Electron-Transfer-Induced Tautomerization in Methylindanones:  Electronic Control of the Tunneling Rate for Enolization

Abstract

The radical cations generated from 4-methyl- and 4,7-dimethylindanone, as well as their deuterated isotopomers, isolated in Argon matrices, were found to undergo enolization to the corresponding enol radical cations at rates that differ by orders of magnitude. It is shown by quantum chemical calculations that the effect of the remote methyl group in the 4-position is of purely electronic nature in that it stabilizes the unreactive pi-radical relative to the reactive sigma-radical state of the 7-methylindanone radical cation. The observed kinetic behavior of the two compounds can be reproduced satisfactorily on the basis of calculated height and width of the thermal barrier for enolization, using the Bell model for quantum mechanical tunneling. High-level calculations on the methylacrolein radical cation show that barriers for enolization in radical cations are overestimated by B3LYP/6-31G.

Tunneling Lifetime of the ttc/VIp Conformer of Glycine in Low-Temperature Matrices

“…One of the most suitable experimental methods to study conformational conversions by hydrogen-atom tunneling is matrix isolation (MI) spectroscopy. Although hydrogen-atom tunneling in many small organic molecules has been studied by this method, − very few recent works investigated biomolecules; these included cytosine, NADH mimicking model compounds, and in a preliminary study by our group, glycine . It is, however, now widely accepted in biochemistry that enzyme-catalyzed hydrogen-transfer reactions cannot be fully understood and modeled without taking into account quantum tunneling effects. , MI studies can thus add valuable information of understanding tunneling in biomolecules and probably in even larger biochemical systems.…”

Photochemistry of Reactive Intermediates

“…Thus, the difference in reactivity of these two very similar compounds are a result of the stabilization of the unreactive -radical state by the additional methyl group [28].…”

“…In this way we have studied several photoinduced ring-opening reactions of radical cations containing three- [17][18][19][20][21] or four-membered rings [22][23][24][25], phototautomerizations of ionized ketones and enols [26][27][28], and rotamer inter-conversions in polyene radical cations [29][30][31]. I will use two examples to illustrate the photochemistry of radical cations:…”

Matrix Isolation

“…Nevertheless, if applied with due circumspection, CASPT2 is a very reliable method and has been applied succesfully to the modeling of electronic spectra of a wide variety of compounds, including many matrix-isolated reactive intermediates in the author's laboratory. 8,[134][135][136][137][138][139][140][141][142] An entirely different approach to ab initio calculations of excited states is to look at the response of the ground-state electron distribution to oscillating electric fields of different frequencies. If the frequency of such an oscillation is in resonance with an electronic excitation, then the polarizability will increase discontinuously, that is, the frequency-dependend polarizability will show a pole whose intensity is proportional to the electronic transition moment.…”