Internal Vs. External Referencing in NMR Studies
phenyl-N-tert-butylnitroneused in this work, and Dr. Ryusei Of Research for helpful discussions.
References and Notes (1) Ann. Rep. Osaka Lab. Radiat. Chem., JAERI-M, 6260,4 (1975); JAERI-M, 6702, 4 (1976); JAERI-M, 7355, 4 (1977).
The Journal of Physical Chemistry, Vol. 82, No. 3, 1978 325 ( 2 ) W. H. Beattie, Report of the Los Alamos Scientific Laboratory,
University of California, LA-4658 (1971). (3) E. G. Janzen, “Creation and Detection of the Exclted State”, W. R. Ware, Ed., Marcel Dekker, New York, N.Y., 1976, Chapter 3. (4) A. R. Forrester, J. M. Hay, and R. H. Thomson, “Organic Chemistry of Stable Free Radicals”, Academic Press, New York, N.Y., 1968, Chapter 5. (5) A. L. Bluhm and J. Weinstein, J . Am. Chem. SOC.,92, 1444 (1970).
Internal Vs. External Referencing in Nuclear Magnetic Resonance Studies of Complex Formation. Complexes of Acetylenes with Benzenes, Thiophenes, and Furans Wayne C. Appleton+ and James Tyrrell” Department of Chemistry and Biochemistry, Southern Illinois University at Carbondale, Carbondale, Illinois 6290 1 (Received August 8, 1977)
Complex formation of benzoylacetylene and phenylacetylene with a number of methylated benzenes, furans, and thiophenes as well as their parent compounds has been investigated using NMR techniques. The relative effectiveness of internal vs. external referencing methods are investigated for the different solvent systems. Equilibrium constants obtained using the external referencing procedure are calculated. Variable temperature studies on the benzoylacetylene-toluene and benzoylacetylene-2-methylthiophenecomplexes provide estimates of the enthalpy of formation of these complexes.
Introduction The ethynyl proton in acetylenic compounds is well known to act as an electron acceptor in hydrogen bonding,’ the electron donor being, most commonly, the lone pair of a highly electronegative atom such as oxygen or nitrogen or the 7 electrons of an aromatic or unsaturated molecule. The principal physical technique utilized for the study of this form of complex formation has been nuclear magnetic resonance spectroscopy (NMR), observing the variation of the position of the ethynyl proton resonance signal, relative to some reference, as a function of the changing concentration of the electron donor solvent in some “inert” solvent.2 The “inert” solvents used have been principally chloroform, carbon tetrachloride, and cyclohexane with a preference in more recent work for cyclohexane because of evidence that chloroform and carbon tetrachloride are themselves capable of complex f ~ r m a t i o n .The ~ referencing technique used in the vast majority of studies has involved an internal reference where the reference is dissolved in the solution being studied. In particular, cyclohexane is often utilized both as an inert solvent and as the internal reference. This technique has the advantage of simplicity of operation and eliminates the need for correction of the data obtained for differences in the bulk magnetic susceptibility. The use of an internal reference presupposes that either the position of the proton resonance signal of that reference will be invariant to changing solution composition or that at worst any changes in its position will be insignificant relative to the shifts in the signal of the hydrogen under investigation. Becker4 has shown that the proton resonance signals of commonly used references such as chloroform, cyclohexane, and tetramethylsilane (TMS) show significant chemical shifts in the presence of increasing concentrations of aromatic solvents due to the changing magnetic anisotropy of the solvent surrounding the reference molecule. Though Becker4 has indicated, Present Address: Velsicol Chemical Corp., A n n Arbor, Mich.
