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Journal of the American Chemical Society
(22)Reference 14 describes a modification of HMTSeF in which sulfur replaces the @-methylene(i.e., P-DTTSeF). (23)Other syntheses of HMTTF: (a) E. Fanghanel, L. van Hink, and G. Schukat, 2.Chem., 16, 317,360(1976):(b)H. K. Spencer, M. P. Cava, F. G. Yamagishi, and A. F. Garito, J. Org. Chem., 41, 730 (1976):P. Delhaes, S. Flandrois, J. Amiell, G. Keryer, E. Toreilles, J. M. Fabre, L. Giral, C. S.Jacobsen, and K. Bechgaard, J. Phys., 233 (1977). (24)For earlier references on this cyclization see E. Campaigne, R. D. Hamilton. and N. W. Jacobsen, J. Org. Chem., 29, 1708 (1964);I. D. Rae, ht. J. Sulfur Chem., 1,59(1971): A. K. Bhattacharyaand A. G. Hortrnann, J. Org. Chem., 39, 95 (1974),and references cited therein: R . R. Schumaker and E. M. Engler. J. Am. Chem. SOC., 99,5521 (1977). (25)(a) F. Wudi, G. M. Smith, and E. J. Hufnagel, Chem. Commun., 1435 (1970): (b) D. L. Coffen, J. 0. Chambers, D. R. Williams, P.E. Garrett, and N. D. Canfield, J. Am. Chem. SOC., 93,2258 (1971):(c) S.Hunig, G. Kiesslich, H. Quast, and D. Scheutzow, Justus Liebigs Ann. Chem., 310 (1973). (26)E. Kiingsberg, J. Am. Chem. Soc., 86, 5290 (1964). (27)For the naming of the heterofulvalenes presented in this paper, the 1,4,5,8-tetraheterofulvalenesystem is chosen as the parent ring. (28)A convenient, high-yield synthesis of 1,2,3-selenadiazoleshas been discovered by I. Lalezari, A. Shafiee, and M. Yalpani. Tetrahedron Left., 5105 (1969);I. Lalezari, A. Shafiee. and M. Yalpani, J. Org. Chem., 38, 2836
(1971). (29)Phosphorus base coupling is currently the only effective means of coupling the selenium analogues of TTF. See ref 17a for discussion. See also ref 11. (30)E. M. Engler, R. R . Schumaker, and V. V Patel, J. Chem. Soc., Chem. Commun., 835 (1977). (31)E. M. Engler and V. V. Patel, Tetrahedron Lett.. 423 (1976). (32) 'H NMR was able to show the presence of cis and trans isomers in DSeDTF (ref 5)and in DMDSeDTF (ref 10). (33)The crystal structure of P-MeHMTSeF has recently been determined by
S.J. LaPlaca, IBM Research (unpublished work). It confirms the presence of the methyl groups in the @ positions. In the samples investigated, crystallization apparently has lead to preferential enrichment in the syn isomer.
