Molecular Models The utility of Dreiding molecular models is well known to chemists. However, the very rigidity and resistance to certain types of rotation that make this type of model so useful in t h e study of stereochemistry make i t impossible t o make models of certain interesting strained or "twistcd" species, such as the triptyeyl radical (I) and the triene (11) in which the double bond in the eight-membered ring is t,rans. The combination of stainless steel Dreiding stereomodels (Swissco, available in &heU.S. from Rinco Instrument Co., Inc., Greenville, Illinois) with inexpensive plastic Framework Molecular Models (available from Prentice-Hall, Englewaod Cliffs, New Jersey) makes i t possible to prepare models of strained species such as I and I 1 (see photo). The eomhinstion of t h e s e t w o types of models also permits the representation of atomic and molecular arbit,als in an unambiguous fashion. The orbitals of the "twisted" transdouble band of I1 can be represented by colored tubing, with dincrent colors corresponding to the sign of the wave function in different regions of spare. (In the model of I1 in the photograph the tubing representiug orbitals is longer than scale to emphasize what would be more obvious in a color photograph.) The plastic tubing of the plastic ,. -..* - "," models can be cut t o scale using t h e ,, .,..,,,, Angstrom scale available with t h e slninless steel models or t h e plestic tubing can be used simply as a eonnector. I n cutting t o scale allowsnce should be made for the fact t h a t the H tubing will contact the soldered conI II nection of the stainless steel models. I n some highly strained molecules i t is advisable to wrap the rod of the stainless .steel model with transparent tape; the tube does not require this precantian. Models af this typo are very useful in illustrating t,he principle of conservation of orbit,al symmctry (the Woadward-Hoftrnan rules) recently elaborated hy R. B. Woodward and Roald Flolrrnall [See H O F F M . ~II., , .AX" W O O D N . \R. ~ , R., Accounts 0.f Chemical Resea~ch,1, 17 (1968) W ~ M OB.N CHIPMAN and the articles cited therein.]
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118 / Journal o f Chemicol Educofion
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