Orientation Of Aromatic Rings Research Articles

Palladium(II) and Platinum(II) Bis(Stibinidene) Complexes with Intramolecular Hydrogen-Bond Enforced Geometries.

The coordination capability of two N,C,N pincer coordinated stibinidenes, i. e. bis(imino)- [2,6-(DippN=CH)2C6H3]Sb (1) or imino-amino- [2-(DippN=CH)-6-(DippNHCH2)C6H3]Sb (2) toward palladium(II) and platinum(II) centers was examined. In the course of this study, seven new square-planar bis(stibinidene) complexes were synthesized and characterized by NMR, IR, Raman, UV-vis spectroscopy and single crystal (sc)-X-ray diffraction analysis. In all cases, both stibinidene ligands 1 or 2 adopt trans positions, but differ significantly in the torsion angle describing mutual orientation of aromatic rings of the stibinidenes along the Sb-Pd/Pt-Sb axes. Furthermore, majority of complexes form isomers in solution most probably due to a hindered rotation around Sb-Pd/Pt bonds caused by bulkiness of 1 and 2. This phenomenon also seems to be influenced by the absence/presence of a pendant -CH2NH- group in 1/2 that is able to form intramolecular hydrogen bonds with the adjacent chlorine atom(s) attached to the metal centers. The whole problem was subjected to a theoretical study focusing on the role of hydrogen bonds in structure architecture of the complexes. To describe the UV-vis spectra of these highly coloured complexes, TD-DFT calculations were employed. These outline differences between the stibinidene ligands, the transition metals as well as between the charge of the complexes (neutral or anionic).

Structure Guiding Supramolecular Assemblies in Metal-Organic Multi-Component Compounds of Mn(II): Experimental and Theoretical Studies

Two multi-component coordination compounds of Mn(II), viz. [Mn(H2O)6](2-Mepy)2(4-NO2bz)2·2H2O (1) and [Mn(H2O)6][Mn(2,3-PDCH)3]2 (2) (where, 2-Mepy = 2-methylpyridine, 4-NO2bz = 4-nitrobenzoate, 2,3-PDC = 2,3-pyridinedicarboxylate), have been synthesized and characterized using elemental, spectroscopic (FT-IR and electronic), TGA and single-crystal X-ray diffraction analyses. Complex 1 is a co-crystal hydrate of Mn(II) involving uncoordinated 2-Mepy, 4-NO2bz and water molecules; while compound 2 is a multi-component molecular complex salt of Mn(II) comprising cationic [Mn(H2O)6]2+ and anionic [Mn(2,3-PDCH)3]−complex moieties. The uncoordinated 2-Mepy and 4-NO2bz moieties of 1 are involved in lone-pair (l.p)-π and C–H⋯π interactions which stabilize the layered assembly of the compound. The crystal structure of compound 2 has been previously reported. However, we have explored the unusual enclathration of complex cationic moieties within the supramolecular host cavities formed by the molecular assembly of complex anionic moieties. The supramolecular assemblies obtained in the crystal structure have been further studied theoretically using DFT calculations, quantum theory of atoms-in-molecules (QTAIM) and non-covalent interaction plot (NCI plot) computational tools. Theoretical studies reveal that the combination of π-staking interactions (l.p-π, π-π and C–H···π) have more structure-guiding roles compared to the H-bonds. The large binding energy of π-stacking interactions in 2 is due to the antiparallel orientation of aromatic rings and their coordination to the metal centers, thereby increasing the contribution of the dipole–dipole interactions.

New insights into controlling radical migration pathways in heme enzymes gained from the study of a dye-decolorising peroxidase.

