Low-temperature Crystal Structure Research Articles

X-ray diffraction and inelastic neutron scattering study of 1:1 tetramethylpyrazine chloranilic acid complex: temperature, isotope, and pressure effects

The x-ray diffraction studies of the title complex were carried out at room temperature and 14 K for H/D (in hydrogen bridge) isotopomers. At 82 K a phase transition takes place leading to a doubling of unit cells and alternation of the hydrogen bond lengths linking tetramethylpyrazine (TMP) and chloranilic acid molecules. A marked H/D isotope effect on these lengths was found at room temperature. The elongation is much smaller at 14 K. The infrared isotopic ratio for O-H(D)...N bands equals to 1.33. The four tunnel splittings of methyl librational ground states of the protonated complex required by the structure are determined at a temperature T=4.2 K up to pressures P=4.7 kbars by high resolution neutron spectroscopy. The tunnel mode at 20.6 microeV at ambient pressure shifts smoothly to 12.2 microeV at P=3.4 kbars. This is attributed to an increase of the strength of the rotational potential proportional to r(-5.6). The three other tunnel peaks show no or weak shifts only. The increasing interaction with diminishing intermolecular distances is assumed to be compensated by a charge transfer between the constituents of deltae/e approximately 0.02 kbar(-1). The phase transition observed between 3.4 and 4.7 kbars leads to increased symmetry with only two more intense tunneling bands. In the isotopomer with deuterated hydrogen bonds and P=1 bar all tunnel intensities become equal in consistency with the low temperature crystal structure. The effect of charge transfer is confirmed by a weakening of rotational potentials for those methyl groups whose tunnel splittings were independent of pressure. Density functional theory calculations for the model TMP.(HF)2 complex and fully ionized molecule TMP+ point out that the intramolecular rotational potential of methyl groups is weaker in the charged species. They do not allow for the unequivocal conclusions about the role of the intermolecular charge transfer effect on the torsional frequencies.

Magnetic-field-induced structural transformation inEr5Si4

Magnetization and magnetostriction measurements in pulsed magnetic fields reaching $600\phantom{\rule{0.3em}{0ex}}\mathrm{kOe}$ reveal strong magnetoelastic effects in ${\mathrm{Er}}_{5}{\mathrm{Si}}_{4}$ at low temperature. A clear hysteretic behavior of the magnetization isotherms and sharp first-order-type anomalies of magnetostriction are observed. Neutron powder diffraction experiments carried out in steady magnetic fields ranging from $0\phantom{\rule{0.3em}{0ex}}\text{to}\phantom{\rule{0.3em}{0ex}}50\phantom{\rule{0.3em}{0ex}}\mathrm{kOe}$ confirm that both the magnetization and magnetostriction anomalies are associated with a reversible magnetic-field-induced crystallographic transformation at low temperature. The low-temperature crystal structure of ${\mathrm{Er}}_{5}{\mathrm{Si}}_{4}$ is monoclinic in a zero field but the application of a strong magnetic field ($\mathrm{H}g80\phantom{\rule{0.3em}{0ex}}\mathrm{kOe}$ at $\mathrm{T}=5\phantom{\rule{0.3em}{0ex}}\mathrm{K}$) induces an orthorhombic phase, which is known as the ground state of numerous other compounds in the extended 5:4 family of materials. The existence of a magnetic-field-induced crystallographic transformation without the change of the magnetic state of the material is an unusual property of magnetic solids, and to our knowledge this is the first time that such an anomaly is observed in the 5:4 intermetallic materials.

