Purity analysis of hydrogen cyanide, cyanogen chloride and phosgene by quantitative (13)C NMR spectroscopy

Hydrogen cyanide, cyanogen chloride and phosgene are produced in tremendously large quantities today by the chemical industry. The compounds are also particularly attractive to foreign states and terrorists seeking an inexpensive mass-destruction capability. Along with contemporary warfare agents, therefore, the US Army evaluates protective equipment used by warfighters and domestic emergency responders against the compounds, and requires their certification at > or = 95 carbon atom % before use. We have investigated the (13)C spin-lattice relaxation behavior of the compounds to develop a quantitative NMR method for characterizing chemical lots supplied to the Army. Behavior was assessed at 75 and 126 MHz for temperatures between 5 and 15 degrees C to hold the compounds in their liquid states, dramatically improving detection sensitivity. T(1) values for cyanogen chloride and phosgene were somewhat comparable, ranging between 20 and 31 s. Hydrogen cyanide values were significantly shorter at 10-18 s, most likely because of a (1)H--(13)C dipolar contribution to relaxation not possible for the other compounds. The T(1) measurements were used to derive relaxation delays for collecting the quantitative (13)C data sets. At 126 MHz, only a single data acquisition with a cryogenic probehead gave a signal-to-noise ratio exceeding that necessary for certifying the compounds at > or = 95 carbon atom % and 99% confidence. Data acquired at 75 MHz with a conventional probehead, however, required > or = 5 acquisitions to reach this certifying signal-to-noise ratio for phosgene, and >/= 12 acquisitions were required for the other compounds under these same conditions. In terms of accuracy and execution time, the NMR method rivals typical chromatographic methods.

Effect of spectroscopic properties on photostability of tamoxifen citrate polymorphs

The photostability of tamoxifen citrate polymorphs, forms A and B, was investigated by chromatographic and spectroscopic analyses including high-pressure liquid chromatography (HPLC), colorimetry and UV/vis solid-state absorption spectroscopy. On the basis of the results of photostability studies under irradiation by visible light and both UVA (320-400 nm) and a fraction of UVB (290-320 nm) light, form A was chemically unstable, whereas form B was stable against light irradiation. The surface color of pellets prepared with any of these crystal forms turned from white to brown; however, the extent of color change in cross-sections of form A pellet was deeper than that of form B pellet. The maximum peak of UV/vis solid-state absorption spectra of form A was observed at 337 nm within the UVA range and was in longer wavelength regions than form B, which exhibited the strong UV absorption mainly in UVB and UVC region. The results obtained suggested that the photodegradation followed by surface color change of form A crystal was caused by the selective absorption of photoenergy of UVA light irradiated by a xenon lamp.

NMR studies of organic polymorphs and solvates

This review article describes the applications of NMR to the study of polymorphs and related forms (solvates) of organic (especially pharmaceutical) compounds, for which it is of increasing academic and practical importance. The nature of the systems covered is briefly introduced, as are the techniques constituting solid-state NMR. The methodologies involved are then reviewed under a number of different headings, ranging from spectral editing through relaxation times to shielding tensors and NMR crystallography. In each case the relevant applications are described. Whilst most studies concentrate on structural matters, motional effects are not neglected. A special section discusses studies of solvates (especially hydrates), and another reviews quantitative analysis.

Crystalline solids

Many drugs exist in the crystalline solid state due to reasons of stability and ease of handling during the various stages of drug development. Crystalline solids can exist in the form of polymorphs, solvates or hydrates. Phase transitions such as polymorph interconversion, desolvation of solvate, formation of hydrate and conversion of crystalline to amorphous form may occur during various pharmaceutical processes, which may alter the dissolution rate and transport characteristics of the drug. Hence it is desirable to choose the most suitable and stable form of the drug in the initial stages of drug development. The current focus of research in the solid-state area is to understand the origins of polymorphism at the molecular level, and to predict and prepare the most stable polymorph of a drug. The recent advances in computational tools allow the prediction of possible polymorphs of the drug from its molecular structure. Sensitive analytical methods are being developed to understand the nature of polymorphism and to characterize the various crystalline forms of a drug in its dosage form. The aim of this review is to emphasize the recent advances made in the area of prediction and characterization of polymorphs and solvates, to address the current challenges faced by pharmaceutical scientists and to anticipate future developments.

Characterization of the solid state: quantitative issues

Quantitative analysis of solid state composition is often used to ensure the safety and efficacy of drug substances or to establish and validate the control of the pharmaceutical production process. There are a number of common techniques that can be applied to quantify the phase composition and numerous different methods for each technique. Each quantitative option presents its own issues in ensuring accuracy and precision of the solid state method. The following article describes many of the common techniques that are used for quantitative phase analysis and many of the considerations that are necessary for the development of such methods.

Investigation of the polymorphs of dimethyl-3,6-dichloro-2,5-dihydroxyterephthalate by (13)C solid-state NMR spectroscopy

Two of the three conformational polymorphs of dimethyl-3,6-dichloro-2,5-dihydroxyterephthalate are studied by solid-state NMR techniques. The structural differences between the polymorphs have previously been studied by X-ray. In these two polymorphs named white and yellow due to their color, the major structural difference is the torsional angle between the ester group and the aromatic ring. The yellow form has a dihedral angle of 4 degrees between the plane of the aromatic ring and the plane of the ester group, while the white form has two different molecules per unit cell with dihedral angles of 70 degrees and 85 degrees. This change greatly affects the conjugation in the pi-electronic system. In addition, there are differences in the hydrogen-bonding patterns, with the white form having intermolecular hydrogen bonds and the yellow form having intramolecular hydrogen bonds. In this work, the carbon isotropic chemical shift values and the chlorine electric field gradient (EFG) tensor information are extracted from the (13)C MAS spectra, and the principal values of the chemical shift tensors of the carbons are obtained from 2D FIREMAT experiments. Quantum chemical calculations of the chemical shift tensor data as well as the EFG tensor are performed at the HF and DFT levels of theory on individual molecules and on stacks of three molecules to account for the important intermolecular interactions in the white form. The differences between the spectral data on the two polymorphs are discussed in terms of the known electronic and structural differences.

Thermal polymorphic transformation of p-tert-Butylcalix

Studies were made on the polymorphism of p-tert-butylcalix[4]arene derivatives having amino acid groups on the lower rim to stabilize their hydrophilic pseudocavity by circular intramolecular hydrogen bonding. The calixarenes exhibit a polymorphic transformation upon heating in the solid state. This transformation is controlled by the thermal history of solids, accompanying the change on the conformation of the calixarene skeleton and also the change of hydrogen bonding in the hydrophilic pseudocavity.