In heme enzymes, such as members of the dye-decolorising peroxidase (DyP) family, the formation of the highly oxidising catalytic Fe(iv)-oxo intermediates following reaction with hydrogen peroxide can lead to free radical migration (hole hopping) from the heme to form cationic tyrosine and/or tryptophan radicals. These species are highly oxidising (∼1 V vs. NHE) and under certain circumstances can catalyse the oxidation of organic substrates. Factors that govern which specific tyrosine or tryptophan the free radical migrates to in heme enzymes are not well understood, although in the case of tyrosyl radical formation the nearby proximity of a proton acceptor is a recognised facilitating factor. By using an A-type member of the DyP family (DtpAa) as an exemplar, we combine protein engineering, X-ray crystallography, hole-hopping calculations, EPR spectroscopy and kinetic modelling to provide compelling new insights into the control of radical migration pathways following reaction of the heme with hydrogen peroxide. We demonstrate that the presence of a tryptophan/tyrosine dyad motif displaying a T-shaped orientation of aromatic rings on the proximal side of the heme dominates the radical migration landscape in wild-type DtpAa and continues to do so following the rational engineering into DtpAa of a previously identified radical migration pathway in an A-type homolog on the distal side of the heme. Only on disrupting the proximal dyad, through removal of an oxygen atom, does the radical migration pathway then switch to the engineered distal pathway to form the desired tyrosyl radical. Implications for protein design and biocatalysis are discussed.

Influence of Surface Chemistry on the Surfactant Organization and Interfacial Structure of Mesoporous Silica and Organosilica

Mesoporous silica (SiO2) and periodic mesoporous organosilica (PMO) with divinylbenzene (DVB)- and divinylaniline (DVA)-bridge groups are studied by solid-state NMR and molecular dynamics simulations to examine the influence of the surface chemistry on the interfacial interactions and the assembly structures of the surfactants. The NMR signals of the surfactants inside the mesopores appear at different chemical shift values depending on the organic bridging groups. Two-dimensional NMR spectroscopy shows that the surfactants assemble as micellar structures, where the polar head groups directly interact with the pore walls, and the interaction strength of the surfactants with pore wall is in the order of SiO2 >DVA-PMO > DVB-PMO. To elucidate the functional dependence of the interfacial structure of SiO2 and PMOs, the radial distribution function is calculated from molecular dynamics simulations, which provides individual atomic correlations quantitatively. For SiO2, the surfactant head is preferentially adsorbed to the inorganic layer with extended aliphatic tail group packing, whereas the adsorption of the surfactant head group to the pore wall is weaker for PMOs. We find that the hydrophobic interaction between DVB and the surfactant tail, which is stronger than that in the case of polar DVA, weakens the interaction between the surfactant head and organic layer for DVB-PMO more than for DVA-PMO. From the conformational analysis, we directly observe a more abundant gauche defect of the surfactant in DVA-PMO than in DVB-PMO. The upfield NMR shift in DVB-PMO is revealed by more planar aromatic ring orientation, which is caused by the steric effect between the organic groups. Our study suggests that the hydrophobicity and bulkiness of the organic bridge can be exploited in the design of PMOs for molecular machines, absorbents, and enzyme immobilization materials.

Regioselective synthesis of 1,2,4-trisubstituted imidazole from a mechanistic and synthetic prospective

The highly substituted 1,2,4-trisubtituted imidazole was regioselectively synthesized via base mediated domino reaction of amidine scaffold with phenacyl bromide followed by in situ N-alkylation of imidazole. The reaction conditions were optimized in order to access three phenomenon (neutralization, condensation, and N-alkylation) in a single pot. A mechanism for the formation of N–H imidazole and its subsequent N-alkylation is depicted by isolation of reaction of intermediates. Electron donating group facilitate N-alkylation, whereas no N-alkylation was observed in the presence of electron withdrawing (NO2) group. The synthesized compounds and reaction intermediate were characterized using NMR, FTIR, and HRMS. Exact orientation of various aromatic rings in regioselective N-alkylated product was confirmed using single crystal X-ray analysis.

