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

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 N2 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 N2 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 N2 matrix (due to OH···N2 interactions) allows this species to survive long enough to allow the IV → III over-the-barrier conversion to be competitive with the IV → II tunneling, justifying the observed net conversion of the NIR excited conformer II into form III in N2 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 N2 matrix medium that has been found before for other molecules of the same family.