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Electronic Determinants of Photoacidity in Cyanonaphthols Noam Agmon,*, Wolfgang Rettig, and Christian Groth
 

Summary: Electronic Determinants of Photoacidity in Cyanonaphthols
Noam Agmon,*, Wolfgang Rettig, and Christian Groth
Contribution from the Department of Physical Chemistry, The Hebrew UniVersity,
Jerusalem 91904, Israel, and the Institute of Chemistry, Humboldt-UniVersity of Berlin,
Bunsenstrasse 1, D-10117 Berlin, Germany
Received November 6, 2000
Abstract: We present semiempirical AM1 calculations for the ground and excited state of 2-naphthol and
some of its cyano derivatives in the gas phase. Following photoexcitation, the Mulliken electron density on
the oxygen diminishes slightly for the acid and more conspicuously for the anionic conjugated base. This
agrees with the measured solvatochromic parameters for 2-naphthol. In both electronic states, we find a
nice correlation with the measured pK values in water. The electronic charge distribution on the distal ring
of the anion agrees with the experimental acidity order in both S0 and S1. Upon excitation, it increases
predominantly in positions 3, 5, and 8. The ring system of the anion assumes an alternant quinoidal structure
in the ground state of the anion, which becomes more symmetric in the relaxed excited state. This suggests
that the enhanced aromatic character of a 4n electron system in the excited state allows for better
delocalization of the oxygen charge within the ring.
Introduction
Hydroxyaryls become strong acids in their excited singlet state
(S1).1,2 For example, the pK of 2-naphthol (2OH, Chart 1) in
its ground state is 9.5, dropping to pK* ) 2.8 in S1, nearly 7

  

Source: Agmon, Noam - Institute of Chemistry, Hebrew University of Jerusalem

 

Collections: Chemistry