Triplet-Tuning: A Novel Family of Non-Empirical Exchange-Correlation Functionals

In the framework of DFT, the lowest triplet excited state, T$_1$, can be evaluated using multiple formulations, the most straightforward of which are UDFT and TDDFT. Assuming the exact XC functional is applied, UDFT and TDDFT provide identical energies for T$_1$ ($E_{\rm T}$), which is also a constraint that we require our XC functionals to obey. However, this condition is not satisfied by most of the popular XC functionals, leading to inaccurate predictions of low-lying, spectroscopically and photochemically important excited states, such as T$_1$ and S$_1$. Inspired by the optimal tuning strategy for frontier orbital energies [Stein, Kronik, and Baer, {\it J. Am. Chem. Soc.} {\bf 2009}, 131, 2818], we proposed a novel and non-empirical prescription of constructing an XC functional in which the agreement between UDFT and TDDFT in $E_{\rm T}$ is strictly enforced. Referred to as "triplet tuning", our procedure allows us to formulate the XC functional on a case-by-case basis using the molecular structure as the exclusive input, without fitting to any experimental data. The first triplet tuned XC functional, TT-$\omega$PBEh, is formulated as a long-range-corrected hybrid of PBE and HF functionals [Rohrdanz, Martins, and Herbert, {\it J. Chem. Phys.} {\bf 2009}, 130, 054112] and tested on four sets of large organic molecules. Compared to existing functionals, TT-$\omega$PBEh manages to provide more accurate predictions for key spectroscopic and photochemical observables, including but not limited to $E_{\rm T}$, $E_{\rm S}$, $\Delta E_{\rm ST}$, and $I$, as it adjusts the effective electron-hole interactions to arrive at the correct excitation energies. This promising triplet tuning scheme can be applied to a broad range of systems that were notorious in DFT for being extremely challenging

Theoretical and Experimental Investigations of High Spin Ionic Intermediates

In order to identify high-spin organic intermediates that could potentially be used as building blocks for the construction of high-spin organic ferromagnets, density functional theory (DFT) computations were performed to assess the singlet-triplet state energy gaps for a number of substituted aryl ionic intermediates. The quantitative accuracy of these DFT computations was benchmarked by high-level multireference second order perturbation theory (CASPT2) computations for representative species. These computations led to the discovery of a novel meta pi donor effect, wherein substituting the meta positions of aryl cationic species such as arylnitrenium ions (Ar-N-H+), arylsilylenium ions (Ar-SiH2+), aryloxenium ions (Ar-O+), and benzyl cations (Ar-CH2+), with pi donors stabilizes a π,π* triplet state analogous to the electronic state of the m-xylylene diradical. Two of these benzylic cations were generated experimentally through photochemical methods and analyzed by laser flash photolysis, chemical trapping studies, and product analysis. The experimental results were consistent with the existence of low-energy triplet states. Additionally, species with an inverted connectivity (e.g. an anionic donor with two pi electron withdrawing groups) were also found to have low-energy triplet states by density functional theory computations. These anions were generated chemically and studied by NMR and EPR spectroscopy as well as quenching studies. The preliminary results of the experimental studies are consistent with the intermediacy of triplet ground state benzyl anions, in line with the theoretical predictions. Vinyl cations substituted with β pi donors were also found to have triplet ground states, as computed by DFT and CBS-QB3 methods. In many cases, the singlet vinyl cations are anticipated to have facile rearrangement pathways, but incorporating the pi donors into rings appears to discourage obvious rearrangement pathways. To permit the photogeneration of congested arylnitrenium ions, a new method for photochemically generating these species was developed through photolysis of protonated 1,1-diarylhydrazines. Additionally, the carbazolyl nitrenium ion was generated photochemically and studied by laser flash photolysis, chemical trapping studies, product analysis and computational studies. This nitrenium ion is found to be more short-lived and reactive than similar diarylnitrenium ions as a likely result of destabilizing antiaromatic character

The influence of solvent representation on nuclear shielding calculations of protonation states of small biological molecules

In this study, we assess the influence of solvation on the accuracy and reliability of isotropic nuclear magnetic shielding calculations for amino acids in comparison to experimental data. We focus particularly on the performance of solvation methods for different protonation states, as biological molecules occur almost exclusively in aqueous solution and are subject to protonation with pH. We identify significant shortcomings of current implicit solvent models and present a hybrid solvation approach that improves agreement with experimental data by taking into account the presence of direct interactions between amino acid protonation state and water molecules

Beyond Density Functional Theory: the Multiconfigurational Approach to Model Heterogeneous Catalysis

