摘要:

llB-Chemical Shifts of Diboranes, Polyboranes, Carboranes and Coupling Constants V(llBIH),V(llBllB) BY JWGEN AND BERND WRACKMEYER* KRONER Institut fur Anorganische Chemie der Universitat Miinchen, Meiserstraae 1, 8 Miinchen 2, W. Germany Received 2nd July, 1976 Linear relationships are observed between calculated charge densities or qg at boron and Sl1B for a considerable number of boron compounds. The term p&2s)Hfrom the density matrix correlates linearly with lJ(IIBIH). The results obtained by CNDO/S calculations were confirmed by ab initio calculations. By correlation of J(IIB1'B) withp&2s)B(2sl reasonable values forJ("B"B) in two centre two electron bonds can be estimated. Extensive theoretical work has elucidated much about the structure, bonding and reactivity of boranes, polyboranes and carboranes,I in which connection IlB n.m.r.spectroscopy is a valuable tool.2 However, this requires that the available IlB- chemical shifts (611B)and coupling constants [J(IIBX)] are consistently interpreted. Calculations of chemical shifts or coupling constants by Ramsey's equations are still of questionable accuracy because knowledge of exact wave functions in the ground and excited states is required. Therefore, most discussions dealing with chemical shifts of nuclei other than hydrogen are restricted to the local paramagnetic term 0;" which reflects the influence of electrons closely associated with the nucleus in q~estion.~ In the same way, discussions of coupling constants using the mean excitation energy approximation are more instructive when the properties of the nuclei concerned allow for this.4 Studies on compounds of tricoordinated boron,5 some carboranes and polyboranes have shown that IB-chemical shifts are dominated by the contribution of the paramagnetic term (a,) to the nuclear screening constant 0.An empirical relationship between s-character at boron and IJ(IIBIH) was found by Williams et aL8 No successful attempt has been made so far to relate charge densities at boron with PIB for boron atoms with a coordination number > 3. The purpose of the present work is to show that many l'B-chemical shifts as well as coupling constants of diboranes, polyboranes, carboranes and selected organoboranes are consistently interpretable on the basis of density matrix changes.RESULTS AND DISCUSSION Table 1 includes the PIB-data and coupling constants IJ(IIBIH) used for the correlations as well as charge densities at boron and BH bond orders obtained by CNDO/S-calculations. 9t Fig. 1 shows a correlation between SllB and total electron density at boron obtained by CNDO/S and ab initio models lo$ respectively. CNDO/S calculations have been performed using a model described elsewhere in ref. (9). The basis functions contain phosphorus, sulphur and chlorine 3d-orbitals. Underlying structural data are cited in table 1. 1For ab initio calculations we used the program " Gaussian 70" with minimum basis sets and standard atom exponents (STO-3G) [ref. (lo)], neglecting 3d-orbitals.2283 l B-CHEMIc A L SHIFTS 1.-b1 lB-DA~A, COUPLING CONSTANTS 'J("B'H), &zs)~, qEta1TABLE AND qg OF DIBORANES, POLYBORANES, CARBORANES, ANIONIC BORONHYDRIDES, BH3-~~~~~~~AND SOME TRICO-ORDINATED BORON COMPOUNDS references~~ ~~ no. compound /P.P.m. /Hz 2 'B(2s)Ht 2 'B(2s)Hb ptalB q; 611B J(BH) structure 1 H.BH2BH2 -17.5 135 0.327 0.094 2.806 1.988 a a A 2 HzBH2BHCH3(1)(2) (1) -8.8 (2) -26.7 -20.5 127 127 131.2 0.327 0.3 14 0.314 0.095 0.089 0.090 2.828 2.009 2.763 1.960 2.782 1.979 a a a a B B (1) -3.6 (2) -36.4 125.5 - 0.326 - 0.096 0.084 2.845 2.026 2.731 1.943 a a B (1) -13.6 (2) -29.2 -24.8 133.7 -- 0.314 -- 0.090 0.084 0.085 2.800 2.747 2.764 1.997 1.958 1.975 a U a fl B B (1) -7.7 24.8 (2) -23.0 139 167 141 0.327 0.315 0.332 0.093 0.096 0.107 2.797 1.984 2.775 2.001 2.894 2.053 b C 0 C c, D D,1-7 26.0 130 0.3 18 0.092 2.894 2.090 a a F 14 1.5-CzB3H5 (1) 41.8 (2) 6.9 (1) 52.7 (2) 13.1 (1) 55.3 (2) -7.5 (3) -0.5 (2) 35.8 (1) -11.3 (5) -0.7 (6) -9.7 1.4 156 132 176 166 152 132 160 136 159 165 159 189 0.351 0.309 0.379 0.357 0.340 0.295 0.363 0.313 0.339 0.322 0.334 0.379 0.103 0.067 0.095 0.143 0.087 0.088 ---0.094 0.075 - 2.946 2.181 2.840 2.057 3.142 2.348 2.874 2.121 3.131 2.375 2.908 2.106 2.844 2.086 2.914 2.134 3.026 2.250 2.936 2.138 2.819 2.023 2.795 1.946 a a e U f n d e a f G G G G H 15 16 17 1 8 1.6-C2B4Hrj 1.2-C2B4H6 2.3-C2B4Hs 2.4-CzBsH7 (3) (4)(1)(4) (5)(1)(3) (5) 18.7 1.6 15.3 53.3 2.0 0.0 23.5 -5.0 -2.0 41.2 188 185 162 179 159 164 179 182 169 80 0.408 0.403 0.388 0.377 0.355 0.35 1 0.388 0.383 0.379 0.245 ----0.116 0.063 ---- 2.903 2.082 2.932 2.102 2.950 2.102 2.980 2.203 2.911 2.123 2.823 2.022 2.859 2.061 2.825 2.025 2.964 2.139 3.267 2.408 9 I1 i i a 9 h i i a H H, 1 G G G 25.3 106 0.280 0.098 3.050 2.201 U k G 30.1 33 0.198 3.024 2.247 a a J 21.1 - - - 3.044 2.315 a - K 23.3 93 0.267 - 3.147 2.309 a a L 42.7 32.3 -86.0 -86.5 103 99 -- 0.252 0.260 -- ---- 3.170 2.358 3.171 2.386 2.708 1.812 2.713 1.805 a a a n U a -- D, M D,N 0 0 -86.0 -81.0 -114 -0.307 -- 2.736 2.706 1.822 1.788 I m -m 0 0 -28.6 130 0.335 - 2.654 1.818 a U 0 -28.3 130 0.356 - 2.688 1.847 n N P -20.9 -26.1 165 141 0.398 0.336 -- 2.765 2.627 1.925 1.818 a U a a Q0 -28.1 173 0.373 - 2.646 1.823 n a R -61.3 -22.0 140 21 1 0.347 0.420 -- 2.772 2.091 1.859 1.323 U a U a D, S T -29.2 133 0.328 - 2.648 1.795 a a U -32.3 133 0.328 - 2.665 1.806 a n U -37.9 113 0.335 - 2.757 1.859 a (1 0 (A) L.E. Sutton, Tables oJ'hterntomic Distances and Configuration in Molecules and Ions (Special Publications No. 11 and 18, The Chemical Society, London, 1958/65). (B) All methyl substituted diboranes have been calculated with the same standard geometry (A) and BC distances of 156 pm (1.56 A). (C) The molecular structure corresponds to that of bromodiborane (A) with a BC1 distance of I80 pm (1.80A).(D)With inclusion of d-orbitals in the CNDO/S basis set. (E)BB = 233.8pni,BS = 190pm,BN=119.3pm,135.5prn,SH= 133prn. (F)K.K. Lau,A.B.BurgandR.A.Beaudet, Imrg. Chem., 1974,13,2787. (G)See ref. (1). (H) E. A. McNeill, K. L. Gallaher, F. R.Scholer and S. H. Bauer, Inorg. Chem., 1973,12, 2108; I. R.Epstein, T. F. Koetzle, R. M. Stevens and J. KRONER AND B. WRACKMEYER 2285 W. N. Lipscomb, J. Amer. Chem. SOC.,1970, 92, 7019. (I) R. A. Beaudet and R. L. Poynter, J. Chem. Phys., 1970, 53, 1899; E.A.McNeill and F. R. Scholer, Inorg. Chem., 1975, 14, 1081. (J)C.R.Peters and C. E. Nordman, J. Amer. Chem. SOC.,1960, 82, 5758. The calculated data given are average values. (K) D. Groves, W. Rhine and G. D. Stucky, J. Amer. Chem. Suc., 1971, 93, 1553 (BC = 156pm).