phane (1) system (8). Thiscompound, originally prepared by Shinmyozu, Inazu and Yoshito (9) has been recently thoroughly studied hy Semmelhack et al. (lo), who devised a hieh-vield svnthesis. determined its crvstal structure and dynamic beliavior, and performed mole&lar mechanics calculations on it using the MM2 force field as implemented in the program BIGSTRN-3. Cyclophane (I) is an almost ideal system for a computational experiment since the molecule can in principle exist in several conformations, yet, despite the conformational complexity, the molecule is of a relative small size which enables fast calculation of the different conformations. E n t e r Range t o be Prlnted A18 F17
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TRAN, and PASCAL. Although the cell manipulation and screen writing modules of SSIO are relatively complex, they are transparent to the user who needs only to understand how to use the API. The interaction between the user program and the SSIO API must he in the C language, hut FORTRAN or PASCAL can be used for the application subroutines since Microsoft C, Fortran, and PASCAL all support mixed language usage. T o use SSIO, one requires an IBM PC (640 k) with DOS 2.1 or higher, Microsoft C v5.0, andlor Microsoft QuickC 772.0. Additional Microsoft languages such as FORTRAN are required only if the application module uses a language other than C.
Molecular Mechanics Calculations of [3.3]metacyclophane Sllvlo E. Biall The Hebrew Unlverslty of Jerusalem Jerusalem 91904, Israel Most undergraduate chemistry curricula lack computational experiments in organic chemistry. This is rather unfortunate, since computational chemistry is rapidly becoming a commonplace research tool and it is highly desirable that students should become familiar with this powerful technique as early as possible in their studies. Recently, Lipkowitz and others described in this Journal molecular mechanics experiments (6).We would like to describe in this article an undergraduate computational experiment introduced last year in the Advanced Organic Chemistry Laboratory at the Hebrew University. The goal of the computational experiment is to familiarize the students with molecular mechanics (MM) calculations (7). In choosing the chemical problem to he solved by molecular mechanics, we were guided by the following considerations: Firstly, the target system should in principle exist in several conformations, but the energetically preferred conformation should not he directly inferred by simple examination of molecular models. Secondly, the chosen system should be of sufficiently small size to assure fast calculations of the different conformations.'I'hirdly, the input ronf.ormations should be of sufficient complexity to guarantee a thorough examination of moleculk models G.g., Dreiding or Framework Molecular Models) in order to extract the atomic coordinates. Finally, the crystal structure of the molecule should be known in order to allow comparison of the calculated geometry of the preferred conformation vis-a-vis the crystal structure. With all these considerations in mind, we chose for the computational experiment the [3.3]metacyclo1038
Journal of Chemical Education
Background
Cyclophane (1) can exist in principle in five different conformations, three (chair-chair (2), boat-chair (3), and boatboat (4)) resulting from a syn arrangement of the two aromatic rings, and two (5 and 6) resultingfrom an anti arrangement of the rings. Conformations 2-6 have ideal point group symmetries Cz,, C,, Cz,, C,, and Ci. Mitchell and co-workers showed that the predominant conformation of 2,ll-dithia[3.3]metacyclophanes is syn 2 (11). Solution as well as X-ray data show that 1 exists in a syn chair-chair conformation 2 (10). A similar conformation has been recently reported for the 1,1,10,10-tetramethyl derivative of 1 (12). Although MM correctly predicts that the lowest energy conformation of 1is the syn chair-chair, the calculated structural parameters of the conformation are not in complete agreement with the experimental (X-ray) values. Most notably, 1 displays in the crystal a twist of -15' about an axis that passes through the centers of both aromatic rings and that is not reproduced by the MM calculations (10). The Experlmeni For the experiment we used both Allinger's MM2(85) program and t h e MM2 implementation of t h e program BIGSTRN-3 (13), hut earlier versions of the programs can he used if necessary. All calculations were performed on a VAX 750 computer. In the case of the MMZ(85) program, the pi system of the rings was not calculated, hut instead the parameters recommended by Allinger were used (14). This saves computer time, since no self-consistent field calculations are carried out. The transformations from internal to Cartesiancoordinatesand the plotting ofthe initialand final conformations were carried our using the "RaII and Stick"
