Spectroscopic moments and the question of d-orbital participation in

SPECTROSCOPIC

MOMENTS AND d-ORBITAL PARTICIPATION

431

Spectroscopic Moments and the Question of d-Orbital Participation in the Elements of Groups IV and VI1

by W. Kenneth Musker and George B. Savitsky

Downloaded by UNIV OF CALIFORNIA SANTA BARBARA on August 29, 2015 | http://pubs.acs.org Publication Date: January 1, 1967 | doi: 10.1021/j100861a035

Department of Chemistry, University of California, Davis, California 96616 (Received March 8, 1966)

The question of d-orbital participation by M(CH3)3 groups (M = C, Si, Ge, Sn) and the halogens is discussed on the basis of an analysis of the change in intensity of the symmetry-forbidden Ale + BzUtransition in the ultraviolet spectrum of benzene derivatives. The extent of overlap between the d orbitals of the group IV metals and the a orbital of the benzene ring in para-substituted anisoles appears to decrease in the series silicon > germanium > tin. The decrease in spectroscopic moment with increasing size of M which is observed for the group IV elements seems to parallel the trend observed in the group VI1 elements.

Introduction The spectroscopic moment’ of benzene substituents can be measured by an analysis of the change in intensity of the symmetry-forbidden Ale + BZu transition2 which appears near 2600 A in the ultraviolet absorption spectrum. The sign of the moment may be either positive or negative depending on whether the group is an electron donor or an electron acceptor by resonance interaction with the benzene ring. However, the sign of the moment of certain groups may be changed as a result of the donor or acceptor ability of another substituent in para position. An inversion or change of sign of the moment of bromine and iodine was observed by Goodman and Frolen3 in a study of p-haloanisoles. I n monosubstituted benzenes, bromine and iodine have positive spectroscopic moments indicating that they are electron donors. However, the oscillator strength4 of pbromo- and p-iodoanisoles was found to be smaller than the oscillator strength of anisole itself indicating that the bromo- and iodo-groups behaved as electronwithdrawing groups when a very strong donor such as the methoxy group was present in the para position. Consequently the sign of the spectroscopic moment of the halogens was inverted. This inversion was attributed to d-orbital participation by the halogens involving resonance structures of type I.

1

The oscillator strengths were estimated by these authors on the basis of the absorption maximum (emax) of these compounds in the 2600-A region. Using this estimate the spectroscopic moment of chlorine appeared to be reduced to zero, but not inverted. However, a more accurate estimate of the oscillator strength was determined by one of us5 which was based on the integrated intensity of the absorption band. This method revealed that an actual inversion of the spectroscopic moment was observed for the chloro group. This can be seen from the results recorded in Table I which lists the ratio of the oscillator strength of the p-haloanisoles to that of anisole. (1) J. R. Platt, J. Chem. Phys., 19, 263 (1951). (2) A. L. Sklar, ibid., 7,984 (1939);ibid., 10, 135 (1942). (3) L.Goodman and L. J. Frolen, ibid., 30, 1361 (1959). (4) The strength is related to the spectrycopic moment by the equation lm,/2 = m,a = k(f, - f x o ) where m, is the spectroscopic moment of substituent x, f x is the experimentally measured oscillator strength of the monosubstituted benzene, CsHsX, f x o is the vibrational contribution to the oscillator strength, and k is a constant of proportionality. ( 5 ) G. B. Savitsky, “Some Studies on Spectroscopic Moments of Polysubstituted Benzene,” University Microfilms, Ann Arbor, Mich., S.C. Card No. Mic. 59-6108.

oscillate:

Volume 71,Number S January 1967

W. KENNETHMUSKERAND GEORGEB. SAVITSKY

432

/(p-X-CsHs-OCHd f(CsHa0CHs)

Group X


F

1.58

c1

0.92

Br I

0.91

0.9lU

Corrected for the atomic absorption by iodine which also falls in the same region: T. M. Dunn and T. Iredale, J. Chem. Soc., 1592 (1952).

Only in the case of the fluoro group is the ratio of intensities greater than unity. Therefore among the halogens only fluorine retains its positive spectroscopic moment in the presence of a strong donor in para position. This observation is consistent with the fact that there are no low-lying d orbitals available in fluorine which could interact with the benzene nucleus. Recently, d-orbital participation in p-methoxyphenylsilane was observed by Goodman, Konstam, and Sommel.6 and the theory of this interaction was thoroughly analyzed. The ratio of the intensity of the silane to that of anisole was found to be 0.89 which is of the same order of magnitude as the ratio of the halogen compounds. I n this work the scope of d-orbital participation' by the group IV elements has been examined by measuring the oscillator strength of two series of compounds.

