Computational studies, NMR and IR spectral analysis, normal coordinate analysis, and thermodynamic properties of 2-fluoro-4-pyridineboronic acid

Abstract

The conformational stability, vibrational frequencies, and thermodynamic parameters of 2-fluoro-4-pyridineboronic acid (FPB, C5H5BFNO2) are theoretically predicted using DFT-B3LYP with 6-31G(d), 6-31+G(d), and 6-311++G(d,p) basis sets employing the Gaussian 09 software. According to the internal rotation of two hydroxyl groups around the C—B bond, four conformers are suggested: TC, CT, TT, and CC (C/T refer to the cis/trans positions of O10—H15 and O9—H14 groups towards the C—B bond). The results of calculations favor the TC conformer with a 70 % relative population compared with low values of 28 %, 1.5 %, and 0.06 % for CT, TT, and CC, respectively. An additional calculation in the solid state (CASTEP) are performed for a molecule per unit cell (C1 symmetry) for all conformers using the DFT-PBE method. The TC conformer is also favored by 2.5 kJ/mol, 2.9 kJ/mol and 16.3 kJ/mol for conformers CT, TT and CC, respectively. TC is the most stable conformer owing to O9…H15, O10…H12 and F7…H11 intramolecular hydrogen bonding. Additionally, 13C NMR chemical shifts are predicted for the TC conformer by means of B3LYP/6-311++G(d,p) calculations utilizing the GIAO approximation and the PCM solvation model. To precisely assign the observed IR bands vis. the estimated vibrational frequencies, a normal coordinate analysis (NCA) is carried using the calculated force constants. The calculated wavenumbers and IR intensities of the TC form are in good similarity with those experimentally observed. With the aid of the potential energy surface scan (2D and 3D), the OH barrier to internal rotations is estimated using the optimized structural parameters from the B3LYP method with the 6-311++G(d,p) basis set. The results are debated herein and compared with similar available molecules.