0022-365417812082-0325$0 1.OO/O
based on his investigation, that these shifts in the position of the internal reference can cause serious error particularly in dealing with weak hydrogen bonds where the shifts may be of the order of 10 Hz, and that internal references should be avoided in systems containing aromatics at high concentration there is little evidence that this caution has been heeded by the majority of the large body of workers who have utilized NMR as a tool to study hydrogen bonding or, more generally, complex formation in solution. Most of the work done in the area of NMR studies of complex formation particularly in aromatic solvents utilizes internal references for the reasons mentioned earlier but without, in general, any indication of concern for errors introduced by the shift in the reference position with changing concentration. This is particularly true in the study of the so-called “aromatic solvent induced shift” (ASIS) which has been the subject of extensive investig a t i ~ n .There ~ have been relatively few attempts made to take account of the medium shift of the internal reference. Rummens and Krystynak6 discussed the relationship of the ASIS to the chosen internal reference and attempted to determine a modified ASIS independent of the nature of the reference. Appleton and Tyrrel17 considered the relative merits of internal vs. external references in the case of complex formation between acetylenes and anisoles where the perturbation due to the medium effect on the internal reference is substantial. The use of an external reference in the case of complex formation between the acetylenes and the anisoles allowed the results to be interpreted in terms of a 1:l complex whereas the data obtained using the internal reference did not appear to fit any meaningful model. The present work is an attempt to extend the method applied in the case of the anisoles to complexes formed between the same acetylenic compounds and a variety of benzenes, thiophenes, and furans and to again compare and contrast the use of an external vs. an internal reference. The model used to interpret, the data is based on a method devised by Landauer and McConnelP using the 0 1978 American Chemical Society
W. C. Appleton
The Journal of Physical Chemistry, Vol. 82,Me. 3, 1978
and S. Tyrrell
Ethynyl Proton Chemical Shiftsa (Nz)for B e n z ~ y ~ ~ c ~ tin y ~Various e n e Benzenes
TABLE E: I
XR
Active solvent
0.05
0.10
0.20
0.30
0.40
0.50
16.00 14.97 17.36 19.36 20.81 23.77 20.92 20.97 24.68 20.16
28.68 30.52 34.00 32.34 33.68 37.86 36.24 35.40 37.60 36.16
39.57 41.23 45.36 43.61 44.49 51.02 47.86 45.98 47.26 50.23
48.29 50.56 55.85 53.04 53.83 59.31 56.41 55.57 56.38 57.54
58.44 60.09 62.16 59.63 63.02 68.19 65.69 63.16 65.44 65.45
0.60
0.70 -__
_ _ I I
Benzene 10.00 Toluene 9.84 o-Xylene 10,45 m-Xylene 11.09 p-Xylene 13.07 1,2,3-Trirnethylbenzene 12.27 1,2,4-Trimethylbenzene 12.33 Mesitylene 9.25 1 , 2 ~ 3 , 4 - ~ e t r a ~ e t h y l b e n z e12.35 ~e 1,2,3,5-'I'etramethylbenzene 12.34
a Shifts measured relative to TMS as an external reference. hexane.
65.75 68.41 69.50 68.93 69.20 74.45 72.42 70.64 71.47 71.91
69.36 75.39 74.87 78.55 75.70 80.55 78.31 76.49 77.27 78,33
0.80
0.90
1 _ 1 _ -
77.22 81.45 81.62 81.74 81.28 85.48 82.94 83.03 82.87 82.29
87.75 85.65 84.16 87.02 90.12 88.29 86.88 87.98 87.26
Mole fraction of active solvent. The inert solvent is cyclo-
~A~~~ 11: Ethynyl Proton Chemical Shiftsa (Nz)for B~~zoylacetylene in Various Furans and Thiophenes
XB
-----------.---_
Active solvent Furan 2-methyl furan 2,5-Dimethylfuran Thiophene 2-Methylthiophene 2,5-Dimethylthiophene a
0.05
0.10
0.20
0.30
0.40
0.50
0.60
0.70
0.80
0.90
3.82 5,lO 2.28 5.06 5,17 6.99
6.31 7.05 4.64 9.33 8.97 10.40
7.91 11.26 7.22 17.93 17.33 17.79
11.29 13.82 9.36 26.73 23.75 24.51
14.29 15.24 12.17 33.29 29.15 29.48
16.88 17.99 13.61 39.64 34.64 34.83
20.65 20.10 15.00 46.04 39.79 39.33
23.15 21.77 16.93 51.98 44.41 43.50
25.63 23.67 18.72 57.77 46.80 47.46
28.58 25.38 19.90 63.09 49.47 50.35
Shifts measured relative to TM8 a5 an external reference, b The inert solvent is cyclohexane.