Lattice Distortion in Antiferromagnetic CoO under High Magnetic Fields

Cobaltous monoxide, CoO, is an archetypal antiferromagnet, with a Neel temperature TN 1⁄4 290K, which has been studied for more than half a century. Tombs and Rooksby found that CoO exhibits a change from the high temperature NaCl type structure to a tetragonal one below room temperature. Greenwald and Smart pointed out that this structural change is related to the magnetic order. A detailed measurement showed that the lattice constant c contracts with decreasing temperature, T , up to about 1%. Later, the low temperature crystal structure was found to be monoclinic, rather than tetragonal. A lattice distortion associated with a magnetic ordering is called the magnetostriction. Magnetostriction depends on the direction of the magnetic moments and should be distinguished from an exchange striction which does not depend on the moment direction. The magnetic structure of CoO was determined by Shull et al. The structure is such that Co moments in a ð111Þ plane point in the same direction with an antiparallel arrangement of moments in the neighboring ð111Þ planes. Roth determined the direction of the magnetic moments from a detailed neutron diffraction measurement on a powder sample of CoO. The spin direction is parallel to 1⁄2 1 17 which makes an angle 11 300 with the 1⁄2001 axis. Kanamori studied theoretically the magnetic anisotropy and the magnetostriction in CoO. The essential point in the Kanamori’s theory is that the orbital degrees of freedom of Co2þ ion are not quenched in a cubic crystal field and magnetic ordering induces a lattice distortion along the spin direction through the spin–orbit interaction. From a microscopic calculation, Kanamori was successful in explaining the lattice contraction along the spin direction. Despite these extensive studies, the magnetism of CoO, in particular under applied magnetic fields, remains enigmatic. One reason for this is that the exchange interaction is very strong, judging from the Neel temperature, and a conventional magnetic field is insufficient strength to study the field induced phenomena in CoO. In this paper, we report the results of synchrotron X-ray diffraction measurements on a powder sample of CoO under strong magnetic fields. Using an X-ray diffractometer in conjunction with a pulsed magnetic field, we measured the magnetic field dependence of the lattice constants of CoO in the magnetically ordered phase. We find the lattice constants expand with magnetic field, H. Because magnetic ordering in CoO strongly influences the lattice, so we expect that an applied magnetic field will induce a lattice distortion. Nakamichi measured macroscopic deformation of a single crystal of CoO up to 1.1 T using a strain gauge and interpreted the results as due to a movement of the antiferromagnetic domain walls. The present X-ray diffraction measurement gives a direct evidence that the phenomenon takes place at an atomic level. The X-ray diffraction experiment was performed at the beamline BL19LXU at SPring-8. The experimental setup and the method are described in ref. 9. The pulsed magnetic field was applied perpendicular to the scattering plane. The magnetic field dependence of the d spacing at the ð002Þ and ð110Þ reflections obtained from the analysis of the diffraction data is shown in Figs. 1 and 2, respectively. Since the difference between the tetragonal and the monoclinic structures of CoO below TN is small, 4) we use the tetragonal structure to discuss our results. In the tetragonal structure, dð002Þ 1⁄4 c=2 and dð110Þ 1⁄4 a= ffiffiffi

Ionic Motion in Crystalline Cryolite

The character of the ion dynamics in crystalline cryolite, Na(3)AlF(6), a model double perovskite-structured mineral, has been examined in computer simulations using a polarizable ionic potential obtained by force-fitting to ab initio electronic structure calculations. NMR studies, and conductivity measurements, have indicated a high degree of mobility, in both Na(+) ion diffusion and reorientation of the AlF(6) octahedral units. The simulations reproduce the low-temperature (tilted) crystal structure and the existence of a transition to a cubic structure at elevated temperatures, in agreement with diffraction measurements, though the calculated transition temperature is too low. The reorientational dynamics of the AlF(6) octahedra is shown to consist of a hopping motion between the various tilted positions of the low-temperature form, even above the transition temperature. The rate of reorientation estimated by extrapolation to the temperature régime of the NMR measurements is consistent with the experimental data. In addition, we report a novel cooperative "tilt-swapping" motion of the differently tilted sublattices, just below the transition temperature. The perfect crystals show no Na(+) diffusion, in apparent disagreement with observation. We argue, following previous analyses of the cryolite phase diagram, that the diffusion observed in the experimental studies is a consequence of defects that are intrinsic to the thermodynamically stable form of cryolite. By introducing defects into the simulation cell, we obtain diffusion rates that are consistent with the NMR and conductivity measurements. Finally, we demonstrate a link between diffusion of the Na(+) ions and the reorientation of AlF(6) units, though the correlation between the two is not very strong.

High- and Low-Temperature Crystal and Magnetic Structures of ε-Fe2O3 and Their Correlation to Its Magnetic Properties

The crystal and magnetic structures of orthorhombic e-Fe2O3 have been studied by simultaneous Rietveld refinement of X-ray and neutron powder-diffraction data in combination with Mossbauer spectroscopy, as well as magnetization and heat-capacity measurements. It has been found that above 150 K, the e-Fe2O3 polymorph is a collinear ferrimagnet with magnetic moments directed along the a axis, whereas the magnetic ordering below 80 K is characterized by a square-wave incommensurate structure. The transformation between these two states is a second-order phase transition and involves subtle structural changes mostly affecting the coordination of the tetrahedral and one of the octahedral Fe sites. The temperature dependence of the e-Fe2O3 magnetic properties is discussed in light of these results.

