The cycloadditions of cyclopentadiene with diphenylketene and dichloroketene are studied by a combination of kinetic and product studies, kinetic isotope effects, standard theoretical calculations, and trajectory calculations. In contrast to recent reports, the reaction of cyclopentadiene with diphenylketene affords both [4 + 2] and [2 + 2] cycloadducts directly. This is surprising, since there is only one low-energy transition structure for adduct formation in mPW1K calculations, but quasiclassical trajectories started from this single transition structure afford both [4 + 2] and [2 + 2] products. The dichloroketene reaction is finely balanced between [4 + 2] and [2 + 2] cycloaddition modes in mPW1K calculations, as the minimum-energy path (MEP) leads to different products depending on the basis set. The MEP is misleading in predicting a single product, as trajectory studies for the dichloroketene reaction predict that both [4 + 2] and [2 + 2] products should be formed. The periselectivity does not reflect transition state orbital interactions. The 13 C isotope effects for the dichloroketene reaction are well-predicted from the mPW1K/6−31+G** transition structure. However, the isotope effects for the diphenylketene reaction are not predictable from the cycloaddition transition structure and transition state theory. The isotope effects also appear inconsistent with kinetic observations, but the trajectory studies evince that non-statistical recrossing can reconcile the apparently contradictory observations. B3LYP calculations predict a shallow intermediate on the energy surface, but trajectory studies suggest that the differing B3LYP and mPW1K surfaces do not result in qualitatively differing mechanisms. Overall, an understanding of the products, rates, selectivities, isotope effects, and mechanism in these reactions requires the explicit consideration of dynamic trajectories.Selectivity in cycloadditions may take many forms, e.g., endo/exo stereoselectivity, regioselectivity, facial stereoselectivity, and diene/dienophile role selectivity. When two distinct formally allowed processes are possible, as in the [4 + 2] versus [6 + 4] cycloadditions of cyclopentadiene with tropone, 1 their differentiation is referred to as periselectivity. The underlying framework within which chemists usually understand any of these forms of selectivity is transition state theory (TST). The preferred product would be that involving the lowest-energy transition state, and the degree of selectivity would be determined by the relative energies for separate transition states. Even when there is no enthalpic barrier, reactivity and selectivity can be discussed in terms of free-energy barriers. 2 Qualitative theories of selectivity such as FMO theory may be thought of as a simplified surrogate for TST, easing the task of predicting which cycloaddition barrier is lowest in energy.singleton@mail.chem.tamu.edu. Publisher's Disclaimer: This PDF receipt will only be used as the basis for generating PubMed Central (PMC) documents. PMC ...

The mechanism of the heterolytic solvolysis of p-tolyldiazonium cation in water was studied by a combination of kinetic isotope effects, theoretical calculations, and dynamics trajectories. Significant (13)C kinetic isotope effects were observed at the ipso (k(12)C/k(13)C = 1.024), ortho (1.017), and meta (1.013) carbons, indicative of substantial weakening of the C(2)-C(3) and C(5)-C(6) bonds at the transition state. This is qualitatively consistent with a transition state forming an aryl cation, but on a quantitative basis, simple S(N)1 heterolysis does not account best for the isotope effects. Theoretical S(N)2Ar transition structures for concerted displacement of N(2) by a single water molecule lead to poor predictions of the experimental isotope effects. The best predictions of the (13)C isotope effects arose from transition structures for the heterolytic process solvated by clusters of water molecules. These structures, formally saddle points for concerted displacements on the potential energy surface, may be described as transition structures for solvent reorganization around the aryl cation. Quasiclassical dynamics trajectories starting from these transition structures afforded products very slowly, compared to a similar S(N)2 displacement, and the trajectories often afforded long-lived aryl cation intermediates. Critical prior evidence for aryl cation intermediates is reconsidered with the aid of DFT calculations. Overall, the nucleophilic displacement process for aryldiazonium ions in water is at the boundary between S(N)2Ar and S(N)1 mechanisms, and an accurate view of the reaction mechanism requires consideration of dynamic effects.

This paper reports on the performance of several ionic liquids for use in separating an olefin (1-octene) from a paraffin (n-octane). To quantify the effectiveness of an ionic liquid in a given separation, a measurement of the activity coefficients of the solutes in the solvent can be used as a guide. This measurement permits the calculation of selectivities and capacities that are important factors in judging the suitability and potential commercial viability of an ionic liquid for a separation application. Several different ionic liquids were tested and compared to a traditional, conventional solvent, N-methyl-pyrrolidone (NMP), and to each other. One ionic liquid tested, silver(I)/N,N-dimethylbenzoamide bis(trifluoromethylsulfonyl) imide [Ag(DMBA)2 +Tf2N−], showed selectivities 24−32 times higher than NMP while maintaining a comparable mass capacity as the nonionic solvent. The experimental data showed that this particular ionic liquid was also superior to any of the other ionic liquids tested in this series of experiments. This ionic liquid has the potential to be used in an economically viable commercial process to recover valuable olefinic components from streams where they are mixed with paraffins.