Molecular Orbltal Calculations Using the Simple Huckel Method Jonathan H. Reeder New College Sarasota, FL 34243 MOINT and MOOBJ are interactive programs that solve the eigenvalue/eieenvector ~ r o h l e mof the Simnle Huckel ~ a t r i x(17), using an iterative procedure when degenerate energy levels are encountered. MOINT is written in BASICA for the IBM-PC; MOOBJ is a compiled version that is much faster. The compiled program must be usrd in conjunction with the IBMsystemsprogram HASRC'h'.EXEand therefore requires access t u the IBM BASIC COMPILER package. The programs are designed to he user-friendlv bv beine error tripped at all inpuistxements and by i n k k i n g the usrr when incorrecr. data can be expected. Also,. thev . afford the user, both before and after execution of the program, the opportunity to correct any inadvertent input data entries. The main and most extensive input is the Simple Huckel Matrix. This is entered as individual matrix elements (aU) and therefore requires the user, not the computer, to construct the matrix elements through Huckel theory. MOINT has been successfully tested in the Physical Chemistry 11: Quantum Mechanics class, an upper-division advanced course a t New College, and was found to he a constructive, educational tool to supplement a standard quantum chemistry textbook. MOINT and MOOBJ require a total of 256K RAM to calculate a 24-center molecule. Output requires a printer. Documentation includes listings (with references contained in REM lines), complete input instructions, and several example calculations. Available from Project SERAPHIM.
Figure 6. Block diagram of Interface for SnIPb phase dlagram experiment.
The Tin/Lead Solid/Liquid Phase Diagram: A Computer-Controlled Experiment Kathyrn R. Williams7,John R. Eyler, and Samuel 0. Colgate University of Florida Gainesville, FL 3261 1 In designing an interfaced experiment for the undergraduate physical chemistry laboratory, it is important that the microcom~uterserve as a tool to relieve the tedium of data ~ . ~ ncqui.;ilion, while nnt obscuring fundanieiital chemical principles. An excellent choice in this regard is determination of the solidtliquid phase diagram for the tin~leadsystem. Since tin and lead furm the nlluss of standard soft solders.. these components present an important practical system for student exposure. The phase diagram is known (181, and the regions of solid/solid solution formation are limited. Thus, the system is reasonably simple if mixtures close to zero and one mole fraction tin are avoided. The construction of the cooling curves (acquisition of thermocouple readings and subsequent plotting) can he performed with aid of an APP L E microcomputer. This leaves the students with adequate time outside the laboratory period t o locate the break and arrest points, plot the phase diagram, and perform the error analysis. The tinilead mixtures are prepared for the students in advance by melting together weighed samples of pure tin and pure lead. The melts are poured into stainless steel tubes, each e q u i u ~ e with d an inner stainless steel closed-end tuhe. T o ot&n'a cooling curve, a thermocouple (Chrornell Constantan, is inserted into the inner tubeand the assemhlv is heated over a Bunsen flame until the contents have melt~
~~~
~
~~
Author to whom correspondence should be addressed.
~
ed. The heated tuhe is then placed in a glass wool-lined Dewar flask to cool slowlv. A block diagram of theinterfaced experiment is shown in Figure 6. The thermocouple readings are acquired for two stident pairs by an ~ ~I1 Plus ~ m~crocomp&er l e equipped with an Adalab interfacing card and an Adamux multiplexer (Interactive Microwave, State College, PA). Although small amplifiers could be built to boost the millivolt level output from the thermocouples, the signals are instead fed through Keithley model 177 DVM's (analog output of *2 V full scale). The DVM's serve a dual nurnose. because thev also allow the students to know when the tuhe contents have melted. Students are nrovided with a table to indicate the millivolt range over &ich readings must be made for the particular tinilead mixture. A toggle switch is included in each thermocouple circuit to allow the ground level of the ADC to he determined. Each student pair must indicate to the computer that its tube has been heated and that readings should commence. or, if necessary, that arunmust be aborted. These signahare accomplished via push buttons on a Universal Designer Board (Etronix, Redmond, WA), which is connected to the interfacing chip on the Adalah card. The board is also with eight logic-level LED's, four for each student - equipped . pair, which are used to indicate the status of a run. The software for the experiment consists of a main program written in BASIC, which calls several short machine language routines to check the status of pushbuttons, turn LED's on and off, initiate AID conversions and set and read the timer. Listings of these programs, as well as a more complete description of the hardware configuration, may be ohtained from the authors on reauest. A cooling curve, plotted using Interactive Microware's SCIENTIFIC PLOTTERsoftware, for the 35/65 wlw S n P b mixture is shown in Figure l a . The students are shown how t o vary the plot format to expand the break and arrest regions: an exnanded olot is shown in Fieure 7h. A comnlete l p h a s e diagram is ;resented in ~ i ~ u 8.r eNote that the solidus lines for the solid/solid solutions are not observed using this experimental setup. The values of the eutectic temperature and com~osition(182" C and 0.748 mole fraction-tin) compare favorably to the literature values (18)(182 "C and 0.739 mole fraction tin). Prior to the apparatus being interfaced to the computer, recording and plotting of the many thermocouple readings was very tedious and obscured the ultimate goal of the experiment. Results tended to he poor, because readings could not be acquired often enough for proper definition of the break regions. Now the experimental phase diagrams are in Volume 64
Number 6 June 1987
499
