Maximum Hardness Principle Research Articles

Antagonistic activity of hydroxycoumarin-based antioxidants as possible singlet oxygen precursor photosensitizers

Coumarins are phenolic-type compounds with efficient antioxidant activity due to their ability to scavenge reactive oxygen species. Nevertheless, their ability to behave as photosensitizers capable of generating reactive oxygen species, such as singlet oxygen, has been less studied. In this work, the photosensitizing ability of seven hydroxycoumarins was evaluated through the photooxidation of ergosterol by quantifying the conversion of ergosterol into ergosterol peroxide. In our experimental conditions, we found that almost every tested antioxidant coumarin promotes the peroxidation of ergosterol. The results suggest that the hydroxycoumarins exhibit potential photosensitizing activity by promoting singlet oxygen generation by a Type II photochemical mechanism. Density functional theory (DFT) calculations were also performed to obtain further insight into the chemical reactivity of tested compounds; the observed tendency in the group of antioxidant coumarins to promote the reaction was their hardness due to the principle of maximum hardness. To evaluate our conclusion, we performed the reaction using a highly polarizable coumarin as a photosensitizer, which resulted in an increased photosensitizing capacity supported with DFT calculations, which reinforces our analysis. Finally, we found that hydroxycoumarins can be potentially pro-oxidants since some of them can act as photosensitizers and generate singlet oxygen in the presence of UV–Vis light, a characteristic that must be considered when these compounds are used as antioxidants.

A first principle study of hydrogen storage in titanium-doped small carbon clusters (C2nTin, n = 2–6)

Hydrogen storage in Ti-doped small carbon clusters, C2nTin (n = 2–6), has been studied using density functional theory. Using the principle of maximum hardness (η) and minimum electrophilicity (ω), stabilities of the clusters are confirmed. The average adsorption energies of all complexes are found in the range of 0.2–0.5 eV/H2 and average Ti-H2 bond length is in the range of 1.953 to 2.145 A, inferring the adsorption process to be physisorption type. All the studied clusters are found to adsorb hydrogen in molecular form through Kubas type of interaction with H2 uptake capacity in the range of 5.31–10.09 wt% which is well above the target set by US-DOE (5.5 wt%). Bader’s topological analyses confirm the closed-shell type interaction among H2 molecules and Ti atoms. Desorption temperature calculated using Van’t-hoff equation is found above the liquid nitrogen temperature.

The Connection between Pseudo-Jahn–Teller Effect and Electron Delocalization for Proving the Origin of Equilibrium Geometry of Nitrosyl Halides (Halides = Cl, Br, and I)

The theoretical calculations on triplet linear and non-linear structures of nitrosyl halides XNO (X = Cl, Br, and I) confirm that the origin of instability and symmetry breaking of XNO linear structures is a pseudo-Jahn–Teller effect (PJTE). However, the PJTE associated with electron delocalization may prove the stability of XNO non-linear structures best. The distortion of high-symmetry (C∞v) configurations in XNOs emerges from mixing the PJT with ground 3Σ– and the excited 3Π states in the Qπ direction, which consequently make the ground state unstable. The PJT problem is formulated in the form (3Σ– + 3Π) $$ \otimes $$ π. The energy difference between high-symmetry C∞v geometries and low-symmetry Cs geometries energy gap between the ground and excited states decreases from ClNO to INO. Also, ΔEPJT (i.e., the difference of energy between the highest occupied molecular orbital (HOMO; LPX), and the lowest unoccupied molecular orbital (LUMO; $$\pi _{{{\text{NO}}}}^{*}$$ )) plays an important role in the instability of structures. Electron delocalization decreases and PJT instability increases from ClNO to INO in high-symmetry C∞v geometries. According to the results of the canonical molecular orbital (CMO) analysis, the contribution of the LPX non-bonding orbitals in the vibronic coupling constant (F) increases from ClNO to INO. Also, the NBODEL results indicate that the instability of compounds increases from ClNO to INO. Because the primary force constant is positive, the curvature of ground electronic state configuration curves in adiabatic potential energy surfaces (APESs) decreases from ClNO to INO. The variations of the chemical hardness differences between the non-linear (Cs) and linear (C∞v) structures decrease from ClNO to INO and of chemical potential differences increase from ClNO to INO, justifying the variation trend of ΔEPJT. Furthermore, based on the obtained results, Δ[η(Cs) – η(C∞v)] obeys the maximum hardness principle.

