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Communications to the Editor
(2) P. 0. P. Ts’o in “Fine Structure of Proteins and Nucleic Acids,” G. D. Fasman and S. M. Timesheff, Ed., Marcel Dekker, New York, N. Y., 1970, pp 49-190. (3) P. 0. P. Ts’o and S. I . Chan, J. Amer. Chem. SOC.,86,4176 (1964). (4) A. D. Broom, M. P. Schweizer, and P. 0. P. Ts’o, J. Amer. Chem. SOC.,89, 3612 (1967). (5) S. J. Gill, M. Downing, and G. F. Sheats, Biochemistry, 6, 272 (1967). (6) F. Garland, R . C. Patel, and G. Atkinson, J. Acoust. SOC.Amer., 54, 996 (1973). (7) J. Rassing and 6.M. Jensen, Acta Chem. Scand., 24,855 (1970). (8) J. Rassing and F. Garland, Acta Chem. Scand., 24, 2419 (1970). (9) D. Pdrschkeand F. Eggers, Eur. J. Biochem., 26,490 (1972).
Department of Chemistry University of Oklahoma Norman, Oklahoma 73069
Frank Garland* Ramesh C. Patel
D/s
‘zH4
fNF3 F6
0.3-
I
F4
-
0.2
NF3 F6
Received November 5. 1973
0.1
Isotope Effect in Energy Losses for Deactivating Collisions of Tetrafluoromethane with Chemically Activated Fluoroethyl Radicals C H 2 1 8 F C H 2 and CD218FCD2 Publication costs assisted by the Dwsion of Research, U S. Atomic
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0 0
0.5
1.0
2.0
1.5
0.89. 1.78 2.67 1.29 2.58
5.87
CF4
lO3/P
NF3 F6
(TORR)
Figure 1.. Graph of decomposition/stabilization vs. (pressure)for CH218FCH2 and CD2”FCD2 in sF6, CF4, and NF3. Different (pressure)-’ scales are used for each gas, chosen to give the same visual slope for CD218FCD2 in all three gases.
Energy Cornmiss/on
Sir: The magnitude of collisional energy transfer from chemically activated molecular species is often evaluated from the deviations from linearity at very low pressures of graphs of decomposition/stabilization (D/S) ratios us. (pressure)-l, and can be expressed in terms of a characteristic energy loss, e.g., step size in a step-ladder deexcitation mode1.l As yet, few measurements of step size exist for isotopic activated species in collision with the same bath gas, and these measurements have not indicated any isotopic differences in energy 1 0 ~ s . ~We have formed CHzisFCH2 and CFz18FCD2 by addition of thermal i8F to C2H4 and to C2D4 in the presence of excess NF3, CF4, and SFg,4-6 and have observed a different ratio (4.2 i 0.4) of (D/S)H/(D/S), for CF4 from that (6.2 f 0.7) found for NF3 and SFe, as summarized in Table I and Figure 1. The (D/S) graphs have been characterized by the slopes of linear plots, and are expressed as extrapolated pressures for half-stabilization. This difference in magnitude of the isotope effect requires that there be an isotope effect in the energy removal by collision with either CF4 or with SF6 and NF3 (or with all three). The molecules NF3, CF4, and SFe increase in size in that order and should show a corresponding decrease in the lifetimes of excited species toward stabilization. However, since the pressure for halfstabilization in CF4 is appreciably higher than for SF6, we conclude that (a) CF4 is a “weak collider,” i.e., that activated species are frequently still able to undergo decomposition despite one collision (or more) with CF4 as bath gas; and (b) that the average energy loss per collision with CF4 (corresponding to step size for the step-ladder model) is less for CD2l8FCD2 than for its isotopic counterpart CHz18FCH2. The experimental procedures have been described in detail earlier for the addition of 18F to C2H4 in SFs or CF4,e and are essentially unchanged with the substitution of C2D4 and/or NF3. The (D/S) ratios are determined through radio gas chromatographic measurements of the relative yields of CH2=CH18F(D) us. CH3CH218F (S, 3 3
The Journal of Physical Chemistry, Vol. 78, No. 8, 1974
TABLE I: Extrapolated Pressures for Half-Stabilization for Fluoroethyl Radicals i n NF,, CFa,and SFBBath Gases Extrapolated half-stabilization preeaures, Torra Bath gas
CHz’aFCHz radical
CDz’aFCD2 radical
Ratio H / D
NFI CFa SFe
117 f. 6 144 f 10 80 f 6
19 f 2. 34 f 3 13 f. 1 . 5
6 . 2 i. 0 . 7 4.2 & 0.4 6.2 f 0.8
a Pressure at which (D/S) = 1.0, if linear extrapolations are made for data in Figure 1. Note that none of the measurements have been made below 250 Torr.
