15min:

J. E. DEL BENE, Department of Chemistry, Youngstown State University, Youngstown, Ohio 44555; J. ELGUERO, Instituto de Química Médica,CSIC, Juan de la Cierva, 3, E-28006 Madrid, Spain.

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 which are indicative of the nature of the hydrogen bond. All two-bond spin-spin coupling constants (^2hJ_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 (^1hJ_H-N) are positive (^1hK_H-N are negative) for traditional hydrogen bonds, but ^1hJ_H-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 ^2hJ_N-N and ¹J_N-H, and the signs of ^1hJ_H-N for those complexes that are too large for EOM-CCSD calculations.