5-甲基胞嘧啶及胞嘧啶无辐射失活的半经典动力学模拟和CASSCF计算

采用半经典动力学方法模拟了5-甲基胞嘧啶(5m-Cyt)和胞嘧啶(Cyt)在267 nm紫外光辐射下的光物理失活过程. 模拟发现, 5m-Cyt 和Cyt 的激发态通过C₅－C₆键的扭曲以及甲基(或H₅)和H₆原子平面外振动失活.失活时, 甲基(或H₅)和H₆原子几乎与环面垂直并指向不同方向, 形成“双自由基态”. 由于甲基的体积比H原子的大, 振动频率较小, 使得C₅原子的变形受到抑制, 导致5m-Cyt 的激发态寿命比胞嘧啶更长. 完全活性空间自洽场(CASSCF)计算显示, 5m-Cyt的圆锥交叉(CI)点能量比Cyt 高0.3 eV, 这也证明5m-Cyt演化至CI 需要克服更大的能垒, 因此激发态寿命长于胞嘧啶.

Semiclassical Dynamics Simulation and CASSCF Calculation for 5-Methyl Cytosine and Cytosine

We performed a semiclassical dynamics simulation study of the photophysical deactivation of 5m-cytosine (5m-Cyt) and cytosine (Cyt) induced by ultraviolet radiation of 267 nm. The results show that deactivation of the excited state of 5m-Cyt and Cyt results from the distortion of the C₅－C₆ bond and the out-of-plane vibration of methyl (or H₅) and the H6 atom. A so-called“biradical state”, in which the methyl (or H₅) and H₆ atoms are nearly perpendicular to the average ring plane and displaced in opposite directions, is formed at the decay point. The vibration frequency of the methyl derivative is less than that of the H atom derivative because of its increased volume relative to the H atom. The results indicated that molecular deformation at the C₅ atom of 5m-Cyt will be weakened and will result in a longer excited lifetime of 5m-Cyt. Complete active space self-consistent field (CASSCF) calculations show that the energy of the conical intersection (CI) of 5m-Cyt is 0.3 eV higher than that of Cyt. This suggests that promotion to the CI point for 5m-Cyt requires the molecule to overcome a larger energetic barrier, which results in a longer excited state lifetime than Cyt.