Department of Chemistry, University of Ottawa, 10 Marie-Curie, Ottawa, Canada K1N 6N5.
J Phys Chem A. 2010 Jan 21;114(2):867-78. doi: 10.1021/jp9096524.
Ab initio molecular orbital calculations demonstrate that ionizing alkyl nitriles produces a dramatic geometry change involving lengthening of a C-CH(2)CN bond. The experimental determination of the adiabatic ionization energy of these species is thus very difficult. In addition, there are generally low barriers for 1,2-H shift reactions in the molecular ions leading to RCHCHN(+) and RCHCNH(+) isomers, which makes generating pure ionized alkyl nitrile in a mass spectrometer a challenge. Threshold photoelectron spectroscopy and threshold photoelecton photoion coincidence spectroscopy were employed to study the ionization and dissociation of two alkyl nitriles, in particular, pentanenitrile and 2,2-dimethylpropanenitrile. Threshold ionization is shown to result not in the respective molecular ions, but rather in isomeric forms, resulting in dissociation thresholds that lie below the calculated adiabatic ionization energies of the two molecules. Appearance energies for all observed fragment ions are reported and compared to available literature values. Charge separation in the dissociation of doubly ionized 2,2-dimethylpropanenitrile is observed as fragment-ion time-of-flight peak broadening at high photon energies.
从头算分子轨道计算表明，烷基腈的电离会导致涉及 C-CH(2)CN 键伸长的剧烈几何变化。因此，这些物质的绝热电离能的实验测定非常困难。此外，在分子离子中通常存在 1,2-H 迁移反应的低势垒，导致 RCHCHN(+)和 RCHCNH(+)异构体，这使得在质谱仪中产生纯电离的烷基腈具有挑战性。采用阈光电离光谱和阈光电离-光电子符合光谱研究了两种烷基腈，特别是戊腈和 2,2-二甲基丙腈的电离和离解。结果表明，阈电离不是产生相应的分子离子，而是产生异构形式，导致离解阈低于这两个分子的计算绝热电离能。报道了所有观察到的碎片离子的出现能，并与可用的文献值进行了比较。在双离子化 2,2-二甲基丙腈的离解中观察到电荷分离，表现为在高光子能量下碎片离子飞行时间峰展宽。
