Centre for Theoretical and Computational Chemistry, Department of Chemistry, University of Oslo, P.O. Box 1033 Blindern, NO-0315 Oslo, Norway.
J Phys Chem A. 2012 Dec 13;116(49):12073-81. doi: 10.1021/jp309552m. Epub 2012 Nov 29.
The microwave spectra of a mixture of cis- and trans-H-N-C-O forms of N-vinylformamide, (H(2)C═CHNHC(═O)H), have been measured at room temperature in the 18-75 GHz spectral range. The spectra of two forms were assigned. The first of these forms has a cis arrangement for the H-N-C-O chain of atoms, whereas the second form has a trans arrangement. The C-C-N-C chain of atoms is antiperiplanar (180°) in both forms. The inertial defect of the ground vibrational state of cis is -0.142(5) × 10(-20) u m(2), whereas this parameter is -0.087098(26) × 10(-20) u m(2) for trans. It is concluded that the equilibrium structures of both cis and trans are completely planar. The dipole moment determined from Stark effect measurements is μ(a) = 9.96(8), μ(b) = 2.22(3), μ(c) = 0 (by symmetry), and μ(tot) = 10.20(8) × 10(-30) C m [3.06(2) D], for cis, and μ(a) = 7.64(16), μ(b) = 9.24(10), μ(c) = 0 (by symmetry), and μ(tot) = 12.0(2) × 10(-30) C m [3.59(5) D] for trans. The spectrum of one vibrationally excited state, presumably the first excited state of the torsion about the C-N bond of cis, was assigned and the frequency of this state was determined to be 76(15) cm(-1) by relative intensity measurements. The spectra of two vibrationally excited states of trans were assigned. These states are assumed to be the first excited state of the torsion about the C-N bond, and a low bending vibration. Relative intensity measurements yielded 101(20) and ca. 300 cm(-1), respectively, for the frequencies of these normal vibrations. Accurate values of the quartic centrifugal distortion constants, the dipole moments, and the vibration-rotation constants have been obtained for both cis and trans. The experimental work has been augmented by high-level quantum chemical calculations at the B3LYP/cc-pVTZ and CCSD(T)/cc-pVTZ levels of theory. The theoretical calculation performed without symmetry restrictions correctly predict that cis and trans are both planar. The CCSD(T) rotational constants are in excellent agreement with their experimental counterparts, whereas the B3LYP quartic centrifugal distortion constants and the vibration-rotation constants are in fairly good agreement with experiments. The CCSD(T) dipole moments deviate more than expected from the experimental dipole moments. It is estimated that further conformers of cis and trans must be at least 4 kJ/mol higher in energy.
顺式和反式 H-N-C-O 形式的 N-乙烯基甲酰胺（(H(2)C═CHNHC(═O)H)）混合物的微波光谱在室温下在 18-75 GHz 光谱范围内进行了测量。分配了两种形式的光谱。其中一种形式的 H-N-C-O 链具有顺式排列，而第二种形式具有反式排列。原子的 C-C-N-C 链在两种形式中均为反式（180°）。顺式基态振动的惯性缺陷为-0.142（5）×10(-20) u m(2)，而反式的此参数为-0.087098（26）×10(-20) u m(2)。可以得出结论，顺式和反式的平衡结构均完全为平面结构。从 Stark 效应测量确定的偶极矩为 μ(a) = 9.96（8），μ(b) = 2.22（3），μ(c) = 0（对称），μ(tot) = 10.20（8）×10(-30) C m [3.06（2）D]，对于顺式，μ(a) = 7.64（16），μ(b) = 9.24（10），μ(c) = 0（对称），μ(tot) = 12.0（2）×10(-30) C m [3.59（5）D]，对于反式。分配了一个振动激发态的光谱，大概是顺式中 C-N 键扭转的第一激发态，并且通过相对强度测量确定了该状态的频率为 76（15）cm(-1)。还分配了反式两种振动激发态的光谱。假设这些状态是 C-N 键扭转的第一激发态和低弯曲振动。相对强度测量分别给出了这些简正振动的频率为 101（20）和约 300 cm(-1)。顺式和反式的四次离心力畸变常数、偶极矩和振转常数均已准确获得。实验工作通过 B3LYP/cc-pVTZ 和 CCSD(T)/cc-pVTZ 理论水平的高精度量子化学计算得到了补充。没有对称性限制的理论计算正确地预测了顺式和反式均为平面。CCSD(T) 转动常数与实验值非常吻合，而 B3LYP 四次离心力畸变常数和振转常数与实验值吻合得相当好。CCSD(T) 偶极矩与实验偶极矩的偏差超出预期。据估计，顺式和反式的其他构象必须至少高出 4 kJ/mol。
