Carolyn Pratt Brock

Temperature dependence of thermal motion in crystalline naphthalene

Single-crystal data for naphthalene have been measured at five temperatures between 90 and 240 K. Positional and thermal parameters for C and H atoms at each temperature were refined by conventional least-squares techniques. The effect of varying the weighting scheme was examined. Contributions of internal molecular modes to the motions of the atoms turn out to be important. They were estimated for the C atoms at each temperature from a standard force field and subtracted from the experimental U~/ values. The corrected UiTs were then analysed to determine rigid-body translational and librational tensors for the naphthalene molecule. The absolute magnitudes and temperature dependence of these quantities have been compared with values calculated from atom-atom potentials and from spectroscopic data.

A list of organic kryptoracemates.

A list of 181 organic kryptoracemates has been compiled. This class of crystallographic oddities is made up of racemic compounds (i.e. pairs of resolvable enantiomers) that happen to crystallize in Sohnke space groups (i.e. groups that include only proper symmetry operations). Most (151) of the 181 structures could have crystallized as ordered structures in non-Sohnke groups. The remaining 30 structures do not fully meet this criterion but would have been classified as kryptoracemates by previous authors. Examples were found and checked with the aid of available software for searching the Cambridge Structural Database, for generating and comparing InChI strings, and for validating crystal structures. The pairs of enantiomers in the true kryptoracemates usually have very similar conformations; often the match is near-perfect. There is a pseudosymmetric relationship of the enantiomers in about 60% of the kryptoracemate structures, but the deviations from inversion or glide symmetry are usually quite easy to spot. Kryptoracemates were found to account for 0.1% of all organic structures containing either a racemic compound, a meso molecule, or some other achiral molecule. The centroid of a pair of enantiomers is more likely (99.9% versus 99% probability) to be located on an inversion center than is the centroid of a potentially centrosymmetric molecule.

Rigid-Body Disorder Models for the High-Temperature Phase of Ferrocene

Ferrocene, [Fe(C5H5)2], which crystallizes at room temperature in space group P21/a with Z = 2, is described in many textbooks as having D5d symmetry. Previous work has shown, however, that the librational amplitude associated with motion about the fivefold axis does not decrease with temperature and that the crystals are probably disordered. Ferrocene molecules in triclinic crystals grown below 169 K have approximate D5 symmetry and an almost eclipsed conformation; the low- and high-temperature phases may be related by an order–disorder transition, during which the number of independent atoms changes by a factor of 4. The structure of the high-temperature phase has been reinvestigated with rigid-body refinements of the neutron diffraction data collected at 173 and 298 K by Takusagawa & Koetzle [Acta Cryst. (1979), B35, 1074–1081]. The C5H5 ring was treated as a rigid group of C5 symmetry; C—C and C—H distances were allowed to vary, as was the displacement of the H atoms from the C5 plane. The rigid-body motion of the C5H5 ligand was described by the TLS model. All the rigid-body disorder models fit better than conventional independent-atom models. A disorder model that includes three sites for each C5H5 ring is the best of the models that were investigated, which indicates that the structure of the high-temperature phase cannot be described by the superposition of the two independent ferrocene molecules in the low-temperature phase. The phase transition between the high- and low-temperature phases is not a simple order–disorder transition.

INVESTIGATIONS OF THE SYSTEMATICS OF CRYSTAL PACKING USING THE CAMBRIDGE STRUCTURAL DATABASE

Several studies that used the Cambridge Structural Database to elucidate principles of packing in molecular crystals are described. Some possible sources of bias in the statistical distributions are discussed.

