The computer program offers the starting material and a data base for the paper and subsequent debate. In addition to thekormation sipplied by the computer oroeram..our students are eiven a brief introduction to risk assessment, risk management, and an explanation of the term LDs0 . To generate controversy and debate about the BCTC problem the students are assigned specific roles. Each student assumes a specific role and writes a five-oaee . paper. The role-playing continues, more effectively as the students present their papers in an oral presentation that usually results in lively debates. The instructions for the paper given to each student state: A
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There mav be a ~mblemin a small town in South Carolina. You willgain informarion from a task force that hns bwnenllrd in to help study this potential prohlem and perform a risk arsessment evaluatxun uf the situation. Achernical called HCTC is involved Several items should be included in all papers: What is BCTC? Is BCTC a health hazard? Is BCTC in the environment? Will BCTC effect the health of the people in the city below the plant? What effect does it have on animals or humans? What will it cast to dean it up and operate the plant? Although you are being assigned only one role, you cannot totally ignore the other factors. The followingis the list of characters available: reoorters from liberal and conservative and domestic and foreign newspapem and Loeal and national television; scientists including a physician, veterinarian, chemist, and environmentalist;businessmen from the company including the plant's vice president, economic advisor, and public information officer; political leaders all of whom are up for reelection including the mayor, the mayor's economic advisor, a member of the county industrial development board, the district coneressman. a local lawver runnine.. aeainat ., the congressman, and clttrens lnrludlnfi a farmer w h o llvea downstream from the plant, a downtown busmessman, a mcmber of Greenpeace, and the head of the lural unwn I n writing the report, most of the students fall into their assigned roles with ease. The most descriptive writing comes from the students given the role of television reporter. For example: "As our boat moves slowly up the river we see a group of unsuspecting Boy Scouts playing in the river downstream from the deadly plant. Will this be the last summer the camp will be open? This is I.D. Rather reporting for 50 Minutes from trouble-ridden South Carolina" (10).Once the students have written their papers and read them during the followine laboratorv " period. it is amazing how they keep their preconceived ideas even aRer the ~rofessorhas exolained that the results of the computer program are meant to be ambiguous. This exercise has been effectivein teaching students how scientists research. collect. and analvze data.The student' 'papers emphasize their inkrpretatiGe skills and points toward their strengths and weaknesses in their risk assessment, which is the main objective of the exercise. The oral resenta at ions allow all the different views to be presented. Pinally, the instructor must pull i t all together and put the exercise in its proper perspective by giving a summary to the students of what risk assessment and risk management are. along with a brief lecture in the importance of s c i e n t i f i c ' o b j e ~and ~ t ~ethics. Employing such teaching methods in the classroom not only makes chemistry tangible, but also it gives the students a more comprehensive learning bv brin!zing together interdisciplinary - experience studies.
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A Potential Energy Surface Experiment for the Undergraduate Computational Chemistry D. C. Buss and K. R. ~ountain' hortheast miss our^ State Ln vers ry Klrksv le. M~sso~r 63501
Recently several authors have noted the desirability of placing computational chemistry in the undergraduate curriculum (11. 12). At NMSU we have two vears ofexoerience in teaching computational chemis&y in our i d vanced Organic Chemistry course. The course is well received by students, as shown by student questionnaires, and produces a deep understanding and appreciation of theoretical material presented in previous courses. Concrete experiments using the computer as an instrument are embhasized, usingmodern modelling methods. One important concept that students fmd hard to masp when presented in theabstract is the notionofa potential-energy surface (PESI. Two-dimensional representations, such as reaction energy profiles, are more easily grasped, but have limited usefulness when it is necessary actually to deal with n three-dimensiunal tor . hieher) - . relationshio. Such relationships oRen are encountered when calculating transition states with MO oackaees. A computerized animation of a three-dimensional PES has been reoorted (1.71. but it is soecificallv related onlv to simple chekical reactions. The present ekperiment $vex experience in m a ~ ~-i an PES g and uses a research (discovery) mode. Local minima, saddle points, and hill tops often are useful, because they allow students exposureto them in some concrete way. This paper describes such a n exercise. Students const&ct t h e - ~ for ~ srotations around the 1 , 2 and 2 , 3 bonds of 2-hydroxy methyl acetoacetate, 1.
