Thermal Aromatizations of 2-Vinylmethylenecyclopropane and 3-Vinylcyclobutene

A comprehensive theoretical investigation of thermal rearrangements of 2-vinylmethylenecyclopropane and 3-vinylcyclobutene is carried out employing density functional theory and high level ab initio methods, such as the complete active space self-consistent field, multi-reference second-order Møller–Plesset perturbation theory, and coupled-cluster singles and doubles with perturbative triples. In all computations, Pople’s polarized triple-ζ split valence basis set, 6-311G­(d,p), is utilized. The potential energy surface for the relevant system is explored to provide theoretical insights for the thermal aromatizations of 2-vinylmethylenecyclopropane and 3-vinylcyclobutene. The rate constant for each isomerization reaction is computed using the transition state theory. The simultaneous first-order ordinary-differential equations are solved numerically for the considered system to obtain time-dependent concentrations, hence the product distributions at a given temperature. Our results demonstrate that at high temperatures thermal aromatizations of 2-vinylmethylenecyclopropane (at 700 °C and higher) and 3-vinylcyclobutene (at 500 °C and higher) are feasible under appropriate experimental conditions. However, at low temperatures (at 500 °C and lower), 2-vinylmethylenecyclopropane yields 3-methylenecyclopentene as a unique product, kinetically, and the formation of benzene is not favorable. Similarly, at 300 °C and lower temperatures, 3-vinylcyclobutene can only yield <i>trans</i>-1,3,5-hexatriene (major) and <i>cis</i>-1,3,5-hexatriene (minor). At 300 < <i>T</i> < 500 °C, 3-vinylcyclobutene almost completely yields 1,3-cyclohexadiene. Hence, our computations provide a useful insight for the synthesis of substituted aromatic compounds. Further, calculated energy values (reaction energies and activation parameters) are in satisfactory agreement with the available experimental results

Theoretical Study of Thermal Rearrangements of 1-Hexen-5-yne, 1,2,5-Hexatriene, and 2-Methylenebicyclo[2.1.0]pentane

In this research, a comprehensive theoretical investigation of the thermal rearrangements of 1-hexen-5-yne, 1,2,5-hexatriene, and 2-methylenebicyclo[2.1.0]­pentane is carried out employing density functional theory (DFT) and high level <i>ab initio</i> methods, such as the complete active space self-consistent field (CASSCF), multireference second-order Møller–Plesset perturbation theory (MRMP2), and coupled-cluster singles and doubles with perturbative triples [CCSD­(T)]. The potential energy surface (PES) for the relevant system is explored to provide a theoretical account of pyrolysis experiments by Huntsman, Baldwin, and Roth on the target system. The rate constants and product distributions are calculated using theoretical kinetic modelings. The rate constant for each isomerization reaction is computed using the transition state theory (TST). The simultaneous first-order ordinary-differential equations are solved numerically for the relevant system to obtain time-dependent concentrations, hence the product distributions at a given temperature. Our computed energy values (reaction energies and activation parameters) are in agreement with Roth’s experiments and the product distributions of Huntsman’s experiments at 340 and 385 °C with various reaction times, while simulated product fractions are in qualitative accordance with Baldwin’s experiment

