Kim Joonghan, Kim Jeongho, Ihee Hyotcherl
Department of Chemistry, The Catholic University of Korea , Bucheon, 420-743, Republic of Korea.
J Phys Chem A. 2013 May 9;117(18):3861-8. doi: 10.1021/jp401128k. Epub 2013 May 1.
We investigate the ground and low-lying excited states of unsaturated chromium carbonyls, Cr(CO)2 and Cr(CO)3, using multiconfigurational ab initio perturbation theory. Unlike other chromium carbonyls, there are discrepancies between the experiment and theory on the identity of the ground states of Cr(CO)2 and Cr(CO)3. From multireference ab initio calculations considering the full valence orbitals of Cr(CO)2 and Cr(CO)3, the differences in the molecular structures of their various electronic states are explained by the electronic structure analysis. On the basis of the result from CASPT2 and MS-CASPT2 calculations, we propose that the ground states of Cr(CO)2 and Cr(CO)3 are the (5)Πg and (1)A1 states, respectively, addressing the ambiguity regarding their ground states. In addition, the multiconfigurational ab initio perturbation theory calculations reveal that (1) the energy gaps between the ground and first low-lying excited states of Cr(CO)2 and Cr(CO)3 are quite small and (2) the first low-lying excited states of Cr(CO)2 and Cr(CO)3 have the same spin multiplicities as the ground states of CrCO and Cr(CO)2, respectively, which are the products of ligand dissociation. As a result, the apparent spin-forbidden dissociation of Cr(CO)2 and Cr(CO)3 into CrCO and Cr(CO)2, respectively, are likely to be facilitated by thermal excitation of the ground states of Cr(CO)2 and Cr(CO)3 into their first low-lying excited states, which then actually undergoes the spin-allowed dissociation to the ground states of CrCO and Cr(CO)2 with the same spin multiplicities.
我们使用多组态从头算微扰理论研究了不饱和羰基铬Cr(CO)₂和Cr(CO)₃的基态和低激发态。与其他羰基铬不同，Cr(CO)₂和Cr(CO)₃基态的归属在实验和理论之间存在差异。通过考虑Cr(CO)₂和Cr(CO)₃的全价轨道进行多参考从头算计算，通过电子结构分析解释了它们各种电子态分子结构的差异。基于CASPT2和MS - CASPT2计算结果，我们提出Cr(CO)₂和Cr(CO)₃的基态分别是(5)Πg和(1)A₁态，解决了它们基态的不确定性问题。此外，多组态从头算微扰理论计算表明：(1) Cr(CO)₂和Cr(CO)₃基态与第一低激发态之间的能隙相当小；(2) Cr(CO)₂和Cr(CO)₃的第一低激发态分别与配体解离产物CrCO和Cr(CO)₂基态具有相同的自旋多重度。因此，Cr(CO)₂和Cr(CO)₃分别向CrCO和Cr(CO)₂的明显自旋禁阻解离可能通过将Cr(CO)₂和Cr(CO)₃基态热激发到它们的第一低激发态来促进，然后这些激发态实际上经历自旋允许解离到具有相同自旋多重度的CrCO和Cr(CO)₂基态。
