Stretching Overtone Spectra Research Articles

Intramolecular hydrogen-bonding effects on O[sbnd]H stretch overtone excitation for fluorinated hydroperoxides

Applying computational methods for predicting gas-phase OH stretch overtone spectra to the series of fluorinated hydroperoxides CF3OOH, CHF2OOH, and CH2FOOH reveals how intramolecular hydrogen bonding changes their overtone photochemistry, relative to their un-fluorinated analog CH3OOH. Intramolecular hydrogen bonding between fluorine and the hydroxyl hydrogen atom decreases OH stretch frequencies and raises the barrier for torsion about the OO bond. Whereas OO torsion accompanies OH stretch overtone excitation for CH3OOH, this is not the case for the fluorinated analogs that have intramolecular hydrogen bonding. The comparison highlights the molecular features that contribute to the feasibility of atmospheric overtone-induced OO bond dissociation for CH3OOH, but not for its fluorinated analogs.

Thermochemistry of HO2 + HO2 → H2O4: Does HO2 Dimerization Affect Laboratory Studies?

Self-reaction is an important sink for the hydroperoxy radical (HO2) in the atmosphere. It has been suggested (Denis, P. A.; Ornellas, F. R. J. Phys. Chem. A, 2009, 113 (2), 499-506) that the minor product hydrogen tetroxide (HO4H) may act as a reservoir of HO2. Here, we compute the thermochemistry of HO2 self-reactions to determine if either HO4H or the cyclic hydrogen-bound dimer (HO2)2 can act as reservoirs. We computed electronic energies using coupled-cluster calculations in the complete basis set limit, CCSD(T)/CBS[45]//CCSD(T)/cc-pVTZ. Our model chemistry includes corrections for vibrational anharmonicity in the zero-point energy and vibrational partition functions, core-valence correlation, scalar relativistic effects, diagonal Born-Oppenheimer, spin-orbit splitting, and higher-order corrections. We compute the Gibbs energy of dimerization to be (-20.1 ± 1.6) kJ/mol at 298.15 K (2σ uncertainty), and (-32.3 ± 1.5) kJ/mol at 220 K. For atmospherically relevant [HO2] = 10(8) molecules per cm(3), our thermochemistry indicates that dimerization will be negligible, and thus H2O4 species are atmospherically unimportant. Under conditions used in laboratory experiments ([HO2] > 10(12) molecules per cm(3), 220 K), H2O4 formation may be significant. We compute two absorption spectra that could be used for laboratory detection of HO4H: the OH stretch overtone (near-IR) and electronic (UV) spectra.

The most stable conformer of benzyl alcohol

We have recorded room temperature OH stretching overtone spectra of benzyl alcohol in the second and third overtone regions and jet-cooled OH stretching spectra in the fundamental and third overtone region. The spectra were recorded using photoacoustic and nonresonant ionization techniques, respectively. The spectra are characterized by a single band for each OH stretching overtone region suggesting the existence of one dominant stable conformer. In addition, our mass selected resonance enhanced multi photon ionization spectrum shows only one origin band. Ab initio calculations for benzyl alcohol yielded a lowest energy conformer adopting a gauche cis conformation around the alcoholic group, which is supported by our calculated OH stretching intensities.

Local mode and normal mode models for molecules with two non-equivalent C–H bonds

For molecules with C–H bonds, a local mode description of the bonds is often adopted to interpret the C–H stretching overtone spectra, although a representation in terms of normal modes is also possible. The equivalence between the two alternative representations demands simple numerical equations between the normal mode anharmonicity constants x rs and the quartic anharmonic coupling constants K rstu , the so-called ‘x–K relations’. The relations appropriate for low symmetry molecules with two non-equivalent C–H bonds (ν1, ν2), such as XHC = CHY, are derived in this paper. The consequences of these relations are illustrated with a detailed analysis of the C–H stretching overtone region, where the naive application of the relations obtained for more symmetric molecules could lead to a different interpretation of the overtones 2v 1 and 2ν2 and of the combination band ν1 + ν2. The relations are tested with theoretical calculations for difluoro- and chlorofluoro-substituted ethenes, propadiene, and butatrienes.

Erratum: “Effects of torsion on O–H stretch overtone spectra and direct overtone photolysis of methyl hydroperoxide” [J. Chem. Phys. 123, 234306 (2005)

First Page

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.

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.

