J . Phys. Chem. 1986,90, 5333-5337
so that other miscellaneous factors could also play significant roles. When an atom is removed from a closest hexagonal configuration for instance, ELincreases substantially, Le., ca. 8.8%. The variation in the LRDF, on the other hand, is restricted to a change in the height of the first peak of the RDF and is almost negligible. Conversely, if the same operation is exercised on model D which is already fairly disordered, the change in E L is more or less the same as that for model A, whereas the change in based on the LRDF is quite different and can even be negative. - - This difference qualitatively explains the lower value of dE,/ddZ at smaller extent of disorderliness and higher value in the highly disordered region. This is inconsistent with the explanation of the difference between Figures 1B and 6A mentioned in Figure 5.1. A more significant difference between the topochemical distribution for model E (Figures 1E and 6B) should also be attributed to the conceptual differences given above. A detailed explanation, however, is still
5333
open to furture elucidation of the problem.
6. Concluding Remarks The local fluctuation of the potential energy or atomic arrangement is represented by the LPE or LRDF shown in the present study. Apart from the restriction of the two dimensionality, the main difficulty in verifying assumptions incorporated in the present study is the lack of experimental data with which the calculated values can be compared. The experimentally obtained data hitherto could be compared only with the average values of those calculated here. A refined model experimental study with respect to the structural and energetical distribution by using a well-defined solid, e.g., a single crystal of pure substance, is strongly desired. Acknowledgment. The authors express their appreciation to Professor H. Kuno for valuable discussions.
Heats of Solvation of Semiquinone-Sodium Ion Pair in Hexamethylphosphoramide Antonio E. Alegda,* Ada N. Medina,t Raymond Tirado,? and Francisco Sotot Department of Chemistry, University of Puerto Rico, Humacao Campus, Humacao, Puerto Rico 00441 (Received: September 19, 1985; In Final Form: May 13, 1986)
The heats of solvation of methyl-1,4-benzosemiquinone(TQ'-), 1,4-benzosemiquinone(BQ'-), 1,4-naphthosemiquinone(NQ'-), 9,lO-phenanthrosemiquinone(PQO-), and 9,lO-anthrasemiquinone (AQ'-) plus that of Naf in hexamethylphosphoramide (HMPA) have been determined by utilizing solution calorimetry and ESR spectroscopy techniques. The thermodynamic parameters controlling the electron-transfer process from BQ- to TQ, NQ, PQ, and AQ have also been determined. Solvation is more exothermic in the order TQ'- C BQ'- < NQ'- < PQ'- < AQ'-. These differences in heats of solvation are not attributed to differences in the net charge solvation but to differences in solvation of the molecular framework of the semiquinones. The thermodynamic stability of the semiquinones (4'3 studied, relative to BQ'-, is governed by differencesin solvation entropies between Q'- and BQ'-.
Semiquinones (Q'-) are postulated as important intermediates in electron transfer occurring in photosynthesis,' artificial photochemical solar energy conversion,2 and antitumor3 and radical scavenging p r o c e ~ s e s . ~Determining the external and intramolecular factors affecting the thermodynamic stability of semiquinones is mandatory in order to understand the role of these intermediates in these electron-transfer processes. Since the classical work of Brauman and Blair: there has been an increasing interest in determining the role of the solvent in controlling the thermodynamic stability of solvated species. It was proved in that work that the role of the solvent can be as important or more important than intramolecular factors. One way of probing the nature of solvation processes is through the measurement of the heat evolved in these processes. This type of measurement has been extensively used by Stevenson and co-workers in studying radical anions ~ o l v a t i o n . ~These ~ ~ studies has been mostly performed for polyacene radical anions solvated in ethereal solvents.6 Radical anions are extensively ion paired in ethereal solvents8 Thus, differences in heats 9f solvation have been interpreted in terms of differences in the solvation of the corresponding ion pairs.6 pSemiquinones are observed to be essentially free of ion pairing in hexamethylphosphoramide.8 This is specially true if sodium is used as the c o ~ n t e r i o n . ~Therefore, the heat of solvation of Q',Na+ can be considered as a summation of the heat of solvation of Q'- plus the heat of solvation of Na', eq 1. Consequently,
differences in the heats of solvation of Q'-,Na+ in HMPA should
'NIH-MBRS
student participants.
be ascribed to differences in the strength of solvation of the corresponding semiquinones. It is the purpose of this work to present the heat of solvation of several semiquinones in hexamethylphosphoramide (HMPA), with sodium as the counterion, and its dependence on the semiquinone structure. The thermodynamic parameters controlling the electron transfer from BQ'- to Q, eq 2, have also been determined.
