Structural, Electric and Dynamic Properties of (Pyrrolidinium)3[Bi2I9] and (Pyrrolidinium)3[Sb2I9]: New Lead-Free, Organic-Inorganic Hybrids with Narrow Band Gaps.

Hybrid organic-inorganic iodides based on Bi(III) and Sb(III) provide integrated functionalities through the combination of high dielectric constants, semiconducting properties and ferroic phases. Here, we report a pyrrolidinium-based bismuth (1) and antimony (2) iodides of (NC4H10)3[M2I9] (M: Bi(III), Sb(III)) formula which are ferroelastic at room temperature. The narrow band gaps (~2.12 eV for 1 and 2.19 eV for 2) and DOS calculations indicate the semiconducting characteristics of both materials. The crystal structure consists of discrete, face-sharing bioctahedra [M2I9]3- and disordered pyrrolidinium amines providing charge balance and acting as spacers between inorganic moieties. At room temperature, 1 and 2 accommodate orthorhombic Cmcm symmetry. 1 displays a complex temperature-induced polymorphism. It is stable up to 525 K and undergoes a sequence of low-temperature phase transitions (PTs) at 221/222 K (I ↔ II) and 189/190 K (II ↔ III) and at 131 K (IV→III), associated with the ordering of pyrrolidinium cations and resulting in Cmcm symmetry breaking. 2 undergoes only one PT at T = 215 K. The dielectric studies disclose a relaxation process in the kilohertz frequency region, assigned to the dynamics of organic cations, described well by the Cole-Cole relation. A combination of single-crystal X-ray diffraction, synchrotron powder diffraction, spin-lattice relaxation time of 1H NMR, dielectric and calorimetric studies is used to determine the structural phase diagram, cation dynamics and electric properties of (NC4H10)3[M2I9].

Progressive Structural Complexity in Ferroelectric 1,2,4-Triazolium Hexabromoantimonate(III): Interplay of "Order-Disorder" and "Displacive" Contributions to the Structural Phase Transitions.

Halobismuthates(III) and haloantimonates(III) with the R3MX6 chemical composition create a new and broadly unexplored class of ferroelectric compounds. In this paper, we report the haloantimonate(III) ferroelectric comprising an aromatic (1,2,4-triazolium) cation, i.e., (C2N3H4)3[SbBr6] (TBA). Temperature-resolved structural and spectroscopic studies indicate that TBA undergoes two solid-solid phase transitions between tetragonal [P42/m (I)] and monoclinic [P21/n (II) and P21 (III)] phases. TBA experiences a paraelectric-ferroelectric phase transition at 271/268 K (II-III) driven by "order-disorder" and "displacive" molecular mechanisms. The ferroelectric properties of phase III have been confirmed by hysteresis loop measurement, and additionally, the acentric order has been further supported by second-harmonic generation measurements. Insight into the molecular origins of the ferroelectric polarization was provided by periodic ab initio calculations using the Berry phase approach at the density functional theory (DFT-D3) method level employed for calculations of spontaneous polarization.

Spatiotemporal Studies of the One-Dimensional Coordination Polymer [Fe(ebtz)2 (C2 H5 CN)2 ](BF4 )2 : Tug of War between the Nitrile Reorientation Versus Crystal Lattice as a Tool for Tuning the Spin Crossover Properties*.

