M06-2X Functionals Research Articles

Hydrogen bonding to graphene surface: A comparative computational study

Theoretical computations employing the M06-2X, PBE0-D3, ωB97X-D, and B3LYP-D3 density functionals combined with the 6-311++G(d,p) basis set are applied in investigating the adsorption of methanol on pristine graphene. The fragment approach in modeling the graphene surface is applied. Three types of fragments are considered: C36H16, C54H18, and C66H22. Comparative studies on the interactions of benzene and coronene with methanol reveal the electronic factors of the monomeric species governing the adsorption process. The shifts of methanol OH and methyl stretching frequencies upon complex formation indicate that the adsorption is realized via OH…π and CH… π hydrogen bonds with the π-electronic systems of these structures. Comparison with experimental Δν(OH)exp shifts for benzene shows that the basicity of the graphene surface is slightly lower than that of benzene. Computed Hirshfeld charges and electrostatic potentials at the ring carbon atoms of the monomeric species reveal the electronic factors associated with the frequency shifts resulting from the complex formation. The effects of the aromaticity of the participating rings on interaction energies and the spectroscopic properties are also considered.

Theoretical investigation of intermolecular interactions between CNT, SiCNT and SiCGeNT nanomaterials with vinyl chloride molecule: A DFT, NBO, NCI, and QTAIM study

Vinyl chloride (VCM) or chloroethylene is reactive in the gaseous state and its adsorption with pristine armchair (5,5) carbon nanotube (CNT) and silicon carbide nanotube (SiCNT) has been studied. Germanium (Ge) element as an impurity has been introduced to silicon carbide structure (i.e. SiCGeNT) so as to increase the reactivity of the surface of SiCNT and thus increase the sensitivity of the adsorbent during the adsorption process. The B3LYP-D3(GD3BJ) hybrid functional has been used to optimize the isolated nanostructures and vinyl chloride as well as their clusters (gas/nanotube). In addition, energy single-point calculations were performed using ωB97XD, PBE0, B3LYP-D3(GD3BJ), and M06-2X functionals. The selected basis function is 6-311G(d), which is a good approximation to cover the integration space with respect to the constituent atoms of the molecules studied in this research. Wave function analysis including non-covalent interaction (NCI), natural bond orbital (NBO), and quantum theory of atoms in molecules (QTAIM), have also been done to reveal the intermolecular interactions nature. The results of these analyses, which are calculated using B3LYP-D3(GD3BJ)/6-311G(d) level of theory, are in agreement with each other and emphasize that among the mentioned nanotubes, silicon carbide nanotube doped with Ge element is more sensitive with respect to gas adsorption. Therefore, it will be a good option to design and build a nanosensor.

In silico screening of ethyl 4-[(E)-(2-hydroxy-4-methoxyphenyl)methyleneamino]benzoate and some of its derivatives for their NLO activities using DFT.

The nonlinear optical (NLO) properties of ethyl 4-[(E)-(2-hydroxy-4-methoxyphenyl)methyleneamino]benzoate (EMAB) and some of its derivatives are investigated herein using the density functional theory (DFT) and time-dependent (TD)-DFT methods. The density functionals B3LYP, CAM-B3LYP, M06-2X and ωB97XD, and basis sets 6-31 + G**, 6-311 + + G** and Def2-TZVPP have been used. From the results, EMAB and its substituted derivatives studied are promising candidates for NLO materials. In all cases, the static first and second hyperpolarizabilities (31.7-86.5 × 10-30 and 84.4-273 × 10-36 electrostatic units (esu), respectively) and the frequency-dependent NLO properties are found to be significantly larger (about 43-103 and 28-76 times greater) than those of the NLO prototypical molecule, para-nitroaniline. Furthermore, the maximum absorption wavelengths of the molecules fall within the UV region of the electromagnetic spectrum. Relative to EMAB, the derivatives have shown improved transparency-nonlinearity trade-offs. Natural bond orbital (NBO) and density of states (DOS) analyses herein revealed effective charge transfer within the molecules studied, especially those with stronger electron donors than that in EMAB (methoxy group). Among the molecules studied, the derivative obtained by substituting EMAB's methoxy group with the pyrrolyl group was found to exhibit the best NLO properties. Conclusively, the NLO activities of EMAB can be significantly improved through the substitution of its methoxy group with stronger electron donors.

