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The charge density distribution in a novel cocrystal (1) complex of 1,3-dimethylxanthine (theophylline) and propanedioic acid (malonic acid) has been determined. The molecules crystallize in the triclinic, centrosymmetric space group P1̅, with four independent molecules (Z = 4) in the asymmetric unit (two molecules each of theophylline and malonic acid). Theophylline has a notably high hygroscopic nature, and numerous cocrystals have shown a significant improvement in stability to humidity. A charge density study of the novel polymorph has identified interesting theoretical results correlating the stability enhancement of theophylline via cocrystallization. Topological analysis of the electron density highlighted key differences (up to 17.8) in Laplacian (∇2ρ) between the experimental (EXP) and single-point (SP) models, mainly around intermolecular-bonded carbonyls. Further investigation via molecular electrostatic potential maps reaffirmed that the charge redistribution enhanced intramolecular hydrogen bonding, predominantly for N(2′) and N(2) (61.2 and 61.8 kJ mol–1, respectively). An overall weaker lattice energy of the triclinic form (−126.1 kJ mol–1) compared to that of the monoclinic form (−133.8 kJ mol–1) suggests a lower energy threshold to overcome to initiate dissociation. Future work via physical testing of the novel cocrystal in both dissolution and solubility will further solidify the correlation between theoretical and experimental results.

We report the accelerated molecular dynamics (aMD) simulation of amyloid-β (Aβ) peptides of four different lengths (16, 28, 40, and 42 residues) and their complexes when bound to Cu(II), Fe(II), or Zn(II). 600 ns equilibrated trajectory data were analyzed for each structure from three independent 200 ns aMD simulations, generating 16 aMD trajectories. We show that the presence of a metal ion leads to reduced size and decreased mobility relative to the free peptide due to the anchoring effect of the ions. The reduced mobility was shown largely to be due to the restricted movement in N-terminal residues, most notably Asp1 and His6 that are involved in the metal-ion coordination in all cases. Significant disruption of the secondary structure and patterns of salt bridge interactions arise on the coordination of metal ions. In this regard, similarities were noted between results for Zn(II) and Fe(II), whereas results for Cu(II) are more comparable to that of the free peptides. Reweighting of free energy surfaces was carried out from aMD data to identify the properties and descriptions of local minima structures.

An asymmetric bipy/NHC ligand L has been used to construct Au/Au, Au/Ag and Au/Cu bimetallic complexes through prior coordination of the NHC to Au(I) and subsequent introduction of the second group 11 metal ion at the bipy donor of the hybrid ligand. The complex [Au(κC-L)2]BF4, 1, has been used as the precursor for the formation of [AuAg(κ-CAu,κ2-N,N′Ag-1)2](BF4)2, 2a, [AuCu(κ-CAu,κ2-N,N′Cu-1)2](BF4)2, 2b and [AuAu′(κ-CAu/Au′,κ1-NAu/Au′-1)2](BF4)2, 3.

Two polymorphic forms of caffeine (CAF)–glutaric acid (GLU) cocrystals have been studied via high-resolution X-ray crystallography and Bader’s quantum theory of atoms in molecules (QTAIM). For both the monoclinic, 1, and triclinic, 2, systems the experimental charge density distributions of the 1:1 ratio of CAF and GLU polymorphs have been determined and compared. Previous studies have determined that 1 is less stable than 2, in relative humidity (RH) testing. A topological analysis of the electron density distribution (EDD) revealed little difference between the two polymorph internal systems. The packing densities (0.76 vs 0.74) and lattice energies (−101.1 vs −107.1 kJ mol–1) of 1 and 2, respectively, are nearly equivalent, implying that the differences in hygroscopicity between the two polymorphs are not due to crystal lattice porosity or stability. A topological analysis of the number and strength of hydrogen bonds for 1 and 2 revealed nine hydrogen bonds in both polymorphs. “Classical” (O–H···X) hydrogen bonds were similarly present in both polymorphs, stabilizing the cocrystals. However, the sum of the stability produced from the “nonclassical” (C–H···X) bonds is higher in 2: −27.6 vs −38.2 kJ mol–1 for 1 and 2, respectively. One of the nine hydrogen bonds in 1 and 2 varies from the others, caused by the torsional rotation of the aliphatic carbon chain in GLU. This bond is critical for packing stabilization, creating a parallelogram-like packing arrangement in 2 in comparison to ribboning in 1. A Hirshfeld surface analysis found that the percentages of O–H···X hydrogen bonds were nearly identical in 1 and 2 (23.9% vs 22.1%); however, the H···H contacts were higher in 2 (61.4% vs 65.8% for 1 and 2, respectively), suggesting that more hydrogen-based contacts require competitive displacement by water in the hydration of 2 in comparison to 1. Additionally, a stabilizing aromatic cycle stack between CAF molecules is present in 2 due to the varied parallelogram packing arrangement, which was absent in 1; this provided ∼11.3 kJ mol–1 of stability to the system of 2. The solid-state entropies and molecular dipole moments (MDMs) of 1 and 2 supported the relative stability of the individual polymorphs, with 1 having a higher entropy and dipole moment in comparison to 2 (123.2 vs 112.8 J K–1 mol–1 and 7.45 and 4.93 D for 1 and 2, respectively), implying that it has the potential to hydrate more rapidly. These findings are in good agreement with previous experimental RH stability studies, giving further insight into the information gained from thermally averaged ground-state crystal electron density data.

Although copper based complexes have been widely used in homogeneous catalysis, more recently they are attracting considerable attention as pharmaceutical therapeutic agents. Of paramount importance in their efficacy of use is their structure and electronic properties, which can be thoroughly probed using advanced EPR techniques. In this study, a series of [Cu(acac)(N-N)]+ Casiopeina type complexes were investigated, bearing a series of diimine N-N ligands (including bipy, phen, Py-bipy and dppz). All complexes displayed rhombic g and CuA tensors, although the extent of rhombicity was dependent on the N-N ligand. Greater Cu(II)-N2 in-plane distortion, away from the square planar arrangement, was detected by CW W-band EPR for the smaller bipy and phen ligands compared to the larger Py-bipy and dppz ligands. Changes in ligand spin density distributions (over the 1H and 14N nuclei) were revealed by CW Q-band ENDOR. The largest components of the 1H imine and 14N hyperfine coupling decreased as the ligand size increased, following the trend bipy > phen > Py-bipy > dppz. These results indicate how even small structural and electronic (spin density) perturbations within the Casiopeina family of Cu(II) complexes can be probed by advanced EPR methods.

We report coupled cluster, MP2 and DFT data on the relative energy and geometry of α-sheet and β-strand conformations of model peptides. We show that ionic effects have a strong effect on energy balance through formation of multiple cation-oxygen contacts in the α-sheet form. Such effects are markedly dependent on both sequence and ion: two peptides considered favour α-sheet in the presence of cations, whereas a third, non-polar one favours β-strand in the same conditions.

