Marius Lewerenz

Vibrational spectrum and potential energy surface of the CH chromophore in CHD3

The rovibrational spectrum of trideutero‐methane has been measured at resolutions mostly close to the Doppler limit on an interferometric Fourier transform spectrometer from the lowest fundamental vibration to high overtones of the CH stretching vibration (wave numbers from 900 to 12 000 cm−1). The CH chromophore spectrum is fully assigned and interpreted by means of the tridiagonal Fermi resonance Hamiltonian for the coupled CH stretching and bending vibrations. The Hamiltonian predicts and also fits the visible spectrum up to 19 000 cm−1 measured by Scherer et al., Perry et al., and Campargue et al. The effective tridiagonal Hamiltonian is derived ab initio by means of MRD‐CI and full CI calculations of the potential surface of methane, a variational vibrational calculation in a normal coordinate subspace of the coupled CH stretching and bending motions and an approximate similarity transformation to tridiagonal form. Fits of the experimental results by the tridiagonal and the variational Hamiltonian le...

Tridiagonal Fermi resonance structure in the vibrational spectrum of the CH chromophore in CHF3. II. Visible spectra

The near IR and visible vibrational absorption spectra of CHF3 were recorded up to wave numbers of 17 500 cm−1 providing complete frequency coverage, together with paper I, from the low frequency fundamentals to the N=6  CH stretching–bending overtone multiplet. All strong bands in the high overtone spectra could be predicted and assigned by means of the tridiagonal Fermi resonance Hamiltonian, including a few combinations with intense CF3 stretching vibrations already observed for the low overtones. Improved vibrational Fermi resonance constants are presented on the basis of a fit to 35 assigned bands. An analysis of the rotational fine structure of the 2ν4 (E) overtone component and several Fermi resonance component bands result in values for αb and αs, which allow us to determine Be. In the high overtone bands no rotational fine structure is observed. The bands can be understood by introducing additional homogeneous rovibrational structures of phenomenological widths Γ≊1 to 10 cm−1. The results are dis...

Spectroscopy and dynamics of the isolated CH chromophore in CD3H: Experiment and theory

Abstract The vibrational spectrum of CD 3 H between 900 and 1200 cm −1 has been measured at high resolution. The CH stretching and bending vibrations are found to be coupled by a strong, tridiagonal Fermi resonance. The vibrational dynamics can be understood on the basis of a potential surface from ab inito (MRD CI) calculations.

Vibrational spectrum, dipole moment function, and potential energy surface of the CH chromophore in CHX3 molecules

The Fermi‐resonance overtone spectra of the CH chromophore up to about 18 000 cm−1 are evaluated by variational vibrational calculations for the CHX3 molecules trideuteromethane (CHD3), trifluoromethane (CHF3), chloroform (CHCl3) and 1,1,1,3,3,3‐hexafluoro‐2‐trifluoromethylpropane [(CF3)3CH]. Using appropriate model potential functions in a normal coordinate subspace, one can derive parameters for the CH chromophore potential and empirical dipole moment functions. For CHD3 and CHF3 ab initio (SCF‐CI and vibrational variational) calculations are presented, the results of which compare well with the experiments and for CHD3 also with previous (MRD‐CI) ab initio results. For all cases an accurate similarity transformation to the equivalent tridiagonal form of the effective hamiltonian can be made and the corresponding spectroscopic parameters agree with previous results. Comparison is also made with results from an internal coordinate model Hamiltonian.

Vibrational overtone intensities of the isolated CH and CD chromophores in fluoroform and chloroform

Abstract Absolute integrated band strengths are reported for the Fermi resonance overtone polyads ( N = 1 to 4) of the isolated CH chromophore in CHF 3 , CDF 3 , CHCl 3 , CDCl 3 . The CH data are evaluated with the Mecke function for the local dipole moment μ( r )α r exp(-α r ) and the resulting first predictions for CD agree with experiment. The results are discussed in relation to electronic structure calculations, IR multiphoton excitation, and IR photochemistry.

Overtone intensities and dipole moment surfaces for the isolated CH chromophore in CHD3 and CHF3: Experiment and ab initio theory

The band strengths of fundamentals (N=1) and overtones (up to N=6) of the strongly coupled CH stretching and bending vibrations in CHD3 and CHF3 are calculated using high level ab initio (SCF‐CI) dipole moment functions and potential surfaces in one and two (three) dimensions. The calculations are performed in approximate normal coordinate and internal coordinate subspaces, the former giving generally superior results. The overall prediction of relative and absolute intensities ranging over many orders of magnitude is often accurate to within a factor of 2, but not to within experimental accuracy. Different dipole model functions and potential surfaces are investigated and an empirical adjustment of the dipole function to experiment is proposed for CHF3. The comparison of experimental and ab initio overtone intensities for the Fermi resonance system is discussed in some detail, as well as the importance of the results for IR spectroscopy and IR multiphoton excitation.

Group additivity and overtone intensities for the isolated CH chromophore

Abstract Integrated band strengths for CH fundamental and overtone transitions (up to N = 4) are reported for the isolated CH chromophore in six compounds C n F m Cl k H. The experimental (vapour) data are analyzed by means of one-dimensional anharmonic oscillator transition moments for the Mecke dipole moment function μ( r ) ∝ r m c −α r . The recently established triagonal Fermi resonance of the CH chromophore is taken into account in the analysis.

Isotope effects in the fermi resonance of the CH chromophore in CHX3 molecules

Abstract High-resolution FTIR spectra have been recorded between 800 and 12000 cm−1 for 13CHCl3 and 13CHF3. Twenty bands for 13CHF3 and sixteen bands for 13CHCl3 could be assigned to CH chromophore absorption. A first theoretical analysis is given for the 13C isotope effect in the CHX3 Fermi resonance spectra of the CH chromophore and is discussed in relation to the CH chromophore potential and CD isotope effects.

High resolution FTIR spectra of CDF3 in the CD stretching fundamental and overtone regions

FTIR spectra of 12CDF3 are reported at resolutions up to 0.006 cm−1 for the CD stretching fundamental and overtone transitions. For the band associated most closely with the CD stretching fundamental, the rotational J and K structure is largely resolved and analysis gives νeff1=2261.2637 cm−1, A’=B’=0.329 5858 cm−1, C’=0.189 0758 cm−1, D’J=3.006×10−7 cm−1, DJK=−3.62×10−7 cm−1, and D’K=1.94×10−7 cm−1. (For further constants see Tables I and 2). The large value of B‘−B’=0.001 347 cm−1 is interpreted in terms of a strong Fermi resonance between ν1 and 2ν2 (symmetrical CF3 stretching vibration) with an effective two‐state coupling constant ‖k122‖≂80 cm−1. Alternatives to this assignment are discussed and it is shown that the two‐state coupling model is only part of a larger coupling scheme involving several states in addition to 2ν2. The rotational structure of the second Fermi resonance component shows severe perturbations for values of K>33. Analysis of 157 assigned lines gives a band center at 2152.2796 c...

Band strengths of fundamentals and overtones of the CF and CH chromophores in CHD2F

Abstract Large quantities of CHD2F have been synthesized by a newly developed method. A complete survey of the fundamental and high overtone spectrum of CHD2F covering the frequency range from 700 to 14000 cm−1 has been measured in part at close to Doppler-limited resolution. The overtone spectrum is found to be dominated by multiple Fermi resonances between CH-stretching and -bending states. The integrated band strengths of the fundamentals and overtones of the CF and CH chromophores have been measured and are interpreted using group additivity models and the Mecke dipole function.