Receiued 2 I st June, 1973 An i.r. study of nitric oxide adsorption on high area MgO (previously degassed at 800°C) at room temperature is reported. The adsorption of NO onto MgO produces i.r. bands at 800-900 cm-I, 110@1250crn-', and 1350-1450cm-'. These absorption bands were assigned to several surface species : NO; ions with various configurations and NOand N202-ions, co-ordinated to Mg2+ ions. The adsorption (3.3 x l O I 3 molecule cm-2, at 10.4 x lo3 N m-2 of NO) occurs in limited surface zones.

Infrared spectra of pyrazole in the vapour and solid states and in solution are reported and discussed. From these, and by comparison with the spectra of N-deuterio-, 3-methyl-, and 3,5-dimethyl-pyrazole, and other simple fivemembered heterocycles, a vibrational assignment is proposed.A VIBRATIONAL assignment for the unsubstituted pyrazole nucleus has not yet been discussed. Bonino and Manzoni-Ansidei reported the Raman spectra of pyrazole and some methyl derivatives,l Mirone and Vampiri the infrared spectrum of 3,5-dimethylpyra~ole,~ and Zerbi and Alberti the infrared spectrum of some monoand poly-alkylpyrazoles without a complete discussi~n.~~The structure of pyrazole in the solid and vapour phases is well established, from X-ray analysis5 and microwave spectroscopy,6 respectively. EXPERIMENTALInfrared spectra (4000-600 cm.-l) are reported for pyrazole (m. p. 70") in the vapour phase, in solution, and in the solid phase at room and liquid-nitrogen temperatures, for N-deuteriopyrazole and 3-methylpyrazole (b. p. 2 0 0 -204") in the vapour and liquid phases and in carbon tetrachloride solutions, and for 3,5-dimethylpyrazole (m. p. 107") in the solid phase and in carbon tetrachloride.N-Deuteriopyrazole was obtained by exchange in D,O solution, followed by evaporation under reduced pressure.

The adsorption of CO at room temperature on well outgassed specimens of MgO gives rise to a large number of bands in the 2200-1000 cm-' range, which can be divided into two main groups. The bands of the former group are destroyed by oxygen at room temperature : some react instantaneously and are associated with a marked pink colour of the sample ; the others are less reactive as they require prolonged contact time in order to be completely oxidized at room temperature (r.t.). The bands of the latter group, far from being destroyed by oxygen, grow when the oxygen-sensitive species are depleted. The oxygen-sensitive species are thought to be negatively charged polymeric CO structures (CO clusters) of the type (CO)",-, where x = 2 or 4 and n is > 2. The simplest CO clusters (dimers) can be transformed into larger polymers by further CO addition. Under the correct conditions the reverse process can also be carried out. The oxygen-insensitive species have a carbonate-like structure and are present on the surface in fairly constant ratios with respect to the former group species. A chemisorption mechanism leading both to oxidized (carbonate-like) and to reduced (negative CO polymers) species is proposed. The active centres for CO chemisorption consist of groups of ions (both positive and negative) in strongly uncoordinated situations.

The nineteenth-century history of the mole concept, of the determination of the Loschmidt number and of the Avogadro constant, is interesting in several respects. While Avogadro's hypothesis (1811) played a key role in the development of the mole concept, it was only one of the epistemic tools that chemists used in their long search for a consistent system of molecular and atomic weights. This line of research and thought was fully mature in the 1880s. Following a Laplacian programme until the 1880s, physicists tried to guess the range of the so-called molecular forces, but the development of the kinetic theory of gases introduced a new interest in the determination of the number of "particles" present in gaseous systems. In this respect Loschmidt's papers (1865) represented the transition between the two types of interest in the molecular reality. However, it was only in the first decade of the twentieth century that the theoretical and experimental determination of physical quantities directly related to the Avogadro constant received full attention by scientists such as Planck, Einstein, Rutherford and Perrin.

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