4170
J. Phys. Chem. 1982, 86, 4170-4175
(a’3Z+,u’=3-6),and the more similar amounts of CH(A2A) and CO(a’3Z+). So far, all of our results derive from products which require an interaction of the terminal oxygen atom with the carbon ?r-electron system. Although we have not yet sampled what we believe may be the major product channels, we have found nothing unusual in the internal states of those species which we have observed.
future research. The interaction of the radical center on the terminal oxygen atom with the carbon ?r-electron system can be significant, since the high vibrational energy content of the 02C2Hintermediate can move the terminal oxygen atom toward the carbon-carbon *-bond system with considerable velocity. For species which derive from this interaction, we see little reason for specificity in the product channels leading to CO + HCO, 2CO + H, and CH + C02,and it is possible that phase space predictions of the chemically distinct product channels and product excitations will provide realistic estimates. The OH + C20 product channel requires that the terminal oxygen seek out the hydrogen atom in preference to its other opportunities, and we will look for this product channel in future experiments. The channel leading to HCCO + 0 is interestingg in that reaction is via a loose transition state, and RRKM estimates of k(E)for this channel may be more accurate than for the other product channels. In our experiments, we found that C2Hvibrational excitation did not influence its rate of removal, but this vibrational excitation was carried over into product degrees of freedom. Vibrational excitation of C2H enhanced both the amount of CO(a’3Z+,u’=5)relative to CH(A2A)and the production of CO(a’3Z+,u’=6),thus showing a tendency to excite more energetic species. The constancy of kR is in agreement with the radical-radical nature of the initial encounter, and such constancy has been noted for systems as simple as CN(v) + H2, in which the reagent vibration does not involve the reaction c ~ o r d i n a t e .The ~ ~ tendency for reagent vibrational excitation to carry over into product excitations, both electronic and vibrational, is in qualitative agreement with phase space predictions, as is the large amount of C02+ relative to both CH(A2A) and CO-
Conclusion We have studied in some detail the chemiluminescent product channels of the reaction of C2Hwith OF The C2H species which is responsible for the products that we obterve is in either the ground X2Z+electronic state or the A211low-lying electronic state which can be produced by both UV photolysis as well as IR MPD of the C2H precursors. Reaction probably proceeds via a peroxy intermediate, whose lifetime may be sufficiently long to allow significant nuclear motion to occur. The reactions of C2H continue to be of interest to us, and we plan to detect ground-state products (CH, OH, C20) by LIF and to optically pump the 02C2Hintermediate, thereby altering the product channels. We would like ultimately to determine the potential energy surface in the regions near the barriers and to apply our experimental techniques to other prototypical systems germane to combustion. Acknowledgment. We have benefited from discussions with J. Tiee, J. McDonald, Y. Haas, and S. Filseth and from the expert technical assistance of R. Senaha. This research was supported by the Air Force Office of Scientific Research and the Office of Naval Research. We acknowledge the generous loan of equipment by the San Francisco Regional Laser Center.
Kinetic Study by Electron Paramagnetic Resonance and Mass Spectrometry of the Elementary Reactions of Phosphorus Tribromide with H, 0, and OH Radicals J. L. Jourdaln, 0. Le Bras,’ and J. Combourleu Centre de Recherches sur la Chimie de la Combustion et des Hsutes Temphratures (C.N.R.S.), 45045 Orlhans Cedex, France (Received: February 10, 1982; In Final Form: June 21, 1982)
- -
The reactions of phosphorus tribromide with H, 0, and OH were studied in a discharge flow reactor coupled to an EPR and a mass spectrometer. The following rate constants were obtained at 295 K: (1)H + PBr3 products, kl = (1.7 f 0.3) X lo-”; (2) 0 + PBr3 products, k 2 = (3.6 f 0.5) X (3) OH + PBr3 products, k 3 = (8.5 f 0.5) X (in units of cm3 molecule-’ s-l). The products were analyzed by EPR and mass spectrometry. Br atoms were detected in reactions 1-3 and P atoms in reaction 1. The mechanism of the initial step of each reaction, deduced from the analysis of the reactants and products curves, showed that reaction 1 proceeds via an abstraction mechanism while reactions 2 and 3 proceed via a mechanism of atom exchange.
