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Lischka, Hans

Publications and source records attributed to Lischka, Hans.

11 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2024

Facile Access to Organostibines via Selective Organic Superbase Catalyzed Antimony‐Carbon Protonolysis

Abstract The selective formation of antimony‐carbon bonds via organic superbase catalysis under metal‐ and salt‐free conditions is reported. This novel approach utilizes electron‐deficient stibine, Sb(C 6 F 5 ) 3 , to give upon base‐catalyzed reactions with weakly acidic aromatic and heteroaromatic hydrocarbons access to a range of new aromatic and heteroaromatic stibines, respectively, with loss of C 6 HF 5 . Also, the significantly less electron‐deficient stibines, Ph 2 SbC 6 F 5 and PhSb(C 6 F 5 ) 2 smoothly underwent base‐catalyzed exchange reactions with a range of terminal alkynes to generate the stibines of formulae PhSb(C≡CPh) 2 , and Ph 2 SbC≡CR [R=C 6 H 5 , C 6 H 4 ‐NO 2 , COOEt, CH 2 Cl, CH 2 NEt 2 , CH 2 OSiMe 3 , Sb(C 6 H 5 ) 2 ], respectively. These formal substitution reactions proceed with high selectivity as only the C 6 F 5 groups serve as a leaving group to be liberated as C 6 HF 5 upon formal proton transfer from the alkyne. Kinetic studies of the base‐catalyzed reaction of Ph 2 SbC 6 F 5 with phenyl acetylene to form Ph 2 SbC≡CPh and C 6 HF 5 suggested the empirical rate law to exhibit a first‐order dependence with respect to the base catalyst, alkyne and stibine. DFT calculations support a pathway proceeding via a concerted σ‐bond metathesis transition state, where the base catalyst activates the Sb‐C 6 F 5 bond sequence through secondary bond interactions.

Culvyhouse, Jacob↗
DOE OSTI2024

Facile Access to Organostibines via Selective Organic Superbase Catalyzed Antimony‐Carbon Protonolysis

The selective formation of antimony-carbon bonds via organic superbase catalysis under metal- and salt-free conditions is reported. Here, this novel approach utilizes electron-deficient stibine, Sb(C 6 F 5 ) 3 , to give upon base-catalyzed reactions with weakly acidic aromatic and heteroaromatic hydrocarbons access to a range of new aromatic and heteroaromatic stibines, respectively, with loss of C 6 HF 5 . Also, the significantly less electron-deficient stibines, Ph 2 SbC 6 F 5 and PhSb(C 6 F 5 ) 2 smoothly underwent base-catalyzed exchange reactions with a range of terminal alkynes to generate the stibines of formulae PhSb(C≡CPh) 2 , and Ph 2 SbC≡CR [R=C 6 H 5 , C 6 H 4 -NO 2 , COOEt, CH 2 Cl, CH 2 NEt 2 , CH 2 OSiMe 3 , Sb(C 6 H 5 ) 2 ], respectively. These formal substitution reactions proceed with high selectivity as only the C 6 F 5 groups serve as a leaving group to be liberated as C 6 HF 5 upon formal proton transfer from the alkyne. Kinetic studies of the base-catalyzed reaction of Ph 2 SbC 6 F 5 with phenyl acetylene to form Ph 2 SbC≡CPh and C 6 HF 5 suggested the empirical rate law to exhibit a first-order dependence with respect to the base catalyst, alkyne and stibine. DFT calculations support a pathway proceeding via a concerted σ-bond metathesis transition state, where the base catalyst activates the Sb-C 6 F 5 bond sequence through secondary bond interactions.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2023

A Perspective on Sustainable Computational Chemistry Software Development and Integration

The power of quantum chemistry to predict the ground and excited state properties of complex chemical systems has driven the development of computational quantum chemistry software, integrating advances in theory, applied mathematics, and computer science. The emergence of new computational paradigms associated with exascale technologies also poses significant challenges that require a flexible forward strategy to take full advantage of existing and forthcoming computational resources. In this context, the sustainability and interoperability of computational chemistry software development are among the most pressing issues. In this perspective, we discuss software infrastructure needs and investments with an eye to fully utilize exascale resources and provide unique computational tools for next-generation science problems and scientific discoveries.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Non-Kasha fluorescence of pyrene emerges from a dynamic equilibrium between excited states

