest that MOR and MCD spectroscopy as analytical techniques are applicable to a different variety of problems than those which can be studied by ORD and CD methods. These include (1) assignments of the positions of excited states in complex spectra 8-1 (2) the determination of magnetic moments of excited states of molecules 9* lo and (3) the determination of the polarization of magnetic moments of excited states.8 In addition there are a number of interesting structure property correlations which can be deduced from studies of the MOR spectra of certain biologically active mole-cules.These experimental investigations are reviewed in this paper as examples of special cases where conformational changes might cause an indirect influence on the MOR spectra. EXPERIMENTAL M E T H O D OF MEASUREMENT Fig. 2 shows a diagram of the instrument l 2 used for measuring the MOR spectra. The equipment is designed to measure the magnetic rotation induced by a 5000 G field as a function of wavelength with an angular sensitivity of ~0.001" and ~k0.003" at the visible and u.-v.spectral regions respectively. In the operation of the instrument a plane polarized light beam is rotated by an angle (@+a) in the first solenoid where 8 is the angle of rotation POLAR IZER SOLUTION MAGNET I SOLVENT MAGNET II MAG N E Ti: FIELD El YAGNETIC FIELD PHOTO -MULTIPLIER FIG. 2.-Diagram of the MOR spectropolarimeter. Thc operation of the i nstrumcnt is illustrated in the inset 8 solvent+sample cell rotation; a solute rotation and 4 modulation angle in the Faraday coil. by the solvent plus sample cell and o! is the rotation of the solute. The light beam then passes through a second solenoid with a magnetic field polarized in the opposite direction. This contains a sample cell with pure solvent to give a rotation of (-0). The net rotation of the polarized light after passing through the two magnetic fields is CI the solute rotation.The light beam then passes through a third a.c. magnetic field which modulates the light beam by using the induced Faraday rotation in a water sample 20 c ni long. This provide V l C l O K E . SHASHOUA 63 the carrier frequency for activating the angle sensing and recording sections of the apparatus. The light source was a 150 W Xe arc lamp. A Cay model 14 double monochromator l3 provided the spectral purity of wavelength selection with a stray light level of less than 1 part in lo6. The sample temperature was maintained to within 0.1"C. Under the experimental conditions used the light beam was along the north-south direction in the first magnetic field. Measurement of a clock-wise rotation of the plane of polarized light for observations opposite to the direction of travel of the light beam was designated as a positive rotation.This is in the same sense as the Faraday rotation for a pure solvent outside its absorption band region. The specific magnetic rotation [a], is defined by where 8 is the angle of rotation in degrees I the path length in dm c the concentration in g/ml and H i s the magnetic field in G. = 10 000 e l m MATERIALS The bovine haemoglobin were commercial grade samples from Sigma Chemical Company, St. Louis Missouri. The cytochrome c samples were obtained from Dr. R. W. Estabrook of the University of Pennsylvania. The chlorophyll and methyl pheophorbide a samples were obtained from Dr. M. Calvin and J.Anderson. RESULTS AND DISCUSSION MOR SPECTRA OF HAEMOGLOBIN A N D METHAEMOGLOBIN The haemoglobin chromophore is an octahedral complex with four nitrogen ligands provided by the planar porphyrin nucleus a globin at the fifth position and various substituent ligands at the sixth position about the central iron atom. Table TABLE 1 .-MOR SPECTRAL DATA FOR HAEMOGLOBIN pH = 6.8 10°C magnetic absorpt. MOR data P) moment maxima Il/nm [=Is 12/nm [akp B.M.(d) nm 554 550 45 585 150 4.9 540 538 1 00 574 510 0 540 530 210 0 572 570 650 542 535 118 577 576 785 no. unpaired electrons 4 0 0 0 (a) The [aIsp values of the MOR data are the sum of the positive and negative components of the type I11 MOR spectra as illustrated by A in fig. 5 ; (b) commercial grade bovine haemoglobin ; (c) horsc haemoglobin data ; (d) data obtained from ref.(15). 1 and fig. 3 show the MOR and ORD spectral data obtained for a number of ferro-haem~globins,'~ with HzO 0 and CO as the ligands. The natural optical rotations of these molecules are quite small for the spectral regions 450-650 nm whereas the MOR data show substantial magnetic rotations with dispersion features characteristic of a type 111 spectrum. The magnitude of the specific magnetic rotation at the 570 nm band of bovine ferrohaemoglobin increases about four fold from 150 to 650 for replacement H20 with CO as the ligand. As shown in table 1 there seems to be no relationship between the specific magnetic rotation data to paramagnetism of the samples. However a large magnetic rotation is obtained for substituents with a large ligand field.I n addition the magnitude of the specific magnetic rotatio 64 M O K S I’ I C 1’KOS CO Y Y - B 1 0 LOG 1 C A L MOLECULES *0° I----- 7 0 -200 0 -200 B -U -400 0 -200 1.0 0 pjfzj Hb-CO 0 500 600 wavelength nm FIG. 3.-MOR and ORD spectra of bovine ferrohaemoglobins with HzO O2 and CO ligands. seems to change with the source of the oxyferrohaemoglobin. The magnetic rotation for horse oxyferrohaemoglobin is 50 % higher than for bovine oxyferrohaemoglobin at the 576 nm band; however at the 535 nm region the magnetic rotation is about the same for the two proteins. This may be indicative of a ligand field change at the chromophore of the two proteins. Table 2 summarizes the results for methaemoglobin (ferrihaemoglobin) and its derivatives.In these molecules the four-absorption-band system characteristic TABLE 2.-MOR SPECTRAL DATA FOR METHAEMOGLOBIN pH 6.8 10°C band I band I1 band I11 band IV magnetic unpaired ligand Al/nm [a] sp 12/nm [a] sp 13/nm [m] sp A4/nm [a] sp moment (a) electrons 630 50 567 36 540 20 500 28 5.20 (4) F 606 95 - - 550 30 485 38 5.92 4 HzO OH@) 607 32 575 95 538 38 - - 4.47 4 CN - - 565 470 540 129 - - 2.5 1 (0) The magnetic moment data were obtained from ref. (15) (16); (b) this was studied at pH 10. of the high spin state of methaemoglobin with water as the ligaiid changes to the two absorption band system of methaemoglobin cyanide with a low spin state. The ORD spectra l4 of these molecules for the 450-650 nm region are small and show no substantial changes for various substituents.The MOR spectra however show large magnetic rotation changes. A comparison of A as defined in fig. 5 for th VICTOR E . SHASHOUA 65 band I11 system (540 nm) shows an increase in the specific magnetic rotation from 20 for H,O as the lilrand to 129 for cyanide as the limand. Similar changes occur for the band I1 system at the 570nm region i.e. from 36 to 470. All the MOR spectra are of type 111. A large magnetic rotation appears to be characteristic of methaemoglobin cyanide (low spin state) while small magnetic rotations are observed for the high spin haemoglobin compounds with H20 OH and F as the ligands. These results suggest a correlation of the magnitude of magnetic rotation with the spin state of the compounds.MOR SPECTRA OF CYTOCHROME C The MOR spectrum of cytochrome c is very sensitive to the oxidation state of the m01ecule.l~ The specific magnetic rotation changes from - 8000 to - 150 at 546 nm. The type I11 magnetic rotation at the 549 nm region (see fig. 4) of the spectrum was used for the study of the kinetics of oxidation and reduction of the 0 0 . 2 0 -so2 0 d 2 .- -.04 Y -.O 6 .-o 8 - - I 0 5 0 0 5 5 0 5 0 0 5 5 0 wavelength nm FIG. 4.-MOR spectra of ferrocytochrome c derived from yeast and pigeon breast. TABLE 3.