Conformational Equilibrium of Cytochrome P450 BM-3 Complexed with N-Palmitoylglycine: A Replica Exchange Molecular Dynamics Study

UV-vis absorbance measurements and associated studies of cytochrome P450 BM-3 in complex with N-palmitoylglycine (NPG) indicate that a conformational change occurs in the active site of the complex where the terminal atoms of the ligand move from a site distant from the heme iron, as seen in the low temperature crystal structure to a site proximal to the heme iron at biological temperatures. We employ replica exchange molecular dynamics simulations to study this conformational change. The population of the proximal state is found to increase with temperature in agreement with UV-vis absorbance and NMR measurements. In addition to the conformations characterized by X-ray crystallography and computer modeling, this study shows that a new conformational state is significantly populated at room temperature. The observed increase in the population of conformations where the terminal atoms of NPG are proximal to the heme iron with increasing temperature indicates that the proximal state is stabilized by conformational entropy. A proposal for the origin of this entropic stabilization is provided on the basis of the structure of the newly identified state. We use the temperature weighted histogram (T-WHAM) method to characterize the transition state regions of the conformational ensemble and propose a mechanism of interconversion between these low free energy conformational states.

In Situ Cryo-Crystallization of Fluorinated Amines: A Comparative Study of Cooperative Intermolecular Interactions Involving Ordered and Disordered Fluorine

The low-temperature crystal structures of 4-fluoroaniline and 2-fluoroaniline, determined following in situ crystal growth from the liquid, reveal the importance of both strong hydrogen bonds and weak intermolecular interactions involving fluorine generating different packing motifs in the crystalline solid in the isomeric molecules.

The second orthorhombic polymorph of zwitterionic metanilic acid at 130 K

The low-temperature (130 K) crystal structure of the second ortho­rhom­bic polymorph of metanilic acid (3-amino­benzene­sulfonic acid), C6H7NO3S, shows the presence of two independent and conformationally similar mol­ecules of the zwitterionic acid (3-ammoniobenzenesulfonate) in the asymmetric unit; these inter­act through hydrogen bonding to give a cyclic R22(14) association. Peripheral hydrogen bonding gives a three-dimensional framework polymer, such as was found in the earlier polymorph [Hall & Maslen (1965), Acta Cryst. 18, 301–306], but the overall structural make-up is significantly different.

Structural systematics of the [Cu(chelate)3][Y]2series. An interesting crystallographic structural insight involving vibronic coupling and the Jahn–Teller effect (JTE). The syntheses and low temperature crystal structures of tris(2,2′bipyridyl)copper(ii) tetraphenylborate and tris(2,2′bipyridyl)zinc(ii) tetraphenylborate

The crystal structures of [Cu(bipy)(3)][BPh(4)](2), 1, and [Zn(bipy)(3)][BPh(4)](2), have been determined at low temperature. 1 and 2 are closely related, but are not isostructural. Both contain a two-dimensional supramolecular construct (SC) involving a sandwich structure. 1 has a six-coordinate CuN(6) chromophore with a regular elongated octahedral stereochemistry and rhombic in-plane bond lengths. The associated tetragonality value, T, of 1 is 0.8868. 2 involves a six-coordinate octahedral chromophore. Differences between 1 and 2 relate to the tendency of copper(II) complexes to undergo a Jahn-Teller (JT) distortion. The zinc(II) cation feels solely the host site strain, whereas the copper(II) cation also involves vibronic JT type coupling. The copper polyhedron geometry is characterized by both phenomena, with the vibronic interaction dominating. Scatter plot analysis involving the tris-chelate copper(II) series suggests that neither pure Q(theta) or Q(epsilon) components or the a(2u) mode operate in isolation over the entire series. All three operate in combination with varying quantifiable contributions, leading to distortion from the regular tetragonal octahedral stereochemistry.