~ that the amount s of ~the donor ~solvent in ~the c o ~ p ~ e x eform d is insignificant relative to the uncomplexed donor. The chemical shift due to complex formation Is taken as the difference, 6, between the position of the ethynyl proton signal in the pure inert solvent, cyclohexane, and in the mixed donor-cyclohexane solvent. The mole fraction of the donor solvent in cyclohexane is given by Xg. The relationship between the 6 and XBis given by ~
6 = KxB8*B/(I--IKXB)
(1)
is the shift which would be observed if complex formation were complete (at XB = 1) and of the form AB where A refers to the acetylenic compound and B to the donor solvent. The acetylenic compound is always used at concentrations low enough to prevent self-association as verified by dilution studied thus allowing the elimination from consideration of complexes of the form A$, (where n > 1). K is the equilibrium constant for the process 668
A+B+AAB
The values of K and 6AB can then be evaluated using the experimental 6's and XB's by means of a non-linear least-squares analysis procedure. Where an external reference is employed with a coaxial sample tube the measured chemical shift must be corrected for dif€erences in the bulk magnetic susceptibility between the reference and the sample 6 = 6 0 f (2n/3)AX (2) where 6 and 6o are the corrected and uncorrected chemical shifts respectively and AX = X,- X,is the difference in bulk magnetic susceptibilities of the reference and the sample. Bulk magnetic susceptibilities can be obtained from the literature: from experiment,1°or theoretically.ll
e~~~~~~~~~ Section Benzoylacetylene, @G€-15@8C=CH, was prepared by the chromic acid oxidation of 1-phenylprop-2-yn-1-01using a method developed by Bowden et a1.12 giving a product which upon multiple sublimation yielded a white solid, mp
48-48.5 ~ "C. The ~ phenylacetylene o and ~ all of the benzenes, thiophenes, and furans were obtained commercially, dried, and fractionally distilled before use. All of the investigations were carried out at an acetylene concentration of 0.2 M, a concentration low enough to prevent self-association. The solvent ranged from pure cyclohexane through 0.9 mole fraction donor in cyclohexane. NMR spectra were recorded on a Varian HA-100 spectrometer equipped with a variable temperature probe. The ambient temperature runs were carried out a t 30 f 2 "C. The variable temperature runs were carried out a t a series of controlled temperatures in the range 10-60 "C. For the internal reference runs cyclohexane was used both as the inert solvent and as the internal reference and the ethynyl proton resonance signal was determined five times for each solution. The external reference experiments were carried out using coaxial tubing with TMS as the external reference. In these latter experiments the cyclohexane proton resonance signal was measured relative to that of the external reference and the shifts observed for the ethynyl proton resonance signal obtained using the internal reference adjusted for the observed variation in the cyclohexane signal. The uncertainty in chemical shift measurements was of the order f0.02 Hz.
Results Tables I and I1 give the ambient temperature, external reference chemical shifts for the ethynyl proton of benzoylacetylene in various mole fractions of benzenes, furans, and thiophenes in cyclohexane. Similar data for the ethynyl proton in phenylacetylene are listed in Tables I11 and IV. The equilibrium constants obtained from this data using the model illustrated by eq 1 and the fit of the data to the model as measured by the sum of the squares of the residuals are presented in Table V. The chemical shifts of the cyclohexane proton signal as a function of changing mole fraction of the various donor solvents used are listed in Table VI. Finally Table VI1 lists the equilibrium constants for complex formation of benzoylacetylene with toluene and 2-methylthiophenerespectively over a range of temperatures along with the enthalpies of formation calculated from this data. Figure 1 shows the
