A study of AB initio computational techniques and their application to cis-trans photoisomerization of retinal analogs

Yadav, Arpita (1989) A study of AB initio computational techniques and their application to cis-trans photoisomerization of retinal analogs. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

Ab initio Hartree Fock self consistent field calculations are performed on retinal analogs using the STO-3G basis set. Complete geometry optimizations are performed. The ground state properties (bond lengths, bond orders, net atomic charge distribution) of various cis and trans isomers of retinal analogs are studied in detail. Where necessary correlational effects are also taken into account by a simple model of strictly localized geminals. The effect of protonation on the properties of retinal Schiff base analogs is studied Convergence in various properties is studied with increasing chain length justifying the use of smaller analogs mimicking retinal. Convergence, is however slower for retinal protonated Schiff base analogs. The retinal protonated Schiff base analogs show an increased conjugation in the vicinity of the NH₂⁺ group. The cyclohexene ring is shown to have little effect on bonding and other properties of the molecule, but causes local torsional distortions. Methyl groups also cause torsional distortions as they introduce steric hindrance. The potential energy surfaces for conformational change around the 6-s-bond and 12-s-bond are studied in detail. The introduction of a methyl group at C13 leads to a skewed geometry around C12-C13 and C10-C11 single bonds. The results indicate that for retinal PSB the preferred conformation is planar 11-cis, 12-s-trans as compared to 11-cis, skewed 12-s-cis in retinal. The introduction of methyl groups on the cyclohexene ring leads to two stable skewed 6-s-cis conformations, one at 58.5ﾟ above the plane and the other at 65.0ﾟ below the plane, plane being defined by C1-C6-C5-C4. Planar 6-s-trans conformation is predicted to be a transition state for the isolated chromophore. -- After a complete study of ground state properties, geometry relaxation studies are performed in the lowest lying triplet (³B) excited state utilizing RHF and CHF methods. Since the UHF results suffer from significant spin contamination (even with the split valence 3-21G basis set), only the RHF results are considered. The RHF results on the other hand, lead into the methodological difficulties concerning the intrinsic RHF instabilities. The ³B excited state results are also discussed from the solid state physics point of view. The results suggest highly delocalized spin density in the vicinity of the NH₂⁺ group. π → π∙ excitation energies are calculated using 'Singlet-triplet Approximation' and all singles doubles configuration interaction (SDCI) calculations, π → π∙ excitation energies are studied as a function of chain length and rotation around the single bond. A red shift in the lowest π → π∙ excitation energy is obtained on going from planar trans to planar cis conformation around a single bond or a double bond. Twisting around single bond leads to blue shift in accordance with the torsion model. SDCI calculations predict the lowest lying excited state to be ¹Ag-like corresponding to the forbidden transition from the ground state. -- Finally, based on ground and excited state results mechanisms for photocyclcs of rhodopsin, isorhodopsin and bacteriorhodopsin are discussed. Structural information on certain intermediates (bathorhodopsin, lumirhodopsin, metarhodopsin) is presented.

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