Gas-phase Density Functional Theory Research Articles

Discarding metal incorporation in pyrazole macrocycles and the role of the substrate on single-layer assemblies.

Pyrazole derivatives are key in crystal engineering and liquid crystal fields and thrive in agriculture, pharmaceutical, or biomedicine industries. Such versatility relies in their supramolecular bond adaptability when forming hydrogen bonds or metal-pyrazole complexes. Interestingly, the precise structure of pyrazole-based macrocycles forming widespread porous structures is still unsolved. We bring insight into such fundamental question by studying the self-assembled structures of a bis-pyrazole derivative sublimed in ultra-high-vacuum conditions (without solvents) onto the three (111) noble metal surfaces. By means of high-resolution scanning tunneling microscopy that is validated by gas phase density functional theory calculations, we find a common hexagonal nanoporous network condensed by triple hydrogen bonds at the molecule-metal interface. Such assembly is disrupted and divergent after annealing: (i) on copper, the molecular integrity is compromised leading to structural chaos, (ii) on silver, an incommensurate new oblique structure requiring molecular deprotonation is found and, (iii) on gold, metal-organic complexes are promoted yielding irregular chain structures. Our findings confirm the critical role of these metals on the different pyrazole nanoporous structure formation, discarding their preference for metal incorporation into the connecting nodes whenever there is no solvent involved.

Controlling Spin Crossover in a Family of Dinuclear Fe(III) Complexes via the Bis(catecholate) Bridging Ligand.

Spin crossover (SCO) complexes can reversibly switch between low spin (LS) and high spin (HS) states, affording possible applications in sensing, displays, and molecular electronics. Dinuclear SCO complexes with access to [LS-LS], [LS-HS], and [HS-HS] states may offer increased levels of functionality. The nature of the SCO interconversion in dinuclear complexes is influenced by the local electronic environment. We report the synthesis and characterization of [{FeIII(tpa)}2spiro](PF6)2 (1), [{FeIII(tpa)}2Br4spiro](PF6)2 (2), and [{FeIII(tpa)}2thea](PF6)2 (3) (tpa = tris(2-pyridylmethyl)amine, spiroH4 = 3,3,3',3'-tetramethyl-1,1'-spirobi(indan)-5,5',6,6'-tetraol, Br4spiroH4 = 3,3,3',3'-tetramethyl-1,1'-spirobi(indan)-4,4',7,7'-tetrabromo-5,5',6,6'-tetraol, theaH4 = 2,3,6,7-tetrahydroxy-9,10-dimethyl-9,10-dihydro-9,10-ethanoanthracene), utilizing non-conjugated bis(catecholate) bridging ligands. In the solid state, magnetic and structural analysis shows that 1 remains in the [HS-HS] state, while 2 and 3 undergo a partial SCO interconversion upon cooling from room temperature involving the mixed [LS-HS] state. In solution, all complexes undergo SCO from [HS-HS] at room temperature, via [LS-HS] to mixtures including [LS-LS] at 77 K, with the extent of SCO increasing in the order 1 < 2 < 3. Gas phase density functional theory calculations suggest a [LS-LS] ground state for all complexes, with the [LS-HS] and [HS-HS] states successively destabilized. The relative energy separations indicate that ligand field strength increases following spiro4- < Br4spiro4- < thea4-, consistent with solid-state magnetic and EPR behavior. All three complexes show stabilization of the [LS-HS] state in relation to the midpoint energy between [LS-LS] and [HS-HS]. The relative stability of the [LS-HS] state increases with increasing ligand field strength of the bis(catecholate) bridging ligand in the order 1 < 2 < 3. The bromo substituents of Br4spiro4- increase the ligand field strength relative to spiro4-, while the stronger ligand field provided by thea4- arises from extension of the overlapping π-orbital system across the two catecholate units. This study highlights how SCO behavior in dinuclear complexes can be modulated by the bridging ligand, providing useful insights for the design of molecules that can be interconverted between more than two states.

Crystal structure and cation-anion interactions of potassium (Difluoromethanesulfonyl) (trifluorome thanesulfonyl)imide.

