Intramolecular reactions hydrogen tunneling

The triplet excited state of deuterated and non-deuterated tetralone (370), and the triplet 1,4-biradicals formed by intramolecular hydrogen abstraction have been studied by flash photolysis at different temperatures. The authors propose that the rate of hydrogen transfer is largely governed by tunnelling effects since they observe a very large isotope effect as well as curved Arrhenius plots for the rate constant for the hydrogen abstraction reaction of the triplet excited state. 

In summary, both experiment and calculations demonstrate low barriers for tautomerization in porphycene as compared to porphyrin. This may explain the tunneling effects readily observed for 1 and its derivatives, as well as the change in the reaction mechanism, from stepwise in 2 towards synchronous in 1. Moreover, the cis structures, postulated, but never observed for porphyrin, may be present in porphycenes, their population increasing with the strength of the intramolecular hydrogen bond, i.e., with the decrease in the distance between hydrogen-bonded nitrogen atoms. 

Corrections for H-atom tunnelling are applied for the intramolecular hydrogen atom transfer reactions of the transition state structures using Wigner 2" order correction [198]. In this study the rate constants of three of our calculated transition states, TSPH OOH and TSC C4DO and TSCDCC DO identified in Table 6.3, are corrected for H-atom tunnelling. 

The previous studies of concerted hydrogen atom and proton transfer in hydrogen-bonded systems have been limited to studies of reaction pathways for simple model systems [4-8] with simple, reduced dimensionality methods for including quantum tunneling [13,14,34,35]. The applicability of modem computational methods to such systems is exemplified by more recent studies of the energetics of intramolecular hydrogen atom transfer in molecules such as malonaldehyde [36-39] and models of... 

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