Precise Semi-Experimental Equilibrium ( <i>r</i> _<i>e</i> ^SE ) 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 (<i>r</i>_<i>e</i>^SE) of pyridine. The structural parameters of pyridine are in generally good agreement with those of previously published <i>r</i>_<i>e</i>^SE 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.