Diversities in the chelation of aroylhydrazones towards cobalt(II) salts: Synthesis, spectral characterization, crystal structure and some theoretical studies

Five cobalt complexes synthesized from two aroylhydrazones were characterized by elemental analyses, thermogravimetric analysis, molar conductivity, magnetic susceptibility measurements, IR and electronic spectra. Single crystal X-ray structure of one of the complex is also reported and it got crystallized in triclinic space group P1¯ and the crystal structure shows a distorted octahedral geometry around the metal center. Spectral data reveal that both the aroylhydrazones are tridentate and coordinate through the azomethine nitrogen, hydrazonic oxygen, and pyridyl nitrogen. Magnetic susceptibility measurements confirm the paramagnetic nature of the Co(II) complexes and one of the complex was found to be diamagnetic in nature. Additionally, HF/6-311G(d,p)/LANL2DZ calculations were performed to predict the possible intramolecular interactions contributing to the lowering of the stabilization energy. Accordingly, π→ π* transitions were found to be responsible for the stabilization energy for the ligands and their cobalt complexes. To describe and discuss the chemical reactivity and stability of synthesized complexes, quantum chemical parameters like frontier orbital energies, hardness, softness, energy gap, electronegativity, chemical potential, electrophilicity, polarizability and dipole moment were calculated. Also, the main electronic structure principles such as maximum hardness, minimum polarizability, and minimum electrophilicity principles were considered to evaluate the stability of the complexes.

A Density Functional Theory Study of the Isomerization of Substituted 2and#39;-hydroxychalcones to the Corresponding Flavanones

The transformation of 2and#39;-hydroxychalcones to their corresponding flavanones was studied theoretically by the use of the density functional theory (DFT) with B3-LYP/ 6-311G basis set to get important information about the role of both of electronic and structural properties in this process. The obtained energies were found to be in agreement with our previous results that obtained from HPLC studies. The estimated hardness, polarizability, and electrophilicity profiles were found to obey the maximum hardness principle (MHP), minimum polarizability principle (MPP), and the minimum electrophilicity principle (MEP) respectively. Flavanone ring closure was found to be the rate-determining step.

A Density Functional Theory Study of the Isomerization of Substituted 2'-hydroxychalcones to the Corresponding Flavanones

The transformation of 2'-hydroxychalcones to their corresponding flavanones was studied theoretically by the use of the density functional theory (DFT) with B3-LYP/ 6-311G basis set to get important information about the role of both of electronic and structural properties in this process. The obtained energies were found to be in agreement with our previous results that obtained from HPLC studies. The estimated hardness, polarizability, and electrophilicity profiles were found to obey the maximum hardness principle (MHP), minimum polarizability principle (MPP), and the minimum electrophilicity principle (MEP) respectively. Flavanone ring closure was found to be the rate-determining step.

A Density Functional Theory Study of the Isomerization of Substituted 2and#39;-hydroxychalcones to the Corresponding Flavanones

The transformation of 2and#39;-hydroxychalcones to their corresponding flavanones was studied theoretically by the use of the density functional theory (DFT) with B3-LYP/ 6-311G basis set to get important information about the role of both of electronic and structural properties in this process. The obtained energies were found to be in agreement with our previous results that obtained from HPLC studies. The estimated hardness, polarizability, and electrophilicity profiles were found to obey the maximum hardness principle (MHP), minimum polarizability principle (MPP), and the minimum electrophilicity principle (MEP) respectively. Flavanone ring closure was found to be the rate-determining step.

