Hans-Rolf Dübal

Tridiagonal Fermi resonance structure in the IR spectrum of the excited CH chromophore in CF3H

The absorption spectrum of trifluoromethane has been recorded between 900 and 14 000 cm−1 with resolutions between 0.004 and 0.5 cm−1 (pressure broadened). 22 bands were assigned as arising from the interacting CH stretching and bending manifolds, which account for most of the absorption in the overtone region. The results can be understood quantitatively with an effective, tridiagonal many‐level Fermi resonance Hamiltonian. The experimental and theoretical results are summarized in Table II. The Hamiltonian is given in Table III and shows a very large stretching–bending interaction constant ‖ksbb‖=106 cm−1, which is even larger than the diagonal anharmonic constant for the stretching vibration ‖x′ss‖=62 cm−1. This leads to extensive vibrational redistribution between stretching and bending motions at high levels of excitation. The time dependent redistribution is calculated with the spectroscopic Hamiltonian. A rotational analysis is presented for some of the bands involved in the Fermi resonance. The ef...

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...

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.

Time-dependent processes in polyatomic molecules during and after intense infrared irradiation

The time-dependent processes occurring in polyatomic molecules during and after intense infrared irradiation, for example in infrared photochemistry, are discussed in terms of underlying structures in the absorption spectra. Origins of homogeneous and inhomogeneous spectral structures are identified in high-resolution spectra of small molecules (CF3H and CF3CCH). Temperature-dependent bandshapes of large polyatomic molecules, (CF3)3CH and (CF3)3C—CCH are evaluated in terms of the most relevant parameters of the vibrational structures, using a new method. The bandshape parameters can be interpreted in terms of intramolecular vibrational processes on the picosecond and subpicosecond time-scales. Characteristic differences in the dynamic coupling behaviour of the acetylenic CH stretching and the saturated CH stretching vibrations are identified in the fundamental and the overtone spectra.

Multiple anharmonic resonances in the vibrational overtone spectra of CHClF2

The vibrational spectra of chlorodifluoromethane (Freon 22) have been measured at intermediate and high resolutions between 800 and 12 000 cm-1 by interferometric (FTIR) techniques. The CH overtone spectra exhibit a pronounced multiple-resonance structure which can be understood in terms of the ‘universal’ anharmonic CH stretching-bending hamiltonian. Quantitative predictions from this hamiltonian agree well (without adjustments) with previous experimental data on the visible spectra, which could not be correctly assigned before.

Spectral bandshape and intensity of the C-H chromophore in the infrared spectra of CF3H and C4F9H

Abstract The vapour phase IR spectra and integrated band intensities of the fundamental and first-overtone transition of the C-H chromophore in CF 3 H and (CF 3 ) 3 CH are reported. The vibrational coarse bandshapes for (CF 3 ) 3 CH are consistent with lorentzians of width Γ = 3 cm −1 (fundamental) and Γ = 12 cm −1 (overtone). The results are discussed in connection with time-dependent molecular processes and IR photochemistry.

Vibrational overtone spectra and vibrational dynamics of CFHCl2 and (CH3)2CFH

The vibrational spectra of fluoro-dichloromethane and 1,1,1,2,3,3,3-heptafluoropropane have been measured at intermediate and high resolutions between 1200 and 12 000 cm-1. The results are interpreted quantitatively in terms of multiple anharmonic resonance interactions for the stretching and bending motions of the CH-chromophore. The relationship to intramolecular vibrational redistribution and relaxation is discussed briefly.

High-resolution IR spectrum of fluoroform: a close resonance

Abstract A very close triple resonance is observed in the Q-branch region of the C-H stretching fundamental transition in CF3H. The vibrational perturbations are analysed in terms of the rotational structure and of the vibrational zero-order spectrum of totally symmetric states. The relationship to time-dependent intramolecular processes is discussed.

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.

Sequence structure in the high-resolution infrared spectrum of trifluoropropyne

Abstract Rotational structure and dense vibrational sequence structure is resolved in the C–H stretching fundamental of trifluoropropyne (CF 3 CCH) at 3329.85 cm −1 and of two overtone bands in the near infrared. The results are discussed in terms of effective coupling between low- and high-frequency vibrations and in their relation to the dynamics of the IR multiphoton excitation of polyatomic molecules.