Molecular self-recognition: rotational spectra of the dimeric 2-fluoroethanol conformers

Fluoroalcohols show competitive formation of intra- and intermolecular hydrogen bonds, a property that may be crucial for the protein-altering process in a fluoroalcohol/water solution. In this study, we examine the intra- and intermolecular interactions of 2-fluoroethanol (FE) in its dimeric conformers by using rotational spectroscopy and ab initio calculations. Three pairs of homo- and heterochiral dimeric FE conformers are predicted to be local minima at the MP2/6-311++G(d,p) level of theory. They are solely made of the slightly distorted most stable G+g-/G-g+ FE monomer units. Jet-cooled rotational spectra of four out of the six predicted dimeric conformers were observed and unambiguously assigned for the first time. All four observed dimeric conformers have compact geometries in which the fluoromethyl group of the acceptor tilts towards the donor and ensures a large contact area. Experimentally, the insertion of the O-H group of one FE subunit into the intramolecular O-H...F bond of the other was found to lead to a higher stabilisation than the pure association through an intermolecular O-H...O-H link. The hetero- and homochiral combinations were observed to be preferred in the inserted and the associated dimeric conformers, respectively. The experimental rotational constants and the stability ordering are compared with the ab initio calculations at the MP2 level with the 6-311++G(d,p) and aug-cc-pVTZ basis sets. The effects of fluorination and the competing inter- and intramolecular hydrogen bonds on the stability of the dimeric FE conformers are discussed.