Nicole Borho

Glycidol dimer: anatomy of a molecular handshake

Chiral self-recognition, the ability of a molecule to distinguish between a copy and a mirror copy of itself via intermolecular interactions, is demonstrated for dimers of glycidol (oxiranemethanol) in a supersonic jet expansion. The infrared OH-stretching spectra of homochiral and heterochiral dimers differ from each other and exhibit an unexpected spectral complexity. A systematic quantum chemical study of the conformational degrees of freedom reveals two important types of hydrogen bond topology in glycidol dimers and up to 10 important conformations in the adiabatic expansion. These dimer conformations derive from two out of eight monomer conformations which are preformed and stabilized by intramolecular hydrogen bond contacts. All important conformations have two intermolecular OH–O hydrogen bonds. In the most stable conformations, identical copies of glycidol appear to interact more strongly with each other than with optical antipodes. Secondary interactions such as CH–O contacts are predicted to contribute importantly to chiral discrimination. The spectral complexity in the OH-stretching region can be rationalized qualitatively by harmonic predictions at HF, B3LYP and MP2 levels using small basis sets. Higher level calculations based on this conformational landscape exploration are initiated. They should become increasingly feasible for such a small prototype and will be desirable in order to achieve a quantitative understanding of chiral recognition. Experimentally, the addition of small amounts of Ar to the He expansion is shown to enhance conformational relaxation in the jet.

Dimerization of Pyrazole in Slit Jet Expansions

Abstract Pyrazole dimer is observed for the first time in a free jet expansion. Its IR-active N–H stretching vibration is shifted by −269 cm−1 relative to the monomer. Along the 600 mm slit jet expansion, the average number density of pyrazole dimers is ∼ 1011 cm−3. Exploratory quantum chemical calculations including electron correlation are in good agreement with the observed frequency shift and confirm reciprocal hydrogen bonding with bent hydrogen bonds in a planar C2h structure, as postulated by W. Hückel 65 years ago in this journal. Nanomatrix isolation spectra can be obtained by using Ar as the carrier gas. The more strongly coupled vibrational dynamics in pyrazole trimer is illustrated.

Intra- vs. intermolecular hydrogen bonding : dimers of alpha-hydroxyesters with methanol

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.

Tailor-made aggregates of α-hydroxy esters in supersonic jets

Hydrogen-bonded clusters of methyl glycolate and methyl α-hydroxyisobutyrate are investigated by ragout-jet FTIR-spectroscopy to elucidate the influence of α-C hydrogens on the unusual aggregation of α-hydroxy esters. Highly structured OH-stretching spectra, stagnation pressure studies, and argon coating experiments show that two α-C hydrogens enable the formation of small oligomers such as trimers and tetramers, whereas the lack of α-C hydrogens stops selective aggregation beyond dimers. Methyl lactate with a single chiral α-C hydrogen is suggested to be intermediate in its clustering behavior, with a homochiral preference for trimers and a strong heterochiral preference for tetramers.

Chiral recognition between lactic acid derivatives and an aromatic alcohol in a supersonic expansion: electronic and vibrational spectroscopy

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.

Chiral self-recognition in the gas phase: the case of glycidol dimers

Supersonic jet FTIR spectra of the OH-stretching bands of glycidol monomers and clusters are presented. Chiral discrimination leads to marked differences in the absorption patterns of RR (SS) s. RS glycidol dimers. The dominant absorption peaks are located at 3492 (RR, SS) and 3488 cm−1 (RS) within a rich line spectrum with sizeable variations between enantiomerically pure and racemic dimers. A spectral difference technique is used to emphasize the intermolecular diastereomeric effects. Glycidol is possibly the first and likely the smallest molecule for which chiral self-recognition has been experimentally demonstrated in the gas phase. It thus lends itself very well to accurate quantum chemical calculations of the chiral discrimination effect. Qualitative results of exploratory calculations are reported.

