J. Phys. Chem. 1985,89, 3277-3279
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Dissociation of IC1 by Energy Transfer from Singlet Oxygen A. T. Pritt, Jr., R. K. Home, and D. J. Benard* Rockwell International Science Center, Thousand Oaks, California 91 360 (Received: February 22, 1985) The dissociation of IC1 in the presence of 02('A) was retarded significantly by trace amounts of H 2 0 and required the deactivation of a large number of 02('A) molecules.
Introduction Molecular iodine is known to dissociate in the presence of metastable 02('A) and the mechanism of the dissociation process has been the subject of a number of investigations.' Since the excitation energy of 0 2 ( l A ) is less than the dissociation potential of 12,as shown in Figure 1, some form of energy pooling is required to accomplish the bond cleavage. When I2 is dissociated by O2(IA), metastable I(2Pl 2) atoms are formed by rapid resonant energy transferZfrom Oj'A) to ground state I(2P3/2)atoms and subsequent energy pooling3between I(2P1/2)and 0 2 ( l A ) produces which has sufficient the more highly excited metastable 02('2), excitation to dissociate 12. Derwent and Thrush4 observed that the dissociation rate of I2 in 02('A) increased as I(2Pl12)appeared in the system. This behavior was attributed to dissociation of I2 by 02('2)in which the formation of 02('2)was the rate-limiting step of the dissociation process. The branched chain reaction was initiated by the slow energy pooling of two 0 2 ( l A ) molecules to yield O,(lX). Recent experiments, however, have shown that the rate of 02(*Z) quenchings by I2 is too small to account for all of the dissociation that occurs, and dissociation of I2 by 0 2 ( l A ) has also been observed in the presence of H20,6 which rapidly quenches O2('2).' Therefore, while the Derwent-Thrush or 02(12) mechanism may be important for initiating the I2 dissociation processes, other mechanisms are apparently responsible for the propagation of the chain reaction. Heidner' has recently proposed that energy transfer from I(2P1/2)pumps I2 to a vibrationally excited state which may be dissociated by further energy transfer from O#A) on subsequent collisions. This model explains the observations of Derwent and Thrush and accounts for the dissociation of I2 in the absence of 0,('2). A number of experiments were recently performed in our laboratory to determine if either of these mechanisms or another is capable of dissociating other iodine parent molecules (RI) by energy transfer from singlet oxygen. In addition to the energy transfer mechanisms discussed above, the dissociation of RI molecules may also proceed by chemical pathways. In these cases, I2 is formed by reactions between iodine atoms and the RI, with subsequent dissociation of the I2 by 0 2 ( l A ) . In most cases, the RI are too tightly bound relative to I2 to permit reaction with I(2P3/2);however, in some cases the reaction of I(2P1/2)with the RI may be exothermic. This mechanism, like the 0 2 ( ' 2 )and sequential pumping models, therefore relies on 1(2P1/2)as the carrier of a branched chain reaction. Our experiments were performed by coflowing trace amounts of each RI candidate species with singlet oxygen at low pressure, while observing the rate of appearance of I(2Pl 24P3/2) chemiluminescence at 1315 nm. The appearance of d2Pli2)measures thecate of RI dissociation because the iodine atoms resulting from the dissociation are rapidly equilibrated with the 02('A) by the ~~~
(1) Heidner, R. F.; Gardner, C. E.; Segal, G. I.; El-Sayed, T. M. J . Phys. Chem. 1983, 87, 2348, and references therein. (2) Derwent, R. G.; Thrush, B. A. Discuss. Faraday SOC.1972,53, 162. (3) Heidner, R. F.; Gardner, C. E.; El-Sayed, T. M.; Kasper, J. V. V. J . Chem. Phys. 1981, 74, 5618. (4) Derwent, R. G.; Thrush, B. A. J. Chem. Soc., Faraday Trans. 2,1972, 68, 720. ( 5 ) Aviles, R. G.; Muller, D. F.; Houston, P. L. Appl. Phys. Lett. 1980, 37, 358. (6) Heidner, R. F., private communication. (7) Stuhl, F.; Niki, K. Chem. Phys. Leu. 1979, 7, 473
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energy-transfer reaction. The RI species tested included ICl, ICN, CH31, CF31, C6HJ, and (CH,),SiI. The experiments were performed both with and without addition of H 2 0 to the flow to assess the role of 02(12)in the dissociation process, and both with and without I2present to determine the importance of I(2P1,2) to the dissociation kinetics. The results were compared to I(zP1 2) profiles that were taken by substituting an equivalent flow 0f/12 for the RI candidate. It was found that only IC1 was substantially dissociated by the singlet oxygen. This paper reports our findings in regard to the rate of IC1 dissociation in singlet oxygen, its sensitivity to the presence of H20, and the number of 0 2 ( l A ) molecules that are deactivated per each IC1 dissociation. Experimental Section The dissociation experiments were carried out in a 4 cm i.d. X 100 cm long flow tube reactor similar to the apparatus used by Heidner' to study the dissociation of 12. Dilute mixtures of IC1 and/or I2 in Ar carrier gas were radially injected into the flow tube through twelve 1-mm-diameter holes that were equally spaced around the circumference of the flow tube at an upstream location. The I2 and IC1 were initially entrained by the passage of Ar carrier gas through a bed of I2 crystals or IC1 liquid at room temperature. The pressure in the I2 and IC1 saturators was controlled by a vacuum regulator and the molar flow of the carrier gas was measured