Nano-level resolution determination of aromatic nucleus in coal

Based on the oxidative cutting method of coal-based graphene quantum dots (GQDs), a new idea to study the aromatic nuclei in coal is proposed: the dispersed aromatic nuclei can be obtained by eliminating the weak aliphatic and functional cross-links. The structural characteristics of coal aromatic nuclei were visualized based on transmission electron microscopy with nano-resolution. Distinct differences in structural characteristics exist among aromatic nuclei, which depends on rank. With increasing rank, the number of condensed rings along with the diameter of the singular aromatic nucleus and the aggregates increased significantly. In high-volatile bituminous coal, most of the aromatic carbon rings are curved and poorly orientated, and internal defects could be observed. The carbon rings in low-volatile bituminous coal are densely arranged and exhibit improvement in crystal orientation. Most heterocyclic compounds and amorphous carbon structures in the anthracite disappeared, and the arrangement and orientation of aromatic rings is the strongest, which closely resembles graphite structure. X-ray diffraction (XRD) and Raman spectra of raw coals showed the average crystallite diameter (La) and the minimum aromatic nuclei sizes are in agreement to those observed under HRTEM, the aromatic nuclei had undergone a process of evolution from small to large and from disorder to order. The first direct observation and quantitative analysis of the geometric and the polycondensation characteristics of the aromatic nuclei structure in various rank coals will provide a new perspective for further research on the molecular structure of coal.

Phosphine-Ligated Gold Clusters with Core+ exo Geometries: Unique Properties and Interactions at the Ligand-Cluster Interface.

Over recent years, research on the structures and properties of ligand-protected gold cluster molecules has gained significant interest. The crystal structure information accumulated to date has revealed the structural preference to adopt closed polyhedral geometries, but the use of multidentate ligands sometimes leads to the formation of exceptional structures. This Account describes results of our studies on diphosphine-coordinated [core+ exo]-type gold clusters featuring extra gold atoms outside the polyhedral cores, highlighting (1) their distinct optical properties due to the unique electronic structures generated by the exo gold atoms and (2) electronic/attractive ligand-cluster interactions that cause definite perturbation effects on the cluster properties. Subnanometer gold clusters with [core+ exo]-type geometries (nuclearity = 6, 7, 8, and 11) commonly displayed single absorption bands in the visible region, which are distinct in patterns from those of conventional polyhedral-only homologues. Theoretical studies demonstrated that the exo gold atoms are critically involved in the generation of unique electronic structures characterized by the HOMO-LUMO transitions with dominant oscillator strengths, leading to the appearance of the isolated absorption bands. On the basis of the frontier orbital distributions, the HOMO and LUMO were shown to be localized around the polyhedral cores and exo gold atoms, respectively. Therefore, the HOMO-LUMO transitions responsible for the visible absorptions occur in the core → exo direction. The HOMO-LUMO gap energies showed no clear trends with respect to the nuclearity (size), indicating that the individual geometric features of the inorganic framework primarily govern the clusters' electronic structures and properties. Systematic studies using octagold clusters bearing various anionic coligands revealed that electronic or attractive interactions between the gold framework and ligand functionalities, such as π-electron systems and heteroatoms, cause substantial perturbations of the wavelength of the visible absorption band due to the HOMO-LUMO transitions. Especially, significant red shifts were observed as a result of the electronic coupling with specific π-resonance contributors. It was also found that the orientation of aromatic rings around the inorganic framework is a factor that affects the cluster photoluminescence. These findings demonstrate the utility of the ligand moieties surrounding the gold frameworks for fine-tuning of the optical properties. During these studies, unusual but definite attractive interactions between the gold framework and C-H groups of the diphosphine ligand were found in the hexagold clusters. On the basis of careful crystallographic and NMR analyses, these interactions were deemed as a certain kind of M···H hydrogen bonds, which critically affect the maintenance of the cluster framework. Such unique interaction activities are likely due to the valence electrons in the gold framework, which serve as the hydrogen-bond acceptor for the unfunctionalized C-H groups. Overall, these observations imply the uniqueness of the ligand-cluster interface associated with the partially oxidized gold entities, which may expand the scope of ligand-protected clusters toward various applications.

Conformation and Visual Distinction between Urea and Thiourea Derivatives by an Acetate Ion and a Hexafluorosilicate Cocrystal of the Urea Derivative in the Detection of Water in Dimethylsulfoxide.

