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Mechanistic implications from the substitution reactions with methyl fluoride of photochemically produced hot tritium atoms
Volume 20, number 1
1 b:ay 1973
CHEMICAL PHYSICS LETTERS
MECHANISTIC IMPLICATIONS FROM THE SUBSTITUTION REACTIONS WITH METHYL FLUORIDE OF PHOTOCHEMICALLY PRODUCED’HOT TRITIUM ATOMS* C.C. CHOU, D.D. WILKEY and F.S. ROWLAND Department
of Clremistry, University of Caiifomia, Irvine, Gdifornia 92664,
Received 12 March
US4
1973
The reactions with CHjF of photochemically produced tritium atoms from TBr have been studied at various v+xvelengths. The energy threshold is lower and the relative yield higher for F replacement than for H repkement CHaF. These near-threshold substitution reactions are consistent with an inversionmechanism.
We have measured the wavelength dependence of the relative yields of the energetic T-for-H and T-for-F substitution reactions in CH3 F for tritium atoms produced by the photolysis of TBfi. From these yield variations versus initial T atom kinetic energy, information can be gained about the energy thresholds for these substitution reactions. The threshold energy for replacement of F is significantly lower than for the replacement of H, as shown in fig. 1 by the extrapolated yield ratios of CH,T/HT and CH,TF/HT. Furthermore, the absolute yield for replacement of F is consistently higher than for replacement of H, despite the 3.fold statistical facror favoring the latter. In earlier experiments, photochemically produced hot tritium atoms have been shown to react with methane both by abstraction of, and by substitution for, H or D atoms [l-3]. Experiments with variously deuterated methanes have shown a decrease of the total yield from the substitution reactions with increasing deuteration. However, the relative yields from T-for-H versus T-for-D substitution in intramolecular-competition for partially-deuterated * This research wzs supported by A.E.C. Contact
No. AT(04-3>34, Agreement No. 126. i Experimental details are sim.iIar to those of refs. [l-d]. Light sources: nitrogen (1743 A, 1745 A; 3.2 eV); mercury (1849 A; 2.8 eV); carbon (1931 A; 2.5 eV); zinc (2026 A, 2062 A, 2139 A; 2.2-1.9 eV; filtered zinc (2139 A, 1.9 eVJ; cadmium (2145 A, 2265 A, 2288 A; 1.9-1.6 eV).
1.0
30
2.0
INlllAL
TRITIUM
in
ATCM
ENERGY
(E.U)
Fig. 1. Relative yields of CHST and CHzTF versus HT from photochemicztlly initiated reactions of T with CHsF, measured versus initial tritium atom kinetic energy.
methanes is neartjr statistical [4]. These isotope effects led to the suggestion that the substirution reaction in methane is a concerted process involving the simultaneous motion of most or all of the hydrogenic substituents. A limited “6-particle” trajectory calculation for the system T + CH4 also led to the concIusion that the substitution reaction couid mt involve a relatively simple “3-particle” event in which only the attacking 53
Volume 20:number
1
CHEMICAL PHYSICS LETTERS
tritium and the displaced H or D atom (plus the C atom) were appreciably involved [5, 6]*. More elaborate trajectory calculations have now been carried out for T + methane with a more flexible 6-particle potential energy surface, and have shown substitutionwith-inversion as the more prominent low energy substitution process [8]. Our lower observed threshold for F replacement than for H suggests a special energetic ease for the F replacement. An attractive explanation for this situation involves the preferential loss of F along a linear T-C-F axis which undergoes Walden inversion during reaction. The necessarily concerted nature of such a substitution would be facilitated by the mobility of the three hydrogenic substituents. On the other hand, the substitution of H by T involves a presumably offcenter heavy fluorine substituent which would be much less responsive in adjusting to changes in configuration. The observation of both higher threshold and iower absolute yield for the replacement of H is consistent with such dynamic hindrance to a successfi11substitution reaction. The substitution reactions initiated by the much more energetic tritium atoms from nuclear recoil have been reguiarly shown to proceed with essentially comp!ete retention of configuration at asymmetric carbon positions (e.g., T-for-H in CHFCICHFCI) [9] _However, if inversion were to occur in some, but not all, systems, then systems with several hydrogenic substituents should provide the most favorable opportunity for the occurrence of substitution with inversion. The substitution reactions of nuclear recoil tritkm atoms usually occur at appreciably higher kinetic energies (as evidenced by internal excitation energies > 3 eV for the substitution products [ lo]) and other reaction.pathways are probably available and may be dominant. The ratio of T-for-F versus T-for-H replacement in CH,F is only 0.33 for T atoms from nuclear recoil, in contrast to the low energy data of fig. 1 [I 1, 121. The most recent trajectory calculations for T t CH, show a retention mode which dominates the
substitution processes at higher kinetic energies [Sj . Photochemical measurements of the reactions of 2.8 eV tritium atoms with CHFClCHFCl indicated ,* Such Zpar&le trajeciory
substitutions were observed in a 3-particle calculation [7], but the corresporiding reactions
1 May 1973
that the direct substitution yields were negligibly small at this energy [9]. Theoretical ab initio calculations for the H f CH4 r&tential energy surface [ 13, 143 have shown substitution with inversion to be the minimum energy pathway, suggesting that this mode should be important in the threshoid energy region. Corresponding calculations on H + CH, F have not yet been carried out. Altogether, a plausible hypothesis for these substitution reactions can be’offered involving inversion reactions at energies near threshold for substrates with several hydrogenic substituents, together with strongly predominating retention Teactions at higher energies for all substrates. No laboratory test for retention/ inversion with CH,, CH,F, etc. will be possible without the introduction of some entirely new experimental technique. However, trajectory calculations with progressively more fluorinated methanes should be possible.