by an electronic mass flowmeter. The IC1 and I2 saturators were operated in series with an intervening cold trap and valves to bypass the carrier flow around either saturator. The purpose of the cold trap was to reduce the IC1 concentration in the flow to a level commensurate with the I2 concentration at room temperature. The concentrations of IC1 and I2 injected into the flow tube were calculated from the measured flow of carrier gas, the vapor pressure of the ICIBand IZ9 at the saturating temperature, the pressure in the saturator, and the corresponding pressure rise in the flow tube, which was measured by a capacitance manometer. A second flow of Ar carrier gas was bubbled through a room temperature H 2 0 saturator and was admitted to the flow tube along with the 02('A) upstream of the I2/IC1 injector. The concentration of H2O in the flow tube was determined in a manner similar to the 12 and IC1 concentrations. The flow tube was exhausted to vacuum through a large glass stopcock which served as a throttling valve. The plug flow velocity in the flow tube was calculated from the measured molar flows, the cross-sectional area, and the pressure in the flow tube. Two sources of 02(lA) were used in the flow tube experiments. The first was a microwave discharge in pure oxygen, which yields approximately 10% 02(1A).10To catalytically recombine 0 atoms formed in the discharge, a HgO film ring was deposited downstream of the discharge. The absence of significant amounts of 0 atoms ( 5 1012/cm3)in the flow tube was confirmed by addition of NO and noting the lack of NO2chemi1uminescence.I' The second 0 2 ( ' A ) source was a chemical generator12in which C12 was bubbled through a 1-in.-diameter vertical column of basic hydrogen peroxide solution under vacuum. This arrangement converts typically 90%of the C12 gas to oxygen, of which a sig(8) Stern, J.; Gregory, N. J . Phys. Chem. 1957, 61, 1225. (9) Calder, G. V.; Giauque, W. F. J . Phys. Chem. 1965, 69, 2443. (10) Benard, D. J.; Pchelkin, N. R. Reu. Sci. Instrum. 1978, 49, 794. (1 1) Fontijn, A.; Mayer, C. B.; Schiff, H. I. J . Phys. Chem. 1964.10, 40. (12) McDermott. W. E.; Pchelkin, N. R.; Benard, D. J.; Bousek, R. R. Appl. Phys. Lett. 1978, 32, 469.
0 1985 American Chemical Society
3270 The Journal of Physical Chemistry, Vol. 89, No. 15, 1985
Pritt et al.
- ' 0 2 x3z
TIME, lMSl
X11
02
I c1
12
Figure 1. Energy level diagram for ground and metastable excited states of I, 02,12, and ICI.
nificant fraction is in the IA state. The quiescent liquid column was approximately 5 cm deep a t the point of Clz injection. The generator was charged by slowly admitting 60 mL of 90% H202and 24 mL of 6 N NaOH, while bubbling N2 through the Clz inlet under dynamic vacuum conditions. After evaporative cooling to approximately 0 OC, approximately 1 mmol/s of C12 was admitted to the generator through a mass flowmeter. After passage through the liquid column, the reactor effluent rose through a 1 in. i.d. X 10 in. vertical cold trap chilled by a -78 O C dry ice-methanol bath. A gas-liquid separator was located immediately below the cold trap and about 25 cm above the quiescent liquid column to prevent splashing of the generator liquids into the cold trap. Upon exiting the cold trap, the gas passed through approximately 1 m of 1-in. i.d. Pyrex tubing, where it was tied into the upstream end of the flow tube in parallel with the microwave discharge source of O,(lA). The yield of 02(lA) from the chemical generator was determined to be 35 f 5% of the total gas flow by a comparison to the microwave discharge source of O,(lA), using a liquid nitrogen cooled intrinsic Ge detector that was filtered to respond to Oz(a X) radiation a t 1270 nm. The chemical generator was operable up to 5 min before the O,(lA) concentration began to degrade significantly. A second liquid nitrogen cooled intrinsic Ge detector, filtered to respond at 1315 nm, was tracked along the flow tube downstream of the radial injector to record the I ( 2 P I / 2 ~ P&3emission profiles. Reference data were generated by reacting d.085 mtorr of I2with 68 mtorr of H20, 1024 mtorr of microwave discharged 02,and 1507 mtorr of Ar carrier gas. These conditions closely approximate similar experiments1 by Heidner in which the I2is totally dissociated in approximately 100 ms. Comparison of the reference data to the analytical modeling expression derived by Heidner gave good agreement for an assumed O#A) yield equal to 9% of the O2that was passed through the microwave discharge. Less satisfactory agreement was obtained for assumed 02('A) yields of 8 and 10%. When the chemical generator was used as the 02(lA) source, the concentration of H 2 0in the flow tube was determined by a correlation between the intensities of the 0 2 ( a X) and 02(b X) radiation at 1270 and 760 nm, respectively. In the absence of I atoms, O2(lZ) is produced by slow O2(IA) energy pooling and is destroyed by quenching due to H20. Therefore, the intensity of 0 2 ( b X) radiation is proportional to the square of the O,(lA) concentration and inversely proportional to the H 2 0 concentration. The Oz(b X) emission was detected by a filtered GaAs photomultiplier tube that was located at the downstream end of the flow tube; the calibration factor was determined by using the microwave discharge source of O,(lA) and known flows of HzO from the water saturator. The concentration of HzO in the effluent of the chemical generator was approximately 10 mtorr at the start of a run and increased with time to a maximum of 70 mtorr.
