Rotational isomers of small molecules in noble-gas solids: From monomers to hydrogen-bonded complexes

Radical reactions on interstellar icy dust grains: Experimental investigations of elementary processes

Molecular clouds (MCs) in space are the birthplace of various molecular species. Chemical reactions occurring on the cryogenic surfaces of cosmic icy dust grains have been considered to play important roles in the formation of these species. Radical reactions are crucial because they often have low barriers and thus proceed even at low temperatures such as ∼10 K. Since the 2000s, laboratory experiments conducted under low-temperature, high-vacuum conditions that mimic MC environments have revealed the elementary physicochemical processes on icy dust grains. In this review, experiments conducted by our group in this context are explored, with a focus on radical reactions on the surface of icy dust analogues, leading to the formation of astronomically abundant molecules such as H₂, H₂O, H₂CO, and CH₃OH and deuterium fractionation processes. The development of highly sensitive, non-destructive methods for detecting adsorbates and their utilization for clarifying the behavior of free radicals on ice, which contribute to the formation of complex organic molecules, are also described.

On the vibrations of formic acid predicted from first principles

In this article, we review recent first principles, anharmonic studies on the molecular vibrations of gaseous formic acid in its monomer form. Transitions identified as fundamentals for both cis- and trans form reported in these studies are collected and supported by results from high-resolution experiments. Attention is given to the effect of coordinate coupling on the convergence of the computed vibrational states.

CC-stretched formic acid: isomerisation, dimerisation, and carboxylic acid complexation

The cis-trans-isomerism of the propiolic acid monomer (HC[triple bond, length as m-dash]C-COOH) is examined with linear Raman jet spectroscopy, yielding the first environment-free vibrational band centres of a higher-energy cis-rotamer beyond formic acid (HCOOH) in addition to all fundamentals and a large number of hot and combination/overtone bands of the trans-conformer. Two near-isoenergetic trans-fundamentals of different symmetry (CC[double bond, length as m-dash]O bend and OH torsion) prove to be a sensitive benchmarking target, as their energetic order is susceptible to the choice of electronic structure method, basis set size, and inclusion of vibrational anharmonicity. For the infrared- and Raman-active C[double bond, length as m-dash]O stretching fundamentals of the cyclic (C_2h) trans-propiolic acid dimer, resonance couplings are found that in part extend to the C_s-symmetric heterodimer of trans-propiolic and trans-formic acid. Exploratory vibrational perturbation theory (VPT2) calculations show that all perturbing states involve displacements of the OH moieties located on the doubly hydrogen bonded ring. The comparison of the infrared spectra of the propiolic acid dimer and its heterodimer with formic acid to that of several other carboxylic acid dimers from the literature reveals a notable similarity regarding a non-fundamental dimer band around 1800 cm^-1, which in most cases is so far unassigned. VPT2 calculations and a simple harmonic model suggest an assignment to a combination vibration of the symmetric and antisymmetric OH torsion.

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.

S–H rotamerizationviatunneling in a thiol form of thioacetamide

Rotamerization of the S–H groupviahydrogen tunneling is reported for the first time.

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.

Infrared spectra of 3-hydroxy-(1H)-pyridinium cation and 3-hydroxy-(1H)-pyridinyl radical isolated in solid para-hydrogen

