Revealing Internal Rotation and 14N Nuclear Quadrupole Coupling in the Atmospheric Pollutant 4-Methyl-2-nitrophenol: Interplay of Microwave Spectroscopy and Quantum Chemical Calculations

Abstract

The structure and interactions of oxygenated aromatic molecules are of atmospheric interest due to their toxicity and as precursors of aerosols. Here, we present the analysis of 4-methyl-2-nitrophenol (4MNP) using chirped pulse and Fabry–Pérot Fourier transform microwave spectroscopy in combination with quantum chemical calculations. The rotational, centrifugal distortion, and 14N nuclear quadrupole coupling constants of the lowest-energy conformer of 4MNP were determined as well as the barrier to methyl internal rotation. The latter has a value of 106.4456(8) cm−1, significantly larger than those from related molecules with only one hydroxyl or nitro substituent in the same para or meta positions, respectively, as 4MNP. Our results serve as a basis to understand the interactions of 4MNP with atmospheric molecules and the influence of the electronic environment on methyl internal rotation barrier heights.

Rotational spectroscopy of methyl tert-butyl ether with a new Ka band chirped-pulse Fourier transform microwave spectrometer

“…In addition, the intrinsically narrow Doppler widths achievable at low frequency allows for resolution of small splittings arising from large-amplitude motion and intramolecular interactions. [2][3][4][5][6][7][8] CP techniques are routinely used to study systems of fundamental interest such as clusters, [9][10][11][12] radicals, 13 and astrochemically-relevant species, 14 often in conjunction with a high-resolution, narrow-bandwidth cavity FTMW spectrometer in the centimeter band. In such investigations, the CP spectrum of the system of interest is directly compared against a predicted spectrum from quantum chemical calculations to yield a quick initial fit of experimental spectroscopic constants, followed by targeted measurements with a cavity instrument to explore small splittings as needed.…”