Bun Chan

Through-Space Intervalence Charge Transfer as a Mechanism for Charge Delocalization in Metal–Organic Frameworks

Understanding the nature of charge transfer mechanisms in 3-dimensional metal-organic frameworks (MOFs) is an important goal owing to the possibility of harnessing this knowledge to design electroactive and conductive frameworks. These materials have been proposed as the basis for the next generation of technological devices for applications in energy storage and conversion, including electrochromic devices, electrocatalysts, and battery materials. After nearly two decades of intense research into MOFs, the mechanisms of charge transfer remain relatively poorly understood, and new strategies to achieve charge mobility remain elusive and challenging to experimentally explore, validate, and model. We now demonstrate that aromatic stacking interactions in Zn(II) frameworks containing cofacial thiazolo[5,4- d]thiazole (TzTz) units lead to a mixed-valence state upon electrochemical or chemical reduction. This through-space intervalence charge transfer (IVCT) phenomenon represents a new mechanism for charge transfer in MOFs. Computational modeling of the optical data combined with application of Marcus-Hush theory to the IVCT bands for the mixed-valence framework has enabled quantification of the degree of charge transfer using both in situ and ex situ electro- and spectro-electrochemical methods. A distance dependence for the through-space electron transfer has also been identified on the basis of experimental studies and computational calculations. This work provides a new window into electron transfer phenomena in 3-dimensional coordination space, of relevance to electroactive MOFs where new mechanisms for charge transfer are highly sought after, and to understanding biological light-harvesting systems where through-space mixed-valence interactions are operative.

Unravelling Some of the Key Transformations in the Hydrothermal Liquefaction of Lignin

Using both experimental and computational methods, focusing on intermediates and model compounds, some of the main features of the reaction mechanisms that operate during the hydrothermal processing of lignin were elucidated. Key reaction pathways and their connection to different structural features of lignin were proposed. Under neutral conditions, subcritical water was demonstrated to act as a bifunctional acid/base catalyst for the dissection of lignin structures. In a complex web of mutually dependent interactions, guaiacyl units within lignin were shown to significantly affect overall lignin reactivity.

Correlation functional in screened-exchange density functional theory procedures

In the present study, we have explored several prospects for the further development of screened‐exchange density functional theory (SX‐DFT) procedures. Using the performance of HSE06 as our measure, we find that the use of alternative correlation functionals (as oppose to PBEc in HSE06) also yields adequate results for a diverse set of thermochemical properties. We have further examined the performance of new SX‐DFT procedures (termed HSEB‐type methods) that comprise the HSEx exchange and a (near‐optimal) reparametrized B97c (cOS,0u2009=u2009cSS,0u2009=u20091, cOS,1u2009=u2009−1.5, cOS,2u2009=u2009−0.644, cSS,1u2009=u2009−0.5, and cSS,2u2009=u20091.10) correlation functionals. The different variants of HSEB all perform comparably to or slightly better than the original HSE‐type procedures. These results, together with our fundamental analysis of correlation functionals, point toward various directions for advancing SX‐DFT methods.

Construction of Challenging Proline–Proline Junctions via Diselenide–Selenoester Ligation Chemistry

Polyproline sequences are highly abundant in prokaryotic and eukaryotic proteins, where they serve as key components of secondary structure. To date, construction of the proline-proline motif has not been possible owing to steric congestion at the ligation junction, together with an n → π* electronic interaction that reduces the reactivity of acylated proline residues at the C-terminus of peptides. Here, we harness the enhanced reactivity of prolyl selenoesters and a trans-γ-selenoproline moiety to access the elusive proline-proline junction for the first time through a diselenide-selenoester ligation-deselenization manifold. The efficient nature of this chemistry is highlighted in the high-yielding one-pot assembly of two proline-rich polypeptide targets, submaxillary gland androgen regulated protein 3B and lumbricin-1. This method provides access to the most challenging of ligation junctions, thus enabling the construction of previously intractable peptide and protein targets of increasing structural complexity.

Accurate Thermochemical and Kinetic Stabilities of C84 Isomers

Accurate double-hybrid density functional theory and isodesmic-type reaction schemes are utilized to report accurate estimates of the heats of formation (Δf H) for all 24 isolated-pentagon-rule isomers of the third most abundant fullerene, C84. Kinetic stabilities of these C84 isomers are also considered via C-C bond cleavage rates ( Pcleav) calculated using density functional theory. Our results show that the relative abundance of C84 fullerene isomers observed in arc discharge synthesis is the result of both thermochemical and kinetic factors. This provides timely insight regarding the characterization of several C84 isomers that have been obtained experimentally to date. For instance, the established assignments of C84 isomers of (using the Fowler-Manolopoulos numbering scheme) 22 [ D2(IV)], 23 [ D2 d(II)], 19 [ D3 d], 24 [ D6 h], 11 [ C2(IV)], and 4 [ D2 d(I)] are consistent with the relative Δf H and Pcleav values for these structures. However, our thermochemical and kinetic stabilities of C s isomers 14, 15, and 16 indicate that the two experimentally isolated C s isomers are 15 and 16, contrary to some previous assignments. Of the remaining isolated isomers of symmetry C2 and D2, definitive assignment was not possible with consideration of only Δf H and Pcleav.

