Crystal Packing Forces Research Articles

Excimer Formation in a Terbium Metal–Organic Framework Assists Luminescence Thermometry

Photoluminescent isotypical phosphonate-based metal–organic frameworks, [Ln(H5btp)]·2H2O [where Ln3+ = Tb3+ (1) or Gd3+ (2) and H8btp = [1,1′-biphenyl]-3,3′,5,5′-tetrayltetrakis(phosphonic acid)] prepared under solvo(hydro)thermal conditions exhibit a three-dimensional supramolecular structure built up from phosphonate linkers bridging Ln3+ ions. Single-crystal X-ray diffraction of 1 revealed hydrogen bonds along the a-axis between the crystallization water molecules and the phosphonate groups. The network features lozenge-shaped tubular channels with intermolecular interactions between adjacent organic ligand molecules. Crystal packing forces bring together two ligand biphenyl groups in a parallel arrangement, forming an excimer. A ratiometric luminescent thermometer operative close to room temperature was built based on the excimer and the 5D4 → 7F5 Tb3+ emissions, exhibiting a maximum relative thermal sensitivity of 1.26%·K–1 and a minimum temperature uncertainty of 0.75 K at 319 K. This performance is...

Charge-transport properties of 4-(1,2,2-tri-phenyl-vinyl)-aniline salicylaldehyde hydrazone: tight-packing induced molecular 'hardening'.

Based on first-principles calculations, the relationship between molecular packing and charge-transport parameters has been investigated and analysed in detail. It is found that the crystal packing forces in the flexible organic molecule 4-(1,2,2-triphenylvinyl)-aniline salicylaldehyde hydrazone (A) can apparently overcome the dynamic intramolecular rotations and the intramolecular steric repulsion, effectively enhancing the molecular rigidity and decreasing the internal reorganization energy. The conducting properties of A have also been simulated within the framework of hopping models, and the calculation results show that the intrinsic electron mobility in A is much higher than the corresponding intrinsic hole mobility. These theoretical investigations provide guidance for the efficient and targeted control of the molecular packing and charge-transport properties of organic small-molecule semiconductors and conjugated polymeric materials.

Crystal-Packing-Driven Enrichment of Atropoisomers.

Crystal-packing forces can have a significant impact on the relative stabilities of different molecules and their conformations. The magnitude of such effects is, however, not yet well understood. Herein we show, that crystal packing can completely overrule the relative stabilities of different stereoisomers in solution. Heating of atropoisomers (i.e. "frozen-out" conformational isomers) in solution leads to complex mixtures. In contrast, solid-state heating selectively amplifies minor (<25 mole %) components of these solution-phase mixtures. We show that this heating strategy is successful for compounds with up to four rotationally hindered σ bonds, for which a single stereoisomer out of seven can be amplified selectively. Our results demonstrate that common supramolecular interactions-for example, [methyl⋅⋅⋅π] coordination and [C-H⋅⋅⋅O] hydrogen bonding-can readily invert the relative thermodynamic stabilities of different molecular conformations. These findings open up potential new avenues to control the folding of macromolecules.

Different Antiferromagnetic Coupling between 5,5′‐ and 10,10′‐Linked Iron(III) Corrole Dimers

Two FeIII corroles were connected through a single meso–meso linkage at different positions, that is, 5,5′‐linkage and 10,10′‐linkage, which led to the observations of position‐dependent antiferromagnetic interactions between the two iron(III) corrole units. The iron(III) centers were found to exhibit an intermediate‐spin state, S = 3/2, under ambient conditions on the basis of 1H NMR spectra, Curie plots, Mössbauer spectroscopy, and super‐conducting quantum interference device (SQUID) measurements. In previous reports, the X‐ray structure of dipyridine tris(pentafluorophenyl)corrolato iron(III) was assigned as a six‐coordinate species, but the 10,10′‐linked and 5,5′‐linked dimers preferred five‐coordinate geometry in the solid state. In addition, a mixed five‐coordinate/six‐coordinate dimer was also obtained, implying a susceptibility of the coordination mode to the crystal packing force. By summarizing the results, it can be concluded that pyridinate–iron(III) tris(pentafluorophenyl)corroles can be better described as five‐coordinate, intermediate‐spin (S = 3/2) species under ambient conditions in toluene, whereas a dominant contribution of six‐coordinate low‐spin (S = 1/2) iron(III) complex appears when coordinated with stronger axial ligands such as imidazole at low temperature. Complementary to these interpretations, the 10,10′‐linked dimer exhibited a larger antiferromagnetic interaction between the two iron(III) corroles (J = –2.04 cm–1) than that of the 5,5′‐dimer (J = –0.64 cm–1).

