IR-induced and tunneling reactions in cryogenic matrices: the (incomplete) story of a successful endeavor

In this article, IR-induced and tunneling-driven reactions observed in cryogenic matrices are described in a historical perspective, the entangling of the two types of processes being highlighted. The story of this still ongoing fascinating scientific endeavor is presented here following closely our own involvement in the field for more than 30 years, and thus focuses mostly on our work. It is, because of this reason, also an incomplete story. Nevertheless, it considers a large range of examples, from very selective IR-induced conformational isomerizations to IR-induced bond-breaking/bond-forming reactions and successful observations of rare heavy atom tunneling processes. As a whole, this article provides a rather general overview of the major progress achieved in the field.

Inducing molecular reactions by selective vibrational excitation of a remote antenna with near-infrared light

We demonstrate here that selective vibrational excitation of a moiety, remotely attached in relation to the molecular reaction site, might offer a generalized strategy for inducing bond-breaking/bond-forming reactions with exquisite precision. As a proof-of-principle, the electrocyclic ring-expansion of a benzazirine to a ketenimine was induced, in a cryogenic matrix, by near-IR light tuned at the overtone stretching frequency of its OH remote antenna. This accomplishment paves the way for harnessing IR vibrational excitation as a tool to guide a variety of molecular structure manipulations in an exceptional highly-selective manner.

Switching on H-Tunneling through Conformational Control

H-tunneling is a ubiquitous phenomenon, relevant to fields from biochemistry to materials science, but harnessing it for mastering the manipulation of chemical structures still remains nearly illusory. Here, we demonstrate how to switch on H-tunneling by conformational control using external radiation. This is outlined with a triplet 2-hydroxyphenylnitrene generated in an N₂ matrix at 10 K by UV-irradiation of an azide precursor. The anti-orientation of the nitrene's OH moiety was converted to syn by selective vibrational excitation at the 2ν(OH) frequency, thereby moving the H atom closer to the vicinal nitrene center. This triggers spontaneous H-tunneling to a singlet 6-imino-2,4-cyclohexadienone. Computations reveal that such fast H-tunneling occurs through crossing the triplet-to-singlet potential energy surfaces. Our experimental realization provides an exciting novel strategy to attain control over tunneling, opening new avenues for directing chemical transformations.

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

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.

Conformational isomerizations triggered by vibrational excitation of second stretching overtones

Vibrational excitation using frequency-tunable IR laser light has been developed as a powerful tool for selective manipulation of molecular conformations. In this methodology, vibrational excitation has been typically applied to the first stretching overtones (∼80 kJ mol^-1) but also to the fundamental modes (∼40 kJ mol^-1). Here, we demonstrate that selective conformational isomerizations are also achieved using excitation to second stretching overtones (∼120 kJ mol^-1). The extremely weak absorptions of the second stretching overtones of molecules isolated in low-temperature matrices were measured for the first time; here using three prototype molecules: hydroxyacetone (HA), glycolic acid (GAc) and glycolamide (GAm). Benchmarking of computed anharmonic IR spectra showed that the B3LYP/SNSD method provides the best agreement with experimental frequencies of the ν(OH), 2ν(OH) and 3ν(OH) modes for the studied molecules in argon matrices. Selective irradiation at the 3ν(OH) frequencies (9850-10 500 cm^-1) of HA, GAc and GAm monomers in argon matrices at 15 K successfully triggers their conformational isomerization. These results open the door to extend control over conformations separated by higher barriers and to induce other transformations not energetically accessible by excitation to the fundamental or first stretching overtone modes.

Nunes C, Tarczay G, Fausto R. Selective conformational control by excitation of NH imino vibrational antennas

We provide experimental evidence for the occurrence of selective and reversible conformational control over the SH group by vibrational excitation of remote NH groups. Using an imino group that acts as a molecular antenna has no precedents.