(L) BN = 166 pm, BH = 120pm,NH = 101.4pm. (M) C.E. Nordman, Acta Cryst., 1960, 13, 535 (PH = 141.8pm). (N) G. J. Bullen and P. R. Mallinson, J.C.S. Dalton, See ref. (5). (P)H. M. Seip, R. Seip and K. Niedenzu, J. Mol. Struct., 1973,17,1973, 1295. (0)361. (Q)C. H.Chang, R. F. Porter and S. H. Bauer, Znorg. Chem., 1969, 8, 1677. (R) BO = 138 pm, BH = 119 pm, CO = 143 pm, CC = 152 pm, OBO = 110",BOC = 110",OCC = 105". (S) BS = 178 pni, BH = 119 pm, CS = 181 pni, CC = 152pm, SBS = llO', BSC = 102.3', SCC = 112.7'. (T)BF = 129.5pm, BH = 119 pm. (U)See ref. (9). a See ref. (2). b See ref. (2)and (8). C P. C.Keller, Chem. Comrn., 1969, 209. d A. 0.Clouse, D. C. Moody, R. R. Rietz, T. Roseberry and R. Schaeffer, J. Amer. Chem.SOC.,1973, 95, 2496. e R. R.Rietz, R. Schaeffer and L. G. Sneddon, J. Amer. Chem. SOC.,1970,92,3514. f R.N. Grimes, J. Airier. Chem. Soc., 1966,88, 1895 and ref. (12a). T. Onak, F.J. Gerhart and R. E. Williams, J. Amer. Chem. SOC.,1963, 85, 3378. hT. Onak, R. P. Drake and G. B. Dunks, Znorg. Chem., 1964,3, 1686. i J. W.Akitt and C. G. Savory, J. Magnetic Resonace, 1975,17,122. j R.Warren, D. Paquin, T. Onak, G. B. Dunks and J. R. Spielmann, Inurg. Chem., 1970,9,2285. k R. K.Hertz, H. D. Johnson, IT and S. G. Shore, Znorg. Chem., 1973, 12, 1875. *B. Wrackmeyer, unpublished results. m S1'B and 'J("B'H) were observed for bis-(2,3-dimethylbutyl)borane,H. C. Brown, J. J. Katz and E. Negishi, J. Amer. Chem. SUC., 1972, 94, 4025. n H.FuBstetter, personal communica- tion.In fig. 2 the correlation between 611B and boron charge densities summed over all p-orbital contributions pi (again from CNDO/S and ab initio calculations) is presented. As can be seen from fig. l(a) there is a linear relationship for many 611B data, given by eqn (1) G"B(p.p.m.) = 382.263 q'BO(al-1083.563. (1) We regard it as very significant that the rather large shift differences for the various alkyldiboranes are well reflected. It is further shown that many GllB-data for boron atoms having quite different environments are within the scope of eqn (1). The only conditions seem to be that the coordination number of boron is 4 and that no formal negative charge is at the boron atom. When these conditions are fulfilled a constant diamagnetic term ad and a constant average excitation energy (AE) may be assumed.Thus the range of 611B (N 80 p.p.m.) as observed for the compounds studied must be explained by changes of the orbital expansion term and/or bond order term in the paramagnetic term op. Large deviations are found for boron atoms with coordination number > 4 or < 4 and in charged molecules like [BH,]-or H3N.BH3because then the relative contributions of o, and o, are bound to change. It is interesting to note that the deviations for the anions decrease with increasing size ([BH,]-to [B3Hs]-) and increasing delocalisation of the negative charge. A further linear relationship as shown in fig. 2(a) and (b) including 6'lB data of boron atoms which do not fit eqn (1) is obtained when the s-electron density at boron is neglected.Even 611B of trialkylboranes and dialkylboranes can be considered. 6"B data of tricoordinated boranes in which n-bonding is likely to occur (e.g., R,BX; X = OR, NR2, F etc.) are not suitable. Obviously a more constant AE value is required.5d Therefore, we believe that an influence of coordination number and boron charge density upon bd is indicated. It should be noted that the relalion- ship in fig. 2(a) and (b) covers almost the whole range of llB-chemical shifts (with the exception of 6'lB far [BI,]- and paramagnetic compounds). The magnitude of the coupling constants 'J(' 'B'H) has been discussed previously. .~Williams et ~2 found a relationship between s-character at boron and 'J(''B1H) "/o s-orbital (l'B) = 0.31 ['J(llB'H)].(2) "B-cHE M.ICAL SHIFTS The analogous interpretation of lJ(13C1H)has been critically reviewed l2 and the same criticism naturally applies in the case of eqn (2). According to Pople and Santry's molecular orbital treatment the Fermi contact term will dominate the coupling mechanism. Assuming that only the lowest excited state is important, it is possible to use the mean excitation energy (AE)approximation, where the mutual polarizability term ~III~I~ leading to eqn (3)is substituted by p&2s,H/dE in which lK is the reduced coupling constant, Y;@) is the valence shell s-electron 2.9 -m* Ern CF 2.8 2.7 1.3c L.6171 6' 'B/p.p .m. FIG.1.-Correlation.of 6' 'B-chemical shifts with boron total electron densities qgta*,obtained by CNDO/S (a)and abrinitio (b)calculations.The numbers are the same as used for the compounds in table 1. Symbols are 0= diboranes, A = polyboranes, 0= carboranes, A = anionic boron hydrides, 0 = BH, adducts, = organoboranes ; (a) qzta*= 0.00266"B+2.8346, standard error = 0.0112, correlation coefficient = 0.98 ; (b)qktal= 0.00296''B+4.8913, standard error = 0.0636, correlation coefficient = 0.70. density at boron and p$zS)Hrefers to the corresponding element of the charge density bond order matrix. If the relationship of Williams et aZ.* is valid a linear relationship between lJ(llBIH)and p&2s)H is expected. As is shown in fig. 3(a) and (h) the expected trend is well reproduced by both CNDO/S and ab initio calculations.However, it should be mentioned that the relationship between members of a class of J. KRONER AND B. WRACKMEYER 2.1 !. 2.0 i #lB/p.p.m. FIG.2.-Correlation of GLIB-chemical shifts with the boron p-electron densities qfl, obtained by CNDO/S (a) and ab initio (b) calculations. For the meaning of numbers and symbols see fig. 1. (a) qg = 0.0039P1B+2.0899, standard error = 0.0762, correlation coefficient = 0.88 ; (6) qi = 0.0034611B+2.0156, standard error = 0.0617, correlation coefficient = 0.85. TABLE 2.