-MOR SPECTRAL DATA-CYTOCHROME C pH = 7 pH = 10 source A B A B yeast 1.77 0.22 1.70 0.37 Tuna 1.75 0.21 1.80 0.21 horse H. 1.79 0.23 1.71 0.15 pigeon B. 1.75 0.23 1.71 0.15 A is the ratio of optical densities at 550 to 520nm.B is the A MOR divided by the optical density. molecule.ls One interesting feature of the MOR spectrum of cytochrome c was observed in a study of samples derived from different sources.19 Fig. 4 compares the MOR results for yeast and pigeon breast ferrocytochrome c at pH 10 for the a and p band absorption regions. The ORD measurements are quite small for this region. Table 3 presents an analysis of the 549 nm MOR rotation for different samples at pH 7.0 and pH 10. All the samples were first converted to the fully reduced state and the spectra were determined in the presence of reducing agent s3-66 MOR SPECTROSCOPY-BIOLOGICAL MOLECULES (formamidine sulphinic acid 20) under a nitrogen atmosphere. The ratio 1.7 to 1.8 of the optical densities of the a and j3 bands was used as a criterion of the fully reduced state of ferrocytochrome c.As shown in table 3 the magnetic rotation per optical density unit (B in table 3) varied for the different samples when the measurements were carried out at pH 10 near the isoelectric point of ferrocytochrome c ; but no change occurred at pH 7.0. Thus yeast cytochrome c had over twice the magnetic rotation of pigeon breast cytochrome c. There are a number of explanations which might account for the observed data. Since all the molecules have the same chromo-phoric group with a ferrous atom at the centre of an octahedral complex the changes in the observed magnetic rotation cannot be due to a major structural change. One possibility similar to the data for ferrohaemoglobin is the use of different ligands in the various cytochrome c molecules.Another alternative is that the known different polypeptide sequences near the chromophoric group can provide a change in the effective pH around the porphyrin nucleus. It is known that pH changes can vastly influence the MOR spectrum of cytochrome c,l* thus it would 500 510 520 530 540 550 560 570 wavelength nm 1 ~ 1 1 1 1 1 1 1 1 10 20 . 30 40 50 60 70 80 temp. "C FIG. 5.-Teniperature effects on the MOR spectrum of ferro and ferricytochrome c. Fig. 5~ shows the spectra for ferrocytochrome c at 16 and 71.5"C. Fig. 5~ is a plot of A for a 0.05 % solution for ferrocytochrome c and A at 0.18 % solution for ferricytochrome c. not be surprising that such a change could result in a conformation change and that this would more easily occur at the isoelectric point of the molecule.This example illustrates the possibility that structural changes can influence the observed MOR spectrum; Another example of indirect stereochemical effects on MOR spectra is illustrated in fig. 5. A study of the magnetic rotation of ferrocytochrorne c. in the presence o 67 METHYL PHEOPHORBIDE a in CCL,, -48-I ' / / I 1 I ;ABSORPTION -\ ' \ -,AT 0.0031 O/o 1 I I I \ \ \ L A - __A_- -r'Lt-- ' f? I 2 3 77 X Y FIG. n I 2 X a T Y .- s .c - a 1.0 FIG. -1.0-- 2-0 METHYL PHEOPHORBIDE a in NMP 350 400 450 500 550 600 650 700 750 wavelength nm 7.-MOR and ORD spectra of methylpheophorbide a in N-methylpyrrolidone (NMP).,/'.\ ABSORPTION ' \\ AT 0.002 O/o _-___. T A- _ _ _ _ _ _ _ _ - - _--- I_ I . 300 350 400 450 500 550 600 650 700 wavelength nm FIG. 8.- MOR and ORD spectra of chlorophyll a 68 MOR SPECTROSCOPY-BIOLOGICAL MOLECULES reducing agent as a function of temperature showed that the magnetic rotation can decrease at elevated temperatures. Thus a plot of A (as defined in fig. 5) as a function of T shows a progressive decrease from 15 up to 71.5"C. Moreover upon cooling the rotation returns to approximately its original value indicating that no permanent configurational change had occurred. This type of temperature effect was not observed for the oxidized ferricytochrome c. It is possible that a change in the ligand field around the octahedral complex of the porphyrin nucleus can occur in ferrocytochrome c but not in ferricytochrome c where the extra valence of the iron atom provides stability.The ligand field change could arise from indirect effects of temperature-induced conformational changes in the polypeptide fragments of the molecule. These could produce a decrease in the ligand field in the ferro compound but may not affect the more tightly bound ferri structure of the molecule. 0 - 1.0 - 2.0 I ) I \ -/// ABSORPTION - 1.0 I , \ /,' \,AT 0.002 Yo x. _ _ _ - ~ - _ - - --. - - - - - -I z-.-L_ '-t-wavelength nm FIG. 9.-MOR and ORD spectra of chlorophyll 6. TABLE 4.-MOR SPECTRAL DATA FOR METHYLPHEOPHORBIDE a AND CHLOROPHYLL a AND b A/nm type Ilnm type A/nm type A/nm type methylpheophorbide n chlorophyll a chlorophyll b CC14 solvent NMP solvent CCI4 solvent CC14 solvent I - - _ L - l L 67 1 I1 668 I1 665 I1 646 I1 61 5 I11 610 I11 618 111 ? 600 I11 ? 537 I 538 I 585 I 508 I11 ? 509 111 ? 460 IV 417 IV 41 5 IV 432 IV NMP N-methyl pyrrolidone 0 MOR SPECTRA OF METHYLPHEOPHORBIDE a AND CHLOROPHYLL a AND b Fig.6-9 and table 4 summarize the MOR spectral data for methylpheophorbide a and chlorophyll a and b. The magnetic rotations are of the same order of magnitude as the ORD data. At the dilute solutions studied the MOR spectral types seem to follow the same general pattern for all three compounds. Methylpheophorbide a as the chromophoric group for the chlorophylls has the most distinct features with a type I1 MOR spectrum at the 671 nm band and a type I at the 537 nm in CC14.By analogy to other studies in the porphyrins,8 these two bands should have magnetic dipole transitions polarized at right angles and in the plane of the porphyriii nucleus VICTOR E . SHASHOUA 69 Briat and Djerassi 21 have reported the MCD spectrum for niethylpheophorbide a. These seem to have different relative intensities from the observed MOR data. Such variations might arise from the known association of the molecules at higher concentra-tion.22 In fact MOR and MCD spectroscopy may be useful in studying such inter-molecular association^.^^ The results described in this paper summarize a number of structural effects in special types of molecular interactions that can be observed by MOR spectroscopy.' M. Faraday Phil. Trans. 1846 3 1. for a general review see J. R. Partington Advanced Treatise on Physical Chemistry (Longmans, Green and Company London 1954) vol. IV pp. 592-632. A. Cotton and M. Scherer Compt. rend. 1932 195,1342. V. E. Shashoua J. Amer. Chem. SOC. 1960 82 5505. V. E. Shashoua J. Amer. Chem. SOC. 1964,86,2109. D. A. Schooley E. Bunnenberg and C . Djerassi Proc. Nat. Acad. Sci. 1965,53 579. V. E. Shashoua J. Amer. Chem. SOC. 1965,87,4044. B. Briat D. A. Schooley R. Records E. Bunnenberg and C. Djerassi J . Amer. Chem. SOC., 1967,89,7062. J. G. Foss and M. E. McCarville J. Amer. Chem. SOC. 1967 89,30. ' A. D. Buckingham and P. J. Stephens Ann. Rev. Phys. Clzem. 1966 17 399. lo A. J. McCaf€ery P. J. Stephens and P. N. Schatz Inorg. Chem. 1967 6,1614. l 2 J. G. Forsythe R. Kieselbach and V. E. Shashoua J . Appl. Optics 1967 6,699. l3 H. Cary R. C. Hawes P. B. Hooper J. J. Duffield and K. P. George Appl. Opt. 1964,3 329. l4 V. E. Shashoua in Hemes and Hemoproteins B. Chance R. W. Estabrook and T. Yonetani ed. l5 C. D. Coryell F. Slitt and L. Pauling J. Amer. Chem. SOC. 1952 198 33. l6 D. Keilin and E. F. Hartree Biochem. J. 1951,49,88. (Academic Press New York 1966) p. 93. V. E. Shashoua Nature 1964 203,972. V. E. Shashoua Arch. Biochem. Biophys. 1965,111,550. and T. Yonatani ed. (Academic Press New York 1966) p. 427. l9 V. E. Shashoua and R. W. Estabrook in Hemes and Hemoproteins B. Chance R. W. Estabrook 2o V. E. Shashoua Biochem. 1964,3,1719. 21 B. Briat and C. Djerassi Nature 1968 217,918. 22 R. L. Clayton The Chluruphylls. L. P. Vernon and G. R. Seely ed. (Acacemic Press N. Y., 23 E. A. Dratz Ph.D. Thesis (University of California Berkeley Calif. 1966). 1966). p. 610