Synthesis, a case of isostructural packing, and antimicrobial activity of silver(i)quinoxaline nitrate, silver(i)(2,5-dimethylpyrazine) nitrate and two related silver aminopyridine compounds

The synthesis and low temperature crystal structures of [Ag(quinoxaline)]n(NO3)n, 1, [Ag(2,5-dimethylpyrazine)(NO3)]n, 2 and [Ag4(3-aminopyridine)4(NO3)4]n 3 are presented. The quinoxaline compound forms a 1D coordination polymer with the characteristic linear 2-coordination figure of silver(I), the N-Ag-N angle being 164.2(1) degrees, and only weak silver-nitrate interactions. In addition there is an interaction giving pairs of parallel chains as the main structural theme. The 2,5-dimethylpyrazine compound has approximately trigonal-planar coordination, also binding one nitrate at the relatively short Ag-O distances 2.444(3) angstroms and 2.484(3) angstroms, respectively, for the two crystallographically different silver atoms. This also results in a 1D coordination polymer that, despite the large differences in the Ag(I) coordination environment, is isostructural with 1. [Ag4(3-aminopyridine)4(NO3)4]n 3 forms a 2D coordination polymer by bridging nitrate ions. The antimicrobial activity of 1-3, and also of [Ag3(2-aminopyridine)4](NO3)3, 4 was screened for 13 different pathogens and substantial activity was shown for 1 against Escherichia coli and Pseudomonas aeruginosa (MIC 4 microg cm(-3)) and somewhat lower activity was registered against Sarcina lutea and Salmonella typhi for 1, Bordetella bronchiseptica for 2, Salmonella typhi and Pseudomonas aeruginosa for 3, and Escherichia coli and Shigella sonnie for 3 (MIC 8 microg cm(-3)). Only low activity was shown against the yeast Candida albicans for 1, 2 and 4 whereas no activity against this pathogen was registered for 3.

Energy Ranking of Molecular Crystals Using Density Functional Theory Calculations and an Empirical van der Waals Correction

By combination of high level density functional theory (DFT) calculations with an empirical van der Waals correction, a hybrid method has been designed and parametrized that provides unprecedented accuracy for the structure optimization and the energy ranking of molecular crystals. All DFT calculations are carried out using the VASP program. The van der Waals correction is expressed as the sum over atom-atom pair potentials with each pair potential for two atoms A and B being the product of an asymptotic C(6,A,B)/r(6) term and a damping function d(A,B)(r). Empirical parameters are provided for the elements H, C, N, O, F, Cl, and S. Following Wu and Yang, the C(6) coefficients have been determined by least-squares fitting to molecular C(6) coefficients derived by Meath and co-workers from dipole oscillator strength distributions. The damping functions d(A,B)(r) guarantee the crossover from the asymptotic C(6,A,B)/r(6) behavior at large interatomic distances to a constant interaction energy at short distances. The careful parametrization of the damping functions is of crucial importance to obtain the correct balance between the DFT part of the lattice energy and the contribution from the empirical van der Waals correction. The damping functions have been adjusted to yield the best possible agreement between the unit cells of a set of experimental low temperature crystal structures and their counterparts obtained by lattice energy optimization using the hybrid method. On average, the experimental and the calculated unit cell lengths deviate by 1%. To assess the performance of the hybrid method with respect to the lattice energy ranking of molecular crystals, various crystal packings of ethane, ethylene, acetylene, methanol, acetic acid, and urea have been generated with Accelrys' Polymorph Predictor in a first step and optimized with the hybrid method in a second step. In five out of six cases, the experimentally observed low-temperature crystal structure corresponds to the most stable calculated structure.

The low-temperature and high-pressure crystal structures of cyclobutanol (C4H7OH)

The low-temperature and high-pressure crystal structures of cyclobutanol (C4H7OH) have been determined using single-crystal X-ray diffraction techniques. At temperatures below 220 K, cyclobutanol crystallizes in the Aba2 space group (Z' = 2) and its crystal structure is composed of pseudo-threefold hydrogen-bonded molecular catemers [assigned as C_2;2(4) in graph-set notation], which lie parallel to the crystallographic a axis. At a pressure of 1.3 GPa, the crystal symmetry changes to Pna2(1) (Z' = 1) and the molecular catemers [expressed as C2 in graph-set notation] adopt a pseudo-twofold arrangement. This structural behaviour is in agreement with our previous observations for phenol and its halogenated derivatives 2-chlorophenol and 4-fluorophenol, where pressure was found to favour a molecular packing more closely associated with small alkyl groups rather than that of relatively bulky alkyl groups. In addition, an examination of the molecular coordination environment in the low-temperature and high-pressure structures of cyclobutanol reveals that the change in structure on application of pressure appears to be driven by the molecules assuming a packing arrangement which more closely resembles that adopted in hard-sphere structures.

