Nov. 20, 1955
KINETICSOF [CONTRIBUTION FROM
THE VANADIUM( II)-(III) THE
ISOTOPIC EXCHANGE REACTION
5921
DEPARTMENT O F CHEMISTRY, \vASHINCTON UNIVERSITY]
Kinetics of the Vanadium(I1)-Vanadium(II1) Isotopic Exchange Reaction1 BY KOTRAV. KRISHNANURTY AND ARTHUR C. W A I ~ L RECEIVED JULY 7, 1958 I n 1.00 f HClOl and ionic strength 2.00 the rate of exchange between VI1 and VI11 is consistent with the law rate = k. At 25” the value of k is 0.82 f-’ min.-l. The experimental activation energy is 13.2 kcal./mole. The rate was not affected measurably by a fivefold increase in the surface area of the reaction vessel, by the absence of ordinary diffuse light, by a n increase in the ionic strength to 4.00 or by replacement of KaC1o4 by LiCIOa for adjustment of ionic strength. The rate is increased by lowering the hydrogen ion concentration below 0.5 Jf and by tlie presence of chloride ion. The rate k?/(H+) kt(Cl-), the values of the constants at 2.5’ being kl = 0.61 j--1 data are consistent with the expression k = k l tnin.-’, k? = 0.21 min.-’ and kl = ~ 8 5 f nun.-’. - ~ (V1I)(WI).
+
+
Kinetic data for the vanadium(I1)-vanadium(111) isotopic exchange reaction are of interest for comparison with existing data for the chromium (II)-chromium (111), 2 - 4 iron (11)-iron (111),6,6 cobalt (11)-cobalt (111)’ and europium(11)-europium(I 11) reactions. King and Garnerg found complete exchange in their experiments a t 2’ involving reaction times of one to five minutes, total vanadium concentrations of -0.2 M , and two different separation methods. Since the iron(I1)-iron(II1) and cobalt(II)-cobalt(II1) exchange reactions would also have been complete under these conditions, the problem was reopened with the hope that rapid mixing and quenching procedures’O - I 2 would make rate measurements possible. However, once quenching procedures were found that did not induce complete exchange, it was learned that the rate was sufficiently small to be studied by more conventional techniques. Apparently. the separation procedures used by King and Garnerg induced complete exchange, as did several procedures that we tried. Experimental Radioactivity.-The tracer used was the 16-day V4* made by the Ti48(d,2n)V48reaction in the Washington University cyclotron. The decay of V4* is accompanied by 2.22, 1.320, 0.990 and 0.511 MeV. y-rays.13 which were conveniently detected with a scintillation counter. Ten milliliter samples of vanadium (117) solutions were counted in tubes selected to give the same counting efficiencies. A stilbene crystal m-as used for some runs and a well-type sodium iodide crystal for others. The radiochemical purification procedure, which was similar to one of Haymond, et a1.,I4 was designed t o minimize sulfate and chloride contamination. Purification after addition of carriers included a sodium carbonate fusion,16 which resulted in water soluble sodium vanadate, (1) This work N X supported by t h e National Science Foundation (Grant-3287). T h e paper was abstracted from the Ph.D. thesis of Kotra T‘. Krishnamurty, Washington University, 1958. (2) A. Anderson and N. A. Banner, THIS J O U R N A L , 76, 3826 (1954). (3) E. L. Ring and H . Taube, ibid,,76, 4053 (1954). (4) D. L. Ball and E. L. King, ibid., 8 0 , 1091 (1958). ( 5 ) J. Silverman and R. 1%’. Dodson, J . P h y s . C h e m . , 56, 846 (1952). ( C ) J. Hudis and A. C. Wahl, THISJ O U R N A L , 7 5 , 4158 (1953). ( 7 ) J. P. H u n t and X. A . Banner, i b i d . , 74, 1866 (1952). ( 8 ) D. J. Meier and C. S. Garner, J . P h y s . C h e m . , 56, 853 (1952). (9) W. K. King, Jr., and C. S. Garner, THIS JOURNAL,74, 3709 (1952). (10) A. C. Wahl and C. F. Deck, i h i d . , 76, 4054 (1954). (11) J. C. Sheppard and A. C. Wahl, ibid., 79, 1020 (1957). (12) B. M . Gordon and A. C. Wahl, ibid.. 80, 273 (1958). (13) P. L. Roggenkamp, C. H. Pruett and R. G. Wilkinson, P h y s . Rev., 8 8 , 1262 (1952). (14) H. R. Haymond, R. D . hfaxwell, W. &I. Garrison and J. G. Hamilton, J . C h e m . P h y s . , 18, 756 (1950). (15) Shohei Uno and Toshio Inokuma, J . Chein. Soc. J a p a n . I n d . Chem. Soc., 54, 758 (1951).