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2-hydmxy methylacetoacetate The computer used was either a 286 PC clone or a Zenith 386 (16 mHz). The software was PCMODEL 4.0 (Serena Software, Box 3076, Bloomington, IN 47402-3076) and SURFER (Golden Software, 809 14th Street, P.O. Box 281, Golden, CO 80402-0281). PCMODEL uses a variant of the molecular mechanics orneram bv Allineer (MM2-87) with (MM% for the extension of gene;al k r m fiild groups not included in the MM2 force field. The ability to compute conformational energies is greater than that of MM2-87 due to these eeneralized oarameters. The program uses standard grld search methods to minimize a classical mechanics e n e m function consisting of a sum of terms for each mode of freedom for a given molecule (14, 15). Surfer allows input in a spreadsheet format. The input from this project was put into SURFER as a set of threecolumn vectors summarized in the table. The orocess of obtainine the PES beeins bv minimizing n-hexke in the all trans-conformatior;, usiGg the DRAG and MINIM routines of PCMODEL 4.0. Redacine the aopmpriate ki atums with 0 atums, double bonded to the C , and the 62 C atom to get the ester linkage gives the hegin-
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'Author to whom correspondence should be addressed. Volume 70 Number 4 Aprii 1993
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Data Values for Three ROT-E Studies" ot
ox Energy (kcals)
0.03 0.03 0.03 0.03 0.03 0.03 0.03 0.03 0.03 0.03 0.03 0.03 0.03 0.03 0.03 0.03 0.03 0.03 0.03 0.03 0.03 0.03 0.03 0.03
15 30 45 60 75 90 105 120 135 150 165 180 195 210 225 240 255 270 285 300 315 330 345 360
-0.4 -0.24 0.22 0.67 1.05 1.8 4.5 11.39 18 15.44 9.59 5.15 2.32 0.65 -0.3 4.65 -0.35 0.69 2.31 3.64 3.63 2.45 1.03 0
mt
wz Energy (kcals)
15.03 15.03 15.03 15.03 15.03 15.03 15.03 15.03 15.03 15.03 15.03 15.03 15.03 15.03 15.03 15.03 15.03 15.03 15.03 15.03 15.03 15.03 15.03 15.03
15 -0.08 30 0.25 45 0.75 60 1.17 75 1.38 90 1.6 105 2.28 120 3.59 135 4.55 150 4.56 165 3.98 180 3.05 195 2.01 210 0.98 225 0.15 240 4.27 255 0.15 270 0.72 285 1.99 300 2.95 315 2.75 330 1.67 345 0.6 360 0
at
oxEnergy (kcals)
30.03 30.03 30.03 30.03 30.03 30.03 30.03 30.03 30.03 30.03 30.03 30.03 30.03 30.03 30.03 30.03 30.03 30.03 30.03 30.03 30.03 30.03 30.03 30.03
15 0.01 30 0.37 45 0.85 60 1.2 75 1.31 90 1.28 105 1.3 120 1.51 135 1.88 150 2.3 165 2.58 180 2.52 195 2.01 210 1.14 225 0.23 240 4 . 3 1 255 -0.26 270 0.47 285 1.68 300 2.61 315 2.42 330 1.42 345 0.47 360 0
through 360' the experiment is over. Atable of data results recording both angles and the corresponding energies (see table). This table is entered into SURFERS GRID INPUT spread sheet and a .GRD file is generated. The result is
Figure 1. The conformation from the first minimizatidn
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These are benchmark values to guide the reproducibility of readers attempts. Afull table of all the values is available from the editor of this column or from the principal author. (emall sct8@NEMOMUS) Vhese three columns are directly from the first three student ROT-E studies used in wnstructlng Figure 3.
