Rotranslational dynamics of confined water. II. Spectroscopic evidence of confinement effects on the far-infrared spectra of water isotopologues in argon and krypton matrices.

Water molecules trapped in rare gas matrices exhibit conspicuous shifts in their far-infrared (FIR), rotranslational spectral features compared with the corresponding transitions observed in the gas phase. These confinement-induced perturbations have been related not only to the quantization of translational motion but also to the coupling between the orientational and positional degrees of freedom: the rotation-translation coupling (RTC). As the propensity displayed by the nuclear spin isomers (NSI) of water to undergo interconversion in confinement is intimately related to how its nuclear spin degrees of freedom are coupled with those for intra- and intermolecular motions, confinement-induced RTC should also strongly impact the NSI interconversion mechanisms and rates. Insight into the rotranslational dynamics for H2 16O, H2 17O, and H2 18O, confined in argon and krypton matrices, is provided here based on the evolution of rotranslational spectra induced by NSI interconversion while a definitive assignment is provided from the transition energies and intensities calculated using the confined rotor model [Paper I, Wespiser et al., J. Chem. Phys. 156, 074304 (2021)]. In order to build a complete rotranslational energy diagram of confined water, which is fundamental to understand the NSI interconversion rates, the energy difference between the ground ortho and para rotranslational states is derived from the temperature dependence of the intensity ratio of mid-infrared lines emerging from these states. These investigations should provide deeper insight of the factors that control NSI interconversion of water isotopologues under extreme confinement.

Infrared Spectroscopy and Density Functional Theory Investigations of the Reactivity of Titanium Atoms with Carbon Monoxide and Water Isolated in Solid Argon: Sequential Evolution from Triplet to Singlet State.

Thermally evaporated titanium atoms reacted with carbon monoxide and water in solid argon at 12 K to produce the HTiOH-CO and HTiOH-(CO)2 molecules, which were characterized using infrared spectroscopy on the basis of CO, Ti, and water concentration variations and of isotopic substitutions. The insertion product, HTiOH, resulting from the reaction of a titanium atom with a water molecule reacts with CO spontaneously to give the HTiOH-CO molecule, which in turn reacts with a second CO molecule to give HTiOH-(CO)2 The density functional theory calculations were performed to elucidate the geometrical and electronic structures and support the spectral assignments. The topological analysis of the charge density within the experimentally observed molecules allowed us to rationalize the coordination sphere as well as the electron pairing on the titanium center.

Jet-cooled rovibrational spectroscopy of methoxyphenols using two complementary FTIR and QCL based spectrometers.

Methoxyphenols (MPs) are a significant component of biomass burning emissions which mainly exists in our atmosphere in the gas phase where they contribute to the formation of secondary organic aerosols (SOAs). Rovibrational spectroscopy is a promising tool to monitor atmospheric MPs and infer their role in SOA formation. In this study, we bring a new perspective on the rovibrational analysis of MP isomers by taking advantage of two complementary devices combining jet-cooled environments and absorption spectroscopy: the Jet-AILES and the SPIRALES setups. Based on Q-branch frequency positions measured in the Jet-AILES Fourier-transform infrared (FTIR) spectra and guided by quantum chemistry calculations, we propose an extended vibrational and conformational analysis of the different MP isomers in their fingerprint region. Some modes such as far-IR out-of-plane -OH bending or mid-IR in-plane -CH bending allow us to assign individually all the stable conformers. Finally, using the SPIRALES setup with three different external cavity quantum cascade laser sources centered on the 930-990 cm-1 and the 1580-1690 cm-1 ranges, it was possible to proceed to the rovibrational analysis of the ν18 ring in-plane bending mode of the MP meta isomer providing a set of reliable excited state parameters, which confirms the correct assignment of two conformers. Interestingly, the observation of broad Q-branches without visible P- and R-branches in the region of the C-C ring stretching bands was interpreted as being probably due to a vibrational perturbation. These results highlight the complementarity of broadband FTIR and narrowband laser spectroscopic techniques to reveal the vibrational conformational signatures of atmospheric compounds over a large infrared spectral range.

Vibrational study in neon matrix of H2S-H2O, H2S-(H2O)2, and (H2S)2-H2O complexes. Identification of the two isomers: HOH-SH2 (H2O proton donor) and HSH-OH2 (H2S proton donor).

