Conformational dependence of long-range spin-spin coupling constants in 2,6-dichlorobenzylfluoride

Abstract

Double resonance experiments on the proton magnetic resonance spectrum of 2,6-dichlorobenzyltl~loridc yield the signs of the long-range coupling constants between the ring protons and the fluorinc nuclei and protons in the fl~~oromethyl group. The signs and nlagnitudes of the long-range couplings are discussed in terms of their dependence on the conformation of the fluoromethyl group.

Signs of methyl–methyl proton spin–spin coupling constants in xylene derivatives (interbenzylic coupling)

“…Barfield and Chakrabarti have suggested (7) that the magnitude of the n-electron contribution to JcH3,sH3m calculated by his valence-bond method is too large because of errors inherent in that method (over-correlation of electrons in the same subset of an alternant system). Both experimental (8,12,13) and theoretical (25) considerations suggest the existence of substantial oelectron contributions to JH,,,,". Comparing the value of IJ,H3,cH3mI found in 2,4,6-trimethylbenzaldehyde with the observed range of I JH,cH3mJ in toluenes in the light of the methyl group replacement technique for assessing K-electron contributions to J values (26), it has been concluded (4) that o-electron contributions to JH,,,," amount to approximately one-third of the observed magnitude.…”

“…A substantial measure of agreement between theory (5-7) and experiment (1)(2)(3)(4)(8)(9)(10)(11) has led to the conclusion that the signs and magnitudes of the ortho and para benzylic coupling constants are primarily determined by n-electron contributions, transmitted to the benzylic proton by o -7 c exchange polarization. For the meta coupling constant, although agreement between theory and experiment is reasonable for an unsubstituted methyl group (1-7), experimental results indicate substantial o-electron contributions, at least in situations where a benzylic proton is constrained to lie in the nodal plane of the n-electron system (8,12,13).…”

Hydrogen bonding and long-range proton coupling constants in some ortho disubstituted aniline derivatives. Intramolecular proton exchange and degree of nonplanarity of the amino group

“…0.04 Hz compares with 0.37 $. 0.02 Hz in benzaldehyde (2) and a number of its halogen and nitro derivatives (9), and with 0.36 Hz in toluene (1) and some of its derivatives (3,4,12,43). In all these cases, 5~:*XH is evidently largely independent of the state of hybridization of X or of the substituents in the ring.…”

“…The coupling to a ring proton in the para position over six bonds, 6J, is apparently dominated by a o-n mechanism and its magnitude is therefore at a maximum when the C-H bond in the side-chain lies perpendicular to the plane of the benzene ring (10,11). The coupling over five bonds to the meta proton, ,J, appears to have contributions from both o and o-7c mechanisms (9,12). It is largest when the two C-H bonds are arranged as in 1 (X = C, N, or 0 ) and is smallest in 2 (8,9,(12)(13)(14).…”

“…The coupling over five bonds to the meta proton, ,J, appears to have contributions from both o and o-7c mechanisms (9,12). It is largest when the two C-H bonds are arranged as in 1 (X = C, N, or 0 ) and is smallest in 2 (8,9,(12)(13)(14). Neither 'J nor ,J are markedly &, o., sensitive to the electronic nature of other ring substituents (1,3,7,9) but may be influenced by the steric effects of the substituents on conforn~ational preferences.…”