Strategies to Calculate Fukui Functions and Applications to Radicals with SOMO-HOMO Inversion.

This work explores an efficient and numerically accurate procedure to obtain the Fukui function from fractional orbital occupation calculations. The energy- and density-linearity conditions are investigated in the context of using optimally tuned range-separated hybrid functionals for the calculation of the Fukui function. The methodology is then used to study the reactivity of organic radicals exhibiting energetic inversion between the singly occupied molecular orbital (SOMO) and the highest occupied molecular orbital (HOMO), that is, SOMO-HOMO inversion (SHI). The Fukui function correctly identifies the reactive sites of the molecules investigated, but additional computed quantities, such as radical reaction energies and vertical ionization potentials, are needed to distinguish SHI systems from conventional radicals.

Experimental and Computational Determination of a M-Cl Homolytic Bond Dissociation Free Energy: Mn(III)Cl-Mediated C-H Cleavage and Chlorination.

This study confirms the hypothesis that [MnCl3(OPPh3)2] (1) and acetonitrile-solvated MnCl3 (i.e., [MnCl3(MeCN)x]) can be used as synthons to prepare Mn(III) chloride complexes with facially coordinating ligands. This was achieved through the preparation and characterization of six new {MnIIICl} complexes using anionic ligands TpH (tris(pyrazolyl)borate) and TpMe (tris(3,5-dimethylpyrazolyl)borate). The MnIII-chloride dissociation and association equilibria (Keq) and MnIII/II reduction potentials were quantified in DCM. These two thermochemical parameters (Keq and E1/2), in addition to the known Cl-atom reduction potential in DCM, enabled the quantification of the Mn-Cl bond dissociation (homolysis) free energy of 21 and 23 ± 7 kcal/mol at room temperature for R = H and Me, respectively. These are in reasonable agreement with the bond dissociation free energy (BDFEM-Cl) of 34 ± 6 kcal/mol calculated using density functional theory. The BDFEM-Cl of 1 was also calculated (25 ± 6 kcal/mol). These energies were used in predictive C-H bond reactivity.

Density-Corrected Density Functional Theory for Molecular Properties.

Density-corrected (DC) density functional theory (DFT) has been proposed to overcome difficulties related to the self-interaction error. The procedure uses the Hartree-Fock electron density (matrix) non-self-consistently in conjunction with an approximate functional. DC-DFT has so far mainly been tested for total energy differences, whereas other types of molecular properties have not been evaluated systematically. This work focuses on the performance of DC-DFT for molecular properties, namely, dipole moments, static polarizabilities, and electric field gradients (EFGs) at atomic nuclei. Accurate reference data were generated from coupled-cluster theory to assess the performance of DC and self-consistent DFT calculations for twelve molecules, including diatomics with transition metals. DC-DFT does no harm in dipole moment calculations, but it negatively impacts the polarizability in at least one case. DC-DFT performs well for EFGs, even for the difficult case of CuCl.

Comparison of the Performance of Density Functional Methods for the Description of Spin States and Binding Energies of Porphyrins.

This work analyzes the performance of 250 electronic structure theory methods (including 240 density functional approximations) for the description of spin states and the binding properties of iron, manganese, and cobalt porphyrins. The assessment employs the Por21 database of high-level computational data (CASPT2 reference energies taken from the literature). Results show that current approximations fail to achieve the "chemical accuracy" target of 1.0 kcal/mol by a long margin. The best-performing methods achieve a mean unsigned error (MUE) <15.0 kcal/mol, but the errors are at least twice as large for most methods. Semilocal functionals and global hybrid functionals with a low percentage of exact exchange are found to be the least problematic for spin states and binding energies, in agreement with the general knowledge in transition metal computational chemistry. Approximations with high percentages of exact exchange (including range-separated and double-hybrid functionals) can lead to catastrophic failures. More modern approximations usually perform better than older functionals. An accurate statistical analysis of the results also casts doubts on some of the reference energies calculated using multireference methods. Suggestions and general guidelines for users are provided in the conclusions. These results hopefully stimulate advances for both the wave function and the density functional side of electronic structure calculations.

Low-Temperature Luminescence in Organic Helicenes: Singlet versus Triplet State Circularly Polarized Emission.

