D.L. Snavely

Spectroscopic investigation of ground state pyrrole (12C4H5N): the NH stretch

Abstract We have recorded the infrared absorption spectrum of pyrrole at 0.005 cm −1 spectral resolution using a Fourier transform interferometer. The rotational analysis of the fundamental NH stretch (1 1 0 ) at 3530.811343(82) cm −1 was performed. A set of 13 upper state rovibrational parameters was determined, allowing the 2715 assigned rovibrational lines to be reproduced with a standard deviation of 1.3 10 −3 cm −1 . An attempt to record the fundamental band under slit-jet conditions is reported. The role of hot bands accompanying the series of the NH stretch excitation is investigated. Effective vibrational parameters — ω 0 1 , X 0 11 , Y 111 , X 1,24 — are obtained. The lower level in the hot band series is unambiguously identified as the V 24 = 1 level, by retrieving X 1,24 independently, from other spectral data. The observation of the complex band pattern accompanying the NH series in the higher overtone range is discussed with the help of new data, recorded around the 1 5 0 band at different temperatures using intracavity laser optoacoustic spectroscopy.

Hot bands in overtone absorption of pyrrole, methyl isocyanide and isobutane: Photoacoustic spectra at 140 C

The intracavity laser photoacoustic spectra of the 4νN–H and 4νC–H overtones of pyrrole, the 5νC–H of methyl isocyanide and acetonitrile, the 5νC–H and 6νC–H of isobutane, and the 6νC–H of cyclohexane were measured at temperatures up to 140 C (T140) and compared to the room temperature (Trt) spectra. Hot bands were identified by their temperature dependence in pyrrole, methyl isocyanide, and isobutane. From the temperature dependence of the relative intensities of the hot band compared to the main band, the following average wave numbers of the thermally populated states were deduced: 340, 300, and 208 cm−1 for pyrrole, isobutane, and methyl isocyanide, respectively. These values compare well to the bending motion frequencies in these molecules. Large redshifts of the hot bands from the main peak were observed, as expected for high overtone transitions due to large anharmonicities. The anharmonic constants determined from these shifts are −25 cm−1 for the N–H in pyrrole and −20 cm−1 for the C–H in methyl ...

Vibrational Overtone Spectroscopy of Pyridine and Related Compounds

Abstract The vibrational overtone spectra of gaseous pyridine, 3-fluoropyridine, and 2,6-difluoropyridine were recorded using photoacoustic absorption spectroscopy. The pyridine, 3-fluoropyridine, and 2,6-difluoropyridine overtone spectra exhibit two local mode progressions. The appearance of these two progressions in the difluorinated compound results from a splitting of the C(3,5)-H and C(4)-H stretches which overlapped in pyridine. In 3-fluoropyridine, ab initio CH bond lengths were used to assign the lower energy progression to the C(2,6)-H stretches and the higher energy progression to the C(4,5)-H stretches. Alternatively these CH stretch absorptions may be mixed with other vibrational modes and cannot be assigned to the separate CH oscillators at the (2,6), (3,5) and 4 positions. Comparisons are made to the spectra of benzene and pyrrole which exhibit only one overtone peak at each quantum level.

THE GROUND ELECTRONIC STATE OF 1,2-DICHLOROETHANE . II. EXPERIMENTAL INVESTIGATION OF THE FUNDAMENTAL AND OVERTONE VIBRATIONS

The absorption spectrum of 1,2-dichloroethane has been recorded under various experimental conditions, between 50cm-1 and 10 000cm-1 using Fourier transform spectroscopy, and between 10 800cm-1 and 17 400cm-1 using optoacoustic laser spectroscopy. The assignment of the fundamental bands of the trans and gauche isomers was confirmed using in particular the results of recent ab initio calculations (El Youssoufi et al., 1998, Molec. Phys., 94, 461). The first overtone and combination bands were assigned for the first time using a variety of criteria including the band type and, in some cases, the spacing between clumps of rotational lines. Emphasis is placed on the CH progression of bands. Most of the results concern bands from the more abundant trans isomer, up to the near infrared range. Bands are tentatively assigned to the gauche species in the mid and near infrared regions and to the trans rotamer in the visible range. Vibrational frequencies and anharmonic parameters are obtained.

