D. W. Beistel
1
A
Computer-Oriented Course Chemical Spectroscopy
The digital computer is becoming as essential to modern, spectroscopy as the spectrometer. Applications range from data acquisition to solution of the secular equations. Yet most courses in chemical spectroscopy make little use of the computer as a teaching aid. That is not too surprising because introductory courses in chemical spectroscopy tend to emphasize the interactions of electromagnetic radiation with simole molecules and the eauations for these interactions can be solved manually in most instances. While the spectroscopist is concentrating on simple molecules his colleague in introductory organic chemistry is makine use of such w t e n t tools as Woodward's rules (I), taburations of outpit from molecular orbital calculations (2), and plots of nuclear magnetic resonance spectra (3). These help make the course relevant to modern research in oreanic chemistm and of immediate use to students. perhaps that is whi most institutions list a course in chemical spectroscopy in their catalog, but seldom offer it. The organic chemist will be the first t o suggest that spectroscopy should be taught by a spectroscopist. He has no intention of pre-empting a colleague's area of expertise. But if his students are not able to obtain a working knowledge of spectroscopy from a formal course in that subject, he must use his own forum to introduce the topics relevant to his discipline. A computer-oriented course in chemical spectrogcopy can combine the mathematical rigor of spectroscopy with the study of relevant molecules. A brief discussion of one such course may he useful to instructors interested in a modern approach t o the topic. No attempt will be made to discuss the lectures involved. Rather, emphasis will be placed on the programming used to illustrate the principles of spectroscopy. A summary of the syllabusis given in Table 1. The level of presentation was that of the text by King (4), supplemented when necessary with illustrations from Ballhausen and Gray (5) and Herzberg (6). Because no prerequisites were imposed, a review of quantum mechanics established the formalism required to consider the mathematical models for each spectral region. In discussing each spectral region the input requirements for the appropriate computer programs were considered after the treatment of "simple molecules." No attempt was made to teach programming; instead the programs were used to give numerical solutions for secular equations that could not he solved in lecture in a reasonable period of time. They extended application of the theoryrather than existing as separate theoretical entities. The Hiickel (7) and self-consistent field (8)approximation methods were employed in conjunction with uv spectroscopy. A 'wider range of Fortran programs was available (see Table 2), but the choice of programming wss dictated by core size and the amount of computer
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Based on a paper presented to rhe Symptmrum on the Status of Cumputer-Assisted lnstrurtiun In Chemistry. .Joint Cunferenw of the Chemical Institute of Canada and the American Chemical Society, Toronto, May, 1970.
Table 1. Course Content and Approximate Emphasis for Each Topic Topic
Lecture Hours
Techniqurr of spectnl3copy Principles of quantum mrrhanirs Electnmic sorrtra (a) diatoke molecules (b) group theory ( c ) polyatomic molecules Vibrational spectra: ir and Raman Nmr suectra
5.5 2.5 10.5 6.0
Table 2.
