Kinetic isotope effects for adiabatic proton transfer reactions in a polar environment

Journal of Physical Chemistry A

Volumn 107, Issue 42, 2003, Pages 9022-9039

  Kiefer, Philip M ^a   Hynes, James T ^a,b  

^a UNIVERSITY OF COLORADO   (United States)

^b ECOLE NORMALE SUPÉRIEURE   (France)

Author keywords

[No Author keywords available]

Indexed keywords

ACTIVATION ENERGY; FREE ENERGY; ISOTOPES; PROTONS; QUANTUM THEORY; REACTION KINETICS; TRANSPORT PROPERTIES;

ADIABATIC PROTON TRANSFER REACTIONS; KINETIC ISOTOPE EFFECTS; POLAR ENVIRONMENT;

MOLECULAR DYNAMICS;

EID: 0242489464     PISSN: 10895639     EISSN: None     Source Type: Journal    
DOI: 10.1021/jp030893s     Document Type: Article

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74. For low T and a surrounding environment with minimal fluctuation capacity (cf. Limbach, H.-H.; Scherer, G.; Meschede, L.; Aguilar-Parrilla, F.; Wehrle, B.; Braun, J.; Hoelger, C.; Bendict, H.; Buntkowsky, G.; Fehlhammer, W. P.; Elguero, J.; Smith, J. A. S.; Chaudret, B. In Ultrafast Reaction Dynamics and Solvent Effects; Guaduel, Y., Rossky, P. J., Eds.; AIP Press: New York, 1994, 225), the standard picture would possibly be a more appropriate description.
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109. For example, in model calculations we have included the T dependence of the dielectric constant ε of water (T = 0-100 °C; see: Riddick, J. A.; Bunger, W. B. Organic Solvents: Physical Properties and Methods of Purification, 3rd ed.; Wiley-Interscience: New York, 1970; pp 67-68) and find (i) a ∼20% reduction of the rate Arrhenius slopes and (ii) only a slight effect (<5%) on the KIE Arrhenius slope, due to an almost complete cancellation of parallel effects on H and D transfer. However, caution is required for lower polarity systems. For example, model calculations for solvents with ε in the range of 20-45 (selected to model experimental systems, i.e.: Shenderovich, I. G.; Burtsev, A. P.; Denisov, G. S.; Golubev, N. S.; Limbach, H. H. Magn. Reson. Chem. 2001, 39, S91) show that reaction asymmetries (and activation free energies) can be drastically altered, such that even the description of the PT as activated no longer holds. All of the above will be discussed in detail elsewhere.
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117. This statement could possibly be used with the standard picture to explain the small symmetric reaction KIE magnitude, presented above, but the plausibility of such a bending vibration has yet to be addressed for the these systems. Furthermore, model calculations by the present authors indicate that the increase in bending frequency would not significantly alter the magnitude of the KIE.
118. Similar nonadiabatic behavior in which bending motion was explicitly included may be found in ref 23; for the corresponding free energy curves of proton stretch and H-bond vibrational excited states, see ref 35.
119. -1, where the parameters have been chosen tobe similar to those of ref 23e. The resulting curves are similar to those of Figure 9b, in that the ground-state reaction is nonadiabatic and bend-stretch mixing must be accounted for in excited state reactions. The former feature is largely caused by the large Q value, with some additional effect arising from the slightly bent geometry.
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125. Here we take the perspective introduced by Johnston for gas-phase H atom transfer, where the TS position is determined by the MEP maximum, and the TS ZPE is evaluated there. Obviously, a variational TS theory approach, where the TS position is determined by the maximum of the MEP plus the transverse ZPE, would dictate different isotopic TS positions, analogous to the procedure of the present perspective (see section 2b.2). However, even in that case, the TS position difference between isotopes will be small except when the TS position significantly deviates from the classical MEP maximum, which is indicative of a predominantly tunneling reaction.
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133. RXN-independent.
134. RXN range, using the full Marcus eq 4.8 (with the intrinsic barriers for H and D determined from the previous calculations) gives the exact KIE maximum, but the KIE only drops to 6.7, i.e., by only ∼2%, a very significant disparity.