The OH-stretching and OOH-bending overtone spectrum of HOONO

We have simulated the HOONO vibrational overtone spectrum with use of a local mode Hamiltonian that includes the OH-stretching, OOH-bending, and NOOH-torsional modes and coupling between all three modes. The local mode parameters and the dipole moment function are calculated with coupled-cluster ab initio theory and an augmented Dunning-type triple-zeta basis set. We investigate the accuracy of the local mode parameters obtained from two different potential-energy fitting routines, as well as the sensitivity of these parameters to the basis set employed. We compare our simulated spectra to previously published action spectra in the first and second OH-stretching overtone regions. In addition we have recorded the spectrum in the OH-stretch and OOH-bend combination region around 7700 cm-1 and we also compare to this. Our simulated spectrum is in qualitative agreement with experiment in the first and second OH-stretching overtone and in the stretch-bend regions.     

Influence of intramolecular hydrogen bonding on OH-stretching overtone intensities and band positions in peroxyacetic acid

Vapor phase absorption spectra and integrated band intensities of the OH stretching fundamental as well as first and second overtones (2ν(OH) and 3ν(OH)) in peroxyacetic acid (PAA) have been measured using a combination of FT-IR and photoacoustic spectroscopy. In addition, ab initio calculations have been carried out to examine the low energy stable conformers of the molecule. Spectral assignment of the primary features appearing in the region of the 2ν(OH) and 3ν(OH) overtone bands are made with the aid of isotopic substitution and anharmonic vibrational frequency calculations carried out at the MP2/aug-cc-pVDZ level. Apart from features associated with the zeroth-order OH stretch, the overtone spectra are dominated by features assigned to combination bands composed of the respective OH stretching overtone and vibrations involving the collective motion of several atoms in the molecule resulting from excitation of the internal hydrogen bonding coordinate. Integrated absorption cross section measurements reveal that internal hydrogen bonding, the strength of which is estimated to be ～20 kJ/mol in PAA, does not result in a enhanced oscillator strength for the OH stretching fundamental of the molecule, as is often expected for hydrogen bonded systems, but does cause a precipitous drop in the oscillator strength of its 2ν(OH) and 3ν(OH) overtone bands, reducing them, respectively, by a factor of 165 and 7020 relative to the OH stretching fundamental.     

Vibrational OH-stretching overtone spectroscopy of jet-cooled resorcinol and hydroquinone rotamers

We have measured the OH-stretching fundamental and overtone spectra of resorcinol and hydroquinone in a supersonic jet using nonresonant ionization detected infrared/near-infrared spectroscopy. Anharmonic oscillator local mode calculations of the OH-stretching frequencies and intensities and Boltzmann populations of the stable rotamers have been calculated at the B3LYP/6-311++G(3df,2pd) level to help interpret the observed spectra. Resorcinol has three stable rotamers and in the recorded second and third OH-stretching overtone spectra there is evidence of two distinguishable rotamers. Hydroquinone has two stable rotamers; however, the OH-stretching oscillators of each rotamer are so similar in nature that even up to the fourth OH-stretching overtone the transitions coincide. These results place a limit on the ability of the jet-cooled overtone spectroscopy technique to distinguish between rotamers.     

Vibrational structures in the photoelectron spectrum of formic acid

He I photoelectron spectra of formic acid and its deuterium substituted derivatives have been measured. The first, second and fifth bands of each molecule exhibit very fine vibrational structures. The characters of molecular orbitals from which the photoelectron bands originate are discussed by the use of results of vibrational analyses and molecular orbital calculations.     

Weak intramolecular interactions in ethylene glycol identified by vapor phase OH-stretching overtone spectroscopy

Vapor phase OH-stretching overtone spectra of ethylene glycol were recorded to investigate weak intramolecular hydrogen bonding. The spectra were recorded with conventional absorption spectroscopy and laser photoacoustic spectroscopy in the first to fourth OH-stretching overtone regions. The room-temperature spectra are dominated by two conformers that show weak intramolecular hydrogen bonding. A less abundant third conformer, with no sign of hydrogen bonding, is also observed. Vapor phase spectra of the ethylene-d(4) glycol isotopomer were also recorded and used to identify an interfering resonance between CH-stretching and OH-stretching states in the fourth overtone. Anharmonic oscillator local mode calculations of the OH-stretching transitions have provided an accurate simulation of the observed spectra. The local mode parameters were calculated with coupled cluster ab initio methods. The calculations facilitate assignment of the different conformers in the spectra and illustrate the effect of the intramolecular hydrogen bonding.     