The Journal of Physical Chemistry, Vol. 82,No. 3, 1978 327
Internal Vs. External Referencing in NMR Studies
TABLE 111: Ethynyl Proton Chemical Shiftsu (Hz) for Phenylacetylene in Various Benzenes XB
Active solvent
0.10
0.05
0.20 9.45 11.45 13.97 12.91 13.01 15.43 15.38 15.20 16.83 16.80
0.30
13.55 Benzene 16.37 Toluene 19.11 o-Xylene 17.75 m-Xylene 18.27 p-Xylene 21.60 1,2,3-Trimethylbenzene 20.86 1,2,4-Trimethylbenzene 20.41 Mesitylene 22.93 1,2,3,4-Tetramethylbenzene 23.05 1,2,3,5-Tetramethylbenzene Mole Shifts measured relative to TMS as an external reference. hexane. 2.70 3.30 4.28 3.62 3.80 5.35 4.71 4.62 5.99 5.58
5.20 5.78 7.69 6.85 7.24 9.15 8.49 8.44 10.66 9.61
0.40
0.50 0.60 0.70 0.80 0.90 29.45 32.75 21.99 23.96 25.64 17.76 25.53 37.29 39.83 30.25 33.55 21.43 38.01 40.65 27.84 31.26 33.63 24.02 37.82 38.83 25.93 29.48 35.29 21.88 37.12 40.29 26.91 30.81 33.72 22.65 40.27 42.64 30.68 34.27 37.21 26.44 40.89 44.22 30.54 34.16 37.57 25.75 45.58 37.95 41.90 30.24 34.61 25.87 41.09 43.55 31.79 35.60 38.47 27.90 42.12 45.04 31.96 35.77 38.82 27.56 fraction of active solvent. The inert solvent is cyclo-
TABLE IV: Ethynyl Proton Chemical Shiftsu (Hz) for Phenylacetylene in Various Furans and Thiophenes
XB Active solvent
0.05
0.10
0.20 1.13 2.82 1.84 4.11 7.40 6.51
0.30 2.02 3.85 2.40 6.77 9.33 9.64
1.39 1.45 Furan 0.84 1.80 2-Methylfuran 0.35 1.05 2,5-Dimethylfuran 2.11 Thiophene 1.31 4.91 2-Methylthiophene 2.18 2.29 3.77 2,5-Dimethylthiophene Shifts measured relative t o TMS as an external reference. hexane.
0.40 0.50 0.60 0.70 0.80 0.90 7.64 9.33 5.87 6.03 3.23 4.17 6.32 7.31 8.23 9.13 4.18 5.29 7.35 8.19 8.50 5.74 4.66 5.42 13.74 16.68 19.52 8.66 10.76 17.01 18.98 19.95 20.96 11.56 14.36 17.73 20.32 22.69 24.95 12.42 15.18 Mole fraction of active solvent. The inert solvent is cyclo-
TABLE V: Equilibrium Constantsu and Sum of Squares of Residuals (c i r i 2 )for Benzoylacetylene and Phenylacetylene in Various Benzenes, Furans, and Thiophenes Phenylacetylene Benzoylacetylene
K
ciri2
K
1.02 -i 0.04 0.90 f 0.02 1.33 f 0.02 1.17 f 0.05 1.31 f 0.05 1.44 * 0.03 1.77 i: 0.03 1.54 f 0.02 1.78 * 0.05 1.68 i 0.04 0.40 f 0.04 1.16 ?: 0.04 1.05 t 0.04 0.49 ?: 0.01 0.93 f 0.02 1.03 f 0.02
47.5 31.7 40.8 138 127 40.7 33.8 30.2 122 54.9 59.6 29.3 12.8 11.3 17.1 38.4
0.55 f 0.45 f 0.88 i. 0.68 0.73 ?: 0.77 f 1.18 f 0.92 ?r 1.58 f 1.21 * 0.14 * 0.32 f 0.11 + 0.51 i 0.83 i 0.30 *
Active solvent Benzene Toluene o-Xylene m-Xylene p-Xylene Mesitylene 1,2,3-Trimethylbenzene 1,2,4-Trimethylbenzene 1,2,3,4-Tetramethylbenzene 1,2,3,5-Tetramethylbenzene Furan 2-Methylfuran 2,5-Dimethylfuran Thiophene 2-Methylthiophene 2,5-Dimethylthiophene
*
ciriz 0.04 0.01 0.03 0.04 0.01 0.02 0.02 0.01 0.03 0.02 0.04 0.05 0.07 0.03 0.04 0.01
18.3 5.76 14.6 36.2 4.04 9.07 4.83 4.83 23.2 7.01 13.1 10.6 25.5 21.6 22.7 3.25
(Mole fraction)-'. TABLE VI: Chemical Shifts of Cyclohexane (Hz) Relative t o External Reference in Various Benzenes, Furans, and Thiophenes XEU Solvent Benzene Toluene o-Xylene m-Xylene p-Xylene 1,2,3-Trimethylbenzene 1,2,4-Trimethylbenzene Mesitylene 1,2,3,4-Tetramethylbenzene
0.1 4.13 3.74 4.58 4.13 4.05 5.00 4.58 4.97 5.51
0.2 7.22 7.90 8.66 8.05 7.62 8.55 9.07 9.46 8.93
1,2,3,5-Tetramethylbenzene
4.88
8.88
Furan 2-Methylfuran 2,5-Dimethylfuran Thiophene 2-Methylthiophene 2,5-Dimethylthiophene Mole fraction of active solvent.
