Park Min Hee, Choi Kyo-Won, Choi Sunyoung, Kim Sang Kyu, Choi Young S
Department of Chemistry and School of Molecular Sciences (BK21), Korea Advanced Institute of Science and Technology, Daejeon 301-750, Korea.
J Chem Phys. 2006 Aug 28;125(8):084311. doi: 10.1063/1.2338322.
Mass-resolved two-photon (1+1) resonance-enhanced multiphoton ionization spectra of the ~A-X transitions of various methylamine isotopomers (CH(3)NHD, CD(3)NH(2), CD(3)NHD, and CD(3)ND(2)) cooled in the supersonic jet expansion have been measured and analyzed. The band analysis using the Hamiltonian for the internal and overall rotational motions provides the accurate vibrational band positions, allowing for unambiguous assignments for all observed vibrational bands of methylamine isotopomers in the ~A states. Amino wagging (nu(9)) and methyl rocking (nu(7)) modes are found to be Franck-Condon active, and associated anharmonicity constants are precisely determined to give the detailed shape of the potential energy surface in the vicinity of the minimum electronic molecular structure. The barrier height for the nearly free internal rotation about the C-N bond in the ~A state is calculated to be strongly dependent on the excitation of the other higher-frequency vibrational modes, and it is found that the trend is consistent with the experiment. Experimentally measured spectroscopic constants are compared with ab initio calculations, confirming all vibronic assignments. Experimental and theoretical results on all possible HD isotopomers of methylamine in this work, with the earlier report on CH(3)NH(2) and CH(3)ND(2) Baek et al., [J. Chem. Phys. 118, 11026 (2003)], provide the complete spectroscopic characterization of the A state of methylamine.
测量并分析了在超声速射流膨胀中冷却的各种甲胺同位素异构体（CH(3)NHD、CD(3)NH(2)、CD(3)NHD和CD(3)ND(2)）的A - X跃迁的质量分辨双光子(1 + 1)共振增强多光子电离光谱。使用哈密顿量对内部和整体转动运动进行的能带分析给出了准确的振动带位置，从而能够明确归属甲胺同位素异构体在A态中所有观测到的振动带。发现氨基摇摆（ν(9)）和甲基摇摆（ν(7)）模式是弗兰克 - 康登活性的，并精确确定了相关的非谐常数，以给出在最小电子分子结构附近势能面的详细形状。计算得出在~A态中围绕C - N键几乎自由的内旋转的势垒高度强烈依赖于其他高频振动模式的激发，并且发现该趋势与实验一致。将实验测量的光谱常数与从头算计算结果进行比较，证实了所有振转归属。本工作中关于甲胺所有可能的HD同位素异构体的实验和理论结果，以及之前Baek等人[《化学物理杂志》118, 11026 (2003)]对CH(3)NH(2)和CH(3)ND(2)的报道，提供了甲胺A态的完整光谱表征。