Ligand Field Molecular Mechanics (LFMM), Density Functional Theory (DFT) and Semi-Empirical methods are used to study Cu(II) binding to the tetrapeptide Asp-Ala-His-Lys (DAHK). Two conformational searching tools, LFMM/AMBER and CREST/xTB, are used to predict the energy and geometry of Cu-DAHK, using DFT as a benchmark. In addition, DFT-predicted electronic spectra are used to evaluate the binding modes found. LFMM and DFT reproduce the experimentally determined coordination, a distorted square planar arrangement of 4 nitrogen ligands with axial coordination to a fifth, oxygen ligand. However, CREST conformational search was unsuccessful in predicting the coordination mode of Cu-DAHK, changing the bonding equatorial ligands from 4 N to 3N1O.

A series of complexes [Et4N][Ln(NCS)4(H2O)4] (Ln = Pr, Tb, Dy, Ho, Yb) have been structurally characterized, all showing the same structure, namely a distorted square antiprismatic coordination geometry, and the Ln–O and Ln–N bond lengths following the expected lanthanide contraction. When the counterion is Cs+, a different structural motif is observed and the eight-coordinate complex Cs5[Nd(NCS)8] isolated. The thorium compounds [Me4N]4[Th(NCS)7(NO3)] and [Me4N]4[Th(NCS)6(NO3)2] have been characterized, and high coordination numbers are also observed. Finally, attempts to synthesize a U(III) thiocyanate compound has been unsuccessful; from the reaction mixture, a heterocycle formed by condensation of five MeCN solvent molecules, possibly promoted by U(III), was isolated and structurally characterized. To rationalize the inability to isolate U(III) thiocyanate compounds, thin-layer cyclic voltammetry and IR spectroelectrochemistry have been utilized to explore the cathodic behavior of [Et4N]4[U(NCS)8] and [Et4N][U(NCS)5(bipy)2] along with a related uranyl compound [Et4N]3[UO2(NCS)5]. In all examples, the reduction triggers a rapid dissociation of [NCS]− ions and decomposition. Interestingly, the oxidation chemistry of [Et4N]3[UO2(NCS)5] in the presence of bipy gives the U(IV) compound [Et4N]4[U(NCS)8], an unusual example of a ligand-based oxidation triggering a metal-based reduction. The experimental results have been augmented by a computational investigation, concluding that the U(III)–NCS bond is more ionic than the U(IV)–NCS bond.

We report replica exchange molecular dynamics (REMD) simulations of the complex formed between amyloid-β peptides and platinum bound to a phenanthroline ligand, Pt(phen). After construction of an AMBER-style forcefield for the Pt complex, REMD simulation employing temperatures between 270 and 615 K was used to provide thorough sampling of the conformational freedom available to the peptide. We find that the full length peptide Aβ42, in particular, frequently adopts a compact conformation with a large proportion of α- and 3,10-helix content, with smaller amounts of β-strand in the C-terminal region of the peptide. Helical structures are more prevalent than in the metal-free peptide, while turn and strand conformations are markedly less common. Non-covalent interactions, including salt-bridges, hydrogen bonds, and π-stacking between aromatic residues and the phenanthroline ligand, are common, and markedly different from those seen in the amyloid-β peptides alone.

We report conventional and accelerated molecular dynamics simulation of Zn(II) bound to the N-terminus of amyloid-β. By comparison against NMR data for the experimentally determined binding mode, we find that certain combinations of forcefield and solvent model perform acceptably in describing the size, shape and secondary structure, and that there is no appreciable difference between implicit and explicit solvent models. We therefore used the combination of ff14SB forcefield and GBSA solvent model to compare the result of different binding modes of Zn(II) to the same peptide, using accelerated MD to enhance sampling and comparing the free peptide simulated in the same way. We show that Zn(II) imparts significant rigidity to the peptide, disrupts the secondary structure and pattern of salt bridges seen in the free peptide, and induces closer contact between residues. Free energy surfaces in 1 or 2 dimensions further highlight the effect of metal coordination on peptide’s spatial extent. We also provide evidence that accelerated MD provides improved sampling over conventional MD by visiting as many or more configurations in much shorter simulation times.

We report a set of theoretical calculations designed to examine the potential of model uranyl complexes to participate in hydrogen- and halogen-bonding. Potential energy scans for the interaction of [UO2Cl2(H2O)3] and [UO2(NCSe)2(H2O)3] with a single water molecule demonstrate that uranyl is a weak hydrogen bond acceptor, but that equatorially coordinated water is a strong hydrogen bond donor. These predictions are supported by a survey of contacts reported in the Cambridge Structural Database. At the minima of each scan, we show that the interaction energy is only weakly dependent on the choice of the theoretical method, with standard density functional theory methods comparing well with coupled-cluster, MP2 and double-hybrid DFT predictions. Geometry optimisation of a 1 : 1 uranyl : water complex results in a cyclic structure, in which vibrational frequencies, atoms-in-molecules and natural bond orbital analysis support the weakness of U-Oyl as an acceptor. The origin of this behaviour is traced to the electronic structure of uranyl, and in particular covalency in the U-Oyl bonds resulting from donation into formally empty 5f and 6d orbitals on U.

A series of uranyl thiocyanate and selenocyanate of the type [R4N]3[UO2(NCS)5] (R4 = nBu4, Me3Bz, Et3Bz), [Ph4P][UO2(NCS)3(NO3)] and [R4N]3[UO2(NCSe)5] (R4 = Me4, nPr4, Et3Bz) have been prepared and structurally characterized. The resulting noncovalent interactions have been examined and compared to other examples in the literature. The nature of these interactions is determined by the cation so that when the alkyl groups are small, chalcogenide···chalcogenide interactions are present, but this “switches off” when R = nPr and charge assisted U═O···H–C and S(e)···H–C hydrogen bonding remain the dominant interaction. Increasing the size of the chain to nBu results in only S···H–C interactions. The spectroscopic implications of these chalcogenide interactions have been explored in the vibrational and photophysical properties of the series [R4N]3[UO2(NCS)5] (R4 = Me4, Et4, nPr4, nBu4, Me3Bz, Et3Bz), [R4N]3[UO2(NCSe)5] (R4 = Me4, nPr4, Et3Bz) and [Et4N]4[UO2(NCSe)5][NCSe]. The data suggest that U═O···H–C interactions are weak and do not perturb the uranyl moiety. While the chalcogenide interactions do not influence the photophysical properties, a coupling of the U═O and δ(NCS) or δ(NCSe) vibrational modes is observed in the 77 K solid state emission spectra. A theoretical examination of representative examples of Se···Se, C–H···Se, and C–H···O═U by molecular electrostatic potentials and NBO and AIM methodologies gives a deeper understanding of these weak interactions. C–H···Se are individually weak but C–H···O═U interactions are even weaker, supporting the idea that the -yl oxo’s are weak Lewis bases. An Atoms in Molecules study suggests that the chalcogenide interaction is similar to lone pair···π or fluorine···fluorine interactions. An oxidation of the NCS ligands to form [(UO2)(SO4)2(H2O)4]·3H2O was also noted.

