Systematic ab Initio Study of ~(15)N-~(15)N and ~(15)N-~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

摘要

A systematic ab initio EOM-CCSD study of ~(15)N-~(15)N and ~(15)N-~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 (~(2h)J_(N-N))are positive and decrease as the N-N distance increases.All one-bond N-H coupling constants (~1J_(N-H))are negative (~1K_(N-H)are positive),~1J_(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_(N-H))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 A,respectively.Predictions are made of the values of ~(2h)J_(H-N)and ~1J_(H-N),and the signs of ~(1h)J_(H-N),for those complexes that are too large for EOM-CCSD calculations.