Biomedical subjects

Luis Manuel Frutos

Publications and source records attributed to Luis Manuel Frutos.

5 recordsLinked to original sources

PubMed2005

Correlated MO study of the low-barrier intramolecular motions in donor-acceptor ethenes.

Correlated MP2 and MCSCF MO calculations of several model push-pull ethenes in most cases indicate no great participation of excited singlet and triplet electronic configurations in either the minima or the transition structures for the suggested facilitated intramolecular rotation about the polarized C=C bond. This situation changes significantly only in molecules with sulfur atoms in the molecule as either donors or acceptors. The outstanding contribution of sulfur atoms as either donors or acceptors is a significant increase of push-pull ethene molecular polarizabilities. Thus, within the studied small series of mostly planar push-pull ethenes, polarizability appears a better indicator of rapid intramolecular motions about the C=C bond than straight polarity. Substituents with larger steric demands around the C=C bond are shown to likely preclude its complete turnaround, thus prompting a ramification of the interpretations of dynamic NMR phenomena in sterically constrained push-pull ethenes as large-amplitude librations resulting from strong rovibrational and relatively weak electronic coupling. These librations, as shown by complete vibrational mode analysis of corresponding rotational transition structures, cover in fact certain sectors of the intuitively suggested full rotations similar to those about C-C single bonds.

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PubMed2005

A new algorithm for predicting triplet-triplet energy-transfer activated complex coordinate in terms of accurate potential-energy surfaces.

The new algorithm presented here allows, for the first time, the determination of the optimal geometrical distortions that an acceptor molecule in the triplet-triplet energy-transfer process undergoes, as well as the dependence of the activation energy of the process on the triplet energy difference of donor and acceptor molecules. This algorithm makes use of the complete potential-energy surfaces (singlet and triplet states), and contrasts with the first-order approximation already published [L. M. Frutos, O. Castano, J. L. Andres, M. Merchan, and A. U. Acuna, J. Chem. Phys. 120, 1208 (2004)] in which an expansion of the potential-energy surfaces was used. This algorithm is gradient based and finds the best trajectory for the acceptor molecule, starting from S(0) ground-state equilibrium geometry, to achieve the maximum variation of the singlet-triplet energy gap with the minimum energy of activation on S(0). Therefore, the algorithm allows the determination of a "reaction path" for the triplet-triplet energy-transfer processes. Also, the algorithm could also serve eventually to find minimum-energy crossing (singlet-triplet) points on the potential-energy surface, which can play an important role in the intersystem crossing process for the acceptor molecules to recover their initial capacity as acceptors. Also addressed is the misleading use of minimum-energy paths in T(1) to describe the energy-transfer process by comparing these results with those obtained using the new algorithm. The implementation of the algorithm is illustrated with different potential-energy surface models and it is discussed in the frame of nonvertical behavior.

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PubMed2004

A theory of nonvertical triplet energy transfer in terms of accurate potential energy surfaces: the transfer reaction from pi,pi* triplet donors to 1,3,5,7-cyclooctatetraene.

Triplet energy transfer (TET) from aromatic donors to 1,3,5,7-cyclooctatetraene (COT) is an extreme case of "nonvertical" behavior, where the transfer rate for low-energy donors is considerably faster than that predicted for a thermally activated (Arrhenius) process. To explain the anomalous TET of COT and other molecules, a new theoretical model based on transition state theory for nonadiabatic processes is proposed here, which makes use of the adiabatic potential energy surfaces (PES) of reactants and products, as computed from high-level quantum mechanical methods, and a nonadiabatic transfer rate constant. It is shown that the rate of transfer depends on a geometrical distortion parameter gamma=(2g(2)/kappa(1))(1/2) in which g stands for the norm of the energy gradient in the PES of the acceptor triplet state and kappa(1) is a combination of vibrational force constants of the ground-state acceptor in the gradient direction. The application of the model to existing experimental data for the triplet energy transfer reaction to COT from a series of pi,pi(*) triplet donors, provides a detailed interpretation of the parameters that determine the transfer rate constant. In addition, the model shows that the observed decrease of the acceptor electronic excitation energy is due to thermal activation of C=C bond stretchings and C-C bond torsions, which collectively change the ground-state COT bent conformation (D(2d)) toward a planar triplet state (D(8h)).

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PubMed2002

Cyclooctatetraene computational photo- and thermal chemistry: a reactivity model for conjugated hydrocarbons.

We use ab initio CASSCF and CASPT2 computations to construct the composite multistate relaxation path relevant to cycloocta-1,3,5,7-tetraene singlet photochemistry. The results show that an efficient population of the dark excited state (S(1)) takes place after ultrafast decay from the spectroscopic excited state (S(2)). A planar D(8)(h)-symmetric minimum represents the collecting point on S(1). Nonadiabatic transitions to S(0) appear to be controlled by two different tetraradical-type conical intersections, which are directly accessible from the S(1) minimum following specific excited-state reaction paths. The higher-energy conical intersection belongs to the same type of intersections previously documented in linear and cyclic conjugated hydrocarbons and features a triangular -(CH)(3)- kink. This point mediates both cis --> trans photoisomerization and cyclopropanation reactions. The lowest energy conical intersection has a boat-shaped structure. This intersection accounts for production of semibullvalene or for double-bond shifting. The mapping of both photochemical and thermal reaction paths (including also Cope rearrangements, valence isomerizations, ring inversions, and double-bond shifting) has allowed us to draw a comprehensive reactivity scheme for cyclooctatetraene, which rationalizes the experimental observations and documents the complex network of photochemical and thermal reaction path interconnections. The factors controlling the selection and accessibility of a number of conjugated hydrocarbon prototype conical intersections and ground-state relaxation channels are discussed.

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PubMed2002

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 D(8h) and D(4h) symmetries. This result is of capital importance for the correct understanding of the bond shifting and ring inversion processes in this compound.

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