The effects of symmetry within the local mode picture: A reanalysis of the overtone spectra of the dihalomethanes

Abstract

The overtone spectra of dichloromethane, dibromomethane, and di-iodomethane are measured in the spectral region from 1770 to 850 mμ. The spectra in this region, and the CH stretching fundamentals, are analyzed in terms of a local mode picture. The spectra display the splittings predicted between the symmetrized combinations of local mode states arising from coupling of the two CH oscillators. We calculate the coupling matrix elements directly using a ’’ladder’’ approximation based on the properties of harmonic oscillator wave functions. The validity of this approximation is demonstrated by a comparison to matrix elements of Morse oscillators. Through a direct calculation of the kinetic energy coupling in dichloromethane from the observed bond angle, we show that kinetic energy provides the principal source of coupling in all of these molecules. Differences in overtone spectral structure between the three molecules are attributed to a Fermi resonance involving two quanta of HCH bending. Adiabatic changes in this bending frequency with increased local CH stretching excitation are discussed.

Internal Methyl Rotation in the CH Stretching Overtone Spectra of Toluene-α-d2, -α-d1, and -d0

“…calc a (-d0) 1 2.5 × 10 -5 3.1 × 10 -5 3.7 × 10 -5 2 5.2 × 10 -7 4.1 × 10 -7 4.7 × 10 -7 4.7 × 10 -7 5.6 × 10 -7 5.3 × 10 -7 3 5.8 × 10 -8 5.4 × 10 -8 6.2 × 10 -8 6.2 × 10 -8 6.7 × 10 -8 7.1 × 10 -8 4 3.1 × 10 -9 4.5 × 10 -9 3.6 × 10 -9 5.2 × 10 -9 4.0 × 10 -9 5.8 × 10 -9 5 7.1 × 10 -10 8.1 × 10 -10 9.0 × 10 -10 a Calculated with the local mode parameters of Tables 3 (toluened0) and 4 (best fit ω and ω x) and the HF/6-311+G(d,p) dipole moment function. cedure.…”

Rotation–vibration energy level clustering in the ground electronic state of PH2

“…As explained, for example, in Ref. [11], in order to approximately account for interactions between the two bond stretches in the local mode Hamiltonian, we use the harmonically coupled anharmonic oscillator (HCAO) model [27,28] in which diagonal matrix elements are given by Eq. (15) and the only non-vanishing off-diagonal matrix elements are taken to be (in cm À1 )…”

State-resolved spectroscopy of high vibrational levels of water up to the dissociative continuum

“…For the highly excited levels with predominantly stretching excitation, the local-mode approach [28,29] gives a more convenient description [8,30,31]. In our experiments, electric dipole transitions excite molecules in such states, because the transition dipole moment for bend overtones is much smaller than for stretch overtones.…”