-cOUPLING CONSTANTS 'J(' 'B1 'B), 'J( 'B' C), REDUCED COUPLING CONSTANTS 'K(''B1 'B), 1K(''B13C) AND SQUARED BOND ORDERS &2s)x(2s) FROM CNDo/s CALCULA-TIONS 1J( 11B11B) 1J(llB13C) lK(1IBX) 2 no.compound /Hz /Hz pB(2s)X(2s) ref. 1 0 -0 0.0184 a 10 25.O(BllB3) -2.02 0.0436 a 11 19.4(BllB2) -1.57 0.0363 b 12 17.O(Bi)B3) -1.37 0.0306 b 13 18.7(BzIBs) -1.51 0.0402 a 14 -18.0 1.86 0.0504 C 17 26.6(B1[ B5) -2.15 0.0528 d 22 -22 2.27 0.0605 e 26 -52 5.36 0.0800 f78.7 -6.36 0.0968 9 * J. D. Odom, personal communication. b Note (d), table 1. C See ref. (12a). d Note (i),table 1. e See ref. (14i). fSee ref. (14g). gThe coupling constant has been extrapolated from fig. 4. 2288 ' ' B-CHEMIcA L S H I FT S closely related compounds is not always linear. Caution is necessary in using IJ(llBIH) for the estimation of s-or p-character in various bonding as has been attempted recently.' A more sophisticated treatment l4 might prove more useful for this purpose, when changes of the mutual polarizability term nllglHa.nd/or changes of the valence shell s-electron densities are taken into account..I mI I JBHIH~ FIG.3.-Corrdation of coupling constants 'J("B'H) with obtained by CNDOlS (a)and ab initio (b) calculations. The bridging hydrogens are not included in the calculation of r. For the meaning of numbers and symbols see fig. 1. (O)&~~~H= 0.0013J~~+0.1519,standard etror = 0.0179, correlation coefficient = 0.92 ; (b) p&2s)~= 0.0009J~~-0.0032. standard error = 0.0089, correlation coefficient = 0.96. Coupling constants of boron to elements other than hydrogen have recen tty received much attention ;I2* l5 especially the coupling constants lJ(llB1lH) in polyboranes and carboranes could yield much information about the bonding conditions.Some coupling constants lJ(''B'lB) and the p&zS)x(2S)values are listed in table 2 arid fig. 4 shows a correlation between 'K('lBX) and p&2S)x(2s)obt;tined 2289J. KRONER AND B. WRACKMEYER from CNDO/S calculations. No coupling constant between two boron atoms in a two centre two electron bond has yet been measured. As shown in fig. 4 we get a reasonable value of -80 Hz for lJ("B21B) between two sp2-Iiybridized boron atoms, by extrapolation, using the calculated B-B bond order of a hypothetical I Ksx/nrn-FIG.4.-Correlation of reduced coupling constants 'K("BX) (X = 'B, I3C) with ~&2~)~(2~,, obtained by CNDO/S calculations.For the meaning of numbers and symbols see fig. 1and table 2. p&2s)x(2s)= 0.0117K~x+0.0226,standard error = 0.0069, correlation coefficient = 0.93. (CH3)2B.B(CH3)2. The lJ(' lB1lB) data of B4H1 strongly support the interpretation of bonding in B4H10. Lipscomb et al. suggested a BlIB3 orbital which is 20 % delocalized and formed of sp3 hybrid orbitals,l in agreement with our calculations. The similar lJ(llB1lB)-data of B,Hl0 and 2,3-C2B,H8 lead to the conclusion that the bonding situation between the boron ztoms concerned is Comparable. The technical assistance of the Leibiiiz Computer Centre, Muinchen, (Telefun ken TR 440), of the Bayerische Akademie der Wissenschaften ad of the Bavarian Ministry of Education (IBM 370/158) is gratefully acknowledged.W. N. Lipscomb in Boron Hydride Clleniistry, ed. E. L. Muetterties (Academic Press, New York, 1975), pp. 39-77. G. R. Eaton and W. N. Lipscomb, N.M.R. Sludies of Boron Hydrides and Related CON?~O~~JI~S (Benjamin, New York, 1969). (a) N. F. Ramsey, Phj~Rer., 1950, 78, 699 ; N. F. Ramsay and E. M. Purcell, Php. Rer., 1952, 85, 143. (a) J. A. Pople, Disc. Faraday Soc., 1963, 34, 7 ; (6) J. A. Pople, J. Chem. Phys., 1962, 37, 53, 60 ; (c) J. A. Pople, 1Mol. Phys., 1964, 7, 301 : (d) J. A. Pople and D. P. Santry, Mol. Phys., 1964, 8, 1. (a)J. Kroner, D. Nolle and H. Noth, 2.Naturfbrsch., 1973, 28b, 416 ; (b) J. Kroner, H. Noth and K. Niedenzu, J. Orgnnometallic Cheni., 1974, 71, 165 ; (c) J. Kroner, D. Nolle, H. Noth and W.Winterstein, 2. Naturforsch., 1974, 29b, 476 ; (d) J. Kroner, D. Nolle, H. Noth and W. Winterstein, Chem. Ber., 1975, 108, 3807 ; H. 0. Berger, J. Kroner and H. Noth, Chem. Ber., 1976, 109, 2266. (a)E. Switkes, I. R. Epstein, J. A. Tosell, R. M. Stevens and W. N. Lipscomb, J. Amer. Chem. SOC.,1970,92, 3837 ; (h) J. H. Hall, Jr., D. S. Marynick and W. N. Lipscomb, J. Amer. Chem. Soc., 1974, 96, 770. F. P. Boer, R. A. Hegstrom, M. D. Newton, J. A. Potenza and W. N. Lipscomb, J. Anier. Chem. SOC.,1966, 88, 5340; D. S. Marynick and W. N. Lipscomb, J. Anrer. Chem. Soc., 1972, 94, 8692 ; D. S. Marynick and W. N. Lipscomb, J. Amer. Cliem. Soc., 1972, 94, 8699. R. E. Williams, K. M. Harmon and J. R. Spielmann, OTS, AD, 603782 (1964). J. Kroner, D.Proch, W. FuB and H. Bock, Tetmhedron, 11972, 28, 1585 ; J. Kroner slid D. Proch, Terruhedrcin Letters, 1972, 2537 ; J. Kroner, W. Strack, F. Holsboer and W. Kosbahn, Z. Nntirrforsch.. 1973, 28b, 188. 2290 B-CHEMIcAL SHIFTS lo W. J. Hehre, R. F. Stewart and J. A. Pople, J. Chem. Phys., 1969, 51,2657; M. D. Newton, W. A. Lathan, W. J. Hehre and J. A. Pople, J. Chem. Phys., 1969, 51, 3927. A paper appeared discussing hybrid orbital character of 'J ("B 'H) and 'J(' 'B lB)after this work was submitted; T. On&, J. B. Leach, S. Anderson, H. J. Frish and D. Marynick, J. Magnetic Resonance, 1976, 23, 237. l2 V. M. S. Gil and C. F. G. C. Geraldes in N.M.R. Spectroscopy of Nuclei Other than Protons, ed. T. Axenrod and G. A. Webb (John Wiley, New York, 1974), p.219. l3 (a) T. Onak and E. Wan, J.C.S. Dalton, 1974, 665 ; (b) T. Onak and E. Wan, J. Magnetic Resonance, 1974, 14,66. l4 G. E. Maciel, J. W. McIver, Jr., N. S. Qstlurid and J. A. Pople, J. Amer. Chem. SOC.,1970, 92, 1. l5 (a) J. D. Qdom, P. D. Ellis and H. C. Walsh, J. Amer. Chem. SOC.,1971, 93, 3529 ; (b) J. D. Odom, P. D. Ellis, D. W. Lowman and M. N. Gross, Znorg. Chem., 1973, 12, 95; (c) D. W. Lowman, P. D. Ellis and J. D. Odom, Znorg. Chem., 1973, 12, 681 ; (d) D. W. Lowman, P. D. Ellis and J. D. Odom, J. Magnetic Resonance, 1972,?3,189;(e) P. D. Ellis,J. D. Qdom, D. W. Lowman and A. D. Cardin, J. Amer. Chem. SOC.,1971,93,6704 ;(f)L. W. Hall, D. W. Lowman, P. D. Ellis and J. D. Odom, Znorg. Chem., 1975,14,580 ; (g)W. McFarlane, B. Wrackmeyer and H. Noth, Chem. Ber., 1975, 108, 3831 ; (h) B. Wrackmeyer and H. Noth, Chem. Ber., 1976, in press; (i) V. V. Negrebetski, V. S. Bogdanov, A. V. Kessenikh, P. V. Petrovskii, Yu. N. Bubnov and B. M. Mikhailov, Zhur. Obsch. Khim., 1974, 44, 1882. l6 E. Switkes, W. N. Lipscomb and M. D. Newton, J. Amer. Chem. Soc., 1970, 92, 3847. (PAPER 6/1285)