We report microsecond timescale molecular dynamics simulation of the complex formed between Pt(II)-phenanthroline and the 16 N-terminal residues of the Aβ peptide that is implicated in the onset of Alzheimer’s disease, along with equivalent simulations of the metal-free peptide. Simulations from a variety of starting points reach equilibrium within 100 ns, as judged by root mean square deviation and radius of gyration. Platinum-bound peptides deviate rather more from starting points, and adopt structures with larger radius of gyration, than their metal-free counterparts. Residues bound directly to Pt show smaller fluctuation, but others actually move more in the Pt-bound peptide. Hydrogen bonding within the peptide is disrupted by binding of Pt, whereas the presence of salt-bridges are enhanced.

Experimental charge density distribution studies, complemented by quantum mechanical theoretical calculations, of a host-guest system comprised of a macrocycle (1) and barbital (2) in a 1:1 ratio (3) have been carried out via high resolution single crystal X-ray diffraction. The data was modelled using the conventional multipole model of electron density according to the Hansen-Coppens formalism. The asymmetric unit of macrocycle (1) contained an intraannular ethanol molecule and an extraannular acetonitrile molecule, while the asymmetric unit of (3) also contained an intraannular ethanol molecule. Visual comparison of the conformations of the macrocyclic ring shows the rotation by 180° of an amide bond attributed to competitive hydrogen bonding. It was found the intraannular and extraannular molecules inside were orientated to maximise the number of hydrogen bonds present, with the presence of barbital in (3) resulting in the greatest stabilisation. Hydrogen bonds ranging in strength from 4-70 kJ mol-1 were the main stabilising force. Further analysis of the electrostatic potential between (1), (2) and (3) showed significant charge redistribution when co-crystallisation occurred, which was further confirmed by a comparison of atomic charges. The findings presented herein introduce the possibility of high resolution X-ray crystallography playing a more prominent role in the drug design process.

Experimental charge density distribution studies complemented by quantum mechanical theoretical calculations of 4-hydroxybenzoic acid (4HBA) (1), 4,4'-bipyridine (44BP) (2) and one polymorphic form of the co-crystal containing 4HBA and 44BP molecules in a 2:1 ratio (3), have been carried out via high resolution single-crystal X-ray diffraction. Synthon formation was found to be the main driving force for crystallisation in both (1) and (3) with a carboxylic acid homosynthon present in (1) and a heterosynthon in (3) comprised of a carboxylic acid from 4HBA and a pyridine nitrogen and aromatic hydrogen from 44BP. Topological analysis revealed the bonding in the homosynthon to be stronger than the heterosynthon (305.88 versus. 193.95 kJ mol-1) with a greater number of weak interactions in (3) helping to stabilise the structure. The distance from the hydrogen and hydrogen bond acceptor to the bond critical point (bcp) was also found to be a significant factor in determining bond strength, potentially having a greater effect than lone pair directionality. Two different methods of lattice energy calculations were carried out and both methods found (1) to be more stable than (3) by ~40 and 10 kJmol-1 for the LATEN and PIXEL methods respectively. Energy framework diagrams reveal (1) to be dominated by coulombic forces while both coulombic and dispersion forces are prominent in (3) contributing equally to the lattice energy. This study examined the utility of homosynthons and heterosynthons in future crystal engineering endeavours and concluded that although in this case the single molecule crystal was more thermodynamically stable, the asymmetry of the co-crystal system allowed it to form a wider range of interactions resulting in only a small reduction in stability. This highlights the potential of using heterosynthons to develop co-crystals to improve pharmaceuticals. These findings highlight the utility of high resolution single crystal X-ray crystallography in rationalising observed physical properties.

The adducts of bis(acetylacetonato)–copper(II), [Cu(acac)2], formed with a range of nitrogen heterocycles including pyridine (2), methylpyridines (3,4,5), amino-methylpyridines (6,7) and diazines (8,9,10) were investigated in frozen solution using X-band EPR and 1H ENDOR spectroscopy. The small perturbations to the EPR spin Hamiltonian parameters (g and CuA) were consistent with the axial coordination of the nitrogen bases to Cu(II), and found to be dependent on both the basicity and steric influence of the coordinating substrate. The detailed structure of two adducts was then investigated by angular selective (1)H ENDOR and DFT. For the [Cu(acac)2](pyridine) adduct, axial coordination of the substrate was found to occur via the pyridine nitrogen as expected, producing a characteristic (1)H hyperfine coupling ((H)Ai = −2.6, −2.04, 4.7 MHz; β = 36°; aiso = 0.2 MHz) arising from the ortho-(1)H in the ring 2 or 6 position. These results were confirmed by DFT. However, in the [Cu(acac)2](2-amino-6-methyl-pyridine) adduct, the ENDOR data revealed a substantially different (1)H hyperfine coupling ((H)Ai = −4.52, −3.35, 6.47 MHz; β = 14°; aiso = −0.47 MHz) arising from the –NH2 amino protons. Analysis of this experimentally derived tensor in conjunction with the calculated DFT tensors, revealed that the 2-amino-6-methyl-pyridine substrate binds to Cu(II) via the exocyclic amino pyridine nitrogen, but with a tilt angle of 20° of the pyridine ring away from the geometry optimised structure. These results reveal how important structural information on the coordination geometry of Cu(II) adducts can be obtained by (1)H ENDOR, but only when the complete angular dependency profile of the ENDOR data is thoroughly considered.

The dimeric magnesium(I) compound [{(DipNacnac)Mg}(2)] [1, DipNacnac = (DipNCMe)(2)CH, Dip = 2,6-diisopropylphenyl] has previously been shown to exhibit a non-nuclear attractor (NNA) between the two Mg atoms in the molecular ground state by both experimental and theoretical electron-density studies. To study the stability and characteristics of this topological entity, we have determined the molecular structure of 1 in the pressure range from 0.4 to 1.9 GPa by high-pressure single-crystal X-ray diffraction. The central Mg-Mg bond contracts significantly in this range to 97% of the value at ambient pressure and a temperature of 100 K. High-level single-point theoretical calculations with the resulting atomic coordinates show that the NNA is persistently present in the topology at all pressures and that the difference between the value of the electron density at the NNA and the Mg-NNA bond critical point (bcp) is close to a maximum at the Mg-Mg separation that is found experimentally.

