Spin-spin coupling constants for iminoboranes RBNH, HBNR, and RBNR and comparisons with corresponding isoelectronic acetylenes RCCH and RCCR, for R = H, CH3, NH2, OH, and F

Ab initio equation-of-motion coupled-cluster singles and doubles method calculations have been performed on iminoboranes RBNH, HBNR, and RBNR, for R = H, CH3, NH2, OH, and F, to evaluate substituent effects on one- and two-bond N-15-B-11, B-11-H-1, and N-15-H-1 spin-spin coupling constants. For comparison purposes, C-13-C-13 coupling constants were evaluated for corresponding isoelectronic molecules RCCH and RCCR. The absolute values of (1)J(N-B) and (1)J(C-C) increase in the series HBNR and RCCH as the sigma-electron-withdrawing ability of R increases. In contrast, NH2 substitution at B leads to a decrease in the absolute value of (1)J(N-B), but OH and F substitution increase (1)J(N-B). Disubstitution has dramatically different effects on coupling constants for RCCR and RBNR. In the former, (1)J(C-C) more than doubles relative to the corresponding RCCH and HCCH; in the latter, disubstitution of OH and F decreases (1)J(N-B) relative to the corresponding monosubstituted derivatives, while NH2 substitution increases (1)J(N-B). Changes in one- and two-bond B-H and N-H coupling constants upon substitution are similar to changes observed for (1)J(N-B) in the corresponding monosubstituted derivatives RBNH and HBNR.