Using an adiabatic separation of the NH stretching vibration from the remaining vibrational molecular motions, the NH fundamental frequencies and absolute intensities of several keto/enol and 7/9NH tautomers of guanine are evaluated ab initio within the framework of a one-dimensional "semirigid" stretching Hamiltonian. The frequencies (calculated by means of the standard MP2, CCSD(T) and DFT procedures) are in a close one-to-one harmony with their experimental counterparts, thus evidencing the adequacy of the used separation for reliable assigning of the NH stretches in the vibrational spectra of very large molecular systems.