Chiral recognition and subsequent selective self-organisation into hydrogen-bonded n-mers is observed in supersonic methyl lactate expansions. The nu(OH) and nu(C=O)-vibrations are investigated by ragout-jet FTIR-spectroscopy and lead to the assignment of homo- and heterochiral clusters of at least three different cluster sizes. Whereas homo- and heterochiral dimers are formed in similar amounts in the racemic mixture, prominent absorptions due to different homochiral and heterochiral lactate trimers and tetramers indicate highly specific chiral self-recognition beyond molecular pairs. Chemical modification of the ester-group (methyl-, ethyl- and isopropyl-lactate) and argon admixture to the helium expansion contribute importantly to an understanding of the cluster spectra and topology.

In this report, we describe rotational spectroscopic and high-level ab initio studies of the 1:1 chiral molecular adduct of propylene oxide dimer. The complexes are bound by weak secondary hydrogen bonds, that is, the O(epoxy)...H-C noncovalent interactions. Six homochiral and six heterochiral conformers were predicted to be the most stable configurations where each monomer acts as a proton acceptor and a donor simultaneously, forming two six- or five-membered intermolecular hydrogen-bonded rings. Rotational spectra of six, that is, three homochiral and heterochiral conformer pairs, out of the eight conformers that were predicted to have sufficiently large permanent electric dipole moments were measured and analyzed. The relative conformational stability order and the signs of the chiral recognition energies of the six conformers were determined experimentally and were compared to the ab initio computational results. The experimental observations and the ab initio calculations suggest that the concerted effort of these weak secondary hydrogen bonds can successfully lock the subunits in a particular orientation and that the overall binding strength is comparable to a classic hydrogen bond.

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Intermolecular hydrogen bonding competes with an intramolecular hydrogen bond when methanol binds to an alpha-hydroxyester. Disruption of the intramolecular OH...O=C contact in favour of a cooperative OH...OH...O=C sequence is evidenced by FTIR spectroscopy for the addition of methanol to the esters methyl glycolate, methyl lactate and methyl alpha-hydroxyisobutyrate in seeded supersonic jet expansions. Comparison of the OH stretching modes with quantum-chemical harmonic frequency calculations and 18O labelling of methanol unambiguously prove the insertion of methanol into the intramolecular hydrogen bond. This is in marked contrast to UV/IR hole burning studies of the homologous system methyl lactate: (+/-)-2-naphthyl-1-ethanol, where only addition complexes were found and the intramolecular hydrogen bond was conserved. This switch in hydrogen bond pattern from aliphatic to aromatic heterodimers is thought to reflect not only a kinetic propensity but also a thermodynamic preference for addition complexes when dispersion forces become more important in aromatic systems.

Jet-cooled diastereoisomeric complexes formed between a chiral probe, (+/-)-2-naphthyl-1-ethanol, and chiral lactic acid derivatives have been characterised by laser-induced fluorescence and IR fluorescence-dip spectroscopy. Complexes with non chiral alpha-hydroxyesters and chiral beta-hydroxyesters have also been studied for the sake of comparison. DFT calculations have been performed to assist in the analysis of the vibrational spectra and the determination of the structures. The observed 1 : 1 complexes correspond to the addition of the hydroxy group of the chromophore on the oxygen atom of the hydroxy in alpha-position relative to the ester function. Moreover, (+/-)-methyl lactate and (+/-)-ethyl lactate complexes with (+/-)-2-naphthyl-1-ethanol show an enantioselectivity in the size of the formed adducts: while fluorescent 1 : 1 complexes are the most abundant species observed when mixing (S)-2-naphthyl-1-ethanol with (R)-methyl or ethyl lactate, they are absent in the case of the SS mixture, which only shows 1 : 2 adducts. This property has been related to steric hindrance brought by the methyl group on the hydroxy-bearing carbon atom.

Supersonic jet expansions of racemic methyl lactate show a prominent OH‐stretching infrared absorption at 3401 cm−1 which is not present in the enantiopure compound. Experimental results and quantum chemical calculations show that the underlying tetrameric cluster has 2:2 stoichiometry of the R and S isomers and S4 symmetry (see picture; C orange, O red, H gray).

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Molecular recognition: The binding between propylene oxide (PO) and ethanol resembles the lock‐and‐key principle, with (R)‐PO as a rigid lock and G−, G+, and T conformers of EtOH as different keys (see picture; C yellow, O red). Six hydrogen‐bonded PO⋅⋅⋅EtOH conformers were studied using rotational spectroscopy and ab initio calculations, revealing which EtOH key fits best into the PO lock.