-
Introduction Elementary reactions of H,0, and OH with some halogenated compounds of boron and $”rus, BC13,BBr, and PC13 have recently been studied in our laboratory1-3 as these compounds, and also the presently considered compound PBr3, are potential flame inhibitors.4 Also, (1) J. L. Jourdain, G. Laverdet, G. Le Bras, and J. Combourieu, J. Chim. Phys., 77, 9, 80. (1980). (2) J. L. Jourdain, G. Laverdet, G. Le Bras, and J. Combourieu, J . Chim.Phys., 78, 3, 253 (1981). (3) J. L. Jourdain, G. Le Bras, and J. Combourieu, J.Phys. Chem.. 85, 655 (1981). 0022-3654/82/2086-4170$01.25/0
these reactions might be related to flame retardant processes since it is now suggested that the inhibiting effect of several flame retardants added to flammable materials takes place at least in part in the gas phase by reactions involving the flame Propagating radicals H, 0, and O K 5 Then the rate constants and pathways of these reactions need to be known for modeling purposes in order to estimate the potential inhibiting effect of such compounds. (4) G. Lask and H. Gg- Wagner, Symp. (Znt.) Combust., [Proc.], 12th, 8, 432 (1960). ( 5 ) J. W. Hastie and C. L. Mc Bee, ACS S y m p . Ser., No. 16, 118 (1975).
0 1982 American Chemical Society
Reactlons of H, 0, and OH whh PBr,
The Journal of Physical Chemistry, Vol. 86, No. 21, 1982 4171 k : ( 16" cm'
1,
0I
I
II
1
moiecuie' i')
3-
2-
Figure 1. Diagram of the apparatus.
In this work, we report the rate constants at room temperature and also the mechanisms for the reactions of PBr3 with H, 0, and OH in a discharge flow reactor. EPR was used to analyze the reactants H, 0, and OH and also some reaction products. With this new set of determinations we now have a complete comparative knowledge of the reactivities of BC13,PC13,BBr,, and PBr, toward H, 0, and OH.
Experimental Section The EPR spectrometer (Varian E 112, equipped with a 1-in. cylindrical cavity) and the data acquisition system have been described el~ewhere.~ The reactor (Figure 1, 22-mm i.d.) is fabricated from teflon in order to limit some important wall reactions which were encountered in previous studies?$ This reactor crossed the EPR cavity which was working under the q,' mode ,at a frequency of 9.003 GHz. A quadrupole mass spectrometer was interfaced downstream from the EPR cavity at the end of the reactor. It was used to study the profiles of reactants and products and to test the purity of reactants. Kinetic data and physical parameters (concentrations, flow rate, pressure) were processed on-line by a 32K microcomputer. EPR signals could be doubly integrated and compared to reference spectra of NO (for OH radicals) or O2 (for H, 0, Br, and P atoms) for quantitative measurements by using the transition probabilities reported by Westenberg? The flow rate in the reactor was set at about 40 m/s at P = 0.7 torr, in order to work with a reaction zone as extensive as possible and much longer than the absorption domain of the EPR cavity (90% of the absorption occurred on a 2-cm axial distance) as previously mentioned. The reaction zone had a length of between 10 and 30 cm depending on the initial conditions. Radial diffusion was fast enough to justify the assumption of plug flow. The experimental rate constants were corrected for axial diffusion with the coefficients of diffusion of H, 0, and OH in he1ium6s7 The gases flowed through two concentric movable inlets, the central one being used for PBr, and the external one for pressure measurements (MKS baratron) or for NO2flow. The pressure was taken as the average of measurements made at each end of the reaction zone so that the effect on the rate measurements of the pressure drop due to viscosity would be negligible. H and 0 atoms were generated from the dissociation of the diatomic molecules diluted in helium (which was first passed through a liquid nitrogen trap to remove impurities) in a microwave discharge. They then flowed into the reactor via a side arm located upstream from the reaction zone. PBr3 is a liquid at room temperature with a low vapor pressure (2 torr at 20 "C) and particular care was taken with the pressure (6)B. Khouw, J. E. Morgan, and H.I. Schiff, J . Chem. Phys., 50,66 (1969). (7) R. S.Yolles and H.Wise, J. Chem. Phys., 48,5109 (1968).
I
I
I
I
I
I
I
4
6
8
10
12
14
+
Figure 2. Reaction H PBr, products (1). Plot of the experimental value of the rate constant as a function of the initiil ratio of reactants. -+
measurements to avoid errors in the concentrations (PBr3 was diluted to 20 torr by argon in a 10-L bulb).
