14050
J. Phys. Chem. A 2006, 110, 14050-14053
Calorimetrically Measurable Enthalpic Isotope Effect Kent Ballard,† Richard C. Reiter, and Cheryl D. Stevenson* Department of Chemistry, Illinois State UniVersity, Normal, Illinois 61790-4160 ReceiVed: September 8, 2006; In Final Form: October 24, 2006
Calorimetric techniques have revealed that the enthalpy of reaction with water is more exothermic by about 2.2 kcal/mol, for the perdeuteriated naphthalene anion radical (K+C10D8•-(s) + H2O(liq) f 1/2C10D8H2(s) + 1 /2C10D8(s) + KOH(aq)) than it is for the perprotiated system. These results, when coupled with the known enthalpy of electron transfer between naphthalene and its perdeuteriated analogue imply that the heat of hydrogenation of naphthalene decreases by about 1.8 kcal/mol upon perdeuteriation of the naphthalene.
Introduction There have been literally thousands of observations of isotope effects, some upon chemical kinetics, the kinetic isotope effect (KIE),1 and some upon chemical equilibria, the equilibrium isotope effect (EIE).2 This latter effect can be broken down into entropic and/or enthalpic effects. EIEs can serve as a probe of nonbonding interactions,2a yet there have been no direct measurements of nonbonding isotopic heat (enthalpic) effects in chemical reactions. By definition, an enthalpic EIE (qIE) would have to be detected calorimetrically as a perturbation in exo- or endothermicity due to isotopic substitution. Over a decade ago, we observed a very large, secondary (nonbonding) EIE for the electron transfer between aromatic hydrocarbon anion radicals and their neutral counterparts.3 For example, at 173 K, reduction of a mixture of benzene and perdeuteriated benzene with potassium metal in THF in the presence of 18-crown-6 was found to favor formation of C6H6•over C6D6•- by a factor of 3.85.4e The equilibrium constant for the reaction C6H6•- + C6D6 a C6H6 + C6D6•- is 0.26 at 173 K, which corresponds to a 0.47 kcal/mol smaller solution electron affinity for C6D6 than for C6H6. This should be reflected quantitatively in the enthalpy of electron transfer. B3LYP/631+G* density functional calculations reproduce this experimental measurement.4 Similarly to the situation with the benzene system, when mixtures of naphthalene and perdeuteriated naphthalene were partially reduced with potassium metal in THF, the resulting EPR spectra indicated that the solution electron affinity (EA) of the isotopically light material is larger than that of the heavy isotopic isomer; see reaction 1.3g,h Since that time, the deuterium
corresponds to a ∆G° of 410 cal/mol for reaction 1.3a,h Because there is no reason for an entropic contribution to ∆G°, it should (in principle) be possible to observe a very small difference in heat release from reactions involving these two isotopic isomers (K+C10H8•- and K+C10D8•-). Here we report a measurable difference in the heat released by the well-known Birch reduction (reaction 2) of naphthalene and of its perdeuteriated
analogue. This enthalpic effect is small, but it implies a rather large isotope effect upon the enthalpy of hydrogenation (∆H°H2). Previously, we have made use of the Birch reduction for the experimental determination of crystal lattice energies for organic anion radicals.7 The method involves the measurement of the heat of reaction of the solid anion radical salt with water (reaction 2) to produce the neutral hydrocarbon, hydrogenated hydrocarbon, and aqueous alkali metal hydroxide.7 Crushing thin-walled evacuated bulbs, under water, containing the solid salts of the naphthalene anion radical, results in a rise in the temperature of a calorimeter due to reaction 2. Plots of the rise in the temperature of the calorimeter vs the amount of anion radical salt in the bulbs are linear, and the slopes of the lines are proportional to the enthalpies of the reactions.7 As our calorimetric techniques and instrumentation improved, we were motivated to attempt to directly measure the “heat content” differences due to the number of neutrons in the organic substrates. The isotope effect upon the electron transfer between C10H8 and C10D8 (reaction 1) is well explained in terms of zero point energy (ZPE) effects,3,4 eq 3 (cation effects neglected),3b where i)48
( effect upon solution EA has also been observed via electrochemical techniques,5 and an analogous effect for cation radicals has been noted.6 At -120 °C, the equilibrium constant for electron transfer between the two ion pairs in reaction 1, determined from the relative EPR intensities of the two anion radicals (C10H8•- and C10D8•-), in equilibrated mixtures, was reported to be 0.26. This * Corresponding author. E-mail:
[email protected]. † Present address: Prairie Analytical Systems, Inc., Springfield, IL.
Keq )
e ∏ i)1
i)48
-i/2kT
)C10D8•-(
e- /2kT)C ∏ i)1
i)48
(
e- /2kT)C ∏ i)1
i
10H8
(3)
i
i)48
(
•-
10H8
e- /2kT)C ∏ i)1 i
10D8
the values for i represent the 48 vibrational eigenvalues for each of the four species involved in reaction 1. The situation is simplified by the absence of an entropic contribution, ∆G° ) ∆H°.3,4 Nevertheless, the very small reaction enthalpy (
