6555
J. Phys. Chem. 1992, 96,6555-6558
Influence of Steric Effects on the Excited Triplet-State Lifetime of 2,2‘-Dinltrostilbene and 2,2’,4,4’,6,6’-Hexanltrostilbene In Acetonltrile Solution K.J. Smit Materials Research Laboratory (MRL)-DSTO, (Received: March 16, 1992)
P.O. Box 50, Melbourne, Victoria 3032, Australia
Solutions of 4,4’-dinitrostilbene, 2,2’-dinitrostilbene, 2,2’,4,4’,6,6’-hexanitrostilbene(HNS), and deuterated derivatives of HNS in acetonitrile have been studied by laser flash photolysis. Transient species with decay lifetimes of 125, 5 5 , and 21 ns, respectively, were observed for the nondeuterated compounds. The decay lifetimes for these species provide evidence that the excited triplet-state lifetime is being reduced as steric hindrance to molecular planarity is increased. This is partly attributed to a reduction in the So-TI energy gap of the perpendicular configuration with increasing intramolecular steric hindrance although, for HNS, rapid formation of photoproducts also occurs. The degree of intramolecular steric hindrance associated with nitro groups ortho to the central double bond is indicated by singlet ground-state absorption measurements. Introduction The photochemistry of nitrated stilbenes has received considerable attention, much of which has been directed toward understanding the trans-cis photoisomerization process.’-7 The presence of nitro groups on the aromatic molecule introduces na* states which can result in high intersystem crossing quantum yields (@,sc).8-13 For 4,4’-dinitrostilbene (4,4’-DNS), dissolved in methanol and benzene, the values of Olsc for the excited trans isomer (‘tr*-3tr*) are 0.69 and 0.81, respectively.6 The fluorescence quantum yield (af)of 4,4’-DNS in methanol is below This is substantially below the afof trans-stilbene derivatives, which are not symmetrically nitrated.2J4.15Several studies have established that the excited trans triplet state of 4,4’-DNS and other nitrated stilbenes decays via a twisted perpendicular triplet state (3p*) which is in equilibrium with 3tr*.4-637While 4,4’-DNS has been widely studied, there is little known about the excited states of nitrostilbenes that are more sterically hindered. 2,2’,4,4’,6,6’-Hexanitrostilbene(HNS) is very sterically hindered and does not adopt a planar configuration in the solid state.I6J7 In solution, steric interactions between the double bond and ortho nitro groups in HNS and possibly 2,2’-dinitrostilbene (2,2’-DNS) may influence the ground-state singletsinglet absorption spectrum of these compounds. Steric hindrance to molecular planarity may also be present in the excited triplet state, and this will affect the excited triplet state’s potential energy surfaces, as recently noted for methyl substitution of 1,3,5-hexstriene.l8 If steric distortion of the excited triplet-state (T,) potential energy surface results in the closer approach of T I to the ground state (So), as the molecule is twisted about the double bond, we would expect to see a reduction in triplet state lifetime relative to that of transstilbene or ~ , ~ ’ - D N S . I V ’ ~ * ~ ~ It should also be noted that the excited triplet state of 2,4dinitrostilbene2Iand other nitrated c ~ m p o u n d s l ~is. ~possibly ~.~~ involved in shock-induced chemistry. HNS is used widely as a thermally stable energetic compound, and studies on its excitedstate chemistry are therefore of particular interest. In this study, a comparison is made between both the ground-state singletsinglet absorption spectra and the excited triplet-state lifetimes of 4.4’-DNS, 2,2’-DNS, and HNS in acetonitrile solutions. This has been camed out to observe the effect of intramolecular steric hindrance of nitro groups, ortho to the central double bond, on both the ground-state and excited-state processes. Deuterated HNS derivatives, HNS(Dz) with the a-H atoms on the double bond replaced and fully deuterated HNS(D6), were also studied to aid in the identification of the HNS transient species. Acetonitrile was chosen as the solvent for HNS both because of solubility considerations and its transparency at the excitation wavelengths.