-
Results and Discussion Reaction H + PBr3 Products (1). Two mechanisms can be considered for this reaction, the abstraction process leading to HBr and PBr2 and the atom exchange mechanism leading to Br and PHBr2. Occurrence of one mechanism or the other cannot be decided from thermodynamic considerations, because both steps appear to be exothermic and consequently may have a nonnegligible rate constant at room temperature. The heat of reaction cannot be calculated from the heat of formation of the species since these data are not known for PBr2and PHBr,. However, the heat of reaction AH can roughly be estimated from the bond energies. For the reaction H + Br-PBr2 H-Br + PBr, (la) AH = E(P-Br) - E(H-Br) = -25 kcal/mol. For the alternative channel H + Br-PBr2 H-PBr, + Br Ob) AH = -15 kcal/mol. E(P-Br) and E(P-H) are taken as one-third of the total atomization energy of PBr3 and PH,, respectively. So the two channels are possible at room temperature and previous have shown that the two kinds of mechanisms are likely to occur. The experiments were carried out under the pseudofirst-order conditions with initial concentrations [PBr3], >> [HI,. The initial concentrations of H atoms were 2.4 X 1012-6.6X 10l2cm-3 and [PBr,] was varied from 1.87 X lo1, to 4.44 X 1013~ m - ~ The . ratio R = [PBr,],/[H], was varied between 4 and 16. The pressure was 0.5 torr and the flow velocity 47 m/s. Results (Figure 2) show an increase of the rate constant k = (-d In [H]/dt)/[PBr,], for the lower values of the ratio R. For R higher than 8, k remains constant. Such a result, already noticed for the reaction between H and PC13,1 can be explained by the occurrence of fast secondary reactions of H with products of the initial and/or secondary steps. These reactions would be fast and the PBr3 concentration is undoubtedly not, high enough to keep their effects negligible compared to the initial step. The rate constant k, of the initial step was calculated at room temperature as the average of the values corresponding to R > 8. kl was not calculated by means of the slope of the straight-line d/dt In [HI = f ([PBr3],), as is usually done, because of the too-restricted range of the initial concentrations of PBr3 which could be considered. With 20 values, the rate constant was found to be kl = (1.7 f 0.3) X lo-'' cm3 molecule-' s-' (these units will be used for all the bimolecular rate constants reported in this study). The error is two standard deviations. This value represents the actual value of k , since it cannot in
-
Jourdain et ai.
The Journal of Physical Chemistry, Vol. 86, No. 21, 1982
4172
I
I
‘
-
I
1 11, I
u
2-
I
I’
I
I
2
4
6
L 8
:IC?,) I
I
10
I 2
I
4
+
Flgure 4. Reaction H PBr, the fdlowing initial conditions: [H],/[PBr,], = 1.8.
-
I
I
I
6
8
10
I
products (1). Br and P curves with
[HI, = 3.6 X
lo1,, [PBr,], = 2 X
For a modeling purpose, a numerical program of integration was used. The predictor-corrector method of Haming was used to solve the system of ordinary first-order differential equations elaborated by the program from a set of elementary reactions and initial conditions. The simulation of P atom formation under conditions of Figure 3 unambiguously shows that these atoms are formed in an abstraction mechanism and not an exchange one. The following gas-phase mechanisms were tried: for abstraction: PBr,
-+ -- + H
H
PBr2
HBr
H
PBr
H2
H
P
Br
for exchange: PBr,
- - - - H
PHBr,
H
PH2Br
H
PH,
H
PHz
H
H
PH-P
For the exchange mechanism, the rate constant of the reaction H + PH3 was taken as 3.3 X (ref 10) and the value 5 X 10-l’ was assigned to the rate constants of the other reactions. As the H atom concentration was higher than that of PBr,, only reactions involving H were considered in the mechanism under study. Even with such high rate constants the P concentration calculated (which is an upper limit) was much lower than that observed in the first milliseconds of the reaction (at t = 2 ms, [P]c&d = 0.6 X 10l2 whereas [ P I o b d = 2.8 X 10l2). Then the simulation qualitatively shows that this mechanism has to be disregarded in favor of the abstraction one which led to better results. Moreover, several experimental data agree better with the abstraction mechanism (stoichiometry, Br atom formation). However, the mechanism does not seem to consist only of the three stripping reactions of PBr, by H and the H + HBr step. Reactions of P and Br also occur because P and Br concentrations are lower than expected from the amount of PBr, reacted. For instance in Figure 3, P and Br yields are three times lower than expected from the simple abstraction mechanism and Figure 4 shows decay curves of P and Br after a few milliseconds. The reactions of P atoms could be successive recombinations. M
P+P-Pp, P2
+ P*
M
P,
P4 vapor could condense into white phosphorus which would be transformed into red phosphorus. This mechanism is based on the P2+detection by mass spectrometry (8) A. A. Westenberg, h o g . React. Kinet., 7, 24 (1973). (9)M. A. A. Clyne, P. B. Monkhouse, and L. W. Townsend, Znt. J . Chem. Kinet., 8, 425 (1976).
