Novel Patterns of Torsion-Inversion-Rotation Energy Levels in the ν11 Asymmetric CH-Stretch Spectrum of Methylamine

The high-resolution infrared spectrum of methylamine (CH3NH2) has been recorded using slit-jet direct absorption spectroscopy in the ν11 CH-stretch region (2965–3005 cm−1) with a resolution of 0.0025 cm−1. The 621 lines assigned by ground state combination differences represent 27 substates with |K′| ≤ 2 for the A, B, E1, and E2 symmetries. The spectrum of CH3NH2 is complicated by torsion and inversion tunneling connecting six equivalent minima. The upper states K′ = 0, ± 1 for E1 and E2 are substantially perturbed by “dark” states. The result in the spectrum is multiplets of 2 or 3 states with mixed bright/dark character. The analysis of the spectrum reveals two qualitative differences in the energy level pattern relative to the vibrational ground state and relative to available data on the lower frequency vibrations (NH2 wag and CN stretch). First at J′ = 0, there is a different ordering of the levels connected by torsion-inversion tunneling. Second, the low-J splittings indicative of torsion-rotation coupling are greatly reduced in the ν11 excited state relative to the vibrational ground state for both the E1 and E2 species, suggesting the partial suppression of torsional tunneling in the ν11 CH-stretch excited state

INFRARED SPECTRA OF METHYLAMINE IN THE ASYMMETRIC C-H STRETCH REGION

Author Institution: Department of Chemistry, The University of AkronHigh-resolution IR spectra of methylamine were obtained using slit jet absorption spectroscopy. A total of 1330 lines were recorded in the asymmetric C-H stretch region ($2965-3005 cm^{-1}$). Hindered internal rotation about the C-N bond and inversion of amino group complicate the analysis of the observed spectra. The subbands are found by fitting the lines to quartic polynomials and these assignments are confirmed by comparison with ground state combination differences based on microwave and far infrared data

SPECTROSCOPY AND DYNAMICS OF HIGHLY VIBRATIONALLY EXCITED METHANOL

Author Institution: Laboratoire de chimie physique mol\'{e}culaire, Ecole Polytechnique F\'{e}d\'{e}rale de Lausanne; Department of Chemistry, University of AkronWe have used single- and double-resonance vibrational overtone excitation, together with Infrared Laser Assisted Photofragment Spectroscopy (IRLAPS) dectection to measure highly resolved spectra of jet-cooled methanol molecules in levels ranging from $V_{OH}=2-8$. The single resonance spectra provide the overall picture of the strongest anharmonic resonances affecting OH stretch vibration, and studies on a number of deuterated isotopomers confirm the assignments of these resonances. These strong couplings define the ultrafast dynamics that the molecule would undergo if certain OH stretch bands were coherently excited. Double-resonance vibrational overtone excitation of jet-cooled methanol produces fully rotationally resolved spectra which reveal finer splittings reflecting the longer time dynamics. These spectra show that the longer time dynamics are a sensitive function of energy. Double resonance studies of the $^{13}C$ substituted molecule confirm that the energy sensitivity arises from the relative sparsity of low order resonances, even at vibrational energies where the total density of states is high

OVERTONE AND COMBINATION BAND SPECTROSCOPY OF JET COOLED METHANOL

[1] H.L. Fang, D.M. Meister, R.L. Swofford, J. Phys. Chem. 88, 405-409 (1984). [2] O.V. Boyarkin, L. Lubich, R.D.F. Settle, D.S. Perry, T.R. Rizzo, J. Chem. Phys. 107, 8409-8422 (1997).Author Institution: Lab. de Chimie Physique Mol\'eculaire, INSTITUT DE CHIMIE PHYSIQUE MOLECULAIRE; Lab. de Chimie Physique Mol\'eculaire, UNIVERSITY OF AKRON; DEPARTMENT OF CHEMISTRY, UNIVERSITY OF AKRONOvertone spectra of jet-cooled methanol have been recorded from 5,000 to $14,000 cm^{-1}$ using Infrared Laser Assisted Photofragment Spectroscopy (IRLAPS) for the detection of the vibrationally excited molecules. In addition to the OH stretch overtones $(n\nu_{1})$, the major components of the spectra are the overtones of the CH stretch (up to $5\nu_{CH}$) as well as combinations of the OH stretch with the CO stretch $(n\nu_{1}+\nu_{8})$, the COH bend $(n\nu_{1}+\nu_{6})$, and both $(n\nu_{1}+\nu_{6}+\nu_{8})$. These data, together with photoacoustic data [1] up to $18,250 cm^{-1}$ and previously reported IRLAPS data [2] up to $22,166 cm^{-1}$ have been fit to an anharmonic Hamiltonian. In this Hamiltonian, the CH stretch vibrations are treated as a pair of local modes, $v_{a}$ and $v_{6}$ for $v_{CH} = 3$ and higher. The other modes are treated as the usual normal vibrations

TORSION-ROTATION ANALYSIS OF TORSIONAL COMBINATION BANDS BUILT ON THE METHANOL OH STRETCH OVERTONE: 2ν12\nu_{1}, 2ν1+ν122\nu_{1}+\nu_{12} AND 2ν1+2ν122\nu_{1}+2\nu_{12}

Author Institution: INSTITUT DE CHIMIE PHYSIQUE MOLECULAIRE, EPFL, CH - 1015, LAUSANNE; Department of Chemistry, UNIVERSITY OF AKRON, AKRON, OHIO 44325The spectrum of the first overtone OH stretch band $(2\nu_{1})$ in jet-cooled methanol has been measured using Infrared Laser Assisted Photofragment Spectroscopy (IRLAPS) and detailed assignments have been made. In addition to the torsional fundamental band $(\delta v_{t}=0)$ it was possible to observe and assign the torsional combination band with ($\delta v_{t}=1$ and 2). These transitions are very week but can be clearly resolved in the spectra. In total, 131 transitions reaching 19 different K levels in the $2\nu_{1}$ state have been fit to a global torsion-rotation Hamiltonian and the leading torsion-rotation parameters have been determined. Assignments for the much weaker and irregular torsional band structure for $3\nu_{1}+\nu_{12}$ and $3\nu_{1}+2\nu_{12}$ are in progress

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