Microwave Spectrum, Conformational Composition, and Dipole Moment of (Fluoromethyl)cyclopropane (C₃H₅CH₂F)

The microwave spectrum of (fluoromethyl)­cyclopropane, C₃H₅CH₂F, has been investigated in the whole 12–75.6 GHz spectral range. Many measurements were also performed in the 75.6–120 GHz region. The spectra of two conformers were assigned. The H–C–C–F chain of atoms is antiperiplanar in the conformer denoted <b>ap</b> and synclinal in the <b>sc</b> rotamer. The <b>sc</b> conformer has a lower energy than <b>ap</b>. The internal energy difference was determined to be <i>E</i>_ap – <i>E</i>_sc = 1.7(8) kJ/mol from relative intensity measurements. The spectra of the ground vibrational state and seven vibrationally excited states belonging to two different normal modes were assigned for <b>sc</b>. The frequencies of these two modes were determined by relative intensity measurements. The dipole moment of this conformer was determined to be μ_a = 5.520(17), μ_b = 3.475(29), μ_c = 0.35(13), and μ_TOT = 6.532(40) × 10^–30 C m [1.958(12) D]. The spectrum of the ground vibrational state was assigned for <b>ap</b>. The microwave work was supported by quantum chemical calculations at the CCSD/cc-pVQZ, MP2/cc-pVTZ, and B3LYP/cc-pVTZ levels of theory