Gas phase measurements of mono-fluoro-benzoic acids and the dimer of 3-fluoro-benzoic acid

Abstract

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The gas phase homodimer of 3-fluorobenzoic acid was detected and the spectra showed evidence of proton tunneling. Experimental rotational constants are A(0$^+$)= 1151.8(5), B(0$^+$)=100.3(5), C(0$^+$)= 87.64(3) MHz and A(0$^-$)=1152.2(5), B(0$^-$)= 100.7(5), C(0$^-$)=88.85(3) MHz for the two ground vibrational states split by the proton tunneling motion. The tunneling splitting ($\Delta$E) is approximately 560 MHz. This homodimer appears to be the largest carboxylic acid dimer observed with F-T microwave spectroscopy. Additionally, the microwave spectra of the mono-fluoro-benozic acids, (2-fluoro, 3-floro and 4-fluoro) benzoic acid have been measured in the frequency range of 4-14 GHz using a pulsed beam Fourier Transform microwave spectrometer. Measured rotational transition lines were assigned and fit using a rigid rotor Hamiltonian. Assignments were made for 3 conformers of 2-fluorobenzoic acid, 2 conformers of 3-fluorobenzoic acid and 1 conformer of 4-fluorobenzoic acid.