Department of Chemical Sciences, Tata Institute of Fundamental Research, Homi Bhabha Road, Colaba, Mumbai, India 400 005.
J Phys Chem A. 2010 Jul 8;114(26):6944-55. doi: 10.1021/jp102346n.
In this paper, the effect of alkyl substitution at the hydrogen bond acceptor and its chain length on the strength and nature of hydrogen bonding is presented. In the present study we combine both experimental and computational methods to investigate the characteristics of O-H...O and O-H...S hydrogen bonding in the complexes of p-cresol (p-CR) with methanol (MeOH), ethanol (EtOH), methanethiol (MeSH), and ethanethiol (EtSH). The results indicate that, with an increase in the alkyl chain length, both O-H...O hydrogen bonding and O-H...S hydrogen bonding become stronger. Energy decomposition analysis emphasizes the dispersive nature of O-H...S hydrogen bonding. In addition, it revealed that the dispersion energy contribution in O-H...O hydrogen bonding increases with an increase in the alkyl chain length of the hydrogen bond acceptor. In the case of O-H...S hydrogen bonding, however, the dispersion energy contribution decreased from 68% for the H(2)S complex to 53% in the case of the MeSH complex; it remained unchanged with a further increase of the alkyl chain length. It was also observed that the red shifts in the OH stretching frequency did not correlate with the proton affinities of the O-centered acceptor vs the S-centered H-bond acceptor, in contrast with the known trend for the conventional H-bonded complexes. The IR/UV double resonance study enabled the assignments of the anti and gauche conformers of p-CR-EtOH and p-CR-EtSH.
本文介绍了氢键受体上的烷基取代及其链长对氢键强度和性质的影响。在本研究中，我们结合实验和计算方法研究了对甲酚（p-CR）与甲醇（MeOH）、乙醇（EtOH）、甲硫醇（MeSH）和乙硫醇（EtSH）形成的复合物中 O-H…O 和 O-H…S 氢键的特性。结果表明，随着烷基链长的增加，O-H…O 氢键和 O-H…S 氢键都变得更强。能量分解分析强调了 O-H…S 氢键的分散性质。此外，它揭示了 O-H…O 氢键中色散能贡献随着氢键受体的烷基链长度的增加而增加。然而，在 O-H…S 氢键的情况下，色散能贡献从 H(2)S 配合物的 68%降低到 MeSH 配合物的 53%；随着烷基链长度的进一步增加，它保持不变。还观察到，OH 伸缩振动频率的红移与 O 中心受体与 S 中心 H 键受体的质子亲和力不相关，这与传统氢键复合物的已知趋势相反。IR/UV 双共振研究能够对 p-CR-EtOH 和 p-CR-EtSH 的反式和 gauche 构象进行分配。