followed by abstraction of H from HI), or of their tetradeuterio counterparts. The physical characteristics of the experiment (chiefly, the path length required to thermalize l8F atoms with > l o a eV initial kinetic energy us. the dimensions of the fast neutron generator irradiation facility) make accurate experiments impractical below about 400 Torr total p r e s s ~ r e , ~ so- that ~ all (D/S) measurements have been carried out under conditions in which stabilization is heavily dominant. The pressure range of measurement is approximately 400-4000 Torr for all systems. Experiments with varying ratios of CF4/C2H4, etc., have confirmed the earlier finding in the SFs/C2H4 systeme that nonthermallsF atom addition is of negligible quantitative importance to (D/S) ratio measurements when the mole fraction of bath gas is >0.90. The experimental data have been graphed in Figure 1 with different reciprocal pressure scales for each bath species, chosen so that the slopes of the graphs of CD218FCD2 will all be visually the same. The differences in slope for CH218FCH2 with CF4 and with SFe/NF3 are then clearly shown. In each system, the observed (D/S) ratio reflects the competition between the rate constant for decomposition, kdHSFs, and that for stabilization, kSHSFe, such that the isotopic ratio of (D/S) values therefore includes four rate
851
Communicationsto the Editor constants
(6) R. L. Williams and F. S. Rowland, J. Phys. Chem., 76, 3509 (1972). (7) RRKM calculations of decomposition rate constants show a monotonic decrease in the percentage increase in decomposition rate constant with increasing energy for a fixed increment in energy content. Therefore, if average step sizes for collisional energy loss were indentical for two isotopic species, one would anticipate that the percentage decrease in rate might be less for the protonated species than for the deuterated, since at each energy the protonated species reacts faster and is thus further along toward its asymptotic limit. While this effect would not be large, it is in the opposite direction from that experimentally observed in Table I. (8) Heats of formation in kcal/mol, literature references given in ref 6: F (18.9), H (52,1), C2H4 (12.5), CHz=CHF (-28). (9) B. S. Rabinovitch and B. A. Thrush, J. Phys. Chem., 75, 3376 (1971). (10) See, for example, J. H. Georgakakos, B. S. Rabinovitch, and E. J. McAlduff, J. Chem. Phys., 52, 2143 (1970). (11) R. Atkinson and B. A. Thrush, Proc. Roy. Soc., Sei. A, 316. 123, 131, 143 (1970). (12) M. G.Topor and R. W. Carr, Jr., J. Chern. Phys., 58,757 (1973). (13) W. Mutch and J. W. Root, private communication. W.Mutch, Ph.D. Thesis, University of California, Davis, 1973. (14) T. Shimanouchi, Nat. Stand. Ref. Data Ser., Nat. Bur. Stand., No. 39 119721. (15) Bond dissociation energies: NF2-F, 57 kcal/mol, J. K. Ruff, Chem. Rev., 67, 665 (1967). CF3-F, 125 kcal/mol, B. Darwent, “Bond Dissociation Energies in Simple Molecules, Nat. Stand. Ref. Data Ser., Nat. Bur.’Sfand., No. 31 (1970). SF5-F, 78 kcal/rnol, ibid. (16) This research was supported by AEC Contract No. AT (04-3)-34, Agreement No. 126.
