EFFECT OF IMPURITIES ON ACTIVITY OF OXYGEN CHEMISORBED ON SILVER
3881
The Effect of Impurities on the Activity of Oxygen Chemisorbed on Silver1
by Y. L. Sandler, S. Z. Beer, and D. D. Durigon Westinghouse Research Laboratories, Pittsburgh, Pennsylvania
15855
(Received June 17, 1966)
+
A previous study of the isotopic exchange reaction 0% 0 3 --+ 2016018 and the desorption of oxygen from pure silver showed that evacuation and oxygen pretreatment at high temperatures of a well-cleaned silver powder reversibly increases and reduces the desorption rate of oxygen below 200" without a change in activation energy (32.5 kcal/mole). This is due to reversible changes in the topography of the surface. It is now shown that pretreatment of silver with hydrogen reduces the activation energy of oxygen desorption. The presence of residual hydrogen appears to be the reason for many discrepancies found in the literature. Isotopic dilution experiments reveal that residual hydrogen is firmly held a t centers containing strongly bound oxygen. A silver powder containing AlgO or alloyed with gold, even when pretreated with hydrogen, gives the same activation energy of desorption as hydrogen-free pure silver. Exchange experiments on Ag-MgO also show tshat no firmly bound oxygen or hydrogen exists in or on these materials. The activity changes produced by the additives, as are those produced by high-temperature oxygen pretreatment, are due to changes in the topography of the silver surface and not to changes in the work function or to lattice expansion as previously suggested in the literature.
Introduction I n a previous communication2 (paper I) it was shown that at temperatures above 160" at least two modes of chemisorption of oxygen exist on pure silver. Pretreatment with oxygen at 500" changes the relative amounts of the two types of chemisorption. The rate of desorption is thus changed but not the activation energy. It was concluded that the observed phenomena must be caused by a change in surface topography. The experiments described in paper I were carried out on silver particles of high analytical purity which had been carefully freed of carbonaceous impurities and of trapped hydrogen. It became apparent in the course of this work that hydrogen is very hard to remove even at temperatures as high as 500" and that it has a profound influence on the properties of the adsorbed oxygen. This paper discusses the effect of the residual hydrogen on the oxygen adsorption. The desorption of oxygen and t,he homonuclear exchange reaction Olez O1*z-+ 2Ol60l8were studied. As shown in paper I, the rate of the two reactions is the same, but additional information can be obtained by studying the isotopic exchange. Also investigated were the effects of other types of impurities : gold (alloyed) and magnesium oxide.
+
These impurities are frequently used to modify the catalytic properties of silver catalyst^.^^^ A study of the effect of MgO was also of interest in view of its effect on the properties of silver as an oxygen elect r ~ d e . ~As ? ~in paper I, all experiments were carried out under conditions at which silver oxide is not formed as a separate phase.
Experimental Section The reaction system was the same as previously described.2i7 It consisted of 2-4 g of metal powder in a quartz vessel of 18-30-cc volume which could be isolated from the gas-handling systems, diffusion pumps, and liquid nitrogen traps by means of a metal valve. (1) Work sponsored in part by the U. S. Army Electronics Command and the Office of Naval Research. (2) Y. L. Sandler and D. D . Durigon, J . P h y s . Chem., 69, 4201 (1965). (3) Cj. L. Ya. Margolis, Advan. Catalysis, 14, 479 (1963). (4) K. E. Hayes, Can. J . Chem., 37, 583 (1959). (5) S. 2. Beer and Y. L. Sandler, J . Electrochem, SOC., 112, 1133 (1965). (6) Y. L. Sandler and E. A. Pantier, Extended Abstract, Theoretical Division, Cleveland Meeting of The Electrochemical Society, May 1966, p 57. (7) Y. L. Sandler and D. D . Durigon, Trans. Faraday SOC.,62, 215 (1966).