B

A

M =C,'Si, Ge, Sn Experimental Section

Materials. Most of the compounds used in this study were synthesized by modifications of reported procedures. Isolation and purification were accomplished by gas-liquid chromatography using a 3-ft silicone column. The appropriate fraction was collected and rechromatographed prior to ultraviolet analysis to ensure that decomposition had not occurred. All products were determined to be >99% pure by this technique. The infrared spectrum of all products was consistent with the structure of expected compounds. Trimethylphenyl~ilane,~ n 2 5 ~ 1.4906, lit. ,9 nZoD 1.4880; triinethylphenyl germane, n z 5 J ~1.5044, 12251) 1.5045; trimethylphenylstannane,lO n Z 5 1.5365, ~ 1.5002, 1 k J l 1 .3125D 1.5330; p-t-butylanisole, 72% s9

The Journal of Physical Chemistry

m-bistrimethylsilylbenzene,l5 n z 5 1.4870, ~ lit., n * 5 ~ 1.4867. p-Methoxyphenyltrimethylgermane. The synthesis used for the germane was identical with the process used in the preparation of the corresponding stannane. l 4 Trimethylbromogermane was prepared according to the procedure of Dennis and Patnodel6 from tetramethylgermane (City Chemical Corp., ?Jew York, N. Y.) and bromine. A solution of p-methoxyphenyllithium in diethyl ether was prepared17 and added slowly at room temperature to a solution of trimethylbromogermane in diethyl ether. After neutralization and ether extraction the p-methoxyphenyltrimethylgermane was collected, chromatographed, ~ Anal. Calcd for C,oHls and analyzed, n Z 5 31.5158. OGe: C, 53.42; H, 7.17. Found: C, 53.66; H, 7.16. Tris-l,S,5-trimethyEsilylbenzene.This compound was prepared by a modification of the methods of Burkhard's and of Clark15 for the preparation of bistrimethyl si1yl-subs titu ted benzenes . 1-Trimethy1si1yl-3,5-dich1orobenzene,bp 117-123" (20 mm), was synthesized from l-bromo-3,5-dichlorobenzene (K and K Laboratories) by preparing the Crignard reagent in diethyl ether and treating this intermediate with trimethylchlorosilane in the usual way. Further silylation was accomplished by treatment of l-trjmethylsilyl-3,5-dichlorobenzenewith sodium and excess trimethylchlorosilane in refluxing toluene. After stirring for 5 hr the solution was filtered while hot and neutralized with 10% "&I. The toluene layer was separated and dried with sodium sulfate. The -~~

(6) L. Goodman, A. H. Konstam, a n d L . H. Sommer, J . Am. Chem. SOC., 87, 1012 (1965). (7) It is convenient to refer only to d-orbital participation; however, in heavier atoms other low-lying orbitals may be available for interaction with the benzenoid system. (8) R. A. Benkeser and A. Torkelson, J . Am. C h a . Soc., 76, 1252 (1954). (9) J. D. Roberts, E. A. McElhill, and R. Armstrong, ibid., 71, 2923 (1949). (10) R. H. Bullard and W. B. Robinson, ibid., 49, 1368 (1927). (11) D. Seyferth and D. L. Alleston, Inorg. Chena., 2, 417 (1963). (12) W. T. Olson, H. F. Hipsher, C. &Buess, !I. I. A. Goodman, I. Hart, J. H. Lamneck, Jr., and L. C. Gibbons, J . Am. Chem. Soc., 69, 2451 (1947). (13) C. Eaborn, J . Chem. Soc., 3148 (1953). (14) 0. Buohman, M.Grosjean, and N. Nasielski, Bull. SOC.Chim. Belges, 71, 467 (1962). (15) H. A. Clark, A. F. Gordon, C. W. Young, and M.J. Hunter, J . Am. C h a . SOC.,73, 3798 (1951). (16) L. M.Dennis and W. I. Patnode, i b i d . , 52, 2779 (1930). (17) H. Gilman and R. A. Benkeser, ibid., 69, 123 (1947). (18) C. A. Burkhard, ibid., 68, 2103 (1946).


SPECTROSCOPIC

JfOMENTS AND

toluene was removed by means of a rotary evaporator and the residue was injected into the gas chromatograph. The major component was collected, rechromatographed, and analyzed. n Z 6 b1.4834. Anal. Calcd for Cl6H30Si3: C, 61.13; H, 10.26. Found: C, 62.16; H, 10.37. The infrared spectrum (liquid film) was consistent with the highly symmetrical structure expected for tris-l,3,5-trimethylsilylbenzene. Spectral Measurements. The ultraviolet absorption spectra were obtained in cyclohexane solvent (Fisher Spectroquality) using a Cary Model 14 recording spectrophotometer. The spectrum obtained for each compound was replotted in wavenumbers (cm-') vs. absorbance and the area under the curve in the 38,500cm-1 (2600-A) region was determined with a planimeter. There is some inaccuracy inherent in this treatment owing to the presence of a much stronger band a t about 50,000 em-' (2000 A). The overlap between these bands was generally small and the tail of the 2600-A band could be extrapolated to zero absorbance. The shape of the curves for the series of para-substituted anisoles and for the series of monosubstituted derivatives are similar and the uncertainty within each of these series should be small. For this reason the ratio of the oscillator strength is generally discussed. In most compounds the h - 0 band was well resolved and its position is reported in Table 11.