Structures of different solvates and solute–solvent interactions of 4-(3-(4-nitrophenyl)urido)benzoate (L1) and methyl-4-(3-(4-nitrophenyl)thiourido)benzoate (L2) with different solvents are analyzed. The solution of L1 with tetrabutylammonium acetate (TBAA) in dimethylsulfoxide (DMSO) is colorless, but a similar solution of L2 with TBAA is orange. On the other hand, respective solutions of these urea and thiourea derivatives with tetrabutylammonium fluoride (TBAF) in DMSO are orange. Urea derivative L1 facilitates the reaction of TBAF with glass to form tetrabutylammonium hexafluorosilicate, which on further interaction with L1 forms cocrystal 2L1·(TBA)2SiF6. Reorganization of hydrogen-bonded self-assembly of 2L1·(TBA)2SiF6 in DMSO caused by water is established by a dynamic light scattering study. With an increase in the amount of water in the solution, visual color changes from orange to colorless, and the color changes are reversed upon the addition of a dehydrating agent such as molecular sieves. Solvates of L1 with DMSO, dimethylformamide (DMF), and dimethylacetamide are quasi-isostructural. The respective self-assembly of these solvates differs due to orientations of aromatic rings and the carbomethoxy group across the thioamide/amide bond. Significant differences in self-assemblies of the respective DMSO solvate of L1 and L2 are observed; self-assembly of the former has dimeric subassemblies as repeat units, whereas the latter has monomeric subassemblies. DMF solvates of L1 and dimethylacetamide of L1 are built by dimeric subassemblies to form self-assembled structures, but these subassemblies differ in the orientation of the carbomethoxy group across the urea units.

1H-Detected REDOR with Fast Magic-Angle Spinning of a Deuterated Protein.

Rotational echo double resonance (REDOR) is a highly successful method for heteronuclear distance determination in biological solid-state NMR, and 1H detection methods have emerged in recent years as a powerful approach to improving sensitivity and resolution for small sample quantities by utilizing fast magic-angle spinning (>30 kHz) and deuteration strategies. In theory, involving 1H as one of the spins for measuring REDOR effects can greatly increase the distance measurement range, but few experiments of this type have been reported. Here we introduce a pulse sequence that combines frequency-selective REDOR (FSR) with 1H detection. We demonstrate this method with applications to samples of uniformly 13C,15N,2H-labeled alanine and uniformly 13C,2H,15N-labeled GB1 protein, back-exchanged with 30% H2O and 70% D2O, employing a variety of frequency-selective 13C pulses to highlight unique spectral features. The resulting, robust REDOR effects provide (1) tools for resonance assignment, (2) restraints of secondary structure, (3) probes of tertiary structure, and (4) approaches to determine the preferred orientation of aromatic rings in the protein core.

Platinum bisphosphine complexes of 1,8-naphthosultone

A series of three platinum(II) bisphosphine complexes 1–3 [Pt(1-(SO2),8-(O)-nap)(PR3)2] (where R3=Ph3, Ph2Me, Me2Ph) have been prepared by metathesis from cis-[Pt(PR3)2Cl2)] and the dilithium salt of 1,8-naphthosultone. The novel compounds were fully characterised by X-ray crystallography, multinuclear NMR, IR and MS. The molecular structures of 1–3 were compared by measuring the peri-distance, splay angle magnitude, peri-atom displacement, naphthalene ring torsions, aromatic ring orientations and the geometry around the platinum centre. The platinum metal adopts a distorted square planar geometry in all three complexes which causes deformation of the naphthalene system. The degree of molecular deformation does not decrease upon going from 1 to 3 as anticipated, instead a competition between steric effects and intramolecular interactions causes 3 to display distortion intermediate of 1 and 2.