Programs Used by the Class
Programs Considered
Soectral reeion Electronic
Nuclear magnetic resonance
by Each Member of the Class Fortran .roer- ram
HkkelMO (Wiberg) Pople SCF (Wiberg) Extended Hiickel (Hoffman) NMRIT (0)(Swalen& Riley)
Programs Studied by Selected Aduanced Students
Program
COORD (PROXYZ)
IR Prism Search
I.AOCOONIIII, I'OPLE rextended) PPP SCF, LCAO \I0
Description
Calculates x,y,z coordinates of atoms Used to identifv similar sueetra in the ~ a d t l & prism IR'catalog A :-spin nmr program 'l'hr I'opir SCF rugr ram mudrfred t o ronsdrr C, N. and 0. t\lrCrsnshanl An ad\.dncrdvl-electron 110 calculationCalculates vibrational-rotational transitions for diatomic molecules
time allocated. The heteroatom Huckel and CND0/2 (9) .Droprams would offer the best choices for studv to those fortunate enough to have large computer facilities a t their disposal. Typical time requirements are given in Table 3. A possible weakness in the state of the art of advanced LCAOIMO oromammine is the lack of identification given the ei&nf;nctions a n d eigenvalues. Only the P P P / SCF/LCAO/MO program (10) contains subroutines to calculate the transition frequencies of the lowest singletsinelet transitions. One must deduce this information for thebther approximation methods. The other weakness is the choice of atomic orbitals and potential functions. Students were surprised to learn that the methods are aoproximate a t best. and did not expect the computer to e& by hundreds of kcal. Yet .the same students attack road map problems in oreanic svnthesis without regard to yield in-intermediate steps. perhaps the computer output for the promam. EXTHUC (11).. might . contain a cryptic message such as, "Caution, these eigenvalues are in units of Hoffman kilocalories! " To introduce the selection rules for transitions in the ultraviolet region, i t was necessary to study elementary group theory. Linear and non-linear diatomic and triatomic molecules were considered with emphasis on the construction of LCAO/MO's, and 'the united atom-separated atoms correlation approach of Mulliken (12) was Volume 50, Number 2, February 7973 / 145
found useful. Term symbols and selection rules, while never trivial. can be develooed loeicallv " " from these tooics and no difficulties were encountered in discussion. Having considered the formalism of group theory the next topic was vibrational spectra. Homonuclear and heteronuclear diatomic molecules were studied using the program, VIBROT (13). It was not possible to treat a range of complex molecules exactly using a library routine because none is available. Instead, complex spectra were considered qualitatively using the Sadtler Spec Finder (14) and spectral search programming in conjunction with files of soectral data. One aspect of a data file approach to ir is obvious but may not be considered. It is possible to study spectroscopy for a semester without looking beyond the classroom assignments for additional examples. But only the most disinterested student could ignore the volumes of spectra shown in the Sadtler (14) or A.P.I. (15) files after learning how to search them systematically. The other topic important to modem spectroscopy, nmr, was taught quickly and effectively in the latter part of the course. The mathematical formalism is straightforward and solutions to complex molecules can be obtained with small com~nters(16). Both LAOCN3 (17) and/or NMRIT (18) may be used with equal ease in treating'representative molecules and NMRIT gives a plot on the high speed printer. Most students, particularly those who had not taken introductory quantum chemistry prior to chemical spectroscopy, encountered difficulties in reading the text. The text, by King (49, had been selected because it used correct notation throughout and introduced the topics in a reasonable sequence. The students experienced little difficulty in absorbing the molecular orbital method preliminary to the study of electronic spectra. King's approach is very thorough. Group theory was chall&giigy however, because of the formalism of matrix algebra. In that regard a student familiar with the Drago approach (19) encountered problems not experienced by the uninitiated. Instead of merely using the simplistic Drago approach to check their work, a few students chose to disregard matrix formalism altogether. Having failed to "learn the language" they fell behind. One might prefer a text such as Brittain, George, and Wells (20) if prerequisites are not imposed, or Chang (21). Other observations and comments by students and instructor are summarized in Table 4. These should provide a basis for objective evaluation of the suitability of a course of this type a t other institutions. Specific information such as listings, typical output, goodness of fit to spectral data, and the like are available on request. Programs are available through the Quantum Chemistry Program Exchange for a nominal fee. Acknowledgment
The author wishes to thank the students of Chemistry 345 for their interest, patience, and understanding. Much was learned by student and instructor alike. I also thank Mr. Michael Martin, UMR Computer Center, and Mr. Vernon Kloster. Sadtler Research Laboratories, for valuable assistance' and the UMR Computer center for approximately 43 hr of IBM 360JMOD 50 Computer time. Literature Cited
W. H.F~eemansndCompany,New Y0~k.1965. 131 Bhacca, N. S., Johnson. L. F., and Shmlery. J. N.. ,'NMR Spectra Cata1og;'Varian Assoeiaies, Palo Alto Calif.. 1962.
