edned by
FRANK DeHAAN Occidental College
LOSAngeles. California 90041
Analysis of Complex NMR Spectra A dry-lab spectral analysis experiment H a r o l d M. Bell Virginia Polytechnic Institute and S t a t e University Blacksburg, Virginia 24061 First-order n m r spectra a r e ohserved for spin systems having a chemical shift difference hetween t w o sets of magnetically equivalent nuclei which are large relative to the coupling c o n s t a n t between t h e chemically shifted nuclei. T h e s e spectra c a n he analyzed h y inspection using t h e procedures outlined in m o s t elementary discussions of n m r . As t h e coupling constant becomes larger relative to t h e difference in chemical shifts t h e s p e c t r u m becomes more complicated in the following ways 1) Line intensities deviate from first-order predictions 2) Line positions deviate from first-order positions. 3) Extra lines may appear. A s a consequence of t h i s behavior, t h e s e "non-first-order" spectra c a n n o t he analyzed b y simple inspection. To determ i n e chemical shifts and coupling constants, less direct procedures m u s t h e aoplied.' T h i s d w - l a b e x ~ e r i m e n affords t students t h e oppor&ty to obtain p r a c t i c a l k ~ ~ e r i e n in c ethe techniques of spectral analysis of s o m e of the more common complex systems.
the calculated and actual spectra. Would the 30 MHz spectrum alone give somewhat misleading results? 2) Dimethoxyphenol. Is the splitting pattern of this 100 MHz spectrum A2X,A2B, AMX, ABX, or ABC? Analyze for chemical shifts and coupling constants. Can you use the information thus obtained to pedict the structure of this compound? A 60-MHz spectrum is also provided. Analyze this spectrum and compare the parameters with those obtained from the 100 MHz case. Do the two analyses agree? Comment, qualitatively, on the behavior of this splitting pattern a t 30 MHz and 300 MHz. 3) I,l,l-Triehloro-3-bromo-3-phenylpropane in DMSO-as. AnJ A Jex, ~, alyze the ABX pattern in this 100 MHzspectrumfor JAB, "a, u ~ and , vx. Treat the spectrum as an AMX case and analyze. How do the parameters compare with the ABX treatment? The spectrum of this compound was also determined in carbon tetrachloride. Can you analyze this spectrum? Why is it so different from the one run in DMSO. is it correct to classifv this soedrum as A,IX? Discuss. 4) Extra Credit ~ r o b l e m : ~ ~ n o l & the 100 M H ~Spectrum of o-Dibromobenzene. This AA'BB' pattern can be analyzed using procedures found in the literature references, but these are somewhat tedious. Alternately, treat the spectrum as if it were AA'XX' and analyze. Your instructor has the actual parameters for comparison with your approximate solution. Computer programs have been written to assist in the analvsis of eamolex soectra. Your instructor give you some appreciation for this spectral analysis technique; further information is available in Abraham's book. Reference ( 3 ) .above. Materials Provlded Consistent with the editorial policy of the "Interpretive Experiments Seriesn.:' full-scale cooies of all soectra are available from the
The Experiment Pre-lab Assignment Familiarity with spin system notation and the concepts of magneticand chemical equivalence is needed before undertaking this experiment.'The ability to construct, identify, and analyze AX, AIX, and AMX spin systems is also required. Procedures for analyzing nonfirst-order spectra are found in the references below. Look a t one or more of these in order to become familiar with the treatment of AB, A2B,ABX, and AA'XX' eases, and use the worked problems in these references for practice before attempting the unknown spectra in this experiment. References for Spectral Analysis I l l Carhirch.E. W...lr.."Analyrir~,fC~~mplex N M R S w t r a firthe OnanicChemirt,"Pan 1. .I.CHEM. F.DUC.. 45. :I11 119681: P A R T 11. 45, 402 119681: Part 111, 45, 4SO i,l.9. .f.Y.,I. I21 Rihle, It. H., JI.. "lnterprotstim d N M R Spectra," Plenum Press, New Ysrk, 1965. (:I1 Abraham. K. .I."Analysis s l H i ~ h Kesulutiun N M R Spectra: Elrevier. New Ysrk. ,a,,
Prem.
ademic New Y d . 1967. 15) Hwey, P.A . " N M K Spectnmetry," Academic Press. New York, 1969. I61 Paudler.W. W.. "Nuclear Ma~neticRescmance,"Allyn and Hanm. B o s ~ m .1971. 171 I'wlo, d. A . Schnoider. W. 1;. and Hemstoin, H. d.. Can. J. C h m . , 35, 65and 1060
The Spectra 1) 1,2,3-Tribromo-2-methylpropane. Analyze the AB pattern u ~B e an, ,, shown in the 100 MHz spectrum of this compound ~ o ~ JAA and 68. Which nuclei in the molecule give rise to this AB pattern? Why? Examine the 30 MHz spectrum of this compound. Is the spectrum consistent with the one obtained a t 100 MHz? Using the 100 MHz parameters, calculate the 30 MHz spectrum and compare 180 1 Jwrnal 01 Chemical Education
also be provided. Spectral Analysls The diastereatopic methylene protons of 1,2,3-tribromo-2-methylpropane give rise to an AB pattern whieh is easily seen and analyzed a t 100 MHz. At 30 MHz, the spectrum becomes "deceptively simple". The outer two lines become too weak to be detected and the inner two lines merge to the point that they cannot be resolved. The experimental 30 MHz spectrum is quite consistent with the spectrum whieh is calculated from parameters derived from the 100 MHz case. Students should recognize the splitting pattern in the dimethoxyphenol spectrum as the ArB case. This narrows the structural possibilites to 2,6-dimethoxyphenol and 3,5-dimethoayphenol. which correlates well with ortho praAnalysls gives a value for JAR ton-proton coupling constants, indicating $6-dimethoxyphenol. Bath the 60 and 100 MHz spectra should give the same coupling constant, whereas A v will be larger by a factor of 1.67 in the 100 MHz case. The ABX spectrum shown by l,l,l-triehlaro-3-bromo-3-phenylpropane in DMSO-dn is rather typical, and can be analyzed with little
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Garbisch, E. W., Jr., J. CHEM. EDUC., 45, 311, 402, and 480 (1968). "ilverstein, R. M. and Silberman. R. G., J. CHEM. EDUC., 50, 484 (1973). :'DeHaan. F. P.. Thibeault. J. C.. and Ottesen, D. K.. J. CHEM. EDUC., 51, i s 3 (19743. "othner-By, A. A,, and Castellano, S. M., "LAOCN3", in "Computer Programs for Chemistry", (Editor: DeTar, D. F )Benjamin, New York, 1968, Vol. 1, Chapter 3.
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