A density functional study of the coronene-pyrrole system in relation to its possible application as NO2 and NH3 sensors

According to recent research on the application of graphene materials as sensors and particularly polypyrrole-graphene materials, which are especially promising, the functionalization of graphene with a pyrrole molecule might be considered a viable alternative as a NO2 and NH3 sensor. In this way, a graphene sheet simulated as a coronene molecule was used in order to test whether this kind of functionalization could be useful for detecting the NO2 and NH3 toxic gases with a relatively high sensitivity. NO2 was studied as an example of an electron acceptor molecule, and NH3 as an electron donor molecule. Both molecules were adsorbed on two different regions of the functionalized adsorbent, and the energy ranges found for adsorption were reported and compared with those of the pristine graphene. The results indicated that in the coronene-pyrrole system, pyrrole tends to lie almost parallel to the coronene sheet in a π-π stacking interaction between the two conjugated systems, being the closest distances of 3.0 and 3.2 Å. The use of Δ (ΔHOMO-LUMO) as a descriptor confirmed that the coronene-pyrrole system is a good option as a NO2- and NH3-sensor; therefore, it might be an easy and suitable descriptor for characterizing the performance of a sensor; all calculations were made using a Density Functional formalism, through a functional M06-2X in combination with the 6-31G(d,p) basis set.

TDDFT versus GW/BSE Methods for Prediction of Light Absorption and Emission in a TADF Emitter.

Design concepts for organic light emitting diode (OLED) emitters, which exhibit thermally activated delayed fluorescence (TADF) and thereby achieve quantum yields exceeding 25%, depend on singlet-triplet splitting energies of order kT to allow reverse intersystem crossing at ambient temperatures. Simulation methods for these systems must be able to treat relatively large organic molecules, as well as predict their excited state energies, transition energies, singlet-triplet splittings, and absorption and emission cross sections with reasonable accuracy, in order to prove useful in the design process. Here we compare predictions of TDDFT with M06-2X and ωB97X-D exchange-correlation functionals and a GoWo@HF/BSE method for these quantities in the well-studied DPTZ-DBTO2 TADF emitter molecule. Geometry optimization is performed for ground state (GS) and lowest donor-acceptor charge transfer (CT) state for each functional. Optical absorption and emission cross sections and energies are calculated at these geometries. Relaxation energies are on the order of 0.5 eV, and the importance of obtaining excited state equilibrium geometries in predicting delayed fluorescence is demonstrated. There are clear trends in predictions of GoWo@HF/BSE, and TDDFT/ωB97X-D and M06-2X methods in which the former method favors local exciton (LE) states while the latter favors DA CT states and ωB97X-D makes intermediate predictions. GoWo@HF/BSE suffers from triplet instability for LE states but not CT states relevant for TADF. Shifts in HOMO and LUMO levels on adding a conductor-like polarizable continuum model dielectric background are used to estimate changes in excitation energies on going from the gas phase to a solvated molecule.

Theoretical Study on Abstraction and Addition Reaction Kinetics for a Medium-Size Unsaturated Methyl Ester: Methyl-3-hexenoate + H/OH Radicals.

Combustion chemistry of methyl esters with long alkyl chains and different degrees of unsaturation has been of particular interest for biodiesel combustion. Methyl-3-hexenoate (mhx3d) with a medium-size unsaturated aliphatic chain is regarded as a valuable surrogate of biodiesel components. Here, the abstraction and addition reaction kinetics of mhx3d + H/OH radicals are comprehensively investigated. An accurate and efficient exchange-correlation density functional M06-2X/ma-TZVP is validated by the DLPNO-CCSD(T)/CBS(T-Q) benchmark calculations and is used for the multistructural search, geometry optimization, potential energy profiles, and direct dynamic calculations. The multistructural torsional (MS-T) anharmonicity, variational effects, and tunneling effects including one-dimensional Winger tunneling, multidimensional zero-curvature tunneling (ZCT), and small-curvature tunneling (SCT) are also evaluated in rate coefficient calculations. The existence of reactant complexes of the abstraction and addition reactions is neglected for mhx3d + H due to weak van der Waals interaction but is considered for mhx3d + OH. Therefore, a pre-equilibrium model is employed to obtain the rate coefficients for mhx3d + OH. The calculation results show that the MS-T factor ranges from 0.29 to 11.41, the tunneling transmission coefficient calculated by SCT is in the range of 1.0-4.0, and the recrossing transmission coefficient is between 0.65 and 1.0. Moreover, the two OH-addition reactions exhibit negative temperature coefficient behavior. The branching ratios show that the H/OH-addition reactions are dominant at lower temperature, especially for mhx3d + H. Rate coefficients of the title reactions are fitted in terms of a modified three-parameter Arrhenius expression.