Pyrene fluorescence after a high-energy electronic excitation exhibits a prominent band shoulder not present after excitation at low energies. The standard assignment of this shoulder as a non-Kasha emission from the second-excited state (S 2 ) has been recently questioned. To elucidate this issue, we simulated the fluorescence of pyrene using two different theoretical approaches based on vertical convolution and nonadiabatic dynamics with nuclear ensembles. To conduct the necessary nonadiabatic dynamics simulations with high-lying electronic states and deal with fluorescence timescales of about 100 ns of this large molecule, we developed new computational protocols. The results from both approaches confirm that the band shoulder is, in fact, due to S 2 emission. We show that the non-Kasha behavior is a dynamic-equilibrium effect not caused by a metastable S 2 minimum. However, it requires considerable vibrational energy, which can only be achieved in collisionless regimes after transitions into highly excited states. This strict condition explains why the S 2 emission was not observed in some experiments.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2021

Pathways to fluorescence via restriction of intramolecular motion in substituted tetraphenylethylenes

The design of materials with enhanced luminescence properties is a fast-developing field due to the potential applicability of these materials as light-emitting diodes or for bioimaging. A transparent way to enhance the emission properties of interesting molecular candidates is blocking competing and unproductive non-radiative relaxation pathways by the restriction of intramolecular motions. Rationalized functionalization is an important possibility to achieve such restrictions. Using time-dependent density functional theory (TD-DFT) based on the ωB97XD functional and the semiempirical tight-binding method including long-range corrections (TD-LC-DFTB), this work investigates the effect of functionalization of the paradigmatic tetraphenylethylene (TPE) on achieving restricted access to conical intersections (RACI). Photodynamical surface hopping simulations have been performed on a larger set of compounds including TPE and ten functionalized TPE compounds. Functionalization has been achieved by means of electron-withdrawing groups, bulky groups which block the relaxation channels via steric hindrance and groups capable of forming strong hydrogen bonds, which restrict the motion via the formation of hydrogen bond channels. Most of the investigated functionalized TPE candidates show ultrafast deactivation to the ground state due to their still existing structural flexibility, but two examples, one containing –CN and –CF 3 groups and a second characterized by a network of hydrogen bonds, have been identified as interesting candidates for creating efficient luminescence properties in solution.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2020

The Generality of the GUGA MRCI Approach in COLUMBUS for Treating Complex Quantum Chemistry

The core part of the program system COLUMBUS allows highly efficient calculations using variational multireference (MR) methods in the framework of configuration interaction with single and double excitations (MR-CISD) and averaged quadratic coupled-cluster calculations (MR-AQCC), based on uncontracted sets of configurations and the graphical unitary group approach (GUGA). The availability of analytic MR-CISD and MR-AQCC energy gradients and analytic nonadiabatic couplings for MR-CISD enables exciting applications including, e.g., investigations of π-conjugated biradicaloid compounds, calculations of multitudes of excited states, development of diabatization procedures, and furnishing the electronic structure information for on-the-fly surface nonadiabatic dynamics. With fully variational uncontracted spin-orbit MRCI, COLUMBUS provides a unique possibility of performing high-level calculations on compounds containing heavy atoms up to lanthanides and actinides. Crucial for carrying out all of these calculations effectively is the availability of an efficient parallel code for the CI step. Configuration spaces of several billion in size now can be treated quite routinely on standard parallel computer clusters. Emerging developments in COLUMBUS, including the all configuration mean energy multiconfiguration self-consistent field method and the graphically contracted function method, promise to allow practically unlimited configuration space dimensions. Spin density based on the GUGA approach, analytic spin-orbit energy gradients, possibilities for local electron correlation MR calculations, development of general interfaces for nonadiabatic dynamics, and MRCI linear vibronic coupling models conclude this overview.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