Torsional deformation effect on the N—H bond dissociation energy in diphenylamine

Abstract In this work, the influence of the inter-ring dihedral angles and their deformation on the energetics of diphenylamine molecule and its radical is investigated by the B3LYP/6-311++G** approach. Our approximated bond dissociation enthalpy of diphenylamine is 370.0 kJ mol-1 and it is in good agreement with the recently published experimental data. The potential functions of both the molecule and the radical with respect to the mutual aromatic ring orientations are presented. The potential function for the molecule is of a double-barrier type, whereas the radical possesses a single-barrier function. The calculated total electronic energies are used to approximate the change of the bond dissociation enthalpy with the twisting of the dihedral angle. The dependence of the bond dissociation enthalpy on the dihedral angle is represented by a single-barrier type function. The dependence of the nitrogen atom spin density on the studied dihedral angle is also discussed.

Impact of the Flexible Character of MIL-88 Iron(III) Dicarboxylates on the Adsorption of n-Alkanes

Adsorption of n-alkane vapors was performed to probe the unusual highly flexible character of a series of iron(III) dicarboxylate materials of the MIL-88 structure type. In agreement with the presence of strong intraframework interactions within the dried closed pores form of MIL-88, it appears first that an increase of the size and aromaticity of the spacer makes it more difficult to adsorb alkanes at room temperature. Thus, this led to a high level of adsorption in the iron fumarate MIL-88A and poor levels in the terephthalate and naphthalenedicarboxylate based MIL-88(B and C, respectively). Second, upon increase in the length of the alkane, diffusion limitations of the guest occur within the very narrow pores, also illustrated through kinetics of adsorption measurements, which result in an overall decrease in the adsorption capacity. Noteworthy, the swelling of the flexible non modified MIL-88 solids occurs only for the MIL-88A sample, because of the number and orientation of aromatic rings that are arranged in trimers within the MIL-88 structures and, therefore, making those more difficult to open than those where the rings are arranged in dimers such as the metal terephthalate MIL-53 structures. Interestingly, modification of the organic linkers by grafting several bulky functional groups (2CF3, 4CH3) makes the adsorption of n-alkanes easier because of a strong decrease in interactions within these trimers associated with a lower pore contraction upon drying, while the substitution of a hydrogen atom by a bromine one on the spacer proved to be not sufficient for an improvement of the adsorbed amounts.

NMR Analysis of Cross Strand Aromatic Interactions in an 8 Residue Hairpin and a 14 Residue Three Stranded β-Sheet Peptide

Cross strand aromatic interactions between a facing pair of phenylalanine residues in antiparallel β-sheet structures have been probed using two structurally defined model peptides. The octapeptide Boc-LFV(D)P(L)PLFV-OMe (peptide 1) favors the β-hairpin conformation nucleated by the type II' β-turn formed by the (D)Pro-(L)Pro segment, placing Phe2 and Phe7 side chains in proximity. Two centrally positioned (D)Pro-(L)Pro segments facilitate the three stranded β-sheet formation in the 14 residue peptide Boc-LFV(D)P(L)PLFVA(D)P(L)PLFV-OMe (peptide 2) in which the Phe2/Phe7 orientations are similar to that in the octapeptide. The anticipated folded conformations of peptides 1 and 2 are established by the delineation of intramolecularly hydrogen bonded NH groups and by the observation of specific cross strand NOEs. The observation of ring current shifted aromatic protons is a diagnostic of close approach of the Phe2 and Phe7 side chains. Specific assignment of aromatic proton resonances using HSQC and HSQC-TOCSY methods allow an analysis of interproton NOEs between the spatially proximate aromatic rings. This approach facilitates specific assignments in systems containing multiple aromatic rings in spectra at natural abundance. Evidence is presented for a dynamic process which invokes a correlated conformational change about the C(α)-C(β)(χ(1)) bond for the pair of interacting Phe residues. NMR results suggest that aromatic ring orientations observed in crystals are maintained in solution. Anomalous temperature dependence of ring current induced proton chemical shifts suggests that solvophobic effects may facilitate aromatic ring clustering in apolar solvents.

