Measuring Solute Concentration with Terahertz Time-Domain Spectroscopy in Single and Multiphase Systems.

A versatile setup based on a microfluidic platform allows investigation of liquid samples at various temperatures with terahertz time-domain spectroscopy. The setup is applied to develop a novel method that performs temperature and concentration calibrations of liquid samples at terahertz frequencies. Other than measuring the concentration of pure liquid phase solutions, it enables the studies of local concentration of semicrystalline systems. An equivalent solute concentration during crystallization can be calculated from the extracted absorption at low frequencies. The MgSO4-water system is discussed as an example to illustrate the idea of this method.

Are the Crystal Structures of Enantiopure and Racemic Mandelic Acids Determined by Kinetics or Thermodynamics?

Mandelic acids are prototypic chiral molecules where the sensitivity of crystallized forms (enantiopure/racemic compound/polymorphs) to both conditions and substituents provides a new insight into the factors that may allow chiral separation by crystallization. The determination of a significant number of single crystal structures allows the analysis of 13 enantiopure and 30 racemic crystal structures of 21 (F/Cl/Br/CH3/CH3O) substituted mandelic acid derivatives. There are some common phenyl packing motifs between some groups of racemic and enantiopure structures, although they show very different hydrogen-bonding motifs. The computed crystal energy landscape of 3-chloromandelic acid, which has at least two enantiopure and three racemic crystal polymorphs, reveals that there are many more possible structures, some of which are predicted to be thermodynamically more favorable as well as slightly denser than the known forms. Simulations of mandelic acid dimers in isolation, water, and toluene do not differentiate between racemic and enantiopure dimers and also suggest that the phenyl ring interactions play a major role in the crystallization mechanism. The observed crystallization behavior of mandelic acids does not correspond to any simple "crystal engineering rules" as there is a range of thermodynamically feasible structures with no distinction between the enantiopure and racemic forms. Nucleation and crystallization appear to be determined by the kinetics of crystal growth with a statistical bias, but the diversity of the mandelic acid crystallization behavior demonstrates that the factors that influence the kinetics of crystal nucleation and growth are not yet adequately understood.

Describing hydrogen-bonded structures; topology graphs, nodal symbols and connectivity tables, exemplified by five polymorphs of each of sulfathiazole and sulfapyridine.

BACKGROUND: Structural systematics is the comparison of sets of chemically related crystal structures with the aim to establish and describe relevant similarities and relationships. An important topic in this context is the comparison of hydrogen-bonded structures (HBSs) and their representation by suitable descriptors. RESULTS: Three different description methods for HBSs are proposed, a graphical representation, a symbolic representation and connectivity tables. The most comprehensive description is provided by a modified graph of the underlying net topology of an HBS which contains information on the multiplicity of links, the directionality and chemical connectivity of hydrogen bonds and on symmetry relations. By contrast, the alternative symbolic representation is restricted to essential properties of an HBS, i.e. its dimensionality, topology type and selected connectivity characteristics of nodes. A comparison of their connectivity tables readily identifies differences and similarities between crystal structures with respect to the intermolecular interaction modes adopted by their functional groups. The application of these methods to the known polymorphs of sulfathiazole and sulfapyridine is demonstrated and it is shown that they enable the rationalisation of previously reported and intricate relationships. CONCLUSIONS: The proposed methods facilitate the comprehensive description of the most important relevant aspects of an HBS, including its chemical connectivity, net topology and symmetry characteristics, and they represent a new way to recognise similarities and relationships in organic crystal structures. Graphical AbstractGraphical Representation of mixing of structures StzIV and StzV to give structure StzIII.

Eight isostructural 4,4'-disubstituted N-phenylbenzenesulfonamides.

The isostructural crystals of 4-cyano-N-(4-methoxyphenyl)benzenesulfonamide, C(14)H(12)N(2)O(3)S, (I), N-(4-methoxyphenyl)-4-(trifluoromethyl)benzenesulfonamide, C(14)H(12)F(3)NO(3)S, (II), 4-iodo-N-(4-methoxyphenyl)benzenesulfonamide, C(13)H(12)INO(3)S, (III), 4-bromo-N-(4-methoxyphenyl)benzenesulfonamide, C(13)H(12)BrNO(3)S, (IV), 4-chloro-N-(4-methoxyphenyl)benzenesulfonamide, C(13)H(12)ClNO(3)S, (V), 4-fluoro-N-(4-methoxyphenyl)benzenesulfonamide, C(13)H(12)FNO(3)S, (VI), N-(4-chlorophenyl)-4-methoxybenzenesulfonamide, C(13)H(12)ClNO(3)S, (VII), and 4-cyano-N-phenylbenzenesulfonamide, C(13)H(10)N(2)O(2)S, (VIII), contain infinite chains composed of N-H···O(sulfonyl) hydrogen-bonded molecules. The crystal structures of (I)-(VIII) have been compared using the XPac software and quantitative descriptors of isostructurality were generated [Gelbrich, Threlfall & Hursthouse (2012). CrystEngComm, 14, 5454-5464]. Certain isostructural relationships in this series involve molecules with substantially different spatial demands, e.g. (VI) and (VIII) are related by the simultaneous interchange of F→CN on the benzenesulfonamide ring and OMe→H on the N-phenyl ring, which indicates that the geometry of the three-dimensional crystal-packing mode of (I)-(VIII) is unusually adaptable to different molecular shapes.