A Density Functional Theory Study of the Isomerization of Substituted 2and#39;-hydroxychalcones to the Corresponding Flavanones

The transformation of 2and#39;-hydroxychalcones to their corresponding flavanones was studied theoretically by the use of the density functional theory (DFT) with B3-LYP/ 6-311G basis set to get important information about the role of both of electronic and structural properties in this process. The obtained energies were found to be in agreement with our previous results that obtained from HPLC studies. The estimated hardness, polarizability, and electrophilicity profiles were found to obey the maximum hardness principle (MHP), minimum polarizability principle (MPP), and the minimum electrophilicity principle (MEP) respectively. Flavanone ring closure was found to be the rate-determining step.

Theoretical insights about inhibition efficiencies of some 8-Hydroxyqionoline derivatives against the corrosion of mild steel

ABSTRACT In the present study, we theoretically analysed the corrosion inhibition performances of some novel 8-Hydroxyqionoline derivatives, namely 5-(((2-hydroxyethyl) thio) methyl) quinolin-8-ol (Q-OH) and 5-(((2-aminoethyl) thio) methyl) quinolin-8-ol (Q-NH2) against the corrosion of mild steel. In the section about DFT and HF calculations of the study, quantum chemical descriptors like frontier orbital energies, HOMO–LUMO energy gap, chemical hardness, electronegativity, softness, chemical potential, electrophilicity, nucleophilicity, proton affinity, electrons transferred from inhibitor to metal surface, electron-donating power, electron-accepting power, dipole moment, polarisability regarding protonated and neutral forms of the mentioned molecules were calculated and commented. In the section including the use of Molecular Dynamics Simulation approach of the analysis, adsorption behaviours of studied organic molecules on Fe (110) surface were investigated. To detect the power of the interactions between novel 8-Hydroxyqionoline derivatives and the mentioned metal surface, adsorption energy and binding energy values were calculated. It is important to note that these resuls determined are in good agreement with experimentally observed results. In addition to these analyses made, the validities of chemical reactivity principles such as Maximum Hardness, Minimum Polarisability and Minimum Electrophilicity Principles in corrosion inhibition studies were also reported.

Theoretical Study for Isomerization of Some 2´-Hydroxychalcone Derivative by Using DFT Calculation

In this work we present a study about the molecular structure and electrochemical behavior of a series of substituted 2´-hydroxychalcones and their conversion to the corresponding flavanones. The study was performed using density function theory (DFT) based on B3YLP with 6-311G basis set was used in order to investigate the effect of the electronic and structure properties for the isomerization process. Reactants, intermediates and products have an equilibrium state and one transition state that possess one imaginary frequency. The optimized structure, total energies ,hardness ,polarizability ,chemical potential and electrophilicity index were calculated. The relative energy calculated using DFT corresponds with those calculated in a practical way by the use of HPLC technique, the hardness profile along the reaction path in agreement with maximum hardness principle, and the minimum polarizability principle is obeyed in all cases.

Conformer selective monohydrated clusters of 1,2,3,4 –tetrahydroisoquinoline in S0: I-Potential energy surface studies, vibrational signatures and NBO analysis

The global minima in the potential energy surface of 1, 2, 3, 4-tetrahydroisoquinoline (THIQ):(H2O) prefer the twisted equatorial orientation of the NH group. It is characterized by a strong OH⋅⋅⋅N and a weak O-H⋅⋅⋅π hydrogen bonds. The computations at the ab initio level and DFT methods with M06–2X and ωB97X-D functionals generated consistent results. The monohydrate with the axial orientation of NH having a single OH⋅⋅⋅N bond in the twisted configuration of THIQ is higher by 240 ± 40 cm−1. The N⋅⋅⋅H bond length is smaller in the latter. All the clusters are located in the potential energy surface of THIQ :(H2O) drawn with the variation of the dihedral angles of the saturated ring of THIQ. Various kinds of hydrogen bonds are observed. Binding energy of each structure is computed incorporating BSSE corrections. Interestingly, the largest of them correspond to a conformer of THIQ, where it was a transition state in the bare molecule. The vibrational analysis of the clusters indicates the reassignment of a number of bands in the observed IR spectra. The red-shifts observed in the different stretching modes of water in the two lowest energy clusters are suitably explained. This is further verified from NBO computations. The changes in s-character, NBO energies and OH bond lengths with the variation of hydrogen bond distances clearly signify the dominant nature of hyperconjugation mechanism. The natures of each OH bonds of water are investigated thoroughly. The bond which is not taking part in hydrogen bonding shows some apparently unexpected behaviors. HOMO and LUMO plots and their energy differences are employed to qualitatively explain the experimental results observed earlier. Changes in Molecular Electrostatic Potentials (MEP) on cluster formation are explained and confirm our computations. Maximum hardness principle (MHP) is followed by the most stable monohydrate only. Hardness values display a certain arrangement depending on the type of hydrogen bonding sites. We suggest further experiments to confirm our key observations.

