title: 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.
authors.affiliation: ⊥Institute of Organic Chemistry, Justus-Liebig University, Heinrich-Buff-Ring 58, D-35392 Giessen, Germany.
authors.forename: Peter R
authors.initials: PR
authors.lastname: Schreiner
authors.affiliation: ⊥Institute of Organic Chemistry, Justus-Liebig University, Heinrich-Buff-Ring 58, D-35392 Giessen, Germany.
authors.forename: J Philipp
authors.initials: JP
authors.lastname: Wagner
authors.affiliation: ⊥Institute of Organic Chemistry, Justus-Liebig University, Heinrich-Buff-Ring 58, D-35392 Giessen, Germany.
authors.forename: Hans Peter
authors.initials: HP
authors.lastname: Reisenauer
authors.affiliation: ⊥Institute of Organic Chemistry, Justus-Liebig University, Heinrich-Buff-Ring 58, D-35392 Giessen, Germany.
authors.forename: Dennis
authors.initials: D
authors.lastname: Gerbig
authors.affiliation: ⊥Institute of Organic Chemistry, Justus-Liebig University, Heinrich-Buff-Ring 58, D-35392 Giessen, Germany.
authors.forename: David
authors.initials: D
authors.lastname: Ley
authors.affiliation: ‡Laboratory of Molecular Structure and Dynamics, Institute of Chemistry, Eötvös University, PO Box 32, Budapest 112, Hungary, H-1518.|§MTA-ELTE Complex Chemical Systems Research Group, Eötvös University, Budapest, Pázmány Péter Sétány 1/A, Hungary, H-1117.
authors.forename: János
authors.initials: J
authors.lastname: Sarka
authors.affiliation: ‡Laboratory of Molecular Structure and Dynamics, Institute of Chemistry, Eötvös University, PO Box 32, Budapest 112, Hungary, H-1518.|§MTA-ELTE Complex Chemical Systems Research Group, Eötvös University, Budapest, Pázmány Péter Sétány 1/A, Hungary, H-1117.
authors.forename: Attila G
authors.initials: AG
authors.lastname: Császár
authors.affiliation: ∥Center for Computational Quantum Chemistry and Department of Chemistry, University of Georgia, Athens, Georgia 30602, United States.
authors.forename: Alexander
authors.initials: A
authors.lastname: Vaughn
authors.affiliation: ∥Center for Computational Quantum Chemistry and Department of Chemistry, University of Georgia, Athens, Georgia 30602, United States.
authors.forename: Wesley D
authors.initials: WD
authors.lastname: Allen