Combination of an electron-rich molecule (e.g. chloride anion or nitrile group) with a chlorinated cyclohexasilane ring produces a supramolecular inverse sandwich complex formed by two guests (Cl(-) or RC≡N) strongly bonded to both faces of a planar host (Si6 ring). In-depth theoretical studies were carried out to investigate the nature of the bonding interactions that generate such a stable complex. Second-order Møller-Plesset perturbation theory (MP2) calculations confirmed that the presence of the Cl substituents is fundamental to the stability of the supramolecular assemblies. The density functional theory (DFT) functional wB97XD gave an estimation of the contribution of dispersion interactions to the binding energy. These interactions become more important as the Cl atoms of the rings are systematically replaced by methyl groups or hydrogen atoms. Analysis of the topology of the electron density and the reduced density gradient gave insight into the binding of the studied supramolecular assemblies.

Ligand field molecular mechanics (LFMM) and semi-empirical Parametric Model 7 (PM7) methods are applied to a series of six PtII-Ligand systems binding to the N-terminal domain of the amyloid-β (Aβ) peptide. Molecular dynamics using a combined LFMM/Assisted Model Building with Energy Refinement (AMBER) approach is used to explore the conformational freedom of the peptide fragment, and identifies favourable platinum binding modes and peptide conformations for each ligand investigated. Platinum coordination is found to depend on the nature of the ligand, providing evidence that binding mode may be controlled by suitable ligand design. Boltzmann populations at 310 K indicate that each Pt-Aβ complex has a small number of thermodynamically accessible states. Ramachandran maps are constructed for the sampled Pt-Aβ conformations and secondary structural analysis of the obtained complex structures is performed and contrasted with the free peptide; coordination of these platinum complexes disrupts existing secondary structure in the Aβ peptide and promotes formation of ligand-specific turn-type secondary structure.

Experimental charge density distribution studies of two polymorphic forms of piroxicam, β-piroxicam (1) and piroxicam monohydrate (2), were carried out via high-resolution single crystal X-ray diffraction experiments and multipole refinement. The asymmetric unit of (2) consists of two discrete piroxicam molecules, (2a) and (2b), and two water molecules. Geometry differs between (1) and (2) due to the zwitterionic nature of (2) which results in the rotation of the pyridine ring around the C(10)-N(2) bond by approximately 180°. Consequently, the pyridine and amide are no longer co-planar and (2) forms two exclusive, strong hydrogen bonds, H(3)O(4) and H(2)O(3), with bond energies of 66.14 kJ mol(-1) and 112.82 kJ mol(-1) for (2a), and 58.35 kJ mol(-1) and 159.51 kJ mol(-1) for (2b), respectively. Proton transfer between O(3) and N(3) in (2) results in significant differences in surface electrostatic potentials. This is clarified by the calculation of atomic charges in the zwitterion that shows the formally positive charge of the pyridyl nitrogen which is redistributed over the whole of the pyridine ring instead of concentrating at N-H. Similarly, the negative charge of the oxygen is distributed across the benzothiazine carboxamide moiety. The multipole derived lattice energy for (1) is -304 kJ mol(-1) and that for (2) is -571 kJ mol(-1), which is in agreement with the experimentally determined observations of higher solubility and dissolution rates of (1) compared to (2).

Iridium(III) complexes were synthesized with the general form [Ir(L1–6)2(bpy)]PF6 (bpy = 2,2′-bipyridine), where ligands (LH1–6) are based on the N-functionalization of 2-phenyl-N-aryl/alkyl-quinoline-4-carboxamides. Single crystal X-ray diffraction studies were undertaken on two complexes, which show that each adopts a distorted octahedral coordination geometry with retention of the expected trans-N, cis-C arrangement of the cyclometalated ligands. Electrochemical studies confirmed the subtle perturbing of theIrIII/IV redox couple as a function of ligand structure. Scalar relativistic DFT studies provided qualitative descriptions of the HOMO and LUMO energy levels of the six complexes. The calculated HOMO is generally located over the Ir(5d) centre (about 45 %) and the amide-substituted 2-phenylquinoline ligand, whilst the LUMO is localized over the ancillary 2,2′-bipyridine ligand. Similar calculations for [Ir(L6)2(bpy)]PF6 revealed a different HOMO depiction with locale on the pendant chromophores. A companion calculation, using an alternative relativistic approach (i.e. incorporating spin–orbit coupling effects) conducted on a simplified model compound, provided HOMO/LUMO depictions that are essentially identical to the non-relativistic calculation, which predicts long-lived phosphorescent emission from the HOMO–LUMO transition. Luminescence studies showed the predictable and tunable phosphorescent emission wavelengths between 585–627 nm. The experimental and theoretical studies suggest that the electronic nature of the pendant amide substituent influences the energy of the emitting state – the strongly electron-withdrawing groups bathochromically shift the luminescence wavelength.

The experimental electron density distribution (EDD) in 8-hydroxyquinoline cocrystallized with salicylic acid, 1, has been determined from a multipole refinement of high-resolution X-ray diffraction data collected at 100 K. The experimental EDD is compared with theoretical densities resulting from high-level ab initio and BHandH calculations using Atoms in Molecules theory. 1 crystallizes in the triclinic crystal system, and the asymmetric unit consists of a neutral salicylic acid molecule, a salicylate anion, and an 8-hydroxyquinolinium cation exhibiting a number of inter- and intramolecular hydrogen bonds and π-π interactions. Topological analysis reveals that π-π interactions are of the "closed-shell" type, characterized by rather low and flat charge density. In general, the agreement of the topological values (ρ(bcp) and ∇(2)ρ(bcp)) between experiment and theory is good, with mean differences of 0.010 e Å(-3) and 0.036 e Å(-5), respectively. The energetics of the π-π interactions have been estimated, and excellent agreement is observed between the relative energy and the strength of π-stacking derived from the Espinosa approach, with an average difference of only 4.4 kJ mol(-1).

The solid state structures of three compounds that contain a perfluorinated chain, CF3(CF2)(5)CH2CH(CH3)CO2H, CF3(CF2)(5)(CH2)(4)(CF2)(5)CF3 and {CF3(CF2)(5)CH2CH2}(3)P=O have been compared and a number of C-F center dot center dot center dot F-C and C-F center dot center dot center dot H-C interactions that are closer than the sum of the van der Waals radii have been identified. These interactions have been probed by a comprehensive computational chemistry investigation and the stabilizing energy between dimeric fragments was found to be 0.26-29.64 kcal/mol, depending on the type of interaction. An Atoms-in-Molecules (AIM) study has confirmed that specific C-F center dot center dot center dot F-C interactions are indeed present, and are not due simply to crystal packing. The weakly stabilizing nature of these interactions has been utilized in the physisorption of a selected number of compounds containing long chain perfluorinated ponytails onto a perfluorinated self-assembled monolayer, which has been characterized by IRRAS (Infrared Reflection Absorption Spectroscopy).

