2469
NMRSPECTRAL PROPERTIES OF SOMEN,N-DIMETHYLAMIDES AND -THIOAMIDES
Concentration Dependence of the Nuclear Magnetic Resonance Spectral Properties of Some N,N-Dimethylamides and -thioamidesl by Robert C. Neuman, Jr., William Snider, and Violet Jonas Department of Chemistry, Universitv of California, Riuerside, California 9?260R (Received December 18, 1967)
The effect of concentration on the nmr spectral properties of the amides N,N-dimethylacetamide (DMA) and N,N-dimethylformamide (DMF) and the thioamides N,N-dimethylthioacetamide (DMTA) and N,N-dimethylthioformamide (DMTF) in carbon tetrachloride have been studied and the results have been analyzed using a self-association equilibrium model. Evidence for hydrogen-bondinginteractions involving the formyl protons of DMF and DMTF is presented. activity of D J l A in carbon tetrachloride.'* That the relative shieldings of the two NCH3 groups in such a In the planar ground-state configuration of N,N-dicompound might be different in the same solvent demethylamides (I, X = 0), N,N-dimethylthioamides pending on whether the compound was a monomer or (I, X = S), and related systems, the two NCH3 groups self-association dimer, however, has only recently been X CH3(c) proposed.2 This suggested that the apparent anom\ / alous behavior of DMTA compared to the other C-N compounds shown in Figure 1 might be a direct result / \ of the extent of self-association in each system.ll R CH3(t) In an attempt to answer this question, we have stud1 ied the effect of concentration on the chemical shifts of the various proton-resonance signals for DMA, DMF, are in nonequivalent magnetic environments. Since DMTF, and DRITA in carbon tetrachloride. Althe energy barriers to rotation about the central CN though a previous extensive study of DhIF in several bond are relatively high owing to electron delocalsolvents has been reported,* we felt obliged to repeat ization, two discrete NCH3 resonances are often obthe experiments in carbon tetrachloride in order to served in nmr spectra of these compounds.lbN2 When verify our own observations and to extend the conthe group R contains protons CY to the C=X carbon, l2 centration range. spin-spin coupling with NCHa(c) and NCH3(t) has ) unavoidable aspect of studies of self-association been frequently observed, and it seems that J R , N C H ~ ( ~An
Introduction
T
This unequal coupling acts as a > label for the proton resonances corresponding to NCH3(c) and KCH3(t),and, in general, it has been found that the proton resonance for NCHa(c) is at higher field than This can be seen in Figure 1 that for NCH3(t).lb~~-~ for N, N-dimethylacetamide (DJIA), N, N-dimethylformamide (DMF), and N,N-dimethylthioformamide (DMTF). However, the relative shieldings of the NCH3protons in N,N-dimethylthioacetamide(DMTA) (Figure 1) appear to be r e ~ e r s e d . ~I n~ ~addition, similar inversions have been observed for amides with more highly branched alkyl groups attached to nitrogene7 It is well recognized that the nonexchanging chemical shift b(NCH3) between the two NCH3 groups of N,N-dimethylamides is significantly temperature, solvent, and concentration dependent, and this has been rationalized by proposing self-association of amides in s o l ~ t i o n . ~Possible ~ ~ ~ - ~supporting ~ ~ ~ ~ evidence includes cryoscopic molecular weight studies of DMA in benzene9 and a large concentration dependence of the
