Excited State Intramolecular Proton Transfer in Anionic Analogues of Malonaldehyde

Abstract

The transfer of a proton in malonaldehyde takes place within an intramolecular H-bond involving a five-membered ring. This process is compared via ab initio methods with the transfer in analogous systems in which the size of the ring is altered to four and to six and in which the system bears an overall negative charge. In addition to the ground state, calculations are applied to the singlet and triplet ππ* states, as well as to 1nπ* and 3nπ*. The barriers to proton transfer are found to correlate strongly with various geometric and energetic markers of the strength of the H-bond. The H-bond is weakened by n → π* excitation, particularly for the neutral molecule, resulting in a higher transfer barrier. In the case of the two anions, excitation to 3ππ* strengthens the H-bond, while the result is more ambiguous for the 1ππ* state. This trend is reversed in malonaldehyde where the singlet is strengthened by the excitation and the triplet weakened. Some of these patterns are traced directly to the nature of the pertinent orbitals and the density shifts arising from the excitation.

Strength of Si–H ⋯ B charge-inverted hydrogen bonds in 1-silacyclopent-2-enes and 1-silacyclohex-2-enes

“…Nevertheless, in spite of this, some additive schemes leading to numbers understood as interaction energies have been proposed. Among them, the so-called open-closed method [45,46] seems to be both the most popular [26][27][28][29][45][46][47][48][49][50][51][52][53][54][55][56][57][58] and straightforward. According to this method, the interaction energy is just a difference between total energies of the closed form of a molecule, i.e.…”

Photophysical properties of substituted intramolecularly hydrogen bonded compounds: A combined experimental and theoretical study

“…This is indeed a common feature to construct the PES of large and complicated systems as mentioned elsewhere [43]. Although it is well known that the CIS usually overestimates the energy barrier [44], this method has been employed because it was demonstrated that it provides a qualitative correct characterization of the ESIPT process [31]. …”

Proton transfer reactions in solution

“…Experimentally 1–11, the proton transfer reaction has been studied on ultrafast time scale both in gas phase and condensed phase. Theoretically, only semiempirical calculations 11 have been reported for large photochemical molecules while ab initio calculations 3–12 are limited to relatively small often model systems. Both the theoretical and the experimental research of the photoinduced tautomerization are dealing with the explanation of the reaction mechanism of proton transfer.…”

“…The ring of the type OCCCO can either be symmetric or asymmetric. Molecules exhibiting symmetric intrinsic intramolecular PT in an OCCCO ring are, for example, the enol‐keto tautomerization of malonaldehyde 3, 4 and the anionic analogs of malonaldehyde 4. Scheiner et al 3, 4 obtained the reaction path for enol‐keto tautomerization for the ground state at the Hartree–Fock level, whereas the potential energy surface for the excited state was generated by the configuration interaction (CI) singles method (CIS).…”