Volume 70,Number 18 December 1966
3882
Two thermistor gauges attached to either side of the metal valve served to measure gas pressures. Surface areas were determined by the BET method with krypton. For pure silver these varied from 600 cm2/g after initial oxygen pretreatment to 230 cm2/g after continued hydrogen and oxygen pretreatment. A small mass spectrometer was coupled to the system to monitor gaseous impurities desorbing from the metals and to analyze the oxygen isotopes. Only the isotopic dilution experiments with silver at 500" were carried out on a larger system. This system, which consisted of 20 g of silver in a vessel of 135 cc, was degassed on the vacuum system and transferred to an analytical mass spectrometer of higher accuracy. The silver powder was supplied by Handy and Harman, gold by American Smelting and Refining Co. Though both were of 99.999% guaranteed purity, the carbon content' was considerably higher than the given impurity limit. Silver-gold alloys were produced by repeated levitation melting of compressed powder mixtures. They were then formed by swaging to provide wires approximately 1 mm in diameter. To assure homogeneity of the wires and to lower the carbon content, the wires were kept for 2 days at 750" in a flow of purified oxygen. The alloy powders were then prepared by spark erosion under triple-distilled water. Figure 1 shows the automatic spark erosion milling machine (Servomet, Metals Research Ltd., Cambridge) adapted for the purpose. The alloy wire to be sparked, extended by a gold wire welded to it, is connected to the anode of the Servomet output and is fed down the vertical capillary of the vessel by means of the servo-controlled arm. The feed of the vertical wire toward the cathode is controlled by the machine in such a way that a fixed voltage of 140-180 v is maintained across the gap. One gram of powder was produced in about 2 hr. To oxidize any carbonaceous impurity present, pure oxygen was bubbled through the water. The colloidal particles coagulated rapidly, and the water could be decanted. ii silver-1.7 mole % magnesium powder was produced by precipitating a mixture of the carbonates from an aqueous solution of the nitrates with ammonium carbonate, followed by thermal decomposition in vacuo. All powders were degassed a t a temperature that was gradually raised to 520" with frequent contact with low-pressure oxygen until the desorbed impurities were reduced to a negligible level. Desorption rates were determined after first allowing oxygen a t a certain pressure to equilibrate with the powder. The gas phase was then rapidly removed by pumping, the valve was closed, and the pressure rise was recorded. The technique is accurate and simple; The Journal of Physical Chemistry
Y. L. SANDLER, S. Z. BEER,AND D. D. DURIGON
Pede
Polyethylen Sleeve
--___
Figure 1. Setup for producing powders from wires by sparking.
however, if the measured rate is to be equal to the desorption rate at equilibrium with the oxygen (before its removal), it must first be shown that no weakly chemisorbed gas is removed during the pumping period. This was done in paper I.
Results The E$ect of Residual Hydrogen in Silver on the Desorption Rate of Oxygen. The previous experiments2 showed that the activation energy of the homonuclear oxygen-exchange reaction is high, about 32 kcal/ mole of 02,if the silver contains no hydrogen. The rate-determining step in the exchange reaction is the desorption of oxygen. The desorption rate was then measured directly by the rate of pressure increase in the reaction vessel after rapid removal of the gas phase. I n Figure 2, the two broken lines represent the previously measured2 desorption rates, in molecules/cm2 sec, as a function of the inverse absolute temperature. The lower curve was obtained when the sample contained a high concentration of strongly bound oxygen and was obtained by cooling the sample in 5 torr of oxygen from 500'. The upper curve was obtained after cooling from 500' in vacuo before contact of oxygen at a lower temperature. As shown by the sequence of the measurements, the change in the state of the surface is completely reversible. The independence of the activation energy of desorption of the relative amount of
EFFECT OF IMPURITIES ON ACTIVITY OF OXYGEN CHEMISORBED ON SILVER
3883
, L y 7 "High Temperature Ox)rJen" Concentration
High J
I
PC
I 1 0
2. 1
I
I
I
To K
I 2.3
2.2
I
2.4
x 103
Figure 2. Effect of gas pretreatment a t 500' on the oxygen desorption between 160 and 190' : oxygen pretreatment, broken lines; hydrogen pretreatment, solid line.