Synthesis and crystal structure of the phenyl-substituted diantimonide [formula omitted] in the ammoniate [Cs 6(PhSb 2) 2] · 11NH 3

The reaction of caesium and triphenylantimony in liquid ammonia in a molar ratio of 5:1 yields crystals of the ammoniate [Cs 6(PhSb 2) 2] · 11NH 3. A low-temperature single crystal X-ray structure analysis of the thermally extremely unstable compound shows it to contain PhSb 2 3 - anions, in which the aromatic substituent is attached to one atom of a single-bonded diantimonide dumbbell. Ionic interactions between the anion and the counter cations lead to the formation of two-dimensionally infinite layers, which lie parallel to the crystallographic ab-plane.

Diethylaminium 3,5-dinitrosalicylate at 130 K

The low-temperature (130 K) crystal structure of the 1:1 proton-transfer compound of 3,5-dinitro­salicylic acid (DNSA) with diethyl­amine, viz. diethyl­aminium 3,5-dinitro­salicylate, C4H12N+·C7H3N2O7−, shows the presence of conformationally extended diethyl­aminium cations in which both H atoms of the aminium centres participate in three-centre hydrogen-bonding associations [one R12(4) and the other R12(6)] with carbox­yl, phenol and nitro O-atom acceptors of two separate DNSA anion species [N⋯O = 2.813 (3)–3.320 (3) A]. These result in 12-membered cyclic R64(12) associations linking the two sets of hetero-species into discrete centrosymmetric tetra­mers which have no significant peripheral extensions.

Spin-State Tuning at Pseudotetrahedral d7Ions: Examining the Structural and Magnetic Phenomena of Four-Coordinate [BP3]CoII−X Systems

Electronic structure, spin-state, and geometrical relationships for a series of pseudotetrahedral Co(II) aryloxide, siloxide, arylthiolate, and silylthiolate complexes supported by the tris(phosphino)borate [BP(3)] ligands [PhBP(3)] and [PhBP(i)()(Pr)(3)] ([PhB(CH(2)PPh(2))(3)](-) and [PhB(CH(2)P(i)()Pr(2))(3)](-), respectively) are described. Standard (1)H NMR, optical, electrochemical, and solution magnetic data, in addition to low-temperature EPR and variable temperature SQUID magnetization data, are presented for the new cobalt(II) complexes [PhBP(3)]CoOSiPh(3) (2), [PhBP(3)]CoO(4-(t)()Bu-Ph) (3), [PhBP(3)]CoO(C(6)F(5)) (4), [PhBP(3)]CoSPh (5), [PhBP(3)]CoS(2,6-Me(2)-Ph) (6), [PhBP(3)]CoS(2,4,6-(i)()Pr(3)-Ph) (7), [PhBP(3)]CoS(2,4,6-(t)()Bu(3)-Ph) (8), [PhBP(3)]CoSSiPh(3) (9), [PhBP(3)]CoOSi(4-NMe(2)-Ph)(3) (10), [PhBP(3)]CoOSi(4-CF(3)-Ph)(3) (11), [PhBP(3)]CoOCPh(3) (12), [PhBP(i)()(Pr)(3)]CoOSiPh(3) (14), and [PhBP(i)()(Pr)(3)]CoSSiPh(3) (15). The low-temperature solid-state crystal structures of 2, 3, 5-10, 12, and 15 are also described. These pseudotetrahedral cobalt(II) complexes are classified as featuring one of two limiting distortions, either umbrella or off-axis. Magnetic and spectroscopic data demonstrate that both S = (1)/(2) and S = (3)/(2) ground-state electronic configurations are accessible for the umbrella distorted structure type, depending on the nature of the X-type ligand, its denticity (eta(1) versus eta(3)), and the tripodal phosphine ligand employed. Off-axis distorted complexes populate an S = (1)/(2) ground-state exclusively. For those four-coordinate complexes that populate S = (1)/(2) ground states, X-ray data show two Co-P bond distances that are invariably shorter than a third Co-P bond. The pseudotetrahedral siloxides 2, 10, and 11 are exceptional in that they display gradual spin crossover in the solid state. The diamagnetic cobalt(III) complex {[PhBP(3)]CoOSiPh(3)}{BAr(4)} ({16}{BAr(4)}) (Ar = Ph or 3,5-(CF(3))(2)-C(6)H(3)) has also been prepared and structurally characterized. Accompanying electronic structure calculations (DFT) for complexes 2, 6, and {16}(+) support the notion of a close electronic structure relationship between these four-coordinate systems and octahedral, sandwich, and half-sandwich coordination complexes.

