Effective one-dimensional dipole moment function for the OH stretching overtone spectra of simple acids and alcohols

To gain insight on the absorption intensities, as well as the direction of the transition moment for the OH stretching vibration in alcohols and acids, we performed detailed analyses for nitric acid, acetic acid, methanol, tert-butyl alcohol, water, and OH radical. We obtained both the potential energy surface and the dipole moment function (DMF) by the B3LYP method and performed quantum mechanical vibrational calculation using the grid variational method based on the local mode model. In this work, we employed the sum rule of the absorption intensities for the one-dimensional (1-D) vibrational Hamiltonian to construct an effective 1-D DMF, which is responsible for the total sum of the overtone intensities. The direction of this effective DMF was found to be tilted away from the OH bond by about 30 degrees for the polyatomic molecules. The nonlinearity of the DMFs in the directions parallel and perpendicular to the OH bond is discussed to rationalize the tilting. Furthermore, we analyzed the effective 1-D DMFs with the vibrational wave function expansion method and derived the effective portion of the 1-D DMF that is responsible for the overtone transition moment.     

A simple rule for prediction of intensities of vibrational overtone spectra

The intensities of vibrational overtone absorption transitions are described in terms of vibronic coupling of the ground molecular state to excited electronic configurations. Model calculations indicate an important role of nuclear geometry of excited electronic states relative to the ground state in determination of molecular overtone spectra. A simple rule for qualitative predictions of the overtone spectra for diatomic molecules or local bond modes of polyatomic molecules is proposed.     

Theoretical calculation of the OH vibrational overtone spectra of 1,5-pentanediol and 1,6-hexanediol

It is well-known that intramolecular hydrogen bonding affects the relative energetics of conformers, as well as the OH stretching peak positions, intensities, and width. In this study we simulated the Δv(OH) = 3, 4 overtone spectra of 1,5-pentanediol (PeD) and 1,6-hexanediol (HD) using the peak positions, intensities, and width calculated from the B3LYP/6-31+G(d,p) method. Furthermore, room temperature free energy calculations were performed using B3LYP/6-31+G(d,p) MP2/6-31+G(d,p), and MP2/6-311++G(3df,3pd) to obtain the relative population of the conformers. From the calculation of 109 and 381 distinct conformers for PeD and HD, respectively, we find that for these long chain diols the intramolecular hydrogen bonded conformers are not the most dominant conformation at room temperature. This is in stark contrast with shorter chain diols such as ethylene glycol for which the hydrogen bonded conformer dominates the population at room temperature. On the other hand, we found that the correlation between the hydrogen bonded OH red shift versus the homogeneous width, Γ = 0.0155(Δω)(1.36), which was derived for shorter chain diols, is valid even for these longer chain diols. We also showed that the intramolecular hydrogen bonded OH initially decays through the CCOH torsion and COH bending mode no matter how long the alkanediol chain length is for 1,n-alkanediols for n up to 6.     

Experimental and theoretical study of the OH vibrational spectra and overtone chemistry of gas-phase vinylacetic acid

In this study we present the gas-phase vibrational spectrum of vinylacetic acid with a focus on the nu = 1-5 vibrational states of the OH stretching transitions. Cross sections for nu = 1, 2, 4 and 5 of the OH stretching vibrational transitions are derived on the basis of the vapor pressure data obtained for vinylacetic acid. Ab initio calculations are used to assist in the band assignments of the experimental spectra, and to determine the threshold for the decarboxylation of vinylacetic acid. When compared to the theoretical energy barrier to decarboxylation, it is found that the nu OH = 4 transition with thermal excitation of low frequency modes or rotational motion and nu OH = 5 transitions have sufficient energy for the reaction to proceed following overtone excitation.     

Bandwidths in the gas phase overtone spectra of neopentane

The overtone spectrum of neopentane vapor is measured from 63 to 670 Torr. The bandwidth for δ_vCH = 5 is substantially smaller for the gas than the liquid. The local mode overtone bandwidths are much smaller than for other molecules. The results are discussed in terms of local mode coupling and vibrational state dynamics.     