Reaction of 1,2-di(tetrazol-2-yl)ethane (ebtz) with Fe(BF4 )2 ⋅6 H2 O in different nitriles yields one-dimensional coordination polymers [Fe(ebtz)2 (RCN)2 ](BF4 )2 ⋅nRCN (n=2 for R=CH3 (1) and n=0 for R=C2 H5 (2) C3 H7 (3), C3 H5 (4), CH2 Cl (5)) exhibiting spin crossover (SCO). SCO in 1 and 3-5 is complete and occurs above 160 K. In 2, it is shifted to lower temperatures and is accompanied by wide hysteresis (T1/2 ↓ =78 K, T1/2 ↑ =123 K) and proceeds extremely slowly. Isothermal (80 K) time-resolved single-crystal X-ray diffraction studies revealed a complex nature for the HS→LS transition in 2. An initial, slow stage is associated with shrinkage of polymeric chains and with reduction of volume at 77 % (in relation to the difference between cell volumes VHS -VLS ) whereas only 16 % of iron(II) ions change spin state. In the second stage, an abrupt SCO occurs, associated with breathing of the crystal lattice along the direction of the Fe-nitrile bonds, while the nitriles reorient. HS→LS switching triggered by light (808 nm) reveals the coupling of spin state and nitrile orientation. The importance of this coupling was confirmed by studies of [Fe(ebtz)2 (C2 H5 CN/C3 H7 CN)2 ](BF4 )2 mixed crystals (2 a, 2 b), showing a shift of T1/2 to higher values and narrowing of the hysteresis loop concomitant with an increase of the fraction of butyronitrile. This increase reduces the capability of nitrile molecules to reorient. Density functional theory (DFT) studies of models of 1-5 suggest a particular possibility of 2 to adopt a low (140-145°) value of its Fe-N-C(propionitrile) angle.

1,2,3-propanetriol radicals formed during oxidative stress.

The presented results attempt to approximate the proper structure of the radical formed as a result of the oxidation of 1,2,3-propanetriol. To fulfil the aim unstable radical originated in 1,2,3-propanetriol was trapped by PBN. Resulted spin adduct was measured using EPR spectroscopy and the isotropic hyperfine coupling constants aiso (14 N) and aiso (1 H) were obtained by simulation of the EPR spectrum. The next step consisted of conducting a comparative analysis of EPR parameters, based on the calculations conducted at the DFT and MP2 methods level in open-shell formalism including solvent effects. For comparison, calculations were also carried out at the level of combined methods (UB3LYP/QCISD and UMP2/QCISD) in terms of the ONIOM formalism. Comparison of the experimental EPR data of the isotropic hyperfine coupling constants aiso (14 N) and aiso (1 H) with the calculated parameters indicate that oxidation of 1,2,3-propanetriol leads to a carbon centred radical where unpaired electron is situated on the second (middle) carbon of 1,2,3-propanetriol. What is important, this conclusion could be made regardless of the chosen calculation method. However, it could be stated that for calculation of the isotropic hyperfine coupling constants aiso (14 N) and aiso (1 H) of PBN/gly• adducts, UMP2 polarisable conductor calculation model with two ethanol molecules is explicitly defined.

Investigations of the hydrogen bond in the crystals of tropolone and thiotropolone via car-parrinello and path integral molecular dynamics.

Car-Parrinello and path integrals molecular dynamics (CPMD and PIMD) simulations were carried out for the 10π-electron aromatic systems: 2-hydroxy-2,4,6-cycloheptatrien-1-one, commonly known as Tropolone (I) and 2-hydroxy-2,4,6-cycloheptatriene-1-thione, called Thiotropolone (II) in vacuo and in the solid state. The extremely fast proton transfer (FPT) and "prototropy" tautomerism in the keto-enol (thione-enethiol) systems have been analyzed on the basis of CPMD and PIMD methods level. Comparisons of two-dimensional (2D) free-energy landscapes of reaction coordinate δ-parameter and RO…O or RO…S distances shows that the OH… tautomer to be more favorable in the Thiotropolone. The hydrogen between the oxygen and the sulfur atoms adopts a starkly asymmetrical position in the double potential well. The values of the energy barriers for the FPT were calculated and suggested a strong hydrogen bond with low barrier for FPT mechanism. These studies and the 2D average index of π-delocalization 〈λ〉 landscape of time evolutions of RO1…O2 and RC7O2 or RC7S1 distances for the both crystals indicate that hydrogen bonds in the crystals of Tropolone (I) and Thiotropolone (II) have characteristic properties for the type of bonding model resonance-assisted hydrogen bonds and also low-barrier hydrogen bonds. In the crystal of the Thiotropolone (II), we found the hydrogen bond OH…S existing without the equilibrium of the two tautomers whereas in the crystal of the Tropolone (I) has been confirmed the hydrogen bond OH…O existing with the equilibrium of the two tautomers. It was also found the significant differences in frequency, speed, and the image of the FPT in the studied crystals. © 2018 Wiley Periodicals, Inc.