Dimorphism in iron(II) methylphosphonate: Low-temperature crystal structure and temperature-dependent Mössbauer studies of a new form of the layered weak ferromagnet Fe[(CH 3PO 3)(H 2O)

A second form of the literature-known layered weak ferromagnet Fe[(CH 3PO 3)(H 2O)] has been isolated. The crystal structure determination of this new form ( 2) has been carried out at T = 300 , 200 and 130 K. It crystallizes in the orthorhombic space group Pmn2 1: a = 5.7177 ( 11 ) , b = 8.8093 ( 18 ) , c = 4.8154 ( 10 ) Å , while form ( 1) crystallizes in the space group Pna2 1: a = 17.58 ( 2 ) , b = 4.814 ( 1 ) , c = 5.719 ( 1 ) Å . Mössbauer spectroscopy on form ( 2) has been performed in the temperature range 4–300 K; and, at T ∼ 160 K , a drastic change in the quadrupole splitting (Δ E) and a broadening of the doublet components is noticed. But surprisingly, on cooling the crystal, no structural change is observed, which could account for the increase in Δ E . Below T = 25 K , 57Fe spectra transform into hyperfine splitting patterns which reveal a magnetically ordered state in agreement with the results of earlier magnetic susceptibility studies.

Temperature dependence of the crystal structure and charge ordering inYb4As3

Resonant and nonresonant x-ray scattering, combined with x-ray absorption data, are presented on ${\mathrm{Yb}}_{4}{\mathrm{As}}_{3}$. The nonresonant data allow a detailed determination of the low-temperature crystal structure and its evolution as a function of temperature. Bond-valence-sum calculations are performed and compared with theoretical predictions. Using the structural knowledge, the energy dependence, near the Yb ${\mathrm{L}}_{3}$ edge, of particular x-ray reflections are calculated and compared with experiment, and the temperature dependence of the charge order is extracted. A united picture of the temperature-dependent crystal structure and charge ordering in ${\mathrm{Yb}}_{4}{\mathrm{As}}_{3}$ emerges and is compared with theory.

Realizing Predicted Crystal Structures at Extreme Conditions: The Low-Temperature and High-Pressure Crystal Structures of 2-Chlorophenol and 4-Fluorophenol

A crystal of 2-chlorophenol was grown from the liquid at ambient pressure by laser-assisted zone refinement; 4-fluorophenol was crystallized from ethanol. Different polymorphs were obtained at high pressure by compression of the liquids in a Merrill-Bassett diamond-anvil cell (crystallization pressures 0.12 and 0.28 GPa, respectively). The structures of all phases are characterized by OH···OH hydrogen-bond formation. In the ambient-pressure polymorph of 2-chlorophenol, a hydrogen-bonded chain is formed about a 32 screw-axis; the ambient-pressure phase of 4-fluorophenol contains hexameric rings located on 3̄ sites. In crystallizing in high-symmetry space groups, these two compounds conform to typical behavior for bulky monoalcohols. By contrast, at high-pressure both compounds form zigzag chains disposed about 21 screw-axes, behavior more characteristic of small monoalcohols. The halophenol moiety thus behaves as a bulky group at ambient pressure but a small group at high pressure. We show that Crystal Structure Prediction methodologies reproduce all four phases, even though the potentials used were developed using ambient-pressure data. This is especially encouraging as the ambient-pressure phase of 2-chlorophenol contains three molecules in the asymmetric unit, while the high-pressure phase of 4-fluorophenol is disordered.

The Through-Bond Interaction of a Sulfur Lone Pair with Oxygenated Substituents in the Thiacyclohexane Framework

Low-temperature X-ray crystal structures were determined on a range of derivatives of 4-thiacyclohexanol 5a of varying electron demand with a view to finding evidence for a through-bond interaction between the sulfur lone pair and the oxygenated substituent. In contrast to earlier suggestions, plots of C–OR bond distance versus pKa (ROH) showed that any interaction between the sulfur and the OR group is unlikely to be of a through-bond origin. Furthermore, unimolecular solvolysis rate measurements on the nosylate ester derivative 5g showed that the sulfur actually retards the reaction slightly in comparison with the corresponding sulfur-free analogue 6.

Determining the σ-donor ability of the cyclopropane C–C bond

The low temperature crystal structures of ester and ether derivatives of varying electron demand, derived from cyclopropylmethanol and dicyclopropylmethanol , have been determined. These structures show a very strong response of the C-OR bond distance to the electron demand of the OR substituent, demonstrating the strong sigma-donor ability of the strained C-C bonds in the cyclopropane ring.