(1) (a) Part I V : Studies of Chemical Exchange by Nuclear Magnetic Resonance; (b) part 111: R. C. Neuman, Jr., and V. Jonas,
J. Amer. Chem. Soc., 90, 1970 (1968). (2) R. C. Neuman, Jr., D. N. Roark, and V. Jonas, ibid., 89, 3412 (1967), and references therein. (3) J. V. Hatton and R. E. Richards, Mol. Phys., 3 , 253 (1960). (4) R. C. Neuman, Jr., and L. B. Young, J . Phys. Chem., 69, 1777 (1965). (5) F. A. L. Anet and A. J. R. Bourn, J . Amer. Chem. Soc., 87, 5250 (1965). (6) B. B. Wayland, R. S. Drago, and H. F. Henneike, ibid., 88, 2455 (1966). (7) A. G. Whittaker, D. Mi. Moore, and 9. Siegel, J . Phys. Chem., 68, 3431 (1964). (8) A. G. Whittaker and S. Siegel, J . Chem. Phys., 42, 3320 (1965). (9) H. 0. Chaplin and L. Hunter, J . Chem. Soc., 1114 (1937). (10) R. S. Drago, R. L. Carlson, N. J. Rose, and D. A. Wenz, J . Amer. Chem. SOC.,83, 3572 (1961). (11) This possibility was also recognized independently by R. 8 . Drago and H. F. Henneike, private communication. (12) The data previously reported8 were given in terms of mole fraction of DMF in carbon tetrachloride. Although this precludes an exact comparison with our results, approximate conversion of
mole fraction to molarity shows that our data are essentially identical. It should be noted that the previous study* was performed a t 66.4 Mops. Volume 73, Number 7 July 1968
R. C. NEUMAN, JR.,W. SNIDER, AND V. JONAS
2470
Table I : Chemical Shifts for the Proton Resonances of DMF, DMA, DMTF, and DMTA in Carbon Tetrachloride"
Cmpd D M F (36 f l o ) d
DNIA (36.5 f 0.5')
D M T F (34.5 f 0.5")
Concnb
CCHa
12.0 5.99 2.99 1.50 0.707 0.354 0.177 0.088 9.44 8.66 6.99 5.91 4.72 3.49 2.36 1.75 1.18 0.590 0.295 0.148 0.074
DMTA (36 5 -I 0.5°)e I
4.89 2.44 1.22 0.611 0.305 0.153 0.076
172.0 171.6 172.0 172.3 172.2 172.5 171.8 172.4
NCHa(c)
NCHs(t)
481.6 475.7 474.8 473.7 473.2 472.8 473.2 473 4
167,l 166.8 167.3 168.0 168.2 168,7 167.9 169.0
176.8 176.4 176.6 176.5 176.2 176.2 175.6 175.8
170.0 169.8 169.5 169.2 169.6 169.6 170.1 170.1 170.7 171.7 171.7 171.4 171.5
180.3 180.3 180.2 179.8 180.5 180.2 180.5 180.2 180.2 180.4 179.6 179.3 179.4
192.5 193.0 193.5 193.6 193.7 193.8 193.7 193.8
200.5 200.7 201.0 200.5 200.2 199.6 198.9 198 0
8.03 k 0.10 7.72 f 0.10 7.53 0 . 1 3 6.87 i 0.10 6 . 4 5 i 0.10 5.82 k 0.10 5 . 2 4 i.0.10 4.98 k 0.10
196.5 196.9 197.3 197.1 197.0 196.7 196.3 195.9
203.9 204.3 204.4 204.6 204.8 204.6 204.7
200.1 200.1 199.5 199.2 198.5 198.1 197.9
-3.70i0.20 -4.07 1 0 . 1 0 -4.67f0.10 -5.27i0.12 -5.83 k O . 1 0 -6.41i.0.10 -6.59k0.10
202.0 202.2 202.0 201.9 201.7 201.4 201 * 2
118.4 118.1 117.5 117.4 117.4 117.2 117.1 117.1 117.4 117.9 117.7 117.5 117.4 552.3 548.3 547.3 547.2 546.9 547.1 547.6 547.5
I
9.72 i.0.10 9.92 f 0.10 9.48 k 0.11 8.95 i.0.10 8.00 i.0.10 7.62 f 0.20 7.00 k 0.10 6.78 f 0.10
CH
I
10.19 5.09 2.55 1.27 0.637 0.318 0.159 0 080
6u(NCHa)'
NCHa doublet center
152.9 153.7 153.7 154.0 153.8 154.0 153.9
I
10.45 10.55 10.79 10.91 10.85 10.71 10.46 1O.ly 9.64 8.79 8.28 7.84 7.74
175.2 175.1 174.9 174.5 175.1 174.9 176.3 175.1 175.5 176.1 175.5 175.4 175.5
a Chemical shifts, in cps, a t 60 Mcps; calibrated by the audio-side-band method. Molar concentration assuming no aggregation. Values of Sv(NCH8) are not the differences of the preceding two columns; see the text. See text for discussion of error limits. Measured temperature range for the series of spectral measurements; see the text. ' Significance of negative values discussed in the text.
