G. D. Nivellini

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.

The empirical harmonic potential function of diborane

Abstract A total of 114 spectroscopic data accumulated from 6 isotopic species of diborane have been used to permit a precise determination of the empirical harmonic potential function to be made for the first time. Of the 33 independent force constants, 30 can be determined with numerical significance. The physical significance of the values is probably best demonstrated by the rather close correspondence throughout with scaled ab initio force constants from two sources, only one rather large disagreement in magnitude being found. The potential function reproduces all known frequency, isotopic frequency shift, Coriolis ζ, and centrifugal distortion Δ data to high accuracy. It is used to calculate parameters which will be of value to further spectroscopic and structural studies, to enable accurate analyses to be made of a number of major Fermi resonance interactions present in the spectra, and to explain the anomalously small Coriolis interaction between the infrared active rocking and wagging fundamentals. The advantages of alternative symmetry coordinate definitions are considered, particularly in relation to making comparisons between diborane and ethylene.

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.

High resolution vibration—rotation analyses for methylene chloride

Abstract The ν 7 C -type CH 2 rocking and ν 8 A -type CH 2 wagging fundamentals of CH 2 Cl 2 have been recorded at a resolution of ∼ 0.04 cm −1 , and the ν 2 + ν 8 A -type combination at a resolution of ∼ 0.005 cm −1 . All three bands are analysed in terms of the quartic asymmetric rotor model, yielding precise vibrational and rotational parameters. Although each band appears to suffer minimal perturbation effects, assignment proved difficult due mainly to the effects of the overlapping spectrum of CH 35 2 Cl 37 Cl, but also to the small magnitude of the B and C rotational constants (∼ 0.1 cm −1 ). In the case of ν 7 , band contour simulations were required in order to confirm or refute possible assignments. The data obtained for ν 8 and ν 2 + ν 8 enable parameters to be predicted for the extremely weak ν 2 CH 2 deformation fundamental, only recently observed in the gas phase spectrum.

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.

High resolution FTIR spectroscopy of chlorofluoromethane near 13 µm: rovibrational analysis and resonances of ν5 and 2ν 6 bands in ClF and ClF

The FTIR spectra of CH2ClF (natural isotopic mixture) and ClF (isotopically enriched sample) were investigated in the ν5 and 2ν6 region between 700 and 800 cm−1 at a resolution of 0.004 cm−1. The ν5 and 2ν6 vibrations of A′ symmetry give rise to a/b hybrid bands with a very predominant a-type component. Due to the proximity of their band origins, the v 5 = 1 and v 6 = 2 levels perturb each other by Fermi and Coriolis resonances. The interaction mechanisms, previously investigated in the rotational spectra of ClF, were extended to the less abundant isotopic species ClF and to higher J and Ka values in the main isotopologue. The spectral analysis resulted in the identification of 4188 and 5392 transitions for ClF and ClF, respectively. All the assigned data were simultaneously fitted using the Watsons A-reduction Hamiltonian in the I r representation and perturbation operators. Excited state parameters, band origins and coupling terms for the ν5/2ν6 dyad of both isotopologues were determined.

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 ?1 and ?3 band system of 15NH3

The infrared spectrum of 15NH3 has been investigated by high-resolution Fourier transform infrared spectroscopy in the region of the stretching fundamentals. A large number of ro-vibration transitions in the 3050–3650 cm−1 spectral range has been recorded and assigned to the fundamentals ν 1 and ν 3, and to the 2ν 4 overtone bands. In total, 1606 transitions involving the (s) and (a) inversion–rotation–vibration levels have been identified and assigned. They include 256 perturbation-allowed transitions with selection rules ΔK = ±2, Δl = −1 in ν 3 and Δl = +2 in , and ΔK = ±3, Δl = 0 in ν 1 and . All assigned transitions were fitted simultaneously to a model Hamiltonian that includes all symmetry-allowed interactions between and within the excited state levels in order to obtain accurate sets of spectroscopic parameters for both inversion states. The standard deviation of the fit, 0.034 cm−1, is about 70 times larger than the estimated measurement precision. This result is similar to that reported for the same band system in 14NH3 by Kleiner et al. [J. Mol. Spectrosc. 193, 46 (1999)] and is a consequence of the neglect of vibration and ro-vibration interactions between the analysed states and vibrationally excited states with close energies.