Density functional theory (DFT) calculations have been performed to determine the strength and geometry of intermolecular interactions of "piano-stool" ruthenium arene complexes, which show potential as anticancer treatments. Model complexes with methane and benzene indicate that the coordinated arene has C-H···π acceptor ability similar to that of free benzene, whereas this arene acts as a much stronger C-H donor or partner in π-stacking than free benzene. The source of these enhanced interactions is identified as a combination of electrostatic and dispersion effects. Complexes of Ru-arene complexes with base-pair step fragments of DNA, in which the arene has the potential to act as an intercalator, have also been investigated. Binding energies are found to be sensitive to the size and nature of the arene, with larger and more flexible arenes having stronger binding. π-stacking and C-H···π interactions between arene and DNA bases and hydrogen bonds from coordinated N-H to DNA oxygen atoms, as well as covalent Ru-N bonding, contribute to the overall binding. The effect of complexation on DNA structure is also examined, with larger rise and more negative slide values than canonical B-DNA observed in all cases.

The stereoelectronic features of the pentacyclic phosphane (1S,4R,4aS,5aR,6R,9S,9aS,10aR)-4,6,11,11,12,12-hexamethyl-10-phenyldodecahydro-1,4:6,9-dimethanophenoxaphosphinine (phenop) have been explored through a range of empirical methods including single-crystal X-ray structure determinations of the sulfide derivative phenopS (1), the selenide phenopSe (2), [Fe(CO)4(κ1-phenop)] (3), [W(CO)5(κ1-phenop)] (4) and trans-[Rh(κ1-phenop)(CO)Cl] (5). Cone angles derived from the structural data range from 164–203° with the smaller values being observed for the compounds possessing a phenyl group that is orthogonal to the P–Z bond and the larger values for the compounds expressing a parallel phenyl ring orientation. The cone angle data suggest a moderately bulky phosphane comparable, in steric terms, to PCy3. This is further borne out on inspection of the M–P bond lengths which tend towards the longer end of the known scale. Some flexibility is observed in the central ring which approximates to a boat conformation at one extreme and an envelope at the other depending on the nature of the P-substituent. The electronic properties of κ1-phenop have been assessed using a combination of infrared and NMR spectroscopy. The absolute value of the one-bond coupling constants 1JP-Se and 1JP-Rh are very close to those reported for PPh3, suggesting a close analogy between κ1-phenop and the well known triphenylphosphane. In addition, relevant υ(CO) stretches in the IR spectra of the metal carbonyl complexes also closely mimic those for the analogous complexes containing PPh3. These conclusions are supported by molecular electrostatic potential calculations at the DFT level which place phenop close to PPh3 in terms of lone pair availability

Details of the complex bonding environment present in the molecular centre of an alkyne-bridged dicobalt complex have been examined using a combination of experimental and theoretical charge-density modelling for two compounds which share a central Co2C2 tetrahedral moiety as their common motif. Topological analysis of the experimental electron density illustrates the problem of separating the Co—C bond-critical points (b.c.p.s) from the intervening ring-critical point (r.c.p.), due largely to the flat nature of the electron density in the CoC2 triangles. Such a separation of critical points is immediately obtained from a topological analysis of the theoretical electron density as well as from the multipole-projected theoretical density; however, the addition of random noise to the theoretical structure factors prior to multipole modelling leads to a failure in consistently distinguishing two b.c.p.s and one r.c.p. in such close proximity within the particular environment of this Co2C2 centre.

A series of calculations employing hybrid quantum mechanics/molecular mechanics methods to explore the binding of square-planar complexes to fragments of DNA are reported. Methylated analogues of the parent compound cis-[Pt(en)Cl(2)], where en is ethylenediamine, show considerable variation in in vitro cytotoxicity depending on the number and position of methyl groups. Calculations reveal variations in the structure and the binding energy of adducts to single- and double-stranded fragments of DNA. Most such variations are relatively small, but the introduction of three or four methyls on nitrogen of ethylenediamine significantly changes the structure and reduces the binding energy, owing to replacement of strong N-H...O interactions by much weaker C-H...O contacts. Close correlation between measured activity and binding energy is observed, whereas little or no correlation is found with structural parameters or with estimates of the octanol/water partition coefficient, either alone or in combination via multiple linear regression.

The syntheses of nine new monometallic heteroleptic platinum complexes [Pt(L1-4)(acac)], [Pt(L1)(hmacac/hfacac)], [PtCl(L1)(py)], [Pt(L1)(8-Q)], [Pt(L1)(bpy)](PF6) (where L1 = 2-phenyl-4-ethyl-quinolinecarboxylate; L2/L3 = N-functionalization of 2-phenyl-N-aryl/alkyl-quinoline-4-carboxamides; L4 = 2-phenyl-4-quinolinecarboxylic acid (cinchophen); acac = acetylacetonato; hmacac =2,2,6,6-tetramethyl-3,5-heptanedionate; hfacac = hexafluoroacetylacetonate; py = pyridine; 8-Q = 8-quinolinato; bpy =2,2'-bipyridine) are described from precursor dimeric Pt(II) species via an intermediate DMSO adduct of the general form [PtCl(L1-4)(DMSO)]. Single crystal X-ray diffraction studies were undertaken on three complexes, [Pt(L1)(acac)], [PtCl(L1)(DMSO)], and [Pt(L1)(bpy)](PF6). The structures show that the complexes each adopt a distorted square planar geometry (most severely in the case of [Pt(L1)(bpy)](PF6)) with indications of intermolecular Pt-Pt interactions in one example. The complexes were investigated using (195)Pt{(1)H} NMR spectroscopy, revealing varied chemical shifts that were strongly dependent upon the specific coordination environment of Pt(II). Luminescence studies showed the complexes possess a phosphorescent character with tunable emission wavelengths between 605 and 641 nm and luminescent lifetimes up to ∼450 ns. Supporting TD-DFT studies provided descriptions of the HOMO and LUMO energy levels of the key complex types, confirming an MLCT contribution to the lowest energy absorption that generally correlated well with the experimental spectra. The contribution of the Pt(5d) center to the calculated HOMOs was strongly ligand dependent, whereas the LUMOs are generally localized over the quinoline component of the cyclometalated ligand.

Structural properties of 2,4,6-trimethoxybenzaldehyde, 2,4,6-trimethoxybenzyl alcohol and 2,4,6-trimethoxyacetophenone have been determined directly from powder X-ray diffraction data, using the direct-space Genetic Algorithm (GA) technique for structure solution followed by Rietveld refinement. Structural similarities and contrasts within this family of materials are elucidated. The work illustrates the value of utilizing information from other sources, including spectroscopic data and computational techniques, as a means of augmenting the structural knowledge established from the powder X-ray diffraction data.