Delta-sulfanilamide.

The delta polymorph of sulfanilamide (or 4-aminobenzenesulfonamide), C(6)H(8)N(2)O(2)S, displays an overall three-dimensional hydrogen-bonded network that is dominated by a two-dimensional substructure with R(2)(2)(8) rings; these result from dimeric N-H...O interactions between adjacent sulfonamide groups. This study shows how the polymorphism of sulfanilamide is linked to its versatile hydrogen-bonding capabilities.

Structural systematics of 4,4'-disubstituted benzenesulfonamidobenzenes. 1. Overview and dimer-based isostructures.

One hundred 4,4'-disubstituted benzenesulfonamidobenzenes, X-C(6)H(5)-SO(2)-NH-C(6)H(5)-Y, where X, Y = NO(2), CN, CF(3), I, Br, Cl, F, H, Me, OMe, have been synthesized and their crystal structures determined. The resulting set of 133 structures, which includes polymorphic forms, is used to make a comparative study of the molecular packing and the nature of the intermolecular interactions, including the formation of hydrogen-bonding motifs and the influence of the two substituents X and Y on these features. Nine distinct supramolecular connectivity motifs of hydrogen bonding are encountered. There are 74% of all the structures investigated which exhibit one of two motifs based on N-H...O=S interactions, a dimer or a chain. There are three other, infrequent motifs, also employing N-H...O=S links, which exhibit more complexity. Four different chain motifs result from either N-H...O=N, N-H...C[triple-bond]N or N-H...OMe interactions, arising from the presence of a nitro (position Y), nitrile (X or Y) or methoxy (Y) substituent. The program XPac [Gelbrich & Hursthouse (2005). CrystEngComm, 7, 324-336] was used to systematically analyse the packing relationships between crystal structures. Similar discrete (zero-dimensional) and extended (one-dimensional and two-dimensional) structure components, as well as cases of isostructurality were identified. A hierarchy for the classification of the 56 distinct structure types of this set is presented. The most common type, a series of 22 isostructures containing the simple centrosymmetric N-H...O=S-bonded dimer, is discussed in detail.

Polymorph VI of sulfapyridine: interpenetrating two- and three-dimensional hydrogen-bonded nets formed from two tautomeric forms.

Polymorph VI of 4-amino-N-(2-pyridyl)benzenesulfonamide, C(11)H(11)N(3)O(2)S, is monoclinic (space group P2(1)/n). The asymmetric unit contains two different tautomeric forms. The structure displays N-H...N and N-H...O hydrogen bonding. The two independent molecules form two separate two- and three-dimensional hydrogen-bonded networks which interpenetrate. The observed patterns of hydrogen bonding are analogous to those in polymorph I of sulfathiazole.

Further errors in polymorph identification: furosemide and finasteride.

Reassessment of the reported single-crystal X-ray diffraction characterization of polymorphs of furosemide and finasteride shows that, in each case, incomplete data collections have resulted in the mistaken identification of two forms that are, in fact, identical.

Effects of polymorphic differences for sulfanilamide, as seen through 13C and 15N solid-state NMR, together with shielding calculations.

We recorded both carbon-13 and nitrogen-15 NMR spectra of the three solid forms of sulfanilamide most commonly known. This study led to an interpretation of the solid-state effects seen in cross-polarization magic angle spinning spectra. Relaxation times for the different forms were measured. These show different behaviour for the three forms, arising from mobility variations. To obtain information on local environments, static spectra and spinning sideband manifolds were recorded and analysed for the 15N resonances, using isotopically enriched samples. Shielding asymmetries and anisotropies for the two nitrogen nuclei were obtained, showing very different behaviour for the two sites. Shielding calculations were carried out for both 13C and 15N nuclei, and the results are discussed in relation to the experimental values.