Computational assessment of electron density in metallo‐organic nickel pincer complexes for formation of PC bonds

Hydrophosphination is an atomically efficient method for introducing new carbon‐phosphorous bonds in organic synthesis. New late‐transition metal catalytic complexes are proposed to facilitate this process. These nickel‐based complexes are analyzed using semiempirical (SE), Hartree–Fock (H–F), and density functional theory (DFT) models. H–F proves to be ineffective, while the SE approach has limited, qualitative use. DFT shows electron density at the metal center suitable for catalyzing bond formation in the proposed, reductive hydrophosphination mechanism. It also shows that the pincer complexes under investigation are relatively insensitive to solvent dielectric constant and to the chemical character of the monodentate ligand, both in terms of electron distribution and in terms of molecular orbital energies. © 2015 Wiley Periodicals, Inc.     

New Insights into the Band‐Gap Narrowing of (N,P)‐Codoped TiO2 from Hybrid Density Functional Theory Calculations

The electronic properties of anatase‐TiO₂ codoped by N and P at different concentrations have been investigated via generalized Kohn–Sham theory with the Heyd–Scuseria–Ernzerhof (HSE06) hybrid functional for exchange‐correlation in the context of density functional theory. At high doping concentrations, we find that the high photocatalytic activity of (N, P)‐codoped anatase TiO₂ vis‐à‐vis the N‐monodoped case can be rationalized by a double‐hole‐mediated coupling mechanism [Yin et al., Phys. Rev. Lett. 2011, 106, 066801] via the formation of an effective N? P bond. On the other hand, Ti³⁺ and Ti⁴⁺ ions’ spin double‐exchange results in more substantial gap narrowing for larger separations between N and P atoms. At low doping concentrations, double‐hole‐coupling is dominant, regardless of the N? P distance.     

Conjugation‐Promoted Reaction of Open‐Cage Fullerene: A Density Functional Theory Study

Density functional theory calculations are performed to study the addition mechanism of e‐rich moieties such as triethyl phosphite to a carbonyl group on the rim of a fullerene orifice. Three possible reaction channels have been investigated. The obtained results show that the reaction of a carbonyl group on a fullerene orifice with triethyl phosphite most likely proceeds along the classical Abramov reaction; however, the classical product is not stable and is converted into the experimental product. An attack on a fullerene carbonyl carbon will trigger a rearrangement of the phosphate group to the carbonyl oxygen as the conversion transition state is stabilized by fullerene conjugation. This work provides a new insight on the reactivity of open‐cage fullerenes, which may prove helpful in designing new switchable fullerene systems.     

Theoretical Shaping of Femtosecond Laser Pulses for Molecular Photodissociation with Control Techniques Based on Ehrenfest′s Dynamics and Time‐Dependent Density Functional Theory

The combination of nonadiabatic Ehrenfest‐path molecular dynamics (EMD) based on time‐dependent density functional theory (TDDFT) and quantum optimal control formalism (QOCT) was used to optimize the shape of ultra‐short laser pulses to achieve photodissociation of a hydrogen molecule and the trihydrogen cation H₃⁺. This work completes a previous one [A. Castro, ChemPhysChem, 2013 , 14, 1488–1495], in which the same objective was achieved by demonstrating the combination of QOCT and TDDFT for many‐electron systems on static nuclear potentials. The optimization model, therefore, did not include the nuclear movement and the obtained dissociation mechanism could only be sequential: fast laser‐assisted electronic excitation to nonbonding states (during which the nuclei are considered to be static), followed by field‐free dissociation. Here, in contrast, the optimization was performed with the QOCT constructed on top of the full dynamic model comprised of both electrons and nuclei, as described within EMD based on TDDFT. This is the first numerical demonstration of an optimal control formalism for a hybrid quantum–classical model, that is, a molecular dynamics method.     

