H atom and heavy atom tunneling processes in tropolone

Abstract

The minimum energy pathway leading between the tautomers of tropolone was calculated using molecular orbital (MO) methods. This, with various 1D and 2D cuts of the potential energy surface (PES) topography, reveals the {tunneling skeleton}/{tunneling H atom} mechanism for tautomerization. In the zero-point states the H atom is localized to one of the O atoms until the tropolone skeleton becomes sufficiently vibrationally displaced towards C2v configurations that near-equal double-minimum potential energy functions (PEFs) arise for the H atom vibration. The resulting delocalization of the H atom between the two O atom sites allows the skeletal displacement to proceed through the barrier and the tautomerization process to be completed. The v1 (OH stretching) energies in quantum states N1 are strongly dependent on the skeletal geometry and, adiabatically separated from the slow v22 vibration, they contribute to markedly different 1D effective potential energy functions V22eff[N1] for v22. V22eff[N1=0] is a normal equal double minimum PEF while V22eff[N1≠0] have more complex shapes. Expressed as a function of the v22 skeletal displacement ΔS, the v1 states show a nonadiabatic curve crossing E1(1)→E1(2) contributing to the V22eff[N1=1→2] effective PEF for v22 vibration in the lowest excited OH stretching state. This function, rather than V22eff[N1=1], is strongly supported by the IR observations on v1. The computed effective energy barriers on the “model” tunneling path for the zero point states are 4.97 kcal/mol for the skeletal motion, and 3.22 kcal/mol for the H atom vibration at C2v skeletal geometry. Overall, the independent computational model predicts the major spectroscopic features observed for S0 tropolone(OH) and tropolone(OD): (a) similar IR tunneling doublets with ∼10 cm−1 splittings for the v22 skeletal vibration; (b) weak v1 IR absorbance with 20 and 5 cm−1 tunneling doublet separations for the isotopomers; (c) small tunneling splittings of the zero point states; and (d) unresolved vibrational state-specific IR tunneling doublets for all other fundamentals.