CH stretching overtone spectra of a fast rotating methyl group: 2-CH3 and 2-CHD2 Pyridines

CH stretching overtone spectra of a fast rotating methyl group: 2-CH3 and 2-CHD2 Pyridines

CH Stretching Overtone Spectra of Fluorine Substituted Toluenes

We have measured and analyzed the vapor phase CH stretching overtone spectra of 3,5-difluorotoluene, 2,3,5,6-tetrafluorotoluene, and 2,3,4,5,6-pentafluorotoluene. The aryl bands are complex due to Fermi resonance. We have used a two-level system model to analyze the Fermi resonance within the aryl bands. The aryl regions of the overtone spectra are “corrected” for Fermi resonance and interpreted as uncoupled stretching oscillators. The methyl band profiles are complex due to the coupling between CH stretching and methyl torsion. The methyl band profiles are simulated with a CH stretching methyl torsion model. The CH stretching modes are described by a harmonically coupled anharmonic oscillator local mode model, and the methyl torsion is described by a rigid rotor model. Simulation parameters are obtained from ab initio calculations with the HF/6-31G(d) method. Relative intensities of aryl and methyl contributions in the CH stretching overtone spectra in the region ΔvCH = 2−5 are measured and calculated for these molecules. We have found that the methyl bands of 3,5-difluorotoluene are very similar to corresponding bands in toluene, 4-methylpyridine, p-xylene, and m-xylene. The methyl bands of 2,3,5,6-tetrafluorotoluene and 2,3,4,5,6-pentafluorotoluene are virtually identical to those of 2,6-difluorotoluene. Thus, the main factor that determines the structure in methyl CH stretching overtone spectra of these molecules is the nature of the groups adjacent to the methyl group. The aryl bands of these molecules are primarily affected by the nearest substituents.

CH stretching overtone spectra of trimethyl amine and dimethyl sulfide

Trimethyl amine (TMA) exhibits the largest known difference in CH bond lengths within a methyl group, due to what is known as the lone pair trans effect. Dimethyl sulfide also exhibits this effect, but to a far lesser extent, making it ideal for comparison to TMA. In this paper, the first through fourth overtone spectra of N(CH3)3, N(CD3)3, N(CD2H)(CD3)2, N(CH3)(CD3)2, N(CD3)(CH3)2 and S(CH3)2 are reported and all major bands are assigned. The intensities of the observed bands are compared to intensities predicted by the harmonically coupled anharmonic oscillator local mode model. Good correlation is found between the experimental intensities and those predicted with the local mode model and HF/6-311++G(2d,2p) calculated dipole moment functions. An increase in the ability to resolve peaks as methyl groups are deuterated suggests that the lone pair mediates increased coupling between methyl groups.

Effect of the Methyl Internal Rotation Barrier Height on CH−Stretching Overtone Spectra

The coupling of CH stretching with methyl internal rotation (torsion) causes complexity in methyl CH stretching overtone spectra. We have used a CH stretching methyl torsion model to simulate the methyl band overtone profiles. Room-temperature vapor phase overtone spectra of acetic acid (ΔvCH = 4−5) and 2-methylfuran (ΔvCH = 4−5) have been recorded in the methyl CH stretching regions. These combined with previously studied molecules acetone, acetaldehyde, toluene, methylpyridines, and xylenes provide a series of molecules with gradually varying barrier heights.

Local modes

The historical background for the development of the local mode approach is reviewed, including the critical role of the radiation field. The harmonically coupled anharmonic oscillator model is described. Local modes have been used in the interpretation of XH stretching overtone spectra. These spectra can be used as sensitive probes of molecular properties such as bond length and molecular conformation. Examples of these spectral features are discussed. An intensity theory has been developed that permits the accurate calculation of relative and absolute overtone intensities. In molecules with low barriers to methyl rotation, torsional states contribute to the XH stretching band profile. A successful theoretical approach is presented that accounts for these effects and provides insight into the vibrational dynamics. The application of calculated local mode overtone spectra to atmospheric chemistry is discussed.Key words: local mode, overtone, vibrational intensity.

Theoretical Analysis of the CH Stretching Overtone Vibration of 1,2-Dichloroethylene

The intensity of the CH stretching overtone spectra of liquid cis-dichloroethylene is known to be greater than that of the trans isomer, even though the transition energies are almost the same. To obtain theoretical insight on this feature, we performed a vibrational calculation under the local mode model, which is preferred for describing vibration of light−heavy bonds, using the grid method and the potential energy surface (PES) and the dipole moment function (DMF) calculated by hybrid density functional theory method. It was determined that the DMF, in the direction perpendicular to the CC bond, was significantly distorted from linearity as a function of the CH bond length. This distortion, which is regarded as the electric anharmonicity, was greater for the cis isomer, thus giving a stronger overtone absorption intensity for this isomer. In addition, the numerical accuracy in representing the PES and DMF was discussed for the overtone vibrational calculation.

