# Rotational Detection of the Missing Conformers of 2‑Chloropropionic Acid

## TL;DR

This study identifies new conformers of 2-chloropropionic acid using advanced spectroscopy techniques and compares their properties with theoretical calculations.

## Contribution

The discovery of two new conformers stabilized by intramolecular interactions and the role of relativistic effects in accurately modeling their properties.

## Key findings

- Two new conformers of 2-chloropropionic acid were identified using millimeter-wave and Fourier transform microwave spectroscopy.
- Intramolecular OH···Cl hydrogen bonding stabilizes one conformer, evidenced by changes in chlorine nuclear quadrupole coupling constants.
- Relativistic calculations are necessary to accurately model the electronic properties of the conformers.

## Abstract

The conformational space of 2-chloropropionic acid was
reinvestigated
using millimeter-wave and chirp-polarization Fourier transform microwave
spectroscopies under isolated conditions in supersonic expansions.
Besides the global minimum (carbonyl oxygen eclipsed with the methyl
group), two additional conformers were identified: one with the carbonyl
eclipsed relative to the α-C–H bond, and another stabilized
by an intramolecular OH···Cl hydrogen bond. The latter
interaction is evidenced by a decrease in |χ
zz
|: the chlorine nuclear quadrupole coupling constant associated
with the principal axis lying along the C–Cl bond and an increase
in the asymmetry parameter η = (χ
xx
– χ
yy
)/χ
zz
of the chlorine quadrupole coupling tensor.
Rotational constants, geometries, and relative energies were determined
and compared with quantum-chemical calculations, revealing larger
deviations for conformers with intramolecular interactions. Calculations
also show that chlorination markedly modifies the conformational landscape
relative to propionic acid, affecting both minima and interconversion
barriers. The accurate reproduction of the electronic properties encoded
in the determined complete nuclear quadrupole coupling tensors requires
the inclusion of relativistic approaches.