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(34)For earlier references on this cyclization see (a) A. Takamizawa and K. Hirai, Chem. Pharm. Bull., 17, 1924,1931 (1969):(b) ref. 8. (35)For an analogous cyclization involving replacement of oxygen by sulfur see P. Rioult and J. Vialle, Bull. Soc. Chim. Fr., 4478 (1968). (36)For earlier references on this cyclization see E. Campaigne and N. W. Jacobsen, J. Org. Chem., 29, 1703 (1964);E.Fanghanel, Z.Chem., 1,58 (1967). (37)A. Roxenbaum, H. Kirchberg, and E. Leibnitz, J. Prakt. Chem., 19, 1 (1963). (38)T. Amano. H. Kuroda, and H. Akarnatu, BUN.Chem. SOC.Jpn., 41,83(1968); J. B. Torrance, 8. A. Scott, and F. B. Kaufman, Solidsfate Commun., 17,
1369 (1975). (39)R. Foster "Organic Charge-Transfer Complexes", Academic Press, New York. N.Y., 1969. (40)H. K. Spencer, M. P. Cava, and A. F. Garito, J. Chem. Soc.,Chem. Commun., 966 (1976). (41)F. G. Yamagishi, D. J. Sandman, A. F. Garito, and A. J. Heeger, Abstracts, 167th National Meeting of the American Chemical Society, Los Angeles, Calif., March 1973,No. ORGN-12. (42)R. P. Shibaeva and 0. V. Yarochkina, Sov. Phys. Dokl., 20,304 (1975). (43)J. P. Ferraris and F. I. Mopsick, Chem. Eng. News (Sept 16,1974);see also K. G. R Sundelin, ibid. (Aug 5 , 1974). (44)For the preparation of less dangerous hexafluorophosphate salts see L. R. Melby. H.D. Hartzler, and W. A. Sheppard. J. Org. Chem., 39,2456
(1974). (45)L. Henriksen and E. S.S.Kristiansen, Inst. J. SuifurChem., Part A, 2, 133 (1972). (46)C. C. Price and J. M. Jusge. Org. Synth., 45, 22 (1965). (47)For an overview of some of these techniques see J. R. Andersen, E. M. Engler, and K. Bechgaard in the Proceedings of the New York Academy of Sciences "Conference on Synthesis and Properties of Low Dimensional Materials", New York, N.Y., June 13-16, 1977,to be published. (48)For information on this technique see L. B. Coleman. Rev. Sci. Instrum.,
48, 1125 (1975). (49) For a descriptionof this method as applied to the analysis of organic charge transfer complexes see ref 17a.
Oxygen- 17 Magnetic Resonance Study of Oxygen Exchange between Arsenite Ion and Solvent Water William C. Copenhafer and Philip H. Rieger* Contribution from the Department of Chemistry, Brown University, Providence, Rhode Island 0291 2. Received June 30, 1977 Abstract: Oxygen-17 magnetic resonance spectra of aqueous solutions of sodium arsenite were measured for 0.6 < [As(III)] < 4.1 m , 10 < t < 90 "C. Line broadening of the water resonance in the 40-90 O C range gives the rate of oxygen exchange between AsO(0H)r- and solvent water. Exchange takes place via a first-order pathway, AS'* = - I20 f 3 J K-l mol-', AH1* = 25 f 1 kJ mol-', and a pathway second-order in arsenite, AS** = -102 f J K-I mol-', AH2* = 33 f 4 kJ mol-I. Rates were measured using solutions of pH -10.2; exchange appears to be only slightly faster in solutions of lower pH.
Introduction Despite the use of arsenic(II1) oxide, As406, by generations of analysts as a redox standard, remarkably little is known of the aqueous chemistry of As(II1). Indeed, less than a decade has passed since the structures of arsenous acid and arsenite ion in aqueous media were finally established by Raman spectroscopy: trigonal pyramidal As(OH)3 and AsO(OH)2-. An attempt was made in 1940 to measure the rate of oxygen exchange between arsenite ion and solvent water, but the exchange proved too fast for the isotopic exchange technique used.3 Apparently because nucleophilic displacements on arsenite are very rapid, only two kinetic studies of As(II1) substitution reactions have come to our attention and both involve As(II1) as a catalyst: AsO(OH)2- catalyzes the hydration of carbon d i ~ x i d e and , ~ As(OH)3 (and perhaps AsO(OH)z-) catalyzes the exchange of oxygen between dihydrogen arsenate, As02(0H)2-, and water.5 As the first of a series of studies of equilibria and kinetics of reactions involving As(III), we report here the measurement ol' the ratz of oxygen exchange s2
0002-7863/78/1500-3776%01 .OO/O
between arsenite ion and solvent water by onance line broadening.
