Molecular modelling studies of side-chain rotation in substituted triazine rings q Helen E. Birkett a , Julian C. Cherryman b , A. Margaret Chippendale b , Paul Hazendonk a , Robin K. Harris a, * a Department of Chemistry, Science Laboratories, University of Durham, South Road, Durham DH1 3LE, UK b Avecia Ltd, Hexagon House, Blackley, Manchester M9 8ZS, UK Received 22 January 2001; accepted 28 January 2001 Abstract Molecular modelling computation using the gaussian 94 package of programs was carried out to estimate four different barrierstointernalrotationaboutaminenitrogentosp 2 carbonbondsfortwocompoundswithtriazinerings.Whereastwoofthe processes appear to retain a planar side-chain nitrogen environment throughout the internal rotation, the other two involve pyramidalisation. In the latter two cases, after the barrier is passed, the process may or may not include a transient inversion at nitrogen. The internal rotation for cyclotriazine systems is discussed in relation to those for aniline and formamide. The four barrier magnitudes calculated for the triazines are in the same order as though somewhat larger than) those measured by variable-temperature NMR bandshape analysis for a relevant compound. q 2002 Elsevier Science B.V. All rights reserved. Keywords: Molecular modelling; Internal rotation; Energy barriers; Cyclotriazine; Formamide; Aniline 1. Introduction In this paper, molecular modelling is used to inves- tigate the energy barriers to rotation of amine side chains in 1,3,5-triazine rings to interpret the experi- mental barriers to internal rotation in compound 1 determined using NMR [1]. Previous dynamic NMR barrier determination on similar systems quote DG ³ that are in line with those of compound 1 Scheme 1) [2±4]. Aminotriazines, and indeed triazines in general, have not been greatly studied by molecular modelling, so it is of interest to explore their beha- viour in order to relate it to that of related known systems, thus increasing the understanding of interac- tions and processes involving nitrogen. Amine nitrogens are sp 3 hybridised,withalonepair, and therefore are in general tetrahedral. Consequently, they can undergo inversion processes [5±10]. The hybridisation re¯ects the electron conjugation and has anin¯uenceonthedynamicbehaviourofamines.Tria- zines may behave as simple alkylamines, in which the nitrogen environment is pyramidal. Alternatively, they may resemble amides, with the triazine ring being comparable to the carbonyl p bond. Arylamines, e.g. anilines, form an intermediate class. Microwave studies of the gas phase and X-ray diffraction of the solid state have shown the nitrogen environment in aniline to be non-planar [11±16]. In fact, the nitrogen is out of the plane of the phenyl ring Journal of Molecular Structure 602±603 2002) 59±70 0022-2860/02/$ - see front matter q 2002 Elsevier Science B.V. All rights reserved. PII: S0022-286001)00726-8 www.elsevier.com/locate/molstruc q Dedicated to Professor Graham A. Webb on the occasion of his 65th birthday. * Corresponding author. Tel.: 144-191-374-3121; fax: 144-191- 386-1127. E-mail address: r.k.harris@durham.ac.uk R.K. Harris).