CH-stretching overtone spectra of a fast rotating methyl group. II. Toluenes C6D5CH2D and C6D5CHD2

The CH-stretching overtone spectra of the methyl group in gaseous toluene C6D5CH2D and C6D5CHD2 are recorded with conventional Fourier transform near infrared spectroscopy in the Δv=1–4 regions and by intracavity laser photoacoustic spectroscopy in the Δv=5 and 6 regions. The spectra are analyzed with a theoretical model that takes into account, within the adiabatic approximation, the coupling of the anharmonic CH stretch with the quasifree internal rotation of the methyl group and with isoenergetic combination states involving methyl bending and rocking modes. A simultaneous successful reconstruction of the CH stretching overtone spectra of three isotopic derivatives of the methyl group of toluene is obtained with a single set of parameters from Δv=1–6 (18 spectra). The modifications caused by the partial deuteration of the rotating methyl group on its IVR mechanisms are analyzed. In both methyl deuterated compounds, the CH/CD interbond coupling induces a shift of the overtone spectra towards high frequencies and the appearance of additional features in the high energy overtone spectra corresponding to [(n−1)νCH+1νCD] resonant combination states. In toluene C6D5CHD2, this shift is progressively amplified until Δv=4 by Fermi resonance phenomena involving mainly HCD bending modes. From Δv=3, the effect of these anharmonic interactions appears as an extra structure in the low energy side of the spectra. The “tuning” of these interacting states into resonance is reached at Δv=5 and causes a strong intramolecular vibrational energy redistribution. In toluene C6D5CH2D, at Δv=1 and 2, the CH stretching spectra are perturbed by Fermi resonance with HCH bending mode “doorway” states. A transitional regime between normal and local mode is detected at Δv=2, which requires a more elaborate model. At higher energy, the HCH bending mode combinations move farther out of resonance. From Δv=4–6, the perturbation of the spectra is then increasingly due to Fermi resonance phenomena involving HCD bending modes.

Vibrational Overtone Spectroscopy and Overtone Intensities of Cyclohexadiene Iron Tricarbonyl and 1,3-Cyclohexadiene

The CH stretching overtone spectra of liquid cyclohexadiene iron tricarbonyl (CHDIT) have been recorded for the first, second, and third overtone regions (Δv = 2−4). The gaseous spectrum of CHDIT has also been obtained for the first overtone region (Δv = 2). Observed transitions are assigned to the overtones of different nonequivalent CH bonds and to the local mode−normal mode combination bands. Oscillator strengths have been calculated with the harmonically coupled anharmonic oscillator (HCAO) local mode model and ab initio dipole moment functions. The theoretical values have been compared to the experimental oscillator strengths and agree well with experiment. The comparison of the overtone spectrum of CHDIT to the corresponding spectrum of uncomplexed 1,3-cyclohexadiene reveal changes that occur in the ligand upon complexation with iron tricarbonyl. The presence of the metal lengthens the terminal olefinic CH bond by 0.002 Å and shortens the nonterminal olefinic CH by 0.001 Å. The axial and equatorial CH bonds of the free ligand become similar in length in the complex. The implications of the observed spectral features to the structure and bonding in the complex are discussed.

Relative Intensities of Nonequivalent CH Bonds in the Local Mode Overtone Spectra of 1,3- and 1,4-Cyclohexadiene

The CH stretching overtone spectra of 1,3- and 1,4-cyclohexadiene have been recorded in the regions corresponding to Δv = 2−6. Bands are assigned as overtones of the various nonequivalent CH bonds. Oscillator strengths have been calculated according to the harmonically coupled anharmonic oscillator (HCAO) local mode (LM) model, with ab initio dipole moment functions. The theoretical values have been compared to experimental values, and good agreement is found between predicted intensities and measured relative intensities within an overtone band. For 1,4-cyclohexadiene, shoulders were observed approximately 100 cm-1 above and below the pure LM transition for Δv ≥ 4. These are assigned as (±) combinations with a low-frequency ring-puckering mode.

Anharmonic stretching vibrations expressed as local modes

Various models used to interpret X-H stretch (or X-D or other weakly coupled bond modes) overtone spectra in terms of local modes are considered. For general systems of coupled X-H stretching modes we demonstrate systematically the origin of local mode anharmonic coupling terms (in a block-diagonal effective Hamiltonian) via the application of perturbation theory to vibrational Hamiltonians expressed in internal (local) coordinates and which include anharmonic effects as well as quadratic interbond coupling. Two approaches are presented: extension of the approximate anharmonically coupled anharmonic (Morse) oscillator (AACAO) model and a perturbation theory treatment of a generalized local mode model Hamiltonian. These results are tested by application of the derived formulae to model potentials. X-H stretching vibrational energy levels are calculated for ammonia and silane and in the latter case compared with experimental data. Local mode anharmonicity constants are calculated for water and methyl bromid...