1'0
magnetic res-
Experimental Section Reagent grade sodium arsenite (Baker and Adamson) was used without purification for preparation of As(II1) solutions. Analysisof this material showed that 23.6 mol % of the As(II1) was present as A s ~ Othe ~ ,remainder as NaAsO2. Solutions for 170N M R spectra were prepared on a vacuum line. A weighed amount of dry sodium arsenite was placed in one section of a Y-shaped sample tube. An aliquot of sodium hydroxide or p toluenesulfonic acid solution was pipetted into the other section of the sample tube and the water pumped off. About 2 g of I70-enriched water (ca. 5% enrichment) was then distilled under vacuum into the sample tube. The I7O water reservoir was weighed before and after the distillation so that the molal concentration of As(II1) could be computed. The enriched water was recovered after running the spectra and used for subsequent samples. I7ON M R spectra were recorded at 7.5 MHz (1 3 kG) on a Varian wide-line N M R spectrometer equipped with a 12-in. magnet and flux stabilizer. To ensure rf stability, the Varian V-4210A variable frequency unit was locked to an external trequency synthesizer (Syntest
0 1978 American Chemical Society
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Copenhafer. Rieger
Oxygen Exchange between Arsenite Ion and Solvent Water
Table I. Concentrations of
Samoles
Sample no.
CAS,m
PLYa
1
0.608 0.610 1.11 2.03 3.08 3.08 4.10
0.03 18 0.03 19 0.0566 0.0989 0.1427 0.1427 0.1814
2' 3 4 5 6d 7
3777 t ('C)
looof
Xacid
'P
30
50
TO
90
R
0.08 0.61 0.24 0.10 0.10 0.30 0.09
Fraction of oxygen at arsenite sites. Fraction of As(II1) in form of As(OH)3. 0.23 m in p-toluenesulfonic acid. 0.20 m in p-toluenesulfonic acid. Corp.). The sideband method of detection was employed under conditions of high frequency, low amplitude field modulation using a modulation frequency of 1000 Hz. The center band of the first a c component was rejected by detecting in phase with the modulation frequency. The two side bands served as an internal calibration for each spectrum. Sample temperature was controlled with a Varian variable temperature accessory employing heated or cooled nitrogen and was measured by placing a copper-constantan thermocouple between the sample tube and the Dewar walls. Viscosities and densities of arsenite solutions in normal water were measured as functions of temperature using an Ostwald viscometer and a pycnometer.
Results I7O NMR spectra were measured at temperatures ranging from 10 to 90 OC for seven different samples having A s ( I I 1 ) concentrations up to 4.10 m. Sample concentrations are given in Table I. Signal-to-noise ratios ranged from about 8: 1 to more than 1OO:l depending on the line width. However, only the resonance due to solvent water was observed; the arsenite I7O resonance was probably too broad to be detectable. All spectra were recorded in quadruplicate; line widths were measured for each recording and averaged. Average observed line widths are given in Table I1 and are plotted vs. temperature in Figure 1. The viscosities of solutions comparable to samples 3, 4, 5, and 7 were measured and fitted to the empirical equation In ( q / T ) = a
+ @ / T+ y / T 2
(1)
/
280
300
520
S O
?BO
T(K)
Figure 1. Measured ''0NMR line widths as a function of temperature. Closed circles, samples I , 3,4, 5, and 7; open circles, samples 2 and 6.
