F. Tullini

The emprical general harmonic force field of ethane

Abstract A total of 175 spectroscopic data, accumulated from 10 isotopic species of ethane, are used to define all 22 parameters of the harmonic potential function within narrow limits. Before calculation, numerous Fermi resonances have been identified and quantified through infrared and Raman spectroscopic studies of CH3CD3 and its 13C isotopic species. This is an essential prerequisite to such an investigation, without which a self-consistent empirical data set cannot be achieved from which to determine physically meaningful force constants. Comparison of the empirical force constants with those predicted by scaled ab initio calculations shows an excellent degree of correspondence in all force constants, and confirms that both approaches can lead to essentially identical results. Calculated values of spectroscopic data of reliable quality are listed. These should be of value to future spectroscopic investigation of isotopic ethanes and for resolving the many resonance perturbations which are present.

Methylene chloride: The mid-infrared spectrum of an almost vibrationally unperturbed molecule

Abstract The infrared gas phase spectra of 12CH2Cl2, 13CH2Cl2, and 12CD2Cl2 have been studied in the region below 6200 cm−1 under conditions of high resolution. Some 30 vibrational levels can be identified for each isotopic species and assigned unequivocally in terms of the band contours displayed. Direct observation has been made of the very weak ν2 fundamentals in all species, and of the “inactive” torsion fundamental of CD2Cl2. Rotational analyses have been performed on the observed Q-branch features of over 30 bands. For each isotopic species, it is found, with one exception, that all vibration levels fit accurately the simple second-order perturbation expression involving ν′s and x′s. The sole exception in each species is the overtone region of the CH2(CD2) stretching vibrations. Here anharmonicity effects bring vibrationally interacting levels into close enough proximity for resonance effects to become just slightly more than of second-order importance. Full analyses including Fermi resonance are made. The effects of the Darling-Dennison resonance between the overtones of the CH stretching fundamentals are observed and corrected for in terms of a simple assumption. Most of the resulting anharmonicity constants bear isotopic relationships similar to those established for H2O and D2O. It is concluded that, with the exception of the CH(CD) stretching overtone region, methylene chloride isotopomers behave as vibrationally unperturbed molecular systems in the mid-infrared region.

The empirical general harmonic force field of methylene chloride

Abstract The general harmonic force field of methylene chloride has been calculated without the necessity of imposing constraints, through the use of a set of 74 observables over eight isotopic species. These include vibration frequencies, 37 Cl and 13 C isotopic frequency shifts, and quartic centrifugal distortion constants. All force constants are well defined by the data with one exception, and this takes a value close to that predicted by transfer from methyl chloride, although with an uncertainty larger than itself. The force field is used to predict precise isotopic distortion constants, in terms of differences from the accurate experimental values for CH 2 35 Cl 2 and CD 2 35 Cl 2 , and to calculate Coriolis interaction constants which will be of assistance in high-resolution rovibration studies. Alternative definitions which may be used for the deformation coordinates of an XY 2 Z 2 molecule are considered. That which associates each deformation uniquely with either the XY 2 or XZ 2 group is clearly preferred.

INTEGRATED BAND STRENGTHS OF BENZENE VAPOUR IN THE 600-1900 CM-1 REGION

Abstract Infrared absorbance cross sections and integrated band strengths for benzene pure vapour have been determined at 273, 298, and 323 K in the region between 600 and 1900 cm−1 by Fourier Transform spectroscopy. This region includes the relevant atmospheric window between 700 and 1200 cm−1. Air broadened spectra of benzene vapour have also been recorded at room temperature (298 K), using mixtures containing 20 kPa and 80 kPa of dry air. Spectra were recorded in the region of the ν4 band (600–800 cm−1) at a resolution of 0.03 and 1.0 cm−1 for the pure vapour and at a resolution of 0.05 cm−1 for the benzene/air mixture, and in the region 900–1900 cm−1 at a resolution of 0.1 cm−1 for both pure benzene and air containing mixtures.