References [I] C.C. Chou and F.S. Rowland, J. Am. Chem. Sot. 88 (1966) 2612. [2] C.C. Chou and F.S. Rowland, J. Chem. Phys. 50 (1969) 2763. [3] CC. Chou and F.S. Rowland, J. Chem. Phys. 50 (1969) 5133. [4] CC. Chou and F.S. Rowland, 3. Phys. Chem. 75 (1971) 1283. [ 51 D.L. Bunker and M. Patter@, Chem. Phys. Letters 4 (1969) 315. [6] D.L. Bunker and hf. Patrengifl, J. Chem. Phys 53 (1970) 3041. [ 71 P.J. Kuntz, E.hl. Nemeth, J.C. Polanyi and W.H. Wang, J. Chem. Phys. 52 (1970) 4654.
[ 81 T. Valencich and D.L. Bunker, Chem. Phis. Le?ters 19 (1973) 50. [ 9 ] G.F. Palino and F.S. Rowland, J. Phys. Chem. 75 (1971) !.299. [lo] F.S. Rowland, in: MTP international review of science, Physical chemistry, Vol. 9 (chemical kinetics), ed. J. Polanyi, p. 109. [ 111 H.C. Jurgeieit and R. Wolfgang, J. Am. Chem. Sot. 85 (1963) 1057. [ 12) E.K.C. Lee, G. Miller and F.S. Rowland, J. Am. Chem.
Sot. 87 (1965: 190. [13] K. Morokuma and R.E. Davis, J. Am. Chem. Sot. 94 (1972) 1060. [14] 5. Ehrenson and M.D. Newton, Chem. Phys. Letters 13 (1972) 24.
1 b:ay 1973
CHEMICAL PHYSICS LETTERS
MECHANISTIC IMPLICATIONS FROM THE SUBSTITUTION REACTIONS WITH METHYL FLUORIDE OF PHOTOCHEMICALLY PRODUCED’HOT TRITIUM ATOMS* C.C. CHOU, D.D. WILKEY and F.S. ROWLAND Department
of Clremistry, University of Caiifomia, Irvine, Gdifornia 92664,
Received 12 March
US4
1973
The reactions with CHjF of photochemically produced tritium atoms from TBr have been studied at various v+xvelengths. The energy threshold is lower and the relative yield higher for F replacement than for H repkement CHaF. These near-threshold substitution reactions are consistent with an inversionmechanism.
We have measured the wavelength dependence of the relative yields of the energetic T-for-H and T-for-F substitution reactions in CH3 F for tritium atoms produced by the photolysis of TBfi. From these yield variations versus initial T atom kinetic energy, information can be gained about the energy thresholds for these substitution reactions. The threshold energy for replacement of F is significantly lower than for the replacement of H, as shown in fig. 1 by the extrapolated yield ratios of CH,T/HT and CH,TF/HT. Furthermore, the absolute yield for replacement of F is consistently higher than for replacement of H, despite the 3.fold statistical facror favoring the latter. In earlier experiments, photochemically produced hot tritium atoms have been shown to react with methane both by abstraction of, and by substitution for, H or D atoms [l-3]. Experiments with variously deuterated methanes have shown a decrease of the total yield from the substitution reactions with increasing deuteration. However, the relative yields from T-for-H versus T-for-D substitution in intramolecular-competition for partially-deuterated * This research wzs supported by A.E.C. Contact
No. AT(04-3>34, Agreement No. 126. i Experimental details are sim.iIar to those of refs. [l-d]. Light sources: nitrogen (1743 A, 1745 A; 3.2 eV); mercury (1849 A; 2.8 eV); carbon (1931 A; 2.5 eV); zinc (2026 A, 2062 A, 2139 A; 2.2-1.9 eV; filtered zinc (2139 A, 1.9 eVJ; cadmium (2145 A, 2265 A, 2288 A; 1.9-1.6 eV).
