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

TLDR: 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 hydrogen-Bonded systems.

Abstract: The N−H···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···O (M·H2O; M = indole, 3-methyl indole) σ-type and N−H···Φ (M·benzene) π-type hydrogen-bonded complexes. It was shown that the S1−S0 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 cluster...