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-
-
-
-
Results and Discussion The microwave discharge source of 02('A) was used to investigate the initial rate of IC1 dissociation. In these experiments,
Figure 2. I(2P1/2)appearance profiles following addition of 0.15 mtorr of I2 (open symbols) and 0.30 mtorr of IC1 (solid symbols) to approximately 50 mtorr of O,('A). The circles, triangles, squares, and hexagons refer to 0.0, 3.8, 5.6, and 32 mtorr of H 2 0 present in the flow tube,
respectively. trace amounts of IC1 (typically 0.3 mtorr or less) were added to approximately 50 mtorr of O2(IA). The IC1 concentration was maintained at a low level to prevent a significant loss of O2(IA). As a result, the I('P1/2) profiles rose to a nearly constant plateau which did not decay significantly along the length of the flow tube. Under these conditions, measurable I(2P1/2)concentrations were only obtained for HzOconcentrations less than approximately 40 mtorr. The dissociation of IC1 was found to be insensitive to the addition of C12 flows that were comparable to the initial 02(A) concentration. Figure 2 shows that the concentrations of I(2P1j2) that were obtained upon addition of either IC1 or half as much Iz to the O2(lA) were nearly equivalent, indicating that the IC1 was completely dissociated. The actual rate of dissociation of the ICl, however, was much slower than the rate of I2 dissociation. In the absence of HzO, the dissociation of I2 was essentially complete in less than 10 ms, whereas the dissociation of IC1 was 90% complete at approximately 40 ms. The presence of trace quantities of HzO in the flow tube also retarded the dissociation of IC1 more severely than the dissociation of I2 The time required to dissociate 50%of the IC1 increased at the rate of approximately 25 ms/mtorr of HzO in the flow, whereas the equivalent rate for dissociation of 1 2 under similar conditions is approximately 1 ms/mtorr of H20. This result implies that the dissociation of IC1 does not proceed primarily through a reactive channel to form Iz, since in that case the rate of the IC1 dissociation process would depend upon the rates of the reaction of I(2P1/2)with IC1 and the subsequent dissociation of Iz by 02(lA), neither of which are significantly retarded by trace quantities of H20. The effect of H20on the dissociation of IC1 is more easily explained by a greater dependence on O2(I2) than is characteristic of Iz dissociation by 02(lA). Although 02(lZ) cannot dissociate IC1 directly, as shown in Figure 1, energy transfer from O,(lZ) can populate the 3111 and 3112states of ICl, which may then be pumped to dissociation by subsequent collisions with 0 2 ( A). Alternatively, quenching of I(2Pl/z)by IC1 may produce vibrationally excited IC1 molecules that could also be pumped to dissociation by Oz(12). The initial rate of IC1 dissociation was substantially accelerated by the addition of small amounts of I2 to the flow, indicating that I(2Pl/z)plays a rate-limiting role in the dissociation process. When no Iz was added, the dissociation of IC1 was undoubtedly initiated by the I(2Pl,z) that resulted from the dissociation of the Iz which is present as a 1% impurity in the ICl. Figure 2 also shows that, once initiated, the rate of IC1 dissociation increased with time until all of the IC1 was dissociated, which suggests that the dissociation of IC1 proceeds by a branched chain reaction. These results are consistent with the dissociation of IC1 through energy transfer from O,(IZ) that is produced by energy pooling between O,(lA) and I(2P,/z). The chemical generator was used as the source of 0 2 ( l A ) for investigating the efficiency of the IC1 dissociation process. Much larger concentrations of IC1 were more rapidly dissociated by the increased 0 2 ( l A ) concentrations that were obtained from the chemical generator. In these experiments, the peak of the I(zPl,2)
J . Phys. Chem. 1985,89, 3279-3285
0.030
I
I
10
20
30
1
I
40
60
I
60
TIME (MSEC)
Figure 3. 1(2P1,2)decay profiles following addition of IC1 to 310 mtorr of 02(lA) with 10-70 mtorr of H20. The circles, triangles, squares, and hexagons refer to 9.8, 6.5, 3.2, and 1.6 mtorr of ICl, respectively.
