Dipartimento di Scienze Molecolari e Nano-sistemi, Università Ca' Foscari Venezia, Calle Larga S. Marta 2137, I-30123 Venezia, Italy.
J Chem Phys. 2012 Jun 7;136(21):214302. doi: 10.1063/1.4720502.
Difluoromethane (CH(2)F(2), HFC-32) is a molecule used in refrigerant mixtures as a replacement of the more environmentally hazardous, ozone depleting, chlorofluorocarbons. On the other hand, presenting strong vibration-rotation bands in the 9 μm atmospheric window, it is a greenhouse gas which contributes to global warming. In the present work, the vibrational and ro-vibrational properties of CH(2)F(2), providing basic data for its atmospheric modeling, are studied in detail by coupling medium resolution Fourier transform infrared spectroscopy to high-level electronic structure ab initio calculations. Experimentally a full quantum assignment and accurate integrated absorption cross sections are obtained up to 5000 cm(-1). Ab initio calculations are carried out by using CCSD(T) theory and large basis sets of either the correlation consistent or atomic natural orbital hierarchies. By using vibrational perturbation theory to second order a complete set of vibrational and ro-vibrational parameters is derived from the ab initio quartic anharmonic force fields, which well compares with the spectroscopic constants retrieved experimentally. An excellent agreement between theory and experiment is achieved for vibrational energy levels and integrated absorption cross sections: transition frequencies up to four quanta of vibrational excitation are reproduced with a root mean square deviation (RMSD) of 7 cm(-1) while intensities are predicted within few km mol(-1) from the experiment. Basis set performances and core correlation effects are discussed throughout the paper. Particular attention is focused in the understanding of the anharmonic couplings which rule the vibrational dynamics of the |ν(1)>, |2ν(8)>, |2ν(2)> three levels interacting system. The reliability of the potential energy and dipole moment surfaces in reproducing the vibrational eigenvalues and intensities as well as in modeling the vibrational and ro-vibrational mixings over the whole 400-5000 cm(-1) region is also demonstrated by spectacular spectral simulations carried out by using the ro-vibrational Hamiltonian constants, and the relevant coupling terms, obtained from the perturbation treatment of the ab initio anharmonic force field. The present results suggest CH(2)F(2) as a prototype molecule to test ab initio calculations and theoretical models.
二氟甲烷（CH(2)F(2)，HFC-32）是一种用于制冷剂混合物的分子，作为对更具环境危害性、消耗臭氧层的氯氟碳化物的替代品。另一方面，由于它在 9μm 大气窗口中呈现出强烈的振动-旋转带，因此是一种温室气体，会导致全球变暖。在本工作中，通过将中分辨率傅里叶变换红外光谱与高精度电子结构从头计算相结合，详细研究了 CH(2)F(2)的振动和 ro-vibrational 性质，为其大气模拟提供了基础数据。实验上，通过量子全分配和精确的积分吸收截面，直至 5000cm(-1) 获得了完整的量子分配和精确的积分吸收截面。从头计算是通过使用 CCSD(T)理论和相关一致或原子自然轨道层次的大基组来进行的。通过使用振动微扰理论到二阶，从 ab initio 四次非谐力场中推导出一套完整的振动和 ro-vibrational 参数，与实验中检索到的光谱常数很好地比较。理论与实验在振动能级和积分吸收截面上达到了极好的一致性：在振动激发的四个量子上的跃迁频率的重现精度为 7cm(-1)，而强度的预测值与实验值相差不到几 km mol(-1)。本文讨论了基组性能和核心相关效应。特别关注的是理解规则 |ν(1)>、|2ν(8)>、|2ν(2)>三个能级相互作用系统振动动力学的非谐耦合。通过使用 ro-vibrational Hamiltonian 常数和相关耦合项进行壮观的光谱模拟，还证明了势能和偶极矩曲面在重现振动本征值和强度以及在整个 400-5000cm(-1) 区域模拟振动和 ro-vibrational 混合方面的可靠性，这些常数和耦合项是通过对 ab initio 非谐力场的微扰处理得到的。目前的结果表明 CH(2)F(2)是测试从头计算和理论模型的原型分子。
