Influence of hydrogen bonding on the conformation of ortho-aminomethylphenol S.M. Melikova a , A. Koll b , A. Karpfen c , P. Wolschann c, * a Faculty of Physics, St. Petersburg University, Russian Federation b Institute of Chemistry, University of Wroclaw, ul. Joliot-Curie 14, PL-50383, Poland c Institut fu ¨r Theoretische Chemie und Strahlenchemie, Universita ¨t Wien, Wa ¨hringer Straße 17, Vienna, A-1090 Austria Received 18 August 1999; accepted 11 October 1999 Abstract Intramolecular hydrogen bonding in ortho-aminomethylphenol is analysed with the aid of ab initio and density functional theory calculations. Selected properties of the components, phenol and benzylamine, are compared to those of the hydrogen- bonded form of the Mannich base as well as that of the non-hydrogen-bonded structure. Additionally, the gas phase infrared spectrum of benzylamine is analysed in terms of the possible conformations of the molecule.  2000 Elsevier Science B.V. All rights reserved. Keywords: Ab initio calculations; Mannich bases; Benzylamine; Ortho-aminomethylphenol; Intramolecular hydrogen bonding 1. Introduction Phenol derivatives are very convenient model systems for the study of hydrogen bonding, and, there- fore, they have been investigated by numerous theo- retical and experimental studies [1–5]. As a special case of phenol derivatives, ortho-Mannich bases were synthesized, and they appear to be very suitable for a detailed study of polarized intramolecular hydrogen bonds [6–10]. The main advantage of these systems is the large thermodynamic stability of the hydrogen bond and the fact that additional intermolecular asso- ciation steps can be neglected. In these systems the phenolic component is connected by a methylene bridge with an aliphatic amine in a proper geometry to build up a relatively strong intramolecular hydrogen bond. The feature which makes this type of compound particularly interesting is the absence of a pronounced electronic coupling between proton donor and acceptor moiety. The conformation of these Mannich bases was investigated by solid state X-ray analysis [10–13] and theoretical considerations [9,11,14]; and it was found that the ring system which contains the hydrogen bridge is not coplanar with the aromatic ring. All these experimentally accessible Mannich bases are tertiary amines. The structure of the unsubstituted primary amine, ortho-methylamino- phenol could so far not be investigated experimentally. Therefore, it was of interest to study the molecular geometry of the parent compound of this series in greater detail. With this goal in mind, ab initio and density functional theory (DFT) calculations were performed on phenol (PH) and benzylamine (BA) molecules, and on the intramolecularly hydrogen-bonded ortho-aminomethylphenol (AP). Journal of Molecular Structure 523 (2000) 223–239 MOLSTR 11208 0022-2860/00/$ - see front matter  2000 Elsevier Science B.V. All rights reserved. PII: S0022-2860(99)00415-9 www.elsevier.nl/locate/molstruc * Corresponding author. Tel.: +43-1-4277-52772; fax: +43-1- 4277-9527. E-mail address: karl.peter.wolschann@univie.ac.at (P. Wolschann).