Experimental Section The compounds 1,4-benzoquinone (BQ), methyl- 1,4-benzoquinone (TQ), 1,4-naphthoquinone (NQ), 9,lO-phenanthroquinone (PQ), 9,lO-anthraquinone (AQ), and HMPA were purchased from Aldrich. All the quinones were sublimed prior to use with the ( 1 ) Clayton, R. K. Photosynthesis: Physical Mechanisms and Chemical Patterns; Cambridge University Press: London, 1980. (2) Fendler, J. H. J . Phys. Chem. 1985, 89, 2730. (3) Gutierrez, P. L.; Egorin, M. J.; Fox, B. M. Biochemical Pharmacology 1985, 34, 1449. (4) Almgren, M.; Frieser, F.; Thomas, J. K. J . Phys. Chem. 1979,83, 3232. (5) Brauman, J. I.; Blair, L. K. J . Am. Chem. SOC.1970, 92, 5986. (6) Stevenson, G. R.; Shock, L. E.; Reiter, R. C. J . Phys. Chem. 1983,87,
4004. (7) Stevenson, G. R.; Hashim, R. T. J . Am. Chem. SOC.1985,107,5794
and references therein. (8) Szwarc, M. Ions and Ion Pairs in Organic Reactions; Wiley: New York, 1974. (9) (a) Martir, W.; Alegria, A. E.; Stevenson, G. R. J . Am. Chem. SOC. 1976, 98, 7955. (b) Stevenson, G. R.; Ocasio, I. J . Am. Chem. SOC.1976, 98, 890. (10) Stevenson, G. R.; Williams, E., Jr.; Caldwell, G. J . Am. Chem. SOC. 1979, 101, 520. ( 1 1) Flame Emission and Atomic Absorption Spectroscopy; Dean, J. A., Rains, T. C., Eds.; Marcel Dekker: New York, 1975; Vol 2.
0022-3654/86/2090-5333$01.50/00 1986 American Chemical Society
5334 The Journal of Physical Chemistry, Vol. 90, No. 21, 1986
Alegria et al.
THIN-WALLED T386
J
exception of NQ, which was recrystallized from low boiling petroleum ether. H M P A was distilled from calcium hydride and stored over molecular sieves (4A). It was then distilled from potassium metal into the evacuated apparatus for anion radical preparation. The procedure, including the apparatus, utilized in the generation of thin-walled bulbs of BQ'-,Na+ solutions is similar to that described by Stevenson et al.,'* with the following modifications, Figure 1. A weighed amount of BQ was placed into tube A. A deficient amount of sodium metal was also placed into tube B. A sodium mirror was generated in bulb E. HMPA was distilled from potassium metal under vacuum from bulb C to the calibrated bulb D. Bulb C was sealed off, and the solvent was transferred into bulb E. Tube D was sealed off, and the HMPA solution of the quinone was stirred over the Na mirror. The presence of semiquinone was detected by means of an ESR sample tube. After several hours, when a strong ESR signal of BQ'- was detected, the solution was passed through glass frits into three thin-walled glass bulbs (F). The thin-walled glass bulbs were sealed from the apparatus and submitted to solution calorimetry where the heat of reaction 3 was determined.' The modified Parr
-
BQ*-(HMPA) + N ~ + ( H M P+A ')/ Z I Z ( ~ , , ~ ~ ) BQ(HMPA) + N~I(HMPA (3)) solution calorimeter utilized in the determination of the heat of reaction 3 is analogous to that reported in the literature.IO The same amount of I2 (3.100 g) in 100.00 mL of HMPA was utilized for all the determinations. A reaction product sample was transferred into a 3-mm glass tube and submitted to ESR analysis. No ESR signal was observed. Proton N M R analysis was performed on the reaction product solution (with 1% Me,Si as internal standard), indicating the presence of BQ and HMPA only. The amount of anion radical in each bulb was determined by flame photometry of Na, a t 489 nm, utilizing NaCl standards. LiCl(621 nm) was used as an internal standard in order to correct for differences in matrix interferences upon changing from standard to sample so1utions.l' The thin-walled glass bulbs contained BQ'-,Na+ in HMPA and nonreduced BQ. It was found that the heat of solution of solid BQ in HMPA (6.7 f 0.8 kJ/mol) is slightly different from that corresponding to dissolved solid BQ in the I,-HMPA solution (14 (12) Stevenson, G . R.; Williams, W., Jr. J . Am. Chem. SOC.1979, 101, 5910.
