The trigonal planar geometry of the nitrogen atom in commonly used phosphoramidite ligands is not in line with the traditional valence shell electron pair repulsion (VSEPR) model. In this work, the effects governing nitrogen configuration in several substituted aminophosphines, A₂PNB₂ (A or B = H, F, Cl, Br, Me, OMe, BINOP), are examined using modern computational analytic tools. The electron delocalization descriptions provided by both electron localization function (ELF) and block localized wavefunction analysis support the proposed relationships between conformation and negative hyperconjugative interactions. In the parent H₂PNH₂, the pyramidal nitrogen configuration results from nitrogen lone pair electron donation into the σ* P — H orbital. While enhanced effects are seen for F₂PNMe₂, placing highly electronegative fluorine substituents on nitrogen (i.e., Me₂PNF₂) eliminates delocalization of the nitrogen lone pair. Understanding and quantifying these effects can lead to greater flexibility in designing new catalysts.

Density functional theory (DFT) has progressively emerged in the last 40 years as a leading methodology for the modelling and simulation of chemical systems. In this paper, some historical landmarks in the development of this method are outlined, emphasizing on its main characteristic being an electron density-based theory. This is in contrast with wavefunction-based methodologies which were exclusively employed previously. Interestingly, DFT has been first applied to solids, with a rather late recognition by chemists and molecular scientists. After this historical survey, several applications of DFT to the structure and properties of zeolites are reviewed as a tribute to Dr Annick Goursot.

The C₂-symmetric electron-poor ligand (R)-BINOP-F (4) was prepared by reaction of (R)-BINOL with bis(pentafluorophenyl)-phosphorus bromide in the presence of triethylamine. The iodo complex [CpRu((R)-BINOP-F)(I)] ((R)-6) was obtained by substitution of two carbonyl ligands by (R)-4 in the in situ-prepared [CpRu(CO)₂H] complex followed by reaction with iodoform. Complex 6 was reacted with [Ag(SbF₆)] in acetone to yield [CpRu((R)-BINOP-F)(acetone)][SbF₆] ((R)-7). X-ray structures were obtained for both (R)-6 and (R)-7. The chiral one-point binding Lewis acid [CpRu((R)-BINOP-F)][SbF₆] derived from either (R)-7 or the corresponding aquo complex (R)-8 activates methacrolein and catalyzes the Diels−Alder reaction with cyclopentadiene to give the [4 + 2] cycloadduct with an exo/endo ratio of 99:1 and an ee of 92% of the exo product. Addition occurs predominantly to the methacrolein C_α-Re face. In solution, water in (R)-8 exchanges readily. Moreover, a second exchange process renders the diastereotopic BINOP-F phosphorus atoms equivalent. These processes were studied by the application of variable-temperature ¹H, ³¹P, and ¹⁷O NMR spectroscopy, variable-pressure ³¹P and¹⁷O NMR spectroscopy, and, using a simpler model complex, density functional theory (DFT) calculations. The results point to a dissociative mechanism of the aquo ligand and a pendular motion of the BINOP-F ligand. NMR experiments show an energy barrier of 50.7 kJ mol^-1 (12.2 kcal mol^-1) for the inversion of the pseudo-chirality at the ruthenium center.

The components of nucleus-independent chemical shift (NICS) tensors for D_nhn-annulenes are discussed as indexes of the aromatic character of electronic π systems. The component corresponding to the principal axis perpendicular to the ring plane, NICS_zz, is found to be a good measure for the characterisation of the π system of the ring. Isotropic NICS values at ring centres contain large influences from the σ system and from all three principal components of the NICS tensor. At large distances away from the ring center, NICS_zz, which is dominated by contributions from the π system, characterizes NICS well.

Entirely unlike the aromatic closo B_nH_n^2- borane dianions, isoelectronic Si₆^2- and Si₁₂^2- are antiaromatic. Their O_h and I_h symmetries are responsible, as the other deltahedral silicon dianion clusters do not exhibit this behavior. These high symmetries prevent mixing among the degenerate lone pair and skeletal orbitals, leading to paratropic behavior.

