It is shown using the PM3 method that 1-methyladenine possesses four low-lying tautomeric forms, three of which are of the imino type. One of the imino forms, Ia is predominant and not energetically equivalent to the other ones. The effect of protonation on the spectra of these forms has been studied. By a polar environment modeled by Cramer-Truhlar SM3 solvation model, the amino form is favored. The assignment of vibrations of all low-lying tautomeric forms of 1-methyladenine in vacuo is elaborated.