Rotational Spectra of 2-Ethynylpyridine and Its Monohydrate: Influence of the Ortho-Substitution on Ring Geometry and Intermolecular Hydrogen Bonds

Rotational spectra of the 2-ethynylpyridine monomer and its monohydrate have been characterized by pulsed jet Fourier transform microwave spectroscopy complemented with quantum chemical calculations. The measurements of rotational transitions of the 2-ethynylpyridine monomer and its eight monosubstituted isotopologues (N-15 and C-13) in natural abundances allow an accurate structural description of the skeleton of 2-ethynylpyridine. For the monohydrate, only the most stable isomer, stabilized by an O-H center dot center dot center dot N and a secondary C-H center dot center dot center dot O hydrogen bonds, has been observed in the supersonic jet. Johnson's noncovalent interaction and quantum theory of atoms in molecules analyses have been performed and compared with results for several ortho-substituted pyridine derivatives to elucidate the general trend in their binding energies.