The vibrationally excited C2HDC_{2}HD molecule

Author Institution: Laboratoire de Chimie Physique Mol\'{e}culaire, Universit\'{e} libre de BruxellesWe have recorded spectra of $C_{2}HD$, $X^{1}\Sigma^{+}_{g}$, at high resolution using a Bruker Fourier transform interferometer IFS12OHR. The emphasis was set on energy ranges as excited as possible, up to the near infrared and possibly visible ranges, using a multiple pass absorption cell allowing up to 49m total path. Various excited levels were observed, rotationally characterised, vibrationally assigned, absolute transition strength from the ground level were estimated and perturbations were considered."

VIBRATIONAL DEPENDENCE OF THE TORSIONAL BARRIER HEIGHT AND THE A/B INTENSITY EVOLUTION IN THE OH OVERTONE SPECTRA OF METHANOL.

$^{a}$O. V. Boyarkin, T. R. Rizzo and David S. Perry, J. Chem. Phys. 110, 11359 (1999). $^{b}$M. Quack and M. Willeke, J. Chem. Phys. 110, 11958 (1999).Author Institution: Department of Physical Sciences, University of New Brunswick; Department of Chemistry, University of AkronRizzo and co-$workers^{a}$ have used supersonic-jet infrared-laser-assisted photofragment spectroscopy (IRLAPS) to record the O---H stretching overtone spectra of $CH_{3}OH$. Their analysis of the rotation-torsion structures revealed the following interesting features: (i) the torsional A-E splitting decreases monotonically as $\nu_{OH}$ increases, indicating increase of the torsional barrier height $V_{3}$, (ii) a-type transitions become dominant at higher excitations of the OH stretching vibration, (iii) a 1:1 anharmonic resonance occurs between the OH stretch and CH stretch vibrations, reaching its maximum in the $5\nu_{1}$ region. The third observation has been recently studied by Quack and $Willeke^{b}$ for the case of $CD_{2}HOH$, by means of ab initio five-dimensional potential energy and dipole moment surfaces. The present contribution explores possible ab initio explanations for the first two observations. At the MP2 level with 6-311G+(3df,2p) basis set, effective one-dimensional functions for the potential energy, dipole moment ($a$ and $b$ directions), barrier height and torsional constant $F$ have been obtained by scanning the O---H bond length in order to take into account the mechanical and electrical anharmonicities. All ab initio quantities have been expressed as Taylor expansions in the dimensionless coordinate, q. Calculations have been carried out in the harmonic basis set to yield vibrational energies and eigenfunctions. The latter have been used to compute the patterns of the barrier height $V_{3}$, the torsional constant F, and the evolution of the infrared intensity ratio $I_{a}/I_{b}$, as functions of the OH vibrational quantum number. All our ab initio results agree with the experimental observations in points (i) and (ii) above. Details of the calculations, the corresponding results and the comparison to experimental data will be presented

A ROTATION-TORSION-VIBRATION 3-D INTERNAL COORDINATE TREATMENT FOR THE CH3CH_{3}-BENDING FUNDAMENTALS OF METHANOL

Author Institution: Department of Physical Sciences, University of New BrunswickA theoretical model has been developed to account for certain features of $CH_{3}$-bending subbands of methanol observed between 1450 and $1570 cm^{-1}$. The observed features in $v_{4}$ include (i) an apparent inversion of the rotationless E-A torsional splitting with respect to the ground state, (ii) a pronounced upward slope in the K-reduced torsion-vibration energy pattern for the subband origins, and (iii) $A_{1}-A_{2}$ inversion of the K = 2A and 3A J-rotational levels leading initially to ambiguity in identifying the vibrational mode as $\nu_{4} (A_{1})$ or $\nu_{10} (A_{2})$. The model is an effective internal coordinate Hamiltonian constructed in $G_{6}$ molecular symmetry with the $CH_{3}$-bends coupled to each other and to torsion and including a- and $\gamma$-type Coriolis coupling terms. Experimental upper state energies for $\nu_{4},\nu_{10}$, and $\nu_{5}$ have together been fitted successfully employing 12 adjustable parameters to give a standard deviation of $\pm0.13 cm^{-1}$. J-dependence is introduced via a rotational Hamiltonian including molecular asymmetry, plus b- and c-type Coriolis terms which account for the observed $A_{1}-A_{2}$ inversion of the rotational levels at low K. The computer program for our model was set up in a CH-stretch/$CH_{3}$-bend local mode polyad scheme, with polyad number $p=2(v1+v2+v3)+(v4+v5+v6)$, for ready extension to the $3 \mu$m CH-stretching region in future

THE 3μm3 \mu m VIBRATION-TORSION-ROTATION ENERGY MANIFOLD OF METHANOL

Author Institution: Department of Physical Sciences, University of New Brunswick; Department of Chemistry, University of AkronThe $3\mu m$ spectrum of methanol is an important gateway to the understanding of molecular dynamics and to the modeling of cometary spectra. The region is extremely complicated due to a dense vibrational structure and network of interactions among the three CH-stretch fundamentals, $\nu_{2}, \nu_{g}, \nu_{3}$, six overtones and combinations of the three $CH_{3}$-bending modes, $\nu_{4}, \nu_{10}, \nu_{5}$, and a variety of overtone combinations of the torsion $\nu_{12}$, with the remaining lower-lying vibrations. We have obtained FT spectra for the $3 \mu m$ region under various conditions. The structure is dense with few easily recognized features above the $v_{3}$ symmetric CH-stretch. However, in an extension of the color-center-laser slit-jet beam spectrum from 2945 to $2975 cm^{-1}$, low K states could be identified, then allowing further assignment and confirmations of the medium K states from FTS. Altogether, about 25 vibration-torsion-K-rotational states have now been firmly assigned up to K = 4. Plots of K-reduced energies place these states into three distinguishable groups assigned as $\nu_{9}, 2\nu_{4}$, and $\nu_{4}+\nu_{10}$, although there are a number of extra subbands in the spectrum arising possibly from interactions with other states. Spectroscopic findings at the present time are: (i) the torsional A/E ordering is inverted for $\nu_{9}$, normal for $2\nu_{4}$, and apparently normal for the presently observed K = 2 states of $\nu_{4}+\nu_{10}$; (ii) the K = 0 torsional A/E splittings are -5.48 and $8.28 cm^{-1}$ for $\nu_{9}$ and $2\nu_{4}$, respectively, and an estimated much lower than ground state value for the $\nu_{4}+\nu_{10}$ combination; (iii) the $\nu_{9}$ and $2\nu_{4}$ states have virtually identical upper state term values around $3092 cm^{-1}$, but show almost equal and opposite linear shifts with K with slopes of $2-3 cm^{-1}$/K-value; (iv) the $\nu_{4}+\nu_{10}$ combination is about $20 cm^{-1}$ lower in energy than $\nu_{9}$ and $2\nu_{4}$, $10 cm^{-1}$ lower than the previous estimates for the band center