Rational Design of Heterogeneous Silver Catalysts by Exploitation of Counteranion‐Induced Coordination Geometry

AbstractThe coordination geometry of silver complexes of the green fluorescence protein chromophore is highly diverse and dependent on the size and shape of the counteranion in the complexes. Only complexes that have vacancies at the silver atom prove to be heterogeneous catalysts in the “green” synthesis of 1,4‐benzodioxines. The results reveal an interesting strategy of exploiting crystal packing forces to enhance catalytic efficiency by just changing the counteranion.

Crystal structures of meso-tetrakis(2′,6′/3′,5′-difluorophenyl)porphyrins and their metal complexes: Influence of position of the fluoro groups on their structural properties

Two series of non-solvated structures of 5,10,15,20-tetrakis(2′,6′/3′,5′-difluorophenyl)porphyrins, H2T(2′,6′/3′,5′-DFP)P and their metal (Cu(II) and Zn(II)) complexes were determined by single crystal XRD. H2(2′,6′-DFP)P, 1 and its metal (Cu(II), 2 and Zn(II), 3) complexes showed near planar geometry of the porphyrin ring with the maximum root-mean-square (r.m.s.) which is the average deviation of the 24 core atoms from their least-squares plane by 0.056(3) Å while MT(3′,5′-DFP)P (Cu(II), 4 and Zn(II), 5) structures feature significant distortion of the macrocycle (r.m.s. = 0.22(2) Å). Nearly planar structures 1-3 revealed more number of intermolecular short contacts than that observed in nonplanar structures 4 and 5. Normal coordinate structure decomposition analysis of the structures 1-3 revealed negligible wave distortion of the macrocycle but the corresponding nonplanar 4 and 5 structures feature mainly ruffled combined with minimal saddled distortions. The increased nonplanarity of the less sterically hindered derivatives 4 and 5 has been ascribed to intermolecular interactions and/or crystal packing forces. Two series of non-solvated 5,10,15,20-tetrakis(2′,6′/3′,5′-difluorophenyl)porphyrins, H2T(2′,6′/3′,5′-DFP)P and their (Cu(II) and Zn(II)) complexes have been examined by single crystal XRD and they showed near planarity of the macrocycle in the MT(2′,6′-DFP)Ps and enhanced nonplanar distortions in the MT(3′,5′-DFP)Ps structures.

A family of heterotetrameric clusters of chloride species and halomethanes held by two halogen and two hydrogen bonds

Two previously reported 1,3,5,7,9-pentaazanona-1,3,6,8-tetraenate (PANT) chloride platinum(II) complexes [PtCl{HNC(R)NCN[C(Ph)C(Ph)]CNC(R)NH}] (R = tBu 1, Ph 2) form solvates with halomethanes 1·1¼CH2Cl2, 1·1⅖CH2Br2, and 2·CHCl3. All these species feature novel complex-solvent heterotetrameric clusters, where the structural units are linked simultaneously by two C–X⋯Cl–Pt (X = Cl, Br) halogen and two C–H⋯Cl–Pt hydrogen bonds. The geometric parameters of these weak interactions were determined using single-crystal XRD, and the natures of the XBs and HBs in the clusters were studied for the isolated model systems (1)2·(CH2Cl2)2, (1)2·(CH2Br2)2, and (2)2·(CHCl3)2 using DFT calculations and Bader's AIM analysis. The evaluated energies of the weak interactions are in the range 0.9–3.0 kcal mol−1. The XBs and HBs in the reported clusters are cooperative. In the cases of (1)2·(CH2Cl2)2 and (1)2·(CH2Br2)2, the contribution of the HBs to the stabilization of the system is dominant, whereas for (2)2·(CHCl3)2 contributions of both types of the non-covalent interactions are almost the same. Crystal packing and other forces such as, e.g. dipole–dipole interactions, also affect the formation of the clusters.

Copper, indium, tin, and lead complexes with fluorinated selenolate ligands: precursors to MSex.

