Department of Chemistry, University of Georgia, Athens, Georgia 30602, United States.
J Phys Chem A. 2012 Nov 29;116(47):11477-91. doi: 10.1021/jp309037d. Epub 2012 Nov 13.
Small carbocations are of longstanding interest in mass spectrometry, organic chemistry and astrophysics, but there are few measurements of their spectroscopy in the gas phase. Of existing infrared measurements, few are available across the full range of IR frequencies. In new work described here, a pulsed-discharge supersonic-nozzle ion source produces higher densities of carbocations at low temperatures (20-100 K). Mass-selected photodissociation spectroscopy and the method of rare gas "tagging", together with new broadly tunable infrared OPO lasers, produce IR spectra for a variety of small carbocations including C(2)H(3)(+), C(2)H(5)(+), C(3)H(3)(+), C(3)H(5)(+), the tert-butyl cation, protonated benzene, and protonated naphthalene. Spectra in the frequency range 600-4500 cm(-1) provide new IR data for these ions and evidence for the presence of coexisting isomeric structures (e.g., C(3)H(3)(+) is present as both cyclopropenyl and propargyl). Protonated naphthalene has bands at 6.2, 7.7, and 8.6 μm matching prominent features in the interstellar unassigned infrared (UIR) emission spectra.
小碳正离子在质谱学、有机化学和天体物理学中一直备受关注，但在气相中对其光谱的测量却很少。在现有的红外测量中，很少有能够覆盖整个红外频率范围的。在本文所描述的新工作中，脉冲放电超声速喷嘴离子源在低温（20-100 K）下产生更高密度的碳正离子。质量选择光解光谱学和稀有气体“标记”方法，以及新的宽调谐红外 OPO 激光器，为包括 C(2)H(3)(+)、C(2)H(5)(+)、C(3)H(3)(+)、C(3)H(5)(+)、叔丁基阳离子、苯质子化和萘质子化在内的各种小碳正离子产生红外光谱。在 600-4500 cm(-1) 的频率范围内的光谱为这些离子提供了新的红外数据，并证明了共存的异构结构的存在（例如，C(3)H(3)(+) 同时以环丙烯基和炔丙基的形式存在）。萘质子化在 6.2、7.7 和 8.6 μm 处有带，与星际未分配红外（UIR）发射光谱中的显著特征相匹配。
