R. J. Seyse,' H. 1. P e ~ r c e , ~ and T. 1. Rose Texas A&M University College Station, Texos 77843
Calculation of Complex NMR Spectra in the Undergraduate Laboratory
Part of the integrated laboratory program at Texas A&M University ( I ) includes an experiment adapted from Angelici (2) on the preparation and characterization of tetraethyltin, (CH&Hz)&. In the discussion of the characterization of this compound, Angelici states that, "Its nmr spectrum is too complex for interpretation without a detailed treatment of nmr spectroscopy." Since we are always on the lookout for ways to refine and expand existing experiments, we suggested that the spectrum be analyzed by one of our students as a special project. Homer Pearce accepted the challenge and developed an interesting report. We thought that a presentation of these results would prove useful as an illustration of a technique to interpret such spectra which is simple enough to be used in the undergraduate laboratory. A sample of tetraethyltin was prepared according to the procedure in Angelici (2). Its nmr spectrum, taken on a Varian Associates HA-100 nmr spectrometer, is shown in the figure. By noting the overall intensities of the badly perturbed multiplets, it becomes obvious that the methyl resonances appear a t lower field than the methylene. This shift ordering might be predicted after considering the electropositive nature of the Sn atom and the resultant shielding of the methylene protons. Complex nmr spectra which cannot he understood in terms of the first-order approximation may sometimes be resolved hy the use of the LAOCN3 computer program. This program has been discussed in several sources (3-5). I t calculates a data set of freauencies. and intensities for all theoretical transitions of a given spin. Since the calculations follow an iterative techniaue, it is ~ossibleto use approximate values for the input (chemical shifts and coupling constants) and still obtain g ~ o dresults. This data may then he used to produce a "stick plot" (6,7) of the calculated spectrum. In the case of tetraethyl tin a complicating factor in computing the nmr spectrum is the presence of three tin isotopes with nuclear spin of % which couple with the protons of the ethyl gmups. These isotopes (1'5Sn, 1"Sn, "9Sn) comprise a total of 16% of the ten stable isotopes. Therefore we mav start the analvsis bv assumine that the observable effect" of tin coupling is small due & its low isotodc abundance and treat the svstem as a simde AnBz typefor the three magnetically eqiivalent methyi prot&s and the two equivalent methylene protons. Using, the standard coupling constant JAB = 7.5 Hz, the spectrum was calculated and the resulting data set plotted to give the stick plot shown in part (b) of the figure. I t is evident that the calculated spectrum using this simplified model agrees very well with the experimental spectrum in part (a) of the figure. The stick plot is normally sufficient to prove the identity of the compound prepared. However, to make the calculated spectrum more realistic, the calculation was repeated on a Nicolet 1080 series data system coupled to a 'Present address: Howard College at Big.Spring, Big Spring, Texas 79720. 2Present address: Department of Chernistly, Harvard Univenity, Cambridge, Massachusetts 02138. 194
/
Journal of
Chemical Education
Hewlett-Packard, x-y recorder. This system permits inclusion of a line width function and resulted in a calculated spectrum for the A3B2 system shown in part (c) of the figure. The next step in calculating an accurate spectrum is to introduce the coupling due to the tin isotopes. To do this, the system A3B2X was calculated, where X represents the coupled tin nuclei. The assumed coupling constants (8) used in this computation were JBX= 30 H z , JAX= 65 Hz, and JAB = 7.5 Hz. This calculated spectrum is shown in part (d) of the figure. The final calculated spectrum is oh-
Experimental and calculated spectra (100 MHz. Sweep Width = 250 Hz) of tetraethyltin. (a1 Experimental (20% w/v in CClr); (b) Slick plot of calculated spectrum tar A& system. JAB = 7.5 Hz; (c) Calculated Spectrum far A& system employing 0.4 Hz line width function; (d) Calculated spectrum tor A&X System assuming 100% Sn isotopes with spin 'h. J A X = 65 Hz. Jsx = 30 Hz. JAB = 7.5 Hz; (a) Calculated spectrum (A& 0.16 X A&X) corrected for isotopic abundance of tin.
+
tained by adding point by point the A ~ B zspectrum to 0.16 x (A3B2X) spectrum, the factor of 0.16 accounting
for the natural abundance of the tin isotopes having nuclear spin $. The resulting spectrum (part (e) of the figure) is nearly identical with the experimental spectrum. We have demonstrated a very powerful technique for the interpretation of complex nmr spectra that produces gwd results and that can be used by the undergraduate student.3 It is also instructive that the nmr spectrum of tetraethyltin may be interpreted as an A& system and 3The execution time for computing these spectra an the IBM
*
360 at Texas A&M University was for the and l4 s for the AsBzX system. The prowam was loaded from a load
module.
that the coupling due to the tin isotopes introduces only a minorperturbation. Acknowledgment
The assistance of Dr. M. D. Johnston, Jr., with the Nicolet data system was gratefully appreciated. Literature Cited (11R~~~,T.L.,~~~s~~~F,R.J.,J.cHEM.EDuc.,sI. 127(1974). (21 A"@ii. J. J.. "S~nthdSandTeehnlqu ill Inorganic Chembtry: W . B. Sawden Co.,Philadelphia, 1969,p. 153. (31 Ccutellano,S., and Bother-By, A. A . , J C h m . Phya., 41,3863(1%). (1) Wilkinr. C . L., and Klopfeostein,C. E., J. CHEM.EDUC.. IS. 10 (1966). (5) ~ e t s rD. F.. "Computer Pmssrnn for Chemistry," W. A. Benjamin. Ine., New York, 1968,Vol. 1, p. 10. (6) Rondesu,R.E.,andRushm,H.A..J.CHEM.EDUC., 4,,l%(lSlO). (7)Claa,P.E.,and~er~in, K.D.,J. CHEM.EDUC.43.382 (19721. (81 Staiford, S.L., and Bsldenehwicl~r,J. D., J.A m e r Cham. Sgc., 83,4473(19611.
Volume 52,Number 3, March 1975
/
191
