Department of Chemical Sciences, Tata Institute of Fundamental Research, Homi Bhabha Road, Colaba, Mumbai 400 005.
J Phys Chem A. 2009 Nov 19;113(46):12763-73. doi: 10.1021/jp907658w.
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.
色氨酸（吲哚和 3-甲基吲哚）和蛋氨酸（二甲硫醚，Me(2)S）的模型化合物的 N-H...S 氢键复合物已通过共振双光子电离（R2PI）、共振离子陷红外光谱（RIDIRS）和荧光陷红外光谱（FDIRS）等实验技术以及从头算电子结构计算、原子在分子（AIM）、自然键轨道（NBO）和能量分解分析等计算方法进行了表征。结果与 N-H...O（M.H(2)O；M=吲哚，3-甲基吲哚）sigma 型和 N-H...Phi（M.苯）pi 型氢键复合物进行了比较。结果表明，N-H...S 氢键复合物的 S(1)-S(0)带起源红移与受体的极化率而非质子亲和力密切相关，这与大多数 X-H...Y（X、Y=O、N、卤素等）氢键系统中观察到的趋势相反。N-H...S HB 团簇（Me(2)S 作为 HB 受体）中 N-H 伸缩振动频率的红移发现比 N-H...O 氢键复合物（H(2)O 作为 HB 受体）大 1.8 倍，尽管两个复合物的结合能相当。对所有 N-H...S 氢键复合物的能量分解分析表明，相关（或色散）能对总结合能有显著贡献。指出 N-H...S 复合物的结合能也与吲哚苯复合物相当，后者完全由色散相互作用主导。原子在分子（AIM）和自然键轨道（NBO）分析表明，氢键相互作用中存在重要的静电分量。与 N-H...O 氢键复合物相比，N-H...S 复合物的稳定性能的色散贡献更大，N-H 伸缩的红移也更大，这使得吲哚二甲硫醚复合物在同时受到色散力和静电力的影响方面具有独特性。为了进行比较，指出在对甲酚.Me(2)S 和对甲酚.H(2)O 复合物的情况下，O-H...S 和 O-H...O 氢键复合物中 O-H 伸缩的红移几乎相同（J. Chem. Phys. 2008, 128, 184311. 和 J. Phys. Chem. A 2009, 113, 5633 - 5643）。这表明 N-H...S 氢键的强度强于 N-H...O 氢键。首次在喷射冷却条件下观察到 N-H...S 氢键，本研究最有趣的特点是，N-H...S“sigma 型”氢键在总相互作用能中，色散主导方面的行为更类似于 C-H...Phi 或 N-H...Phi“pi 型”氢键。