by the method of least squares. The parameters cy, P, and y are given in Table 111. Equation 1 and the parameters of Table 111 fit the measured viscosities to well within 1%, the approximate experimental uncertainty. Assuming that oxygen exchange occurs between arsenite and solvent water, the width of the water peak is given by6
(2) where T2,o-I is the line width in the absence of exchange, p a is the fraction of oxygen nuclei a t arsenite sites, 7, is the life-
Table 11. " 0 N M R Line Widths"
T,K
a
T2-l
T, K
T2-I
T,K
T2-'
Sample no. 1 28 1.9 278 f 9 293.4 203 f 5 303.2 I54 f 5 313.4 122 f 3 324.5 98 f 5 335.1 92 f 2 343.6 87 f 2 355.0 87 f 1 365.7 94 f 2
Sample no. 2 282.5 257 f 22 290.7 204 f 14 303.2 I50 f 4 3 13.2 I23 f 4 323.3 105 f 5 333.3 92 f 2 343.4 89f 1 351.5 87 f 2 358.2 89 f 2 363.1 93 f 2
Sample no. 3 285.0 265 f 12 293.0 220 f 5 303.2 168 f 2 313.7 I39 f 3 323.1 I26 f 5 333.5 121 f 2 343.4 I23 f 2 352.8 I37 f 3 365.3 155 f 8
Sample no. 4 283.0 385 f 12 293.1 275 f I O 302.6 226 f 8 313.5 195 f 6 323.3 185 f 6 333.4 192 f 2 344.1 214 f 7 354.3 252 f 12 365.5 315 f 11
Sample no. 5 281.2 582 f 17 293.1 402 f 17 302.9 305 f 10 313.4 262 f 12 322.6 250 f 15 333.5 276 f 4 343.9 338 f 12 354.3 411 f 5 365.7 519 f 15
Sample no. 6 282.2 642 f 30 291.1 448 f 23 297.2 367 f 21 306.7 339 f 7 313.4 292 f I O 322.6 300 f 12 333.3 341 f 12 343.2 389 f 26 352.8 478 f 18 568 f 17 365.1
In units of rad s-'.
T ,K T2-1 Sample no. 7 281.7 900 f 39 292.3 650 f 18 303.1 456 f 22 313.7 378 f 5 323.9 363 f 4 332.4 410 f 11 343.5 477 f 11 354.3 633 f 5 365.7 788 f 21
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Journal of the American Chemical Society
Table 111. Empirical Viscosity Parametersa
a
10-3p
10-6y
1.02 2.03 3.05 4.01
- 13.978 - 13.967 - 12.806 -11.774
- 1.459
0.574 0.598 0.747 0.892
a
For 7 in units of kg
100:12
/
June 7 , 1978
4000
C A ~m,
-1.433 -2.164 -2.830
/
T/
s-' and T in K, see eq 1.
m-l
Table IV. Least-Sauares Line Width Parametersa 10-5~,
SamDle no.
K-l
AS*, J K-' mol-'
296 f 3 273 f 3 425 f 5 377 f 2 353 f 6 379 f 14 352 f 8
1 2 3 4 5 6 7 a
m ke-]
-104 f 12 -134 f 6 -106 f 6 -103 f 1 -95 f 4 -104 f 7 -93 f 5
AH+, kJ mol-' 30 f 4 19f2 28 f 2 28 f 1 31 f 1 27 f 3 31 f 2
See eq 4. 0 0
time of an oxygen nucleus a t an arsenite site, T2, is the transverse relaxation time of an oxygen nucleus a t an arsenite site, and Am, is the chemical shift, relative to water, of oxygen a t a n arsenite site. In the slow exchange limit, where T,-' is small compared with either Tz,-l or IAw,I, eq 2 reduces to T2-l =
T2,O-I
+
POT,-'
(3)
Values of T,-', obtained assuming the validity of eq 3, range up to about 4 X lo3 s-I. T2,-l is certainly greater than T2,0-', but it seems unlikely to be even as large as T , - ~ , a t least a t the higher temperatures. Thus the validity of eq 3 relies on the assumption that I Aw,l >> T,-'. Oxy anion I7Ochemical shifts are generally downfield from water by 200 ppm or more;7 thus a t 7.5 MHz, lAw,l is probably greater than lo4 rad S-I. However, if the arsenite chemical shift is as small as -200 ppm, T,-~ could be underestimated by as much as 25%. In the concentration range studied, 0.6-4.1 m , the solution viscosity is strongly dependent on arsenite concentration, so that T2,o-l cannot be measured independently on arsenite-free solutions. Thus, as a first step in the data analysis, it was assumed that T2,o-I is proportional to v/T and that T,-I is given by an absolute rate theory expression. The measured line widths were then fitted to the equation Tl-l = A(v/T) p , ( k ~ T / h ) exp(AS*/R) exp(-AH*/RT)
+
(4)
