The Use of Model and QCPE 506 in the Undergraduate Instrumental Laboratory Semiempirical Calculations Thomas S. Lillie' and Kirk Yeager Lafayette College, Easton. PA 18042 The interfacing of computational methods with experimental techniques has evolved to the point that each can be used to heln understand the other. There are. however. few undergrad;ate experiments in the literature that combine these techniques into a cohesive laboratory experiment (I, 2). In recent years the advancement in computers has drasticallv reduced the reauired comoutational time for compoun& of interest. At t h e same time the cost of the necessarv software to do so~histicatedcalculations has been greatly reduced by special iducational discounts. These two factors alone have made the ~ossibilitvof interfacing computational chemistry with insirumentd methods feaiible a t the undergraduate level. Computational methods themselves have grown rapidly in the last decade as evidenced by the increasing number of iournals that deal exclusivelv with comnuter methods in chemistry. Semiempirical andforce field techniques are two of the more orominent methods cited in these iournals. The Quantum ~ L e m i s t r yProgram Exchange a t 1niiana Universitv is the source of manv modestlv nriced Droarams that are available to run on either mainframe cornput& or personal com~uters. 1n.our senior-level laboratory course at Lafayette College we have combined many computational techniques with experimental $11 literature assignmenu to emphasize the strengths and weakness of these programs. In this paper I wish to describe a successfully implemented laboratory ex-
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periment that uses AMPAC calculations in conjunction with infrared spectroscopy. The lecture portion of the class discusses normal modes of vibration, symmetry, group frequencies, isotope effects, internal coordinates. and calculation techniques, including a discussion of the computer programs AMPAC and ~ o d k l . Since the rer requisite for thisclass is physicalchemistry,the students have already been exposed to the theory behind these methods. In the first experiment the students run infrared spectra and do aemiempirical ralculationv on benzene, benzene.de, chloroform. and chloroform-d. Benzene and chloroform were chosen because of the minimal number of fundamental freauencies observed. The exoerimentallv determined frequekies are compared to the calculated values, and individual neaks in the IR soectra are assigned t o the aooro~riate .. . stretching or bending vibrational modes. The internal coordinates or z-matrix input for AMPAC were generated with the use of Model.2 hey may also he generated with other structural input programs that are a ~ a i l a h l eModel, .~ originally written by Clark Still a t Colum-
' Author to whom correspondence should be addressed. ?Model is available for the VAXIVMS computer from Kosta Steliou at Montreal University. The PC verslon s availab e from Senna SOHware. QCPE offersa number of programs to aid in the input to AMPAC and MM2.
Volume 66
Number 8
August 1969
675
hia Universitv and since modified hv Kosta Steliou a t Montreal ~ n i v e r i i t y ,allows the mo1ec;le to he preminimized with molecular mechanics before input to AMPAC. Preminimization with Model has avoided many of the problems associated with the modeling of riug-containing compounds by semiempirical techniques. The students are each assigned a different compound for which vihrational frequencieihave been calculatedusing the MNDO Hamiltonian (3). They run infrared spectra and AMPAC calculations a t the AM1 level. Each student assigns peaks in the spectra with respect to their functional group, knd the resultsof the classarecompiled constructinga group frequency tahle. Assignment of peaks iiaided by the Natiunal Hureau of Standards Tables of blolecular Vibrational Frequencies (4). These values are compared to those calculated bv Dewar et al. usine MNDO and blIN1)0/3 Hnmiltunians (5).
Table 1. A Comparlson of Calculated Vlbratlonal Frequencies to Experlmenlal Data for Benzene, Benzene-d., Chlorolorm, and Chloroform-d Frequency
Chloroform Chloroformd
C-H C-H
Sbetch Bend
302013003a 121511119
225612206 9081894
C=C
Stretch Shetch
7561856
7331894
C