profile moved forward in the flow tube as the IC1 flow was increased. In this respect, the behavior of IC1 is similar to 12,which also dissociates more rapidly in larger concentrations. This result is expected whenever I(2P1/2)plays a rate-limiting role in the dissociation process. Upon adding up to 10 mtorr of IC1 to 310 mtorr of chemically generated 02('A), the I(2PI,2)profiles decayed
3279
significantly within the flow tube and the peak I(2P1/2)concentrations did not scale linearly with the initial concentration of IC1. These results implied that a substantial fraction of the initial 02(lA) was consumed in the process of dissociating the ICI. To quantify this effect, the I(zPl/2)concentrations were normalized to the IC1 flows and plotted as a logarithmic function of time in the flow tube. As shown in Figure 3, the I(2Pl/2)decays following the dissociation of IC1 were found to be exponential in character, with decay rates that were proportional to the IC1 concentration with a slope of approximately 9000 torr-' s-I. This loss of I(2P,/2) reflects a much greater loss of OZ('A), since these species are maintained in equilibrium by the rapid energy-transfer reaction. The loss of I(2P1/2)and O,(lA) at long times in the flow tube can be explained by the quenching13of I(zP1/2)by H20. There is also a substantial loss of I(2P1/2)and 0 2 ( l A ) at early tiimes in the flow tube, as can be seen in Figure 3 by the extrapolations of the exponential decays back to t = 0. The reduction of the t = 0 intercept with increasing IC1 concentrations corresponds to the deactivation of approximately 20 02('A) molecules for each IC1 molecule added to the flow. Under similar conditions, the dissociation of I2 is known to occur with a much smaller loss of 02(1A).6 The inefficiency of the IC1 dissociation process may be explained either by a low branching ratio in one of the energytransfer steps or by significant deactivation or quenching of 0 2 ( l Z ) or one of the intermediate states of IC1. In summary, the dissociation of IC1 by O,(lA) appears to proceed primarily through O,('Z) as an intermediate and is markedly slower and less efficient than the dissociation of I2 by 02(lA).
Acknowledgment. This work was supported by the Air Force Weapons Laboratory under Contract AFWL-29610-82-C-0082. Registry No. ICI, 7790-99-0; 02,7782-44-7; H 2 0 , 7732-18-5. (13) Grimely, A. J.; Houston, P. L. J . Chem. Phys. 1978, 69, 2339.
Langmuir Constants for Spherical and Linear Molecules In Clathrate Hydrates. Validity of the Cell Theory Vijay T. John* Department of Chemical Engineering, Tulane University, New Orleans, Louisiana 70118
and Gerald D. Holder Department of Chemical and Petroleum Engineering, University of Pittsburgh, Pittsburgh, Pennsylvania 15261 (Received: August 13, 1984)
The Lennard-Jones and Devonshire (LJD) cell theory as applied to the calculation of Langmuir constants for spherical and linear guest molecules in hydrate cavities is critically examined. Using the crystallographic locations of the host water molecules, and modeling binary guest-host interactions by Kihara type. potentials, we carried out more precise calculations for Langmuir constants, and the results are compared to Langmuir constants obtained by smearing the host water molecules over the surface of a sphere (the LJD approach). The disparity between the two methods of calculation is especially pronounced for the large cavity of structure I hydrate, which is the most asymmetric of hydrate cavities. Further, the variation of Langmuir constants obtained from the two methods is dependent on the Kihara effective size and energy parameters used. The results may be significant to the precise determination of gas hydrate phase equilibria.
Introduction Gas Hydrates are clathrate inclusion compounds formed by water (the host) and low molecular weight gases (the guest). Thermodynamic stability of the inclusion complex is due to dispersion interactions betwen the enclathrated guest molecule and the host lattice made up of hydrogen-bonded water molecules. 0022-3654/85/2089-3279$01.50/0
Hydrates of nonpolar gases crystallize in one of two structures commonly referred to as structures 1 and 11. Lattice properties of these structures are shown in Table I which indicates that both structures are characterized by small and large cavities. The formation of either structure I or structure 11is dependent on the relative stabilities of the structures when a given gas (or gas 0 1985 American Chemical Society
