The short-chain acroleiniminium and pentadieniminium cations: towards a model for retinal photoisomerization. A CASSCF/PT2 study

In this communication we report the results obtained in a computational stu dy Of the Minimum Energy Paths (MEP) found in the first excited state S-1 a nd in the ground state S-0 of two short-chain protonated Schiff bases (PSB) : the s-cis 1-iminium-2-propene cation H2C=CH-CH=NH2 + and the tZt 1-iminiu m-2,4-pentadiene cation H2C=CH-CH=CH-CH=NH2 +. This computational study has been performed at an high ab-initio level where the geometries of the rele vant points have been optimized at the CAS-SCF level and the energetics hav e been refined via single-point computations at the CAS-PT2 level. This com munication provides the important information that the photochemistry of th e two studied PSB is driven by the spectroscopic 1B ionic state which remai ns the lowest excited state along all the optimized MEP. Both PSB show a S- 1/S-0 Conical Intersection which is reached through a low barrier (barrierl ess) relaxation path for the shorter (longer) system and has an almost 90 d egrees twisted double bond (the CH2=CH- double bond fbr the shorter and the central double bond for the longer PSB) which provides a route for fully e fficient non-adiabatic cis- > trans isomerization; In both PSB the crossing involves also a charge transfer between the two twisted fragments and the isomerization reaction coordinates on SI are dominated by a stretching plan ar mode in the initial part of the MEP. (C) 1999 Published by Elsevier Scie nce B.V. All rights reserved.