Hartree-Fock, density functional, and MP2 methods are applied to the problem of designing neutral, bicyclic C3-symmetric cages incorporating interacting anion- and cation-binding sites which strongly bind NaCl as an ion contact pair. A large number of trial ligands L and their complexes L:NaCl are tested, with the focus on maximizing binding by (i) optimizing the cavity size and shape and (ii) varying the nature of the anion- and cation-binding functionalities. The corresponding complexes L:Cl(-) and L:Na(+) are also studied in some detail. An analysis of their structures and charge distributions helps to build a consistent picture of the requirements for a successful NaCl binding. The 'best' candidate ligand utilizes a tripodal triether-substituted amine N(CH2CH2OR-)3 to bind the sodium cation; three thiourea groups in a tripodal arrangement with a 1,3,5-trisubstituted benzyl spacer group {C6H3(CH2NHC [Formula: see text] XNH-)3 X=O,S} to bind chloride; and a -CH2CH2- spacer linking the two binding sites. A simple Quantitative Structure-Property analysis suggests that the binding cavity shape and size is near to the optimal one for this system.

Density functional calculations and Atoms in Molecules analysis are used to investigate the role of covalent and hydrogen bondings in determining the binding of transition metal complexes to guanine, and the subsequent effect on pairing with cytosine. Hydrogen bonding is ubiquitous, and typically contributes ca. 10% to overall binding, a value that varies with the coordination site on guanine, as well as metal and ligands. Early transition metals show a clear preference for the O6 position, while later ones prefer N7, the crossover point coming at the vanadium group. Metallation at N7 causes a redistribution of hydrogen bonding strength between guanine and cytosine, but does not greatly affect the overall pairing energy. In contrast, metallation at O6 strongly reduces the pairing energy, as may be expected given the role of O6 in pairing guanine with cytosine. This effect can be quantified using electron density properties, and seems to be due to both electrostatic repulsion from the positive metal centre and a redistribution of electron density within guanine itself. Qualitative agreement with experimental mass spectroscopic results is obtained.

The experimental electron density distributions (EDD) in tetrafluorophthalonitrile and tetrafluoroisophthalonitrile have been determined from a multipole refinement with use of accurate X-ray diffraction data collected at 100 K. As hydrogen atoms pose a serious challenge to a successful X-ray based EDD determination due to their diminishing diffraction power, the absence of any such atoms in the studied complexes is an important advantage. The experimental EDD is compared with theoretical densities resulting from high-level ab initio and DFT calculations using the atoms in molecules theory. Topological analysis of the electron density was used to compare the two different isomers and to estimate the similarity of identical functional groups in different crystalline environments. Comparison with theoretical results reveals subtle differences in C−F bonds. The experimental molecular electrostatic potential satisfactorily explains the observed differences in the patterns of weak intermolecular interactions.

The experimental charge density distribution in crystalline 9-ethynyl-9-fluorenol has been determined in a refinement of a pseudo-atomic multipolar expansion (Hansen−Coppens formalism) against extensive low-temperature (T = 100 K) single-crystal X-ray diffraction data and compared with a selection of theoretical DFT calculations on the same complex. The molecule crystallizes in the centrosymmetric space group C2/c with two independent molecules in the asymmetric unit. A range of multipole models of varying complexity were tested and found to describe the observed intensities almost equally well in terms of the usual residual values presented; however, the resulting total density distributions and derived properties, such as atomic charges and molecular electrostatic potentials, showed significant differences and it is emphasized that care has to be taken when parameters used for the refinements are selected. In addition to the two medium-strength OH---O hydrogen bonds, which are found to differ between mo...

The triphosphorus macrocycle 1,5,9-triethyl-1,5,9-triphosphacyclododecane, 12[ane]P3Et3, reacts with [V(CO)6]− under photolytic conditions to produce fac-[(12[ane]P3Et3)V(CO)3]− (1) which has been isolated as its Et4N salt. This can be oxidised to the neutral 17 electron complex fac-(12[ane]P3Et3)V(CO)3 (2), which has been characterised by EPR spectroscopy. Density functional calculations support the spectroscopic observations and give insight into the bonding. The V(0) compound cannot be further oxidised, but the macrocycle reacts with CpV(CO)4 thermally or photolytically to produce cis-[(η2-12[ane]P3Et3)V(η5-Cp)(CO)2] (3). The diamagnetic compounds have been characterised by IR and multinuclear (1H, 13C, 31P and 51V) NMR spectroscopy.

The charge density distribution in 2,2'-Dihydroxy-1,1'-naphthalazine (Pigment Yellow 101; P.Y.101) has been determined using high-resolution X-ray diffraction and multipole refinement, along with density functional theory calculations. Topological analysis of the resulting densities highlights the localisation of single/double bonds in the central C=N-N=C moiety of the molecule in its ground state. The density in the N—N is examined in detail, where we show that very small differences between experiment and theory are amplified by use of the Laplacian of the density. Quantification of hydrogen bonds highlights the importance of the intramolecular N—H…O interaction, known to be vital for retention of fluorescence in the solid state, relative to the many but weak intermolecular contacts located. However, a popular method for deriving H-bond strengths from density data appears to struggle with the intramolecular N—H…O interaction. We also show that theoretical estimation of anisotropic displacements for hydrogen atoms brings little benefit overall, and degrades agreement with experiment for one intra-molecular contact. NH&MRC

Thiation of N , N -biphthalimide in a search for new axially chiral ligands gave a mixture of oligothiobiphthalimides; X-ray crystallography shows that the achiral isomer of dithiodioxobiphthalimide, 1,3-dithioxo-1H-3H-[2,2 ' ]biisoindolyl-1 ' ,3 ' -dione is formed, which may imply a change in the mechanism of thiation between mono- and di-phthalimides.

Ten cationic heteroleptic iridium(III) complexes, [Ir(emptz)2(N^N)](PF6) were prepared from a cyclometalated iridium bridged-chloride dimer involving two ethyl-4-methylphenylthiazole-5-carboxylate (emptz) ligands. One X-ray crystallographic study was undertaken where the ancillary N^N ligand was 4,7-diphenyl-1,10-phenanthroline and revealed the anticipated structure, showing a distorted octahedral coordination geometry at Ir(III). The complexes were visibly luminescent with modestly structured emission at 540-590 nm and lifetimes (60-340 ns) consistent with phosphorescence. TD-DFT calculations suggest that strong MLCT character contributes to the visible absorption characteristics, whilst the moderately structured emission profiles indicate a (3)MLCT/(3)IL admixture of states to the phosphorescence.

