Nature of the N-H...S hydrogen bond.

The N-H...S hydrogen-bonded complexes of the model compounds of tryptophan (indole and 3-methylindole) and methionine (dimethyl sulfide, Me(2)S) have been characterized by a combination of experimental techniques like resonant two-photon ionization (R2PI), resonant ion dip infrared spectroscopy (RIDIRS), and fluorescence dip infrared spectroscopy (FDIRS) and computational methods like ab initio electronic structure calculations, atoms-in-molecules (AIM), natural bond orbital (NBO), and energy decomposition analyses. The results are compared with the N-H...O (M.H(2)O; M = indole, 3-methyl indole) sigma-type and N-H...Phi (M.benzene) pi-type hydrogen-bonded complexes. It was shown that the S(1)-S(0) band origin red shifts in the N-H...S hydrogen-bonded complexes correlated well with the polarizability of the acceptor rather than their proton affinity, contrary to the trend observed in most X-H...Y (X, Y = O, N, halogens, etc.) hydrogen-bonded systems. The red shift in the N-H stretching frequency in the N-H...S HB clusters (Me(2)S as HB acceptor) was found to be 1.8 times greater than that for the N-H...O hydrogen-bonded complexes (H(2)O as HB acceptor), although the binding energies for the two complexes were comparable. The energy decomposition analyses for all of the N-H...S hydrogen-bonded complexes showed that the correlation (or dispersion) energy has significant contribution to the total binding energy. It is pointed out that the binding energy of the N-H...S complex was also comparable to that of the indole.benzene complex, which is completely dominated by the dispersion interaction. Atoms-in-molcules (AIM) and natural bond orbital (NBO) analyses indicated a nontrivial electrostatic component in the hydrogen-bonding interaction. Greater dispersion contribution to the stabilization energy as well as greater red shifts in the N-H stretch relative to those of N-H...O hydrogen-bonded complexes makes the indole.dimethylsulfide complex unique in regard to the simultaneous influence of both the dispersion and electrostatic forces. For the sake of comparison, it is pointed out that the red shifts in the O-H stretch for O-H...S and O-H...O hydrogen-bonded complexes were almost the same in the case of para-cresol.Me(2)S and para-cresol.H(2)O complexes ( J. Chem. Phys. 2008 , 128 , 184311. and J. Phys. Chem. A 2009 , 113 , 5633 - 5643 ). This suggests that the strength of the N-H...S hydrogen bonding is stronger than the N-H...O hydrogen bonding. The N-H...S hydrogen bonding was observed for the first time using jet-cooled conditions, and the most interesting feature of this study is that N-H...S "sigma-type" hydrogen bonding behaves more like C-H...Phi or N-H...Phi "pi-type" hydrogen bonding in regard to the dispersion domination in the total interaction energy.