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.

Reactions that involve tunneling by carbon and the role that calculations have played in their study

“…Subsequent, more sophisticated, calculations on the automerization of CBD‐d 2 and experiments by Michl and coworkers on the isomerization of CBD‐1,2‐ 13 C 2 in matrix isolation, confirmed Carpenter's conclusions about the important role that tunneling by carbon plays in the CBD automerization reaction.…”

Isotope Effects and Heavy-Atom Tunneling in the Roush Allylboration of Aldehydes

“…Large kinetic isotope effects (KIEs) in hydrogen transfer reactions have often been interpreted as a diagnostic for tunneling through the reaction barrier. 1,2 The extent of tunneling is mass dependent and decreases with increasing mass of the atom. The much larger mass of carbon decreases its tunneling probability and consequently the effect of carbon tunneling on reaction rates is often ignored.…”

“…As a result, carbon tunneling is much less understood and has been addressed for only a handful of chemical reactions. 2 It has been observed that allowance for tunneling improves the prediction of heavy-atom isotope effects, but tunneling corrections in most reactions not involving hydrogen transfer are very small. As a result, heavy-atom KIE predictions based solely on TST, not allowing for tunneling, are still usually reasonably accurate 3 and sufficient for mechanistic interpretation.…”

“…Outside of reactions at exceedingly low temperatures, 2 however, the role of heavy-atom tunneling in organic reactions has been generally been assumed to be trivial. In the case of the ring-opening of cyclopropylcarbinyl radical, we recently demonstrated that there were relatively large effects of heavy-atom tunneling at −100 °C, 2h but the cyclopropylcarbinyl radical system has been postulated to be an exceptional case owing to an unusually narrow barrier in the reaction. 23 The allylboration reaction here is a perfectly ordinary organic reaction and the temperature, while low, is one that is commonly used in organic chemistry.…”

Modeling high pressure reactivity in unsaturated systems: Application to dimethylacetylene

“…Two bands around 760 and 1640 cm 21 are not been predicted by CASSCF. These vibrational modes are related to the automerization transition, 45 and therefore, they cannot be calculated with the harmonic approximation. In a recent publication, 46 these bands have been calculated using the present CASSCF calculations and an anharmonic model, confirming that the transformed product can be identified as TMCBD.…”