is that the concentration of solute must be continuously increased in order to observe potential shifts in the equilibrium distribution of monomer and self-association aggregates. This leads to changes in the macroscopic properties of the system, such as viscosity, dielectric constant, etc., and such changes could well cause changes in Gv(NCH3) independent of the existence of any true self-association equilibrium that might exist. However, independent of the quantitative significance of the association equilibrium constants which we have derived, the effect of concentration on Gv(NCH,) is important, since studies of the magnetic anisotropy of the carbonyl and thiocarbonyl groups have been based on these relative chemical shifts6t13-'6 and little attention has been paid to the question of whether The Journal of Physical Chemistry
or not they reflect the values for the unassociated monomer.
Results Nmr Spectral Data. The data are tabulated in Table I. All of the chemical shifts were calibrated by superposition of the low-field audio side band of an internal tetramethylsilane sample, except for those listed as b(NCH3). These latter values were obtained from 50-cps sweep width traces and were calibrated using (13) P. T. Narasimhan and M. T. Rogers, J . Phys. Chem., 63, 1388 (1959). (14) D.L. Hooper and R. Kaiser, Can. J. Chem., 43, 2383 (1965). (15) H.Paulsen and K. Todt, Angew. Chem. Int. Ed. En&, 5, 899 (1966).
2471
N m SPECTRAL PROPERTIES OF SOME N,N-DIMETHYLAMIDES AND -THIOAMIDES
I/
-3,
!
fI
-7
-*I DMA
I
t
I
.I
1
.o
0.0
log
I
I .o
c
DMTA
Figure 1. Typical nmr spectra of the NCHa resonance signals for carbon tetrachloride solutions of DMA, DMF, DMTF, and DMTA a t ambient probe temperature. The magnetic field increases from left to right.
their own audio side bands. Although these values should be identical with the differences between the corresponding values of v(NCH3)t and v(NCH3), (Table I), they are not, and this reflects the lower experimental accuracy associated with these latter two quantities determined from 250-cps sweep width spectra. The values for Gv(nTCH3) represent the average of several determinations. Although the range of values observed in any given experiment was found to be less than 0.2 cps in many cases, we have assigned minimum error limits of *0.1 cps in these cases because smaller values seem to be unjustifiable. Larger error limits associated with the remaining values reflect the actual spread of the experimental data. The data for all concentrations of a given compound were not determined on a single day. The temperature range reported thus reflects inaccuracies in measuring temperatures using the Varian-supplied standards and the temperature variation in the spectrometer between the various days in which the data were accumulated. Although the effect of temperature on 6v(NCH3) for these compounds in carbon tetrachloride was not determined, available temperature-variation data for D N F in trichlorofluoromethane* indicate that variations in Gv(NCH3) over the temperature range for DMF in our studies are well within the error limits quoted in Table I. Values of 8v(NCH3) have been arbitrarily assigned as negative numbers when the spectra showed the apparent NCH3 shielding inversion as seen for DRSTA (Figure 1). The values of Bv(NCH3) for each compound have been plotted vs. log C in Figures 2-5. The
Figure 2. Experimental (points) and calculated (lines) dependence of 8v( NCHa) on concentration in carbon tetrachloride solutions of DMTA; see text.
log c Figure 3. Experimental (points) and calculated (lines) dependence of av( NCHs) on concentration in carbon tetrachloride solutions of D M T F ; see text.
curves shown are calculated and will be discussed below. Molecular Weight Determinations. Attempts to determine the apparent molecular weights of DMF and DMA in the solvent carbon tetrachloride by vapor pressure osmometry were not successful (see the Experimental Section). The nature of the results indicated that the vapor pressures of the solutes were too high, resulting in mass transfer of solute vapor into the initially pure reference solvent. This would lead to spuriously high molecular weights. The analogous Volume 78, Number 7
Julg 1968
2472
R. C. NEUMAN, JR.,W. SNIDER,AND V. JONAS we have found the average molecular aggregate to be about 1.4 molecules a t formal concentrations in the region of 0.05 M . 2 II
.
