摘要:

CONTENTS .*. Vlll Self-recording Microspectrography and its Applications on Problems regarding the Endocellular Formation of Haemo- globin. By Bo Thorell Microspectrographic Studies on the Yellow Pigment in Nerve Cells. By Holger Hydh and Bo Lindstrom . Ultra-violet Microspectrography of Living Tissue Culture Cells- Ultra-violet Absorption Spectra of Structures in Surviving Ultra-violet Microspectrography of Living Tissue Culture Cells- Part 11.-Microspectrographic Studies of Living Cells and Ultra-violet-Irradiated Chick Fibroblasts. By P. M. B. Walker and H. G. Davies . Components of Normal and Abnormal Liver Cells Studied by Ultra-violet Microscopy and by Differential Centrifugation. By Hubert R. Catchpole and Isidore Gersh The Ultra-violet Absorption of Living Cells. By J.R. G. Bradfield . GENERAL DIscussIoN.-Dr. R. Barer, Dr. E. Schauenstein, Dr. G. H. Beaven, Dr. E. R. Holiday, Prof. B. Commoner, Dr. I. MacArthur, Mr. P. M. B. Walker, Dr. E. M. F. Roe, Dr. B. Thorell, Dr. D. F. Cole, Dr. H. R. Catchpole, Dr. I. Gersh, Dr. J. R. G. Bradfield Part 1.-Radiation Measurements. Cells. By Barry Commoner . By H. G. Davies . . . PAGE 432 436 442 449 471 481 491CONTENTS .*. Vlll Self-recording Microspectrography and its Applications on Problems regarding the Endocellular Formation of Haemo- globin. By Bo Thorell Microspectrographic Studies on the Yellow Pigment in Nerve Cells. By Holger Hydh and Bo Lindstrom . Ultra-violet Microspectrography of Living Tissue Culture Cells- Ultra-violet Absorption Spectra of Structures in Surviving Ultra-violet Microspectrography of Living Tissue Culture Cells- Part 11.-Microspectrographic Studies of Living Cells and Ultra-violet-Irradiated Chick Fibroblasts. By P. M. B. Walker and H. G. Davies . Components of Normal and Abnormal Liver Cells Studied by Ultra-violet Microscopy and by Differential Centrifugation. By Hubert R. Catchpole and Isidore Gersh The Ultra-violet Absorption of Living Cells. By J. R. G. Bradfield . GENERAL DIscussIoN.-Dr. R. Barer, Dr. E. Schauenstein, Dr. G. H. Beaven, Dr. E. R. Holiday, Prof. B. Commoner, Dr. I. MacArthur, Mr. P. M. B. Walker, Dr. E. M. F. Roe, Dr. B. Thorell, Dr. D. F. Cole, Dr. H. R. Catchpole, Dr. I. Gersh, Dr. J. R. G. Bradfield Part 1.-Radiation Measurements. Cells. By Barry Commoner . By H. G. Davies . . . PAGE 432 436 442 449 471 481 491

ISSN:0366-9033    

DOI:10.1039/DF95009FX001

出版商:RSC    

年代:1950

数据来源: RSC

摘要:

HETEROGENEOUS CA’I’ALYSI S ERRATA Furaduy SOC. DasczIssaons, No. 6, 1950. P. 36, 1. 9 : E(Ta -= N ) should be 8913, making Qcalc. = - 74.4 kcal./mole-~ for the process, H, + 2’Ta 3 N + 2’Ta = N - H. The observed value chemisorption of hydrogen onto a nitrogen filiii on tantalum is 27 kcal. /mole-l, and so the conclusion is IIOW that the niodel above cannot describe the process. 1’. 183, Table 111, 1. r : AEade. For CV- . . . H+ should be 8.76 not lo199 eV.

ISSN:0366-9033    

DOI:10.1039/DF950090X003

出版商:RSC    

年代:1950

数据来源: RSC

摘要:

HETEROGENEOUS CA’I’ALYSI S ERRATA Furaduy SOC. DasczIssaons, No. 6, 1950. P. 36, 1. 9 : E(Ta -= N ) should be 8913, making Qcalc. = - 74.4 kcal./mole-~ for the process, H, + 2’Ta 3 N + 2’Ta = N - H. The observed value chemisorption of hydrogen onto a nitrogen filiii on tantalum is 27 kcal. /mole-l, and so the conclusion is IIOW that the niodel above cannot describe the process. 1’. 183, Table 111, 1. r : AEade. For CV- . . . H+ should be 8.76 not lo199 eV.HETEROGENEOUS CA’I’ALYSI S ERRATA Furaduy SOC. DasczIssaons, No. 6, 1950. P. 36, 1. 9 : E(Ta -= N ) should be 8913, making Qcalc. = - 74.4 kcal./mole-~ for the process, H, + 2’Ta 3 N + 2’Ta = N - H. The observed value chemisorption of hydrogen onto a nitrogen filiii on tantalum is 27 kcal. /mole-l, and so the conclusion is IIOW that the niodel above cannot describe the process. 1’. 183, Table 111, 1. r : AEade. For CV- . . . H+ should be 8.76 not lo199 eV.

ISSN:0366-9033    

DOI:10.1039/DF95009BX004

出版商:RSC    

年代:1950

数据来源: RSC

摘要:

THE LOW EXCITED STATES OF SIMPLE AROMATIC HYDROCARBONS BY D. P. CRAIG Received 18th July, 1950 Lack of experimental knowledge of the polarization of spectral band systems on the one hand, and disagreements between theoretical predictions on the other, have allowed no spectral assignment in the aromatic series to be securely held except that of singlet A,, - Bzu for the 2600 A band system of benzene. This position has recently become clearer. On the experimental side a key assignment in naphthalene is strongly, even if not conclusively, indicated by Kasha and Nauman's finding that the lowest transition is extremely weak, comparable with the symmetry forbidden system in benzene. And on the theoretical side the major disagreements between molecular-orbital (MO) and valence-bond (VB) calculations are resolved if in the former, used with non- empirical energy terms, configurations are allowed to interact, and if in the latter some polar structures are added to the familiar non-polar ones.In the simplest cases of benzene and cyclobutadiene the effect of configuration interaction is greatly to diminish the importance of polar structures from the large value associated with conventional MO wave functions. This brings the state wave functions, as well as the sequence of their energies, much nearer to those of VB theory. The general support given to the VB theory by its agreement in simple cases with a non-empirical theory allows some confidence in its use for more complex molecules. In 'acenes, polar structures being excluded, low-lying A , and B,, states ought t o be calculable without serious error, Low-lying B1, and B,, states will only be so calculable when polar structures are included.The assign- ments in 'acene spectra which these considerations support are set out and discussed. Comparisons between theory and experiment in the ultra-violet spectroscopy of aromatic molecules are severely limited because in very few casesis it possible to assert definitely that the experimental band system is that one to which the calculations refer. Indeed there is only a single assignment, that of singlet A,, - Bau for the 2600 benzene band, which stands firmly on an experimental basis,'. although there are others to be mentioned supported by rather convincing but not yet conclusive evidence. We may for example identify a second benzene band system, the intense system at 1750 A, as A,, - El, on the ground that this is the only allowed transition within the r-electron shell starting from an A,, state.These two assignments in benzene, together with an assignment in naphthalene to be detailed later, form the more precise part of the ex- perimental basis for theories of a-electron states and no theory which fails to account for them, at least approximately, can be satisfactory. The numerous wavelength and intensity measurements of band systems of unknown assignment do not allow critical tests of such theories although they may give them some support if these two quantities can be correctly calculated. Two theoretical methods, one empirical and one non-empirical, appear at present to deal satisfactorily with the three assigned band systems already mentioned. The intention of this paper is to discuss these two, and some aspects of the relation between them, and in the course of this to indicate the position and prospects from the theoretical point of view.1 Sponer, Nordheim, Teller and Sklar, J . Chem. Physics, 1939, 7, 207. * Ingold et al., J . Chem. SOC., 1948, 406, el seq. 56 AROMATIC HYDROCARBONS A general problem which prompts detailed study of cases such as those discussed here is that of justifying simple ways for dealing with molecules too complex for direct non-empirical calculations, Broadly, it will be concluded that the agreement in simple cases between these two methods is enough to give considerable support to the empirical one in calculations of certain of the energy levels in complex molecules.1. Benzene.-The two theoretical methods whose performance is to be discussed are the empirical valence-bond (VB) 3, method and the non- empirical method of interacting molecular-orbital configuration^.^^ (I An empirical method is one in which the results for a series of related molecules are expressed in terms of energy parameters. These are given values by equating calculated and experimental values in one or more band systems which are not themselves to be the subject of the theoretical study. The common practice has been to use the 2600 benzene transition in this way and so to calculate the energies of other band systems in aromatic molecules relative to it. In non-empirical methods the terms appearing in the energy calculations are derived mathematically, the only experi- mental quantity taken for granted being the C-C internuclear distance.Simplifying assumptions underlie both methods, but this fact needs no detailed repetition here being set out in the references already cited. In the non-empirical method the starting-point is a set of one-electron molecular orbitals formed by combining atomic orbitals situated at the carbon atoms. Molecular wave functions are formed by assigning the six electrons to these molecular orbitals and making the whole a n t i s p - metric to electron interchange. No molecular orbital may appear more than twice in such a wave function but with this restriction it is still possible to place the electrons in 141 different ways, each of which is an electron configuration.The wave functions describing actual molecular states are linear combinations of configuration wave functions with co- efficients chosen to minimize the energy.6 The implied variation problem is expressed in a secular equation whose roots are the approximate energy eigenvalues. TABLE I.-THE LOW-LYING STATES OF BENZENE The results are shown in the third column of Table I. Expt. Energy (eV)9 State Calc. (ev) (non- Calc. (ev) empirical)@, 7 (emp. VB)a A comparison with the experimental values shows adequate agreement on two essential points. Firstly, the lowest excited singlet state is correctly found to be B2u. Secondly, an E,, state is found in the expected energy region and the calculated 7 intensity of transitions leading to it (f = 0.81) agrees tolerably well with the experimental (f = 0 .6 g ) . ~ Agreement of this general sort is as much as may be looked for in the non-empirical method, because three- and four-centre integrals, even if included, are somewhat inaccurate, and also because the calculated energies come out Huckel, 2. Physik, 1930, 70, 204. Sklar, J . Chem. Physics, 1937, 5, 669. 5 Goeppert-Mayer and Sklar, ibid., 1938, 6, 645. Craig, Proc. Roy. SOC. A , 1950, zoo, 474. ' Bevan and Craig (to be published). sCraig, Proc. Roy. Soc. A , 1950, 200, 401. Klevens and Platt, J - Chem. Physics, 1949, 17, 470.D. P. CRAIG 7 as differences between much larger numbers. In the calculated levels of Table I there are two which fall between the B,, and E,, already referred to.The lower of these is an E,, state and this suggests that the band system at 2000 This assignment is as- sumed in Table I but it is not yet supported by experimental evidence. The right-hand column of Table I gives values calculated by the em- pirical VB method, based on the 5 nonpolar structures and 24 of the polar structures. The bracketed value for B,, is assumed in the course of the calculation and hence affords no test of the theory. An El, state is found in the correct energy region and the calculated intensity for transitions leading to it is f = 0.22 (expt. 0.69). The small calculated value is due to the neglect of polar structures with charges on remote atoms, which contribute very strongly to the transition moment even though energeti- cally not so important.Using this method as a guide to the assignment of the band system at 2000 A we again conclude that it is A,, - E,,. It is unlikely that band systems in the far ultra-violet beyond 1750 %i are to be understood as transitions between v-electron states, and the present discussion therefore embraces only three band systems. As has been explained two of these are assigned independently of detailed theo- retical considerations as A,, - B,, and A,, - El, and the third system, at 2000 A, is assigned as A,, - E,, from two independent theoretical methods. In the triplet series there is qualitative agreement between the em- pirical and non-empirical methods that the lowest and first excited triplet states are 3B1, and 3Elu.These will not, however, be discussed in detail because the states have not yet been finally identified experimentally. 2. Configuration Interaction and the Role of Structures.-The pur- pose of the next few paragraphs is to compare the two methods and to demonstrate that the agreement between their results reflects a strong similarity, which is not at once obvious, in the character of the wave functions used to describe molecular states. VB structures may be classified into nonpolar and polar structures. Nonpolar structures assign one a-electron to each atomic z p orbital; they alone are used in the VR method in its simplest form. Polar structures have doubly occupied and unoccupied orbitals as illustrated below for cyclobutadiene, and a refinement to the VB method is to include some of these in the description of molecular states.In highly symmetrical is to be assigned A,, - E,,. + - /1* Nonpolar structure. Polar structure Polar structure (singly charged) (doubly charged) FIG. I. molecules this classification is associated with a distinction of symmetry. Thus in benzene the familiar five nonpolar structures are capable of being combined to give molecular wave functions of only three out of the possible six different symmetry types, viz., A,,, B,, and E2,. The others, A z g , B,, and El,, can therefore be represented in VB theory only by combinations of polar structures. This distinction is one of energy as well as of sym- metry since the assumptions of the method make the states admitting nonpolar structures come out to be more stable than the others.This simply means that within the framework of VB theory it is energetically very unfavourable to place two n-electrons on the one atomic centre, as required in a polar structure. In MO theory the formally analogous dis- tinction is between symmetries which do and do not arise in a chosen configuration, and it is easily. verified that the symmetries which group together naturally in this scheme are not the same as those in the VB theory. However, when the MO configurations are allowed to interact8 AROMATIC HYDROCARBONS and so to effect energetically favourable changes in the composition of the wave functions, we at once notice that it is the states with symmetries compatible with nonpolar structures which are the most reduced in energy and which, finally, become lowest of all.This suggests that the pre- dominance of nonpolar structures taken for granted in the empirical theory is supported by the non-empirical one. Two aspects of this situation may' be illustrated in benzene and cyclo- butadiene both of which allow the necessary clear distinction between the symmetry types with respect to nonpolar structures. The first aspect is, that, of the states which arise in a chosen configuration, those for which nonpolar structures may be written are the most changed by configuration interaction, and the second is that, after configuration interaction has been allowed for, states of these symmetries are the lowest of their multiplicity. Table I1 illustrates the first point, TABLE II.--ENERGY CHAXGES (IN eV) IN CONFIGURATIONAL INTERACTION States Compat- ible with Non- polar Structures Others (a) Benzene.' (b) Cyclobutadiene.l o 4'4 6.1 5'9 - - - 1'2 1'1 - Results for the triplet states of benzene are based on configurations within a 10 eV range only as in ref. (6). Otherwise all the important configurations are used. States similarly marked in Table I1 by stars or daggers occur in the same NO configuration. Those admitting nonpolar arrangements of the electrons are more, and usually much more, depressed in energy than the others and this must surely be due to the more complete separation of the electrons that such arrangements allow. This interpretation also fits the differences in Table I1 between singlet and triplet states. A triplet state implies two electrons with parallel spins and these two can never be found on the same atomic centre.Thus the separation of electrons is already partly achieved by the spin symmetry even in an MO wave function, and the further effect possible by configuration interaction is less than in singlet states. Specifically, a given triplet is less depressed than the correspond- ing singlet. In two cases, El, of benzene and E, of cyclobutadiene, this effect and that connected with structure type are in opposition and no definite interpretation is possible. *t States similarly marked by stars or daggers arise in the same configuration. Thus SBlu < lBzu, S B z u < lBlu, 3AZg < lAlg.D. P. CRAIG 9 The second point, that after configuration interaction is allowed for in the non-empirical theory the lowest states are those compatible with nonpolar structures, is illustrated for benzene in Table I.It will be noticed that the three lowest states are of this type. In cyclobutadiene, Table 111, the four lowest states are those for which nonpolar structures may be written. TABLE III.-THE STATES OF CYCLOBUTADIENE IN THE NON-EMPIRICAL THEORY 10 For the next stage of the comparison a more precise statement is needed of the relationship between the wave functions used in the two theories. It is the case, as Longuet-Higgins 11 first pointed out, that a complete set of antisymmetric LCAO molecular orbital configuration wave functions as used in the non-empirical theory, and a complete set of VB structures properly mathematically expressed, are equivalent bases for representing molecular states.This means that a wave function expressed in one basis may be transformed into the other provided only that both are simply built up from the same set of atomic orbitals, as they are in practical cases. The fundamental elements in the one basis are MO configuration wave functions with proper space and spin symmetry and in the other are antisymmetric combinations of atomic orbital products conventionally represented as valence-bonded structures. In both cases it is practicable in small molecules to work with complete sets of the fundamental elements, and if energies were calculated in the same way in each, the results would be identical ; but for calculations in large molecules it is convenient in both theories to work only with a very few such elements. Thus in the one case the aim is to use a single configuration in molecular orbitals and in the other to use a single configuration in atomic orbitals. The energetic circumstance that the lowest MO configuration is closed (all orbitals, or all but one, doubly filled) but the lowest atomic orbital one is multiple degenerate, makes the difference that the first contributes a single element to its basis while the second contributes a number, viz., all the structures of a given polar character.But the restriction is the same, in an exact sense, in both cases. A comparison of the wave functions under the restriction is, very nearly, a comparison between those of the simple MO theory and the nonpolar VB theory, and this is familiar ground.It is well known that there is a striking difference in the importance these theories associate with accumulations of charge about a single centre. Now it is already clear, from the improved agreement between the results of the calculations, that abandoning single configurations greatly diminishes this difference, and this may be illustrated more directly by examining the wave function for a particular state. We choose for this the lowest state of cyclobutadiene which belongs to the B2, representation. The starting point in the non-empirical theory is the set of four LCAO molecular orbitals, written as follows : a, b, c and d are atomic 2p wave functions and the 0's are normalizing coefficients : $ 0 = I / O o ( a + b + c + d), $h*l = I/Ul(U & ib - c q= id), dZ = I/U,(U - b + c - d ) .l o Craig, Proc. Roy. Soc. (in press). * Only the lowest state of each symmetry type is shown. l1 Longuet-Higgins, Pvoc. Physic. SOC., 1948. 60, 270. States marked with a star are compatible with nonpolar structures. A*I 0 AROMATIC HYDROCARBONS In conventional MO theory we should represent the lowest state as having two v-electrons in $o and the remaining two in dl or $-l. This configuration is denoted by (+o)2 (&J2. and (&) 2(42) also admit 1B,, symmetry and the essence of the configuration interaction procedure is to represent the actual state as a mixture of all three. The starting points in the empirical theory are the structures com- patible with lB2, symmetry. Examples of the three types are shown in Fig.I and the complete set may be found by rotating and reflecting until the possibilities are used up. Two nonpolar, 8 singly-charged polar and 4 doubly-charged polar structures make the complete set. By a straightforward expansion 11a the configuration wave functions of the first basis can be expressed in terms of the structures of the second, with the results shown in Table IV. The upper numbers in Table IV are the coefficients with which the struc- tures entei the configuration wave functions. The total contributions by structures of the various types are best measured by their “weights ” which are found from the squares of these coefficients, due account being taken of the nonorthogonality of the nonpolar structures. These weights normalized to IOO yo, are the bracketed numbers in Table IV.We see But two other configurations ($1) ($-1) TABLE IV.-COEFFICIENTS OF STRUCTURES IN B,, CONFIGURATIONS OF CYCLOBUTADIENE I Configuration Energy-minimized mixture of configurations . Non-polar Structures Singly -charged Polar Structures Doubly-Charged Polar Structures that in the simple MO theory, where the first configuration done is taken to be the state, the wave function is 37-5 yo nonpolar while in the simple VB theory it is IOO yo nonpolar. Now it is necessary to know how the configurations are combined in the non-empirical theory to give the state of lowest energy. If the first configuration has the coefficient unity it is found that the second must have - 0.329 and the third - 0-145 ; and referring to Table IV this means that to get the Actual state the first configuration has to be modified by subtracting from it small fractions of the second and third.The effect of this will clearly be to increase the weight of nonpolar structures at the expense of polar ones. An actual calculation, in which the normalizing coefficients u must enter as well as the coefficients of Table IV, shows that in the state wave function the weight of nonpolar structures has been increased to 88 yo, singly-charged polar structures have been reduced to 11 yo and doubly-charged polar structures have been almost entirely eliminated. The non-empirical wave function is thus seen to be dominated by nonpolar structures, and this fact supports the assumption underlying the empirical method. It is tempting therefore to say that the assumption brings the latter method close to physical reality. With properly chosen coefficients the configurations may be combined to eliminate all lla Wheland, Proc.Roy. SOC. A , 1938, 164, 464. This line of argument may be taken one stage further.D. P. CRAIG I1 polar structures, making the wave function wholly nonpolar. This wave function though formally expressed in MO terms, is the wave function for the two Kekulk structures of cyclobutadiene, and so it is the same, except trivially,* as the simple VB wave function. The energy of the wave function with coefficients so chosen can be calculated. It proves to be 0-9 eV less stable than the energy-minimized wave function but 1.5 eV more stable than the lowest MO configuration wave function. In other words, the nonpolar wave function is considerably more stable than any single configuration wave function and 0.9 eV less stable than the best wave function that can be formed from atomic z p orbitals.The conclusions from this section are that, in benzene, the non-empirical and VB methods are substantially in agreement for the low states. This agreement depends strongly on the inclusion of configuration interaction in the non-empirical method, and somewhat less strongly on the inclusion in the empirical method of some polar structures. The fact that in both methods the low states of benzene, and also of cyclobutadiene, are just those for which nonpolar structures may be written reflects that nonpolar structures play the dominating part, and suggests that the VB theory in its simple form is likely to be correct in at least the lowest states of more complex molecules.3. Naphthalene.-In naphthalene there is no assignment so well founded experimentally as either of those referred to in benzene at the beginning of this paper. But a very good indication toward one assign- ment exists in the discovery by Kasha and Nauman18 that the weak naphthalene band system at 3200 records a distinct electronic transition, and that its upper state is so long-lived as to indicate strongly that the transition is symmetry forbidden. Since the ground state is A , the upper state can be A , or B,, if this indication is correct, but beyond this the experimental material does not yet go. Granted this, the theoretical calculations allow a clear choice between the two possibilities and so allow an assignment to be made.The numerical work in theoretical calculations increases manyfold in going from benzene to naphthalene and the calculations so far reported use only a very small part of the basis of configuration wave functions, or structures, as the case may be. A non-empirical calculation by Miss Jacobs 13 shows that the lowest forbidden transition is A , - A , but does not make this the lowest of all the transitions in the approximation to which the calculation is taken. It is necessary therefore to assume the forbidden character in order to deduce the assignment from this calculation. The VB theory,l4, 15 however, even without polar structures, does find the first excited state to be 1A , and so directly favours the same assignment without a similar assumption ; the lowest B,, state is 4 eV in energy above this first excited A , state.Some considerable weight thus attaches to the assignment 1A , - lA for the 3200 and 2200 A a key experimental fact is wanting. This is the polarization of the transitions, i.e. the direction of the active electric vector relative to the molecular axes. The twofold symmetry (Table V) of the 'acenes admits two allowed transitions, A , - B,, (transverse polarization) and A , - Bsu (longitudinal polarization). It is no longer possible to appeal to agreemerk between the theories to assign the shorter wavelength band systems to one or other of these allowed transitions because agreement is lacking at the stage to which the calculations have been taken. We shall therefore simply mention the results in the two theories and leave some more general * The only difference is that the overlap integral is neglected in the VB theory.12 Kasha and Nauman, J. Chew. Physics, 1949, 17, 516. l 3 Jacobs, Proc. Physic. SOC., 1949, 62, 710. l4 Rlumenfeld, J. Physic. Chem. SOC. RUSS., 1947, 21, 529. 15 Craig, Proc. Int. Congr. Chem. (London, 1948). naphthalene band system. In the remaining ultra-violet systems of naphthalene at 2900I E I A , I B, I B, I B , I Den Representation CIS czu cz= I I I I -1 -1 -1 I -1 -1 -1 I :orresponding Da Representation l6 Craig, Natuve, 1946, 158, 235. 17 Sponer and Nordheim, ONR Contract N60rz-107, T.O.1 (1st June, 1945). Forster, 2. physik. Chem. B, 1938, 41, 287.D.P. CRAIG I 3 The states of benzene may now be labelled according to the representations of DZA. In effect this is as if benzene were the natural first member of the 'acene series. The upward sequence of energy levels now reads (i) A,, ground, (ii) B,, (from B,, of Den), (iii) A , and B,, (degenerate, from E,,), (iv) B,, and B,, (degenerate, from El,). To carry the argument on de- mands an appeal to the physical similarity between benzene and the 'acenes. But it must appear at the least very likely that in the 'acenes the lowest B,, state (deriving from the first excited state of benzene) will fall below the lowest B,, (deriving from the third excited state). It is thus likely that the first allowed band systems in 'acenes are to be assigned A , - B,,, polarized longitudinally.This conclusion agrees with, but is derived independently of, that from actual VB calculations mentioned earlier in this section. The degeneracies noted above arising from the sixfold symmetry of benzene are split if the hexagon is distorted. This could be done by bringing to single bond distance two atoms para to one another making an imaginary molecule which physically as well as by its symmetry is the precursor of the 'acene series. The effect of the splitting is to make the A , component of (iii), which arises from the E,, state of benzene, more stable than the B,, component. But the B,, level (ii), which is nonde- generate even in the regular hexagon, is less affected. Thus the low excited states in the distorted model are A , and B3,.In actual 'acenes, by analogy, these two symmetries but not B,, or B,, are expected as low- lying states. These considerations, supported by the VB calculations already referred to, suggest the following assignments for the lower transitions in some 'acenes. Molecule Naphthalene (see $3) . Anthracene . Naphthacene . Pentacene . Experimental Absorption 0 (cm.-1) 32000 34600 45400 26400 39000 21100 36700 17100 24000 32300 Proposed Assignment The A I - A band systems in anthracene and naphthacene are presumably hidden by the more intense A , - B,, systems, as suggested by Sponer and Nordheim. l7 This paper may well conclude with a comment on the practical question that arises in energy level calculations by empirical methods. The work of 0 z and 0 3 may be taken as general support for the physical basis of the VB method in excited states. But the method cannot be equally reliable for all states even though in polynuclear hydrocarbons all are compatible with nonpolar structures. It is relevant that although all three A,,, B,, and E,, of benzene are compatible with nonpolar structures, the most stable, Kekul6 structures, appear only in the first two and it is found that these two alone can be calculated reasonably accurately when polar struc- tures are neglected in the VB method. Likewise in 'acenes the two cor- responding symmetries A , and B,, alone may be constructed from Kekul6 structures and we must expect that only states of these two symmetries will be at all reliable calculable by the VB theory in its purely nonpolarI 4 COMPLEX MOLECULES form. will be essential to include polar structures. a Turner and Newall Research Fellowship tenable at University College. The Sir William Ramsay and Just as for E,, and El, of benzene so for B1, and B,, of 'acenes it The writer acknowledges the award by the University of London of Ralph Forster Laboratories, University College, London.