Experimental Section Materials. 4-Nitrobenzyl chloride (Sigma) and 2-nitrobenzyl chloride (Aldrich) were used in the preparation of 4,4’-DNS and 0022-3654/92/2096-6555%03.00/0
2,2’-DNS, both isolated as pure trans isomers. The preparation of 4,4’-DNS involved minor modifications of the literature proced~re.2~ The Cnitrobenzyl chloride was dissolved in 98% ethanol, and alcoholic potassium hydroxide was added dropwise to the rapidly stirred solution. The precipitate was washed with hot distilled water, warm ethanol, and hot glacial acetic acid; the product was then recrystallized twice from acetonitrile, yielding yellow needles, mp 301-302 “C (lit.25303-304 “C). The 2,2’DNS was similarly prepared from 2-nitrobenzyl chloride26and recrystallized from glacial acetic acid and then chloroform, yielding light yellow needles, mp 197-198 “C (lit.25196 “C). HNS-I1 was obtained from the Naval Surface Warfare Center, White Oak, MD, and recrystallized twice from acetonitrile. This yielded yellow needles, mp 322-323 “C (lit.27318-324 “C). The preparation of the a,a’-(D2) derivative of HNS and the fully deuterated a,d,3,3’,5,5’-(D6) derivative of HNS (both >97% isotopic purity), used in some experiments, has been described previously.28 The acetonitrile (BDH) used was HPLC grade. Apparatus. The flash photolysis apparatus employed has previously been described.29 The Q-switched output of a Nd3+/silicate glass laser at 354 or 265 nm was used to excite acetonitrile solutions of the nitrated stilbenes with pulses of 14-ns duration (fwhm). The quartz cuvette, of 1-cm path length, was monitored at right angles to the laser excitation using a stabilized 150-W Xe arc lamp (Rofin), filtered to remove UV light, and pulsed to increase lamp intensity over 8 ms with a Xe lamp pulsing unit (Applied Photophysics). A holographic grating monochromator (Jobin-Yvon, HIV UVf/3.5) isolated the wavelength of the monitoring light. The detection system consisted of an RCA 81-52 photomultiplier tube and a Tektronix 7633 storage oscilloscope. Solutions were degassed, using at least five freezepump-thaw cycles, and ranged in concentration from 5 X lo4 M for 4,4’-DNS and HNS to 1.5 X lW3 M for 2,2’-DNS. Data were digitized from the original photographs and averaged using the program KGRAPH.~ A Varian Superscan 3 W-vis absorption spectrometer was used to record the ground-state absorption spectra.
Results and Discussion Singlet Ground-State Absorption Spectra. So-Sl absorption spectra for the 4,4’-DNS, 2,2’-DNS, and HNS dissolved in acetonitrile are presented in Figure 1. The high molar absorptivities for each of the maxima are indicative of m r * transitions, although low-intensity n-r* transitions may be present in the A comparison may long-wavelength tail of the HNS ~peCtrum.~~* be made with tram-stilbene, which is almost planar in the ground state and has strong conjugation between the rings through the central double bond. For trans-stilbene in benzene, the long‘Agtransition (A-band) is centered at 294 nm, wavelength IB, while for 4,4’-DNS and 2,2’-DNS the corresponding absorption peaks are red shifted to 357 and 335 nm, respectively. Solvent effects on the trans-stilbene A-band are relatively minor,32and we can therefore attribute the observed red shifts to resonance
-
Published 1992 by the American Chemical Society
.
6556 The Journal of Physical Chemistry, Vol. 96, No. 16, 1992
.-
I
h
I
I
a
0
i
200
I
250
300 350 400 Wavelength (nm) Figure 1. Singlet ground-state absorption spectra for acetonitrile solutions of (a) 4,4’-DNS, (b) 2,2’-DNS, and (c) HNS.
0.81
I
I
,
,
i
Smit
Time (ns) Figure 3. In (absorbance) versus time for 4,4’-DNS at 500 nm excited at 354 nm, taken from the absorption maximum in (a) degassed acetonitrile solution ( T = 125 ns) and (b) air-saturated acetonitrile solution (7 = 55 ns). Experimental data are shown by dashed lines; the lines of best fit are shown by continuous lines.
0.03I
L
I
m
I1 0
1 0
: ,I ;i
1
I
80
160
- - _ _--....
