The Tunnelling Mechanism in Proton Transfer Reactions

A calculation of proton tunnelling frequencies in the following reactions shows the importance of symmerty for the effectiveness of tunnelling  

CH30H2 + HOCH3 CH3OH + H20CH3 H20-H20H + OH2-OH2 H20-H20 + H0H20H2 

In the calculations of the PES in the first two reactions, the symmetry of the potential was upset by rotating the methyl group in the symmetrical H-complex XVI(s) giving thus rise to an asymmetrical H-complex XVI(A)  

A similar effect of the symmetry of the double-well potential is predicted by calculations on the systems with an intramolecular hydrogen bonding. Thus, in the case of 3-oxy-2-methyl-2-propenal XVII, a proton tunnelling with the frequency of 7 x 10 s is expected only for the conformation XVII(s) of the transition state, while a 90° rotation of the 2-methyl group totally prevents the operation of this transfer mechanism [75]  

An interesting example of a combined experimental and theoretical study of the influence the symmetry has on the mechanism of proton transfer is provided by the system of naphthazarin XVIII. According to the data of the 

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