ISSN:0366-9033    

DOI:10.1039/DF9500900005

出版商:RSC    

年代:1950

数据来源: RSC

摘要:

I 4 COMPLEX MOLECULES CHARACTERIZATION OF ELECTRONIC TRAN- SITIONS IN COMPLEX MOLECULES BY MICHAEL KASHA *p Received 3rd July, I950 A summary is given of the types of conclusions which may be drawn from a study of the emission properties of complex molecules under optical excitation in suitable systems. The types of radiationless transitions in complex mole- cules are discussed, and a resultant spectroscopic criterion stated : The emitting level of a given multiplicity is the lowest excited level of that multiplicity. Inter- combinations in complex molecules are described, and their importance is shown t o arise from the high probability of the excitation of triplet states-a con- clusion which runs counter t o the trend of spectroscopic thought of a few years ago. An atomic number criterion is stated (after McClure) which permits the identification of intercombinations by a study of intensity of the transition with heavy atom substitutions in a complex molecule.Finally, a listing of empirical criteria is made which permits a distinction between pure n-electron transitions (T -+ n-*) and transitions involving excitation of non-bonding N, 0, and S atom electrons t o anti-bonding T molecular orbitals (n 3 n*). The most definitive of these criteria is the disappearance of n -+ n* transitions in acid media. An unusual enhancement of the spin-orbit coupling process for n --f n* transitions is reported and a possible interpretation is given. Much useful information concerning the electronic states of complex molecules can be gained from a study of the emission properties of such molecules under optical excitation.In particular cases it may be necessary to study the molecule in rigid glass solutions at the temperature of liquid nitrogen in order to observe its light emission properties. How- ever, since there is hardly a case in which light emission can not be ob- served with a moderate quantum efficiency, the method of emission spectroscopy can be applied almost universally. The information derived from such studies forms an essential supplement to that obtained from absorption spectra in the vacuum ultra-violet, high resolution spectra of vapours, and absorption spectra of solutions. In a series of forthcoming publications by the writer a detailed study of emission spectroscopy of complex molecules is reported .1 The present paper summarizes the type of information revealed by such investigations.Radiationless Transitions in Complex Molecules .-Two basic types of radiationless transitions are observed in most complex molecules. These axe merely defined here, a full discussion and references being given e1sewhere.W 1~ * U.S. Atomic Energy Commission Research Fellow. t Present address : Department of Chemistry, The University, Manchester, England. Emission Spectroscopy of Complex Molecules, M. Kasha, to be published soon. (a) Relations between Lifetime and Quantum Yield. (b) Internal Conversion of Electronic Energy. (c) Intersystem Crossing of Potential Surfaces,MICHAEL KASHA I 5 INTERNAL CONVERSION may be defined as the rapid radiationless combination of excited electronic states of like multiplicity (combination in the spectroscopic sense of undergoing transition between).In general, internal conversion to the ground state does not occur with a high proba- bility, although in certain types of molecules the latter process is sig- nificant and diminishes the total intrinsic quantum yield of luminescence.1' The phenomenon of internal conversion is manifested by the appearance of a unique luminescence, regardless of which state of a given multiplicity is excited. It yields a most useful spectroscopic criterion : The emitting electronic level of a given multiplicity i s the lowest excited level of that multiplicity. An important recent application of this criterion is the re-assignment by Shull a of Sklar's lowest calculated triplet level of benzene to the emitting triplet level ; Sklar arbitrarily has assigned the observed triplet level to one of the higher calculated levels.INTERSYSTEM CROSSING is the spin-orbit-coupling-dependent internal conversion. In most cases this is the radiationless transition from the lowest excited singlet level to the lowest triplet level of the molecule. Analogous to radiative transitions involving n-electrons (e.g. in aromatic hydrocarbon molecules), a prohibition factor of about 106 distinguishes inter-system crossing from internal conversion. Moreover, since spin- orbit coupling forces are involved, the introduction of heavy 10 (i.e. high atomic number) and paramagnetic a atoms into the molecule will strongly increase the probability of the transition, as will strong external electric and magnetic fields.Despite the large prohibition factor, however, inter- system crossing is a rather probable process in most molecules. In fact, although usually observed only very weakly as a direct absorption from the ground (assumed singlet) state, the lowest triplet state is readily excited to emission (under conditions of low quenching, viz., rigid glass solutions). Thus, the lowest triplet state has been located in several hundred complex molecules through a study of their phosphorescence s p e ~ t r a . ~ ~ The facile excitation of the lowest triplet state (by light absorption to the lowest excited singlet state) is really a consequence of the very high rate of internal conversion (e.g. between singlet states).The follow- ing analysis of the rates involved may help to make this clear. A molecule is excited optically to its nth excited singlet state, 3. Under steady excitation a spectrum is photographed in search for the spontaneous S" -t- emission (this has not been observed thus far in the cases studied). The ratio of the rate constant (reciprocal " lifetime ") for the spontaneous Sfi +- luminescence to the rate constant for the radiationless Sfi -t S' internal conversion is given by the intensity ratio since the intensity of the fluorescence S' 4 S indicates the number of internal conversions. Numerous observations of this sort have placed the upper limit at the value indicated. Thus, the internal conversion process takes place at least 104 times as fast as the spontaneous Sn +- emission.The intrinsic lifetime of the latter can be calculated from the integrated absorption of the band corresponding to the S a c S absorption: we shall assume this to be (d) T- and %-electron Transitions in N-heterocyclics (with C. Reid). (e) %-Electron Transitions in Molecules Containing - Y = X : Groups. If) Internal Conversion t o the Ground State. (g) Summation of the Quantum Yield. ZShull, J . Chem. Physics, 1949, 17, 295. 6 Nauman, Thesis (University of Califorpia, Berkeley, 1947). Yuster and Weissman, ibid., 1949, 17, 1182. Lewis and Kasha, J . Amer. Chem. SOC., 1944, 66, 2100.16 COMPLEX MOLECULES I O - ~ sec., a value commonly found for intense u.-v. absorptions. the reciprocal rate of the internal conversion process is l / R l c Q IO-" x / k , 6 10-l~ sec.The natural limit on the rate of internal conversion may well be the time of a vibrational period; shorter estimates are actually obtained from spectroscopic data in some cases. As a consequence of the very high rate it is to be expected that " internally converting levels " will be somewhat broadened in a band width relative to the band width of the " fluorescing level " with a natural lifetime of 10-7 to 10-9 sec. In other words, the lowest absorption band of a given multiplicity manifold will be better vibrationally resolved than the higher energy bands of the same manifold (e . g . singlet -singlet transit ions). For intersystem crossing we apply an arbitrary prohibition factor of I 06, giving an intersystem crossing reciprocal rate Then I/k, < 108 .I/kIc < 10-7 sec. The probability of intersystem crossing in a molecule is measured by the intersystem crossing ratio x, which is defined as the ratio of quantum yields of phosphorescence to fluorescence under steady simultaneous excitation 1 c A fluorescence lifetime of 10-8 sec. is arbitrarily assumed here for illustration. The actual observed values of x are of the order of magnitude of I for molecules with a-electron energy levels,lc indicating that the general assumptions made in the rate analyses are approximately correct. Intercombinations in Complex Molecules .-Having observed the long-lived luminescence corresponding to the triplet-singlet emission of the molecule, we can locate the converse singlet-triplet absorption band with confidence.(If there are two luminescences, the long-lived one is naturally identifiable as the intercombination process. However, if only a single luminescence is observed, even under conditions of minimum quenching, the criteria to be described below must be applied.) The intensity (integrated absorption) of the converse absorption band mag be calculated from the measured lifetime of the luminescence by the well-known expression 6 , 7 From the integrated absorption and an assumed band width for the type of transition in question, the peak molar absorption coefficient can be estimated. Using the latter and the frequency of the 0, o-band of the phosphorescence emission spectrum, the singlet-triplet absorption can be found.7 Since this is usually about I O ~ times weaker than the normal singlet-singlet transitions, rather long optical paths must be used, or else concentrated solutions or even the pure liquid (or solid) substance must be studied.Under these conditions absorption of light by an impurity easily could be misinterpreted as a forbidden electronic transition, were it not for the double check of correspondence with the 0, o-band of the emission spectrum, and with the integrated absorption intensity calculated from the mean lifetime of the emission process. To identify spectroscopically the long-lived luminescence and converse absorption bands as intercombinations, use is made of the character- istics of the spin-orbit coupling process. As is known from the theory of 'Ladenburg, Veyh. dtsh. physik. Ges., 1914, 16, 769; 2.Physik, 1921, 4, 451. Tolman, Physic. Rev., 1924, 23, 693. Perrin, J . Physique Rad., 1926, 7 , go ; Ann. physique, 1929, 12, 169. Cf. Lewis and Kasha, J . Amer. Chem. SOC., 1945, 67, 994.MICHAEL KASHA 17 atomic spectra, the probability of spin-orbit coupling increases rapidly with increasing atomic number of the atom. This principle has not been applied generally in molecular spectroscopy, although it is well known that, e.g. in the halogen series of molecules, the probability of the singlet- triplet transition in the visible region increases greatly in intensity in the order Cl,, Br,, I,. McClure 8 made the valuable extension of this principle to heavy-atom substituted . complex molecules, showing that like the (radiationless) intersystem crossing process,1c the radiative intercombination process likewise increases rapidly in probability with increasing atomic number of the substituent.Thus, the rate constant for phosphorescence k , = I / T , as well as the oscillator strength f (or the integrated absorption, Jedv) increase rapidly with increasing atomic number. This is shown clearly in Fig. I, which is a plot of McClure’s data for the monohalo-benzenes FIG. th the effect of atomic number on intercombinations in the spectra of substituted complex molecules. and naphthalenes. The solid line indicates the probability of the triplet-singlet emission process measured as the phosphorescence lifetime. The dotted lines show how one of the singlet-singlet transitions behaves for the same series of halo-derivatives. (The increase in f for bromo- benzene and especially iodobenzene is probably due to the approach of the non-bonding halogen transition to the benzene 2600 A absorption ; the dotted line is arbitrarily drawn downward to suggest the correction required.) Obviously the atomic number is the wrong parameter against which to plot the probability of singlet-singlet transitions : some other property of the halogen substituent would give a more rational dependence.@ The atomic number effect is particularly valuable for the identification of intercombinations because the frequency of the transition changes only 8 McClure, J .Chem. Physics, 1949. 17, 905. S Matsen (Symposium on Molecular Structure and Sfiectroscopy, Ohio State University, Columbus, Ohio, I 3th June, 1950) has presented a molecular orbital treatment of the problem which explains the anomalous f-values of the 2600 A transition of the halo-benzenes in terms of an inductive effect.18 COMPLEX MOLECULES slightly while the intensity changes by a large factor (e.g.the factor is 1000 for the lowest singlet-triplet of the mono-iodonaphthalenes com- pared with naphthalene). Armed with this increased knowledge on the nature of intercombin- ations in complex molecules, great progress in this branch of molecular spectroscopy may be anticipated. n +- n* and n +- ‘ZF* Transitions.-A study of the emission spectral properties of the N-heterocyclics, which the writer has carried out to- gether with Dr. C. Reid,ld has indicated the optical similarities of some of these molecules to such molecules as carbonyl, nitro, and nitroso com- pounds.In, e.g., the carbonyl compounds, the lowest singlet-singlet transition has been characterized tentatively as a transition corresponding to the excitation of a (nearly) non-bonding (oxygen) electron to an anti- bonding 7~ molecular orbital.10 We have obtained definite physical evidence for this interpretation and have found analogous transitions (involving the non-bonding nitrogen electrons) in various N-heterocyclics (e.g. pyridine, pyrazine, phenazine) . For convenience we designate non- bonding electrons as n-electrons, and the corresponding transitions as n + T* transitions. These transitions are characterized by remarkably different properties in comparison with -.rr 3 T* transitions as shown, e+., by the spectra of the aromatic hydrocarbons.A summary of dis- tinguishing empirical criteria is offered. VIBRATIONAL BAND WIDTH (VAPOUR SPECTRUM) .--.rr -+ m* transi- tions : moderately broad, owing to unresolved rotational structure. n + T* transitions : “ atomic ” in sharpness, a fact commented upon by numerous observers. This seems to be due to the weakness of the P and I? rotational branches, the strong Q branch predominating. VIBRATIONAL BAND WIDTH (SOLUTIONS).--.~~ +- T* transitions : vibra- tional fine structure preserved (e.g. the 2600 n +- -P* transitions : very‘ blurred, usually only slight vibrational structure re- mains, even in hydrocarbon solvents. This is all the more striking in view of the sharpness of the vapour absorption lines.Probably it is a characteristic of the non-bonding orbitals which leads to an unusual perturbation in solutions. In hydroxylic and other solvents, vibrational structure is completely blurred. Complex formation, probably involving hydrogen bonding in most cases, occurs and changes the non-bonding character of the orbitals. ACIDIC SOLVENTS.--P +- -P* transitions : the transition is preserved, although some blurring of fine structure occurs ; usually there is little change in intensity. n +- -P* transitions : the transition “ disappears ” (moves to very high energies). Addition of a proton to the non-bonding pair of the hetero-atom greatly increases the binding energy of the “ lone pair ”. This behaviour offers the clearest proof of the excitation of non- bonding electrons in such molecules.complex vibrational envelope, many normal vibrations excited. n --f -P* transitions : usually a unique vvib excited ; this can probably be explained in terms of the localized character of the non-bonding electron orbital. RATIO OF TRANSITION PROBABILITIES, P11/P31 (singlet-singlet against singlet-triplet) .-v --f T* transitions : the oscillator strength ratio f i l / f s l ~ roS for the radiative process. For the radiationless process oP/QF H I, which implies an equal probability of transition to the lowest triplet (and consequent triplet-singlet emission) and of spontanteous singlet-singlet emission. For N --+ T* t-ransitions f l r l f s l ~ 104, and > 1000. Here is evidence for a striking increase in the probability of the spin-orbit coupling process in such transitions. The increase by a factor IOO to 1000 is much band of benzene). VIBRATIONAL ENVELOPE.--^ + -P* transitions : IOMulliken, J . Chem. Physics, 1935, 3, 564. McMurry and Mulliken, Prac. McMurry, J.Chem. Physics, 1g41,9, 231 and 241. Nat. Acad. Sci., 1g40,26,312.MICHAEL KASHA 19 more than can be accounted for by an increase in atomic number in going from C to N to 0. It is to be noted that the djP/Op property here means that fluorescence is scarcely to be observed in these molecules (some N-heterocyclics, all carbonyl, thiocarbonyl, nitro, nitroso, azo compounds) : the " forbidden " conversion to the triplet state is essentially complete. A possible interpretation of the greatly enhanced spin-orbit coupling for n --f T* transitions is that sp-hybridization which is present or can be plausibly introduced into non-bonding orbitals brings the optical electron much closer to the nucleus than in the case of an electron in a Ir-orbital ; the spin-orbit coupling process occurs near the electric field of the nucleus of an atom. There are difficulties in this interpretation,ld which, however, must be resolved by theoretical computations. Department of Physics, University of Chicago.

