Heavy atoms and tunneling in the X̃ state of tropolone

Publisher Website
Google Scholar
Abstract
Large (6.9 to 16.3 cm−1 ) tunneling splittings are uniquely observed for the ν27 (OD stretch), ν31 (carbonyl stretch), and ν34 (C=C–C stretch) fundamentals of tropolone‐OH and tropolone‐OD in the X̃ 1A1 (ground) electronic state. These same three modes are predicted by the molecular geometry to interact strongly with tunneling because the dominant vibrational and tunneling displacements involve the same atoms. The heavy atom tunneling displacements (≊0.07 Å) are small enough to plausibly consider heavy atom tunneling phenomena—especially in appropriate excited vibrational states—and the tunneling splittings appear consistent with behavior expected at zero order for adiabatic reaction surface theory with a 2D reaction surface defined by C=O/C–O and C=C–C heavy atom coordinates. This model attributes tunneling in the X̃ state of tropolone to heavy atom motion followed adiabatically by H atom motion rather than the reverse. Energy balance equations are used to obtain estimates for the vibrational state‐specific tunneling barrier heights of tropolone‐OH (13.7 and 9.3 kcal/mol for the zero‐point and ν27 states) and tropolone‐OD (14.3 and 11.0 kcal/mol for the zero‐point and ν27 states).