Why is the N-1-H stretch vibration frequency of guanine shifted upon dimerization to the red and the amino N-H stretch vibration frequency to the blue?

Three isomers of the guanine dimer (K9K9-1, K9K7-1 and K7K7-1) possessing two N-H...O=C H-bonds were investigated theoretically using the ab initio calculations at the HF/6-31G** level. Amino groups in these isomers were not directly involved in H-bonding. Besides the red shifts of the N-H stretch vibrations (supporting the H-bonded character of these contacts) an unexpected blue shift of amino N-H stretch vibrations was found and this conclusion fully agrees with published experimental results. The blue shift of the amino group N-H stretch vibrations in all guanine dimer structures was clarified by the planarization of the guanine amino group. Absolute values of harmonic amino N-H stretch frequencies and their shifts upon planarization were verified by performing the two-dimensional anharmonic vibrational analysis. The planarization of the guanine amino group cannot be interpreted on the basis of an electrostatic model and is due to redistribution of electron density in subsystems upon dimerization. The redistribution occurred at the aromatic ring as well as at the amino group nitrogen and leads to formation of a new resonant state. The electron density decrease in the lone electron pair of amino nitrogen gives rehybridization of the respective atomic orbitals (change from the sp(3) to the sp(2) state). The (amino) H...O=C contacts can be thus described as an improper, blue-shifting H-bond. An increased amino N-H stretch frequency represents the fingerprint of the planarization of the guanine amino group and is the first spectroscopic manifestation of the fact that the amino group in nucleic acid bases is nonplanar. Blue shifts of the amino N-H stretch frequencies occurred only if the amino group is bifurcated. The guanine dimer is the first complex where one proton acceptor (C=O group) is simultaneously connected with two proton donors (NH and NH2) by H-bond and improper, blue-shifting H-bond.