Mixed ligand copper(II) chelates derived from an O, N, S- donor tridentate thiosemicarbazone: Synthesis, spectral aspects, FMO, and NBO analysis

Five new copper(II) chelates [(Cu(bmct))2] (1), [Cu(bmct)(phen)] (2), [Cu(bmct)(bipy)] (3), [Cu(bmct)(4,4′-dmbipy)] (4) and [Cu(bmct)(5,5′-dmbipy)] (5) with 5-bromo-3-methoxysalicylaldehyde-N(4)-cyclohexylthiosemicarbazone (H2bmct) as the chelating ligand and 1,10-phenanthroline, 2,2′-bipyridine, 4,4′-dimethylbipyridine, 5,5′-dimethylbipyridine as coligands have been synthesized and characterized by different physicochemical techniques like CHNS analysis, molar conductivity and magnetic studies, IR, UV/Vis and EPR spectral studies. In all the complexes, the thiosemicarbazone exists in thioiminolate form and coordinates to the metal through azomethine nitrogen, thioiminolate sulfur, and phenolate oxygen. EPR spectra in polycrystalline state at 298 K showed that compounds 1, 4, and 5 are isotropic, 2 is axial and 3 is rhombic in nature. In DMF at 77 K, compound 1 showed hyperfine lines in the parallel and perpendicular regions as well as superhyperfine lines due to the interaction of copper center with azomethine nitrogen of the ligand. Complex 2, in which g||>g⊥> 2.0023 suggests a distorted square pyramidal structure. To analyze the stability of the complexes, quantum chemical parameters like hardness, softness, polarizability, electrophilicity, electronegativity, and dipole moment were calculated and discussed within the framework of electronic structure principles known as Maximum Hardness, Minimum Polarizability and Minimum Electrophilicity Principles. Besides, the intramolecular donor–acceptor interactions for all complexes were evaluated by using NBO analysis. All calculations proved that Compound 3 is the most stable chelate among them.

Tautomers of homophthalic anhydride in the ground and excited electronic states: analysis through energy, hardness and vibrational signatures.

The keto-enol tautomerisation in homophthalic anhydride (HA) is investigated in the ground (S0) and excited (S1) electronic states. The keto form with a dicarbonyl structure is found to be the most stable form in S0 and enol form with a monocarbonyl structure in S1 indicating an excited state intramolecular proton transfer (ESIPT) process. The computed results show consistency with the change in basis sets and methods of calculations. Apart from the two tautomers, transition states are also identified. The barrier to interconversion is found to reduce substantially in S1. Internal reaction coordinate (IRC) calculations confirm the pathway of interconversion between the two forms in S0 and S1. The observed FT-IR spectra corroborate well with our computed spectra. The appearance of two strong lines around 1800cm-1 confirms the lowest energy structure to be the keto tautomer with a dicarbonyl form in S0. Our computations corroborate well with the crystal structure data for an analogous molecule. Electron distribution in HOMO and LUMO indicate the excitation process as π → π* in nature. The qualitative chemical concepts like hardness and electrophilicity are calculated to estimate the stability of the tautomers. The energy and hardness profiles with the variation of IRC are opposite to each other, verifying the principle of maximum hardness.

