550
MOHAMMED ALEI, JR.,AND ALANE. FLORIN
Proton Magnetic Resonance Studies of Proton Exchange in Ethylenediaminewater and Ammonia-Water Systems‘ by Mohammed Alei, Jr., and Alan E. Florin University of California, Lo8 Alamos Scientific Laboratory, Los Alamos, New Mexico 87644 (Received July 17, 1967)
The exchange of HzO protons with the NH2 protons of ethylenediamine (en) is rapid in the nmr sense over the entire range of composition from pure HzO to pure en. The same is true for NH3 protons and the NHz protons in en in the NH3-en liquid system. In rather striking contrast, however, the exchange of protons is slow in the Hz0-NH3 liquid system a t room temperature in solutions containing 0 to -50 mole % HzO. Addition of small amounts of NH4+ markedly accelerates proton exchange in this system. In both the H20-en and H20-NHa systems, the averaged HzO-amine proton resonance displays significant paramagnetic deviations from the straight line which assumes weighted averaging of the proton shifts for each pure component. The deviation is a maximum in the region of composition corresponding to one HzO per amine group. These results suggest an amine-hydrate liquid structure in which hydrogen bonding is stronger than a simple average between the hydrogen bonding in pure HzO and in pure amine. A study of the kinetics of Hz0-NH3 proton exchange at low Hz0 concentrations in the presence of added NH4+ shows that the proton exchange between HzO and NH3 is best fit by a kinetic expression of the form k [“I+] [HzO] in which k = 3.48 X lo6 (moles/l.) -l sec-I, with a standard deviation of -1%. Although more than one interpretation is possible, these results are consistent with a mechanism involving proton exchange between an NH4+ ion and a water-ammonia solvate.
Introduction I n previously reported nmr and epr studies of Cu(I1) in the water-ethylenediamine (en) system2 covering the entire range of composition from pure HzO to pure en, an understanding of the behavior of the protons in the pure solvent system was essential to the over-all interpretation. I n an earlier study3 of aqueous Cu(I1) solutions containing up to 0.1 M concentrations of free en, the results were compatible with the assumption that HzO protons exchange rapidly enough with the NHz protons of en to produce a single averaged nmr peak for these protons. It was by no means clear, however, that this exchange would continue to be rapid over the entire range of composition from pure H 2 0 to pure en. I n fact, on extending the study of Gutowsky and Fujiwara4 to NHX-HzO solutions of high NH3 content, we found that whereas NH3-Hz0 proton exchange is rapid at low concentrations of NH3 in H20, it is slow, on the nmr time scale, at compositions between 0 and -50 mole % HzO in liquid NH3 at room temperature. The work of Birchall and Jolly5 had shown that in H20-NH3 solutions containing NaOH, separate HzO and NH3 proton resonances are observed at an H2O ’ HzO (0.86 M ) and preconcentration of -2.5 mole % sumably also at -16 mole % H 2 0 (6 M ) . Our indeThe Journal of Physical Chemistry
pendent study of this system clearly shows that proton exchange is not rapid at over twice that HzO concentration and that addition of NaOH is not a necessary condition for slow proton exchange, provided that extraneous addition of NH4+ ion is carefully avoided. We have, moreover, studied the effect of NH4+ ion as an accelerator for proton exchange in this system and the results serve to indicate probable species involved in the exchange process. In view of the fact that, contrary to our observations and those of Birchall and Jolly, Ogg6t7had postulated extremely rapid proton exchange between HzO and NH3 at very low HzO concentrations in liquid “3, we have also examined the basis for Ogg’s conclusion
(1) Work done under the auspices of the U.S. Atomic Energy Commission. (2) M. Alei, Jr., W. B. Lewis, A. B. Denison, and L. 0. Morgan, J . Chem. Phys., 47, 1062 (1967). (3) L. 0. Morgan, J. Murphy, and P. F. Cox, J . Am. C h e n . Soc., 81, 5043 (1959). (4) H. S. Gutowsky and S. Fujiwara, J. Chem. Phys., 2 2 , 1782 (1954). (5) T. Birchall and W. L. Jolly, J . Am. Chem. SOC.,87, 3007 (1965). (6) R. A. Ogg, Jr., J . Chem. Phys., 2 2 , 560 (1954). (7) R. A. Ogg, Jr., Discussions Faraday Soc., 17, 215 (1954).
55 1
PMRSTUDIES IDF PROTON EXCHANGE and offer an 8lternative explanation for his observations.
