Precise dipole moments and quadrupole coupling constants of the cis and trans conformers of 3-aminophenol: Determination of the absolute conformation

TL;DR

This study precisely measures the rotational constants, quadrupole coupling, and dipole moments of cis and trans 3-aminophenol conformers using microwave spectroscopy and Stark effect, aiding in conformational assignment.

Contribution

The paper provides accurate experimental data on dipole moments and quadrupole coupling constants for both conformers, combined with high-level ab initio calculations to support conformational analysis.

Findings

Dipole moments differ significantly between conformers, enabling their absolute configuration determination.

Rotational constants and quadrupole coupling constants alone are insufficient for conformer assignment.

High-level ab initio calculations validate experimental measurements and estimate errors.

Abstract

The rotational constants and the nitrogen nuclear quadrupole coupling constants of cis-3-aminophenol and trans-3-aminophenol are determined using Fourier-transform microwave spectroscopy. We examine several $J=2\leftarrow{}1$ and $1\leftarrow{}0$ hyperfine-resolved rotational transitions for both conformers. The transitions are fit to a rigid rotor Hamiltonian including nuclear quadrupole coupling to account for the nitrogen nucleus. For cis-3-aminophenol we obtain rotational constants of A=3734.930 MHz, B=1823.2095 MHz, and C=1226.493 MHz, for trans-3-aminophenol of A=3730.1676 MHz, B=1828.25774 MHz, and C=1228.1948 MHz. The dipole moments are precisely determined using Stark effect measurements for several hyperfine transitions to $\mu_a=1.7735$ D, $\mu_b=1.5195$ D for cis-3-aminophenol and $\mu_a=0.5563$ D, $\mu_b=0.5376$ D for trans-3-aminophenol. Whereas the rotational constants and quadrupole coupling constants do not allow to determinate the absolute configuration of the two conformers, this assignment is straight-forward based on the dipole moments. High-level \emph{ab initio} calculations (B3LYP/6-31G^* to MP2/aug-cc-pVTZ) are performed providing error estimates of rotational constants and dipole moments obtained for large molecules by these theoretical methods.