(10)J. H.Lee, J. V. Michael, W. A. Payne, D. A. Whytock, and L. J. Stief, J. Chem. Phys., 66, 8, 3280 (1976).
Reactions of
H, 0, and OH with PBr3
The Journal of Physical Chemistry, Vol. 86, No. 21, 1982 4173
0 + PBr, or
0 + PBr, followed by
-
0 + BrO
-
1
+
2
3
Flgure 5. Reaction 0 PBr3 products (2). Least-squares plot of the apparent first-order rate constant vs. [PBr,],: k , = (3.6 f 0.5) X lo-'' and k2(0)= (28 f 50)s-'.
which could be representative of P2 and/or P4,and the observation of a red deposit on the wall of the reactor after a series of experiments. For Br atoms, we can suggest possible successive additions to P, PBr, PBr,, and PBr,:
+ Br PBr, + Br PBr3
(M)
PBr,
(+M)
PBr5
PBr5 is a solid and can exist as a transient species in the gas phase at low pressure. These assumptions are in agreement with the curves of Figures 3 and 4. In Figure 3 after 1ms, when no more PBr3 remains, Br is constant, while in Figure 4, where PBr, is still present after several milliseconds, a decay of Br is observed. Also crossed reactions between radicals cannot be totally excluded in the mechanism. But under conditions of high excess of H over PBr3 these reactions and Br atom steps would become negligible. Reaction 0 PBr3 Products ( 2 ) . In these experiments, the pressure in the reactor was increased to 0.7 torr in order to work with an unresolved sextet line of 0 atoms which gives a better sensitivity for detection. The concentration of 0 atoms was in the range 1.2 X 10l2to 2.1 X lo', cm-, and PBr3 was varied from 1.18 X lo1, to 3.19 X lo', cm-,, so that the ratio [O],/[PBr,], varied from 10 to 22 and 27 decay constants were measured. The rate constant was deduced from the slope of the straight line 4 In [ O ] / d t = f([PBr,],) (Figure 5) and the following value was found: k2 = (3.6 f 0.5) X 1O-l1, the value of the intercept being k,!,, = (28 f 50) s-l. The precision is two standard deviations and the maximal correction due to axial diffusion was 8%. We point out that the value of the intercept is included in the error domain so that no particular significance was given to its value concerning the occurrence of wall reactions of oxygen atoms. The study of the products was done by EPR and by mass spectrometry. Br atoms were detected by EPR and were found to appear after a short reaction time, their concentration remaining constant afterward. The kinetics of Br atom production and oxygen atom consumption were well correlated. The yields of Br production and 0 consumption were the same for the different ratios [O],/ [PBr,], considered (excess up to 113 of 0 over PBr3 or PBr3 over 0). P atoms were also looked for by EPR but no signal could be observed. Mass spectrometry showed a decrease of the P+peak which was probably representative of PBr, and an increase of the PO+ peak when the reaction was occurring under conditions where 0 atoms were in excess compared to PBr3 From the analysis of the products, Br atoms could be produced from
+
-
+ Br
(2a)
+ PBr,
(2b)
POBr,
BrO Br
+ 0,
(BrO could also partly recombine if 0 atoms are not in sufficient excess over PBr,.) A numerical simulation of both mechanisms showed that is was not possible to get correct profiles of Br atoms with reaction 2b followed by 0 + BrO. The calculated production of Br was too low by a factor of 15 compared to the experimental value for a ratio [PBr,],/[O], = 13. Exchange reaction 2a gave a better agreement. The absence of P atoms with both EPR and mass spectrometric detection also supports mechanism 2a. The production of P atoms via reaction 2b followed by Br abstraction steps by 0 atoms would not occur. However, even if P atoms were produced in this way, they probably would not be detected as a result of their reaction with 0,: P
