The infrared spectra of CH4 and CD 4 , present as substitutional impurities in crystals of argon, krypton and xenon, were studied at temperatures ranging from 5 to 40 0 K.Both v3 and v4 of CH 4 in xenon showed a simple four line pattern which is consistent with that expected for a slightly hindered rotor.In kryp-.ton and argon a fifth line appeared on Vs as expected from King's theoretical calculations for a tetrahedral rotor in an octahedral field. In addition, argon showed absorption due to pairs or higher aggregates of CH 4 molecules.The rotational spectrum of CD 4 is much more highly perturbed. Detailed assignments to hindered rotational levels could not be made but the general features are in accord with King's model.The frequencies of the CH4 and CD 4 absorption bands. increase steadily in going from xenon to krypton to argon. This can be accounted for by the decrease in the sizes of the cavities occupiedwith corresponding larger repulsive interactions between the CH4 or CD4 and the host atoms.

Communications to the Editor 437 the adsorption characteristics and hence the exchange properties of the heterocycles and nitrobenzene were different from other substituted aromatics and aliphatics.From 7r-complex chemisorption theory, it is significant that aromatic compounds in general are more reactive than aliphatics in "self-activation and exchange. In conclusion, the "self-activation" process offers a convenient tritiation procedure for a large range of compounds without the necessity for hydrogen pre-activation of the catalyst.A full discussion of the process of "self-activation" using a variety of organic compounds and different catalysts as model systems will be published in detail elsewhere.

A considerable low-field shift in C13 carbonyl spectra is usually observed in simple fivemembered ring compounds. The same low-field shift of the sp1 2 carbon is observed in cyclopentanone oxime, but it is much less pronounced in methyleneeyclopentane when compared to its fourand six-membered analogs. It also appears that the low-field carbonyl shift no longer characterizes a five-membered ring when it is fused with another ring in bicyclic ketones. It is suggested that unusually low or high carbonyl chemical shifts in homologous compounds may be explained in terms of their electronic excitation energies. A good linear correlation between the carbonyl shifts in some cyclic and bicyclic ketones and the n -* * transition energies has been established.

It is shown that 70 C13 chemical shifts on sp3 and sp1 2 carbons in simple alkanes, olefins, and polyenes, as well as some aromatic compounds, can be calculated by constitutive additivity using only 10 bond parameters, with 2 p.p.m. standard error. This seems to indicate that the C13 chemical shift is a constitutive property largely dependent on the immediate structural environment of the carbon in question and to a much smaller extent on the remainder of the molecule in simple hydrocarbons. It also emphasizes previous observations in the literature that the C13 chemical shift, unlike other forms of spectroscopy, is relatively insensitive to double bond conjugation in unsubstituted hydrocarbons. Application of the same rule to sp carbon, however, results in a significant deviation from constitutive additivity when the triple bond is conjugated with a benzene ring. The general scope and limitations of the constitutive rule are briefly discussed.

Infrared spectra of CH4 and CD4, in the region of the fundamentals ν3 and ν4, have been obtained in all of their crystalline phases at temperatures ranging from 5° to 40°K. In addition, the spectra of dilute solutions of CH4 and CD4 in one another have been studied through the same temperature range. The spectra of the dilute solutions consist of single sharp lines for both fundamentals, demonstrating that the barriers to molecular rotation are high in all the phases. The fine structure observed in the low-temperature phases of the pure crystals is inconsistent with any of the structural models heretofore proposed but a model for phase II is suggested which is consistent with the spectra and which shows very efficient packing. Phase III of CD4 is probably complex and of low symmetry. Phase I is probably disordered.