O r TMS
Experimental Section All measurements were made on a Varian DP-60 instrument operating at a fixed frequency of 60 MH5. en-HzO solutions were sealed in 5-mm 0.d. thin-walled cylindrical sample holders. NH3-H20 and "3-en solutions were sealed in standard wall (1 mm) 5-mm 0.d. Pyrex tubes which withstand the equilibrium pressure over liquid NH3 a t room temperature. TMS was used as am internal reference for solutions of high amine content and T M P (3-trimethylsilyl-l-propanesulfonic acid, sodium salt) was used in solutions of high HzO content. Pure en was prepared by refluxing Eastman 98% ethylenediamine with Na and distilling under argon. NH3 was usually expanded directly from a tank of Matheson anhydrous NH3 into a Pyrex vacuum system and condensed into the nmr sample tube by cooling with liquid Nz. It was found that purification by distillation from an Na-NH3 solution was generally unnecessary.
- 5 0 ~
-loo!-
/I
--
--'-/
/
-200
/ -250
-
Results
3
o
o
; 2"0 '
I
' 40
60 I
'
80 1
'
MOLE % NH2 PROTONS
The amine protons of en were found to exchange rapidly with those of HzO over the entire range of composition from pure HzO to pure en. The proton nmr spectrum of pure en consists of two sharp resonances of equal intensity, the peak corresponding to the amine (N&) protons lying 65 H5 upfield of that for the methylene (CHz) protons. With increasing addition of HzO, the peak for the averaged NHt-HzO protons shifts progressively downfield toward the position of the proton resonance in pure HzO, while the position of the CHz proton peak remains relatively constant. I n each solution, the ratio of the area under the CH2 peak to the area under the NH2-Hz0 peak was found to equal the ratio men:(men 0.5m~,0)where men and mHpOwere the respective moles of en and HzO used in preparing the solution. The positions of the two proton peaks as a function of solution composition are plotted in Figure 1. I n the NH3--Hz0 system, we found two proton resonances in solutions containing -50 mole % or less HzO in liquid NHa at room temperature. Both peaks were relatively broad and the ratio of the area under the upfield peak to Ghat under the downfield was found, for ~ ~~ ~where :0 each solution, to equal the ratio 3 m ~2 m mNHa and mHIo were the respective moles of NH3 and HZO in the particular solution. I n solutions containing somewhat more than 50 mole % HzO, a single broad peak was observed. With increasing HzO, this peak grew sharper and moved progressively downfield toward the position of the proton resonance in pure H2O. Addition of very small amounts of NH4+ ion to the NH3-H20 solutions produced rapid averaging of pro-
Figure 1. Proton shifts in ethylenediamine-water system: 0, averaged HzO and NH2 protons; A, CHI protons.
-50-
-100-
+
tn 0. 0
-150-
a-
-200-
-250
-
0
3 20
0 40
MOLE Yo NH,
0 60
3 00
PROTONS
Figure 2. Shift of averaged protons in NHs-HZO system. Volume 72, Number 2 February 1968
552
MOHAMMED ALEI, JR.,AND ALANE. FLORIN OrTMS
7
3- WAY PRESSURE STOPCOCK APIEZON - N LUBRICANT
PYREX WOOL
. WALL PYREX
-20011 0
I
I
20
MOLE
I 40 */e
I
I
I
60
I 80
I
I
IO0
NH3 PROTONS
\5mm 0.d. STD. WALL PYREX
Figure 3. Proton shifts in ethylenediamine-ammonia system: 0 , averaged NHs and NH, protonfi; A, CH2 protons.
tons over the entire range of composition. Thus, the data in Figure 2 showing the position of the resonance for the averaged protons in the NH3-H20 system were taken by adding 0.1-0.2 mole % NH&l to each solution. I n solutions of high H 2 0 content, where exchange was rapid enough without added NH4+to produce averaging, the position of the averaged proton peak was not detectably altered by adding 0.2 mole % NHIC1. The data in the region from 0 to 40 mole % NHa protons is in good agreement with the results of Gutowsky and Fujiwaraj4 who carried out a similar study over this composition range only. The position of the averaged amine protons in the en-NH3 system is plotted as a function of composition in Figure 3. Proton exchange was rapid enough to produce averaging of the KH3 protons with the NH2 protons of en over the entire composition range. The CH2protons of en displayed a separate resonance whose position was essentially independent of composition. The ratio of the area under the CH2proton peak to the area under the averaged amine proton peak was equal to men:( 0 . 7 5 m ~ ~ , men), expected for averaging of NHa protons with the ISH2protons of en. The observation of nonaveraging of H2O and NH3 protons in NH3-H20 solutions of high NHa content prompted us to study this system in somewhat greater detail. Since we had observed that small additions of NH4+ion were extremely effective in accelerating proton exchange in this system, we were interested in observing the proton spectrum under conditions which would exclude any NH4+except that which would result from the self-ionization of liquid NHa or the ionization of HzO in liquid ”3. For this purpose, the apparatus shown in Figure 4 was used. The entire apparatus fits between the pole pieces of the magnet with the sample chamber in position in the 5-mm Varian probe insert. I n order first to obtain a sample of pure liquid NHs displaying the expected6J triplet proton spectrum,
+
The Journal of Physical Chemistry
SAMPLE
Figure 4. Apparatus for preparation of ammonia-water mixtures.
