Crystal Packing Effects Research Articles

Solid state photochromism and thermochromism of two related N-salicylidene anilines

The crystalline structure and optical properties of N-salicylidene-p-cyanoaniline and N-salicylidene-p-carboxyaniline were investigated in solid state (microcrystalline powder), with the purpose to connect the effects of substitution and crystal packing with their optical properties. Diffuse reflectance and fluorescence spectroscopy were used to study the absorption and emission properties upon photoirradiation and cooling down to the liquid nitrogen temperature. The Stokes-shifted fluorescence, with a quantum yield of about 10−2–10−1, is given by the tautomeric cis-keto species formed as a result of the excited state intramolecular proton transfer from the initial enol structure. Despite their similar geometrical parameters, only SA-COOH is photochromic, which is in contradiction with literature. On the other side, the thermochromism is especially strong at SA-CN. The study shows the behavior of SA-CN similar to that of a classical anil, while SA-COOH presents interesting features that contradict the presumed behavior based only on its supramolecular structure.

X-ray crystallographic study of diversely substituted isatin derivatives

A series of isatin derivatives were synthesized and studied by XRD. For the first time the peculiarities of the molecule structure and crystal packing effects were analyzed. Indoline heterocycles are planar in all the investigated structures. The molecules bearing the sterically hindered substituents at nitrogen atom of indoline heterocycle have the maximum deviations of two oxygen atoms from the indoline plane. The substituent nature at nitrogen atom does not effect on the bond lengths of indoline moiety. The value of endocyclic bond angle at nitrogen not so strongly depends on the substituent and is almost equal to the one in unsubstituted isatin. In contrast to the unsubstituted isatin, where the system of classical hydrogen NH⋯O-bonds is realized, in crystals of investigated compounds there is no opportunity to reveal ones, and the crystal structure is stabilized by CH⋯O- and π–π-interactions.

Distortional Effects of Noncovalent Interactions in the Crystal Lattice of a Cp*Ir(III) Acylhydroxamic Acid Complex: A Joint Experimental–Computational Study

[Cp*Ir(μ-OH)3IrCp*]OH reacts with PhCONHOH to give [Cp*Ir(η2-ONCOPh)], in which the doubly deprotonated −NHOH unit binds side-on via N and O, an otherwise unrecorded binding mode. The X-ray structure shows pyramidalization at Ir together with secondary bonding between the carbonyl oxygen and Ir (dIr···O = 2.873(8) A). The related o-hydroxyphenylhydroxamic acid gives a conventional chelate structure in which both sp3 O atoms are bound in deprotonated form. In contrast, PhSO2NHOH reacts with S–N cleavage to give the nitrosyl, [Cp*Ir(NO)(SO2Ph)]. A detailed computational analysis identifies noncovalent interactions in the crystal lattice (crystal-packing effects) as responsible for the distortion in [Cp*Ir(η2-ONCOPh)].

NMR crystallography, diffraction and modeling of small organic molecules

Solid-state NMR (SSNMR) is a powerful atomic-level characterization technique able to study the local chemical environment of a nucleus in crystalline/amorphous solids. Toward a better understanding of how small molecules self-assemble in the solid-state and reorganizes to produce its hydrate/anhydrous forms, an experimental SSNMR, X-ray diffraction (XRD), and computational study of the supramolecular assemblies of selected small pharmaceuticals is presented. The effect of crystal packing on the 1H and 13C chemical shifts including nonconventional hydrogen bonds, pi···pi and CH···pi contacts, is studied through computer simulations. It will be shown that NMR chemical shifts are sensitive detectors of hydration/dehydration states in highly insoluble antibiotics.[1] Recently, SSNMR became an important gadget in the process of crystal structure solution in powders. This is a non-trivial task and using powder XRD methods alone may often lead to the wrong structure solution. In this talk, a new hybrid approach for structure determination of crystalline solids, will be presented, based on the combination of SSNMR, XRD and an ensemble of computational-assisted structure solution tools including a genetic algorithm based on evolution-inspired operators repeatedly applied to populations of possible crystal structure solutions that evolve to eventually produce the best new offspring candidates. Such methodologies are successfully applied to challenging cases involving multiple component crystals composed by flexible molecules such as a trihydrate β-lactamic antibiotic [2] and an azole-based co-crystal featuring an hydrogen bond network of α-helixes involving NH···N/CH···π intermolecular interactions. ACKNOLEDGEMENTS: Supported by Fundação para a Ciência e a Tecnologia (FCT), Portuguese National NMR Network (RNRMN), CICECO (PEst-C/CTM/LA0011/2013), FEDER, COMPETE, and University of Aveiro. FCT is greatly acknowledge for the consolidation grant IF/01401/2013.