ISSN:0366-9033    

DOI:10.1039/DF9500900014

出版商:RSC    

年代:1950

数据来源: RSC

摘要:

MICHAEL KASHA DISCUSSION OF THE LOWEST SINGLET TRANSITION I N NAPHTHALENE AS A AND REMARKS ON THE HIGHER SINGLET LEVELS * FORBIDDEN TRANSITION A,, - Al, BY H. SPONER AND (THE LATE) GERTRUD P. NORDHEIM Received 18th July, 1950 The absorption system of naphthalene a t 3200-2goo A is interpreted as IAl, -lAlg transition. The infra-red active vibration of 568 cm.-l is tentatively suggested as vibration that makes the forbidden transition allowed in second order. In making this analysis, experimental data on absorption and fluor- escence in the different states of aggregation of naphthalene and new results obtained for heavy naphthalene were used. The 0-0 band of the next higher absorption system at ~ ~ O O - Z ~ O O A is located at 35910 cm.-l and of the third absorption system at about 45100 cm.-l, both for the vapour.A brief com- parison with theoretical results is given. The near ultra-violet absorption of naphthalene has been studied in the vapour phase,b 2 in liquid 2, 3 and in rigid glass solutions,* and in the solid state 5 (crystal at zoo K). The spectrum consists of two absorption regions lying fairly close together. The long wave part ranging from about ~ ~ O O - Z ~ O O shows in the vapour sharp narrow bands, whereas the bands from 2900-2500 A are broad and diffuse. Moreover, the pattern in which the bands appear is different in the two regions. The measurements in liquid solution give an average extinction coefficient E of 200-300 for the first region and of 5000-6000 for the second region. Furthermore, at 2200 a very strong and broad band with little indication of structure was observed 2 .6 with E N IOO,OOO. The absorption spectrum of the solid * This research was supported by the Office of Naval Research. 1 Henri and de Laszlo, Proc. Roy. SOG. A , 1924, 105, 662. de Laszlo, 2. physik. Chem., 1925. 118, 369. For example, Morton and de Gouveia, J . Chem. Soc., 1934, ~ I I ; Mayneord Kasha and Nauman, J . Chem. Physics, 1949, 17, 516. Klevens and Platt, J . Chem. Physics, 1949, 17, 479. and Roe, Proc. Roy. Soc. A , 1935, 152, zgg. 6 Prjkhotjko, J . Physics, U.S.S.R., 1944. 8, 257.20 SINGLET TRANSITION IN NAPHTHALENE contains discrete relatively sharp bands, but looks different from the vapour spectrum whether obtained with natural or with polarized light. Fluorescence of naphthalene has been studied mostly in liquid and solid solution^.^^ It consists of a number of bands in the region 3000-3650 A.Fluorescence of the vapour was excited 8 by irradiation with wavelengths below 3000 A showing narrow bands between 3000 and 3340 superim- posed on a continuous background. Fluorescence and absorption bands coincide in the overlapping region. The fluorescence of crystalline naph- thalene 0 (at zoo K) consists of a complicated spectrum of many, mostly narrow, bands. From studies of the fluorescence lifetime of naphthalene in rigid glass solution Kasha and Nauman 4s concluded that the weak long wavelength absorption is a forbidden transition. Calculations of the electronic terms of naphthalene have been carried out with the bond orbital method taking into account valence structures with no or one I ‘ long ” bond.1° With this approximation one obtains as the first two excited singlet states a B,, and a B,, level. Transitions from the symmetrical ground state to the B,, state are allowed with the transition moment in the long molecular axis (x axis), and to the B,, state with the moment in the short (y axis).Earlier more complete cal- culations by Blumenfeld, l1 taking all 42 canonical structures into account, give for the first excited level a symmetric one, lA1#. The transition from the ground state to this level is forbidden. Blumenfeld obtains as next excited levels a lBSu and a lBg, term. Craig la also using the valence bond method, has likewise predicted an A,, as lowest excited singlet level, followed by a lBsu, lB1,, 1B,, term, in that order.Platt’s 13 one-dimensional free electron picture leads to the same order of the allowed levels as the bond orbital method, whereas calculations with the standard molecular orbital method (German, l4 Coulson,ls Davydov 16) and a slightly modified molecular orbital method (Simpson 17) reverse this order. In this approach the symmetric level should lie at higher excitation energy. In the most recent treatment of the energy states of naphthalene by Jacobs 1* an antisymmetriced molecular orbital calculation was carried out, taking into account the interaction between the different configurations formed by the molecular orbitals. In this approach a B,, and a B,, are obtained as lowest singlet levels with a very small separation between them.The first excited lAl, state appears 1’4eV above the B,, state. Oscillator strengths were calculated for the first three allowed transitions and compared with experimental values omitting the first weak absorption at 3zoo-2gooA. The agreement is rather good. Analysis of the Lowest Electronic Singlet Transition in Naphthalene at 3200-2900 A.-(a) OCCURRENCE OF 0-1 and 1-0 BANDs.<onsidering the weakness and structure of the first absorption in naphthalene, and the lifetime of the fluorescence, this system can be explained best as a transition forbidden by symmetry. Possibilities of interpreting the low intensity by cancellation of positive and negativz terms of the same order 7 Compare P. Pringsheim, Fluorescence and Phosphorescence (Interscience Publishers, New York, 1949)~ 408.Pringsheim, Ann. Acad. Warsaw, 1938, 5 , 29. Obreimov and Shabaldas, J . Physics U.S.S.R., 1943, 7, 168. Remarks of M. Kasha a t O.N.R. Spectroscopy Conference, Jan., 1948. lo Sponer and Nordheim, ONR Contract N60ri-107, T.O. I (Annual Report, l1 Blumenfeld, J . Physic. Chem. U.S.S.R., 1947, 21, 529. l2 Craig, Proc. XIth Int. Congr. Chem. (Lond.) (July, 1947). l3 Platt, J . Chem. Physics, 1949, 17, 484. 1* German, J . Physics U.S.S.R., 1944, 8, 276. l6 Coulson, Proc. Physic. Soc., 1948, 60, 257. l6 Davydov, J . Expt. Theor. Physics U.S.S.R., 1947, 17, 1106. Simpson, J . Chem. Physics, 1948, 16, 1124. l8 Jacobs, Proc. Physic. SOC. A , 1949, 62, 710. June, 1948)-H. SPONER AND GERTRUD P. NORDHEIM 21 in the transition moment integral have also been mentioned.lol 1 7 If the long wavelength system arises from a transition forbidden by symmetry, it is obvious that absorption and fluorescence bands should show the same charactcristic difference as the near ultra-violet transition in benzene, i.e. a strong 0-1 band and a w-eak 1-0 band in absorption and a weak 0-1 and a strong 1-0 band in fluorescence.Experimental data on ab- sorption and on fluorescence in the different states of aggregation w2re consulted and the interpretation chosen is presented in Table I. There is no doubt that the narrow band group at 3080.1-3081.3 A in the vapour represents the 0-1 transition. It is very strong in absorption and weak in fluorescence. The band at 3080.4 A or 32454 cm.-l is taken as main band.Likewise, the band at 3172-4-3173'4 A occurs strongly in fluorescence and weakly in absorption and is here considered as 1-0 transition. Already Pringsheim * noticed these intensity ratios which would be in agreement with a forbidden transition. 31513 cm.-l is taken as main band of the 1-0 transition. The intensity ratio of the two bands as taken from absorption plates obtained by Cooper la is 1/20. From spectrogram No. 169 in the catalogue of ultra-violet spectrograms a ratio of about I to 30 may be estimated for the solution.20 These ratios will be used later in the evaluation of the 0-0 band. As seen in Table I the band positions in the solution are shifted to the red by about 310 cm.-1 as compared to the vapour. For rigid glass solutions Kasha and Nauman's values have been used.TABLE I.-0-1, 1-0 AND 0-0 BANDS OF LONG WAVELENGTH SYSTEM OF NAPHTHALENE Bands 0-1 1-0 Separation 0-0 Vapour abs., ref. I fluor., ref. 8 32454 cm.-l 31513 crn.-l 941 cm.-l (32080 cm. -l) Liqu. Solution (hexane) abs., ref. I, 20 fluor., ref. 7 32145 crn.-1 31173 cm.-l 972 crn.-l (31757 cm.-l ?) Rigid Glass abs., ref. 4 fluor., ref. 4 32 104 cm. -l 31144 cm.-]. 960 crn.-l 31680 crn.-l Crystal abs.. ref. 5 fluor., ref. g --- - 31062 cm.-1 ? (31062 cm.-1 ?) - I 1 The corresponding assignments in the crystal spectra are much more difficult to achieve. These spectra are extremely complex and not too well understood. Four different electronic transitions were assumed in the absorption analysis and three were used for the explanation of the fluores- cence spectrum.Q The absorption spectra were taken with natural light and with polarized light where the polarization vector was either parallel to the u or b axis of the crystal.In the long wavelength part there are bands which appear weakly or not at all ir, one case and strongly in the other, but above 33400 cm.-l the spectrum is strong only when the polariza- to the a or b axis 21 of the crystal. In the long wavelength part there are bands which appear weakly or not at all in one case and strongly in the other, but above 33400 the spectrum is strong only when the polariza- tion vector is parallel to the a axis and it is practically missing for the other position of the Nicol. In spite of the excitation by polarized light as means of differentiation, we found it not possible to obtain a satisfactory analysis.It was expected that, if the analysis presented here is valid for the vapour and solutions, it would also, at least in its main features, apply to the solid. However, the characteristic features may become obscured because inter- 19 Cooper, Master Thesis (Duke Univ., 1948); 2O American Petroleum Institute Research Project 44 at the National Bureau of Catalogue of UEtra-violet S$ectrograms, Serial No. I 69, Naphthalene Standards. liqu. sol.).22 SINGLET TRANSITION IN NAPHTHALENE action with crystal forces (the molecules are arranged in skew packing "') may cause the breakdown of the selection rule. It will also lead to splitting of the energy levels 22 and the spectrum will look more compli- cated than that in the vapour phase.For naphthalene, the fluorescence spectrum is less complex than the absorption spectrum. Obreimov and Shabaldas stated that the fluorescence spectrum has a sharp beginning with a line at 31062 cm.-l and they succeeded in interpreting all stronger lines with the use of known Raman frequencies belonging to totally symmetric vibrations. Small frequencies u p to about IOO cm.-1 were identified with lattice vibrations found in the Raman effect of naphthalene crystals. 23 From the previous discussion it would seem logical to assign the narrow band at 31062 cm.-l to the 1-0 transition in the crystal spectrum. How- ever, the situation becomes confused when considering the absorption spectrum. The groups in the spectrum do not stand out as clearly as in the fluorescence spectrum and the overlapping of the second absorption region is adding to the complexity of the spectrum.A new analysis by Prikotjke 24 was not available to the authors except a very brief abstract in Chem. Abstr. in which no mention is made of any figures. Interpreting the band 31062 as o--o band would account for the appearance of this band in fluorescence and absorption but it does not lead to a satisfactory analysis of the spectrum. It would also give a very large red shift as compared to the vapour and solution spectra. Work on the spectrum of the crystal is being continued. (b) FIXATION of 0-0 BAND.-The separation between the 0-1 and 1-0 bands in the different states of aggregation are also included in Table I. This separation represents the sum of the vibrational frequencies in the lower and upper electronic state that makes the forbidden transition allowed.Assuming that it is a transition Alg-Alg, it is clear that excita- tion of a ps, vibration will produce a transition moment in the long molec- ular axis and a pZu vibration a moment in the short axis. A moment perpendicular to the molecular plane (along the z axis) could be produced by excitation of a plu vibration. Fixation of the 0-0 band is not easy. The three types of vibration which can produce the necessary moment are Raman inactive. Infra-red measurements are known in the literature Z 5 only above 600 cm.-l. It seems more likely that the separation 941 cm.-l would involve a vibration below that value. Dr. E. K. Plyler and Miss Mary A.Lamb have very kindly taken the infra-red spectrum of naphthalene in solution from 4000 to 360 wave numbers at the National Bureau of Standards. Their absorption curves show four frequencies in the critical region : 618 (m), 568 (w), 476 (vs), 361 (w).* Of these, as far as numerical values are concerned, 568 and 476 seem most plausible. If a 50 yo drop of the frequency in the upper state can be assumed, then the 618 is also eligible. Using these values with the separation 941 cm.-l in the gas (although they have been obtained from solution data), this would give an upper state frequency of 323, or 373 or 465 cm.-l respectively, and correspondingly, would place the 0-0 band at 32131, or 32081 or 31989 crn.-' Although faint and narrow bands have been measured by de Laszlo in all three places, this has not much significance.Cooper, who studied the absorption in dependence of pressure and not of temperature, did not 21 Robertson, Proc. Roy. SOC. A , 1933, 142, 674 : Abrahams, Robertson and White, Acta Cryst., 1949, 2, 238. 22 Davydov, J. Expt. Theor. Physics, U.S.S.R., 1948, 18, ZIO. Gross and Vuks, J . Physique Rad., 1936, 4, 113 ; Nature, 1929, 124, 692. 24Prikhotjko, Nut. Acad. Sci. U.S.S.R., Ser. Phys., 1948, 12, 499, reviewed in Chem. Abstr., 1950, 4, 433. 25 For example, Lambert and Lecomte, Ann. Physique, 1933, 18,329 ; Barnes, Gore, Liddell and Williams, In@-red Spectroscopy (Reinhold, New York, 1944), P. 5:. m = medium, vs = very strong, w = weak,H. SPONER AND GERTRUD P. NORDHEIM 23 observe a band in any of these positions.The solution spectrum obtained by de Laszlo shows a peak at 31757 crn.-l which divides the distance of 970 into 584 + 388 cm.-l, but a similar peak has not been obtained by other authors (for example, ref. 20). The interpretation of the 31757 cm.-l peak as 0-0 band must be considered as uncertain, particularly in view of its intensity. Examining the data obtained in rigid glass solution, it is seen at once that the 0-0 band does occur weakly in absorption and in fluorescence. Interaction with the glass medium causes a breakdown of the selection rule. Unfortunately, the measurements are accurate to only f 30 cm.-l. Actually, the two 0-0 bands deviate by about IOO cm.-l from each other. In the absorption curve the distance from the 0-0 band to the next very strong band is 367 cm.-l, and the corresponding distance in the fluorescence curve is 482 cm.-1.In Table I the values for the 0-1 and 1-0 bands (rigid glass) are the wave numbers of the strongest bands in the spectra and are more reliable than the weak o---o peaks. With this in mind and assuming that the ratio of the distance 367 and 482 has some real significance, the position of the o---o peak is estimated at about 31680 cm.-l which is the averaged value reported in Kasha and Nauman's paper. No additional information can be obtained from the spectra of crystal- line naphthalene as was indicated before. (c) SUPPORT OF THE ANALYSIS AND DETERMINATION OF THE INTER- ACTING VIBRATION FROM THE SPECTRUM OF HEAVY NAPHTHALENE.- Although the preceding analysis (intensities, numerical relations, Boltnnann factor from intensity ratio of 0-1 and 1-0 bands) points to a vibration in the neighbourhood of 600 cm.-l rather than to the strong 476 cm.-l as interacting vibration, the latter cannot be definitely excluded because of the only roughly known positions of absorption and fluorescence peaks in liquid and solid solutions. To facilitate the assignment, a sample of completely deuterated naphthalene was prepared by L.Corrsin * in this laboratory and the ultra-violet absorption spectrum was taken by C. D. Cooper. The first noticeable feature was the greater diffuseness of the spectrum as compared to that of light naphthalene. There was probably a small percentage of not completely deuterated compound in the sample which would produce close lying bands and somewhat obliterate details.However, the edges on the ultra-violet side of the bands would correspond to the completely deuterated substance. It was found that the 0-1 band in heavy naphthalene is shifted by only about IOO cm.-l to the violet as compared to ordinary naphthalene while the corresponding shift in benzene is 180cm.-l. (The shift in mono-deuterobenzene is 31 cm.-1). The shift to the violet results from zero point vibrations. The separation between the 0-1 and 1-0 bands is about 910 cm.-l in heavy naphthalene as compared to 941 in the light compound. The infra-red frequencies were taken from curves obtained again by Dr. E. K. Plyler and Miss Mary A. Lamb at the National Bureau of Standards using our samples in solution.In the region pertinent to our dicussion these vibra- tional frequencies were observed : 594 (m), 566 (vw), 541. (w), 422 (vw), 403 (s), 328 (w). It seems reasonable to assume the following correspond- ences between light and heavy naphthalene vibrations : unlikely that the vibration will have a much larger frequency in the excited Light Heavy 618 (m) 594 (4 568 (w> 541 (4 476 (4 403 (4 361 (w) 328 (w) The separation of 910 cm.-l rules out the participation of the 403 vibration in the mechanism of making the A,,-A,, transition allowed because it is * A paper by L. Corrsin is in the course of preparation.24 SINGLET TRANSITION IN NAPHTHALENE electronic state. For 594 and 541, the ratio of the vibrational frequencies in the two states would be about the same for both naphthalenes.Measure- ments in rigid glass solution agree better with using the 568 than the 618 cm.-1 vibration, but the wide limits of accuracy in these spectra do not entirely exclude the higher value. A consideration of the symmetrics of these vibrations may permit a further selection. The question to which symmetry class the interacting vibration belongs is identical with the question in which direction the “ forbidden ” transition is polarized. Firstly, it is expected and assumed that this vibration is a carbon vibration. It is furthermore assumed that the vibrational transition moment will be in the molecular plane and more specifically, in the long axis ( x ) since the next allowed transition (Alg-B3,J should be polarized in the x direction. This makes the interacting vibra- tion one of type pSu.Either vibration (618 or 568) could belong to this type, From intensity arguments the 618 seems a better choice.* The large drop of almost 50 yo in wave number in the upper electronic state would have to be explained. Although a drop of this magnitude was assumed to occur in benzene 2E for the p2, carbon vibration, it is surprising for the p3% mode. Therefore, until further support in favour of the 618 vibration has been found, the 568 vibration is used here tentatively as the interacting vibration. While this refers to the strong part of the long wavelength system, it is quite possible that weak bands which do not fit in this scheme result from a vibrational moment in the short axis ( y ) and involve a vibration. In Table I1 there are collected the main vibrational frequencies occur- ring in the spectrum produced through interaction of a ,€Isu vibration.All are interpreted as carbon vibrations. Remarks on the Higher Electronic Singlet Levels-As mentioned in the introduction, a second absorption region occurs in naphthalene at zgoo-2500 A. The beginning of this absorption (0-0 band) was given in Kasha and Nauman’s paper as 33736 cm.-1. This value was taken from Prikhotjko’s absorption measurements in the crystal. Klevens and Platt place the onset of the second absorption in solution at about 35,000 cm.-l. From a careful study of our own plates we have chosen the strong broad vapour band at 35910 cm.-1 as 0-0 bandln of a second ab- sorption system. It seems impossible to correlate this value with the 33736 in the solid because of the too large shift. In looking for a possible lower transition we had noticed on our plates a rather strong group of bands located at 34060-33940 cm.-l with the bands spaced about 40 cm.-l apart.They look different from the other naphthalene bands, H group of Henri and de Laszlo appearing strongly in the vapour and in solution. A corresponding group in heavy naphthalene is missing. Dr. Corrsin suggested the group might be due to an impurity which was removed with the sulphonation in the preparation of heavy naphthalene. After re- fluxing the ordinary naphthalene with sodium, the peculiar band group had disappeared.? The spectra lcck now more uniform, and it has been possible to extend the analysis of the lAl,-lAlo system a little further toward shorter waves, and of the second stronger system to longer waves from the 0-0 band 35910 cm.-1.The second system is considered to be the predicted 1A lg-lB 3u transition. Although measurements in the spectrum of heavy naphthalene have not been completed, it may be mentioned here that a preliminary calculation indicates a shift of about *Tentative assignments of the infra-red active vibrations of the two naphthalenes will be published by L. Corrsin soon. 2E Garforth, Ingold and Poole, J. Chem. SOC., 1948, 491. -f Naphthalene which had been sublimed and recrystallized several times still gave a positive test for sulphur before refluxing with sodium. Dr. Corrsin’s suspicion that the impurity might be benzothiophene was recently verified spectroscopically by C.D. Cooper. 3 These results will be published with C. D. Cooper elsewhere.H. SPONER AND GERTRUD P. NORDHEIM 25 130 cm.-l of the 0-0 band in the second system of heavy naphthalene as compared to light naphthalene. The principal vibrational frequencies found in the transition A ,,-B,, are included in Table 11. Except for 930, they are considered as belonging to carbon vibrations. Coming back to the absorption spectrum of the crystal, photograph 4a, Plate I in Prikhotjko’s paper suggests from the intensity distribution of bands that the region 33445-36000 cm.-l may consist of two different transitions. It is tentatively suggested here that the state B,, splits in the crystal into two states, one covering a spectral transition from about 33440-34500, and the other from about 34500 to above 36000 cm.-l.The third absorption region of naphthalene occurs at 2220 A-2000 A (~~ooo-~oooocm.-~). It is more than 10 times stronger than the second. Structure in this system was obtained in the vapour on u.-v. sensitized plates. in this region but shows in addition a few fainter bands on the long wave side which are lost in the sharp rise of the absorption curve in the liquid. It agrees with the general contours of the spectrum of the liquid TABLE II.-VIBRATIONAL FREQUENCIES (CM. -l) OCCURRING IN THE FIRST Two SINGLET SYSTEMS OF NAPHTHALENE. LONG WAVELENGTH SYSTEM Alg-A1, Ground State 1 Excited State 1 Raman I i I- (373) 701 474 I 997 I SHORTER WAVELENGTH SYSTEM AI0-B3, Symmetry of Vibration 512 762 940 1380 I022 Although work on this system is still in progress, a vaIue of 451oocm.-l mav be suggested as possible location of the o--o transition.Comparison with Theory .-The occurrence of a forbidden transition lAlg-lAII as lowest electronic transition in the naphthalene molecule is in agreement with calculations based on the valence bond method.ll9 l2 The tentative position of the o--o band (Table I) is at 3116 (32080 cm.-l), while Blumenfeld calculated 3193 and Craig’s value is 2830 A. The good agreement with Blumenfeld’s value is probably fortuitous. The analysis presented here is also consistent with results obtained from Platt’s free electron model. The location of the 0-0 band fits well on his curve for the lL, transition in polyacenes.6 Both the valence bond and molecular orbital method agree on a B,, level as upper state of the first allowed transition.From these predictions and not from the analysis, the 35910 band was identified with the 0-0 transition of the system 1A1,-1B3u. A distinction between a Bzu and B,, level from the spectrum is difficult as essentially the same vibrational structure may be expected in the two transitions. The close agreement of the position of our lBQu level (2784 A) with Blumenfeld’s calculated position is again more or less fortuitous. Craig’s calculations place it somewhat lower at 2665 A. There seems, however, to be disagreement26 PYRIDINE HOMOLOGUES with the latest results obtained with the molecular orbital method. Cal- culations of Jacobs would place the B,, level at about 3420 A, that is, in the neighbourhood of the forbidden absorption system. These calcula- tions further predict a B,, level very close to the B,, state. Transitions to these levels from the ground state would give overlapping spectra. Careful search was made for a second system between the long wave forbidden spectrum and the allowed system starting at 35910 cm.l This seemed important in view of the appearance of the crystal spectrum in that region, as was discussed before. Now it is true that there are a number of unexplained bands in the critical region of both transitions, but almost all the stronger bands (or rather groups of narrow bands) were found to fit into one of the two systems. There are also a very few fainter bands which look different from the other naphthalene bands in either system and which, remembering the fate of the H group (notation of Henri and de Laszlo), might still belong to an impurity. Taking all this into account, the existence of another system in the transition region between the two identified systems cannot be excluded until all the bands have been accounted for, but this existence is not considered very probable. The third, very intense transition is polarized in the long axis as was shown in previous experimental The authors would like to express their indebtedness to Dr. E. K. Plyler and Miss Mary A. Lamb of the National Bureau of Standards for taking the infra-red spectra of light and heavy naphthalene in solution. and theoretical research. Department of Physics, Duke University, Durham, North Carolina. 27 Jones, Chem. Rev., 1947, 41, 353.