240
320
Time (ns) Figure 2. Transient absorption at 500 nm versus time for 4,4’-DNS excited at 354 nm in (a) degassed acetonitrile solution and (b) air-saturated acetonitrile solution. Inset: molecular configuration of 4,4’-DNS.
effects associated with the nitro groups in positions para and ortho to the central double bond.31b For HNS the indistinct shoulder in the vicinity of 300 nm is attributed to the long-wavelength (A-band) absorption transition of the tram-stilbene ~hromophore.~~ It is apparent, from a comparison of A-band absorption maxima, that relative to 4,4’-DNS, the 2,2’-DNS and HNS molecular configurations are affected by steric crowding of the ortho nitro groups. In the sterically hindered stilbene derivatives, twisting occurs about the 1,a and l’,d essential single bonds, increasing the energy gap to the first excited singlet state and therefore decreasing the A-band absorption wavelength?lC X-ray ~tudies’~J~ have shown that HNS crystals are composed of two independent trans configurations. In both forms the nitrated benzene rings exist in parallel planes, although these are severely twisted with respect to the plane of the central double bond at angles of 104’ and 98’.17 The second absorption band (B-band) for 4,4’-DNS in acetonitrile is at 227 nm. It increases in intensity as the A-band is blue shifted and loses intensity for this scrim of compounds. These observations are consistent with the effects of steric hindrance on ground- and excited-state molecular planarity, as noted for 2,2’,4,4’,6,6’-hexamy~tilbene and cis-stilbene.uIc The resultant has absorption maximum of HNS at 221 nm (41 300 M-’ an-’) a molar absorptivity over twice that of 2,4,6-trinitrotoluene (TNT) ~ ~ is consistent in methanol at 227 nm (19 200 M-I ~ m - l ) .This with the trinitrobenzyl molecular units in HNS absorbing light ~ , ~ ~in stark contrast to independent of one a n o t h ~ r . ~Thus tram-stilbene and 4,4’-DNS, there is little conjugation between the aromatic rings and the central double bond in HNS. Excited TripIet-StateAbrptloll. 4,4’-DNS. When excited at 354 nm, 4,4’-DNS in degassed acetonitrile has a strong transient
I
I
I
I
Excited Triplet-State Lifetimes of Stilbenes
The Journal of Physical Chemistry, Vol. 96, No. 16, I992 6557
II \,
0.031
-
i
-1
I
0.01
-71 0
I
1
I
7
20
40
60
80
Time (ns)
Figure 5. In (absorbance) versus time for 2,2’-DNS at 440 nm excited at 354 nm, taken from the absorption maximum in (a) degassed acetonitrile solution (no baseline subtraction; T = 55 ns) and (b) air-saturated
acetonitrile solution (with 22%of the absorption subtracted to form a new baseline; T = 27 ns). Experimental data are shown by dashed lines; the lincs of best fit are shown by continuous lines.
a
A
I 0 LLL 0 40 80 120 160 Time (ns) Figure 7. (a) Transient absorption rise at 500 nm for HNS in acetonitrile solution with 265- and 354-nm excitation, normalized to an absorbance of 0.0156 at 120 ns. (b) Transient absorption decay at 440 nm for HNS(D6) in acetonitrile solution with 354-nm excitation.
0.051
0.011
-
01 420
I
l
\
Time (ns)
\
l
I
460
500
540
580
620
Wavelength (nm) Figure 6. Transient absorption versus wavelength for HNS in degassed acetonitrile solution with 354-nm excitation.
M-’ s-l. It is considered that the oxygen-sensitive transient absorption is due to the excited triplet state of 2,2’-DNS, with some photoproduct formation in air-saturated solution. It may be noted that while the nitro groups in 4,4’-DNS have resulted in a large increase in the triplet state lifetime compared to trans-stilbene, the presence of nitro groups at the 2,2’ positions has effected a slight decrease in triplet lifetime (55 ns compared to 59 ns). This is despite the expected similarity between the resonance stabilization achieved for aromatic excited states with para- or ortho-directing ~ubstituents.”~It is therefore suggested that the absence of a resonance effect on the 2,2’-DNS triplet lifetime may partly be due to steric hindrance which twists the 2,2’-nitro groups out of the plane of the ring, thereby reducing lone-pair *-orbital interaction with the aromatic ring Steric constraints on the 2,2’-DNS molecular planarity may also be increased due to the presence of the nitro groups, which will reduce the resonance interaction in the molecule. HNS. Using both 265- and 354-nm excitation, degassed acetonitrile solutions of HNS provided transient absorption at 440 and 500 nm (Figure 6). The longer wavelength absorption did not decay on a millisecond time scale, even in air-saturated solutions, and it can be attributed to a long-lived photoproduct. The rise time for the photoproduct, which occurred over 60-80 ns (see F i i 7a), was not exponential, but no correction for the excitation pulse width (14-ns fwhm) was applied. After a period of continuous UV irradiation, acetonitrile solutions of HNS become reddish in color as a resuit of photoproduct formation. This could be related to observed formation of pink coloration when TNT Some TNT in aqueous solution is irradiated with UV light.38*39 phototransformation products that have been identified include 4,6-dinitroanthranil, 2,4,64rinitrobenzaldehyde,2,4,6-trinitroSolid HNS also forms benzoic acid, and 1,3,5-trinitr0bmzene.~~*~~ these products upon irradiation.@
1
0
1
0
,
1
, -1
20
1
40
60
-
I
80
1
Y
100
120
Time (ns) Figure 8. Absorption decay of the short lived HNS(D6) transient at 440 nm in acetonitrile solution, excited at 354 nm. Inset: In (absorbance) versus time plot, taken from the absorption maximum ( T = 19 ns).