Sulfonimide salts are of great interest for battery use thanks to their special properties including sufficient superior chemical/thermal stabilities, structural flexibility, etc. In particular, the hydrogen-containing sulfonimide (difluoromethanesulfonyl)(trifluoromethanesulfonyl)imide anion {[N(SO2CF2H) (SO2CF3)]-, DFTFSI-}, stands out owing to its suppressed anion mobility and superior electrochemical properties. We herein report the structural analyses of potassium (difluoromethanesulfonyl)(trifluoromethanesulfonyl)imide {K [N(SO2CF2H) (SO2CF3)], KDFTFSI} by virtue of single crystal X-ray diffraction and computational approaches. Our results reveal that KDFTFSI crystallizes in a orthorhombic cell (space group: Pbcn) comprising of cationic and anionic layers, which is similar to the conventional sulfonimide salt, potassium bis(trifluoromethanesulfonyl)imide {K [N(SO2CF3)2], KTFSI}. Gas-phase density functional theory calculations show that the conversion from trans to cis DFTFSI- anions is hindered due to the presence of stabilizing intramolecular H-bonding interactions in the trans conformer; yet interaction with K+ substantially minimizes the energy difference between the two conformers due to the formation of strong tridentate K+ coordination with oxygen atoms in the cis KDFTFSI. This work is anticipated to provide further understanding on the structure-property relations of hydrogenated sulfonimide anions, and thus inspire the structural design of new anions for battery research.

Impact of iodide ions on the speciation of radiolytic transients in molten LiCl-KCl eutectic salt mixtures.

The fate of fission-product iodine is critical for the deployment of next generation molten salt reactor technologies, owing to its volatility and biological impacts if it were to be released into the environment. To date, little is known on how ionizing radiation fields influence the redox chemistry, speciation, and transport of iodine in high temperature molten salts. Here we employ picosecond electron pulse irradiation techniques to elucidate for the first time the impact of iodide ions (I-) on the speciation and chemical kinetics of the primary radiation-induced transient radicals generated in molten chloride salt mixtures (eS- and Cl2˙-) as a function of temperature (400-700 °C). In the presence of I- ions (≥ 1 wt% KI in LiCl-KCl eutectic), we find that the transient spectrum following the electron pulse is composed of at least three overlapping species: the eS- and the Cl2˙- and ICl˙- radical anions, for which a deconvoluted spectrum of the latter is reported here for the first time in molten salts. This new transient spectrum was consistent with gas phase density functional theory calculations. The lifetime of the eS- was unaffected by the addition of I- ions. The newly observed interhalogen radical anion, ICl˙-, exhibited a lifetime on the order of microseconds over the investigated temperature range. The associated chemical kinetics indicate that the predominate mechanism of ICl˙- decay is via reaction with the Cl2˙- radical anion. The iodine containing product of this reaction is expected to be ICl2-, which will have implications for the transport of fission-product iodine in MSR technologies.

Gas-phase and solid-state electronic structure analysis and DFT benchmarking of HfCO.

Ab initio multi-reference configuration interaction (MRCI) and coupled cluster singles doubles and perturbative triples [CCSD(T)] levels of theory were used to study ground and excited electronic states of HfCO. We report potential energy curves, dissociation energies (De), excitation energies, harmonic vibrational frequencies, and chemical bonding patterns of HfCO. The 3Σ- ground state of HfCO has an 1σ22σ21π2 electron configuration and a ∼30 kcal mol-1 dissociation energy with respect to its lowest-energy fragments Hf(3F) + CO(X1Σ+). We further evaluated the De of its isovalent HfCX (X = S, Se, Te, Po) series and observed that they increase linearly from the lighter HfCO to the heavier HfCPo with the dipole moment of the CX ligand. The same linear relationship was observed for TiCX and ZrCX. We utilized the CCSD(T) benchmark values of De, excitation energy, and ionization energy (IE) values to evaluate density functional theory (DFT) errors with 23 exchange-correlation functionals spanning GGA, meta-GGA, global GGA hybrid, meta-GGA hybrid, range-separated hybrid, and double-hybrid functional families. The global GGA hybrid B3LYP and range-separated hybrid ωB97X performed well at representing the ground state properties of HfCO (i.e., De and IE). Finally, we extended our DFT analysis to the interaction of a CO molecule with a Hf surface and observed that the surface chemisorption energy and the gas-phase molecular dissociation energy are very similar for some DFAs but not others, suggesting moderate transferability of the benchmarks on these molecules to the solid state.