ISSN:0366-9033    

DOI:10.1039/DF9500900019

出版商:RSC    

年代:1950

数据来源: RSC

摘要:

26 PYRIDINE HOMOLOGUES APPLICATIONS OF THE ELECTRONIC SPECTRA OF PYRIDINE HOMOLOGUES TO QUANTITATIVE ANALYSIS AND TO THE MEASUREMENT OF DISSOCIATION CONSTANTS BY E. F. G. HERINGTON Received 27th June, 1950 The difficulty of preparing pure samples of pyridine bases is indicated. The absorption spectra of highly purified samples of pyridine, a-, P-. and y-picoline and z : 6-lutidine obtained under different conditions are discussed from the point of view of chemical analysis. The spectra of the vapours over a wide range of pressures a t 20° C using cells of length 14-5 cm., 61.2 cm. and 153 cm. are compared. Special attention has been paid t o the effect of small amounts of impurities. The spectra of solutions of these bases in 0-1 N sulphuric acid, 0-1 N sodium hydroxide and cyclohexane are recorded.A method for the analysis of a mixture of 8-picoline, y-picoline and z : 6-lutidine based on the differences in absorption coefficient of acid and alkaline solutions is described. The dissociation constants of these bases have been determined from the spectra of buffered solutions. Compounds of the pyridine series are particularly difficult to prepare in a state of purity, partly because there are no convenient synthetic methods, partly because certain members have boiling points near othersE. F. G. HERINGTON 27 and partly because they are difficult to free from water. The modern technique of fractional distillation will readily separate at-picoline and pyridine from a mixture of bases which was impossible when much of the earlier work on the spectra was undertaken (e.g.see Herrman l), but certain homologues such as jl-picoline (b.p. 144.0~ C), y-picoline (b.p. 145.3~ C) and z : 6-lutidine (b.p. 144.0' C) have boiling points which are so little different that even now special methods have to be used in their separation. a Extensive fractional distillation fails to remove hydrocarbon impurities from pyridine and a-picoline so that chemical treatment is a necessary step in the purification of these bases.a In view of the difficulty of preparing pure samples, it is not surprising that Herrmanl thought that the bands at 2789-4, 2832-4 and 2876.7A obtained by him in the vapour spectra of a-picoline were due to the presence of pyridine in his sample although Purvis at an earlier date had commented upon the large number of near coincidences in the wave- lengths of bands in pyridine and at-picoline.The present work shows that some of these bands do occur in the spectra of pure at-picoline. A more recent example of the difficulty of obtaining pure samples of these bases is afforded by the investigation of the Raman spectra by Herz, Kahovec and Kohlrausch 6 who showed that their jl-picoline sample contained 2 : 6-lutidine even after extensive treatment. have suggested that the ultra-violet spectra of cc-picoline and pyridine vapour should be compared in order to establish whether a band at 33824 cm.-l observed by them and by Henri and Angenot * in pyridine was due to a small quantity of a-picoline. Highly purified samples of these bases have been prepared at the Chemical Research Laboratory in connection with other work, and in view of the uncertainties concerning published spectra it was decided to record the absorption of these authenticated specimens.Interest at the Chemical Research Laboratory centred on the possibility of devising spectroscopic methods for the analysis of mixtures and no vibrational assignments have been attempted. The ultra-violet absorption spectra of pyridine in the vapour and in solutions have frequently been described, e.g. vapour spectra by Henri et aL8 and by Sponer et aZ.% 7 ; heptane solution by Spiers and Wibaut ; aqueous solution by Marchlewski and Wqtrobek and by Hughes, Jellinek and Ambrose.11 The spectra of a-picoline in the vapour has been reported by Purvis and by Herrnan,l and in solution by PuMs and by Baker and Baly.13 The vapour spectra of 2 : 6-lutidine was de- scribed by Purvis 4 and that of jl-picoline by Herrman.l but in view of the spectra reported here it appears probable that his jl-picoline specimen was composed largely of z : 6-lutidine.A note by Sponer and Rush la on the near ultra-violet spectra of a-, jl- and ypicoline appeared when our preliminary work on the spectra had been carried out. Experimental Sponer et d.'* General.-The purities of the samples of bases used were established by a freezing point technique 14 and the following values found : pyridine 99-85 f Herrman, 2. Wiss. Photog., 1919, 18, 233. Coulson and Jones, J . SOL Chem. Ind., 1946, 65, 169. Report Chem. Res. Board (19481, 33. 4Purvis, J . Chem.SOC., 1910, 97, 692. 5 Herz, Kahovec and Kohlrausch, 2. physik. Chem. B, 1943, 53, 124. 6 Sponer, Rev. Mod. Physics, 1944, 16, 224. 7 Sponer and Stticklcn, J . Chem. Physics, 1946, 14, 101. * Henri and Angenot, J . Chim. Phys., 1936, 33, 641. Spiers and Wibaut, Rec. trav. chim., 1937, 56, 573. lo Marchlewski and Wyrobek, Bull. Acad. Polanaise, 1929, A 93 ; 1934. A 22. llHughcs, Jellinck and Ambrose, J . Physic. Chem., 1949, 53, 410. l2 Baker and Baly, J . Chern. SOC., 1907. 91, 1122. l3 Sponcr and Rush, J . Chem. Physics, 1949, 17, 587. l4 Hcrington and Handley, J . Chem. SOC., 1950, 199.28 PYRIDINE HOMOLOGUES 0.07 yo, a-picoline 99-89 0.06 yo, p-picoline 99-97 f 0.02 yo, y-picolinc. 99-75 f 0.13 yo, z : 6-lutidine 99-93 f 0.04 "/o. The water content was less than 0-02 yo by volume.Sensitive colour reactions l6 gave results consistent with the freezing point determinations, thus, for example, the 7-picoline con- tents of the ,9-picoline and 2 : 6-lutidine samples were shown to be less than 0-1 yo by volume. The vapour spectra were obtained by means of a Zeiss quartz spectrometer, slit width 0.01 mm., reciprocal dispersion 7.4 A per mm. in the wavelength range 2400-3000 A, using Ilford Selochrome 30' backed plates. The light source was a high tension, 2500-V hydrogen lamp consuming I kW and exposure times varied from 4 to 16 min. The vapour absorption cells of lengths 14-5 cm.. 61.2 cm. and 153 cm. were of Pyrex glass with quartz windows and carried a side tube containing the liquid. The cells were evacuated with a mercury diffusion pump backed by a rotary oil pump.The pressure of the vapour of the base in the cell was varied by immersing the liquids in the side tube in suit- able baths. The temperature of the vapour in the light path was that of the room. The wavelengths of the absorption bands were measured on photo- graphic enlargements of the spectra using an iron arc as standard l6 and em- ploying the Hartman formula for interpolation. The wavelengths in air recorded in the tables are expressed in International angstrom and are believed t o be correct t o a t least 0.5 A. The letters given in the tables following the wavelengths designate the appearance of various types of band : thus, a, strong band with sharp edge on violet side ; b, strong band most intense in centre ; c, weak narrow band with sharp edges ; d, diffuse band ; e, centre of very broad band ; f, violet edge of very broad band.Some of the solution spectra were measured on a Beckman quartz spectro- photometer and others on a Unicam quartz spectrophotometer. The 0.1 N sulphuric acid used as solvent had a transmission of 95 % a t 2450 A compared with conductivity water while the value for the 0.1 N sodium hydroxide solu- tion was 97-2 yo. The cyclohexane which was used as a solvent was purified by acid treatment followed by percolation through silica gel l 7 and had a trans- mission of go % at 2450 A compared with conductivity water. The appropriate solvent was employed in the blank cell in every instance. Vapour Spectra.-PYRnxNE.-Henri and Angenot 8 who studied the spectra of this material over a wide range of temperatures and pressures recorded 255 bands.Such a wide range of conditions were not examined here and it was not expected that so many bands would be observed, but nevertheless 109 bands were measured and agreement found with those recorded by Henri in most cases. The only bands recorded by the French workers which were not observed al- though it was anticipated that these bands would be found from the recorded intensities and from the respective resolving powers of the spectrometers were bands at 2925.5, 2932.2, 2923.0, 2goo.0, 2886.3, 2833-1 and 2750.8 A. o-Xylene has been separated from some impure samples of pyridine but it was shown that these bands which we have failed to detect could not have been due t o the presence of this hydrocarbon in the pyridine used by other workers.The only bands found in this work which were additional t o those recorded by Henri and Angenot were weak and diffuse bands a t 2497, 2487, 2466, 2447 and 2427 A. Plate I is a photograph of the spectra. The absence of diffuse bands of the type shown by a-picoline (Plate 11) should be noted. These pyridine spectra exhibited a band a t 33824 cm.-1 (29556 A) which was not due to the presence of picoline (see below). experienced diffi- culty in assigning notably because Kline and Turkevich18 had reported a mediumly strong band in the infra-red a t 936 cm.-1 (mean 941 cm.-l). Measure- ments of the infra-red spectra of a sample of pure pyridine carried out a t the Chemical Research Laboratory failed t o reveal the presence of a mediumly strong band a t 941 cm.-l.a-PICOLINE.-The spectra of this base shows fewer bands than pyridine (Plate 11). Com- parison of the wavelengths of these bands with those recorded by Henri8 for pyridine shows a large number of near coincidences. The general intensitv of This is the band which Sponer and Stiicklen The 34 bands which were observed are listed in Table I. l5 Herington (in preparation). l 6 Iron Charts (Adam Hilger Ltd.), 3rd edn. l 7 Maclean, Jencks and Acree. J . Res. Nut. Bur. Stand., 1945, 34, 271. Kline and Turkevich, J . Chem. Physics, 1944. 12, 300.PLATE I.-Vapour spectra of pyridine at 19O C. A Iron arc. D 14-5 cm. cell, liquid at 14' C. cell, liquid at -15" C. B 153 cm. cell, liquid at 19" C. c 61 cm. cell, liquid at 19' C.F 14-5 cm. E 14-5 cm. cell, liquid at 0" C. G 14-5 cm. cell, liquid at -30" C. A B F G PLATE 11.-Vapour spectra of a-picoline at zoo C. B 153 cm. cell, liquid a t 20" C. E '14.5 cm. cell, liquid at 0" C. G 14.5 cm. cell, liquid at -22" C. A Iron arc. D 14-5 cm. cell, liquid a t 18" C. cell, liquid at -17" C. c 61 cm. cell, liquid'at zoo C. F 14.5 -cm. [To face page 28A B C G PLATE 111.-Vapour spectra of P-picoline at I ~ O C. A 1-53 cm. ccll, liquid at 19" C. cell, liquid at oo C:. -30' C. B 61 cm. cell, liquid at 19' C. c 61 cm. E 61 cm. cell, liquid a t n G I cm. cell, liquid at - - I ~ O C,. F 61 cm. cell, liquid at -45" C. G Iron arc. PLATE 1V.-Vapour spectra of y-picoline at 19" C . A 153 cm. cell, liquid at 19" C . B 61 cm.cell, liquid a t 19" C . c 61 cm. cell. liquid a t oo C. D 61 cm. cell, liquid at -15" C. E Iron arc.PLATE V.-Vapour spectra of 2 : 6-lutidine at 19" C. A Iron arc. B 153 cm. cell, liquid at 1 9 ~ C. c 61 crn. cell, liquid at 19" C. D 14.5 cm. cell, liquid at 15" C. E 14.j cm. cell, liquid at - 5" C .E. F. G. HERINGTON 2826.8 d 2823-3 a 2813.3 d 2805-9 d 29 the a-picoline spectra is less than that of pyridine. region 2663-3-25 16.6 A are particularly striking. spectra of p-picoline vapour has not been studied within recent times. The diffuse bands in the ~-PICOLINE.-EXCept. for the note by Sponer and Rush13 the aksorption TABLE ~.-WAVELENGTHS OF ABSORPTION BANDS IN OL-PICOLINE VAPOUR A l 1 - 2928-8 c 2914'7 c 2905.1 c 2898.7 c 2891.7 c 2887-9 G 2921'3 2 2879-2875.9 d A 2875.9 b 2869.7 b 2866.1 c 2862.1 d 2861.3 d 2858.8 d 2856.0 d 2853.8 c A I A 2850.0 c 2847.6 c 2844.1 c 2836.4 G 2823-3 b 2826-2882 d 2816.5 b 2839.7 c 2806.9 c 2803.8 c 2795'9 c 2789.0 b 2783'5 c 2775'5 d 2747'2 d 2736.9 c A Plate I11 shows the spectra and the wavelengths of 50 bands recorded in Table 11. The tendency for bands to occur in small groups gives to this spectra its characteristic appearance.The bands become more diffuse towards shorter wavelengths but wide diffuse bands of the type shown by a-picoline or by 2 : 6- lutidine are absent. TABLE II.-WAVELENGTHS OF ABSORPTION BANDS IN ~-PICOLINE VAPOUR A 2938'2 C 2934'4 c 2925.6 c 2922-6 c 2918.5 C 2912'5 C 2904.5 c 2896.8 c 2892.3 c 2888.9 c 2884.5 b A 2880.8 b 2872.9 a 2867.0 c 2859-6 a 2856.8 b 2851.1 c 2848.1 c 2840.1 a 2837.1 b 2853'5 c 2845.2 c A 2833.0 C 2828-3 c 2823.0 c 2816.8 b 2813.3 a 2809.1 c 2795'2 d 2784.2 c 2779'2 c 2775'3 c 2762-7 c A A 2635'5 d 2594'9 d 2579'9 d 2450-2550 d 2618.1 d 2606.8 d - yPICoLINE.-The absorption bands in the spectra of this base (see Plate IV) are displaced towards shorter wavelengths compared with those in a- and pTpicoline and are much more diffuse particularly below 2800 A although wide diffuse bands of the type which appear in a-picoline and 2 : 6-lutidine are absent.Table I11 gives the wavelengths of 25 bands measured. TABLE III.-WAVELENGTHS OF ABSORPTION BANDS IN Y-PICOLINE VAPOUR A 2884.1 d 2864.4 c 2851.2 c 2841.9 a 2834.6 c A A A 2748.0 d 2720.6 d 2713.6 d 2694.2 d 2727'5 d A 2 : ~-LuTIDINE.-T~~ spectra of this material exhibits very few bands, The wavelengths of the bands measured were : 2866.7 c, 2824-9 c, 2693.2 f, see Plate V.2702.8 el 2651-8 d , 2627.2 d, range 2500-2620 A weak diffuse bands.3 0 PYRIDINE HOMOLOGUES General Comparison of the Vapour Spectra.-Pyridine exhibits the largest number of bands, and successive substitution of methyl groups in the a-positions causes a progressive suppression of fine bands and an enhancement of diffuse bands. Amongst the mono-methyl substituted bases, the /3- and y-homologues exhibit most fine bands and of the five bases studied only a-picoline and z : 6- lutidine exhibit broad diffuseibands. Vapour Spectra of Binary Mixtures of the Bases .-Experiments were carried out with the object of establishing the smallest percentage of one base that can be detected in the presence of another by means of the spectra of the vapours.Pyridine could be detected in a-picoline down t o a concentration of 3 yo by the appearance of a band a t 2788-582 using the 14-5 cm. cell with the liquid in the side arm a t room temperature. /3-Picoline or 2 : 6-lutidine in y-picoline could be detected down t o a concentration of 5 yo by a change in the position of the ‘ I cut off ” using the 14-5 cm. cell and liquid a t room temperature although no F-picoline or z : 6-lutidine bands could be detected. The presence of 50 yo of z : 6-lutidine by volume in p- and y-picoline does not produce any marked change in the spectra of the picolines so that clearly the vapour spectra do not provide a convenient means for the detection of small amounts of these bases in the presence of homologues.Absorption Spectra of Solutions.-Fig. I, 2 and 3 show the molar extinction E plotted against wavelength expressed in A for solutions of these bases in 0-1 N sulphuric acid, 0-1 N sodium hydroxide and cyclohexane respectively. Certain relationships can easily be seen from these figures. Thus the molar ex- tinction coefficient of these bases in acid solution is approximately twice that in alkaline solution and is least in the hydrocarbon solvent. Hartley l9 lsng ago com- mented on the sharper spectra of an aqueous solution of pyridine hydrochloride compared with pyridine, while Purvis 2o reported that acid caused a marked per- sistence in the spectrum of pyridine and or-picoline.The sequence of the main peaks arranged in the order of decreasing wavelength is approximately z : 6- lutidine, a-picoline, j3-picoline, pyridine and y-picoline in all the three solvents, and the magnitudes of the maximum extinction coefficients follow the .same order. All the spectra studied except z : 6-lutidine show the greatest amount of fine structure in 0.1 N sodium hydroxide solution while the lutidine exhibits most structure in cyclohexane. The shift in the position of the main peak t o longer wavelengths on substitution in the ring in the sequence, pyridine, a- picoline, 2 : 6-lutidine, is similar t o that shown by the series benzene, toluene, dimethyl benzenes, but I-picoline is an exception as i t does not exhibit a shift compared with pyridine.Estimation of @-, y-picoline and 2:6-lutidine in Mixtures .-A method for estimating pyridine based on the measurement of the absorption spectra of acid solutions has been described by Hofmann 21 and by Le Rosen and Wiley.22 Study of Fig. I reveals that 2 : 6-lutidine can readily be estimated in a j3-picoline fraction by measuring the absorption at 2800 A of an acid solution. The values of the differences in extinction coefficient (g./l.) in 0-1 N acid and 0-1 N alkali solutions for these bases are plotted in Fig. 4. A very simple method of analysis of the j3-picoline fraction which takes advantage of the occurrence of isobestic points has been developed based on the data in Fig. 4. Thus if measurements of the absorption are made a t 2416A the difference between the extinction co- efficients of acid and alkaline solutions so found is independent of the /3-picoline concentration while at 2780 8, it is independent of the y-picoline concentration.As this technique of analysis which employs the difference between the ab- sorption of the unknown in acid and alkaline media does not appear t o have been very widely used, a short account of the procedure as applied t o mixtures of the three bases j3-, y-picoline, and 2 : 6-lutidine will be given. Analyses can be made rapidly by this spectroscopic method and t o take full advantage of the speed of this method it was decided to carry out measurements on a volume basis, To obtain the necessary calibration points 0.1 ml. of each pure base was made up t o IOO ml. with distilled water and each of these solutions were further diluted by taking I ml. of the appropriate dilute solution and dilut- ing t o roo ml.with 0-1 N sulphuric acid. The alkaline solutions were prepared similarly except that z ml. of the dilute aqueous solution were made up t o IOO ml. 19 Hartley, J . Chem. SOC., 1885, 47, 685. 2o Purvis, ibid., 1909, 95, 294. 21 Hofmann, Arch. Hyg. Baht., 1942, 128, 169. zp Le Rosen and Wiley, Anal. Chem., 1949, 21, 1175.E. F. G, HERINGTON FIG. I .-Solution spectra, solvent 0.1 N sulphuric acid. Pyridine- - - - - - - - - - a-Picoline , y-Picoline- . .- . . -. . - 2 : 6-Lutidine- - - - p-Picoline .............. , \ A A 2400 2500 zqoo 2 ~ 0 0 >\ FIG. 2.-Solution spectra, solvent 0.1 N sodium hydroxide. Pyridine- - - - - - - - a-Picoline I 8-Picoline ...............Y-Picoline- . . - . . - . .- 2 : 6-Lutidine- - - - ;Q5 40 35 FIG. 3.-Solution spectra, solvent c yclohexane. I d i n e - - - - - - - - - - - m: /3-Picoline .............. , a-Picoline s 7-Picoline- . . - . . -. . - z : 6-Lutidine- - - - FIG. 4. 8-Picoline .............. y-Picoline- . .- . .- . .- z : 6-Lutidine- - - -32 B-picoline y-picoline PYRIDINE HOMOLOGUES 2 : 6-lutidine with 0-1 N sodium hydroxide. The absorptions were then measured a t 2416, 2646 and 2780 A and the observed optical densities were multiplied by the factors IOO and 50 for the acid and alkaline solutions respectively t o calculate the ex- tinction coefficients equivalent t o a base concentration of I ml./l. of solution. The differences between these extinction coefficients for acid and alkaline solu- tions so obtained are recorded in Table IV.To minimize errors it is desirable Optical Density Optical Density Acid Soln. Acid Soln. TABLE ~V.-~AVELENGTHS AND THE DIFFERENCES BETWEEN THE EXTINCTION COEFFICIENTS (CONC. I ml./l.) IN ACID AND ALKALINE SOLUTIONS Difference in Extinction Coefficient (I ml.11.) A 0.140 0.412 0.162 2416 2646 2780 0.103 3'7 0.241 17.1 0.027 13'5 0 25'55 0.55 I 4-2 - 2'2 0 -2-8 27'9 40.0 I I I t o carry out this calibration with each spectrometer employing the same volu- metric apparatus that is t o be used for the estimation of the unknown. In a typical analysis the unknown mixture was diluted t o a concentration equivalent t o 0.01 ml. of base per litre in the acid and alkaline solutions and the optical densities measured and the results shown in Table V obtained.TABLE V A ~~ 2416 2646 2780 The concentration of the three bases in the unknown sample were calculated by successive approximation. Let x, y and z be the volume fractions of ,8- picoline, y-picoline and 2 : 6-lutidine respectively and let xl, yl. z1 and x,, yz. 2% be the first and second approximations t o these quantitives. The first approxim- ation to the 2 : 6-lutidine concentration 2, is given by 13-5/40-0 since a t 2780 A, the y-picoline has a zero difference value and the /3-picoline difference is so small (0.55) that it can be ignored to a first approximation, hence z,'= 0-3375. At 2416 A, 8-picoline has a zero difference so that eqn. (I) applies ( I ) At 2646 A, eqn. (2) applies 3-7 = 14.2 yl - 2-8 z,, hence y1 = 14.2 yl - 2-8 x 0-3375 = 0.3274 17.1 = 25'55 X i + 27'9 21 - 2'2 y1 = 25-55 X i + 27'9 X 0.3375 - 2'2 X 0'3274, - - (2) hence X, = 0.3287. centration a, is calculated by the equation hence 2, = 0'334.Using this value of x1 the second approximation t o the 2 : 6-lutidine con- 13.5 = 40.0 Z2 f 0.55 Xl = 40.0 2% + 0.55 X 0.3287, By substituting 2, in eqn. (I) in the place of z1 the second approximation t o the y-picoline concentration, y,, is found to be 0.327. Substituting z2 = 0.-334, y, = 0.327 in the place of z1 and yl in eqn. (2) yields x, = 0-333. There 1s no necessity t o carry the approximations further and these values-are accepted as the final analysis. Table VI gives the results obtained in the analysis of various synthetic mixtures by this method.The solutions where one component is in large excess were chosen to serve as a fairly exacting test of the method, and as can be seen the results for these solutions appear to be as accurate as for the other mixtures. The wavelengthsE. F. G. HERINGTON r-piwbe -0.6 -1.8 -0.7 -0.6 -2.3 -1.3 - - 33 L2 utidme :. 6: -- f o e ~ +I-I +o-4 3-0-8 - - +0'3 -0'1 2416, 2646, 2780 A were selected as being most suitable for mixtures containing approximately equal proportions of each base but other wavelengths may with advantage be used if certain components are in excess. Thus while the wave- length 2780 A is robably always the best choice for the estimation of 2 : 6- lutidine yet 2620 may be more suitable for the estimation of /3-picoline when 2 : 6-lutidine is in excess, while a wavelength of 2491 A may be the most suitable for the determination of y-picoline under these conditions.TABLE VI.-ANALYSIS O F p-, Y-PICOLINE, 2 : 6-LUTIDINE MIXTURES. ALL PERCENTAGES ARE BY VOLUME Synthetic Mixtures (%) #?-picoline 33'3 60.0 20.0 20'0 90.0 90.0 10.0 10'0 y -picoline 33'3 60.0 20.0 20'0 10'0 90-0 - - 2: 6- Lutidine 33'3 20'0 20'0 60.0 - I 10'0 90.0 Analytical Results (%) 33'3 59'1 19.0 18.7 go' I 9.6 91'3 8.3 32'7 18-22 59'3 19'4 7'7 88.7 - - 33'4 20.4 60.8 21'1 - - 10-3 89.9 Differences (%) Lpicoline 0'0 -0.9 - 1-3 +O.I -0.4 +I'3 - 1-7 - 1'0 Pyridine and a-picoline will interfere with the determination of p- and y- picoline by this method and these bases should be removed by fractional dis- tillation before carrying out an analysis of the 8-picoline fraction.Dissociation Constants from the Absorption Spectra.-Table VII taken from a recent paper by Brown and Barbaras z3 shows some of the values which have been reported for the dissociation constants of these bases. TABLE VII Base Pyridine . a-Picoline . . p-Picoline . . y-Picoline . . K B x 109 I I 2-24 2'4 3'0 10-5 45 32 I1 I1 Ref. - 24 25 26 24 25 26 26 26 I The dissociation constants have now been redetermined by studying the change of absorption spectra with pH.* The appropriate expression for the calculation of the logarithm of the thermodynamic dissociation constant KB from absorption measurements is where el, E ~ , and c3 are the extinction coefficients for the base in acid, buffered and alkaline solution respectively, KW is the ionic product of water and yB+ is 23 Brown and Barbaras, J .Amer. Chem. Soc., 1947, 69, 1137. s4 Barron, J . Biol. Chem., 1937, I21,.313. * Goldschmidt and Salcher, 2. physzk. Chem., 1899, 29, 114. 96 Constram and White, Amer. Chem. J., 1903, 29, 46. * See Flexser, Hammett and Dingwall e7 also Hughes, Jellinek and Ambrose 11 87 Flexser, Hammett and Dingwall, J . Amer. Chem. Soc., 1935, 57, 2103. who obtained a value of 1-32 X I O - ~ for pyridine at 20 f zo C. B34 PYRIDINE HOMOLOGUES the activity coefficient of the base ion, from the approximate Debye-Hackel expression. The value of log yB+ was calculated 1'074 1.065 1-077 1.378 1.406 1.407 1.005 0.985 1.000 1.726 1.710 1'753 3.207 3'330 3-356 where I is the ionic strength, z the valency and A has the following values for water, 0.500 a t 15" C, 0.509 a t 25O C and 0.514 a t 30' C.Solutions were made up by volume and the pH of the solution buffered with a sodium acetate + acetic acid mixture (approximately 0.01 M) was measured with a glass-saturated calomel electrode. The pH meter was standardized with M/zo potassium hydrogen phthalate and sodium borate buffers. The temperature of the buffered solution was determined directly after measuring the extinction coefficient and the appropriate values of A and log,, K , (see Harned and Owen 28) were used in the calculation. In order t o obtain the highest accuracy the pH was chosen so that the value of (eZ - cS)/(el - e2) was near unity. Substitution of methyl groups in this series clearly increases the dissociation constant. The results are given in Table VIII. TABLE VIII.-THERMODYNAMIC DISSOCIATION CONSTANTS OF SOME PYRIDINE HOMOLOGUES 5-16 5-16 5-16 5-84 5'84 5-72 5-72 5-72 5.86 5.86 5.84 5-86 6.06 6-06 6.06 Base Pyridine . a-Picoline /3-Picoline y-Picoline 2 : 6- Lutidine I wish Wave- length A 2506 2609 2600 2650 2710 2547 2610 2660 2558 2485 2589 2519 2646 2720 2780 Temp. of Soh. "C 25'5 25'3 25'3 25-2 25-2 25'2 24-6 24.6 24.6 24'3 24'3 24'3 26.7 26.7 26-7 81 - 82 I pH -- I r 0'010 0'0 I 0 0'0 I0 0.013 0.013 0.0 I 3 0.0125 0.0125 0.0125 0.013 0.013 0.013 0.014 0.014 0.014 KB x 14 1-44 1-43 1-45 8.65 8-83 8-83 4'5 8 4'56 4'49 10.59 10.50 10.76 37'3 38.8 39.1 Mean Thermo- dynamic Constant KB x 10' 1-44 8-77 4'54 10.62 38-4 I thank Dr. E. A. Coulson, 1. B. Ditcham, E. C. Holt, A. Sleven for supplying the pure base samples, R. Handley and A. J. Cook for the estimations of the purity by freezing point determinations and J. L. Hales for the determinations of the water contents. Mr. R. Handley carried out much of the preliminary investigations on the vapour spectra while W. Kynaston made the recorded measurements. The work described has been carried out as part of the research pro- gramme of the Chemical Research Laboratory and this paper is pub- lished by permission of the Director, Chemical Research Laboratory, Teddington, Middlesex. Chemical Research Laboratory, Teddington, Harned and Owen, The Physical Chemistry of Electrolytic Solutions (Rein- Middlesex. hold Publishing Corporation, New York, 1g43), p. 485.