The electronic spectrum of chloroformic acid in comparison to formic acid

The electronic spectra of chloroformic acid ClCOOH and formic acid HCOOH are computed in large-scale multireference configuration interaction (MRD-CI) calculations. The computed spectrum of formic acid is in reasonable agreement with prior calculations and experimental data. The first electronic transition of ClCOOH is computed at 6.41 eV (193.4 nm), about 0.5 eV higher than in HCOOH. Together with five strong transitions calculated at 7.66 eV (161.9 nm; 2(1)A' <-- X(1)A'), 8.36 eV (148.3 nm; 3(1)A' <-- X(1)A'), 8.49 eV (146.0 nm; 4(1)A' <-- X(1)A'), 9.00 eV (137.8 nm; 5(1)A' <-- X(1)A'), and 9.44 eV (131.3 nm; 7(1)A' <-- X(1)A'), this can serve as a guideline for experimental search of ClCOOH.     

Measurement of the fourth O-H overtone absorption cross section in acetic acid using cavity ring-down spectroscopy

We report the absolute absorption cross sections of the fourth vibrational O-H (5ν(OH)) overtone in acetic acid using cavity ring-down spectroscopy. For compounds that undergo photodissociation via overtone excitation, such intensity information is required to calculate atmospheric photolysis rates. The fourth vibrational overtone of acetic acid is insufficiently energetic to effect dissociation, but measurement of its cross section provides a model for other overtone transitions that can affect atmospheric photochemistry. Though gas-phase acetic acid exists in equilibrium with its dimer, this work shows that only the monomeric species contributes to the acetic acid overtone spectrum. The absorption of acetic acid monomer peaks at ～615 nm and has a peak cross section of 1.84 × 10(-24) cm(2)·molecule(-1). Between 612 and 620 nm, the integrated cross section for the acetic acid monomer is (5.23 ± 0.73) × 10(-24) cm(2)·nm·molecule(-1) or (1.38 ± 0.19) × 10(-22) cm(2)·molecule(-1)·cm(-1). This is commensurate with the integrated cross section values for the fourth O-H overtone of other species. Theoretical calculations show that there is sufficient energy for hydrogen to transition between the two oxygen atoms, which results in an overtone-induced conformational change.     

The IR spectrum of formic acid in an argon matrix

A new method of preparation of matrix-isolated molecules of formic acid at low temperatures has been developed. A set of experimental frequencies and intensities of the IR spectrum of monomer molecules of formic acid isolated in a low temperature argon matrix has been obtained. It is shown on the basis of analysis of the IR spectrum that the new method eliminates association and decomposition of molecules of the acid during preparation of the sample.     

Role of OH-stretch/torsion coupling and quantum yield effects in the first OH overtone spectrum of cis-cis HOONO

