Phenyl Ring Carbons Research Articles

Crystal structure of cephalexin monohydrate, C16H17N3O4S(H2O)

The crystal structure of cephalexin monohydrate has been solved and refined using synchrotron X-ray powder diffraction data and optimized using density functional techniques. Cephalexin monohydrate crystallizes in space group C2 (#5) with a = 27.32290(17), b = 11.92850(4), c = 16.75355(8) Å, β = 108.8661(4)°, V = 5166.99(3) Å3, and Z = 12. Although the general arrangement of molecules is similar to that in cephalexin dihydrate, the structural differences result in very different powder patterns. The crystal structure is characterized by alternating layers of hydrogen bonds and van der Waals contacts parallel to the bc-plane. The water molecules occur in clusters. Five of the six protons in the water molecules act as donors in O–H⋯O hydrogen bonds. The sixth hydrogen atom acts as a donor to two different phenyl ring carbon atoms to form bifurcated O–H⋯C hydrogen bonds. Each cephalexin molecule is a zwitterion, containing ammonium and carboxylate groups. The ammonium ions form N–H⋯O hydrogen bonds to carboxylate groups and water molecules, as well as to carbonyl groups. The powder pattern is included in the Powder Diffraction File™ as entry 00-065-1417.

High-Resolution Solid State 13C NMR Studies of Bent-Core Mesogens of Benzene and Thiophene

Bent-core mesogens are an important class of thermotropic liquid crystals as they exhibit unusual properties as well as morphologies distinctly different from rodlike mesogens. Two bent-core mesogens with differing center rings namely benzene and thiophene are considered and investigated using high-resolution oriented solid state 13C NMR method in their liquid crystalline phases. The mesogens exhibit different phase sequences with the benzene-based mesogen showing a B1 phase, while the one based on thiophene showing nematic and smectic C phases. The 2-dimensional separated local field (2D-SLF) NMR method was used to obtain the 13C–1H dipolar couplings of carbons in the center ring as well as in the side-wing phenyl rings. Couplings, characteristic of the type of the center ring, that also provide orientational information on the molecule in the magnetic field were observed. Together with the dipolar couplings of the side-wing phenyl ring carbons from which the local order parameters of the different subunits of the core could be extracted, the bent angle of the mesogenic molecule could be obtained. Accordingly, for the benzene mesogen in its B1 phase at 145 °C, the center ring methine 13C–1H dipolar couplings were found to be significantly larger (9.5–10.2 kHz) compared to those of the side-wing rings (1.6–2.1 kHz). From the local order parameter values of the center (0.68) as well as the side-wing rings (0.50), a bent-angle of 130.3° for this mesogen was obtained. Interestingly, for the thiophene mesogen in its smectic C phase at 210 °C, the 13C–1H dipolar coupling of the center ring methine carbon (2.11 kHz) is smaller than those of the side-wing phenyl ring carbons (2.75–3.00 kHz) which is a consequence of the different structures of the thiophene and the benzene rings. These values correspond to local order parameters of 0.85 for the center thiophene ring and 0.76 for the first side-wing phenyl ring and a bent-angle of 149.2°. Thus, the significant differences in the dipolar couplings and the order parameter values between different parts in the rigid core of the mesogens are a direct consequence of the nature of the center ring and the bent structure of the molecule. The present investigation thus highlights the ability of the 13C 2D-SLF technique to provide the geometry of the bent-core mesogens in a straightforward manner through the measurement of the 13C–1H dipolar couplings.

(13)C NMR investigations and the molecular order of 4-(trans-4'-hexylcyclohexyl)-isothiocyanatobenzene (6CHBT).

The static 1D (13)C and 2D Proton Encoded Local Field (PELF) NMR experiments are carried out in the nematic phase of a less viscous liquid crystal 4-(trans-4'-hexylcyclohexyl)-isothiocyanatobenzene (6CHBT) with a view to find orientational order. The PELF spectra provide better resolution which facilitates the assignment of cyclohexyl and phenyl ring carbons relatively easy. For the cyclohexyl unit, four pairs of dipolar splitting are clearly noticed in contrast to earlier reports on structurally similar mesogens where only two pairs of doublets are seen. The linear relationship between anisotropic chemical shifts and orientational order is established and semi-empirical parameters are obtained to aid the study of the order behaviour of 6CHBT over the entire nematic range. The data further fitted to the Haller equation and a reasonably good agreement is observed. The temperature dependence trends of orientational order parameters extracted for various carbons using (13)C-(1)H dipolar couplings with those of (13)C chemical shifts are compared. A gradual decrease in the order parameter is noticed for different molecular segments while traversing from the core to the aliphatic chain via the cyclohexyl ring. The notable decreasing trends of order parameters along the chain are observed similar to those of the corresponding phenyl cyclohexanes reported earlier.

