Biomedical subjects

Carolyn Pratt Brock

Publications and source records attributed to Carolyn Pratt Brock.

11 recordsLinked to original sources

PubMed2006

S,S-1,2-Dicyclohexylethane-1,2-diol and its racemic compound: a striking exception to Wallach's rule.

The structures of enantiopure S,S-1,2-dicyclohexylethane-1,2-diol and its racemic compound (rac-S,S-1,2-dicyclohexylethane-1,2-diol) have been determined at 295 and 173 K. The crystals of the enantiopure material are more than 4% denser than the crystals of the racemic compound, but the melting points indicate that the crystals of the less dense racemic compound are considerably more stable than those of the racemic conglomerate. This apparent exception to the correlation of crystal density and melting point is explained. The enantiopure crystals have four molecules in the asymmetric unit (Z' = 4). Two of the molecules have the conformation observed for the one independent molecule of the racemic compound and two have a higher energy conformation; the overall P2(1) structure is a perturbed version of a P2(1)2(1)2(1) structure with Z' = 2. The enantiopure and racemic crystals have the same hydrogen-bonding motif, but the motif in the former appears to be significantly strained. A reason why crystals of enantiopure material might be systematically less dense than crystals of its racemic compound and to be more likely to have Z' > 1 is suggested.

Journal Article↗

PubMed2005

Three modulation patterns in four related [M(H2O)2(15-crown-5)](NO3)2 structures.

The structures of [M(H2O)2(15-crown-5)](NO3)2, M = Cu, Zn, Mg and Co, and 15-crown-5 = 1,4,7,10,13-pentaoxacyclopentadecane, have been redetermined at 294 and 90 K. The four structures, and a second form of the Cu structure, have been reported in the literature, but are all incorrect in some significant way. The structures, which all have at least two independent formula units (i.e. Z' > or = 2), are related; each water ligand is hydrogen-bonded to two nitrate anions, while each nitrate anion is hydrogen bonded to the water ligands of two cations. In the tetragonal Co structure the hydrogen-bond pattern is three-dimensional; in the monoclinic Cu, Zn and Mg structures the hydrogen-bond patterns are two-dimensional. In the isostructural Zn and Mg structures Z' = 3, while in the Cu structure Z' = 5. The Cu, Zn and Mg structures are modulated variants of a basic structure, which was reported for Cu but which probably does not exist. The conformations of the 11 independent cations are remarkably similar; they all have approximate twofold symmetry and so exist as conformational enantiomers. The most important modulation is imperfect enantiomeric alternation of the cations along the longest cell axis; the independent cations are related by very good pseudotranslation and pseudoinversion operations. The diffraction patterns for all four structures have classes of weak, even very weak, reflections.

Journal Article↗

PubMed2005

The unusual phases of anhydrous and hydrated pinacol.

The structure of highly twinned pinacol (2,3-dimethyl-2,3-butanediol) monohydrate, the existence of which has been known since 1922, has been determined, and the structures of anhydrous pinacol and its two other known hydrates have been reinvestigated. All the phases are unusual. The anhydrous phase is exceptional among molecular crystals in having molecules located on three different symmetry sites (1, \overline 1 and 2). A hexagonal form of pinacol originally described as a second polymorph has been shown to be a solvate of uncertain composition that is very loosely packed. Pinacol hexahydrate, which was originally reported as tetragonal and highly disordered, appears to be described better as having an orthorhombic structure that is both disordered and twinned; the diffraction pattern at 90 K shows structured diffuse scattering that suggests short-range correlations of disordered molecules. The occurrence of this unusual set of structures is attributed to the combination of the hydrogen-bonding requirements of the pinacol molecule with its small size and limited conformational flexibility.

Journal Article↗

PubMed2005

A helical structure with Z' = 10.

The structure of N''-cyano-N,N-diisopropylguanidine, a push-pull nitrile that includes the H(2)N-C=N-CN fragment, has been determined. Although the number of molecules in the asymmetric unit is exceptionally high (Z' = 10), there were no special crystallographic difficulties associated with data collection, structure solution or structure refinement because the individual molecules are small (12 non-H atoms) and because there is no important crystallographic pseudosymmetry. A complete set of N-H...N bonds is formed. Pairs of molecules form dimers, which associate to form ribbons that twist into a helix having five dimers per turn. The helices lie parallel to [101] and fit together with close contacts in three different directions perpendicular to that axis. The problems with optimizing so many different intermolecular contacts lead to a loosely packed structure.