by a nonlinear least-squares procedure. The resulting leastsquares parameters ( A , AS*,and AH*) are given in Table IV. The variation in the values of A probably reflects (1) contributions to the viscosities from acid or base added to the NMR samples and (2) possible errors in extrapolation of viscosities for samples 1 and 2, which lie somewhat outside the range of the viscosity measurements. The temperature dependence of v / T should be nearly correct, however, and the low-temperature fit is well within experimental error in all cases. The parameter A is determined primarily by the low-temperature points and is relatively independent of the kinetic parameters. Although the fit to the high-temperature points is good as well, the activation parameters neither are constant nor do they exhibit any sensible trend. Assuming that A ( v / T ) is a good approximation to Tz,o-l, T,-' can be computed from eq 3. These values of T,-', which may be regarded as pseudo-first-order rate constants, are plotted against the concentration of As(II1) for samples 1 , 3 ,
I
2
3
4
CAs(m)
Figure 2. Observed rate constants, re-', as a function of total As(II1)
concentration at six temperatures. Closed circles and straight lines, samples 1, 3, 4, 5 , and 7; open circles, samples 2 and 6. 4,5, and 7 in Figure 2 for data a t 40 O C and above (60 OC and above for sample 1; the kinetic contribution to the line widths is negligible a t lower temperatures). As is seen, there is an excellent linear correlation between T,-I and arsenic concentration, suggesting that both first-order and second-order exchange pathways are important. Indeed, absolute rate theory plots (Figure 3) of the intercepts ( k l with units of S-I) and the slopes ( k 2 with units of m-I s-l) of Figure 2 give satisfactory straight lines and activation parameters: ASl* = -120 f 3 J K-I mol-I, AH2* = 25 f 1 kJ mol-' for the first-order process; AS** = -102 f 10 J K-I mol-', AH2* = 3 3 f 4 kJ mol-' for the second-order process. These parameters can, of course, be used to compute T,-I and, together with the values of A given in Table IV, are responsible for the solid curves of Figure 1. It should be noted that if systematic error due to the approximate nature of eq 3 is significant, then values of T,-I for the high-concentration, high-temperature points are underestimated; thus, if anything, the second-order pathway is even more important, the activation enthalpy may be larger than 33 kJ mol-', and the activation entropy may be less negative than -102 J K-' mol-'. Values of T,-' for samples 2 and 6, which were slightly acidified with p-toluenesulfonic acid, and thus had significant concentrations of As(OH)3, are also plotted in Figure 2 but were not included in the least-squares fit resulting in the straight lines of the figure. It is apparent that oxygen exchange is somewhat faster in the more acidic solutions, and the rather limited data available suggest that exchange with As(OH), is faster than with AsO(OH)2- by a factor of 3 or 4.However, because of the low solubility of As406, the experiments could not be extended to lower pH, and the data are not sufficiently accurate to establish a rate law which distinguishes between As(OH)3 and AsO(OH)2-. The results presented here thus refer, within reasonable error limits, to exchange between water and AsO(OH)2-.
Discussion The observed first-order oxygen exchange process presumably corresponds to a nucleophilic displacement by water on arsenite. Entropies of activation for bimolecular nucleophilic
/
Copenhafer, Rieger
3779
Oxygen Exchange between Arsenite Ion and Solvent Water
Table V. Rate Constants for Oxygen Exchange at 30 "C
Reactants
+
Ionic strength
k
~ s 0 ~ 3 -H ~ O A s O ~ ( O H ) ~ - H2O As02(0H)2H2O AsO(OH)2- + H 2 0 AsO3(OH)'AsO~(OH)'As03(0H)'A~02(0H)2A~02(0H)2- A~02(0H)2As02(0H)2As(OH)3 As02(OH)2AsO(OH)2A ~ 0 3 ( 0 H ) ~ - As(OH)3 AsO(OH)2- AsO(OH)2-
+ +
+
+ + + + + +
1.5 x 10-6s-1 1.2 x 10-5 s - l 1.0 x 10-4 s-1 167 s-l 8.5 X M-l 6.4 x 10-3 M-1 7.4 X M-l 6.8 M-' s-I
