SPIN-SPIN COUPLING CONSTANTS ACROSS N-H$^+$-N HYDROGEN BONDS

Abstract

A systematic ab initio EOM-CCSD study of $^{15}$N-$^{15}$N and $^{15}$N-$^1$H spin-spin coupling constants has been carried out for a series of complexes formed from 11 nitrogen bases with experimentally measured proton affinities. When these complexes are arranged in order of increasing proton affinity of the proton-acceptor base, and for each proton acceptor, increasing order of proton affinity of the protonated N-H donor, trends in distances and signs of coupling constants are evident which are indicative of the nature of the hydrogen bond. All two-bond spin-spin coupling constants ($^{2h}$J$_{N-N}$) are positive, and decrease as the N-N distance increases. All one-bond N-H coupling constants ($^1$J$_{N-H}$) are negative ($^1$K$_{N-H}$ are positive). $^1$J$_{N-H}$ is related to the N-H distance and the hybridization of the donor N atom. One bond H...N coupling constants ($^{1h}$J$_{H-N}$) are positive ($^{1h}$K$_{H-N}$ are negative) for traditional hydrogen bonds, but $^{1h}$J$_{H-N}$ becomes negative when the hydrogen bond acquires sufficient proton-shared character. The N-N and H...N distances at which $^{1h}$J$_{H-N}$ changes sign are approximately 2.71 and 1.62 \AA, respectively. Predictions are made of the values of $^{2h}$J$_{N-N}$ and $^1$J$_{N-H}$, and the signs of $^{1h}$J$_{H-N}$ for those complexes that are too large for EOM-CCSD calculations.