Exploring the Potential Energy Surface of E2P4 Clusters (E=Group 13 Element): The Quest for Inverse Carbon‐Free Sandwiches

Inverse carbon‐free sandwich structures with formula E₂P₄ (E=Al, Ga, In, Tl) have been proposed as a promising new target in main‐group chemistry. Our computational exploration of their corresponding potential‐energy surfaces at the S12h/TZ2P level shows that indeed stable carbon‐free inverse‐sandwiches can be obtained if one chooses an appropriate Group 13 element for E. The boron analogue B₂P₄ does not form the D_4h‐symmetric inverse‐sandwich structure, but instead prefers a D_2d structure of two perpendicular BP₂ units with the formation of a double B?B bond. For the other elements of Group 13, Al–Tl, the most favorable isomer is the D_4h inverse‐sandwich structure. The preference for the D_2d isomer for B₂P₄ and D_4h for their heavier analogues has been rationalized in terms of an isomerization‐energy decomposition analysis, and further corroborated by determination of aromaticity of these species.     

Density functional theory study of mechanism of epoxy‐carboxylic acid curing reaction

A comprehensive picture on the mechanism of the epoxy‐carboxylic acid curing reactions is presented using the density functional theory B3LYP/6‐31G(d,p) and simplified physical molecular models to examine all possible reaction pathways. Carboxylic acid can act as its own promoter by using the OH group of an additional acid molecule to stabilize the transition states, and thus lower the rate‐limiting barriers by 45 kJ/mol. For comparison, in the uncatalyzed reaction, an epoxy ring is opened by a phenol with an apparent barrier of about 107 kJ/mol. In catalyzed reaction, catalysts facilitate the epoxy ring opening prior to curing that lowers the apparent barriers by 35 kJ/mol. However, this can be competed in highly basic catalysts such as amine‐based catalysts, where catalysts can enhance the nucleophilicity of the acid by forming hydrogen‐bonded complex with it. Our theoretical results predict the activation energy in the range of 71 to 94 kJ/mol, which agrees well with the reported experimental range for catalyzed reactions. © 2017 Wiley Periodicals, Inc.     

Density Functional Characterization of the Electronic Structure and Visible‐Light Absorption of Cr‐Doped Anatase TiO2

To evaluate the electronic and optical properties of Cr‐doped anatase TiO₂, three possible Cr‐doped TiO₂ models, including Cr at a Ti site (model I), Cr at a Ti site with an oxygen vacancy compensation (model II), and an interstitial Cr site (model III), are studied by means of first principles density functional theory calculations. In model I, the splitting behavior of the Cr 3d states and the insulating properties are successfully depicted by the GGA+U method, from which it is proposed that Cr at a Ti site should exist as Cr⁴⁺ instead of the generally believed Cr³⁺. As a result, the electron transitions between these impurity states, the conduction band (CB), and the valence band (VB), as well as the d–d transitions between occupied and unoccupied Cr 3d states, provide a reasonable explanation for the experimentally observed major and minor absorption bands. In models II and III, the impurity states and associated optical transition processes—as well as the corresponding electron configurations—are examined.     

Density functional studies of cation–water complexes

By using density functional ab initio techniques the equilibrium structure, binding energy, and electronic distribution were determined for [X(H₂O)_n]^+k. Specifically, when X = H, Li, and Na, k=+1 and when k=+2, X = Be and Mg. In all cases the number of water molecules varies from one through four. A correlation between the distribution of the positive charge and the binding energy of the complex was encountered. A connection between simple arguments used to describe solvation in the bulk and the results obtained here for clusters was established. © 2000 John Wiley & Sons, Inc. J Comput Chem 21: 63–68, 2000     

Density functional computations of 99Ru chemical shifts: relativistic effects, influence of the density functional, and study of solvent effects on fac-[Ru(CO)3I3]-

Solvent effects on the 99Ru NMR chemical shift of the complex fac-[Ru(CO)3I3]- are investigated computationally using density functional theory. Further, benchmark calculations of the 99Ru shift for a set of ten Ru complexes have been performed in order to calibrate the computational model and to determine the importance of relativistic effects on the 99Ru nuclear magnetic shielding and on the chemical shift. A computational model for fac-[Ru(CO)3I3]- that includes both explicit solvent molecules and a continuum model is shown to yield the best agreement with experiment. Relativistic corrections are shown to be of minor importance for determining 99Ru chemical shifts. On the other hand, the nature of the density functional is of importance. In agreement with literature data for ligand trends of 99Ru chemical shifts, the chemical shift range for different solvents is also best reproduced by a hybrid functional.     

