Computed Spin−Spin Coupling Constants (1JX-Y) in Molecules HmX−YHnfor X and Y =13C,15N, and31P: Comparisons with Experiment and Insights into the Signs of1JX-Y

Abstract

One-bond X-Y spin-spin coupling constants ( 1 J X - Y ) for 18 H m X-YH n molecules, with X and Y = 1 3 C, 1 5 N, and 3 1 P, have been computed using the equation-of-motion coupled-cluster singles and doubles method. The molecules investigated include all possible combinations of these three elements bonded with single, double, and triple bonds. The computed coupling constants are in good agreement with experiment over a range that extends from -250 to +200 Hz. With only two exceptions, the sign of the Fermi-contact (FC) term is the same as the sign of 1 J X - Y , but the FC term may or may not be a good quantitative estimate of 1 J X - Y . When reduced spin-spin coupling constants ( 1 K X - Y ) are used for comparing coupling constants involving different atoms, a linear relationship is observed between 1 K X - N and 1 K X - P . The signs of 1 J X - Y for approximately half of the molecules included in this study are exceptions to the Dirac vector model. The recently proposed NMR triplet wave function model has been used to provide insight into the variation of the signs of these one-bond spin-spin coupling constants.