Effects of the matrix and intramolecular interactions on the stability of the higher-energy conformers of 2-fluorobenzoic acid

Abstract

DFT(B3LYP)/6-311++G(d,p) calculations on 2-fluorobenzoic acid (2FBA) show that the molecule has four conformers: two low energy conformers (forms I and II) with the carboxylic acid group assuming the cis configuration (O=C-O-H dihedral equal to 0°) and two higher-energy conformers (III, IV) with a trans carboxylic group configuration. Isolation of 2FBA monomers in argon or nitrogen matrices allows for the efficient trapping of both low-energy conformers. Narrowband selective near-IR(NIR) excitation of the 2νOH mode of I in both argon and N matrices leads to its efficient conversion into conformer III, which is stabilized by an intramolecular O-H···F interaction. On the other hand, upon identical selective vibrational excitation of II no changes could be noticed in the argon matrix spectra, while experiments carried out on N matrices showed conversion of II into III. In conformer IV (the expected direct product resulting from NIR excitation of II), the stabilizing O-H···F interaction existing in III is replaced by an O-H···H repulsive interaction, which leads to a barrier separating this form from II that is about one third of that separating III from I. Under these circumstances, once formed by vibrational excitation of II, conformer IV can easily convert to the reactant species by fast tunneling, justifying the apparent inefficiency of the II→IV conversion upon vibrational excitation of II in an argon matrix. On the other hand, the stabilization of the initially formed conformer IV by the N matrix (due to OH···N interactions) allows this species to survive long enough to allow the IV→III over-the-barrier conversion to be competitive with the IV→IItunneling, justifying the observed net conversion of the NIR excited conformer II into form III in N matrix. These results demonstrate as the intramolecular local topology may decisively influence the intrinsic kinetic stability of different conformers of the same molecule. They are also a clear additional evidence of the stabilization of higher-energy trans conformers of carboxylic acids by the N matrix medium that has been found before for other molecules of the same family.

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).…”

“…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). However, once trans-II is produced in situ by vibrational excitation (upon infrared irradiation) of the matrix-isolated conformer cis-II, the high energy barrier for the trans-II→cis-II conversion precludes this transformation to take place (the over the barrier thermal process is not possible at all, and the experiments showed that quantum mechanical tunneling is also inneficient in this case).…”

“…The existence of the highest energy trans-I conformer of 2FBA was inferred indirectly in the matrix isolation conformational studies reported in Ref. [16], but it could not be directly observed due to its fast spontaneous conversion to the lowest-energy cis conformers. For 2CBA, in situ UV irradiation of the matrix-isolated cis conformers was shown to lead to their conversion into their trans counterparts, which were then found to decay by quantum mechanical tunneling back to the cis forms [17].…”

“…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 experimental observation of these effects was achieved using matrix isolation infrared spectroscopy in cryogenic Ar, N 2 or Xe matrices. In situ irradiation either by UV [17] (for 2-chlorobenzoic acid) or IR [16] (for 2-fluorobenzoic acid) allowed conversion of the abundant low-energy cis conformers into the higher-energy trans forms, and the kinetics of the subsequent trans→cis decay by quantum mechanical tunneling were then estimated and found to correlate with the size of the trans→cis isomerization barriers [16,17].…”