Matrix Isolation Infrared Studies of Nucleic Acid Constituents. 5. Experimental Matrix-Isolation and Theoretical ab Initio SCF Molecular Orbital Studies of the Infrared Spectra of Cytosine Monomers

Article abstract of DOI:10.1021/ja00233a006

Results are presented from an experimental study of the infrared spectra of cytosine and its deueriated derivatives isolated in an inert Ar matrix and also in an N2 matrix deposited on a window at 15 K.These spectra show that isolated cytosine exists under these conditions as a mixture of the "normal" amino-oxo (a-o) tautomer and the "rare" amino-hydroxy (a-h) tautomer; in fact, the last form predominates in both matrices with an "equilibrium constant" K_t(o/h) = / measured to be about 0.5.Infrared spectra of the crystalline solid are also presented, and they agree with the conclusion from X-ray diffraction studies that only the normal amino-oxo tautomer occurs in the solid.In order to interpret the observed infrared spectra, we have carried out an ab initio molecular orbital calculation of the spectra of both tautomers at the SCF level with a 3-21G basis set.The calculation predicts frequencies, absolute infrared intensities, and potential energy distributions (PEDs) for all normal modes of each tautomer to provide a basis for the assignment of the experimental spectra.The latter problem was aided greatly by the discovery that irradiation of cytosine isolated in an Ar matrix with UV light changes the relative concentrations of the two tautomers in the matrix.Application of this technique in our studies allowed us to separate the absorption spectra to obtain the complete infrared spectrum for each tautomer free from the absorption by the other form.Each separate spectrum was thus assigned with a reasonable degree of confidence by comparison with the calculated spectra.The relatively good agreement between calculated and experimental spectra for each tautomer provides support for confidence in the validity of the use of these calculated vibrational parameters as a basis for predicting spectra of cytosine and its derivat....Finally, the effect of intermolecular interaction upon the spectrum of cytosine is examined in a comparison of the spectra of matrix-isolated samples with the spectrum of the crystalline solid.