CH stretching anharmonicity in CH2CF2 and CH2CCl2

Abstract The CH stretching anharmonicity in CH 2 CF 2 and CH 2 CCl 2 has been investigated through application of the local mode model to vibration levels up to 14500 cm −1 . Good reproduction of observed data is achieved in each case, but whereas a “normal” anharmonicity constant of ≈−58 cm −1 is obtained for CH 2 CCl 2 , an anomalous value of ≈−52 cm −1 is determined for CH 2 CF 2 . By comparison of the spectra, the cause of the problem is traced to two critical misassignments due to the effects of Fermi resonances at different levels of excitation in CH 2 CF 2 . A “normal” anharmonicity constant is then obtained. The Fermi resonances cannot be quantified satisfactorily, due to lack of sufficient observations and their complexity. The local mode parameters determined for each molecule are supported by available structural information.

Vibrational spectrum of 1,1,1-trifluoroethane

IR spectra of 1,1,1-trifluoroethane (HFC-143a) have been recorded at medium and high resolution. Raman spectra in polarization controlled experiments have also been measured. Comparison between the spectra recorded with the two techniques has made possible a reassignment of the vibrational spectrum of CH3CF3 in the range 200–3100 cm-1. Room-temperature and low-temperature gas-phase spectra were compared in order to assign a large number of hot bands due to the low lying fundamentals ν6 (A2, ca. 220 cm-1) and ν12 (E, 366cm-1). An unambiguous assignment for ν8 has been obtained, together with a reliable frequency for ν6, which is forbidden in both the IR and Raman.

The high-resolution infrared spectrum of ethane-1,1,1-D3 rovibration studies of CH3CD3 and 13CH3CD3

Abstract The infrared spectra of a number of fundamentals of CH 3 CD 3 and 13 CH 3 CD 3 were recorded at ∼0.05 cm −1 resolution using a Nicolet FTIR spectrometer. Complete analyses at this resolution were performed for the ν 2 (CD 3 sym. stretch), ν 7 (CH 3 asym. stretch), ν 9 (CH 3 asym. deformation), ν 11 (CD 3 asym. deformation), and ν 12 (CD 3 rock) fundamentals, and sets of upper-state parameters are derived. Perturbations in the ν 9 band are accounted for in terms of an A 1 - E Coriolis interaction with ν 3 (CH 3 sym. deformation), and in the ν 11 band in terms of the combined effects of an A 1 - E Coriolis interaction with ν 4 (CD 3 sym. deformation) and an E (± l ) - E (∓ l ) interaction with ν 10 (CH 3 rock). A small, localized perturbation in ν 7 is identified as due to a higher-order rotational resonance with 2 ν 9 2 . All first- and second-order Coriolis interaction parameters are determined. Limited spectroscopic information is obtained on ν 3 , ν 4 , and ν 10 , all of which are extremely weak in the infrared. The perpendicular band analyses enable the centrifugal distortion D K 0 constant to be estimated.

The high-resolution infrared spectrum of ethane-1,1,1-D3 rovibration analyses of the pseudoperpendicular A1A2ν9 + ν10 band and the parallel ν3 + ν4 and ν5 bands

Abstract A complete vibration-rotation analysis was made of the A 1 A 2 combination band ν 9 + ν 10 of CH 3 CD 3 at 2582 cm −1 . This band exhibits pseudoperpendicular structure due to the large effective Coriolis interaction constant (ζ ≈ 0.7), which couples the almost degenerate A 1 and A 2 vibrational components for all nonzero values of the rotational quantum number K , and gives a subband Q -branch spacing of 2.5 cm −1 . The location of the band center is assisted through an interruption of the perpendicular-like structure, since both K = 0 Q branches are forbidden by the vibrational and rotational selection rules. The conventional A 1 parallel bands ν 3 + ν 4 at 2507 cm −1 and ν 5 (CC stretch) at 905 cm −1 were also analyzed. For ν 5 , a combination of numerical analysis and band contour simulation was used to determine a set of upper-state rotation parameters. Combination of the present results with previous data for ν 9 and 2 ν 3 permits rotational parameters to be derived for the ν 4 and ν 10 fundamentals of CH 3 CD 3 . Neither of these fundamentals are amenable to straightforward analysis, both being very weak in the infrared and overlaid by the intense ν 11 fundamental.