The use and the number of sulfonylurea herbicides have increased since the early 1980s. A good understanding of their degradation is of ecological importance, since environmental pollutants can be issued from them. It is claimed that microbial degradation and chemical hydrolysis present the main degradation pathways but photodegradation cannot be neglected. Time-dependent density functional theory has been used to help in the elucidation of the photochemical behavior of sulfonylureas.

Individual molecular orbital (MO) contributions to the magnetic shielding of atoms as well as to the nucleus-independent chemical shifts (NICS) of aromatic compounds can be computed by the widely used gauge-including atomic orbital (GIAO) method. Detailed analyses of magnetic shielding MO-NICS contributions provide interpretive insights that complement and extend those given by the localized MO (“dissected NICSâ€�, LMO-NICS) method. Applications to (4n + 2) Ï€-electron systems, ranging from [n] annulenes to D_nh S₃, S₅, and N₆H₆²⁺ rings as well as to D_2h cyclobutadiene, show the extent to which their diatropic character results from the σ framework and from the Ï€ orbitals. The diatropicity of both these contributions decreases with the number of nodes of the wave function around the ring. The highest-energy orbitals can become paratropic. This is generally the case with the σ orbitals, but is found only for “electron-richâ€� Ï€ systems such as sulfur rings. MO-NICS contributions, which can be interpreted using London−Hückel theory, correlate with inverse ring size.

As shown by detailed nucleus-independent chemical shift (NICS) analyses of the contributions of each molecular orbital, the very recently reported gas-phase all-metal Al₄Li₃^- anion and its relatives (Kuznetsov, A.E.; Birch, K.A.; Boldyrev, A.I.; Li, X.; Zhai, A.I.; Wang, L.S. Science 2003, 300, 622) are aromatic rather than antiaromatic. The paratropic (antiaromatic) four-Ï€-electron contribution is overcome by the predominating diatropic effects of σ aromaticity. However, true antiaromatic all-metal clusters, such as Sn₆^2- (Schiemenz, B.; Huttner, G. Angew. Chem., Int. Ed. Engl. 1993, 32, 297), do exist.

Quinodimethanes are highly reactive toward dienophiles since Diels−Alder cycloaddition results in an aromatic product. Density functional-based ¹³C, ¹H NMR, NICS, and MO-NICS calculations indicate that the increase of aromatic character of the developing benzenoid ring along the reaction path is especially pronounced after the transition state is reached, even though the number of Ï€ orbitals decreases. The forming aliphatic ring exhibits large ring current effects during the reaction.

Several methods to address aromaticity in terms of nucleus-independent chemical shifts (NICS) are compared. These include NICS at the ring centre NICS(0), NICS 1 Ã… above the ring plane NICS(1), aromatic ring current shielding (ARCS), and dissected NICS, i.e. NICS calculated from selected Ï€ orbitals NICS_Ï€, again in the ring plane and 1 Ã… above. The methods are tested on the basis of density-functional theory (DFT) and the individual gauge for local orbitals (IGLO) technique. Applications include simple organic rings (C₄H₄, C₄H₄²⁺, C₆H₆, C₅H₅^–, C₇H₇⁺) and transition metal carbonyl complexed molecules Fe(CO)₃C₄H₄ and Cr(CO)₃C₆H₆.

Geometries and ²⁹Si NMR chemical shifts are calculated for silanes Si_nH_2n+2, n=1,…,5, methylsilanes SiH_nMe_4−n, methoxysilanes SiH_n(OMe)_4−n, and methylmethoxysilanes SiMe_n(OMe)_4−n, n=0,…,4. Geometries and ²⁹Si NMR chemical shifts are in satisfying agreement with experiment within LCGTO-DFT at the DZVP/LDA level for geometries and IGLO-III/GGA (GGA=PW91,PBE) level for shielding constants, which is an improvement to B88PW86, P86PW86 and B3LYP results. If an auxiliary basis is applied to express the Coulomb potential, g-functions have to be included to reproduce SiOSi angles and ²⁹Si NMR chemical shifts correctly.

Calculations of ¹³C nuclear shieldings for low-energy isomers of C₃₆H_2x (x=2,3) suggest that it should be possible to use experimental¹³C shifts, when these become available, to distinguish the isomeric form of the underlying fullerene cage and, in the case of isomers based on the six-fold symmetrical cylindrical fullerene cage 36:15, the degree of polar hydrogenation.