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~
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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
zood aereement with nublished work. and the students seem to enjoy this experimknt. Presented a t the 33rd Southwestl37th Southeast Reeional ACS Meeting, Memphis, TN, October 9, 1985.
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An Inexpensive Linear Thermistor Thermometer for Cryoscopic and Calorimeter Measurements and Lecture Demonstrations
provides a way to control the cooling rate of the sample. The modifications not only do not detract from the need for constant and careful stirring and the recording of the data for sufficient periods of time to obtain satisfactory cooling curves but em~hasizethese and other needs. A c o ~ of v the thermometer computer program and a report describing the modifications we have made to improve the quality of the results and the ease of performing this cryoscopic rneasurements may he obtained by writing to JWB.
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James W. Beatty and Todd B. Colln Ripon College Ripon. WI 54971 Two years ago, inspired by papers in this Journal by Chan and Ng (19) and Srivastava and Meloan (201, we set out to build a precision computer thermistor thermometer for general use in the physical chemistry laboratory. In doing background work we discovered a commercially available component that is linear over a wider range, is even simpler to build, and for most operations does not require calibration. The use of a commercially available linear thermistor bridge network thermometer has general applications for those wishing a simple economical unbiased precision thermometer with 10-cm-high numerals for chemistry demonstrations and experiments. We have used this thermometer for lecture demonstrations and for calorimeter and cryoscopic measurements. The thermometer, a YSI8 44212 thermolinear component, consists of three thermistors in a composite and three precision resistors in a resistance bridge network that we incorporated in the thermometer leads. The linear component is the heart of expensive precision commercial thermistor thermometers. The component can be used in voltaee or resistance mode and produces a linear output. We used it in the resistance mode where i t has a sensitivity of 129 tl per degree Celsius. Temperature differences were measured with ease to 0.01 OC with our components and differences of 0.001 "C can be measured with the component. Our limitation was the stability of our resistance meter. The use of the network in the voltaae mode reauires a constant voltaee " sunnlv. .. . The reader interested in;sing the component in the voltage mode is referred to YSI literature and the cited papers (19,
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A Keithleyg Model 179A 4 K digit multimeter with IEEE488 option was used to measure the resistance and convert i t to a digital signal. The digital signal was read by a BASIC program using a Commodore 8032 microcomputer. The data was converted to Celsius or Fahrenheit temperature displayed on the screen in 10-cm-high numerals suitable for lecture demonstrations. The temperatures and acquisition time were held in an array and printed out at the end of an experiment. The acquistion time was programmable. We have resisted the temptation to have the computer graph the temperaturetime data for we believe a t this stage the preparation of the graph is an important exercise for the student. In using the thermometer for freezing-point depression measurement we made modifications of the classic experiment (21) using a thermoelectric stirrer-eooler to remove the heat from a specially designed freezing-point depression cell. The thermoelectric cooler effectively and conveniently BYellow Springs Instrument Co., Inc., (YSI) type 44212 linear temperature transducer may be purchased from Newark Electronics for about $40. Our final temperature probe was this component sealed in a 25-cm-long. 3/1&diameter stainless tube obtained directly from YSI for around $100. The special order number was 03444020-8-RP-8-ST for the thermistor composite and 44312 for the resistor composite. A similar multimeter with an RS-232C interface is available from Omega Engineering, Inc. 500
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Journal of Chemical Education
F w r e 7 Sample cwilng cLwe tor 35/65 w!w SnlPb mlxlde (a) Comp ete ID)Plot expanded in the regcon ot the break
cooling Carve
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Figure 8. Sample tinllead phase diagram.
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