A Time‐Dependent Density Functional Theory Investigation on the Origin of Red Chemiluminescence

Is the resonance‐based anionic keto form of oxyluciferin the chemical origin of multicolor bioluminescence? Can it modulate green into red luminescence? There is as yet no definitive answer from experiment or theory. The resonance‐based anionic keto forms of oxyluciferin have been proposed as a cause of multicolor bioluminescence in the firefly. We model the possible structures by adding sodium or ammonium cations and investigating the ground‐ and excited‐state geometries as well as the electronic absorption and emission spectra. A role for the resonance structures is obvious in the gas phase. The absorption and emission spectra of the two structures are quite different—one in the blue and another in the red. The differences in the spectra of the models are small in aqueous solution, with all the absorption and emission spectra in the yellow–green region. The resonance‐based anionic keto form of oxyluciferin may be one origin of the red‐shifted luminescence but is not the exclusive explanation for the variation from green (≈530 nm) to red (≈635 nm). We study the geometries, absorption, and emission spectra of the possible protonated compounds of keto(?1) in the excited states. A new emitter keto(?1)′‐H is considered.     

On the applicability of density functional theory to manganese‐based complexes with catalytic activity toward water oxidation

The present contribution assesses the performance of several popular and accurate density functionals, namely B3LYP, BP86, M06, MN12L, mPWPW91, PBE0, and TPSSh toward manganese‐based coordination complexes. These compounds show promising properties toward application to catalytic water oxidation. Although manganese with N‐ and O‐biding ligands tends to give rise to high spin complexes, the results show that BP86, mPWPW91, and specially MN12L, tend to yield low‐spin complexes. The usage of these functionals for such compounds is, thus, discouraged. All the functionals considered deliver accurate geometries. The present results show, however, that B3LYP delivers geometries deviating from experimental values when compared to the other functionals of the set. M06, PBE0, and TPSSh deliver geometries of similar accuracy, PBE0 outstanding slightly with respect to the other two. © 2017 Wiley Periodicals, Inc.     

On the Prebiotic Synthesis of D‐Sugars Catalyzed by L‐Peptides: Assessments from First‐Principles Calculations

What accounts for a particular chiral selection in the case of a few sugars of prebiotic relevance, thereby mirroring the asymmetry observed in nature? By using first‐principles calculations, the generation of pentoses from glycolaldehyde (the initial product of the autocatalytic formose reaction), which has been detected in outer space), has been modeled by using L ‐Val‐L ‐Val as a primeval catalyst. Our theoretical study provides insight into the mechanism of this reaction and satisfactorily explains a few key molecular events. Our rationale agrees with the reported experimental data and shows that the D ‐configuration is only favored for ribose. L ‐pentoses are usually favored in the presence of L ‐configured dipeptides, as observed experimentally, although no chiral selection could be observed in the case of xylose. These results confirm that a prebiotic sugar soup could be fine‐tuned in the presence of shorter peptides as catalysts and that D ‐ribose would have also resulted in an advantageous imbalance for further amplification and chemical evolution.     

Quantum‐Chemical Study of the FeNCN Conversion‐Reaction Mechanism in Lithium‐ and Sodium‐Ion Batteries

We report a computational study on 3d transition‐metal (Cr, Mn, Fe, and Co) carbodiimides in Li‐ and Na‐ion batteries. The obtained cell voltages semi‐quantitatively fit the experiments, highlighting the practicality of PBE+U as an approach for modeling the conversion‐reaction mechanism of the FeNCN archetype with lithium and sodium. Also, the calculated voltage profiles agree satisfactorily with experiment both for full (Li‐ion battery) and partial (Na‐ion battery) discharge, even though experimental atomistic knowledge is missing up to now. Moreover, we rationalize the structural preference of intermediate ternaries and their characteristic lowering in the voltage profile using chemical‐bonding and Mulliken‐charge analysis. The formation of such ternary intermediates for the lithiation of FeNCN and the contribution of at least one ternary intermediate is also confirmed experimentally. This theoretical approach, aided by experimental findings, supports the atomistic exploration of electrode materials governed by conversion reactions.     

