NOTES
Dec., 1956
1659
zation. This result is most easily explained by -+
reference to Fig. 3 where E is the electric vector of the incident radiation and SI and St are the two spheres constituting the portion of the models pres-+ ently under consideration. In principle, E can be +
-+
E‘ and E”. Since + E” lies along the line of centers of SIand SZ,SI and Sz can be considered to be a capacitor which is +
resolved into two components,
IE
charged by E”. This implies a transfer of energy 4
from E” to S1 and SZ,which in turn means a scat+ tering or attenuation of E”in the direction of propagation leaving E’ dominant in the transmitted beam. This would be equivalent to a rotation of the plane of polarization in the direction of E’. This phenomenon is to be distinguished from optical activity, however, since the sense of the rotation can be changed merely by twisting the model through 90”. In fact, the observed effect corresponds to linear dichroism which is caused by anisotropic true absorption or anisotropic s ~ a t t e r i n g . ~ On the basis of the preceding discussion, it seems likely that the results obtained by Lindman could be partly attributed to the exercise of insufficient care in centering the model and partly to an uncertainty in the plane of polarization of the microwaves. The apparent absorptions observed by Lindman but never observed by the present author probably were due to a local intensification of the radiation field, as a result of diffraction, a t the indicating antenna used to measure the total power (9) W. Heller, Reu. Modern Phys., 14, 406 (1042).
Fig. 3.-The
+
origin of linear dichroism from a specific orientation of spheres.
emitted from the generator. The model itself does not seem to be optically active even though theoretical considerationslo suggest that it may be when the model is very close to the antenna. In this case, the wave front a t the model is no longer planar, that is to say, the electric vector has a longitudinal componentllplz and so the necessary coupling between spheres lying in different planes perpendicular to the direction of propagation could arise. However, since a single sphere yields data which are identical with those obtained for the complete model, it can be assumed that this coupling does not occur to any appreciable extent. Acknowledgment.-The author wishes to express his gratitude to Professor K. M. Mislow for the advice and aid which he gave during the course of this work. (10) B. Y. Oke, Proc. Roy. SOC.(London), A108, 339 (1936). (11) F. E. Terman, “Radio Engineem’ Handbook,” McGraw-Hill Book Co., New York, N. Y., 1943, p. 827. (12) “Radar Electronics Fundamentals,” Bureau of Shipa, Navy Department, Washington, D. C., 1844. p. 412.
NOTES METHYL COMPOUNDS OF THE ELEMENTS BY R.E. RUNDLE Contribution No. 418 from the Department of Chemistry and Institute f o r Atomzc Research, Iowa State College, Amea, Iowa’ Received December 16, 1966
From a novel and instructive approach to electronegativity12Sanderson infers the net charge on a methyl group in simple compounds M(CH3),, and he observes that there is a direct relation between the degree of “polymerization” and the methyl charge thus inferred.3 He has suggested, moreover, that polarity arguments may lead to an explanation for electron deficient bonding in generaL2 Unfortunately, in assembling his correlations Sanderson omitted tetramethylplatinum, which (1) Work was performed in the Amee Laboratory of the Atomic Energy Commission. (2) R. T. Sanderaon, J . Chem. Ed., 3% 538 (1952);81,2,238 (1954). I . A m . Chcm. Soe., 77,4581 (1955). (3) R. T.Sanderson, .
forms a tetramer of known structure.* A solid up to its decomposition point, tetramethylplatinum remains a tetramer in and is relatively unreactive with Oz,COz and HzO. Any reasonable electronegativity for platinum would put tetramethylplatinum far from the class of methyl compounds which polymerize according to Sanderson’s observation. For other reasons, too, Sanderson’s correlation of polymerization with methyl charge seems fortuitous rather than fundamental. In going across the periodic table from groups I to IV electronegativity assuredly increases, but so does the number of methyls per metal atom. Surely, for example, trimethylaluminum is only a dimer while dimethylberyllium is a solid polymer6 because, while both tend to form four bonds to methyl, the methyl-to(4) R . E. Rundle and J. H. Sturdivant, ibid.. 69, 1561 (1947). (5) R. E. Rundle and E. J. Holman,, ibid., ‘71, 3264 (1949). (6) A. I. Snow and R. E. Rundle, Acta Ciyrt., 4,348(1951).