ISSN:0430-0696    

DOI:10.1039/SF9690300061

出版商:RSC    

年代:1969

数据来源: RSC

摘要:

Magnetic Circular Dichroism and the Assignment of Charge Transfer Transitions in Tetrahalide Complexes BY B. D. BIRD,* B. BRIAT,? P DAY * AND J. c. RIVOAL t *Inorganic Chemistry Laboratory South Parks Road Oxford "f.P.C.T. 10 Rue Vauquelin Paris-Ve France. Received 17th October 1969 Magnetic circular dichroism (MCD) spectra are reported for the charge transfer transitions in a large number of tetrahalide complexes of first transition series ions. Spectra of all the compounds have been obtained from solutions at room temperature and also for Co1:- and NiIi- from evaporated films and doped crystals at various temperatures down to liquid helium temperature. Assignments are discussed in terms of models based on Russell-Saunders first-order spin-orbit or j,j-coupling depending on the size of the halogen spin-orbit coupling constant.Precise unambiguous assignments of charge transfer spectra in metal complexes are difficult to obtain. At present there is no theoretical model available which will reliably predict the energies of such transitions so we must seek other observable properties of the spectra. Observables depending on the angular momenta of ground and excited states are likely to be most useful and MCD spectra have already provided valuable information about charge transfer transiti0ns.l. In this paper we report MCD measurements on charge transfer bands in tetra-halide complexes of first transition series ions both on solutions at room temperature and in some cases on films and doped crystals down to liquid helium temperatures.Our theoretical framework for analyzing the MCD data follows the basic approach laid down in our earlier paper on the intensities and spin-orbit splittings in tetra-halide charge transfer spectra i.e. we use vector-coupling methods in the formalism given by Griffith4 to derive expressions for the MCD in terms of the parameters conventionally called A B and C for the various configurations and states concerned. The latter are expressed in terms of reduced matrix elements of orbital and spin angular momentum. At first we derive state reduced matrix elements and then further reduce them to one-electron matrix elements. At that stage one may con-front theory and observation by adopting an LCAO molecular orbital scheme and expressing the one-electron reduced matrix elements in terms of atomic eigenvectors, obtainable from theoretical calculation.Our hope however is that even without making use of the precise numerical values of these matrix elements symmetry arguments alone may yield predictions about signs and magnitudes of the MCD parameters associated with the various possible configurations and states so that definite assignments of the observed bands can be established. EXPERIMENTAL The tetrahalide complexes were prepared as tetraethylammonium salts by standard methods.6 Samples were dissolved in CH2Cl containing a large excess of halide ion to prevent solvolysis. Evaporated films were prepared by the method used previo~sly.~ MCD of the CoIi- solution was essentially similar to that of the evaporated film and hence is not reported here.The MCD and absorption spectrometers and the cryostat used have 7 B . D . BTRD B . BRIAT P . D A Y A N D J. C . RIVOAL 71 been described elsewhere.* Low temperature absorption spectra of the crystals were measured both in Oxford and in Paris with good agreement. Crystals of (N(C2H5)&Zn14 doped with either CoIi- or NiIi- were grown by slow evaporation from CH3N02 solutions. To obtain reliable baselines for the low temperature MCD measurements spectra were first recorded in zero magnetic field. RESULTS MCD is expressed in terms of the magnetic dichroic optical density per unit magnetic field [AD] and absorption in terms of optical density D. At room temperature and even in some cases at liquid helium temperature (e.g. NiTi-) there are many absorption and MCD bands which overlap strongly and hence extraction of parameters from the data is difficult.For CuC1;-(300") and NiIi- (6") the components were extracted by a method described else-where? In all other cases only rough estimates of the magnitudes of the absorption and MCD components were made by considering the halfwidths and peak-trough separations in the spectra. Hence in comparing theory and observation we place little weight on the precise values either of the theoretical or experimentally derived parameters. Only the signs and relative magnitudes are considered significant. The results are presented in fig. 1-6. FIG. 1 . I I 5 0.5 n 1 (nm) CHzCll solution at room temperature. -MCD (full lines) and absorption spectra (dotted lines) of (a) FeCI and (b) FeBr; i 72 Of 0 -05 - I I - I CHARGE TRANSFER IN TETRAHALIDES -___I- c u a,*- solution I I- I FIG.2.-MCD (full lines) and absorption spectra (dotted lines) of (a) CuC1:- and (b) CuBri- in CH2C12 solution at room tem- 7 I 0 - FIG. 3.-MCD (full line) and absorption spectrum (dotted line) of CoBrz- in 9 CH2C12 at room temperature. 2 - I -2 perature. solution B \ \ \ 8 1 \250 I \ \ I 1 3.5 B. D . BIRD B . BRIAT P. DAY A N D J . C. RIVOAL 73 0.5 0 A (nm) [Fig. 4 continued overlea 74 CHARGE TRANSFER I N TETRAHALIDES A (nm) FIG. 4.-MCD (full lines) and absorption spectra (dotted lines) of CoIi- (a) as an evaporated film of the tetra-n-butylammonium salt at 300 K and 100 K and (b) doped into a crystal of [(C2H5)4N]2ZnT4 at 29 9.3 and 7.2 K.A (nrn) (4 FIG. 5.-MCD (full lines) and absorption spectra (dotted lines) of tetrahalogenonickelate(I1) ions in CH,Cl at room temperature (a) NiCIz- (b) NiBri- (c) NiTi-B . D . BIRD B. BRIAT P . D A Y AND J . C. RIVOAL 5 - 5 T = 102 K I I 75 1 0.5 0 I 3.5 0 FIG. 6.-MCD (full lines) and absorption spectra (dotted lines) of Nig- doped into a crystal of [(C2H5)4N]2Zn14 at 300 102 and 6.5 K 76 CHARGE TRANSFER I N TETRAHALIDES THEORY AND ASSIGNMENTS Employing a basis set consisting of the valence-shell s- and p-orbitals of the halogen and the d-shell of the metal the ground-state configurations and terms of the ions we are interested in are as follows : FeX, ( le)4(3 2)6(t 1)6(2e)2(4t2)3 ; 6Al COX - (1 e)4(3 t 2)6( t 6(2e)4(4t 2)3 ; 4A Nix - ( le)4(3t2)6(t 1)6(2e)4(4t2)4 ; Tl CuXz- (le)4(3t2)6(t1)6(2e)4(4t2)5 ; r2 In the CuXi- however there is a lowering of point symmetry from T d to D2d with the result that e+al + bl tl -+a2 + e and t2-+b2 + e.Paramagnetic resonance has shown that the ground state is the orbital singlet 2Bz(. . . e4bi). Our previous work on the intensity and spin-orbit splitting of the charge transfer states in the tetrahalides suggested the orderingof the n-orbitals of mainly ligand character was (le) <(3t2) < ( t l ) and so we are concerned with excited configurations such as ( le)4(3t2)6( t 1)5(2e)3(4t2)3(FeX,) and the terms arising from them. Spin-orbit coupling at the halogen is an important feature lo of the charge transfer spectra of halide complexes because the electrons are excited from orbitals of pre-dominantly ligand character.On the other hand the effect of electron repulsion on the terms of a charge transfer configuration has hardly received attention and little is known about the magnitude of the possible splittings. Thus it seems wise, in attempting to assign a wide range of halide charge transfer spectra to consider both of the extreme situations of negligible and dominant spin-orbit coupling energy, i.e. pure Russell-Saunders and purejj and also as far as possible for such complicated excited states intermediate coupling. We take each of these cases in turn using as illustrations those halides for which we expect each type of approximation to be most valid.(1) RUSSELL-SAUNDERS COUPLING In the Russell-Saunders limit electric-dipole-allowed transitions can occur from 6T2~6Al(FeX:) 4T1 t4A2(CoXi-) 3A2 3E1,3T1 3T2+-3Tl(NiX$-) and 2A1, 2Et2B2(C~Xi-) so that for all except the nickel (11) compounds a single allowed term exists for each charge transfer excited configuration. Furthermore in all cases except Nix:- the ground states are orbital singlets and hence in the Russell-Saunders approximation C-terms need not be considered. On the other hand all the excited states except 3A2 of NIX:- and 2A of CuXz- are orbitally degenerate, so that A-terms are expected in most cases. Referring to fig. 1-6 pronounced A-terms are indeed observed in CuX$- and FeX:. We therefore consider these in greater detail. For FeXz where the transitions are 6T2t6A1 the expression for AID is the same as that given by Schatz et aZ.l for the IT2+- ‘ A transitions in Mn04 because the spin wave functions are separable.Thus * AID = -<i/Js)(T2 II L II T2). {arx I gj I a’rx’) = ( l / ~ / A ( a ) ) ( - l ) a + a ’ + b ( a II gb II a’><ba’pct’ I act), (1) * Throughout this paper we define reduce matrix elements <a jl gb II a’> according to the equation : where A(a) is the degeneracy of the state a B . D . B I R D B . BRIAT P . DAY AND J . C . RIVOAL 77 Values of (Tz I] L 11 T2) are then compiled for each of the orbital excitations e e t , t 2 e t l etc. and are given in table 1. All the excited configurations resulting from such one-electron transitions contain three open-shells so a further generalization of the equation given by Griffith (e.g.10-15 ref. (4)) is required to evaluate them. Finally the one-electron reduced matrix elements in table 1 are expressed in terms of LCAO eigenvectors by expanding the molecular orbitals and eliminating two-centre terms. At the same time the operator I centred on the metal atoms must be transformed to each of the ligand coordinate systems. As before the orbital combina-tions defined by Gray and Ballhausen are also employed giving where c1 and c2 are the coefficients of the metal 4p and 3d and c3 and c4 are those of the ligandpn and p a . Substitution of the appropriate eigenvectors from a self-consistent field MO calculation l2 on FeBri yields the predicted values for AID shown in table 1. The MCD spectra in fig.1 contain a clea-rly-defined negative A-term under the lowest energy transition in agreement with our previous assignment of this band as e t t . As for MnO and TiC14 the sign and magnitude of AID for the e t t , transition does not depend on our choice of eigenvectors. The other clearly defined A-term lies under the fourth transition of FeBr; at 282 nm with the possibility of an analogous band at 243 nm in FeC14. On the grounds of their dipole strength these bands have previously been assigned to 4t2+-3t2 excitation. The observed positive sign of A at 282 nm in FeBri is compatible with this assignment (see table l) but the other MCD terms overlap so strongly that no other firm conclusions can be drawn. To interpret the A-terms observed in CuCIi- and CuBri- calculations must be performed within the D2d point group.A-terms can arise from E t 2 B 2 transitions resulting from excitations which in Td would be 4t2ctl or 4t2c3t,. We must consider for example a transition from the (x y ) components of tl(Td) transforming as e in D2d to xy the [-component of 4t2(Td) which transforms as b in Dad. Hence the calculation of AID for 4 t 2 t t (Td) can be performed without introducing eigen-vectors for 4t2 the degeneracy of the latter having been lifted. Then AID for 2 E e 2B2(4t2 f- t,) is equal to + $ and for ' E f - ,B2(4t2 +-3t2) is - ( J$)(icg - c$ -J2c3c,) where the eigenvectors relate to 3t2(Td). Taking eigenvectors for 3t2 from the calculation performed by R0s,13 AID for 4t2t3t2(Td) is predicted to be +0.60.Experimentally we find positive A-terms under the two major bands in the spectra (410 and 294 nm in CuCIi- and 515 and 350 nm in CuBri-). These bands had previously been assigned as the 2E(4t2ttl) and 2E(4t2t3t2) a con-clusion now reinforced by the MCD spectra. For CuCli- the MCD spectrum has been resolved into its components and we find experimental values of AID equal to +0.15 at 410 nm and +0.60 at 294 nm 78 CHARGE TRANSFER IN TETRAHALIDES The excellent agreement of the observed and calculated parameters suggests that a Russell-Saunders coupling scheme is a good approximation at least for these transitions. (2) FIRST-ORDER SPIN-ORBIT EFFECTS The most straightforward method of including the effect of spin-orbit coupling on charge transfer states is to assume that each Russell-Saunders state to which transitions are electric-dipole-allowed is split in first order only.That is we ignore the possibility of interaction between successive Russell-Saunders terms belonging to different excited configurations and also ignore states which would be dipole-for-bidden in the absence of spin-orbit coupling. Thus we could calculate A-terms for each degenerate spin-orbit component and also the B-terms resulting from mixing between them. There is also the possibility that C-terms could arise from the spin part of the magnetic moment operator in the ground state. in CoBri- where reasonable agreement between theory and observation was also found. A 4T1 state splits into E’ E” Ui and U; E and U& remaining degenerate to first-order.The energy difference between the El’ U& pair and U i is 5k and between U$ and E’ is 3k where for 4t2+tl The reduced matrix elements of the spin-orbit operator su defined previ~usly,~ can be evaluated like those of the magnetic moment operator using theoretically calculated eigenvectors. In this way it was found that the direction and magnitude of the splitting of 4T1 (4tzttl) and also the relative intensities of the spin-orbit components agreed with the assignment of the first three bands in the absorption spectrum. Thus we have calculated the MCD parameters for these transitions. An example to which the first-order theory has been applied in some detail k = (1/36)C(& II su TI 3tl> +(W2 II su II 34t2>l. (3) 3 1 1 3 FIG. 7.4ntensities and polarizations of the components of 4T1(E‘) -f4A2(U’) in a magnetic field.Under spin-orbit coupling the 4A2 ground state becomes U’ and we consider as an example the transition E‘(4T,)t U’(4A2). Both orbital and spin contributions to the diagonal matrix elements ( a 11 p z 11 a) where p z = -pB(L,+2S,) must now be included and in fact the spin part predominates. For example (E’a I 2Sz I E’a) = 5 while (E’cr I L I E‘a) has the magnitude & for a 4t2+-t transition. To determine the relative contributions from A and C to the experimental curve under these circumstances a particularly straightforward approach is that used b B . D . B I R D B . BRIA’T P . DAY A N D J . C . RIVOAL 79 Luty and Mort.14 The situation for E ’ t U’ is shown in fig. 7 which includes the relative intensities and polarizations of the four components.