State-specific vibrational anharmonicities in cyclobutadiene and evidence for fast automerization by C412H4

Abstract

MO computations of vibrational spectra for 1,3-cyclobutadiene (CB) and CB:CO2 van der Waals molecule isotopomers at the MP2/6-311++G(2d,2p) level of MO theory are analyzed together with the extensive Ar/CB matrix-isolation spectra of the Michl group, the low 3.2 kcal/mol (ZPE-corrected) MO-computed energy of the square-planar saddle-point for CB reported by Nakamura et al., and opportune one-dimensional analysis of the automerization energy levels of C412H4. The composite interpretation results in the assignment of the strongly anharmonic Raman transitions of Ar/CB observed at 723 and 1678 cm−1 to the E2-E0 and E4-E0 transitions, respectively, in the automerization coordinate. The C13 and D isotope dependencies of the nominal C=C stretching and δCH wagging vibrational normal modes of the CB valence isomers depend markedly on whether the labeled HC=CH moieties of CB are mass equivalent or not, and the pronounced normal mode differences are viewed as providing different gateways into the global anharmonic PES domains. The automerization coordinate Qa originates as the nominal C=C stretching normal coordinate of ag symmetry, followed by a curving to more efficiently interchange the C=C and C–C bond lengths as the square-planar D4h configuration is approached. The latter is crossed at a geometry dilated with respect to the saddle-point, thereby providing a barrier configuration about 5 kcal/mol above the PES minima (i.e., a dilation energy of about 2 kcal/mol plus the 3.2 kcal/mol saddle-point energy). The computed zero-point levels of C412H4 (1-D modeling) are separated by E1-E0=11 cm−1 to suggest ZP tunneling rates reaching the picosecond time scale. The analysis exposes new aspects of the intramolecular dynamics of CB, and the proposed automerization mechanism accounts for all of the presently known information pertaining to this interesting property of the cyclobutadiene molecule.