Ab Initio Study of the Influence of Trimer Formation on One- and Two-Bond Spin-Spin Coupling Constants Across an X-H-Y Hydrogen Bond: AH:XH:YH3 Complexes for A, X = 19F, 35Cl and Y = 15N,31P
Ab initio equation-of-motion coupled-cluster singles and doubles (EOM-CCSD) calculations have been carried out to investigate the effect of a third polar near-neighbor on one-bond (1JX-H and 1hJH-Y) and two-bond (2hJX-Y) spin−spin coupling constants in AH:XH:YH3complexes, where A and X are 19F and 35Cl and Y is either 15N or 31P. The changes in both one- and two-bond spin−spin coupling constants upon trimer formation indicate that the presence of a third molecule promotes proton transfer across the X−H−Y hydrogen bond. The proton-shared character of the X−H−Y hydrogen bond increases in the order XH:YH3 < ClH:XH:YH3 < FH:XH:YH3. This order is also the order of decreasing shielding of the hydrogen-bonded proton and decreasing X−Y distance, and is consistent with the greater hydrogen-bonding ability of HF compared to HCl as the third molecule. For all complexes, the reduced X−H and X−Y spin−spin coupling constants (1KX-H and 2hKX-Y) are positive, consistent with previous studies of complexes in which X and Y are second-period elements in hydrogen-bonded dimers. 1hKH-Y is, as expected, negative in these complexes which have traditional hydrogen bonds, except for ClH:FH:NH3 and FH:FH:NH3. In these two complexes, the F−H−N hydrogen bond has sufficient proton-shared character to induce a change of sign in 1hKH-Y. The effects of trimer formation on spin−spin coupling constants are markedly greater in complexes in which NH3 rather than PH3 is the proton acceptor.