10 μm High-resolution spectrum of trans-acrolein: Rotational analysis of the ν_(11), ν_(16), ν_(14) and ν_(16) + ν_(18) - ν_(18) bands

摘要

High-resolution Fourier transform spectra of trans-acrolein, H _2CC(H)C(H)O, have been recorded in the 10 μm region at both room and cooled temperatures on the modified Bomem DA3.002 at the National Research Council of Canada and the Bruker IFS 125HR spectrometer at the far infrared beam line of the Canadian Light Source in Saskatoon. Vibrational fundamentals analyzed so far include the ν_(11), ν_(16) and ν_(14) bands centered at 911.3, 958.7 and 992.7 cm~(-1) corresponding respectively to the A′ in-plane CH_2~-rocking mode, the A″ out-of-plane CH_2~-wagging mode, and the A″ wagging mode highly mixed between the ≥CH vinyl and ≥CH formyl groups [Vibrational mode descriptions are based on Y.N. Panchenko, P. Pulay, F. T?r?k, J. Mol. Spectrosc. 34 (1976) 283-289.] As well, the ν_(16) + ν_(18) - ν_(18) hot band centred at 957.6 cm~(-1) has been analyzed, where ν18 is the low-frequency (157.9 cm~(-1)) A″ ≥CC torsional mode. The ν_(11) band is a/b type while the ν_(16), ν_(14) and ν_(16) + ν_(18) - ν_(18) bands are c-type. The assigned transitions of each band have been fitted to a Watson asymmetric rotor Hamiltonian, with ground state parameters fixed to values obtained from rotational analyses in the literature. As well, a combined 3-state fit for ν_(11), ν_(16) and ν_(14) was carried out including Coriolis and Z_1 constants which account for J and ΔK interactions. Transition dipole moments have been calculated for each of the fundamentals using the ab initio B3LYP method and 6-311++G basis set. For the A′ vibrational modes, we have also evaluated transition dipole a- and b-components in the principal axis system from vibrational displacements and dipole moment derivatives. Our ab initio results predict that the ν-(11) in-plane CH_2 rocking mode has an a-type transition strength about three times greater than the b-type, which is consistent with our observations. Our ab initio force field analysis gives vibrational mode descriptions consistent with previously published work for all 13 A′ modes. However, for the five A″ vibrational modes, our ab initio results disagree with two of the descriptions of Panchenko et al. and three of the descriptions of Hamada et al. [Y. Hamada, Y. Nishimura, M. Tsuboi, Chem. Phys. 100 (1985) 365-375].