J . Phys. Chem. 1992, 96, 5669-5670
scheme is the fastest ...” but certainly does not prove that the scheme is correct. In fact, the kinetics of ref 1 are very sensitive to one reaction rate in particular: k= NH3 N O + NH, H N O 8 x 10” exp(-2OOO/Rn cm3/(mol.s) (19) However, this reaction is approximately 59 kcal/mol endothermic, and the chosen Arrhenius parameters would make the reverse rate coefficient at 870 K some 14 orders of magnitude faster than gas kinetic. (The reverse is included specifically as reaction 11; the ratio of k19to k l l differs from the equilibrium constant by as many orders of magnitude.) Muzio et al.3 (the source for reaction 19 and its rate coefficient in ref 1) in fact had acknowledged that their “...attempt at modeling N O reduction with ammonia has not yet proved successful in that the mechanism employed required that the rate constants for [NH3 + N O and NH, NO] are inordinately large.” When the value of this rate coefficient is reduced to a physically possible one, the reaction time is lengthened drastically: the maximum in the slope of dT/dt occurs after hundreds rather than tenths of a microsecond. The contribution of this moleculemolecule reaction to the overall mechanism is greatly reduced, and the contention of Sahu et al. that this pathway would “... proceed immediately ...due to the large availability of N O and 0,” [the latter to react with NH2] is considerably weakened. A second serious flaw in the mechanism is that the only reaction to take C103 to products is k = 1 X l O I 4 exp(-l1930/RT) (6) 2C103 C12 30,
+
+
+
-
+
Thic step converts C103to final products in a single global step, thus obviating the need for directly invoking C10, ClO,, and other chlorinecontaining intermediates other than C1 atoms, whose only role in this mechanism is the equilibrium between C1 atoms and C12 molecules. (Two other included reactions produce C1 atoms, but the mechanism does not indicate that reverse reactions have been taken into account.) In fact, the mechanism has no chainbranching steps a t all involving chlorine species. Reaction 6 and its rate coefficient are taken from Bodenstein et a1.: who postulated the step to explain observations in their study of the Cl2-03 reaction at 15-50 OC; the activation energy was deduced from an assumed preexponential factor and an observed rate at a single temperature. The conclusions have been criticized elsewhere.5 Most importantly, the reaction is not an elementary one and cannot be inserted with confidence in a completely different system. The self-reaction of HNO (10) is assumed to yield directly H20 and N,O-which seems less likely than some alternative processes. The direct generation of observed products thus circumvents some additional radical steps that would contribute to more chain processes. Of course, it is easier to fault the proposed mechanism than to rectify it with confidence. Inclusion of the reverse reactions -7 and -5 will allow the H/Cl/HCl chain reaction to contribute; inclusion of H + 02 OH 0 (23)
-
+
will make the H/O/OH chain reaction a contributor as well. Omission of both of these important chain processes militates seriously against the correctness of the proposed mechanisma6 As it stands, the mechanism includes only three chain-branching steps: attack on N H 3 by N O (19) and by 0 (20) and attack on NH, by 0 2 . The C10, chemistry is not well understood, but a sequence of steps such as (6’) rather than (6) and (24)-(27) is at least reasonable, if not verified: 2C103 C l o d + C1O2 (6’)
-- + c1 + c10, + - c1 + + - +
2c10,
c103
c10
2c10 0 0
c10
c10,
c10
0, 0,
(24) (25) (26) (27)
Rate coefficients for reactions 25,26, and 27 are at least approximately known;’ (6’) and (24) can be estimated to be the same as (25). Finally, one needs some additional OH reactions, which in ref 1 reacts only with H2 (8), H N O (12), and itself (22), the latter process being chain terminating.
References and Notes (1) Sahu, H.; Sheshadri, T. S.; Jain, V. K. J . Phys. Chem. 1990,94,294. (2) Guirao, C.; Williams, F. A. AIAA J . 1971, 9, 1345. (3) Muzio, L. J.; Arand, J. K.; Teixeira, D. P. Symp. (Int.) Combusr., [Proc.],16th 1976, 199. (4) Bodenstein, M.;Padelt, E.; Schumacher, H. J. Z . Phys. Chem. 1929. BS, 209. ( 5 ) Cohen, N.; Heicklen, J. In Comprehensive Chemical Kinetics;Bamford, C. H., Tipper, C. F. H., Eds.; Elsevier: Amsterdam, 1972; Vol. 6, Chapter 1. (6) In an earlier paper [Sheshadri, T. S.; Jain, V. K. Propellants, Explos. Pyrotech. 1989.14, 1931 the authors considered a more detailed mechanism, but the argument that the kinetics are not sensitive to other reactions is
weakened by the unreasonably fast steps noted above. (7) Atkinson, R.; Baulch, D. L.;Cox, R. A.; Hampson, Jr., R. F.; Kerr, J. A.; Troe, J. J . Phys. Chem. Ref.Data 1989, 18, 881.