The 440-nm transient absorption consisted of a fast-decaying transient and a long-lived species (Figure 7b). By assuming the same growth for the 440-nm long-lived absorption and the 5 C ” long-lived absorption, the absorption due to the fast decaying species could be determined from the difference. This produced the following lifetimes: 21 f 4 ns for HNS, 23 f 6 ns for a,d-HNS(D2),and 19 f 2 ns for a,d,3,3’,5,5’-HNS(D6) (Figure 8). From the apparent correspondence between the growth of the HNS photoproduct at 500 nm and the decay of the HNS and deuterated HNS transient absorptions at 440 nm (Figure 7), it is considered likely that the growth of the photoproduct is related to the decay of the short-lived species at 440 nm. It has been reported that deuteration of the ethylenic positions on transstilbene increases the triplet-state lifetime by 30%.4l The absence of an observable increase in the transient lifetime for fully deuterated HNS suggests that the short-lived 440-nm absorbing transient species is not an excited triplet state. No effect of oxygen was observed on the 440-nm transient decay. The 440-nm short-lived transient of HNS may therefore, itself, be a photoproduct derived from an excited triplet state which should be populated rapidly given the very high anticipated BPI=of HNS. If this is the case, the excited triplet state of HNS would have a lifetime 621 ns. The HNS excited triplet-state lifetime is much lower than that observed for 2,2’-DNS, and it is suggested that this is due to the increased steric hindrance to molecular planarity in HNS. As in 2,2’-DNS, the nitro groups are twisted out of the planes of the aromatic rings,I6s1’and this decreases their contribution to resonance stabilization. For 2,2’,3,3’,4,4’,5,5’,6,6‘-dmfluorostilbene at 101 K in glassy ethanol or ethanol-methanol mixtures the
6558 The Journal of Physical Chemistry, Vol. 96, No. 16, 1992
Smit
excited triplet-state lifetime is reduced to 0.5 ms from 12 ms for This is trans-stilbene and 2-5 ms for two tetrafluoro~tilbenes.~~ at least partly due to the bulky fluorine atoms, which result in increased intramolecular steric intera~tion.~~ The consequences of contortion of the nitrated aromatic rings relative to the ethylenic double bond is also likely to be of considerable importance in determining the triplet-state 1ifetime.l’ Twisting of trans-stilbene about the double bond in the excited triplet state has been considered as a means by which the TI and So potential energy surfaces might contact each other, thereby shortening the triplet-state lifetime.’ In crystalline HNS the ground-state structure is in the trans configuration with its aromatic rings already twisted approximately 100° out of the plane of the ethylenic bridge.” Further twisting about the central double bond in the excited state to form a perpendicular triplet can be envisaged, but there is no evidence that a cis isomer of HNS can ultimately form. Overlap between Tl(3p*)and SO(lpo)for HNS may occur, during twisting, to reduce the triplet lifetime. A rigid cis-stilbene analogue, 2,3-diphenylnorbornene, which severely restricts ethylenic double bond torsion, has been shown to have an excited triplet-state lifetime of 65 ps,2O 3 orders of magnitude larger than that for the perpendicular stilbene triplet in benzene solution.5 The increased lifetime has been attributed to the uframework for the rigid stilbene which prevents the approach of the Soand TI surfacts along the central double-bond coordinate.20 Steric interaction, between the ethylenic bridge and ortho nitro groups, increases the probability of photoproduct formation in acetonitrile solution. A recent study on o-nitrobenzyl-p-cyanophenyl ether has shown that in THF a triplet state of lifetime In acetonitrile this triplet decays to form 0.3-0.4 ns is ~roduced.4~ a biradical absorbing at 425 nm with a 7.6-1s lifetime. A further product, the o-quinoid, is formed from the biradical, and subsequently a further anthranilic cyclization product can be formed. The latter anthranilic species may also result directly from the b i r a d i ~ a l .Related ~~ anthranilic species, similar to this, have been identified from both TNT photodegradation in aqueous soluti0n~*3~ and the photodegradation of solid HNS40 It is possible, therefore, that the 440-nm short-lived HNS transient is a ground-state biradical species derived from a shorter lived excited triplet state. This species may subsequently form a long-lived cyclized photoproduct that absorbs at 440 and 500 nm.