The cis-4-Amino-l-proline Residue as a Scaffold for the Synthesis of Cyclic and Linear Endomorphin-2 Analogues

Endomorphin-2 (EM-2: Tyr-Pro-Phe-Phe-NH(2)) is an endogenous tetrapeptide that combines potency and efficacy with high affinity and selectivity toward the μ opioid receptor, the most responsible for analgesic effects in the central nervous system. The presence of the Pro(2) represents a crucial factor for the ligand structural and conformational properties. Proline is in fact an efficient stereochemical spacer, capable of inducing favorable spatial orientation of aromatic rings, a key factor for ligand recognition and interaction with receptors. Here the Pro(2) has been replaced by 4(S)-NH(2)-2(S)-proline (cAmp), a proline/GABA cis-chimera residue. This bivalent amino acid maintains the capacity to influenc the tetrapeptide conformation and offers the opportunity to generate new linear models and unusually constrained cyclic analogues characterized by an N-terminal Tyr bearing a free α-amino group. The results indicate that the new analogues do not show affinity for both δ and κ opioid receptors and bind only poorly to the μ receptors (for cyclopeptide 9: K(i)(μ) = 660 nM; GPI (IC(50)) = 1.4% at 1 μM; for linear tetrapeptide acid 13: K(i)(μ) = 2000 nM; GPI (IC(50)) = 0% at 1 μM; for linear tetrapeptide amide 15: K(i)(μ) = 310 nM; GPI (IC(50)) = 894 nM).

Aromatic interactions in model peptide β‐hairpins: Ring current effects on proton chemical shifts

Crystal structures of eight peptide β-hairpins in the sequence Boc-Leu-Phe-Val-Xxx-Yyy-Leu-Phe-Val-OMe revealed that the Phe(2) and Phe(7) aromatic rings are in close spacial proximity, with the centroid-centroid distance (R(cen)) of 4.4-5.4 Å between the two phenyl rings. Proton NMR spectra in chloroform and methanol solution reveal a significant upfield shift of the Phe(7) C(δ,δ') H(2) protons (6.65-7.04 ppm). Specific assignments of the aromatic protons have been carried out in the peptide Boc-Leu-Phe-Val-(D)Pro-(L)Pro-Leu-Phe-Val-OMe (6). The anticipated ring current shifts have been estimated from the aromatic ring geometrics observed in crystals for all eight peptides. Only one of the C(δ,δ') H proton lies in the shielding zone with rapid ring flipping, resulting in averaging between the two extreme chemical shifts. An approximate estimate of the population of conformations, which resemble crystal state orientation, may be obtained. Key nuclear Overhauser effects (NOEs) between facing Phe side chains provide support for close similarity between the solid state and solution conformation. Temperature dependence of aromatic ring proton chemical shift and line widths for peptide 6 (Boc-Leu-Phe-Val-(D)Pro-(L)Pro-Leu-Phe-Val-OMe) and the control peptide Boc-Leu-Val-Val-(D)Pro-Gly-Leu-Phe-Val-OMe establish an enhanced barrier to ring flipping when the two Phe rings are in proximity. Modeling studies suggest that small, conformational adjustment about C(α)-C(β) (χ(1) ) and C(β)-C(γ) (χ(2) ) bonds of both the Phe residues may be required in order to permit unhindered, uncorrelated flipping of both the Phe rings. The maintenance of the specific aromatic ring orientation in organic solvents provides evidence for significant stabilizing interaction.

Raman spectroelectrochemical study of Meldola blue, adsorbed and electropolymerized at a gold electrode

Surface-enhanced Raman spectroscopy (SERS) was used to probe the structure of adsorbed and electropolymerized Meldola blue (MB) films on roughened gold surfaces in solutions with pH 1.0 and 7.0 by using 785 nm excitation wavelength. Spectral bands were assigned based on density functional theory (DFT) calculations at B3LYP/6-311+G(2d, p) level. The most characteristic band of the oxidized MB form was found to be the breathing vibration of the central ring containing heteroatoms at 596 cm −1. Based on a red shift of bands assigned to vibrations of double C N(C 2H 6) bonds and adjacent ring C C bonds in surface spectra as compared with solution 1 it was suggested that polymerization and interaction with an electrode surface proceed through these moieties. The presence of out-of-plane bands in SERS spectra was attributed to “flat” or slightly “tilted” orientation of aromatic rings at the interface. Potential-dependent spectral changes were followed by SERS spectroscopy. Raman spectra of the reduced MB form were obtained in both pH 1.0 and pH 7.0 solutions by analysis of the potential-difference SERS spectra. Reduced MB form can be recognized by characteristic bands near 1620, 1574, 1374, and 1234 cm −1. By comparing the intensities of 1637 cm −1 (oxidized MB form) and 1374 cm −1 (reduced MB form) bands in experimental spectra of polymerized MB in pH 1.0 solution, a reduction-induced decrease by factor of 7 was estimated. A similar tendency in intensity changes showed calculations indicating that this effect is associated with reduction-induced changes in the molecular structure of the dye.