[AD] is found to be wherefis the shape of the individual lines. The spin-only g-values are 10/3 and 2 for E’ and U‘ respectively so that 2o df 5/’!Bf [AD] cc -pB-+-6 dv k T ’ Assuming that the half-width at room temperature is about 1000 cm-l (see fig. 3) the major contribution to [AD] can be shown to arise from the second term in eqn (4) i.e. the C-term. In table 3 we give the C / D terms for all the spin-orbit com-ponents of 4T1 and also the contribution C/kT to the MCD at 300 K. To compare these predictions with observation it is now necessary to estimate the relative magni-tudes of the C and B terms. In evaluating the B-terms resulting from mixing between the spin-orbit components it is again important to consider both the orbital and spin contributions to the off-diagonal matrix elements ( j ] pz I k).For example, When the eigenvectors from a self-consistent-field calculation l2 on CoBri- are substituted into the expression for (TI 11 L 11 Tl} it is found that the second spin, term in eqn (5) is approximately an order of magnitude greater than the orbital term. In table 2 we give both orbital and spin contributions to B for each pair of TABLE 2.-MCD B-TERMS FROM INTERACTION BETWEEN THE SPIN-ORBIT COMPONENTS OF 4T1 IN COX:-E‘ E“ LJi G E’ - 0 10 ip 20 _ _ _ _ - _ 2 7 4 6 9 0 - 2 ip 4 i 5 a - 5 32 ip 32 +- 1 3 5 4 6 15 - __ spin-orbit components of 4T1 in units of rn2/AE where m = (4A2 11 Y 11 4T1) and AE is the energy difference between the pairs of components.If the orbital contri-bution is neglected and the first-order energies of the components are used to evaluate the various AE we obtain expressions for B which involve only the k defined in eqn (3). E” and U& present a problem since they are accidentally degenerate in first order which would lead to two infinite B-terms of opposite sign i.e. a resultant of zero. We have assumed arbitrarily that they are split by k as a result of second-order effects and then have two predictions for the B-terms of those states depending on the order of energies chosen. k can be taken from the observed separation of the spin-orbit components in the absorption spectrum or estimated from the SCF calculation. Choosing the former alternative k - 500 cni-’ and the resulting value 80 CHARGE TRANSFER I N TETRAHALIDES of B are given in table 3.The dominant feature of the B-term contribution to the MCD is thus expected to be the two bands of opposite sign under the lowest energy absorption band. Two such bands are indeed found in the CoBri- spectrum, though with relative signs which do not agree with the predictions of table 3. The main conclusion to be drawn from table 3 is that at room temperature B and C contributions have comparable magnitudes and hence could only be separately estimated from temperature dependence experiments. However the sign of the U . band to which B and C contribute with the same sign is correctly predicted. TABLE 3.-cONTRIBUTIONS OF c AND B TO THE ROOM-TEMPERATURE MCD OF COX:-B U' lowest t 4T1 4A 2 D Cl D C/kT En lowest 1 1 6 +% +1.25 + 4.00 - 4.58 1 6 - 2.08 + 1.48 + 1.48 U& c U' - +* +2.50 - 4.48 + 4.08 E" t U' --1 - 1.67 - 1-00 - 0.99 E' c U' - - 5 u; t U' -D the dipole strength is in units of m2 where m = ( A 2 11 Y 11 T I ) ; C / D in pug ; C/kT and B in pg cm3 lo3.(3) INTERMEDIATE AND j j COUPLING For CoIi- on the other hand where we have been fortunate in obtaining spectra over a wide temperature range (fig. 4) it is certain that the major contribution to the observed MCD is derived from C-terms. However for iodides (cI = 5040cm-l) the simple first-order spin-orbit theory is unlikely to prove adequate. Its breakdown will be manifested in two ways transitions which would have been dipole- or spin-forbidden in the absence of spin-orbit coupling may appear strongly and interaction between excited states derived from configurations t 24t," and 3t24t," may become important since the separation of the baricentres of these two configurations is comparable to the spread of spin-orbit states arising from each.Our attempts to interpret the data of fig. 4 have therefore centred on diagonalization of the spin-orbit matrices for all the Russell-Saunders terms arising from each excited configuration. As a starting hypothesis we have assumed that interelectron repulsion is in-significant compared with spin-orbit coupling in splitting the charge transfer states, and have hence set all the Russell-Saunders terms of each configuration degenerate. If the complete set of Russell-Saunders terms were included in the spin-orbit diagon-alization this would be equivalent to a j j coupling scheme.The first excited con-figuration t24t," then gives rise to four bands corresponding to the excitations 4t,(y,,y,)tt1(y6,yk) with a major splitting of $cI and a minor one of 4Cmetal. Although two major band systems are indeed seen in the low-temperature absorption spectrum of CoIi- at 25 600 and 30 000 cm-l with separations corresponding to expectation (& = 3900cm-l) the spectrum in fig. 4(b) is clearly more complicated than this. One reason is almost certainly that the j,j-coupling approximation is not adequate for the ground state or for those parts of the excited configurations which are localized on the metal. For example the configuration y ' i y t is probably heavily mixed with y'&Q+ by interelectron repulsion within the d-shell.For the 4t part of the excited configuration it may therefore be more realistic to return to the Russell-Saunders limit. Then one may estimate that the three singlet terms lA1 lE lT2 of 4t; lie at least 6B+ 2C above the only triplet 3T1 a splitting of at least 12 000 cm-l. We have therefore constructed and diagonalized the spin-orbit matrices of all the Russell-Saunders quartets arising from t :4t; and of those doublets arising from 3T1(4t24). The individual matrix elements l 5 were finally expressed in terms of th B . D . B I R D €3. BRIAT P . DAY A N D J . C. RIVOAL 81 one-electron reduced matrix elements ( i t l 11 su 11 $t,) and (34t2 11 su 11 34t2). No MO calculations on CoIi- have been reported so to estimate these matrix elements, LCAO eigenvectors were taken from the same self-consistent-field calculation on CoBri-.The resulting wave-functions were used to calculate the dipole-strengths of all the transitions together with the signs and magnitudes of the C-terms in the MCD spectrum. Since the Russell-Saunders ground state is an orbital singlet we have made the same approximation as in the treatment of CoBri- in the previous section and considered only the spin contribution to pz. Fig. 8 shows the calculated absorption and MCD spectra. Both show some resemblance to fig. 4(b) the transitions are grouped into two regions corresponding to 4t + t l ( y 8 ) and 4t2 .+ t l ( Y 6 ) ~ the former with C-terms of mixed sign the latter with predominantly negative C-terms.The observed and calculated MCD both have negative contributions at lowest energy. Many more trial calculations will have to be performed to clarify the picture further but we believe that for the present a useful qualitative view of this complicated spectrum has begun to emerge. Absorption I I I I I -I-32 31 3 0 29 20 27 I MCD 1 26 I 25 FIG. 8.-Calculated absorption and MCD spectra of GI:-. (4) TETRA I o D ONI c K E LA T E ( I I) For NiIi- the situation is even more complicated for not only must one consider strong spin-orbit coupling in the excited states but also in the orbitally-degenerate ground-state. Furthermore there are several terms of each charge transfer con-figuration having spin-orbit Components to which transitions are allowed from one or other of the spin-orbit components of the ground term.To first order the 3T1 ground term of a tetrahedral d8 complex is split by spin-orbit coupling into A - 3c) Tl( - $5) and E T2( + Pc) where 5 = 2/9(+4t2 11 su 11 t4t2>. The temperature dependences of the magnetic susceptibilities of many tetrahedral nickel(II) complexes including salts of Nil$- have been fitted l 6 to values of 5 i 82 CHAKGE TRANSFER I N TETRAHALIDES the range 130-200 cm-l. On this basis we expect the spin-orbit ground state to be Al with Tl roughly 100-150 cm-l above. Now whilst transitions are allowed from Tl to A Z E TI and T2 states examples of which arise from most terms of each excited configuration from A l they are only allowed to T2. Therefore on lowering the temperature one would expect both the absorption and the MCD spectra to be drastically simplified.Fig. 6 shows that this does not occur. Some small variations in the relative intensitites of the absorption bands are seen between 77 and 6 K but these are much less pronounced than one would expect if the Tl state had been completely depopulated. Furthermore the variation of the MCD spectrum with temperature shows clearly that the major contribution comes from C-terms even at 6 K. Two alternative explanations for this striking observation are first that the multiplet splitting of 3T1 has almost completely collapsed the total spread being no more than about 10 cm-I or secondly that the sign of 5 and hence the multiplet, are inverted. Rationalization of the former might lie in the Ham effect (quenching of orbital angular momentum by a dynamic Jahn-Teller effect 17) or of the latter in the opposing contributions of cNi and the much larger [I to [.Some support for the former hypothesis comes from a calculation of the signs and magnitudes of the C-terms associated with transitions from T1(3T1) (table 4). TABLE 4.-MCD AND ABSORPTION PARAMETERS FOR TRANSITIONS FROM 3T1(T1) OF t f 4 t ; TO ALL STATES OF t:4t; in Nix;-C D CID 1 z 1 -3 T2 1/72 1/36 E - 1/24 1/12 -1 - 1/48 1/24 T1 T2 1/48 1/24 2 3T2 A2 1 /l8 1/18 1 E - 1/72 1/36 ; - 1/48 1/24 _ - Tl T2 1/48 1 124 -1 3Az T2 1 19 2/9 3E TI - 1/24 1/12 Tl -1 1 - -_ -1 C is in units of pm2 where p = -(i/22/6)(4t2 11 1.1 11 4t2) and rn = ( t l 11 r I( 4t2) ; L) is m2.Five out of ten of the spin-orbit states resulting from ts4t25 should have negative C-terms which if 41 is assumed to consist mainly of 3d(Ni) should have magnitudes in the range 0.25-0.5 pB (C/O). A curve analysis of the 6.5 K MCD spectrum using the method described elsewhere in this Symposium * confirms these predictions (fig. 9). Our hope is that accurate measurements of the MCD spectrum at various temperatures between 4 and 77 K will serve to resolve the question. CONCLUSIONS In few if any cases have we reached definitive conclusions about the assignment of these charge transfer spectra. Our account has much the nature of a progress report. On the experimental side temperature dependence experiments are manda-tory for separating B and C-term contributions and more theoretical work is needed on the coupling schemes most appropriate to composite states containing both strong and weak spin-orbit coupling.Nevertheless MCD has proved a subtle and sensitive tool for the assignment of these extremely complicated spectra B . D. B l R D B . BRIAT P . D A Y AND J . C . RIVOAL I n 83 A (nm) FIG. 9. Analysis of the MCD and absorption spectra of Ni12- at 6.5 K (see fig. 7). We are grateful to Dr I. H. Hillier and Dr R. M. Canadine for communicating the results of their MO calculations on FeBrz and CoBri- in advance of publication. The crystals of ((C2H5)4N)2Zn14 doped with CoI$- and NiI$- were grown by Mr G. A. Griffiths who also measured their low-temperature absorption spectra. P. N. Schatz A. J. McCafTery W. Suetaka G. N. Henning A. B. Ritchie and P. J. Stephens, J. Chem. Phys. 1966,45,722. B. Briat and J. C. Rivoal J. Chim. Phys. in press. B. D. Bird and P. Day J. Chem. Phys. 1968,49,393. J. S . Griffiths The Irreducible Tensor Method for Molecular Sytninetry Groups (Prentice-Hall, New York 1962). A. D. Buckingham and P. J. Stephens Ann. Rev. Phys. Chem. 1966,17,399. N. S . Gill and R. S . Nyholm J. Chem. SOC. 1959,3997. J. Badoz M. Billardon A. C. Boccara and B. Briat this Symposium. M. Sharnoff J. Chem. Phys. 1965,42,3383. C. K. Jargemen Mol. Phys. 1959,2,309. C. J. Ballhausen and H. B. Gray Molecular Orbital Theory (Benjamin New York 1964). I. H. Hillier and R. M. Canadine personal communication. ' B. D. Bird and P. Day Chem. Comm. 1967,741. l3 P. Ros and C. G. A. Schuit Theor. Chim. Acta 1966,4,1. l4 F. Luty and J. Mort Phys. Rev. Letters 1964 12,45. l 5 B. D. Bird D.Phi1. Thesis (Oxford 1969). l 6 B. N. Figgis J. Lewis F. E. Mabbs and G. A. Webb J. Chem. SOC. A 1966,141 1 ; M. Gerloch l 7 F. S . Ham Phys. Rev. A 1965,138,1727. and R. C. Slade J. Chem. SOC. A 1969 1022

