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首页> 外文期刊>The journal of physical chemistry, A. Molecules, spectroscopy, kinetics, environment, & general theory >Theoretical Characterization of Dimethyl Carbonate at Low Temperatures
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Theoretical Characterization of Dimethyl Carbonate at Low Temperatures

机译：碳酸二甲酯的低温理论表征

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摘要

Highly correlated ab initio methods (CCSD(T) and RCCSD(T)-F12) are employed for the spectroscopic characterization of the gas phase of dimethyl carbonate (DMC) at low temperatures. DMC, a relevant molecule for atmospheric and astrochemical studies, shows only two conformers, cis-cis and tran-cis, respectively, of C-2v and C-s symmetries. cis-cis-DMC represents the most stable form. Using RCCSD(T)-F12 theory, the two sets of equilibrium rotational constants have been computed to be A(e) = 10493.15 MHz, B-e = 2399.22 MHz, and C-e = 2001.78 MHz (cis-cis) and to be A(e) = 6585.16 MHz, B-e = 3009.04 MHz, and C-e = 2120.86 MHz (trans-cis). Centrifugal distortions constants and anharmonic frequencies for all of the vibrational modes are provided. Fermi displacements are predicted. The minimum energy pathway for the cis-cis -> trans-cis interconversion process is restricted by a barrier of similar to 3500 cm(-1). DMC displays internal rotation of two methyl groups. If the nonrigidity is considered, the molecule can be classified in the G(36) (cis-cis) and the G(18) (trans-cis) symmetry groups. For cis-cis-DMC, both internal tops are equivalent, and the torsional motions are restricted by V-3 potential energy barriers of 384.7 cm(-1). trans-cis-DMC shows two different V-3 barriers of 631.53 and 382.6 cm(-1). The far-infrared spectra linked to the torsional motion of both conformers are analyzed independently using a variational procedure and a two-dimensional flexible model. In cis-cis-DMC, the ground vibrational state splits into nine components: one nondegenerate, 0.000 cm(-1) (A(1)), four quadruply degenerate, 0.012 cm(-1) (G), and four doubly degenerate 0.024 cm(-1) (E-1 and E-3). The methyl torsional fundamentals are obtained to lie at 140.274 cm(-1) (v(15)) and 132.564 cm(-1) (v(30)).
机译：高度相关的从头算方法（CCSD（T）和RCCSD（T）-F12）用于低温下碳酸二甲酯（DMC）气相的光谱表征。 DMC是用于大气和天化学研究的一个相关分子，仅显示C-2v和C-s对称性的两个构象分别为顺式和顺式。顺式-顺式-DMC代表最稳定的形式。使用RCCSD（T）-F12理论，已计算出两组平衡旋转常数分别为A（e）= 10493.15 MHz，Be = 2399.22 MHz和Ce = 2001.78 MHz（顺-顺），并且为A（e ）= 6585.16 MHz，Be = 3009.04 MHz和Ce = 2120.86 MHz（反顺式）。提供了所有振动模式的离心变形常数和非谐频率。费米位移是可以预测的。顺式->反式-顺式互变过程的最小能量途径受到类似于3500 cm（-1）的壁垒的限制。 DMC显示两个甲基的内部旋转。如果考虑到非刚性，则该分子可分为G（36）（顺-顺）和G（18）（反-顺）对称组。对于cis-cis-DMC，两个内部顶部均相等，并且扭转运动受到384.7 cm（-1）的V-3势能垒的限制。反顺式DMC显示631.53和382.6 cm（-1）的两个不同的V-3屏障。使用变分程序和二维柔性模型独立分析了与两个构象异构体的扭转运动有关的远红外光谱。在cis-cis-DMC中，地面振动状态分为九个分量：一个不变质，0.000 cm（-1）（A（1）），四个变质，0.012 cm（-1）（G），四个变质0.024 cm（-1）（E-1和E-3）。获得的甲基扭转基波位于140.274 cm（-1）（v（15））和132.564 cm（-1）（v（30））。

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