A joint theoretical and experimental investigation is undertaken to study the effects of OH-stretch/HOON torsion coupling and of quantum yield on the previously reported first overtone action spectrum of cis-cis HOONO (peroxynitrous acid). The minimum energy path along the HOON dihedral angle is computed at the coupled cluster singles and doubles with perturbative triples level with correlation consistent polarized quadruple zeta basis set, at the structure optimized using the triple zeta basis set (CCSD(T)/cc-pVQZ//CCSD(T)/cc-pVTZ). The two-dimensional ab initio potential energy and dipole moment surfaces for cis-cis HOONO are calculated as functions of the HOON torsion and OH bond length about the minimum energy path at the CCSD(T)/cc-pVTZ and QCISD/AUG-cc-pVTZ (QCISD-quadratic configuration interaction with single and double excitation and AUG-augmented with diffuse functions) level of theory/basis, respectively. The OH-stretch vibration depends strongly on the torsional angle, and the torsional potential possesses a broad shelf at approximately 90 degrees , the cis-perp conformation. The calculated electronic energies and dipoles are fit to simple functional forms and absorption spectra in the region of the OH fundamental and first overtone are calculated from these surfaces. While the experimental and calculated spectra of the OH fundamental band are in good agreement, significant differences in the intensity patterns are observed between the calculated absorption spectrum and the measured action spectrum in the 2nu(OH) region. These differences are attributed to the fact that several of the experimentally accessible states do not have sufficient energy to dissociate to OH+NO(2) and therefore are not detectable in an action spectrum. Scaling of the intensities of transitions to these states, assuming D(0)=82.0 kJ/mol, is shown to produce a spectrum that is in good agreement with the measured action spectrum. Based on this agreement, we assign two of the features in the spectrum to Deltan=0 transitions (where n is the HOON torsion quantum number) that are blue shifted relative to the origin band, while the large peak near 7000 cm(-1) is assigned to a series of Deltan=+1 transitions, with predominant contributions from torsionally excited states with substantial cis-perp character. The direct absorption spectrum of cis-cis HOONO (6300-6850 cm(-1)) is recorded by cavity ringdown spectroscopy in a discharge flow cell. A single band of HOONO is observed at 6370 cm(-1) and is assigned as the origin of the first OH overtone of cis-cis HOONO. These results imply that the origin band is suppressed by over an order of magnitude in the action spectrum, due to a reduced quantum yield. The striking differences between absorption and action spectra are correctly predicted by the calculations.     

Effect of OH internal torsion on the OH-stretching spectrum of cis,cis-HOONO

We show that a simple two-dimensional vibrational model can explain most of the features observed in the first and second OH-stretching overtone region of the room temperature cis,cis-HOONO spectrum. The model uses ab initio calculated parameters and includes the OH-stretching mode coupled to the internal torsion of the OH group.     

Torsion-vibration coupling in methanol: diabatic behavior in the CH overtone region

Through a fit to methanol CH overtone data, a previously developed 4-dimensional torsion-vibration Hamiltonian is extended to high CH stretch excitation as well as to high torsional excitation. The strength of the torsion-vibration coupling is found to increase with CH stretch excitation. Systematic patterns of near degeneracy (3-, 4-, and 6-fold) are found in different regions of quantum number space. In the region of the CH fundamentals, an approximate a diabatic separation of the torsion (slow degree of freedom) from the CH stretches (fast degrees of freedom) accounts for the pattern of the energy levels and for the signs of the torsional tunneling splittings. For the higher CH overtones (v(CH) > or = 4), a diabatic representation accounts for the torsional structure obtained from the fully coupled calculation and for certain trends found in the pattern of the energy levels.     

Vibrational overtone spectrum of matrix isolated cis, cis-HOONO

Cis, cis-peroxynitrous acid is known to be an intermediate in atmospheric reactions between OH and NO2 as well as HOO and NO. The infrared absorption spectra of matrix-isolated cc-HOONO and cc-DOONO in argon have been observed in the range of 500-8000 cm-1. Besides the seven fundamental vibrational modes that have been assigned earlier for this molecule [Zhang et al., J. Chem. Phys. 124, 084305 (2006)], more than 50 of the overtone and combination bands have been observed for cc-HOONO and cc-DOONO. Ab initio CCSD(T)/atomic natural orbital anharmonic force field calculations were used to help guide the assignments. Based on this study of the vibrational overtone transitions of cis, cis-HOONO that go as high as 8000 cm-1 and the earlier paper on the vibrational fundamentals, we conclude that the CCSD(T)/ANO anharmonic frequencies seem to correct to +/-35 cm-1. The success of the theoretically predicted anharmonic frequencies {upsilon} in assigning overtone spectra of HOONO up to 8000 cm-1 suggests that the CCSD(T)/ANO method is producing a reliable potential energy surface for this reactive molecule.     