we found it necessary to condition the sample chamber by many cycles of distilling NHa from the Na-NHs solution reservoir into the sample chamber and pouring back into the reservoir. An even more effective technique was that of pouring the sodium-ammonia solution back and forth between the sample chamber and the reservoir and finally rinsing the sample chamber with NH, distilled from the reservoir until a, colorless sample of liquid NH3 was collected in the sample chamber. After such conditioning, traces of NaOH introduced with the Na-NH3 solution may have remained in the sample tube due to the low solubility of NaOH in liquid “3. I n any event, a liquid NH3 sample collected in this fashion displayed a sharp triplet spectrum which persisted unchanged for many hours. To such a sample of liquid NH3 in the sample chamber, we added sufficient HzO by distillation from a reservoir through a vacuum system to prepare an NH3-H20 solution containing 60.5 mole yo HzO. A series of solutions with progressively decreasing H2O concentration was prepared from this solution by distilling in increments of pure NHa from the Na-NHa solution reservoir. The proton nmr spectra of four solutions in this series are shown in Figure 5 . The H20-NH3 proton exchange accelerated by NH4+ was studied by measuring the exchange rates at room temperature over a range of H 2 0 concentrations from
553
PMRSTUDIES OF PROTON EXCHANGE
If we assume a rate independent of NH, concentration and let rate = k[HzO][NH4+],we obtain the final expression
If, on the other hand, we assume that rate = k’ [HzO][NHd+][NH,], we obtain
Table I lists pertinent variables, line broadenings, and calculated values of k and k’.
Discussion With regard to the position of the average amineHzO proton resonance as a function of composition, the en-HzO and NHrHzO systems (Figures 1 and 2) both display significant paramagnetic deviations from the straight line drawn between the roton resonance positions in pure HzO and pure liq ‘d amine. This suggests a mixed-liquid structure in which, relative to the weighted mean between the average proton in the pure liquid amine structure and the average proton in the pure H2O structure, the hydrogen bonding is stronger or more extensive or both. The fact that the maximum deviation from the straight line occurs in the region of composition corresponding to one H2O molecule per amine group indicates that the effect may be attributable to an amine hydrate structure of exceptional stability. Such a view is supported in the enH 2 0 case2 by pronounced maxima in both the viscosity and the activation energy for viscous flow at this composition. I n the en-”, system (Figure 3) on the other hand, the results indicate that proton magnetic environments in the mixed liquid system are a weighted average of the average proton environment in each of the pure liquid amines.9 I n principle, the observed shift for the average prowhere f r i s the fracton in any system is given by
t
HFigure 5. Proton nmr spectra for ammonia-water mixtures with no added ammonium ion: (a) 60.5 mole % H,O, (b) 37.6 mole % H20, (0) 24.0 mole % HzO, and (d) 16.1 mol’e % HzO.
5.17 to 16.2 M and NH4+ concentrations from 4.43 X to 3.01 X lop2 M. I n this region, proton exchange is rapid enough to produce an average broadened single line for the H20and NH, proton resonances. The broadening of the line relative to the sharp signal, obtained under conditions of very rapid exchange, is related to proton residence times by the relationships
cfcSz i
+
p 2 ~ 2 ~ p 2-~ W~N Hs J ~( ( T~H ~~O z TNHJ ~
(1) where W is the broadening in hertz of the absorption signal width at half-height, and P H ~ O and are the fractions of H20 and NH, protons, respectively, in the system. The measured separation of the H2O and NH, proton resonances at slow exchange rates (no added NH4+) js 240 Hz, thus (WH~O - W N H J ~ = (2n X 240)2. Moreover, since T H ~ O TNHa = T H , O / P E ~ O and 7 ~ = ~ 0 2[H20]/rate, we obtain by substitution in eq 1
+
where rate represents the moles/l. of H 2 0 molecules which exchange a single proton with NH, each second.
