. Both amino-0x0 and amino-hydroxy tautomeric forms of 9-methylguanine have been identified in approximately equal abundance in. infrared studies of these molecules isolated in the hydrophobic environment of an argon matrix at 12 K. The amino-hydroxy tautomer occurs in two different rotamers correlated with the rotation of the OH group. The ratio of concentrations of the two rotamers is sensitive to UV irradiation, and this ratio then relaxes to an equilibrium value after irradiation is stopped. This sensitivity allows us to separate the experimental spectra related to the 0x0 tautomer and to each of the rotamers of the hydroxy tautomer. The relative concentrations of the amino-0x0 and amino-hydroxy tautomers (3) and of the two rotamers (K(h1-h2) = 0.31 & 0.10 in an argon matrix at 12 K and about 30 2 15 in the vapor at 470 K) are estimated from the observed relative infrared absorbances. From these relative concentrations the differences between the free energies of the tautomers (AG47O (0-h) = 0 & 0.5 kJ mol-') and of the two rotamers (AG(h1-h2) = 0.12 ? 0.03 kJ mol-' in the argon matrix at 12 K and AG470 between +2 and -13 kJ mol-' in the vapor at 470 K) have been estimated. The electronic absorption spectrum of 9-methylguanine isolated in the argon matrix at 12 K and the effect of brief ultraviolet irradiation on it have also been studied. In an effort to interpret the experimental results, ab initio calculations of the infrared spectra have been made for 9-methylguanine at the 3-21G//3-21G level. Comparison with the experimental spectra is of some help with the assignment of the infrared spectra for the different tautomers.Key words: 9-methylguanine, tautomerism, infrared and ultraviolet spectra, matrix isolation, ab initio calculation On a aussi CtudiC le spectre d'absorption Clectronique de la 9-mtthylguanine isolCe dans la matrice d'argon B 12 K ainsi que l'effet d'une brkve irradiation avec de I'ultraviolet. Dans le but dlinterprCter les rCsultats exptrimentaux, on a effectuC des calculs ab initio au niveau 3-21G//3-21G des spectres infrarouges de la 9-mtthylguanine. Une comparaison des rCsultats de ces calculs avec les spectres experimentaux est utile pour attribuer les spectres infrarouges des divers tautomkres. Mots clks : 9-mkthylguanine, tautomine, spectres infrarouge et ultraviolet, isolement dans une matrice, calculs ab initio.[Traduit par la rCdaction]

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Experimental infrared spectra of a mixture of 3-hydroxypyridine and water in an argon matrix formed by condensation of a gaseous mixture onto a cold window at 15 K exhibit new bands from the two types of 1:l complexes formed between the two molecules. The first of these is H20--H0(3HP), in which the water molecule acts as a proton acceptor from the OH group of the 3-hydroxypyridine, while the second is HOH-N(3HP), with the water molecule acting as a proton donor to the N atom of the 3-hydroxypyridine. The vibrational spectra of these complexes are recorded and compared with the spectra of the isolated H 2 0 and 3-hydroxypyridine molecules to observe the changes that result from complex formation. The relative concentrations of each complex are estimated from the observed relative intensities and the calculated values of the absolute intensities of the corresponding bands. In order to verify these experimental observations and to aid in their interpretation, ab initio calculations at the HF/6-31G(d) level have been carried out to predict the infrared spectra of the monomers and of the complexes. In general, there is good agreement between the experimental and calculated spectra. The calculations provide a basis for the assignment of the experimental spectra of the monomer and for both complexes. According to the calculations the intramolecular vibrations of the 3-hydroxypyridine do not change appreciably on complex formation, except for those bonds directly involved in the hydrogen bonding. Analysis of the calculated changes in terms of changes in the intensity parameters and in the force constants for the OH stretching mode (for the proton donor group in the hydrogen bond) shows striking similarities for the two complexes studied here and the water dimer.

The substitutional solid solutions of goethite (a‐FeOOH) and diaspore (α‐AlOOH) have been investigated by X‐ray powder diffraction and chemical analysis in order to establish the solubility limits of aluminium in the goethite structure. α‐Fe1‐xAIxOOH samples were obtained by ageing of coprecipitated amorphous (Fe,Al) hydroxides at pH 13 in NaOH and Na[Al(OH)4] solutions at 25°C (G25 series) and 70°C (G70 series), respectively. Chemical analysis reveals that the aluminium content of Al goethites is consistent with Al concentration in the coprecipitated hydroxides up to about 10 mol% Al. In Al goethites obtained from (Fe,Al) hydroxides with higher Al:Fe ratios, the relative Al content is considerably lower. It was found that pure ferrihydrite stored in the aluminate solution is also transformed into Al goethite. The X‐ray diffraction results show a significant divergence of the unitcell parameters from Vegard's law. This points out the limited ranges of the solid solution between goethite and diaspore and suggests the formation of defect, cation‐deficient structures.

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