Insights into Intramolecular Trp and His Side‐Chain Orientation and Stereospecific π Interactions Surrounding Metal Centers: An Investigation Using Protein Metal‐Site Mimicry in Solution

Metal-binding scaffolds incorporating a Trp/His-paired epitope are instrumental in giving novel insights into the physicochemical basis of functional and mechanistic versatility conferred by the Trp-His interplay at a metal site. Herein, by coupling biometal site mimicry and (1)H and (13)C NMR spectroscopy experiments, modular constructs EDTA-(L-Trp, L-His) (EWH; EDTA=ethylenediamino tetraacetic acid) and DTPA-(L-Trp, L-His) (DWH; DTPA=diethylenetriamino pentaacetic acid) were employed to dissect the static and transient physicochemical properties of hydrophobic/hydrophilic aromatic interactive modes surrounding biometal centers. The binding feature and identities of the stoichiometric metal-bound complexes in solution were investigated by using (1)H and (13)C NMR spectroscopy, which facilitated a cross-validation of the carboxylate, amide oxygen, and tertiary amino groups as the primary ligands and indole as the secondary ligand, with the imidazole (Im) N3 nitrogen being weakly bound to metals such as Ca(2+) owing to a multivalency effect. Surrounding the metal centers, the stereospecific orientation of aromatic rings in the diastereoisomerism is interpreted with the Ca(2+)-EWH complex. With respect to perturbed Trp side-chain rotamer heterogeneity, drastically restricted Trp side-chain flexibility and thus a dynamically constrained rotamer interconversion due to π interactions is evident from the site-selective (13)C NMR spectroscopic signal broadening of the Trp indolyl C3 atom. Furthermore, effects of Trp side-chain fluctuation on indole/Im orientation were the subject of a 2D NMR spectroscopy study by using the Ca(2+)-bound state; a C-H2(indolyl)/C-H5(Im(+)) connectivity observed in the NOESY spectra captured direct evidence that the N-H1 of the Ca(2+)-Im(+) unit interacted with the pyrrole ring of the indole unit in Ca(2+)-bound EWH but not in DWH, which is assignable to a moderately static, anomalous, T-shaped, interplanar π(+)-π stacking alignment. Nevertheless, a comparative (13)C NMR spectroscopy study of the two homologous scaffolds revealed that the overall response of the indole unit arises predominantly from global attractions between the indole ring and the entire positively charged first coordination sphere. The study thus demonstrates the coordination-sphere/geometry dependence of the Trp/His side-chain interplay, and established that π interactions allow (13)C NMR spectroscopy to offer a new window for investigating Trp rotamer heterogeneity near metal-binding centers.