tion of protons in a given environment and at the shift for the proton in that environment. Thus, in simple systems, it might be possible uniquely to evaluate the ai and f t (related to over-all composition through appropriate equilibrium constants) from the experimentally observed shifts of the average proton. I n the amine-Hz0 systems considered here, however, one can expect at least six different proton environments (N-H, 0-H, N+H-N, O+H-0, N-H-0, 0-t H-N) with unknown ai and f$. Moreover, both the (8) J. A. Pople, W. G. Schneider, and H. J. Bernstein, “High Resolution Nuclear Magnetic Resonance,” McGraw-Hill Book Co., Inc., New York, N. Y., 1959, p 222. (9) Another possible interpretation is that exchange of protons each having the same occurs between “islands” of en and “8, structure which it would have in its pure liquid state. This seems unlikely.
Volume 72,Number 2 February 1968
MOHAMMED ALEI,JR.,AND ALANE. FLORIN
554 Table I
34.8 34.8 33.7 33.7 33.7 29.1 29.1 29.1
5.17 5.17 9.46 9.46 9.46 16.2 16.2 16.2
4.86 X 9.61 X 4 . 4 3 x 10-8 8.92 X 1.76 X 7 . 5 9 x 10-8 1.53 x 3.01 X IO-*
0,0901 0,0901 0.158 0.158 0.158 0.271 0.271 0.271
and the equilibrium constants determining the fc might well vary with over-all solution composition. I n view of these uncertainties, any numbers derived from the present data would be of questionable value. With regard to the rate of exchange of protons between HzO and NH3 at low concentrations of HzO in liquid NH3, our results (Figure 5) show clearly that in the absence of added NH4+, this exchange is slow in the nmr sense (i.e., the average proton lifetime in the “HzO” environment is long compared with the reciprocal of the frequency separation, expressed in radians/ sec, of the HzO and KH3 proton resonances). This is consistent with the conclusion of Birchall and Jolly: who observed a separate HzO peak and NH3 triplet in 0.86 M HzO in liquid NH3 saturated with NaOH. Ogg69’ had earlier concluded that the reaction HzO NH3 Ft NH4+ OH- was extremely rapid with a rate constant of 4.6 X lo81. mole-’ sec-l. This conclusion was mainly based upon the assumption that traces of HzO had to be removed from the walls of his sample chamber before he could obtain liquid NH3 samples displaying the proton triplet. As indicated earlier, we also found that prior conditioning of our sample chamber was necessary. We also observed, however, that a small crystalline residue remained after distilling NH3 away from a clean, dry sample tube which had been exposed to liquid NH3 for the first time. This nonvolatile material had to be removed by repeated rinsing before the NH3 proton spectrum would split into a triplet. When this was done, relatively large amounts of HzO could be added without collapsing the NH3 proton spectrum to a singlet. We have not identified the solid material formed when glass is exposed to liquid NHa for the first time, but in view of the very strong acceleration of proton exchange by NH4+, we suspect it to be an ammonium salt formed by reaction of NH3 with acidic protons in the glass surface.lO I n support of this assumption, we have observed that small concentrations of OH- added to dilute solutions of HzO in liquid NH3 cause the NH3 proton spectrum to be displayed as a triplet without prior conditioning of the sample tube. Thus it seems clear that what Ogg had interpreted as an effect due to removal of small traces of HzO from sample-chamber walls, was, in fact,
+
The Journal of Physical Chemistry
+
0.9099 0.9099 0.842 0.842 0.842 0.729 0.729 0.729
6.1 3.1 10.6 5.3 2.7 8.2 4.0 2.0
3.63 3.64 3.47 3.44 3.42 3.34 3.41 3.47
1.05 1.05 1.03 1.02 1.01 1.15 1.17 1.19
due to removal of much more strongly acidic constituents from the glass walls. Since the NH3 triplet structure can be collapsed by NH3-NH3 proton exchange, which is very strongly accelerated by NH4+,6J one questions whether small amounts of H 2 0 could collapse the NH3 triplet, not by direct HZO-NHa proton exchange, but by acceleration of NH3-NH3 proton exchange via the equilibrium concentration of NH4+ produced by ionization of HzO in liquid NH3. Since NH4+ concentrations of the order M are sufficient to coalesce the NH3 triplet in of liquid NH3, one might conclude from the spectra in Figure 5 that the ionization of H 2 0 a t -25 mole % HzO in liquid NH3 produces an NH4+ concentration M . However, inadvertent significantly less than introduction of traces of OH-, which we estimate could M , could greatly suppress the be as high as 5 X concentration of NH4+ which might otherwise have resulted from the HzO ionization. The principal conclusion to be drawn from Figure 5 is, therefore, that direct proton exchange between HzO and NH3 is not extremely rapid even at -50 mole % ’ HzOin liquid NHa. Moreover, we should point out that although HzONH3 proton exchange is accelerated by NH4+, this exchange is not nearly as sensitive to traces of NH4+ as NH3-NH3 proton exchange. Thus, separate HzO and NH3 proton signals (the NH3 signal may or may not be a triplet) are readily observed in sample tubes cleaned in conventional fashion with no special conditioning. As noted earlier, proton averaging between H 2 0 and the NHz protons of ethylenediamine was observed to be rapid even at very low concentrations of HzO in en. I n view of the observed acceleration of HzO-XH3 proton exchange by NH4+, one might question whether or not adequate precautions were taken to eliminate acidic impurities in the H20-en system. We have observed, however, that rapid averaging occurs in solutions prepared by mixing distilled HzO with en distilled directly into the nmr sample tube from blue solutions of Na in en on a vacuum system. It thus appears that at low HzO concentrations, H2O-amine proton exchange is definitely more rapid in the H20-en system than in the (10) H. P. Boehm, Angew. Chem., 5 , 533 (1966).
