Teslja Alexey, Dagdigian Paul J, Banck Michael, Eisfeld Wolfgang
Department of Chemistry, The Johns Hopkins University, Baltimore, Maryland 21218-2685, USA.
J Phys Chem A. 2006 Jun 29;110(25):7826-34. doi: 10.1021/jp061578j.
A collaborative experimental and theoretical study of the electronic spectrum and excited-state photochemistry of H(2)CN has been carried out. The absorption spectrum, in the range of 287-278 nm, was measured through cavity ring-down spectroscopy. The radical was prepared by 193 nm photolysis of monomeric formaldoxime vapor. Two diffuse features were observed in the 34800-35800 cm(-1) spectral range, along with the A-X (1,0) band of the OH cofragment. The broad features were assigned through high-level ab initio calculations as vibronic transitions to the ground and 2b(1) (umbrella mode) levels of the second excited B (2)A(1) state from the ground X (2)B(2) state of H(2)CN. Rotational constants for the lower and upper levels of these transitions were computed from the expectation values of the moments of inertia tensor, using the appropriate vibrational wave functions. Experimental and simulated rotational profiles of these bands agree extremely well with each other for an assumed type-B electric dipole-allowed (2)A(1) <-- (2)B(2) transition appropriate to this transition. The former assignment to the dipole-forbidden (2)B(1) <-- (2)B(2) transition can be ruled out by these results. A theoretical investigation of the dissociation pathways for electronically excited H(2)CN is also presented. The upper states of the observed bands cannot dissociate directly but rather decay through internal conversion and subsequent dissociation to H + HCN fragments; higher b(1) levels are above the excited-state dissociation limit.
对H₂CN的电子光谱和激发态光化学进行了一项合作性的实验与理论研究。通过腔衰荡光谱法测量了287 - 278 nm范围内的吸收光谱。该自由基是通过193 nm光解单体甲醛肟蒸气制备的。在34800 - 35800 cm⁻¹光谱范围内观察到两个漫射特征，以及OH共碎片的A - X(1,0)带。通过高水平的从头计算将这些宽特征指定为从H₂CN的基态X(2)B₂态到第二激发态B(2)A₁态的基态和2b₁（伞形模式）能级的振动电子跃迁。利用适当的振动波函数，从惯性张量矩的期望值计算出这些跃迁的上下能级的转动常数。对于适合该跃迁的假设的B型电偶极允许的(2)A₁←(2)B₂跃迁，这些谱带的实验和模拟转动轮廓彼此非常吻合。这些结果可以排除先前对偶极禁戒的(2)B₁←(2)B₂跃迁的归属。还给出了对电子激发的H₂CN解离途径的理论研究。观察到的谱带的上能级不能直接解离，而是通过内转换和随后解离为H + HCN碎片而衰减；更高的b₁能级高于激发态解离极限。