Study of the Electrochemical Behavior of N-Substituted-4-Piperidones Curcumin Analogs: A Combined Experimental and Theoretical Approach

The electrochemical behavior of N-methyl- and N-benzyl-4-piperidone curcumin analogs were studied experimentally and theoretically. The studied compounds present different substituents at the para position in the phenyl rings (-H, -Br, -Cl, -CF3, and -OCH3). We assessed their electrochemical behavior by differential pulse and cyclic voltammetry, while we employed density functional theory (DFT) M06 and M06-2x functionals along with 6-311+G(d,p) basis set calculations to study them theoretically. The results showed that compounds suffer a two-electron irreversible oxidation in the range of 0.72 to 0.86 V, with surface concentrations ranging from 1.72 × 10-7 to 5.01 × 10-7 mol/cm2. The results also suggested that the process is diffusion-controlled for all compounds. M06 DFT calculations showed a better performance than M06-2x to obtain oxidation potentials. We found a good correlation between the experimental and theoretical oxidation potential for N-benzyl-4-piperidones (R2 = 0.9846), while the correlation was poor for N-methyl-4-piperidones (R2 = 0.3786), suggesting that the latter suffer a more complex oxidation process. Calculations of the BDEs for labile C-H bonds in the compounds suggested that neither of the two series of compounds has a different tendency for a proton-coupled electron transfer (PCET) oxidation process. It is proposed that irreversible behavior is due to possible dimerization of the compounds by Shono-type oxidation.

Rhenium Tricarbonyl Complexes of Azodicarboxylate Ligands

The excellent π-accepting azodicarboxylic esters adcOR (R = Et, iPr, tBu, Bn (CH2-C6H5) and Ph) and the piperidinyl amide derivative adcpip were used as bridging chelate ligands in dinuclear Re(CO)3 complexes [{Re(CO)3Cl}2(µ-adcOR)] and [{Re(CO)3Cl}2(µ-adcpip)]. From the adcpip ligand the mononuclear derivatives [Re(CO)3Cl(adcpip)] and [Re(CO)3(PPh3)(µ-adcpip)]Cl were also obtained. Optimised geometries from density functional theory (DFT) calculations show syn and anti isomers for the dinuclear fac-Re(CO)3 complexes at slightly different energies but they were not distinguishable from experimental IR or UV-Vis absorption spectroscopy. The electrochemistry of the adc complexes showed reduction potentials slightly below 0.0 V vs. the ferrocene/ferrocenium couple. Attempts to generate the radicals [{Re(CO)3Cl}2(µ-adcOR)]•- failed as they are inherently unstable, losing very probably first the Cl- coligand and then rapidly cleaving one [Re(CO)3] fragment. Consequently, we found signals in EPR very probably due to mononuclear radical complexes [Re(CO)3(solv)(adc)]•. The underlying Cl-→solvent exchange was modelled for the mononuclear [Re(CO)3Cl(adcpip)] using DFT calculations and showed a markedly enhanced Re-Cl labilisation for the reduced compared with the neutral complex. Both the easy reduction with potentials ranging roughly from -0.2 to -0.1 V for the adc ligands and the low-energy NIR absorptions in the 700 to 850 nm range place the adc ligands with their lowest-lying π* orbital being localised on the azo function, amongst comparable bridging chelate N^N coordinating ligands with low-lying π* orbitals of central azo, tetrazine or pyrazine functions. Comparative (TD)DFT-calculations on the Re(CO)3Cl complexes of the adcpip ligand using the quite established basis set and functionals M06-2X/def2TZVP/LANL2DZ/CPCM(THF) and the more advanced TPSSh/def2-TZVP(+def2-ECP for Re)/CPCMC(THF) for single-point calculations with BP86/def2-TZVP(+def2-ECP for Re)/CPCMC(THF) optimised geometries showed a markedly better agreement of the latter with the experimental XRD, IR and UV-Vis absorption data.