The electron densities in two analogous dimetallic transition metal compounds, namely, [M2(μ-OH2)((t)BuCOO)4((t)BuCOOH)2(C5H5N)2] (M = Co(1), Ni(2)), were determined from combined X-ray and neutron single-crystal diffraction at 100 K. Excellent correspondence between the thermal parameters from X- and N-derived atomic displacement parameters is found, indicating high-quality X-ray data and a successful separation of thermal and electronic effects. Topological analysis of electron densities derived from high-resolution X-ray diffraction, as well as density functional theory calculations, shows no direct metal-metal bonding in either compound, while the total energy density at the bond critical points suggests stronger metal-oxygen interactions for the Ni system, in correspondence with its shorter bond distances. The analysis also allows for estimation of the relative strength of binding of terminal and bridging ligands to the metals, showing that the bridging water molecule is more strongly bound than terminal carboxylic acid, but less so than bridging carboxylates. Recently, modeling of magnetic and spectroscopic data in both of these systems has shown weak ferromagnetic interactions between the metal atoms. Factors related to large zero-field splitting effects complicate the magnetic analysis in both compounds, albeit to a much greater degree in 1. The current results support the conclusion drawn from previous magnetic and spectroscopic measurements that there is no appreciable direct communication between metal centers.

Ab initio and DFT calculations on around 50 hydrogen bond, or Lewis bases and their complexes with hydrogen fluoride are reported. A range of calculated properties of both free bases and complexes are correlated with β, an experimental scale of hydrogen bond basicity. For the entire range of bases, we find that none of the isolated base properties correlate well with β, although some family-dependent models can be constructed for O and N bases separately. In contrast, several properties of the HF complexes and changes on complexation correlate strongly with β, including the H-bond binding energy, changes in the electron density at bond critical points, and the lengthening and weakening of the H–F bond on H-bond formation. Using these models, new values of β are predicted for molecules and functional groups which have no experimentally measured counterparts, including carbenes, C-ylides, phosphines, and amine and phosphine oxides.

Ab initio calculations on N≡C–C≡C–H and its anti-parallel dimer are reported, with a view to studying the intermolecular interactions present. Following initial optimisation of the monomer at the MP2/6-311+G(d,p) level, the PES of the dimer corresponding to variations in chain separation and overlap was explored. Two minima on this surface were found, varying in their degree of overlap, but both having C 2 h symmetry. Topological charge density analysis shows these two arrangements to have qualitatively different structures. Atoms in Molecules decomposition shows the origin of the dimer stabilisation to lie in an increase in nitrogen's population and self-stabilisation.

A comprehensive study of the complexes A4[U(NCS)8] (A = Cs, Et4N, nBu4N) and A3[UO2(NCS)5] (A = Cs, Et4N) is described, with the crystal structures of [nBu4N]4[U(NCS)8]·2MeCN and Cs3[UO2(NCS)5]·O0.5 reported. The magnetic properties of square antiprismatic Cs4[U(NCS)8] and cubic [Et4N]4[U(NCS)8] have been probed by SQUID magnetometry. The geometry has an important impact on the low-temperature magnetic moments: at 2 K, μeff = 1.21 μB and 0.53 μB, respectively. Electronic absorption and photoluminescence spectra of the uranium(IV) compounds have been measured. The redox chemistry of [Et4N]4[U(NCS)8] has been explored using IR and UV–vis spectroelectrochemical methods. Reversible 1-electron oxidation of one of the coordinated thiocyanate ligands occurs at +0.22 V vs Fc/Fc+, followed by an irreversible oxidation to form dithiocyanogen (NCS)2 which upon back reduction regenerates thiocyanate anions coordinating to UO22+. NBO calculations agree with the experimental spectra, suggesting that the initial electron loss of [U(NCS)8]4– is delocalized over all NCS– ligands. Reduction of the uranyl(VI) complex [Et4N]3[UO2(NCS)5] to uranyl(V) is accompanied by immediate disproportionation and has only been studied by DFT methods. The bonding in [An(NCS)8]4– (An = Th, U) and [UO2(NCS)5]3– has been explored by a combination of DFT and QTAIM analysis, and the U–N bonds are predominantly ionic, with the uranyl(V) species more ionic that the uranyl(VI) ion. Additionally, the U(IV)–NCS ion is more ionic than what was found for U(IV)–Cl complexes., We thank TCD (R.J.B. and E.H.) for funding this work. J.A.P. is grateful to UK National Service for Computational Chemistry Software. M.E. and G.L. thank MINECO (MAT2012-38318-C03).

ENDOR spectroscopy and DFT calculations have been used to thoroughly investigate the ligand hyperfine couplings for the bis(acetylacetonato)-copper(ii) complex [Cu(acac)2] in frozen solution. Solutions of [Cu(acac)2] were prepared under anhydrous conditions, and EPR revealed that the g and (Cu)A values were affected by traces of water present in the solvent. The ligand (H)Ai hyperfine couplings were subsequently investigated by CW and pulsed ENDOR spectroscopy. Anisotropic hyperfine couplings to the methine protons ((H)Ai = 1.35, -1.62, -2.12 MHz; a(iso) = -0.80 MHz) and smaller couplings to the fully averaged methyl group protons ((H)Ai = -0.65, 1.658, -0.9 MHz; a(iso) = 0.036 MHz) were identified by simulation of the angular selective ENDOR spectra and confirmed by DFT. Since the barrier to methyl group rotation was estimated to be ca. 5 kJ mol(-1) by DFT, rapid rotation of these -CH3 groups, even at 10 K, leads to an averaged value of (H)Ai. However, variable temperature X-band Mims ENDOR revealed an additional set of hyperfine couplings which showed a pronounced temperature dependency. Using CW Q-band ENDOR, these additional couplings were characterised by the hyperfine parameters (H)Ai = 3.45, 2.9, 2.62 MHz, a(iso) = 2.99 MHz and assigned to a hindered methyl group rotation. This hindered rotation of a sub-set of methyl groups occurs in 120° jumps, such that a large A(dip) and a(iso) component is always observed. Whilst the majority of the methyl groups undergo free rotation, a sub-set of methyl groups experience hindered rotation in frozen solution, through proton tunnelling. This hindered rotation appears to be caused by weak outer-sphere solvent interactions with the complex.

