Applications of multireference coupled cluster theory

Abstract

In this work, Mukherjee multireference and single reference coupled cluster theory has been applied to the cyclobutanetetraone and trimethylene systems. Computations performed on cyclobutanetetraone have found that while the molecule appears to be just another simple, closed-shell molecule, it in fact has at least four low‐lying electronic states with D4h symmetry (Zhou, X.; Hrovat, D. A.; Borden, W. T. J. Phys. Chem. A 2010, 114, 1304). These include an all-symmetric state with eight π-electrons, a open-shell singlet and a triplet state with nine π-electrons, and an all-symmetric state with ten π-electrons. In this work, Mk-MRCCSD and CCSD(T) geometry optimizations with a cc-pVQZ basis have been executed for the low-lying states. These states are spread over 5.95 kcal/mol based upon the focal point approach. A recent photoelectron study (Guo, J.-C.; Hou, G.-L.; Li, S.-D.; Wang, X.-B. J. Phys. Chem. Lett. 2012, 3, 304) of the cyclobutanetetraone anion reported term values of 1.50 ± 0.1 and 3.23 ± 0.1 kcal mol–1 for the 8π all-symmetric and the 9π singlet open-shell states of neutral cyclobutanetetraone respectively; however, our computations indicate that the 10π all-symmetric state lies below the 9π open-shell singlet state. In contrast, the 9π triplet ground state is predicted to lie 4.34 kcal/mol below the 8π all-symmetric state, and the 10π all-symmetric state 1.00 kcal/mol above the 8π all-symmetric state. The trimethylene diradical is involved in the thermal isomerization of cyclopropane and the isomerization to propene. The transition states (2, C2v) and (3, Cs) for the isomerization of cyclopropane possess substantial static and dynamical electron correlation, and thus has been optimized with the Mk‐MRCCSD(T) method. Upon which focal point analyses of the barrier height to isomerization of cyclopropane have been performed. Their character was investigated by computing vibrational frequencies, which have imaginary frequencies corresponding to the conrotatory and disrotatory motions for (2) and (3) respectively, at the Mk-MRPT2 and Mk-MRCCSD levels of theory with the cc-pVDZ basis set. Focal point analyses of the vertical excitation energy of the transition states found that a seam of crossing occurs near the C2v edge-to-edge transition state (2).