Mode of attachment of adsorbed nitrous oxide

Abstract

The application of infrared spectroscopy to surface chemistry, and in particular to the adsorption of the diatomic gases CO, NO, and N~ on various surfaces, has been widely used. The changes in the infrared spectra of these gases on adsorption are similar to those when the gas becomes coordinated in a transition-metal complex--carbonyl, nitrosyl, and dinitrogen complex, respectively (1). In this note we wish to point out the similarity of the infrared spectra of adsorbed and coordinated N20 and to determine how the N20 is bonded. N20 is a linear triatomic molecule and its three fundamental bands: ~1 (the pseudosymmetric stretch), g3 (the pseudoas>mmetric stretch), and ~2 (the doubly degenerate bend), occur at 1286, 2224, and 589 cm -1, respectively (2). Kozirovski and Folman (3) observed all three absorptions of N20 when it was adsorbed on alkali halide films (Table I). They observed that the frequency of the pl and p2 vibrations of N20 were shifted to lower frequencies whereas that of the p3 vibration was shifted to higher frequencies. Normally, red shifts are obtained on adsorption. However, similar shifts in the stretching frequencies v~ and u~ were observed when N~O was adsorbed on synthetic zeolites (4). We have prepared (5) the N20 complexes [-Ru(NH3)sN20~X2, X = Brand I-, and [-Ru(NH3)sN20-]Y~'H20, Y = PF6and BF4-. In these complexes the absorptions corresponding to the stretching vibrations of N20 (~3 and ;1) are easily assigned (Table I). Although the positions of these absorptions depend on the counteranion associated with the cation, their shifts from the frequencies of gaseous N~O are in the same directions, but with greater magnitudes, as those for adsorbed N~O. N20 has a small dipole moment oriented towards the oxygen atcm, but molecularorbital calculations indicate that both terminal atoms are positions of high electron density in the molecule (6). Thus, potentially, N20 has two atoms through which it may bond--the terminal nitrogen atom or the oxygen atom. The u3 absorption of N20 adsorbed on alkali halide films is sharp and narrow and its bandwidth is not temperature dependent. From this, Kozirovski and Folman (3) assumed that the N20 is adsorbed perpendicular to the surface and on the basis of the charge distribution in N20 they proposed that the adsorption is through the oxygen atom to an alkali metal ion at the surface. (The multiple absorptions which are observed for the fundamentals were suggested to arise from adsorption on different sites). However, the rapid reduction of coordinated N20 to coordinated N2 in the complexes has been interl~reted to be, on the basis of certain assumptions, that the N20 is coordinated through the terminal nitrogen atom (7). In order to determine the mode of attachmerit of N~O, complexes containing N20 labeled with nitrogen-15 in the endo(14Nl~NI60) and exo(15N'4N160) positions