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• W2071553905 abstract "An analytical LCAO MO theory of the Fermi contact term in reduced nuclear spin coupling constants 1K(A-F) has been developed for substituted planar trigonal AF3−kL′K and tetrahedral AF4−kL′K, K = 1 to 3, main group element fluorides. The new model is a perturbation extension of the Pople-Santry (P-S) theory of 1K(A-F) for unsubstituted polyhedra AFm. The differences between diagonal matrix elements, δα′s = 〈sL′|H|sL′〉 − 〈sF|H|sF〉 and δα′p = 〈σL′|H|σL′〉 − 〈σF|H|σF〉, have been chosen as perturbations and first-order corrections to the P-S terms (both linear and quadratic in δα′s and δα′p) have been obtained in explicit form. It was shown that both central atoms A and substituents L′ should be divided into two groups displaying quite different regularities of 1K(A-F). Namely, carbon differs from all other atoms A (B, Si, Ge, etc.) in relative energies of the SA and pF orbitals and H, CH3, C6H5, SiH3, etc., differ from other substituents L′ (halogens, OR, NR2, SR, PR2, etc.) in lack of a lone ns2 pair. We suggest naming these two kinds of substituents “one-pronged” and “two-pronged,” respectively. It was shown that changes in 1K(A-F) will be negative and increase monotonically in absolute value as the σ donor ability of the L′ and their numbers k increase only for two-pronged substituents L′ such as halogens. This monotonic increase should be more or less additive for A = B, Si but for A = C noticeable positive deviations from additivity should take place. One-pronged L′ such as H or CH3 can cause nonmonotonic changes in 1K(A-F) along the series AF4−kL′k, k = 0 to 3 for all atoms A. Moreover, for A = C the resulting changes in 1K(C-F) may even be substantially positive. Model results agree with experiment and permit a number of predictions to be made. Extended Hückel calculations were performed as an independent check of model conclusions. All of the expected trends were found but only after it was discovered that a key factor was the magnitude of the overlap between the fluorine 2s orbital and the central atom ns orbital." @default.
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• W2071553905 date "1980-04-01" @default.
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• W2071553905 title "Perturbation theory and spin coupling constants in substituted compounds. II. Main group tetrahedral and planar trigonal fluorides" @default.
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• W2071553905 doi "https://doi.org/10.1016/0022-2364(80)90160-2" @default.
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