ISSN:0300-9238    

DOI:10.1039/F29767202283

出版商:RSC    

年代:1976

数据来源: RSC

摘要:

Spin-lattice Relaxation Studies of Organophosphorus Compounds BY ROBINK. HARRIS" AND ELIZABETHM. MCVICKER School of Chemical Sciences, University of East Anglia, Norwich NR4 7TJ Received 5th July, 1976 Phosphorus-31 spin-lattice relaxation times and 31P-('H} nuclear Overhauser enhancements are reported for three diphosphines and eight diphosphine disulphides. In three instances work was carried out at several temperatures. The (31P,lH) dipolar contributions to relaxation vary with substituent bulk and temperature, particularly for the diphosphines. It is probable that spin- rotation interactions provide the other major relaxation mechanism. In comparison with the increasing quantity of work on spin-lattice relaxation, the phosphorus-3 1 nucleus has received relatively little attention.This is somewhat surprising as 31P has a much greater n.m.r. sensitivity than 13C, although the range of compounds incorporating phosphorus which are suitable for relaxation studies is not as vast as that for 13C. Consequently, we decided to study some organo-phosphorus systems containing two phosphorus atoms per molecule in order to establish the typical range for TI ("P) and to deduce the important relaxation mechanisms. We have examined molecules in two series, viz. diphosphines, I, and diphosphine disulphides, 11. As the 31P nuclei are shielded from external fields by the alkyl or aryl substituent (except, possibly, for the diphosphines), intramolecular relaxation processes will be important. However, none of the compounds studied contains direct P-H bonds so (P, H) dipolar relaxation rates are likely to be fairly long for normal mobile liquids or solutions. Using 31P-(1H}nuclear Overhauser enhancements (NOE) and spin-lattice relaxation times (Tf)the dipolar contributions to T; arising from (P, H) interactions have been isolated and the contributions from other mechanisms evaluated. R R R1 S S R1 \/ \I1 P-P ll/P-P R /\ /\R R2 R2 I I1 PH 0SPHOR US-31 SPI N -LA T T ICE RE LA XA TI0N The major contributions to spin-lattice relaxation rates reported so far for phosphorus in the literature have been from dipole-dipole interactions and from the spin-rotation relaxation mechanism.Asknes, Rhodes and Powles separated the dipolar and spin-rotation contributions to Tlfor PCl,, POC13 and POF,, and the spin-rotation mechanism was also found 2, to contribute to TIin P406,although inter- and intramolecular (P, P) dipolar interactions were also present.Seymour and Jonas 4* found that the (P, H) dipole-dipole and spin-rotation mechanisms were 2291 SPIN-LATTICE RELAXATION STUDIES effective for phenyl phosphines. The contribution to TF from (P, H) interactions has been reported for orthophosphoric acid and some related systems, but the NOE was not measured in these experiments. Recently the NOE enhancements for phosphorus compounds have been reported,' but as yet there has been no quanti- tative use of the NOE in determining the dipolar contribution to TF. A third mechanism which was thought to have importance in 13Prelaxation was chemical shift anisotropy, as found by Dale and Hobbs.' However, other workers 59 have shown that this mechanism makes a negligible contribution to TI in PBr,, PCl,, POCl, and PhPH,.In particular, Gillen has shown that the contribution from chemical shift anisotropy may be ignored for PBr3 at 19 MHz. The expression for the dipole-dipole relaxation rate between two spin-$ particles, I and S, is well known,1° and is given (for the extreme narrowing condition) in terms of an effective correlation time zc, magnetogyric ratios 71 and ys, and the internuclear (I, S) distance rls by eqn (1) :* In the case of the dipole-dipole mechanism operating between two like nuclei the equation becomes* Both eqn (1) and (2) assume isotropic rotation of the relevant internuclear vector.It is possible that in some the molecules studied here the homonuclear (P, P) inter-action will compete with the (P, H) interaction, as the factor r-6 will be greater for the directly-bonded phosphorus nuclei, compensating for the smaller factor due to y. The nuclear Overhauser effect can be used to obtain the dipole-dipole relaxation rate, as the enhancement, y, on irradiating the S spins may be written as = Vmax TllT1dd (3) where qmax,the maximum enhancement, isy,/2y1 (for 31P-(1H) experiments this is 1.24). The contribution to the relaxation rate from other relaxation mechanisms, TI,,,may then be calculated as TT1 = TFA+Tr:. (4) T,, contains the terms arising from mechanisms such as spin-rotation and chemical shift anisotropy.This assumes that scalar relaxation can be ignored. A dominant spin-rotation contribution, Tlsr,may be readily distinguished since TI,,is inversely related to an increase in temperature, in contrast to the other mechanisms. EXPERIMENTAL The samples of diphosphines with R = Me, But and Ph were made up in a dry box under dry nitrogen, using benzene with N 30 % C6D6 (to provide a lock signal) as solvent. A second sample of Me4P2 for low temperature measurements was prepared with toluene/ C6D6 as solvent. The remaining compounds were studied as solutions in CDC13. All samples were in 12mm 0.d. n.m.r. tubes, and were degassed by freezing, pumping and thawing several times prior to sealing.The use of nickel spatulas and syringes was avoided when preparing the samples, as it is known that molecules containing trivalent phosphorus arc easily attacked by paramagnetic impurities. * Eqn (1) and (2) are written in SI form ; po is the permeabilityconstant, 47~x lo-' kgm s--~A-2. R. K. HARRIS AND E. M. MCVICKER Relaxation times were measured using a Varian XLlOO Spectrometer, operating at 40.5 MHz for 31P. An inversion-recovery pulse sequence and program as modified by Freeman and Hill lo was used to measure T,,under conditions of proton noise decoupling. NOE enhancements were recorded at least five times for each sample, at the same temperature, using the gated decoupling method." Signal intensities were obtained 9 from a computer listing of peak heights, and the selected spectral width (250 Hz) ensured sufficient data points for a well-defined peak.Spin-lattice relaxation times were reproducible to within 10 %, generally better than 5 %, although careful temperature control was required to repeat an experiment closely. The reproducibility of NOE results was better than k0.08. The TI and NOE values were recorded at three different temperatures (limited by solubility problems) for [Me2P(S)I2 and [(CH,),P(S)],, and at -55°C (as well as +34"C)for [Me2PI2 in toluene. For such variable-temperature work the temperatures were measured using an alcohol thermometer immersed in CDCl3 in an n.m.r. tube with both r.f. fields on. No corrections were made for the effect of gating on the temperature when measuring NOES.RESULTS It has already been noted l3 that the 31Presonance of some diphosphines is particularly sensitive to temperature ; applying an r.f. pulse or gating the decoupler can cause the signals to move by up to 10 Hz during a pulse delay. The solution of BukP,, which showed the greatest effect, was sufficiently concentrated to allow reproducible TI measurements to be made using a single pulse, for each value of the recovery time, t. The chemical shifts of the other diphosphines chosen in this study remained sufficiently constant at a particular temperature setting for repetitive pulse sequences to be used. TABLE1.