Talapatra Surma, Geva Eitan
Department of Chemistry, University of Michigan , 930 North University Avenue, Ann Arbor, Michigan 48109, United States.
J Phys Chem B. 2014 Jul 3;118(26):7395-404. doi: 10.1021/jp504460q. Epub 2014 Jun 24.
The lifetimes of the first vibrational state of (12)C(14)N(-) and (13)C(15)N(-) dissolved in H2O or D2O were calculated. The calculations were based on the Landau-Teller formula that puts the vibrational lifetimes in terms of the autocorrelation function of the force exerted on the C-N stretch by the remaining degrees of freedom. The force autocorrelation functions were calculated from classical molecular dynamics simulations of the four cyanide/water isotopomer combinations ((12)C(14)N(-)/H2O, (12)C(14)N(-)/D2O, (13)C(15)N(-)/H2O, (13)C(15)N(-)/D2O). The cyanide ion was described by a polarizable force field, and the water was described by either the rigid SPC/E model or the flexible SPC/Fw model, in order to compare two different types of accepting modes, namely, (1) intermolecular (translational and rotational) solvent accepting modes (rigid SPC/E water) and (2) intramolecular (vibrational) solvent accepting modes (flexible SPC/Fw water). Since quantum effects are expected to increase in size with increasing frequency mismatch between relaxing and accepting modes, different quantum correction factors were employed depending on the identity of the accepting modes, more specifically, the harmonic/Schofield quantum correction factor in the case of intermolecular accepting modes and the standard quantum correction factor in the case of intramolecular accepting modes. The lifetimes with either the rigid SPC/E or flexible SPC/Fw water models were found to be in good quantitative agreement with the experimentally measured values for all isotopomer combinations. Our results suggest that taking into account quantum effects on the vibrational energy relaxation of cyanide in aqueous solution can make the intermolecular pathway at least as likely as the intramolecular pathway.
计算了溶解于H₂O或D₂O中的(¹²)C(¹⁴)N⁻和(¹³)C(¹⁵)N⁻第一振动态的寿命。计算基于朗道 - 泰勒公式，该公式根据其余自由度施加在C - N伸缩上的力的自相关函数来确定振动寿命。力自相关函数由四种氰化物/水同位素异构体组合（(¹²)C(¹⁴)N⁻/H₂O、(¹²)C(¹⁴)N⁻/D₂O、(¹³)C(¹⁵)N⁻/H₂O、(¹³)C(¹⁵)N⁻/D₂O）的经典分子动力学模拟计算得出。氰离子由可极化力场描述，水由刚性SPC/E模型或柔性SPC/Fw模型描述，以便比较两种不同类型的接受模式，即：(1)分子间（平动和转动）溶剂接受模式（刚性SPC/E水）和(2)分子内（振动）溶剂接受模式（柔性SPC/Fw水）。由于预计量子效应会随着弛豫模式和接受模式之间频率失配的增加而增大，因此根据接受模式的不同采用了不同的量子校正因子，更具体地说，分子间接受模式采用谐波/斯科菲尔德量子校正因子，分子内接受模式采用标准量子校正因子。对于所有同位素异构体组合，发现采用刚性SPC/E或柔性SPC/Fw水模型计算出的寿命与实验测量值在数量上吻合良好。我们的结果表明，考虑水溶液中氰化物振动能量弛豫的量子效应可使分子间途径至少与分子内途径一样可能。
