A molecular dynamics study of intramolecular proton transfer reaction of malonaldehyde in solution based upon a mixed quantum–classical approximation. II. Proton transfer reaction in non-polar solvent

Solvent Control of Chemical Identity Can Change Photodissociation into Photoisomerization

In solution-phase chemistry, the solvent is often considered to be merely a medium that allows reacting solutes to encounter each other. In this work, however, we show that moderate locally specific solute-solvent interactions can affect not only the nature of the solute but also the types of reactive chemistry. We use quantum simulation methods to explore how solvent participation in solute chemical identity alters reactions involving the breaking of chemical bonds. In particular, we explore the photoexcitation dynamics of Na₂⁺ dissolved in liquid tetrahydrofuran. In the gas phase, excitation of Na₂⁺ directly leads to dissociation, but in solution, photoexcitation leads to an isomerization reaction involving rearrangement of the first-shell solvent molecules; this isomerization must go to completion before the solute can dissociate. Despite the complexity, the solution-phase reaction dynamics can be captured by a two-dimensional energy surface where one dimension involves only the isomerization of the first-shell solvent molecules.

Asymmetric substitution changes the UV-induced nonradiative decay pathway and the spectra behaviors of β-diketones

Asymmetric substitution has not been termed as an essential factor in studying photo-induced ultrafast dynamics of molecular system. Asymmetric 4-hydroxybut-3-en-2-one (HEO), together with symmetric malonaldehyde (MA) and acetylacetone (AA), have been provided as target sample to study the nonradiative decay (ND) processes of β-diketones. An effective ND pathway of the three molecules is presented that their excited second (S₂) states transfer to first (S₁) state by nonadiabatic surface hopping, and then transfer to triplet (T₁) state by crossing minimum energy crossing point (MECP), after which decay to ground (S₀) state through MECP. More importantly, the asymmetric substitution of HEO induces the proton transfer in the S₁ state and generates a proton-transferred conformer with lowest energy, which does not occur for MA and AA. This change exploits a new ND pathway that the S₁ state decays to the proton transferred T₁ state and then undergoes reverse proton transfer to S₀ state through the MECPs between the three states. The two pathways of HEO give detailed energy and geometric information on surface hopping of S₂/S₁ and MECPs of S₁/T₁/S₀, and interpret the reason of the ND pathway while not spectra emission. This result is significantly different from the previous reported ND pathway of photoisomerization or conical intersection between different states. This work shows that asymmetric substitution changes the molecular structure and then changes their spectra behaviors.