titanium and other neighboring elements remaining in the melt residue and two lead vanadate precipitations.l6 The last lead vanadate precipitate was dissolved in 1 f HClO,, the resulting solution treated with hydrogen sulfide, and the lead sulfide removed by centrifugation from the VO + - + tracer solution. Excess hydrogen sulfide was removed by boiling and by sweeping with carbon dioxide. Radiochemical purity was checked by following the decay with the stilbene-crystal scintillation counter. Measured half-lives for the nine different preparations agreed well with the reported value of 16.0 f 0.2 days.” Additional evidence for the radiochemical purity of the first few tracer preparations was the equality of the vanadium(I1) and vanadium(II1) “infinite-time” specific activities and the average specific activity. Chemicals.-Vanadium(1T:) stock solutions in HCIO, were prepared by electrolytic reduction18 of V205 (Fisher Scientific Company, C.P. grade) suspended in HC104. A modification of Banerjee’s cell19 was used, a platinum strip serving as cathode and a platinum wire in a porous cup as anode. Vanadium(I1) stock solutions in HClOl were made just prior to use by electrolytic reduction in a nitrogen atmosphere of vanadium(1V) stock solutions in the cell described above. Completion of reduction was indicated by the gradual disappearance of the blue color and by the characteristic purple color of \’+T. The reducing equivalence of one solution was determined and found to be 2.96. Tagged vanadium(II1) solutions in HClOJ were made by mixing equivalent amounts of the vanadium(I1) and vanadium(1V) solutions after addition of a small volume of V48 tracer solution (as V(IV)) to the latter. The reaction is complete20 and rapid, Reactants prepared by electrolytic or zinc-amalgam reduction2‘q22of vanadium(1V) solutions prepared by H2S reduction of V Z Osuspensions ~ in HClOa gave exchange rates that were larger than those obtained with reactants prepared entirely by electrolytic reduction. Mallinckrodt A.R. HClOa, G. F. Smith NaClOl and Eastman Kodak 2,2’-dipyridyl were used without further purification. All other chemicals were of reagent grade. Analytical Methods.-Vanadium was determined by titrationu of vanadium(1V) with MnO4- at 60 to 80’. The vanadium(I1‘) was prepared for titration by oxidation of lower states to vanadium(V) with (NHr)&Os, boiling to destroy excess oxidant, reduction by SOZ to vanadium(1V) and removal of excess SO2 by boiling and by sweeping with
coz.
The acidity of vanadium solutions was determined by shaking with 15 to 20 g. of the acid form of Dowex 50-X4 resin in a 125-ml. stoppered erlenmeyer flask flushed with nitrogen and the filtrate titrated with standard sodium (16) W. W. Scott. “Standard Methods of Chemical Analysis,” D. Van Nostrand Co., Inc., h7ew York, N. Y., 1939, p. 1034. (17) W. C. Peacock and M. Deutsch, P h y s . Rev., 69, 306 (1946). (18) S. C. Furman and C. S. Garner, T H I S J O U R N A L , 73, 4528 (1951). (19) P. C. Banerjee, J . I n d i a n Chem. SOL.,12, 198 (1435). (20) K for V O + + V++ 2H+ = Z V + + + H 2 0 is 1 X 1010; calculated from d a t a given by W. hf. Latimer. “Oxidation Potentials,” Prentice-Hall Inc., New York, N. Y . , 1953, p. 265. (21) H . W. Stone and D. K. Hume, I n d . Eng. C h e m . , Anal. E d . , 11,
+
+
+
598 (1939). (22) L. Meites, J. C h e m . Educ., 27, 458 (1950). ( 2 3 ) H . H. Willard and P. Young, I n d . Eng. Chein., A n a l . Ed., 6, 48 (1931).
