CH Stretch Overtone Research Articles

Laser vibrational overtone activation of ethyl acrylate/benzoyl peroxide mixture

Intra- and extracavity laser vibrational overtone polymerization of ethyl acrylate/benzoyl peroxide mixture has been demonstrated. Five photolysis wavenumbers on and near the fifth CH stretch overtone absorption of benzoyl peroxide was investigated. The polymer yield was monitored by observing the decrease in the intensity ratio of the olefinic/methyl and methylenic first CH stretch overtone absorptions of ethyl acrylate. The rate of the polymerization did not depend on the photolysis wavenumber. Molecular weights of the overtone initiated polymers were an order of magnitude larger than those obtained by thermal polymerization. The polymerization rate is compared to the intracavity laser vibrational overtone polymerization of methyl methacrylate.

Quasiclassical dynamics of benzene overtone relaxation on an ab initio force field. I. Energy flow and survival probabilities in planar benzene for CH(v=2,3)

Computational studies based upon ensembles of quasiclassical trajectories are presented for 21-mode (planar) benzene, with which we examined relaxation of the first and second (v=2,3) CH stretch overtones. The most complete ab initio force field available for this molecule was used for short time (t<240 fs) and long time (t=2.4 ps) dynamical calculations. Local mode analyses indicate that energy flow from the initially excited CH stretch is fast (t<240 fs) and irreversible. For both overtones examined, energy from the CH chromophore activates the following local modes in sequence: ipso in-plane CH wag, adjacent CC stretch, terminal CC stretch, ortho CH in-plane wag, meta CH in-plane wag, intermediate CC stretch, meta CH stretch, para CH in-plane wag, and para CH stretch. Long time calculations indicate that the in-plane CH wag and CH stretching local modes are at steady-state quantities by 2.4 ps, while the CC stretches and CCC bends continue to take on energy. The ortho and meta CH stretches were of minor importance in intramolecular vibrational energy redistribution in benzene, gaining little or no energy over the time scales we studied. Survival probabilities indicate that the half life of the excited CH stretch decreases by almost two thirds as the overtone energy is raised. The results of our study are compared to other classical and quantum dynamical investigations of benzene and are found to be in good qualitative agreement.

Quantum mechanical study of intramolecular vibrational energy redistribution in the second CH stretch overtone state in benzene

The vibrational level mixing at the second CH stretch overtone state CH(v=3) in benzene has been studied quantum mechanically using a completely symmetrized vibrational basis set in terms of a combined local mode/normal mode description. The employed symmetrized approach has helped to reduce the dimensionality of coupling Hamiltonian matrices and thus allowed for the inclusion of all 30 vibrational modes in the calculations. The absorption spectrum and dynamical intramolecular vibrational redistribution characteristics for initial excitation of a symmetrized local mode “bright” state in the CH(v=3) overtone manifold have been calculated and analyzed in connection with the degree of localization of the CH stretch overtone vibrational system in benzene.

Vibrational overtone spectroscopy and internal dynamics in gaseous nitromethane NO2CH2D

The CH-stretching overtone spectra of the methyl group in gaseous nitromethane NO2CH2D have been recorded with conventional Fourier transform near-infrared spectroscopy in the ΔvCH=1–4 regions and by intracavity laser photoacoustic spectroscopy in the ΔvCH=5 and 6 regions. All spectra exhibit a complex structure; they have been analyzed with a theoretical model which takes into account, within the adiabatic approximation, the coupling of the anharmonic CH stretch vibrations, described by Morse potentials, with the quasifree internal rotation of the methyl group and with isoenergetic combination states involving methyl bending modes. Most of the parameters of this model, and their variation with the internal rotation coordinate (θ), are identical to ones used to account for the overtone spectra of the monohydrogenated and perhydrogenated species. Fermi resonance phenomena, also modeled with θ dependent parameters, lead to only weak intramolecular vibrational energy redistribution. This simple calculation successfully describes the relative intensity and frequency of each peak within a given overtone. The vibrational energy is seen to be localized at lower energy in the dihydrogenated compound [at the second overtone (v=3)] than in the perhydrogenated one [at the third overtone (v=4)]. The overtone vibrations obtained from the calculations can be considered as normal modes up to Δv=2 and as local modes from Δv=3 to 6. However, at Δv=3, the existence of a transitional regime, where normal and local modes coexist, can be demonstrated. The CH/CD interbond coupling shifts the overtone spectra toward high for some additional weaker features in the high overtone spectra (Δv=5 and 6).