the two modes of adsorption was interpreted as showing that the surface sites for the weaker chemisorption are unchanged. The oxygen pretreatment causes a change in the distribution of crystallographic planes (faceting). At the end of the experiment just described, it was ascertained that the silver had contained no appreciable amount of hydrogen. A small dose of deuterium was equilibrated with the powder at 500'. The small amount of HD evolved showed that the total amount of light hydrogen in the silver was less than the equivalent of monolayer. The effect of sorbed hydrogen on the desorption rate of oxygen was then tested. The silver was brought into contact with 20 torr of hydrogen for about 0.5 hr at 500' and was then pumped overnight at the same temperature. Oxygen was then adsorbed at 160' and pumped, in the same manner as described2 for the oxygen pretreated samples. Figure 2 shows that the activation energy of desorption of the oxygen was now lower, being 24 kcal/mole between 160 and 180'. A similar result will be described below. As will be shown, residual hydrogen is firmly bound to silver (in the presence of residual oxygen) and it modifies the adsorption properties of the surface for oxygen. Residual Oxygen and Hydrogen in Pure Silver. Oxy-
gen cannot be completely removed from silver by prolonged pumping at 500' (paper I). In part, the residual oxygen resides at the surface and rapidly exchanges with chemisorbing oxygen at temperatures as low as 160' (the lowest temperature at which desorption was fast enough to make measurement of the isotopic exchange possible). The current experiments show that contact of hydrogen produces changes in the surface which affect the reversibility of the oxygen adsorption. In the following, isotopic dilution experiments are summarized. These were carried out at 500°, a temperature high enough to make the diffusion of both hydrogen and oxygen through a silver particle fast so that the oxygen in the gas phase rapidly equilibrates with the oxygen in the silver. The amounts of residual oxygen and hydrogen in the silver were determined after different hydrogen and oxygen pretreatments. It will be seen that hydrogen is as strongly retained as is oxygen. In the oxygen dilution experiments a mixture of was admitted to a argon and 0l8(containing 6% 0l6) pure silver sample after certain pretreatments with oxygen (Ola) and hydrogen and pumping at 500'. The amounts of the different isotopes (in cc atm) in the gas phase at any given time were determined by measuring the composition of small samples and the total amount of argon present; the latter was measured by expansion into a large standard volume and by a pressure reading on a capacitance micromanometer. For analyzing the hydrogen content of the samples a known pressure (usually 2 torr) of 98.4% deuterium was admitted at 500'. No inert gas was used in this case and the results are somewhat less accurate. The amount admitted was calculated (1) from the approximate pressure of the admitted gas and the volume of the reaction vessel and (2) by expansion of the gas at the end of an experiment into the standard volume and reading the pressure. In all cases, the two methods agreed to better than 30%. Table I summarizes some of the experiments. The pretreatment before starting the dilution experiment is stated in each case, then the type and amount of gas added, the time of equilibration of the gas with the metal before measurement, the isotopic ratio found, and the total residual gas, for 20 g of silver. In the first example, the sample was first contacted several times with 0l6and pumped, to remove 0l8from previous experiments and to remove carbonaceous impurities. After repeated contact with hydrogen and pumping, the oxygen exchange with 01*was carried out. From the samples taken between 5 and 160 min it may be seen that, as expected, equilibrium is rapidly attained. The residual 0l6found in the sample volume 70,Number 18 December 1966
3884
Y. L.SANDLER, S. Z. BEER,AND D. D. DURIGON
Table I: Isotopic Dilution Experiments with Pure Silver at 500’ Ag sample 9 run no.
1
2
3
Pretreatment at 500’
Gas added at 1 = 0, cc
Repeated contact with 016~ and pumping; pumped 1 hr. Contacted two times with 5 torr of H2 for 15 min; pumped 5 hr a t 500‘
01*, 0.106
Pumped 2 hr
Time 1, min
(01‘/019ea
or (H/D)eq
Residual gas (20 g of A d , cc
5 20 160
2.2 2.4 2.5
D, 0.13
30
0.93
Admitted 10 torr of OlS2 and pumped; four times in 3 hr. Then admitted 30 torr of HZand pumped; seven times in 5 hr; pumped 17 hr Pumped 2 hr
D, 0.32
30 60
2.45 3.26
0.041
30
0162,
0.21
Admitted 30 torr of H2 and pumped; seven times in 26 hr (in H2 overnight). Then pumped 45 min
01*,0.038
30
0l62,
0.062