Reductive cleavage of C6F5SeSeC6F5 with elemental M (M = Cu, In, Sn, Pb) in pyridine results in the formation of (py)4Cu2(SeC6F5)2, (py)2In(SeC6F5)3, (py)2Sn(SeC6F5)2, and (py)2Pb(SeC6F5)2. Each group adopts a unique structure: the Cu(I) compound crystallizes as a dimer with a pair of bridging selenolates, two pyridine ligands coordinating to each Cu(I) ion, and a short Cu(I)-Cu(I) distance (2.595 Å). The indium compound crystallizes as monometallic five-coordinate (py)2In(SeC6F5)3 in a geometry that approximates a trigonal bipyramidal structure with two axial pyridine ligands and three selenolates. The tin and lead derivatives (py)2M(SeC6F5)2 are also monomeric, but they adopt nearly octahedral geometries with trans pyridine ligands, a pair of cis-selenolates, and two "empty" cis-positions on the octahedron that are oriented toward extremely remote selenolates (M-Se = 3.79 Å (Sn), 3.70 Å (Pb)) from adjacent molecules. Two of the four compounds (Cu, In) exhibit intermolecular π-π stacking arrangements in the solid state, whereas the stacking of molecules for the other two compounds (Sn, Pb) appears to be based upon molecular shape and crystal packing forces. All compounds are volatile and decompose at elevated temperatures to give MSex and Se(C6F5)2.The electronic structures of the dimeric Cu compound and monomeric (py)2M(SeC6F5)2 (M = Sn, Pb) were examined with density functional theory calculations.

Investigating the flap dynamics of HIV‐1 Protease

The potency of the HIV virus arises from the unknowing spread of the disease. Within the United States, it is estimated that one in four people newly infected with HIV is between the ages of thirteen and twenty‐four. (1) The HIV protease is an essential enzyme that cleaves three previral polyproteins leading to the maturation of the virus. The HIV virus mutates at a rapid pace, resulting in the emergence of HIV resistance to protease inhibitors. In 2003, it was suggested by Wu et al. that the flap tips of the HIV protease control enzyme activity. (2) Although the HIV‐1 protease flap dynamics have been studied extensively using both experimental and computational methods, rarely have all four flap tip conformations been observed in a single computer simulation. In addition, it has been suggested that one of the observed conformations was an artifact of crystal packing forces. (3) By performing long‐time scale simulations with two different force fields (CHARMM27 and AMBERff99SB) we were able to observe the existence of all four conformational states: closed, semi‐open, open, and wide‐open states. The lifetimes and percent occupancies of the conformations were determined through energy and structural analyses. Further analyses were performed to quantify stability and sampling of all simulations. These results will be presented and comparisons made with experimental results.(1) AIDS.gov 2013.(2) Wu, T. D.; Schiffer, C. A.; Gonzales, M. J.; Taylor, J.; Kantor, R.; Chou, S.; Israelski, D.; Zolopa, A. R.; Fessel, W. J.; Shafer, R. W. J Virol 2003, 77, 4836.(3) Layten, M.; Hornak, V.; Simmerling, C. J. Am. Chem. Soc. 2006, 128, 13360.

The Power of Nonconventional Phenyl C–H···N Hydrogen Bonds: Supportive Crystal-Packing Force and Dominant Supramolecular Engineering Force

The role of phenyl C–H···N interactions in crystal engineering is explored with a variety of fluorinated phenyl-containing compounds. In particular, we show that this interaction can guide the formation of one-dimensional phenyl C–H···N hydrogen-bonded ribbons with, for example, 4-(2,3,5,6-tetrafluorophenylethynyl)pyridine. The interaction is shown to also control the formation of self-complementary homodimers with 3-(2,3,4,5-tetrafluorophenylethynyl)pyridine. We also demonstrate that the phenyl C–H···N hydrogen bond interaction is capable of enticing co-crystallization of molecules such as 2,3,5,6,2′,3′,5′,6′-octafluorobiphenyl and 4,4′-dipyridyl. Finally, we describe the use of an intramolecular scaffold to evaluate the effect of electron-withdrawing substituents on the strength of a phenyl C–H···N hydrogen bond.

Backbone resonance assignments of the 54kDa dimeric C-terminal domain of murine STING in complex with DMXAA.