Environmental Monitoring and Technology Department Space and Environment Technology Center The Aerospace Corporation P.O. Box 92957 Los Angeles, California 90009
N. Cohen
Received: February 3, 1992; In Final Form: March 26, 1992
Halide Ion Quadrupole Relaxation In Aqueous Solutions Containing Organic Cations Sir: Several years ago we observed that, contrary to simple inorganic ions, cations containing hydrophobic groups dramatically increase the rate of quadrupole relaxation of halide ions in aqueous solution.’-3 Other research groupsH made analogous observations and the effects of a large number of cosolutes on C1, Br, and I relaxation were investigated under different conditions. As summarized in ref 7, a consensus was reached attributing these peculiar effects to a type of hydrophobic interaction, where the anion interacts with the apolar chains of the organic ions. Recently Suezawa et al.s in reexamining halide ion quadrupole relaxation in aqueous solutions containing organic cations, have questioned the previous view and argued that the relaxation effect is due to direct interaction between nitrogen cations and the halide ions and make an analysis in terms of contact ion pairing. In our opinion, Suezawa et al. have failed to consider a large number of previous findings, which appear to be inconsistent with their view. While the arguments for attributing the dramatic effect of organic cations on halide ion quadrupole relaxation to an ion-hydrophobic solute interaction rather than direct ion-ion interactions are reviewed in some detail in ref 7, we list the most significant observations here: 1. Originally, these effects were discovered with symmetrical tetraalkylammonium ions, which cause a halide ion relaxation effect orders of magnitude larger than for simple inorganic ions. The effect increases dramatically with the number and length of the alkyl groups (and when the cationic center is closed for approach), as well as with the halogen atomic number (as also found by Suezawa et al.), contrary to what is generally found for ion pairing and direct ion-ion interactions. In nonaqueous solvents, the dependence on cation size is very different and qualitatively consistent with ion-ion effects. 2. Substitution of the cationic atom, for example changing from nitrogen to phosphorus, does not produce any significant change. 3. If the organic groups in the cation are made more polar, the effect is reduced. 4. Direct evidence that the dominant line broadening is due to the nonpolar groups was obtained from the observations that
5670 The Journal of Physical Chemistry, Vol. 96, No. 13, 1992 these effects are present also when the organic solute is uncharged or even when it is negatively charged. 5. The effect is not caused by aggregation. As we have shown in a large number of paper^,^ micellization of long-chain alkylammonium halides causes a dramatic increase in the halide ion relaxation. However, symmetric tetraalkylammonium salts do not form micelles and are yet more effective in promoting relaxation than a corresponding (equal number of carbons) compound with a single long chain, provided the measurement is made below the cmc of the latter. 6 . Symmetrical tetraalkylammonium halides give a relaxation rate that increases monotonically and nearly linearly with concentration up to remarkably high (ca. 3 mol/kg) concentrations. This is in strong contrast to the dramatic increase in the relaxation rate that one finds above the cmc for associating compounds. We also note in passing that the relaxation rates found by Suezawa et a1.* show the same near-linear dependence on concentration. However, the authors have chosen to divide the observed relaxation rate by the viscosity of the solution, probably with the hope of obtaining a more invariant property. However, this procedure is inappropriate (as shown clearly in the “reduced” relaxation rates of I4N, which may show an unphysical decrease with concentration). Thus, for the motion of small molecules, the macroscopic viscosity is irrelevant and there are numerous examples of rapid molecular rotation and translation for highly viscous systems. If one wants to analyze the dynamics of small molecules and ions in a hydrodynamic picture,1° the best one can do is to use the viscosity of the solvent, while that of the solution is irrelevant. A main motivation for writing this Comment is to point out that, in our opinion, the halide ion relaxation effect discussed above is a manifestation on the molecular level of an anion binding phenomenon of great generality and importance. Halide, as well as many other small anions, shows a certain degree of surface activity, which increases according to the so-called Hofmeister or lyotropic series, SQ4*- < F C C1- < Br- < NO3- < I- < SCN-. The weak surface activity shows up directly in surface tension measurements both for the air-water” and the hydrocarbon-water interfaces.I2 It is observed in a more indirect way as nonspecific binding to proteins,I3 polysaccharide^,^^ nonionic polymer^,'^ and nonionic micelles,16 to give a few examples. The molecular mechanism that leads to this surface activity has not been well established. Sometimes the effects are explained
Additions and Corrections in terms of “makers” and “breakers” of bulk water structure. It seems to be a much more likely explanation that anions prefer an inhomogeneous solvation because of their large polarizability. When the halide ion is asymmetrically solvated, there is a reaction field from the solvent acting on the ion. A polarizable ion interacts favorably with this field.