Discussion
IO 6V
9-
8-
71
I
1
-1.0
0.0
1
I.o
log c
Figure 4. Experimental (points) and calculated (lines) dependence of 6 v ( NCHa) on concentration in carbon tetrachloride solutions of DMA; see text. I
1 I1
I
DMF
The general trends in Gv(NCH3) for all four compounds as a function of concentration are quite similar (Figures 2-5). In each case the NCH3(c) resonance signals moved downfield as concentration was increased, and those for NCHa(t) moved upfield, Table I. Since the signal positions for DMTA are inverted compared with those of the other compounds, the net result is that the absolute value 6v(NCH3) for DMTA decreased while those for DMTF, DMA, and DlKF increased with increasing concentration. The relative shifts of NCHr (c) and NCH3(t) were such as to make the position of the doublet center reasonably independent of concentration (see the last column in Table I). Although conclusive supporting evidence is not available, the trends observed for 8v(nTCH3) as a function of concentration could be associated with a selfassociation equilibrium between amide or thioamide molecules in solution. The self-association dimer could be held together by dipolar interactions. If such a monomer-dimer equilibrium existed, the values of 8v(NCH3) would be described by eq 1, in which 6vM and 6vD are the nonexchanging chemical shifts between
+
~ v ( N C H= ~ )( M ~ v M 2D6vn)/(M
+ 20)
(1)
the NCH3 groups in pure monomer and dimer, respectively, and 144 and D are the respective molar concentrations. Since the association equilibrium constant K is equal to D / M 2 and the formal concentration, C, of 2 0 , eq solute in carbon tetrachloride is equal to M 1may be rearranged to give eq 2, which is useful for the computer analysis of the nmr data (Table I).17
+
[(8KC
I
I
-1.0
0.0
log
t
I.o
I
c
Figure 5. Experimental (points) and calculated (lines) dependence of Gv(NCHa) on concentration in carbon tetrachloride solutions of DMF; see text.
thioamides DMTF and DNTA should have significantly lower vapor pressures, and molecular weight data obtained by Henneike by vapor pressure osmometry on carbon tetrachloride solutions of DMTF and DMTA indicate average molecular aggregates of 1.7 and 1.1 molecules/unit at formal concentrations of 0.03 and 0.05 M , respectively.lB Additionally, we have previously determined the apparent molecular weight of the thioamide N,N-dimethylthiocarbamoyl chloride, ClCSN(CH3)2, in carbon tetrachloride by the same technique with no obvious experimental problems and The Journal of Phgsical Chemistry
+ 1)’” + lISv(NCH3) = 28VM + [(8KC + 1)’”- 1 ] 8 V J J
(2)
The solid lines in Figures 2-5 represent the best-fit theoretical curves to the experimental data based on a monomer-dimer equilibrium model and the resulting , ~ v D are given (the numbers not parameters K , 6 ~ 1 1 and in parentheses) in Table 11. The signs of ~ V Mand ~ V are both negative for DMTA, while they are positive for the other compounds, and this implies that the extent of aggregation of the amide or thioamide systems is not responsible for the shielding inversion observed for DMTA (Figure 1). The value of ~ V for D DMTA is,
D
(16) H. F. Henneike, private communication. (17) For a given value of K , a series of simultaneous equations can
be generated by substitution of each C studied and the corresponding observed value Sv(NCHa) (Table I). These equations may then be solved to obtain the best values of Svnr and S V D corresponding to the value K . In addition the standard deviations of these best values S u x and S V D can be calculated, as well as an over-all standard deviation for the fit. Repetition of this procedure for a series of values K then allows a choice of the K giving values of Sunr and ~ V which show the smallest standard deviations.
D
NMRSPECTRAL PROPERTIES OF SOMEN,N-DIMETHYLAMIDES AND -THIOAMIDES Table I1 : Calculated Nonexchanging Chemical Shifts for the NCHa Proton Resonances in Monomer and Dimer and the Association Equilibrium Constants
Compd
DMTA DMTF DMA DMF
1.