A quantum mechanical description of vibrational motion in benzene in terms of a completely symmetrized set of complex vibrational coordinates and wave functions

In this work a fully symmetrized quantum mechanical description of vibrational motion in terms of complex vibrational coordinates and complex basis wavefunctions is outlined, designed for studying vibrational level mixing and intramolecular vibrational energy redistribution (IVR) around CH stretch overtone states in benzene. Symmetrized local mode (LM) formalism has been applied to the CH stretch system, while the remaining benzene vibrations (including out-of-plane modes) were treated as normal modes (NM). Using the outlined approach a model calculation of the absorption spectrum of the first overtone state CH (n=2) at ∼6000 cm−1 has been carried out.

CH-stretching overtone spectra and internal methyl rotation in 2,6-difluorotoluene

Vapor phase overtone spectra of 2,6-difluorotoluene are recorded in the ΔvCH=2 and 3 regions by conventional near-infrared spectroscopy and in the ΔvCH=4–6 regions by intracavity dye/titanium: sapphire laser photoacoustic spectroscopy. The spectra are interpreted on the basis of ab initio calculations at the HF/6-31G* level. The methyl regions of the spectra are complex due to coupling between the nearly freely rotating methyl rotor and CH stretching. A model has been developed to predict the methyl spectral profiles, which uses the harmonically coupled anharmonic oscillator local mode model and the rigid rotor model for stretching and torsion, respectively. A dipole moment function is formulated which combines a Taylor series for CH stretching and a Fourier series for torsion. The dipole moment function includes both angular dependence and higher order expansion terms in the CH-stretching coordinate. The model is successful in predicting the methyl overtone spectral profiles and attributes these profiles to a very large number of transitions that arise from terms involving torsion–stretching coupling, both in the Hamiltonian and in the dipole moment function.

Vibrational Overtone Spectroscopy of Butadiene Iron Tricarbonyl and 1,3-Butadiene

The photoacoustic vibrational overtone spectrum of gaseous butadiene iron tricarbonyl (BDIT) at the third overtone region was recorded and compared to that of 1,3-butadiene. The effect of the metal, determined by an interpretation of the overtone spectrum of the complex, is to lengthen the terminal trans CH bond of the 1,3-butadiene ligand by 0.0024 A while the terminal cis bond does not change significantly. The assignments of the terminal cis, terminal trans, and nonterminal CH stretch overtone absorptions are supported by ab initio geometry optimization calculations of the trans and cis butadiene and the BDIT complex. The spectrum of the complex possessed three additional bands not seen in the free ligand. These bands originate from vibrational coupling induced by the presence of the metal.

Spectroscopic Investigation of Subtle CH-Bond Changes between Solid and Vapor Phase Naphthalene

We have recorded CH-stretching overtone spectra of naphthalene-d0 (N-d0) and selectively deuterated naphthalene-2,3,6,7-d4 (N-βd4) both in the vapor and in the solid phase. We have used intracavity...

Internal dynamics contributions to the CH stretching overtone spectra of gaseous monohydrogenated nitromethane NO2CHD2

The methyl CH-stretching overtone spectra of gaseous monohydrogenated nitromethane NO2CHD2 have been recorded with conventional near infrared spectroscopy in the ΔvCH=1 to 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. These spectra have been analyzed with a theoretical model which takes into account in the adiabatic approximation the coupling between the anharmonic CH stretch described by a Morse potential and the quasifree internal rotation of the methyl group. All the parameters of this model (the zero point energy, the CH stretch frequency, the expansion coefficients of the dipole moment function) and their variation with the internal rotation coordinate have been determined from HF/6-31G** ab initio calculations. This simple calculation, which contains no adjustable parameters, successfully describes the relative intensity and frequency of each peak within a given overtone and accounts for the variation of the dipole moment function as the vibrational energy increases. Owing to the relative localization of the wave functions within the effective potential wells, the spectral features can be assigned to particular pseudoconformers. The outer bands correspond to rovibrational transitions associated with the parallel and perpendicular conformation of the CH bond versus the molecular plane, the central band is generated by ‘‘free rotor’’ rovibrational transitions. Fermi resonance phenomena lead to no sizeable IVR until the fifth overtone. The CH/CD interbond coupling shifts the overtone spectra toward high frequency and is responsible for some additional weaker features in the high overtone spectra (Δv=5 and 6).