ISSN:0430-0696    

DOI:10.1039/SF9690300070

出版商:RSC    

年代:1969

数据来源: RSC

摘要:

Magnetic Circular Dichroisrn of the Tetrachlorocobaltate Ion BY R. G. DENNING AND J. A. SPENCER W. A. Noyes Laboratory University of Illinois Urbana Illinois U.S.A. Received 6th October 1969 The absorption and magnetic circular dichroism spectra of the tetrachloro-cobaltate ion have been measured as a function of temperature in a single crystal. The major contribution to the Faraday effect of the 4Az -t4T1(P) transition is temperature dependent. The source of this effect and the assignment of the spectra are discussed. A theoretical treatment of the Faraday effect of CoCl;4- by Stephens stimulated the measurement of the magnetic circular dichroism (MCD) of Co Xi-(X = Cl-, Br- I-). Stephens’s work primarily a demonstration of the potential of the tech-nique assumed spin-orbit coupling to be zero and predicted the major MCD for the transition to 4T1(P) to be a B term arising from the 4A2 4T2 second-order Zeeman interaction and an A term associated with orbital angular momentum in the excited state.5 The experimental work showed this approach to be inadequate.The fine structure precluded a single A term analysis and the integrated B term was opposite in sign to that predicted. We show here that the inclusion of spin-orbit coupling accounts for the main features of the MCD spectrum which has now been studied as a function of tempera-ture. EXPERIMENTAL Single crystals of tetra-ethylammonium tetra-chloro zincate containing ca. 0.1 of cobalt(I1) by weight were grown by evaporation from nitromethane solution. These crystals are isomorphous with the analogous nickel(I1) salt whose crystal structure belongs to the tetragonal space group P4/nrnc6 The CoCIi- ion is detectably tetragonally distorted from tetrahedral.The optic axis which is perpendicular to a large rectangular face is coincident with the S4 axis of the tetrahedron. For spectroscopy the crystals were mounted on this face. The crystal used for absorption spectroscopy contained 0.051 % Co(I1) by eight,^ was 1.89 mm thick and had a density of 1.26 g CM-~. We can therefore obtain molar values for extinction coefficients dipole strengths and MCD parameters. The crystal was mounted on a copper disc in a variable temperature cell closed by quartz windows. Measurements were made in the range 224-423 K. Cooling the crystal below 220 K invariably shattered it.A phase transition ascribed to ordering of the ethyl groups has been observed at ca. 218 K by Gerloch * in the isomorphous nickel salt where it results in a large decrease in the anisotropy of the magnetic susceptibility. Spectra were obtained with the apparatus previously de~cribed.~ CD was calibrated with naturally optically active standards and the magnetic field with a Rawson rotating coil gaussmeter. Magnetic fields were about 42,000 G. A series of measurements was made at nine different temperatures without changing the magnet current. RESULTS The absorption and MCD spectra at some representative temperatures are shown Four principal bands are apparent and are numbered. 84 in arbitrary units in fig. 1 R . G . DENNING AND J . A.SPENCER 85 Both types of spectra suggest small extra components. Extracting meaningful parameters for each band is the most uncertain part of our procedure. The absorp-tion spectrum was subjected to an iterative curve-fitting routine involving gaussian components based on method (V) of Jones.9 Band energies half-widths and in-tensities were obtained and the dipole strength of the band envelope was also obtained directly from the experimental points by incremental integration. With these band energies as fixed parameters and with the Faraday parameters and half-widths as variables the same procedure was applied to the MCD spectra. This procedure was repeated at each temperature. The most reliable parameters obtained are those for the total band envelope. As a numerical check the values at room temperature of (B+ C/kT) and D the dipole strength can be compared against the solution values of ref.(4). We find D = 0.692 (B+C/kT) = 2.37 x compared with solution values of D = 0.687 (B+ C/kT) = 0.615 x The large difference in the Faraday parameters is due to a factor of 3 in the definition used here,5 compared to the defini-tions used in ref. (4).3 700 6 0 0 wavelength (nm) FIG. 1 .-Experimental absorption and MCD spectra at 423 K . . . . 399 K - . -7 290°K - - - and 229 K -. In analyzing bands (1)-(4) progressively more scattered results are obtained. In particular uncertainties arise in band (4) due to a small band at 605 nm which increases in intensity at high temperatures. The intrinsic variation of the dipole strength with temperature complicates the identification of C terms.For simplicity we assume that the Franck-Condon factors have parallel effects on the amplitude of both the dipole strength and the MCD parameters. We consequently take the experimental values of (B+ C/kT)/D and AI 86 TETRAHEDRAL COBALT as suitable quantities for interpretation. For bands (1) and (2) we find AID to be small and reasonably constant with temperature as expected; but its value in bands (3) and (4) is greatly dependent on the shortcomings of the curve-fitting procedure. In fig. 2 we plot (B+ C/RT)/D for the total band envelope and for bands (I) (2) and (3). The intercept at 1/T = 0 gives B / D and the slope C/D. The results for band (4) were too erratic to plot but inspection clearly shows a negative C term and a positive A term.The values obtained or estimated are collected in table 1. Fig. 2 demonstrates that most of the MCD at room temperature is C term in origin. 1 1 I I FIG. 2.-Temperature dependence of the MCD parameters of (3 band 1 ; @ band 2; & band 3 ; @ total band envelope. TABLE 1 .-OBSERVED MCD PARAMETERS band CID(8) BlmBlm-1) AID(B) 1 + 1.45 +o.i x 10-3 +0.1 2 + 0.98 + 0.0 x 10-3 +0.1 3 - 2.50 +9.5 x 10-3 + 0.9 4 - ve ? + 4.0 total + 0.51 + 0.4 x 10-3 -DISCUSSION The cobalt (11) ion in the host lattice is in a slightly distorted tetrahedral environ-ment. The S axis bisects a Cl-Co-C1 angle of 106.8°.6 The electronic mani-festations of the distortion are neglected here on the following grounds. The same ion in the tetragonal crystal Cs,CoCl has the S axis bisecting an angle of 106.1°,10 and the zero-field splitting is 4.31 cm-l.ll We therefore assume a similar or smaller splitting in the present lattice.At the temperatures above 210 K used here the C term summations will not be affected. Further a complete crystal-field calculation with the exact geometry for the ion predicts e.g. that 5/2 U'(4T1(P)) will be split by less than 20 cm-1 by the low symmetry field.12 With band widths of about 500 cm-l this splitting need not concern us R . G. DENNING AND J . A . SPENCER 87 for 4T1(P) for the tetrahedral cobalt ion in zinc oxide. We give this in fig. 3. The El’ *Ul degeneracy is resolved by second-order spin-orbit interactions. Weakliem calculated relative intensities on a d - p mixing or a-bonding model finding that the components of ’G, which are close in energy to 4T,(P) are nevertheless very weak.We therefore assume with him that the main features of the spectrum observed near 15,000 cm-l are transitions to 4T,(P). In this we differ from Ferguson l4 who believed that com-plex excited state vibronic effects prohibited any meaningful assignment. We rely on the energy level scheme calculated by Weaklieni E’ ‘ / / / ‘ fEa I FIG. 3.Excited state energy level diagram. We now calculate the predicted Faraday parameters for the spin-orbit components of 4T1(P). We present results for two levels of approximation calculated (i) under first-order spin-orbit coupling giving quantities subscripted zero ; (ii) under second-order spin-orbit coupling giving quantities subscripted I and 11.We work in the strong-field basis using unspecified symmetry adapted orbitals. We later make the assumption of a d orbital basis set in order to give results for the weak-field limit and an appropriate intermediate field. We use the double group TJFd and expand the strong-field states in space-spin product functions using the coupling coefficients of Griffith.15 This procedure uses the isomorphism of the Td and 0 groups. After spin integration the electric dipole and orbital magnetic moment matrix elements are expressed in terms of seven electron reduced matrix elements using the same coupling coefficient^.'^ The spin magnetic moments can be obtained after spatial integration. With these matrices which are independent of the explicit form of wavefunction the Faraday parameters are readily obtained.In table 2, spin-dependent quantities are given in units of p the Bohr magneton while orbitally dependent quantities are given in units of seven-electron reduced matrix elements. These units can be expressed in terms of one-electron reduced matrix elements by explicit evaluation of a typical element using the seven-electron strong-field wave-functions of Griffith.16 These relations are summarized for all the strong-field quartet states in table 3. We now discuss Co/Do. The presence of these terms is an example of C terms arising from spin-orbit coupling in the excited state but not in the ground state. The use of coupled excited state wave-functions forces spin selection on the electric dipole transition moments such that the C term for individual components need not vanish.However Co vanishes when summed over all components consistent with the limit of zero spin-orbit coupling. We give Co/Do in table 2 for comparison with experiment. The signs and magnitudes are in general agreement with the assignment ; band (I) 88 TETRAHEDRAL COBALT 5 U’ ; band (2) E’’ ; band (3) 3 U’ ; band (4) E’. This is the order given by a crystal field calculation (fig. 3). Values of Ao/Do are given separately for the spin and orbital contributions to the angular momentum. The former should be inde-pendent of bonding factors while the latter requires more careful consideration. Unlike the Co terms the A . spin terms need not sum to zero since inter-component B terms also dependent on spin angular momentum originate from A terms in the DO CO COlDO TABLE 2.