Bond-Breaking/Bond-Forming Reactions by Vibrational Excitation: Infrared-Induced Bidirectional Tautomerization of Matrix-Isolated Thiotropolone

Abstract

Infrared vibrational excitation is a promising approach for gaining exceptional control of chemical reactions, in ways that cannot be attained via thermal or electronic excitation. Here, we report an unprecedented example of a bond-breaking/bond-forming reaction by vibrational excitation under matrix isolation conditions. Thiotropolone monomers were isolated in cryogenic argon matrices and characterized by infrared spectroscopy and vibrational computations (harmonic and anharmonic). Narrowband near-infrared irradiations tuned at frequencies of first CH stretching overtone (5940 cm–1) or combination modes (5980 cm–1) of the OH tautomer, the sole form of the compound that exists in the as-deposited matrices, led to its conversion into the SH tautomer. The tautomerization in the reverse direction was achieved by vibrational excitation of the SH tautomer with irradiation at 5947 or 5994 cm–1, corresponding to the frequencies of its CH stretching combination and overtone modes. This pioneer demonstration of bidirectional hydroxyl ↔ thiol tautomerization controlled by vibrational excitation creates prospects for new advances in vibrationally induced chemistry.

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

“…For ease of identification, we computed anharmonic IR spectra of 1 and 2 at B3LYP/6–311++G(3df,3pd). DFT in conjunction with a large Pople basis set generally reproduces matrix IR spectra very reasonably; recent precedence is provided in a study on thiotropolone [49] . Herein, the inclusion of anharmonicities leads to an overall good agreement between computations and measurements (see Supporting Information for details).…”

Structural Relevance of Intramolecular H-Bonding in Ortho-Hydroxyaryl Schiff Bases: The Case of 3-(5-bromo-2-hydroxybenzylideneamino) Phenol

“…Matrix isolation has also many practical advantages compared with other possible approaches (e.g., molecular beams), in particular when the probing method is a spectroscopic technique [ 31 , 32 ]. Moreover, the concerted use of matrix isolation infrared spectroscopy and quantum chemical electronic structure calculations has been proven to be a very powerful tool for structural studies on conformationally flexible and multi-isomeric complex systems [ 31 , 32 , 33 , 34 , 35 , 36 , 37 ].…”

Effect of a Solid-Hydrogen Environment on UV-Induced Hydrogen-Atom Transfer in Matrix-Isolated Heterocyclic Thione Compounds

“…The possibility of intramolecular hydrogen-atom transfer induced by excitation of higher vibrational states of the ground electronic state has recently been experimentally proven. 52 It was demonstrated that near-IR excitation of thiotropolone leads to the transformation of the thio-hydroxy tautomer into the oxo-thiol tautomeric form. This transformation occurs by a hydrogen-atom shift from the oxygen atom to the vicinal sulfur atom of the thiotropolone molecule.…”

“…This transformation occurs by a hydrogen-atom shift from the oxygen atom to the vicinal sulfur atom of the thiotropolone molecule. 52 This mechanism may be operative especially for cases where the energy barriers for tautomerization in the ground electronic state are relatively low. For molecules, where the tautomers are separated by very high barriers, the PIDA mechanism involving dissociation of a hydrogen atom, followed by its association with another heteroatom, seems to be more adequate.…”