For the first time, the investigation of water molecules complexed with hydrogen sulfide in solid neon was performed from 80 to 6000 cm-1 using Fourier transform infrared spectroscopy. In the first step, we identify the ν1 and ν3 frequencies of the proton donor in the H2S dimer. From concentration effects and with the help of theoretical results, we have highlighted the presence of the two stable isomers, HOH-SH2 where H2O is the proton donor and HSH-OH2 where H2S is the proton donor. We also identify several transitions for (H2S)2-H2O and H2S-(H2O)2 complexes, the first step of the microsolvation of H2S, and we propose structures for these complexes with the help of theoretical calculations at the second-order Møller-Plesset level.

Infrared Spectroscopy and Density Functional Theory Investigations of PdTi Heterodimer Reactivity with Carbon Monoxide Isolated in Solid Argon.

The reactivity of diatomic palladium-titanium toward carbon monoxide has been studied in solid argon by infrared spectroscopy (Fourier transform infrared) in the carbonyl stretching frequency region. Our technique of sublimation of Ti and Pd atoms from two filaments heated separately allowed the identification of five new molecules. Small polynuclear carbonyl clusters, PdTi(CO) n ( n = 1-3), have been characterized on the basis of isotopic substitutions, metal atom and CO concentration variations, and irradiation effects. Selective irradiation in the visible region leads to conversion between two isomers distinguished by the stretching frequency of the diatomic CO: PdTiCO Ti-eo (νCO = 1908.4 cm-1) and PdTiCO Pd-eo (νCO = 2009.3 cm-1). The density functional theory calculations have been carried out to elucidate the geometrical and electronic structures and support the spectral assignments. The nature of the metal-carbon bonding has been discussed using the topology of the Laplacian of the electron density.

First vibrational investigations of N2O-H2O, N2O-(H2O)2, and (N2O)2-H2O complexes from the far to the near-infrared spectral region by neon matrix isolation and ab initio calculations.

We present for the first time the investigation of water molecules complexed with dinitrogen monoxide, two abundant molecules in atmosphere, in solid neon using Fourier transform infrared (IR) spectroscopy. We identify at least three complexes from concentration effects, N2O-H2O, N2O-(H2O)2, and (N2O)2-H2O, by observation of new absorption bands close to the monomer fundamental modes from the far to the near IR region. We highlight the presence of isomers for the N2O-H2O complex with the help of theoretical calculations at second order Møller-Plesset (MP2) and coupled-cluster single double triple-F12a/aug-cc-pVTZ levels. The observed frequencies for the N2O-(H2O)2 and (N2O)2-H2O complexes are compared with MP2/aug-cc-pVTZ harmonic data. Anharmonic coupling constants have been derived from the observations of overtones and combination bands.

First infrared investigations of OCS-H2O, OCS-(H2O)2, and (OCS)2-H2O complexes isolated in solid neon: Highlighting the presence of two isomers for OCS-H2O.

For the first time, complexes involving carbonyl sulfide (OCS) and water molecules are studied by FTIR in solid neon. Many new absorption bands close to the known fundamental modes for the monomers give evidence for at least three (OCS)n-(H2O)m complexes, noted n:m. With the help of theoretical calculations, two isomers of the 1:1 complex are clearly identified. Concentration effects combined with a detailed vibrational analysis allow for the identification of transitions for the 1:1, 1:2, and 2:1 complexes. Anharmonic coupling constants have been derived from the observations of overtones and combinations.

Conformational Landscape of the 1/1 Diacetyl/Water Complex Investigated by Infrared Spectroscopy and ab Initio Calculations.

The complexes of diacetyl with water have been studied experimentally by Fourier transform infrared (FTIR) spectroscopy coupled to solid neon matrix and supersonic jet, and anharmonic ab initio calculations. The vibrational analysis of neon matrix spectra over the 100-7500 cm-1 infrared range confirms the existence of two nearly isoenergetic one-to-one (1/1) diacetyl-water S1 and S2 isomers already evidenced in a previous argon matrix study. A third form (S3) predicted slightly less stable ( J. Mol. Mod. 2015 , 21 , 214 ) is not observed. The correct agreement obtained between neon matrix and anharmonic calculated vibrational frequencies is exploited in several cases to derive band assignments for the vibrational modes of a specific isomer. Thereafter, theoretical xij anharmonic coupling constants are used for the attribution of combination bands and overtones relative to the 1/1 dimer. Finally, the most stable isomer of the one-to-two (1/2) diacetyl-water complex is identified in the OH stretching region of water on the grounds of comparison of experimental and calculated vibrational shifts between water dimer and the three most stable 1/2 isomers.