-cALCULATED CONTRIBUTIONS TO MCD a 116 113 116 113 1 - 512 -113 114 1 / 2 0 - 512 - 1 312 312 0 E’ 312 U’ E” 5/2U’ total C TABLE 3 .-ANGULAR MOMENTUM AND ELECTRIC DIPOLE MOMENT MATRICES 4A2 4 T ~ T I (tfe3) 4Tl(tZe2) 4Az 0 ‘d2 - lb -2/2.mb 0 [ = 0 limit.The intrastate B terms are given in table 2 as &/Do with spin and orbital contributions again separated. The energy denominators are the separations given in fig. 3. The B term arising from 4A2 4T2 interaction considered by step hen^,^ is given for each spin-orbit component and for the band as a whole as Bo/Do in table 2. To include the orbital contributions to B&/Do and Ao/Do we require the quantity pl = (p/ili)(4T1 11 I l l 4T1).This depends on the explicit composition of the 4T1(P) excited state. We write For simplicity we now adopt a d orbital approximation and solve the d7 secular determinant which gives c1 = 2/d5 c2 = - 1/J5. With A/B = 5.0 (a realistic I 47’1(p)> = c1 I 4~1(t:e3))+~2 I 4T1(tze2)) R . G. DENNING AND J . A . SPENCER 89 value) c1 = 0.811 c2 = -0.585. Using the values in table 3 we obtain for the general case, In the weak field limit, Following Stephens we now neglect n-bonding and give values for the one-electron matrix elements in terms of the a-bonding molecular orbital coefficient of Ballhausen and Liehr.17 With ( e 11 I [I t 2 ) = - ,/6ihN,a ( t2 11 I 11 t 2 ) = 4 2 ihNza2 and A/B = 5.0 we obtain p1 = - 1.34N,a p. We now take Naa as 0.80 to imply a dominance of metal d orbitals in the molecular orbitals.We give the values for A o / D calculated with this choice in table 4. TABLE 4.-cALCULATED MCD PARAMETERS E' 3/2U' E" 5/2U' total Ao(spin)lDo 1.66 1.73 - 1.00 - 0.20 Ao(orbital)/Do 0.13 0.10 0.13 0.33 1.79 1.83 - 0.87 0.13 - AOlDO B$spin) /Do 9 . 2 ~ 10-3 - 1.1 x 10-3 - 1 0 . 6 ~ 10-3 1 . 8 ~ 10-3 0 B6(orbital)/Do 3.4 x - 0 . 9 ~ 10-3 osx 10-3 - 1 . 2 ~ 10-3 0 B(p0 1 2 . 6 ~ 10-3 - 2 . 0 ~ 10-3 - 10.1 x 10-3 o . 6 ~ 10-3 0 <co +CI)PO - 2.94 - 1.18 1.76 1.76 0 Cr1IDo 0.25 0.20 0.46 0.46 0.51 C/DO - 2.70 - 0.98 2.22 2.22 0.51 BOD0 o . 4 ~ 10-3 o . 4 ~ 10-3 o . 4 ~ 10-3 o . 4 ~ 10-3 o . 4 ~ 10-3 It is clear that spin contributions dominate and that a cancellation of effects virtually removes the A term in the transition to 3U'.The predicted A terms agree broadly with the results of table 1. We stress the unreliability of the experimental values which are sensitive to the curve-fitting procedure. A visual assessment of the A terms of the three COX:- ions of ref. (4) confirms the dominant positive A terms of the E' and 4U' components especially in the MCD spectrum of CoIz-. These results then strongly support the assignment made on the basis of the C terms. 360cm-l taken from the experimental spectrum the intra-state B terms can be calculated and are given as &/Do in table 4. These results do not agree well with experiment. However since the B term originating from the 4A2 4T2 interaction makes a uniform contribution to Bo/Do for each component it may be significant that the transition to E has the most negative B term both experimentally and theoretically.It is possible that doublet states are contributing to the B terms or that experimental uncertainties are responsible. Intra-state B terms must vanish when summed over the whole band and it follows that BID obtained from the separate integration of (B+C/kT) and D over the band envelope is a measure of inter-state B terms. We examine shortly the possibility that the experimental non-zero value of BID arises from 4A2 4T2 interaction. Using the same value of p1 and with El = 590 cm-l E2 = 700 cm-l E3 90 TETRAHEDRAL COBALT The total band values of C / D and BID are independent of curve-fitting procedures, being obtained by direct integration.With these relatively reliable results we now ask why C / D is experimentally non-zero. We consider therefore the effects of second order spin-orbit coupling. 4T1 (F) and 4T1 (P) are separated by 9000 cm-l, and we assume that doublet inixing will not appreciably affect the transition moments to 4T1(P). We therefore neglect these interactions. 4T2 is the only quartet mixed with 4A2 and we consider this the dominant second-order effect. We expand the ground state as I a> = I ao)+Y I bo) substitute in the expression for C and collect term to first order in y. We find two new contributions to the C term, 2 The C contribution is due to the modification of the ground state magnetic moment, the electric dipole selection rules remaining unchanged.This modification is realized in the experimental g-value. Taking g = 2.37 the mean experimental value,ll we obtain the values of (Co + C,)/Do in table 4. The band summation for this quantity still vanishes but for C, it does not vanish. CII/Do is given in table 2 in units of ym2/ml. Both Bo/Do and CII/Do contain the quantity m2/m1. Their ratio is given by Bo/CI1 = -p21J5 E,Y, where Eo is the 4T2 4A2 energy difference. y is best obtained from the experimental g-value since In the same model as before y = O.179/Naa. Also p 2 = -2 J 3 ~ a a whence g = 2(1 -y(2J2/3 J5)(e I[ I I[ t2)) = 2.37. Bo/CII = 8.65N2a2/Eo. We now assume that we have found the source of both B and C terms. Taking the experimental values from table 1 and Eo = 3130 cm-l from Cotton l1 we obtain N&:a = 0.54.Unfortunately the extrapolation to obtain B gives wide error limits and it is probably fortuitous that this value agrees with that given by Ballhausen and Liehr. However the reasonable value obtained does support the suggestion that Bo and C, are related. With more accurate experimental data over a wider range of temperature an accurate value of Naa could in principle be measured. We now consider the quantity m2/m1. With table 3 it is easily shown that m2h1 = (- 1/J6)[(3/2)((t ll m I1 t2)/<e II m I1 t2>)+cz/c1l. In the weak field limit this gives a value of Bo identical to that of step hen^.^ We work in the intermediate field (A/B = 5.00). From table 2 CII/Do = J10ym2/m and with the same value of y used earlier we obtain a value of the ratio, < t 2 [I m 11 t2)/(e 11 7n I[ 12) = 4 = -1.47Na~+O.48.With Naa = 0.8 q = -0.70. In table 4 we give CII/Do for each component using this value of m2/ml R. G . DENNING AND J . A . SPENCER 91 This value of q is unexpected. A d - p mixing model predicts q = 2 and a a-bonding molecular orbital model predicts a value 0 <q< 2. We do not speculate here on the significance of this figure. It is easy to imagine that B term interactions have been neglected in our treatment. Thus interactions with charge-transfer type excited states could contribute large moments although with small mixing parameters. The non-vanishing positive C term is however more difficult to explain since it involves only the ground and excited state wave-functions. If q is positive the experimental results imply large second-order effects in the excited state which provide C terms of opposite sign to those arising from CII.The role of the *G components in this context must await more detailed variable temperature work, over a wider range in crystals which do not shatter at low temperatures. In summary we present results which demonstrate that the major MCD at room temperature is temperature dependent and whose broad features agree well with A and C terms calculated with a conventional energy level assignment. We believe that this implies more simplicity in the structure of the band than Ferguson l4 thought possible. We demonstrate how other experimental results may be used to obtain molecular orbital parameters and one-electron electric dipole transition moments, although more work is required before these can be fully understood.We are indebted for support to the Materials Research Laboratory University of Illinois and to the National Science Foundation. present address Inorganic Chemistry Laboratory South Parks Road Oxford England. present address University of Virginia Charlottesville Virginia U.S.A. P. J. Stephens J. Chem. Phys. 1965 43,4444. R. G. Denning J. Chem. Phys. 1966,45 1307. For a definition of the Faraday parameters used here see P. N. Schatz A. J. McCaffery, W. Suetaka G. N. Henning A. B. Ritchie and P. J. Stephens J. Chem. Phys. 1966,45,722. G. D. Stucky J. B. Folkers and T. J. Kistenmacher Acta. Cryst. 1967 23 1064. Atomic Absorption Analysis (Materials Research Laboratory University of Illinois). * M. Gerloch and R. C. Slade J. Chem. SOC. A 1969 1022. J. Pitha and R. N. Jones Can. J. Chem. 1966,44,3031. H. G. Belgers P. F. Bongers R. P. Van Stapele and H. Zijlstra Phys. Letters 1964 12 81. l o B. N. Figgis M. Gerlock and R. Mason Acfa. Cryst. 1964 17 506. I2 J. A. Spencer unpublished results. l3 H. A. Weakliem J. Chem. Phys. 1962,36,2117. l4 J. Ferguson J. Chem. Phys. 1963,39 116. l 5 J. S. Griffith The Theory of Transition Metal Ions (Cambridge University Press 1961) Table A.20. ibid. Table A.24. l 7 C. J. Ballhausen and A. D. Liehr J. MoZ. Spectr. 1958 2 342 ; 1960,4 190. l o F. A. Cotton and D. M. L. Goodgame J. Amer. Chem. SOC. 1961 83,4690

