Fourier Transform Microwave Spectroscopic and ab Initio Study of the Rotamers of 2-Fluorobenzaldehyde and 3-Fluorobenzaldehyde.

The rotational spectra of 2-fluorobenzaldehyde (2-FBD) and 3-fluorobenzaldehyde (3-FBD) were recorded using Fourier transform microwave (FTMW) spectroscopy from 4 to 26 GHz. Two planar rotamers were observed for each species that correspond to structures in which the carbonyl bond is directed toward (O-cis) or away from (O-trans) the C1-C2 bond. Observation of transitions due to heavy atom isotopes (13C, 18O) in natural abundance allowed derivation of the ground state effective (r0) structures and mass dependence (rm) structures for the lowest energy rotamer of 2-FBD (O-trans) and both rotamers of 3-FBD, which compare favorably with ab initio estimates of the equilibrium (re) geometries at the MP2/aug-cc-pVTZ level. The resultant parameters are consistent with the introduction of bond length alternation in the benzene ring, which is dependent on the orientation of the aldehyde group. Careful study of the experimental structure and results of natural bond orbital (NBO) analysis do not support the presence of intramolecular hydrogen bonding as the source of its stabilization of O-trans 2-FBD over its cis counterpart. Furthermore, calculations of the interconversion pathways between rotamers suggest that despite being 9.39 kJ/mol higher in energy, the O-cis 2-FBD moiety is metastable in the molecular beam, which has allowed the observation of its microwave spectrum for the first time.