Gas‐phase pyrolysis mechanisms of 3‐anilino‐1‐propanol: Density functional theory study

The gas‐phase pyrolytic decomposition mechanisms of 3‐anilino‐1‐propanol with the products of aniline, ethylene, and formaldehyde or N‐methyl aniline and aldehyde were studied by density functional theory. The geometries of the reactant, transition states, and intermediates were optimized at the B3LYP/6‐31G (d, p) level. Vibration analysis was carried out to confirm the transition state structures, and the intrinsic reaction coordinate method was performed to search the minimum energy path. Four possible reaction channels are shown, including two concerted reactions of direct pyrolytic decomposition and two indirect channels in which the reactant first becomes a ring‐like intermediate, followed by concerted pyrogenation. One of the concerted reactions in the direct pyrolytic decomposition has the lowest activation barrier among all the four channels, and so, it occurs more often than others. The results appear to be consistent with the experimental outcomes. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2009     

Crystal structure and spectroscopic characterization of a cobalt(II) tetraazamacrocycle: completing a series of first‐row transition‐metal complexes

The tetraazamacrocyclic ligand 1,4,8,11‐tetramethyl‐1,4,8,11‐tetraazacyclotetradecane (TMC) has been used to bind a variety of first‐row transition metals but to date the crystal structure of the cobalt(II) complex has been missing from this series. The missing cobalt complex chlorido(1,4,8,11‐tetramethyl‐1,4,8,11‐tetraazacyclotetradecane‐κ⁴N )cobalt(II) chloride dihydrate, [CoCl(C₁₄H₃₂N₄)]Cl·2H₂O or [Co^IICl(TMC)]Cl·2H₂O, crystallizes as a purple crystal. This species adopts a distorted square‐pyramidal geometry in which the TMC ligand assumes the trans‐I configuration and the chloride ion binds in the syn‐methyl pocket of the ligand. The Co^II ion adopts an S = spin state, as measured by the Evans NMR method, and UV–visible spectroscopic studies indicate that the title hydrated salt is stable in solution. Density functional theory (DFT) studies reveal that the geometric parameters of [Co^IICl(TMC)]Cl·2H₂O are sensitive to the cobalt spin state and correctly predict a change in spin state upon a minor perturbation to the ligand environment.     

Density functional study of the conformers of Co4‐based single‐molecule magnet

We present a detailed density functional theory‐based investigation on the geometry and electronic structure of the [Co₄(hmp)₄(MeOH)₄Cl₄] molecule. It is experimentally found to behave as a molecular magnet. The all‐electron electronic structure calculations and geometry optimization of the 88‐atom molecule were carried out within the generalized gradient approximation to the exchange correlation energy. We also study the electronic structures and geometries of a few low‐lying conformers of this molecule. It is found that the magnetic anisotropy energy is highly sensitive to the geometric structure of the molecule. © 2003 Wiley Periodicals, Inc. Int J Quantum Chem 93: 324–331, 2003     

Coupled‐cluster reaction barriers of : An application of the coupled‐cluster//Kohn–Sham density functional theory model chemistry

In this work, we report a theoretical investigation concerning the use of the popular coupled‐cluster//Kohn‐Sham density functional theory (CC//KS‐DFT) model chemistry, here applied to study the entrance channel of the reaction, namely by comparing CC//KS‐DFT calculations with KS‐DFT, MRPT2//CASSCF, and CC//CASSCF results from our previous investigations. This was done by performing single point energy calculations employing several coupled cluster methods and using KS‐DFT geometries optimized with six different functionals, while conducting a detailed analysis of the barrier heights and topological features of the curves and surfaces here obtained. The quality of this model chemistry is critically discussed in the context of the title reaction and also in a wider context. © 2013 Wiley Periodicals, Inc.