References and Notes (1) Lindman, B.; Forsen, S.;Forslind, E. J . Phys. Chem. 1968, 72, 2805. (2) Lindman, B.; Wennerstrom, H.; Forsen, S . J . Phys. Chem. 1970, 74, 754. (3) Wennerstrom, H.; Lindman, B.; Forsen, S. J . Phys. Chem. 1971, 75, 2936. (4) Hertz, H. G.; Holz, M. J . Phys. Chem. 1974, 78, 1002. ( 5 ) Holz, M. Magnetische Relaxation von Ionenkernen und hydrophobe Hydration. Ph.D. Thesis, Karlsruhe, 1973. (6) Maijgren, B. Studies of Internal and External Phase Boundaries in Aqueous Systems. Ph.D. Thesis, Stockholm, 1974. (7) Lindman, B.; Forsen, S. Chlorine, Bromine and Iodine NMR. Physico-Chemical and Biological Applications; Springer Verlag: Heidelberg, Germanv. 1976. (8) Siezawa, H.; Horiike, N.; Yamazaki, S.; Kamachi, H.; Hirota, M. J . Phys. Chem. 1991, 95, 10787. (9) Lindblom, G.; Lindman, B.; Mandell, L. J. Colloid Interface Sci. 1973, 42, 400. Lindblom, G.; Lindman, B. J . Phys. Chem. 1973. 77. 2531. Wennerstrom, H.; Lindblom, G.; Lindman, B. Chem. Scr. 1974, 6, 97. Lindman, B.; Lindblom, G.; WennerstrBm, H.; Gustavsson, H. In Micellizaiion Solubilization and Microemulsions; Mittal, K. L., Plenum Press: New York; Vol. I, p 195. Fabre, H.; Kamenka, N.; Khan, A,; Lindblom, G.; Lindman, B.; Tiddy, G. J. T. J . Phys. Chem. 1980,84, 3428. (IO) Moseley, M. E. Chem. Scr. 1980, 16, 28. (1 1) Johansson, K.; Eriksson, J. C. J. Colloid Interface Sci. 1974, 49,469. Jarvis, N. L.; Scheiman, M. A. J . Phys. Chem. 1968, 72, 746. (12) Aveyard, R.; Saleem, S. M. J. Chem. SOC.,Faraday Trans. I , 1977, 72, 1609. Aveyard, R.; Saleem, S. M.; Heselden, R. J. Chem. Soc., Faraday Trans. I 1977, 73, 84. (13) Forsen, S.; Lindman, B. Chem. Br. 1978, 14, 29. (14) Piculell, L.; Nilsson, S. Prog. Colloid Polym. Sci. 1990, 82, 198. (15) Florin, E.; Kjellander, R.; Eriksson, J. C. J . Chem. Sot., Faraday Trans. I , 1984,80, 2889. Karlstrom, G.; Carlsson, A.; Lindman, B. J . Phys. Chem. 1990, 94, 5005. (16) Firman, P.; Haase, D.; Jen, J.; Kahlweit, M.; Strey, R. Langmuir 1985, I , 718.
Divisions of Physical Chemistry Chemical Center, Lund University POB124 S-221 00 Lund, Sweden
Bjorn Lindman* HIkan Wennerstrom Sture F o d n
Received: February 19, 1992
ADDITIONS AND CORRECTIONS M. Sodupe, Charles W. Bauschlicber, Jr.,* Stephen R. Langhoff, and Harry Partridge: Theoretical Study of the Bonding of the First-Row Transition-Metal Positive Ions to Ethylene. Page 21 18. The following table was inadvertently omitted. TABLE IV. Binding Energies for Transition-Metal-Ethylene Cationsa sc theory experiment Armentroutb BeauchamPC Freiser“
24.8 235.1 f 1.2 40 f 5
Ti 24.2 232
V 25.2
-50
Cr 22.0
Mn 16.1
33 f 5
“In kcal/mol. bReferences 1-7. CReferences8-10. dReferences 1 1 and 12.
Fe 25.7 42f9 34 f 2
co 36.4 >40f 5 37 f 2, 46 f 8
Ni 37.6 >35*5 48 f 8 37f 2
cu 36.0 >26f3 >28f 1