Journal Article↗

PubMed2004

Packing conflicts in the Z' = 5 structure of CF(3)(CF(2))(3)(CH(2))(10)COOH.

Dimers of the partially fluorinated amphiphile 1-(perfluorobutyl)undecanoic acid, CF(3)(CF(2))(3)(CH(2))(10)COOH, crystallize in the same basic packing arrangement found for many n-alkyl carboxylic acids. The unit cell of the fluorinated compound, however, contains five independent molecules rather than the one found for most n-alkyl carboxylic acids including CH(3)(CH(2))(13)COOH. The structures of the two C(15) acids are compared. The structural modulations that lead to the large value of Z' can be understood as resulting from the conflict between the necessity of filling space densely and uniformly and the tendency of unlike groups to be segregated spatially. The few other known structures of compounds with both (CF(2))(n) and (CH(2))(m) regions show evidence of the same conflict.

Crystallization↗

PubMed2004

2-Biphenylol revisited.

The substantially disordered structure of 2-biphenylol [Perrin et al. (1987). Acta Cryst. C43, 980-982; Kapon & Reisner (1988). Acta Cryst. C44, 2039] has been reinvestigated as a function of temperature. The structure was determined at 305, 294, 145 and 90 K; crystals were taken to the lowest temperature both rapidly and slowly (0.1 K min(-1)). The results of the previous room-temperature study were confirmed and no important structural change was found down to 90 K. Comparisons with structures known for other simple 2- and 2,2'-substituted biphenyl derivatives suggest that the molecular stacks found for 2-biphenylol are probable for related molecules. The disorder in 2-biphenylol allows the formation of some O-H...O bonds, but packing efficiency is a more important factor in this structure than is hydrogen bonding.

Journal Article↗

PubMed2003

Isostructural or not? Adducts of some aryltin halides with (E)-1,2-bis(4-pyridyl)ethene.

The title complexes [mu-(E)-4,4'-(ethene-1,2-diyl)dipyridine-kappa(2)N:N']bis[halotris(4-methylphenyl)tin(IV)], [Sn(2)(C(7)H(7))(6)X(2)(C(12)H(10)N(2))], where halo is chloro (X = Cl) and bromo (X = Br) are isostructural. In both crystals, the molecules lie on inversion centers, and there are voids of ca 80 A(3) that could, but apparently do not, accommodate water molecules. The corresponding iodo structure (X = I) is almost, but not quite, isostructural with the other two compounds; when Br is changed to I, the length of the c axis decreases by more than 1 A and the voids are no longer large enough to accomodate any solvent molecule. The related complex [mu-(E)-4,4'-(ethene-1,2-diyl)dipyridine-kappa(2)N:N']bis[chlorotriphenyltin(IV)], [Sn(2)(C(6)H(5))(6)Cl(2)(C(12)H(10)N(2))], crystallizes in a related structure, but the molecules lie on general rather than on special positions. The molecular structures of the four complexes are similar, but the conformation of the phenyl derivative is approximately eclipsed rather than staggered.

Journal Article↗

PubMed2002

Dichloro[1,1'-(5,9-dithia-2,12-diazoniatrideca-1,12-diene-1,13-diyl)dinaphthalen-2-olato-kappa2O,O']dimethyltin(IV) acetonitrile solvate.

Reaction of the potentially hexadentate ligand 1,9-bis(2-hydroxy-1-naphthalenemethylimino)-3,7-dithianonane with dimethyltin chloride gave the title 1:1 adduct, in which the long ligand wraps around the SnCl(2)Me(2) unit and in which the stereochemistry is fully trans. This compound crystallizes from acetonitrile as the 1:1 solvate [Sn(CH(3))(2)(C(29)H(30)N(2)O(2)S(2))Cl(2)].C(2)H(3)N. During the reaction, the hydroxyl protons move to the N atoms. Most of the chemically equivalent bond lengths agree to within experimental uncertainty, but the Sn-Cl bond that is inside the ligand pocket is substantially longer than the Sn-Cl bond that points away from the long ligand [2.668 (1) versus 2.528 (1) A]. The O-Sn-O angle is 166.0 (1) degrees. Comparison of the Sn-O, C-O and aryl C-C bond lengths with those of related compounds shows that the most important resonance forms for the Schiff base aryloxide ligand are double zwitterions, but that the uncharged resonance forms having carbonyl groups also contribute significantly.