555
PMRSTUDIES OR’PROTON EXCHANGE H20-NH3 system though we do not as yet understand why this is so. With regard to the H20-NH3 proton exchange accelerated by N134+,the data in Table I demonstrate that even over the necessarily limited range of variation of [NH,], the constancy of k [average value 3.48 X lo6 (moles/l.)-I sec--’ with a standard deviation of f1%]is significantly better than that of k‘. Moreover, k’ shows a definite trend to high values at the lowest NH3 concentration. We should emphasize, however, that concentrations rather than activities were used in these calculations. Vapor pressure datal1 for the “3H 2 0 system a t room temperature indicate clearly that the activity coefficient for NH3 decreases from 1 to -0.9, while that for H 2 0 remains essentially 1 in going from pure NH, to 16 M H 2 0 in NH3. If the correction for NH3 activity were applied, the trend to higher values in k’ with decreasing [WH3]would be even more pronounced than in Table I. This would clearly suggest an exchange mechanism independent of [NH,] which, in turn, would be consistent with exchange between NH4+ and a rather stable water-ammonia solvate. However, since we do not have any activity coefficient data for NH4+ in these media, we cannot rule out the possibility that the trend in k’ may be due to failure to account properly for a possible increase in the activity coefficient of NH4+ with increasing [HzO]. Thus we cannot claim to have proven the existence of a stable water-ammonia solvate as a kinetic species in these media. However, such a species might, by virtue of stronger hydrogen bonding, provide a more paramagnetic proton environment than the average proton environment between the two pure liquid systems and this could explain the observed paramagnetic deviation from the line in Figure 2. Moreover, Grunwald and Cocivera12 have presented kinetic evidence which indicates the presence of a stable amine-hydrate in aqueous solutions of NH4+ and NH3. From studies of proton exchange between NH4+and HzO in aqueous solutions at varying pH, Meiboom,
Loewenstein, and Alexanderla conclude that the principle exchange between H 2 0 and NH4+ occurs via the process H NH4+
H
I + O-H + NH, ”il,NH3 + H-0 I + NH4+
with a value of IC7 = (0.9 f 1) X 108 sec-’ M-‘ at 21’. Birchall and Jolly6 have suggested that the exchange of protons between H20 and NH3 in unbuffered liquid NH3 solutions may involve the same mechanism. From arguments just presented, it seems to us that a more likely mechanism is
H NH4+
I + O-H
H NH3 +NH3 * H-0
I
+ NH4+
where
H
I
O-H
*
NH3
is meant to denote a species in which a water molecule is associated with at least one NH3 molecule. The results of Meiboom et ul., would also be consistent with such a mechanism operating in aqueous solutions of NH4+ containing small amounts of NH3 in the form of an ammonia-water solvate. If indeed the same mechanism operates in both media, the fact that the reaction in liquid NH3 is -100 times slower than in liquid H20 may be attributable, at least in part, to generally smaller ionic activity coefficients in media of lower dielectric constant. (11) J. H. Perry, Ed., “Chemical Engineers Handbook,” 2nd ed, McGraw-Hill Book Co., Inc., New York, N. Y., 1941,pp 404-405. (12) E. Grunwald and M. Cocivera, Discussions Faraday SOC.,39, 105 (1965). (13) S. Meiboom, A. Loewenstein, and S. Alexander, J. Chem. Phys., 29, 969 (1958).
Volume 78, Number 2 February 1068
