Knowledge Bank

As the smallest organic molecule exhibiting a symmetric hydron migration process mediated by an intramolecular hydrogen bond, malonaldehyde $(HO-CH=CH-CH=O)$ provides a model system for probing the influence of nuclear and electronic degrees of freedom upon unimolecular dynamics. The vibrationless level of the ground electronic potential surface $(X~1A1)$ has been the subject of numerous experimental efforts; however, proton transfer and hydrogen bonding in excited vibrational and electronic states have not been explored extensively. By exploiting polarization-selective Degenerate Four-Wave Mixing (DFWM) spectroscopy to extract information that otherwise would be hidden in the congested room temperature spectrum of bulk (gas-phase) malonaldehyde, this presentation will discuss new evidence for the dramatic changes in hydron dynamics that accompany electronic excitation into the lowest-lying singlet manifold $(A~1B1)$. More specifically, observed $A~1B1$ tunneling splittings are consistent with a decrease in proton-transfer efficiency upon $\pi \ast \leftarrow n$ electron promotion while excitation of O...O stretching motion within the $X~1A1$ potential surface is found to increase the rate of proton transfer substantially.