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.

New findings from old data: A semi-experimental value for the eQq of the nitrogen atom

“…Several studies have reported the observation of structural changes by means of microwave spectroscopy in aromatic rings. [37][38][39][40][41][42] Experimentally, the determination of molecular structures relies on the observation of every possible mono-isotopic substitution, either in natural abundance or using enriched samples, which can be a challenge. The trends in the values of the NQCC χcc of 14 N in heteroaromatic rings can provide further confirmation of resonance and allow us to trace back the electron density even if the molecular structures are not determined.…”

The heavy atom structures and 33S quadrupole coupling constants of 2-thiophenecarboxaldehyde: insights from microwave spectroscopy

“…An enormous number of investigations on the hyperfine structures of many nuclei have been reported, e.g. those of 14 N [22,23], 35 Cl and 37 Cl [24], 79 Br and 81 Br [25], and 127 I [26], but only a few of 33 S [27], most probably because of the low natural abundance of 0.76% and an expensive preparation (if possible). The high resolution and sensitivity of our experimental setup have enabled the observation of this valuable sulfur species in natural abundance.…”

4-(5-Benzyl-3-((4-fluorophenyl)sulfonyl)-5-methyl-4,5-dihydrofuran-2-yl)-2-nitrobenzamide

“…This enhanced stability could be influenced by both the electronic influence of the nitro group and the solvation effect of the ethanol (where hydrogen bond interactions occur) forming a solvent network that could mediate the proton transfer in the course of the reaction [28]. Furthermore, regarding the inductive and mesomeric effects of the nitro group [29,30], the nitrile becomes a more electrophilic center, and therefore a 'harder' electrophile that predominantly undergoes an O-attack from the hydroxylamine in TS-1, since 'hard-hard' (and 'soft-soft') interactions are preferred [31,32]. Meanwhile, the predominant formation of amidoxime observed with DMSO as a solvent could be explained either by a hindered second hydroxylamine attack in the absence of an appropriate solvent network necessary to stabilize the TS-2; then, the intermediary compound c via TS-1 is unable to transform forward d and the process either turns back to an N-attack or to an alternative possibility involving a cyclic intermediate rearrangement [25].…”