Anharmonic stretching vibrations expressed as local modes

Various models used to interpret X-H stretch (or X-D or other weakly coupled bond modes) overtone spectra in terms of local modes are considered. For general systems of coupled X-H stretching modes we demonstrate systematically the origin of local mode anharmonic coupling terms (in a block-diagonal effective Hamiltonian) via the application of perturbation theory to vibrational Hamiltonians expressed in internal (local) coordinates and which include anharmonic effects as well as quadratic interbond coupling. Two approaches are presented: extension of the approximate anharmonically coupled anharmonic (Morse) oscillator (AACAO) model and a perturbation theory treatment of a generalized local mode model Hamiltonian. These results are tested by application of the derived formulae to model potentials. X-H stretching vibrational energy levels are calculated for ammonia and silane and in the latter case compared with experimental data. Local mode anharmonicity constants are calculated for water and methyl bromide and two of its deuterated isotopomers. Comparisons with the harmonically coupled anharmonic oscillator (HCAO) model show that serious systematic errors in the latter may be corrected whilst retaining a relatively simple effective Hamiltonian. The anomalously low magnitude of the HD harmonic interbond (near-resonant) coupling constant determined in an earlier study of CHD2Cl is rationalized by analysis of previously neglected anharmonic effects. This last application demonstrates the relevance of these local mode anharmonic couplings to systems in which Fermi resonances (with non-stretching modes) are also important. Applications to CH3F and CH2D2 are also briefly discussed.

Internal dynamics contributions to the CH stretching overtone spectra of gaseous nitromethane NO2CH3

The methyl CH stretching overtone spectra of gaseous nitromethane NO2CH3 have been recorded with Fourier transform infrared conventional near infrared spectroscopy in the ΔvCH=1–4 regions and by intracavity laser photoacoustic spectroscopy in the ΔvCH=5 and 6 regions. They all exhibit a complex structure with, at ΔvCH=1 and 2, a characteristic asymmetric top vibration-rotation profile which vanishes as vibrational energy increases. In addition, in these two lower energy spectra, the perpendicular stretching vibrations exhibit a widely spaced fine structure profile resulting from a Coriolis coupling induced by the methyl internal rotation. These excited spectra have been analyzed with a theoretical model which takes into account, in the adiabatic approximation, the coupling of the anharmonic CH stretch vibrations, described by a Morse potential, with the quasi-free internal rotation of the methyl group and with isoenergetic combination states involving methyl bending modes. Till Δv=3, the theoretical treatment yields normal modes. From the third overtone, the vibrational energy is seen to be localized and thus the calculations proceed in a local mode basis from Δv=4–6. Most of the parameters of this model and their variation with the internal rotation coordinate θ are the same as those used to account for the overtone spectra of the monohydrogenated species. Fermi resonance phenomena, also modeled with θ dependent parameters, lead to only weak IVR localized at the second overtone where only the two first tiers are effective and at the third overtone where three tiers must be considered to give a good reproduction of the experimental spectral features. This simple calculation successfully describes the relative intensity and frequency of each peak within a given overtone.

State‐to‐state studies of intramolecular dynamics and unimolecular reactions

AbstractDouble‐resonance vibrational overtone excitation spectra of small molecules, in combination with laser‐induced fluorescence product detection, provide information on both their energetics and intramolecular energy redistribution dynamics. Application of this approach to measure OH stretch overtone spectra of HOOH and NH2OH near their dissociation threshold provides estimates of the O‐O and N‐O bond energies of 17051.8±4 cm−1 and 21620±20 cm−1 respectively.In symmetrical molecules, such as HOOH, the double‐resonance excitation technique allows us to prepare molecules in either pure OH stretch overtone levels or combination levels in which the excitation energy is divided between two identical OH stretch oscillators. Comparison of the resulting vibrational overtone linewidths indicates no clear difference between the pure OH stretch and the local‐local combination levels. However, comparison of the energy transfer rates from pure OH stretch vibrations and those in which one quantum of energy is put in a low frequency mode reveals faster rates from the latter. This observation clearly indicates that coupling strengths depend markedly on the vibrational character of the coupling partner and not simply on the total energy.Comparison of the OH stretch overtone spectra of the structurally similar molecules HOOH, NH2OH, and CH3OH reveal similar rates of vibrational energy redistribution, despite the difference in the density of states in these molecules. This observation reinforces the notion that energy flow between the OH stretch and dark background states is not statistical but rather is controlled by coupling to a small subset of background states.