Matrix Isolation FTIR Spectroscopic and Theoretical Study of Dimethyl Sulfite

The preferred conformations of dimethyl sulfite and their vibrational spectra were studied by matrix-isolation
Fourier transform infrared spectroscopy and theoretical methods (density functional theory (DFT) and Moller−Plesset (MP2), with basis sets of different sizes, including the quadruple-ζ, aug-cc-pVQZ basis). Five minima
were found at these levels of theory. At the MP2/6-31++G(d,p) and DFT/B3LYP/aug-cc-pVQPZ levels, the
GG conformer (where the O−S−O−C dihedral angles are 73.2 and 70.8°) resulted in the conformational
ground state. At the highest level of theory used, the GT conformer (O−S−O−C = +68.5 and −173.2°) is
0.83 kJ mol-1 higher in energy than the GG form, while conformer GG‘ (O−S−O−C = +85.7 and −85.7°)
has a relative energy of 1.18 kJ mol-1. The remaining two conformers (G‘T and TT) are high-energy forms
and not experimentally relevant. In consonance with the theoretical predictions, conformer GG was found to
be the most stable conformer in the gaseous phase as well as in the low-temperature matrices. Annealing of
the argon matrices first promotes the GG‘→GT isomerization, which is followed by conversion of GT into
the most stable conformer. There is no evidence of occurrence of GG‘→GG direct conversion in the low-temperature matrices. On the other hand, during deposition of the xenon matrices conformer GG‘ totally
converts to conformer GT. Two observations demonstrated this fact:  no evidence of bands corresponding to
GG‘ were observed in xenon matrices and the GG/GT intensity ratio became similar to the GG/(GT + GG‘)
intensity ratio observed in argon matrices. All these results could be explained by taking into account the
relative values of the theoretically predicted energy barriers for the different isomerization processes:  GG‘→GT,
1.90 kJ mol-1; GT→GG, 9.64 kJ mol-1; and GG‘→GG, 19.46 kJ mol-1.

本研究采用基质隔离傅里叶变换红外光谱（matrix-isolation Fourier transform infrared spectroscopy）结合理论方法——包括不同基组尺寸的密度泛函理论（DFT）与莫勒-普莱塞特微扰理论（MP2），其中涵盖辅助相关一致极化四重ζ（aug-cc-pVQZ）基组——对亚硫酸二甲酯（dimethyl sulfite）的优势构象及其振动光谱展开了研究。在上述理论计算级别下共发现五个极小值结构。在MP2/6-31++G(d,p)与DFT/B3LYP/aug-cc-pVQPZ理论级别下，GG构象（其O−S−O−C二面角分别为73.2°与70.8°）为构象基态。在本研究采用的最高理论级别下，GT构象（O−S−O−C二面角分别为+68.5°与−173.2°）的能量比GG形式高0.83 kJ·mol⁻¹，而GG'构象（O−S−O−C二面角分别为+85.7°与−85.7°）的相对能量为1.18 kJ·mol⁻¹。剩余两种构象（G'T与TT）为高能量构象，无实验相关性。与理论预测一致，GG构象在气相以及低温基质中均为最稳定构象。对氩基质进行退火处理时，首先会促进GG'→GT的异构化反应，随后GT进一步转化为最稳定的GG构象。在低温氩基质中未观测到GG'→GG直接转化的迹象。另一方面，在氙基质沉积过程中，GG'构象会完全转化为GT构象。这一结论可通过两项观测结果佐证：其一，氙基质中未检测到对应GG'构象的特征吸收峰；其二，氩基质中测得的GG/GT强度比与GG/(GT+GG')强度比趋于一致。上述所有结果均可通过不同异构化过程的理论预测能垒相对值得到解释：GG'→GT为1.90 kJ·mol⁻¹，GT→GG为9.64 kJ·mol⁻¹，GG'→GG为19.46 kJ·mol⁻¹。