ning molecule. Minimizing the molecule with the MMX force field of PCMODEL 4.0 gives a conformation similar to Figure 1. Rotate the C-O-H bond to coplanarity and a cisoid relationship to the keto C = 0 , and mark the H atom for hydrogen bonding. Reminimize the structure to get a conformation similar to Figure 2. You should have torsional angles for the wland 02 similar to those in Figure 2. Use ROT-B to move the keto C = 0 and OH angle to near zero degrees, and then perform a ROT-E study ( a rigid rotator) about 02. Arigid rotor assumes only the change of the bond angles ol and Q. All other degrees of freedom are kept constant as the study about the mtated bonds is performed. For this reason results are expected to be greatly different from those obtained by a bond rotation followed by a complete minimization of the remainder of the structure at that particular dihedral (torsional) angle. This latter is also pmvided for by PCMODEL as a D-DRV study. D-DRV pmvides a more complete and much more accurate study of an energy hypersurface, but it takes over 20 h on our Zenith 386 (16mHz)instrument. Thus, it is less suitable for a laboratory session with students where computer time and availability is sharply limited. In the present experiment each student can be assigned a particular vector in the torsional angle study to obtain. Pooling these data in SURFER allows cooperative learning to get an overall class result. Record the energies and the angles, using 15' increments, through a full 360' pathway. Move wl to 15' and repeat the ROT-E study again. Continue to increase wlby 15' increments and follow each increment by a ROT-E study through 360' for Q. When wlhas been advanced 296
Journal of Chemical Education
Figure 2. The conformation after H-Bonding
Figure 3. The potential energy surface for 2-Hydroxy Methyl Acetoacetate.
Figure 3, when it is viewed on the monitor. The geometries corresponding to A, B, C, and D on Figure 3 are in Figure 4. The PES shows most of the features anyone would encounter in computational chemistry. There is a saddle point at A connectiIlg a local minimum at B with a deeper, apparent global minimum at C. A particularly instructive feature is that the saddle point is actually canted so the path from B to C is not a direct one. Often this is a feature of transition states (saddle oointsl that is comoletelv lost in the undergraduate cdculuk. ~ccasionailya Groad plane, where many loosely coupled motions can occur, is found in computational chemistry. This fact also is illustrated. Finally, a hilltop, at D, illustrates situations where gradient searches in such programs as MOPAC 6.0 would give two negative eigenvalues for Hessian matrix diagonalization. This occurs often when locating saddle points with SIGMA or NLLSQ in MOPAC. It is diagnosed by running a FORCE calculation to obtain two or more negative eigenvalues. We also have experienced this problem with the newer routine eigenvector following (Keyword TS in MOPAC). Seeing this feature alerts students to the meaning of two or more negative Hessian eigenvalues. The use of this lab experiment has given students insight into molecular mechanics, and later, the vagaries of transition state computations with MO packages. Such visual connection of the rotational angles q and m in 1with the energies in casting the data into a 3-D representation opens up entirely new prospects for students accustomed to 2-D transition state representations in textbooks.
Acknowledgment We thank the Petroleum Research Fund, administered by the American Chemical Soeiety, for support of this research.
Chemical Checkers on the Computer Y. G. Orlik, P. V. Glyakov, and R. M. Varova
Byelorussian University Minsk 220050, Byelarus The ~ l e ofs a well-known game served as a basis for the computer program "Chemical Checkers", implemented for the IBM personal computer and available from Project SERAPHIM. The program "Chemical Checkers" is intended for mastering the following inorganic chemistry topics: "Chemical properties of the main classes of inorganic compounds", "Chemical Properties of Metals", and Chemical Properties of Acids, Bases, and Salts". The program enables the students to organize and sum up their knowledge of the topics given above. Feedback is a necessarv condition for effective leamine. This im~lies monitoring acquisition of the material s t h i e d and'disolavinr information about the results achieved. This function i'implemented in the learning mode during the course of the game. The rules defining the set of possible moves constitute the basis of any game. The rules of the game "Chemical Checkers" include one characteristic feature-chemical compounds are checker pieces: acids, salts, oxides, metals, water. Their formulas are displayed on the screen. According to the rules, the checker piece can be removed only
C Figure 4. The geometries corresponding to A, B, C, and D in Figure 3 Volume 70 Number 4 A~ril1993
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