Diels–Alder Cycloaddition of Cyclopentadiene and C60 at the Extreme High Pressure

Diels-Alder Cycloaddition of Cyclopentadiene to C60 and Si60 and Their Endohedral Li+ Counterparts

Both silicon and carbon are elements located in group 14 on the periodic table. Despite some similarities between these two elements, differences in reactivity are important, and whereas carbon is a central element in all known forms of life, silicon is barely found in biological systems. Here, we investigate the Diels-Alder cycloaddition reaction of cyclopentadiene (CP) and cyclopentasildiene (CPSi) with fullerenes C60, Li+@C60, Si60, and Li+@Si60 using density functional theory methods. The results reveal distinct kinetic and thermodynamic trends that govern the reactivity and selectivity. For C60, the [6,6] pathway is kinetically and thermodynamically favored, whereas for Si60, the [5,6] pathway is preferred thermodynamically but not kinetically. The introduction of lithium cations increases the reactivity of both C60 and Si60. Energy decomposition analysis (EDA) unveils the importance of the components of the interaction energy between CPSi and the corresponding fullerenes. The findings provide insights into the interplay of electronic structure, substrate reactivity, and fullerene electrophilicity in cycloaddition reactions.

An algorithm for very high pressure molecular dynamics simulations

We describe a method to run simulations of ground or excited state dynamics under extremely high pressures. The method is based on the introduction of a fictitious ideal gas that exerts the required pressure on a molecular sample and is therefore called XP-GAS (eXtreme Pressure by Gas Atoms in a Sphere). The algorithm is most suitable for approximately spherical clusters of molecules described by quantum chemistry methods, Molecular Mechanics or mixed QM/MM approaches. We compare the results obtained by the algorithm here presented and by the XP-PCM approach, based on a continuum description of the environment. As a test case, we study the conformational dynamics of 1,3-butadiene either as an isolated molecule ("naked" butadiene) or embedded in a cluster of argon atoms, under pressures up to 15 GPa. Overall, our results show that the XP-GAS QM/MM simulation method is in good agreement with the XP-PCM QM/Continuum model (Cammi model) in describing the effect of the pressure on static properties as the equilibrium geometry of butadiene in the ground state. Furthermore, the comparison of XP-GAS simulations with naked butadiene and butadiene in argon shows the importance, for XP-GAS and related methods, of a realistic representation of the medium in modelling pressure effects.

Bond Lengths and Dipole Moments of Diatomic Molecules under Isotropic Pressure with the XP-PCM and GOSTSHYP Models

While high-pressure chemistry has a well-established history, methods to simulate pressure at the single-molecule level have been somewhat lacking. The current work aims at comparing two static models (XP-PCM and GOSTSHYP) to apply isotropic pressure to single molecules, focusing on the equilibrium bond length and electric dipole moment of diatomic molecules. Numerical challenges arising in the potential energy surface using the XP-PCM method were examined, and a pragmatic approach was followed to mitigate these. The definition of the cavity was scrutinized, and two approaches to retrieve the isotropic character that could potentially be lost when using the standard methodology were suggested. Subsequently, equilibrium bond lengths under pressure were evaluated, showing reasonable agreement between GOSTSHYP and XP-PCM, but some discrepancies persist. A Taylor series analysis introduced elsewhere was then applied to rationalize the observed trends in terms of the bond surface. Finally, the dipole moment was shown to be highly sensitive to the cavity definition, and qualitative agreement necessitates the use of our adapted procedure.

Ionic liquid post-modified carboxylate-rich MOFs for efficient catalytic CO2 cycloaddition under solvent-free conditions

The synthesis of cyclic carbonates through cycloaddition reactions between epoxides and carbon dioxide (CO2) is an important industrial process. Metal-Organic Frameworks (MOFs) have functional and ordered pore structures, making them attractive catalysts for converting gas molecules into valuable products. One approach to enhance the catalytic activity of MOFs in CO2 cycloaddition reactions is to create open metal sites within MOFs. In this study, the amino-functionalized rare earth Gd-MOF (Gd-TPTC-NH2) and its ionic liquid composite catalysts (Gd-TPTC-NH-[BMIM]Br) were synthesized using 2'-amino-[1,1':4',1''-terphenyl]-3,3'',5,5''-tetracarboxylic acid (H4TPTC-NH2) as the ligand. The catalytic performance of these two catalysts was observed in the cycloaddition reaction of CO2 and epoxides. Under the optimized reaction conditions, Gd-TPTC-NH-[BMIM]Br can effectively catalyze the cycloaddition reaction of a variety of epoxide substrates with good to excellent yields of cyclic carbonate products. Comparatively, epichlorohydrin and epibromohydrin, which possess halogen substituents, promote higher yields of cyclic carbonates due to the electron-withdrawing nature of Cl and Br substituents. Additionally, the Gd-TPTC-NH-[BMIM]Br catalyst demonstrated good recyclability and reproducibility, maintaining its catalytic activity without any changes in its structure or properties after five reuse cycles.

