Theoretical study of excited state proton transfer in 3,6-bis(benzoxazolyl)pyrocatechol (BBPC)

  1. Get@NRC: Theoretical study of excited state proton transfer in 3,6-bis(benzoxazolyl)pyrocatechol (BBPC) (Opens in a new window)
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Journal titleThe Journal Of Chemical Physics
Pages74867494; # of pages: 9
Subjectab initio calculations; chemical exchanges; configuration interactions; density functional theory; excited states; fluorescence; isomerisation; isotope effects; organic compounds
AbstractAb initio density functional theory and approximate instanton methods are used to study proton transfer processes in the first excited electronic state of 3,6-bis(benzoxazolyl)pyrocatechol (BBPC). Geometries of di-enol, keto-enol, and di-ketone tautomers as well as transition states for single and double proton transfer processes and the corresponding force fields are obtained with the CIS/6-31G* method and verified with CISD/6-31G* single point calculations. It is shown that keto-enol tautomer is the most stable in the S1 state while the least stable is di-ketone. The single proton transfer in the 2A1 state of di-enol leads to a somewhat more stable keto-enol tautomer. This result nicely reproduces the experimental assignment stating that BBPC, a symmetric molecule with two equivalent proton transfer reaction sites, undergoes a single proton transfer in the S1 state. The excited system has to overcome the barrier of about 9 kcal/mol and proton transfer is therefore dominated by tunneling. Dynamics calculations with the instanton method yield the rate of transfer of 9.8�1010 s�1, again in a very good agreement with the experimental value of kPT = (5.1�0.4)�1010 s�1 [Chem. Phys. Lett. 169, 450 (1990)]. Theory predicts a large kinetic isotope effect on this process. It is also shown that the reverse proton transfer leading back to di-enol has the rate strongly dependent on the stabilization energy of keto-enol. It effectively competes with the radiative decay of the latter, resulting in the observed weak di-enol fluorescence of BBPC. Finally, the calculations demonstrate why BBPC is not a photochrome unlike many typical Schiff bases.
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AffiliationNational Research Council Canada; NRC Steacie Institute for Molecular Sciences
Peer reviewedNo
NPARC number12338465
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Record identifier3887c3f6-7fce-4f65-b71f-bb1edb08ab53
Record created2009-09-10
Record modified2016-05-09
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