Valence-Bond Studies of the Dependence upon Substituents of C13-H and Si29-H Coupling
An interpretation is presented for the additivity of substituent effects on the C/sup 13/-- H coupling constant, which was previously observed in the high- resolution NMR spectra of substituted methanes. Each atom or group X is assigned a characteristic affinity'' for s character in the carbon hybrid orbital of the C--X bond. Distribution of s character among the carbon orbitals in accord with the relative s affintties of the four substituents leads to the observed additivity relation provided that the total s character is conserved. The valence-bond approach used with the model gives a linear relation between the s character of the carbon hybrid orbital involved in the C-- H bond ( alpha /sub H/ sup 2/) and the observed C/sup 13/-- H coupling constart (J/sub CH/ = 500 alpha / sub H/sup 2/). Als o, it allows the determination of the s character of the other carbon orbitals. The dependence of the s character of the C--X bond on the electronegativity of X is discussed in terms of electron spin and charge correlation. It is noted that the hybridization changes should affect not only J/ sub CH but also JHH, which is consistert with the observed proportionality between JcH (CH/sub 3/X) and the cis and trans H--H coupling constants in CH/sub 2/= C/sup 13/HX. The treatment developed for methanes was extended to J/sub CH/ in substituted ethylenes and to the Si/sup 29/--H coupling in silanes. For the former, introduction of the s-electron affinities, obtained from the values of J/ sub CH/(CH/sub 3/X) observed for the substituted methanes, led to the result that J/sub CH/(CH/sub 2/ = C/sup 13/HX) = J/sub CH (CH/sub 2/ = CH/su b 2/) + d in this manner are systematically larger than those observed, which implies that there is a small, negative pi -electron cortribution of 5 to 10 cps to J/sub CH/ (CH/sub 2/=C/sup 13/HX). Such a contribution is compatible with current theories for proton and C/sup 13/ hyperfine splittings in ESR spectra of free radicals. The Si/sup 29/--H coupling constarts observed in substituted silanes exhibit large, systematic deviations from the simple additivtty found in the methanes. The deviations are explicable qualitatlvely in terms of changes in the Si--H-bond polarity.
- Research Organization:
- Univ. of Illinois, Urbana
- Sponsoring Organization:
- USDOE
- NSA Number:
- NSA-17-008110
- OSTI ID:
- 4725859
- Journal Information:
- Journal of Chemical Physics, Vol. 37, Issue 10; Other Information: Orig. Receipt Date: 31-DEC-63; ISSN 0021-9606
- Publisher:
- American Institute of Physics (AIP)
- Country of Publication:
- Country unknown/Code not available
- Language:
- English
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