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Infrared studies have been made of several oxides of nitrogen trapped at liquid-helium temperatures in matrices of argon, N2, O2, CO2, H2, and N2O. The novel techniques permitting these studies are described. NO is found as the monomer and two forms of dimer, ``cis'' and ``trans''; the cis is more stable. NO2 is found as the monomer, the known stable dimer of planar structure, and two new dimers; one of these has the ONO–NO2 structure, and the other may be the twisted Vd form of the stable dimer. N2O3 is found in its stable form ON–NO2 and in a second form whose spectrum is consistent with the structure ONONO. Observations on covalent N2O5 are also reported.

Explains the shape of simple molecules using the distribution of atomic s character.

Chemical combination frequently disguises essential resemblances. The molecules C3H4 and N20, for example, have very different chemical formulas. Yet, to judge by their shapes and by their color (in the infrared, both molecules are transparent in the visible part of the spectrum), these molecules have very similar electronic structures, sufficiently similar to be called "isoelectronic."As a general rule, or principle, molecules are isoelectronic with each other when they have the same number of electrons and the same number of heavy-atoms.1 Usually, then, they have similar heavy-atom geometries --and, by inference, similar electronic structures. The molecules H2CCCH2 (allene), H2CCO (ketene), HNCO (isocyanic acid), OCO (carbon dioxide), NNO (nitrous oxide), HNNN (hydrazoic acid), and H2CNN (diazomethane), for example, all have 22 electrons and three heavy atoms; and all have linear, heavy-atom skeletons.Similarly, the molecules (CH3)2CCH2 (isobutylene), (CH3)2CO (acetone), (H2N)2CO (urea), (HO)2CO (carbonic acid), (CH3)(HO)CO (acetic acid), F2CO (carbonyl fluoride), FN02 (nitryl fluoride), H0N02 (nitric acid), H2NN02 (nitramide), and CH3NO2 (nitromethane), all have 32 electrons and four heavy atoms; and all have planar, Y-shaped, heavy-atom skeletons.(Frequently molecules have similar heavy-atom geometries if they have the same number of heavy atoms and the same number of valence electrons. Examples are NFa and PF3 (pyramidal), S02 and 03 (bent), and XeF2 and I012-(linear).)Even the general locations of bound protons are similar in molecules that have the same number of electrons and the same number of heavy atoms. The bound protons of ketene (H2CCO), for example, lie with respect to each other and the heavy-atom skeleton as do two of the bound protons in allene (H2CCCH2). Similarly, the bound proton of isocyanic acid (HNCO) lies approximately at one of the proton positions of allene, or ketene. Even more striking, perhaps, is the prediction, based on the statement above and the geometry of methane, that ammonia should be pyramidal and water bent. Figure 1 shows how much alike are the shapes of these three molecules.2These facts may be related to each other by imagining that removal of a proton from a hydrogen-heavy-atom bond leaves behind an electron pair-an unshared pair, which possibly occupies about as much space in the heavy-atom's valence shell as did the original bonding pair-and that the remainder of the electron cloud is essentially unchanged.3 Ketene's structure, for example, may be obtained from that of allene by replacing two of the C-H bonds This study was supported by a grant from the National Science Foundation and by the grant from the University of Minnesota of a Faculty Single Quarter Leave.1 "Heavy atom" means here any atom other than hydrogen or helium, or lithium. 4 With very careful work absolute structure factors can be determined with X-rays to an accuracy of about 1%. This accuracy has been achieved, however, in only a very limited number of cases (the noble gases and a few solids such as...