The experimental measurement of the photophysical and chiroptical properties of helicene-based π-conjugated emitters with electron-accepting (-CN, -py, -NO2) or donating (TMS, NMe2, NH2) moieties is reported at low temperature (77 K). The samples exhibit strong circularly polarized phosphorescence in frozen solution of 2-MeTHF, with a luminescence dissymmetry factor reaching 1.6 × 10-2 and a lifetime of over 0.46 s for the most active molecule, the nitro compound. The theoretical investigation shows that although the singlet (S1) and triplet (T1) excited-state emissions mainly arise from the helicene core, the rotatory strengths of the spin-allowed versus spin-forbidden emission have opposite signs. Further analysis of the spin-orbit coupling matrix elements shows that there is no strong mixing between S1 and T1, justifying the different signs of the rotatory strengths. In the case of the nitro compound, the enhanced phosphorescence emission is due to an efficient intersystem crossing.

Hydrogenative Catalysis with Three-Coordinate Zinc Complexes Supported with PN Ligands is Enhanced Compared to PNP Analogs.

This work details the synthesis, characterization, and catalytic activity of reactive low-coordinate organozinc complexes. The complexes activate hydrogen and they appear to be more active in hydrogenation of ketones and imines than their tridentate pincer analogs. This is thought, in part, to be due to the lack of trailing third phosphorus arm present in previous work. DFT computations reveal a sigma-bond metathesis mechanism is comparable to an alternative aromatization/dearomatization metal-ligand cooperative mechanism.

Comparative Study of Vibrational Raman Optical Activity with Different Time-Dependent Density Functional Approximations: The VROA36 Database.

A new database, VROA36, is introduced to investigate the performance of computational approaches for vibrational Raman optical activity (VROA) calculations. The database is composed of 36 molecules with known experimental VROA spectra. It includes 93 conformers. Normal modes calculated with B3LYP-D3(BJ)/def2-TZVP are used to compute the VROA spectra with four functionals, B3LYP-D3(BJ), ωB97X-D, M11, and optimally tuned LC-PBE, as well as several basis sets. SimROA indices and frequency scaling factors are used to compare calculated spectra with each other and with experimental data. The four functionals perform equally well independently of the basis set and usually achieve good agreement with the experimental data. For molecules in near- or at-resonance conditions, the inclusion of a complex (damped) linear response approach is important to obtain physically meaningful VROA intensities. The use of any of the tested functional approximations with the def2-SVPD Gaussian-type basis set, or a basis of similar flexibility, can be recommended for efficient and reliable theoretical VROA studies.

Zinc Ammonio-dodecaborates: Synthesis, Lewis Acid Strength, and Reactivity.

Two series of zinc salts, [EtZn][A] and Zn[A]2, with weakly coordinating anions [A]- as counterions have been prepared, and their activities as catalysts for hydrosilylation reactions of 1-hexene, benzophenone, and acetophenone have been investigated. The counterions and per- and partially chlorinated 1-ammonio-closo-dodecaborate anions [Me3NB12Cl11]- [1]-, [Pr3NB12H5Cl6]- [2]-, [Bu3NB12H4Cl7]- [3]-, and [Hex3NB12H5Cl6]- [4]- were chosen as potential and more readily available alternatives to carborate anions such as [CHB11Cl11]- and [HexCB11Cl11]-. The basicity of anion [4]- was determined as being close to that of the triflimide anion [N(SO2CF3)2]-, and the fluoride ion affinities (FIAs) of compounds [EtZn][2] and Zn[2]2 are lower than those of the Lewis acids B(C6F5)3 and Zn[HexCB11Cl11]2. The higher anion basicity and the resulting lower Lewis acidity of the zinc centers result in low activity in 1-hexene hydrosilylation catalysis and only moderate activity in the hydrosilylation catalysis of benzophenone and acetophenone.

Competition between cyclization and unusual Norrish type I and type II nitro-acyl migration pathways in the photouncaging of 1-acyl-7-nitroindoline revealed by computations.

The 7-nitroindolinyl family of caging chromophores has received much attention in the past two decades. However, its uncaging mechanism is still not clearly understood. In this study, we performed state-of-the-art density functional theory calculations to unravel the photo-uncaging mechanism in its entirety, and we compared the probabilities of all plausible pathways. We found competition between a classical cyclization and an acyl migration pathway, and here we explain the electronic and steric reasons behind such competition. The migration mechanism possesses the characteristics of a combined Norrish type I and a 1,6-nitro-acyl variation of a Norrish type II mechanism, which is reported here for the first time. We also found negligible energetic differences in the uncaging mechanisms of the 4-methoxy-5,7-dinitroindolinyl (MDNI) cages and their mononitro analogues (MNI). We traced the experimentally observed improved quantum yields of MDNI to a higher population of the reactants in the triplet surface. This fact is supported by a more favorable intersystem crossing due to the availability of a higher number of triplet excited states with the correct symmetry in MDNI than in MNI. Our findings may pave the way for improved cage designs that possess higher quantum yields and a more efficient agonist release.