Intensities of CH-stretching overtones in 2-butenes

The gas phase vibrational overtone spectra of trans-2-butene and cis-2-butene are recorded in the ΔνCH=1−6 regions by Fourier transform infrared and conventional near infrared-visible spectroscopy. Overtone spectra in the ΔνCH=5−9 regions are recorded by intracavity dye/titanium: sapphire laser photoacoustic spectroscopy. Absolute oscillator strengths are obtained from the conventional spectra. Oscillator strengths are calculated with a harmonically coupled anharmonic oscillator local mode model and ab initio dipole moment functions. The experimental and calculated oscillator strengths are compared. Spectral absorptions that are not due to transitions to pure local mode states or to local mode combination states, are observed in the spectra of both molecules, and are believed to be due to absorptions that involve methyl group torsion.

Evidence that irreversible intramolecular vibrational energy redistribution is slow for ring modes up to 8200 cm −1 in S 0 benzene

High-resolution dispersed fluorescence spectra have been used to probe intramolecular vibrational energy redistribution (IVR) for ring vibrations in S 0 benzene up to ≈ 8200 cm −1. All bands monitored show sharp structure (≈ 1 cm −1 fwhm) with resolved rotational features. An upper limit to the contribution from irreversible IVR to these linewidths is 0.5 cm −1, corresponding to rate constants < 4.7 × 10 10 s −1 (τ IVR > 21 ps). The ring mode linewidths appear to be narrower than CH stretch overtone linewidths at comparable energies, indicating that irreversible IVR is slower for ring modes.

Overtone spectroscopy by vibrationally assisted dissociation and photofragment ionization

A new method for obtaining overtone spectra in the gas phase is presented: IR and UV laser light is focused into a flow cell. The IR excites v = 3 or v = 4 CH-stretch overtones in the molecules studied, CHCl 3, CH 3Cl, CF 2HCl and (CH 3) 3CCl. Subsequent excitation by UV photon absorption dissociates the molecule. The photofragments are probed by multiphoton ionization using the same frequency as the UV dissociation laser. By scanning the IR laser while monitoring the ionization yield, overtone spectra have been obtained. The new technique requires sample pressures below 1 mbar, and the absorption path is the focus region, only. The method is thus suitable for overtone spectroscopy in a supersonic jet expansion, or to study molecules with low vapour pressures.

Dipole moment function and equilibrium structure of methane in an analytical, anharmonic nine-dimensional potential surface related to experimental rotational constants and transition moments by quantum Monte Carlo calculations

The pure rotational spectrum in the far-infrared and its absolute intensity in the vibrational ground state of CHD3 and CH3D, and the integrated band strength of the N=5 CH-stretching overtone of CHD3 in the near infrared to visible were measured by high-resolution interferometric Fourier transform techniques. The far-infrared data result in permanent electric dipole moments (‖μz0‖=(5.69±0.14)×10−3 D for CHD3, ‖μz0‖=(5.57±0.10)×10−3 D for CH3D), consistent with previous experimental data. The integrated N=5 overtone cross section is found to be (0.828±0.068) fm2. The overtone data are used, together with previous data, to derive a new, nine-dimensional, isotopically invariant dipole moment function for CH4 within the chromophore model for the CH chromophore in CHD3. With this function, the experimental data can be reproduced to an averaged factor of 1.2, in the best case. In the vibrational ground state, a nine-dimensional calculation of expectation values on a new, fully anharmonic potential surface was performed using the solution of the rovibrational Schrödinger equation by diffusion quantum Monte Carlo methods. The results for the rotational constants of several isotopomers, which include significant contributions from rovibrational interactions, indicate that the equilibrium CH bond length of methane is re=108.6 pm. The calculated value for the vibrationally averaged permanent dipole moment from these nine-dimensional vibrational quantum calculations, using the dipole moment function consistent with the analysis of the overtone bands, is μz0=−(6.6±0.4)×10−3 D for CHD3 (with positive z coordinate for the H atom) and μz0=(6.8±0.5)×10−3 D for CH3D (with positive z coordinate for the D atom) in essential agreement with the far-infrared rotational intensities. The sign could be determined unambiguously by comparison with ab initio data. We predict the permanent dipole moment of several further methane isotopomers. The polarity of the CH bond in methane is C−–H+, within our simple bond dipole model, but is discussed to be a model dependent (not purely experimental) quantity.