Catalytic processes are crucially important for many practical chemical applications. Heterogeneous catalysts are especially appealing because of their high stability and the relative ease with which they may be recovered and reused. Computational modeling can play an important role in the design of more catalytically active materials through the identification of reaction mechanisms and the opportunity to assess hypothetical catalysts in silico prior to experimental verification. Kohn-Sham density functional theory (KS-DFT) is the most used method in computational catalysis because it is affordable and it gives results of reasonable accuracy in many instances. Furthermore, it can be employed in a “black-box” mode that does not require significant a priori knowledge of the system. However, KS-DFT has some limitations: it suffers from self-interaction error (sometime referred to as delocalization error), but a greater concern is that it provides an intrinsically single-reference description of the electronic structure, and this can be especially problematic for modeling catalysis when transition metals are involved. In this perspective, we highlight some noteworthy applications of KS-DFT to heterogeneous computational catalysis, as well as cases where KS-DFT fails accurately to describe electronic structures and intermediate spin states in open-shell transition metal systems. We next provide an introduction to state-of-the-art multiconfigurational (MC; also referred to as multireference (MR)) methods and their advantages and limitations for modeling heterogeneous catalysis. We focus on specific examples to which MC methods have 2 been applied and discuss the challenges associated with these calculations. We conclude by offering our vision for how the community can make further progress in the development of MC methods for application to heterogeneous catalysis

λ-Density Functional Valence Bond: A Valence Bond-Based Multiconfigurational Density Functional Theory With a Single Variable Hybrid Parameter

A new valence bond (VB)-based multireference density functional theory (MRDFT) method, named λ-DFVB, is presented in this paper. The method follows the idea of the hybrid multireference density functional method theory proposed by Sharkas et al. (2012). λ-DFVB combines the valence bond self-consistent field (VBSCF) method with Kohn–Sham density functional theory (KS-DFT) by decomposing the electron–electron interactions with a hybrid parameter λ. Different from the Toulouse's scheme, the hybrid parameter λ in λ-DFVB is variable, defined as a function of a multireference character of a molecular system. Furthermore, the EC correlation energy of a leading determinant is introduced to ensure size consistency at the dissociation limit. Satisfactory results of test calculations, including potential energy surfaces, bond dissociation energies, reaction barriers, and singlet–triplet energy gaps, show the potential capability of λ-DFVB for molecular systems with strong correlation

Software for the frontiers of quantum chemistry: An overview of developments in the Q-Chem 5 package

This article summarizes technical advances contained in the fifth major release of the Q-Chem quantum chemistry program package, covering developments since 2015. A comprehensive library of exchange–correlation functionals, along with a suite of correlated many-body methods, continues to be a hallmark of the Q-Chem software. The many-body methods include novel variants of both coupled-cluster and configuration-interaction approaches along with methods based on the algebraic diagrammatic construction and variational reduced density-matrix methods. Methods highlighted in Q-Chem 5 include a suite of tools for modeling core-level spectroscopy, methods for describing metastable resonances, methods for computing vibronic spectra, the nuclear–electronic orbital method, and several different energy decomposition analysis techniques. High-performance capabilities including multithreaded parallelism and support for calculations on graphics processing units are described. Q-Chem boasts a community of well over 100 active academic developers, and the continuing evolution of the software is supported by an “open teamware” model and an increasingly modular design

Software for the frontiers of quantum chemistry : An overview of developments in the Q-Chem 5 package

This article summarizes technical advances contained in the fifth major release of the Q-Chem quantum chemistry program package, covering developments since 2015. A comprehensive library of exchange–correlation functionals, along with a suite of correlated many-body methods, continues to be a hallmark of the Q-Chem software. The many-body methods include novel variants of both coupled-cluster and configuration-interaction approaches along with methods based on the algebraic diagrammatic construction and variational reduced density-matrix methods. Methods highlighted in Q-Chem 5 include a suite of tools for modeling core-level spectroscopy, methods for describing metastable resonances, methods for computing vibronic spectra, the nuclear–electronic orbital method, and several different energy decomposition analysis techniques. High-performance capabilities including multithreaded parallelism and support for calculations on graphics processing units are described. Q-Chem boasts a community of well over 100 active academic developers, and the continuing evolution of the software is supported by an “open teamware” model and an increasingly modular design.This article summarizes technical advances contained in the fifth major release of the Q-Chem quantum chemistry program package, covering developments since 2015. A comprehensive library of exchange-correlation functionals, along with a suite of correlated many-body methods, continues to be a hallmark of the Q-Chem software. The many-body methods include novel variants of both coupled-cluster and configuration-interaction approaches along with methods based on the algebraic diagrammatic construction and variational reduced density-matrix methods. Methods highlighted in Q-Chem 5 include a suite of tools for modeling core-level spectroscopy, methods for describing metastable resonances, methods for computing vibronic spectra, the nuclear-electronic orbital method, and several different energy decomposition analysis techniques. High-performance capabilities including multithreaded parallelism and support for calculations on graphics processing units are described. Q-Chem boasts a community of well over 100 active academic developers, and the continuing evolution of the software is supported by an "open teamware" model and an increasingly modular design.Peer reviewe