Vibrational investigations of CO2-H2O, CO2-(H2O)2, and (CO2)2-H2O complexes isolated in solid neon.

The van der Waals complex of H2O with CO2 has attracted considerable theoretical interest as a typical example of a weak binding complex with a dissociation energy less than 3 kcal/mol. Up to now, experimental vibrational data are sparse. We have studied by FTIR the complexes involving CO2 and water molecules in solid neon. Many new absorption bands close to the well known monomers fundamentals give evidence for at least three (CO2)n-(H2O)m complexes, noted n:m. Concentration effects combined with a detailed vibrational analysis allow for the identification of sixteen, twelve, and five transitions for the 1:1, 1:2, and 2:1 complexes, respectively. Careful examination of the far infrared spectral region allows the assignment of several 1:1 and 1:2 intermolecular modes, confirmed by the observation of combinations of intra + intermolecular transitions, and anharmonic coupling constants have been derived. Our results demonstrate the high sensibility of the solid neon isolation to investigate the hydrogen-bonded complexes in contrast with the gas phase experiments for which two quanta transitions cannot be easily observed.

The cyclic ground state structure of the HF trimer revealed by far infrared jet-cooled Fourier transform spectroscopy.

The rovibrationally resolved Fourier transform (FT) far infrared (FIR) spectra of two intermolecular librations of (HF)3, namely the in-plane ν6 and out-of-plane ν4 bending fundamentals centered, respectively, at about 494 cm(-1) and 602 cm(-1), have been recorded for the first time under jet-cooled conditions using the supersonic jet of the Jet-AILES apparatus. The simultaneous rotational analysis of 245 infrared transitions belonging to both bands enabled us to determine the ground state (GS), ν6 and ν4 rotational and centrifugal distortion constants. These results provided definite experimental answers to the structure of such a weakly bound trimer: firstly the vibrationally averaged planarity of cyclic (HF)3, also supported by the very small value of the inertia defect obtained in the GS, secondly the slight weakening of the hydrogen bond in the intermolecular excited states evidenced from the center of mass separations of the HF constituents determined in the ground, ν6 = 1 and ν4 = 1 states of (HF)3 as well as the decrease of the fitted rotational constants upon excitation. Finally, lower bounds of about 2 ns on ν6 and ν4 state lifetimes could be derived from the deconvolution of experimental linewidths. Such long lifetimes highlight the interest in probing low frequency intermolecular motions of molecular complexes to get rid of constraints related to the vibrational dynamics of coupled anharmonic vibrations at higher energy, resulting in loss of rotational information.

The far infrared spectrum of naphthalene characterized by high resolution synchrotron FTIR spectroscopy and anharmonic DFT calculations.

Using synchrotron radiation, we performed the rotationally resolved Fourier transform infrared absorption spectroscopy of three bands of naphthalene C10H8, namely ν(46)-0 (centered at 782 cm(-1), 12.7 µm), ν(47)-0 (centered at 474 cm(-1), 21 µm), and ν(48)-0 (centered at 167 cm(-1), 60 µm). The intense CH bending out of plane ν(46)-0 band was recorded under supersonic jet-cooled conditions using a molecular beam (the Jet-AILES apparatus) and the low frequency ν(47)-0 and ν(48)-0 bands were measured at room temperature in a long absorption path cell. The simultaneous rotational analysis of these bands permitted us to refine the ground state (GS) and ν(46) rotational spectroscopic constants and to provide the first sets of constants for the ν(47) and ν(48) modes. The experimental rotational constants were then used as reference data to calibrate theoretical models in order to provide new insights into the accuracy of anharmonic calculations. The B97-1 functional associated with the cc-pVTZ and ANO-RCC basis sets gave a consistent set of results, for rotational constants and fundamental frequencies. The data presented here pave the way for the search of naphthalene through its far-infrared spectrum in different objects of the interstellar medium.