amounted to 0.27 cc atm. This corresponds to 17 ppm (weight) of the bulk (or about 1.5 monolayers; the surface area was 230 cmz/g). The isotopic hydrogen dilution experiment carried out immediately after pumping the sample for 2 hr showed that the residua1 hydrogen concentration was of the same order as the residual oxygen concentration, about half its amount. In the next experiment, run no. 2, the procedure was reversed; the hydrogen was exchanged first and then the oxygen. A more thorough hydrogen pretreatment was given here, as indicated in the column headed “Pretreatment at 5 O O O . ” Nevertheless, the result was almost the same: the residual oxygen concentration was only slightly lower than in the first run and the amount of hydrogen was again about half of the oxygen concentration. Other dilution experiments with hydrogen, not carried out in conjunction with oxygen dilution, also showed that at least the order of magnitude of the retained hydrogen is the same as found for oxygen. This leaves little doubt that the strong retention of the hydrogen in the silver is caused by the presence of firmly bound oxygen in the solid. The result is analogous to a previous conclusion’ that firmly bound hydrogen, formed on the siIver surface at high temperatures, is caused by strongly chemisorbed oxygen. I n run no. 3, the silver was pretreated with hydrogen for 26 hr; this included contact with hydrogen overnight. A liquid nitrogen trap was used to keep the HzO pressure low. In this way the residual oxygen concentration could be reduced to 0.003 cc/g of Ag, The Journal of Physical Chemistry
01*,
O’*z, 0.27 (= 17 ppm
(wt)) HI, 0.12
H2, 0.10
or 4 ppm, corresponding in the present case to onethird of a monolayer. The best way to remove residual hydrogen appears to be continued contact with more hydrogen. It removes the residual oxygen which causes retention of the hydrogen. Example 2, as well as other exchange experiments at 500’ with hydrogen, showed a slow increase with time of the amount of exchanged hydrogen after the initial fast exchange. The silica walls of the reaction vessel may have been responsible for the slow exchange. Another vessel without silver powder, treated in similar fashion with oxygen and hydrogen, showed no noticeable exchange after 16 hr. However, it is possible that water formed during the initial phase of hydrogen pretreatment of the silver may have produced exchangeableOH in the silica walls. The Homonuclear Oxygen Exchange on Silver with and without Additives. The rate of the reaction O1‘jZ 0%4 2016018 was measured with pure silver, silvergold alloys, and a silver-magnesium oxide (coprecipitated). The powders received a degassing pretreatment at temperatures up to 520’ for several days with frequent oxygen and hydrogen contact at decreasing pressures. All samples were pumped at 450-500O overnight, immediately before measurements were made. The results are presented in Figure 3 where the logarithm of the rate of exchange (in molecules cm-1 sec-’) is plotted against the inverse absolute temperature. The curve marked “Ag(O)” was oxygenpretreated only and was taken with the same sample as used for the desorption experiments before contact
+
EFFECT OF IMPURITIES ON ACTIVITY OF OXYGENCHEMISORBED ON SILVER
27 5
2% I
225 1
ToC
200
150
175 I
I
I
I
3885
was different. A silver-75 at. % ’ gold alloy was pretreated with hydrogen and oxygen, similar to the silver sample “Ag(H,O).” However, it gave the same slope as the silver sample “Ag(0)” containing no hydrogen. The surface area was 1200 cm2/g. The points were taken in random sequence, as indicated by t.he numbers in Figure 3. When it is considered that only one-fourth of all atoms are silver, the absolute rate appears to be the same as for oxygen-treated pure silver (Ag(0)). However, this conclusion is not reliable. The thermal pretreatment may have decreased somewhat the silver content of the surface.
Table 11: Reaction of 3 Torr of 0%
+
01*? on
Ag,
Pretreated with 0 %after H, 1.8
1. 9
2.0
2.1 1/PK x lo3
2.2
2. 3
24
Figure 3. Isotopic oxygen exchange rate for oxygen-pretreated silver, Ag(O), for hydrogen- and oxygen-pretreated pure silver, Ag(H,O), alloy Ag-Au, and Ag-MgO.
with hydrogen. Rates and activation energy are the same as obtained in the desorption experiment (Figure 1,upper solid line). All the other samples used in the experiments of Figure 3 were pretreated with both hydrogen and oxygen. “Ag(H,O)” was a 1.8-g sample produced by spark erosion for comparison with the similarly prepared silver-gold alloy. It was cleaned with oxygen up to 420’ and treated with 10 doses of hydrogen for 2 hr at 500’. On pumping, a slow desorption of hydrogen was first observed, then mainly of water vapor. After 3 hr, no measurable desorption was obtained at 400’. After this, the parahydrogen conversion at -195’ was measured’ and the sample was found to have a strongly paramagnetic surface. After pumping off the parahydrogen at low temperatures, the sample was pumped for 16 hr at room temperature. The homonuclear oxygen exchange was then measured as a function of temperature. The activation energy was roughly 17 kcal/mole. Since the rate at a given temperature strongly decreased with time, the sample was further treated with oxygen. After alternate oxygen contact and pumping for 24 hr at 500’, the run was made. The result of this run is presented in Figure 3, marked “Ag(H,O),” and in Table 11. The activation energy was still 17 kcal/ mole, as compared to 32.5 kcal/mole obtained with the samples which were pretreated with oxygen only. The behavior of silver containing impurity additions