-31P RELAXATION DATA FOR DIPHOSPHINES R2PPR2 R solvent temperature/"(= Tl/s 17 Tlcidls Tlo/S Me C6H6/C6DG -35 9.9 0.05 245 10 CH3C6H4/C6D6 34 12.3 0.15 101 14 CHJC 6H4/C6D6 -55 11.3 0.85 17 36 BU' C~H~/C~DO -38 19.2 1.12 21 198 Ph CGH~/C~D~ -35 29.0 0.65 46 62 TABLE2.-RELAXATION DATA FOR DIPHOSPHINE DISULPHIDES [R1R2P(S)]2 IN CDC13 AT -35°C RiRZ Tl/S '7 T]dd/S TlolS Me2 17.1 0.55 38 31 MeEt meso 21.9 0.56 48 40 racemic 21.6 0.57 48 40 Et, 9.5 0.68 17 21 Pr,' 15.4 0.7 27 35 MeBut a 19.5 0.4 60 29 MePh 14.2 0.8 22 39 CH2CH2CH2CH2 17.6 0.65 34 37 a Isomer configuration unknown ; b racemic isomer Tables 1-3 list the relaxation times and NOE data obtained for the compounds studied.Using eqn (3) and (4) the (P, H) dipolar contributions to Tlhave been evaluated separately, and hence Tl,also obtained.The contribution to TI of the spin-rotation mechanism was investigated by variable temperature experiments on certain of the compounds (see tables 1 and 3). Solubility of the disulphides was a limiting factor for low temperature measurements. SPIN-LATTICE RELAXATION STUDIES TABLE3.-vARIABLE TEMPERATURE 31PRELAXATION DATA FOR Me&'(S)P(S)Me2 AND (CH2)4P(S)P(S)(CH& temperature/'C a Tl/s r) T~ddls T~ols 11, R1= R2 = Me 10 14.9 0.68 27 33 33 16.9 0.48 44 28 48 17.3 0.37 57 25 11, R1R2= (CH2)4 9 13.5 0.79 21 37 33 17.5 0.65 33 37 48 17.7 0.56 39 32 a measured as described in the text DISCUSSION Despite the inherent inaccuracy of some of these measurements, it is interesting to compare the relative efficiencies of different relaxation mechanisms.In the diphosphines there is a remarkable increase in NOE at ambient probe temperature from near zero for Me,P, to 90 % of qmaxfor BukP,, indicating that (P, H) dipolar interaction plays a negligible role in relaxation of the former compound, but is the dominant mechanism for the latter compound. At low temperature, however, the dipolar contribution in T, for Me4P, is dominant, and Troldecreases, implying that spin-rotation effects deterniiiis TI,, and give the dominant mechanism at ambient temperatures. The low moment of inertia of the smaller molecule (and possibly relatively low barriers to internal rotation) will render this mechanism more effective than in other diphosphines. The changes in TI, and Tldd for Me,P, as temperature decreases are compensatory (as required by Hubbard's relation if Tl, is dominated by the spin-rotation mechanism) such that Tlat -55" is within experimental error of the value at ambient probe temperature.It may be that variations in the extent of chemical interactions between diphosphine molecules with temperature contribute to the rapid change in y by providing additional dipolar terms. At all temperatures, of course, the (P, P) dipolar interaction will also contribute to the relaxation time, T,,,but this will be of greater relative importance at low temperature. A comparison of the effect of heteronuclear and homonuclear inter- actions may be calculated using eqn (1) and (2). No structural data are available for Me,P,, but a molecular model was constructed, using the sum of covalent radii l4 for bond lengths, with P-P = 0.22 nm, P-C = 0.177 nm and C-H = 0.109 nm.Tetrahedral coordination was assumed for the phosphorus and carbon nuclei. From this model, the P-H distance was calculated to be 0.24 nm. The ratio of dipolar relaxation rates is given by where n is the number of protons at distance rPHfrom phosphorus. Only interactions from protons in one R2P group are considered, as interactions across P-PR2 are assumed to be very small due to the r-6 factor. From the approximate internuclear distances described above, and taking n = 6, the following ratio was obtained Tldd(p, = 0.06. R. K. HARRIS AND E. M. MCVICKER Thus homonuclear dipolar contributions to 31Prelaxation are probably negligible for Me,P,.The ratio calculated above is obviously only correct to an order of magnitude due to the approximations made, namely (i) the molecular geometry will almost certainly deviate from the standard bond lengths and bond angles, due to lone pair interactions ; (ii) isotropic motion has been assumed, with the same correlation times for (P, H) and (P, P) interactions ; (iii) internal rotation has been ignored ; (iv) inter- molecular (P, H) and (P, P) interactions have been ignored. The effect of rapid internal rotation of the methyl groups in 13Crelaxation studies has been shown to reduce the effective relaxation rate by l6 [+(1-3 cos ")I2 in cases where the correlation time for internal rotation, z~~,is much less than zc, the overall orientation correlation time.Such effects may be relevant for 31Prelaxation, and for the cases studied here the angle 8 is between the (P, H) vector and the axis of internal rotation (P-C bond). As the rate of internal rotation is reduced z~~ increases, and when qR 9 zc, the internal rotation has little effect on dipolar relaxation. If there is rapid rotation of methyl groups in Me,P,, then TGfl (P, H) is reduced. Using the assumed geometry for the inolecule Me4P2, with standard bond lengths and bond angles, the calculated angle 6' is 26", which reduces Ti& by a maximum factor of 3. Such a reduction in the dipolar contribution to T1may help to allow TG1to dominate the relaxation rate at room temperature. The other possible mechanism which has been discussed for 31Prelaxation, chemical shift anisotropy, has the same temperature dependence as the dipole-dipole mechanism, but experiment shows T1, has the opposite temperature dependence. Hence T& does not make any significant contribution to the relaxation rate.Glonek and Van Wazer 6b have recently suggested that intermolecular (P, H) dipolar inter- actions play an important role in 31Prelaxation for aqueous solutions, but we have not investigated this possibility. In BukP,, the magnitude of the NOE is perhaps surprising, as