No abstract

The nature of the chemical bond in nonpolar molecules has been investigated by energy-partitioning analysis (EPA) of the ADF program using DFT calculations. The EPA divides the bonding interactions into three major components, that is, the repulsive Pauli term, quasiclassical electrostatic interactions, and orbital interactions. The electrostatic and orbital terms are used to define the nature of the chemical bond. It is shown that nonpolar bonds between main-group elements of the first and higher octal rows of the periodic system, which are prototypical covalent bonds, have large attractive contributions from classical electrostatic interactions, which may even be stronger than the attractive orbital interactions. Fragments of molecules with totally symmetrical electron-density distributions, like the nitrogen atoms in N(2), may strongly attract each other through classical electrostatic forces, which constitute 30.0 % of the total attractive interactions. The electrostatic attraction can be enhanced by anisotropic charge distribution of the valence electrons of the atoms that have local areas of (negative) charge concentration. It is shown that the use of atomic partial charges in the analysis of the nature of the interatomic interactions may be misleading because they do not reveal the topography of the electronic charge distribution. Besides dinitrogen, four groups of molecules have been studied. The attractive binding interactions in H(n)E-EH(n) (E=Li to F; n=0-3) have between 20.7 (E=F) and 58.4 % (E=Be) electrostatic character. The substitution of hydrogen by fluorine does not lead to significant changes in the nature of the binding interactions in F(n)E-EF(n) (E=Be to O). The electrostatic contributions to the attractive interactions in F(n)E-EF(n) are between 29.8 (E=O) and 55.3 % (E=Be). The fluorine substituents have a significant effect on the Pauli repulsion in the nitrogen and oxygen compounds. This explains why F(2)N-NF(2) has a much weaker bond than H(2)N-NH(2), whereas the interaction energy in FO-OF is much stronger than in HO-OH. The orbital interactions make larger contributions to the double bonds in HB=BH, H(2)C=CH(2), and HN=NH (between 59.9 % in B(2)H(2) and 65.4 % in N(2)H(2)) than to the corresponding single bonds in H(n)E-EH(n). The orbital term Delta E(orb) (72.4 %) makes an even greater contribution to the HC triple bond CH triple bond. The contribution of Delta E(orb) to the H(n)E=EH(n) bond increases and the relative contribution of the pi bonding decreases as E becomes more electronegative. The pi-bonding interactions in HC triple bond CH amount to 44.4 % of the total orbital interactions. The interaction energy in H(3)E-EH(3) (E=C to Pb) decreases monotonically as the element E becomes heavier. The electrostatic contributions to the E-E bond increases from E=C (41.4 %) to E=Sn (55.1 %) but then decreases when E=Pb (51.7 %). A true understanding of the strength and trends of the chemical bonds can only be achieved when the Pauli repulsion is considered. In an absolute sense the rep...

The recent literature has shown an increase in the number of co-crystals reported to be polymorphic, with at least 45 such systems identified thus far. The question of whether cocrystals, defined as any multicomponent neutral molecular complex that forms a crystalline solid, are inherently less prone to polymorphism than the individual components is shown to be untrue in four sets of polymorphic co-crystals. The co-crystal formers in this study, acridine, nicotinamide, 3-hydroxybenzoic acid, 2,4-dihydroxybenzoic acid, malonic acid, and pimelic acid, are all polymorphic in their unimolecular states and are shown to be dimorphic in the following combinations: (3-hydroxybenzoic acid)•(acridine) [1(I) and 1(II)], (2,4-dihydroxybenzoic acid)• (nicotinamide) [4(I) and 4(II)], (malonic acid)•(nicotinamide) [5(I) and 5(II)], and (pimelic acid)•(nicotinamide) [6(I) and 6(II)]. These co-crystals are assembled primarily using carboxylic acid and phenol hydrogen bond donors that hydrogen bond to pyridine N or amide carbonyl acceptors. Two different combinations of donors and acceptors are primarily responsible for the formation of polymorphs in 1 and 4, whereas conformational differences within the malonic and pimelic acid molecules lead to different packing arrangements using the same combination of hydrogen bonded interactions in 5 and 6. The 1:2 co-crystal of (3hydroxybenzoic acid)•(acridine) 2 (2) displays both the phenol O−H•••N hydrogen bond observed in 1(I) and the carboxylic acid O−H•••N hydrogen bond observed in 1(II). In addition, a methanol solvate of (2,4-dihydroxybenzoic acid)•(nicotinamide) (3) is reported. DFT calculations show that the carboxylic acid•••pyridine hydrogen bond is strongest and one of co-crystallization's most useful interactions.