The interplay between hydrogen and halogen bonding: substituent effects and their role in the hydrogen bond enhanced halogen bond.

The hydrogen bond enhanced halogen bond (HBeXB) has recently been used to effectively improve anion binding, organocatalysis, and protein structure/function. In this study, we present the first systematic investigation of substituent effects in the HBeXB. NMR analysis confirmed intramolecular HBing between the amine and the electron-rich belt of the XB donor (N-H⋯I). Gas-phase density functional theory studies showed that the influence of HBing on the halogen atom is more sensitive to substitution on the HB donor ring (R1). The NMR studies revealed that the intramolecular HBing had a significant impact on receptor performance, resulting in a 50-fold improvement. Additionally, linear free energy relationship (LFER) analysis was employed for the first time to study the substituent effect in the HBeXB. The results showed that substituents on the XB donor ring (R2) had a competing effect where electron donating groups strengthened the HB and weakened the XB. Therefore, selecting an appropriate substituent on the adjacent HB donor ring (R1) could be an alternative and effective way to enhance an electron-rich XB donor. X-ray crystallographic analysis demonstrated that intramolecular HBing plays an important role in the receptor adopting the bidentate conformation. Taken together, the findings imply that modifying distal substituents that affect neighboring noncovalent interactions can have a similar impact to conventional para substitution substituent effects.

CO2 adsorption and activation on Ag(1 1 1) surfaces in the presence of surface charge density: A static gas phase DFT study

The CO2 reduction reaction on Ag in the presence of surface plasmons are of strong interest in photocatalysis. Here, two Density Functional Theory (DFT) approaches are proposed to localize charges at the Ag surface to mimic surface plasmon excitation. These calculations predict charge localization at the outermost surface layer, a result confirmed via light excitation at the plasmon resonance modeled by Density Functional Tight Binding Hamiltonian (DFTB) theory. The CO2(gas) reduction initial steps are studied by DFT models showing that bonded species can be created on top of an Ag-atom in the presence of extra charges. A second CO2 molecule can assist the first molecule, decreasing the charge carrier density requirement for CO2 reduction bonded species. A Molecular Dynamics (MD) study shows a possible interaction among CO2 molecule. These results show that a static DFT simulation can mimic charge localization resulting from surface plasmon effects, thus enabling studies on surface plasmon-enhanced chemical reactions, paving the way for future time-dependent (TD) studies.

Why Monoamine Oxidase B Preferably Metabolizes N-Methylhistamine over Histamine: Evidence from the Multiscale Simulation of the Rate-Limiting Step.

Histamine levels in the human brain are controlled by rather peculiar metabolic pathways. In the first step, histamine is enzymatically methylated at its imidazole Nτ atom, and the produced N-methylhistamine undergoes an oxidative deamination catalyzed by monoamine oxidase B (MAO-B), as is common with other monoaminergic neurotransmitters and neuromodulators of the central nervous system. The fact that histamine requires such a conversion prior to oxidative deamination is intriguing since MAO-B is known to be relatively promiscuous towards monoaminergic substrates; its in-vitro oxidation of N-methylhistamine is about 10 times faster than that for histamine, yet this rather subtle difference appears to be governing the decomposition pathway. This work clarifies the MAO-B selectivity toward histamine and N-methylhistamine by multiscale simulations of the rate-limiting hydride abstraction step for both compounds in the gas phase, in aqueous solution, and in the enzyme, using the established empirical valence bond methodology, assisted by gas-phase density functional theory (DFT) calculations. The computed barriers are in very good agreement with experimental kinetic data, especially for relative trends among systems, thereby reproducing the observed MAO-B selectivity. Simulations clearly demonstrate that solvation effects govern the reactivity, both in aqueous solution as well as in the enzyme although with an opposing effect on the free energy barrier. In the aqueous solution, the transition-state structure involving histamine is better solvated than its methylated analog, leading to a lower barrier for histamine oxidation. In the enzyme, the higher hydrophobicity of N-methylhistamine results in a decreased number of water molecules at the active side, leading to decreased dielectric shielding of the preorganized catalytic electrostatic environment provided by the enzyme. This renders the catalytic environment more efficient for N-methylhistamine, giving rise to a lower barrier relative to histamine. In addition, the transition state involving N-methylhistamine appears to be stabilized by the surrounding nonpolar residues to a larger extent than with unsubstituted histamine, contributing to a lower barrier with the former.