K,
8*Y,
6VD,
m-1
CPS
OPE
0.64 (0.24) 1.22 (13.0) 0.37 0.60
-7.1 (-6.7) +4.3 (-0.3) +7.3 $6.3
-1.8 (0.0) +8.9 (+8.5) +13.8 +11.6
however, very close to zero. I n order to see if the experimental data might also reasonably fit a theoretical curve which led to inversion of shielding in going from monomer to dimer, best-fit curves just barely leading to such an inversion for DRITA and DMTF were calculated and are represented by the dashed lines in Figures 2 and 3. For DMTA, the dashed curve represents the results for ~ V = D 0.0 (6v;lr = -6.7), and for DMTF, the dashed curve was obtained with ~ V M= -0.3 ( 6 v ~= 8.5) (numbers in parentheses, Table 11). I n both cases, these dashed lines are not as good fits of the data as the solid curves not leading to inversion; however, the differences involved are quite small, especially for DMTA. The strongest supporting evidence for suggesting that inversion does not occur is the rather marked deviation from the dashed line of the highest concentration point for DnITA and the lowest concentration point for DMTF. Although these analyses lead to numbers which accommodate the majority of the data reasonably well, it may be that this correspondence is fortuitous. Clearly, our simple model will not fit the high-concentration data for DMA and DMF (Figures 4 and 5). The behavior in these regions may be due to major deviations from ideality or could reflect the onset of additional association equilibrium processes. Additionally, the trends at lower concentrations might also be due to factors other than those considered here. The available molecular weight data for DMTF and DMTA give values of K for DMTF and D N T A in carbon tetrachloride of 440 and 2.7 M-l, respectively, compared with the values of 1.22 and 0.64 M-’ obtained from the nmr data. The greater disparity for the K’s of DMTF, however, compared with those for DMTA suggests that the major discrepancy may reside in the molecular weight data rather than in the nmr results. While DMTA is a white, almost odorless, crystalline solid, DMTF is a liquid with a noticeable odor. Since solutes with finite vapor pressures will give rise to abnormally high molecular weights, the large K (440 M-l) for DRSTF may be due to such a problem. Additionally, this latter value of K is based on the apparent molecular weight a t only one concentration. Independent of the validity of the equilibrium anal-
2473
ysis, it is important to note (see Figures 2-5) that the values of Gv(NCH3) for all of these compounds at the lowest measured concentrations (0.07-0.08 M ) do not yet represent the limiting chemical shifts for infinite dilution. This fact must be borne in mind when such data are used for magnetic anisotropy studies. While no significant trends are apparent in the C-CH3 resonance signals for DMA and DMTA as a function of concentration, there is a ,sudden relatively large downfield shift in the formyl proton resonance signal for both DMF and DMTF above concentrations of 5-6 M . We suggest that this is evidence for the participation of the formyl proton in hydrogen bonding
X
B- -H-C
// \
N(CH31-2
I1 with some base represented by B.IS I n these systems, B must be another DMF or DMTF molecule and the basic site most certainly is the oxygen or sulfur atom, re~pective1y.l~A previous study of DMF over a more limited concentration range led to the conclusion that such an interaction was not important for D,1/F.8 Experimental Section
Materials. The amides DMF and DMA were commercial samples which had been purified and dried over molecular sieves. The syntheses of the thioamides DMTF and DMTA have been previously described.4 An initial high-concentration sample of amide or thioamide in carbon tetrachloride was prepared and the lower concentration samples were prepared by volumetric dilution. Nrnr Spectyal Data. The nmr spectra were determined at ambient probe temperature using a Varian A-60 nmr spectrometer. All resonance lines were calibrated by the audio-side-band method. Tetramethylsilane was used as an internal standard. Temperatures were determined using the Varian chemical thermometers. Apparent Molecular Weight Studies. Attempts were made to determine the apparent molecular weights of DMA and DWIF in carbon tetrachloride solution using a Mechrolab vapor pressure osmometer. Although no difficulty was encountered in calibration of the instrument with carbon tetrachloride solutions of b e n d , the AT readings for samples of DRlA or DMF in carbon (18) Protons engaged in hydrogen bonding generally become deshielded relative to the nonhydrogen-bonded state: J. w. Emsley, J. Feeney, and L. H. Sutcliffe, “High Resolution Nuclear Magnetic Resonance Spectroscopy,” Vol. I, Pergamon Press Ltd., London, 1985, pp 534-549. (19) See, for example, D. P.Eyman and R. S. Drago, J . Amer. Chem. SOC.,88, 1617 (19GG). Volume 78, Number 7