13C NMR relaxation and computational study of anisole and derivatives in the solution state

13C NMR spin–lattice relaxation times and nuclear Overhauser effects were measured at several temperatures for the methoxyl methyl carbon and the phenyl ring carbons in neat samples and in dilute cyclohexane solution for anisole, 4‐methylanisole, and 4‐chloroanisole. Similar measurements were made for 2‐methylanisole, 2‐methyl‐4‐bromoanisole, and 2,4,6‐trimethylanisole in dilute cyclohexane solution. Density functional theory (DFT) computations were performed on anisole, 4‐chloroanisole and 2,4,6‐trimethylanisole to obtain the minimum energy structures and the potential energy barriers to the internal rotations of the methoxyl group. The shortest distance between a methoxyl methyl hydrogen and the ortho hydrogen in anisole is 1.920 Å. The DFT results point to steric interactions that arise thereof as the principal source of the energy barriers to the internal rotation of the methyl or of the methoxyl group. The carbon relaxation data are consistent with the existence of noncovalent intermolecular interaction, especially π − π stacking interaction. The nuclear magnetic resonance and DFT results are discussed with reference to the rotational characteristics of the methoxyl methyl and the anisotropy in the reorientational motion of anisole and its derivatives in dilute cyclohexane solution. Copyright © 2012 John Wiley & Sons, Ltd.

EPR and 1H NMR spectroscopy and DFT study of pentaammineruthenium(III)phenylcyanamide complexes

The EPR and 1H NMR spectroscopy of seven [Ru(NH3)5L]2+ complexes, where L=3,5-dimethoxyphenylcyanamide (MeO2pcyd), 3,4,5-trimethoxyphenylcyanamide (MeO3pcyd), 4-nitrophenylcyanamide (NO2pcyd), 2,3-dichlorophenylcyanamide (Cl2pcyd), 2,4,6-trichlorophenylcyanamide (Cl3pcyd), 2,3,5,6-tetrachlorophenylcyanamide (Cl4pcyd) and pentachlorophenylcyanamide (Cl5pcyd), was performed. EPR spectra of the complexes showed an axial signal with g|| and g⊥ at high and low field, respectively. The g|| axis is suggested to lie along the Ru–cyanamide bond. Gas-phase DFT calculations of [Ru(NH3)5 phenylcyanamide]2+ showed spin density localized mostly on the phenylcyanamide ligand, in disagreement with EPR data. DFT/polarizable continuum model (PCM, water solvation) calculations shifted spin density towards ruthenium so that spin density was shared between ruthenium and phenylcyanamide ligand. Proton contact shifts were determined from NMR and EPR data and were used to estimate spin density distributions on phenyl ring carbons. The results showed that the DFT/PCM calculation overestimated spin density on phenyl ring carbons by approximately one order of magnitude. Donor–acceptor interactions between the solute and solvent that are not fully accounted for in the DFT/PCM method are suggested to stabilize the Ru(III) oxidation state.

NEXAFS Depth Profiling of Surface Segregation in Block Copolymer Thin Films

NEXAFS spectroscopy was used to probe the surface composition and under-water surface reconstruction of thin films of comb-like diblock copolymers with cylindrical and spherical microphases. The polymers consisted of a polystyrene block, and a second block prepared from a styrenic monomer grafted with fluoroalkyl-tagged poly(ethylene glycol) side chains. Compositional depth profiling of the microphase separated block copolymer films, in the top 1−3 nm of the film, was performed to understand the role of block copolymer microstructure and self-assembly on surface composition. Using experimentally determined concentration profiles, the surface concentration of phenyl ring carbon atoms was quantified and compared with those of homopolymer and random copolymer controls. The carbon atoms from the relatively high surface energy phenyl groups were depleted or excluded from the surface, in favor of the low surface-energy fluoroalkyl groups. While it is expected that block copolymer surfaces will be completely covered by a wetting lamellar layer of the lower surface energy block, a significant amount of the higher surface energy polystyrene block was found to be present in the surface region of the cylinder-forming block copolymer. Evidently, the spontaneous formation of the cylindrical polystyrene microdomains in the near-surface region compensated for the lowering of the free energy that could have been achieved by completely covering the surfaces with a lamellar layer of the lower surface energy fluorinated block. All surfaces underwent molecular reconstruction after immersion in water. The experimental concentration depth profiles indicated an increased surface depletion of phenyl ring carbon atoms in the water-immersed thin films, due to the tendency of hydrophilic PEG side groups to be present at the polymer−water interface. Such a detailed characterization of the outermost layers of the block copolymer surfaces was possible because of the exceptional depth resolution of the NEXAFS depth profiling technique.

