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MODELS OF PLANE MOLECULES' BRYANT HARRELL and ALSOPH H. CORWIN The Johns Hopkins University, Baltimore, Maryland
CERTAIN of the atom-model sets presently available commercially, such as the Fisher-Hirschfelder-Taylor sets, suffer from the limitation that some of the more complex ring systems such as porphyrin, phthalocyanine, and certain aromatic, fused rings cannot he satisfactorily constructed. We have found that useful models of such ring systems can be made relatively easily by taking advantage of the fact that they are planar. I n brief, the outline of the molecule is drawn on a plank having the scalar thickness of the carbon atom and then is cut out as a unit. Provision is made for attaching side chains, which may be constructed from the atoms of the commercial kits.
(1) All distances were measured to the nearest millimeter. (2) The central hole was altered slightly to accommodate iron instead of nickel. (Ferric iron was assigned a bond length of 1.2 (3) The molecule was drawn completely centrosymmetric. (4) The pyrrole rings were rendered somewhat more symmetrical, as might be expected with the removal of the fused benzene rings. The skeleton of Figure 1 resulted. The intersections of the lines in Figure 1 give the locations of the centers of the various atoms. About these centers were inscribed circles having radii corresponding to those employed in the Fisher-HirschfelderTaylor sets. However, it was found that the pyrrole carbons tended to overlap in the center of the pyrrole ring, which is logically undesirable, so the carbon radius was shortened slightly to 1.1 em. so as to practically eliminate the overlap. The radius of nitrogen was taken as 1.2 cm. Faces between the atoms within the molecule mere drawn as illustrated in Figure 1 (lower right pyrrole ring). External bonds for carbon and nitrogen were extended 0.7 cm. from the center of the atom4. The direction was assumed so as to make the angles a equal (Figure 1 ; only one external bond is shown). Erasure of unneeded lines now gave the drawing illustrated in part in Figure 2. This was transferred to the surface of a smooth plank of maple wood, 7/8 in. thick (approximately the adjusted diameter of the carbon atom), and the molecule was cut out on the jigsaw, removing wood inside the ring as indicated by the shading of Figure 2. A sharp-pointed tool was employed to inscribe the lines marking the C-C and C-N bond faces. Ti1. Louting C e n t m and Fsc- in A t o m of Pomhmin Modd Shallow holes were drilled in the centers of the exThe construction of the porphyrin ring is described ternal bond faces of the molecule to receive the standard t o illustrate the method of constructing molecular Fisher-Hirschfelder-Taylor clips (obtained from Fisher models of planar rings. The ring was designed for Scientific Company, Pittsburgh, Pennsylvania) using a simple homemade tool which cut a hole the shape of chelation with iron. The model is scaled one om. t o one A. to conform the clip. (The clips should fit snugly.) A small with the Fisher-Hirschfelder-Taylor models. Bond dis- screw through the central hole of each clip was used t o tances and angles were taken from those determined hold it tight. The molecule was then sanded and painted to match for nickel phthalocyaninez with the following modithe commercial atoms: nitrogen blue, carbon black. fications: Since cobalt, the only metal atom available in our kit, 'This investigation was supported by a research grant (RG- is quite close to (covalent) iron in size, we have used it
3752) from the National Institutes of Health, Public Health Service. 'ROBERTSON, J. M., AND I. W O O D W ~ J. C DChem. , Soc., 1937, 221.
a PAWLING, L., "The Nature of the Chemical Bond," 2nd ed., Cornell University Press, Ithaca, N. Y., 1942, p. 182. ' Ibid.,p. 164.
APRIL, 1955
187
instead of iron. The hole in the center of the porphyrin ring was carefully shaped so that the metal atom is held tightly by friction, yet may be removed when required. In the same manner we have constructed models representing pyrrole, pyrromethene, imidazole, and purine. Except for pyrrole6 there are no X-ray data available for these molecules, so we have had to assume what their shapes are. For instance, nitrogen is substituted for a carbon of the pyrrole ring to make imidazole, and pyrromethene is drawn from two pyrrole rings and a bridge carbon of the porphyrin ring. I t is interesting that when two pyrrole rings are joined by the C4(aromatic; benzene) carbon atom from the Fisher-Hirschfelder-Taylor set, the molecule corresponds quite closely to the pyrromethene molecule derived from phthalocyanine. Four pyrrole rings may he hooked together in this way to form a porphyrin ring. I t will be seen in Figure 3 in which our pyrrole is compared with the Fisher-Hirschfelder-Taylor benzene, that very little is lost so far as utility is concerned by not using rounded atoms to construct the rings. This holds so long as the atoms in the ring have very nearly the same diameters, as is the case with carbon and nitrogen. Were, say, thiophene to he constructed, in which case the sulfur has a diameter appreriably greater than that SCHOMAKER, V., AND L. PAULINC, J . Am. Chem.Soc.,61,1776 (1930).
Figure 3.
F i w s 2.
Pi-1
D.D.D.~"~ of Po,,,h*in
Model
of carbon, it would he necessary to leave a gap in the ring for sulfur, shape the faces of a two-bond sulfur from the Fisher-Hirschfelder-Taylor models to fit the gap, and glue the sulfur in. Obviously, the type of model described here is not limited to use with Fisher-Hirschfelder-Taylor atoms. The construction can be easily modified for use with any similar kit.
Pmole Model with One Hydrogen Remound. Compared with
P-H-T Benzene
