Domino Tunneling

Abstract

Matrix-isolation experiments near 3 K and state-of-the-art quantum chemical computations demonstrate that oxalic acid [1, (COOH)2] exhibits a sequential quantum mechanical tunneling phenomenon not previously observed. Intensities of numerous infrared (IR) bands were used to monitor the temporal evolution of the lowest-energy O-H rotamers (1cTc, 1cTt, 1tTt) of oxalic acid for up to 19 days following near-infrared irradiation of the matrix. The relative energies of these rotamers are 0.0 (1cTc), 2.6 (1cTt), and 4.0 (1tTt) kcal mol(-1). A 1tTt → 1cTt → 1cTc isomerization cascade was observed with half-lives (t1/2) in different matrix sites ranging from 30 to 360 h, even though the sequential barriers of 9.7 and 10.4 kcal mol(-1) are much too high to be surmounted thermally under cryogenic conditions. A general mathematical model was developed for the complex kinetics of a reaction cascade with species in distinct matrix sites. With this model, a precise, global nonlinear least-squares fit was achieved simultaneously on the temporal profiles of nine IR bands of the 1cTc, 1cTt, and 1tTt rotamers. Classes of both fast (t(1/2) = 30-50 h) and slow (t(1/2) > 250 h) matrix sites were revealed, with the decay rate of the former in close agreement with first-principles computations for the conformational tunneling rates of the corresponding isolated molecules. Rigorous kinetic and theoretical analyses thus show that a "domino" tunneling mechanism is at work in these oxalic acid transformations.

Characterization of the Simplest Thiolimine: The Higher Energy Tautomer of Thioformamide

“…This might on the one hand be due to the IR Globar irradiation from the spectrometer (the activation barrier for the reverse reaction 2 ct → 2 tt is only 3.7 kcal mol −1 ). On the other hand, such phenomena have been assigned to matrix effects before, namely that specific matrix environments (or matrix sites) inhibit QMT [50] . Recent precedence is provided in a study on thioacetamide [20] .…”

Atom Tunneling in Chemistry

“…[174] The cis-trans isomerization reactions of carboxylic acids were studied extensively. [175][176][177][178][179][180][181][182] These reactions are mostly carried out at cryogenic temperatures in noble gas or N 2 matrix environments.Itwas shown that such an environment can influence the tunneling rates of the isomerization reactions significantly.T his might be taken as ac aveat when comparing with, for example,q uantum chemical gas-phase calculations. [178,179,[183][184][185][186] In larger, bio-organic molecules,t unneling supports radical reactions such as in the autoxidation of tetraline.…”

“…Rotation of the OH group in phenol was shown by FTIR spectroscopy to happen through tunneling, while deuteration of the ortho or meta positions suppresses the tunneling motion at low temperature . The cis–trans isomerization reactions of carboxylic acids were studied extensively . These reactions are mostly carried out at cryogenic temperatures in noble gas or N 2 matrix environments.…”

Tunnelling in carbonic acid

“…45 To achieve highquality fits of the data within the experimental error bars, the assumption of three classes of matrix-isolated carbonic acid molecules was required: a frozen class, in which tunnelling is entirely precluded by the environment; a slow class with seemingly hindered conformational tunnelling and half-lives (t 1 ) of 11-26 h; and a fast class, characterized by half-lives (t 2 ) of 2-5 h, for which the matrix atoms appear to interfere to only a limited extent with the conformational tunnelling. 45 To achieve highquality fits of the data within the experimental error bars, the assumption of three classes of matrix-isolated carbonic acid molecules was required: a frozen class, in which tunnelling is entirely precluded by the environment; a slow class with seemingly hindered conformational tunnelling and half-lives (t 1 ) of 11-26 h; and a fast class, characterized by half-lives (t 2 ) of 2-5 h, for which the matrix atoms appear to interfere to only a limited extent with the conformational tunnelling.…”