Reliable gas-phase tautomer equilibria of drug-like molecule scaffolds and the issue of continuum solvation.

Accurate calculation of relative tautomer energies in different environments is a prerequisite to many parameters of relevance in drug discovery. This work provides a thorough benchmark of the semiempirical methods AM1, PM3 and GFN2-xTB, the force-field OPLS4, Hartree-Fock and HF-3c, the density functionals PBEh-3c, B97-3c, r2SCAN-3c, PBE, PBE0, TPSS, r2SCAN, ω-B97X-V, M06-2X, B3LYP, B2PLYP, and second-order perturbation theory MP2 versus the gold-standard coupled-cluster DLPNO-CCSD(T) using the def2-QZVPP basis set. The outperforming method identified is M06-2X, whereas r2SCAN-3c is the best-perfoming one in the set of cost-optimized methods. Application of the two methods on a challenging subset from the SAMPL2 challenge provides evidence that deviations from experiment are caused by deficiencies of current continuum solvation methods.

Metal-decorated BN monolayer as potential chemical sensors for detection of purinethol drug

Using the density functionals B3LYP, M06-2X, and B97D, we found that the interaction of boron nitride nano-sheet (BNNS) with purinethol (PT) drug is a physical adsorption with a small sensing response (SR) of 7.9. To increase the sensitivity, we decorated Ni, Pd, Ag, and Au metals on the sheet and then we found that the adsorption energy of PT changed from −4.9 on pristine BNNT to −18.3, −18.8, −19.4, and −19.9 kcal/mol, respectively. The corresponding SR dramatically increased to 41.2, 50.3, 65.1, and 79.3. This shows that the sensitivity of the metal-decorated BNNS ([email protected]) increased by increasing the atomic number of metals. However, we predicted that Au atom increased the sensitivity of BNNS to PT more than other metals. The SR of the metal[email protected] slightly decreased in the water solvent. Finally, based on our theoretical results, the metal-decorated BN nano-structures have further practical applications.

Sarin chemical warfare agent detection by Sc-decorated XN nanotubes (X = Al or Ga)

In order to scrutinize the impact of the decoration of Sc upon the sensing performance of an XN nanotube (X = Al or Ga, and XNNT) in detecting sarin (SN), the density functionals M06-2X, τ-HCTHhyb, and B3LYP were utilized. The interaction of the pristine XNNT with SN was a physical adsorption with the sensing response (SR) of approximately 5.4. Decoration of the Sc metal into the surface of the AlN and GaN led to an increase in the adsorption energy of SN from −3.4 to −18.9, and −3.8 to −20.1 kcal/mol, respectively. Also, there was a significant increase in the corresponding SR to 38.0 and 100.5, the sensitivity of metal decorated XNNT ([email protected]) is increased. So, we found that Sc-decorating more increases the sensitivity of GaNNT toward SN compare to AlNNT. Also, the recovery time for SN to be desorbed from the [email protected] surface was found to be short, i.e., 4.4 s. Based on the energy decomposing analysis, the interaction between the SN and [email protected] was of electrostatic nature, which is also called a cation-lone pair interaction.