As pyridine and its derivatives are regarded as building blocks of nitrogen-containing polycyclic aromatic hydrocarbons, spectral identifications of their protonated and hydrogenated species are important. The infrared (IR) absorption spectra of the 3-hydroxy-(1H)-pyridinium cation, 3-C₅H₄(OH)NH⁺, and the 3-hydroxy-(1H)-pyridinyl radical, 3-C₅H₄(OH)NH, produced on electron bombardment during deposition of a mixture of 3-hydroxypyridine, 3-C₅H₄(OH)N, and para-H₂ to form a matrix at 3.2 K were recorded. Intense IR absorption lines of trans-3-C₅H₄(OH)NH⁺ at 3594.4, 3380.0, 1610.6, 1562.2, 1319.4, 1193.8, 1167.5, and 780.4 cm^-1 and eleven weaker ones decreased in intensity after the matrix was maintained in darkness for 20 h, whereas lines of trans-3-C₅H₄(OH)NH at 3646.2, 3493.4, 3488.7, 1546.7, 1349.6, 1244.1, 1209.1, 1177.3, 979.8, and 685.2 cm^-1 and nine weaker ones increased. The intensities of lines of trans-3-C₅H₄(OH)NH decreased upon irradiation at 520 nm and diminished nearly completely upon irradiation at 450 nm, whereas those of trans-3-C₅H₄(OH)NH⁺ remained unchanged upon irradiation at 370, 450, and 520 nm. Observed vibrational wavenumbers and relative intensities of these species agree satisfactorily with the scaled harmonic vibrational wavenumbers and IR intensities predicted with the B3LYP/aug-cc-pVTZ method. The observed 3-C₅H₄(OH)NH⁺ cation and 3-C₅H₄(OH)NH radical are predicted to be the most stable species among all possible isomers by quantum-chemical calculations.

Formic acid dimers in a nitrogen matrix

Formic acid (HCOOH) dimers are studied by infrared spectroscopy in a nitrogen matrix and by ab initio calculations. We benefit from the use of a nitrogen matrix where the lifetime of the higher-energy (cis) conformer is very long (∼11 h vs. 7 min in an argon matrix). As a result, in a nitrogen matrix, a large proportion of the cis conformer can be produced by vibrational excitation of the lower-energy (trans) conformer. Three trans-trans, four trans-cis, and three cis-cis dimers are found in the experiments. The spectroscopic information on most of these dimers is enriched compared to the previous studies in an argon matrix. The cis-cis dimers of ordinary formic acid (without deuteration) are reported here for the first time. Several conformational processes are obtained using selective excitation by infrared light, some of them also for the first time. In particular, we report on the formation of cis-cis dimers upon vibrational excitation of trans-cis dimers. Tunneling decays of several dimers have been detected in the dark. The tunneling decay of cis-cis dimers of formic acid as well as the stabilization of cis units in cis-cis dimers is also observed for the first time.

Nuclear dynamics and phase polymorphism in solid formic acid

We apply a unique sequence of structural and dynamical neutron-scattering techniques, augmented with density-functional electronic-structure calculations, to establish the degree of polymorphism in an archetypal hydrogen-bonded system - crystalline formic acid. Using this combination of experimental and theoretical techniques, the hypothesis by Zelsmann on the coexistence of the β₁ and β₂ phases above 220 K is tested. Contrary to the postulated scenario of proton-transfer-driven phase coexistence, the emerging picture is one of a quantitatively different structural change over this temperature range, whereby the loosening of crystal packing promotes temperature-induced shearing of the hydrogen-bonded chains. The presented work, therefore, solves a fifty-year-old puzzle and provides a suitable framework for the use neutron-Compton-scattering techniques in the exploration of phase polymorphism in condensed matter.

VUV photochemistry of the H2O⋯CO complex in noble-gas matrices: formation of the OH⋯CO complex and the HOCO radical

VUV photolysis of the H₂O⋯CO complexes leads to the formation of the OH⋯CO radical–molecule complexes and trans-HOCO radicals.

Theoretical investigation of gas-phase molecular complex formation between 2-hydroxy thiophenol and a water molecule

A systematic study of the interaction energies and hydrogen bonding interaction of a gas-phase molecular complex between 2-hydroxy thiophenol and a water molecule.

Acetic acid-water complex: The first observation of structures containing the higher-energy acetic acid conformer

Non-covalent interaction of acetic acid (AA) and water is studied experimentally by IR spectroscopy in a nitrogen matrix and theoretically at the MP2 and coupled-cluster with single and double and perturbative triple excitations [CCSD(T)]/6-311++G(2d,2p) levels of theory. This work is focused on the first preparation and characterization of complexes of higher-energy (cis) conformer of AA with water. The calculations show three 1:1 structures for the trans-AA⋯H2O complexes and three 1:1 structures for the cis-AA⋯H2O complexes. Two trans-AA⋯H2O and two cis-AA⋯H2O complexes are found and structurally assigned in the experiments. The two cis-AA⋯ ⋅ H2O complexes are obtained by annealing of a matrix containing water and cis-AA molecules prepared by selective vibrational excitation of the ground-state trans form. The less stable trans-AA⋯H2O complex is obtained by vibrational excitation of the less stable cis-AA⋯H2O complex. In addition, the 1:2 complexes of trans-AA and cis-AA with water molecules are studied computationally and the most stable forms of the 1:2 complexes are experimentally identified.