Sub-Doppler, infrared laser spectroscopy of the propyne 2ν1 band: Evidence of z-axis Coriolis dominated intramolecular state mixing in the acetylenic CH stretch overtone

The eigenstate-resolved 2ν1 (acetylenic CH stretch) absorption spectrum of propane has been observed for J′=0–11 and K=0–3 in a skimmed supersonic molecular beam using optothermal detection. Radiation near 1.5 μm was generated by a color center laser allowing spectra to be obtained with a full-width at half-maximum resolution of 6×10−4 cm−1 (18 MHz). Three distinct characteristics are observed for the perturbations suffered by the optically active (bright) acetylenic CH stretch vibrational state due to vibrational coupling to the nonoptically active (dark) vibrational bath states. (1) The K=0 states are observed to be unperturbed. (2) Approximately 2/3 of the observed K=1–3 transitions are split into 0.02–0.25 cm−1 wide multiplets of two to five lines. These splittings are due to intramolecular coupling of 2ν1 to the near resonant bath states with an average matrix element of 〈V2〉1/2=0.002 cm−1 that appears to grow approximately linearly with K. (3) The K subband origins are observed to be displaced from the positions predicted for a parallel band, symmetric top spectrum. The first two features suggest that the coupling of the bright state to the bath states is dominated by parallel (z-axis) Coriolis coupling. The third suggests a nonresonant coupling (Coriolis or anharmonic) to a perturber, not directly observed in the spectrum, that itself tunes rapidly with K; the latter being the signature of diagonal z-axis Coriolis interactions affecting the perturber. A natural interpretation of these facts is that the coupling between the bright state and the dark states is mediated by a doorway state that is anharmonically coupled to the bright state and z-axis Coriolis coupled to the dark states. Z-axis Coriolis coupling of the doorway state to the bright state can be ruled out since the ν1 normal mode cannot couple to any of the other normal modes by a parallel Coriolis interaction. Based on the range of measured matrix elements and the distribution of the number of perturbations observed we find that the bath levels that couple to 2ν1 do not exhibit Gaussian orthogonal ensemble type statistics but instead show statistics consistent with a Poisson spectrum, suggesting regular, not chaotic, classical dynamics.

Intensities of CH- and CD-stretching overtones in 1,3-butadiene and 1,3-butadiene-d6

Gas phase vibrational overtone spectra of 1,3-butadiene are recorded in the ΔvCH=2–6 regions by conventional near infrared–visible spectroscopy, and in the ΔvCH=4–7 regions by intracavity dye/titanium:sapphire, laser photoacoustic spectroscopy (ICL-PAS). Gas phase vibrational overtone spectra of 1,3-butadiene-d6 are recorded in the ΔvCD=2–5 regions with conventional spectroscopy and in the ΔvCD=5–8 regions by ICL-PAS. Oscillator strengths are calculated from wave functions that are obtained from a harmonically coupled anharmonic oscillator (HCAO) local mode model and from a dipole moment function that is obtained from ab initio calculations. The experimental oscillator strengths are compared to the values that are calculated for both the CH- and CD-stretching components of the spectrum. Our simple calculations, which contain no adjustable parameters, are in very good agreement with the relative intensities of the peaks corresponding to the three different CH oscillators in 1,3-butadiene. As expected, the local mode description is not as good for the CD oscillators in 1,3-butadiene-d6. Nonetheless, the calculations can provide a reasonable explanation of the CD-stretching intensity distribution in the higher overtone spectra of 1,3-butadiene-d6. Small hydrogen impurities in the fully deuterated sample give rise to isolated CH-stretching overtones. The relative intensities of the CD peaks and the CH impurity peaks in the 1,3-butadiene-d6 sample spectra are predicted by the calculations. A comparison of the 1,3-butadiene-d6 sample spectra in the CH-stretching region with the CH-stretching overtone spectra in 1,3-butadiene dramatically illustrates the effects of vibrational coupling between CH oscillators.