Axial and equatorial hydrogen-bond conformers between (CH2)3S and H(D)F: Fourier transform infrared spectroscopy and ab initio calculations.

The coexistence of axial and equatorial hydrogen-bonded conformers of 1 : 1 (CH(2))(3)S-HF (and -DF) has been observed in the same adiabatic expansion of a supersonic jet seeded with argon and in a static absorption cell at room temperature. High level calculations computed the axial conformer to be the most stable one with a small energy difference with respect to the equatorial one, in full agreement with previous microwave experiments. On the grounds of band contour simulations of FTIR spectra and ab initio energetic and anharmonic vibrational calculations, two pairs of ν(s) HF donor stretching bands, observed in a series of jet-FTIR spectra at 3457.9 and 3480.5 cm(-1) have been respectively assigned to the axial and equatorial forms of the 1 : 1 complex. In the jet-FTIR spectra series with HF, the assignment of an additional broad band (about 200 cm(-1) higher in frequency with respect to ν(s)) to a 1 : 2 complex has been supported by theoretical investigations. Experimental detection of both axial and equatorial forms of a cyclic trimer has been confirmed by calculated energetic and vibrational properties. The nature of hydrogen bonding has been examined within topological frameworks. The energetic partitioning within the 1 : 1 dimers has been elucidated with SAPT techniques. Interestingly, the interconversion pathway between two 1 : 1 structures has been explored and it was seen that the formation of the 1 : 1 complex affects the interconversion barrier on the ring puckering motion. The band contour analysis of gas phase FTIR experiments provided a consistent set of vibrational frequencies and anharmonic coupling constants, in good agreement with ab initio anharmonic vibrational calculations. Finally, from a series of cell-FTIR spectra recorded at different partial pressures of (CH(2))(3)S and HF monomers, the absorption signal of the 1 : 1 complex could be isolated which enabled to estimate the equilibrium constant K(p) = 0.023 at 298 K for the dimerization.

Bonding nature and vibrational signatures of oxirane:(water)(n=1-3). Assessment of the performance of the dispersion-corrected DFT methods compared to the ab initio results and Fourier transform infrared experimental data.

Spectroscopic properties of 1:n complexes (n = 1, 2, and 3) formed between an oxirane molecule and water clusters have been evaluated using experimental techniques (FTIR spectroscopy using a new supersonic jet experiment coupled to the infrared AILES beamline of synchrotron SOLEIL and also cryogenic neon matrix device) and theoretical approaches (SAPT, ab initio, DFT, and topological analyses). From a systematic comparison between the theoretical results (obtained with both wave function based methods and several newly hydrogen bonded adapted functionals) with the available experimental results on the studied compounds, it was concluded that only the wave function based methods (particularly coupled clusters ones) are able to well describe these compounds, while the newly hydrogen bonded adapted functionals (long-range and/or dispersion-corrected ones and also double hybrids) cannot adequately describe all the spectroscopic properties in a systematic way. The MP2 method, although more expensive than DFT, still offers a reliable method to study both isolated molecules and hydrogen bonded complexes provided the contribution of the dispersion energy in total energy is properly taken into account. The nature of interaction between oxirane and water molecules has been analyzed using the symmetry adapted perturbation theory (SAPT) method. It was evidenced that the water-oxirane interaction corresponds to the hydrogen-bonded systems with a large contribution of the dispersion energy. The nature of the oxirane-water bonding has been studied using two topological methods: atoms in molecules and electron-localization function (ELF). Geometrical structures of the titled complexes were rationalized from the spatial arrangement of ELF attractors. Secondary interaction was also accounted for the bond critical points found at H(oxirane)···O(water) bond paths.

The (CH2)2O-H2O hydrogen bonded complex. Ab Initio calculations and Fourier transform infrared spectroscopy from neon matrix and a new supersonic jet experiment coupled to the infrared AILES beamline of synchrotron SOLEIL.

