| Abstract
| - The structural and conformational properties of 1-fluorocyclopropanecarboxylic acid have been explored bymicrowave spectroscopy and a series of ab initio (MP2/6-311++G(d,p) level), density functional theory(B3LYP/aug-cc-pVTZ level), and G3 quantum chemical calculations. Four “stable” conformers, denotedconformers I−IV, were found in the quantum chemical calculations, three of which (conformers I −III) werepredicted to be low-energy forms. Conformer I was in all the quantum chemical calculations predicted tohave the lowest energy, conformer III to have the second lowest energy, and conformer II to have the thirdlowest energy. Conformers II and III were calculated to have relatively large dipole moments, while conformerI was predicted to have a small dipole moment. The microwave spectrum was investigated in the 18−62GHz spectral range. The microwave spectra of conformers II and III were assigned. Conformer I was notassigned presumably because its dipole moment is comparatively small. Conformer II is stabilized by anintramolecular hydrogen bond formed between the fluorine atom and the hydrogen atom of the carboxylicacid group. Conformer III has a synperiplanar orientation for the FCCO and HOCO chains ofatoms. Its dipole moment is: μa = 3.4(10), μb = 10.1(13), and μc = 0.0 (assumed) and μtot = 10.6(14) ×10-30 C m [3.2(4) D]. Several vibrationally excited states of the lowest torsional mode of each of II and IIIwere also assigned. The hydrogen-bonded conformer II was found to be 2.7(2) kJ/mol less stable than III byrelative intensity measurements. Absolute intensity measurements were used to show that the unassignedconformer I is the most abundant form present at a concentration of roughly 65% at room temperature.Conformer I was estimated to be ca. 5.0 kJ/mol more stable than the hydrogen-bonded rotamer (conformerII) and ca. 2.3 kJ/mol more stable than conformer III. The best agreement with the theoretical calculations isfound in the MP2 calculations, which predict conformer I to be 5.1 kJ/mol more stable than III and 1.7kJ/mol more stable than II.
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