Fourier transform spectroscopy of CH3OH: rotation-torsion-vibration structure for the CH3-rocking and OH-bending modes

High-resolution Fourier transform spectra of CH₃OH have been investigated in the infrared region from 930 to 1450 cm⁻¹ in order to map the torsion-rotation energy manifolds associated with the ν₇ in-plane CH₃ rock, the ν₁₁ out-of-plane CH₃ rock, and the ν₆ OH bend. Upper-state term values have been determined from the assigned spectral subbands, and have been fitted to power-series expansions to obtain substate origins and effective B-values for the three modes. The substate origins have been grouped into related families according to systematic trends observed in the torsion-vibration energy map, but there are substantial differences from the traditional torsional patterns. There appears to be significant torsion-mediated spectral mixing, and a variety of “forbidden” torsional combination subbands with |Δυ_t|>1 have been observed, where υ_t denotes the torsional quantum number (equivalent to υ₁₂). For example, coupling of the (υ₆,υ_t)=(1,0) OH bend to nearby torsionally excited (υ₇,υ_t)=(1,1) CH₃-rock and (υ₈,υ_t)=(1,1) CO-stretch states introduces (υ₆,υ_t)=(1,0)←(0,1) subbands into the spectrum and makes the ν₇+ν₁₂−ν₁₂ torsional hot band stronger than the ν₇ fundamental. The results suggest a picture of strong coupling among the OH-bending, CH₃-rocking, and CO-stretching modes that significantly modifies the traditional energy structure and raises interesting and provocative questions about the torsion-vibration identity of a number of the observed states.     

New assignments, line intensities, and HITRAN database for CH3OH at 10 μm

The Fourier transform spectrum of CH₃OH in the 10 μm region has been re-examined at higher pressure and path length than heretofore, as part of a program to provide comprehensive CH₃OH spectral data for astrophysical and atmospheric applications. With the increase in spectral sensitivity, it has been possible to assign new torsionally excited ν₁₂=1 and ν₁₂=2 subbands plus further high-K, ν₁₂=0 subbands of the ν₈ CO-stretching band. Upper-state term values have been determined, and have been fitted to J(J+1) power-series expansions in order to obtain the excited ν₈ substate origins. A variety of weaker subbands from other modes has also been identified in the 10 μm spectrum including ν₁₂=0, ν₁₂=1, and ν₁₂=0←1 torsional subbands of the ν₇ in-plane CH₃ rock, ν₁₂=0←1 and ν₁₂=0←2 torsional combination subbands of the ν₆ OH bend, and ν₁₂=0←2 subbands of the ν₅ symmetric CH₃ bend. Line intensities have been retrieved line-by-line from the spectra. A large set of “unperturbed” ν₈ transitions has been modeled using the same type of multi-parameter effective Hamiltonian employed successfully for the ground state, with inclusion of the intensities of a subset of the stronger ν₈ spectral lines in the fitting in order to obtain appropriate transition dipole terms. Together, a 10 μm methanol database in HITRAN format has been generated.     

A rotation-torsion-vibration treatment with three-dimensional internal coordinate approach and additional FTIR spectral assignments for the CH3-bending fundamentals of methanol

A theoretical model has been developed to account for certain features of both newly observed and previously reported CH₃-bending subbands between 1450 and 1570 cm⁻¹ in the high-resolution Fourier transform infrared spectrum of CH₃OH [Can. J. Phys. 79 (2001) 435]. The features include (i) an apparent inversion of the rotationless E-A torsional splitting with respect to the ground state, i.e., the A state located above the E state, (ii) a pronounced upward slope in the K-reduced torsion-vibration energy pattern for the subband origins, and (iii) unexpected A₁/A₂ inversion of the K=2A and K=3AJ-rotational levels that led to ambiguity in identifying the vibrational mode as or . The model is an effective internal coordinate Hamiltonian constructed in G₆ molecular symmetry with the CH₃-bends coupled to each other and to torsion and including a- and γ-type Coriolis coupling. With this model, 33 out of 36 experimental upper-state K-term values for newly assigned , and ν₁₀ subbands plus previous ν₄ subbands have together been fitted successfully, employing 9 adjustable parameters and 17 fixed parameters to give a standard deviation of 0.14 cm⁻¹. The P_γ Coriolis term appears to be the leading cause of the upward shift in the K-reduced energies. When J-dependence is introduced via a rotational Hamiltonian including b- and c-type Coriolis terms in addition to molecular asymmetry, the observed A₁/A₂ inversion of the K=2A and 3A rotational levels can also be reproduced. Predictions using the fitted K-rotation-torsion-vibration Hamiltonian show an interesting Coriolis-induced crossover and mixing of the ν₅ and ν₁₀ torsion-vibration energy patterns. These predictions played a role in identifying two of the new ν₅ subbands in the crossing region, thereby helping to validate the model.