Department of Chemistry, University of Georgia, Athens, Georgia 30602-2556, USA.
J Chem Phys. 2010 Feb 7;132(5):051101. doi: 10.1063/1.3298881.
C(3)H(3)(+) ions produced with a pulsed discharge source and cooled in a supersonic beam are studied with infrared laser photodissociation spectroscopy in the 800-4000 cm(-1) region using the rare gas tagging method. Vibrational bands in the C-H stretching and fingerprint regions confirm the presence of both the cyclopropenyl and propargyl cations. Because there is a high barrier separating these two structures, they are presumed to be produced by different routes in the plasma chemistry; their relative abundance can be adjusted by varying the ion source conditions. Prominent features for the cyclopropenyl species include the asymmetric carbon stretch (nu(5)) at 1293 cm(-1) and the asymmetric C-H stretch (nu(4)) at 3182 cm(-1), whereas propargyl has the CH(2) scissors (nu(4)) at 1445, the C-C triple bond stretch (nu(3)) at 2077 and three C-H stretches (nu(2), nu(9), and nu(1)) at 3004, 3093, and 3238 cm(-1). Density functional theory computations of vibrational spectra for the two isomeric ions with and without the argon tag reproduce the experimental features qualitatively; according to theory the tag atom only perturbs the spectra slightly. Although these data confirm the accepted structural pictures of the cyclopropenyl and propargyl cations, close agreement between theoretical predictions and the measured vibrational band positions and intensities cannot be obtained. Band intensities are influenced by the energy dependence and dynamics of photodissociation, but there appear to be fundamental problems in computed band positions independent of the level of theory employed. These new data provide infrared signatures in the fingerprint region for these prototypical carbocations that may aid in their astrophysical detection.
用脉冲放电源产生的 C(3)H(3)(+)离子，并在超音速束中冷却，用稀有气体标记法在 800-4000 cm(-1) 区域用红外激光光解光谱进行研究。在 C-H 伸缩和指纹区域的振动带证实了环丙烯基和炔丙基阳离子的存在。由于这两种结构之间存在很高的势垒，因此它们被认为是在等离子体化学中通过不同的途径产生的；通过改变离子源条件可以调整它们的相对丰度。环丙烯基物种的突出特征包括 1293 cm(-1)处的不对称碳伸缩（nu(5)）和 3182 cm(-1)处的不对称 C-H 伸缩（nu(4)），而炔丙基具有 CH(2)剪刀（nu(4)）在 1445 cm(-1)，C-C 三键伸缩（nu(3)）在 2077 cm(-1)，以及三个 C-H 伸缩（nu(2)、nu(9)和 nu(1)）在 3004、3093 和 3238 cm(-1)。对带有和不带有氩标记的两种异构体离子的振动光谱进行密度泛函理论计算，定性地再现了实验特征；根据理论，标记原子仅略微扰动光谱。尽管这些数据证实了环丙烯基和炔丙基阳离子的公认结构图像，但理论预测与测量的振动带位置和强度之间无法获得完全一致。带强度受到光解的能量依赖性和动力学的影响，但在不考虑所使用理论水平的情况下，计算出的带位置似乎存在根本问题。这些新数据为这些原型碳阳离子在指纹区域提供了红外特征，这可能有助于它们在天体物理中的检测。