ISSN:0430-0696    

DOI:10.1039/SF9690300084

出版商:RSC    

年代:1969

数据来源: RSC

摘要:

GENERAL DISCUSSION Mr. P. A. Cox (Oxford University) (communicated) Schatz and his coworkers have attempted to interpret the MCD of hexachloro-osmate on the basis of the ligand-field assignment due to D0rain.l Now the optical spectrum of hexachloro-osmate is very similar to that of hexachloro-iridate and if the spectra are due to ligand-field transitions this is difficult to understand since the osmium compound is d4 and the iridium compound d5. Jarrgensen has shown however that the similarity can be explained if it is assumed (i) that both spectra arise from charge-transfer transitions, and (ii) that because of the high spin-orbit coupling constant of osmium we may apply pure j-j coupling to the osmate.2 In this interpretation the intense band at about 26 kK in the osmate spectrum like the first intense band in the iridate spectrum is due to transitions from the U’ (r,) spin-orbit component of a t l orbital of pre-dominantly ligand pi character into the E” (I?,) component of the f 2 g orbital.The intense band starting at 29 kK contains two overlapping transitions from the E” and U’ components of a t2u ligand pi orbital into the E component of t2g. The charge transfer interpretation of the hexachloro-iridate spectrum has been confirmed by MCD meas~rernents.~ We therefore calculated the MCD expected for hexachloro-osmate on the basis of this assignment. For t l to t2s (U’ to E ) we predict AID = -0.25 BM and for t2 to t2g we find for E” to E” AID = - 1.7 BM, and for U’ to E” AID = + 1.0 BM all values being approximate. In addition, large B terms are expected from mixing the two states arising from the t2 to tZg transition in the magnetic field.If the E” to E” transition is at lower energy than the U’ to E” transition we expect for the former BID = -0.8/AE and for the latter, BID = + 1.7/AE where AE is the energy separation. If the energies are the other way round the signs will be reversed. No experimental parameters have been reported by Schatz and the complexity of the spectrum especially in the band starting at 29 kK must make the extraction of these a difficult task. However we believe that our calculated parameters are in qualitative agreement with the MCD observed for the intense bands of hexachloro-osmate thus supporting the charge-transfer assignment. We are doubtful however, that it is a good approximation to apply pure j-j coupling to this compound and are at present engaged in more elaborate calculations in intermediate coupling which we hope will throw more light on this spectrum.Prof. P. N . Schatz (University of Virginia) (commzmicated) In reply to the com-ment of Cox we would emphasize that we do not believe that our OsC1;- spectrum can be interpreted on the basis of predominantly ligand-field assignments and the preliminary analysis in our paper shows that Dorain’s detailed attempt to do this is contradicted in many particulars by our MCD data. As indicated in our oral presenta-tion we believe that a fruitful starting point for the interpretation is a j-j coupling formulation in terms of charge-transfer transitions as has also just been suggested by Cox.We outline here briefly some of the general features of such an interpretation. Jsrgensen 4 first noted the similarity of Ir4+ and Os4+ hexahalide solution spectra P. B. Dorain et al. J. Chem. Phys. 1968,49 3845. C. K. Jsrgensen Mol. Phys. 1959 2 309. (a)G. N. Henning et aZ. J. Chern. Phys. 1968,48 5656. (6) A. J. McCaffery et a!. J. Chem. Phys. 1969 50 379. C . K. Jsrgensen 2. Naturforsch. 1967 22 945. 9 GENERAL DISCUSSION 93 and attributed this to the predicted similarity of the d4 and d5 systems in the j-j coupling limit. For example both the t&tz configuration of OsC1,2- and the r,5,t,6, configuration of IrCl2- in the j-j limit give two allowed states separated by a[,-,-for the tZu-+tzg transition. (When electrostatic repulsion is included the d4 system gives many more states than d5.) As discussed in our paper Dorain et a1.l concluded from a study of the extensive fine structure in their low temperature Cs2ZrC16 Os4+ crystal spectrum that all transitions below 35,000 cm-1 were d-d.We follow Jarrgensen's general j-j charge-transfer interpretation but do not rule out the presence of d-d transitions in addition. The helium temperature Cs2ZrC16 Ir4+ crystal spectrum has such striking simi-larities with that of CsZZrCl6 Os4+ that it seems clear that the main absorption bands in the spectra arise from analogous transitions. Our MCD of IrC1;- has shown unequivocally that the first and second strong bands in the IrCl2- spectrum arise respectively from tlu+t2g and t2,-+t2 ligand-to-metal charge-transfer transitions.By analogy then we assign the 26,000 cm-l band in Cs2ZrC16 Os4+ to and the 29,000 cm-l band to and A lg-+Tl ,3[(~3~(e3~(24)~+ (~:)~(e':)~(u;)"(e;)]. We assume that these allowed transitions account for the high extinction coefficients observed. Electrostatic interactions will separate the states of the j-j configurations in an unknown manner but we assume these interactions are small since the Ir4+ spectrum is so similar to that of Os4+. The forbidden states undoubtedly account in part for the temperature dependence of the bands while the T, states are responsible for the gross features of the absorption and MCD. with eel- = 590 cm-l 5oS4+ = 2400 cm-l and assuming that all two-centre integrals are zero we find AID = - 1.67 for A1,+T1,2 and A / D = 0.92 for A1,-+T1,3.The value of A / D for Alg-+Tlul varies between -0.25 and - -0.50 assuming a realistic range of 0-IT mixing for the tl orbital. These values are in good qualitative agreement with experiment. Thus the broad-line progression in the totally symmetric vibration through the 26 000 cm-l band and the corresponding progression of negative A terms in the MCD fits the assignment to the allowed Alg+Tlul transition. Likewise the lower energy end of the MCD for the 29 000 cm-l band is reasonably assigned to a totally symmetric progression accompanying the Alg+TlU2 transition since the MCD shows a pro-gression of large negative A terms again in agreement with our calculation. The higher energy end of the 29 000 cm-I band is dominated by a progression of positive A terms in agreement with the assignment to the A1,+TlU3 transition.The fine structure in the MCD of the 29 000 cm-l band corresponds to the fine structure in the absorption spectrum but is not obviously related to the Alg+2&2 and TlU3 transitions. The spacing between the various sharp progressions beginning at - 28 500 cm-1 suggests assignment to parity-forbidden transitions interacting with odd vibrational modes.4 These parity-forbidden transitions together with the A 1 g -+ Tl u 1 E(u:)4(e:)2(~~)4-+(uI) (e32(44(~31, A 1,-+~lu2C(u:)4(ec)2(~~)4-)(U:)4(e~)(~~)4(e~)l Using Griffith's irreducible tensor methods P. B. Dorain H. H. Patterson and P. C . Jordan J. Chenz. Phys. 1968 49 3845. G. N. Henning A. J. McCaffery P. N. Schatz and P.J. Stephens J. Chem. Phys. 1968 48, 5656; A. J. McCaffery P. N. Schatz and T. E. Lester J. Chem. Phys. 1969,50,379. J. S. Grfith The Irreducible Tensor Method for Molecular Syinrnerry Groups (Prentice-Hall Inc. Englewood Cliffs New Jersey 1962). * I. N. Douglas J. Chem. Phys. 1969 51 3066 94 GENERAL DISCUSSION orbitally-forbidden charge-transfer transitions arising from the t2u-) t2g excitation probably account for the temperature-dependent behaviour of the 29 000 cm-I band. Both our temperature dependent data on OsBri- doped into crystals and that of Day supports the assignment of the strong bands in the Os4+ spectra as charge-transfer transitions. In the bromide the strong bands either gain in intensity or stay about the same as the temperature is lowered.Since the solution MCD of 0sBr;-is very similar to that of OsCl;- we are confident that we are observing analogous transitions. Finally assignment of all bands below 35 000 cm-I in Cs,ZrCl Os4+ as d-d transitions is unreasonable on intensity grounds. d-d transitions cannot gain intensity by mixing with states from $tzg (7 = tlu t2J configurations but only by mixing with the states from the higher energy yzttgeg config~rations.~ If one assumes with Dorain et al. that all charge-transfer transitions are above 35 000 cm-l the $t&eg states will be extremely high in energy ; thus the high oscillator strengths of the strong OsC12- bands would seem to be incompatible with their assignment as d-d transitions. However d-d bands almost certainly are present below 35 000 cm-l in the Cs,ZrCl Os4+ spectrum and may account for many of the sharp lines observed.Dr. W. A. Runciman (A.E.R.E. Harwell) said It is known from tables of atomic energy levels that the 6s6d levels of T1+ are at considerably higher energy and will not greatly affect the 6s6p levels. Also the multiplet splitting rule can be compared with the Russell-Saunders value. It is not greatly different being 3.17 compared with the expected value of 2. The Land6 g-factor can be found from the Zeeman spectra. There are many examples which show that charge compensation sometimes does and sometimes does not affect the spectrum of an impurity replacing an ion of different charge. Dr. E. J. Bowen (Oxford University) said Changes will depend on whether the compensating ion is adjacent to or somewhat removed from the excited ion.Dr. P. Day (Oxford University) said In view of Schatz’s discovery that the angular momenta of the 3P1 excited states in a number of B-subgroups ions are substantially quenched it may be worth pointing out that in the lattices of their pure salts the coordination geometries of such ions are frequently distorted. Thus e.g. TI1 is 5-coordinate and TlF has a tetragonally distorted NaCl ~tructure,~ whilst the unusual structures of PbO and Bi203 were commented on in Dunitz and Orgel’s review.6 Under a distortion the s2 ground state might interact with the totally symmetric component of slpl mixing p into former and some more s into the latter. Perhaps it is this type of mixing which Schatz has detected.Prof. P. J. Stephens (Uniu. of S. Calfornia LA.) said The ions Sez+ and Te:+ discussed in my paper had already been identified prior to the MCD work. More recently I have studied the MCD of solutions of S in oleum and obtained evidence for the new species S:+ therein.’ I believe this to be the first instance in which a new P. Day and E. A. Grant Chem. Comm. 123; B. D. Bird P. Day and E. A. Grant J. Chem. SOC. A 1970 100. G. N. Henning Diss. (University of Virginia June 1968). R. F. Fenske J. Amer. Chem. SOC. 1967 89,252. C. E. Moore Atomic Energy Levels NBS circ. no. 467 1958 vol 3 p. 204. A. F. Wells Structural Inorganic Chemistry 3rd ed. (Oxford University Press 1962) p. 900. J. D. Dunitz and L. E. Orgel Adv. Inorg. Chem. Radiochem. 