f 3 kJ/mol). Thus, the heat of reaction 3 was corrected for the heat of transfer of the nonreduced BQ from HMPA into the I,-HMPA solution (see below). The heat of dilution of the I,-HMPA solution, utilizing similar amounts of HMPA to those used in the semiquinone solutions, was found to be negligible within the experimental error invoved in these determinations. In order to correct for the heat of transfer of BQ, the total amount of nonreacted BQ was determined in each thin-walled glass bulb as follows. The amount of BQ'- was determined as indicated above. The total amount of solution in each bulb was determined in the same manner as described by Stevenson and co-workers,I2Le., by weighing the solution containing bulbs before breaking them and weighing the broken pieces of glass after the bulb is broken. The ratio of the summation of the Q'- and BQ masses in the thin-walled bulbs to the total initial BQ mass should be the same as the ratio of the summation of the HMPA masses in the thin-walled bulbs to the total initial HMPA mass, eq 4,
(MI + * J / M = MHMPA,~/MHMPA
(4)
where MI and a,are the masses of BQ and BQ'-, respectively, in each bulb i , MHMPA,r is the mass of HMPA in each bulb i, and M and MHMPA are the masses of total initial BQ and HMPA, respectively. The ratio of the semiquinone mass in each bulb to the semiquinone mass in an arbitrary bulb number 1 should be the same as the ratio of the corresponding BQ masses, eq 5. When eq 4 and 5 and the expression for the mass in bulb 1 are utilized, M I and consequently the other masses were obtained. (*l/*l) = (Ml/Ml) (5) From the obtained M I value and the heat of transfer of BQ from HMPA to I,-HMPA, the corresponding AT for this transfer was calculated and subtracted from the observed change in temperature corresponding to reaction 3. Thus, a corrected AT for reaction 3 was finally obtained. The heat of sublimation of T Q was determined as described in the literature by utilizing an isotenis~ope.l~AQ, TQ, PQ, and N Q are all soluble in HMPA. The heats of solution of these compounds in HMPA were determined by breaking thin-walled bulbs with different amounts of the solution calorimeter containing 100.0 mL of HMPA. The heat capacity of the calorimeter containing 100.0 mL of water was determined to be 515 f 12 J/OC. The heat capacity of the calorimeter containing 100.0 mL of HMPA was calculated from that of the calorimeter containing
( 1 3 ) Solsky, J. F.; Grushka, E. J . Phys. Chem. 1974, 78, 275
The Journal of Physical Chemistry, Vol. 90, No. 21, 1986 5335
Heats of Solvation of Semiquinones
Figure 2. Simultaneous first derivative ESR spectra utilized in the determination of the {Q*-]/{BV-]ratio. A (Q = NQ), B (Q = AQ), and C (Q = PQ) are low-resolved simultaneous spectra. The BQ' lines indicated are free from overlap with other lines. D (Q = TQ) is a well-resolved simultaneous spectrum. The lines indicated in D are those utilized in the determination of the {TQ'-}/{BQ'-] ratio by measuring the ratio of the corresponding heights times their square peak-to-peak width. These line intensities were corrected by their corresponding intensity factors in order to obtain the total one. The high-resolved ESR spectra corresponding to all of the semiquinones shown in this work, in HMPA, are identical with those previously reported.26
water utilizing the reported heat capacity of HMPA.I4 The heats of solution are depicted in Table I1 and correspond to averages or heats which are proportional to slopes of linear graphs of the change in temperature vs. the millimoles of the corresponding solid compound. ESR measurements were made with an X-band IBM ER/2OOD EPR spectrometer. Variable-temperature measurements were performed with the aid of an IBM E R 4111 VT temperature controller. The equilibrium constant determinations corresponding to the electron transfer from BQ'- to Q in HMPA, reaction 6 ,
*
QHMPA + BQO-HMPA Q*-HMPA + BQHMPA (6) at each temperature were made by determining the ratio (Q*-)/ (BQ'-), and then when a spin standard, galvinoxyl in benzene, was utilized, the total spin concentration, (Q'-) (BQO-), was determined. From these two equations the concentrations of each semiquinone and, consequently, of the neutral species were determined. The ratio {Q*-)/{BQ*-)was obtained either by determining the ratio of corresponding double-integrated Lorentzian first derivative ESR lines (height times the square of the peakto-peak width), as in the case of TQ, or by double integrating low-resolved simultaneous ESR spectra, as implied by eq 7, Figure 2, where A , is the double-integrated area of a free-from-overlap
+
WI/PQ*-I = (AI - 4 C ? / A , C
0.151 0.20
I
1
0.35
0.50 MMOLES OF
I
I
0.8
0,65
BQ:
Figure 3. Plot of the AT corresponding to reaction 3 vs. the millimoles of BQ'- contained in the thin-walled bulbs. From the slope of this line a heat of -88 f 4 kJ/mol is obtained.
