Raúl Palmeiro

Organic Thermochemistry at High ab Initio Levels. 1. A G2(MP2) and G2 Study of Cyclic Saturated and Unsaturated Hydrocarbons (Including Aromatics)

With the purpose of exploring the reliability of the enthalpies of formation calculated using G2 methods, we have examined a series of saturated and unsaturated alicyclic hydrocarbons varying the size and the number of formal double bonds in the molecule. Heats of formation have been calculated at the G2(MP2) and G2 levels through both atomization reactions and bond separation isodesmic reactions, and comparison with experimental values has been made. A linear relationship between the differences between experimental and calculated (from atomization reactions) heats of formation and the number of formal double bonds is obtained.

Potential energy surface of cyclooctatetraene

We present a theoretical study of the cyclooctatetraene (COT) molecule. Seven COT structures are located on the singlet ground state potential energy surface. Four of them, which present D2d (tub), Cs (bicyclo[4.2.0]octa-2,4,7-triene or BOT), C2h (chair) and D4 (crown) symmetries are stable species, and the other three are transition state structures showing Cs, D4h, and D8h symmetry. We discuss the symmetry of wave functions for these stationary points. Geometries, energies, and harmonic vibrational frequencies of these structures, and energy gaps between singlet–triplet states and low-lying singlets are presented. For the planar D4h and D8h structures, Jahn–Teller and tunneling effects have also been discussed. Ring inversion, bond shifting and valence isomerization reactive channels from the tub COT conformer are discussed from the point of view of the corresponding transition state structures. Where possible, in order to lend support to this theoretical information comparisons with recent transition s...

Role of bifurcation in the bond shifting of cyclooctatetraene

The present study of the cyclooctatetraene potential energy surface shows the presence of a bifurcation (valley ridge inflection point) in the intrinsic reaction coordinate path between the two transition states of D8h and D4h symmetries. This result is of capital importance for the correct understanding of the bond shifting and ring inversion processes in this compound.

Optomechanical Control of Quantum Yield in Trans-Cis Ultrafast Photoisomerization of a Retinal Chromophore Model

The quantum yield of a photochemical reaction is one of the most fundamental quantities in photochemistry, as it measures the efficiency of the transduction of light energy into chemical energy. Nature has evolved photoreceptors in which the reactivity of a chromophore is enhanced by its molecular environment to achieve high quantum yields. The retinal chromophore sterically constrained inside rhodopsin proteins represents an outstanding example of such a control. In a more general framework, mechanical forces acting on a molecular system can strongly modify its reactivity. Herein, we show that the exertion of tensile forces on a simplified retinal chromophore model provokes a substantial and regular increase in the trans-to-cis photoisomerization quantum yield in a counterintuitive way, as these extension forces facilitate the formation of the more compressed cis photoisomer. A rationale for the mechanochemical effect on this photoisomerization mechanism is also proposed.

Mechanical Forces Alter Conical Intersections Topology.

Photoreactivity can be influenced by mechanical forces acting over a reacting chromophore. Nevertheless, the specific effect of the external forces in the photoreaction mechanism remains essentially unknown. Conical intersections are key structures in photochemistry, as they constitute the funnels connecting excited and ground states. These crossing points are well known to provide valuable information on molecular photoreactivity, including crucial aspects as potential photoproducts which may be predicted by just inspection of the branching plane vectors. Here, we outline a general framework for understanding the effect of mechanical forces on conical intersections and their implications on photoreactivity. Benzene S1/S0 conical intersection topology can be dramatically altered by applying less than 1 nN force, making the peaked pattern of the intersection become a sloped one, also provoking the transition state in the excited state to disappear. Both effects can be related to an increase in the photostability as the conical intersection becomes more accessible, and its topology in this case favors the recovery of the initial reactant. The results indicate that the presence of external forces acting over a chromophore have to be considered as a potential method for photochemical reactivity control.