School of Chinese Materia Medica, Beijing University of Chinese Medicine, Beijing, PR China 100029.
J Phys Chem A. 2013 Mar 14;117(10):2152-9. doi: 10.1021/jp311681u. Epub 2013 Feb 27.
The ground triplet state and lowest singlet state of formylmethylene have been proposed as important intermediates in the Wolff rearrangement of α-diazo ketones into ketenes. The ground triplet state of formylmethylene has been examined experimentally, but the lowest singlet state has yet to be observed. We predict equilibrium geometries, energies, bonding, dipole moments, and harmonic vibrational frequencies for these two lowest states of formylmethylene at the cc-pVQZ CCSD(T) level of theory. The singlet-triplet energy difference [ΔE(S-T)] is quite sensitive to the level of theory. The highly accurate cc-pVQZ CCSD(T) level of theory yields the most reliable result of only 2.0 kcal mol(-1). An estimate based on the experimentally characterized CH2 molecule yields ΔE(S-T) = 1.27 kcal mol(-1). In addition, accurate quartic force fields have been determined at the cc-pVTZ CCSD(T) level of theory. Fundamental vibrational frequencies, anharmonic constants, and vibration-rotation coupling constants were determined using vibrational second-order perturbation theory (VPT2). Our results should aid in experimental detection and characterization of the lowest singlet state of formylmethylene, which is highly desirable for better understanding the mechanism of the Wolff rearrangement.
甲酰基亚甲基的基态三重态和最低单重态被认为是α-重氮酮转化为烯酮的Wolff 重排中的重要中间体。甲酰基亚甲基的基态三重态已经过实验检验，但最低单重态尚未被观察到。我们在 cc-pVQZ CCSD(T)理论水平上预测了这两种最低态甲酰基亚甲基的平衡几何形状、能量、键合、偶极矩和谐振振动频率。单重态-三重态能量差[ΔE(S-T)]对理论水平非常敏感。高度准确的 cc-pVQZ CCSD(T)理论水平仅产生最可靠的结果，即 2.0 kcal mol(-1)。基于实验表征的 CH2 分子的估算得出的ΔE(S-T)为 1.27 kcal mol(-1)。此外，还在 cc-pVTZ CCSD(T)理论水平上确定了精确的四次力场。使用振动二级微扰理论 (VPT2) 确定了基本振动频率、非谐常数和振动-旋转耦合常数。我们的结果应该有助于实验检测和表征甲酰基亚甲基的最低单重态，这对于更好地理解 Wolff 重排的机制非常重要。