Rotational spectrum of a chiral α-hydroxyester: conformation stability and internal rotation barrier heights of methyl lactate

High resolution spectrum of methyl lactate, a chiral alpha-hydroxyester, has been investigated using a molecular jet Fourier transform microwave spectrometer. High level ab initio calculations were employed to study the conformational isomerism of methyl lactate. The observed rotational spectrum confirms that the most stable conformer has an intramolecular hydrogen bond of OH...O==C type, as predicted by the ab initio calculations. The internal rotation barrier heights of the ester methyl group and the alpha-carbon methyl group were calculated to be 5.4 and 14.5 kJ mol(-1) at the MP2/aug-cc-pVDZ level of theory for the most stable conformer. The internal rotation splittings due to the ester methyl group were observed and analyzed and the ester methyl group tunneling barrier height was determined experimentally to be 4.762 (3) kJ mol(-1).

Tailoring the Key in a Molecular Lock-and-Key Model System : The Propylene Oxide···2-Fluoroethanol Complex

The conformational isomerism of the propylene oxide (PO)...2-fluoroethanol (FE) complex has been investigated using molecular beam Fourier-transform microwave spectroscopy complemented with high level ab initio calculations. Rotational transitions of three different binary conformers have been observed experimentally. On the basis of the agreement of the experimental and calculated rotational constants, they could be identified as the three most stable structures, anti G-g+, anti G+g-, and syn G+g-. All the observed structures exhibit a primary O-H...O hydrogen bond, an intramolecular O-H...F hydrogen bond and two secondary intermolecular C-H...F contacts. The two anti conformers, with FE and the PO methyl group on the opposite sides of the oxirane ring, show higher abundances than the syn conformer. In all three observed conformers, FE remains approximately in its favorable compact gauche conformation. The monofluorination of the molecular lock-and-key model system PO...ethanol increases not only the number of possible binary conformers, but also the discrimination energy among them. The superior discrimination ability of FE as compared to ethanol classifies it as a tailored key to the PO lock.

Molecular recognition in 1 : 1 hydrogen-bonded complexes of oxirane and trans-2,3-dimethyloxirane with ethanol: a rotational spectroscopic and ab initio study

High resolution rotational spectroscopy complemented by ab initio calculations has been used to elucidate the diastereomeric interactions in 1 : 1 complexes of ethanol, a transient chiral alcohol, hydrogen-bonded to oxirane (achiral) or trans-2,3-dimethyloxirane (DMO, 2 stereocenters). Two conformers of oxirane[dot dot dot]ethanol and three conformers of DMO[dot dot dot]ethanol have been identified, and their structures as well as their stability ordering have been determined. This completes, together with previous results on the propylene oxide...ethanol complex (N. Borho and Y. Xu, Angew. Chem., 2007, 119, 2326-2329; Angew. Chem., Int. Ed., 2007, 46, 2276-2279.), the study of a set of model systems with zero, one, and two methyl functional groups at the hydrogen bond acceptor oxirane. The dependence of the observed rotational line intensities on pressure, nozzle temperature, and different carrier gases has been investigated for the case of DMO[dot dot dot]ethanol. This provides insight into the kinetical and thermodynamical influence on the formation of different conformers. Comparison of the subtle energy differences among the complexes and within each set of conformers allows for a detailed analysis of molecular recognition in this benchmark system.

Chirality Recognition in the Glycidol⋅⋅⋅Propylene Oxide Complex: A Rotational Spectroscopic Study

Chirality recognition in the hydrogen-bonded glycidol···propylene oxide complex has been studied by using rotational spectroscopy and ab initio calculations. An extensive conformational search has been performed for this binary adduct at the MP2/6-311++G(d,p) level of theory and a total of 28 homo- and heterochiral conformers were identified. The eight binary conformers, built of the two dominant glycidol monomeric conformers, g-G+ and g+G-, were predicted to be the most stable ones. Jet-cooled rotational spectra of six out of the eight conformers were observed and unambiguously assigned for the first time. The experimental stability ordering has been obtained and compared with the ab initio predictions. The relative stability of the two dominant glycidol monomeric conformers is reversed in some cases when binding to propylene oxide. The contributions of monomeric energy, deformation energy, and binary intermolecular interaction energy to the relative stability of the binary conformers are discussed.