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, ⁿJ(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.