A series of urea-derived heterocycles, 5N-substituted hexahydro-1,3,5-triazin-2-ones, has been prepared and their structures have been determined for the first time. This family of compounds only differ in their substituent at the 5-position (which is derived from the corresponding primary amine), that is, methyl (1), ethyl (2), isopropyl (3), tert-butyl (4), benzyl (5), N,N-(diethyl)ethylamine (6), and 2-hydroxyethyl (7). The common heterocyclic core of these molecules is a cyclic urea, which has the potential to form a hydrogen-bonding tape motif that consists of self-associative equation image(8) dimers. The results from X-ray crystallography and, where possible, Laue neutron crystallography show that the hydrogen-bonding motifs that are observed and the planarity of the hydrogen bonds appear to depend on the steric hindrance at the α-carbon atom of the N substituent. With the less-hindered substituents, methyl and ethyl, the anticipated tape motif is observed. When additional methyl groups are added onto the α-carbon atom, as in the isopropyl and tert-butyl derivatives, a different 2D hydrogen-bonding motif is observed. Despite the bulkiness of the substituents, the benzyl and N,N-(diethyl)ethylamine derivatives have methylene units at the α-carbon atom and, therefore, display the tape motif. The introduction of a competing hydrogen-bond donor/acceptor in the 2-hydroxyethyl derivative disrupts the tape motif, with a hydroxy group interrupting the N[BOND]H⋅⋅⋅O[DOUBLE BOND]C interactions. The geometry around the hydrogen-bearing nitrogen atoms, whether planar or non-planar, has been confirmed for compounds 2 and 5 by using Laue neutron diffraction and rationalized by using computational methods, thus demonstrating that distortion of O-C-N-H torsion angles occurs to maintain almost-linear hydrogen-bonding interactions.

A recent study of organic crystalline compounds containing nitro groups, which concentrated on the intermolecular approach of oxygen to the N of the NO 2 group, has revealed typical N…O distances much shorter than would be expected from the van der Waals radii. Ab initio dimer calculations reported here substantiate this finding, and indicate that these interactions are comparable in strength with weak hydrogen bonds. Dimer binding energies of 10–13 kJ mol −1 are obtained at the MP2/6–31++G ∗∗ level, and the optimal N…O distance is ≈ 2.85 A. The difference between one-electron (Hartree-Fock) and correlated treatments, combined with an atoms-in-molecules decomposition of the charge distribution, suggests that this interaction is dominated by dispersive forces.

The syntheses of five new heteroleptic iridium complexes [Ir(L(1-4))(2)(Diobpy)]PF(6) (where Diobpy = 4,4'-dioctylamido-2,2'-bipyridine) and [Ir(L(3))(2)(bpy)]PF(6) (where L = para-substituted 2,3-diphenylquinoxaline cyclometalating ligands; bpy = 2,2'-bipyridine) are described. The structures of [Ir(L(3))(2)(Diobpy)]PF(6) and [Ir(L(3))(2)(bpy)]PF(6) show that the complexes each adopt a distorted octahedral geometry with the expected trans-N, cis-C arrangement of the cyclometalated ligands. Electrochemical studies confirmed subtle perturbation of the Ir(III/IV) redox couple as a function of ligand variation. Luminescence studies showed the significant contribution of (3)MLCT to the phosphorescent character with predictable and modestly tunable emission wavelengths between 618 and 636 nm. DFT studies provided approximate qualitative descriptions of the HOMO {located over the Ir(5d) center (11-42%) and the phenylquinoxaline ligand (54-87%)} and LUMO {located over the ancillary bipyridine ligands (ca. 93%)} energy levels of the five complexes, confirming significant MLCT character. TD-DFT calculations indicate that UV-vis absorption and subsequent emission has substantial MLCT character, mixed with LLCT. Predicted absorption and emission wavelengths are in good general agreement with the UV-vis and luminescence experiments.

The experimental charge density distributions in a host-guest complex have been determined. The host, 1,4-bis[[(6-methylpyrid-2-yl)amino]carbonyl]benzene (1) and guest, adipic acid (2). The molecular geometries of 1 and 2 are controlled by the presence in the complex of intermolecular hydrogen bonding interactions and the presence in the host 1 of intramolecular hydrogen bonding motifs. This system therefore serves as an excellent model for studying noncovalent interactions and their effects on structure and electron density, and the transferability of electron distribution properties between closely related molecules. For the complex, high resolution X-ray diffraction data created the basis for a charge density refinement using a pseudoatomic multipolar expansion (Hansen-Coppens formalism) against extensive low-temperature (T = 100 K) single-crystal X-ray diffraction data and compared with a selection of theoretical DFT calculations on the same complex. The molecules crystallize in the noncentrosymmetric space group P2(1)2(1)2(1) with two independent molecules in the asymmetric unit. A topological analysis of the resulting density distribution using the atoms in molecules methodology is presented along with multipole populations, showing that the host and guest structures are relatively unaltered by the geometry changes on complexation. Three separate refinement protocols were adopted to determine the effects of the inclusion of calculated hydrogen atom anisotropic displacement parameters on hydrogen bond strengths. For the isotropic model, the total hydrogen bond energy differs from the DFT calculated value by ca. 70 kJ mol(-1), whereas the inclusion of higher multipole expansion levels on anisotropic hydrogen atoms this difference is reduced to ca. 20 kJ mol(-l), highlighting the usefulness of this protocol when describing H-bond energetics.

The structures of 1,8-bis(dimethylamino)naphthalene and its protonated forms have been determined by ab initio geometry optimization. The title compound is found to possess C 2 symmetry in the gas phase, and the N(C 10 H 6 )N fragment is markedly nonplanar. Protonation causes the molecule to become more planar, as it approaches (without quite attaining) C 2v symmetry. Asymmetric protonation at one nitrogen is energetically preferred to the symmetric N-H-N bridged form. The predicted proton affinity is within a few kilojoules per mole of an experimental value. The changes in structure and the electron distribution are discussed in terms of the topology of the charge density g and of ⊇ 2 g, for this and related amines

The structures of 1,8-bis(dimethylamino)naphthalene and its protonated forms have been determined by ab initio geometry optimization. The title compound is found to possess C 2 symmetry in the gas phase, and the N(C 10 H 6 )N fragment is markedly nonplanar. Protonation causes the molecule to become more planar, as it approaches (without quite attaining) C 2v symmetry. Asymmetric protonation at one nitrogen is energetically preferred to the symmetric N-H-N bridged form. The predicted proton affinity is within a few kilojoules per mole of an experimental value. The changes in structure and the electron distribution are discussed in terms of the topology of the charge density g and of ⊇ 2 g, for this and related amines

A series of ab initio calculations are used to determine the C--Hpi and pipi-stacking interactions of aromatic rings coordinated to transition-metal centres. Two model complexes have been employed, namely, ferrocene and chromium benzene tricarbonyl. Benchmark data obtained from extrapolation of MP2 energies to the basis set limit, coupled with CCSD(T) correction, indicate that coordinated aromatic rings are slightly weaker hydrogen-bond acceptors but are significantly stronger hydrogen-bond donors than uncomplexed rings. It is found that pipi stacking to a second benzene is stronger than in the free benzene dimer, especially in the chromium case. This is assigned, by use of energy partitioning in the local correlation method, to dispersion interactions between metal d and benzene pi orbitals. The benchmark data is also used to test the performance of more efficient theoretical methods, indicating that spin-component scaling of MP2 energies performs well in all cases, whereas various density functionals describe some complexes well, but others with errors of more than 1 kcal mol(-1).