Car-Parrinello and Path Integral Molecular Dynamics Study of the Proton Transfer in the Intramolecular Hydrogen Bonds in the Ketohydrazone-Azoenol System.

Car-Parrinello (CPMD) and path integral molecular dynamics (PIMD) simulations were carried out for 1-(phenylazo)-2-naphthol (I) and 1-(4-F-phenylazo)-2-naphthol (II) (Sudan I) in vacuo and in the solid state at 298 K. The fast proton transfer (FPT) and tautomerism in the ketohydrazone-azoenol systems have been analyzed on the basis of CPMD and PIMD methods level. The two-dimensional free-energy landscape of reaction coordinate δ-parameter and RN···O distances shows the NH tautomer to be more favorable in the gas phase as well as in the solid state according to the CP and PI results, respectively. The hydrogen between the nitrogen and the oxygen atoms adopts a starkly asymmetrical position in the double potential well. The molecular geometry and energy barrier for the intramolecular proton transference were calculated, and the value found suggested a strong hydrogen bond with low barrier for FPT mechanism. These studies and the two-dimensional average index of π-delocalization ⟨λ⟩ landscape of time evolutions of RN1···O1 and RC1═O1 distances for both the crystals indicate that the hydrogen bonds in the crystals of 1-(phenylazo)-2-naphthol (I) and 1-(4-F-phenylazo)-2-naphthol (II) have characteristic properties for the type of bonding model: resonance-assisted hydrogen bonds and low-barrier hydrogen bonds, without the existence of equilibrium in the two tautomers. The infrared spectrum has been calculated, and a comparative vibrational analysis has been performed. The CPMD vibrational results appear to qualitatively agree with the experimental ones.

Car-Parrinello and path integral molecular dynamics study of the intramolecular hydrogen bonds in the crystals of benzoylacetone and dideuterobenzoylacetone.

The dynamics of the intramolecular short hydrogen bond in the molecular crystal of benzoylacetone and its deuterated analogue are investigated using ab initio molecular dynamics simulations. A study on intramolecular hydrogen bonding in 1-phenyl-1,3-butadione (I) and 1-deuteroxy-2-deutero-1-phenylbut-1-en-3-one (II) crystals has been carried out at 160 K and 300 K on the CPMD method level and at 300 K on the PIMD method level. The analysis of the two-dimensional free-energy landscape of reaction coordinate δ-parameter and ROO distances shows that the hydrogen (deuter) between the two oxygen atoms adopts a slightly asymmetrical position in the single potential well. When the nuclear quantum effects are taken into account, very large delocalization of the bridging proton is observed. These studies indicate that hydrogen bonds in the crystal of benzoylacetone have characteristic properties for the type of bonding model resonance-assisted hydrogen bonds (RAHB) without existing the equilibrium of the two tautomers. The infrared spectrum has been calculated, and a comparative vibrational analysis has been performed. The CPMD vibrational results appear to qualitatively agree with the experimental ones.

Investigations of the very short hydrogen bond in the crystal of nitromalonamide via Car-Parrinello and path integral molecular dynamics.

In this paper are presented the results of theoretical studies of the structure in proton motion in a very short O···O and two weak N-H···O intramolecular hydrogen bonds in the nitromalonamide crystal. The dynamics of proton motion in hydrogen bonds were investigated in the NVT ensemble at 298 K using the Car-Parrinello and the path integral molecular dynamics. A very large delocalization of proton in the slightly asymmetrical single well of free energy potential of O-H···O intramolecular hydrogen bond was noted especially in the path integral simulation where quantum effects are taken into account. This hydrogen bond is very strong with the estimated energy of hydrogen bond ca. -27 kcal/mol. The nature of intra- and intermolecular interactions was studied by means of quantum theory of atoms in molecules. The infrared spectra were calculated and compared with available experimental data. CPMD vibrational results appear to be in good agreement with the experimental ones.

Ab Initio Molecular Dynamics Study of the Very Short O-H···O Hydrogen Bonds in the Condensed Phases.

