Ab initio post-HF study of guanine dimer isomers involving imino forms in gas phase

We have optimized some guanine dimer (GG) isomers including imino forms with MP2/6-31++G(d,p) level. Contrary to the Hartree-Fock level of calculation, our optimized GG structures are found to be slightly non-planar due to the electron correlation effect. Though the interaction energy of the GG isomer including imino forms is slightly greater than that of keto form dimer, our calculated vibrational frequencies for GG isomers including imino forms are not in reasonable agreement with the spectra of the resonance enhanced multiphoton ionization (REMPI) method by Nir et al. Our MP2 results suggest that the species observed by their REMPI method should be keto form dimer.