Generalized bond orders

We introduce generalized bond orders defined in terms of weighted Kekule valence structures. The weights were determined by the contributions of linearly independent and minimal conjugated circuits in individual Kekule valence structure. When special values for the contributions of conjugated circuits of different size are assumed, one obtains quantities that show considerable similarity to the Pauling and the Clar's bond orders. Pauling bond orders are obtained when one assumes that all conjugated circuits make equal contribution to bond orders. © 1994 John Wiley & Sons, Inc.     

NMR self-diffusion studies in heterogeneous systems

The principles and main limitations of the application of the NMR field gradient technique to self-diffusion measurements are discussed. Due to their higher sensitivity for small diffusion coefficients and due to several advantages with the analytical treatment of the experimental data, the pulsed gradient methods are of special importance for the investigation of heterogeneous systems. Examples of the application of these techniques to diffusion studies in multicomponent systems and bounded media are given.     

Molecular orbital studies of the NMR hydrogen bond shift

Magnetic shielding constants are calculated for the protons in XOH and XOH…OH₂ (XH, CH₃, NH₂, OH and F) molecules using a slightly extended set of atomic functions modified by gauge factors. These results are used to determine theoretical values for the NMR hydrogen bond shifts in the XOH…OH₂ systems. Such theoretical data are consistent with the few available experimental data. An analysis of the theoretical results reveals that there are three major types of shielding contribution to the NMR hydrogen bond shift; (a) a deshielding change due to the variation of the local currents on the hydrogen bonded proton; (b) a reduction in shielding from currents localized on the oxygen atom of the proton donor; (c) a deshielding contribution from currents induced on the oxygen atom of the proton acceptor. Except for the water dimer, contributions (a), (b) and (c) are of comparable importance for changes in isotropic shielding. For (H₂O)₂ contributions (a) and (c) are somewhat more important than contribution (b). Contribution (c) is almost totally responsible for the changes in the anistropies of the shielding tensors associated with the hydrogen bonded protons. The proton shielding anisotropy changes which occur on hydrogen bond formation are generally much larger than the corresponding variations in the isotropic values of the shielding tensors. This suggests that proton magnetic shielding anisotropies may be more sensitive measures of features of hydrogen bonding than are isotropic proton shielding constants.     

Electronic structure, 1H NMR spectra,and reactivity in certain tricyclic heteroaromatic systems

The simple MO LCAO method was used for calculations on 4H-imidazo[5,1-b]benzimidazole, imidazo[5,1-b]benzoxazole, imidazo[5,1-b]benzothiazole, and a number of their derivatives. Inclusion of the unshared pairs of the hetero atoms at positions 4 and 9 in the -electron system has a considerable effect on the stability of compounds of this type. The ¹H NMR spectra were recorded for 4-methyl-imidazo[5,1-b]benzimidazole and imidazo[5,1-b]benzoxazole derivatives, and a similarity was found between the chemical shifts of the imidazole ring protons and the -electron densities at the adjacent carbon atoms. Calculation of a series of models for the cations and measurements of the basicity constants show protonation of 4-methylimidazo[5,1-b]benzimidazole takes place at the nitrogen atom in position 2. The calculated values for the electronic structure and reactivity indices in the investigated series of tricyclic systems were compared with their chemical properties.     

Solid-state NMR studies of weak interactions in supramolecular systems

The field of application of solid-state NMR to the study of supramolecular systems is growing rapidly, with many research groups involved in the development of techniques for the study of crystalline and amorphous phases. This Feature Article aims to provide an overview of the recent contributions of our research group to this field, paying particular attention to the study of the weak interactions such as hydrogen bonds in supramolecular systems through solid-state NMR investigations. The structure and dynamic behaviour of selected host-guest systems will be also discussed.     

Conformational studies by dynamic NMR—VI: Torsional barriers in 5-membered heteroaromatic carbamides and thiocarbamides

Thermodynamie parameters involved in the rotation of the partial C-N double bond in a series of 5-membered heteroaromatic N,N-dimethyl-carbamides and -thiocarbamides have been measured by total line shape (TLS) analysis of their NMR. Lanthanide induced shift (LIS) has been applied in two cases to determine the conformation of the CO moiety with respect to the heteroatom. The energy barriers of thiazolyl carbamides and thiocarbamides were found to be larger than those of furyl and thienyl analogues, and explained in terms of a perturbational molecular orbital analysis.     

Electrophoretic NMR studies of polymer and surfactant systems

The aim of this tutorial review is to introduce to a broader readership the emerging technique of electrophoretic NMR (eNMR). The "electrophoretic" element of the technique refers to the fact that charged particles are induced to flow by the application of an electric field. This flow is measured using pulsed-gradient spin-echo NMR (PGSE-NMR). The great potential of this experimental approach is the fact that NMR is chemically selective and non-invasive. eNMR, especially when combined with the more established PGSE-NMR experiment, may therefore be used to quantify the structure of multi-component systems via the dynamics and charge of each species within a complex mixture. Accordingly, eNMR is likely to be of great significance for colloid scientists, biologists, technologists and formulation scientists.     

