Krasnoshchekov Sergey V, Craig Norman C, Boopalachandran Praveenkumar, Laane Jaan, Stepanov Nikolay F
Lomonosov Moscow State University , Leninskiye Gory, 119991, Moscow, Russian Federation.
Department of Chemistry and Biochemistry, Oberlin College , Oberlin, Ohio 44074, United States.
J Phys Chem A. 2015 Oct 29;119(43):10706-23. doi: 10.1021/acs.jpca.5b07650. Epub 2015 Oct 14.
A quantum-mechanical (hybrid MP2/cc-pVTZ and CCSD(T)/cc-pVTZ) full quartic potential energy surface (PES) in rectilinear normal coordinates and the second-order operator canonical Van Vleck perturbation theory (CVPT2) are employed to predict the anharmonic vibrational spectra of s-trans- and s-gauche-butadiene (BDE). These predictions are used to interpret their infrared and Raman scattering spectra. New high-temperature Raman spectra in the gas phase are presented in support of assignments for the gauche conformer. The CVPT2 solution is based on a PES and electro-optical properties (EOP; dipole moment and polarizability) expanded in Taylor series. Higher terms than those routinely available from Gaussian09 software were calculated by numerical differentiation of quadratic force fields and EOP using the MP2/cc-pVTZ model. The integer coefficients of the polyad quantum numbers were derived for both conformers of BDE. Replacement of harmonic frequencies by their counterparts from the CCSD(T)/cc-pVTZ model significantly improved the agreement with experimental data for s-trans-BDE (root-mean-square deviation ≈ 5.5 cm(-1)). The accuracy in predicting the rather well-studied spectrum of fundamentals of s-trans-BDE assures good predictions of the spectrum of s-gauche-BDE. A nearly complete assignment of fundamentals was obtained for the gauche conformer. Many nonfundamental transitions of the BDE conformers were interpreted as well. The predictions of multiple Fermi resonances in the complex CH-stretching region correlate well with experiment. It is shown that solving a vibrational anharmonic problem through a numerical-analytic implementation of CVPT2 is a straightforward and computationally advantageous approach for medium-size molecules in comparison with the standard second-order vibrational perturbation theory (VPT2) based on analytic expressions.
采用量子力学方法（混合MP2/cc-pVTZ和CCSD(T)/cc-pVTZ）在直线型正规坐标下构建全四次势能面（PES），并运用二阶算符正则范弗莱克微扰理论（CVPT2）预测反式和顺式丁二烯（BDE）的非谐振动光谱。这些预测结果用于解释它们的红外和拉曼散射光谱。给出了气相中新的高温拉曼光谱，以支持对顺式构象体的归属。CVPT2解基于在泰勒级数中展开的PES和电光性质（EOP；偶极矩和极化率）。使用MP2/cc-pVTZ模型通过二次力场和EOP的数值微分计算了比Gaussian09软件常规可得的更高阶项。推导了BDE两种构象体的多量子数整数系数。用CCSD(T)/cc-pVTZ模型对应的频率取代谐波频率，显著改善了与反式-BDE实验数据的吻合度（均方根偏差≈5.5 cm⁻¹）。对研究较为充分的反式-BDE基频光谱的准确预测确保了对顺式-BDE光谱的良好预测。获得了顺式构象体基频的近乎完整的归属。还解释了BDE构象体的许多非基频跃迁。在复杂的C-H伸缩区域中多个费米共振的预测与实验结果相关性良好。结果表明，与基于解析表达式的标准二阶振动微扰理论（VPT2）相比，通过CVPT2的数值解析实现来解决振动非谐问题对于中等大小分子是一种直接且计算上有利的方法。
