The chirality controlled conformational landscape of the trimer of propylene oxide (PO), a prototypical chiral molecule, was investigated using rotational spectroscopy and a range of theoretical tools for conformational searches and for evaluating vibrational contributions to effective structures. Two sets of homochiral (PO)3 rotational transitions were assigned and the associated conformers identified with theoretical support. One set of heterochiral (PO)3 transitions was assigned, but no structures generated by one of the latest, advanced conformational search codes could account for them. With the aid of a Python program, the carbon atom backbone and then the heterochiral (PO)3 structure were generated using 13C isotopic data. Excellent agreement between theoretical and experimental rotational constants and relative dipole moment components of all three conformers was achieved, especially after applying vibrational corrections to the rotational constants.

Using chirped-pulse Fourier transform microwave spectroscopy, we have experimentally identified the most abundant dimer of an alcohol analogue of naphthalene, 1naphthol. The 1-naphthol dimer features a V-shaped, partially overlapping π−π stacked structure with no canonical hydrogen bonds between the subunits. This structural assignment is in contradiction to an earlier study of the 1-naphthol dimer using UV-IR dip double resonance spectroscopy in the O−H stretch region, which assigns a π-stacked but also canonically hydrogen-bonded structure. We use an improved theoretical analysis to resolve this discrepancy and show that the new, V-shaped structure is also consistent with the previously measured UV-IR data. These new computational and spectroscopic results shed light on the complicated nature of evaluating energetics and structures for larger, dispersionbound systems.

The 1:1 adduct of 1-phenyl-2,2,2-trifluoroethanol (PhTFE), a chiral fluoroalcohol, with water was investigated using chirped pulse Fourier transform microwave spectroscopy and computational methods. While PhTFE itself was predicted to have three minima, I (gauche+), II (trans), and III (gauche−), only I and II were stable and only I was observed experimentally. A systematic search of the PhTFE···H2O conformational landscape identified 110 stable minima, 14 of which are within a 15 kJ mol–1 energy window. Rotational spectra of the two PhTFE···H2O conformers along with several deuterium and 18O isotopologues were assigned, and the isotopic data were used to verify the corresponding structures. In the two observed monohydrate conformers, one contains PhTFE I where the water subunit is inserted into the existing intramolecular OH···F contact of I, and the binary adduct is stabilized by two intermolecular contacts: OH···OW and HW···F, whereas the other contains PhTFE II where the water subunit interacts with both the alcohol hydrogen and phenyl ring of II, demonstrating that interaction with water sufficiently stabilizes II for its observation in a jet expansion. Interestingly, the predicted electric dipole moment components at the identified minima deviate considerably from the experimental ones. Such deviations were analyzed in terms of dynamic effects associated with the large amplitude motions of the unbound HW. In addition, tunnelling effects associated with the exchange of the bonded and nonbonded HW were also discussed.

A chiral adduct formed between a chiral carboxylic acid, tetrahydro‐2‐furoic acid (THFA), and a chiral ester, propylene oxide (PO), was investigated using rotational spectroscopy and DFT calculations. Isolated THFA exists dominantly as three different conformers: I, II, and III in a jet, with I and II taking on the trans‐COOH configuration and III having the cis‐COOH configuration. We utilized CREST, a conformational ensemble space exploration tool, to identify the possible conformations of the binary adduct, THFA⋅⋅⋅PO. Subsequent DFT geometry optimizations predicted about two hundred homochiral and heterochiral binary structures with 28 low energy structures within an energy window of 15 kJ mol−1. A rich broadband rotational spectrum was obtained with a mixture of trace amounts of THFA+PO in neon in a supersonic jet expansion. Six THFA⋅⋅⋅PO conformers were identified experimentally. Kinetically favored binary products which contain trans‐COOH I dominate among the observed conformers, while thermodynamically more stable adducts were also detected. Detailed analyses of the structures of the observed conformers show interesting chirality‐controlled structural preferences. Such non‐covalently bound chiral contact pairs are the foundation of chiral‐tag rotational spectroscopy, an exciting new analytical application of rotational spectroscopy for determination of enantiomeric excess. Enantiomeric excess analyses were performed and the results are discussed.

Rotational spectra of the 1,1,1,3,3,3-hexafluoro-2-propanol (HFIP)···water complex were measured using a chirped pulse Fourier-transform microwave spectrometer. The spectral analyses, aided by density functional theory calculations, reveal two HFIP···water isomers: one previously reported, trans HFIP (HFIP t )···water (Phys. Chem. Chem. Phys. 2015, 119, 5650–5657), and a new isomer, gauche HFIP (HFIP g )···water. To confirm the identity of the new isomer, rotational spectra of seven of its deuterated species were also measured and analyzed. Both the experimental and theoretical pieces of evidence indicate that the intermolecular interaction with water preferentially stabilizes the HFIP g monomer configuration over the global minimum configuration, HFIP t . The relative energy difference between these monomeric forms is 4.1 kJ mol–1 and decreases to 2.5 kJ mol–1 in the respective monohydrates at the B3LYP-D3­(BJ)/def2-QZVP level of theory. Both rigid and relaxed potential energy surface scans were carried out to gain insights into the large-amplitude water motions in HFIP g ···water. The nonobservation of a water tunneling splitting in HFIP t ···water has been explained to be a result of a barrier-less (after zero-point-energy correction) pathway for the water motion, whereas in HFIP g ···water, a relatively large water tunneling barrier was identified as the cause of barely resolved water tunneling splittings. Noncovalent interaction and quantum theory of atoms and molecule analyses were used to evaluate the changes in HFIP g ···water when going from the minimum to the transition state in terms of attractive interactions such as the OH···H and OH···F contacts. The effect of fluorination is discussed by comparing the vastly different binding topologies of isopropanol···water and HFIP···water.