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The CNDO/2 and INDO MO FPT values for 5Jm(F,CH3) in 3-fluoro- and 3,5-difluorotoluene are exactly reproduced by Acos2θ + Bsin2θ + Csin2(θ/2). Here θ is the angle by which the α C—H bond twists out of the benzene plane. Adjustment of A,B, and Cto give an agreement with experiment for 3,5-difluorotoluene yields an equation, best considered empirical, which is tested by 5J(F,CH) in 3,5-difluoroethylbenzene and 3,5-difluoroisopropylbenzene. The equation reproduces 5J(F,CH) in these two compounds, the values of being derived from 6Jp(H,CH), the spin–spin coupling constants over six bonds between the α and pararing protons. 5J(F,CH) is obtained for the asymmetrical compounds, 2,3-difluorobenzylidene diacetate and 2-bromo-5-fluorobenzylidene diacetate. It is shown how 5J(F,CH) in the latter can discriminate between two conformers, each of which, on the basis of 6J(H,CH), will be characterized by a large degree of torsion about the bond.

The 1H nuclear magnetic resonance spectra and their analyses are reported for benzyltrimethyl X (X = C, Si, Ge, Sn, Pb) and for the 2,6-dichloro derivatives where X = Si, Ge, Sn, Pb. The steric barrier to rotation about the Csp2—Csp3bond for X = C, based on the long-range proton–proton coupling constant over six bonds, 6J(H,CH2), must be greater than 20 kJ/mol. Unexpectedly, 6J(H,CH2) implies that the barriers for X = Si, Ge, Sn, Pb have relatively minor steric components. If these barriers are dominated by hyperconjugative interactions of the C—X bond with the aromatic π system, then they are probably mainly twofold in nature. Furthermore, their magnitude is likely to be near 9.5 kJ/mol for X = Si, Ge, Sn and may be as high as 12 kJ/mol for X = Pb. Keywords: NMR, benzyl X(CH3)3, X = C, Si, Ge, Sn, Pb; NMR, conformations; NMR, hyperconjugation.

6J(H,CHO), the long-range coupling constant between the aldehydic and paraprotons in benzaldehyde, has not been detected, possibly because the σ–π interaction giving rise to a negative coupling is intrinsically rather small and because the internal barrier to rotation about the bond is large. However, 6J(H,CHO) in some metasubstituted derivatives is actually positive and as large as 0.09 Hz in 3,5-difluorobenzaldehyde; the barrier to internal rotation in this molecule is some 4 kJ/mol lower than in the parent molecule. The magnitude of 6J(H,CHO) in these derivatives correlates well with σR.STvalues, a recent set of substituent constants derived from 13C nuclear magnetic resonance chemical shifts of the β carbon in styrene derivatives. It is hypothesized that the substituents with negative σR.STvalues stabilize an ionic valence bond structure that has a positive 6J(H,CHO). A brief discussion of 4J(H,CHO) in some of these molecules is also presented. Keywords: NMR, spin coupling NMR, benzaldehyde.