Theoretical study of the electronic structure and electrical properties of Al-doped niobium clusters

A methodology previously suggested for niobium clusters is here applied to the prediction of the ground spin-state of the niobium clusters doped with aluminum atoms. By using all-electron density functional theory with Douglas-Kroll-Hess correction, the spin-state, geometry, hardness, binding energy, ionization potential, electron affinity, and mean static dipole polarizability of the NbxAly clusters are examined. The ground spin-states obtained from the energy minimization were confronted with the results provided by the maximum hardness principle (MHP) and by the minimum polarizability principle (MPP). In general, the MHP and MPP have yielded the expected results, i.e., in agreement with the ones obtained from the minimum energy. Additionally, were compared the theoretical results for polarizability, ionization potential, and reactivity with the available experimental values.

Role of π-electron conjugation in determining the electrical responsive properties of polychlorinated biphenyls: a DFT based computational study

Global reactivity descriptors e.g. average polarizabilty (αav), chemical hardness (η), electrophilicity index (ω) of some donor–acceptor substituted polychlorinated biphenyl (PCBs) i.e. 2,2′,5,5′-tetrachloro-1,1′-biphenyl (PCB-1a), 2,5,2′,5′-Tetrachloro-4′-dimethylamino-biphenyl-4-carbonitrile (PCB-1b), Dimethyl-(2,5,2′,5′-tetrachloro-4′-nitro-biphenyl-4-yl)-amine (PCB-1c), 3,3′,5,5′-tetrachloro-1,1′-biphenyl (PCB-2a), 3,5,3′,5′-Tetrachloro-4′-dimethylamino-biphenyl-4-carbonitrile (PCB-2b) and Dimethyl-(3,5,3′,5′-tetrachloro-4′-nitro-biphenyl-4-yl)-amine (PCB-2c) were computed along the torsional potential of biphenyl ring. Density functional based hybrid functional CAM-B3LYP with 6-31G(d) basis were used to all the computational study. Out of the six compounds, variation of the global reactivity descriptors e.g. αav, η, ω as a function of torsional angle of biphenyl ring, three compounds are not in conformity with the respective optimum principle i.e. minimum polarizability, maximum hardness and minimum electrophilicity principle. In this present research article, we have raised this issue that apart from electronically stable conformation, localization of π-electron plays a major role in determining the optimum values of global reactivity descriptors, αav, η & ω. On the other hand computation of first average hyper polarizability (βav) as a function of torsional angle shows that the variation of βav are in good agreement with the optical gap as well as electronic spatial extent ( ) of the conformers of all the six compounds.

Conformers of 1,2,3,4 –tetrahydroisoquinoline in S0 and S1: An analysis through potential energy surface, hardness principles and vibrational spectroscopy

The conformational search for 1, 2, 3, 4-tetrahydroisoquinoline (THIQ) yields a number of conformers in S0 and S1 states. DFT calculations with different functionals along with ab-initio calculations indicate that the structure of the global minima in S0 strongly depends on basis sets and methods of calculation. The two close lying low energy conformers are the twisted ones with i) the axial position of NH and ii) equatorial position of NH. Their energy differs by 60 ± 30 cm−1 over all possible variations. The computed potential energy surfaces (PES) show three inequivalent minima in S0 and two in S1. Three transition states are found in S0 and two in S1. The twisted conformer with axial hydrogen (TA) is found to be the global minima in S1. The twisting angle for the optimized TA conformer is ∼31° in S0 and ∼32° in S1. The excited state is probed through CIS and TDDFT methods. The characteristic low-frequency vibrations appear at 93 cm−1 and 146 cm−1 in S0. The observed FT-IR spectra and the earlier observations in molecular beam experiments corroborate well with our calculated spectra. Frontier molecular orbital calculations associated with the electron density plots clearly identify the changes on the density on excitation. The calculated UV–Vis absorption spectra are explained with the help of these orbitals. Molecular electrostatic potential is calculated and the sites of eletrophilic interactions are noted. Hardnesses and electrophilicities are calculated for all the conformers to check their relative stability. The maximum hardness principle (MHP) is obeyed by the lowest energy conformer in DFT calculations with different functionals, but minimum electrophilicity principles (MEP) are not obeyed.