In the “strong collision” approximation, each decomposition rate constant is a function only of the characteristics of 18F addition to CzH4 or CzD4, and not of the bath gas properties, so that the isotopic ratio of decomposition rate constants, (kdH/kdD),is therefore independent of the bath gas. Since collision cross sections are isotopically invariant to a high degree of accuracy, any variation in the isotopic ratio of (D/S) values among different moderator gases immediately implies the breakdown of the “strong collision” assumption for a t least one of the systems. While one “weak” collision is not sufficient to prevent further decomposition by the activated species, the ratio of isotopic rates will be altered only if the rates of decomposition after one collision are not negligible and are in a __ different ratio from the initial rates.? Thus, a reduction of the isotopic rate ratio from 6.2 to 4.2 requires a substantially greater contribution to decomposition after the first collision with CF4 from CDzl8FCDz than from CHz18FCHz. The data are roughly consistent with average Joan P. Frank Department of ChemistrylG energy losses sufficient to reduce the decomposition rate F. S. Rowland* University of California constant by a factor of 2 per collision for C D Z ~ ~ F C and DZ Irvine, California 92664 a factor of 3 per collision for C H Z ~ ~ F C Quantitative H~. estimates of the differences in average energy loss are deReceived October 9, 1973 pendent upon the models used for energy transfer (step ladder, exponential, etc.)l and for the RRKM calculation of decomposition rates. However, since the decomposition of the newly formed radical is only mildly exothermic (7 kcal/mol, with excitation energy of 44 kcal/mol),s the average energy losses per collision with CF4 by C D Z ~ ~ F C D Z cannot be larger than about 2 kcal/mol in order to obtain Investigation of the Hydrophobic Interaction between numerical agreement with the data. fert-butyl Alcohol and Paramagnetic Solutes in Both NF3 and SF6 are implicitly assumed to be “strong” colliders in our comparisons. Under our low Aqueous Solutions by Proton Nuclear Magnetic (D/S) conditions, this is not a very stringent assumption, Resonance Relaxation’ for an average energy loss of 3 kcal/mol would be strong enough to make decomposition after one collision negligiSir: The net attractive forces operating between hydrobly important. The question of strong us. weak collisions phobic solutes in aqueous solutions are clearly evidenced with polyatomic bath molecules is an active subject of inin many thermodynamic measurements.2 For example, vestigation and some controversy,lj9-13 and energy losses the increased molar solubility of most hydrocarbons in the of this small magnitude are not inconsistent with a t least presence of solutes containing a significant hydrophobic some of the current literature estimates for collisions ingroup3 points to a positive interaction between the solute volving molecular species of comparable size.11-13 species, overwhelming the excluded volume effect and, in Examination of the fundamental vibrational frequenmany cases, the repulsive forces due to the “dielectric cies14 of NF3, CF4, and SFe does not indicate any striking cavity e f f e ~ t . ”Similar ~ interactions are inferred from the difference between CF4 and the other two that might acexcess free energies of aqueous solutions of salts containcount simply for the less efficient energy removal by the ing large organic ions.s In both cases, model computaformer. It is worth noting that complete rupture of a C-F t i o n ~agree ~ with the existence of such solute-solute atbond requires approximately double the amount of energy traction, now generally referred to as “hydrophobic interneeded to break the N-F or S-F bond.l5 Although all of action.”2 the bond dissociation energies are far above the energies To specify the molecular origin of these interactions, transferred in these collisions, we believe that there may and their consequence for the properties of aqueous solube a correlation between less efficient transfer of energy tions including biological systems, one requires physical and more tightly bound fluorine atoms, as measured by methods capable of monitoring the interaction between a bond dissociation energies. given pair of solute species. Likewise, to understand the kinetic aspects of these phenomena, methods should be References and Notes sought to elucidate the effect of the hydrophobic interac(1) A recent review of chemical activation systems has been given by tion on the dynamic behavior (rotational and translationD. W. Setser in “MTP International Review of Science, Physical Chemistry,” Vol. 9, Butterworths. London, 1972, p 1. al) of the solute particles involved. (2) K. Deesand D. W. Setser, J. Chem Phys., 49,1193 (1968). The dynamic effects have been studied recently6 using (3) W. G. Clark. D W. Setser. and K. Dees. J. Amer. Chem. Soc.. 93. hydrophobic nitroxide radicals, namely, 2,2,6,6-tetra5328 (1971) (4) T Small, G E Miller, and F S Rowiand, J Phys Chem, 74, 4080 methylpiperidine N-oxide (TEMPO), as probe molecules in (1970) aqueous solutions of various other hydrophobic solutes, (5) f. Smail, R. S. lyer, and F. S. Rowland, J. Amer. Chem. Soc., 94, 1041 (1972). Following a detailed analysis of the electron spin reso.
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The Journal of Physical Chemistry, Voi. 78, No. 8, 1974