Double deuterated acetylacetone in neon matrices: infrared spectroscopy, photoreactivity and the tunneling process

Open enol conformers of double deuterated acetylacetone are produced by UV and IR irradiation, allowing the analysis of the tunnelling effect between them.

Understanding the ionic liquid [NC4111][NTf2] from individual building blocks: an IR-spectroscopic study

This study explores the interactions underlying the IR spectra of the ionic liquid [NC4111][NTf2] and its deuterated isotopomer [d9-NC4111][NTf2] by first isolating the spectra of charged ionic building blocks using mass-selective CIVP spectroscopy and then following the evolution of these bands upon sequential assembly of the ionic constituents. The spectra of the (1,1) and (2,2) neutral ion pairs are recorded using superfluid helium droplets as well as a solid neon matrix, while those of the larger charged aggregates are again obtained with CIVP. In general, the cluster spectra are similar to that of the bulk, with the (2,2) system displaying the closest resemblance. Analysis of the polarization-dependent band intensities of the neutral ion pairs in liquid droplets as a function of external electric field yields dipole moments of the neutral aggregates. This information allows a coarse assessment of the packing structure of the neutral pairs to be antiparallel at 0.37 K, in contrast to the parallel arrangement found for the assembly of small, high-dipole neutral molecules with large rotational constants (e.g., HCN). The role of an extra anion or cation attached to both the (1,1) and the (2,2) ion pairs to form the charged clusters is discussed in the context of an additional remote, more unfavorable binding site intrinsic to the nature of the charged IL clusters and as such not anticipated in the bulk phase. Whereas for the anion itself only the lowest energy trans conformer was observed, the higher clusters showed an additional population of the cis conformer. The interactions are found to be consistent with a minimal role of hydrogen bonding.

Matrix-isolation studies of noncovalent interactions: more sophisticated approaches

Noncovalent interactions are crucial for many physical, chemical, and biological phenomena. Matrix isolation is a powerful method to study noncovalent interactions, including hydrogen-bonded species, and it has been extensively used in this field. However, there are difficult situations, such as in the case of species that are impossible to prepare in the gas phase. In this article, we describe some advanced approaches allowing studies of complexes that are problematic for the traditional methods. Photolysis of a suitable precursor in a matrix can lead to a large concentration of 1:1 complexes, which are otherwise very difficult to prepare (e.g., the H2O···O complex). Photolysis of species combined with annealing can lead to complexes of molecules with mobile atoms (e.g., the same H2O···O complex). Simultaneous photolysis of two species combined with annealing can produce complexes of radicals via reactions of the photogenerated complexes with mobile atoms (e.g., the H2O···HCO complex). Interaction of noble-gas (Ng) hydrides with other species is another topic (e.g., the N2···HArF complex) and very large blue shifts of the H-Ng stretching modes are normally observed for these systems. Complexes and dimers of the higher-energy conformer of formic acid have been prepared by using selective vibrational excitation of the ground-state conformer. The higher-energy conformer of formic acid can be efficiently stabilized in the complexes with strong hydrogen bonding. We also consider some problematic cases when the changes in the vibrational frequencies of the 1:1 complexes are very small (e.g., the phenol···Xe complex) and when the complex formation is prevented by strong solvation in the matrix (e.g., species in solid xenon).