Intermolecular Interactions and Second-Harmonic Generation Properties of (E)-1,5-Diarylpentenyn-1-ones

Investigation of crystal structures of eight quasi-planar (E)-1,5-diarylpentenyn-1-ones has been carried out. Five of these compounds crystallize in chiral space groups. Intermolecular interactions responsible for the loss of planarity and steric hindrances that prevent molecules to form centrosymmetric dimers were suggested to be the reasons of the crystallization in chiral groups. Intermolecular bonding has been investigated by means of the Hirshfeld surfaces, and the role of the C–H···O interactions as driving force for crystal structure formation has been demonstrated. Three types of C–H···O bonded synthons have been suggested for this family. Those synthones are found to be in accordance with the charge distribution along the conjugated system estimated with natural population analysis. Nonlinear optical (NLO) properties for five (E)-1,5-diarylpent-2-en-4-yn-1-ones crystallizing in chiral space groups have been investigated both theoretically and experimentally. It was shown that crystalline NLO susceptibility of the Br-derivative is comparable to such an NLO material as N-(4-nitrophenyl)-l-prolinol. The crystal packing effects on calculated NLO properties have been estimated based on the recently proposed Charge Model. Second-harmonic generation measurements of selected (E)-1,5-diarylpent-2-en-4-yn-1-ones confirmed a validity of the Charge Model and demonstrated an efficiency of this family for potential application as materials for nonlinear optics.

Second‐order Nonlinear Optical Susceptibilities and Refractive Indices of Organic Crystals from a Multiscale Numerical Simulation Approach

The performance of a simple multiscale approach to rapidly screen the second‐order nonlinear optical (NLO) susceptibilities [χ(2)] as well as the refractive indices of organic crystals solely from the knowledge of their crystal structure is demonstrated. The method combines first principles calculations of the molecular properties with a classical electrostatic interactions scheme to account for the crystal environment effects. The calculated values reproduce closely the measured χ(2) response for 10 organic crystals, including ionic crystalline salts, covering a χ(2) range from 10 to 600 pm V–1. The agreement between calculations and measurements is also excellent in the case of the refractive indices. Moreover, the method allows to analyze the relationship between the macroscopic responses [χ(2)] and the molecular first hyperpolarizabilities, highlighting the effects of crystal packing and chromophore density as well as the role played by the crystal dressing field.

Three different fluoro- or chloro-substituted 1′-deoxy-1′-phenyl-β-D-ribofuranoses

Crystal structures are reported for three fluoro- or chloro-substituted 1'-deoxy-1'-phenyl-β-D-ribofuranoses, namely 1'-deoxy-1'-(2,4,5-trifluorophenyl)-β-D-ribofuranose, C11H11F3O4, (I), 1'-deoxy-1'-(2,4,6-trifluorophenyl)-β-D-ribofuranose, C11H11F3O4, (II), and 1'-(4-chlorophenyl)-1'-deoxy-β-D-ribofuranose, C11H13ClO4, (III). The five-membered furanose ring of the three compounds has a conformation between a C2'-endo,C3'-exo twist and a C2'-endo envelope. The ribofuranose groups of (I) and (III) are connected by intermolecular O-H···O hydrogen bonds to six symmetry-related molecules to form double layers, while the ribofuranose group of (II) is connected by O-H···O hydrogen bonds to four symmetry-related molecules to form single layers. The O···O contact distance of the O-H···O hydrogen bonds ranges from 2.7172 (15) to 2.8895 (19) Å. Neighbouring double layers of (I) are connected by a very weak intermolecular C-F···π contact. The layers of (II) are connected by one C-H···O and two C-H···F contacts, while the double layers of (III) are connected by a C-H···Cl contact. The conformations of the molecules are compared with those of seven related molecules. The orientation of the benzene ring is coplanar with the H-C1' bond or bisecting the H-C1'-C2' angle, or intermediate between these positions. The orientation of the benzene ring is independent of the substitution pattern of the ring and depends mainly on crystal-packing effects.