the protons are separated by three bonds from phosphorus. However, a molecular model makes it clear that some protons are situated quite close to the 31Pnucleus, and the dipolar mechanism may, therefore, be fairly efficient.Moreover, the overall rotational correlation time should be significantly longer than for the tetramethyl compound. Using the same method as for Me4P,, the (P, H) distances in BukP, were measured, although the assumed bond angles and lengths are likely to be in greater error for this compound, due to steric interactions of the t-butyl groups. Methyl conforma- tions such that a C-H bond in each CH3 is trans to the P-C bond were assumed. In a totally rigid molecule there are three distinct (P, H) distances, of the order of 0.30,0.35and 0.40 nm, and six protons at each distance. The ratio of heteronuclear and homonuclear dipolar relaxation may be estimated, assuming (P, H) and (P, P) interactions have the same correlation time, to give TLdd(P, N 0.16, Tldd(P, Rotation of the methyl groups will increase this ratio, although completely free rotation of the substituent groups is unlikely in this sterically crowded molecule.Indeed, it is possible to prevent P-C internal rotation in t-butyl phosphines at low temperatures. From experiment the ratio of TI, to T1dd was TO : 1, where T1dd refers to (P, H) interaction. From the above calculation it is evident that the (P, P) interaction may have a significant effect on Tlo,although from the approximations made in calculating the ratio, it would be too great an assumption to say that Tldd (P,P) was the only other mechanism competing with the (P, H) dipole-dipole mechanism.SPIN-LATTICE RELAXATION STUDIES Tetraphenyl diphosphine had a longer Tl than any of the other compounds studied. The dipolar mechanism accounts for just over half of the relaxation rate, and the spin-rotation mechanism may dominate T,,,although the bulk of this molecule will render this latter mechanism less efficient. Using standard bond lengths and bond angles, a molecular model of Ph4P2 showed that the angle 8 between the C-P axis of rotation of the phenyl group, and the (P, H) vector for the ortho position, was close to the magic angle of 54'44'. In this position the expression [+(l-3 cos ")I2 is zero, and when z~~ < z~,the dipolar interaction is ineffective. Although the bond lengths and bond angles will differ from those of the model, it is possible that the reduction in T& could be caused by rapid internal rotation, but it seems unlikely that z~~will be much less than ,rc, due to steric interactions of the two phenyl groups hindering the internal rotation.The greater (P, H) distance will reduce the efficiency of Tldd, and, as the spin-rotation interaction is also likely to be small, the longer overall relaxation time of this moiecule is not unexpected. The relaxation data for diphosphine disulphides cover a very small range both for T, and q, despite considerable changes in bulk of substituents. The lowest TI value, 9.5 s for [Et2P(S)I2, is surprising since the smaller molecule, [Me,P(S)],, has a longer T1, whereas the isopropyl compound has an intermediate value.The anomalies are still present when values of Tldd are considered, so they cannot be explained by the presence of paramagnetic impurities. Presumably the effects arise from ii com-bination of factors such as changes in the number of protons, the values of rCH,and the correlation times, but a quantitative explanation is not currently feasible. The two compounds studied at different temperatures, [Me,P(S)] and [(CH2)4P(S)]2, behaved in the same manner, with the overall T1 decreasing at low temperatures, while TI, increased. Thus TI,is strongly influenced by the spin-rotation mechanism. The structures of both compounds are known,"* and estimates of 2, were made using eqn (1). In [Me,P(S)], the rPpand rpH distances are 0.218 nm l8 and 0.234 nm respectively (the latter was calculated assuming the C-H bond length is 0.109 nm).In [(CH,),P(S)], the bond length rppis 0.221 nm and, for the or-CH, groups, rPH was calculated to be 0.241 nm, assuming LPCH = 109.5'. The correlation times for the two molecules were calculated, ignoring internal rotation, using the values of Tldd given in table 2. For [Me,P(S)], 2, = 0.76 x lo-'' s and for [(CH,),P(S)], Z, = 1.59 x lo-'' s. The expected ratio of heteronuclear and homonuclear dipolar interactions is *ldd(p' H, = 0.06 for [Me2P(S)I2 Tldd(P, = 0.07 for [(CH,),P(S)],. Rotation ofthe methyl groups in [Me,P(S)], will increase this ratio, and the contribution of the (P, P) dipolar relaxation may be more important in determining the overall relaxation time, although the expErimenta1 results demonstrate the importance of spin-rotation in TI,.In conclusion it can be seen that these molecules containing two 31Pnuclei are relaxed by two competing mechanisms-dipolar and spin-rotation interactions, with a smaller contribution from the (P, P) dipole-dipole mechanism. The relative effects of the iiiechanisms depend on the temperature, size of substituent and possibility of intcrnal rotation of the molecule. R. K. HARRIS AND E. M. MCVICKER We are grateful to Dr. M. Fild for supplying most of the compounds used. One of us (E. M. McV.) thanks the Northern Ireland Department of Education for the award of a research studentship. D. W. Asknes, M. Rhodes and J. G. Powles, Mol. Phys., 1968, 14, 333.D. J. Mowthorpe and A. C. Chapman, Mol. Phys., 1968, 15,429. D. W. Asknes, Acta Chem. Scand., 1969, 23, 1078.'S. J. Seymour and J. Jonas, J. Chem. Phys., 1971, 54,487. S. J. Seymour and J. Jonas, J. Magnetic Resonance, 1972, 8, 376. (a)W. E. Morgan and J. R. Van Wazer, J. Amer. Chem. Suc., 1975, 97, 6347; (h) T. Glonek and J. R. Van Wazer, J. Phys. Chenz., 1976, 80, 639.'P. L. Yeagle, W. C. Hutton and R. B. Martin, J. Amer. Chem. Soc., 1975, 97, 7175. S. W. Dale and M. E. Hobbs, J. Phys. Chem., 1971, 75, 3537. K. T. Gillen, J. Chem. Phys., 1972, 56, 1573. lo R. Freeman and H. D. W. Hill, J. Chem. Phys., 1971, 54,3367. R. Freeman, H. D. W. Hill and R. Kaptein, J. Magnetic Resonance, 1972, 7, 327. l2 R.Freeman and H. D. W. Hill, J. Magnetic Resonance, 1971,5,275. l3 S. Aime, R. K. Harris, E. M. McVicker and M. Fild, J.C.S. Dalton, 1976, in press. L. Pauling, The Chemical Bond (Oxford U.P., London, 3rd edn., 1967). l5 K. F. Kuhlmann and D. M. Grant, J. Chem. Phys., 1971, 55, 2998. l6 A. Allerhand, D. Doddrell and R. Komoroski, J. Chem. Phys., 1971, 55, 189. C. H. Bushweller and J. A. Brunelle, J. Amer. Chem. Soc., 1973, 95, 5949. C. Pedone and A. Sirigu, J. Chem. Phys.. 1967, 47, 339. lg J. D. Lee and G. W. Goodacre, Acta Cryst., 1970, 25,2127. (PAPER 6/1298)