hydroxide. T h e titer was corrected for the hydrogen-ion equivalents released by the resin (2/V(II), 3/V(III), 2/V(IV)). The purity of a vanadium(1V) stock solution was checked by testing aliquots for \'(III) by addition of KIOl and for y ( V ) by addition of KI. The absence of iodine color in CClr shaken with these solutions showcd the absence of V(II1) and V(V). Chloride ion could not be detected in the vnnadium solutions by addition of AgNOa and hence was less than lo-' AT. Quenching-Separation Method .-Six milliliters of reaction solution was added to the quench solution prepared just prior to use by mixing 2 ml. of 15 f XH4OH and 20 ml. of 2y0 dipyridyl solution in 1:1 ethanol-water mixture. Mechanical stirring started before addition of the reaction mixture was continued for one minute after addition, then the quenched mixture was allowed to stand for six minutes before centrifugation for three minutes. T h e supernatant containing V(dipy), + + was taken for specific-activity determination. Zero-time exchange was about 25Yo1,,but increased to about 50% for reaction mixtures of low acid concentration. Heterogeneous exchange between V(dipy) + - and V(OH)s during the stirring, standing and centrifugation was about 12% and varied little for standing times ranging from 3 to 15 minutes. Only ~ 2 5 of 7 ~the vanadiuni(I1) was recovered, and a t the lower hydrogen ion concentrations the recovery was even less. Other quenching procedures investigated were less satisfactory. Mixing the reaction and dipgridyl solutions and then addition of NH40H, a procedure used by King and G a r ~ i e r but , ~ under different conditions, gave -40y0 zerotime exchange and -305% recovery of vanadium(I1). Quenching with a n acetate buffered dipyridyl solution and then addition of NH40H, as was done in the iron(I1)iron(II1) exchange,5 increased the vanadium(I1) yield t o 430y0 and the zero-time exchange to -85y0. Attempts to precipitate V(dipy), + + with various anions from acid solution were unsuccessful. Extraction of vanadium(II1) from acid thiocyanate solution into ethyl acetate resulted in complete zero-time exchange. Procedure.-The vanadium(II1) reactant solution in the reaction vessel, a pear-shaped vessel6 of about 125-ml. capacity equipped with a n automatic pipet and a side tube through which nitrogen was passed, and the vanadium(I1) reactant solution in a glass stoppered tube mere brought t o the desired temperature in a constant temperature (f0.2') water-bath and then mixed. Necessary amounts of standardized HCIOn and/or NaC104 solutions were added to either or both reactant solutions to adjust the acidity and the ionic strength. hliquots mere removed with the automatic pipet and quenched. Two aliquots of the reaction mixture were taken for average specific-activity determinations. Reactant concentrations were known from the concentrations of the stock vanadiurn(I1) and vanadium(II7) solutions used. The supernatant from the quenched mixture was evaporated t o dryness, and the residue was fumed with a H2S04HC104-HN03 mixture to destroy organic material. After conversion to vanadium(IV), a sample was diluted to 10 nil., counted and titrated with KMnOg. The fraction exchange was obtained by division of the specific activity (counts/min. per ml. KMnOl solution) by the average specific activity. Data were plotted as illustrated in Fig. 1, and half-times lvere read from the plots. '
TABLE I RATEI~EPESDENCE ON \ ' ( I I ) AND \.(III) COSCENTRATI~INS (25.0", 1.000 .li I1 +,ionic strength atljusterl ti, 2.00 wit11 NaC104)
v(rr),
r i), 043
r'irr),
1,
oxti
. 04:j
9. 5 ii,I)
,129 .08ti ,129 . OX6
1) 4 3
4.8
. IM; . (Mi . 12-1
5. 1 :j. 9 3 , $1
,
12 ,
I,
I
mill
0 11.4:;
'
:111111
0 , 85
.7x
.s1 .79 . 83
83 Av. 0 . 8 2
The dependence of rate on temperature is shown in Fig. 2 . From the slope of the line an experimental activation energy of 13.2 =t0.3 kcal.,/mole was obtained. The value of the experimental entropy of activationz5is - 24.9 cal.,/deg./mole. Above 0.5 d l H + the rate was not measurably affected by variation of the hydrogen ion concentration. Below 0.5 ;If H + the rate increased with decreasing hydrogen ion concentration. The rate data for the lower hydrogen ion concentrations are somewhat less accurate than for the higher concentrations because of greater zero-time exchange and lower vanadium(I1) yields. The data can be fit reasonably well by a k us. l,/(H+) plot as shown in Fig. 3, suggesting a rate expression of the form Rate
= ( 1 7 * I ) ( T T I 1 1 ) {,ki
+ ke/(H')}
(:3)
The values of kl and k2 are 0.61 f-' rnin.-' and 0.21 min.-l, respectively. The data in Table I1 show that the exchange rate is not measurably affected by increasing the surface of the reaction vessel fivefold (by the addition of glass beads), by the absence of ordinary diffuse light, by increasing the ionic strength to -1.00 or by replacement of Xaf ion by Li+. TARLE I1 ~IISCELIASEOLS RaTE D A T A (P5.0", 0.086f V ( I I ) , 0.086 f i 7 ( I I I ) , 1.000 d 1 H-, ionic strength adjusted t o 2.00 with SaC104, except as noted) tl!?,
fz , .f-1 min.