Failures of fractional crystallization: ordered co-crystals of isomers and near isomers

A list of 270 structures of ordered co-crystals of isomers, near isomers and molecules that are almost the same has been compiled. Searches for structures containing isomers could be automated by the use of IUPAC International Chemical Identifier (InChI™) strings but searches for co-crystals of very similar molecules were more labor intensive. Compounds in which the heteromolecular A···B interactions are clearly better than the average of the homomolecular A···A and B···B interactions were excluded. The two largest structural classes found include co-crystals of configurational diastereomers and of quasienantiomers (or quasiracemates). These two groups overlap. There are 114 co-crystals of diastereomers and the same number of quasiracemates, with 71 structures being counted in both groups; together the groups account for 157 structures or 58% of the total. The large number of quasiracemates is strong evidence for inversion symmetry being very favorable for crystal packing. Co-crystallization of two diastereomers is especially likely if a 1,1 switch of a methyl group and an H atom, or of an inversion of a [2.2.1] or [2.2.2] cage, in one of the diastereomers would make the two molecules enantiomers.

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.

Structure and thermal motion of triphenylboroxin

Abstract : The molecular dimensions determined are very similar to those found previously for the related hydrocarbon 1,3,5-triphenylbenzene, but the conformations of the two molecules differ. The phenylrings in the hydrocarbon adopt a propeller-like arrangement, while in the boroxin they are nearly coplanar with the central 8303 ring. Possible relationships between thermal-motion descriptions and the intramolecular potential-energy surfaces for these two compounds are considered. Keywords: Boroxin, X-ray crystal structure, Benzenes.

The elusive [Ni(H2O)2(15-crown-5)]2+ cation and related co-crystals of nickel(II) hydrates and 15-crown-5.

Initial attempts to make [Ni(H2O)2(15-crown-5)](NO3)2, i.e. to insert the Ni2+ ion into the 15-crown-5 macrocycle, gave the mono- (two polymorphs) and dihydrate of a co-crystal of[Ni(H2O)6](NO3)2 and 15-crown-5 (1,4,7,10,13-pentaoxacyclopentadecane=15C5). Synthetic routes designed to restrict the water available to the Ni2+ cation gave three additional compounds, [Ni(H2O)6](NO3)2-trans-[Ni(H2O)4(MeOH)2]-(NO3)2x2(15C5), cis-[Ni(H2O)4(NO3)2]-trans-[Ni(H2O)4-(NO3)2]x2(15C5) and [Ni(H2O)2(MeCN)(NO3)2]x15C5x-MeCN. All five compounds contain Ni2+ ions with two trans aqua ligands. In all six structures these aqua ligands make hydrogen bonds to the 15C5 molecules to form stacks in which the Ni complexes and 15C5 molecules alternate. The structures are surprisingly complicated: all are co-crystals,some are also solvates, and most have Z0 > 1. The target compound was finally prepared by heating pale green crystals of [Ni(H2O)6](NO3)2x15C5x2H2O to over 350 K and then cooling the resulting mixture of yellow crystals and solution to room temperature. Formation of the target compound seems to be favored at higher temperatures by a positive Delta(rxn)S(o) and an increased rate of ligand exchange.

An enantioselective approach to ring a of taxol using the wieland-miescher ketone

Abstract The selective protection of the S-(+)-enantiomer of the Wieland Miescher ketone ( 2 ) as the tert-butyldimethylsilyl-protected cyanohydrin 7 and the oxidative cleavage of an α-ketol 8 that was derived from 7 provided a model 10 for the A ring of the taxol ( 1 ) possessing the correct C-1 stereochemistry of taxol and functionality suitable for further elaboration.

A synthesis of C-23 and C-24 diastereomers of 5α-dinosterane

Abstract Stereoselective routes for the preparation of C-23 and C-24 diastereomers of the C30 biological marker, 5α-dinosterane (1), involved the alkylation of (20S)-20-(iodomethyl)-4α-methyl-5α-pregnane (7) with either a saturated ester, methyl 3,4-dimethylpentanoate (9), followed by reduction to give principally the erythro-diastereomers or the alkylation of 7 with an α,β-unsaturated ester, methyl 3,4-dimethylpentenoate (12), followed by reduction to give principally the threo-diastereomers.