Predicted organic compounds derived from rare gas atoms and formic acid

Organic insertion compounds of rare gas atoms into formic acid were investigated at the MP2(full)/aug-cc-pVTZ level. There are two configuration isomers for each molecule based on the location of H atoms: trans- and cis-HCOORgH (Rg = Ar, Kr, Xe). Their structures, harmonic frequencies, and decomposition energies have been calculated using the above ab initio method. Using trans-HCOOXeH as an example, natural bond orbital (NBO) and atom-in-molecules (AIM) analyses were also carried out to explore the binding nature of the rare gas atoms. The formation mechanism of molecular orbitals is also presented in this paper. The presented results indicate that HCOOXeH and HCOOKrH are potential candidates for experimental observation.

Matrix isolation spectroscopy and nuclear spin conversion of NH3 and ND3 in solid parahydrogen

We present matrix isolation infrared absorption spectra of NH3 and ND3 trapped in solid parahydrogen (pH2) at temperatures around 1.8 K. We used the relatively slow nuclear spin conversion (NSC) of NH3 and ND3 in freshly deposited pH2 samples as a tool to assign the sparse vibration-inversion-rotation (VIR) spectra of NH3 in the regions of the ν2, ν4, 2ν4, ν1, and ν3 bands and ND3 in the regions of the ν2, ν4, ν1, and ν3 fundamentals. Partial assignments are also presented for various combination bands of NH3. Detailed analysis of the ν2 bands of NH3 and ND3 indicates that both isotopomers are nearly free rotors; that the vibrational energy is blue-shifted by 1-2%; and that the rotational constants and inversion tunneling splitting are 91-94% and 67-75%, respectively, of the gas-phase values. The line shapes of the VIR absorptions are narrow (0.2-0.4 cm(-1)) for upper states that cannot rotationally relax and broad (>1 cm(-1)) for upper states that can rotationally relax. We report and assign a number of NH3-induced infrared absorption features of the pH2 host near 4150 cm(-1), along with a cooperative transition that involves simultaneous vibrational excitation of a pH2 molecule and rotation-inversion excitation of NH3. The NSCs of NH3 and ND3 were found to follow first-order kinetics with rate constants at 1.8 K of k = 1.88(16) × 10(-3) s(-1) and k = 1.08(8) × 10(-3) s(-1), respectively. These measured rate constants are compared to previous measurements for NH3 in an Ar matrix and with the rate constants measured for other dopant molecules isolated in solid pH2.

Experimental (FT-IR, FT-Raman, NMR) and theoretical spectroscopic properties of intermolecular hydrogen bonded 1-acetyl-2-thiohydantoin polymorphs

In this work, use of FT-Raman, FT-IR and (13)C NMR spectroscopies have been made for the full characterization of 1-acetyl-2-thiohydantoin (ACTH). A detailed interpretation of the vibrational spectra was carried out with the aid of normal coordinate analysis using single scaling factor. Our results support the hydrogen bonding pattern proposed in the reported crystalline structure. Good reproduction of experimental values is obtained and % error is small in majority of the cases. Isotropic chemical shifts were calculated using gauge-invariant atomic orbital (GIAO) along with several thermodynamic parameters.

Formic and acetic acids in a nitrogen matrix: Enhanced stability of the higher-energy conformer

Formic acid (HCOOH, FA) and acetic acid (CH(3)COOH, AA) are studied in a nitrogen matrix. The infrared (IR) spectra of cis and trans conformers of these carboxylic acids (and also of the HCOOD isotopologue of FA) are reported and analyzed. The higher-energy cis conformer of these molecules is produced by narrowband near-IR excitation of the more stable trans conformer, and the cis-to-trans tunneling decay is evaluated spectroscopically. The tunneling process in both molecules is found to be substantially slower in a nitrogen matrix than in rare-gas matrices, the cis-form decay constants being approximately 55 and 600 times smaller in a nitrogen matrix than in an argon matrix, for FA and AA respectively. The stabilization of the higher-energy cis conformer is discussed in terms of specific interactions with nitrogen molecule binding with the OH group of the carboxylic acid. This model is in agreement with the observed differences in the IR spectra in nitrogen and argon matrices, in particular, the relative frequencies of the νOH and τCOH modes and the relative intensities of the νOH and νC=O bands.