Origin of significant solvent effects on 1J(CC) spin-spin coupling in some acetylenes: hydrogen bonding and solvent polarity

It is demonstrated that some acetylenes, those of the R C-CH structure, display anomalously high sensitivity to solvent effects of their (1)J(C C) coupling while R-C CR acetylenes fail to show that. The solvent-induced variation in the latter coupling does not exceed 3 Hz; this seems to be the upper limit of variation of any J(CC) and J(CH) coupling in the molecular system studied which included: acetylene (in 13 solvents), phenylacetylene (in 12 solvents), 1-phenylpropyne, and 2-hexyne (two solvents each), and the only exceptions are (1)J(CC) in acetylene, which is shown to vary within about 13 Hz, and that in phenylacetylene where the range amounts to about 8 Hz. These apparent anomalies are explained in the present study in terms of two effects of prime importance, solvent polarity and the solute-to-solvent hydrogen bonds where the CH moiety in R-C CH acetylenes acts as a donor of hydrogen bonds to acceptor sites in the solvent concerned. Copyright (C) 2009 John Wiley & Sons, Ltd.