We studied the absorption spectrum of 7-aminocephalosporanic acid between 312 and 238 nm (32,000-42,000 cm<sup>−1</sup>) at different pH values. The spectrum was deconvoluted into log-normal curves that showed three different transitions; the first two bands are due to π-π* transitions between the HOMO and LUMO, and from the HOMO to the LUMO+1, respectively. We also recorded the spectrum of the acid in dioxane/water mixtures and found band I to shift to shorter wavelengths with an increase in the medium polarity. Theoretical calculations allowed us to ascribe this behavior to the fact that the β-lactam nitrogen atom possesses a high electron density in the ground state. As the medium polarity increases, so does the stability of this state, which results in the band being shifted to higher energy values. The theoretical calculations (MNDO) performed yielded 36.8 and 39.8 kK (1 kK = 10<sup>3</sup> cm<sup>−1</sup>) for the first two electron transitions, i.e., very close to the experimental values (∼36 and 39 kK).

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