steric hindrance, in which twisting about the ethylenic double bond toward a perpendicular configuration can result in overlap between the So and T I potential energy surfaces. The formation of a photoproduct from the 440 nm short-lived HNS transient is also a result of crowding between the 2,2’ and 6,6’ nitro groups and the ethylenic bridge.
Conclusion The introduction of nitro groups in the 2,2/ positions, as opposed to the 4,4’ positions, in stilbenes is shown to result in decreased raonance stabilization of the excited singlet state relative to the ground state. This is attributed to steric hindrance to planarity of the 2,2’-nitro groups with respect to the aromatic ring and distortion of the molecule’s configuration to reduce resonance between the ring and double bond. In acetonitrile solution, HNS has a So to SIabsorption maximum at shorter wavelength than that observed for 2,2’-DNS; this reflects the highly contorted configuration of HNS observed in crystallographicstudies. It is likely that there is little effective conjugation between the aromatic rings and the double bond of HNS in the ground state. The triplet-state lifetime for 2,2’-DNS (55 ns) is substantially shorter than that for 4,4’-DNS (125 ns), and it is similar to that for tram-stilbene. This is attributed to steric effects which reduce the influence of the nitro groups on the triplet excited-state conjugation, and which may decrease conjugation between the aromatic ring and the ethylenic bridge. W e intersystem Crossing in HNS is expected to be highly efficient, the observed HNS transient at 440 nm with a 214s lifetime may be derived from a still unobserved shorter lived triplet state. Justification for this is associated mainly with the absence of a deuterium isotope effect on the lifetime of the transient species. The reduction in triplet-state lifetime in HNS can be attributed to the high degree of
Acknowledgment. I thank Dr. I. J. Dagley (DSTO-MU) for valuable advice during the course of this work and for the provision of deuterated HNS. Useful discussions have also been held with Dr. R. J. Spear (DSTO-MRL) and Dr. K. P. Ghiggino (School of Chemistry, University of Melbourne). The assistance of the School of Chemistry, University of Melbourne, in providing facilities and the help of P. G. Spizzirri is much appreciated. References and Notes (1) Schulte-Frohlinde, D.; Blume, H.; Gusten, H. J. Phys. Chem. 1962, 66, 2486. (2) Gegiou, D.; Muszkat, K. A.; Fischer, E. J . Am. Chem. Soc. 1968,90, 3907. (3) Bent, D. V.; Schulte-Frohlinde, D. J . Phys. Chem. 1974, 78, 451. (4) Schulte-Frohlinde, D.; Gorner, H. Pure Appl. Chem. 1979, 51, 279. (5) Gorner, H.; Schulte-Frohlinde, D. J. Phys. Chem. 1981, 85, 1835. (6) Gomer, H. Ber. Bunsen-Ges. Phys. Chem. 1984.88, 1199. (7) Gomer, H.; SchulteFrohlinde, D. Ber. Bunsen-Ges.Phys. Chem. 1984, 88, 1208. (8) El-Sayed, M. J. Phys. Chem. 1963, 38, 2834. (9) Bent, D. V.; Schulte-Frohlinde, D. J . Phys. Chem. 1974, 78, 446. (10) Pisanias, M. N.;Schulte-Frohlinde, D. Ber. Bunsen-Ges.Phys. Chem. 1975, 79, 662. (1 1) Wilkinson, F. Organic Molecular Phorophysics; Birks, J. B., Ed.; Wiley: London, 1975; Vol. 2, Chapter 3. (12) Capellos, Ch.; Iyer, S.Inr. Annu. ConJ ICT 1979, 61 1. (13) Gorner, H. J . Phys. Chem. 1989, 93, 1826. (14) Gorner, H. J. Phorochem. 