Furfural (furfuraldehyde, C{sub}4H{sub}3OCHO) belongs to a class of heterocyclic aromatic compounds characterized by five-membered furan ring structure consisting of four CH{sub}2 groups and one oxygen atom. It is commonly used as a solvent. In our recent paper [1] the results of investigation of microwave spectra of furfural were reported. We have assigned and analyzed the states which give the most intensive lines in the spectrum: ground and first torsional states of trans- isomer and ground state of cis- isomer which is about 286 cm{sup}(-1) less stable. Using the rotational spectra of single 13{sup left}C and 18{sup left}O species we have also determined accurate structural parameters for both conformers. However, broadband records of the furfural spectra obtained also contain a lot of other relatively strong lines belonging to the lowest vibrationally excited states. Since the furan ring is rather rigid structure all the lowest vibrational modes in furfural arise from displacements of the atoms of aldehyde group CHO. According to the most recent FIR data [2] the frequencies of the lowest vibration - ring-CHO torsion for trans- and cis-furfural are: 146.25 cm{sup}(-1) and 127.86 cm{sup}(-1) respectively and the frequencies of ring-CHO out-of-plane (antisymmetric) bending vibration are: 235.9 cm{sup}(-1) and 280.2 cm{sup}(-1). In the same work [2] the FIR bands corresponding to ring-CHO in-plane (symmetric) bending vibration were found to be overlapped providing a single slightly distorted band with a centre at 212.0 cm{sup}(-1). Therefore in our analysis we used other available data such as relative intensity measurements for cis-furfural [3] which yields the frequency of symmetric vibrational mode of 195 cm{sup}(-1) and IR measurements of v{sub}a=2 band of trans-furfural at 415 cm{sup}(-1) providing correspondingly the frequency of 207.5 cm{sup}(-1) [4]. In the present contribution we report the results of investigation of microwave spectra of excited vibrational states of furfural in the frequency range between 49 and 149 GHz. The broadband records obtained allowed us to assign and analyze 13 lowest excited vibrational states of furfural (7 - for trans and 6 - for cis isomer), 5 of them - newly assigned ones. The rotational analysis including transitions with the values of quantum number J up 45 and K{sub}a up to 30 has enabled accurate determination of the full sets of quartic centrifugal distortion constants for all states. For some states several sextic and octic constants were also needed in order to fit data within experimental accuracy.
展开▼
