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Intramolecular Proton Tunneling in 9-Hydroxyphenalen-1-One, Methyl Substituted Naphthazarins, and Other Systems

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Description: Ab initio SCF calculation for the interconversion of the two tautomers in 2,6- and 2,7- dimethylnaphthazarins indicate the presence of two asymmetric hydrogen bonds with symmetric double minimum potentials as has been indicated for naphthazarin. The rate of exchange of 26 to 25 MHz is consistent with the ('1)H-NMR spectra of these compounds. Similar calculations in monomethylnaphthazarin indicated a slightly asymmetric double minimum potential. The interconversion between the two tautomers is not detected by ('1)H-NMR. This provides additional support to the claim that a small departure from the symmetry in a double minimum potential with high barrier eliminates tunneling. Rates of exchange in 9-hydroxyphenalen-1-one have been reported to be 127 (+OR-) 10 cm('-1) for the normal and 9 (+OR-) 10 cm('-1) for the deuterated species. Destruction of the symmetry of the profile by the addition of a methyl group producing the 2- and 8- methyl derivatives has been reported not to eliminate tunneling. The ab initio SCF calculations for the proton exchange in 9-hydroxyphenalen-1-one indicate a barrier of 5.25 kcal/mol. With a direct transfer path between the initial and final structures, the interminimal distance is 0.64 Angstroms. The calculated rate of exchange is in agreement with the experimental value when the effective mass used is 1.0 amu. The addition of the methyl substituent introduces an energy difference between the two tautomers of 0.34 kcal/mol. The low barrier and small interminimal distance explain the result that tunneling is not eliminated in this system.
Language: English
Format: Degree Work