1980, 13, 269. (15) Smit, K. J.; Ghiggino, K. P. Chem. Phys. Lett. 1985, 122, 369. (16) Chang, H.-C.; Tang, C.-P.; Chen, Y. J.; Chang, C.-L. Int. Annu. ConJ ICT 1987, 51-1. (17) Zengguo, F.; Boren, C.; Zuccai, L. Acta Armamentarii Sinica 1990, 42. (18) Negri, F.; Orlandi, G.; Brouwer, A. M.; Langkilde, F. W.; Moller, S.; Wilbrandt, R. J . Phys. Chem. 1991, 95, 6895. (19) Calvert, J. G.; Pitts, Jr., J. N . Photochemistry; Wiley: New York, 1967; p 301. (20) Gorman, A. A.; Beddoes. R. L.; Hamblett, 1.; McNeeney, S. P.; Prescott, A. L.; Unett, D. J. J . Chem. SOC.,Chem. Commun. 1991, 963. (21) Justus, B. L.; Merritt, C. D.; Campillo, A. J. Chem. Phys. Lett. 1989, 156, 64. (22) Capellos, C. Fast Reactions in Energetic Systems; Capella, C., Walker, R. F., Eds.; Reidel: Dordrecht, 1981; p 33. (23) Capellos, C.; Iyer, S. In ref 22, p 401. (24) Walden, P.; Kernbaum, A. Ber. Dtsch. Chem. Ges. 1890, 23, 1959. Buckingham, J., Dona(25) Dictionary of Organic Compounds, 5th 4.; ghy, S. M., Eds.;Chapman and Hall: New York, 1982. (26) Bischoff, C. A. Ber. Dtsch. Chem. Ges. 1888, 21, 2071. (27) Zeng-guo, F.; Boren, C. Propellants, Explos., Pyrotech. 1991,16, 12. (28) Dagley, I. J. Ausr. J . Chem. 1988, 41, 855. (29) Smit, K. J.; Ghiggino, K. P. Dyes Pigm. 1990, 13, 45. (30) Kennett, S. R. DSTO-MRL Technical Note 1985. MRL-TN-485. (31) Jaffe, H. H.; Orchin, M. Theory and Applications of Ultraviolet Spectroscopy; Wiley: New York, 1962; (a) pp 111-146, (b) p 256, (c) pp 424-434, (d) pp 248-268. (32) Dyke, R. H.; McClure, D. S.J. Chem. Phys. 1962,36,2326. (33) Smit, K. J. J. Energetic Mater. 1991, 9, 81. (34) Yinon, J.; Zitrin, S.The Analysis of Explosives; Pergamon: Oxford, 1981; p 145. (35) Berlman, I. B. J . Chem. Phys. 1970,52, 5616. (36) Berlman, I. B. J . Phys. Chem. 1970, 74, 3085. (37) Saltiel, J.; Thomas, B. Chem. Phys. Lett. 1976, 37, 147. (38) Spanggod, R. J. Toxicity of Nitroaromatic Compounds; Rickert, D. E., Ed.;Hemisphere Publishing: Washington, 1985; pp 20-27. (39) Yinon, J. Toxicity and Metabolism of CRC Press: Boca - Explosives; Raton, FL, 1990. (40) Kayser, E. G.USARO Technical Report; 1989, NSWC-TR-90-60. (41) Saltiel. J.: D’Aaatino. J. T.: Herhtroeter. W. G.: Saint-Ruf, G.: Buu-Hoi, N. P. J . Am.?hem. SOC.1973, 95, 2543. (42) Muszkat, K. A.; Castel, N.; Jakob, A,; Fischer, E.; Luettke, W.; Rauch, K. Photochem. Photobiol. A-Chem. 1991, 56, 219. (43) Yip. R. W.; Wen, Y. X.; Gravel, D.; Giasson, R.; Sharma, D. K. J . Phys. Chem. 1991, 95,6078.
