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Attention is focused on the quantitative relationships between the parameters of the normal mode and the local mode effective Hamiltonian models of anharmonic stretching vibrations. R. G. Della Valle (1988, Molec. Phys., 63, 611) has demonstrated the general relationship between the harmonically coupled anharmonic (Morse) oscillators (HCAO) model and the normal mode model of X-H (or X-D or other weakly coupled) stretching vibrations of polyatomic molecules. Here we consider more fully the relationship between the normal mode and the anharmonically coupled local mode models for X-H stretching vibrations. The local mode Hamiltonian is expressed in terms of boson shift operators and these operators are then transformed into the corresponding normal mode model operators. Such relationships have already been derived for the special cases of H2X (Baggott, J. E., 1988, Molec. Phys., 65, 739), XH3 (C3v) and XH4 (Td) (Law, M. M., and Duncan, J. L., 1994, Molec. Phys., 83, 757) and here they are derived systematica...

Attention is focused on the relationship between the normal mode and the local mode descriptions of the stretching vibrations of polyatomic molecules. The harmonically coupled local mode model requires that the equivalent normal mode model parameter set obeys certain fixed interrelationships, the so called ‘x,K relations’. In this paper, a boson shift operator formalism is used, following J. E. Baggott (1988, Molec. Phys., 65, 739), to transform the unconstrained normal mode models for XH4 (Td) and XH3 (C3v) molecules to equivalent local mode effective Hamiltonians which include anharmonic as well as harmonic couplings between the bond modes. Relationships between the unconstrained normal mode and anharmonically coupled local mode models are established which allow for more reliable schemes for the relaxation of the x,K relations than those suggested hitherto for systems containing three or four equivalent stretching vibrations. The magnitudes of the anharmonic coupling terms in the local mode model are e...

The ν5 + ν9 A-type infrared combination band of C2D4 at 4627 cm-1 has the superficial appearance of an unperturbed absorption. Assignment, however, reveals the effects of Coriolis interaction with an inactive vibration level, ν2 + ν8 + ν9, which lies within 0·4 cm-1 of ν5 + ν9. Through the K dependence of the interaction, the K a = 0 subband J structure of ν5 + ν9 is completely unperturbed, while higher K a subband J structures assigned are displaced varyingly up to 3·5 cm-1. Due to almost exact degeneracy of interacting rovibration levels, for the K a = 3–7 subbands, two series of J structures are identified in the spectrum, and two series of subbands are identified in the overall Q branch for K a = 1–8, the ‘inactive’ transitions stealing intensity from those of ν5 + ν9 through the Coriolis interaction. Analysis of the assignments made yield unperturbed upper state parameters and the magnitude of the Coriolis interaction parameter.

Four fundamental bands in the infrared spectrum of 1,1-ethylene-d2 have been studied at a resolution of ca. 0·05 cm-1: the two type-B bands v 6 and v 10 and the two type-C bands v 7 and v 8. Analysis of a set of 409 ground state combination differences enables a complete set of ground state rotational and quartic distortion constants to be determined. The effect of the a-axis Coriolis interactions between the four bands is analysed, and upper state vibrational and rotational parameters for all four bands are obtained. Two localized higher order Coriolis perturbations are identified and characterized, and the band centre of the inactive twisting vibration v 4 is estimated. From the overlapping and weak type-A band v 3, a few upper state parameters are determined.

The availability of twelve precisely determined ground state spectroscopic rotational constants for isotopic diboranes and a physically realistic harmonic potential function has enabled the ground state r 0, substitution r s, zero-point average r z and equilibrium r e structure parameters to be calculated. Vibrational effects render the ground state and substitution structures of doubtful physical significance. Their removal leads to zero-point average and equilibrium structure parameters which are entirely compatible with, but more accurately determined than, those available from electron diffraction observations. The equilibrium structure is fully substantiated by parameters calculated from a recent extension to the Watson mass-dependence method.

The effects of first order Coriolis interactions and vibrational l type resonances in the infrared spectrum of the band system 2ν9 ±2, ν9 ±1+ν10 ±1, 2ν10 ±2 of gaseous allene-d4 have been studied at a resolution near 0·05 cm-1. Sets of spectroscopic constants for each of the vibrations are determined, as well as the Coriolis interaction parameter which couples all three levels. The relative magnitudes and signs of the intensity parameters of the band system have been derived from contour simulations. A number of local perturbations are discussed qualitatively.