Synthesis and Crystal Structure of a Novel Silver Sulfonate Involving Ag–C Interactions

A new coordination polymer [Ag(C6H5NHC6H4SO3)] (1) was synthesized by the reaction of sodium diphenylamine sulfonate, [C6H5NHC6H4SO3Na] (NaL, L = C6H5NHC6H4SO 3 − ) and silver nitrate and its structure was determined by X-ray diffraction analysis. The crystals are triclinic Pī space group with cell parameters a = 5.640(1) A, b = 10.711(2) A and c = 10.737(2) A, α = 117.70(1)°, β = 90.95(1)°, γ = 94.51(1)°, V = 571.5 (4) A3, and Z = 2. The X-ray analysis shows that each silver cation is 4-coordinated by three oxygen atoms and one phenyl ring carbon atom. Silver cations are bridged by sulfonate groups to form an 8-membered closed ribbon structure, each ribbon is connected to the two neighboring ribbons through the organometallic bonds between the phenyl carbon atoms and the silver ions of the neighboring ribbons to form two-dimensional layer. Reaction of sodium diphenylamine sulfonate, [C6H5NHC6H4SO3Na] (NaL, L = C6H5NHC6H4SO 3 − ), with silver nitrate affords a novel silver sulfonate, [Ag(C6H5NHC6H4SO3)] (1) involving Ag–C interactions.

Toward a computed structure database: methodology for effective molecular orbital computations

This work provides a concise description and presentation of selected results emerging from the application of an established modular definition of molecular structure to molecular orbital computations. Perturbations in the relative spatial orientation of the constituent phenyl ring carbon nuclei in benzene, phenol and conformers of Ac-Phenylalanine-NH 2, Ac-Phenylalanine-NHMe and Ac-Tyrosine-NHMe, each geometry optimized at the B3LYP/6-31G(d) level of ab initio theory, were investigated. Several stable conformers were selected for the peptide models. The influence of basis-set refinement was investigated using the Ac-Phenylalanine-NH 2 system, applying the 6-31G(d,p) and 6-31+G(d) levels of theory. The deviation of the phenyl structural elements from that of benzene was statistically negligible in all cases, with standard deviations of approximately 0.001 Å, 0.15°, 0.20°, for the phenyl ring carbon bond lengths, bond angles, dihedral angles, respectively. Total and relative energies are also presented, showing that the phenyl ring structures of high energy model peptide conformers are also immune to perturbation.

PET versus PEN: what difference can a ring make?

Poly(ethylene terephthalate) (PET) and poly(ethylene 2,6-naphthalate) (PEN) are structurally related polyesters. In each polymer, the ethylene glycol diesters are separated by rigid rings and are attached to the 1,4-positions of the phenyl and the 2,6-positions of the naphthyl rings in PET and PEN, respectively. Because neighboring ethylene glycol units of each polyester are separated by phenyl or naphthyl rings, their conformations are independent of each other. As a consequence, their RIS conformational models should be identical, with the same populations of trans, gauche +, and gauche− conformations about the –O–CH2–, –CH2–CH2–, and –CH2–O– bonds. This means that PET and PEN are equally flexible as judged by their conformational partition functions. However, because they differ geometrically, properties such as the mean-square end-to-end distance (〈r2〉0) or characteristic ratio (Cr=〈r2〉0/n〈l2〉), though averaged over identical conformations, are not expected to be coincident. The terephthaloyl portion of PET can be considered to consist of the ▪, the –C1⋯C4–, and ▪ bonds, which are collinear and only the conformations about the carbonyl carbon to phenyl ring carbon bonds may be altered. This results in the terephthaloyl unit acting as a freely rotating link in both the statistical and dynamic senses. In the naphthaloyl residue, on the other hand, the carbonyl carbon to C2 and C6 to carbonyl carbon bonds are connected to a collinear, non-rotatable virtual bond between C2 and C5 and to the non-collinear, non-rotatable real bond between C5 and C6, respectively. These geometrical differences between PET and PEN result in distinctly different values for properties like 〈r2〉0 and Cr, even though they are averaged over the same conformational populations. Additionally, volumes occupied by their segments when confined to extended conformations and interconversions between these extended conformers were found to be particularly sensitive to the geometrical distinctions between PET and PEN and several differences in their physical properties are discussed in this context.

