Rotamers of Isoprene: Infrared Spectroscopy in Helium Droplets and Ab Initio Thermochemistry
Rotamers of Isoprene: Infrared Spectroscopy in Helium Droplets and Ab Initio Thermochemistry
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DOI：
10.1021/acs.jpca.7b10260
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发表时间：
2018-01-11
影响因子：
2.9
通讯作者：
Douberly, Gary E.
中科院分区：
文献类型：
--
作者：
Franke, Peter R.;Douberly, Gary E.
异戊二烯(C5H8)是一种丰富的、反应性的对流层碳氢化合物，来自生物排放。因此，详细了解异戊二烯的光谱是可取的。在氦液滴中分离到了异戊二烯单体，并在CH伸缩区测量了其红外光谱。采用CCSD(T)力场，用ANO1(ANO0)基组计算了二次(三次和四次)力常数，通过VPT2+K模拟预测了非简谐频率。在这些计算的基础上，绝大多数光谱特征可以归属于反式异戊二烯。与使用加热异戊二烯的实验相比，高能Guche构象的一些特征也是可以指定的。对于异构化途径，提出了收敛的从头算热化学方法，对于该途径，配分函数显式地解释了与分离的、分离的一维势能面有关的本征态，用于甲基扭转和旋转体之间的内旋转。298.15和2.52K的反式/高切能差分别为2.82千卡/摩尔和2.52千卡/摩尔，表明室温高支布居数为2.8%。
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%.