Sample
New O2 mixture
Time, min
Temp, OC
----Mole 0%
70 of0%
016018
016/01a
0 1065
24
1
48.2 47.1
6.2 8.5
45.6 44.4
1.05 1.05
2 3
0 345
222
46.9 47.7
9.9 11.5
43.2 40.8
1.08 1.15
4 5
0 53
256
48.5 51.2
14.6 21.6
36.9 27.2
1.26 1.63
0 60 985
280
48.4 47.3 45.0
6.4 21.2 43.9
45.2 31.5 11.1
1.07 1.38 2.07
New 0% mixture
1 2
This was clearly seen in an experiment with a second silver-gold alloy that contained 80 at. % ’ silver. It was heated in oxygen and hydrogen to a higher temperature, 520’ (instead of about 480’ for the first alloy), and was found to be inactive up to 500’. A gold film visibly covered the inside of the reaction vessel and must have covered the alloy surface. The rate per silver surface atom for the silver-gold alloy in Figure 3 may be higher than for pure silver. The silver-magnesium oxide (7 mole %) sample was also pretreated with hydrogen and oxygen up to 500’ and pumped for 24 hr. It had a surface area of 1200 cm2/g. Again there was no effect of the hydrogen pretreatment. The curve is shown in Figure 3 and marked “Ag-MgO.” After pumping at 500°, a relatively high activity was found at low temperatures. The activity here is due to the presence of defect magnesium oxide. It is measurable down to -130’ and is due to the existence of a very weak Volume 70, Number I d
December 1966
3886
Y. L. SANDLER, S. Z. BEER,AND D. D. DURIGON
chemisorption on this and other defect oxides.8,g The activity is suppressed by contact with oxygen a t 300'. On lowering the temperature from 300 to 160°, the slope is the same as obtained with oxygentreated pure silver or the silver-gold alloy. The higher specific activity is significant. It is connected with the fact that the silver contained no firmly bound oxygen, which would reduce the activity as seen in the desorption experiments (Figure 2). The absence of firmly bound oxygen is proved by the experiment shown in Table 111. The table shows data of an exchange experiment made with an 0162 Ols2 mixture at various temperatures on the same material after prior contact with 0 l 6 2 at 500'. The ratio of 0l6/O1*can be seen to be constant up to the highest temperature, 300'. With pure silver, all experiments of this type showed an increase in the ratio. This is due to exchange with residual, firmly bound oxygen in and on the surface of the silver which cannot be removed by pumping at 500' before the start of an experiment (compare Table I1 and paper I). The impure silver evidently contains no firmly bound oxygen and therefore no firmly bound hydrogen. (The latter, as shown earlier, requires the presence of firmly bound oxygen.)
+
+ 0 %on Ag-MgO,
Table 111: Reaction of 11 Torr of 0% Pretreated with 0 %after Hz Sample
New 02 mixture
Time, min
Temp, O C
-----Mole 0'6:
7c of-0'60's
0'82
O'B/O's
1
0 1061
24 24
46.1 42.4
5.6 13.8
48.3 43.8
0.95 0.97
2 3
0 4320
100
100
41.4 32.4
15.5 32.0
43.1 35.6
0.97 0.94
4 5
n
200 200
32.0 29.3
33.2 41.9
34.8 28.8
0.95
90
New 0 2 mixture 1
0 16
300 300
45.1 24.9
6.6 49.0
48.3 26 1
0.94 0.97
1.01
Conclusions retained in pure Hydrogerl is just as is oxygen. No such strong retention of hydrogen would be expected in a pure s metal. Actually, the t,rue (endothermal) equilibrium solubility1° of hydrogen " - in silver at 500" and 10 torr is about 100 times lower than t,he residual amounts of hydrogen The Journal of Physical Chemistry
found in our experiments (Table I). The fact that the amount of residual hydrogen is of the same order as the residual oxygen shows that the very strong binding of the hydrogen is caused by the presence of the oxygen. The presence of hydrogen at the surface changes the binding energy of the more weakly chemisorbed oxygen, as seen from the change in the activation energy of desorption. The good reproducibility of the present results with hydrogen-free silver stands in sharp contrast to the often-noticed" lack of reproducibility of previous data. The latter were usually taken with catalysts that were pretreated with hydrogen and evacuated at relatively low temperatures. Presumably, the low value found by -21argolis12and co-workers for the activation energy of the homonuclear