1960 2 1 . Cheni. Comnt.1969 1496 GENERAL DISCUSSION 95 molecular species has been initially identified by MCD and to substantiate further the potential value of MCD in synthetic inorganic chemistry. Dr. W. A. Runciman (A.E.R.E. Harwell) said For anisotropic centres in cubic crystals consisting of trivalent rare-earth ions the orientation can be best determined by the Zeeman effect. Earlier theoretical calculations contain errors and recent articles l* based on a lecture to the Zeeman Centennial Conference described the corrected results. The Zeeman effect is also useful for isotropic centres as illustrated by results for divalent europium in calcium fluoride. Dr. B. Briat (L’l?cole Supkrieure de Physique et Chirnie Paris) said We agree with Dr. Runciman that Zeeman effect experiments do provide the answer to the problem of the orientation (and thus the local symmtery) of anisotropic centres.Such experiments however are restricted to spectral lines having half-widths in the range 1-10 cm-l. This is not so when one considers the Faraday or Voigt-Cotton-Mouton effects. Both allow the study of broad bands. It has been established in particular (see our ref. (la)) that half-widths of a few hundred cm-l would result in magnetic linear dichroism signals which could be easily detected with our apparatus (using a 50 kG field). As far as MCD (or MORD) is concerned bands as broad as a few thousand wavenumbers can be investigated (e.g. in cobaltous salts). Dr. P. Day (Oxford University) said In addition to the bands whose MOR parameters are listed in table 2 of Shashoua’s paper high-spin ferric haemoproteins, though not low-spin ones also exhibit a further band in the near infra-red at about 10 000 cm-l.The latter with Shashoua’s band I have been assigned as transitions from the highest occupied n-orbitals (aln and a,,) of the porphyrin to the d-shell of the iron(III) and they have identical polarizations to the visible hands in a series of metmyoglobin derivatives.5 As the charge transfer states thus have a symmetry E, common to the locally excited n-n* states configuration interaction will be possible between them. Such an effect has been used to explain the change in the visible region of the absorption spectrum on going from low-spin to high-spin in the met-myoglobins and may also account for the large decrease in MOR intensity.Prof. M. Sharnoff (University o j Delaware) said I would remark that the theory developed in the paper by Bird Briat Day and Rivoal should find fruitful application in the study of d-d spectra of transition metal ions. The magnetic susceptibility tensors of the states excited by d-d absorption are often predictable with considerable precision from non-magneto-optical measurements which are inherently free of the complications which the combination of A B and C terms impose upon the analysis of MCD spectra. The comparison of such predictions with the results of MCD studies would not only provide a severe test of the analysis which you have developed in this paper but would also deepen our insights into the bonding of transition metal ions.As an example I would cite the tetrachlorocuprate ion where the results of an e.s.r. study of the ground state taken in conjunction with the energies and polariza-tions of the d-d transitions enable one to predict (ref. (9) of their paper) the magnetic W. A. Runciman Proc. Iizt. Adu. Summer Physics Institute,-1969 Crete (New York Plenum Press) p. 344. D. F. Johnston S. Marlow and W. A. Runciman J. Phys. C. (Proc. Phys. SOC.) 1968,1,1455. W. A. Runciman and C . V. Stager J. Chem. Phys. 1963 38,279. P. Day D. W. Smith and R. J. P. Williams Bioclzenz. 1967 6 1963. ‘ P. Day G. Scregg and R. J. P. Williams Biopolymers Symp. 1963 1 271 96 GENERAL DISCUSSION properties of all the excited d-levels. The principal values of the g-tensor of the upper-most level (the 2A1 level at 9050 cm-l above the ground level) are predicted to be 911 = 2.002 g1 = 1.591.While MCD measurements on d-d bands of tetra-halide complexes necessarily entail the awkwardness of working in the near infra-red, it seems that current technology makes MCD experiments in this region quite feasible. Dr. A. J. McCaffery (University of Sussex) J. A. Spencer and P. N. Schatz (University of Virginia) said The purpose of this contribution is to draw attention to the use of magneto-optical measurements in investigating a wider variety of physical problems than is suggested by the more conventional spectroscopic applications described in the MCD papers presented so far at this Symposium. In this we utilize the unique property of magnetic optical activity viz. that it measures the ground and excited state magnetic moments for each individual electronic transition.Thus if we have a large number of thermally populated ground states we can measure the magnetic moments for each of these provided transitions from them are resolved. This situation occurs when there is exchange coupling between pairs of paramagnetic ions and MCD may be used as a very sensitive probe into the nature of the inter-actions between unpaired spins in solids. The technique is illustrated here to determine the signs of the magnetic interactions between Cr3+ ions in ruby. Oxygen o ~ r 3 ' o r ~ P FIG. 1 .-Spatial ariangement of nearest neighbours in the M203 lattice. Fig. 1 shows the spatial relationships of metal ions in the Al,03 lattice. In a concentrated ruby crystal there will be an appreciable number of chromium ion pairs and they may be classified as first second third etc.nearest neighbours as shown in fig. 1. Each Cr3+ site has one nearest neighbour three equivalent second neighbours, three equivalent third neighbours and six equivalent fourth neighb0urs.l At distances greater than that of the fourth neighbours Cr3+ sites are linked by at least two oxygen atoms and the coupling between these is much weaker. The A1203 Cr3+ system N. Laul-ance E. C. McIrvine and J. Lambe J. Phys. Chem. Solids 1962,23,5 15 GENERAL DISCUSSION 97 has been investigated by a variety of techniques and thus provides a good test of the method. The exchange interaction between unpaired spins on coupled Cr3+ ions may be approximately accounted for by a term in the Hamiltonian of the form H = J,S s2.This may be ferro- or antiferromagnetic in nature depending on whether the exchange constant for the nth nearest neighbour J, is negative or positive. The two possible couplings are shown in fig. 2 with the energies in terms of the exchange constant J. In fig. 3 we show the states which arise from the various couplings of the S = 3 FIG. 2.Ctates produced by ferro-and antiferro-magnetic coupling of the S = 4 spins of two Cr3+ ions. anti-fe r romag n et i c f er rorn ag net i c s=3 s=o *; 3 .... -ground state of Cr3+. The energies of these levels have been determined by stress experiment^,^'^ Stark splitting~,~ temperature dependence of absorption and emis-sion,6s 7 s 2 * * and other techniques.l Here we show only the ground states to illustrate the method.Inclusion of the excited states which are also coupled leads to a much 1st 2nd 3 rd 4t h 2 3 Nearest neighbour 400 . nteradions in A'zo3' Cr3' FIG. 3 . 4 t a m produced by coup-2 ling of first four nearest neighbour pairs in A1201 Cr3+. I - 'i 0- 0 1 0 - 1 0 0 1 2 3 P. Kisluik N. C. Chang P. L. Scott and M. H. L. Pryce Phys. Rev. to be published. L. F. Mollenauer and A. L. Schawlow Phys. Rev. 1968,168 309. A. A. Kaplyanskii and A. K. Przevuskii Soviet Physics Doklady 1962 7 37. A. A. Kaplyanskii and A. K. Przevuskii Soviet Physics-Solid State 1967 9 190. A. A. Kaplyanskii V. N. Medvedev and A. K. Przevuskii JETP Letters 1967 5 347. P. Kisliuk A. L. Schawlow and M.D. Sturge Advances In Quantum Electronics (Columbia Press N.Y. 1964) p. 725. ' P. Kisliuk and W. F. Krupke Appl. Phys. Letters 1963,3,215. * R. C. Powell B. DiBartole B. Birang and C. S. Naiman Phys. Reu. 1967,155,296. s3-98 GENERAL DISCUSSION more complicated picture. The absorption spectrum of a very concentrated ruby crystal shows 110 resolved lines in the region 6850-7065 A and we shall focus on the various groups of lines originating from the different exchange coupled ground states. Fig. 4 shows the MCD of a relatively concentrated ruby sample at low temperature. The signals are very large for very low fields-a characteristic of sharp intra-con-figurational transitions 2-though the lines were too weak to be seen in absorption in our sample. The magnitude of the MCD signal for each transition is sensitive to the magnetic moment of the ground state of that transition.If the transition is 40 kg 0 It II FIG. 4.-MCD of moderately concentrated ruby at approximately 6 K. Only the R1 and R2 lines show up in absorption in this sample. from a ferromagnetically coupled ground state the MCD will increase rapidly with decrease in temperature-more rapidly that is than the 1/T dependence of a para-magnetic state. If from an antiferromagnetic ground state the MCD will conversely decrease with reducing temperature. Thus from the temperature dependence of the MCD we may determine the sign and the magnitude of the exchange constants J, for the various interactions in the solid. Our preliminary temperature- and field-dependence experiments on ruby in the temperature range of around 6-15 K indicate that the groups of bands at 7041 and 6995 A are from antiferromagnetically coupled ground states in agreement with increasing assignments form other experiments as from the third and second nearest neighbours respectively.The group of lines around 6980 A however decreases rapidly with temperature and can be assigned as from the fourth neighbour interaction, in agreement with Stark experiment^.^ The first neighbour interaction is so strongly antiferromagnetic that we could detect no MCD in the known region of these lines in our sample. Ruby is clearly a very complex system and it may be impossible to analyze the complete spectrum due to the large number of overlapping lines. However from S. F. Jacobs Doctoral Thesis (The John Hopkins University Baltimore Maryland 1956). A. J. McCaffery P. J. Stephens and P. N. Schatz Inorg. Chem. 1967 6 1614. P. Kisliuk N. C. Chang P. L. Scott and M. H. L. Pryce Phys. Rev. to be published GENERAL DISCUSSION 99 these and continuing experiments we have been able to confirm the nature of the ground-state couplings in ruby. Since the excited state moment also contributes to the MCD of each transition we can hope to determine the signs and magnitudes of the excited states interactions. One particular region which appears to be tractabla is 6959-6970& where the MCD is relatively simple. Here we have been able to evaluate theg-value for one of the third nearest neighbour excited states and an analysis of other regions may be possible. Through this and continuing work we hope to comment in more detail on the excited state coupling mechanism

ISSN:0430-0696    

DOI:10.1039/SF9690300092

出版商:RSC    

年代:1969

数据来源: RSC