Intermolecular interaction network in liquid acetone

Molecular dynamics simulations, employing the transferable potentials for phase equilibria-united atom force field, have been performed to explore the structural features of liquid acetone. Topological and dynamical network analyses have been performed on the intermolecular interaction (IMI) networks, which are constructed based on a set of pair energy cutoffs. Topological analysis revealed that the acetone pairs interacting stronger than −10 kJ/mol exist as isolated pairs in a sense that the members of the pair do not interact with other molecules with an energy smaller than −10 kJ/mol. As such, those strongly interacting pairs do not constitute a three-dimensional extended network. As the pair interaction energy cutoff becomes larger than −10 kJ/mol, the IMI network starts to become a three-dimensional extended network and its connectivity increases gradually. Dynamical analysis revealed that the decay function for intermolecular interaction lifetimes is characterized by a nonexponential decay for all the IMI networks such that it decays faster at short-times in comparison to the longer-times. Yet, the duration of the intermolecular interactions can go beyond 1.0 ps for the pairs with the interaction energies larger than −6 kJ/mol. Furthermore, the orientational correlations in the liquid acetone have been examined for a more detailed picture of its structure. It is clearly shown that as the distance between the carbonyl carbons of the two acetones gets smaller, (i) they tend to adopt an anti-parallel dipole orientation, (ii) the planes of the acetones tend to adopt a parallel orientation, and, (iii) such acetone pairs particularly interact more strongly. In the light of these findings, two dimeric structures, stack-type and planar-type, are highlighted. Using density functional theory (DFT) at the mPW2PLYP/def2-TZVP level, the interaction energy in the gas-phase is computed to be −19.85 kJ/mol for the stack-type dimer, which is more stable than the planar-type dimer by 7.7 kJ/mol. Both the gas-phase DFT interaction energy and the computed acetone dipole moment, 2.97 D, are in great agreement with the corresponding experimental data. Furthermore, in accordance with the gas-phase results, the stack-type dimers are found to be more stable than the planar-type dimers also in the liquid-phase as revealed from both the liquid-phase DFT interaction energies and the MD based mean interaction energies. Finally, trimeric structure search in the simulated liquid acetone revealed that of all trimers, 94% belongs to the chain-like trimers, while the remaining 6% belongs to the cyclic trimers.

Structures and Spin States of Iron(II) Complexes of Isomeric 2,6-Di(1,2,3-triazolyl)pyridine Ligands.

Iron(II) complex salts of 2,6-di(1,2,3-triazol-1-yl)pyridine (L1) are unexpectedly unstable in undried solvent. This is explained by the isolation of [Fe(L1)4(H2O)2][ClO4]2 and [Fe(NCS)2(L1)2(H2O)2]·L1, containing L1 bound as a monodentate ligand rather than in the expected tridentate fashion. These complexes associate into 44 grid structures through O-H···N hydrogen bonding; a solvate of a related 44 coordination framework, catena-[Cu(μ-L1)2(H2O)2][BF4]2, is also presented. The isomeric ligands 2,6-di(1,2,3-triazol-2-yl)pyridine (L2) and 2,6-di(1H-1,2,3-triazol-4-yl)pyridine (L3) bind to iron(II) in a more typical tridentate fashion. Solvates of [Fe(L3)2][ClO4]2 are low-spin and diamagnetic in the solid state and in solution, while [Fe(L2)2][ClO4]2 and [Co(L3)2][BF4]2 are fully high-spin. Treatment of L3 with methyl iodide affords 2,6-di(2-methyl-1,2,3-triazol-4-yl)pyridine (L4) and 2-(1-methyl-1,2,3-triazol-4-yl)-6-(2-methyl-1,2,3-triazol-4-yl)pyridine (L5). While salts of [Fe(L5)2]2+ are low-spin in the solid state, [Fe(L4)2][ClO4]2·H2O is high-spin, and [Fe(L4)2][ClO4]2·3MeNO2 exhibits a hysteretic spin transition to 50% completeness at T1/2 = 128 K (ΔT1/2 = 6 K). This transition proceeds via a symmetry-breaking phase transition to an unusual low-temperature phase containing three unique cation sites with high-spin, low-spin, and 1:1 mixed-spin populations. The unusual distribution of the spin states in the low-temperature phase reflects "spin-state frustration" of the mixed-spin cation site by an equal number of high-spin and low-spin nearest neighbors. Gas-phase density functional theory calculations reproduce the spin-state preferences of these and some related complexes. These highlight the interplay between the σ-basicity and π-acidity of the heterocyclic donors in this ligand type, which have opposing influences on the molecular ligand field. The Brønsted basicities of L1-L3 are very sensitive to the linkage isomerism of their triazolyl donors, which explains why their iron complex spin states show more variation than the better-known iron(II)/2,6-dipyrazolylpyridine system.