+ 0, -*PO + 0
(4)
for which k4 = 1X lo-', (ref 11) (0,which is undissociated in the discharge is an important constituent of the reacting medium). In the case of occurrence of mechanism 2b the PO+ peak could be representative of the PO radical formed in reaction 4 and not of POBr, or other compounds produced by the mechanism initiated by reaction 2a. However, the Br atom simulation appears to favor reaction 2a. This conclusion is also supported by thermochemical considerations: reaction 2a would be exothermic by about 78 kcal/mol, whereas reaction 2b would be endothermic by about 6 kcal/mol. (The heats of reaction were calculated by assuming that the Br-PBr, bond energy in PBr, was equal to one third of its atomization energy.) Thus we can conclude that reaction 2 proceeds via the initial step 2a which can be either an initial step or the result of the following sequence: 0 PBr, POBr3* POBr, Br. The addition step giving POBr, would be analogous to the mechanism found for the reaction of 0 with PC13 where POCl, was identified by mass spectrometry as a product of the reaction.' The mechanism appears to be different with PBr, because of the fast generation of Br atoms which indicates that reaction 2a would be an elementary step or that POBr, would decompose very rapidly under the conditions used in the reactor. Consequently, even if POBr, is formed, its concentration would be very low and in any case POBr, could not be detected by mass spectrometry because of the restricted mass range. Reaction OH PBr3 Products (3). OH radicals were generated by the classical method H + NOz NO OH. NOz flowed through the intermediate inlet, in excess compared to H atoms so that no H atom remained in the reactor zone where PBr3 was added. The reaction was studied at a pressure of 0.5 torr. the OH concentration was 1.1X 10"-1.35 X 10" cm-, downstream of the production zone, and the PBr, concentration was varied from 2.8 X 10l2 to 1.63 X lo1, cm-,so that the ratio [PBr3],/ [OH], varied from 21 to 141. Thirtyone decay constants were measured and the rate constant, derived from the slope of the straight line -d In [OH]/dt = f([PBr,],), was found to be k3 = (8.5 f 0.5) X 10-l' and the intercept k3(0, = (47 f 56) s-l (Figure 6). The precision is twice the standard deviation and the correction due to axial diffusion
+
+
-
-
+
+
+
+
(11) Y. 0.Connor and M. A. A. Clyne, J . Photochem., 17, 1/2, 44 (1981).
Jourdain et ai.
The Journal of Physical Chemistty, Vol. 86, No. 27, 1982
4174
TABLE I : Rate Constantsa Obtained at 295 K for the Reactions of BCl,, BBr,, PCl,, and PBr, with H , 0, and OH BC1, BBr, PC1, e PBr, f
~~
H 1 x 10-15
k E
2 7 0 0 i. 5 0 0 ( 2 9 5 - 8 0 7
k E
5.2 x 10-13 2 2 0 0 I500 ( 2 9 5 - 7 9 3 K )
(5.9 t 0.8)
k
( 4 . 1 i 0.6) X
(1.4
(2.8 i. 0.5) X 10.'' 1 8 0 0 i. 500 ( 2 9 5 - 4 8 8 K )
K)
(2.4 t 0.4)
x
(1.7
i.
0.3)
x 10-l'
( 3 i. 0.5) X
lo-''
(3.6
i.
0.5)
x
(3.8 i. 0.5)
x
(8.5 i. 0.5)
0
x
OH ?:
0.4) x lo-''
10'''
lo-''
x lo-''
k,(,) = 6 2 8 S-' kNteflon) = 2 2 0 S"
k , is the intercept of the straight line -d/ a Units are cm3 molecule-' s-' for k and calimol for the activation energy E . dt In [ O H ] = f ( [BBr,],). Reference 2. Reference 3 . e Reference 1. f This work,
I
I 12
Br
10
10
/ I I
I / 8- 1 I I
2
1
I
1
4
6
8
+
-
I I
1
10
12
I
14
I
products (3). Least-squares plot Flgure 6. Reaction OH PBr, of the apparent firstorder rate constant vs. [PBr,],: k, = (8.5 f 0.5) X kqo, = (47 f 56) s-'.