Computational high-frequency overtone spectra of the water-ammonia complex

We have computed vibrational high-frequency overtone spectra of the water-ammonia complex, H(2)O-NH(3), and its isotopomers. The complex has been modeled as two independently vibrating monomer units. The internal coordinate Hamiltonians for each monomer unit have been constructed using exact gas phase kinetic energy operators. The potential energy and dipole moment surfaces have been calculated with the explicitly correlated coupled cluster method CCSD(T)-F12A and the valence triple-ζ VTZ-F12 basis around the equilibrium geometry of the complex. The vibrational eigenvalues have been calculated variationally and the eigenfunctions obtained have been used to compute the intensities of the absorption transitions. In H(2)O-NH(3), the water molecule acts as the proton donor and its symmetry is broken. The hydrogen-bonded OH bond oscillator undergoes a large redshift and intensity enhancement compared to the free hydrogen bond. Broken degeneracy of the asymmetric vibrations, quenched inversion splittings, and blueshift of the symmetric bending mode are the most visible changes in the ammonia unit.     

Effects of torsion on O-H stretch overtone spectra and direct overtone photolysis of methyl hydroperoxide

We use laser photoacoustic spectroscopy to obtain overtone spectra at three through six quanta of O-H stretch excitation (3nu(OH)-6nu(OH)) for methyl hydroperoxide (MeOOH). Extending the spectral regions beyond our previous work reveals new features that can be attributed to transitions involving torsion about the O-O bond. Experimental spectral profiles (3nu(OH)-6nu(OH)) and cross sections (3nu(OH)-5nu(OH)) at room temperature show a good agreement with the simulated spectra that we obtain from ab initio calculations employing a vibration-torsion model at 298 K. A Birge-Sponer analysis yields experimental values for the O-H stretch frequency (omega=3773+/-15 cm(-1)) and anharmonicity (omegax=94+/-3 cm(-1)). We also detect OH radicals by laser-induced fluorescence and present photodissociation action spectra of MeOOH in the regions of 4nu(OH) and 5nu(OH). While the spectral profile at 5nu(OH) mimics the photoacoustic spectrum, the peak intensity for transitions to torsionally excited states is relatively more intense in the action spectrum at 4nu(OH), reflecting the fact that the 4nu(OH) excitation energy is below the literature dissociation energy (D0=42.6+/-1 kcal mol(-1)) so that features in the action spectrum come from thermally populated excited states. Finally, we use our calculations to assign contributions to individual peaks in the room-temperature spectra and relate our findings to a recent dynamics study in the literature.     

Fundamental and overtone infrared spectra of CF3I

The i.r. spectra of CF₃I between 250 and 5000 cm⁻¹ have been measured with a resolution of 0.3–1 cm⁻¹ and with optical path lengths up to 4 bar·10 m. A few bands of a sample cooled down to 180 K and several bands of a sample enriched in ¹³CF₃I were also recorded. Values are assigned to most of the anharmonic x_ij and g_ii constants of ¹²CF₃I. Serious perturbations occur however by Fermi resonance between ν₁ and ν₂ + ν₃ and between ν₁ and 2ν₅, whose analysis is still very preliminary.     

CH-stretching overtone spectra of cis- and trans-1,3-pentadiene

The room-temperature vapor-phase overtone spectra of cis- and trans-1,3-pentadiene (piperylene) have been recorded in the 5000-17500 cm(-1) region with the use of conventional and intracavity laser photoacoustic spectroscopy. The presence of five nonequivalent olefinic CH bonds and one methyl group in each molecule complicates assignment of the spectra. We have used a harmonically coupled anharmonic oscillator local mode model with one oscillator for each of the nonequivalent CH bonds to calculate the CH-stretching overtone spectra and thus facilitate assignment of the spectra. Our calculated spectra are in good agreement with the observed spectra. The observed high overtone spectra are distinctively different despite the similarity of the two molecules and the numerous and broad transitions.     