1660
NOTES
VOl. 60
metal ratio is 3/1 in the former, 2/1 in the latter, and the methyl charge has little to do with the degree of “polymerization.” It is easily seen that coordination number us. methyl-metal ratio will explain most of Sanderson’s table. For the least electronegative metals the metalcarbon bonding is no doubt so ionic as to justify using the ionic coordination numbers to explain the “polymerization.” But even for dimethylberyllium and trimethylaluminum the structures in the solid state are not characteristic of ionic structures (large Madelung constants) , and the coordination number for the metal is strictly equal to the number of low energy orbitals of the metal. (For group IVB metals and non-metals coordination number, number of electrons for bonding and number of low energy orbitals are all equal, so there is no polymerization problem.) A comparison of tetramethyllead and tetramethylplatinum illustrates the importance of the point. The lead atom is larger than the platinum atom, as judged by its covalent radius, and probably has a lower electronegativity, yet experimentally the coordination number of lead for methyl is four, for platinum it is six. Lead IV has tetrahedral spa orbitals for bond formation, platinum IV has octahedral daspSorbitals for bond formation. I n both cases all the bond orbitals are used, resulting in a monomer for tetramethyllead and a tetramer for tetramethylplatinum. The principle that bond delocalization arises when atoms with more low energy orbitals than valence electrons (always metals) are combined with atoms or groups with no unshared electron pairs is easily justified by simple quantum mechanical arguments,’ and leads naturally to the expectation that bonds will become more delocalized as the metal-non-metal ratio increases until the highly delocalized bonding in metals is reached. Surely polarity is not a necessary requirement for “polymerization” of this type.
Each of these theories ascribes the carbonyl doubling to hydrogen bonding, one1 through the mechanism of enolization followed by hydrogen bonding to a second carbonyl group, and the other218 to an activation of the a-hydrogens such that they become acidic enough t o bond to a second carbonyl group, perhaps in a cyclic dimerization. On either basis, the bonded carbonyl gives rise to the lower frequency band and the free keto to the higher frequency band. I have obtained additional information which I believe can best be interpreted by a dipole interaction of carbonyl groups rather than by either of the above hydrogen bonding theories. Infrared and Raman spectra6 of cyclopentanone (I),cyclohexanone (11), and several straight-chain ketones were run in this Laboratory. The doubled carbonyl band of cyclopentanone is clearly resolved in the Raman spectrum and verifies the doublet separation of 20 cm.-l. However, this band could not be resolved in the infrared spectrum although a slight shoulder wm observed a t less than 20 cm. -l. Upon dilution with a non-polar solvent such as carbon tetrachloride, the lower frequency carbonyl band of I decreases relative to the higher frequency band and disappears a t a concentration of about 10%. On the other hand, the infrared frequency is only slightly affected by dilution, and is nearly identical in frequency to the higher frequency band in the Raman spectrum. It appears probable from this behavior that the vibrational selection rules for the dimer may be different from those for the monomer; if the dimer is symmetrical, the infrared frequency may correspond to the higher (asymmetric) and the Raman to the lower (symmetric) vibration. On this basis, one would therefore expect the infrared frequency to be very nearly equal to the monomeric frequency and little or no shift would be expected as concentraton is changed. In pure I, association amounts to nearly 50% (7) R. E. Rundle, J . Am. Chem. Soc., 69, 1327 (1947): J . Chem. since the Raman carbonyl bands are of nearly Phys., 11, 671 (1949). equal intensity (ratio of free t o bonded is 1.15). This rules out enolization as the cause of splitting because Ingold6 reports an experimentally determined enol concentration in I of only 0.0048%. INTERMOLECULAR ASSOCIATION OF Furthermore, the enol content of I1 is 0.02% so CARBONYL COMPOUNDS that, if enolization was the cause, splitting should be greater in I1 than in I. BY ALVINW. BAKER Hirschfelder and Courtauld molecular model Rsreaich Departmsnt, Western Division, Th? Dow Chemical Company, Pithburg. Cdafornzo construction of I and I1 indicates that there is Received February 7. 1868 probably small difference between steric or inducThree recent have discussed the tive effects involving activation of the a-hydrogens. Raman spectra of aliphatic and alicyclic ketones, If the extent of ring strain due to hydrogen-hydroand the authors have come to different conclusions gen repulsion is considered, the constellation theory regarding the causes affecting the broadening or of Brown and Borkowsk? predicts 3 4 kcal./mole splitting of the carbonyl band. Their data show for I and less than 1-2 kcal./mole for 11. This is that the splitting is of the order of eight4 wave due to the fact that the methylene groups are numbers (cm.-1) for all ketones which were exam- alternately staggered above and below the plane of ined except for the larger splitting of 20 cm.-l in the molecule in I1 while I is more nearly planar. cyclopentanone. (5) Raman spectra were obtained with a Hilger reoording Raman (1) E. Gray and M. M. Bottreau, Compt. rend., SH), 2134 (1955). (2) W. Suetaka, Gam. chim. id.,8% 768 (1962). (3) W. Suetaka, J . Chum. Soo. Japan, Pure Chum. Sect., 74, 318 (1963). (4) C. A., It, 962b (1956), incorrectly statea 18 om.-’.
spectrometer. (6) C. K. Ingold, “Structure and Mechanism in Organic Chemistry,” Cornell University Press, Ithsca. N. Y., 1963, p. 566. (7) H. C. Brown and M. Borkowski, J. Am. Chsm. Soc., 74, 1894 (1952).