A series of hydrogen bonded complexes involving oxirane and water molecules have been studied. In this paper we report on the vibrational study of the oxirane-water complex (CH(2))(2)O-H(2)O. Neon matrix experiments and ab initio anharmonic vibrational calculations have been performed, providing a consistent set of vibrational frequencies and anharmonic coupling constants. The implementation of a new large flow supersonic jet coupled to the Bruker IFS 125 HR spectrometer at the infrared AILES beamline of the French synchrotron SOLEIL (Jet-AILES) enabled us to record first jet-cooled Fourier transform infrared spectra of oxirane-water complexes at different resolutions down to 0.2 cm(-1). Rovibrational parameters and a lower bound of the predissociation lifetime of 25 ps for the v(OH)(b) = 1 state have been derived from the rovibrational analysis of the ν(OH)(b) band contour recorded at respective rotational temperatures of 12 K (Jet-AILES) and 35 K (LADIR jet).

Intermolecular vibrations of (CH2)2O-HF and -DF hydrogen bonded complexes investigated by Fourier transform infrared spectroscopy and ab initio calculations.

A series of Fourier transform infrared spectra (FTIR) of the hydrogen bonded complexes (CH(2))(2)O-HF and -DF have been recorded in the 50-750 cm(-1) range up to 0.1 cm(-1) resolution in a static cell maintained at near room temperature. The direct observation of three intermolecular transitions enabled us to perform band contour analysis of congested cell spectra and to determine reliable rovibrational parameters such as intermolecular frequencies, rovibrational and anharmonic coupling constants involving two l(1) and l(2) librations and one σ stretching intermolecular motion. Inter-inter anharmonic couplings could be identified between ν(l(1)), ν(l(2)), ν(σ) and the two lowest frequency bending modes. The positive sign of coupling constants (opposite with respect to acid stretching intra-inter ones) reveals a weakening of the hydrogen bond upon intermolecular excitation. The four rovibrational parameters ν(σ) and x(σj) (j = σ, δ(1), δ(2)) derived in the present far-infrared study and also in a previous mid-infrared one [Phys. Chem. Chem. Phys. 2005, 1, 592] make deviations appear smaller than 1% for frequencies and 12% for coupling constants which gives confidence to the reliability of the data obtained. Anharmonic frequencies obtained at the MP2 level with Aug-cc-pvTZ basis set agree well with experimental values over a large set of frequencies and coupling constants. An estimated anharmonic corrected value of the dissociation energy D for both oxirane-HF (2424 cm(-1)) and -DF (2566 cm(-1)) has been derived using a level of theory as high as CCSD(T)/Aug-cc-pvQZ, refining the harmonic value previously calculated for oxirane-HF with the MP2 method and a smaller basis set. Finally, contrary to short predissociation lifetimes evidenced for acid stretching excited states, any homogeneous broadening related to vibrational dynamics of (CH(2))(2)O-HF and -DF has been observed within the three highest frequency intermolecular states, as expected with low excitation energies largely below the dissociation limit as well as a negligible IVR contribution.

Influence of the geometry of a hydrogen bond on conformational stability: a theoretical and experimental study of ethyl carbamate.

Spectra of ethyl carbamate (urethane) in the gas phase have been recorded in the microwave (4-20 GHz), millimeter-wave (49-118 GHz and 150-235 GHz) and mid-infrared (1000-1900 cm(-1)) regions. At the same time, high level ab initio calculations have been performed in order to both predict the experimental results and help in understanding the physical properties of the system. An extensive set of spectroscopic constants for the two most stable conformers in the gas phase, that might be useful for astrophysical databases, has been derived from the observed signals. The most stable conformer has been unambiguously identified. Then, the influence of a weak intramolecular hydrogen bond on the conformational stability has been discussed on the basis of theoretical and experimental results.

Evidence of an isomeric pair in furan...HCl: Fourier transform infrared spectroscopy and ab initio calculations.

For the first time the coexistence of a sigma- and a pi-complex in the C(4)H(4)O:HCl system has been observed, in the same supersonic expansion of a molecular jet seeded with argon (or helium) or in a flow-cooled cell at 240 K. This is an exception to the third of the Legon-Miller rules which claims the sigma-structure to be the only one to exist. On the grounds of energetic considerations and band contour simulations, two observed bands at 2787.7 and 2795.5 cm(-1) of the nu(s) HCl stretching frequency are assigned to the two complexes, recorded as Fourier transform infrared spectra with a resolution between 0.2 and 0.5 cm(-1). Complementary calculations show that the use of the standard second-order Moller-Plesset perturbation theory may be erroneous for such a complex, due of the overestimation of the dispersion contribution with respect to the electrostatic term. It is finally established that only a balanced version of the second-order Moller-Plesset perturbation method, spin-component scaled-MP2, or a higher level of theory like a coupled-cluster approach, can provide a reliable energetic analysis for this complex.