Theoretical calculation of the OH vibrational overtone spectra of 1-n alkane diols (n = 2-4): origin of disappearing hydrogen-bonded OH peak

In this theoretical study, we simulated the vibrational overtone spectrum of ethylene glycol (EG), 1-3 propanediol (PD), and 1-4 butanediol (BD). Using the local mode model along with the potential energy curve and dipole moment function calculated by B3LYP/6-31+G(d,p) and QCISD/6-311++G(3df,3pd), we obtained the theoretical peak position and integrated absorption coefficient. Furthermore, the vibrational spectra was simulated using a Voigt function using homogeneous and inhomogenous width obtained from quantum chemical calculation methods. Previously, Howard and Kjaergaard recorded the second and third overtone photoacoustic spectra of the three aforementioned alkane diols in the gas phase and observed that the intramolecular hydrogen bonded OH peak becomes difficult to observe as the intramolecular hydrogen bonding strength increased, that is, as the chain length was increased. In this paper we show that the disappearance of the hydrogen-bonded OH peak for the OH stretching overtone excitation for BD is partly due to the increase in homogeneous width due to the increase in the hydrogen bond strength and partly due to the decrease in the relative population of the intramolecular hydrogen-bonded conformers as the chain length is increased. This latter feature is a consequence of the unfavorable strained geometry needed to form the intramolecular hydrogen bond in longer alkane chains.     

Vibrational overtone spectra of chloroanilines--evidence for intramolecular hydrogen bonding in o-chloroaniline

The near infrared vibrational overtone absorption spectra of liquid phase aniline and chloroanilines are reported. The analysis of the observed CH and NH local mode mechanical frequency values shows that intramolecular hydrogen bonding occurs between NH2 group and chlorine atom in o-chloroaniline. This observation supports the conclusion drawn from microwave spectroscopic studies reported earlier.     

Second OH overtone excitation and statistical dissociation dynamics of peroxynitrous acid

The second OH overtone transition of the trans-perp conformer of peroxynitrous acid (tp-HOONO) is identified using infrared action spectroscopy. HOONO is produced by the recombination of photolytically generated OH and NO(2) radicals, and then cooled in a pulsed supersonic expansion. The second overtone transition is assigned to tp-HOONO based on its vibrational frequency (10 195.3 cm(-1)) and rotational band contour, which are in accord with theoretical predictions and previous observations of the first overtone transition. The transition dipole moment associated with the overtone transition is rotated considerably from the OH bond axis, as evident from its hybrid band composition, indicating substantial charge redistribution upon OH stretch excitation. The overtone band exhibits homogeneous line broadening that is attributed to intramolecular vibrational redistribution, arising from the coupling of the initially excited OH stretch to other modes that ultimately lead to dissociation. The quantum state distributions of the OH X (2)Pi (nu=0) products following first and second OH overtone excitation of tp-HOONO are found to be statistical by comparison with three commonly used statistical models. The product state distributions are principally determined by the tp-HOONO binding energy of 16.2(1) kcal mol(-1). Only a small fraction of the OH products are produced in nu=1 following the second overtone excitation, consistent with statistical predictions.     

Simulation of inversion motion and N-H stretching overtone spectra of aniline

A curvilinear internal coordinate Hamiltonian is used to simulate the N-H stretching overtone spectra and the associated inversion splittings in aniline. A simple local mode type model is applied to the N-H stretching and H-N-H bending modes. Geometric algebra is employed to derive the kinetic energy operator for the large amplitude inversion motion. Electronic structure calculations at the Moller-Plesset second order perturbation theory and correlation consistent aug-cc-pVTZ basis set level are used to obtain model parameters, some of which have been optimized with the least-squares method using experimental vibrational term values as data. The observed N-H stretching overtone vibrational levels and the inversional tunneling splittings are well reproduced with our approach.     

A comparative study of the fundamental and overtone spectra of dioxane

Infrared, near infrared and visible (NIR) spectra were recorded for Dioxane, Tetrahydropyrane and Diethylether in the liquid. The study of the frequencies of the overtones in Dioxane provides parameters that can be used also for interpreting the fundamental spectrum between 2800–3000 cm⁻¹ as long as one takes into account the Fermi resonance. A characteristic trend for intensities of fundamental and overtones is observed and models for appropriate parametrization are discussed.     