ISSN:0300-9238    

DOI:10.1039/F29767202291

出版商:RSC    

年代:1976

数据来源: RSC

摘要:

Centrifugal Distortion of Carbonyl Sulphide in Excited Vibrational States BY JOHN G. SMITH Department of Physical Chemistry, The University, Newcastle upon Tyne Received 21st June, 1976 The millimetre-wave rotational spectra of the OCS molecule and some of its isotopes in excited vibrationalstates have been studied in order to obtain centrifugal distortion constants for comparison with a theoretical treatment. In a recent paper,l the centrifugal distortion constants of the ground and vibra- tionally excited states of several isotopes of carbonyl sulphide were calculated. Table 2 of that paper summarised the calculated distortion constants of some of the lower vibrational states of the four main isotopic derivatives. It was notable that there were several gaps in the column labelled experimental, particularly for the OC34Sspecies. The purpose of this note is to report millimetre-wave measurements on some of these weak transitions which yield reliable centrifugal distortion constants for some of the missing states in table 2 of ref.(1). EXPERIMENTAL All transitions were measured on the Newcastle millimetre-wave spectrometer. The basis of this instrument has been described previously.2 Millimetre frequencies were generated as harmonics of klystrons oscillating between 22 and 28 GHz. All frequencies are measured with reference to a standard crystal which is regularly checked against the BBC Droitwich transmission. All observations were performed at room temperature. ANALYSIS The low frequency measurements (J" < 5) are mainly taken from those reported by Maki3 In all cases a simple least squares fitting procedure was used.In several cases only the upper (or lower) I-doublet of an excited bending vibrational mode was observed ; these have been fitted to an effective rotational constant Beff. The measured transitions are reported in table 1 and the refined constants are given in table 2 together with the values computed by Whiffen.l In the case of the ground state of OC34Sa previous measurement of the centrifugal distortion constant is reported by Maki which disagrees with the calculated value by rather more than the likely error in the computation, although the difference is within one standard deviation of the experimental value.The new measurements result in a distortion constant which is much closer to the calculated value. It can be seen that there is reasonable agreement between all the observations reported here and the corres- ponding calculated quantities given by Whiffen. This lends some support to the method of calculation and also to the force field used in that cal~ulation.~ 2298 J. G.SMITH 2299 TABLE1.-MEASUREDTRANSITIONS oc34s (0 00 0) J' obs. freq. obs.-calc. error in obs. 2 35 596.874 -0.001 0.05 7 94 922.800 -0.009 0.1 9 118 651.732 0.01 3 0.1 13 166 105.747 0.031 0.2 15 189 830.284 -0.041 0.2 17 213 553.021 -0.002 0.2 19 237 273.555 -0.015 0.3 23 284 707.290 0.033 0.4 1 23 760.480 -0.006 0.05 2 35 640.660 0.007 0.05 7 95 039.537 0.005 0.1 9 118 797.584 -0.023 0.15 15 190 063.640 0.007 0.2 oc34s (0 20 0) 1 23 804.970 -0.047 0.05 2 35 707.500 0.038 0.05 7 95 218.014 -0.017 0.1 9 119 021.017 0.006 0.1 oc34s (0 22 o)e 2 35 720.270 0.009 0.05 7 95 251.328 -0.034 0.1 9 119 062.035 0.016 0.1 oc34s (0 22 o)f 2 35 720.270 0.004 0.05 7 95 251.778 -0.016 0.1 9 119 062.910 0.008 0.1 oc34s (1 00 01 1 23 660.560 0.002 0.05 2 35 490.770 0.010 0.05 7 94 639.754 -0.043 0.1 9 11 8 297.942 0.021 0.1 1 24 300.640 0.004 0.05 7 97 199.973 -0.009 0.1 9 121 498.060 0.005 0.1 CENTRIFUGAL DISTORTION OF OCS TABLE1.-Contd.ocs (0 3' 0)f J" obs.freq. obs.-calc. error in obs. 1 24 459.200 -0.047 0.05 7 97 834.951 0.126 0.1 9 122 291.847 -0.063 0.1 ocs (0 33 0) 4 61 128.860 0.004 0.08 7 97 804.350 -0.008 0.1 9 122253.362 0.004 0.1 All frequencies are given in MHz. The e and f labels for states with f = 1 follow the convention given in ref. (5). TABLE2. oc34s Q vib. state Bem/MHz &?/kHz(this work) DJ/kHzref. (1) (0 oo 0)(0 1' 0)e (0 2O 0)(0 22 0)e (0 22 0)f(1 oo 0) 5 932.8348 (5) 5 940.1315 (7) 5 951.2628 (61) 5 953.4041 (34) 5 953.4006 (16) 5 915.1495 (26) 1.244(1) 1.256(2) 1.061(40) 1.5 16(22) 1.278(10) 3.267(17) 1.242 1.260 1.085 1.471 1.290 1.267 013cs (0 1' 0) 6 075.1698 (14) 1.335(8) 1.316 ocs (0 (0 3' 33 0)f 0) 6 114.8208 (183) 6 112.9581 (10) 1.126(109) 1.451(6) 1.214 1.574 a Unless stated otherwise the isotopic species is 1G012C32S; b Beffmay equal B" *(u + 1)qO if I = 1. The author would like to thank Prof. D. H. Whiffen for pointing out the problem. D. H. Whiffen, Mol. Phys., 1976,31, 989. * J. H. Carpenter, J. D. Cooper, J. B. Simpson, J. G. Smith and D. H. Whiffen, J. Phys. E., 1974, 7, 678. A. G. Maki, J.Phys, Chem. Ref: Data, 1974, 3, 221. A. Foord, J. G. Smith and D. H. Whiffen, Muf.Phys., 1975, 29,1685. J. M. Brown, J. T. Hougen, K. P. Huber, J. W. C. Johns, I. Kopp, H. Lefebure-Brion, A. Merer, D. A. Ramsay, J. Rostas and R. N. Zare, J. Mol. Spectr., 1975, 55, 500. (PAPER 6/1191)