oxygen exchange (-12 kcal/mole) is due to the presence of hydrogen in the silver. The current results, in which the total residual oxygen and hydrogen were determined in the same experiment, are too few to say whether the value 2 found for the oxygen to hydrogen ratio is significant. The corresponding complex at the surface was found' to be paramagnetic and might consist of two chemisorbed oxygen atoms with a proton trapped between them. It was found that alloy formation with gold and incorporation of hIgO does not change the activation energy for the oxygen desorption (or the exchange reaction, the rate of which is the same). There is no apparent effect of a possible change in work function8 or change in the dimensions of the lattice by the presence of the additive. No strong oxygen chemisorption was found and, consequently, no residual hydrogen. The added impurities eliminate the sites for the stronger chemisorption and stabilize those planes which cause the weaker O2 adsorption; there is no direct effect on the binding energy of the oxygen. The presence of hydrogen may also cause topographic changes,'$ but here the bond strength of the oxygen is also affected as seen from t,he change in the . activation energy of desorption. It is difficult at the present stage to state definite correlations of the effects of oxygen pretreatments or (8) G . K. Boreskov, Advan. Catalysis, 15, 327 (1964). (9) Y. L. Sandler and D. D. Durigon, t o be published. (10) E. H. Steacie and F. M.G. Johnson, Proc. Roy. Soc. (London), A117, 662 (1928). (11) M. I. Temkin and N. V. Kulkova, Dokl. Akad. Nauk SSSR, 105, 1021 (1955). (12) L. Ya. Mrtrgolis, Iza. Akad. Nauk SSSR, Otd. Khim. Nauk, 225 (1959). (13) B. E . Sunquist, Acta Met., 12, 67 (1964) \-
I
3887
SOLVENT EFFECTS ON 13C-H COUPLING PARAMETERS
purity content on the oxygen reduction at a silver electrode in an alkaline electrolyte was recently found5 which is due to similar causes. Work on a more definite identification of the exposed crystal planes is now in progress.
impurity additions with topographic changes described in the literature. With face-centered-cubic structures the f 111 and f 1001 facets are stabilized by treatment with oxygen, and the rounded edges between the facets become sharper as the oxygen pressure increase^.'^-^^ The changes in shape with pressures are reversible, as are the observed desorption characteristics with oxygen pretreatment. Similarly, the presence of impurities is knownI3 to affect the morphology of the surface. A marked effect of thermal pretreatment and im-
1
Acknowledgment. The authors are indebted to W. &I.Hickam and his group for carrying out the isotopic dilution experiments. (14) R. Y.Meelheim, et al., Actes Congr. Intern. Catalyse, 9e, Paris, 1960,2, 2005 (1961).
Solvent Effects on W-H Coupling Parameters and Chemical Shifts of Some Halomethanes
by V. S. Watts and J. H. Goldstein Department of Chemistry, Emory University, Atlanta, Georgia 30322
(Received June 17, 1966)
Medium effects on the chemical shift and 13C-H coupling of bromoform have been determined in a series of 30 solvents representing a variety of functional groups. Similar observations have been carried out for 13 substituted methanes as the neat liquids and as solutions in cyclohexane, carbon tetrachloride, and dimethylformamide. The observed behavior can in general be correlated with the structure of the solvents and solutes studied. The results are adequately explained in terms of specific molecular interactions, in particular, hydrogen bonding. The advantages of using 13C-H couplings as a criterion for molecular interactions are pointed out.
Introduction The effect of solvent media on nmr coupling parameters has been the subject of considerable recent interest. Variability with solvent and/or concentration has been established for the cases of geminal H-H, 1--5 directly bonded l3C---H69’ geminal P-H (PCH),* and vicinal H-F couplings through C--C and C=C bonds.g At the present time it is not entirely clear whether these changes are primarily produced by specific interactions6 or whether they arise from more general effects such as the reaction fields induced by solutes in the dielectric solvent medium.lOsll
In an effort to clarify this problem further we have carried out solvent-effect studies of two types: (1) the effect of an extended series of both saturated and (1) V. S. Watts, G. S. Reddy, and J. H. Goldstein. J . Mol. Spectry., 11, 325 (1963). (2) B.L.Shapiro, R. M. Kopchik, and S. J. Ebersole. ibid., 11, 326 (1963). (3) B. L. Shapiro, R. M. Kopchik, and S. J. Ebersole, J. Chem. Phys., 39, 3154 (1963). (4) P.Bates, S. Cawley, and S. 5. Dsnyluk, ibid., 40, 2415 (1964). (5) V. S. Watts and J. H. Goldstein, ibid., 42, 228 (1965). (6) D.F. Evans, J. Chem. Soc., 5575 (1963).
Volume 70. Number 18 December 1966