2.82 2.79 1.98 3.01 3.29 3.06
3.96 5.00 3.44 5.68 6.12 5.39
0.3 10.19 11.55 11.95 11.30 11.13 12.58 12.44 13.05 12.66 12.93 6.15 6.97 5.17 8.78 8.07 8.23
0.4 13.80 15.73 15.47 14.32 14.44 16.01 15.99 17.08 16.22 16.00 8.59 8.57 8.31 11.44 10.24 10.93
0.5 17.50 19.05 18.30 17.52 17.74 19.19 19.49 20.35 18.78 19.20 11.08 11.14 10.25 14.26 12.93 13.57
0.6 19.19 23.47 20.99 20.67 20.91 22.16 22.40 24.04 21.56 22.03 14.85 13.61 11.98 17.55 15.90 16.17
0.7 20.92 26.74 23.26 25.84 23.59 24.67 25.64 27.18 24.08 24.63 17.25 16.13 15.00 20.82 18.26 18.94
0.8 24.88 30.34 27.33 28.47 26.82 27.68 28.48 30.61 26.44 27.56 21.07 18.90 17.22 24.10 19.58 22.03
0.9 28.84 33.13 29.79 29.58 29.91 30.17 31.78 34.23 28.85 30.27 25.06 21.76 19.01 27.37 21.08 24.77
320
The Journal of Physical Chemistry, Vol. 82, No. 3, 1978
TABLE VII: Equilibrium Constants and Sum of Squares of Residuals for Complex Formation between Benzoylacetylene and Toluene and Benzoylacetylene and 2-Methylthiophene at Various Temperatures Temp, Equilibrium "C constant, mol-' Xiri2 Toluene
10 20 30 40 60
1.12 * 1.00 i 0.90 It 0.81 i 0.68 *
2-Methylthiophene
20 30 40 50 60
1.02 i 0.04 0.93 t 0.02 0.84 ~t 0.04 0.81 i 0.04 0.67 ~t0.02
0.03 0.03 0.02 0.02 0.02
45.9 37.8 31.7 14.3 13.3 33.2 17.1 29.7 32.3 9.57
- A H ( toluene-benzoylacetylene) = 1.9 kcal/mol -A
H(2-methylthiophene-benzoylacetylene)= 2.0 kcal/mol
jO1
251 20
-1.0
Xb
-20/
Flgure 1. The chemical shift (6) in hertz for benzoylacetylene, phe-
nylacetylene, and cyclohexane in furan as a function of the mole fraction of furan: (0) benzoylacetylene using an internal reference; (0) phenylacetylene using an internal reference; (A)cyclohexane relative
to TMS as an external reference; (W) benzoylacetylene using an external reference; ( 0 )phenylacetylene using an external reference.
chemical shifts as a function of mole fraction of the donor solvent for benzoylacetylene and phenylacetylene in the donor solvent furan. The figure shows the values for the chemical shifts using both internal and external reference as well as the shift in the cyclohexane proton resonance signal relative to the external reference. In the case of the external reference data for benzoylacetylene and phenylacetylene the solid lines represent the theoretical results and allow comparison with the experimental data.