Quantum chemistry for molecules at extreme pressure on graphical processing units: Implementation of extreme-pressure polarizable continuum model.

Pressure plays essential roles in chemistry by altering structures and controlling chemical reactions. The extreme-pressure polarizable continuum model (XP-PCM) is an emerging method with an efficient quantum mechanical description of small- and medium-sized molecules at high pressure (on the order of GPa). However, its application to large molecular systems was previously hampered by a CPU computation bottleneck: the Pauli repulsion potential unique to XP-PCM requires the evaluation of a large number of electric field integrals, resulting in significant computational overhead compared to the gas-phase or standard-pressure polarizable continuum model calculations. Here, we exploit advances in graphical processing units (GPUs) to accelerate the XP-PCM-integral evaluations. This enables high-pressure quantum chemistry simulation of proteins that used to be computationally intractable. We benchmarked the performance using 18 small proteins in aqueous solutions. Using a single GPU, our method evaluates the XP-PCM free energy of a protein with over 500 atoms and 4000 basis functions within half an hour. The time taken by the XP-PCM-integral evaluation is typically 1% of the time taken for a gas-phase density functional theory (DFT) on the same system. The overall XP-PCM calculations require less computational effort than that for their gas-phase counterpart due to the improved convergence of self-consistent field iterations. Therefore, the description of the high-pressure effects with our GPU-accelerated XP-PCM is feasible for any molecule tractable for gas-phase DFT calculation. We have also validated the accuracy of our method on small molecules whose properties under high pressure are known from experiments or previous theoretical studies.

Reactivity of an air-stable dihydrobenzoimidazole n-dopant with organic semiconductor molecules

Summary 1,3-Dimethyl-2-(4-(dimethylamino)phenyl)-2,4-dihydro-1H-benzoimidazole, N-DMBI-H, is widely used as an air-stable n-dopant for organic semiconductors. Here, its reactivity is investigated with a variety of imide- and amide-containing semiconductor molecules that have reduction potentials in the range −0.54 to −1.10 V versus ferrocene. Reaction rates correlate poorly with these potentials. The more reactive of the imides form the corresponding radical anions cleanly, but kinetic isotope studies using N-DMBI-D indicate that the reaction proceeds via an initial hydride- or hydrogen-transfer step. For an amide- and ester-rigidified bis(styryl)benzene derivative the hydride-reduced product is stable under an inert atmosphere and can be observed directly; the radical anion can only be obtained in sub-stoichiometric yield and under certain reaction conditions. On the other hand, (N-DMBI)2 rapidly reduces all the imides and amides examined to the corresponding radical anions. The implications of these findings for dopant selection and use are discussed.

Accurate Thermochemistry of Complex Lignin Structures via Density Functional Theory, Group Additivity, and Machine Learning

A molecular-level understanding of lignin structures and bond dissociation energies could facilitate depolymerization technologies. Still, this information is currently limited due to the lack of databases and the simplification of surrogate models. Here, substitution effects on seven common linkages in lignin polymers are systematically investigated. An automated reaction network generator is employed to create a database of structures. A new group additivity (GA) model based on principal component analysis (PCA) descriptors is introduced and trained on gas-phase density functional theory data of 4100 species at the M06-2X/6-311++G(d,p) level. Hydrogen bonds, local steric, and nonaromatic ring contributions are also incorporated. Finally, we improve the accuracy of the group additivity model to reach the G4 theory by computing a data set of 770 species at this level and using a data fusion approach.