The mammalian ER protein STING (stimulators of interferon genes) is an important innate immunity proteinfor linking detection of novel secondary messengers c-di-GMP, c-di-AMP, cGAMP or cytosolic dsDNA to the activation of TANK kinase 1 and its downstream interferon regulator factor 3. Recently quite a few of crystal structures representing different states of the C-terminal domain (CTD) of human and murine STING (hSTING and mSTING) in complex with c-di-GMP, cGAMP or DMXAA have been reported. However, the C-terminal 42 residues of STING-CTD, which may be important in mediating the downstream reactions, is invisible or absent in all reported X-ray structures. In addition, X-ray crystal structures may be subject to crystal packing force. Hence an alternate method of determining the structure and function of STING in a near physiological condition is essential. We now report the near complete backbone resonance assignments of the 54kDa dimeric mSTING-CTD in complex with DMXAA, which is the first step in determining its complex structure and understanding why DMXAA, which is a very efficient agent for curing mouse cancer, is totally ineffective in human.

Switching the Macrocycle Conformation from Nonplanar to Planar in Cobalt(II) and Copper(II) β‐Tetra‐2′‐thienyl‐meso‐tetraphenylporphyrin Cocrystallates with C60

AbstractCrystal structures of cobalt(II) and copper(II) complexes of 2,3,12,13‐tetra‐2′‐thienyl‐5,10,15,20‐tetraphenylporphyrin/C60 cocrystallates were determined by single‐crystal XRD analysis. The crystal structures of the two cocrystallates, MTPP(2′‐thienyl)4·C60·(toluene)2 (M = CoII and CuII; TPP = tetraphenylporphyrin), compounds 1 and 2, respectively, are isomorphous and reveal a one‐dimensional (1D) supramolecular structure. In these cocrystallates, the macrocycle is almost planar within ±0.08 Å, as evidenced by comparing the value of ΔCb (the average deviation of the β‐pyrrole carbon from the porphyrin ring mean plane) with those for the nonplanar parent structures, CoTPP(2′‐thienyl)4 [±0.710(5) Å] and CuTPP(2′‐thienyl)4 [±0.546(2) Å]. This difference has been ascribed to intermolecular interactions and/or crystal packing forces.

Metal-binding studies of linear rigid-axle [2]pseudorotaxanes with in situ generated anionic metal halide complexes

A systematic study of metal–pseudorotaxane binding, using the linear N,N′-bis(4-pyridyl)-4,4′-bipyridinium axle (PyBP2+) and aromatic crown ethers bis(1,5-naphtho)-32-crown-8 (BN32C8), bis(1,5-naphtho)-38-crown-10 (BN38C10) and bis-para-phenylene-34-crown-10 (BPP34C10), is presented. The three corresponding [2]pseudorotaxanes, including the novel, fully characterized (visible absorption and nuclear magnetic resonance spectra, association constant, electrochemistry, crystal structure) [PyBP/BPP34C10]2+ system, were each reacted with MX2 (M = Zn2+, Cd2+, Hg2+; X = Cl−, Br−, I−). Of the twenty-seven different pseudorotaxane/MX2 combinations explored, fifteen yielded single-crystals containing a pseudorotaxane unit, and were characterized by X-ray diffraction; in eight of those crystals metal–pseudorotaxane binding was observed. The results reflect the lower association constant of [PyBP/BPP34C10]2+ (170 M−1) than the corresponding ones of [PyBP/BN32C8]2+ (870 M−1) and [PyBP/BN38C10]2+ (420 M−1) in solution, where pseudorotaxanes are in equilibrium with the corresponding free axle and wheel components. In all cases, the isolated crystals contain anionic metal halide species, such as singly charged MX3− (M = Zn, Cd; X = Cl, Br, I) or doubly charged CdX42− (X = Br, I) and Hg2X62− (X = Cl, Br, I). Ordered 3D-arrays of perfectly aligned pseudorotaxane units are found in the dichroic mercury-based crystals. Our study demonstrates that although specific intermolecular interactions (collectively called crystal packing forces), may occasionally interfere with the crystallization of metal–pseudorotaxane complexes based on pseudorotaxanes with low association constants, the use of pseudorotaxanes instead of synthetically more challenging rotaxanes is a promising approach for the construction of metal–organic rotaxane frameworks (MORFs).

X-ray vs. NMR structure of N-terminal domain of δ-subunit of RNA polymerase.