Potential Energy Surfaces for Rearrangements of Berson Trimethylenemethanes

In this research, thermal rearrangements of the Berson trimethylenemethanes (Berson-TMMs) have been investigated by employing density functional theory (DFT) and high-level ab initio methods, such as the complete active space self-consistent field (CASSCF), multireference second-order Møller–Plesset perturbation theory (MRMP2), multireference configuration interaction singles and doubles (MRCISD), and coupled-cluster singles and doubles with perturbative triples [CCSD­(T)]. In all computations Pople’s polarized triple-ζ split valence basis set, 6-311G­(d,p), is utilized. The relevant portions of the lowest-energy, singlet-spin potential energy surface of the C₄H₆ (parent TMM), C₆H₈ (Berson-TMMa), and C₈H₁₂ (Berson-TMMc) chemical systems have been explored in order to determine the reaction energies and activation parameters accurately, with the ultimate objective of providing a theoretical account of experiments by Berson on TMMc. The nature of the orthogonal and the planar structures of the parent TMM have been clarified in this study. We have concluded that the orthogonal TMM ¹B₁ minimum has a <i>C</i>_2<i>v</i> symmetry structure, and there is no pyramidalization in the unique methylene group. It lies at 13.9 kcal mol^–1 above the triplet minimum ³B₂ at MRCISD level. The closed-shell ¹A₁ state of the planar TMM is not a true minimum but a transition structure (TS) for 180° rotation of the unique methylene group in the orthogonal TMM minimum. It lies at 3.0 kcal mol^–1 above ¹B₁. The planar structures are also involved in the interchange of equivalent orthogonal TMMs (<b>o</b>₁, <b>o</b>₂, <b>o</b>₃). Many features of the parent TMM are retained in TMMa and TMMc, despite the constraints imposed by the five-membered ring in the latter species. Thus, ring closure to the bicyclic molecules <b>3a</b> (<b>3c</b>) and <b>5a</b> (<b>5c</b>) takes place similarly to that in the parent TMM. Likewise, planar TMMa (TMMc) structures are TSs, while orthogonal ones are true minima. The adiabatic singlet–triplet gaps are also similar, being 14.7 (13.0) and 16.5 (16.2) kcal mol^–1 in the orthogonal (<b>o</b>₁) and planar TMMa (TMMc), respectively. It has been shown here that the substantial reductions in the ring-opening barriers of MCP derivatives <b>3a </b>(<b>3c</b>) and <b>5a </b>(<b>5c</b>) can be largely attributed to ring strain in the former and π-bond strain in the latter species

Philosophical reflexivity and entrepreneurship research

International audienceEntrepreneurship research attracts scholars from a wide spectrum of disciplines. Yet the field is multi-paradigmatic and lacking consensus, even on the nature of core entrepreneurial phenomena. What is recognized is that it is characterized by dynamic and emergent processes - a complex interplay between actors, processes and contexts. As a result, post-positivistic approaches are gaining traction in a field long dominated by positivistic philosophies. This book reflects on the fundamental philosophical basis of entrepreneurship scholarship. It explores the shifting meanings of entrepreneur and entrepreneurship, the unexamined assumptions which lie behind the established discourses which legitimize or dismiss the possibilities for scholarship. Contributing scholars adopt a reflexive approach to entrepreneurship research challenging readers to question their approaches and assumptions and explicitly defend them against competing alternatives. Building on this critical reflection, this book provides space for philosophical reflexivity in the conduct and publication of scholarly enquiry and will be of great interest to scholars, researchers and advanced students in all aspect of entrepreneurship study.<br/

Effect of Charge State on the Equilibrium and Kinetic Properties of Mechanically Interlocked [5]Rotaxane: A Molecular Dynamics Study

Rotaxanes can exhibit stimuli-responsive behavior by allowing positional fluctuations of their rota groups in response to physiochemical conditions such as the changes in solution pH. However, ionic strength of the solution also affects the molecular conformation by altering the charge state of the entire molecule, coupling the stimuli-responsiveness of rotaxanes with their conformation. A molecular-scale investigation on a model system can allow the decoupling and identification of various effects and can greatly benefit applications of such molecular switches. By using atomistic molecular dynamics simulations, we study equilibrium and kinetics properties of various charge states of the [5]rotaxane, which is a supramolecular moiety with four rotaxanes bonded to a porphyrin core. We model various physiochemical charge states, each of which can be realized at various solution pH levels as well as several exotic charge distributions. By analyzing molecular configurations, hydrogen bonding, and energetics of single molecules in salt-free water and its polyrotaxanated network at the interface of water and chloroform, we demonstrate that charge-neutral and negatively charged molecules often tend to collapse in a way that they can expose their porphyrin core. Contrarily, positively charged moieties tend to take more extended molecular configurations blocking the core. Further, sudden changes in the charge states emulating the pH alterations in solution conditions lead to rapid, sub-10 ns level, changes in the molecular conformation of [5]rotaxane via shuttling motion of CB6 rings along axles. Finally, simulations of 2D [5]rotaxane network structures support our previous findings on a few nanometer-thick film formation at oil–water interfaces. Overall, our results suggest that rotaxane-based structures can exhibit a rich spectrum of molecular configurations and kinetics depending on the ionic strength of the solution