ISSN:0366-9033    

DOI:10.1039/DF9500900026

出版商:RSC    

年代:1950

数据来源: RSC

摘要:

THE ULTRA-VIOLET ABSORPTION SPECTRA OF FLUORINATED TOLUENES BY W. T. CAVE AND H. W. THOMPSON Received 30th June 1950 The absorption spectra of some fluorinated toluenes in the vapour phase have been measured in the near ultra-violet. Benzotrifluoride m- and p-fluoro- benzotrifluoride o- m- and p-fluorotoluenes have been studied. A vibrational analysis has been made in each case and although this is incomplete important deductions about the values of molecular vibration frequencies have been made. The latter have been correlated with values obtained from the Raman spectra. The most intense progression for each molecule appears to involve a frequency interval which is characteristic of the structural type and this might be useful for diagnostic purposes with the o- m- and p-isomers.Several workers have recently measured the ultra-violet absorption spectra of polyatomic molecules using high resolution and the theory of the electronic and vibrational transitions permitted with these molecules has become more systematized (Sponer and Teller I). One class of mole- cules of particular interest is the substituted benzenes. Following earlier detailed analysis of the spectrum of benzene in the middle ultra-violet Sponer and Wollman 2 have measured chlorobenzene and Wollman 3 has studied fluorobenzene. Recently Sponer Hall and Stallcup 4 have given results for I 3 5-trichlorobenzene and I 3 5-trimethylbenzene and Sponer has discussed the electronic transitions involved with the tri-substituted benzenes. The spectrum of toluene was studied by Masaki,6 by Ginsburg Robertson and Matson and by Savard,B who also examined the isomers of cresol.Following work in this laboratory on the infra-red spectra of some fluorinated toluenes (Thompson and Temple O) we have now analyzed the ultra-violet band systems of some of these substances Although a complete interpretation of all the bands in the spectra cannot be suggested several useful results have emerged. The present paper in- cludes results for benzotrifluoride m- and p-fluoro benzotrifluoride o- m- and $-fluorotoluenes all measured in the vapour phase. Experimental The spectra were first measured under low dispersion using a Hilger E 315 spectrograph giving about 10 &mm. at 2600 A and then with higher dispersion using a large quartz Lithow spectrometer giving about 3 &mm.at this wave- length. The absorption cell was a cylindrical quartz tube I metre in length to which a side arm was attached for connection t o the sample and t o a pumping system. The source of continuous radiation was a hydrogen discharge tube. Owing to the large variation of extinction coefficient with wavelength it was necessary to take many exposures for each substance using a series of pressures. 1 Sponer and Teller Rev. Mod. Physics 1941 13 75. 2 Sponer and Wollman J . Chem. Physics 1941 9 816. 4 Sponer Hall and Stallcup V. Henri Mem. Vol. (LiBge 1948) p. 211. 3 Wollman ibid. 1946 14 123. 7 Matson Ginsburg and Robertson J . Chem. Physics 1945 13 309. 8 Savard Ann Chim.1929 I I 287. 6 Sponer Chem. Rev. 1947. 41 281. BMasaki Bull. Chem. Soc. Jap. 1936 11 346. B Thompson and Temple J . Chem. SOC. 1948 1432. 35 FLUORINATED TOLUENES Ill Ill I p- f /uorobenro&-if/uoride ' ' a 4bOoO' Cm? I rn-F7~robenz&if Iuoride 961 36 The latter varied from 0-1 mm. t o 20 mm. An iron arc was superposed for wave- length calibration and most of the bands were measured from two or more plates taken at different pressures. The compounds were supplied by Imperial Chemical Industries Ltd. (General Chemicals Division) and were re-distilled before use. The boiling points were as follows benzotrifluoride 102.5~ C pfluorobenzotrifluoride 1 0 5 O C m-fluoro- benzotrifluoride 100.5~ C p-fluorotoluene I 16.5' C m-fluorotoluene I 15-5" C o-fluorotoluene 114.5" C.Freedom from small traces of the corresponding isomers was established where relevant by the infra-red spectrum. Benzotrifluoride.*-The system of bands extends from about 2700 A t o 2400 A. Most of the bands degrade t o the red i.e. have a sharp edge on the higher frequency side ; others are broader and more diffuse. For uniformity the centre of each band has been measured. At the lowest pressures used there were still strong absorption bands the lowest frequency of which was 37822 cm. -1. The positions of the bands are given in Table I and part of the spectrum is shown in Plate I. In Fig. I the arrangement of bands is depicted in order t o clarify the interpretations given but although the significant variations of in- tensity are brought out in this diagram; the intensities shown are not meant t o be quantitative.a' FIG. I. The molecule can be regarded to a first approximation as belonging t o the symmetry group Cza. and the electronic transition will be A,-B, with a transition moment in the plane of :,he ring and perpendicular t o the C-(CF,) bond. It represents an I s allowed transition and the 0-0 band can be confidently fixed a t 37822 cm.-1. Table I gives an interpretation of most of the bands in terms of vibrational levels in the upper (o') and lower (w") states. A satisfactory explanation of nearly all the bands which is plausibly consistent with the in- tensities is obtained using the following values 7.50 1035 cm.-l 1006 ern.".o" 660 598 623 539 337 315 142 927 767 Values found by Pendl and Radinger 10 for the Raman intervals are 139 (7) 618 (3). 656 Ioz7-(4)' 77I (8)J 839 ( O ) Ioo5 (2)S 339 (5)# 3g6 There is therefore close agreement between the strong Raman intervals and the values of W" obtained from the ultra-violet spectrum. We might expect these strong Raman intervals t o be associated with the symmetrical vibrations although no measurements of the depolarization factors have been made. The appearances of the bands suggest that 337 (0'') may correspond to 315 (o') * During this Discussion we learnt that this substance had recently been examined by Sponer and Lowe (J. Opt. Sci. 1949~ 39 840). Our results agree closely with those given there. 10 Pendl and Radinger 1939.W. T. CAVE AND H. W. THOMPSON and 1006 (0”) t o 927 (0‘). This last value of 927 cm.-l is the strongest recurring interval in the excited electronic state and leads t o a well-marked progression and this vibration involves a symmetrical breathing mode of the carbon ring skeleton. It is also likely that the three ground state frequencies 623 660 and 767 cm.-l correspond to 539 598 and 750 cm.-l in the upper electronic level. 37 TABLE 1.-BENZOTRIFLUORIDE Interpretation v(cm.-1) 36487 670 706 741 778 816 36980 3 7020 055 e - 1006-337 e-1006-142 e - 1006-75 -36 e-1006-75 e-1006-36 e - 1006 e-767-75 e -767 -36 e - 767 e-623 e-337-142-36 e-337-142 e 092 126 I 62 I99 305 3 40 375 413 447 485 540 571 605 644 680 711 745 782 37822 37867 904 37998 38030 064 I01 I37 185 e - 337 - 75 - 36 e -337 - 75 e-337-36 e-337 e-(2 x 142) e -142 -75 -36 e - 142 - 75 e - 142 -36 e-142 e-75 -36 e-75 e-36 > - e+ 3 I5 - 142 e+315-75 -36 e+315-75 e+315-36 e+315 e+539-142 -36 e+539 - 142 e+ 5 39 - 75 -36 e+539-75 e+539 -36 e+539 et-598 -36 e+598 e+750-75 e+750 -36 e+750 e+927 - 142 I e+g60 - 142 e+927 -75 -36 e+960-75-36 e+927 -75 e+927 -36 1 218 251 286 323 361 385 420 502 536 5 73 61 I 644 676 711 Interpretation v(cm.-l) 750 :% 858 18892 19067 I02 I39 I75 211 2.50 287 19319 350 389 424 463 500 535 569 604 640 675 712 746 783 819 i9996 .003 I 065 I01 1 39 176 2 I2 246 280 316 355 389 426 462 499 533 568 602 638 672 708 0744 FLUORINATED TOLUENES - - e-75 e-50 e-25 e e+283-75 ef283 -50 e+283 -25 e+283 - e f 4 1 6 - e+538 -75 ef538 -50 e+538 -25 e+538 38 The small differences 36 cm.-1 and 75 cm.-l are less easy to explain.They may arise from v -+ v transitions but both on grounds of relative intensity and also from a consideration of the frequency scheme as a whole this does not seem satisfactory. They always arise in the interpretation of bands lying just to the longer wavelength side of the main transitions as if t o correspond to low vibra- tion frequencies in the ground state.This matter might be further elucidated by studying the effect of temperature on the relative intensities of the bands. The calculated positions of the bands given in Table I usually agree with the measured values within a few cm.-l which is as close as might be expected in view of the diffuse nature of many bands. p-Fluorobenzotrifluoride .-The band system extends from about 2700 t o 2400 A. At longer wavelengths the bands clearly degrade t o the red but below 2600 A they become diffuse making it difficult t o fix their positions very precisely. The measured positions are given in Table 11 and the spectrum is v(cm.-1) 36890 37003 029 I73 I99 224 270 608 631 657 68 I 746 768 791 816 841 37866 38068 104 126 I49 261 282 310 331 358 381 404 424 TABLE ~I.-+-FLUOROBENZOTRIFLUORIDE Interpretation e -837-50 e -837 -25 e -837 e - 642 - 50 e - 642 - 25 e -642 e - 596 e - 185 - 75 e - 185 -50 e-185-25 e-185 v (cm.-l) 449 is475 508 532 552 583 609 63 3 658 693 852 876 896 917 38939 3905 1 076 097 148 172 I94 451 659 690 729 925 39967 40003 247 - depicted in Plate I and Fig.I. As with benzotrifluoride the symmetry class must be approximately CZ0 leading t o an " allowed " transition A,-B,.The 0-0 band lies at 37866 cm.-l and although in this case some bands cannot be interpreted unambiguously several definite vibrational intervals can be seen. Plausible values which give a good fit for the important bands are 283 416 538 792 1030 1231 cm.-l. w" 185 596 642 837 cm.-l. w' The marked progression is associated with the interval 792 in the excited elec- tronic state. Raman data are not available for comparison with these results. With this compound too there are satellite bands lying 25 50 and 75 cm.-l t o the lower frequency side of the main bands. These may arise from v -+ v transitions but this interpretation seems improbable. There is a close parallelism between the spectra of 9-fluorobenzotrifluoride and $-cresol measured by Savard A fresh analysis of Savard's results suggests ~~~ Interpretation W.T. CAVE AND H. W. THOMPSON that for $-cresol frequencies in the excited state are 419 808 and 1187 cm.-l and probably also 210 and 1268 cm.-1. 2750 A to 2480 A. It is shown in Plate I and diagrammatically in Fig. I rn-Fluorobenzotrifluoride. - The band system extends from about and the positions of the bands are given in Table 111. At the longer wavelengths the bands degrade to the red but a t higher frequencies they become diffuse. The strong band at 37355 cm.-l is taken as the 0-0 transition. The most striking progression in the excited state is associated with a vibra- tion interval of 963 cm.-l and another prominent interval is 298 cm.-1. Satis- factory explanation of the main bands can be achieved by assuming the following values wf 298 730 843 963 cm.-1.w’’ 326 756 911 1009 cm.-l. A t the shorter wavelength side of the main bands of the leading progression there occur two bands lying about 40 and 80 cm.-l from the main band. Here again an interpretation in terms of v -f er transitions though possible does not TABLE 111.-FLUOROBENZOTRIFLUORIDE Interpretation e - 1009 -80 e - 1009 -40 e - 1009 e-grr e -756 e -326 -40 e-326 e-(2 x 125)-40 e-(2 x 125) Interpretation Y (cm.-1) 237 38279 318 535 5 74 618 - e-125 e-80 e-40 e - e+298-80 e+298 -40 e+298 v (cm.-l) 36270 307 346 444 599 36989 37029 067 I03 176 230 277 314 37355 37380 518 577 615 653 699 749 824 877 916 37952 38000 046 085 - - e+(z x 298) -125 e+(2 x 298) - 8 0 e+(2 x 298) -40 e+ (2 x 298) e+ 730 - 80 e+730 -40 e+730 e+843 198 seem t o agree with the observed intensities.Another interval of about 125 bination cm.-I may (40 be + a 80). frequency in the ground state though this may be the com- Raman data for comparison with the above are not available. However a re-examination of Savard’s results with m-cresol reveals similarity and the latter suggests excited state intervals of 423 693 963 and 1199 cm.-I. p-Fluorotoluene. -With this substance the variation in the absorption coefficient with wavelength is at times so rapid that many exposures a t different pressures were required t o obtain the full details of the spectrum and using the higher pressures which are necessary to develop some of the weaker bands the more intense bands become so broad as to produce serious overlapping which obscures other features.39 :: 668 718 878 915 38964 39005 046 163 203 240 278 312 347 544 580 627 680 874 39928 40014 242 FLUORINATED TOLUENES W' O" 40 The band system extends from 2800 A t o 2400 A and most bands appear t o degrade t o the red. It is shown in Plate I1 and depicted in Fig. 2 although it must be emphasized again that the intensities shown axe not quantitative. Many of the bands occur in pairs having the appearance of doublets but the relative intensity of the component of such a pair is variable and the doublet appearance may be accidental.The symmetry class is C2,, and the allowed transition gives a 0-0 band at 36876 cm.-1. The positions of the observed bands are given in Table IV from which a few very weak bands have been omitted. The analysis is complicated in this case not only by the larger number of bands but also because of an ambiguity in the allocation of frequencies. Thus (398 + 794) rn 1194 which might mean that the strong interval 1194 is not associated with a fundamental vibrational interval. However it is certain that the most prominent progressions are associated with a vibration frequency in the excited state of 794 cm.-l and consideration of the intensities and combinations suggests that I 194 cm.-l is another strong frequency in the upper state. A good interpretation of all except a few weak bands has been obtained with the following values 3" (O)D 185 398 584 794 8433 1014 1194 1229 2173 3I18 337 453 641D 825 844# 11.56. The Raman spectrum was measured by Kohlrauschll and by Paulsen12 and the following intervals found ('ID 824 (71 342 (8) 843 (91 455 (71 501 (4) 642 (41 698 728 ('1 995 (01 1158 (3)s 1,213 (6b) 1294 (11 1380 (3). I FIG. 2. All the frequencies for the ground state (d') suggested by the present results agree with strong Raman intervals except that there is no Raman interval corresponding t o 217 and the Raman interval 311 is given as weak.As with the other compounds discussed above there are groups of bands on the longer wavelength side of some of the main bands which cannot be explained con- vincingly. In the Table given intervals of 40 and 96 cm.-l are used in this connection. These bands may be associated with v -f v transitions but there are at times peculiar splitting phenomena and the intensities are irregular. There is a marked parallelism between the spectra of p-fluorotoluene and pcresol. As stated above a re-examination of Savard's data for the molecule gives the following frequencies in the excited state 210 419 808 1187 and 1268 cm.-l. rn-Fluoroto1uene.-Part depicts it diagrammatically. The band system lies between 2750 and 2400 A. of the spectrum is shown in Plate 11 and Fig.2 It differs from those discussed above in that most of the strong bands occur in pairs about 20-30 cm.-l apart and with the longer wavelength component slightly the more intense. Also at about 70 cm.-l to the lower frequency side l1 Kohlrausch Sitzber Ahad. Wien. 1933 142 650. l2 Paulsen ibid. 1938 147 395. V (cnL-1) 35636 720 810 844 940 3 5 9 w 36000 J 032 051 I09 140 163 205 235 333 377 423 450 468 497 5 39 565 595 623 659 688 751 780 797 809 82 I 852 36876 36888 91 1 938 36966 3 7004 048 061 097 132 231 274 334 358 425 460 480 510 576 677 B * 41 W. T. CAVE AND H. W. THOMPSON TABLE IV.-+-FLUOROTOLUENE e-1156 e -844 -217 e-825-217 e -844 -96 e-844-40 e-844 e-825 (8-453-3111 e -641 -96 Interpretation (e-453-217) @-(2X337)) e-641 e -453 -96 e - 453 -40 8-453 e-(2 x217) e-311-96 e - 337 - 40 e-337 e-311 e -21 7 -40 e-(2 x96) e -2 17 e -96 e-40 e e+185-96 ef584-453 e+185 e + 794 -40 FLUORINATED TOLUENES TABLE 1V.-(continued) Interpretation v (cm.-1) 719 754 793 821 850 890 222 42 37973 38034 070 105 123 159 258 287 372 425 471 503 564 617 654 686 771 820 867 905 946 38983 39017 040 05 1 081 I 16 184 259 280 298 328 448 39487 569 604 664 697 728 758 834 875 39965 40019 061 096 244 274 309 443 478 522 543 642 676 v(cm.-1) 712 756 852 892 923 947 40985 41024 061 I47 I97 236 41290 319 459 494 63 3 41672 36334 359 I Y (cm.-1) 395 423 4% 517 552 606 631 674703 889 925 36964 37033 37087 145,176 273 37325,346 368 3739% 418 431 214 243 g; W.T. CAVE AND H. W. THOMPSON (e+ (4 x 794) +843) (e+ (4 x 794) + 1014) (e + (4 x 794) + 1 194) @+(4 x 794)+1229) (e+ (6 x 794)) (e+(2 x 794)+1229+1014) (8 + 794+ =29+ 1014+843) @+(5X794) ( e + b x II94)+(2 x794)) Interpret ation TABLE V.-FLUOROTOLUENE Interpretation e-70 e 43 TABLE 1V.-(continued) FLU0 R INATED TOLUENES 44 TABLE V-(continued) v(cm.-1) 484 494 535 558,573 63 1 680 75'J 763J 7g2 828 855,892 924,955 38010 032 38082 108 140 179,208 295 318 343 38x1 393 442 475 495 510 529,541 555 595,613 '59 677D 685 734 752 783 800 828 858 913 981,998 39049,071 104,141 172,201 263,284 39330,358 420 554 Interpretation (e-j-1261-70) (e+ 1261) 5s2 (2)* 728 (Io)* 775 (81 852 (O0)* Ioo3 (I2) 1078 (3).423 650 718,747 821 846 39968 40014,041 106 234 294,327 406 589,618 488,713 751,780 40877.910 40979rI008 41081 41149 41193 41243,291 41365 41437 471 41551s 582 520. 726 and 1003 cm.-1 are obtained for ground state frequencies and other likely values are 311 184 and 253 cm.-1.The Raman spectrum measured by Kohlrausch l1 gave the following intervals 209 (4) 243 (a) 298 (2) 444 (oo) 512 (4)D 527 (6)D W. T. CAVE AND H. W. THOMPSON 1152 (rb) 1254 (3) 1266 (5) 1379 (3). The strongest Raman intervals 243 728 1003 are therefore matched by the values found at 253.520 726 1003 cm.-l. As already stated a fresh analysis of the results of Savard for m-cresol gave as excited state frequencies 423 693 963 and 1199 which are closely parallel t o those just given for m-fluorotoluene. o-F1uorotoluene.-Part of the spectrum is shown in Plate 11. Bands are found between 2750 and 2500 A but the arrangement is much more complicated than with the other isomers.There is a marked similarity in the grouping of bands with that shown by Savard in the spectrum of o-cresol. It has been im- possible to obtain a convincing analysis of the band system but several points may be noted. There is some ambiguity about the choice of the 0-0 transi- tion. It almost certainly occurs at 37576 cm.-l but just t o the lower frequency side of this position there are several intense bands and a broad strong absorption region occurs 60-100 cm.-l below 37576 cm.-l which might be associated with the 0-0 transition. This broad absorption occurs similarly at 38200 cm.-1 just below the band at 38283cm.-l but at higher frequencies it appears t o split and the broad nature may in fact arise from overlapping bands. What- ever the exact position of the 0-0 transition may be it is certain that the most marked interval leading t o a progression is 707 cm.-l a vibration fre- quency in the excited state.Three members of this progression lie a t 37576. 38283 and 38991 cm.-1. Another marked interval is 924 cm.-1 and a third may be 1230 cm.-l. Combinations of these three intervals corresponding to excited state vibrations serve t o explain many of the observed bands. From the bands a t the lower frequency side of the 0- transition prominent intervals 274 and 749 cm. -l can be derived which correspond t o strong Raman frequencies (Kohlrausch) of 274 and 747. Along the series o- m- p - for cresols or fluorotoluenes the position of the 0-0 transition moves towards lower frequencies.With p-fluoro- benzotrifluoride however this regularity breaks down and the alter- ation in electronic structure which must lead to this is a matter worth Discussion TABLE VI Benzene . . Phenol Toluene Benzotrifluoride . Chlorobenzene Flurobenzene . . p-Fluorotoluene . . p-Cresol p-Fluorobenzotrifluoride . . . . m-Fluorotoluene . m-Cresol m-Fluorobenzotrifluoride . o-Cresol o-Fluorotoluene . . . . further theoretical consideration. Moreover there are also some striking variations in the absorption coefficients which have been given elsewhere (Thompson and Miller lS). Another interesting feature is noticed by comparison of the values of the most marked interval found in the progressions. Table VI gives the values for the above and related molecules.The values for chloro- benzene and fluorobenzene have been taken from the work of Sponer and la Thompson and Miller J . Chem. Physics. 1949 17 845. 45 NATURAL HYPSOCHROMIC SHIFTS 46 Wollmann * and Wollman,s and those of toluene and phenol from Ginsberg Robertson and Matson.14 The excited state frequency 923 for benzene is associated with a totally symmetrical carbon ring vibration. Re- moval of the hexagonal symmetry causes this vibration to split into a set of which several components have been detected in the spectra of some of the compounds given in Table VI. It seems significant however that the value of the strongest interval is characteristic of the type of substitution in the aromatic ring.Thus the p-disubstituted benzenes have an interval close to 800 cm.-l; m-disubstituted compounds near 965 cm.-l and o-disubstituted derivatives near 705 cm.-1. Mono- substituted benzenes usually show more than one very marked progression interval but examination of all the data shows that the interval near 925 cm.-l is dominant. Indeed the interval found near 965 cm.-1 is variable in intensity. Its entire absence from the spectrum of phenol where a lower frequency (783 cm.-1) appears is anomalous. This regularity in the value of the interval in the main progressions may provide a useful diagnostic criterion for the structural type and suggests an interesting problem for theoretical consideration by the methods of analytical dynamics similar t o that discussed by Bell Thompson and Vago,16 for other characteristic vibrations of substituted aromatic compounds.We are grateful to the Rhodes Trustees for a scholarship to one of us (W. T. C . ) . The Physical Chemistry Laboratory Oxford. 14 Ginsburg Robertson and Matson J . Chem. Physics 1946 14 511. 