Thermally Stable Nitrothiacalixarene Chromophores: Conformational Study and Aggregation Behavior.

Achieving high thermal stability and control of supramolecular organization of functional dyes in sensors and nonlinear optics remains a demanding task. This study was aimed at the evaluation of thermal behavior and Langmuir monolayer characteristics of topologically varied nitrothiacalixarene multichromophores and phenol monomers. A nitration/azo coupling alkylation synthetic route towards partially O-substituted nitrothiacalixarenes and 4-nitrophenylazo-thiacalixarenes was proposed and realized. Nuclear magnetic resonance (NMR) spectroscopy and X-ray diffractometry of disubstituted nitrothiacalix[4]arene revealed a rare 1,2-alternate conformation. A synchronous thermal analysis indicated higher decomposition temperatures of nitrothiacalixarene macrocycles as compared with monomers. Through surface pressure/potential-molecular area measurements, nitrothiacalixarenes were shown to form Langmuir monolayers at the air–water interface and, through atomic force microscopy (AFM) technique, Langmuir–Blodgett (LB) films on solid substrates. Reflection-absorption spectroscopy of monolayers and electronic absorption spectroscopy of LB films of nitrothiacalixarenes recorded a red-shifted band (290 nm) with a transition from chloroform, indicative of solvatochromism. Additionally, shoulder band at 360 nm was attributed to aggregation and supported by gas-phase density functional theory (DFT) calculations and dynamic light scattering (DLS) analysis in chloroform–methanol solvent in the case of monoalkylated calixarene 3. Excellent thermal stability and monolayer formation of nitrothiacalixarenes suggest their potential as functional dyes.

Advancing Free-Energy Calculations of Metalloenzymes in Drug Discovery via Implementation of LFMM Potentials.

To address some of the inherent challenges in modeling metalloenzymes, we here report an extension to the functional form of the OPLS3e force field to include terms adopted from the ligand field molecular mechanics (LFMM) model, including the angular overlap and Morse potential terms. The integration of these terms with OPLS3e, herein referred to as OPLS3e+M, improves the description of metal-ligand interactions and provides accurate relative binding energies and geometric preferences of transition-metal complexes by training to gas-phase density functional theory (DFT) energies. For [Cu(H2O)4]2+, OPLS3e+M significantly improves H2O binding energies and the geometric preference of the tetra-aqua Cu2+ complex. In addition, we conduct free-energy perturbation calculations on two pharmaceutically relevant metalloenzyme targets, which include chemical modifications at varying proximity to the binding-site metals, including changes to the metal-binding moiety of the ligand itself. The extensions made to OPLS3e lead to accurate predicted relative binding free energies for these series (mean unsigned error of 1.29 kcal mol-1). Our results provide evidence that integration of the LFMM model with OPLS3e can be utilized to predict thermodynamic quantities for such systems near chemical accuracy. With these improvements, we anticipate that robust free-energy perturbation calculations can be employed to accelerate the drug development efforts for metalloenzyme targets.

Crystal structure, Hirshfeld surface analysis and DFT studies of (E)-4-methyl-2-{[(2-methyl-3-nitro-phen-yl)imino]-meth-yl}phenol.

The title compound, C15H14N2O3, was prepared by condensation of 2-hy-droxy-5-methyl-benzaldehyde and 2-methyl-3-nitro-phenyl-amine in ethanol. The configuration of the C=N bond is E. An intra-molecular O-H⋯N hydrogen bond is present, forming an S(6) ring motif and inducing the phenol ring and the Schiff base to be nearly coplanar [C-C-N-C torsion angle of 178.53 (13)°]. In the crystal, mol-ecules are linked by C-H⋯O inter-actions, forming chains along the b-axis direction. The Hirshfeld surface analysis indicates that the most important contributions to the crystal packing are from H⋯H (37.2%), C⋯H (30.7%) and O⋯H (24.9%) inter-actions. The gas phase density functional theory (DFT) optimized structure at the B3LYP/ 6-311 G(d,p) level is compared to the experimentally determined mol-ecular structure in the solid state. The HOMO-LUMO behaviour was elucidated to determine the energy gap.

