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

Abstract

The microwave spectrum of the mono-fluoro-benzoic acids, 2-fluoro-, 3-fluoro-, 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. Additionally, 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 (ΔE) is approximately 560 MHz. This homodimer appears to be the largest carboxylic acid dimer observed with F-T microwave spectroscopy.

Rotational spectra and gas phase structure of the maleimide – Formic acid doubly hydrogen bonded dimer

“…The doubly hydrogen bonded carboxylic acid dimers that exhibited the tunneling motion had C2v(M) symmetry and a symmetric double-well potential for the tunneling motion. Proton tunneling between identical molecules has also been observed for the homodimers of 3-fluoro-benzoic acid 5 and acrylic acid. 6 Calculations have been reported for the problem of tunneling in the asymmetric double-well potentials.…”

Structural Aspects of the Ortho Chloro- and Fluoro- Substituted Benzoic Acids: Implications on Chemical Properties

“…The intramolecularly hydrogen bonded trans-II conformer of 2FBA is also a well-defined minimum with a high energy barrier (40.4 kJ•mol −1 ) of conversion into cis-II (see Figure 4, bottom-left panel), and has an expected population in the gas phase at room temperature of ca. 2% [16], justifying its experimental observation in the microwave experiment [18]. Trans-II was not detected in the deposited cryogenic matrices [16] most probably because its expected population in the matrix is below the sensitivity of the technique (the infrared spectra is complex, showing the vibrational signatures of the two cis conformers and the spectra of trans-II conformer is not much different from those of the cis forms, which complicates its experimental detection by this technique).…”

“…On the other hand, the trans conformer of BA has never been observed experimentally. In the case of 2FBA, the intramolecularly hydrogen bonded conformer of 2FBA (trans-II) was detected in gas phase (together with the two cis conformers) by microwave spectroscopy [18], and it has been produced in argon and N 2 cryomatrices as result of vibrational excitation of the cis conformers (using infrared narrowband in situ irradiation) [16]. The existence of the highest energy trans-I conformer of 2FBA was inferred indirectly in the matrix isolation conformational studies reported in Ref.…”

“…The different intrinsic characteristics of the fluorine versus chlorine substituent atoms have also been shown to be major factors in determining the structures of the conformers of the corresponding isolated molecules of the ortho -halogen substituted benzoic acids [ 16 , 17 , 18 , 19 ]. Likewise, they play a key role in defining other properties of the compounds, as for example relative acidities, cell volumes in crystalline phase, dipole moments, spectroscopic data, and melting points [ 16 , 17 , 18 , 19 , 20 , 21 , 22 , 23 , 24 , 25 , 26 , 27 , 28 , 29 ].…”

Gas Phase Measurements of Mono-Fluoro-Benzoic Acids and the Dimer of 3-Fluoro-Benzoic Acid

“…The measured lines and deviations of lines calculated with the "best fit" parameters are listed in Tables S1, S2, and S3 in the supplementary material. 26 The molecular parameters are given in Table II. The three conformers have been identified as the hydrogen bonded (O-H· · · F) species (2-F conformer 1) and the two hydrogen bonded (O-H· · · O) conformers (2-F conformers 2 and 3) have been predicted to be planar, with the angles α of 0…”