Lone-Pair Orientation Effect of an α-Oxygen Atom on (1)JCC NMR Spin-Spin Coupling Constants in o-Substituted Phenols. Experimental and DFT Study.

The well-known N lone-pair orientation effect on (1)JCC spin-spin coupling constants (SSCCs) in oximes and their derivatives was used to study how negative hyperconjugative interactions of type LP1(O) → σ*CC depend on ortho interactions involving the OH group. This study demanded the following analyses: (i) a qualitative estimation of how (1)JCC SSCCs are affected by hyperconjugative interactions, (ii) a study of similar stereochemical effects to those in oximes, but in (1)JC1C2 and (1)JC1C6in a series of 2-substituted phenols, and (iii) a quantitative estimation, with the natural bond order approach, of some key electron delocalization interactions. A few unexpected results are quoted. LP1(O) → σ*CC interactions are affected by proximity interactions as follows: (a) they are enhanced by hydrogen bonds transferring charge into the (O-H)* antibonding orbital; (b) they are enhanced by proximity interactions of type LP1(O)···H-C; (c) they are inhibited by interactions of type LP(O1)···H-O. Consequences of these observations are discussed.

Substituent effects on scalar J(13C, 13C) couplings in pyrimidines. An experimental and DFT study.

One- two- and three 13C, 13C (n = 1, 2, 3) scalar couplings, (n)J(C,C) in a set of pyrimidine derivatives were studied both experimentally at natural abundance and theoretically by their DFT calculation of all four contributions. Trends of non-contact terms are discussed and substituent effects are rationalized, comparing some of them with the corresponding values in benzene and pyridine. Although substituent effects on non-contact terms are relatively important, the whole trend is dominated by the Fermi contact term. According to the current literature, substituent effects on 1J(C,C) couplings in benzene derivatives are dominated by the inductive effect, which, apparently, is also the case in nitrogen heteroaromatic compounds. However, some differences observed in this work for substituent effects on 1J(C,C) couplings in pyrimidine derivatives suggest that in the latter type of compounds substituent effects can be affected by the orientation of the ring nitrogen lone pairs.