Computational studies on thermo-kinetics aspects of pyrolysis of isopropyl acetate and its methyl, bromide and hydroxyl derivatives

The gas-phase decomposition kinetics of isopropyl acetate (IPA) and its methyl, bromide and hydroxyl derivatives into the corresponding acid and propene were investigated using density functional theory (DFT) with the ωB97XD and M06–2x functionals, as well as the benchmark CBS-QB3 composite method. Transition state theory (TST) and RRKM theory calculations of rate constants under atmospheric pressure and in the fall-off regime were used to supplement the measured energy profiles. The results show that the formation of propene and bromoacetic acid is the most dominant pathway at the CBS-QB3 composite method, both kinetically and thermodynamically. There was a good agreement with experimental results. Pressures greater than 0.01 bar, corresponding to larger barrier heights are insufficient to ensure saturation of the measured rate coefficient when compared to the RRKM kinetic rates.Natural bond orbitals (NBO) charges, bond orders, bond indices, and synchronicity parameters all point to the considered pathways taking place via a homogenous, first-order concerted, as well as an asynchronous mechanism involving a non-planar cyclic six-membered transition state. The calculated data exhibit that the elongation of the Cα−O bond length and subsequent polarization of the Cα+δ…O−δ bond is the rate-determining step of the considered reactions in the cyclic transition state, which appears to be involved in this type of reaction.

DFT calculations of 1H- and 13C-NMR chemical shifts of 3-methyl-1-phenyl-4-(phenyldiazenyl)-1H-pyrazol-5-amine in solution

Geometries of the 3-methyl-1-phenyl-4-(phenyldiazenyl)-1H-pyrazol-5-amine azo-dye compound and its tautomer were optimized using B3LYP and M06-2X functionals in coupling with TZVP and 6–311 + G(d,p) basis sets. The 1H- and 13C-NMR chemical shifts of all species were predicted using 13 density functional theory (DFT) approaches in coupling with TZVP and 6–311 + G(d,p) basis sets at the different optimized geometries by applying the using GIAO method using the eight geometries. The selected functionals are characterized by having different amount of Hartree–Fock exchange. The selected DFT methods were B3LYP, M06-2X, BP86, B97XD, TPSSTPSS, PBE1PBE, CAM-B3LYP, wB97XD, LSDA, HSEH1PBE, PW91PW91, LC-WPBE, and B3PW91. The results obtained were compared with the available experimental data using different statistical descriptors such as root mean square error (RMSE) and maximum absolute error (MAE). Results revealed that the prediction of the 1H-NMR chemical shifts has more significant dependence on the applied geometry than that of the prediction of the 13C-NMR chemical shifts. Among all the examined functionals, B97D and TPSSTPSS functionals were found to be the most accurate ones, while the M06-2X functional is the least accurate one. Results also revealed that the prediction of NMR chemical shifts using TZVP basis sets results is more accurate results than 6–311 + G(2d,p) basis set.

AlCl3-Catalyzed Cascade Reactions of 1,2,3-Trimethoxybenzene and Adipoyl Chloride: Spectroscopic Investigations and Density Functional Theory Studies.

The reaction of 1,2,3-trimethoxybenzene with adipoyl chloride in the presence of AlCl3 gave two isomeric cyclopentene derivatives, 1,6-bis(2,3,4-trimethoxyphenyl)hexane-1,6-dione, and two demethylation products of aryl methyl ethers. The cyclopentene derivatives including unconjugated or conjugated enones are products formed in a cascade reaction resulting from first the Friedel-Crafts acylation reaction and then aldol condensation. All compounds were optimized by density functional theory calculated using two functional levels, B3LYP and M06-2X, with the 6-311+G(d,p) basis set. The structural properties were established, natural bond orbital analysis of donor-acceptor interactions was carried out, and charges on the atoms and quantum chemical reactivity identifiers were determined to compare the strength of the intramolecular hydrogen bonds formed and their stabilities. To compare the experimental 1H and 13C NMR chemical shifts with the calculated values, NMR chemical shift calculations were carried out using the gauge-invariant atomic orbital method.