The reHISS Three-Range Exchange Functional with an Optimal Variation of Hartree-Fock and Its Use in the reHISSB-D Density Functional Theory Method

In the present study, we have reparametrized the HISS exchange functional. The new “reHISS” exchange provides a balance between short‐ and mid‐range Hartree–Fock exchange (HFX) and a large total HFX coverage, with a fast convergence to zero HFX in the long range. The five parameters in this functional (according to equations 3 and 4 in the main text) are cSR = 0.15, cMR = 2.5279, cLR = 0, ωSR = 0.27, and ωLR = 0.2192. The combination of reHISS exchange with a reparametrized B97c‐type correlation functional (Chan et al., J. Comput. Chem. 2017, 38, 2307) and a D2 dispersion term (s6 = 0.6) gives the reHISSB‐D method. We find it to be more accurate than related screened‐exchange methods and, importantly, its accuracy is more uniform across different properties. Fundamentally, our analysis suggests that the good performance of the reHISS exchange is related to it capturing a near‐optimal proportion of HFX in the range of interelectronic distance that is important for many chemical properties, and we propose this range to be approximately 1–4Å.

Formulation of Small Test Sets Using Large Test Sets for Efficient Assessment of Quantum Chemistry Methods

In the present study, we have examined in detail literature data of deviations for a wide range of (mainly) DFT methods for the extensive MGCDB82 set (∼4400 data points) of main-group thermochemical quantities. We use the data and standard statistical techniques (lasso regularization and forward selection) to devise the MG8 model for linearly combining assessment results of a collection of small data sets to accurately estimate the MAD of MGCDB82. The MG8 model contains a total of 64 data points representing noncovalent interactions, isomerization energies, thermochemical properties, and barrier heights. It is thus well suited for rapid evaluation of new quantum chemistry procedures. We propose that a value of ∼4 kJ mol-1 for an estimated MAD by the MG8 model (EMADMG8) to be an initial indicator of a highly robust quantum chemistry method, with large deviations occurring mainly for properties (such as heats of formation) that are known to be difficult to accurately compute. For methods with larger EMADs, we emphasize the importance of more thorough testing, as these methods are likely to have a larger number of outliers, and it may be less trivial to anticipate circumstances under which large deviations occur. In relation to this aspect, we have applied the same generally applicable statistical techniques to further formulate small-data-set models for assessing the accuracy for some properties that are not covered by MG8 nor by MGCDB82. They include the MOR13 model for metal-organic reactions, the SBG5 model for semiconductor band gaps, and MB13 for stress-testing methods with artificial species.

Mechanism for Three-Component Ni-Catalyzed Carbonyl–Ene Reaction for CO2 Transformation: What Practical Lessons Do We Learn from DFT Modelling?

In the present study, we use computational quantum chemistry to examine the nickel-catalyzed three-component coupling for transforming CO2 into a homoallylic alcohol. We find that the reaction is limited by several Ni-assisted atom transfer reactions in the catalytic cycle, in which a new product formation pathway is found from our calculations. Our results also point towards several key factors for an efficient reaction. Thus, substrates that would lead to a stabilized alkene facilitate a key step in the catalytic cycle. The optimal phosphine ligand should provide a good balance between directing stereochemistry with its steric bulk and enabling the reaction without being excessively bulky. Our calculations also highlight the importance of carefully chosen substrates and ligands in order to avoid potential side reactions, and that knowing the conformational preference in the substrate alone may not be sufficient for predicting the stereochemistry.

Modelling the Effect of Conformation on Hydrogen-Atom Abstraction from Peptides

Computational quantum chemistry is used to examine the effect of conformation on the kinetics of hydrogen-atom abstraction by HO• from amides of glycine and proline as peptide models. In accord with previous findings, it is found that there are substantial variations possible in the conformations and the corresponding energies, with the captodative effect, hydrogen bonding, and solvation being some of the major features that contribute to the variations. The ‘minimum-energy-structure-pathway’ strategy that is often employed in theoretical studies of peptide chemistry with small models certainly provides valuable fundamental information. However, one may anticipate different reaction outcomes in structurally constrained systems due to modified reaction thermodynamics and kinetics, as demonstrated explicitly in the present study. Thus, using a ‘consistent-conformation-pathway’ approach may indeed be more informative in such circumstances, and in this regard theory provides information that would be difficult to obtain from experimental studies alone.

Impact of Hydrogen Bonding on the Susceptibility of Peptides to Oxidation

The tendency of peptides to be oxidized is intimately connected with their function and even their ability to exist in an oxidative environment. Here we report high-level theoretical studies that show that hydrogen bonding can alter the susceptibility of peptides to oxidation, with complexation to a hydrogen-bond acceptor facilitating oxidation, and vice versa, impacting the feasibility of a diverse range of biological processes. It can even provide an energetically viable mechanistic alternative to direct hydrogen-atom abstraction. We find that hydrogen bonding to representative reactive groups leads to a broad (≈400u2005kJu2009mol-1 ) spectrum of ionization energies in the case of model amide, thiol and phenol systems. While some of the oxidative processes at the extreme ends of the spectrum are energetically prohibitive, subtle environmental and solvent effects could potentially mitigate the situation, leading to a balance between hydrogen bonding and oxidative susceptibility.