The crystal structure of the N-terminal domain of the RNA polymerase δ subunit (Nδ) from Bacillus subtilis solved at a resolution of 2.0Å is compared with the NMR structure determined previously. The molecule crystallizes in the space group C222(1) with a dimer in the asymmetric unit. Importantly, the X-ray structure exhibits significant differences from the lowest energy NMR structure. In addition to the overall structure differences, structurally important β sheets found in the NMR structure are not present in the crystal structure. We systematically investigated the cause of the discrepancies between the NMR and X-ray structures of Nδ, addressing the pH dependence, presence of metal ions, and crystal packing forces. We convincingly showed that the crystal packing forces, together with the presence of Ni(2+) ions, are the main reason for such a difference. In summary, the study illustrates that the two structural approaches may give unequal results, which need to be interpreted with care to obtain reliable structural information in terms of biological relevance.

Inclusion Complexes and Solvates of Buckycatcher, a Versatile Molecular Host with Two Corannulene Pincers

X-ray crystal structure study of cocrystallites of buckycatcher (2, C60H28) demonstrates a remarkable versatility of the molecular clip to accommodate guest molecules of various sizes and shapes. As demonstrated by the B97-D calculations, concave–concave conformation of the clip seems to be preferred for the gas-phase 1:1 inclusion complexes of 2 with a series of potential guests. However, the unprecedented concave–convex conformations were found in two of the reported solvates [i.e., 2·0.75(p-xylene) and 2·2PhNO2]. Three of the studied inclusion complexes with the highest calculated gas-phase binding energies (1,3,5-trinitrobenzene@2, and the previously reported C60@2 and C70@2) exhibit the analogous 1:1 supramolecular arrangements in the crystal state. On the other hand, p-xylene and PhNO2 solvates as well as DDQ/MeCN cocrystallite exhibit quite different stoichiometries and supramolecular arrangements, presumably enforced by the crystal packing forces. Dramatic variations of the intercentroid distances...

Terahertz Vibrations and Hydrogen-Bonded Networks in Crystals

The development of terahertz technology in the last few decades has made it possible to obtain a clear terahertz (THz) spectrum. THz vibrations clearly show the formation of weak bonds in crystals. The simultaneous progress in the code of first-principles calculations treating noncovalent interactions has established the position of THz spectroscopy as a powerful tool for detecting the weak bonding in crystals. In this review, we are going to introduce, briefly, the contribution of weak bonds in the construction of molecular crystals first, and then, we will review THz spectroscopy as a powerful tool for detecting the formation of weak bonds and will show the significant contribution of advanced computational codes in treating noncovalent interactions. From the second section, following the Introduction, to the seventh section, before the conclusions, we describe: (1) the crystal packing forces, the hydrogen-bonded networks and their contribution to the construction of organic crystals; (2) the THz vibrations observed in hydrogen-bonded molecules; (3) the computational methods for analyzing the THz vibrations of hydrogen-bonded molecules; (4) the dispersion correction and anharmonicity incorporated into the first-principles calculations and their effect on the peak assignment of the THz spectrum (5) the temperature dependence; and (6) the polarization dependence of the THz spectrum.

Synthesis and Structural Data of Tetrabenzo[8]circulene

In 1976, the first attempted synthesis of the saddle-shaped molecule [8]circulene was reported. The next 37 years produced no advancement towards the construction of this complicated molecule. But remarkably, over the last six months, a flurry of progress has been made with two groups reporting independent and strikingly different strategies for the synthesis of [8]circulene derivatives. Herein, we present a third synthetic method, in which we target tetrabenzo[8]circulene. Our approach employs a Diels-Alder reaction and a palladium-catalyzed arylation reaction as the key steps. Despite calculations describing the instability of [8]circulene, coupled with the reported instability of synthesized derivatives of the parent molecule, the addition of four fused benzenoid rings around the periphery of the molecule provides a highly stable structure. This increased stability over the parent [8]circulene was predicted by using Clar's theory of aromatic sextets and is a result of the compound becoming fully benzenoid upon incorporation of these additional rings. The synthesized compound exhibits remarkable stability under ambient conditions-even at elevated temperatures-with no signs of decomposition over several months. The solid-state structure of this compound is significantly twisted compared to the calculated structure primarily as a result of crystal-packing forces in the solid state. Despite this contortion from the lowest-energy structure, a range of structural data is presented confirming the presence of localized aromaticity in this large polycyclic aromatic hydrocarbon.

