Judy L. Hoff and Thomas A. Furtsch Tennessee Technological University Cookeville, Tennessee 38501 and Jerrv L. Mills Texas Tech University Lubbock, 79409
I I I
A Change in the Sign of an NMR Coupling Constant An advanced undergraduate experiment
An understanding of the phenomenon that nmr coupling constants have a sign associated with them is an integral part . . of understanding tKe nuclear magnetic resonance experiment. However, normally the sign of a coupling constant is manifest only either in certain decoupling experiments or in secondorder spectra, both of which are rather abstract to the beginning student. The experiment described herein graphically demonstrates a change in the sign of the P-C-H coupling . trivalent ohosohorus and tetraconstant. ~ J D - Ubetween valent phosphoks. The samples yield first-order 'H spectra that are recorded at ambient s~ectrometertemuerature with no decoupling. Discussion The exchange reaction of the Lewis acid-base complex Me3P:BMea with excess Me3P
MesP:BMea + MesP* = Me3Pe:BMe3 + MesP has been shown to proceed hv a dissociative mechanism ( I , 2 ). ~t ambient temperature these exchange reactions are rapid on the nmr time scale. Thus the 'H nmr spectrum of the complex plus excess Me3P does not yield separate phosphorus methyl proton peaks for both the complexed and the free ~ ~but rather ~ p a single , doublet is that is an aver. age of the quantity of complexed and free Me3P. The doublet arises from snin-snin . c o u ~ l i n ebetween the nrotons and the phosphorus nucleus (for "P, I = 1/2,100% nat&al abundance). Additionally, and of integral importance to the experiment, the coupling between the phosphorus methyl protons and the phosphorus nucleus is also a weighted average between 'JP-H in MesP:BMe3 and z J p - ~for free MeaP. The coupling 'JP-H in trivalent phosphorus compounds such as Me3P has been shown to he positive (2, 31, while 'JP_H for tetravalent phosphor~licc~rnpuundsis of opposite sign, i.6.. ntgntire. Thus the uveraye?Jp-)I in the system \le,,P:H\le., + \Ie~I'in\,ol\,ri huth positivr and negative coupling constants. Fieure l a is an 'Hnmr spectrum a t ambient temperature P &owing the doublet (2.8 Hz) for of M ~ in~ cyclohexane, ~ J P - UFieure . l b shows the s ~ e c t r u mof MenP and BMen in a moie raiio of 112.5, which-can also he considered to-be Me3P:BMen and PMe3 in a mole ratio of 111.5. Only a single doublet is observed for the protons of Me3P due to the rapid averaging of the free and complexed forms. The resonance is shifted downfield due to the deshielding of the protons as the uhosphorus atom coordinates to the Lewis acid BMe3. (The ketkyl protons of BMe3 are upfield and are broadened by the quadrapolar boron nucleus.) Not only are the protons of the phosphorus methyls chemical shift averaged between the amount of free Me# and Me3P:BMe3, but also the coupling constant 2 J p - ~ is averaged. Since 2 J p - ~ undergoes a sign change between three coordinate phosphorus in MesP and four coordinate phosphorus in MesP:BMes, then a t an appropriate stoichiometry when Me3P and Me3P:BMe3 are undergoing rapid exchange, the average coupling constant will H hr zf:r(, i l l I,e zero. An awrage coupling constant V I ' L will a Hhlc,~/PMt., mole ratio of about 1 4 c5inct. ' J p - H in \114' = t 2-. X Hznnd 2 J.u 1 0 in M ~ T P : R M=~-9.0 I Hzr. \\-hen the mole ratio exceeds 114, then the average coupling constant will he aeain manifest. Fieure 2 shows the chanees that occur in " the appearance of the protons of the phosphorus methyl groups with progressive addition of BMe3 to Me3P. Figure 2g
.
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Figure 1. (a) ' H nmr of 1.7 M M ~ S Pin cvclohexane solvent.(b) 'H nmr spectrum of BMe.and Me3P in a mde ratioof 112.5 @Me3-0.80 M). Sweep wldth 500 Hz,
Figure 2. 'H nmr resonance of Me3P in the system BMea + PMes with:
(a)
BMea/PMea.0,100: (b)BMea/MesP,0.1211.00: (c)BMealMeaP. 0.2411.00: (d) BMeaIMe3P,0.4011.00; (e) BMes/MeaP, 0.77/1.00: (f) BMesIMeaP, 1.0011.00: (g) BMeslMerP. 2.0011.00.