Computational modeling of piezochromism in molecular crystals.

Piezochromic materials, whose luminescence responds to external pressure, have recently garnered much experimental attention. Computational modeling of piezochromism is of high theoretical interest, yet currently lacking. Herein, we present a computational effort to predict the piezochromism for a selection of molecular crystals. The current methodology employs a combination of dispersion-corrected solid-state and gas-phase density-functional theory and Becke's virial exciton model. Our study finds that piezochromism is primarily driven by the modification of intermolecular interactions within the molecular crystal and can be understood from the perspectives of changing polarizability or bandgaps upon the application of mechanical pressure.

The TACO Puzzle: A Phase-Transition Mystery Revisited

The organic salt (5-methyl-1-thia-5-azacyclo-octane-1-oxide) perchlorate (TACO) is known to undergo a single-crystal-to-single-crystal phase transition in the 276–298 K T range without a change in the external shape of the sample. Despite extensive computational and experimental investigations, no safe conclusions about the transition mechanism could be drawn till now. The two packing patterns are very similar, and symmetry is conserved, apart from an interchange of cell axes from P21/c (α-TACO, low-T) to P21/a (β-TACO, high-T). Yet, the phase transition implies a significant conformational rearrangement, coupled with ∼180°-wide rotations, of 1/2 of the cations, in conjunction with reorientation of the anions. Here, we analyze the crystal packing of the two phases in terms of pairwise molecule–molecule interaction energies, as derived from the PIXEL approach. Rigid-body molecular reorientations are simulated by solid-state Monte Carlo calculations, while the likelihood of conformational rearrangements is estimated through gas-phase density functional theory M06/6-311G(p,d) simulations. We demonstrate that rotational motion of the cations is not hampered by substantial energetic barriers, while the ring flip can be described as a two-step process with a main kinetic barrier of ∼45 kJ·mol–1, which might explain the metastable behavior of the β phase at low T. A possible mechanism of the phase transition is proposed, accounting for the present computational evidences in the context of the former experimental findings.

Twisted and Bent Out of Shape: Symmetry and Chirality Analysis of Substituted Ferrocenes

Ferrocene symmetry is commonly described as either eclipsed (D5h) or staggered (D5d), but this symmetry does not hold when substitution is involved. Here we examine and quantify the effect of substitution on the geometry of the core structure of ferrocene, and provide means to distinguish between its various distortion paths. Continuous symmetry analysis of the core structure of 7418 substituted ferrocenes extracted from the Cambridge Crystallographic Database was used to explore its twisting and bending distortion paths, as well as asymmetric bond stretch that deforms the symmetry of its cyclopentadienyl rings. Gas-phase density functional theory calculations provided a theoretical background to describe these distortion paths and define symmetry profiles for conformer interconversion processes and the interplay between them. Our results show that the distortion of ferrocene can often be substantial. Furthermore, its geometry is highly flexible, and almost always chiral to some degree. Handedness in term...

The intramolecular hydrogen bonded-halogen bond: a new strategy for preorganization and enhanced binding.

Natural and synthetic molecules use weak noncovalent forces to preorganize structure and enable remarkable function. Herein, we introduce the intramolecular hydrogen bonded-halogen bond (HB-XB) as a novel method to preorganize halogen bonding (XBing) molecules, while generating a polarization-enhanced XB. Positioning a fluoroaniline between two iodopyridinium XB donors engendered intramolecular hydrogen bonding (HBing) to the electron-rich belt of both XB donors. NMR solution studies established the efficacy of the HB-XB. The receptor with HB-XBs (G2XB) displayed a nearly 9-fold increase in halide binding over control receptors. Gas-phase density functional theory conformational analysis indicated that the amine stabilizes the bidentate conformation. Furthermore, gas-phase interaction energies showed that the bidentate HB-XBs of G2XBme2+ are more than 3.2 kcal mol-1 stronger than the XBs in a control without the intramolecular HB. Additionally, crystal structures confirm that HB-XBs form tighter contacts with I- and Br- and produce receptors that are more planar. Collectively the results establish the intramolecular HB-XB as a tractable strategy to preorganize XB molecules and regulate XB strength.