Pyridine and Methylpyridines: Calculations of the Structure, Proton Affinity, Gas-Phase Basicity, and Mobility of Protonated Molecules and Proton-Bound Dimers

Abstract

Accurate calculations of the structure and properties of analytes and their ions are of great interest to the theory and practice of mass spectrometry, ion mobility spectrometry, and related methods. In this work, using accurate quantum chemical methods, we computed the structure of neutral and protonated pyridine, 2-methyl-, 4-methyl-, 2,4-dimethyl-, 2,6-dimethyl-, and 2,4,6-trimethylpyridine molecules and also of proton-bound dimers of pyridine and 2,4-dimethylpyridine. Two stable conformers of the 2,4-dimethylpyridine proton-bound dimer are found. An accurate and economical method is proposed for calculating proton affinity and gas-phase basicity with the calculation error close to the experimental one. The values of proton affinity and gas-phase basicity for 2,4,6-trimethylpyridine are computed. Reduced mobilities of protonated molecules and proton-bound dimers are calculated by the trajectory method in different versions. A calculation version is proposed that allows mobility computation with the error close to the experimental one and adequately reproduces small differences in the mobility of isomers. The versatility of the chosen B3PW91−D3BJ/def2−TZVP method of the density functional theory with the inclusion of dispersion correction is shown. The method (in combination with B2GP−PLYP−D3BJ/def2−TZVPPD calculations) ensures accurate calculations of the molecular structure, proton affinity, and gas-phase basicity, and also charges on atoms for computing ion mobility.