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Breakdown of the first hyperpolarizability/bond-length alternation parameter relationship.

ABSTRACT: We have investigated the dependence of the static first hyperpolarizability on the bond-length alternation (BLA) parameter. Our analysis indicates that the validity of the first hyperpolarizability/BLA parameter relationship is restricted to the no-field, vacuum, limit, while it successively breaks down along with increasing polarity of a surrounding medium, becoming invalid, for instance, in an aqueous solution. This contention is based on a series of TD-DFT, TD-DFT/PCM and hybrid TD-DFT/MM calculations of the first hyperpolarizability for a set of molecular configurations generated from Car-Parrinello hybrid QM/MM simulations of the stilbazolium merocyanine chromophore in chloroform and water solvents, and on a rationalization by means of the two-state model for the first hyperpolarizability. The BLA dependence of the three parameters entering the two-state model is shown to be qualitatively different in vacuum and in solvents. Particularly, in the vacuum case, the difference between ground and excited state dipole moments goes to zero for a vanishing BLA, which is not true in the presence of an aqueous medium. In the aqueous medium, an opposing behavior of the parameters involved in the two-state model results in an almost constant first hyperpolarizability with varying BLA parameter.

SUBMITTER: Murugan NA

PROVIDER: S-EPMC2944713 | biostudies-literature | 2010 Sep

REPOSITORIES: biostudies-literature

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Breakdown of the first hyperpolarizability/bond-length alternation parameter relationship.

Murugan N Arul NA Kongsted Jacob J Rinkevicius Zilvinas Z Agren Hans H

Proceedings of the National Academy of Sciences of the United States of America 20100907 38

We have investigated the dependence of the static first hyperpolarizability on the bond-length alternation (BLA) parameter. Our analysis indicates that the validity of the first hyperpolarizability/BLA parameter relationship is restricted to the no-field, vacuum, limit, while it successively breaks down along with increasing polarity of a surrounding medium, becoming invalid, for instance, in an aqueous solution. This contention is based on a series of TD-DFT, TD-DFT/PCM and hybrid TD-DFT/MM cal ...[more]

PMID: 20823263

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Project description:Quantum chemical calculations were carried out for deprotonated (P-) and protonated purine (PH+) and for adducts with one alkali metal cation (P-M+ and PM+, where M+ is Li+ or Na+) in the gas phase {B3LYP/6-311+G(d,p)}, a model of perfectly apolar environment, and for selected structures in aqueous solution {PCM(water)//B3LYP/6-311+G(d,p)}, a reference polar medium for biological studies. All potential isomers of purine derivatives were considered, the favored structures indicated, and the preferred sites for protonation/deprotonation and cationization reactions determined. Proton and metal cation basicities of purine in the gas phase were discussed and compared with those of imidazole and pyrimidine. Bond-length alternations in the P, PH+, P-M+, and PM+ forms were quantitatively measured using the harmonic oscillator model of electron delocalization (HOMED) indices and compared with those for P. Variations of the HOMED values when proceeding from the purine structural building blocks, pyrimidine and imidazole, to the bicyclic purine system were also examined. Generally, the isolated NH isomers exhibit a strongly delocalized ?-system (HOMED >?0.8). Deprotonation slightly increases the HOMED values, whereas protonation and cationization change the HOMED indices in different way. For bidentate M+-adducts, the HOMED values are larger than 0.9 like for the largely delocalized P-. The HOMED values correlate well in a comprehensive relationship with the relative Gibbs energies (?G) calculated for individual isomers whatever the purine form is, neutral, protonated, or cationized. When PCM-DFT model was utilized for P-, PH+, PM+, and P-M+ (M+?=?Li+) both electron delocalization and relative stability are different from those for the molecules in vacuo. The solvation effects cause a slight increase in HOMEDs, whereas the ?Es decrease, but in different ways. Hence, contribution of particular isomers in the isomeric mixtures of PH+, PM+, and P-M+ also varies. HOMED variations for the favored neutral, deprotonated, protonated, and lithiated forms of purine in the gas phase and aqueous solution.

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Breakdown of the first hyperpolarizability/bond-length alternation parameter relationship.

Publications

Breakdown of the first hyperpolarizability/bond-length alternation parameter relationship.

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