Precise Semi-Experimental Equilibrium (reSE) Structure of Pyridine from 32 Isotopologues: Accurate Assessment of the Effect of Nitrogen-Atom Substitution in Aromatic Rings.

Millimeter-wave rotational spectra of 32 pyridine isotopologues have been measured within the 80-750 GHz frequency range. The rotational constants resulting from analysis of these spectra, corrected for the effects of vibration-rotation interactions and electron-mass distributions using CCSD(T)/cc-pCVTZ calculations, enable the determination of a highly precise semiexperimental equilibrium structure (reSE) of pyridine. The structural parameters of pyridine are in generally good agreement with those of previously published reSE structures, but the precision achieved here is substantially improved: bond lengths are determined to ±0.0002 Å and bond angles are determined to ±0.020° (2σ uncertainties). Such precision approaches the limit of semiexperimental equilibrium structure determination using a contemporary methodology. The current investigation provides critical structural data that enables a detailed analysis of the effects of nitrogen-atom substitution in prototypical aromatic molecules. Only through comparisons among recently determined, highly precise structures for benzene, pyridine, pyridazine, and pyrimidine are the subtle structural effects of nitrogen-atom substitution in the aromatic ring experimentally revealed.