Facile Synthesis and Structure of Heavy Alkali Metal Thiocarboxylates: Structural Comparison with the Selenium and Tellurium Isologues.

Potassium, rubidium, and cesium thiocarboxylates were found to be synthesized by the reaction of thiocarboxylic acid or its O-trimethylsilyl esters with KF, RbF, and CsF. Tetramethylammonium thiocarboxylates were prepared in good yields by the reaction of sodium thiocarboxylates with tetramethylammonium chloride. The structures of potassium benzene-, 2-methoxybenzene-, and 4-methoxybenzenecarbothioates, rubidium 2-methoxybenzenecarbothioate, tetramethylammonium 2-methoxy- and 2-trifluoromethylbenzenecarbothioates, potassium 2-methoxybenzenecarboselenoate, and rubidium 2-methoxybenzenecarbotelluroate were characterized by X-ray structural analysis. All of these alkali metal salts exhibit a dimeric structure in which the oxygen and/or sulfur atom is associated with the metal of the opposite molecule, while the tetramethylammonium thiocarboxylates are monomeric. In potassium 2-methoxybenzenecarbothioate, the two thiocarboxylate groups chelate to the potassium atoms above and below the plane which involves the thiocarboxylate groups. In potassium 4-methoxybenzenecarbothioate, one thiocarboxylate group chelates to potassium. Without exception, the o-methoxy oxygen intermolecularly coordinates to the metal of the opposite molecule. The C-O bond lengths of the thiocarboxylate group are in the range 1.22-1.24 Å, which is slightly longer than those of common thioesters. The C(sp(2))-S distances are in the range 1.70-1.72 Å, which is close to that of a typical C-S single bond and suggests that the negative charge may be somewhat localized on the sulfur atom. The interaction number of the metals with oxygen and sulfur atoms was 7 for K and 8 for both Rb and Cs. The dihedral angle between the thiocarboxyl group and the phenyl ring is substantially increased by the introduction of the o-methoxy group. In these alkali metal thio-, seleno-, and tellurocarboxylates, only cesium 2-methoxybenzenecarbotelluroate showed an intermolecular nonbonding interaction between the cesium and the phenyl ring carbons.

Towards a better understanding of the initial steps in the photocatalyzed mineralization of amino acids at the titania/water interface. An experimental and theoretical examination of l-alanine, l-serine and l-phenylalanine

The pathway to the photomineralization of the three amino acids l-serine ( l-Ser), l-phenylalanine ( l-Phe) and l-α-alanine ( l-Ala) is described experimentally on the basis of CO 2 evolution and conversion of the amino group to NH 4 + and NO 3 − ions, and theoretically on the basis of molecular orbital calculations to define frontier electron densities and point charges on all the individual atoms. The relatively high negative point charges on the carboxylate oxygens are consistent with adsorption of the amino acids to the TiO 2 particle surface through the carboxylate function. Mineralization to carbon dioxide is complete for l-Ser (∼98%) and nearly so for l-Ala (∼90%), whereas for l-Phe the extent of mineralization is 59% corresponding to the total photooxidation of the phenyl ring carbons; for the amine function the extent of conversion is 87% for l-Ser, 97% for l-Ala and 91% for l-Phe. Relative formation yields of NH 4 + and NO 3 − ions depend on the structural fragment R attached to the α-amino carboxylic acid functions, R-CH(NH 2)COOH. Primary attack of the amino acids by the ⋅OH radical is correlated with the frontier electron densities. Ammonia is formed through a photoreductive step by electron attachment onto the zwitterionic form of the amino acids, whereas nitrate is produced through a photooxidative step implicating a very tortuous series of events.

