Ultraviolet spectra hydrogen bonds

Ultraviolet spectra of benzoic acid in sulphuric acid solutions, published by Hosoya and Nagakura (1961), show a considerable medium effect on the spectrum of the unprotonated acid, but a much smaller one in concentrated acid. The former is probably connected with a hydrogen-bonding interaction of benzoic acid with sulphuric acid which is believed to be responsible for a peculiarity in the activity coefficient behaviour of unprotonated benzoic acid in these solutions (see Liler, 1971, pp. 62 and 129). The absence of a pronounced medium effect on the spectra in >85% acid is consistent with dominant carbonyl oxygen protonation. In accordance with this, Raman spectra show the disappearance in concentrated sulphuric acid of the carbonyl stretching vibration at 1650 cm (Hosoya and Nagakura, 1961). Molecular orbital calculations on the structure of the carbonyl protonated benzoic acid have also been carried out (Hosoya and Nagakura, 1964). 

With the exception of l,6-naphthyridin-8-ols, nuclear hdyroxy-1,6-naphthyridines would be expected to exist as the corresponding naphthyridinones. This has been confirmed by infrared,1035 ultraviolet,1026 and nuclear magnetic resonance spectral means.639 Other physicochemical studies of these oxy-1,6-naphthyridines include IR stretching frequencies1019 and ionization constants1040 of 1,6-naphthyr-idin-8-ols the mass spectra of 1,6-naphthyridinones the effect of solvents on the UV spectrum of an 8-hydroxy-1,6-naphthyridine derivative 773 and X-ray analysis of the dilactam, 4-hexyl-3,4,4a,5-tetrahydro-l,6-naphthyridine-2,7(l//,6Z/)-dione (1), in connection with its crystallization as self-assembled hydrogen-bonded polymers.549... 

The intramolecular hydrogen bond is Indicated by a red shift of the ultraviolet absorption maximum, compared to the 2,2 dimethoxy derivative (Table 6). In the absorption spectrum of the latter in toluene, an n-ir band is found as a shoulder around 25,000 cm" which vanishes in alcohol due to the usual blue shift. The same shift should be brought about by intramolecular hydrogen... 

Absorption and fluorescence of the photoproducts, irradiation of SAA and SAT with ultraviolet light In solid solutions produces a new absorption band with a maximum at 21,300 cm l (5,49,53,55,59). The 0-0 peak of this transition has been found at 17,550 cm l in a crystalline dibenzyl matrix at 4°K (53). The extreme sharpness of this spectrum, not expected for Internally hydrogen-bonded molecules. Is an Indication of the trans-gulnold nature of the photoproduct (53). 

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