was less than 6.5%. The products were studied by EPR since no significant results could be obtained by mass spectrometry. As for reactions 1 and 2, Br atoms were detected by EPR and the curves of production were measured for different ratios of initial concentrations of reactants. Figure 7 gives an example of Br atom production and OH consumption curves with [OH], in excess over [PBr,],. It can be seen that important production of Br if we assume that occurs ([Br] reduced = l.7[PBr3]0,reacted) PBr, is totafiy reacted in less than 6 ms under such conditions. This result suggests that reaction 3 proceeds via an exchange mechanism: OH PBr3 POH Br, Br (34
+
-
and not by the abstraction step OH PBr3 PBr,
+
+
+ BrOH
(3b)
This is confirmed by the fact that reaction 3a is expected to be rather strongly exothermic, whereas reaction 3b would be endothermic by 7.5 kcal/mol (value deduced from the Br-OH bond energy and the PBr2-Br bond energy previously defined). Concerning the secondary reactions, we made an attempt to get some further information from the OH and Br profiles. In the first milliseconds of the reaction the interpretation is difficult as a result of undefined mixing conditions and complex mechanisms due to the presence of 0 and H atoms produced by the OH recombination OH + OH HzO + 0 (5) followed by 0 + OH 0 2 + H (6) In the range 3-6 ms, where the mixture was homogeneous and the gas-phase recombination less important, simulation was easier to perform. The calculations considered 4
+
67
I
I
1
3
+
-
1
I
1
5
7
9
Flgwe 7. Reaction OH PBr, products (3). OH decay curve and Br atom production under the following initial conditions: [PBr,], = 6.6 X lo1', [OH], > 1.8 X
steps 5 and 6, the wall recombination of OH, and the three exchange reactions: PBr,
OH
POHBr,
OH
P(OH),Br
OH
P(OH)3
The resulta of the calculations were very dependent on the rates considered for the two secondary exchange steps leading to P(OH),. The rate for wall recombination of OH was in the range 30-50 which is a reasonable value for a teflon wall in the presence of PBr, according to the experimental value of k3,ol. The simulation showed that good agreement between simulated and experimental OH and Br curves was obtained if rate constants of about 2 X were considered for the reactions of OH with POHBr, and P(OH),Br. For higher values, the ratio [Br] produced/[OH] reacted was lower than the experimental one. These data have led us to suppose that the mechanism would be the sequence of the three exchange reactions leading to P(OH),, for the two secondary steps having a rate constant of 2 X But if these rate constants are higher, the occurrence of secondary addition steps of OH and Br with radicals such as POH Br, also has to be considered. Conclusion A summary of the rate constants measured for the reactions of H, 0,and OH with the halogenated compounds of boron and phosphorus BCl,, BBr3, PC13, and PBr3 is given in Table I. The values reported in this work for the reactions of H, 0, and OH with PBr3 confirm the previous conclusions that the reactivity increases for these compounds from chlorine to bromine and from boron to phosphorus. This can be related to thermochemical con-
J. Phys. Chem. 1982, 8 6 , 4175-4178
4175
a halogen atom. For OH reactions, an exchange mechasiderations of the bond energies which decrease from B-Cl, P-C1 to B-Br, P-Br and from B-C1, B-Br to P-C1, P-Br nism was always found and this result is in accordance with (these bonds are broken during reaction for all the studied the thermochemistry. reactions except for one; also B-Cl represents, for example, Application of the different kinetic data obtained to the C1-BC12 bond in BCl,). Thus, as expected, PBr3 was flame inhibition allows us to estimate the potential infound to exhibit the highest reactivity toward H, 0, and hibiting effect of these compounds. Comparative calcuOH. I t can also be noted that, in the flow reactor, halolations of reaction rates show that reactions of H and OH genated compounds of boron, BC13 and BBr,, are able to with PCl,, BBr,, or PBr, and OH with BC13 would compete give, by reaction with OH, important wall p r o ~ e s s e s , ~ ~ ~with branching and propagation reactions in flames at while no such effects were observed with PC13 and PBr,. relatively low temperatures. Furthermore, the identifiIf we consider the 12 cases in Table I, the mechanism cation of the reaction pathways also indicates that the was experimentally found to be atom (or radical) exchange products could