This paper reports the analysis of adenine spectra using both harmonic and anharmonic approximations to the vibrational frequencies reported in matrix isolation studies. The harmonic approximation procedure consists of the application of a scaled ab initio calculated harmonic force field to predict the frequencies, and infrared intensities, of adenine. Theoretical calculations were made using Hartree-Fock density functional theory (DFT) B3-LYP/6-31G* and GGA/DNP computational methods. The equilibrium calculated force constants were scaled according to the method of Pulay (Pulay, P.; Fogarasi, G.; Pang, F.; Boggs, J. E. J. Am. Chem. Soc. 1979, 101, 2550-2560) and compared with the experimentally determined frequencies, and intensities, to assess the accuracy and fit of the theoretical calculation. Good agreement is found except for the in-plane X-H bending or stretching and the out-of-plane X-H bending or wagging modes (X = C and N) which exhibit cubic and quartic anharmonicity, respectively. The NH2 puckering mode, is an out-of plane mode that is poorly modeled by both DFT methods, which is most probably caused by its quartic-quadratic anharmonicity. In this work, we document the anharmonicity and show that intermode coupling can be estimated using harmonic shift analysis. The goal of this study is to compare various DFT approaches with the aim of determining their limits within the harmonic approximation. We present the method of harmonic shift analysis as a tool for the estimation of mode anharmonicity and for the determination of intermode coupling in the DFT calculation of adenine.