The effect of internal rotation on the methyl CH-stretching overtone spectra of toluene and the xylenes

The structure of methyl CH-stretching overtone bands in the vibrational spectra of methylbenzenes was investigated theoretically. The anharmonic CH-stretching vibration, described by a Morse potential, was represented in terms of a harmonic basis while hindered internal rotation of the methyl group was represented by a rigid rotor attached to an infinitely massive frame. Relatively weak coupling between the anharmonic CH vibration and the hindered internal rotation is sufficient to shift the positions of rovibrational lines from a PQR-like rotational contour to patterns similar to those observed experimentally. For high rotational barriers, as in o-xylene, the rovibrational transitions form two bands associated with conformationally nonequivalent CH-bonds, consistent with the conformational preference established by microwave spectroscopy and molecular orbital calculations. For nearly free internal rotation, as in toluene, m-xylene and p-xylene, a prominent middle band is also present. This ‘‘free rotor’’ band corresponds to rotational transitions between states high above the barrier and disappears as the barrier height increases. The outer bands correspond to transitions for which either the initial or the final state is below or near the barrier height in energy. Contrary to earlier suggestions, the band structure is not indicative of the conformational preference of the methyl group in toluene. In fact, the calculated spectra of nearly free internal rotors are insensitive to this preference.

The 3 ← 0 CH stretch overtone of benzene

The second CH stretch overtone of benzene has been studied both experimentally and theoretically. A recent high-resolution opto-thermal experiment showed that the overtone spectrum consists of several absorption bands located in the region between 8730 and 8875 cm −1. In this work, we investigate possible contributions of strong hot bands to the absorption around 8770 cm −1. Our results show that this region of the opto-thermal spectrum is not significantly affected by hot bands. The spectrum has been described by a theoretical calculation based on the algebraic theory. We find good agreement with experimental results. The low-frequency region and minor satellite peaks are reproduced reasonably well. Quantum labels are assigned to some of the observed peaks.

Spectrum and dynamics of the CH chromophore in CD 2HF. I. Vibrational Hamiltonian and analysis of rovibrational spectra

We have measured the complete rovibrational spectrum of dideutero-fluoro-methane (CD 2HF) from the far-infrared (20 cm −1) to the visible (14000 cm −1) largely at Doppler-limited resolution. Rovibrational parameters are reported for CH-stretching and bending fundamentals and overtones. The spectra of the coupled stretching and bending modes of the CH chromophore are analyzed quantitatively both in terms of effective Hamiltonians and a three-dimensional model Hamiltonian. A new numerical method for solving the corresponding eigenvalue problem is described briefly. The strong Fermi and Darling—Dennison resonances lead to fast vibrational redistribution on the femtosecond time scale.

Calculation of infrared intensities of highly excited vibrational states of HCN using Van Vleck perturbation theory

Canonical Van Vleck perturbation theory (CVPT) is used to calculate electric dipole intensities for one-, two-, and three-dimensional models of HCN and a six-dimensional model of H2CO. Lehmann and Smith [J. Chem. Phys. 93, 6140 (1990)] have shown that the intensities of overtone transitions are sensitive to the details of the inner wall of the potential. Dipole intensities calculated for several, similar one-dimensional CH stretch potentials demonstrate that perturbation theory correctly predicts this sensitivity. The perturbation intensities of a two-dimensional ab initio dipole surface indicate the importance of selected stretch–stretch resonance interactions in interpreting the CH stretch overtone spectra of HCN. The inclusion of the bend confirms that this degree of freedom plays a significant role in weakening the intensity of the CN overtones. The CH stretch overtone spectra of H2CO is calculated to illustrate the utility of the perturbative approach for predicting the transition intensities for a system in which there are multiple Fermi interactions.

Intracavity-laser-absorption spectroscopy of the visible overtone transition of methane in a supersonically cooled jet

We report the CH-stretching overtone spectrum of jet-cooled methane in the visible range. Due to the very weak intensity of the transition, the intracavity-laser-absorption-spectroscopy technique (ICLAS) was used with a free jet of methane from a slit orifice placed inside a dye-laser cavity. In this way, an equivalent pathlength through the jet of 800 m was achieved. The rotational cooling drastically reduces the congestion of the spectrum: whereas the room-temperature spectrum is extremely congested and complicated, the jet spectrum is sparse and reduced to well-isolated lines. These results show that rotational congestion is mainly responsible for the overtone band shape of methane and that, in consequence, the contribution of the background vibrational states is not predominant. The experimental method developed in this work can be extended to study the overtone transitions of a wide class of jet-cooled compounds.