Binding Energies of Interstellar Relevant S-bearing Species on Water Ice Mantles: A Quantum Mechanical Investigation

Binding energies (BEs) are one of the most important parameters for astrochemical modeling determining, because they govern whether a species stays in the gas phase or is frozen on the grain surfaces. It is currently known that, in the denser and colder regions of the interstellar medium, sulfur is severely depleted in the gas phase. It has been suggested that it may be locked into the grain icy mantles. However, which are the main sulfur carriers is still a matter of debate. This work aims to establish accurate BEs of 17 sulfur-containing species on two validated water ice structural models, the proton-ordered crystalline (010) surface and an amorphous water ice surface. We adopted density functional theory-based methods (the hybrid B3LYP-D3(BJ) and the hybrid meta-GGA M06-2X functionals) to predict structures and energetics of the adsorption complexes. London’s dispersion interactions are shown to be crucial for an accurate estimate of the BEs due to the presence of the high polarizable sulfur element. On the crystalline model, the adsorption is restricted to a very limited number of binding sites with single valued BEs, while on the amorphous model, several adsorption structures are predicted, giving a BE distribution for each species. With the exception of a few cases, both experimental and other computational data are in agreement with our calculated BE values. A final discussion on how useful the computed BEs are with respect to the snow lines of the same species in protoplanetary disks is provided.

Benchmark of density functional theory methods for the study of organic polysulfides.

Elemental sulfur is often used in organic synthesis as its low cost and high abundance make it a highly desirable source of sulfur atoms. However, sulfur's unpredictable catenation behavior poses challenges to its widespread usage due to difficulties in designing new reactions that can account for its multifaceted reactivity. In order to accurately model sulfur's mechanisms using computational approaches, it is necessary to identify density functional theory (DFT) methods that are accurate on these systems. This study benchmarks 12 well-known DFT functionals that include local, non-local, and hybrid methods against DLPNO-CCSD(T)/aug-cc-pV(Q+d)Z//MP2/aug-cc-pV(T+d)Z/SMD(MeCN) for the accurate treatment of organic polysulfides, taking cyanide as a nucleophile. Our benchmarking results indicate that the M06-2X and B3LYP-D3(BJ) density functionals are the most accurate for calculating reaction energies, while local functionals performed the worst. For activation energies, MN15, MN15-L, M06-2X, and ωB97X-D are the most accurate. Our analysis of structural parameters shows that all functionals perform well for ground state optimizations except B97D3, while MN15-L and M06-2X performed best for transition structure optimizations. Overall, the four hybrid functionals MN15, M06-2X, ωB97X-D, and B3LYP-D3(BJ) appear adequate for studying the reaction mechanisms of polysulfides.

Experimental and Quantum Chemical Approaches for Hydrazide-based Crystalline Organic Chromophores: Synthesis, SC-XRD, Spectroscopic and Nonlinear Optical Properties

Crystalline organic compounds, (E)-N-(2-(2-(2-hydroxybenzylidene)hydrazinecarbonyl)-phenyl) benzamide (NHBPB) and (E)-4-chloro-N-(2-(2-(2-hydroxybenzylidene)hydrazine-carbonyl) phenyl)benzene sulfonamide (CNHBP), were prepared through condensation of hydrazide derivatives of N-benzoylated and N-4-chlorobenzenesulfonylated derivatives of anthranilic acid with 2-hydroxybenzaldehyde. The structural interpretations were accomplished distinctly by single-crystal X-ray diffraction (SC-XRD) analysis and NMR studies. Accompanying the experimental studies, quantum chemical computations were employed to determine the non-linear optical (NLO) properties using the M06 and M06-2X functionals along with 6-311G (d,p) basis set. A comparative analysis was made in order to compare the geometrical parameters between XRD and DFT calculations and a good agreement was found. Frontier molecular orbital (FMO) analysis found a narrower HOMO/LUMO energy gap for NHBPB than CNHBP. Further, UV-Vis investigation revealed a red-shifted behavior for NHBPB (297.894 and 311.594 nm at M06-2X and M06 levels, respectively) owing to its smaller HOMO/LUMO energy gap. From the FMOs findings, Global Reactivity Parameters (GRPs) were also calculated and a relatively lower ionization potential and hardness with larger softness were found for NHBPB. The FMO and UV results indicated the greater polarizability in NHBPB as compared to CNHBP. Therefore, greater second hyperpolarizability response was noticed for NHBPB (2.749 ×105 and 1.945×105a.u at M06-2X and M006 functional, respectively). Additionally, comparative study with urea molecule showed good NLO responses for the foresaid compounds. These findings revealed that hydrazide-based compounds possess extremely high linear and nonlinear polarizabilities compared to most of the previously synthesized compounds which highlights their excellent properties for use in NLO materials.

