Conformational Switching Induced by Near-Infrared Laser Irradiation

Methoxyacetone revisited

Conformational space of methoxyacetone (MA) was studied at the MP2/6-311++G(d,p) and DFT(B3LYP)/6-311++G(d,p) levels of theory. Computations predict MA to adopt four conformations, resulting from internal rotations around the O=C-C-O (Trans, Cis) and C-C-O-C (trans, gauche) dihedral angles. The Tt (Trans-trans) conformer is the most stable. The computed energies of two gauche (Tg and Cg) conformers fall in the 3-8 kJ mol-1 range above Tt and should account for 1/3 of the room-temperature gas-phase equilibrium. The energy of Ct form is 11 kJ mol-1 above Tt, and its expected population is negligible (below 1 %). In our earlier work, MA monomers were isolated in cryogenic argon matrices and characterized by infrared spectroscopy. In the experiment, only the most stable Tt conformer was detected in the sample. Signatures of the other conformers were not detected, either in freshly deposited samples, or in samples subjected to different UV irradiations. We rationalize those observations in terms of computed barriers for intramolecular torsions, indicating occurrence of conformational cooling during deposition. The experimental infrared spectrum of the Tt form is now assigned with the aid of anharmonic DFT computations. Exposure of MA to UV irradiation in the 300-260 nm range led to photolysis, according to the Norrish type II mechanism, resulting in dimer between enol acetone and formaldehyde observed as a cage-confined intermediate photoproduct. The subsequent photolysis resulted in the formation of carbon monoxide as the dominating photoproduct, formed in the Norrish type I photoreaction. Mechanistic interpretation of this photo decarbonylation reaction is presented.

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.

Vibrationally Induced Conformational Isomerization and Tunneling in Pyrrole-2-Carboxylic Acid

The conformational behavior of carboxylic acids has attracted considerable attention, as it can be used as a gateway for the study of more complex phenomena. Here, we present an experimental and computational study of pyrrole-2-carboxylic acid (PCA) conformational space and the vibrational characterization of the compound by infrared spectroscopy. The possibility of promoting conformational transformations using selective vibrational excitation of the 2ν(OH) and 2ν(NH) stretching overtones is explored. Two conformers, exhibiting the cis configuration of the COOH group (O═C-O-H dihedral angle near 0°) and differing by the orientation of the carboxylic group with respect to the pyrrole ring (i.e., showing either a cis or a trans NCC═O arrangement), were found to coexist initially for the compound isolated in a cryogenic nitrogen matrix, in an 86:14 ratio, and were characterized by infrared spectroscopy. A third conformer, with the COOH group in the trans configuration, was produced, in situ, by narrowband near-infrared (NIR) excitation of the most stable PCA form (with a cis NCC═O moiety). The photogenerated PCA conformer was found to decay back to the most stable PCA form, by H-atom quantum mechanical tunneling, with a characteristic half-life time of ∼10 min in the nitrogen matrix at 10 K. Tunneling rates were theoretically estimated and compared for the observed isomerization of pyrrole-2-carboxylic acid and for the structurally similar furan-2-carboxylic acid. This comparison showcases the effect of small modifications in the potential energy surface and the implications of quantum tunneling for the stability of short-living species.

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.

Model Behavior: Characterization of Hydroxyacetone at the Air-Water Interface Using Experimental and Computational Vibrational Sum Frequency Spectroscopy

