Phase barrier

These composite data strongly suggest that the presence of adventitious water or other hydrogen donors can markedly affect the observed rate of epoxidation. For example, Murray and Gu have reported AH = 5.0 kcalmol" for the DMDO epoxidation of cyclohexene in CDCI3 and 7.4 kcalmol" in acetone as solvent . Curci and coworkers also reported an a value of 9.3 kcalmol" for the DMDO epoxidation of isobutylene in acetone . These barriers are significantly lower than the 13-18 kcalmoD gas-phase barriers reported " at the B3LYP level of theory (Tables 3 and 4). Activation barriers of 12.6,... 

In an earlier report Bach and coworkers suggested that MesN and MesP had nearly the same gas-phase barriers with peroxyformic acid (AA 5 = 0.5 kcalmol", MP4//MP2/6-31G ) and that their relative reactivity in protic solvent is better attributed to a much greater ground-state solvation of the more basic tertiary amine than the polarizability of the phosphorus. In a more recent study, a 3.3 kcalmol" barrier difference was observed with HCO3H in favor of the P nucleophile at B3LYP/6-31- -G(d,p) (3.0 versus —0.3 kcalmol". Table 12). Thus, the less reactive f-BuO—OH is a more discriminating oxidant with a later TS and the difference in barrier heights for N versus P oxidation widens. These data also point out that rate differences should be compared for a common set of nucleophiles and that when you use oxidants of different reactivity you should... 

Solution studies (Albery and Kreevoy, 1978), gas phase data (Pellerite and Brauman, 1980, 1983) and theoretical calculations (Wolfe et al., 1981) all indicate an enormous variation in the barriers for identity exchange of CH3X (113). For example, in (113) the experimental gas-phase barrier for... 

HICKMAN ET AL.—CENTRIFUGAL PHASE-BARRIER RECOMPRESSION DISTILLATION... 

Theoretical studies of dication 92 have shown it to reside in a deep potential energy well, with a gas-phase barrier to deprotonation estimated to be about 42 kcal/mol (MP4/6-311G level).45... 

Sampler Sorption Phase Barrier to Diffusion3 Analytes Sampling Purpose Period for Chemical Analysis Reference... 

An experimental Acan be derived from the temperature dependence of the second-order rate constant, which yielded a value of 25.9 kcal/mol.59 Although it appears that this disagrees with the computed free energy of activation (16.6 kcal/ mol) for the reaction of H3N + CH3SH2 in water, the difference actually originates from the intrinsic reactivity of the two reactions. The additional methyl group substitutions both on the nucleophile and substrate raise the gas-phase barrier by 10 kcal/mol to a value of 10.5 kcal/mol at the HF/6-31G(d) level. Taking the methyl substitution effect into account, the computed solvation effect in fact is in accord with experiment,59 which is about 15 kcal/mol (25.9 — 10.5 kcal/mol). 

The calculated gas-phase barriers to internal rotation225 around the C1=C2 bond decrease by 56.60, 47.01 and 41.59 kcal mol-1 in the sequence CN > CHO > N02, which parallels the increase in electron-attracting power of these three substituents. The transition state of this rotation is best described by a zwitterionic rotated configuration of the type indicated in 98b to 98d. The barrier to internal rotation around the C1—N1 bond varies in the reversed order from 21.82, 17.56 to 12.77 kcal mol-1 for 129, 130, and 131, respectively. Both trends are consistent with the variations of the electron-acceptor properties of the substituents which decrease in the sequence N02 > CHO > CN. The rotational barriers of / -CH3 and / -F substituted vinylamines 127 and 128 are calculated in the 6-31G basis set to be much lower 3.13 kcal mol-1 for the E-CH3 substituent 127a, (in parentheses, the value for the higher barrier to type 120 4.64 kcal mol-x), 2.80 (4.64 kcal mol-1) for Z-CH3 (127b) for E-F 0.33 (2.89 kcal -mol-1), and for 128b a reversal of the two barriers. That barrier to type 121 is now 4.55 kcal mol-1, and that to 120 is 3.89 kcal mol-1. 

Solvent tends to attenuate the a-branching effect in Sj,f2 reactions (see Table 6.6). The gas-phase barrier increases twice as rapidly with the addition of each methyl group compared to that in acetone. Jensen" performed B3LYP/6-31-I-G optimizations of the reactants and TSs in the gas phase and then obtained free energies in water using the polarized continuum mediod (PCM). These results are also listed in Table 6.6. The gas-phase computational values match up well with the experimental values. However, the relative activation barriers computed with PCM are quite similar to the gas-phase values and, therefore, overestimate the solution-phase barriers. 

Within the studies of the effect of p-substitution on the gas-phase S 2 reaction described in the previous section was an examination of the solution-phase effect. Listed in Table 6.8 are the experimental values of the relative activation barriers of two simple substitution reactions with increasing p-substitution. The solution-phase barriers for the chloride exchange reactions were computed by adding the C-PCM/B3LYP/6-31G(d) solvation energies to the CBS-QB3 gas-phase energies or using the SM8/M06-2x/6-31-l-G(d,p) method. These computed relative activation barriers are also listed in Table 6.8. The trend of... 

OOH. The barrier that was 0.59 eV in gas phase is now 1.23 eV in solvent. HO-OH dissociation is also hardly affected by solvation just as O-OH was not (gas phase barrier = 0.35 eV, solvent phase barrier = 0.43 eV). Overall, the new rate determining step for this pathway is H O formation. 

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