Synthetic and Structural Studies of 1,8‐Chalcogen Naphthalene Derivatives

Four novel 1,8-disubstituted naphthalene derivatives 4-7 that contain chalcogen atoms occupying the peri positions have been prepared and fully characterised by using X-ray crystallography, multinuclear NMR spectroscopy, IR spectroscopy and MS. Molecular distortion due to noncovalent substituent interactions was studied as a function of the bulk of the interacting chalcogen atoms and the size and nature of the alkyl group attached to them. X-ray data for 4-7 was compared to the series of known 1,8-bis(phenylchalcogeno)naphthalenes 1-3, which were themselves prepared from novel synthetic routes. A general increase in the E...E' distance was observed for molecules containing bulkier atoms at the peri positions. The decreased S...S distance from phenyl-1 and ethyl-4 analogues is ascribed to a weaker chalcogen lone pair-lone pair repulsion acting in the ethyl analogue due to the presence of two equatorial S(naphthyl) ring conformations. Two novel peri-substituted naphthalene sulfoxides of 1, Nap(O=SPh)(SPh) 8 and Nap(O=SPh)(2) 9, which contain different valence states of sulfur, were prepared and fully characterised by using X-ray crystallography and multinuclear NMR spectroscopy, IR spectroscopy and MS. Molecular structures were analysed by using naphthalene ring torsions, peri-atom displacement, splay angle magnitude, S...S interactions, aromatic ring orientations and quasi-linear O=S...S arrangements. The axial S(naphthyl) rings in 8 and 9 are unfavourable for S...S contacts due to stronger chalcogen lone pair-lone pair repulsion. Although quasi-linear O=S...S alignments suggest attractive interaction is conceivable, analysis of the B3LYP wavefunctions affords no evidence for direct bonding interactions between the S atoms.

Sterically Crowded peri‐Substituted Naphthalene Phosphines and their PV Derivatives

Three sterically crowded peri-substituted naphthalene phosphines, Nap[PPh(2)][ER] (Nap=naphthalene-1,8-diyl; ER=SEt, SPh, SePh) 1-3, which contain phosphorus and chalcogen functional groups at the peri positions have been prepared. Each phosphine reacts to form a complete series of P(V) chalcogenides Nap[P(E')(Ph(2))(ER)] (E'=O, S, Se). The novel compounds were fully characterised by using X-ray crystallography and multinuclear NMR spectroscopy, IR spectroscopy and MS. X-ray data for 1, 2, nO, nS, nSe (n=1-3) are compared. Eleven molecular structures have been analysed by naphthalene ring torsions, peri-atom displacement, splay angle magnitude, X...E interactions, aromatic ring orientations and quasi-linear arrangements. An increase in the congestion of the peri region following the introduction of heavy chalcogen atoms is accompanied by a general increase in naphthalene distortion. P...E distances increase for molecules that contain bulkier atoms at the peri positions and also when larger chalcogen atoms are bound to phosphorus. The chalcogenides adopt similar conformations that contain a quasi-linear E...P-C fragment, except for 3O, which displays a twist-axial-twist conformation resulting in the formation of a linear O...Se-C alignment. Ab initio MO calculations performed on 2O, 3O, 3S and 3Se reveal Wiberg bond index values of 0.02 to 0.04, which indicates only minor non-bonded interactions; however, calculations on radical cations of 3O, 3S and 3Se reveal increased values (0.14-0.19).

Synthetic and Structural Studies of 1-Halo-8-(alkylchalcogeno)naphthalene Derivatives

A series of eight 1-halo-8-(alkylchalcogeno)naphthalene derivatives (1-8; halogen=Br, I; alkylchalcogen=SEt, SPh, SePh, TePh) containing a halogen and a chalcogen atom occupying the peri positions have been prepared and fully characterised by using X-ray crystallography, multinuclear NMR spectroscopy, IR spectroscopy and MS. Naphthalene distortion due to non-covalent substituent interactions was studied as a function of the bulk of the interacting chalcogen atoms and the size and nature of the alkyl group attached to them. X-ray data for 1, 2, 4 and 5-8 were compared. Molecular structures were analysed in terms of naphthalene ring torsions, peri-atom displacement, splay angle magnitude, X...E interactions, aromatic ring orientations and quasi-linear X...E-C arrangements. A general increase in the X...E distance was observed for molecules that contain bulkier atoms at the peri positions. The I...S distance of 4 is comparable with the I...Te distance of 8, and is ascribed to a stronger lone pair-lone pair repulsion due to the presence of an axial S(naphthyl) ring conformation. Density functional theory (B3LYP) calculations performed on 5-8 revealed Wiberg bond index values of 0.05-0.08, which indicate minor interactions taking place between the non-bonded atoms in these compounds.