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Keto-enol tautomerism o f m o n o -s u b s t i t u t e d phenylpyruvic acids has been s t u d i e d by t h e 'H and 13C NMR s p e c t r a . The e q u i l i b r i u m c o n s t a n t s and t h e k i n e t i c p a r a m e t e r s for t h e tautomerism w e r e o b t a i n e d from t h e s p e c t r a l d a t a . The e q u i l i b r i u m c o n s t a n t s are s t r o n g l y dependent on t h e p o s i t i o n of t h e s u b s t i t u t i o n ; t h e v a l u e s f o r t h e o -s u b s t i t u t e d PPAs are s e v e r a l times g r e a t e r t h a n t h o s e of t h e m-or p -s u b s t i t u t e d d e r i v a t i v e s . The PM3 c a l c u l a t i o n s havebeencarriedouttoobtaintheinformation on t h e p r e f e r r e d c o n f o r m a t i o n s of t h e t a u t o m e r s and on t h e mechanism f o r t h e tautomerism. The r e s u l t s s u g g e s t t h e involvement of a s o l v e n t molecule i n t h e e q u i l i b r i u m p r o c e s s .

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Two polymorphic forms (I and 11) of 4-fluorophenylpyruvic acid (F-PPA) were obtained by crystallization from different solvents, showing a melting point at 163.2 and 171 .O "C. Crystal structures of polymorphs I and 11 were determined by X-ray crystallography. IR and Raman spectra of the two polymorphs were measured and the spectral characteristics were compared with those of phenylpyruvic acid. The two polymorphs show similar molecular and crystal structures to each other, except for the molecular geometries of the enol and the carboxylic acid moieties. Distinct IR spectral differences which result from the crystal field splitting were observed between the two polymorphs.