The Active Form of E6-associated protein (E6AP)/UBE3A Ubiquitin Ligase Is an Oligomer

Employing 125I-polyubiquitin chain formation as a functional readout of ligase activity, biochemical and biophysical evidence demonstrates that catalytically active E6-associated protein (E6AP)/UBE3A is an oligomer. Based on an extant structure previously discounted as an artifact of crystal packing forces, we propose that the fully active form of E6AP is a trimer, analysis of which reveals a buried surface of 7508Å2 and radially symmetric interacting residues that are conserved within the Hect (homologous to E6AP C terminus) ligase superfamily. An absolutely conserved interaction between Phe(727) and a hydrophobic pocket present on the adjacent subunit is critical for trimer stabilization because mutation disrupts the oligomer and decreases kcat 62-fold but fails to affect E2 ubiquitin binding or subsequent formation of the Hect domain Cys(820) ubiquitin thioester catalytic intermediate. Exogenous N-acetylphenylalanylamide reversibly antagonizes Phe(727)-dependent trimer formation and catalytic activity (Ki12 mM), as does a conserved-helical peptide corresponding to residues 474–490 of E6A Pisoform 1 (Ki22M) reported to bind the hydrophobic pocket of other Hect ligases, presumably blocking Phe(727) intercalation and trimer formation. Conversely, oncogenic human papillomavirus-16/18 E6 protein significantly enhances E6AP catalytic activity by promoting trimer formation (Kactivation 1.5 nM) through the ability of E6 to form homodimers. Recombinant E6 protein additionally rescues the kcat defect of the Phe(727) mutation and that of a specific loss-of-function Angelman syndrome mutation that promotes trimer destabilization. The present findings codify otherwise disparate observations regarding the mechanism of E6AP and related Hect ligases in addition to suggesting therapeutic approaches for modulating ligase activity.

Coiled-coil dimerization of the LOV2 domain of the blue-light photoreceptor phototropin 1 fromArabidopsis thaliana

A key role in signal transduction and dimerization mediated by Per-Arnt-Sim (PAS) domains is played by α-helical linkers that flank the structurally similar α/β cores of these domains. However, crystal-packing forces and the different construct lengths and sequences of the PAS domains influence the final length and orientation of the linkers relative to the core and create uncertainty in the exact mechanism of the linker function. Thus, structural characterization and comparison of the linkers within isolated PAS-domain constructs and/or full-length PAS-containing proteins is important for clarification of the mechanism. The plant blue-light photoreceptors phototropins possess two N-terminal flavin mononucleotide-based light, oxygen or voltage (LOV) domains (LOV1 and LOV2) that comprise a subclass of the PAS family and one C-terminal serine/threonine kinase domain whose enzymatic activity is regulated by blue light. The dark-adapted state crystal structures of the Arabidopsis thaliana phototropin 1 and phototropin 2 LOV1-domain constructs flanked by an N-terminal A'α helix and the structure of the phototropin 2 core LOV2 domain are known. Here, the crystal structure of the A. thaliana phototropin 1 LOV2 domain has been determined in its dark-adapted state. The core is flanked by an N-terminal A'α helix and a C-terminal Jα helix similar to those in the previously reported structure of Avena sativa phototropin 1 LOV2. In contrast to the monomeric A.sativa LOV2, A.thaliana LOV2 is a dimer in which two A'α helices adopt a scissor-like orientation at the dimer interface and form a short α-helical coiled coil. The Jα helix predominantly interacts with the β-sheet and plays a role in coiled-coil formation and dimerization.

Tetrabenzo[8]circulene: Aromatic Saddles from Negatively Curved Graphene

An aromatic saddle was designed from the hypothetical three-dimensional graphene with the negative Gaussian curvature (Schwarzite P192). Two aromatic saddles, tetrabenzo[8]circulene (TB8C) and its octamethyl derivative OM-TB8C, were synthesized by the Scholl reaction of cyclic octaphenylene precursors. The structure of TB8C greatly deviates from planarity, and the deep saddle shape was confirmed by single-crystal X-ray crystallography. There are two conformers with the S4 symmetry, which are twisted compared to the DFT structure (D2d). The theoretical studies propose that the interconversion of TB8C via the planar transition state (125 kcal mol(-1)) is not possible. However, the pseudorotation leads to a low-energy tub-to-tub inversion via the nonplanar transition state (7.3 kcal mol(-1)). The ground-state structure of TB8C in solution is quite different from the X-ray structure because of the crystal-packing force and low-energy pseudorotation. OM-TB8C is a good electron donor and works as the p-type semiconductor.