Departamento de Química y Física Teóricas, Instituto de Estructura de la Materia, CSIC, C/Serrano, 121, 28006 Madrid, Spain.
Phys Chem Chem Phys. 2019 Feb 13;21(7):3597-3605. doi: 10.1039/c8cp04490b.
The far infrared region of three detectable molecules sharing the empirical formula C3H3O2N, acetyl isocyanate CH3CONCO (AISO), methyl cyanoformate NC-COOCH3 (MCN) and acetyl cyanate CH3COOCN (ACN), is explored using explicitly correlated coupled cluster ab initio methods and a variational procedure designed for non-rigid species and large amplitude motions. The three isomeric forms display two conformers, cis and trans, of Cs symmetry that intertransform through the torsion of the central bond. This internal rotation interacts with the methyl group torsion generating a ground electronic state potential energy surface of six minima. Accurate rotational constants, centrifugal distortion constants, potential energy barriers, and surfaces, as well as, the low energy levels and their splittings, are provided. Far infrared energies are calculated up to 600 cm-1 which represent excitations of the torsional and the skeletal bending modes. Below 410 cm-1, 28, 14 and 20 vibrational energy levels and their splittings have been identified and classified for acetyl isocyanate, methyl cyanoformate, and acetyl cyanate, respectively. All the methyl torsion barriers are relatively low (∼300 cm-1) generating relevant tunneling effects. Computed spectroscopic parameters can help further interpretation and assignments of experimental rotational spectra using effective Hamiltonians.
使用显式相关耦合簇从头算方法和针对非刚性物种和大振幅运动设计的变分程序，探索了三个具有相同经验公式 C3H3O2N 的可检测分子的远红外区域，分别为乙酰异氰酸酯 CH3CONCO（AISO）、甲基氰基甲酸酯 NC-COOCH3（MCN）和乙酰氰酸酯 CH3COOCN（ACN）。这三个异构体显示出具有 Cs 对称性的两种构象，顺式和反式，通过中心键的扭转相互转化。这种内部旋转与甲基扭转相互作用，产生了六个最小值的基态电子势能面。提供了准确的转动常数、离心畸变常数、势能势垒和表面，以及低能级及其分裂。远红外能量计算高达 600 cm-1，代表了扭转和骨架弯曲模式的激发。在 410 cm-1 以下，分别为乙酰异氰酸酯、甲基氰基甲酸酯和乙酰氰酸酯确定并分类了 28、14 和 20 个振动能级及其分裂。所有甲基扭转势垒都相对较低（约 300 cm-1），产生了相关的隧道效应。计算出的光谱参数可以帮助使用有效哈密顿量进一步解释和分配实验旋转光谱。