Quantum-chemical investigation of bond orders in symmetric polymethine dyes

The behavior of the bond orders in symmetric model polymethine dyes has been investigated by the HMO, PPP, and CNDO/2 methods. It has been established that the greatest equalization of the bonds is achieved in the case of dyes with moderately basic terminal groups. In other cases, alternation of the bond orders, which diminishes with increasing length of the polymethine chain and attenuates with increasing distance from the end, has been discovered at the ends of the polymethine chain. The amplitude of the alternation of the bond orders in a polymethine chain is greater, the more the basicity of the terminal groups, as characterized by the electron-donor parameter _o (0 _o 90°), deviates in either direction from the mean (_o = 45°). It is considerably smaller in the first excited state than in the ground state. The -electronic approximation is a correct method for describing the electronic structure of a polymethine chain. The neglect of the electrons can distort the bond orders of the terminal groups.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 21, No. 2, pp. 138–146, March–April, 1985.     

NMR and MRI studies of drug delivery systems

Nuclear magnetic resonance imaging and spectroscopy are now commonplace in most academic and industrial research environments. The ability of magnetic resonance techniques to provide the researcher with non-invasive, quantitative, physicochemical information in the disciplines of chemistry, biology, materials science, chemical engineering and medicine is widely known. In the last 10–15 years a variety of magnetic resonance methods have provided the pharmaceutical research community with valuable information, especially in the important area of drug delivery using solid dosage forms. This review will highlight recent advances in magnetic resonance techniques and its specific applications to further our understanding of pharmaceutical drug delivery systems. The review is aimed at non-clinical research and development, and will focus on the behaviour and characterisation of drug release from pellets, tablets and capsules, which are the most commonly used drug delivery systems. In addition to magnetic resonance techniques a number of complementary analytical techniques are mentioned to illustrate the importance of adopting a multi-modal analytical approach to gain a better scientific understanding of the behaviour of drug delivery devices.     

Examination of P-OR bridging bond orders in phosphate monoesters using (18)O isotope shifts in 31P NMR

Evidence indicates that phosphate monoesters undergo hydrolysis by a loose transition state with extensive bond fission to the leaving group. It has been proposed that part of the high dependence of the rate on the leaving group pKa (betalg = -1.23) arises from weaker ester bonds in the reactants as the pKa of the leaving group decreases, on the basis of X-ray structures and calculations. In contrast, IR and Raman studies suggest that the leaving group has little effect on the length of the P-OR bridging bond in solution. To gather additional data on this issue, we have used (18)O isotopic shifts in 31P NMR to monitor the bond order of P-O bonds in a range of phosphate esters with different leaving groups. Using this technique, we have been able to evaluate whether significant changes are observed in the P-O bond orders for the bridging and nonbridging positions of methyl, ethyl, phenethyl, propargyl, phenyl, and p-nitrophenyl phosphate using [(16)O(18)O] labeled species in deuterium oxide. The results indicate that the bridging and nonbridging bond orders to phosphorus in phosphate monoesters are not significantly altered by differences in the pKa of the leaving group or by the counterion of the phosphate ester dianion.     

About direct obtainment of π SC bond orders

A method for direct obtainment of SC bond orders is shown for even systems with a twofold symmetry axis not crossing a center. Two sets of equations are stated, one referring to symmetric and the other to antisymmetric basis functions. Pauli matrices are used for solving the 4-electron case.Consejo Nacional de Investigaciones Cientificas y Técnicas, Argentina.     

NMR studies of sorption and adsorption phenomena in colloidal systems

NMR methods provide chemically selective tools, particularly suitable to detect the molecular environment of molecular species in micro-heterogeneous materials. They are consequently applied to solve many questions in colloid science. The present review covers NMR studies of molecular adsorption onto particle surfaces as well as sorption into colloidal particles. Various methods ranging from liquid or solid state spectral analysis over spin relaxation to pulsed field gradient diffusion NMR have been employed in this field, monitoring either the chemical environment or the restricted dynamics of adsorbed or encapsulated guest molecules. Adsorption systems include surfactant layers, stabilizing ligands, small molecules, polymer layers or polyelectrolyte multilayers at the surface of various types of particles. Sorption into colloidal particles and detection of their position in specific compartments of the colloid are particularly relevant in systems employed as colloidal carriers, such as micelles, vesicles, or hollow polymeric capsules. With guest molecules considered as model compounds for drugs these studies have large relevance for the development of nanoparticle drug delivery systems.     

