Structural study of hydrogen-bonded complexes between 2-aminoethanol derivatives and a chiral aromatic alcohol
Introduction
Intra- and inter-molecular hydrogen bonds play a key role in a number of (bio) chemical processes such as the building of the fundamental molecular assemblies of living organisms as well as the modification of their structure as a function of their environment. The competition between these two types of interactions may be investigated in simple molecules bearing several H bonding sites such as aminoalcohols. In addition to the fact that a lot of natural products contain the aminoalcohol functionality, these compounds present several interesting properties. First, the combination of donor and acceptor groups results in the stabilisation of molecular conformations involving intramolecular H bonding either of the OH⋯N or the NH⋯O type. The conformational isomerism of the most simple aliphatic aminoalcohols and the related intramolecular hydrogen bonding within these compounds has been for this reason extensively studied both experimentally by microwave [1], [2] and IR spectroscopy [3], [4], [5] as well as theoretically using different levels of ab initio calculations [6], [7], [8], [9]. It is widely recognised that the most stable conformer in the isolated conditions adopts a folded gauche geometry favouring the intramolecular OH⋯N hydrogen bond. The conformation of less stable isomers is further dictated by the size of the chain and the proton accepting property of the amino group. In the neat liquid, however, intermolecular OH⋯N bonds become preponderant over intramolecular ones and the dominant conformation of monomeric units involves weak intramolecular NH⋯O hydrogen bond. Second, substitution by an alkyl group on the carbon backbone of 2-aminoethanol introduces an asymmetric center. Many substituted aminoalcohols are thus chiral and some of them are considered as prebiotic precursors of natural aminoacids such as alanine, valine or isoleucine [10]. The existence of enantiomers in derivatives bearing the 2-aminoalcohol skeleton has been exploited in our previous work on gas phase chiral discrimination [11]. We have shown that diastereoisomeric pairs consisting of the association between enantiomers of a chiral chromophore with those of a chiral alcohol or aminoalcohol, formed in a supersonic jet, can be differentiated on the basis of the shift of the S0–S1 transition of the chromophore. The system that has been recently studied, consisted of R- or S-2-naphthyl-1-ethanol (NapOH) as the selectand and alaninol (R-or S-2-amino-1-propanol, 2A1P) as the solvating partner. The structure of diastereoisomeric complexes of 2-naphthyl-1-ethanol with alaninol has been characterised by IR fluorescence dip spectroscopy. By comparing the IR signature of the complexes in the OH stretch region with simulated spectra obtained by DFT calculations, two types of geometries have been evidenced in this system. The homochiral complexes exist in two isomeric forms. In one of them, which is also the solely observed form of the heterochiral complex, alaninol keeps its most stable conformation and is connected to the chromophore through an intermolecular OH(NapOH)⋯O(aminoalcohol) bond. The second homochiral complex involves an open form of the aminoalcohol and is linked to the chromophore through a strong OH(NapOH)⋯N(aminoalcohol) bond and a weaker OH⋯π interaction.
The structure of the phenol/2-aminoethanol (AE) clusters has also been investigated recently using double resonance IR/UV spectroscopic techniques combined with Resonance Enhanced MultiPhoton Ionisation (REMPI) detection methods in free jet expansion conditions [12]. Two types of hydrogen bonded structures have also been identified in this case. In one of them, just as in complexes of NapOH with alaninol, the phenol OH is attached to AE through an intermolecular OH⋯O bond, leaving the intramolecular OH⋯N bond of the most stable conformer of 2-aminoethanol intact. However, in contrast with the complexes of NapOH with alaninol, the second geometry of phenol/2-aminoethanol 1:1 cluster involves the insertion of the more acidic phenolic OH within the N and OH groups of 2-aminoethanol.
With the aim to gain further insight on the role of isomerism in these systems, we report here a comparative spectroscopic study on the complexes of 2-naphthyl-1-ethanol with the aminoalcohols pictured later. The first model system investigated here involves the non-chiral 2-aminoethanol and is studied in detail using fluorescence excitation, UV–UV and IR–UV depletion techniques together with quantum chemistry calculations to help in making structural assignments. The role of the strength of the intramolecular hydrogen bond was examined by studying the influence of methyl substitution of the amino group on complexes of NapOH with 2-(N-methylamino)ethanol (MAE) and 2-(N,N-dimethylamino)ethanol (DMAE) or of the length of the methylene chain in the case of 3-amino-1-propanol (3A1P).
Section snippets
Experimental section
The laser induced fluorescence excitation spectra and UV/UV depletion spectra have been obtained with van der Waals complexes produced in a continuous supersonic expansion of Helium (2–3 atm). The aminoalcohol vapor obtained at room temperature or with a slight heating (40 °C) is mixed with the helium flux and passes through the preheated (90 °C) chromophore sample before the expansion nozzle. The free molecules and complexes present in the cold region of the jet molecules are excited by means
Bare molecules
The conformations of NapOH and 2-aminoethanol have been calculated by using the hybrid HF functional density B3LYP with the standard 6-31G** basis set of gaussian 98 [15] as reported previously for the NapOH/ 2-amino-1-propanol complexes [11].
Complexes
The calculation strategy involves two steps as described in our previous paper [11]. First a global exploration of the whole potential energy surface is carried out by means of a semi-empirical method in which the intramolecular coordinates are kept
Fluorescence excitation spectra of the NapOH chromophore in the presence of aminoalcohols
The laser induced fluorescence spectrum of the NapOH chromophore has already been reported. Its 0–0 transition located at 31738.4 cm−1 is followed by two low-frequency features at+39 and+76 cm−1 assigned to the torsion motion of the CH(CH3)OH group. The latter feature at 0−0+76 cm−1 was shown to consist of two overlapping bands, which were assigned to a second isomer of NapOH and to the third member of the low frequency progression [20]
Fig. 1 shows the fluorescence excitation spectrum (FES) in
Aminoalcohol conformations
As a prototype molecule in the class of 1,2-disubstituted ethanes with intramolecular H-bonding, the conformational structure of 2-aminoethanol has been the subject of considerable attention [2], [3], [5], [6], [7], [8], [9], [12]. The different conformers are labeled according to the notation used in the literature by three letters [3]. The central capital letter (G,G′,T) relates to the N–C–C–O dihedral angle (respectively, ∼+60°, −60°, 180°), whereas the preceding and following g,g′,t,
Discussion
Two structures have been identified in the complexes of NapOH with 2-aminoethanol. One of them involves the disruption of the intramolecular H bond of 2-aminoethanol, the two functional groups of AE making a bridge between the OH and the π system of the chromophore. Similar ‘N addition’ structures have also been characterised in the homochiral complexes of NapOH with 1-amino-2-propanol [23] and 2-amino-1-propanol [11]. The second structure is associated with the intramolecularly bonded
Conclusion
The structure of the complexes formed between the chiral 2-naphthyl-1-ethanol chromophore and 2-aminoethanol derivatives has been studied by spectroscopic techniques and ab initio calculations. This study has shown the formation of two types of addition complexes namely ‘O addition’ and ‘N addition’ complexes. In the first case, the alcoholic OH group of the chromophore acts as a proton donor towards the O atom of the aminoalcohol. This structure keeps and even reinforces the intramolecular H
Acknowledgements
J.S. gratefully acknowledges the CNRS for a visiting scientist fellowship. His research was also supported by the Polish State Committee for Scientific Research (grant 4T 09A 033 22)
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