Investigation of the rotamers of 3-furfural by microwave spectroscopy
van Wijngaarden, Jennifer
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The rotational spectrum of 3-furfural (3-FF) was investigated using Fourier transform microwave spectroscopy from 6 to 19 GHz in order to examine the rotational isomerism properties of the formyl group relative to the furan ring backbone. Two planar rotamers with the formyl group anti or syn to the ring oxygen atom were observed with the latter being identified in the gas phase for the first time. The observed relative intensity of ~8:1 (anti:syn) of transitions is consistent with quantum chemical calculations at the B3LYP-D3(BJ)/aug-cc-pVTZ level which predict that the anti rotamer is more stable than the syn by 4.86 kJ/mol. Transitions due to singly-substituted 13C isotopologues for both rotamers and the 18O isotopologues for the anti form were assigned. The resulting sets of rotational constants allowed derivation of accurate ground state effective (r0) and mass dependent (rm(1)) geometries for the furan ring of each rotamer for comparison with their equilibrium (re) geometries. The syn/anti arrangement of the formyl group appears to have little effect on the geometry of the furan ring itself but the exocyclic group is rotated slightly closer to the oxygen atom of the heterocycle when in the anti arrangement.