Discussion The results shown in Figure 1 and Tables I-IV are typical of the systems studied. It is noted that for benzoylacetylene the pattern of shifts observed using both internal and external reference is similar in benzene and thiophene, i.e., a continuous upfield shift with increasing mole fraction of these solvents tending to level off at higher concentrations of the donor solvents. Both the internal and external reference data can be interpreted on the basis of a 1:l complex using eq 1 for all the benzoylacetylenebenzene and benzoylacetylene-thiophene systems. The equilibrium constants for benzoylacetylene in thiophene, 2-methylthiophene, and 2,5-dimethylthiophene are respectively, 0.08, 1.43, and 2.08, using internal reference
W. C. Appleton and J. Tyrrell
data. These should be compared with the values obtained using external reference data given in Table V which are seen to be significantly smaller. However, the results obtained for benzoylacetylene in furan using the internal reference show first a shift upfield which then levels off and turns back downfield at higher furan concentrations. These results are not interpretable in terms of a 1:l complex or in fact in terms of a variety of possible complex models. The results for phenylacetylene using an internal reference show for benzene an initial upfield shift followed by a leveling off and finally a turn downfield at high benzene concentrations. For both thiophene and furan the results for phenylacetylene show a consistent downfield shift significantly larger in the case of furan than thiophene. The phenylacetylene results again are not capable of being fit by the 1:l complex model. On a superficial level one might interpret the downfield shift in thiophene and furan for phenylacetylene as an indication of complex formation to the heteroatoms where the heteroatoms behave as in an ether. While this interpretation might be acceptable in the case of furan it is extremely unlikely to apply to thiophene. In any case no quantitative fit could be obtained with these data. The effect of taking into account the chemical shift of cyclohexane by use of the external reference is to bring all of the data for both benzoylacetylene and phenylacetylene into a consistent pattern corresponding to an upfield shift showing signs of leveling off at high electron donor concentrations. All of the complexes can thus be thought of as involving complex formation between the ethynyl proton and the H electron system of the donor placing the proton in such a position that the ring current effect leads to an upfield shift. In addition, all of the cases can be fitted using the 1:l complex model. This pattern of behavior is observed for all of the systems studied, the various alkylated donor solvents following essentially the same pattern as the parent compounds. An almost linear upfield shift is observed for cyclohexane, relative to the external reference, the extent of the shift being dependent on the donor solvent but all lying within the relatively narrow range, at 0.9 mole fraction donor solvent in cyclohexane) of 20-30 Hz. These results show that only where the shifts due to complex formation, as obtained using the external referencing process, are in general smaller than those observed for the cyclohexane does one observe the deviations from the expected curve given by a 1:l complex. Where the shifts are consistently smaller over the entire mole fraction range one observes a steady downfield shift using internal referencing as observed for the phenylacetylene system in both furan and thiophene. Where the shifts are initially larger than those of cyclohexane but then drop below those of cyclohexane at higher mole fractions one observes the curvature found for benzoylacetylene in furan and as previously discussed in the anisoles. Where the external reference shifts are consistently only slightly larger than those for cyclohexane, one observes the typical 1:lcomplex curve even using the internal reference as seen, for example, in the case of phenylacetylene in benzene. Thus, in situations as exemplified by complex formation between benzoylacetylene and the benzenes and thiophenes and between phenylacetylene and the benzenes the results will still fit a 1:1 complex model, however, the results obtained for the equilibrium constants, as illustrated by the comparison given earlier between the benzoylacetylene-thiophene series using internal and external reference) differ markedly. The equilibrium constants obtained show a general tendency to increase with increasing methylation and in
Solute-Solvent Interaction in N-Methyl-2-pyrrolidone
the benzene series show some marked positional variations. The results obtained for the benzoylacetylene-toluene and benzoylacetylene-2-methylthiophenesystems at various temperatures are consistent with expectations and lead to reasonable estimates for the enthalpy of complex formation. The present work enlarges on the results reported earlier on the acetylene-anisole systems. It indicates that the unusual data obtained for the furans and for the phenylacetylene-thiophene series using internal referencing can be understood and corrected for by taking into account the concentration dependent shift of the cyclohexane resonance. It further indicates the need to consider this effect in systems where the results appear to behave normally but which if not corrected for the cyclohexane shift will lead to significantly different equilibrium constants.