TABLE I: Thermochemical Cycle Utilized in the Determination of the Heat of Solvation of BQ'- + Na' in HMPA AHo/kJ reaction
mol-'
a
ref
(7)
line corresponding to BQ'-, C is the ratio of the total double-integrated area corresponding to BQ'- to that corresponding to the free-from-overlap line of BQ'-, and A , is the total double-integrated area corresponding to the simultaneous spectrum. The double integrations of the low-resolved ESR spectra were performed by manually digitizing the ESR spectra coupled to a computer integration.I5 (14) Castagnolo, M.; Inglese, A,; Petrella, G.; Sacco, A. Thermochim. Acta 1981, 44, 67. (15) Alegria, A. E.; Lozada, 0.; Rivera, H.; SBnchez, J. J . Organomet. Chem. 1985, 251, 229. (16) Stevenson, G. R., private communication. (17) Weast, R. C.; Astle, M. J. CRC Handbook of Chemistry and Physics, 63rd ed.; CRC: Boca Rat6n, FL, 1984. (18) Hicks, W. T. J . Chem. Phys. 1963, 38, 1873. (19) Cox, J. D.; Pilcher, G. Thermochemistry of Organic and Organometallic Compounds; Academic: New York, 1970. (20) Cooper, C. D.; Naft, W. T.; Compton, R. N. J . Chem. Phys. 1975, 63, 2752.
BQ'- + Na'(,) N~'(HMPA)
-.
BQ'-(HMPA)+
-287.7 f 0.1 31.4 f 0.4 -184.5 f 0.4 -63 f 3 182.4 f 0.8 -495.3 f 0.4
17 16 18 19 20 21
-755 f 10
OThe errors are those reported in the reference or estimated as the unit in the uncertain figure of the reported value.
Results The heat of reaction 1, where Q = BQ, was obtained by determining the heat of reaction 3 in the solution calorimeter and (21) Lotz, W. J . Opt. SOC.Am. 1967, 57, 813.
5336 The Journal of Physical Chemistry, Vol. 90, No. 21, 1986
Alegria et al.
+
Na' in HMPA h H o / k J mol-'
TABLE 11: Thermochemical Cycle Utilized in the Determination of the Heat of Solvation of Q'-
TQ 3 f l -54.0 f 2.5 175 f O.tlb 16 f 1" -182.4 f 0.8 63 f 3 --6.7 f 0.8
reaction Q(HMPA)
+ BQ'-(HMPA)
+
Q'-(HMPA)
+ BQ(HMPA)
Q(s1 + Q(s) Q*-(g) Q(S1
+
Q(g) +
e-(g)
-+Q(HMPA)
BQM + e-(g) -+ BQ'-,g) BQ(~I-+ BQw BQ(HMPA) BQis)
-
NQ 9 f 2 -72.4 f 3.Sd 170.3 f 0.Sb 17 f 2* -182.4 f 0.8 63 f 3 -6.7 f 0.8
PQ 7 f 3 -92 f 4d 176 f 4' 12 f 2" -182.4 f 0.8 63 f 3 -6.7 f 0.8
AQ 17.6 f 0.8 -112 f 5 d 151 f 4' 10 f 6" -182.4 f 0.8 63 f 3 -6.7 f 0.8
aThis work. bThese electron affinities were calculated from the relative electron affinities reported in ref 22 and the electron affinity reported for BQ in ref 20. See text. ?These electron affinities were estimated. See text. dReference 19. 3.23
2.18
1.73 LN
?