15 Bell Thompson and Vago Pmc. Roy. Soc. A 1948 19 498. THE ULTRA-VIOLET ABSORPTION SPECTRA OF FLUORINATED TOLUENES BY W. T. CAVE AND H. W. THOMPSON Received 30th June 1950 The absorption spectra of some fluorinated toluenes in the vapour phase have been measured in the near ultra-violet. Benzotrifluoride m- and p-fluoro-benzotrifluoride o- m- and p-fluorotoluenes have been studied. A vibrational analysis has been made in each case and although this is incomplete important deductions about the values of molecular vibration frequencies have been made.The latter have been correlated with values obtained from the Raman spectra. The most intense progression for each molecule appears to involve a frequency interval which is characteristic of the structural type and this might be useful for diagnostic purposes with the o- m- and p-isomers. Several workers have recently measured the ultra-violet absorption spectra of polyatomic molecules using high resolution and the theory of the electronic and vibrational transitions permitted with these molecules has become more systematized (Sponer and Teller I). One class of mole-cules of particular interest is the substituted benzenes. Following earlier detailed analysis of the spectrum of benzene in the middle ultra-violet, Sponer and Wollman 2 have measured chlorobenzene and Wollman 3 has studied fluorobenzene.Recently Sponer Hall and Stallcup 4 have given results for I 3 5-trichlorobenzene and I 3 5-trimethylbenzene, and Sponer has discussed the electronic transitions involved with the tri-substituted benzenes. The spectrum of toluene was studied by Masaki,6 by Ginsburg Robertson and Matson and by Savard,B who also examined the isomers of cresol. Following work in this laboratory on the infra-red spectra of some fluorinated toluenes (Thompson and Temple O) we have now analyzed the ultra-violet band systems of some of these substances Although a complete interpretation of all the bands in the spectra cannot be suggested several useful results have emerged.The present paper in-cludes results for benzotrifluoride m- and p-fluoro benzotrifluoride o- m-and $-fluorotoluenes all measured in the vapour phase. Experimental The spectra were first measured under low dispersion using a Hilger E 315 spectrograph giving about 10 &mm. at 2600 A and then with higher dispersion using a large quartz Lithow spectrometer giving about 3 &mm. at this wave-length. The absorption cell was a cylindrical quartz tube I metre in length, to which a side arm was attached for connection t o the sample and t o a pumping system. The source of continuous radiation was a hydrogen discharge tube. Owing to the large variation of extinction coefficient with wavelength it was necessary to take many exposures for each substance using a series of pressures.1 Sponer and Teller Rev. Mod. Physics 1941 13 75. 2 Sponer and Wollman J . Chem. Physics 1941 9 816. 3 Wollman ibid. 1946 14 123. 4 Sponer Hall and Stallcup V. Henri Mem. Vol. (LiBge 1948) p. 211. 6 Sponer Chem. Rev. 1947. 41 281. BMasaki Bull. Chem. Soc. Jap. 1936 11 346. 7 Matson Ginsburg and Robertson J . Chem. Physics 1945 13 309. 8 Savard Ann Chim. 1929 I I 287. B Thompson and Temple J . Chem. SOC. 1948 1432. 3 36 FLUORINATED TOLUENES The latter varied from 0-1 mm. t o 20 mm. An iron arc was superposed for wave-length calibration and most of the bands were measured from two or more plates taken at different pressures. The compounds were supplied by Imperial Chemical Industries Ltd. (General Chemicals Division) and were re-distilled before use.The boiling points were as follows benzotrifluoride 102.5~ C pfluorobenzotrifluoride 1 0 5 O C m-fluoro-benzotrifluoride 100.5~ C p-fluorotoluene I 16.5' C m-fluorotoluene I 15-5" C, o-fluorotoluene 114.5" C. Freedom from small traces of the corresponding isomers was established where relevant by the infra-red spectrum. Benzotrifluoride.*-The system of bands extends from about 2700 A t o 2400 A. Most of the bands degrade t o the red i.e. have a sharp edge on the higher frequency side ; others are broader and more diffuse. For uniformity the centre of each band has been measured. At the lowest pressures used there were still strong absorption bands the lowest frequency of which was 37822 cm. -1. The positions of the bands are given in Table I and part of the spectrum is shown in Plate I.In Fig. I the arrangement of bands is depicted in order t o clarify the interpretations given but although the significant variations of in-tensity are brought out in this diagram; the intensities shown are not meant t o be quantitative. p- f /uorobenro&-if/uoride Ill Ill I ' ' a 4bOoO' Cm? I rn-F7~robenz&if Iuoride FIG. I. The molecule can be regarded to a first approximation as belonging t o the symmetry group Cza. and the electronic transition will be A,-B, with a transition moment in the plane of :,he ring and perpendicular t o the C-(CF,) bond. It represents an I s allowed transition and the 0-0 band can be confidently fixed a t 37822 cm.-1. Table I gives an interpretation of most of the bands in terms of vibrational levels in the upper (o') and lower (w") states.A satisfactory explanation of nearly all the bands which is plausibly consistent with the in-tensities is obtained using the following values : o" 142 337 623 660 767 1006 ern.". Values found by Pendl and Radinger 10 for the Raman intervals are 139 (7), There is therefore close agreement between the strong Raman intervals and the values of W" obtained from the ultra-violet spectrum. We might expect these strong Raman intervals t o be associated with the symmetrical vibrations, although no measurements of the depolarization factors have been made. The appearances of the bands suggest that 337 (0'') may correspond to 315 (o'), * During this Discussion we learnt that this substance had recently been examined by Sponer and Lowe (J.Opt. Sci. 1949~ 39 840). Our results agree closely with those given there. 10 Pendl and Radinger 1939. 927 961 1035 cm.-l a' 315 539 598 7.50, 339 (5)# 3g6 618 (3). 656 77I (8)J 839 ( O ) Ioo5 (2)S Ioz7-(4) W. T. CAVE AND H. W. THOMPSON 37 and 1006 (0”) t o 927 (0‘). This last value of 927 cm.-l is the strongest recurring interval in the excited electronic state and leads t o a well-marked progression and this vibration involves a symmetrical breathing mode of the carbon ring skeleton. It is also likely that the three ground state frequencies 623 660 and 767 cm.-l correspond to 539 598 and 750 cm.-l in the upper electronic level. TABLE 1.-BENZOTRIFLUORIDE v(cm.-1) 36487 670 706 741 778 816 36980 3 7020 055 092 126 I 62 I99 305 3 40 375 413 447 485 540 571 605 644 680 711 745 782 37822 37867 37998 38030 064 I37 185 218 251 286 323 361 385 420 502 536 5 73 61 I 644 676 711 904 I01 Interpretation e - 1006-337 e-1006-142 e - 1006-75 -36 e-1006-75 e-1006-36 e - 1006 e -767 -36 e-767-75 e - 767 e-623 e-337-142-36 e-337-142 e - 337 - 75 - 36 e -337 - 75 e-337-36 e-337 e-(2 x 142) e -142 -75 -36 e - 142 - 75 e - 142 -36 e-142 e-75 -36 e-75 e-36 e -e+ 3 I5 - 142 e+315-75 -36 e+315-75 e+315-36 e+315 e+539-142 -36 e+539 - 142 e+ 5 39 - 75 -36 e+539-75 e+539 -36 e+539 et-598 -36 e+598 e+750-75 e+750 -36 e+750 e+927 - 142 e+927 -75 -36 e+960-75-36 e+927 -75 e+927 -36 > I e+g60 - 142 1 v(cm.-l) 750 858 18892 19067 :% I02 I39 I75 211 2.50 287 19319 350 389 424 463 500 535 569 604 640 675 712 746 783 819 i9996 .003 I 065 1 39 176 246 280 316 355 389 426 462 499 533 568 602 638 672 708 0744 I01 2 I2 Interpretatio 38 FLUORINATED TOLUENES The small differences 36 cm.-1 and 75 cm.-l are less easy to explain.They may arise from v -+ v transitions but both on grounds of relative intensity and also from a consideration of the frequency scheme as a whole this does not seem satisfactory. They always arise in the interpretation of bands lying just to the longer wavelength side of the main transitions as if t o correspond to low vibra-tion frequencies in the ground state.This matter might be further elucidated by studying the effect of temperature on the relative intensities of the bands. The calculated positions of the bands given in Table I usually agree with the measured values within a few cm.-l which is as close as might be expected in view of the diffuse nature of many bands. p-Fluorobenzotrifluoride .-The band system extends from about 2700 t o 2400 A. At longer wavelengths the bands clearly degrade t o the red but below 2600 A they become diffuse making it difficult t o fix their positions very precisely. The measured positions are given in Table 11 and the spectrum is TABLE ~I.-+-FLUOROBENZOTRIFLUORIDE v(cm.-1) 36890 37003 029 I73 I99 224 270 608 631 657 746 768 841 37866 68 I 791 816 38068 104 126 I49 261 282 310 331 358 381 404 424 Interpretation e -837-50 e -837 -25 e - 642 - 50 e - 642 - 25 e -642 e - 596 e - 185 - 75 e - 185 -50 e-185-25 e-185 e -837 --e-75 e-50 e-25 e e+283-75 ef283 -50 e+283 -25 e+283 e f 4 1 6 e+538 -75 ef538 -50 e+538 -25 e+538 ---v (cm.-l) 449 is475 508 532 552 583 609 63 3 658 693 852 876 896 917 38939 3905 1 076 097 148 172 I94 451 659 690 729 925 39967 40003 247 ~~~ Interpretation depicted in Plate I and Fig.I. As with benzotrifluoride the symmetry class must be approximately CZ0 leading t o an " allowed " transition A,-B,.The 0-0 band lies at 37866 cm.-l and although in this case some bands cannot be interpreted unambiguously several definite vibrational intervals can be seen. Plausible values which give a good fit for the important bands are : w' 283 416 538 792 1030 1231 cm.-l. w" 185 596 642 837 cm.-l. The marked progression is associated with the interval 792 in the excited elec-tronic state. Raman data are not available for comparison with these results. With this compound too there are satellite bands lying 25 50 and 75 cm.-l t o the lower frequency side of the main bands. These may arise from v -+ v transitions but this interpretation seems improbable. There is a close parallelism between the spectra of 9-fluorobenzotrifluoride and $-cresol measured by Savard A fresh analysis of Savard's results suggest W.T. CAVE AND H. W. THOMPSON 39 that for $-cresol frequencies in the excited state are 419 808 and 1187 cm.-l and probably also 210 and 1268 cm.-1. rn-Fluorobenzotrifluoride. - The band system extends from about 2750 A to 2480 A. It is shown in Plate I and diagrammatically in Fig. I, and the positions of the bands are given in Table 111. At the longer wavelengths the bands degrade to the red but a t higher frequencies they become diffuse. The strong band at 37355 cm.-l is taken as the 0-0 transition. The most striking progression in the excited state is associated with a vibra-tion interval of 963 cm.-l and another prominent interval is 298 cm.-1. Satis-factory explanation of the main bands can be achieved by assuming the following values : wf 298 730 843 963 cm.-1.w’’ 326 756 911 1009 cm.-l. A t the shorter wavelength side of the main bands of the leading progression there occur two bands lying about 40 and 80 cm.-l from the main band. Here again an interpretation in terms of v -f er transitions though possible does not v (cm.-l) 36270 307 346 444 599 36989 37029 067 I03 176 230 277 314 37355 37380 518 577 615 653 699 749 824 877 916 37952 38000 046 085 198 TABLE 111.-FLUOROBENZOTRIFLUORIDE Interpretation e - 1009 -80 e - 1009 -40 e - 1009 e-grr e -756 e -326 -40 e-326 e-(2 x 125)-40 e-(2 x 125) e-125 e-80 e-40 e --e+298-80 e+298 -40 e+298 e+(z x 298) -125 e+(2 x 298) - 8 0 e+(2 x 298) -40 e+ (2 x 298) e+ 730 - 80 e+730 -40 e+730 e+843 --Y (cm.-1) 237 38279 318 535 5 74 618 668 ::: 718 878 915 38964 39005 046 163 203 240 278 312 347 544 580 627 874 39928 40014 242 680 Interpretation seem t o agree with the observed intensities.Another interval of about 125 cm.-I may be a frequency in the ground state though this may be the com-bination (40 + 80). Raman data for comparison with the above are not available. However, a re-examination of Savard’s results with m-cresol reveals similarity and the latter suggests excited state intervals of 423 693 963 and 1199 cm.-I. p-Fluorotoluene. -With this substance the variation in the absorption coefficient with wavelength is at times so rapid that many exposures a t different pressures were required t o obtain the full details of the spectrum and using the higher pressures which are necessary to develop some of the weaker bands the more intense bands become so broad as to produce serious overlapping which obscures other features 40 FLUORINATED TOLUENES The band system extends from 2800 A t o 2400 A and most bands appear t o degrade t o the red.It is shown in Plate I1 and depicted in Fig. 2 although it must be emphasized again that the intensities shown axe not quantitative. Many of the bands occur in pairs having the appearance of doublets but the relative intensity of the component of such a pair is variable and the doublet appearance may be accidental.The symmetry class is C2,, and the allowed transition gives a 0-0 band at 36876 cm.-1. The positions of the observed bands are given in Table IV from which a few very weak bands have been omitted. The analysis is complicated in this case not only by the larger number of bands but also because of an ambiguity in the allocation of frequencies. Thus (398 + 794) rn 1194 which might mean that the strong interval 1194 is not associated with a fundamental vibrational interval. However it is certain that the most prominent progressions are associated with a vibration frequency in the excited state of 794 cm.-l, and consideration of the intensities and combinations suggests that I 194 cm. -l is another strong frequency in the upper state. A good interpretation of all except a few weak bands has been obtained with the following values : W' 185 398 584 794 8433 1014 1194 1229 O" 2173 3I18 337 453 641D 825 844# 11.56.The Raman spectrum was measured by Kohlrauschll and by Paulsen12 and the following intervals found : 3" (O)D 342 (8) 455 (71 501 (4) 642 (41 698 ('ID 728 ('1, 824 (71 843 (91 995 (01 1158 (3)s 1,213 (6b) 1294 (11 1380 (3). I FIG. 2. All the frequencies for the ground state (d') suggested by the present results agree with strong Raman intervals except that there is no Raman interval corresponding t o 217 and the Raman interval 311 is given as weak. As with the other compounds discussed above there are groups of bands on the longer wavelength side of some of the main bands which cannot be explained con-vincingly.In the Table given intervals of 40 and 96 cm.-l are used in this connection. These bands may be associated with v -f v transitions but there are at times peculiar splitting phenomena and the intensities are irregular. There is a marked parallelism between the spectra of p-fluorotoluene and pcresol. As stated above a re-examination of Savard's data for the molecule gives the following frequencies in the excited state 210 419 808 1187 and 1268 cm.-l. rn-Fluoroto1uene.-Part of the spectrum is shown in Plate 11 and Fig. 2 depicts it diagrammatically. The band system lies between 2750 and 2400 A. It differs from those discussed above in that most of the strong bands occur in pairs about 20-30 cm.-l apart and with the longer wavelength component slightly the more intense.Also at about 70 cm.-l to the lower frequency side l1 Kohlrausch Sitzber Ahad. Wien. 1933 142 650. l2 Paulsen ibid. 1938 147 395 W. T. CAVE AND H. W. THOMPSON 41 V (cnL-1) 35636 720 810 844 940 3 5 9 w 36000 J 032 051 I09 140 163 205 235 333 377 423 450 468 497 5 39 565 595 623 659 688 751 780 797 809 82 I 852 36876 36888 91 1 938 36966 3 7004 048 061 097 132 231 274 334 358 425 460 480 576 510 677 B * TABLE IV.-+-FLUOROTOLUENE Interpretation e-1156 e -844 -217 e-825-217 e -844 -96 e-844-40 e-844 e-825 (8-453-3111 e -641 -96 (e-453-217) @-(2X337)) e-641 e -453 -96 e - 453 -40 8-453 e-(2 x217) e-311-96 e - 337 - 40 e-337 e-311 e -21 7 -40 e -2 17 e-(2 x96) e -96 e-40 e e+185-96 ef584-453 e+185 e + 794 -4 42 v (cm.-1) 719 754 793 821 850 890 37973 38034 070 105 123 159 222 258 287 372 425 471 503 564 617 654 686 771 820 867 905 946 38983 39017 040 05 1 081 I 16 184 259 280 298 328 448 39487 569 604 664 697 728 758 834 875 39965 40019 061 096 244 274 309 443 478 522 543 642 676 FLUORINATED TOLUENES TABLE 1V.-(continued) Interpretatio W.T. CAVE AND H. W. THOMPSON 43 TABLE 1V.-(continued) 712 756 852 892 923 947 40985 41024 061 I47 I97 236 41290 319 459 494 63 3 41672 v(cm.-1) I Interpret ation (e+(2 x 794)+1229+1014) (8 + 794+ =29+ 1014+843) @+(5X794) ( e + b x II94)+(2 x794)) (e+ (4 x 794) +843) (e+ (4 x 794) + 1014) (e + (4 x 794) + 1 194) @+(4 x 794)+1229) (e+ (6 x 794)) TABLE V.-FLUOROTOLUENE Y (cm.-1) 36334 359 395 423 4% 517 552 606 631 674703 889 925 36964 37033 37087 145,176 214 243 273 37325,346 368 3739% 418 431 g; Interpretation e-70 44 FLU0 R INATED TOLUENES TABLE V-(continued) v(cm.-1) 484 494 535 558,573 63 1 680 828 855,892 924,955 38010 032 38082 108 75'J 763J 7g2 140 179,208 295 318 343 38x1 393 442 475 495 510 529,541 555 595,613 734 752 783 800 828 858 913 981,998 39049,071 104,141 172,201 263,284 39330,358 420 554 423 650 718,747 821 846 39968 40014,041 106 234 294,327 406 589,618 488,713 751,780 40877.910 40979rI008 '59 677D 685 41081 41149 41193 41243,291 41365 41437 471 41551s 582 Interpretation (e-j-1261-70) (e+ 1261) 520.726 and 1003 cm.-1 are obtained for ground state frequencies and other likely values are 311 184 and 253 cm.-1. The Raman spectrum measured by Kohlrausch l1 gave the following intervals 209 (4) 243 (a) 298 (2) 444 (oo), 512 (4)D 527 (6)D 5s2 (2)* 728 (Io)* 775 (81 852 (O0)* Ioo3 (I2) 1078 (3) W. T. CAVE AND H. W. THOMPSON 45 1152 (rb) 1254 (3) 1266 (5) 1379 (3). The strongest Raman intervals 243, 728 1003 are therefore matched by the values found at 253.520 726 1003 cm.-l. As already stated a fresh analysis of the results of Savard for m-cresol gave as excited state frequencies 423 693 963 and 1199 which are closely parallel t o those just given for m-fluorotoluene.o-F1uorotoluene.-Part of the spectrum is shown in Plate 11. Bands are found between 2750 and 2500 A but the arrangement is much more complicated than with the other isomers. There is a marked similarity in the grouping of bands with that shown by Savard in the spectrum of o-cresol. It has been im-possible to obtain a convincing analysis of the band system but several points may be noted. There is some ambiguity about the choice of the 0-0 transi-tion. It almost certainly occurs at 37576 cm.-l but just t o the lower frequency side of this position there are several intense bands and a broad strong absorption region occurs 60-100 cm.-l below 37576 cm.-l which might be associated with the 0-0 transition.This broad absorption occurs similarly at 38200 cm.-1 just below the band at 38283cm.-l but at higher frequencies it appears t o split and the broad nature may in fact arise from overlapping bands. What-ever the exact position of the 0-0 transition may be it is certain that the most marked interval leading t o a progression is 707 cm.-l a vibration fre-quency in the excited state. Three members of this progression lie a t 37576. 38283 and 38991 cm.-1. Another marked interval is 924 cm.-1 and a third may be 1230 cm.-l. Combinations of these three intervals corresponding to excited state vibrations serve t o explain many of the observed bands. From the bands a t the lower frequency side of the 0- transition prominent intervals 274 and 749 cm.-l can be derived which correspond t o strong Raman frequencies (Kohlrausch) of 274 and 747. Discussion Along the series o- m- p - for cresols or fluorotoluenes the position of the 0-0 transition moves towards lower frequencies. With p-fluoro-benzotrifluoride however this regularity breaks down and the alter-ation in electronic structure which must lead to this is a matter worth TABLE VI Benzene . Phenol Toluene . Benzotrifluoride . Chlorobenzene . Flurobenzene . p-Cresol . . . p-Fluorotoluene . . p-Fluorobenzotrifluoride . m-Cresol . m-Fluorotoluene . m-Fluorobenzotrifluoride o-Cresol . . . o-Fluorotoluene . further theoretical consideration. Moreover there are also some striking variations in the absorption coefficients which have been given elsewhere (Thompson and Miller lS). Another interesting feature is noticed by comparison of the values of the most marked interval found in the progressions. Table VI gives the values for the above and related molecules. The values for chloro-benzene and fluorobenzene have been taken from the work of Sponer and la Thompson and Miller J . Chem. Physics. 1949 17 845 46 NATURAL HYPSOCHROMIC SHIFTS Wollmann * and Wollman,s and those of toluene and phenol from Ginsberg, Robertson and Matson.14 The excited state frequency 923 for benzene is associated with a totally symmetrical carbon ring vibration. Re-moval of the hexagonal symmetry causes this vibration to split into a set of which several components have been detected in the spectra of some of the compounds given in Table VI. It seems significant however, that the value of the strongest interval is characteristic of the type of substitution in the aromatic ring. Thus the p-disubstituted benzenes have an interval close to 800 cm.-l; m-disubstituted compounds near 965 cm.-l and o-disubstituted derivatives near 705 cm.-1. Mono-substituted benzenes usually show more than one very marked progression interval but examination of all the data shows that the interval near 925 cm.-l is dominant. Indeed the interval found near 965 cm.-1 is variable in intensity. Its entire absence from the spectrum of phenol, where a lower frequency (783 cm.-1) appears is anomalous. This regularity in the value of the interval in the main progressions may provide a useful diagnostic criterion for the structural type and suggests an interesting problem for theoretical consideration by the methods of analytical dynamics similar t o that discussed by Bell, Thompson and Vago,16 for other characteristic vibrations of substituted aromatic compounds. We are grateful to the Rhodes Trustees for a scholarship to one of us (W. T. C . ) . The Physical Chemistry Laboratory, Oxford. 14 Ginsburg Robertson and Matson J . Chem. Physics 1946 14 511. 15 Bell Thompson and Vago Pmc. Roy. Soc. A 1948 19 498

