i
mean residence time independent variable in regression estimated dependent variable in regression 4 U,O transformation coefficients, eq 1 subscript 0 inlet or initial conditions x
NCIN LINEAR REGRESSION
1 1.060E-0011 Maxllnun v a l u e of kA-
error Interval
Maxmum
:
1.59BE-0011 : L 5.2991-0051 :
[
Results: Number of icerations . . . . Final ~ n t e r v a l. . . . . . . . . . kA . . . . . . . . . . . . . . . . . . . . . . S u m of square error... . .
21 3.88508-005 : 1.3190E-001 : 1.2275E-008 : :
Figwe 5. An example of the final examination
THE ERROR SUN OF SQUARES AS R NWCTlOW OF M Linkage of Laboratory Exercises with Theoretical Calculations on a Personal Computer Ronald A. Balley Rensselaer Polytechnic Institute Troy, NY 12181
IHE MDEL COMPARED TO M P E R I ~ A LDATA 0.8890
.
7
L
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M " ' ~
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l
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0.6040 0.60
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15.88
line, nin Did you find the nininun of the sun oE square e m m ? Figure 4. Nonlinear regression of
rate data. Case: saponification of
acetate.
838
Journal of Chemical Education
ethyl
With the availabilitv of oromams that will perform reasonable calculations moiec& properties i n a personal cornouter. it is practical to bring- a~vlications of theoretical .. chemistry into ;he undergraduate program in a realistic way. When such cnlculntions arc linked with a laboratory measurement, the connection between theory and experiment can be reinforced. The dipole moment of a molecule is one property that is readily measured in a physical chemistry laboratory (6) and that can he calculated from available nroerams. f h e molecule that we have dealt with experimentally in our junior undergraduate laboratory is methyl acetate, but many others could be used. Methyl acetate is readily visualized in either a "cis" or "trans" configuration, and the measured dipole moment should provide information on the conformation that actually predominates. Expected dipole moments for each conformer are easily calculated from the vector sum of bond moments, but a calculation based on more advanced theoretical approaches not only serves to introduce and illustrate molecular modeling techniques but also provides other information ahout the energy relationships of the two forms and is a convenient tie to other lecture to~ics. A useful and inexpensive program for these calculations is availableon theSERAPHIM disks IB 1401,1402,and 1406.' Using the first two disks, the approximate structure of the molecule can be entered and the energy minimized to give the structure with the first local minimum. The coordinates of the minimum energy structure can be transferred to the program on the 1406 disk that performs an energy minimizing calculation that gives, among other parameters, the di-
' SERAPHIM disks 1401 and 1402; programs wrlnen by J. Gajewskland K. Gilbert. 1984. Seraphim disk 1406; program wrinen by G. S. Owen. 1984. Disks are available from Project SERAPHIM.
pole moment. (Transfer can be either by printing the data file and re-entering the coordinates manually, or more conveniently by a file transfer program). The results, 4.89 D for the "cis", and 1.8 D for the "trans" conformations are realistic, while the energy difference provides a quantitative explanation for the expected predominance of the "trans" form. (The experimental dipole moment is 1.78 D (7).)Requirements for running these programs, an IBM-PC with 512 K memory and an 8087 coprocessor, are not excessive, while SERAPHIM programs themselves are available at nominal cost. Typical time requirements have been 2 h for a given conformation. Although we employed the SERAPHIM programs initially and found them to be useful, we have now switched to a faster and more convenient, although more expensive, program, PCMODEL from Serena Software2 (academic price $200). In addition to the results eiven bv the SERAPHIM disks, this program readily additional information, such as plots of enerw as a function of rotation about a bond. that permits the student easily to explore the details of conformational changes and rotational barriers. Figure 6 illustrates one type of information that can be displayed. This shows the variation in energy on rotation about the CH&-OCH3 bond. This program also gives a simple readout of the structural parameters of the conformation. Students find the data input easy and the entire exercise fast enough to permit extension to other molecules in a reasonable time period. This approach provides a useful linkage of experiment to theorv,as well asa valuableop~ortunitvLO link this material further to lecture topics deaiing with molecular conformations and theoretical chemistry. It is important to make clear the approximations and limitations of the calculations to show that they are not a replacement for experiment, but a guide and supplement.
Microcomputer Interfacing Programs Shahmkh Ghaffarl Mount Marly College Yankton, SD 57078 With recent emphasis on computer interfacing and automation of chemical techniaues and instruments it is inevitable for chemists, especialliin the field of analytical chemistrv. to overlook the application of computers and their place iiboth undergraduate teaching and research. To extend the capabilities of our instruments such as DBG Beckman spectrophotometer and Gow-Mac gas chromatograph and to introduce students to the applications and the advantages of microcomputers in analytical laboratories, Microcomputer Interfacing Programs were developed. These programs are designed to work with most instruments, and required no computer programming or electronic experiences. The package consists of three programs. The first program is desimed to work with spectrophotometers. In the first section-of this program students are able to scan for sample cell comparisons (in double-beam instruments) andlor find the maximum absorbance. Data are plotted on the screen as they are collected and the program finds the wavelength of maximum absorbance (Fie. 7). Then the snectrum and other information can be stor& on a disk an'dlor printed on a printer. Scanning can be done in two ways, automatically or Available from Serena Software, Box 3076, Bloomington, IN '47402.
Figure 6. -gy
Screen display hom the FCMODEL (Serena) program. as a function of mtation about CH3-OCHs bond.
0
7 b
600
500
400
showing
300
Wavelength (nm) Flwe 7. Wavelengm scan for (a) sample cell comparison, (b) finding the wavelength of the maximum absotbance.
manually, from low wavelength to high wavelength or vice versa. Further scanning is possible using the same plot or selecting different conditions. The second part of the program is designed to collect data from the instrument through an A D for a set of standards and unknown(s). Data are collected and saved as both absorbance and percent transmittance. Results are tabulated, and the best fitted line is plotted. Also other statistical information such as slope, least-square line equation, etc., are presented (Table 1). This program is able to calculate the concentration of one or more unknowns using the same set of standards. If more than one unknown is run, average or separate values for concentration of unknowns can be obtained. Table of data, result of calculations, and calibration curve (Fig. 8) are printed on the printer. Also a simple interface circuitry for Spectronic-20 is available in the supplementary material. The second program, which is developed to work with a Gow-Mac gas chromatograph, also can be used with most other chromatographs. This program can be used to obtain a simple chromatogram for a single injection or to run standardlunknown solutions for a quantitative and/or qualitative experiment. In the latter case after initialization, which includes entering the operation conditions, the computer Volume 66 Number 10 October 1989
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