Microwave three‐wave mixing has emerged as a novel approach for studying chiral molecules in the gas phase. This technique employs resonant microwave pulses and is a non‐linear and coherent approach. It serves as a robust method to differentiate between the enantiomers of chiral molecules and to determine the enantiomeric excess, even in complex chiral mixtures. Besides such analytical applications, the use of tailored microwave pulses allows us to control and manipulate chirality at the molecular level. Here, an overview of some recent developments in the area of microwave three‐wave mixing and its extension to enantiomer‐selective population transfer is provided. The latter is an important step towards enantiomer separation—in energy and finally in space. In the last section, we present new experimental results on how to improve enantiomer‐selective population transfer to achieve an enantiomeric excess of about 40 % in the rotational level of interest using microwave pulses alone.

Chiral molecules with low enantiomer interconversion barriers racemize even at cryogenic temperatures due to quantum tunneling, forming a racemic mixture that is impossible to separate using conventional chemical methods. Here we both experimentally and theoretically demonstrate a method to create and probe a state-specific enantiomeric enrichment for such molecular systems. The coherent, non-linear, and resonant approach is based on a microwave six-wave mixing scheme and consists of five phase-controlled microwave pulses. The first three pulses induce a chiral wavepacket in a chosen rotational state, while the consecutive two pulses induce a polarization for a particular rotational transition (listen transition) with a magnitude proportional to the enantiomeric excess created. The experiments are performed with the transiently chiral molecule benzyl alcohol, where a chiral molecular response was successfully obtained. This signal demonstrates that enantiomeric excess can be induced in a quantum racemic mixture of a transiently chiral molecule using the developed microwave six-wave mixing scheme, which is an important step towards controlling non-rigid chiral molecular systems.

The initial stages of the gas‐phase nucleation between CO₂and monoethanolamine were investigated via broadband rotational spectroscopy with the aid of extensive theoretical structure sampling. Sub‐nanometer‐scale aggregation patterns of monoethanolamine‐(CO₂)_n,n=1–4, were identified. An interesting competition between the monoethanolamine intramolecular hydrogen bond and the intermolecular interactions between monoethanolamine and CO₂upon cluster growth was discovered, revealing an intriguing CO₂binding priority to the hydroxyl group over the amine group. These findings are in sharp contrast to the general results for aqueous solutions. In the quinary complex, a cap‐like CO₂tetramer was observed cooperatively surrounding the monoethanolamine. As the cluster approaches the critical size of new particle formation, the contribution of CO₂self‐assembly to the overall stability increases.

The rotational spectrum of the parent isotopic species of HCCNCS, along with those of three ¹³C singly substituted variants and one ³⁴S minor isotopologue, has been observed with high resolution using Fourier transform microwave spectroscopy from 4 to 26 GHz. Based on the observed spectral pattern and assignment of the ¹⁴N nuclear quadrupole hyperfine structure, the geometry of HCCNCS is effectively linear, which is further supported by the observation of $\mathit{\ell }$ -type doubling for rotational transitions in an excited bending state of the parent HCCNCS. The experimental observations are supported by high level quantum chemical calculations using the coupled-cluster singles and doubles model augmented by a perturbative correction for triple excitations, CCSD(T), level of theory that reveal an extremely shallow potential energy well along the CNC bending angle with a surprising basis set dependence.