CH stretching vibrational overtone spectra of 1,3,5,7-cyclooctatetraene and 1,1,1-trichloroethane

Local mode frequencies, omega, and anharmonicities, omegax, are obtained from the delta upsilon(CH) = 2-7 spectral regions of 1,3,5,7-cyclooctatetraene (COT) and 1,1,1-trichloroethane. In 1,1,1-trichloroethane omega and omega x are used in conjunction with ab initio potential energy surfaces to calculate local mode anharmonicity-torsion coupling terms, delta(omega x), and frequency-torsion coupling terms, delta(omega). Blue-shifting of sterically hindered CH oscillators in 1,1,1-trichloroethane indicates nonbonded, through-space intramolecular interactions with Cl. Multiple, complex Fermi resonances are observed in 1,1,1-trichloroethane and in COT between local mode states and local mode/normal mode combination states. Intensities of vibrational overtone transitions are calculated in the range delta upsilon(CH) = 3-9 using ab initio dipole moment functions and the harmonically coupled anharmonic oscillator (HCAO) model. HCAO intensities are compared to experimental intensities at delta upsilon(CH) = 3.     

Energy level distributions in spectra of coupled harmonic oscillators

The spectral density distribution of a hamiltonian which represents a system of N coupled harmonic oscillators, and hence may approximately describe molecular vibrations in the local mode picture, is analyzed. The spectral density moments are expressed as linear combination of products of coefficients which depend on the molecular structure and of one-particle moments describing individual bonds and interactions between them. Detailed expressions for linear and tetrahedral molecules are analyzed. Moreover, general formulae for matrix elements of powers of momentum in the harmonic oscillator basis are given.     

NIR vibrational overtone spectra of toluidines-evidence for steric and electronic effects in o-toluidine

The near infrared vibrational overtone absorption spectra of liquid phase toluidines are reported. The analysis of the observed CH and NH local mode mechanical frequency values shows that there exists steric and electronic interaction between the amino and methyl groups in o-toluidine. This observation supports the conclusions drawn from structural studies of toluidines by resonance two-photon ionization (R2PI) spectroscopy, ab initio calculations and laser induced fluorescence studies reported earlier.     

Anharmonic oscillator model of overtone spectra for D3h symmetry molecules

The bond stretching vibrations of XF₅ molecules with D_3h symmetry are treated computationally on the Morse oscillator model in which the bond oscillators are coupled harmonically. Each calculation involves four parameters for two types of Morse potential and three parameters for the kinetic-energy, potential coupling terms. The eigenvalue formula for overtone and combination states up to three are presented and can be used to predict all the vibrational energy levels from local mode molecules through normal mode molecules. For PF₅, AsF₅ and VF₅, the coupled Morse oscillator model gives a prediction in good agreement with the experimental data.     

Long-wave Raman spectra of some normal alcohols

Long-wave Raman spectra of some normal alcohols (from n-pentanol to n-decanol) in the liquid phase were registered. The regularities in the dependencies of Raman bands frequencies on the number of carbon atoms in the hydrocarbon chain were deduced. The calculations of Raman spectra of the studied molecules, their equilibrium structures and possible conformers were carried out in the approximation B3LYP/cc-pVDZ. These results in combination with the analysis of literature data allowed to explain the observed regularities in Raman band positions in the spectral range of 200–600 сm⁻¹ and their shifts upon increasing length of the chains. It was found that the plane configurations dominate in the liquid phase for molecules with short- and moderate-chain lengths. The elongation of the chain leads to the decrease of the fraction of plane conformers and in n-decanol the plane structure is completely absent.     

Mass spectra of branched long-chain aliphatic alcohols

The mass-spectral behaviour of long-chain aliphatic primary alcohols of the normal, iso-and anteiso-series have been discussed. The mass-spectral fragmentation of individual alcoholic types have been found to be structurally specific. The mass spectra of some deuterio derivatives have shown, however, the complicated nature of the fragmentation processes.