In this paper are presented the results of theoretical studies of the structure and proton motion in very short O···O intramolecular hydrogen bonds in two molecular crystals. A comparison was conducted between 3-cyano-2,4-pentanedione (I) and 4-cyano-2,2,6,6-tetramethyl-3,5-heptanedione (II) in the solid state. The dynamics of proton motion in the O-H···O hydrogen bond were investigated in he NVT ensemble at 298 and 50 K, respectively, for crystals I and II using Car-Parrinello and path integral molecular dynamics. Very large delocalization of the bridging proton was noted especially in the path integral simulation where quantum effects are taken into account. The infrared spectrum was calculated, and a comparative vibrational analysis was performed. CPMD vibrational results appear to be in qualitative agreement with the experimental ones.

Car-Parrinello and path integral molecular dynamics study of the hydrogen bond in the acetic acid dimer in the gas phase.

In the paper are described studies of the double proton transfer (DPT) processes in the cyclic dimer of acetic acid in the gas phase using Car-Parrinello (CPMD) and path integral molecular dynamics (PIMD). Structures, energies and proton trajectories have been determined. The results show the double proton transfer in 450 K. In the classical dynamics (CPMD) a clear process mechanism can be identified, where asynchronized DPT arises due to coupling between the O-H stretching oscillator and several low energy intermolecular vibrational modes. The DPT mechanism is also asynchronic when quantum tunneling has been allowed in the simulation. It has been found that the calculated values of barrier height for the proton transfer depends very strongly on the used approaches. Barrier received from the free-energy profile at the CPMD level is around 4.5 kcal mol(-1) whereas at the PIMD level is reduced to 1 kcal mol(-1). The nature of bonding in acetic acid dimer and rearrangement of electron density due to the proton movement has been also studied by the topological analysis of Electron Localization Function and Electron Localizability Indicator function.

Proton transfer dynamics in the propionic acid dimer from path integral molecular dynamics calculations.

The double proton transfer process in the cyclic dimer of propionic acid in the gas phase was studied using a path integral molecular dynamics method. Structures, energies and proton trajectories were determined. Very large amplitude motions of the skeleton of a propionic acid molecule were observed during the simulations, and almost free rotation of the C(2)H(5) group around the C(α)-C bond. A double-well symmetric potential with a very small energy barrier was determined from the free energy profile for the proton motions. Infrared spectra for different isotopomers were calculated, and comparative vibrational analysis was performed. The vibrational results from CPMD appear to be in qualitative agreement with the experimental ones.

A Car-Parrinello and path integral molecular dynamics study of the intramolecular lithium bond in the lithium 2-pyridyl-N-oxide acetate.

Lithium bonding in lithium 2-pyridyl-N-oxide acetate has been investigated using classic Car-Parrinello molecular dynamics (CPMD) and the path integral approach [path integrals molecular dynamics (PIMD)]. The simulations have been performed in 300 K. Structures, energies, and lithium trajectories have been determined. The CPMD results show that the lithium atom is generally equidistant between heavy atoms in the (O...Li...O) bridge. Applying quantum effects through the PIMD leads to similar conclusion. The theoretical lithium 2-pyridyl-N-oxide acetate infrared spectrum has also been determined using the CPMD calculations. This shows very good agreement with available experimental results and reproduces well the broad low-frequency band observed experimentally. In order to gain deeper understanding of the nature of the lithium bonding topological analysis of the electron localization function has been applied.

Car-Parrinello and path integral molecular dynamics study of the hydrogen bond in the chloroacetic acid dimer system.

We have studied the double proton transfer (DPT) reaction in the cyclic dimer of chloroacetic acid using both classical and path integral Car-Parrinello molecular dynamics. We also attempt to quantify the errors in the potential energy surface that arise from the use of a pure density functional. In the classical dynamics a clear reaction mechanism can be identified, where asynchronized DPT arises due to coupling between the O-H stretching oscillator and several low energy intermolecular vibrational modes. This mechanism is considerably altered when quantum tunneling is permitted in the simulation. The introduction of path integrals leads to considerable changes in the thermally averaged molecular geometry, leading to shorter and more centered hydrogen bond linkages.