Extending on-surface synthesis from 2D to 3D by cycloaddition with C60

As an efficient molecular engineering approach, on-surface synthesis (OSS) defines a special opportunity to investigate intermolecular coupling at the sub-molecular level and has delivered many appealing polymers. So far, all OSS is based on the lateral covalent bonding of molecular precursors within a single molecular layer; extending OSS from two to three dimensions is yet to be realized. Herein, we address this challenge by cycloaddition between C60 and an aromatic compound. The C60 layer is assembled on the well-defined molecular network, allowing appropriate molecular orbital hybridization. Upon thermal activation, covalent coupling perpendicular to the surface via [4 + 2] cycloaddition between C60 and the phenyl ring of the molecule is realized; the resultant adduct shows frozen orientation and distinct sub-molecular feature at room temperature and further enables lateral covalent bonding via [2 + 2] cycloaddition. This work unlocks an unconventional route for bottom-up precise synthesis of three-dimensional covalently-bonded organic architectures/devices on surfaces.

High-Pressure Reaction Profiles and Activation Volumes of 1,3-Cyclohexadiene Dimerizations Computed by the Extreme Pressure-Polarizable Continuum Model (XP-PCM)

Quantum chemical calculations are reported for the thermal dimerizations of 1,3-cyclohexadiene at 1 atm and high pressures up to the GPa range. Computed activation enthalpies of plausible dimerization pathways at 1 atm agree well with the experiment activation energies and the values from previous calculations. High-pressure reaction profiles, computed by the recently developed extreme pressure-polarizable continuum model (XP-PCM), show that the reduction of reaction barrier is more profound in concerted reactions than in stepwise reactions, which is rationalized on the basis of the volume profiles of different mechanisms. A clear shift of the transition state towards the reactant under pressure is revealed for the [6+4]-ene reaction by the calculations. The computed activation volumes by XP-PCM agree excellently with the experimental values, confirming the existence of competing mechanisms in the thermal dimerization of 1,3-cyclohexadiene.

The second derivative of the electronic energy with respect to the compression scaling factor in the XP-PCM model: Theory and applications to compression response functions of atoms

We present the analytical theory for the second derivative of the electronic energy with respect to the scaling factor of the compression cavity within the eXtreme pressure polarizable continuum model (XP-PCM) for the study of compressed atomic and molecular systems. The theory has been exploited to study compression response functions describing how the atomic/molecular properties are effected by an external pressure. The response functions considered include the atomic compressibility and the pressure coefficients of the ionization energy (IE) and electron affinity (EA). The theory has been validated by numerical application to compressed neon, argon, and krypton atoms.

Multireference Perturbation Theory Combined with PCM and RISM Solvation Models: A Benchmark Study for Chemical Energetics

The polarizable continuum model (PCM) has been one of the most widely used approaches to take into account the solvation effect in quantum chemical calculations. In this paper, we performed a series of benchmark calculations to assess the accuracy of the PCM scheme combined with the second-order complete-active-space perturbation theory (CASPT2) for molecular systems in polar solvents. For solute molecules with extensive conjugated π orbitals, exemplified by elongated conjugated arylcarbenes, we have incorporated the ab initio density matrix renormalization group algorithm into the PCM-CASPT2 method. In the previous work, we presented a combination of the DMRG-CASPT2 method with the reference interaction site model (RISM) theory for describing the solvation effect using the radial distribution function and compared its performance to the widely used density-functional approaches (PCM-TD-DFT). The work here allows us to further show a more thorough assessment of the RISM model compared to the PCM with an equal level of the wave function treatment, the (DMRG-)CASPT2 theory, toward a high-accuracy electronic structure calculations for solvated chemical systems. With the exception that the PCM models are not capable of properly describing the hydrogen bondings, accuracy of the PCM-CASPT2 model is in most cases quite comparable to the RISM counterpart.

