Kumar P Padma, Marx Dominik
Lehrstuhl für Theoretische Chemie, Ruhr-Universität Bochum, Germany.
Phys Chem Chem Phys. 2006 Feb 7;8(5):573-86. doi: 10.1039/b513089c. Epub 2005 Dec 2.
A comprehensive study of the properties of protonated methane obtained from ab initio molecular dynamics simulations is presented. Comparing computed infrared spectra to the measured one gives further support to the high fluxionality of bare CH(5)(+). The computational trick to partially freezing out large-amplitude motion, in particular hydrogen scrambling and internal rotation of the H(2) moiety, leads to an understanding of the measured IR spectrum despite the underlying rapid hydrogen scrambling motion that interconverts dynamically structures of different symmetry and chemical bonding pattern. In particular, the fact that C-H stretching modes involving the carbon nucleus and those protons that form the H(2) moiety and the CH(3) tripod, respectively, result in distinct peaks is arguably experimental support for three-center two-electron bonding being operative at experimental conditions. It is proposed that hydrogen scrambling is associated with the softening of a mode that involves the bending of the H(2) moiety relative to the CH(3) tripod, which characterizes the C(s) ground-state structure. The potential energy surface that is mapped on to a two dimensional subspace of internal coordinates provides insight into the dynamical mechanism for exchange of hydrogens between CH(3) tripod and the three-center bonded H(2) moiety that eventually leads to full hydrogen scrambling.
本文展示了一项对从头算分子动力学模拟得到的质子化甲烷性质的全面研究。将计算得到的红外光谱与实测光谱进行比较，进一步支持了裸CH₅⁺的高通量性。尽管存在快速的氢交换运动，这种运动能动态地互变具有不同对称性和化学键模式的结构，但通过部分冻结大幅度运动，特别是氢交换和H₂部分的内旋转的计算技巧，仍能对实测红外光谱有所理解。特别是，分别涉及碳原子核以及形成H₂部分和CH₃三脚架的那些质子的C - H伸缩模式会产生不同的峰，这可以说是在实验条件下三中心两电子键起作用的实验证据。有人提出，氢交换与一种模式的软化有关，该模式涉及H₂部分相对于CH₃三脚架的弯曲，这是Cₛ基态结构的特征。映射到内部坐标二维子空间的势能面，为CH₃三脚架和三中心键合的H₂部分之间氢交换的动力学机制提供了见解，这种氢交换最终导致完全的氢交换。
