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12a. LISTRIBUTION, AVAILAILIT '.TýTA ENT 12IW. DSTRIBUTION CODE Approved for public release: distribution unlimited (I:) 2.*STRACT ' trunn ',Iv \woasi The vibrational overtone spectra of gaseous pyridine, 3-fluoropyridine, 2,6-difluoropyridine, ()and pyrazole have been recorded using photoacoustic absorption spectroscopy. The pyridine, 3-fluoropyridine, and 2,6-difluoropyridine overtone spectra exhibit two local mode progressions. These CH local modes are mixed and cannot be assigned to the separate CH oscillators at the 2,6 and _ 3,4 and 5 positions. The transition wavenumbers and peak widths are compared to those of benzene ~ and pyrrole, both of which exhibit only one overtone peak at each quantum level. The fourth 0 overtone spectrum of gaseous pyrazole displays two peaks. These spectra indicate that the presence _ _ of a lone pair perpendicular to the aromatic 11 system in aromatic heterocycles removes the local- %J mode equivalency of the CH oscillators. 0,4. SUBJECT TERM