Volume 56. Number 2 February 1979 / 125
represents total complexation of MeaP with BMe3 with a full mole ratio excess of MenB, i.e. a 211 mole ratio of BMe3 t o MeaP. Figure 3a is a plot of t h e coupling constant Vp-" versus t h e mole ratio of MenB t o MenP. It is clear t h a t the average coupling constant p&ressesfrom +2.8 H z f i r free Me:rP, t o zero for a 114 mole ratio a n d finally t o -9.0 H z for the totally complexed MesP. T h e chemical shift (from internal cyclohexane solvent) also undergoes a linear change a s t h e Me:# is complexed (Fig. 3 6 ) . It should h e noted also t h a t t h e protons of the horon methyl groups in Me8P:BMes are shifted upfield from free BMe3 d u e t o the extra shielding provided by the phosphorus lone pair of electrons. However, when there is more BMe2 than MenP. as in F i.. e u r e z r . the horon methvl resmanw wdl be shiited downtield Iwci~uiethe resonance wfll he aweirhred averareof - thcchumiml shiftsoffrecand cornplexed RMQ. Experimental Trimethylhwane, BMe:r (4), and trimethylphosphine, Me3P (5), can be either synthesized or purchased. (CAUTION: Trimethylborane and trimethylphosphine are pyrophoric and toxic. They should he used only on a standard high vacuum line.) The compound BMex and MesP are introduced into an accurately calibrated high vacuum line usingstandard techniques (6). very similar to the experiment in Angelici's text describing the preparation of the Lewis acid-base complex MeN:BP:, (7). The appropriate quantities of MenP and HMes are measured using the ideal gas law and are transferred by condensation with liquid Nn into an nmr tube that is attached to the vacuum line. Appropriate quantitiesof MerP and BMe3 that will yield high resolution spectra in a medium wall nmr tube are: tube 1,0.50 mmole Me& tube 2,0.08 mmole BMes and 0.67 mmole Me2P, tube 3,0.16 mrnole RMe.4 and 0.67 mmole MesP; tube 4,0.27 mmole BMes and 0.67 mmole MenP; tube 5, 0.52 mmole BMen and 0.67 mmole MqP; tube 6,0.67 mmole BMeo and 0.67 mmale Me& and tuhe 7, 0.80 mmole BMea and 0.40 mrnole Me:?P. Sufficient solvent should be condensed into each nmr tube tu yield solutions that are 1.0-2.0 M. in our samples, we used 0.33 ml of purified cyclohexane. Cyclohexane is an ideal inert solvent because of high solubility uf the acid, base, and complex, and because it provides a good internalstandard for chemical shift measurements. Chlorinated hydrocarbon solvents have been shown to react with MexP, and benzene often causes anisotropic chemical shifts. Sealed nmr tubes prepared as described are stable indefinitely. Other Lewis acids besides BMe,? that undergo rapid exchange in solution with MepPcan be used also, such as other buranes or organoaluminums (8).However, BMes is ideal because it is convenient to use on the vacuum line, and it has no 'H nmr resonances that interfere with the Me# resonance. Each nmr sample should be recorded on two scales, one of which allows for accurate measurement of the chemical shift of the MelP resonance relative to the internal standard cyclohexane,e.g. 500 Hz, and one ofwhich allows for accurate measurement of the MeiP coupling constant, e.g. 100 Hz. A plot similar to Figure 3 should he made using the data collected. Conclusion This experiment demonstrates t h e very important phenomena in nmr spectroscopy of the sign of coupling constants. No decoupling or variable temperature accessories a r e re-
126 / Journal of Chemical Education
Mole R a t i o
IMe3/Me3*
Figure 3. (a1 Plot of '.L,(Hz) and (b) Chemical shin (Hz) from cyclohexane Solvent versus mole ratio BMeslMerP in the system BMes Me3P.
+
quired. T h e nmr experiment can be performed i n conjunction with high vacuum line preparations, or t h e samples can he prepared in advance in sealed n m r tubes and stored indefinitely, requiring t h e student simply t o record the spectra and graph the results. Acknowledgment J. L. M. would like t o acknowledge t h e generous support of t h e Robert A. Welch Foundation, and T.A.F. gratefully acknowledges Tennessee Technological University for a Faculty Research Grant. Literature Cited I l l Cowiey, A. H., and Mills, J. I.., J. Amer. Cham. Snc.. 91,2911, 119691. (2) Alfwd. K. J., Birhw, E.0.. Carey. P. R.. and Smith,d. P., J. Chrm. Soc. (A). 2574 (1Y7LI.
(,?I Nixon,.l. F , a d Pkimk, A . "Annual RevkwofNMR SpeetrcwopyI.. iEXtur: Mmney.
8.F.l.Vol. 2,Acadeniic Press. New Y o ~ k 1969,p.345. , (41 Lohmann. W. J.. Wilron.C.O.,and Sharpiro. J. Cham. Phya, 28.777 (19581. 151 Markhnm, R. T., Uiets, dr., E. A,. and Martin, U. R., Inm8anic Syntheses, 16. 151 lI9761. I61 Shriuer, D.F.. "The Manipulstinn of Air-Sensitive Compounds," McCraa-Hill Rook Ce.. New York. 1969. (71 Angelici, R. d., 'Pynlhesis and Technique in Inorganic Chemistry." 2nd Ed.. W. B. SaundersCo., Philsdelphis. 1 9 7 1 ~188. . 181 Cullinpwurth. A.R.. Pidn~ck.A..andSmithJ. D..Chem. C'