Implications of reactive oxygen species (ROS) in initiating chemical reactions

This thesis presents a series of scientific studies exploring the initiation of various chemical reactions with reactive oxygen species (ROS), mainly singlet oxygen. These studies have revealed new mechanistic insights in environmental, industrial and biological systems, have described the associated set of reactions, have illustrated the detection of new radicals i.e., environmentally persistent free radicals (EPFR), and have provided a new insight explaining the spontaneous fire in coal mines. Comprehensive experimental and quantum-mechanical calculations afforded the investigation of oxidation reactions of singlet oxygen with wastewater organic contaminants, for example, the photodegradation of Phenol and Aniline in water. Detailed experimental studies on modelled surrogates, i.e., Anisole, resolved the fundamentals of thermal interaction of coal with iron oxide Fe2O3 nanoparticles. Along the same line of interest, enhancing the combustion efficiency of fuel constitutes a mainstream strategy in the pursuit of meeting the ever-increasing energy demand. Therefore, this thesis also provides a comprehensive mechanistic and thermo-kinetic accounts underpinning the reaction of fuel surrogates, namely Toluene, with singlet oxygen in the internal combustion (IC) engines. Finally, this work extends insights into biological systems, mapping the Alloxan-Glutathione redox cycle to expose the formation of ROS, species that eventually cause necrosis of the pancreatic insulin-producing beta cells and prompt the insulin-dependent diabetes mellitus (IDDM). The methodology involve customised LED-photoreactors, thermal packed-bed reactor, and various reaction product-monitoring systems, e.g., Fourier transform infrared spectroscopy (FTIR) to quantitate the ignition temperatures of fuel surrogates, in-situ electron paramagnetic resonance (EPR) to elucidate the formation of environmentally-persistent free radicals (EPFR) as well as intermediate radical species, diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) to monitor the chemisorption of organic substrates on the nanoparticles, X-ray diffraction for particles characterisation, as well as broad-scan UV-Vis spectroscopy and high-performance liquid chromatography (HPLC) to identify and quantify the intermediate and product species in solutions. Results obtained in this thesis elucidate, for the very first time, the formation of para-semibenzoquinone anion (PSBQ) supporting the reaction pathway leading to the formation of para-benzoquinone during the reaction of phenol (and aniline) with singlet oxygen. These results have practical application to quantify the degradation of organic pollutants in wastewater. Investigations regarding combustion applications shows that the presence of singlet oxygen considerably lowers the activation energy of the initiation channels of aromatic hydrocarbons (e.g., in IC engines), resulting in an energetically improved combustion process, the relative reactivity of singlet oxygen, based on the reaction rate constants, follows the order of OH > H > CH3 > 1O2 > HO2 > 3O2. Furthermore, the chemisorption of anisole on α-Fe2O3 surfaces has been elucidated to follow a direct dissociation of the O–CH3 (and OCH2–H), leading to the formation of surface-bound phenoxy radicals and gaseous species at temperatures as low as 25 °C. This insight applies to free-radical chain reactions that induce spontaneous fires of coal, as low-ranked coal comprises ferric oxide nanoparticles, and equally, to coexistence of aromatic fuels with thermodynamically reactive Fe2O3 surface, e.g., in fly ash, at the cooled-down tail of combustion stacks. Results from alloxan-glutathione redox cycle clarified, for the first time, the direct synchronised generation of dialuric acid radical (DA˙) and glutathione radical (GS˙), assigning the nature of the mysterious “compound 305” to the DA˙- GS˙ complex. These results explain the alloxan-induced diabetes on precise molecular bases. This thesis provides new perspectives on opportunities in understanding the influence of ROS, mainly singlet oxygen (1O2) and superoxide (O2−) in germane chemical reactions. Such attempts will advance the existing ROS-related technologies, and improve the fundamental theories in supports of environmental management and application decisions

DESIGN OF RADIAL/LINEAR HYBRID PI-CONJUGATED SYSTEMS WITH DISJOINT SUBSTITUTION PATTERN

The understanding of pi conjugation in organic electronics has largely grown from linear conjugated oligomers and polymers. Recent research enabled the synthesis of [n]cycloparaphenylenes (CPPS) which show unique electronic properties from radial pi conjugation. Previous work in this lab incorporated CPPs into linear conjugated systems to investigate fundamental properties of the curved pi surface. This dissertation details the design of disjointly-substituted CPP incorporated into linear small molecule and polymeric systems, and synthetic attempts to extend functionalized CPPs with various aryl groups and fused polycyclic aromatic hydrocarbons (PAHs). Chapter 1 describes the design and optoelectronic characterization of [8]CPP with disjointly substituted di-alkyne subunits in collaboration with Dr. Ramesh Jasti that allows for pi extension primarily through Sonogashira cross couplings to afford small molecule and polymer systems. New electronic states arise from multiple operative radial/linear conjugation pathways, as the disjoint pattern results in both ortho and meta connections to the CPP ring. Chapter 2 details oxidation studies of linearly extended CPPs and progress made on post-construction cyclization chemistry to form fused PAH/CPP hybrids. Chapter 3 discusses progress made to synthesize CPPs with directly linked aryl groups, including the design of terphenyl model compounds, and ends with proposed CPP materials targets. Chapter 4 introduces a design theory for organic diradicals through computational studies of linear conjugated diradicals with lower symmetry patterns, as well as CPP diradicals. Synthetic progress toward some of these low symmetry molecules are included alongside considerations for designing and characterizing stable and persistent organic diradicals