Electron-withdrawing effects on the molecular structure of 2- and 3-nitrobenzonitrile revealed by broadband rotational spectroscopy and their comparison with 4-nitrobenzonitrile

Abstract

The rotational spectra of 2- and 3-nitrobenzonitrile were recorded via chirped-pulse Fourier transform microwave spectroscopy in the frequency range of 2–8 GHz. These molecules each display large dipole moments, making them viable candidates for deceleration and trapping experiments with AC-electric fields. For both molecules, the main isotopologues and all isotopologues of the respective ¹³C-, ¹⁵N-, ¹⁸O-monosubstituted species in their natural abundance were assigned. These assignments allowed for the structural determination of 2- and 3-nitrobenzonitrile via Kraitchman's equations as well as a mass-dependent least-squares fitting approach. The experimentally determined structural parameters are then compared to those obtained from quantum-chemical calculations for these two molecules and 4-nitrobenzonitrile. Structural changes caused by steric interaction and competition for the electron density of the phenyl ring highlight how these strong electron-withdrawing substituents affect one another according to their respective positions on the phenyl ring.