The traditional picture of bonded fluorine as strongly δ-suggests that it can only interact with electrophilic centers [1] and does not form halogen bonds,[2] however this view does not take polarization into account. In trifluoromethyl groups negative hyperconjugation (anomeric polarizability) results in two of the fluorine atoms becoming more polarizable and thus more able to form σ-holes.[3] The unique combination of the anomeric effect and the group-polarization process associated with it in trifluoromethyl groups allows the most negative molecular electrostatic potential (MEP) on the surface in contact with a nucleophile to become zero, so that the area of positive MEP on the backside of the carbon atom becomes dominant. The unusual group polarizability therefore results in the trifluoromethyl groups exhibiting amphiphilic behavior, i.e. acting not only as nucleophiles (as expected) but also as electrophiles and thus as halogen bond donors. A survey of experimental crystal structures obtained from the Cambridge Structural Database (CSD) and the Protein Databank (PDB) as well as MP2/aug-cc-pVDZ calculations on model systems demonstrate these interactions. In particular, a survey of structures in the CSD containing the trifluorotoluene moiety shows that the trifluoromethyl group forms more, and stronger, interactions with neighboring species than the F in fluorobenzene moieties, which does not experience anomeric polarization.

Quantum chemical calculations have been performed for the dicoordinated carbon compounds C(PPh(3))(2), C(NHC(Me))(2), R(2) C=C=CR(2) (R = H, F, NMe(2)), C(3)O(2), C(CN)(2)(-) and N-methyl-substituted N-heterocyclic carbene (NHC(Me)). The geometries of the complexes in which the dicoordinated carbon molecules bind as ligands to one and two AuCl moieties have been optimized and the strength and nature of the metal-ligand interactions in the mono- and diaurated complexes were investigated by means of energy decomposition analysis. The goal of the study is to elucidate the differences in the chemical behavior between carbones, allenes and carbenes. The results show that carbones bind one and two AuCl species in η(1) fashion, whereas allenes bind them in η(2) fashion. Compounds with latent divalent carbon(0) character can coordinate in more than one way, with the dominant mode indicating the degree of carbone or allene character. The calculated structures of the mono- and diaurated tetraaminoallenes (TAAs) reveal that TAAs exhibit a chameleon-like behavior: The bonding situation in the equilibrium structure is best described as allene [(R(2)N)(2)]C=C=C[(NR(2))(2)] in which the central carbon atom is a tetravalent C(IV) species, but the reactivity suggests that TAAs should be considered as divalent C(0) compounds C{C[(NR(2))(2)]}(2), that is, as "hidden" carbones. Carbon suboxide binds one AuCl preferentially in the η(1) mode, whereas the equilibrium structures of the η(1)- and η(2)-bonded diaurated complex are energetically nearly degenerate. The doubly negatively charged isoelectronic carbone C(CN)(2)(2-) binds one and two AuCl very strongly in characteristic η(1) fashion. The N-heterocyclic carbene complex, [NHC(Me)(AuCl)], possesses a high bond dissociation energy (BDE) for the splitting off of AuCl. The diaurated NHC adduct, [NHC(Me)(AuCl)(2)], has two η(1)-bonded AuCl moieties that exhibit aurophilic attraction, which yield a moderate bond strength that might be large enough for synthesizing the complex. The BDE for the second AuCl in [NHC(Me)(AuCl)(2)] is clearly smaller than the values for the second AuCl in doubly aurated carbone complexes.

Without destruction of monocrystallinity: The conformational switching of a dinuclear metal complex between four distinct states (see picture) occurs without destroying the single crystal. This observation implies a substantial degree of cooperativity between host molecules during guest uptake, release, or exchange.

Crystallographic pressure-lapse snapshots of a porous material responding to gas loading were used to investigate the stepwise uptake of carbon dioxide and acetylene molecules into discrete confined spaces. Based on the data, a qualitative statistical mechanical model was devised that reproduces even subtle features in the experimental gas sorption isotherms.

Heterocyclic mono(thione), trans-bis(thione), cis-bis(thione), trans-(carbene-thione), cis-(carbene-thione), trans-(phosphine-thione) and mono(imine) complexes of rhodium(I) have been prepared and fully characterised. Chloro(eta(4)-1,5-cyclooctadiene)(L)rhodium(I)(1a, L = 1,3-diisopropyl-4,5-dimethyl-2,3-dihydro-1H-imidazol-2-thione; 1b L = 1,3,4,5-tetramethyl-2,3-dihydro-1H-imidazol-2-thione) appear as isomers at room temperature due to slow coordination exchange on the S-donor atom. In the three structures determined, the substituent on the sulfur appears syn to Cl. Hindered rotation about the Rh-carbene bond is revealed in the NMR spectra of seven new complexes with isopropyl substituents on the heterocyclic carbene ligands. The trans influence of the thione ligands is smaller than that of carbenes but larger than that shown by imines and chloride. Thione complexes are better catalyst precursors than the carbene complexes for the hydroformylation of 1-hexene under the chosen reaction conditions: 80 degrees C, 8 MPa CO-H2(1:1), 16 h, 1:1000 catalyst to 1-hexene ratio.