Tautomerism, normal coordinate analysis, vibrational assignments, calculated IR, Raman and NMR spectra of adenine

The Raman (3700-100 cm^-1), infrared (4000-200 cm^-1), mass spectrum and ¹H NMR temperature-dependent study of adenine have been recorded. Quantum chemical calculations were carried out for N(9)H-amino, N(7)H-amino, N(9)H-imino and N(7)H-imino adenine tautomers using RHF, B3LYP and MP2 with full electron correlation up to 6-311++G(d,p) basis set. The computational results reveal the non-planar N(9)H-amino conformer of adenine to be the most stable structure of adenine. The planar (C_s) form of N(9)H-amino adenine is found to represent a transition state, lying 8 cm^-1 above the non-planar conformer and with one imaginary frequency. Comparison between theoretical and experimental ¹H NMR spectra favors the assignment of the signals to N9(H) and N7(H)-amino tautomers over the corresponding imino tautomers. On the other hand, the recorded IR, Raman and ¹³C NMR spectra were fully consistent with N9(H)-amino adenine tautomer, therefore it is the only tautomer in both the gas and solid phases and in solution. Moreover, the results of NH₂ potential surface scans utilizing B3LYP and MP2 = full methods at 6-31G(d) basis also support the non-planarity of N(9)H-amino adenine. The mass spectral measurements indicate the presence of 3% adenine dimmer which indicates weak inter-molecular hydrogen bonding interactions in adenine. Additionally, intramolecular hydrogen bonding is also predicted between N₁ and H₁₅ atoms. The theoretical infrared and Raman spectra have been successfully simulated by means of both DFT and MP2 calculations, allowing the interpretation of the complex bands observed. Aided by normal coordinate analysis, potential energy distributions and the calculated force constants, a revised and accurate vibrational assignment for all fundamentals has been provided for the non-planar N(9)H-adenine tautomer. The results are reported herein and compared with similar molecules whenever appropriate.