Condition>
min.
Surfacc iticrcasetl fi\-c.fiiItl \\.it11glais 11e;ids Total darkness Ionic strength -1.00 0.198 -11H + 0.198 3 1 H-, ionic strength t o 2.00 with I.iCiO,
5.0 4.9 ,5,2 2.1
0.81 .82
1.9
2.12
-1
.78 1.92
AS shown in Table I the values of k do not vary with the reactant concentrations, indicating the validity of equation 1.
The exchange rate was found to be larger in the presence of chloride ion. The rate data are not very accurate because of low vanadium(I1) recoveries, and their interpretation is not straight-forward because the exchange curves exhibited some curvature, especially a t the higher chloride-ion concentrations. (At the lower hydrogen ion concentrations there also was a suggestion of curvature in some of the exchange curves.) The curvature suggests that exchange occurred a t more than one rate, as would happen if there were exchange between more than two species not in rapid equilibrium. The exchange data were not sufficiently accurate for
(21) S e e , f o r example, 0. E . hleyeri and R. J. I'restwood, "IZadiy, J . Phys. Ch?m., 5 6 , 8(GX (19.i2). ?) R . J. Zivolinsky, R . J. N a r c u s and IT. Eyring, C h e f n . R E B S .66, , 137 (1'333). (33) H. Taube, €i, LIek-er ani1 R. I.. Rich, Tms JOL-RNXI., 76, 41 18 (ICli3). (RL) I3. Keisch antl Kotra \'. Kri.;hnnmiirtj-, reported j n reference 1.
SAINT T,OLW,
[CONTRII3UTIOX F R O M THE T)EPARTMENT O F CfIEMISTKT,
MISSOL-RI
THEJorisi
I T O P K I N S USITERSITT.]
The Vibrational Spectra of Trisilylamine and Trisilylamine-d, BY DEAN1%'. ROBIKSON RECEIVED JUNE 16, 1955 The infrared spectra between 4008 arid 60 cni.-' of tridylamine antl trisilylamine-d9 are reported as well as some Raman data. The observed spectra are interpreted on the basis of a CShpoint group with three of the four skeletal frequencies observed a t 996, 490 and ca. 230 c m - ' in trisilylamiue arid 963, 439 and cn. 250 cm.-' in trisilylamine-&. Silicon-hydrogen motions group together a t three silyl-group frequencies in tlie regions 2167, 941 and 748 c m - ' in the light compound and 1575, 608 and 587 cni.-' in hea'i'y trisilylaniine.
Structural studies of silyl Lewis bases indicate large bond angles, shorter bond lengths and higher force constants than are expected from comparison with methyl compounds. Aside from the wellknown short bond distances in silyl halides, there is evidence' for a much greater bond angle in disiloxane than in dimethyl ether, although its exact (1) R. C . Lord, D. W. Robinson and W. C. Schumh, THISJOURNAL, 78, 1327 (1956). See also R . F. Curl and K. S. Pitzer, ibid., 80, 2371 (19383,who studied H3SiOSiHs in matrices a t 20OK. Their interpretation of their d a t a is questionable b u t yields an SiOSi angle of 15015So in the solid phase, a n unexpectedly wide angle in agreement with the previous investigators.
angle has not yet been reported. Trisilylaniine on the other hand has been the subject of a careful electron diffraction investigation by Hedberg2 and has been shown to have Si-N-Si bond angles of ll9.G f. 1.O0, a nearly planar skeletal structure. The present paper is a description of some vibrational spectroscopic work that has given results in agreement with the electron diffraction study, that is, that trisilylamine is indistinguishable from a 111olecule. ('2) K , €?edhcrg, tbtd
, 77, 6401 (1053)