Diheteroarylmethanes. 5.1 E−Z Isomerism of Carbanions Substituted by 1,3-Azoles: 13C and 15N π-Charge/Shift Relationships as Source for Mapping Charge and Ranking the Electron-Withdrawing Power of Heterocycles

Previously proposed pi-charge/shift relationships have been applied to (13)C and (15)N shifts of the carbanions of 2-benzylazoles (thiazole, oxazole, and imidazole), their corresponding benzo-fused analogs, and bis(2-azolyl)methanes (azolyl groups as above). In this way it is possible to rank the pi electron-withdrawing power of these heterocycles in terms of charge demands c(X), a quantity representing the fraction of pi negative charge withdrawn (delocalized) by the ring. The results indicate that c(thiaz) > c(oxaz) > c(imidaz); furthermore, benzoazoles are more efficient than monocyclic systems in delocalizing the negative charge. The charge demand c(X) of imidazole is the smallest among the heteroaromatics so far considered, being even smaller than that of the phenyl ring. As a consequence, the negative charge in the anion of 2-benzyl-N-methylimidazole is predominantly transferred from the carbanionic carbon to the phenyl group rather than to the imidazolyl residue. The high double bond character of the bond linking the carbanionic and ipso phenyl ring carbons leads to room temperature (13)C shift anisochrony of the meta and meta' and ortho and ortho' positions of the phenyl ring. In all of the other cases, hindered rotation is observed at room temperature between the carbanionic carbon and position 2 of the heterocycle. A single set of resonances is presented by the bis(heteroaryl)methyl carbanions. pi-Charge/shift relationships allow for the accurate pi-charge mapping in these carbanionic systems, and the results point to considerable delocalization of the electron pair(s) of the oxygen and pyrrolic nitrogen atoms at position 1 in oxazole and imidazole toward the pyridic nitrogen at position 3 of the rings (in both the neutrals and the carbanionic species). On the contrary, not only does the sulfur atom in thiazole derivatives not delocalize any negative charge in the anions but it is barely involved in any pi-donation to the pyridic nitrogen atom at position 3 also in the neutrals.

An unusual pathway for the elimination of hydrogen cyanide from benzonitrile upon electron impact as revealed by 13C and 15N labelling

AbstractIt is shown by 15N and specific 13C labelling that ∼50% of the molecules of hydrogen cyanide, eliminated within ∼10−6 s upon electron impact of benzonitrile, contains the original cyano carbon atom, whereas the remaining percentage contains one of the phenyl ring carbon atoms at random. This is even more dramatic for the molecular ions of benzonitrile which decompose in the first and second field‐free regions of the VG Micromass ZAB‐2F high‐field mass spectrometer used. Then only 5–7% of the eliminated molecules of hydrogen cyanide contains the original cyano carbon atom. A cycloaddition‐cycloreversion process in the molecular ions, leading to ionized 1‐cyano‐1,3‐hexadien‐5‐yne as an intermediate in the hydrogen cyanide loss, is proposed to explain this.

Conformation and reorientation of acetophenone in solution. A proton and deuterium magnetic resonance study in a nematic solvent

The deuterium spectrum of 2,3,4,5,6-pentadeuteroacetophenone dissolved in the nematogen Merck Phase V has been recorded and the quadrupolar splittings used to determine elements of the ordering matrix. Two kinds of multiple resonance experiment are used to obtain DCH3, the dipolar coupling between protons in the methyl group. In one experiment both halves of a quadrupolarsplit doublet in the deuterium spectrum are irradiated whilst observing the proton spectrum, and experiments on a sample of CH3CD2Br dissolved in Phase V are used to show the improved efficiency of this triple resonance experiment at removing proton–deuterium spin–spin coupling. In the second experiment, which is also demonstrated on the CH3CD2Br sample as well as on the acetophenone, one half of a quadrupolar-split doublet in the deuterium spectrum is irradiated whilst observing the proton-spectrum. In this case the proton spectrum is shown to yield DCH3 and also the relative sign of a quadrupole and dipolar coupling. The information obtained from these experiments is used to show that acetophenone exists predominantly in the two forms with the CCOC and phenyl ring carbons coplanar. It is also shown that the molecules reorients in the liquid crystal on each internal rotation about the phenyl—C(O) bond.

13C spin-lattice relaxation in substituted benzenes

Spin-lattice relaxation of phenyl ring carbons in mono-substituted benzenes indicates that the tumbling of these molecules in solution is anisotropic; relaxation is dominated by dipole–dipole interactions with contributions from other mechanisms.