What does graphitic carbon nitride really look like?

Graphitic carbon nitrides (g-CNs) have become popular light absorbers in photocatalytic water splitting cells. Early theoretical work on these structures focused on fully polymerized g-C3N4. Experimentally, it is known that the typically employed melamine polycondensation does not go toward completion, yielding structures with ∼15 at% hydrogen. Here, we study the conformational stability of "melon", with the [C6N9H3]n structural formula using DFT. Referencing to a 2D melon sheet, B3LYP-dDsC and PBE-MBD computations revealed the same qualitative trend in stability of the 3D structures, with several of them within 5 kJ mol-1 per tecton. Fina's orthorhombic melon is the most stable of the studied conformers, with Lotsch' monoclinic melon taking an intermediate value. Invoking a simple Wannier-Mott-type approach, Fina's and Lotsch' structures exhibited the lowest optical gaps (2.8 eV), within the error margin of the experimental value (2.7 eV). All conformers yielded gaps below that of the monolayer's (3.2 eV), suggesting Jelley-type ("J") aggregation effects.

Towards a simplified description of thermoelectric materials: accuracy of approximate density functional theory for phonon dispersions.

We calculate the phonon-dispersion relations of several two-dimensional materials and diamond using the density-functional based tight-binding approach (DFTB). Our goal is to verify if this numerically efficient method provides sufficiently accurate phonon frequencies and group velocities to compute reliable thermoelectric properties. To this end, the results are compared to available DFT results and experimental data. To quantify the accuracy for a given band, a descriptor is introduced that summarizes contributions to the lattice conductivity that are available already in the harmonic approximation. We find that the DFTB predictions depend strongly on the employed repulsive pair-potentials, which are an important prerequisite of this method. For carbon-based materials, accurate pair-potentials are identified and lead to errors of the descriptor that are of the same order as differences between different local and semi-local DFT approaches.

A DFT study of the formation of xanthydrol motifs during electrophilic poly(aryl ether ketone) synthesis.

The reaction pathway of the cyclization of 2-phenoxybenzophenone into 9-phenyl-9H-xanthen-9-ol in the presence of acid and an excess of AlCl33 was studied using density functional theory. This type of reaction is known to occur during the Friedel-Crafts polycondensation of poly(aryl ether ketones) following the undesired benzoylation of nucleophilic positions ortho- to the growing polymer's ether groups. The formed defect acts as an undesired terminator of the polymer chain, causing severe problems in the polymer's melt state. A branched, multistep mechanism reminiscent of the Friedel-Crafts acylation reaction is discovered; the reaction starts with the protonation of the carbonyl oxygen, followed by intramolecular electrophilic attack on the carbonyl carbon that determines the turnover frequency of the catalytic cycle and ends by deprotonation of the Wheland intermediate.

Electronic properties of PbX3CH3NH3 (X = Cl, Br, I) compounds for photovoltaic and photocatalytic applications.

Since the discovery of their excellent performance as the light-absorbing semiconducting component in photovoltaic cells, the PbX3CH3NH3 (X = I, Br, Cl) perovskites have received renewed attention. The five polymorphs stable above 200 K - the tetragonal phases for X = I, Br, Cl and the cubic phases for X = I, Br - were studied using periodic DFT calculations involving hybrid functionals (PBE0 and HSE), employing Gaussian-type orbitals as well as plane waves and including relativistic effects (spin-orbit coupling). The influence of the halogen substitution and of the crystal phase on these properties is analysed by comparing the properties obtained in this study to the experimental ones and to the theoretical ones computed using other methods. We show that an accurate treatment of these systems requires the description of dispersion forces and spin-orbit coupling. The different time scales for the electronic and vibrational components of the polarizability inspire the hypothesis that several interfacial charge transfer mechanisms are encountered in the working principle of the photovoltaic devices involving these perovskite materials. The heavy elements in the structure (Pb, I) play a major role in the high polarizability and the low effective charge carrier masses and hence in the low exciton binding energies and the high charge mobility. This systematic work on the PbX3CH3NH3 family offers to theoreticians an overview of the landscape of quantum chemical methods to enable a reasonable choice of methodology for studying these systems.

A DFT study of the Al2Cl6-catalyzed Friedel-Crafts acylation of phenyl aromatic compounds.

The reaction pathways of several Friedel-Crafts acylations involving phenyl aromatic compounds were studied using density functional theory. The reactions were related to the Friedel-Crafts polycondensation of polyaryletherketones. In particular, the acylation of benzene with benzoyl chloride to form benzophenone and variations on this reaction were investigated. The acylation of benzene by one molecule of terephthaloyl chloride or isophthaloyl chloride as well as acylations at the m-, o-, and p-positions of diphenyl ether with one molecule of benzoyl chloride were studied. Adding an additional acyl chloride group to the electrophile appeared to have little influence on the reaction pathway, although the activation energy for the C-C bond-forming steps that occurred when isophthaloyl choride was used was different to the activation energy observed when terephthaloyl chloride was used. Upon changing the nucleophile to diphenyl ether, the reactivity changed according to the trend predicted on based on the o-, p-directing effects of the ether group. The deprotonation step that restored aromaticity varied widely according to the reaction. The rate-determining step in all of the studied reactions was the formation of the acylium ion, followed in importance by either the formation of the Wheland intermediate or the abstraction of hydrogen, depending on the reactivity of the nucleophile.

Study of N1-alkylation of indoles from the reaction of 2(or 3)-aminoindole-3-(or 2)carbonitriles with DMF-dialkylacetals.

The condensation of 2-aminoindole-3-carbonitriles and their 3-aminoindole-2-carbonitrile isomers with various DMF-dialkoxyacetals was investigated under microwaves. The appearance of reactive and versatile alkoxyiminium species allowed convenient access to indole precursors of building blocks with potential biological activity. The experimental results have been rationalised using DFT calculations of theoretical descriptors based on the electrostatic potential.