0.98
0.23 182
/m
r
-
1
I
/
3.00
3,23
5.12
3.38
4
dm
1 0 ~ 0 ~ 1 ~
Figure 5. Graph utilized in the determination of the heat of sublimation of TQ. Different symbols correspond to different determinations. E A B < C T ! i f l n3i:
Figure 4. Linear correlation obtained between absolute electron affinities (EAA)20,22and average values of electron affinities obtained from charge-transfer energy data for several quinones (EAB).23The reported EA values are converted to kJ/mol units. The slope of the line is 1.26 with a correlation factor of 0.982. The points correspond, in a decreasing order of magnitude in the absolute EA (ordinate axis), to tetrachlorop-benzoquinone, trichloro-p-benzoquinone,methyl-trichloro-p-benzopquinone, 2,6-dichloro-p-benzoquinone,2,5-dichloro-p-benzoquinone, benzoquinone, methyl-p-benzoquinone, 1,4-naphthoquinone, 2,6-dimethyl-p-benzoquinone, and tetramethyl-p-benzoquinone. All the quinones with EAA and EABdata are included. The interpolated points for AQ and PQ are (151, 157) and (171, 176). respectively.
utilizing a convenient thermochemical cycle, Figure 3 and Table I. The heat of reaction 1 corresponding to the other semiquinones was obtained by determining the heat of electron exchange between BQ'- and Q in HMPA, eq 6, coupled to the heat of solvation of BQ'- + Na' in HMPA and a convenient thermochemical cycle, Table 11. The electron affinities of DQ, TQ, and N Q were calculated from their relative electron affinities reported by McIver and coworkers,22and these values are anchored to the electron affinity reported for BQ by Compton et aLZ0 This reported value for the electron affinity of BQ (139 eV) was selected because of its good agreement with theoretical calculations.20 The electron affinities corresponding to AQ and PQ were estimated by an interpolation of their values from a linear graph of reported absolute electron affinities20.22vs. average values of electron affinities obtained from charge-transfer energy data for ( 2 2 ) Fukuda, E. K.; McIver, R. T., Jr. J. Am. Chem. SOC.1985,107,2291.
1
2.93 3
I ~
,
co
3.11
i I
3 23
I
3 34
i J 45
1O3'KiT
Figure 6. Representative van't Hoff plots utilized in the determination of the heat of electron transfer from BQ'- to Q in HMPA, reaction 6. (A) Q = AQ; ( 0 )Q = TQ; (W Q = NQ; ( 6 ) Q = PQ. TABLE 111: Equilibrium Constants Determined for the Electron-Exchange Process between BQ" and Q in HMPA, Reaction 6
Q TQ NQ AQ PQ '25 'C
Kq/1O2a 0.26 f 0.02 2.3 f 2.0 4.8 f 0.1 2.8 f 2.0
The Journal of Physical Chemistry, Vol. 90, No. 21, 1986 5337
Heats of Solvation of Semiquinones
TABLE IV: Thermochemical Cycle Utilized in the Determination of the Heat of Solvation of the Negative Charge plus Na+ in HMPA AHo /kJ mol-' TQ
BQ
NQ
PQ
19 -16.3 f 1.3 54.0 2.5
-755 f 10 -6.7 f 0.8 62.8 f 3.3
-757 f 22 -17.2 f 2.9 72.4 f 3.7
-778 f 27 -11.7 f 1.7 92.0 f 4.2
AQ -814 f 30 -10.0 6.6 112.1 f 5.9
-699 f 6
-702 f 15
-698 f 21
-712 f 18
reaction
Q'-(g)
-. -. -. + Na+(g)
Q.-(HMPA)
+ N ~ + ( H M P A ) -741
Q(HMPA) Qw QW Q'-(g)
*
Q(s)
+ Q(HMPA) + Na+(g)
f
--*
Q*-(HMPA)
+
-703
f
15
*
N ~ + ( H M P+A Q(g) ) "The errors from the second and third reactions are subtracted from the first one since the former reactions are included in the thermochemical cycle utilized to obtain the first reaction.
several quinones as reported by Chen and W e n t ~ o r t h Figure ,~~
4. The heat of sublimation of TQ was determined by plots of In
Pvs. 1 / T , Figure 5 . The heats of reaction 6 were obtained from Figure 6 . The equilibrium van't Hoff plots of In Kes vs. T', constants, at 25
O C ,
for reaction 6 are summarized in Table 111.