The Journal of Physical Chemistry, Vol. 82, No. 3, 1978 329
References and Notes (1) J. C. D. Brand, G. Eglinton, and daTyrrell, J. Chem. Soc., 5914 (1965). (2) P. Laszlo in "Progress in NMR Spectroscopy", Vol. 111, J. W. Emsley, J. Feeney, and L. H. Sutciiffe, Ed., Pergamon Press, London 1967, Chapter 6. (3) T. Schaefer, B. Richardson, and R. Schwenk, Can. J. Chem., 40, 2775 (1968). (4) E. D. Becker, J . Phys. Chem., 03, 1379 (1959). (5) T. N. Huckerby, Annu. Rep. NMR Spectrosc., 4, 1 (1971). (6) F. H. A. Rummens and R. H. Krystynak, J . Am. Chem. Soc., 94, 6914 (1972). (7) W. C. Appleton and J. Tyrreli, J . Phys. Chem., 81, 1201 (1977). (8) J. Landauer and H. McConneil, J. Am. Chem. Soc., 84, 1221 (1952). (9) J. W. Emsley, J. Feeney, and L. H. Sutcliffe, "High Resolution Nuclear Magnetic Resonance Spectroscopy", Pergamon Press, Oxford, 1965, Appendlx C. (10) H. J. Bernstein and K. Frei, J. Chem. Phys., 37, 1891 (1962). (1 1) H. Suhr, "Anwendugen Der Kernmagnetischen Resonanz in Der Organischen Chemie", Springer-Veriag, Berlin, 1965. (12) K. Bowden, I. M. Heilbron, and 0. C. L. Weedon, J . Chem. SOC., 39 (1946).
Investigation of Solute-Solvent Interaction in N-Metkyl-2-pyrrolidone Using Proton Magnetic Resonance Joseph Rosenfarb" and Ronald J. Baker Department of Chemistry, Universlty of Florida, Gainesville, Florida 326 11 (Received March 28, 1977)
Proton magnetic resonance was used to study a series of alkali metal and ammonium perchlorates, thiocyanates, chloride, and tetraphenylborates in N-methyl-Bpyrrolidone,a heterocyclic solvent of moderate dielectric constant (32.0). The results were compared with similar studies in two other five-membered heterocyclic solvents of dielectric constants 37.6 and 77.5. Solvent peak shifts were generally linear with solute concentrations in the range 0.19-0.75 M for all salts investigated; which indicated that the solvent molecule was shared by no more than one cation in this concentration range. Chloride, perchlorate, and thiocyanate solution downfield shifts for solvent protons were cation dependent in the order Li+, NH4+> Na+, K+, and were anion independent. This result signified the lack of extensive contact ion pairing for these salts in N-methyl-2-pyrrolidone. Tetraphenylborate salts gave upfield shifts for solvent protons which were attributable to direct anion-solvent interaction.
Introduction
structure and solvation behavior.
Solution structure, although generally a relatively less well-defined system, has been investigated by quite a variety of te~hniques.l-~ Among these, proton magnetic resonance has been an established and convenient method for probing solute as well as solvent environments in Proton containing solute and solvent proton resonance peaks have been observed to shift with increasing solute concentrations and it appears possible to interpret such results on the basis of solute-solvent and solute-solute interaction^.^ N-Methyl-2-pyrrolidone (NM2P) of dielectric constant 32.0,9has been previously investigated by conductance," electroanalytical," and spectroscopic techniques,12J3and it's versatility as a solvent and ligand has made it an important compound for many fundamental and applied studies.ll Several similar five-membered heterocyclic solvents have been recently investigated by utilizing lH NMR spectra of solutions containing various alkali metal and ammonium salts.14J5 The present investigation utilizes a similar series of alkali metal and ammonium salts in NM2P to investigate more fully the solution behavior of these salts in NM2P and to compare NM2P to similar solvents of differing dielectric constant in order to make possible correlations between 0022-3654/78/2082-0329$0 1.OO/O
Experimental Section Reagents. Potassium tetraphenylborate was prepared from aqueous solutions of sodium tetraphenylborate (Fisher) and potassium chloride. Ammonium tetraphenylborate was similarly prepared using the corresponding iodide. The salts thus prepared were thoroughly washed with triply distilled water and then recrystallized from acetone with water portions. All other salts used were the purest available commercial grade reagents and were used without further purification. Salts were generally dried overnight at 120 "C except for the tetraphenylborates and ammonium perchlorate which were dried at 60 "C in vacuo. N-Methyl-2-pyrrolidone (MCB) was refluxed over granular barium oxide for 6 h after which the middle fraction of about 500 mL was collected at 78 "C (ca. 10 Torr). This fraction had a water content of less than 0.01 % as determined by Karl Fischer titrations. Measurements. Spectra were obtained using a Varian A-60 NMR spectrometer. All solvent peaks were determined with respect to 5% tetramethylsilane (TMS) as the internal standard. Since an internal reference was used, 0 1978 American Chemical Society