Click Chemistry with Cyclopentadiene

Cyclopentadiene is one of the most reactive dienes in normal electron-demand Diels-Alder reactions. The high reactivities and yields of cyclopentadiene cycloadditions make them ideal as click reactions. In this review, we discuss the history of the cyclopentadiene cycloaddition as well as applications of cyclopentadiene click reactions. Our emphasis is on experimental and theoretical studies on the reactivity and stability of cyclopentadiene and cyclopentadiene derivatives.

Solvation effects drive the selectivity in Diels-Alder reaction under hyperbaric conditions

High pressure effects on the Diels-Alder reaction in condensed phase are investigated by means of theoretical methods, employing advanced multiscale modeling approaches based on physically grounded models. The simulations reveal how the increase of pressure from 1 to 10 000 atm (10 katm) does not affect the stability of the reaction products, modifying the kinetics of the process by lowering considerably the transition state energy. The reaction profile at high pressure remarkably differs from that at 1 atm, showing a submerged TS and a pre-TS structure lower in energy. The different solvation between endo and exo pre-TS is revealed as the driving force pushing the reaction toward a much higher preference for the endo product at high pressure.

Diels-Alder Cycloaddition on Nonisolated-Pentagon-Rule C2v(19 138)-C76 and YNC@C2v(19 138)-C76: The Difference in Regioselectivity Caused by the Inner Metallic Cluster

Diels-Alder reactions of cyclopentadiene to C_2v(19 138)-C₇₆ and YNC@C_2v(19 138)-C₇₆ violating the isolated pentagon rule have been systematically studied by means of density functional theory calculations. As for the free fullerene, the pentalene-type [5,5]-bond in the adjacent pentagon pair is the most favorable from thermodynamic and kinetic viewpoints, which is attributed to the highly strained carbon atoms accompanied by the suitable lowest unoccupied molecular orbital shape with a large distribution to interact with cyclopentadiene. Upon encapsulating the YNC cluster, a corannulene-type [5,6]-bond and a pyracylene-type [6,6]-bond become the two most reactive addition sites under thermodynamic and kinetic conditions, which possess similar reaction energies and energy barriers. Especially, the [5,6]-bond exhibits a larger reaction energy and a lower energy barrier than that on the free fullerene, which should be ascribed to its shorter bond length and larger π-orbital axis vector value after trapping the metallic cluster. The suitable unoccupied molecular orbital lobes with large distributions on the [5,6]- and [6,6]-bonds are also an advantage of cycloadditions. This work presents the first example that the most favorable addition site is remote from the adjacent pentagon pair in the fullerene cage after encapsulating a metallic cluster.

Linear chains of hydrogen molecules under pressure: An extreme-pressure continuum model study

New analytical gradients of the electronic energy of a confined molecular system within the extreme-pressure continuum model are presented and applied to the study of the equilibrium geometries of linear chains of hydrogen molecules nH₂ under pressures. The decrease in inter- and intramolecular H-H distances with the increase in the pressure has been studied up to 80 GPa. We have also shown that the compression of the bond-lengths can be interpreted in terms of the effect of the confining potential of the electron density of the molecular systems.

Analytical calculation of pressure for confined atomic and molecular systems using the eXtreme-Pressure Polarizable Continuum Model

We show that the pressure acting on atoms and molecular systems within the compression cavity of the eXtreme-Pressure Polarizable Continuum method can be expressed in terms of the electron density of the systems and of the Pauli-repulsion confining potential. The analytical expression holds for spherical cavities as well as for cavities constructed from van der Waals spheres of the constituting atoms of the molecular systems. © 2018 Wiley Periodicals, Inc.

Insights on the Realgar Crystal Under Pressure from XP-PCM and Periodic Model Calculations

The spectroscopic properties of As₄S₄ with pressure have been computed by the quantum mechanical XP-PCM method and by density functional theory periodic calculations. The comparison has allowed the interpretation of the available experimental data. By comparison of the two methods and with experiments, we show that the XP-PCM method is able to reproduce the same behavior of the periodic calculations with much lower computational cost allowing to be adopted as a first choice computational tool for a qualitative interpretation of molecular crystals properties under pressure.