11 Search Results

The Bergman cyclization of (Z)-hexa-3-ene-1,5-diyne to form the aromatic diradical p-benzyne has garnered attention as a potential antitumor agent due to its relatively low cyclization barrier and the stability of the resulting diradical. Here, we present a theoretical investigation of several ionic extensions of the fundamental Bergman cyclization: electrocyclizations of the penta-1,4-diyne anion, hepta-1,6-diyne cation, and octa-1,7-diyne dication, leveraging the spin-flip formulation of the equation-of-motion coupled cluster theory with single and double substitutions (EOM-SF-CCSD). Though the penta-1,4-diyne anion exhibits a large cyclization barrier of +66 kcal mol^–1, cyclization of both the hepta-1,6-diyne cation and octa-1,7-diyne dication along a previously unreportedmore » triplet pathway requires relatively low energy. We also identified the presence of significant aromaticity in the triplet diradical products of these two cationic cyclizations.« less

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Abstract This Review brings a new perspective to the study of chemical reactions in compressed fluid media. We begin by reviewing the substantial insight gained from more than 50 years of experimental studies on organic reactions in solution under pressure. These led to a proper estimation of the critical roles of volume of activation (Δ ^≠ V) and reaction volume (ΔV) in understanding pressure effect on rates and equilibria of organic reactions. A recently developed computational method, the XP‐PCM (extreme pressure polarizable continuum model) method, capable of carrying out quantum mechanical calculations of reaction pathways of molecules under pressure, is introduced.more » A case study of the Diels–Alder cycloaddition of cyclopentadiene with ethylene serves, in pedagogical detail, to describe the methodology. We then apply the XP‐PCM method to a selection of other pericyclic reactions, including the parent Diels–Alder cycloaddition of butadiene with ethylene, electrocyclic ring‐opening of cyclobutene, electrocyclic ring‐closing of Z‐hexatriene, the [1,5]‐H shift in Z‐pentadiene, and the Cope rearrangement. These serve as examples of some of the most common combinations of Δ ^≠ V and ΔV. Interesting phenomena such as a shift in a transition state along a reaction coordinate, a switch of rate‐determining step, and the possible turning of a transition state into a stable minimum are revealed by the calculations. A reaction volume profile, the change in the volume of the reacting molecules as the reaction proceeds, emerges as being useful.« less

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Abstract Dieser Aufsatz zeigt eine neue Perspektive für die Untersuchung chemischer Reaktionen in komprimierten flüssigen Medien auf. Wir beginnen, indem wir die wesentlichen Einsichten zusammenfassen, die in über 50 Jahren experimenteller Studien an organischen Reaktionen in Lösung unter Druck gewonnen wurden. Diese Arbeiten führten zu einer zuverlässigen Beurteilung der kritischen Rolle der Aktivierungs‐ (Δ ^≠ V) und Reaktionsvolumen (ΔV) beim Verständnis von Druckeffekten auf Geschwindigkeiten und Gleichgewichte organischer Reaktionen. Eine kürzlich entwickelte Computermethode, die XP‐PCM‐Methode (XP‐PCM=extreme pressure polarizable continuum model), die quantenchemische Berechnungen von Reaktionswegen von Molekülen unter Druck vornehmen kann, wird eingeführt. Eine Fallstudie der Diels‐Alder‐Cycloaddition von Cyclopentadien mitmore » Ethylen soll – in pädagogischem Detail – der Beschreibung der Methode dienen. Anschließend wenden wir die XP‐PCM‐Methode auf eine Auswahl anderer pericyclischer Reaktionen an, einschließlich der Diels‐Alder‐Cycloaddition von Butadien mit Ethylen, der elektrocyclischen Ringöffnung von Cyclobuten, dem elektrocyclischen Ringschluss von Z‐Hexatrien, der [1,5]‐H‐Verschiebung in Z‐Pentadien und der Cope‐Umlagerung. Diese Reaktionen dienen als Beispiele für einige der häufigsten Kombinationen von Δ ^≠ V und ΔV. Interessante Phänomene, wie die Verschiebung eines Übergangszustands entlang der Reaktionskoordinate, ein Wechsel des geschwindigkeitsbestimmenden Schrittes und die mögliche Umwandlung eines Übergangszustands in ein stabiles Minimum, werden durch diese Berechnungen aufgedeckt. Ein Profil des Reaktionsvolumens, entsprechend der Änderung des Volumens des reagierenden Moleküls im Verlauf der Reaktion, erweist sich als nützlich.« less

We describe a general, symmetry-conditioned way of enumerating isomers of saturated singly substituted one-dimensional nanothreads of the (CH)₅E and (CH)₅CR type, where E is a heteroatom and R is a substituent. Four nanothreads — so-called tube (3,0), polytwistane, the zipper polymer, and polymer I, are treated in detail. The methodology, combining symmetry arguments and computer-based enumeration, is generally applicable to isomerism problems in polymers.

Though hydrogen and lithium have been assigned a common column of the periodic table, their crystalline states under common conditions are drastically different: the former at temperatures where it is crystalline is a molecular insulator whereas the latter is a metal that takes on simple structures. On compression, however, the two come to share some structural and other similarities associated with the insulator-to-metal and metal-to-insulator transitions, respectively. To gain a deeper understanding of differences and parallels in the behaviors of compressed hydrogen and lithium, we performed an ab-initio comparative study of these systems in selected identical structures. Both elements undergomore » a continuous pressure-induced s-p electronic transition, though this is at a much earlier stage of development for H. The valence charge density accumulates in interstitial regions in Li but not in H in structures examined over the same range of compression. Moreover, the valence charge density distributions or electron localization functions for the same arrangement of atoms mirror each other as one proceeds from one element to the other. Application of the viral theorem shows that the kinetic and potential energies jump across the first-order phase transitions in H and Li are opposite in sign because of non-local effects in the Li pseudopotential. Finally, the common tendency of compressed H and Li to adopt three-fold coordinated structures as found is explained by the fact that such structures are capable of yielding a profound pseudogap in the electronic densities of states at the Fermi level, thereby reducing the kinetic energy. Lastly, these results have implications for the phase diagrams of these elements and also for the search for new structures with novel properties.« less