Discussion Even though the errors in the equilibrium constants for reaction 6 are large, they are all much greater than 1. This observation is in contrast with the enthalpy changes corresponding to these same reactions, which are positive and small. The combination of these two effects produce positive entropy changes, indicating that reaction 6 is entropy controlled. The enthalpy changes for reaction 6 in the gas phase, eq 8, which can be calculated from the relative electron a f f i n i t i e ~ are , ~ ~also all positive and small. Thus, the enthalpy changes in reaction 6 can be ascribed essentially to differences in the gas-phase electron affinities between Q and BQ, while the entropy changes are due mainly to a more disordered state of solvation of Q'- relative to that corresponding to BQ'-. From the results in Table I1 one major trend is observed. The relative heats of solvation, those corresponding to reaction 9, or
the actual heats of solvation are more negative as Q'- gets less sterically hindered (less number of methyl groups), although only one example is presented, or more benzene rings are added to the quinonoid moiety. This increase in the magnitude of the heat of solvation with the size of the semiquinone is analogous to that observed by Stevenson and co-workers in the case of polyacene radical anions, with Na+ as the countenon, solvated in THF.6 Two main differences exist between Stevenson's systems and ours; Le., the gas-phase EA'S of the polyacene increase with the number of benzene rings: in contrast to our case, and the polyacene radical anions are ion paired to Na+ in THF,* which is not the case in (23) (a) Chen, E. C. M.; Wentworth, W. E. J . Chem. Phys. 1975, 63, 3183. (b) The absolute electron affinity of AQ (1.15 eV) reported in ref 23a is lower than that of BQ (1.34 eV) as determined by the magnetron technique. (24) The entropy change in reaction 7 should be very small due to the small variation of the corresponding partition functions with temperature in the gas
phase.22
HMPA. Furthermore, the number of polyacenes studied is also greater than the number of semiquinones with expanded electron delocalization studied in the present work. The fact that the gas-phase EA'S of the p-benzosemiquinones decrease as the electron delocalization increases has been observed has been given to this p r e v i o ~ s l y although , ~ ~ ~ ~ no ~ ~explanation ~~ observation. Interestingly, if the heat of solvation of the neutral quinone is subtracted from the heat of solvation of Q'- Na', the heat of solvation of the negative charge plus that of Na+ should be obtained, Table IV. Essentially no difference is observed in this heat of solvation upon changing the structure of Q'-. This observation indicates that the observed differences in heats of solvation, or in the relative heats of solvation, are mainly due to differences in the strength of solvation of the molecular framework of Q'- and not to differences in the solvation of the net negative charge. Thus, differences in the strength of solvation due to unequal charge density distributions in Q'- are all leveled off probably due to an increase in the number of weak HMPA-net negative charge interactions, while the number of strong HMPA-net negative charge interactions decreases as the Aelectron delocalization increases. According to these observations, the heat corresponding to reaction 6 would be close to zero if only heats of solvation of the corresponding species were considered. It is not zero due to the differences in the gas-phase electron affinities. In summary, even though the solution thermodynamic stability of Q'- as compared to that of BQ'- is not favored by differences in the gas-phase electron affinities, it is favored by entropies and heats of solvation differences.
+
Acknowledgment. We express appreciation to the NIH-MBRS Program for support of this work with Grant R R 08216. We thank Dr. G. R. Stevenson from Illinois State University and Dr. J. R. Miller from Argonne National Laboratories for helpful suggestions. Registry No. Na', 17341-25-2; TQ'-, 3998-67-2; BQ'-, 3225-29-4; NQ'-, 20261-01-2; PQ'-, 24490-51-5; AQ'-, 3426-73-1; HMPA, 68031-9. ( 2 5 ) Grimsrud, E. P.; Caldwell, G.; Chowdhury, S.;Kebarle, P. J . Am. Chem. SOC.1985. 107. 9627. ( 2 6 ) Stevenson, G. R.; Block, A. McB.; Alegrra, A. E. J . Am. Chem. SOC. 1975, 97, 4859.