Ligand stabilized transient “MNC” and its influence on MNC → MCN isomerization process: a computational study (M = Cu, Ag, and Au)

A theoretical investigation of the binding ability of different ligands [L = CO, H2O, H2S, N2, NH3, 1,3-dimethylimidazole (DMI), C2H2 and C2H4] with metal isocyanide and cyanide (MNC and MCN; M = Cu, Ag, Au) compounds has been carried out using quantum chemical computations. In order to analyze the thermochemical stability of these complexes, we have calculated the changes in the related dissociation energies and free energies by considering different possible dissociation pathways (four two-body and one three-body) such as (a) LMCN(/NC) = L + MCN(/NC); (b) LMCN = LM + CN; (c) LMCN = L + M + CN; (d) LMCN = LM+ + CN−; and (e) LMCN = L− + MCN+. The possible dissociation processes are endothermic in nature at room temperature suggesting non-spontaneity at 298 K. Our inspection suggests that MNC have higher binding ability than the MCN compounds in all of the L-bonding cases and both of them follow similar trends as Au > Cu > Ag. The natural bond orbital analysis, topological analysis of the electron density from atoms in molecules technique, and energy decomposition analysis have been carried out to characterize the nature of interaction between L and MCN which shows that the L‐M bonds acquire some degree of covalent character. Furthermore, in order to check the validity of the conceptual DFT-based electronic structure principles like maximum hardness and minimum electrophilicity principles, the change in the relevant global reactivity descriptors like chemical hardness (η), chemical potential (μ), and electrophilicity index (ω) is also studied along the isomerization path, LMNC → LMCN.

The Reactivity of Ambident Nucleophiles: Marcus Theory or Hard and Soft Acids and Bases Principle?

The model reactions CH3 X + (NH-CH=O)M ➔ CH3 -NH-NH═O or NH═CH-O-CH3 + MX (M = none, Li, Na, K, Ag, Cu; X = F, Cl, Br) are investigated to demonstrate the feasibility of Marcus theory and the hard and soft acids and bases (HSAB) principle in predicting the reactivity of ambident nucleophiles. The delocalization indices (DI) are defined in the framework of the quantum theory of atoms in molecules (QT-AIM), and are used as the scale of softness in the HSAB principle. To react with the ambident nucleophile NH═CH-O- , the carbocation H3 C+ from CH3 X (F, Cl, Br) is actually a borderline acid according to the DI values of the forming C…N and C…O bonds in the transition states (between 0.25 and 0.49), while the counter ions are divided into three groups according to the DI values of weak interactions involving M (M…X, M…N, and M…O): group I (M = none, and Me4 N) basically show zero DI values; group II species (M = Li, Na, and K) have noticeable DI values but the magnitudes are usually less than 0.15; and group III species (M = Ag and Cu(I)) have significant DI values (0.30-0.61). On a relative basis, H3 C+ is a soft acid with respect to group I and group II counter ions, and a hard acid with respect to group III counter ions. Therefore, N-regioselectivity is found in the presence of group I and group II counter ions (M = Me4 N, Li, Na, K), while O-regioselectivity is observed in the presence of the group III counter ions (M = Ag, and Cu(I)). The hardness of atoms, groups, and molecules is also calculated with new functions that depend on ionization potential (I) and electron affinity (A) and use the atomic charges obtained from localization indices (LI), so that the regioselectivity is explained by the atomic hardness of reactive nitrogen atoms in the transition states according to the maximum hardness principle (MHP). The exact Marcus equation is derived from the simple harmonic potential energy parabola, so that the concepts of activation free energy, intrinsic activation barrier, and reaction energy are completely connected. The required intrinsic activation barriers can be either estimated from ab initio calculations on reactant, transition state, and product of the model reactions, or calculated from identity reactions. The counter ions stabilize the reactant through bridging N- and O-site of reactant of identity reactions, so that the intrinsic barriers for the salts are higher than those for free ambident anions, which is explained by the increased reorganization parameter Δr. The proper application of Marcus theory should quantitatively consider all three terms of Marcus equation, and reliably represent the results with potential energy parabolas for reactants and all products. For the model reactions, both Marcus theory and HSAB principle/MHP principle predict the N-regioselectivity when M = none, Me4 N, Li, Na, K, and the O-regioselectivity when M = Ag and Cu(I). © 2019 Wiley Periodicals, Inc.