Electronic interactions in aromatic an heteroaromatic two-ring systems

The chemical shifts of the methyl protons ( ) in the PMR spectra and the integral intensities (A^1/2) of the band of the symmetrical CH stretching vibration of the methyl group in the IR spectra were measured for an extensive series of methyl derivatives of aromatic and heteroaromatic compounds including 3-, 4-, 5-, and 6-substituted 1- and 2-methylnaphthalenes, 4-, 5-, 6-, and 7-substituted 2-methylquinolines, and 5- and 6-substituted 2-methylbenzothiazoles, as well as a number of two- and three-ring systems that do not contain substituents. The changes in the chemical shifts are interpreted with allowance for two principal contributions: the contribution from the ring currents and the contribution from the changes in the electron densities (q) on the H and C atoms of the methyl group. The fact of the existence of a linear relationship between the corrected (for the ring current) values and the calculated (by the CNDO/2 method) electron charges q_C and q_H (the latter plays the dominant role) indicates that the electronic effects of the substituent make the principal contribution to the changes in the shielding constants of the protons of the methyl group in most of the investigated compounds. Similarly, the charges on the H and C atoms of the methyl group convey the trend of the integral intensities of the IR bands, thereby confirming the primarily electronic nature of the effects of structural factors on the A^1/2 values.See [2] for communication 2.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 3, pp. 390–396, March, 1979.     

Proton NMR relaxation studies on several nitroxyl-water systems

Proton spin-lattice and spin-spin relaxation times of several nitroxyl radicals in aqueous solutions have been measured between 10 kHz and 90 MHz. There are two regions where the relaxation times of the solvent water protons are frequency dependent. It is possible to extract from these experimental data structural and dynamical parameters such as the number of solvated water protons and/or their mean lifetime in the first hydration sphere around the paramagnetic center. These results were found to differ considerably from corresponding data reported in the literature which have been deduced mainly from chemical shift measurements. To whom correspondence should be addressed.     

Conformational studies by dynamic NMR. 98. Stereodynamics of bond rotation in beta-hydroxyesters

The dynamic processes due to restricted intramolecular motions have been observed in a number of variously substituted beta-hydroxyesters by variable-temperature NMR spectroscopy. The barriers for three such processes (i.e. Ph-C, t-Bu-C, and C(alpha)-C(beta) bond rotation) have been measured.     

Covalent bond orders revisited: the open-shell case

This article states the concept of covalent bond order for open-shell systems from the invariance properties of the first- and second-order reduced density matrices for all the components of a multiplet state. A general bond order definition is formulated in the framework of the electronic population analyses in the Hilbert space of atomic orbitals.     

2H NMR studies of oriented bacteriorhodopsin membranes to determine single bond orientations

For the structural analysis of oriented polymers and other macromolecular systems, we present a solid state ²H NMR technique capable of measuring the orientations of individual chemical bond vectors with high accuracy. When an immobilized uniaxially oriented sample is aligned along the spectrometer magnetic field direction, the angle between a deuterium-labelled methyl group and the axis of ordering can be calculated from the spectral quadrupole splitting. However, since positive and negative splittings are indistinguishable, there may be two solutions. We show how these may be discriminated by acquiring additional spectra at different sample inclinations (Ref 1). The analysis of the resulting complex lineshapes is aided by computer simulation, which furthermore allows a characterization of the orientational distribution function of the sample. The ²H NMR method has been developed around the membrane protein bacteriorhodopsin as a biological model system that can be prepared in uniaxially oriented films. The overall orientation and molecular conformation of the retinal chromophore were resolved from the individual orientations of its five labelled methyl groups (Refs 2, 3). Structural changes were detected in an intermediate of the photocycle (M), which provide insight into the mechanism of the protein as a proton pump (Ref 4).     

Natural abundance 17O NMR spectroscopy of heteroaromatic Nitro compounds

The ¹⁷O chemical shift data, at natural abundance, for selected nitroquinolines, nitroindoles, nitroindazoles and nitrothiophenes are reported. In the absence of a peri or a lone-pair repulsion effect, the nitroquinolines' chemical shifts differ little from those of their carbocyclic analogs. However, the signal for 5-nitroquinoline, 2 , is deshielded by 25 ppm compared to 6-nitroquinoline, 1 , and the ¹⁷O nucleus in 8-nitroquinoline, 3 , is deshielded by 49 ppm compared to that in 1 . Both these shifts are attributed to rotation of the nitro group from the plane of the heteroaromatic ring arising from peri hydrogen interaction and lone pair repulsion, respectively. The signals for nitro groups on electron excessive ring systems (e.g., indoles and thiophenes) are shielded relative to corresponding ones in electron deficient heterocyclic ring system analogs. The chemical shifts for the π-excessive systems are interpreted in terms of electronic effects.     

Condensed heteroaromatic systems including a thiophene ring

A number of complexes based on 3-hydroxy-2-benzo[b]formylthiophene and its Schiff bases were obtained and investigated as stabilizers for polycaproamide fibers. It was established that {N,N-bis(3-hydroxy-2-benzo[b]thenylidene)ethylenediaminato}-copper (II) which makes it possible to simultaneously improve the thermal stability and photostability of polycaproamide fibers, is the most effective stabilizer of the investigated compounds.