1.0
30
2.0
INlllAL
TRITIUM
in
ATCM
ENERGY
(E.U)
Fig. 1. Relative yields of CHST and CHzTF versus HT from photochemicztlly initiated reactions of T with CHsF, measured versus initial tritium atom kinetic energy.
methanes is neartjr statistical [4]. These isotope effects led to the suggestion that the substirution reaction in methane is a concerted process involving the simultaneous motion of most or all of the hydrogenic substituents. A limited “6-particle” trajectory calculation for the system T + CH4 also led to the concIusion that the substitution reaction couid mt involve a relatively simple “3-particle” event in which only the attacking 53
Volume 20:number
1
CHEMICAL PHYSICS LETTERS
tritium and the displaced H or D atom (plus the C atom) were appreciably involved [5, 6]*. More elaborate trajectory calculations have now been carried out for T + methane with a more flexible 6-particle potential energy surface, and have shown substitutionwith-inversion as the more prominent low energy substitution process [8]. Our lower observed threshold for F replacement than for H suggests a special energetic ease for the F replacement. An attractive explanation for this situation involves the preferential loss of F along a linear T-C-F axis which undergoes Walden inversion during reaction. The necessarily concerted nature of such a substitution would be facilitated by the mobility of the three hydrogenic substituents. On the other hand, the substitution of H by T involves a presumably offcenter heavy fluorine substituent which would be much less responsive in adjusting to changes in configuration. The observation of both higher threshold and iower absolute yield for the replacement of H is consistent with such dynamic hindrance to a successfi11substitution reaction. The substitution reactions initiated by the much more energetic tritium atoms from nuclear recoil have been reguiarly shown to proceed with essentially comp!ete retention of configuration at asymmetric carbon positions (e.g., T-for-H in CHFCICHFCI) [9] _However, if inversion were to occur in some, but not all, systems, then systems with several hydrogenic substituents should provide the most favorable opportunity for the occurrence of substitution with inversion. The substitution reactions of nuclear recoil tritkm atoms usually occur at appreciably higher kinetic energies (as evidenced by internal excitation energies > 3 eV for the substitution products [ lo]) and other reaction.pathways are probably available and may be dominant. The ratio of T-for-F versus T-for-H replacement in CH,F is only 0.33 for T atoms from nuclear recoil, in contrast to the low energy data of fig. 1 [I 1, 121. The most recent trajectory calculations for T t CH, show a retention mode which dominates the
substitution processes at higher kinetic energies [Sj . Photochemical measurements of the reactions of 2.8 eV tritium atoms with CHFClCHFCl indicated ,* Such Zpar&le trajeciory
substitutions were observed in a 3-particle calculation [7], but the corresporiding reactions
1 May 1973
that the direct substitution yields were negligibly small at this energy [9]. Theoretical ab initio calculations for the H f CH4 r&tential energy surface [ 13, 143 have shown substitution with inversion to be the minimum energy pathway, suggesting that this mode should be important in the threshoid energy region. Corresponding calculations on H + CH, F have not yet been carried out. Altogether, a plausible hypothesis for these substitution reactions can be’offered involving inversion reactions at energies near threshold for substrates with several hydrogenic substituents, together with strongly predominating retention Teactions at higher energies for all substrates. No laboratory test for retention/ inversion with CH,, CH,F, etc. will be possible without the introduction of some entirely new experimental technique. However, trajectory calculations with progressively more fluorinated methanes should be possible.
References [I] C.C. Chou and F.S. Rowland, J. Am. Chem. Sot. 88 (1966) 2612. [2] C.C. Chou and F.S. Rowland, J. Chem. Phys. 50 (1969) 2763. [3] CC. Chou and F.S. Rowland, J. Chem. Phys. 50 (1969) 5133. [4] CC. Chou and F.S. Rowland, 3. Phys. Chem. 75 (1971) 1283. [ 51 D.L. Bunker and M. Patter@, Chem. Phys. Letters 4 (1969) 315. [6] D.L. Bunker and hf. Patrengifl, J. Chem. Phys 53 (1970) 3041. [ 71 P.J. Kuntz, E.hl. Nemeth, J.C. Polanyi and W.H. Wang, J. Chem. Phys. 52 (1970) 4654.
[ 81 T. Valencich and D.L. Bunker, Chem. Phis. Le?ters 19 (1973) 50. [ 9 ] G.F. Palino and F.S. Rowland, J. Phys. Chem. 75 (1971) !.299. [lo] F.S. Rowland, in: MTP international review of science, Physical chemistry, Vol. 9 (chemical kinetics), ed. J. Polanyi, p. 109. [ 111 H.C. Jurgeieit and R. Wolfgang, J. Am. Chem. Sot. 85 (1963) 1057. [ 12) E.K.C. Lee, G. Miller and F.S. Rowland, J. Am. Chem.
Sot. 87 (1965: 190. [13] K. Morokuma and R.E. Davis, J. Am. Chem. Sot. 94 (1972) 1060. [14] 5. Ehrenson and M.D. Newton, Chem. Phys. Letters 13 (1972) 24.