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Rotamers of Isoprene: Infrared Spectroscopy in Helium Droplets and Ab Initio Thermochemistry
The Journal of Physical Chemistry A ( IF 2.9 ) Pub Date : 2017-12-28 00:00:00 , DOI: 10.1021/acs.jpca.7b10260
Peter R. Franke 1 , Gary E. Douberly 1
Affiliation  

Isoprene (C5H8) is an abundant, reactive tropospheric hydrocarbon, derived from biogenic emissions. A detailed understanding of the spectroscopy of isoprene is therefore desirable. Isoprene monomer is isolated in helium droplets and its infrared spectrum is measured in the CH stretching region. Anharmonic frequencies are predicted by VPT2+K simulations employing CCSD(T) force fields with quadratic (cubic and quartic) force constants computed using the ANO1 (ANO0) basis set. The vast majority of the spectral features can be assigned to trans-isoprene on the basis of these computations. Some features of the higher energy gauche conformer are also assignable, by comparison to experiments using heated isoprene. Convergent ab initio thermochemistry is presented for the isomerization pathway, for which the partition function explicitly accounts for the eigenstates associated with separate, uncoupled one-dimensional potential surfaces for methyl torsion and internal rotation between rotamers. The respective 0 and 298.15 K trans/gauche energy differences are 2.82 and 2.52 kcal/mol, which implies a room temperature gauche population of 2.8%.

中文翻译:

异戊二烯的旋转器:氦气滴中的红外光谱和从头算热化学

异戊二烯(C 5 H 8)是一种丰富的反应性对流层碳氢化合物,来源于生物成因排放。因此,需要对异戊二烯的光谱学有详细的了解。异戊二烯单体被分离成氦滴,并在CH拉伸区测量其红外光谱。通过使用CCSD(T)力场的VPT2 + K模拟可预测非谐频率,该场具有使用ANO1(ANO0)基集计算的二次(三次和四次)力常数。绝大多数的光谱特征可以被分配到反式异戊二烯这些计算的基础上。与使用加热的异戊二烯的实验相比,还可以指定高能薄纱构象异构体的某些功能。收敛的从头开始提出了异构化途径的热化学,对于该途径,分配功能明确地说明了与甲基扭转和旋转异构体之间的内部旋转的独立的,未耦合的一维潜在表面相关的本征态。0和298.15 K反式/薄纱的能量差分别为2.82和2.52 kcal / mol,这意味着室温薄纱的人口为2.8%。
更新日期:2017-12-28
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