Small atmospheric aldehydes and ketones are known to play a significant role in the formation of secondary organic aerosols (SOA). However, many of them are difficult to experimentally isolate, as they tend to form hydration and oligomer species. Hydroxyacetone (HA) is unusual in this class as it contributes to SOA while existing predominantly in its unhydrated monomeric form. This allows HA to serve as a valuable model system for similar secondary organic carbonyls. In this paper the surface behavior of HA at the air-water interface has been investigated using vibrational sum frequency (VSF) spectroscopy and Wilhelmy plate surface tensiometry in combination with computational molecular dynamics simulations and density functional theory calculations. The experimental results demonstrate that HA has a high degree of surface activity and is ordered at the interface. Furthermore, oriented water is observed at the interface, even at high HA concentrations. Spectral features also reveal the presence of both cis and trans HA conformers at the interface, in differing orientations. Molecular dynamics results indicate conformer dependent shifts in HA orientation between the subsurface (∼5 Å deep) and surface. Together, these results provide a picture of a highly dynamic, but statistically ordered, interface composed of multiple HA conformers with solvated water. These results have implications for HA's behavior in aqueous particles, which may affect its role in the atmosphere and SOA formation.

Nunes C, Fausto R, Reva I. Conformational control over an aldehyde fragment by selective vibrational excitation of interchangeable remote antennas

We apply vibrational antennas (OH or NH₂ group) to achieve unprecedented conformational control over the heavy aldehyde fragment in 2-formyl-2H-azirine, using selective vibrational excitations of the OH or NH₂ stretching overtones and combination modes.

Conformational changes in matrix-isolated 6-methoxyindole: Effects of the thermal and infrared light excitations

Conformational changes induced thermally or upon infrared excitation of matrix-isolated 6-methoxyindole were investigated. Narrowband near-infrared excitation of the first overtone of the N-H stretching vibration of each one of the two identified conformers is found to induce a selective large-scale conversion of the pumped conformer into the other one. This easily controllable bidirectional process consists in the intramolecular reorientation of the methoxy group and allowed a full assignment of the infrared spectra of the two conformers. Matrices with different conformational compositions prepared by narrow-band irradiations were subsequently used to investigate the effects of both thermal and broadband infrared excitations on the conformational mixtures. Particular attention is given to the influence of the matrix medium (Ar vs. Xe) and conformational effects of exposition of the sample to the spectrometer light source during the measurements.

Conformers of β-aminoisobutyric acid probed by jet-cooled microwave and matrix isolation infrared spectroscopic techniques

β-aminoisobutyric acid (BAIBA) has been studied in isolation conditions: in the gas phase and trapped into a cryogenic N2 matrix. A solid sample of the compound was vaporized by laser ablation and investigated through their rotational spectra in a supersonic expansion using two different spectroscopic techniques: broadband chirped pulse Fourier transform microwave spectroscopy and conventional molecular beam Fourier transform microwave spectroscopy. Four conformers with structures of two types could be successfully identified by comparison of the experimental rotational and (14)N nuclear quadruple coupling constants with those predicted theoretically: type A, bearing an OH⋯N intramolecular hydrogen bond and its carboxylic group in the trans geometry (H-O-C=O dihedral ∼180°), and type B, having an NH⋯O bond and the cis arrangement of the carboxylic group. These two types of conformers could also be trapped from the gas phase into a cryogenic N2 matrix and probed by Fourier transform infrared (IR) spectroscopy. In situ irradiation of BAIBA isolated in N2 matrix of type B conformers using near-IR radiation tuned at the frequency of the O-H stretching 1st overtone (∼6930 cm(-1)) of these forms allowed to selectively convert them into type A conformers and into a new type of conformers of higher energy (type D) bearing an NH⋯O=C bond and a O-H "free" trans carboxylic group.

Conformational switching in pyruvic acid isolated in Ar and N₂ matrixes: spectroscopic analysis, anharmonic simulation, and tunneling

