Mil'nikov G V, Kühn O, Nakamura H
Department of Theoretical Studies, Institute for Molecular Science, Myodaiji, Okazaki 444-8585, Japan.
J Chem Phys. 2005 Aug 15;123(7):074308. doi: 10.1063/1.2000257.
The previously developed instanton theory [G. V. Mil'nikov and H. Nakamura, J. Chem. Phys. 122, 124311 (2005)] is applied to the calculation of vibrationally assisted tunneling splitting of the deuterated formic acid dimer (DCOOH)2 with all the degrees of freedom taken into account. The ground-state tunnel splitting is determined by the density-functional theory combined with coupled cluster level of quantum chemistry to be 0.0038 cm(-1) which is comparable to the experimental value of 0.0029 cm(-1). Further, the tunnel splittings of fundamental excitations are estimated for frequencies below 300 cm(-1). In this energy range it is found that the excitation modes may either enhance or suppress tunneling as compared to the ground state. For the higher-frequency modes a rapid growth of the tunnel splitting is observed. At frequencies above 1000 cm(-1) the semiclassical solution becomes unstable and no reliable tunneling splittings can be obtained. This is in vast contrast to the adiabatic approximation to the instanton theory in which the tunnel splittings can be retrieved up to 3000 cm(-1). We discuss this disparity from the viewpoint of the multidimensional character of tunneling in hydrogen bonds and the adiabatic approximation is concluded to be inaccurate.
将先前发展的瞬子理论[G. V. 米尔尼科夫和H. 中村，《化学物理杂志》122, 124311 (2005)]应用于计算考虑了所有自由度的氘代甲酸二聚体(DCOOH)₂的振动辅助隧穿分裂。通过密度泛函理论结合量子化学的耦合簇水平确定基态隧穿分裂为0.0038 cm⁻¹，这与0.0029 cm⁻¹的实验值相当。此外，估计了频率低于300 cm⁻¹的基本激发的隧穿分裂。在这个能量范围内发现，与基态相比，激发模式可能增强或抑制隧穿。对于高频模式，观察到隧穿分裂迅速增长。在频率高于1000 cm⁻¹时，半经典解变得不稳定，无法获得可靠的隧穿分裂。这与瞬子理论的绝热近似形成了巨大对比，在绝热近似中，隧穿分裂可以恢复到3000 cm⁻¹。我们从氢键中隧穿的多维特征的角度讨论了这种差异，并得出绝热近似不准确的结论。