Steps toward Rationalization of the Enantiomeric Excess of the Sakurai-Hosomi-Denmark Allylation Catalyzed by Biisoquinoline N,N'-Dioxides Using Computations.

Allylation reactions of aldehydes are chemical transformations of fundamental interest, as they give direct access to chiral homoallylic alcohols. In this work, we focus on the full computational characterization of the catalytic activity of substituted biisoquinoline-N,N'-dioxides for the allylation of 2-naphthaldehyde. We characterized the structure of all transition states as well as identified the π stacking interactions that are responsible for their relative energies. Motivated by disagreement with the experimental results, we also performed an assessment of 34 different density functional methods, with the goal of assessing DFT as a general tool for understanding this chemistry. We found that the DFT results are generally consistent as long as functionals that correctly account for dispersion interactions are used. However, agreement with the experimental results is not always guaranteed. We suggest the need for a careful synergy between computations and experiments to correctly interpret the data and use them as a design tool for new and improved asymmetric catalysts.

Statistically representative databases for density functional theory via data science.

The amount of data and number of databases for the assessment and parameterization of density functional theory methods has grown substantially in the past two decades. In this work, we introduce a novel cluster analysis technique for density functional theory calculations of the electronic structure of atoms and molecules with the goal of creating new statistically significant databases with broad chemical scope, and a manageable number of data-points. By analyzing without a priori chemical assumptions a population of almost 350k data-points, we create a new database called ASCDB containing only 200 data-points. This new database holds the same chemical information as the larger population of data from which it is obtained, but with a computational cost that is reduced by several orders of magnitude. The labelling of the significant chemical properties is performed a posteriori on the resulting 16 subsets, classifying them into four areas of chemical importance: non-covalent interactions, thermochemistry, non-local effects, and unbiased calculations. The analysis of the results and their transferability shows that ASCDB is capable of providing the same information as that of the larger collection of data-such as GMTKN55, MGCDB84, and Minnesota 2015B-for several density functional theory methods and basis sets. In light of these results, we suggest the use of this new small database as a first inexpensive tool for the evaluation and parameterization of electronic structure theory methods.

Aerobic CuCl2-Catalyzed Dehydrogenative Cross-Coupling of Tertiary Amines. A Combined Computational and Experimental Study.

The generation of an iminium from amines is a way to functionalize the α carbon by coupling reactions. The reaction mechanism of the conversion of tertiary amines to iminium with CuCl2(H2O)2 as catalyst in aerobic conditions has been computationally and experimentally studied in this work. This process is initiated by the oxidation of the amine to a radical cation dichlorocuprate through the reduction of CuII to CuI. Then, the iminium dichlorocuprate is generated from the radical-ion through a hydrogen-transfer. The H atom can be accepted by molecular oxygen or by a second molecule of catalyst (in anaerobic conditions). Therefore, O2 also assumes the important role of acceptor along with that of regenerator of the catalyst. The experimental study confirmed the computational study and lead to the synthesis of four new molecules from the cross-coupling of N, N-diethyl- and N, N-diisoproylaniline with nitromethane and dimethylmalonate.

ACCDB: A collection of chemistry databases for broad computational purposes.

The importance of databases of reliable and accurate data in chemistry has substantially increased in the past two decades. Their main usage is to parametrize electronic structure theory methods, and to assess their capabilities and accuracy for a broad set of chemical problems. The collection we present here-ACCDB-includes data from 16 different research groups, for a total of 44,931 unique reference data points, all at a level of theory significantly higher than density functional theory, and covering most of the periodic table. It is composed of five databases taken from literature (GMTKN, MGCDB84, Minnesota2015, DP284, and W4-17), two newly developed reaction energy databases (W4-17-RE and MN-RE), and a new collection of databases containing transition metals. A set of expandable software tools for its manipulation is also presented here for the first time, as well as a case study where ACCDB is used for benchmarking commercial CPUs for chemistry calculations. © 2018 Wiley Periodicals, Inc.

Axial-Chiral Biisoquinoline N, N'-Dioxides Bearing Polar Aromatic C-H Bonds as Catalysts in Sakurai-Hosomi-Denmark Allylation.

The design, synthesis, and evaluation of axial-chiral biisoquinolines bearing polar aromatic C-H bonds as Lewis base catalysts are reported. Lewis bases containing the 3,5-bis(trifluoromethyl)phenyl group were found to be significantly more enantioselective for a wider range of substrates than those bearing aromatic residues that are not strongly electron-deficient in the allylation of aldehydes with allyltrichlorosilane. Also, optically pure 3,3'-dibromo-1,1'-biisoquinoline N, N'-dioxide that has not been previously reported was synthesized as a common catalyst precursor to facilitate the study.