ISSN:
1089-7690
Source:
AIP Digital Archive
Topics:
Physics
,
Chemistry and Pharmacology
Notes:
Results of ab initio two-configuration CI-SD/[3s2p1d/2s], MBPT(4), CCSD+T(CCSD), and CCSDT-1 calculations are reported for the rectangular D2h equilibrium and square D4h transition structures of cyclobutadiene. The latter is a classic example of a multireference correlated method. The optimum CC and CH bond lengths found for the D4h transition structure are 1.448 and 1.093 A(ring), respectively. The activation barrier for the automerization is 9.0 kcal/mol at the two-reference GVB-CISD level while the single reference CCSD gives 19.9, 14.4 for CCSD+T(CCSD) and finally a dramatic change to 9.5 at the highest CCSDT-1 level. The importance of triples in overcoming the multireference character at the transition state is apparent. On the other hand, GVB-CISD is simpler than CCSDT-1 which attests to the importance of a qualitatively correct multireference starting point for this example. A less sophisticated computational method, GVB/4-31G, which also gives a reasonable barrier of 10.2 kcal/mol was used for the construction of the two-dimensional potential surface of automerization. The following lowest vibrational energies were obtained for this surface (v1 and v2, the symmetric and antisymmetric CC stretches in D4h symmetry, are given in parentheses): 0 and 4.2 cm−1 (00+; 00−); 1526.1 and 1607.6 cm−1 (01+; 01−), 1480.9, and 1485.5 cm−1 (10+; 10−), and 791.6 cm−1 for the zero-point energy (00+). The computed splitting of the vibrational ground state implies the rate of automerization is k=2.5×1011 s−1 for temperatures close to absolute zero.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1063/1.454956
Permalink