VIBRATIONAL OVERTONE ENHANCEMENT OF METHYL METHACRYLATE POLYMERIZATION INITIATED BY BENZOYL PEROXIDE DECOMPOSITION

Abstract Vibrational overtone initiated polymerization has been demonstrated using intracavity photolysis of a benzoyl peroxide/methyl methacrylate mixture. Excitation of the 6 ν CH overtone transition of the ground electronic state of benzoyl peroxide creates radicals which subsequently begin the polymerization process. Polymer yield was monitored by comparison of the 2 ν CH overtone absorptions for the methyl, methylenic and olefinic CH stretches at 5946 and 6170 cm −1 , respectively. Plots of polymer yield versus time demonstrate an autoacceleration of the polymerization rate commencing many hours after the photolysis period. The delay before autoacceleration depends on the duration of the photolysis.

Direct correlation method for OH, NH and CH local modes: vibrational overtone spectroscopy of biphenyl, anthracene, isobutanol, 2-chloroethanol and ethylenediamine at the third overtone region

Abstract The vibrational overtone spectra of gaseous biphenyl, anthracene, isobutanol, 2-chloroethanol and ethylenediamine at the Δv=4 vibrational overtone region have been recorded and assigned using the local mode model. For the spectral assignments, linear correlations were determined between the ab initio bond lengths (calculated in this work) and the overtone transition energies (taken from the literature) of aromatic and nonaromatic hydrocarbons, alcohols and amines. The bond lengths were obtained from ab initio geometry optimizations using both the 3-21G * and the 6-31G ** basis sets. The bond length–overtone transition wavenumber correlations for the OH, NH and aromatic, methyl and methylenic CH oscillators are presented for the Δv=4 region for both basis sets. The bond lengths (CH for biphenyl and anthracene, OH for isobutanol and 2-chloroethanol and NH for ethylenediamine) were also determined. These bond lengths were used to predict the transition wavenumbers for Δv=4 transitions which could then be compared to the experimental values. These predictions which accounted for the possibility of various conformations agreed well with the experiment.

Two‐photon photodissociation of gaseous azulene at 325 nm

The two‐photon photodissociation of gaseous azulene at 325 nm is reported. Acetylene is the major photodissociation product. The quantum yields for acetylene produced by irradiation of neat azulene at its normal vapor pressure and as a function of argon bath gas pressure are reported. The quantum yield of acetylene produced from irradiated azulene at its normal vapor pressure (∼10 mTorr) is ∼(5.85±1.46)×10−6. Using a mechanism by which internal conversion precedes the absorption of the second photon, the two‐photon absorption cross section is calculated.

Vibrational overtone activation of methylcyclopropene

Laser vibrational overtone activation has been used to investigate the reaction channel competition in the isomerization of 1‐methylcyclopropene (MCPene). The vibrational overtone activation of three types of CH stretches (methyl, methylenic, and olefinic) in the 5νCH and 6νCH transitions initiated the isomerization and all three products (2‐butyne, 1,3‐butadiene, and 1,2‐butadiene) were detected by gas chromatography. Stern–Volmer plots were constructed for the appearance of each individual product and the derived experimental specific rate coefficients were compared to those of the Rice–Ramsperger–Kassel–Marcus (RRKM) theory. The rate coefficients for the 6νCH transitions were in good agreement with the predicted values but those for the 5νCH transition were as much as a factor of 5 too large. Product ratios of 1,3‐butadiene to 2‐butyne and 1,2‐butadiene to 2‐butyne were independent of pressure. In general, these ratios were lower than the RRKM predicted ratios due to collisional deactivation. No eviden...

Laser vibrational overtone activation of ethyl acrylate/benzoyl peroxide mixture

Abstract 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.

MASTER EQUATION SIMULATIONS OF THE VIBRATIONAL OVERTONE ACTIVATION OF METHYLCYCLOPROPENE

The complete kinetic data for the vibrational overtone activation of methylcyclopropene have been simulated using master equation calculations. The simulation included photoactivation, collisional energy transfer, and reaction into three unimolecular channels. A good fit to the Stern–Volmer plots for all the products at six different photolysis energies was obtained. The fit required an adjustment of the thermal activation barriers input into the RRKM calculation. The best fit barriers were 12 833 cm−1 for 2‐butyne, 14 547 cm−1 for 1,3‐butadiene, and 14 685 cm−1 for 1,2‐butadiene. The collisional deactivation was fit with a single exponential energy transfer distribution function with an average amount of energy transferred down per collision of 1000 cm−1. This average value fit all of the Stern–Volmer plots. The product yield ratios were examined for local mode specific effects, but none were found. Previously obtained thermal data can be fit if log A is changed from 12.72 to 12.30. Stern–Volmer plots we...