f x loa = H = C(CHa)a = Si(CH& = Ge(CH& = Sn(CH3)a

XO-0,

-Av,

cm-1

cm -1

0

1.6" 1 . 92b 3.32 3.42 2.85

37,850" 37,450" 36,970 37,740 37,690

4.93

37,240'

(610)

35,950 35,310 35,590 35,550 35,510

1900 2540 2260 2300 2340

400"

880 110 260

2

X = H X = C(CHa)a X = Si(CH& X = Ge(CH3)a X = Sn(CH&

22.9 23.2 18.3 19.8 23.1

Listed in Table I11 is the ratio of the oscillator strength of the 2600-A bands of para-substituted anisole derivatives to that of anisole. All of the group IV derivatives have an oscillator strength higher than that of benzene, and even if a correction for the vibraTable I11 : Ratio of the Oscillator Strength of p-Substituted Anisoles to Anisole

M

c

1.00 0.80 0.86 1.00

Si Ge Sn

tional contribution of the oscillator strength is made (see discussion below), a small, finite, spectroscopic moment is associated with these groups. If these compounds involve only hyperconjugative electron-donor structures of type 11, then the moment can be assumed to have a positive sign. I n para-substituted anisoles

+

',HC

xp X = Y = Z = Si(CH3)s

Results

:L+-=J+

Table I1 : Spectral Properties of Substituted Benzenes

X X X X X

433

d-ORBITAL PARTICIPATION

a J. Petruska, J . Chem. Phys., 34, 1120 (1961). * Reference 5. The reported value represents the maximum of the band envelope.

I1

the positive sign of the moment for aU the elements of group VI1 is inverted except for fluorine; however, all of the elements in group IV show either a significant inversion or a reduction t o zero. The extent of overlap between the d orbital of the metal and the T orbital of the benzene ring would be expected to decrease in the series silicon > germanium > tin. Therefore silicon should exhibit a higher negative moment than the rest of the group IV elements in the presence of a strong donor as observed in Table 111. This would indicate that resonance structures of type I are decreasingly important in the series silicon > germanium > tin. The trend of spectroscopic moment of the M(CH3)3 groups, where n1 = carbon, silicon, germanium, and tin, is of interest since apparently few systematic physical measurements related to the resonance parameters of these groups have been made. The oscillator strengths of phenyltrimethyl derivatives are listed in Table 11. These values are relatively small and seem to pass through a maximum at germanium. However, to Volume 71I Number 2

January 1967


434

estimate that part of the oscillator strength which is associated with the actual spectroscopic moment, a vibrational contribution must be subtracted from the observed oscillator strength. These vibrational contributions for various groups are never known accurately, but, they can be estimated from the oscillator strength of 1,3,5-trisubstituted benzenes. The osciland that for lator strength of benzene is 1.6 X most 1,3,5.-trisubstituted benzenes is from 2.0 X 10-3 indicating a negligible vibrational conto 2.1 X tribution of about 0.15 X per substituent to the oscillator strength. However, for the t-butyl group the oscillator strength of the 1,3,5-trisubstituted benzene is significantly higher, amounting to 2.9 x and indicating a vibrational contribution of about 0.4 X 10-~ per t-butyl group to the oscillator strength. We have prepared only one trisubstituted benzene, tris-l,3,5-trimethylsilylbenzene,and found that its than oscillator strength is even higher (4.93 X the t-buty: analog. This would indicate that the vibrational contribution amounts to about 1.1 X per trimethylsilylgroup. Thus the oscillator strengths corrected for the vibrational contribution of the t-butyl. group and the trimethylsilyl group are about 0.9 and 0.6, respectively. If it is assumed that the vibrational contributions of the other -M(CH& groups are similar to the silyl group or increase with an increase in the atomic weight of 14, then the spectroscopic moments of the trimethylgermyl group be similar or slightly smaller than the trimethylsilyl

The Journal of Physical Chemistry

W. KENNETHMUSKERAND GEORGE B. SAVITSKY

group and that of the trimethylstannyl group would be practically negligible. The generally decreasing trend of the spectroscopic moment with an increasing size of the atom M, which is observed for the group IV elements, seems to parallel the trend observed in the group VI1 elements. I n Table I11 the maximum of the 0-0 band has been recorded as well as its frequency shift ( - A v ) with respect to benzene. The red shift of this transition in substituted benzenes can be related to its intensification when there is constructive interferen~e.~"~ However, when there is strong destructive interference the red shift-intensification rule will be violated.6 Although all of the p-substituted anisole derivatives exhibited a red shift with respect to anisole, this observation cannot be regarded as the most reliable criterion for dorbital participation. The sign of the spectroscopic moment should be more definitive. The data on the methylated silanes reported here parallel the data reported by Goodman, Konstam, and Sommel.6 for the simple silanes.

Acknowledgments. We thank Mr. E. AI. Chen, Mr. J. Thomas, and Mr. L. A. Lafferty for synthesizing several of the compounds used in this study and for carrying out some of the spectroscopic measurements. We also thank the National Science Foundation (GP5716) for support of part of this research. (19) J. Petruska, J . Chem. Phys., 34, 1120 (1961).

Spectroscopic moments and the question of d-orbital participation in

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