July 1968
JOHN C. SYNNOTT AND JAMES N. BUTLER
2474 tetrachloride did not remain constant after initial calibration but decreased rapidly toward zero. For example using a 0.1 M sample of DMF in carbon tetrachloride, the AT reading 2 min after the sample drop was placed on the thermistor was 5.20,l min later it was 3.20, and after 15 min AT was 0.03. The rate of AT decrease for DRlA solutions was slightly lower. This behavior would be expected with solutes whose vapor
pressures were too high for study by this method and would lead to spuriously high molecular weights. Acknowledgment. Support by the n'ational Institutes of Health (General Medical Sciences, GM 13342) is gratefully acknowledged. The authors wish to thank Dr. H. Fred Henneike for helpful discussions and for permission to use a portion of his molecular weight data prior to publication.
The Mean Activity Coefficient of Sodium Sulfate in Aqueous Sodium Sulfate-Sodium Chloride Electrolytes by John C. Synnott and James N. Butler Tyco Laboratories, Inc., Waltham, Massachusetts OS154 (Received December 18, 1967)
The activity coefficient of NazSOc in aqueous NazS04-h'aC1electrolytes at 25" and total ionic strength 1.0 has been measured using the cell Pb(Hg)IPbSOl(s)/Na+,S04*-, C1-, HzO/glasselectrode where the glass electrode is reversible to Na+. If care is taken to exclude oxygen from the cell during preparation and measurements, reproducibility of the order of &0.05 mV is obtained. Harned's rule was found to be obeyed within experimental error for Na2S04 (component 2) in these mixtures, and the coefficient aZlat I = 1 was calculated by a least-squares method to be -0.035 i 0.005. This agrees with the value of azl calculated from published osmotic coefficients and activity coefficients of NaCl (component 1) in the corresponding mixtures.
Introduction We have published' measurements of the activity coefficients of NaCl in NaCl (component l)-NazS04 (component 2) electrolytes and have compared these with the measurements made using cation-sensitive glass electrodes.2 From these measurements and from the known osmotic coefficients of the pure aqueous solutions of the pure components, we calculated the Harned rule coefficient for Na2SO4in these mixtures (az1). Since there was some discrepancy between the a12 values of various workers at ionic strength 1, it is of interest to measure aZlby an independent method. We have made some measurements using a sulfatereversible cell Pb(Hg)IPbS04(s)INa+,Sod2-,C1-, HzOl sodium ion-sensitive glass electrode whose potential is given by
E
=
E"
+ RT - In 2P
The Jeurnal of Phgsicnl Chemistry
( ~ N ~ ~ ~ S O , Y Z I ~ )
(1)
where yzl is the mean activity coefficient of Na2S04in the mixed electrolytes, and the other symbols have their usual meanings. The lead amalgam-lead sulfate electrode should not respond to chloride ion since the solubility of PbClz is much larger than that of PbS04. The standard potential E" includes the asymmetry potential of the glass electrode, and is expected to vary slowly with time. Therefore, the measurements were made by transferring the same glass electrode between two cells, one of which contained a reference solution of NazS04without any added chloride.a This measurement is, in principle, the same as one using two sodium amalgam electrode cells, but is easier to apply because of the simplicity of working with the glass electrode. According to the work of Lanier,2aresults of comparable precision should be obtained. (1) J. N. Butler, P. T. Hsu, and J. C. Synnott, J . Phys. Chem., 72, 910 (1967). (2) (a) R. D. Lanier, ibid., 69, 3992 (1965); (b) J. M,Gieskes, 2. Physik. Chem. (Frankfurt), 5 0 , 78 (1966). (3) -4.J. Zielen, J . Phys. Chem., 67, 1474 (1963).
