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

Abstract

2 Ec + AH* for temperatures below rollover and E,,, = Ec for temperatures above rollover.The selectivity for the hydrogenolysis of n-butane on the two surfaces has been identified with the occurrence of particular, adsorbed reaction intermediates on each surface. When our results are compared to those reported previously on supported iridium catalysts of varying metallic particle size,22 a direct correlation Registry No. C2H6,, C4Hlo,Supplementary Material Available: Details of mechanistic modeling of the hydrogenolysis of alkanes on iridium and the microscopic significance of kinetic parameters (1 1 pages). Ordering information is given on any current masthead page.Abstract: Results are presented from an experimental study of the infrared spectra of cytosine and its deuteriated 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-axo (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, a-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 derivatives. 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.Vibrational spectra of cytosine and of its corresponding nucleoside and nucleotide, as well as of several of its derivatives, have been the subject of numerous experimental studies in solution and in the solid phase (ref 1 and references given therein). These studies establish the predominance of the amino-oxo form for the cyt...

Absolute cross sections for electronic excitations of cytosine by low energy electron impact

“…27 To prevent any insidious effect such as decomposition of the cytosine molecule by the Pt surface in the present experiment, a 3-layer film of Ar was systematically used as a spacer. 39 Since, it is generally accepted from VUV, 40 photoelectron, 41 IR, 42 and Raman 42 spectroscopy studies that cytosine does not decompose upon heating up to 220°C in vacuum; the present sublimation temperature is expected to preserve the integrity of the molecule. After several days of experiments, the sample holder and the electron spectrometer needed to be heated at about 50°C during 48 hours in order to remove the insulating deposits that deteriorate the measurements.…”

Time-resolved photoelectron imaging of the isolated deprotonated nucleotides

“…The origin of this confusion is almost certainly due to multiple tautomers present in the molecular beam. Microwave 80 and high resolution IR 81,82 spectroscopies reveal that there are (at least) three tautomers of Cyt present within a molecular beam. These are the biological keto tautomer, and the nonbiological enol and imino tautomers, which are present in a ratio of approximately 4:4:1, respectively (from microwave experiments).…”

UV-Excitation from an Experimental Perspective: Frequency Resolved

“…The keto form is the biologically important one, with Watson-Crick base pairing in DNA, and predominant in solution. In matrix isolation Szczesniak et al observed both keto and enol forms with higher abundances for the latter and small contributions from the imino form 79 82 . They conclude that the deactivation pathways are quite different for the different tautomers, with three time constants for keto-cytosine and two for the others.…”