Mass spectral and semi-empirical studies on chiral discrimination in gaseous aggregation products of protonated dialkyl tartrates

The enantiomers (M) of isopropyl threo- and erythro-3cyclohexylglycerates (4 and 5), 2,3-butanediol (6), trans-1,2-cyclohexanediol (7) and 1,4-dimethoxy-2,3-butanediol (8) were studied in the gas phase by chemical ionization mass spectrometry with enantiomer-labelled diisopropyl tartrates (1c) as chiral additive. The protonated mixed dimers M · 1c · H+ of 4 and 8, but not those of 5–7, showed small but distinct chiral discriminations. The smaller number of H-bond acceptors sites in 4–8 compared with 1c reduces or cancels the chirality effect. For the dimethyl tartrates, semi-empirical calculations predicted homochiral and heterochiral dimer structures RR · H+ and RS · H+ with four and three intermolecular H bonds, respectively. The stability increase of −0.22 kcal/mol for RR · H+ agrees with the chirality effect KRR / KRS ≈ 1.5. Protonated trimers of 1c showed a chiral effect KRRS / KRRR ≈ 1.6 in favour of the heterochiral RRS · H+ and RSS sd H+ ions compared with RRR · H+ and SSS · H+. Association of a methanol, ethanol or diol molecule (X) in heterogenous trimers X · 1c · 1c≈ H+ affected the initial chiral discrimination which occurs in the unsolvated 1c · 1c · H+ dimers.