Spectroscopic and structural consequences of intramolecular hydrogen bond formation in ortho-dimethylaminomethylphenol A. Koll a, * , S.M. Melikova b , A. Karpfen c , P. Wolschann c a Institute of Chemistry, University of Wroclaw, ul. Joliot-Curie 14, PL-50383, Wroclaw, Poland b Faculty of Physics, St. Petersburg University, St. Petersburg, Russian Federation c Institut fu ¨r Theoretische Chemie und Molekulare Strukturbiologie, Universita ¨t Wien, Wa ¨hringer Straße 17, Vienna, A-1090 Austria Received 17 May 2000; revised 22 June 2000; accepted 22 June 2000 Abstract Ab initio and density functional calculations are applied to analyse the structure of the Mannich base ortho-dimethylami- nomethylphenol with emphasis on a detailed description of the properties of the intramolecular hydrogen bond. The calculated structures of the components, phenol and dimethylbenzylamine, are compared to the geometries of the hydrogen-bonded and open forms of the Mannich base. Additionally, the gas-phase infrared spectra of the Mannich base and of dimethylbenzylamine are presented. The experimental spectra are confronted with theoretical infrared spectra and interpreted with the aid of a detailed normal coordinate analysis. Trends in the modifications of computed harmonic force constants upon closure of the intramolecular hydrogen bond are monitored and discussed.  2001 Elsevier Science B.V. All rights reserved. Keywords: Ab initio calculations; DFT methods; Mannich bases; Dimethylbenzylamine; Ortho-dimethylaminomethylphenol; Intramolecular hydrogen bonding; Gas-phase IR spectra 1. Introduction The interactions between phenols and aliphatic amines are frequently investigated as model systems for hydrogen bonding and proton transfer [1–5]. In particular, Mannich bases are well-suited for the study of intramolecular hydrogen bonding between the phenolic component as proton donor and an aliphatic amine, bound via a methylene bridge to the phenol ring, as proton acceptor [6–11]. In general, ortho-substituted benzenes with intra- molecular hydrogen bonds have a quite high thermo- dynamic stability and a uniform structure [6–20]. This stability and the structural inflexibility of the intramo- lecular hydrogen bond is often explained by the exis- tence of an indirect electronic coupling between the interacting acid and base groups, mediated by the p electrons of the benzene ring, leading to the model of the so-called resonance assisted hydrogen bonds [12– 20]. Due to the presence of a methylene bridge, this type of coupling is strongly reduced in Mannich bases [9–11,21]. In a previous work [22] (paper I of this series), the properties of the intramolecular hydrogen bond of the simplest possible Mannich base, ortho-aminomethyl- phenol (AP), were studied, applying ab initio and density functional methods. In that work, we have monitored the structural and vibrational spectroscopic changes taking place upon formation of the Journal of Molecular Structure 559 (2001) 127–145 0022-2860/01/$ - see front matter  2001 Elsevier Science B.V. All rights reserved. PII: S0022-2860(00)00682-7 www.elsevier.nl/locate/molstruc * Corresponding author. Tel.: +43-1-2775-2772; fax: +43-1- 2779527. E-mail address: karl.peter.wolschann@univie.ac.at (A. Koll).