Conformational analysis of dimethylbis(methyldithiocarbonato)stannum(IV) revisited: Application of cluster method, dispersion and counterpoise corrections

This investigation extends the previous determinations on the lowest energy conformation of dimethylbis(methyldithiocarbonato)stannum (IV) [Me2Sn(S2COMe)2]. In the previous investigations, calculations were performed only on single molecules, hence the crystal packing effects in the calculations were neglected. In this study, we performed systematic investigations on this compound by employing the molecular orbital cluster method. The largest cluster is an 11-molecule system. Methods from ab initio and density functional theory (DFT) were used, with empirical dispersion energy included to account for the intra- and intermolecular energy; and basis set superposition error (BSSE) is corrected with geometrical counterpoise scheme. The results showed that the neglect of crystal packing effects for 1- and 2-molecule clusters was unable to be rectified by the corrective energies, and we showed that the many-molecule cluster is needed to obtain a good agreement with the experimental results. Using the cluster method, our results showed agreement with the SS: SO conformation found in the solid state structure of Me2Sn(S2COMe).

Solid state effects on the electronic structure of H2OEP.

We present the results of a joint experimental and theoretical investigation concerning the effect of crystal packing on the electronic properties of the H2OEP molecule. Thin films, deposited in ultra high vacuum on metal surfaces, are investigated by combining valence band photoemission, inverse photoemission, and X-ray absorption spectroscopy. The spectra of the films are compared, when possible, with those measured in the gas phase. Once many-body effects are included in the calculations through the GW method, the electronic structure of H2OEP in the film and gas phase are accurately reproduced for both valence and conduction states. Upon going from an isolated molecule to the film phase, the electronic gap shrinks significantly and the lowest unoccupied molecular orbital (LUMO) and LUMO + 1 degeneracy is removed. The calculations show that the reduction of the transport gap in the film is entirely addressable to the enhancement of the electronic screening.

Reply to “Structural requirements for RNA degradation by HIV-1 reverse transcriptase”

