TY - JOUR
T1 - Automerization reaction of cyclobutadiene and its barrier height
T2 - An ab initio benchmark multireference average-quadratic coupled cluster study
AU - Eckert-Maksić, Mirjana
AU - Vazdar, Mario
AU - Barbatti, Mario
AU - Lischka, Hans
AU - Maksić, Zvonimir B.
N1 - Funding Information:
The authors acknowledge financial support by the WTZ treaty between Austria and Croatia (Project Nos. 9/2004 and 1/2006). The work in Austria (H.L. and M.B.) was also supported by the Austrian Science Fund within the framework of the Special Research Program F16, while the work in Zagreb (M.E.-M., M.V., and Z.B.M.) was supported by the Ministry of Science and Technology of Croatia (Project Nos. 0098056 and 0098058). One of the authors (M.B.) thanks the Brazilian agency CNPq for financial support. This work was also sponsored by the COST D26 action, working groups 0014/03 and 0006/02. The calculations were performed in part on the Schroedinger III LINUX cluster of the Vienna University Computer Center.
PY - 2006
Y1 - 2006
N2 - The problem of the double bond flipping interconversion of the two equivalent ground state structures of cyclobutadiene (CBD) is addressed at the multireference average-quadratic coupled cluster level of theory, which is capable of optimizing the structural parameters of the ground, transition, and excited states on an equal footing. The barrier height involving both the electronic and zero-point vibrational energy contributions is 6.3 kcal mol -11, which is higher than the best earlier theoretical estimate of 4.0 kcal mol-1. This result is confirmed by including into the reference space the orbitals of the CC σ bonds beyond the standard π orbital space. It places the present value into the middle of the range of the measured data (1.6-10 kcal mol-1). An adiabatic singlet-triplet energy gap of 7.4 kcal mol-1 between the transition state 1Btg and the first triplet 3A2g state is obtained. A low barrier height for the CBD automerization and a small ΔE(3A2g, 1B1g) gap bear some relevance on the highly pronounced reactivity of CBD, which is briefly discussed.
AB - The problem of the double bond flipping interconversion of the two equivalent ground state structures of cyclobutadiene (CBD) is addressed at the multireference average-quadratic coupled cluster level of theory, which is capable of optimizing the structural parameters of the ground, transition, and excited states on an equal footing. The barrier height involving both the electronic and zero-point vibrational energy contributions is 6.3 kcal mol -11, which is higher than the best earlier theoretical estimate of 4.0 kcal mol-1. This result is confirmed by including into the reference space the orbitals of the CC σ bonds beyond the standard π orbital space. It places the present value into the middle of the range of the measured data (1.6-10 kcal mol-1). An adiabatic singlet-triplet energy gap of 7.4 kcal mol-1 between the transition state 1Btg and the first triplet 3A2g state is obtained. A low barrier height for the CBD automerization and a small ΔE(3A2g, 1B1g) gap bear some relevance on the highly pronounced reactivity of CBD, which is briefly discussed.
UR - http://www.scopus.com/inward/record.url?scp=33747253731&partnerID=8YFLogxK
U2 - 10.1063/1.2222366
DO - 10.1063/1.2222366
M3 - Article
AN - SCOPUS:33747253731
SN - 0021-9606
VL - 125
JO - Journal of Chemical Physics
JF - Journal of Chemical Physics
IS - 6
M1 - 064310
ER -