Fourier transform microwave spectrum of the CO2-propylene oxide complex

The rotational spectrum of the carbon dioxide-propylene oxide (CO2-PO) complex was measured in the frequency region from 5 up to 24 GHz by Fourier transform microwave spectroscopy. For the normal species 82 a-type, 53 b-type, and 43 c-type transitions were observed, while a-type and b-type transitions were assigned for the four isotopic species with one C-13 in each of the PO or CO2 moiety in natural abundance. Enriched (COO)-O-18 and (CO2)-O-18 samples were employed to record a-type, b-type, and c-type transitions for the complexes with the respective isotopic CO2 species bound to PO, including the two different (COO)-O-18-PO complexes: the inner (OCO)-O-18-PO and the outer (OCO)-O-18-PO. All the observed transition frequencies were analyzed for the normal and the three O-18-carbon dioxide complexes using a one CH3 group internal rotation and overall rotation Hamiltonian. The potential barrier height V-3 to internal rotation of the CH3 in the PO was determined to be 859.8 (62) cm(-1). The rotational constants derived for the CO2-PO complex led to the structure in which the CO2 moiety was located in one side of the PO ether plane opposite to that of the CH3 group, namely an anti-conformer. The intermolecular bonding of the CO2-PO was found weaker than those of the CO2-EO and the CO2-DME; by assuming a Lennard-Jones-type potential, the force constant of the van der Waals stretching mode and the dissociation energy were estimated to be 5.0 N m(-1) and 5.7 kJ mol(-1), respectively, which were to be compared with those of the CO2-EO and the CO2-DME: 8.0 N m(-1) and 7.1 kJ mol(-1) and 10.9 N m(-1) and 9.7 kJ mol(-1), respectively. It is interesting to note that the intensities of the inner (OCO)-O-18-PO spectra are stronger than those of the outer (OCO)-O-18-PO ones. This observation was explained by the zero-point energy of the inner conformer being a little smaller than that of the outer. (C) 2011 Elsevier Inc. All rights reserved.