In their correspondence, Das and colleagues 1 raise a number of issues about our work, which we address below. Firstly, we have not claimed that the RNA scissile phosphate is situated in the RNase H active site of HIV-1 RT ready for cleavage. Instead we reported 2 that our structures are “compatible with RNA degradation” (not “catalytically relevant” as incorrectly quoted by Das et al. 1), whereas all previous RT-nucleic acid (NA) complex structures are incompatible with RNA degradation. The incompatibility with RNA cleavage in the previous RT-NA complexes lies in the orientation of the NA substrate, not in the distance between NA and the RNase H active site, as we clearly demonstrated in Figure S5 2. In the previous RT-NA complexes, one DNA strand can be connected with the RNA/DNA hybrid positioned for RNA cleavage (Fig. 1a) according to the human and bacterial RNase H1-RNA/DNA hybrid structure 3,4, but the second strand (RNA equivalent) cannot be connected because of a 14 A gap. This gap cannot be closed by any amount of bending or unwinding of the duplex 3. In the three RT-RNA/DNA-NNRTI complex structures we reported 2, the RNA/DNA hybrids are oriented such that there is no long a gap (Fig. 1b), and a slight adjustment of the RNA strand would permit hydrolysis as illustrated in Figures 5 2. Indeed, the nearest RNA phosphate in our structures is 8.8 A from the active site carboxylate as depicted in Figure 5 2. We should clarify that the distance between the scissile phosphate and the active site in the human RNase H1-RNA/DNA complex is 4.6A (PDB: 2Q39) 3. Thus the scissile phosphate in the 4B3O structure needs to move 4.2A to be positioned for cleavage. It is not the distance but rather the NA orientation that makes our RNA/DNA hybrid compatible with RNA degradation, which is also evident in Fig. 1a of the correspondence by Das and colleague 1. Figure 1 Differences between previous RT-NA complexes and our RT-RNA/DNA structures. (a) All RT-NA structrues previously reported are similar 2 and represented here by the RT-DNA-dATP ternary complex (PDB:3KK2 6) in the polymerization mode. Human RNase H1-RNA/DNA structure ... Secondly, we disagree with the statement that our structure (4B3O) is most closely related to the RT-DNA/DNA-nevirapine structure (PDB 3V8I)5. Both the protein and nucleic acid of the 3V8I structure are more similar to that of RT-DNA complexes than to our RT-RNA/DNA hybrid complexes, as we showed in Figure 3 2. For example, the RNA/DNA hybrid in our structure has the A-form conformation, while the DNA duplex in 3V8I as well as all previously reported RT-NA complexes are largely B-form 2 (Fig. 1). Thirdly, with regard to the alleged crystal packing effects, we had noticed the lattice contact all along but found it irrelevant to RT-RNA/DNA hybrid complex formation, based on three different crystal forms of RT-RNA/DNA hybrid complexes (Figures 1 and S1 2). The region near the p66 thumb domain, with which Das and colleagues are concerned 1, is not involved in crystal packing in at least one of our three crystal structures 2, and the A-form conformation of our entire RNA/DNA is independent of this lattice contact. Fourthly, on the issue of the nick in the RNA strand used in one of the three RT-RNA/DNA complex structures presented in our paper, the nick was engineered by design as reported in the main text and Methods section, and depicted in Figures 1 and S1 2. However, the other two RT-RNA/DNA complex structures (PDB: 4B3P and 4B3Q) contain a continuous RNA strand 2. The statement by Das and colleagues that a continuous RNA/DNA duplex would not be able to adopt the conformation or trajectory adopted by the nicked hybrid 1 is simply incorrect. The RNA/DNA hybrids in all three of our structures, including two with a continuous RNA strand are in similar conformations (Figures 1d and S3 2). Finally, regarding cross-linking between RT and nucleic acid in previous RT complexes, we have not questioned the validity of this strategy in capturing HIV-1 RT in the DNA polymerization mode. Rather, we respectfully pointed out that in the more than 20 RT-DNA crosslinked structures, the DNAs are all in a similar conformation, and one that is incompatible with RNA degradation.

Packing interface energetics in different crystal forms of the λ Cro dimer

Variation among crystal structures of the λ Cro dimer highlights conformational flexibility. The structures range from a wild type closed to a mutant fully open conformation, but it is unclear if each represents a stable solution state or if one may be the result of crystal packing. Here we use molecular dynamics (MD) simulation to investigate the energetics of crystal packing interfaces and the influence of site-directed mutagenesis on them in order to examine the effect of crystal packing on wild type and mutant Cro dimer conformation. Replica exchange MD of mutant Cro in solution shows that the observed conformational differences between the wild type and mutant protein are not the direct consequence of mutation. Instead, simulation of Cro in different crystal environments reveals that mutation affects the stability of crystal forms. Molecular Mechanics Poisson-Boltzmann Surface Area binding energy calculations reveal the detailed energetics of packing interfaces. Packing interfaces can have diverse properties in strength, energetic components, and some are stronger than the biological dimer interface. Further analysis shows that mutation can strengthen packing interfaces by as much as ∼5 kcal/mol in either crystal environment. Thus, in the case of Cro, mutation provides an additional energetic contribution during crystal formation that may stabilize a fully open higher energy state. Moreover, the effect of mutation in the lattice can extend to packing interfaces not involving mutation sites. Our results provide insight into possible models for the effect of crystallization on Cro conformational dynamics and emphasize careful consideration of protein crystal structures.