An ab initio gradient calculation of the general harmonic force field (GHFF) of methylene chloride at the HF-SCF/6-31 G** level has been undertaken. The results compare favourably with those determined empirically, thus demonstrating a good predictive capability at this level of sophistication for a molecule possessing two third row halogen atoms. The ab initio results reveal small inadequacies in the empirical force constants relating to the CH2 moiety, which may be corrected by a careful re-appraisal of the accumulated spectroscopic data. The resulting empirical and scaled quantum mechanical (SQM) sets of force constants are then in very good accord. Predicted ab initio cubic force constants for methylene chloride are also calculated.

Five bands in the infrared spectrum of ethylene-D4 have been studied with a resolution of 0·02 cm-1, the A-type ν11 fundamental and and ν1 + ν11 combination, and the B-type ν9 fundamental and ν5 + ν11, ν2 + ν9 combination bands. From a set of 635 combination differences the ground state rotation and distortion constants are determined. Assignments for transitions with J ⩽ 30, Ka ⩽ 20 for B-type bands and Ka ⩽ 14 for A-type bands enable upper state vibration frequencies and rotational constants to be determined for all bands studied. The effects of Coriolis interaction through ζ1,5 c are observed in the ν1 + ν11 and ν5 + ν11 bands, and a value of the Coriolis constant is determined. A number of other higher order resonances in upper vibrational states are identified and characterized.

The infrared active v 7 CD wagging fundamental band of ethylene-D4, has been studied with a resolution of ∼0·05 cm-1. A full upper state rovibration analysis is achieved, once the various effects of the indirect Coriolis interaction with the inactive v 4 torsion vibration are taken into account. Several localized perturbations are observed in the spectrum, all of which are identified. A number of these are shown to derive from energy level crossings between v 4 and v 7. A simultaneous fit to these allows the frequency of the inactive v 4 vibration to be estimated accurately.

General harmonic and hybrid orbital force fields have been calculated for methyl chloride, bromide and iodide using the best available frequency, Coriolis coefficient, and centrifugal distortion data, including the recent 13C frequencies for the chloride and iodide. Anharmonicity corrections were made to the fundamental frequencies using Dennison's rule and x values of 0·04 for CH stretching, 0·02 for CH bending, and 0·01 for CX stretching modes. These were the most reasonable values which did not seriously over-compensate for the product rule deficiencies in the 12C and 13C frequencies. The 13C and CD3X frequency information was input in the form of frequency shifts from the 12CH3X frequencies. All 12 parameters in the most general harmonic force fields are determined with significance for the first time, although the previously uncertain parameters F12 and F13 are only determined with good precision when the 13C frequency data are utilized in addition to the other information. The signs of F12, F45 and ...

A reinvestigation of the Fermi and Coriolis interacting band system ν 3, ν 6, ν 7 + ν 8 of methyl cyanide has been performed with a resolution better than 0·3 cm-1. The observed J structure in some of the K sub-bands enables an unambiguous numbering of the sub-band Q branches to be made. The resulting assignment, which differs from Amat and Nielsen's previous assignment, is used to re-analyse the band system. A band contour has been calculated by computer simulation, including the effects of both the Fermi resonance of ν 6 with ν 7 + ν 8 and the Coriolis resonance of ν 6 with ν 3; both the frequency and intensity perturbations which result have been accurately reproduced. The molecular parameters obtained from our analysis are summarized in table 2.

Analyses of the v 3, 2v 3, and (predominantly) v 1 parallel bands, and of the v 4 and v 6 perpendicular fundamentals have been made for 13CH3F in terms of the rotational structure observed with a resolution of ∼0·2 cm-1. In addition, the band centres of the strongly Coriolis-interacting v 2 and v 5 fundamentals are accurately located. Some elucidation of the complex Fermi resonance interactions in the 3000 cm-1 region is achieved through study of spectra of crystalline samples. This enables all three components of the v 1, 2v 2, 2v 5 0 triad to be observed for both 12CH3F and 13CH3F, and estimates to be made for the unperturbed vibration frequencies. The 13C frequency shifts determined for all six fundamentals are used in conjunction with existing frequency, Coriolis ζ, and centrifugal distortion data for CH3F, CD3F, CHD2F and CH2DF, to determine the general harmonic force field for methyl fluoride. The extra shift data enable all 12 parameters of the force field to be fixed within narrow limits for the f...