Nature of the N-H•••S Hydrogen Bond

The N-H center dot center dot center dot S hydrogen-bonded complexes of the model compounds of tryptophan (indole and 3-methylindole) and methionine (dimethyl sulfide, Me2S) 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 center dot center dot center dot O (M center dot H2O; M = indole, 3-methyl indole) sigma-type and N-H center dot center dot center dot Phi (M center dot benzene) pi-type hydrogen-bonded complexes. It was shown that the S-1-S-0 band origin red shifts in the N-H center dot center dot center dot 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 center dot center dot center dot Y (X, Y = O, N, halogens, etc.) hydrogen-bonded systems. The red shift in the N-H stretching frequency in the N-H center dot center dot center dot S HB clusters (Me2S as HB acceptor) was found to be 1.8 times greater than that for the N-H center dot center dot center dot O hydrogen-bonded complexes (H2O as HB acceptor), although the binding energies for the two complexes were comparable. The energy decomposition analyses for all of the N-H center dot center dot center dot 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 center dot center dot center dot 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 center dot center dot center dot O hydrogen-bonded complexes makes the indole center dot 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 center dot center dot center dot S and O-H center dot center dot center dot O hydrogen-bonded complexes were almost the same in the case of para-cresol center dot Me2S and para-cresol center dot H2O 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 center dot center dot center dot S hydrogen bonding is stronger than the N-H center dot center dot center dot O hydrogen bonding. The N-H center dot center dot center dot 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 center dot center dot center dot S "sigma-type" hydrogen bonding behaves more like C-H center dot center dot center dot Phi or N-H center dot center dot center dot Phi "pi-type" hydrogen bonding in regard to the dispersion domination in the total interaction energy.