Supramolecular step in design of nonlinear optical materials: Effect of π…π stacking aggregation on hyperpolarizability

Theoretical estimation of nonlinear optical (NLO) properties is an important step in systematic search for optoelectronic materials. Density functional theory methods are often used to predict first molecular hyperpolarizability for compounds in advance of their synthesis. However, design of molecular NLO materials require an estimation of the bulk properties, which are often approximated as additive superposition of molecular tensors. It is therefore important to evaluate the accuracy of this additive approximation and estimate the extent by which intermolecular interactions influence the first molecular hyperpolarizability β. Here we focused on the stacking aggregates, including up to 12 model molecules (pNA and ANS) and observed enhancement and suppression of molecular hyperpolarizability relative to the additive sum. We found that degree of nonadditivity depends on relative orientation of the molecular dipole moments and does not correlate with intermolecular interaction energy. Frenkel exciton model, based on dipole-dipole approximation can be used for qualitative prediction of intermolecular effects. We report on inaccuracy of this model for the molecules with long π-systems that are significantly shifted relative to each other, when dipole-dipole approximation becomes inaccurate. To obtain more detailed information on the effect of intermolecular interactions on β we proposed electrostatic approach which accounts for the mutual polarization of the molecules by each other. We measure the induced polarization of each molecule in the aggregate by the charge of its donor (or acceptor) group. The proposed approach demonstrates linear correlation β(FF) vs β(elm) (estimated by finite field theory and electrostatic model, respectively) and allows decomposition of the hyperpolarizability for a molecular aggregate into separate molecular contributions. We used this decomposition to analyze the reasons of deviation of aggregate β from additivity, as well as the cooperative effect of intermolecular interactions on hyperpolarizability for stacks of growing size. In cases of positive cooperativity (enhancement), we found 6-8 molecules to be necessary to reach the asymptotic limit. In more frequent cases of negative cooperativity two opposite factors play role. The first one consists of direct lowering of β due to repulsive dipole-dipole interactions. The second factor is originated in a decrease of molecular dipole moments, which in turn leads to a decrease of dipole-dipole repulsion, and therefore increases β. For strong intermolecular repulsive dipole-dipole interactions these effects nearly cancel each other. In such cases the trimers and even dimers are sufficient to reach the asymptotic limit of the infinite stacks. Based on the observed trends we estimated non-additive correction to β for well known NLO crystals NPAN and MNMA. In the case of NPAN, stacking effect on molecular hyperpolarizability represents the leading component of the crystal packing effect and improves the agreement between calculated and experimental data which is further improved when frequency dependence is taken in account.

Theoretical interpretations of electronic and fluorescence spectra of new 2(1H)-pyridone derivatives in solution and solid state

A combined experimental and computational study was performed for the spectroscopic properties of novel 2(1H)-pyridones. The compounds were found to be virtually non-fluorescence in solution while modestly fluorescent in solid state. The solvent effects on the UV–vis and fluorescence maxima were estimated by means of a series of ab-initio quantum chemical calculations in conjunction with Polarizable Continuum Model (PCM) method. Influence of structural displacements and intermolecular interactions in crystalline state were examined in details on the spectra of two representative compounds by using Fragment Molecular Orbital (FMO) scheme. The FMO pair interaction analysis of the spectra indicate that (1) intermolecular hydrogen bonds provoke bathochromic shifts (2) electrostatic interactions induce hypsochromic shifts (3) crystal packing effects induce hypsochromic shifts in total from the maxima in vacuo.