Journal Article↗

PubMed2002

Crystal packing in vicinal diols CnHm(OH)2.

The O-HtriplebondO bonds in vic-diols C(n)H(m)(OH)(2) have been studied using data retrieved from the Cambridge Structural Database. About half of these diols form complete, or almost complete, sets of intermolecular O-HtriplebondO bonds (i.e. two satisfied donors per molecule). For this half of the structures the frequencies of high-symmetry space groups and of structures with Z' > 1 (more than one molecule in the asymmetric unit) are substantially elevated. The most common motif among fully bonded structures is an R(2)(2)(10) dimer, which can be linked in a variety of ways to form one-, two- or even three-dimensional patterns. Most of the other half of the vic-diols form simple O-HtriplebondO chains in which each OH group participates in only one intermolecular hydrogen bond. The space-group frequencies for this second group of structures are unexceptional. The most important factor determining the extent of O-HtriplebondO bond formation is the degree of substitution of the vic-diol. The spatial segregation of OH groups that is necessary for the formation of O-HtriplebondO bonds is found to make the dense filling of space more difficult because the intermolecular spacings that are appropriate for the O-HtriplebondO bonds may be inappropriate for the rest of the molecule.

Journal Article↗

PubMed2002

Two phases of C(9)H(12)O(4): why is the structure at 295 K so complicated?

Molecules of 4,4'-dimethyl-2-hydroxy-6-oxocyclohexene-1-carboxylic acid, C(9)H(12)O(4), crystallize at 295 K in a modulated superstructure with five half-molecules in the asymmetric unit; each molecule is located on one of the mirror planes of the space group Cmc2(1). Reflections with k not equal 5n are systematically weak; a satisfactory refinement can be obtained in a Cmcm pseudocell having b' = b/5. The important modulation involves small rotations of the molecules around axes perpendicular to the mirror plane; there is also an up/down disorder of the CMe(2) fragment in four of the five molecules (two molecules with occupancy factors ca 4:1; two with occupancy factors ca 3:2). The modulation is a response to packing problems that can be traced to the differences between the thin, electron- and oxygen-rich 'head' of the molecule and the thicker, methyl-rich 'tail'. At 130 K the length of b is reduced by 2/5 and the Pmnb structure is ordered. Both structures can be described as modulated variants of the Cmcm substructure; the wavevectors are 2b'*/5 for the room-temperature structure and b'*/2 for the low-temperature structure, where b'* is the reciprocal axis of the subcell. The structure at room temperature can also be understood as a hybrid of the fully disordered pseudocell structure and the ordered structure that is found at low temperature.

Journal Article↗

PubMed2001

Z' = 4 structure without obvious pseudosymmetry: implications for the formation of solid-state compounds.

Crystals of 4-chloro-2'-biphenylol grown at 263 K and studied at both 293 and 142 K are loosely packed and contain OH.O bonded tetramers composed of four crystallographically independent molecules (Z' = 4). Approximate symmetry relationships between independent molecules are apparent in many, perhaps even most, structures with Z' > 1, but are not obvious in this structure. The inefficient crystal packing, the surprisingly large value of Z' and the absence of pseudosymmetry reflect the difficulty of finding a dense molecular arrangement that also allows for hydrogen-bond formation. Impediments to good packing are the steric congestion around the ortho hydroxyl groups and the difficulty in filling space with significantly twisted biphenyl fragments. The packing arrangement adopted is strongly reminiscent of the structure type found for eight simple 4,4'-biphenyl derivatives, but one of the four independent 4-chloro-2'-biphenylol molecules has an orientation that is significantly different from the orientations of the other three and that does not fit the 4,4'-XPhPhY pattern. It is suggested that molecules that crystallize with Z' > 1 are good candidates for solid-state compound formation.

Biphenyl Compounds↗