Ab initio molecular dynamics simulation of the UV absorption spectrum of b-ionone Christophe Raynaud, Romuald Poteau, Laurent Maron * , Franck Jolibois Laboratoire de Physique Quantique (UMR 5626 du CNRS), IRSAMC, Universite ´ Paul Sabatier, 118 route de Narbonne, 31062 Toulouse Cedex, France Available online 10 May 2006 Abstract Ab initio molecular dynamics have been performed at the ONIOM (B3LYP/3-21G(d):AM1) level of calculation in order to study the link between the exploration of interconversion pathways of the b-ionone compound and its UV absorption spectrum. Absorption spectra were obtained by TDDFT (B3LYP/6-31CG*) calculations which account for bulk solvent effects performed on geometries picked on different molecular dynamic trajectories. We show that, beyond the usual theoretical interpretation of UV spectra in terms of vertical excitation energies and associated oscillator strengths obtained on a single geometry, the coupling of various strategies (i.e. ab initio molecular dynamics, hybrid QM/MM methods, solvent described with polarizable continuum models and TDDFT) is now easily feasible on large molecules and provides theoretical absorption spectra which take into account dynamical effects. In that context, our code based on ab initio molecular dynamics using Gaussian-type orbitals, is able to integrate several theoretical methods. q 2006 Elsevier B.V. All rights reserved. Keywords: TDDFT; Molecular dynamics; Ionone 1. Introduction Ionone is used in perfumery, flavoring, and vitamin A (retinol) production for cosmetics and toiletries, the b isomer showing a less floral odor than the a. b-ionone shares with retinal some structural and bonding features and can be considered as a relevant model of retinal for the purpose of evaluating dynamical processes on such systems. a-ionone (4-(2,6,6-trimethyl-2-cyclohexen-1-yl)-3-buten-2-one) and b-ionone (4-(2,6,6-trimethyl-1-cyclohexen-1-yl)-3-buten-2- one) exhibit very different UV absorption spectra (Fig. 1), with a single band for a-ionone at l max Z227 nm (e max Z 14,250 L mol K1 cm K1 ) and two bands for b-ionone at l max Z 219 and 295 nm (e max Z5750, and 9450 L mol K1 cm K1 , respectively). This can be related to different chromophores, since a-ionone has two chromophores, i.e. ethylene-like (CaC) and enone (CaC–CaO), whereas in b-ionone the conjugation of the two p systems leads to a single dienone chromophore (CaC–CaC–CaO). By the way, Woodward-Fieser rules provide theoretical l max which, among several effects, take into account the extent of conjugation in b-ionone. Application of these rules yields l max Z227 and 299 nm for a- and b-ionone, respectively, thus being unable to explain the high- energy band of the second compound. Our goal is firstly to assign the experimental bands by a theoretical study of the lowest excited states of ionone, and secondly to estimate the modification of the theoretical absorp- tion spectrum when taking into account the effect of temperature on the motion of nuclei. As a matter of fact, b-ionone can undergo three types of conformational inter- conversions: rotation around the diene single bond; rotation around the enone single bond; cyclohexene ring inversion. According to 1 H NOE NMR, the s-cis diene conformation of b-ionone in solution is dominant, but this compound probably also exists in its s-trans form (Fig. 2) [1]. It is also interesting to mention that the isolation of the s-trans and the s-cis confor- mers as inclusion complexes with different hosts compounds has been reported [2]. We shall see in the following that several stationary points (isomers, stereoisomers and the corre- sponding saddle points) can be identified on the potential energy surface (PES), and b-ionone can thus be considered as a structurally versatile compound, considering the rather small barrier heights between isomers. Accordingly, a coupled ‘excited states calculation/ab initio molecular dynamics’ approach on such systems is certainly desirable. This was already suggested in Ref. [3] in the case of Schiff bases of b-ionone, but to our knowledge it has not been accomplished. We have used the following methods: (i) density functional theory (DFT) for calculating the electronic ground state [4], Journal of Molecular Structure: THEOCHEM 771 (2006) 43–50 www.elsevier.com/locate/theochem 0166-1280/$ - see front matter q 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.theochem.2006.03.038 * Corresponding author. E-mail address: laurent.maron@irsamc.ups-tlse.fr (L. Maron).