Hingeless negative linear compression in the mechanochromic gold complex [(C6F5Au)2(μ-1,4-diisocyanobenzene)

The compression of a crystalline material under hydrostatic pressure always results in a reduction in volume of the solid, with the observed reduction typically occurring in all crystallographic cell parameters.1 However, there are a number of exceptional materials that have been shown to expand in a specific direction upon hydrostatic compression while maintaining a positive volume compression. The phenomenon of uni-or biaxial expansion under compression is known as negative linear compressibility (NLC).1 Rare examples of NLC have attracted attention recently owing to the potential applications in a range of materials, including body armor, artificial muscle actuators, and pressure sensors.2

Crystal Structure of a Human IκB Kinase β Asymmetric Dimer

Phosphorylation of inhibitor of nuclear transcription factor κB (IκB) by IκB kinase (IKK) triggers the degradation of IκB and migration of cytoplasmic κB to the nucleus where it promotes the transcription of its target genes. Activation of IKK is achieved by phosphorylation of its main subunit, IKKβ, at the activation loop sites. Here, we report the 2.8 Å resolution crystal structure of human IKKβ (hIKKβ), which is partially phosphorylated and bound to the staurosporine analog K252a. The hIKKβ protomer adopts a trimodular structure that closely resembles that from Xenopus laevis (xIKKβ): an N-terminal kinase domain (KD), a central ubiquitin-like domain (ULD), and a C-terminal scaffold/dimerization domain (SDD). Although hIKKβ and xIKKβ utilize a similar dimerization mode, their overall geometries are distinct. In contrast to the structure resembling closed shears reported previously for xIKKβ, hIKKβ exists as an open asymmetric dimer in which the two KDs are further apart, with one in an active and the other in an inactive conformation. Dimer interactions are limited to the C-terminal six-helix bundle that acts as a hinge between the two subunits. The observed domain movements in the structures of IKKβ may represent trans-phosphorylation steps that accompany IKKβ activation.

Solution structures of Mycobacterium tuberculosis thioredoxin C and models of intact thioredoxin system suggest new approaches to inhibitor and drug design.

Here, we report the NMR solution structures of Mycobacterium tuberculosis (M. tuberculosis) thioredoxin C in both oxidized and reduced states, with discussion of structural changes that occur in going between redox states. The NMR solution structure of the oxidized TrxC corresponds closely to that of the crystal structure, except in the C-terminal region. It appears that crystal packing effects have caused an artifactual shift in the α4 helix in the previously reported crystal structure, compared with the solution structure. On the basis of these TrxC structures, chemical shift mapping, a previously reported crystal structure of the M. tuberculosis thioredoxin reductase (not bound to a Trx) and structures for intermediates in the E. coli thioredoxin catalytic cycle, we have modeled the complete M. tuberculosis thioredoxin system for the various steps in the catalytic cycle. These structures and models reveal pockets at the TrxR/TrxC interface in various steps in the catalytic cycle, which can be targeted in the design of uncompetitive inhibitors as potential anti-mycobacterial agents, or as chemical genetic probes of function.

Relationship between crystal structure and mechanical properties of ranitidine hydrochloride polymorphs

Polymorphism plays a critical role during pharmaceutical development, as it helps in the selection of optimal solid form. In present study, mechanical properties of ranitidine hydrochloride polymorphs were studied using instrumented tablet press, to understand the effect of crystal packing on the compaction behaviour. Out-of-die compressibility plot and Heckel analysis confirmed greater plastic deformation of form II over form I. Detailed crystallographic examination revealed that form I has several weak C–H⋯O interactions across the ‘proposed slip plane’ parallel to (−2 0 2) that prevent slip under compaction pressure. On the other hand, crystal structure of form II was relatively more open and multiple slip were possible under compaction pressure. These crystallographic features offered increased compressibility and deformability to form II. In absence of an active slip plane system, closed crystal structure of form I resists deformation under compaction pressure and hence showed poor compressibility and higher mean yield pressure. However, form I showed greater tabletability at a given compaction pressure, by virtue of its greater bonding strength.