Vibrational dynamics of the hydrogen bonded complexes (CH2)2O-HF and -DF investigated by combined jet- and cell-Fourier transform infrared spectroscopy.

Fourier transform infrared spectra of the Vs stretching bands of HF and DF bonded to (CH2)2O have been recorded at 0.5 cm(-1) resolution in a cooled cell and in a supersonic expansion seeded with argon. The analysis of the congested spectra of this type of medium strength hydrogen bonded complexes exploits a combination of controlled temperature effects in the ranges 25-80 K and 200-300 K and a band contour simulation program accounting for homogeneous and inhomogeneous contributions. Significant anharmonic couplings between the donor stretch mode and three of the low frequency intermolecular modes are found to be responsible for the characteristic hot band patterns in the Vs fundamental region of cell spectra. A global analysis of sum and difference combination bands involving Vs provides reliable values of intermolecular frequencies, anharmonic coupling constants and a good estimate of the dissociation energy of the complex which compares favorably with ab initio results. The effective linewidth provides a lower bound for the predissociation lifetime of 1.5 ps for HF and 7 ps for DF containing complexes, respectively. The correlation between effective linewidths and vibrational densities of states for (CH2)2O-HF and -DF underlines the important role of intramolecular vibrational redistribution in the vibrational dynamics of these complexes while the lifetime decrease for HF (or DF) bonded to oxygenated cyclic ethers with respect to sulfured homologues might be explained by the change in the arrangement of the acid relative to the plane of the acceptor subunit.

Vibrational dynamics of medium strength hydrogen bonds: Fourier transform infrared spectra and band contour analysis of the DF stretching region of (CH2)2S-DF.

Fourier transform infrared spectra of the nu(s) band of the (CH2)(2)S-DF complex have been recorded at 0.1-0.5 cm(-1) resolution in a cooled cell and in a supersonic jet expansion seeded with argon. A sufficient density of (CH(2))(2)S-DF heterodimers is produced by a double injection nozzle device, which limits the possibility of reaction between thiirane and DF before the expansion. The observation of partially resolved PQR branch structures at cell temperatures as high as 252 K indicates relatively small effective line widths, which allow a detailed analysis of the underlying vibrational couplings and of the structural properties of the complex. The analysis of cell and free jet spectra in the temperature range 50-250 K is performed with a software package for the simulation and fitting of multiple hot band progressions in asymmetric rotors. The analysis reveals that the three low frequency hydrogen-bond modes are strongly coupled to the DF stretch with anharmonic coupling constants, which indicates a strengthening of the hydrogen bond upon vibrational excitation of DF. Rovibrational parameters and a reliable upper bound for the homogeneous line width have been extracted.

Rovibrational and dynamical properties of the hydrogen bonded complex (CH2)2S-HF: a combined free jet, cell, and neon matrix-Fourier transform infrared study.

Fourier transform infrared spectra of the nu(s) (HF stretching) band of the (CH(2))(2)S-HF complex have been recorded at 0.1-0.5 cm(-1) resolution in a cooled cell, in a supersonic jet expansion seeded with argon and in a neon matrix at 4.5 K. The combination of controlled temperature effects over a range of 40-250 K and a sophisticated band contour simulation program allows the separation of homogeneous and inhomogeneous contributions and reveals significant anharmonic couplings between intramolecular and intermolecular vibrational modes similar to our previous work on (CH(2))(2)S-DF. The sign of the coupling constants is consistent with the expected strengthening of the hydrogen bond upon vibrational excitation of HF which also explains the observed small variations of the geometrical parameters in the excited state. The analysis of sum and difference combination bands involving nu(s) provides accurate values of intermolecular harmonic frequencies and anharmonicities and a good estimate of the dissociation energy of the complex. Frequencies and coupling parameters derived from gas phase spectra compare well with results from neon matrix experiments. The effective linewidth provides a lower bound for the predissociation lifetime of 10 ps. The comparison between effective linewidths and vibrational densities of states for (CH(2))(2)S-HF and -DF complexes highlights the important role of intramolecular vibrational redistribution in the vibrational dynamics of medium strength hydrogen bonds.