Density functional theory for exploration of chemical reactivity: Successes and limitations

Computational modeling of molecules and their reactions are now essential components of the scientific research in chemistry.

Density functional theory (DFT) is among the most prominent and effective quantum mechanical theories of molecules and materials. Presently, it is commonly employed in calculations of band configuration of solids and binding energies of substances. It appears that these are the initial applications that are pertinent to sciences like biology and minerals, which are typically regarded as being further removed from quantum mechanics. DFT has been used to examine superconducting properties, particles at the forefront of intense laser impulses, relativistic influences in heavy molecules and nuclei of atoms, conventional fluids, and the magnetic characteristics of composites. DFT's adaptability is due to the flexibility of its basic principles and the range of possibilities with which they can be used. Despite this adaptability and applicability, the logical foundation of DFT is relatively inflexible.¹

Through years of debating the many-body challenge, numerous effective strategies to address Schrödinger's equation have been created. For instance, the schematic perturbation concept, which relies on Feynman models and Green's functions, is utilized in physics, whereas configuration interaction (CI) approaches, which depend on methodical development in Slater determinants, are commonly employed in chemistry. There are also a variety of more specialized techniques. These approaches have a drawback in that they exert an enormous demand on computational resources, making it difficult to use them effectively on enormous and intricate systems. Here, DFT offers a strong substitute that may be less precise but is far more adaptable.² The 1998 award for the Nobel Prize in Chemistry, given to Walter Kohn,³ the creator of DFT, and John Pople,⁴ who played a key role in integrating DFT with computational chemistry, is indicative of the level to which DFT has made an impact on the scientific community of computational chemistry and physics. It has been utilized to determine a large portion of the information that is understood concerning the electronic, magnetic, and structural characteristics of substances.^{5, 6}

A computational achievement was subsequently made possible by involving orbital parameters in the framework, as was done in the Kohn–Sham paradigm^{7, 8} and in the beginning of 1995, DFT through the Kohn–Sham approaches in Pople's GAUSSIAN software tool⁹ become the favored wave function computing package at those times as well as currently. In the end of 1970s and beginning of 1980s, eminent scientist R. G. Parr has established another form of DFT known as “conceptual DFT.”¹⁰ In accordance with the notion that the density of electrons is the essential parameter used to characterize the ground states of molecule as well as atom, Parr and his research team, followed by a focused scientific group, succeeded in giving a specific description to the reactivity of chemical substances regarding that was actually popular and existed for a long time in different fields of chemical science (for instance, electronegativity) and enabling for their estimation and mathematical utilization.

DFT provides energies and electron densities of molecular systems in a computationally tractable manner. Though use of DFT in organic chemistry is very popular and well established, the emerging fields, especially at the interface between inorganic, organometallic, and organic chemistry, push the limits of DFT, especially when chemical accuracy is desired in the presence of multiple electronic states of close energy. Similar challenges apply to photochemical and photoredox reactions. DFT is not limited to conceptual density functional theory (CDFT), although CDFT is also quite useful to make a correlation between experimental observations and their theoretical counterparts. There is a vast number of models that allow one to compute various observable properties using DFT: geometries, vibrational frequencies and their intensities (IR/Raman), electronic excitation energies and probabilities (UV-spectra), NMR chemical shifts, circular dichroism, reaction rate constants (Eyring equation), equilibrium concentrations (Boltzmann equation), and product ratios in kinetically controlled reactions (Curtin–Hammett equation). These observables are highly useful for the organic chemists to validate their experimental findings and predict mechanistic feature of any chemical reaction.

This special issue has brought together two groups of scientists. First, it has included physical chemists who develop DFT and can highlight its inherent advantages and conceptual limitations. On the other hand, the special issue has become the appropriate platform for the organic chemists who use DFT for understanding organic structure and reactivity and have encountered these limitations first hand in their own research.

To provide more focused and development in CDFT to the scientific community, this special issue is devoted completely on CDFT parameters especially on chemical reactivity: success and Limitations. In this special issue, we have invited prominent and incipient researchers working in this domain to contribute. Total 18 manuscripts which are focused on CDFT parameters are collected.

Das¹¹ examined stability, reactivity, and aromaticity of imidazolium by using CDFT approach. Authors found that ligands with a stronger electron-withdrawing nature cause imidazolium complexes to evolve into more acidic. Patra et al.¹² reported the local and global electrophilicity index of Fischer and Schrock carbene species. Experimental chemists might anticipate the catalytic usage of transition metal carbene species by examining the reactivity factors in this manner without having to assign them to the Fischer and Schrock type of catalysis. Chakraborty et al.¹³ presented direct dynamics for the purpose of providing atomistic insights into the process of the collisional dynamics of O(3P) and dimethylamine (DMA) across the triplet electronic interface. Kaya et al.¹⁴ predicted the magnetic susceptibilities of substances on the basis of correlation among molar diamagnetic susceptibility and van der Waals constant. Liu et al.¹⁵ studied nine dimeric systems with various alkyl group sizes by using DFT approach. Patra et al.¹⁶ examined the variation in electrophilicity for simple diatomic, triatomic, and tetratomic species on electronic excitation. Jain et al.¹⁷ reported various physicochemical properties of thiazole-based compounds and their copper(II) complexes by using experimental and DFT approach. Catinkaya et al.¹⁸ studied elimination of erythrosine B dye from stormwater by incorporating a chitosan–boric acid hybrid substance with the help of experimental and DFT approach. Politzer et al.¹⁹ reported a successful technique to link electronegativity to the median valence electron ionization energies of atoms, which produces findings that are generally consistent with Pauling's values. Sharma et al.²⁰ investigated intramolecular H-bonded naphthoquinone compounds by using DFT and MP2 approaches. Paul et al.²¹ studied PC31 bm-based for DSSC applications by using DFT approach. Balasubramanian²² reported porous nanographenes by using DFT and graph theory approach. Roy et al.²³ examined the biological activities of thiazolyl-thiadiazines for Alzheimer's ailment by using DFT method. Mondal et al.²⁴ reported CDFT-based descriptors—hardness and electrophilicity of β-D-glucopyranose–silver ion (1:1) complex. Saloni et al.²⁵ reported lead-free perovskites A2BCl6 for photovoltaic applications by using DFT approach. Yabas et al.²⁶ studied optical and electronic properties of MPc and MPc-Go composites with the help of DFT and experimental approaches. Poon et al.²⁷ performed DFT calculations for ionomers consisting of amidinium or imidazolinium groups. Solanki et al.²⁸ studied A2BI6 double perovskite systems for solar cell applications by using CDFT approach.

We hope that such an issue will lead to productive exchange of ideas and expose both parts of the chemical community to concepts and ideas that will be new and mutually beneficial. We also hope that this special issue can provide a forum where top experts can share their ideas and discuss possible solutions to persistent problems.