The harmonic frequencies, IR intensities and the potential energy distributions of the vibrational modes of the keto and enol tautomers of guanine are calculated at the Hartree-Fock 6-31G** level. The results are compared with the experimental spectra obtained in an argon low temperature matrix. Excellent agreement is found between calculated and experimental frequencies. The use of a potential energy decomposition scheme is shown to allow a comparison with the atomic character of the modes that are inferred from experiment. It is found that optimization without symmetry constraints is necessary to obtain a true minimum on the HF/6-31G** potential energy surface. DUE TO their biological importance [l] there are continuing experimental and theoretical investigations of the electronic structure of purine bases and their intermolecular interactions, particularly those involving hydrogen bonds. Infrared (IR) spectroscopy has been utilized extensively to study these nucleic acid constituents, particularly in low temperature matrices, in which it is generally assumed that intermolecular interactions are minimized. This method has yielded information on the structure and tautomeric equilibria of cytosine [2], adenine [3], guanine [4], thymine [S] and uracil [S-7].
Recent IR studies on the tautomers of guanine and 9-methyl-guanine have found that in low temperature matrices the amino-ox0 and the amino-hydroxy tautomers are both present in significant concentrations [8,9], this having important biological implications related to the possible appearance of guanosine-(amino-hydroxy)-thymine base pairs. The ability of theoretical simulations to reproduce accurately and reliably the IR spectra of medium size compounds is of great importance to the identification of the products of chemical or photochemical reactions taking place in low temperature matrices [lo]. Until recently, theoretical simulations of the IR spectra of large polyatomic molecules have been performed at the Hartree-Fock (I-IF) level, with split valence basis sets without polarization functions. A significant deficiency of such calculations is the inaccurate prediction of the out-of-plane modes. Whilst such calculations were useful in interpretation procedures they are still not sufficiently precise to be used as references in spectroscopic identification of compounds.
Recently, IR spectra have been predicted for 6-azauracil, 5-azauracil [ll], uracil [12,13], two tautomers of 2-pyridinone [14] and two tautomers of cytosine [15], at the HF/6-31G** level. Comparison of the theoretically derived spectra at this level with experiment yields quantitative agreement for both the in-plane and out-of-plane modes.
Ab initio theoretical predictions of the harmonic vibrational frequencies and IR intensities of the amino-oxo tautomer of guanine have been performed at the HF/3-21G level by LATAJKA et al. [16]. In this paper we investigate whether an HF/6-31G** theoretical prediction of the IR spectrum of the amino-ox0 and amino-hydroxy tautomers can reproduce the experimental spectrum with sufficiently good accuracy to perform a reliable assignment.
COMPUTATIONAL
We have previously reported predicted molecular energies from geometry optimization calculations at the HF/6-31G** level of the amino-ox0 and amino-hydroxy forms of guanine [17], followed by the inclusion of zero-point vibrational energy effects and of electron correlation calculated at the MP2 level. These calculations predict that the