also act as inhibitors: HC1 and HBr formed for eight reactions, Br abstraction for three reactions, and in halogen abstraction reactions of H atoms are efficient addition for one reaction. H atom reactions apparently in trapping H, 0, and OH propagating radicals in flames. proceed by Br or C1 abstraction except for reaction of H C1 and Br atoms produced in exchange reactions are also with BBr,. Although this result appears to be anomalous, efficient for H atom depletion in the low-temperature reit could be justified by considering that the abstraction combination zone of the flame (H + C1, Br HCl, HBr step is more thermochemically favorable compared to the followed by H, 0, OH + HC1, HBr C1, Br). exchange reactions for all cases except H with BBr,. For Thus the different kinetic data obtained in ref 1-3 and the reactions of 0 atoms, the exchange mechanism was in this work show that BCl,, BBr,, PCl,, and PBr, can be always observed except for the reaction 0 + PCl,, and this potential inhibitors and their specific effect could be esreaction was found to proceed by an addition step. This timated from these data for different kinds of flames. conclusion also agrees with thermochemistry because these These data could be also useful in understanding the flame reactions are exothermic and all of the halogen abstraction retardant chemistry of polymers, provided it is released steps are endothermic and are therefore unlikely in the into the gas phase where the polymer undergoes comtemperature range considered. It can also be noted that bustion. Then these halogenated compounds could inhibit the only reaction leading to addition, 0 + PCl, POCl,, the flame in the gas phase in a way similar to that which gives the only stable product. However, similar species has been observed for the Sb-halogen combination.12 This might have been generated from the reactions of 0 atoms assumption is indeed very speculative and will remain so with the other studied compounds. Clearly, further study until flame structure studies of such systems are carried such as pressure dependence measurements could indicate out. if reactions of 0 atoms with BC13, BBr,, and PBr, really do proceed through one-step exchange or through longer-lived intermediates (BOC13*,BOBr3*,or POBr3*)which (12)W. E.Wilson, J. T. Donovan, and J. R. Fristrom, S y m p . (Int.) respectively decompose into BOC12,BOBr2,or POBr2 and Combust., [ R o c . ] ,12th, 432 (1960).
- -
-
Enthalpies of Decomposition and Heat Capacities of Ethylene Oxide and Tetrahydrofuran Hydrated D. G. Lealst,” J. J. Murray, M. L. Post, and D. W. Davldson National Research Council of Canada, Ottawa, Ontario, Canada K1A OR9 (Received: March 18, 1982; I n Final Form: June 22, 1982)
Heat capacitiesat temperatures between 120 and 260 K and enthalpies of congruent melting have been determined for clathrate hydrates of ethylene oxide and tetrahydrofuran (von Stackelberg’sstructures I and 11, respectively). A Tian-Calvet differential heat conduction calorimeter was used. Enthalpies of decomposition and estimates of the heat capacities of the hydrate lattices and enclathrated guests are reported.
Introduction Clathrate hydrates are nonstoichiometric solids formed from mixtures of water and low molecular weight gases or l i q ~ i d s . l - ~The hydrate structure contains cavities enclosed by a lattice of hydrogen-bonded water molecules. Within the cavities are held loosely bound “guest” molecules which stabilize the “host” framework of water molecules. *Address correspondence to this author at the Department of Chemistry, The University of Western Ontario, London, Ontario, Canada N6A 5B7. +Issued as NRC No. 20217. 0022-3654/82/2086-4175$01.25/0
The p-T conditions necessary for hydrate stability have been extensively studied.“’ From measurements of hydrate decomposition pressures at different temperatures, (1)S. S. Byk and V. I. Fomina, Russ. Chim. Reo., 37, 469 (1968). (2) D. W. Davidson in “Water: A Comprehensive Treatise”,F. Franks, Ed., Plenum Press, New York, Vol. 2, 1973,p 115. (3) J. A. Ripmeester and D. W. Davidson, Mol. Cryst. Liq. Cryst., 43, 189 (1977). (4)D. W. Davidson and J. A. Ripmeester, J. GlacioE., 21,33 (1978). (5) W. R. Parrish and J. M. Prausnitz, Ind. Eng. Chem. Process Des. Develop., 11, 26 (1972). (6)P.B. Dharmawardhana, W. R. Parrish, and E.D. Sloan, Ind. Eng. Chem. Fundam., 19,410 (1980). (7) G. D. Holder, G. Corbin, and K. D. Papadopoulos, 2nd. Eng. Chem. Fundam., 19,410(1980).
0 1982 American Chemical Society