ISSN:0366-9033    

DOI:10.1039/DF9500900035

出版商:RSC    

年代:1950

数据来源: RSC

摘要:

46 NATURAL HYPSOCHROMIC SHIFTS NATURAL HYPSOCHROMIC SHIFTS BY MME. ALBERTE PULLMAN AND BERNARD PULLMAN Received 12th October, 1950 The name of natural bypsochromic shifts is being given to hypsochromic shifts of the longest wavelength of absorption which are being produced by electronic factors alone and are thus independent of steric interference With coplanarity. These natural hypsochromic shifts may be brought about in the same way as the natural bathochromic ones by appropriately choosing the basic molecular skeleton. They may be produced by increasing the size of a conjugated system, by alkylation, by dehydrogenation of a partially saturated system, by replacement of a double bond by a triple one and by internal strain. Numerous examples are quoted and i t is shown that simple molecular orbital calculations account satisfactorily for the observed facts.The name of natural hypsochromic shift is here given to hypsochromic shifts of the longest wavelength of absorption which are brought about by electronic factors alone and the existence of which are thus completely independent of steric factors. In fact it is generally admitted that, in the absence of steric interference with coplanarity, electronic interaction which follow, say, the increase in size of a conjugated system, alkyl sub- stitution of such a system, dehydrogenation of a partially saturated system, etc., always bring about a bathochromic displacement of the longest wavelength of absorption. It is the aim of this communication to show briefly that this conception is erroneous, to point out different cases ofMME.ALBERTE PULLMAN AND BERNARD PULLMAN 47 natural hypsochromic shifts and to show at the same time that simple molecular orbital calculations, carried out with the usual approximation of the linear combination of atomic orbitals, are able to account very well €or the existence of this phenomenon. It has recently been shown by different people, some of whom are present here, that these approxim- ations are insufficient in more detailed studies.l Nevertheless, they seem to account for the majorfeatures of the spectra of conjugated com- pounds a and it is interesting to note that they account for the existence of natural hypsochromic shifts as well as they do for the existence of the more usual bathochromic ones.This is undoubtedly a pleasant state of affairs as it is rather difficult as yet to carry out more complete calcula- tions for large organic molecules. The point to note is that though the importance of these calculations must not be overestimated] neither should they be underestimated. No details of these calculations are given here ; they may be found in the references quoted. The transition energies are expressed in teims of the exchange integral y in calculations in which the overlap integral has been included, and also in terms of the exchange integral ,8 in calculations in which it has been neglected. American authors usually use these parameters in the reverse sense. We shall now consider the principal ways in which natural hypso- chromic shifts may be brought about.Increase in Size of a Conjugated System.-(a) FULVALENES. The fact that the fulvalenes (I), (11) and (111) belong to a hypothetical group (1) (11) (111) of compounds which have not been prepared does not diminish their theoretical importance. The energy of the N --f Vl transition in these three molecules a is respectively o-296~, 0 . 3 5 7 ~ and 0 . 3 9 5 ~ . I t s increase with the lengthening of the central chain of conjugated double bonds indicates that there should be a parallel progressive hypsochromic shift of their longest wavelength of absorption. The same thing should happen in the corresponding series of heptafulvalenes (IV), (V) and (VI) for which (IV) (V) e)=cii--cn=-cH-cH- i-A W (VI) the N --f V , transition is equal ~espectively~ to o .2 6 3 ~ , O - ~ O O Y , o*3z5y. These results are significant as they show that the series of diphenylpolyenes, represented by (VII), which exhibits a bathochromic shift of its longest (VII) 1 See e.g. Sponer and Nordheim, ONR Contract N60ri-107, T.O.I. Report ; Craig, J . Chem. Physics, 1949, 17, 1358 ; Proc. Roy. SOC. A, 1950, 200, 474 ; Jacobs, Proc. Physic. Soc., 194% 62, 710 ; Coulson and Longuet-Higgins, Phzl. Mag., 1949, 40, 1172. See e.g. Klevens and Platt, J . Chem. Physics, 1949, 17, 470 ; Platt, J . Claem. Physics, 1949, 17, 484 ; Platt, ONR Contract N60ri-20, T.O. TX Report, See also Pullman and Pullman, J . Chim. Phys., 1949, 46, 2 1 2 . a Pullman and Berthier, Compt. rend., 1949, 229, 717. * Mayot and Pullman (in preparation).48 NATURAL HYPSOCHROMIC SHIFTS wavelength of absorption with the lengthening of the conjugated chain of double bonds, should not be considered, as it generally is, as a typical example of a general rule.Though the fulvalenes (I), (11) and (111) are unknown, yet their benzenoid homologues, dibiphenylene ethylene (VIII), dibiphenylene butadiene (IX) and dibiphenylene hexatriene (X) are known. Calculations show that, contrary (VIII) (X) to what happens in the hypothetical compounds (I), (11) and (111), the N + Vl transition energy decreases steadily in the series (VIII), (IX), (X) (viz. 0*620y, o - 5 1 5 ~ and 0-41oy). These compounds should thus show a bathochromic shift of Amax with the lengthening of the chain. In reality we observe a hypso- chromic shift on going from (VIII) (Amax, 460 mp) to (IX) (Amax, 446 mp) and a bathochromic one on passing from (IX) t o (X) (Amax, 555 mp).It may thus be concluded that the hypsochromic shift is not in this case a natural one but is due to steric factors. Owing to the low bond order of its central double bond (0.673 in a hypothetical planar molecule) and to steric repulsion between the positions facing each other on the two sides of this bond, the molecule of di- biphenylene ethylene is partially twisted round the central linkage. This dimin- ishes its N --f V, transition energy and makes it absorb a t longer wavelength that i t otherwise would. Actually i t may be concluded from these and other data (dipole moment, diamagnetic susceptibility, reduction potential) that the angle between the two halves of the molecule is6 approximately 60'.(b) FuLvENEs.-Though fulvene (XI), benzofulvene (XII) and di- benzofulvene (XIII) are isomers respectively of benzene, naphthalene and anthracene, it is well know that, contrary to what happens in this last group of molecules, there is a steady hypsochromic shift of their Amax with increasing size of the molecule. This is in complete agreement (XI) (XII) (XIII) (XIV) with theory from which one evaluates the N -+ Vl transition as equal to 0.860~ in (XI), 0.881~ in (XII), and 1.081~ in (XIII).6 Recently the spectra of these compounds have been accurately measured 7 and some more detailed theoretical studies carried out8 An interesting point concerns the dinaphthofulvene (XIV) . Cal- culations show that this compound, which as yet has nct been prepared, should show a bathochromic shift with respect to (XIII).This probably represents a general case : in all the series of molecules mentioned here, the natural hypsochromic shift should probably be limited to the first three or four members of the sexies and should be replaced afterwards by Bergmann, Berthier, Pullman and Pullman, Bull. SOC. Chim. (in press). Pullman, Pullman and Rumpf, Bull. SOC. Chim., 1948, 15, 757. 7 Bergmann and Hirschberg, Bull. Sos. Chim. (in press). 8 Pullman, Berthier and Pullman, BuZZ, SOG. Chim. (in press).MME. ALBERTE PULLMAN AND BERNARD PULLMAN 49 a bathochromic one. The reason for this lies generally in a crossing-over of molecular orbitals.9 The preparation and experimental investigation of (XIV) is being undertaken.A parallel natural hypsochromic shift should also occur in the seri3s of the hypothetical heptafulvenes (XV), (XVI), and (XVII),* the N -+ V, transition energy being equal to 0.6617 in (XV) and to 0.862~ in (XVII). (XV) (XVI) (XVII) (c) QuINoDiMEmmEs.-In agreement with experiment, which shows a continuous hypsochromk displacement of the longest wavelength of absorption with increasing size of the resonating system in the series of tetraphenyl derivatives of 9-benzoquinodimethane (XVIII) (orange), fi-naphthoquinodimethane (XIX) (yellow) and p-anthraquinodimethae (XX) (colouIless), the calculations show a parallel gradual increase of (XVIII) (XIW (XX) (XXV the N +- V , transition energy : this is equal to 0.6267 in (XVIII), to 0.726~ in (XIX) and to 0.860~ in (XX).lo A similar development of colour is observed in the series of the cor- responding quinones : the yellow colour of p-benzoquinone fades away when we p a s to the p-naphtho- and p-anthraquinones.No exact cal- culations of energy levels have as yet been carried out for these com- pounds in which the longest absorption band is due t o an N + A transi- tion. An interesting point is that, in conformity with the observation made in connection with the hypothetical dinaphthofulvene, the hypo- chromic shift seems to be limited to the quinones quoted, and is being replaced from naphthacenequinone (XXI) upwards by a steady deepening of the visible colour. ( d ) BENzoAZuLENES.-It has recently been shown by Plattner and co-workers,ll that while the I : 2-benzazulene (XXII) shows the “ normal ” bathochromic shift of the longest wavelength of absorption with respect to azulene itself, the 5 : 6-isomer (XXIII) shows on the contrary an ‘‘ unusual ” hypsochromic shift.(XXII) (XXIII) This evidently is another case of a natural hypsochromic shift and cal- culations are being carried out in order to verify this assumption. lo Pullman, Berthier and Pullman, Bull. Soc. Chim., 1949, 15, 450. Details will be given in a paper on naphthofulvenes, in preparation. Plattner, Fiirst and Keller, Helv. chim. A d a , 1949, 32, 2464.50 NATURAL HYPSOCHROMIC SHIFTS Aiky1ation.-Methyl substitution which is considered as leading always, in the absence of steric factors, to a bathochromic displacement of Amax may in fact give rise as well to natural hypsochromic shifts.An example of this is given by some methylated derivatives of azulene (XXIV).12 3 4 5 (XXIV) While the I- and 5-methylated derivatives show the usual bathochromic shift, the 2-, 4- and 6-methylated compounds manifest a hypsochromic shift. Calculations confirm the assumption that these are naturaE hypso- chromic shifts : the N -+ V, transition energy which is equal to 0.804 p in azulene itself, to 0*755/3 in the I-methyl derivative and to 0-789p in the 5-methyl derivative is equal to 0.814p in the 2-methyl derivative, to 0.8168 in the 4-methyl derivative and to 0-823,8 in the 6-methyl derivative. The methylazulenes are the only known examples of natural hypso- chromic shifts brought about by alkylation. Nevertheless it can be predicted that similar shifts should occur in compounds (XXV), XXVI), (XXVII).Experiments are being carried out in order to verify this predict ion. (XXV) (XXVI) (XXVII) It may be shown that, as a general rule, natural hypsochromic shifts can only occur, by alkylation, in conjugated compounds containing an odd membered ring. One could thus not expect these to occur in the usual benzenoid molecules. The only thing that may sometimes be observed in this last group of compounds is the absence of the expected bathochromic shift. The 1’-methyl-1 : 2-benzanthracene (XXVIII) seems (XXVIII) to be an example of such a case. This compound absorbs exactly at the same wavelength as the parent unsubstituted hydr0carb0n.l~ It may be thought that this absence of the usual bathochrcmic shift is due to steric interference with the nearby rneso position but in fact cal- culations show that the N -+ V, transition energy of (XXVIII) has exactly the same value (0.916y) as that of the parent I : 2-ben2anthra~ene.l~ A normal bathochromic shift is predicted by calculakion €or all the re- maining methyl derivatives of this compound.(XXVIII) thus represents a particular case of a natural absence of a bathochromic shift. l2 For details see Pullman, Mayot and Berthier, J . Ch,em. Physics, 1950, 18, 257iS Jones, Chenz. Rev., 1943, 32, I. l4 Pullman, Berthier and Pullman, Acta Cancer. (in press).MME. ALBER'I'E PULLMAN AND BERNARD PULLMAN 5 1 Dehydrogenation.-Partial hydrogenation of an unsaturated system generally leads to a hypsochromic displacement of the longest wavelength of absorption while the reverse is true for the dehydrogenation of a par- tially saturated system.That this is not always the case has recently been pointed out by Longuet-Higgins, Rector and Platt; l5 thus while tetrahydroporphyrine (XXIX) absorbs somewhere near 8000 A, por- phyrine itself (XXX) absorbs only up to 6300 A. This is thus a special case of a natural hypsochromic shift produced by increasing the dimen- sions of a resonating system through dehydrogenation. The assump- tion is verified by explicit calculations which give the values of 0 . 3 8 0 ~ and o.391~ for the N -+ V, transition energy of (XXIX) and XXX) respzctively . (XXIX) Replacement of a Double Bond by a Triple One.-Though the re- placement of a double bond by a triple one increases the number of available mobile electrons the comparison of compounds such as styrene (XXXI) (Amax, 282 mp) and phenylacetylene (XXXII) (Am,,, 278 mp) or stilbene (XXXIII) (Amax, 295 mp) and tolane (XXXIV) (Amax, 279 mp) (XXXII) (XXXIII) (XXXIV) shows that it is generally accompanizd by a hypsochromic shift of the longest wavelength of absorption.In (XXXII) and (XXXIV) only one of the two pairs of the T electrons of the triple bond conjugates in fact with the benzene nuclei. The hypsochromic shift observed has never- theless to bc considered as a natural one as it is due to electronic factors alone, namely to the greater localization of the T electrons of triple bond in comparison with the T electrons of a double bond. This conception which has recently been stressed by Moffitt and Coulson 113 and by Walsh l7 is substantiated by the values of the ionization and reduction potentials of similar ethylenic and acetylenic cornpounds.l8 Explicit calculations verify the natural character of the hypsochromic shift referred to as they indicate the following values for the N -+ V, transition energy : lo 1.589~ in (XXXI), 1.870~ in (XXXII), 1.237~ in (XXXIII) and 1.540~ in (XXXIV).Internal Strain-There is a continuous hypsochromic displacement of the longest wavelength of absorption in the series cyclopropane l5 Longuet-Higgins, Rector and Platt, O N R Contract N60ri-20, T.O. I X . Recort. l6 Moffitt and Coulson, Phil. Mag., 1947, 38, 634. l7 IValsh, Ann. Refiorts, 1947, 4, 32 ; Quart. Rev., 1948, 2, 73.lS Berthier and Pullman, Compt. rend., 1949, 228, 397. Pullman, Berthier and Pullman, Bull. SOC. Chim., 1950, 17, 591.52 NATURAL HYPSOCHROMIC SHIFTS (XXXV) ( Amax, 1950 A). cyclobutane (XXXVI), cyclopentane (XXXVII), cyclohexane (XXXVIII) (Amax, 1500 A). As all these compounds, with the exception of (XXXVIII), are planar this hypsochromic shift cannot be considered as due to steric factors in the usual sense associated with this expression but must be regarded as a natural one. In fact it is to be related to the existence of internal strain whose magnitude is decreasing with the increasing size of the molecule. Though no exact calculations of the distribution of molecular orbitals in any of these compounds have as yet been carried out, it has recently been shown 2 0 that the strain is (XXXV) (XXXVI) (XXXVII) (XXXVIII) associated with a bending of the C-C bonds, a deformation of the hybrid- ization ratio at the C atoms and an appreciable electronic delocalization. These phenomena being the most pronounced in the most strained cyclo- paraffins it is possible to understand qualitatively the origin of this hypsochromic shift. Conclusion.-After this short review it seems to be clear that natural hypsochromic shifts have to be regarded as occurring, at least potentially, as frequently as the natural bathochromic ones. In fact, natural hypso- chromic shifts can be produced just in the same way as natural batho- chromic ones with an appropriate choice of the basic molecular skeleton. The examples of odd, five- or seven-membered rings show that the pro- perties of the more usual six-membered rings are exceptional and should not be used as a basis for too wide a generalization. It may be noted that even in the group of compounds composed of six-membered rings there is a pronounced difference in the spectroscopic behaviour of the molecules usually represented by a Kekul6 structure and those containing crossed- conjugated double bonds. Institut du Radium, 31 rue Pierre Curie, Paris ge, France. Coulson and Moffitt, Phil. Mag., 1949, 40, I ; Walsh, Trans. Faraday sm, 19491 45, 179.