ISSN:0300-9238    

DOI:10.1039/F29767202298

出版商:RSC    

年代:1976

数据来源: RSC

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Reviews of Books The 1)ynamics of Spectroscopic Transitions. By JAMESD. MACOMBER.(John Wiley and Sons, New York, 1976). Pp. xxiv+ 332. Price El I SO, $23.00. The purpose of this book is “ to bridge the gap between scientists and engineers with a conventional training in quantum mechanics. . . and those engaged in the study of coherent transient effects ”. It is true that many physical chemists are not familiar with coherent phenomena of the type studied in magnetic resonance and, more recently, in laser spectroscopy. Pulse techniques are of considerable importance in the study of relaxation and other time-dependent processes, so the appearance of this book is timely. It has been written in a fresh and clear style for ‘‘advanced undergraduates, beginning graduate students, and practising research scientists who use spectroscopy as a tool ”.The book is comprised of eight chapters and a short appendix. The first five chapters are introductory and deal with quantum theory, electromagnetic radiation, the interaction of radiation and matter, and the density matrix. The sixth chapter is concerned with magnetic resonance and the seventh with any spectroscopic transition. The final chapter is devoted to the propagation of light through a two-level system. There is a good account of the Bloch equations and their solution, and of the density matrix description of systems that evolve in time. There are numerous helpful little analogies, such as that on pp. 80 and 81 for energy transfer where there are amusing pictures of a parent interacting with a child on a swing.But a few of the analogies are of doubtful value. Thus on p. 72 it is stated that quadrupolar molecules “ always possess a positive or negative ‘waist ’ with head and feet of opposite polarity ”; apart from the ambiguity, this assertion hardly seems applicable to BF3 and C6&. And on p. 79 there is the statement that “ to an accuracy of better than 136 parts in 137. no other types of transitions (than dipolar ones) are possible.” And on p. I86 “ electrons are points, not balls”. There are pedagogical difficulties in chapter 7. Electric dipoles are likened to magnetic dipoles and are even equated to a constant (the electrogyric ratio) times the angular momentum. But such a relation does not represent the nature of the.electric dipole-it is contrary to its behaviour under both time and space reversal. The book is well produced with clear and helpful diagrams. The price is reasonable. A. D. BUCKINGHAM Received 16th June, 1976 Magnetic Resonance of Riomolecules. By D. MARSH,P. F. KNOWLESand H. W. E. RATTLE. (J.Wiley and Sons, London, J976). Pp. 343. Price 29.75, $19.75. Is magnetic resonance spectroscopy of any real value in the study of biological systems? This question must be answered if this book is to be seen to be needed. My own affirmative answer is based on three outstanding observations : (i) Magnetic resonance methods (especially n.m.r.) supply the only detailed methods for the study of structure in solution and of biological processes. For exaniple we know now niuch about proteins which was not apparent and never could have been apparent from solid state X-ray crystal structure work-proteins are dynamic molecules and some proteins have no fixed secondary structure.I believe the conventional biochemist is today as far behind the frontiers of knowledge about the nature of biological macro-molecules as he was in 1960 when X-ray crystal studies brought an explosion of information. (ii) Magnetic resonance methods (especially e.p.r. but more recently n.ni.r.1 are very sensitive analytical detection procedure: for very small amounts of materials particularly metal ions. The whole wcrld of ironlsulphur proteins has been revealed by these techniques. (iii) Magnetic resonance can be used for kinetic purposes and even for following changes of in ~i~osteady states of radicals, or combined forms of atoms, ‘H, *3C, 31P etc.The application to in vivo systems is just developing. If chemists and biochemists are to understand the nature of biological solutions it is vital that simple explanatory books should be written about magnetic resonmce methods but they must be good and attractivelq presented for the subject is not easy. At the beginning of the use cf X-ray diffraction methods in biology, a great deal of ill advised criticism was raised largely because the presentation of material was in the form of poorly defined electron density maps. The same type of cri!icism is levelled at the presentation of n.m.r.and e.s.r. spectra of bio-molecu!es. There is a large gap between the data and the interpretation and it is 2301 REVIEWS OF BOOKS very important that the research worker and student who study biological molecules should be able to bridge the gap with facility. Fortunately this book meets the needs and the high standards demanded. It is equally divided between n.m.r. and e.s.r. and between principles and practicr: applications to real biological systems so that all that needs to be known is introduced. In summary this book will be of great help in achieving the objective of bringing magnetic resonance spectroscopy to the public at large and I recommend it whole-heartedly. R. J. P. WILLIAMS Received 23rd June, 1976 EIectron and Photou Interaction with Atoms.Proceedings of a symposium dedicated to Ugo Fano. Ed. H. KLEINPOPPENand M. R. C. MCDOWELL.(Plenum Press, New York and London, 1976). Pp. xviii+682. Price $54.00. This volume contains 57 articles by authors well known in the different branches of the subject. Progress is reported both on theoretical and experimental fronts in areas such as configuration interaction effects, autoionization, electron attachment, photodetachment, angular distributions and correlations, photon and electrori spin polarization, cross section threshold studies and phase shift analysis. Many of the theoretical concepts basic to the understanding of how electrons and photons interact with matter have been developed by Professor Ugo Fano who has always shown great interest in providing the right theory to explain the experimental observations.In recognition of his many and vaned contributions to the field this international symposium has been dedicated to him. The advent of high resolution electron spectroscopy and the use of synchrotron radiation in photoelectron spectroscopy permits of a more detailed study of subshell properties. These are essential for the refinement of the theory of atomic structure, for determining binding energies, natural level widths etc. This is just one of the many topics discussed in the book all of which are in a rapidly developing state. For those whose research interests lie in the excited states of atoms the book has only one draw-back-its price. w.c.PRXCE Received 13th July, 1976 Vibrational States. By S.CALIFANO.(John Wiley and Sons, Chichester, 1976). Pp. xiif335. Price E16.75, $32.10. The publication, now just over twenty one years ago, of Wilson, Decius and Cross's book formed a landmark in the systematic study of molecular vibrations. The present volume fallows the spirit, and programme, of this distinguished forerunner, and naturally brings to mind the developments that the passing of two decades have brought in this field, and which are reflected in the pages of Califano's book. Perhaps it is more remarkable how little has changed;-the determination of the harmonic force field of the molecule remains the principal challenge, the piquancy being that the information provided by the frequencies of the normal modes is insufficient to uniquely fix the force field parameters.The mathematical methods used, revolving round the "F " and "G " matrices, have been polished, and adapted for computer use, but in essentials remain unchanged. Although the two decades have produced no magical stroboscope that allows us to view molecular vibrations directly, methods have been developed which enable additional information to be obtained about the force field,- from Coriolis constants, mean vibrational amplitudes, and centrifugal distortion. Each of these is discussed, although briefly, and would merit rather more extensive treatment. Should one regard the present work as a worthy epilogue of a trilogy opened by "Herzberg *' and having "W.D. and C " as its centrepiece? One of the greatest satisfactions of science is that there is virtually never a neat and tidy "last word "-witness classical optics, and one feels sure that the study of molecular vibrations may, too, be poised for vigorous new growth. In a further twenty years, what might the successor of the present volume include? Pulsed and double resonance effects, the vibrations of extremely anharmonic and fluxional molecules, inter- molecular force fields, all aspects of kinetic spectroscopy? The present volume then, is a fitting summary of a chapter which is closing-and in so doingalso serves the advances to come. J. L.WOOD Received 26th July, 1976

ISSN:0300-9238    

DOI:10.1039/F29767202301

出版商:RSC    

年代:1976

数据来源: RSC