Vibrationally mediated photodissociation of ethene isotopic variants preexcited to the fourth C-H stretch overtone

H and D photofragments produced via vibrationally mediated photodissociation of jet-cooled normal ethene (C2H4), 1,2-trans-d2-ethene (HDCCDH), and 1,1-d2-ethene (CH2CD2), initially excited to the fourth C-H stretch overtone region, were studied for the first time. H and D vibrational action spectra and Doppler profiles were measured. The action spectra include partially resolved features due to rotational cooling, while the monitored room temperature photoacoustic spectra exhibit only a very broad feature in each species. Simulation of the spectral contours allowed determination of the band types and origins, limited precision rotational constants, and linewidths, providing time scales for energy redistribution. The H and D Doppler profiles correspond to low average translational energies and show slight preferential C-H over C-D bond cleavage in the deuterated variants. The propensities toward H photofragments emerge even though the energy flow out of the initially prepared C-H stretch is on a picosecond time scale and the photodissociation occurs following internal conversion, indicating a more effective release of the light H atoms.     

The overtone oh stretching raman spectrum of liquid water

The very weak Raman spectrum of liquid water has been measured in the overtone OH stretching region. The similarity of tlie observed spectrum to the overtone infrared spectrum, and the weak spectral dependence of the depolarization ratio is in sharp contrast to the fundamental spectra.     

Calculation of the O-H stretching vibrational overtone spectrum of the water dimer

The O-H stretching vibrational overtone spectrum of the water dimer has been calculated with the dimer modeled as two individually vibrating monomer units. Vibrational term values and absorption intensities have been obtained variationally with a computed dipole moment surface and an internal coordinate Hamiltonian, which consists of exact kinetic energy operators within the Born-Oppenheimer approximation of the monomer units. Three-dimensional ab initio potential energy and dipole moment surfaces have been calculated using the internal coordinates of the monomer units using the coupled cluster method including single, double, and perturbative triple excitations [CCSD(T)] with the augmented correlation consistent valence triple zeta basis set (aug-cc-pVTZ). The augmented correlation consistent valence quadruple zeta basis set (aug-cc-pVQZ), counterpoise correction, basis set extrapolation to the complete basis set limit, relativistic corrections, and core and valence electron correlations effects have been included in one-dimensional potential energy surface cuts. The aim is both to investigate the level of ab initio and vibrational calculations necessary to produce accurate results when compared with experiment and to aid the detection of the water dimer under atmospheric conditions.     

The fourth overtone of the free O-H stretching vibrations of alcohols and their solvent effect

The fourth overtones of the free O-H stretch of C_nH_2n+1OH (n = 2,3,4,6,10,14) in the neat liquid slate have been observed by a thermal-lens technique. Those frequencies in the CCl₄ solution were red-shifted with increasing dilution, and this is ascribed to the formation of a weak hydrogen bond O-H·Cl.     

Experimental and theoretical investigation of the first overtone spectrum of 1,3,5-trinitrotoluene

Vibrational overtone spectroscopy is a powerful tool for studying intramolecular and intermolecular interactions. We report on a combined experimental and modeling study of the C-H stretch first overtone of bulk 1,3,5-trinitrotoluene (TNT) and TNT on fumed-silica powder. We recorded the overtone spectra by laser photoacoustic spectroscopy and compared them with those predicted with the harmonically coupled anharmonic oscillator model in the 5600-6600 cm(-1) region. The model systems included single molecules and hybrid quantum and molecular mechanical (QM:MM) clusters to account for the effects of intermolecular interactions on the observed spectra. We performed the hybrid QM:MM calculations at the HF/6-31+G(d,p), B3LYP/6-31+G(d,p), and MP2/6-31+G(d,p) levels of theory and with the universal force field (UFF) to account for van der Waals and electrostatic effects from surrounding molecules. Overtone spectra calculated from the MP2 level of theory, using a HF/3-21+G* calculation to assign molecular charges in the MM layer, and the Merz-Singh-Kollman population analysis for assigning partial charge in the QM layer and determining the transition dipole moment agreed best with the experimental data.