Importance of imposing gauge invariance in time-dependent density functional theory calculations with meta-generalized gradient approximations.

It has been known for more than a decade that the gauge variance of the kinetic energy density τ leads to additional terms in the magnetic orbital rotation Hessian used in linear-response time-dependent density functional theory (TDDFT), affecting excitation energies obtained with τ-dependent exchange-correlation functionals. While previous investigations found that a correction scheme based on the paramagnetic current density has a small effect on benchmark results, we report more pronounced effects here, in particular, for the popular M06-2X functional and for some other meta-generalized gradient approximations (mGGAs). In the first part of this communication, this is shown by a reassessment of a set of five Ni(II) complexes for which a previous benchmark study that did not impose gauge invariance has found surprisingly large errors for excitation energies obtained with M06-2X. These errors are more than halved by restoring gauge invariance. The variable importance of imposing gauge invariance for different mGGA-based functionals can be rationalized by the derivative of the mGGA exchange energy integrand with respect to τ. In the second part, a large set of valence excitations in small main-group molecules is analyzed. For M06-2X, several selected n → π* and π→π⊥ * excitations are heavily gauge-dependent with average changes of -0.17 and -0.28eV, respectively, while π→π‖ * excitations are marginally affected (-0.04eV). Similar patterns, but of the opposite signs, are found for SCAN0. The results suggest that reevaluation of previous gauge variant TDDFT results based on M06-2X and other mGGA functionals is warranted.

Antioxidant Potential of Santowhite as Synthetic and Ascorbic Acid as Natural Polymer Additives.

A wide variety of additives are used to improve specific characteristics of the final polymeric product. Antioxidant additives (AAs) can prevent oxidative stress and thus the damage of polymeric materials. In this work, the antioxidant potential and thus the applicability of Santowhite (SW) as synthetic and ascorbic acid (Asc) as natural AAs were explored by using computational tools. Two density functional theory (DFT) methods, M05-2X and M06-2X, have been applied in combination with the 6-311++G(2d,2p) basis set in gas phase. Three antioxidant mechanisms have been considered: hydrogen atom transfer (HAT), single electron transfer-proton transfer (SET-PT), and sequential proton loss electron transfer (SPLET). Bond dissociation enthalpy (BDE), ionization potential (IP), proton dissociation enthalpy (PDE), proton affinity (PA), and electron transfer enthalpy (ETE) have been computed for each potential hydrogen donor site. The results indicate that the antioxidant potential of Asc is higher than SW. Furthermore, some of the C-H bonds, depending on their position in the structures, are potent radical scavengers, but O-H groups are more prone to donate H-atoms to free radicals. Nonetheless, both additives can be potentially applied to safeguard common polymers and prohibit oxidative stress-induced material deterioration.

Experimental and Computational Study of Novel Pyrazole Azo Dyes as Colored Materials for Light Color Paints

This paper presents the synthesis of eight new pyrazole azo dyes using ethyl 5-amino-3-methyl-1H-pyrazole-4-carboxylate as the diazotization component and various active methylene derivatives as coupling components. These new azo dyes were characterized by spectroscopic (FT-IR, UV-VIS), and spectrometric (1H NMR, 13C NMR, MS) analyses. The dye structures were modeled by the MMFF94s force field and quantum chemical density functional theory (DFT) calculations using the B3LYP functional and the 6-311G(d,p) basis set, in the gas phase. Weak electrostatic hydrogen bonds for the azo and hydrazo dye tautomers were found in the ground state. The CIS, TD (using the B3LYP and M06-2X functionals), and ZINDO methods were used to estimate the dye UV-VIS spectra in ethanol, which were compared with the experimental ones. The anti-configuration arrangement of the π-bonds and the presence of the prevalent hydrazo dye tautomer were supported by the computed 1H NMR and 13C NMR spectra. A good accordance between the experimental and predicted absorption maxima and chemical shifts was observed. Color investigations using the CIEL*a*b* space were conducted for all dyes in powder and for their mixtures in water-based acrylic resins. The results confirm the newly synthesized dyes’ color properties and that they might be used for light color paints in the varnishes industry.