Monomers of pyruvic acid (PA) isolated in cryogenic argon and nitrogen matrixes were characterized by mid- and near-infrared spectroscopy. Interpretation of the experiments was aided by fully anharmonic calculations of the fundamental modes, overtones, and combinations up to two quanta, including their infrared intensities. The initially dominating PA conformer (Tc) has a cis CCOH arrangement and is stabilized by a strong intramolecular H-bond. Selective near-infrared excitation of Tc at the first OH overtone (6630 cm(-1) in Ar, 6643 cm(-1) in N2) induced a large scale conformational conversion to the higher-energy conformer (Tt) with trans CCOH arrangement. Tt was then converted back to Tc by selective NIR irradiation at the first Tt OH overtone (6940 cm(-1) in Ar, 6894 cm(-1) in N2). In N2 matrix, the Tt form was stabilized due to interaction between the OH group and the matrix molecules. This stabilization manifested itself in the absence of Tt → Tc relaxation and in a considerable change of the vibrational Tt signature upon going from argon to nitrogen matrix. In argon, the Tt form spontaneously decayed back to Tc in the dark (characteristic lifetime +16 h). In the presence of broad-band near-infrared light, the Tt → Tc relaxation speed considerably increased. The decay mechanisms are discussed.

Near-infrared laser-induced generation of three rare conformers of glycolic acid

Structural transformations were induced in conformers of glycolic acid by selective excitation with monochromatic tunable near-infrared laser light. For the compound isolated in Ar matrixes, near-IR excitation led to generation of two higher-energy conformers (GAC; AAT) differing from the most stable SSC form by 180° rotation around the C-C bond. A detailed investigation of this transformation revealed that one conformer (GAC) is produced directly from the near-IR-excited most stable conformer. The other higher-energy conformer (AAT) was effectively generated only upon excitation of the primary photoproduct (GAC) with another near-IR photon. Once these higher-energy conformers of glycolic acid were generated in an Ar matrix, they could be subsequently transformed into one another upon selective near-IR excitations. Interestingly, no repopulation of the initial most stable SSC conformer occurred upon near-IR excitation of the higher-energy forms of the compound isolated in solid Ar. A dramatically different picture of near-IR-induced conformational transformations was observed for glycolic acid isolated in N2 matrixes. In this case, upon near-IR excitation, the most stable SSC form converted solely into a new conformer (SST), where the acid OH group is rotated by 180°. This conformational transformation was found to be photoreversible. Moreover, SST conformer, photoproduced in the N2 matrix, spontaneously converted to the most stable SSC form of glycolic acid, when the matrix was kept at cryogenic temperature and in the dark.

Conformational space and photochemistry of tyramine isolated in argon and xenon cryomatrixes

The infrared spectra of tyramine monomers trapped in low-temperature argon and xenon matrixes were recorded. The presence of the flexible ethylamino side chain gives rise to a complex conformational surface that contains several minima of relatively low energies, some of them stabilized by a weak N-H···π hydrogen bond interaction between the amino group and the phenyl ring. The experimental infrared spectra confirm the presence of at least two stable conformers isolated in the matrixes. Annealing experiments performed on the xenon matrix revealed a change in the relative population of the experimentally relevant conformers upon isolation in this polarizable matrix, compared to the gas phase. The general interpretation of the spectra was based on harmonic and anharmonic quantum chemical calculations, undertaken at the DFT/B3LYP and MP2 levels of theory with the 6-311++G(d,p) basis set. The photochemical behavior of the matrix-isolated compound upon narrow-band UV irradiation was also investigated. Identification of ketene species in the spectra of the irradiated matrixes suggests the occurrence of a ring-opening reaction, which in the xenon matrix occurs concomitantly with the conformational isomerization of tyramine.

Photochemical isomerizations of thiosemicarbazide, a matrix isolation study

Two thione conformers of monomeric thiosemicarbazide were trapped from the gas phase into a low-temperature Ar matrix. A phototransformation converting the less stable form of the compound into the most stable conformer was induced by irradiation with near-IR (λ = 1462 nm) or UV (λ > 320 nm) light. This photoeffect allowed separation of the IR spectra of the observed thione forms. The structures of both observed isomers were identified by comparison of the separated experimental IR spectra with the spectra theoretically predicted for two most stable forms of the compound. The population ratio of the two conformers in an Ar matrix, prior to any irradiation, was estimated to be equal ≈2:1. Irradiation of matrix-isolated thiosemicarbazide with shorter-wavelength UV (λ > 270 nm) light induced a phototautomeric reaction generating thiol forms of the compound.