ISSN:0366-9033    

DOI:10.1039/DF9500900046

出版商:RSC    

年代:1950

数据来源: RSC

摘要:

THE ABSORPTION SPECTRA OF SOME SUB- STITUTED BENZENES AND NAPHTHALENES IN THE VACUUM ULTRA-VIOLET BY V. J. HAMMOND,* W. C. PRICE,* J. P. TEEGAN t AND A. D. WALSH t Received 14th August, 1950 Vacuum ultra-violet spectra and ionization potential data are given for the molecules, benzene, toluene, ethyl benzene, isopropyl benzene, o-, m-, p-xylenes, phenol, monofluorobenzene, benzotrifluoride, o-, nz-, 9-monofluorotoluenc, nz-fluorobenzotrifluoride, naphthalene and 2-methyl naphthalene. The effect of the nature and position of the substituting groups is discussed and is com- pared with the effect of solvents in solution spectra. The 2000 A absorption region of benzene and its derivatives is interpreted as a Rydberg transition. In order to understand the spectra of the substituted benzenes in the vacuum ultra-violet it is convenient to begin by describing the main features of the spectrum of benzene itse1f.h The modifications in the spectrum resulting from the substitution can then be related to the nature of the attached groups.The Spectrum of Benzene.-The far ultra-violet absorption spectrum of benzene falls into four main regions (Fig. ~a). The first of these con- sists of a diffuse region of absorption extending from 2070 to 1900 A. This is followed by an extremely strong region of continuous absorption having a maximum at about 1790 A. A third region between about 1650 and 1370 A contains sharp bands which are Rydberg in type. The main bands form two Rydberg series converging to a limit representing the first ionization potential of benzene vo" = 74,495 - R/(n - 1*03)2 n = 3, 4, 5.- - (1) yon = 74,590 - W(n - o-45)5 n = 3, 4, 5. - * (2) The short vibrational progressions accompanying the main electronic transitions indicate that the electron comes from a weakly bonding orbital and there is no doubt that it is of a B type. It seems certain that the terms of series (I) are associated with ns atomic terms and those of series (2) with nj5 terms and the quantum defects have been chosen so as to bring this out. In the original paper by Price and Wood, the n = 3, 4 terms of series (I) and the n = 3 of series (2) were not included in the series. Firstly, the n = 3 and n = 4 terms of series (I), because of the low value of n, have orbitals which are to a large extent still within the dimensions of the molecule. Secondly, because of their s character, these orbitals particularly penetrate the molecular framework.The first reason causes the frequencies to deviate appreciably from the Rydberg formula, which can only hold precisely for orbitals that are sufficiently large for the molecule to be considered as a point. The second causes the bands to become very diffuse as a result of internal collision of the excited electron with other electrons in various bonds of the molecule. Thus the n = 4 member * Wheatstone Laboratory, King's College, London W.C.2. -f Chemistry Department, University of Leeds. The reasons for this are now fairly clear. Price and Wood, J . Chem. Physics, 1935, 3, 439. Price and Walsh, Proc. Roy. Suc. A , 1947, 191, 22. 3 Original value of 9'190 V corrected for new value of electronic charge.5354 SUBSTITUTED BENZENES of series (I) is to be associated with a set of diffuse bands in the neighbour- hood of 61350 cm.-l (1630 -81) in spite of the fact that the calculated series value is 62056 cm.-l. The n = 3 member is to be identified with the 0 2 Methyl N ' h h h / 4 0 Q A -/ I 1 I I I I I I I FIG. I .-Absorption spectra of benzene, naphthalene and derivatives. The ordinates indicate qualitatively the strength of the absorption and are not to be taken quantitatively. diffuse absorption in the region of 2000 A (ca. 50,000 cm.-l observed, 46210 cm.-l calculated), the deviation from the series being about what might be expected for such a low lying orbital. The intensity of absorp-HAMMOND, PRICE, TEEGAN AND WALSH 5 5 tion is also about right, judging from the intensity of the higher series members.The n = 3 member of series (2) has been identified,% beyond any doubt, with a sharp band system near 1790 A, both by the structure of the band and by’ its accompanying vibrational pattern. The other interesting facts about series (2) are that its members retain their sharp- ness even down to the first member and that, apart from the first member, the series is generally weaker and less well developed than series ( I ) . Both these facts are explicable if the excited states are associated with m p atomic orbitals. Such orbitals will not penetrate the plane of the molecule and be involved in internal electron collision which would cause the bands to become diffuse.Furthermore, since transitions of this type are forbidden in the atomic case, the series will be generally weak, especi- ally for the higher members where conditions are of a more atomic character. The assignment of the 2070-1900 A absorption to a Rydberg transition is not in disagreement with the theoretical expectations for benzene, since the only allowed transitions within the electron shell is A,, - El, which is to be identified with intense continuous absorption having a maximum at 1790 A. The fourth region of the benzene spectrum lies below 1360 A. At about 1355 A a fairly strong band occurs which appears to correspond to the first resonance band leading to the ion (xlxi,s) and corresponding to an ionization potential of ca. 11.7 V. Diffuse absorption from electrons in the basic single CC and CH bonds appears to prevent all but the first two or three Rydberg bands converging to this ionization limit being observed.Mono-alkyl Benzenes.-The absorption of toluene is illustrated diagrammatically in Fig. I b. It can be seen that while the spectrum is still very similar to that of benzene it is shifted considerably to longer wavelengths. This shift increases from about 612 cm.-l for the near ultra-violet bands (ca. 2600 A) to about 2250 cm.-l for the 1800 maxima and to 3300 cm.-l for the higher Rydberg bands. It is thought to arise largely from polarization or charge transfer effects which particularly develop in the substituent in the excited state. Such effects are natur- ally greater the nearer the excited state to the ion.The explanation fits with the fact that similar shifts are observed in going from the gaseous phase to solutions in hydrocarbon solvents. For example, bands in the 2600 A system of benzene are shifted by about 150 cm.-l to long wave- length in going from gas to solution in iso-octane, those in the 2000 range by about 1000 cm.-l and those in the 1800 range by GU. 2000 cm.-l (for maxima in solution see ref. ( 3 ) ) . The larger orbitals are clearly much more affected by the solvent molecules. The shifts are attributable to polarization of the solvent. Similar effects for the naphthalenes will be referred to later. The Rydberg bands of toluene are much less sharp than those of benzene, but a series analogous to the strong series ( I ) of benzene can be readily picked out and fitted into the Rydberg formula : 2 .(3) The corresponding ionization potential is 8.820 6 0.005 V, which is a reduction of 0.42 V relative to benzene. Deviations from the formula for the n = 3 and n = 4 members are larger than in benzene because of the more extended structure. In going from toluene to ethyl benzene there is a further slight shift to long wavelengths, which is, however, much smaller than the shift produced by’ the primary’ substitution. The Rydberg bands have in- creased in diffuseness but it is still possible to establish a correlation with the toluene bands and so to obtain the approximate Rydberg series which corresponds to an ionization potential of 8.77 f 0.01 V. yoa = 71,130 - R/(n - 0 - g 5 ) ~ , where n = 3, 4, 5 yon = 70,750 - R(n - I - I O ) ~ , n = 3, 4, 5 .* (4) Platt and Klevens, Chenz. Rev., 1947, 41, 301.56 SUBSTITUTED BENZENES The shifts in going from ethyl benzene to isopropyl benzene are still smaller. Although the bands have become increasingly more diffuse, it is still possible to estimate that the ionization potential of isopropyl benzene is about 0.01 V less than that of ethyl benzene, i.e. ca. 8-76 V. It should be noted that in the near ultra-violet region there is always a short wavelength shift on replacing the hydrogens of an attached methyl group by other methyl groups. This has been linked by Matsen, Robertson and Chuoke,' with the Baker-Nathan effect and explained in terms of hyperconjugation which would raise the highest occupied ground state rn orbital and depress the lowest unoccupied w orbital by an amount greatest for toluene and least for tert.-butyl benzene.The difficulty with this explanation is that it does not account for the Eong wavelength shifts of the shorter wavelength bands of tert.-butyl (or ethyl or isopropyl) benzene relative to toluene. In part these may be due to inductive shifts raising the highest occupied ground state 7~ orbital and increasing from toluene to tert.-butyl benzene. We should like, however, to put forward an additional or alternative explanation in terms of the dimensions of the excited orbital. Polarization stabilization of an excited orbital by an alkyl group (regarded as a small piece of dielectric) is clearly most effective when the stabilizing group lies near to or within the excited orbital.A substituted methyl group has its polarized end groups lying far outside FIG. 2.-Diagram indicating probable dimensions of excited orbitals cor- responding t o the 2600, 2000, 1790 and 1521 A transitions in substituted the excited orbital of the 2600 A systems and they are relatively in- effective on this account, the greater stabilization being brought about by hydrogen atoms attached to the cc carbon atom. The Baker-Nathan effect could be explained along these lines provided an excited electronic state were involved. A diagram indicating the probable dimensions of the excited orbitals corresponding to the 2600, 2000, 1800 and ca. 1600 A transitions in substituted benzenes is given in Fig. 2. These are drawn a t distances 1-2, 2.0, 2.8 and 5-8 A respectively from the ring carbons, being calculated on the simplified assumption that the average distance of the electron from the ring carbons varies inversely as the term value of the state.It is not difficult to deduce from such a diagram the effects referred to above. due pre- sumably to the excitation of a X1 electron has moved to 1380 A in toluene. In ethyl and isopropyl benzene continuous absorption arising from the alkyl groups appears and tends to blot out all detail at wavelengths shorter than 1400 A. Long wave shifts of the bands of mono-alkyl benzenes in going from the vapour to the dissolved state are of the same order as those found for benzene. The Di-alkyl Benzenes .-The substitution of two methyl groups for two hydrogen atoms both attached to the benzene ring produces a further shift in the spectrum towards longer wavelengths, the magnitude of the shift increasing in the order ortho, meta, Para.For the 2600 A systems the shifts relative to toluene are ca. 120, 240, 360 cm.-l respec- tively. The shifts of the maximum of the strong continuum in the 1900 A region are ca. 400, 700, 1200 cm.-1 respectively. The Rydberg bands Matsen, Robertson and Chuoke, Chem. Rev., 1947, 41, 273. benzenes. Outer circles correspond to shorter wavelength bands. In the region below 1400 A the band in benzene at 1355HAMMOND, PRICE, TEEGAN AND WALSH 57 in o-xylene, are rather broad and diffuse, but by comparison with the spectrum of toluene a rough Rydberg series could be established : yon = 69,200 - R(n - 1.08)~~ n = 4, 5, 6.. ' ( 5 ) The limit corresponds to an ionization potential of 8-58 f 0.01 V. In nz-xylene the Rydberg bands are still fewer and more diffuse. How- ever, it appears from their position relative to those of o-xylene that the ionization potential of m-xylene is very close to the value for this mole- cule. For 9-xylene numerous .Rydberg bands were observed. These are somewhat diffuse but a fairly reliable Rydberg series converging to a limit of 8.48 V was established. The appearance of more Rydberg bands is probably to be connected with the greater symmetry of the molecule and it should be pointed out in this connection that in the near ultra- violet para derivatives have the sharpest bands and ortho derivatives the most diffuse absorption bands.Bands corresponding to the excitation of a x, electron appear in the neighbourhood of 1410 for each of the isomers. sorption spectrum of monofluorobenzene is very similar to that of benzene itself though the bands are not so sharp as they are in benzene and are shifted slightly to longer wavelengths. The two following series are analogous to the two series observed for benzene. The Spectrum of Monofluorobenzene.-The far ultra-violet ab Yon = 74,203 - R/(n - I.05)8J n = 3, 4, 5 * (6) yon = 74>3O4 - R/(n - I'5O)', = 3 , 41 5. * * (7) Table I shows the agreement between the observed and calculated fre- quencies. As is usual in molecular Rydberg series, the earlier members TABLE I.-THE OBSERVED AND CALCULATED FREQUENCIES OF BANDS OF SERIES (6) AND (7) Series 6 n 3 4 5 6 7 8 9 I 0 I1 Series 7 n deviate somewhat from the formula but the agreement is good for the higher members.The bands are on the average about 300 cm.-l to the low frequency side of the corresponding benzene bands. The limit cor- responds to an ionization potential of 9.197 f 0.005 V. The difference of ca. IOO cm.-l between the limits of the two series is equal to the differ- ence between the corresponding limits found for benzene and may represent convergence to slightly different states of the molecular ion. The small- ness of the long wave shift of the benzene bands on fluorine substitution as compared with the shift produced for example by chlorine substitution 3 indicates clearly that the high electron affinity of fluorine produces a sufficiently large inductive effect almost to balance the mesomeric effect.Similar conclusions can be reached from infra-red s t ~ d i e s . ~ The spectrum See also 6Torkington and Thompson, Trans. Faraduy SOC., 1945, 41, 237. Price, ibid., 1945, 41, 246.58 yobs. cm.-1 SUBSTITUTED BENZENES Ycalc. of phenol on the other hand shows considerable long wave shifts of about the same order of magnitude as for monochlorobenzene. The near ultra- violet spectra of these substances show the same relative shifts as observed in the far ultra-violet. The Spectrum of Benzotriflu0ride.-The spectrum of this sub- stance is of special interest because, while the absorptions corresponding to the 2000 and 1790 A regions of benzene are shifted to long wave- lengths and occur at practically the same positions as in toluene, the Rydberg bands are shifted to short wavelengths and converge to an ionization potential greater than that of benzene.The series found are v0n = 78,120 - R/(n - 1-05)2, n = 3, 4, 5 v0n = 78,250 - R/(n - 0.50)~, n = 3, 4, 5. . . (8) * (9) . Table I1 shows the agreement between observed and calculated frequencies. The ionization limit obtained from series (8) is 9-683 4 0.005 V. That this ionization potential should be greater than that for benzene is in accord with the strong electron attracting power of the CF, group known n 3 4 5 6 7 TABLE I1 .-OBSERVED AND CALCULATED FREQUENCIES FOR THE BENZO- TRIFLUORIDE BANDS OF SERIES (8) AND (9) Yobs. ern,-' ycalc. c*.-l -55,250 60,692 66,972 69,292 72,838 72,831 74,640 74,622 75,604 75,653 Series 8 n 3 4 5 6 7 8 9 I0 I1 Series g from the strength of trifluoro-acetic acid and dipole moment data.It is also in accord with the absence of complex formation in solutions of iodine in benzotrifluoride. Complexes of iodine and benzene derivatives owe their stability 6, to the donation of electrons from the benzene ring to the iodine molecule and the tendency to such donation will de- crease with increasing T-1 ionization potentiaL8 In benzotrifluoride there is no mesomeric release of electrons as in monofluorobenzene to offset the high inductive pull of the fluorine atoms. Whereas in benzotrifluoride the long wave systems are shifted to long wavelengths and the short wave ones to short wavelengths relative to fluorobenzene, the reverse effect happens on a smaller scale in tert.-butyl benzene and toluene. The latter effect has already been referred to in discussing the spectra of ethyl benzene and isopropyl benzene.This reversal is to be related to the fact that in one case we have an electron- attracting and in the other an electron-releasing substituent. The Spectra of the o-, m-, p-fluoroto1uenes.-The spectra of these substances are given in Fig. I. The Rydberg bands are too diffuse to enable series to be obtained from them. However, the regions show certain interesting features. While the meta and para derivatives have their bands shifted to long wavelengths in much the same way as the m- and p-xylene spectra are moved to the long wavelength side of toluene, 6Benesi and Hildebrand, J. Auner. Chem.Soc., 1949, 71, 2703. * Chamberlain and Walsh, Trans. Faraduy SOC., 1949, 45, 1032. Mulliken, ibid., 1950, 72, 600.HAMMOND, PRICE, TEEGAN AND WALSH 59 the spectrum of the ortho derivative is not shifted by nearly as much as might be expected. This we attribute to local action between the fluorine and the methyl group which prevents the latter from releasing negative charge to the ring. A similar effect on a smaller scale must be operative in o-xylene. The fields of the adjacent polarized methyl groups tend to depolarize each other and to reduce the charge transfer that might occur if these groups w erewell separated. The spectrum of m-fluorobenzotrifluoride has also been obtained. The Rydberg bands are too diffuse to permit an analysis, but the other regions indicate a slight long wavelength shift relative to benzotrifluoride. The Spectra of Naphthalene and 2-methyl Naphthalene.-We con- clude this series with a discussion of the spectra of naphthalene and 2-methyl naphthalene. The spectra were obtained at pressures of about one-tenth of the vapour pressure of these substances in path lengths of 2 m.They are given in Fig. I (n and 0). The main features of the spectra are the very strong absorption systems in the range 2200-2000 A. Again we should like to call attention to the shifts to long wavelengths in going from vapour to solution and to the way these become larger for the shorter wave bands in much the same way as shifts caused by alkyl substitution. For naphthalene in hydrocarbon solvents the shifts are : for the 3200 A system ca.310 cm.-l,O for the 2900-2500 A system ca. 1000 cm.-l (ref. (9) and A.P.I. Spectrogram 169) and for the 2200- 2000 A system ca. 2250 cm.-1. These shifts are clearly due to polar- ization in the molecules of the solvent when excitation occurs (i.e. to Lorentz-Lorenz forces). This stabilizes the excited state and thus causes the absorption to be at longer wavelengths in solution. There is no doubt that a similar process is operative when substituents are directly attached to the molecule. The fact that the solvent effect is of the same order of magnitude as the substituent effect indicates that the long wave shift produced by alkyl substitution is to be explained as being mainly due to polarization of these groups in the excited state. The 2200-2000 A absorption of naphthalene has diffuse structure associated with it.The first diffuse hump, centred at 2108 (47,440 cm.-1), is relatively much stronger than the diffuse bands accompanying it on the short wavelength side and must thus be regarded as the (0-0) band of the transition, which, in spite of its breadth, does not give rise to much change in bonding. We shall not discuss the diffuse accompanying structure here as our plates have so far been taken with the Lyman continuum which has some undesirable emission lines in this region. It is planned to use the hydrogen continuum for this range in order to obtain more satisfactory details of the band envelope. To the short wave side of 1900 A strong sharp bands are obtained which are clearly Rydberg in type.The first set occurs at ca. 1830 A and is followed by a similar weaker set at ca. 1780 A, followed by a set of still weaker bands which are superimposed on a weak continuous absorption (1680-1550 A ; max. ca. 1620 A). The spectrum in the region 1900-1500 A shows isolated bands getting weaker and closer together as they crowd towards a limit in the region of 1500 A. The appearance of the spectrum makes it clear that the first ionization potential of naphthalene is not far from 8.1 V. This is confirmed by the electron impact value of 8-3 V.ll Furthermore, the ionization potential of naphthalene is expected to be appreciably less than that of benzene (9-24 V) and lower even than that of o-xylene (8.58 V) because of the additional conjugation. The low ionization potential means that the early Rydberg transitions should fall in the 2500-2000 A region and their orbitals should lie within the molecular framework.These facts are clearly of considerable im- portance in considering the origin of the bands in the 2500-2000 range. Sponer and Nordheim, this Discussion. loprice, J . Chem. Physics, 1936, 4, 539. l1 Sugden, Walsh and Price, Nature, 1941, 148, 373.60 SUBSTANCES OF SHORT LIFE Below 1500 there is a region of weak continuous absorption fading out again about 1450 A, below which there is no strong absorption down to at least 1100 A. It is possible that the continua in the regions ca. 2100, 1620 and 1485 A may form a Rydberg series going to a second ionization potential at ca. 9-3 V. The ten w electrons in naphthalene fall into five groups each of which should be associated with a different ionization potential. The doublet character of some of the Rydberg bands indicates another possible ionization potential about 400 cm.-l or 0.05 V lower than that at ca. 8-1 V. The spectrum of z-methyl naphthalene is very similar to that of naphthalene. It suffers the usual long wavelength shift and increase in diffusiveness as a result of methyl substitution. The strong 2200-2000 A has more diffuse bands superimposed on it. Differences of ca. 1000 cm.-l, which may correspond to the ring breathing vibration, are observed but the analysis of these together with smaller differences will have to await further experimental work. Observation of shifts relative to the cor- responding naphthalene systems indicates ionization potentials of ca. 8.0 and 9-2 V. We wish to acknowledge financial assistance for this work from the U.S. Office of Naval Research (T.O. IX, Contract N6-ori 20 with the University of Chicago), the Royal Society, the Chemical Society and the Institute of Petroleum. One of us (J. P. T.) wishes to thank the Com- missioners for the Exhibition of 1851 for the award of an Overseas Scholarship (for Eire). Wheatstone Laboratory, Chemistry Department, King’s College, London W.C.2. University of Leeds.

ISSN:0366-9033    

DOI:10.1039/DF9500900053

出版商:RSC    

年代:1950

数据来源: RSC