Departamento de Química y Física Teóricas, I. Estructura de la Materia, IEM-CSIC, Serrano 121, Madrid 28006, Spain.
Laboratoire de Spectroscopie Atomique, Moléculaire et Applications-LSAMA LR01ES09, Faculté des sciences de Tunis, Université de Tunis El Manar, 2092, Tunis, Tunisie.
J Phys Chem A. 2015 Apr 30;119(17):4057-64. doi: 10.1021/acs.jpca.5b01836. Epub 2015 Apr 15.
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 trans-cis, respectively, of C2v and Cs 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 Ae = 10 493.15 MHz, Be = 2399.22 MHz, and Ce = 2001.78 MHz (cis-cis) and to be Ae = 6585.16 MHz, Be = 3009.04 MHz, and Ce = 2120.36 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 ∼3500 cm(-1). DMC displays internal rotation of two methyl groups. If the nonrigidity is considered, the molecule can be classified in the G36 (cis-cis) and the G18 (trans-cis) symmetry groups. For cis-cis-DMC, both internal tops are equivalent, and the torsional motions are restricted by V3 potential energy barriers of 384.7 cm(-1). trans-cis-DMC shows two different V3 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) (A1), four quadruply degenerate, 0.012 cm(-1) (G), and four doubly degenerate 0.024 cm(-1) (E1 and E3). The methyl torsional fundamentals are obtained to lie at 140.274 cm(-1) (ν15) and 132.564 cm(-1) (ν30).
采用高度相关的从头算方法（CCSD(T) 和 RCCSD(T)-F12）对碳酸二甲酯（DMC）在低温下的气相进行光谱特性研究。DMC 是大气和天体化学研究中相关的分子，仅显示出两种构象，分别为 C2v 和 Cs 对称性的顺式-顺式和反式-顺式。顺式-顺式-DMC 代表最稳定的形式。使用 RCCSD(T)-F12 理论，计算了两组平衡旋转常数，分别为 Ae = 10493.15 MHz、Be = 2399.22 MHz 和 Ce = 2001.78 MHz（顺式-顺式）和 Ae = 6585.16 MHz、Be = 3009.04 MHz 和 Ce = 2120.36 MHz（反式-顺式）。提供了所有振动模式的离心变形常数和非谐频率。预测了费米位移。顺式-顺式→反式-顺式互变异构过程的最低能量途径受到约 3500 cm(-1) 势垒的限制。DMC 显示两个甲基基团的内部旋转。如果考虑非刚性，该分子可以归类为 G36（顺式-顺式）和 G18（反式-顺式）对称群。对于顺式-顺式-DMC，两个内部顶端是等价的，扭转运动受到 384.7 cm(-1) 的 V3 位能势垒的限制。反式-顺式-DMC 显示出两个不同的 V3 势垒，分别为 631.53 和 382.6 cm(-1)。两个构象的扭转运动的远红外光谱分别通过变分程序和二维柔性模型进行分析。在顺式-顺式-DMC 中，基态振动分裂为九个分量：一个非简并的，0.000 cm(-1)（A1），四个四重简并的，0.012 cm(-1)（G）和四个二重简并的 0.024 cm(-1)（E1 和 E3）。甲基扭转基频被确定为 140.274 cm(-1)（ν15）和 132.564 cm(-1)（ν30）。
