Solvent Effects on Conical IntersectionsSolvent Effects on Conical Intersections

A Photochemical Analog of Hammond's PostulateA Photochemical Analog of Hammond's PostulateS. Cogan, S. Zilberg and Y. Haas

The Farkas Center for Light Induced Processes, Department of Physical Chemistry, The Hebrew University in Jerusalem

IntroductionIntroductionConical Intersections (CIs) role in Conical Intersections (CIs) role in Photochemical Process was recognized for Photochemical Process was recognized for quite a long timequite a long time11 as they represent a very as they represent a very fast funnels for nonradiative relaxation.fast funnels for nonradiative relaxation.

Phase Change Rule:

According to Lounget-Higgins theorem2 electronic wavefunction will change its sign along any loop enclosing CI within.

We use elementary reaction coordinates (RCs) for such loop construction. In the model the elementary reaction is defined as nuclear transformation between two different spin paring schemes, termed anchors.

There are two types of sign inverting loops:All three reactions are sign inverting: An i3 loop; Two reactions are sign preserving, one sign inverting: An ip2 loop

In general three anchors – defining three different minima on PES, are needed to construct the domain.

In some cases (when the electronic wavefunction at of the stable species is a superposition of two different pairing schemes) the loop can be formed by two reaction coordinates connecting two minima only.

An example of this special case is when one of the spin pairing schemes is zwitterionic and the other is covalent; in this case the two TS will be of quite different polarity.

ObjectivesObjectives- To investigate molecular systems in which Conical Intersection is defined by two independent reaction coordinates connecting two minima.

-To investigate the connection of the Conical Intersection to TSs.

- To check the possibility of photochemical reaction control by solvent polarity.

Model SystemsModel Systems

Computational DetailsComputational Details-CASSCF (12,11)/DZV and CASMP2 (12, 11)/DZV with full π-systems.- All Critical Points were classified by normal modes frequency analysis. - The Conical Intersection was saught along the nuclear coordinate connecting two TSs - Solvent effect on Critical Points and Conical Intersection were estimated by SCRF4 and PCM5 models.

ResultsResults

ConclusionsConclusions- Conical intersections defined by two TSs do exist.

- Conical intersection lies on coordinate that connects two TS, and its structure resembles the highest TS. This is the Hammond Postulate for photochemical reactions.

- The Conical Intersection energy can be manipulated by solvent polarity.

- The large stabilization of Zwitterionic TS can lead to disappearance of Conical Intersection, and may lead to measurable changes in the photoreaction quantum yield.

Our approach3

Figure 1: Conical Intersection

Figure 2: Reaction Coordinates span PES

Figure 3: i 3 (left) and ip2 (right) loops

Figure 4:Two RCs determined by two minima

Figure 5: Model System

Figure 6:Model System in Gas Phase

Figure 7: Model System in Cyclohexane

Figure 8:Model System in AcetonitrileThe change in TSs relative energies by interaction with solvent leads to change in CI energy and Structure in full analogy with Hammond Postulate6 (Figure 9).

Figure 9: A Hammond Postulate analog for CIs: the conical intersection is more similar to the higher lying TS.

The results of CASSCF and CASMP2 as well as SCRF and PCM calculations are in good agreement. The results of these calculations in different solvents are summarized in Figures 6-9.

In our case the strong interaction with a very polar solvent leads to elimination of Biradical TS and therefore of the CI (Figure 10)

This research was supported by The Israel Science Foundation founded by The Israel Academy of Sciences and Humanities . The Farkas Center for Light Induced Processes is supported by the Minerva Gesellschaft mbH.

Acknowledgments

References1. Teller, E., J Phys Chem 1937, 4, 109

2. Longuet-Higgins, H. C. , Proc Roy Soc London A,1975, 344, 147

3. Haas, Y., Cogan, S., Zilberg, S., Int. J. Quant. Chem. (in press)

4. Onsager, L., J. Am. Chem. Soc., 1936, 58, 1486

5. Miertus, S., Scrocco, E., Tomasi, J., Chem. Phys., 1981, 55, 117

6. Hammond, G. S., J. Am. Chem. Soc, 1955, 77,334