146
1Journal of Chemical Education
Table 31 Time Requirementsfor a "Typical" Calculation -. L me
Program Hiickel MO
pOple Extended Hiiekel PROXYZ VIBROT
IRPrism Search NMRIT (oj
COORD
LAOCOON(II1)
Molecule
(see)
1,2-benzypyrene Styrene (2 iterations) Anthracene Ethane (staggered) Hydra en (ground state) Six praflems mixed l-chloro-4-nitrobenzene 1,2-benzanthraeene 3,4-benzo(A)pyrene l-chloro-4-nitrobenzene
14 17 180 199 9 900 18
29 1560 22
IBM360/50 Computer Characteristics Operation Time (msee) Addlsuhtract 3.25 to 9.69 Mukiply/divide 20-45 Read prmt 1000 lines/min Usabje core 195 K bytes
Table 4.
Program HiickelMO
Evaluation of Computer Programs
Comments Raoid: sim~lifiedinout
~xcellenifor spectral work Excellent Much moreuseful than Pople Excellent but does not plot Criticism Too little time available M.O. methods useful hut weak Text too difficult
Criticisms by Students Recommendation Prerequisite of quantum chemistry Hcekel, Pople ( ~ x t CND0/2 ), Ballhausen-Gray, Colthup, et a/.. Roberts
I41 King. G. W., "Spectmscopy and Molecular Structure," Holt, Rinehart and Winrfon. Inc., New York, 1964. Ballhausen. C. .I.. and G u y . H. B.. "Muleculsr O~hitalTheory." W. A. Benjamin. Inc.. New York. 1%. Hercberg, G., "Eleetronie SPY= and E l e e ~ o n i cStructure of Polyafomic Moloculer."O. Van Nortrand. Co., lnc. Princeton, N.J.. 1966. Wihorg, K. B.. "Computer Programming for Chcmisrs." W. A. Benjamin. hc., Ne-York, 1965, DP. 216-225. wibera, op. cit.,pp i2b234. Poplo. J . A. and Segsl. ti. A , J Chem Phys.. 43. 5131 11965): 44. 3289 11436. QCPE 91. QCPE refen t o the catalog number af the program 8s available from the Quantum Chomiary Program Exchange, Chemistry Department, Room 204 Indiana Uniuenily. Rlmminpton, Ind. 4740,. B l m ~ J, . E. and G i h n . B. H., SCFCIO. Franklin Institule. Philadelphia, QCPE 71.2. APL/lvcrrion has been witten by uaandi-.avaiiableonrpquost. Hoffman. R., EXTHUC, Harvard University, Baston. QCPE50. Mulliken, R. S., Reu Mod Phys.. 4.111932l. Schneiderman. S. B., VIBROT. United Axcraft Rosesrch Laboratories. QCPE 113. Sadtler Roseareh Laborarorier. he., "Sadtler Standard Spectra," 3316 Spring Garden Street, Philsdelphia, Pa. 19104 The S a d t l n "Prism Spec Finder" and "Grsfing Spec F~nder" provide rapid identifications using the same approach p r w a m m e d into their computer soarchmutinor. American Petroleum Institute, "Cslalog of Specfrwram~." APT Rosearch Project 44. Carn~gielnsfitufe nf Technolopv, Pittsburgh. Pa. If core sire is limited progrems ~ u c has the "ARC Slnctrum Pmgram." Wiborg. K. R., "Computer Progmmming for Chemistn.'. W. A. Benjsmin Lne., New York, 1 9 5 pp. 169-196, are quite gmd. While re~trietedto three-spin ayrtems, the method can be underrtmd easily by ~tudyingtheprogram and its output. Bothner-By. A. A , and Caitcllano. S.. .'I.AOCU5." Mellon Institute. 4400 Fidh Avenue. Pittsburgh. Pa. 15215. Cooper. J a m s W., "NMHITIIV1,"Daganmenf of Chemistry, Ohio State Univeni~ ty, Columbus,QCPE 126. D r a m R. S.. "Physical Methods in Inorzanie Chemistry." Reinhold Publishing Corp, New York. 1963. pp. IW2:IR. Brittain, E. F. H.. George. W. 0.. and Wells. C. H. J., "Introduction to Molecular SpecLnacnpy."Acadcm~rPress, london. IJ7II. Chang. R.. "Basic PRnciples of Spectroscopy" MeGraw~HillBmk Company, NOW York. 1971.
