Infrared Ion Dip Spectroscopy of a Noradrenaline Analogue:  Hydrogen Bonding in 2-Amino-1-phenylethanol and Its Singly Hydrated Complex

Abstract

The conformational landscapes of 2-amino-1-phenylethanol and its 1:1 water complexes have been investigated by UV band contour, UV−UV hole-burning, and IR−UV ion dip spectroscopy, coupled with ab initio computation. The two molecular conformers observed are both stabilized by an intramolecular hydrogen bond, located in the folded (gauche) OCCN side chain, which links the proton donor OH group to the terminal amino group and leads to a significant constriction of the side chain. In the dominant 1:1 water complex, the intramolecular hydrogen bond is disrupted by the first water molecule, which inserts between the OH group and the nitrogen atom, to form a cyclic H-bonded structure. The side chain expands significantly in order to accommodate the water molecule within the neighborhood of both the hydroxyl and amino groups.

Infrared photodissociation spectroscopy of protonated neurotransmitters in the gas phase

“…A similar trend is found for neutral [1,2] and protonated APE. The most apparent change in bonding following protonation is a reversal in the direction of the intramolecular hydrogen bond of the ethanolamine side-chain from OH → N in the neutral to NH + → OH in the protonated ion.…”

“…In the bare ion, APE.H + , this structure is calculated to lie at an energy ~6 kJ mol -1 below that of the extended, AG conformer, in contrast to the neutral molecule, where the AG conformer is marginally favoured [1,2,46]. In the 'tagged' ion the minimum energy conformer is associated with two, almost isoenergetic, GG conformational structures, GG-NH-add and GG-π-ins.…”

“…To cite this version: The conformational preferences of neutral ethanolamines and their hydrated complexes, isolated in the gas phase, including 2-amino-1-phenylethanol [1,2], the diastereoisomers ephedrine and pseudoephedrine [3,4], and the catecholamines noradrenaline and adrenaline [5,6] (see Scheme 1), have been characterised recently using a combination of quantum chemical computation, infrared ion-dip spectroscopy and ultraviolet rotational band contour analysis [7]. In every case their favoured structures are stabilised through intramolecular hydrogen bonding (OH → N) between the functional groups on the flexible ethanolamine side-chain which preferentially adopts an extended configuration relative to the aromatic ring.…”

“…They all display a band centred at ~3650 cm The widths of the OH and NH band contours, 20-40 cm -1 FWHM, are substantially greater than those recorded in ion-dip experiments of the corresponding jetcooled neutral species, ~5 cm -1 [1][2][3][4][5][6][7]. The broader widths are a consequence of the production method.…”

“…, suggests a perturbed pseudoephedrine. These types of interaction also shape the conformational landscape of the neutral species [1][2][3][4][5][6][7] but the outcome can be radically altered by protonation. The diastereoisomers, ephedrine and pseudoephedrine provide a good case study.…”