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

Tarek A. Mohamed*, Ibrahim A. Shabaan, Wajdi M. Zoghaib, John Husband, Rabei S. Farag, Abd El Nasser M.A. Alajhaz

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

57 Citations (Scopus)


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 (Cs) 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.

Original languageEnglish
Pages (from-to)263-276
Number of pages14
JournalJournal of Molecular Structure
Issue number1-3
Publication statusPublished - Dec 16 2009


  • Adenine
  • DFT calculations
  • NMR spectra
  • Normal coordinate analysis
  • Tautomerism
  • Vibrational assignments

ASJC Scopus subject areas

  • Analytical Chemistry
  • Spectroscopy
  • Organic Chemistry
  • Inorganic Chemistry


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