Conformations and Structures of 3-Oxopentanenitrile and 4-Methyl-3-oxopentanenitrile Revealed by Rotational Spectroscopy

The conformations and molecular structures of two cyano-substituted ketones, namely 4-methyl-3-oxopentanenitrile (MPN) and 3-oxopentanenitrile (PN), have been investigated by using supersonic jet Fourier transform microwave (FTMW) spectroscopy. The rotational spectrum of a single conformation was measured and assigned for 4-methyl-3-oxopentanenitrile (MPN-I) and 3-oxopentanenitrile (PN-I), respectively. Additional measurements were conducted for ¹³C and ¹⁵N singly substituted isotopologues of conformer PN-I in natural abundance. The comprehensive rotational spectroscopic investigations yield accurate values for the rotational constants and ¹⁴N quadrupole coupling constants, which facilitate the structural elucidations of the observed conformers. For conformer PN-I, the near constancy of the <i>P</i>_cc values upon isotopic substitution indicates that the heavy atoms of this conformation are closely located to the <i>ab</i>-plane. Quantum chemical calculations, noncovalent interaction analysis, and molecular electrostatic surface potentials provide further insight into the energetics and structures of the two molecules.