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 15N−11B, 11B−1H, and 15N−1H spin−spin coupling constants. For comparison purposes, 13C−13C coupling constants were evaluated for corresponding isoelectronic molecules RCCH and RCCR. The absolute values of 1J(N−B) and1J(C−C) increase in the series HBNR and RCCH as the σ-electron-withdrawing ability of R increases. In contrast, NH2 substitution at B leads to a decrease in the absolute value of 1J(N−B), but OH and F substitution increase 1J(N−B). Disubstitution has dramatically different effects on coupling constants for RCCR and RBNR. In the former, 1J(C−C) more than doubles relative to the corresponding RCCH and HCCH; in the latter, disubstitution of OH and F decreases 1J(N−B) relative to the corresponding monosubstituted derivatives, while NH2substitution increases 1J(N−B). Changes in one- and two-bond B−H and N−H coupling constants upon substitution are similar to changes observed for 1J(N−B) in the corresponding monosubstituted derivatives RBNH and HBNR.