Systematic ab initio study of15N-15N and 15N-1H spin-spin coupling constants across N-H +-N hydrogen bonds: Predicting N-N and N-H coupling constants and relating them to hydrogen bond type

Abstract

A systematic ab initio EOM-CCSD study of 15N-15N and 15N-1H 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 that are indicative of the nature of the hydrogen bond. All two-bond spin-spin coupling constants (2hJN-N) are positive and decrease as the N-N distance increases. All one-bond N-H coupling constants 1J N-H) are negative 1KN-H are positive). 1JN-H is related to the N-H distance and the hybridization of the donor N atom. One-bond H⋯N coupling constants ( 1hJH-N) are positive (1hKH-N are negative) for traditional hydrogen bonds, but 1hJH-N becomes negative when the hydrogen bond acquires sufficient proton-shared character. The N-N and H⋯N distances at which 1hJ H-N changes sign are approximately 2.71 and 1.62 Å, respectively. Predictions are made of the values of 2hJN-N and 1JN-H, and the signs of 1hJH-N, for those complexes that are too large for EOM-CCSD calculations. © 2006 American Chemical Society.