The Role of Pseudo Jahn–Teller Effect in Geometry and Electronic Parameters of Tetrahalodigermene Ge2X4 (X= Cl, Br, I)

The symmetry breaking of molecules depends on its context and origin. It is well shown the pseudo Jahn – Teller effect (PJTE) describes the origin of symmetry breaking of planar configurations. Based on the PJTE theory, the configurations, electronic parameters and conformational changes in molecular systems are predicted. Optimization of the D2h and C2h configurations of tetrahalodigermene Ge2X4 (X = Cl, Br, I) by ab initio calculations showed that the C2h configurations are more stable than planar ones. The different energy between high-symmetry D2h geometries and low-symmetry C2h geometries decrease from Ge2Cl4 to Ge2I4. Instability of the high-symmetry D2h configurations is arising from the PJT coupling of the ground Ag state and the exited B2g state in the Qb2g direction. The PJTE problem $$({{A}_{g}} + {{B}_{{2g}}}) \otimes {{b}_{{2g}}}$$ creates stable geometries of Ge2X4 (X = Cl, Br, I) analogs with C2h configurations. It is expected that when the energy gap between the desired states with D2h configurations decreases, the PJT stabilization energy (EPJT) increases. Despite this expectation, with increasing the energy gap from Ge2Cl4 to Ge2I4, EPJT decreases. According to the results of the canonical molecular orbital (CMO) analysis, the contributions of the b3u and b1u molecular orbitals in the vibronic coupling constant (F) decrease from Ge2Cl4 to Ge2I4. Consequently, assuming the primary force constant is positive, the negative curvatures of ground state electronic configuration curves in adiabatic potential energy surfaces (APESs) decreases from Ge2Cl4 to Ge2I4. The trend observed for the variations of the chemical hardness (η) of Ge2X4 analogs obey the maximum hardness principle.

Sc and Ti-functionalized 4-tert-butylcalix[4]arene as reversible hydrogen storage material

Abstract Using the idea of metal functionalized material for H2 storage, 4-tert-butylcalix[4]arene (CA) functionalized with Sc and Ti atoms are explored. The first principles density functional theory (DFT) with M06 functional and 6-311G(d,p) basis set is used to explore the hydrogen storage properties of metal functionalized CA. Sc and Ti strongly binds with CA by Dewar coordination with high binding energy. It is found that maximum four hydrogen molecules are adsorbed on each metal site in Sc and Ti functionalized CA. Hydrogen molecules are adsorbed on metals by Kubas and Niu-Rao-Jena mechanism. In Sc functionalized CA system all 4 hydrogen molecules on each Sc bind in molecular fashion while on each Ti in Ti functionalized CA, the first hydrogen molecule binds in dissociative fashion and remaining three hydrogen molecules bind in a molecular form. The stability of Sc and Ti functionalized CA is studied by computing conceptual DFT parameters, which obeys maximum hardness and minimum electrophilicity principle. Hirshfeld charge analysis and electrostatic potential map explore the charge transfer mechanism during the hydrogen adsorption. Born-Oppenheimer molecular dynamics simulations are performed at temperature range 200–473 K to study the stability of the system and the reversibility of adsorbed hydrogen from the system. The calculated H wt% is found to be 10.3 and 10.1, respectively for Sc and Ti functionalized CA systems on complete H2 saturation. This study explores that Sc and Ti functionalized CA systems are efficient reversible hydrogen storage material.