Comparison of the Crystal and Solution Structures of a Cyanobacterial Hemoglobin

GlbN is a monomeric 2/2 hemoglobin found in several cyanobacteria. Physiological data suggest that, in Synechococcus sp. PCC 7002, GlbN protects the cell from damage associated with nitrate metabolism [1]. Under reducing conditions, GlbN undergoes a facile posttranslational cross-linking between His117 and the heme 2-vinyl. In a study of the enzymatic and chemical properties of GlbN, the solution structure of the protein in the ferric, bis-histidine state with heme posttranslational modification was determined by NMR methods. As this protein is paramagnetic (S = 1/2) and can be prepared in the reduced state (S = 0), heme methyl hyperfine chemical shifts were used to determine the orientation of the ligating histidines, and proton pseudocontact shifts were applied in the refinement. The NMR model contains regions that appear to sample multiple conformations, specifically the A helix and EF loop. Here we present the X-ray structure of GlbN. Crystals were obtained by the vapor diffusion hanging drop method, and data were collected under cryogenic conditions (National Synchrotron Light Source, BNL). The structure of the related Synechocystis sp. PCC 6803 GlbN (1RTX) was used as the search model for molecular replacement. Iterative model building yielded a final structure at 1.65 A resolution. The X-ray data revealed a ruffled heme, provided a reliable representation of its environment in solution, and allowed for an assessment of the paramagnetic NMR approach. The X-ray data also gave an indication of the effect of crystal packing on the flexible regions of GlbN. Comparison with the Synechocystis GlbN structure highlighted the resilience of the 2/2 globin fold as well as subtle differences in the solvent accessibility of the heme active site.[1] Scott et al. (2010) Biochemistry 49:7000Supported by NSF MCB 0843439

Unusual Saddle‐like Structure of (2‐MeOC6H4CS)2S: Theoretical Studies and Comparison with its Oxygen Isologues

AbstractCompound (2‐MeOC6H4CS)2S (1) showed an unusual saddle structure in which the –C(=S)–S–C(=S)– moiety is planar and two benzene rings lie a face‐to‐face and the distances (3.07 Å) between the central sulfur atom and two 2‐methoxy oxygen atoms are within the sum of van der Waals radii of the both atoms. However, the results of MO calculation at the B3LYP/6‐311+G(2d, p) level showed no orbital interaction between both atoms. From the results of the calculations at the MP2 level, it was deduced that the crystal packing effect is important for such densely packed crystal, due to its less effective volume. On the other hand, the para‐methoxy derivative (4‐MeOC6H4CS)2S (2) in which the two methoxy oxygen atoms are not able to contact with the central sulfur atom shows an L‐shaped structure in which the –C(=S)–S–C(=S)– moiety is not planar. The –C(=O)–S–C(=S)– moiety in compound (2‐MeOC6H4CO)(2‐MeOC6H4CS)S (3b) shows an L‐shaped structure, though the two methoxy oxygen atoms are in intramolecular contact with the central sulfur atom. The deep blue to green colors of compounds 1 and 2 and the deep violet color of compound 3b are due to transitions of the lone‐pair electrons in the HOMO (ψ87) of the thiocarbonyl sulfur atom to the LUMO (ψ89)and of those in the HOMO (ψ83) of the thiocarbonyl sulfur atom to the LUMO (ψ85), respectively.

Reversible crystal-to-crystal phase transition of a 4,4′-bipyridine-linked dinuclear copper(II) complex

The crystal-to-crystal phase transition of the previously reported dinuclear C i-symmetric complex [{Cu(NO3)2(phen)}2(4,4′-bipy)] (1) (phen = 1,10-phenanthroline; 4,4′-bipy = 4,4′-bipyridine) [Seidel et al. (2011) Z Anorg 637:1545–1554, 10] was studied in detail by differential scanning calorimetry (DSC), powder X-ray diffraction and variable temperature determinations of the unit cell parameters on a single-crystal. A density functional theory (DFT) study was undertaken to elucidate effects of crystal packing on the molecular structure in the solid-state. The DFT study confirmed that the molecular structures of 1 found in the solid-state do not represent the minimum energy conformation of the free molecule, especially with respect to the twist of the 4,4′-bipy bridging ligand. The DSC analysis revealed that the phase transition is a fully reversible process, and suggests that the relationship between the dimorphic forms of 1 is enantiotropic.