Kinetic model of polymorphic transitions in molecular crystals

A polychromous model in the form of an integral equation of first kind is suggested for autocatalytic polymorphic transitions of arbitrary order in molecular crystals. The model relates the observed heat absorption to the distribution of the volume of the crystalline substance over the concentrations of the initial and final modifications. The activation energy of the polymorphic transition has been calculated. The inverse problem of reconstructing the distribution function from experimental data has been solved. The model is in good quantitative and qualitative agreement with calorimetric data.     

Thermal methods for evaluating polymorphic transitions in nifedipine

The thermal behaviour of nifedipine was studied with the view to understand the various phase transitions between its polymorphs. The focus was on polymorph identification, accompanying morphological changes during crystallization and the nature of the phase transformations. These features were compared to the complexity of the crystallization mechanisms, studied by dynamic differential scanning calorimetry (DSC) heating techniques. DSC, thermogravimetry (TG) established the temperature limits for preparation of amorphous nifedipine from the melt. DSC studies identified that metastable form B, melting point ∼163 °C, was enantiotropically related to a third modification, form C, which existed at lower temperatures. Form C converted endothermically to form B at ∼56 °C on heating and was shown by hot stage microscopy (HSM) to be accompanied by morphological changes. Modulated temperature differential scanning calorimetry (MTDSC) showed discontinuities in the reversing heat flow signal during crystallization of amorphous nifedipine (from ∼92 °C) to form B, which suggested that a number of polymorphs may nucleate from the melt prior to form B formation. Identification of the number of nifedipine polymorphs included the use of combined DSC-powder X-ray diffraction (PXRD) and variable temperature powder X-ray diffraction (VTPXRD). The crystallization kinetics studied by dynamic DSC heating techniques followed by analysis using the Friedman isoconversion method where values of activation energy (E) and frequency factor (A) were estimated as a function of alpha or extent of conversion (α). The variations in E with α, from 0.05 to 0.9, for the amorphous to form B conversion could indicate the formation of intermediate polymorphs prior to form B. The form B to form A conversion showed a constancy in E on kinetic analysis from α 0.05 to 0.9, which suggested that a constant crystallization mechanism operated during formation of the thermodynamically stable form A.     

Comparison of DSC and optical transmission measurements of polymorphic transitions

Using optical measurements on single crystals, the speed of first order transitions (K₂Cr207, KNO₃, AgNO₃) was determined and compared with the strongly smeared DSC signals. Such optical measurements are proposed to select materials which are suitable for the determination of the DSC apparatus function.

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Zusammenfassung Mit Hilfe von optischen Messungen wird die Geschwindigkeit von polymorphen Phasenumwandlungen 1. Ordnung (K₂Cr₂O₇, KNO₃, AgNO₃) bestimmt und mit entsprechenden stark verschmierten DSC-Signalen verglichen. Derartige optische Messungen erscheinen geeignet, um Substanzen zu suchen, welche für die Bestimmung der DSC-Apparatefunktion geeignet sind.

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K₂Cr₂O₇, KNO₃ AgNO₃ $ , $ . , $ .

     

Exoelectron emission during polymorphic phase transitions of some ammonium salts

Results of experimental investigations of the photostimulated exoelectron emission (EEE) accompanying polymorphic phase changes in ammonium chloride, ammonium bromide and ammonium iodate are reported for the first time. The temperature-dependences of the intensity of photostimulated EEE from reagent grade powder materials were measured in air at atmospheric pressure, the exoelectrons being detected with an open air point counter with saturated ethanol quenching vapour. The DTA control measurements were performed at the same heating rate, with reagent grade Al₂O₃ as a reference. A comparison of the results of the EEE and DTA measurements shows clearly that the polymorphic phase transitions of the investigated materials are accompanied by peaks of the EEE intensity, thereby allowing the detection of solid-solid phase transitions in inorganic compounds with the EEE technique.

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Zusammenfassung In dieser Arbeit wird zum ersten Mal über die Untersuchung der photostimulierten Exoelektronenemission (EEE) berichtet, die die polymorphen Phasenübergänge von Ammoniumchlorid,-bromid und -jodat begleitet. Die Temperaturabhängigkeit der Intensität der photostimulierten EEE wurde an analysenreinen pulverisierten Proben bei Normaldruck in Luft untersucht, wobei die Detektion der Exoelektronen mit einem offenen Spitzenzähler und äthanol als Löschdampf diente. Als Bezug dienten DTA-Messungen mit analysenreinem Al₂O₃ als Referenz und mit der gleichen Aufheizgeschwindigkeit. Der Vergleich von EEE- und DTA-Ergebnissen zeigt, dass die polymorphen Phasenübergange der untersuchten Substanzen von einer verstärkten Exoelektronenemission begleitet sind. Dies ermöglicht die Anwendung der Exoemissionstechnik zur Detektion von Fest-FestPhasenumwandlungen anorganischer Verbindungen.

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Work sponsored by the Polish Ministry of Science and Education within the Central Project of Fundamental Research CPBP 01.08.A4.     

Radiationless transitions. Quantitative interpretation of some experiments.

The Green function method is used to calculate the rate of radiationless transitions. The obtained formula is applied for quantitative interpretation of the fluorescence quenching of anthracene, pyrene, methylanthracene in various solvents. The vibrational relaxation rates of small molecules substituted to the noble gas crystalline hosts are evaluated. The agreement with experiment is satisfactory.     

Theoretical interpretation of electronic absorption and emission transitions in 9-acridinones

Stationary absorption, fluorescence excitation and fluorescence spectra for 9(10H)-acridinone, 9(10-methyl)-acridinone, 2-methyl-9(10-methyl)-acridinone, 2-nitro-9(10-methyl)-acridinone, 9(10-ethyl)-acridinone and 9(10-phenyl)-acridinone dissolved in 1,4-dioxane, methyl alcohol or acetonitrile, as well as the available spectral characteristics reported by others were compared with those predicted theoretically at the semi-empirical PM3/CI (including the solvent effect within the COSMO model) or PM3/S levels of theory, in order to interpret spectral features of the compounds, i.e. the energies and probabilities of S0-->Sn, S0-->T1, T1-->T2, S1-->S0, T1-->S0 and S1-->T1 transitions. Calculations at the PM3 and PM3/CI levels of theory enabled the structural changes accompanying S0-->S1, S1-->T1 and T1-->S0 transitions to be investigated; they yielded, moreover, basic physicochemical characteristics of the molecules in the ground and excited electronic states. Theoretically predicted dipole moments and charge distributions in the S0, S1 and T1 states provided further insight into the nature of electronic transitions in 9-acridinones. The predicted characteristics correlate quite well with the available experimental ones, thus providing confirmation of the utility of theory in predicting the features of electronically excited molecules and interpreting the electronic transitions occurring in them.     

Nuclear Coordinate dependence of electronic transition moments in orbitally forbidden transitions: A new interpretation of deuterium isotope effects

The nuclear coordinate dependence of electronic transtion moments has been investigated for the purpose of finding new interpretations of deuterium isotope effects on spectral intensities and radiative decay rates in orbitally forbidden electronic transitions. By using “AO following nuclei” wavefunctions as the building block for the electronic wavefunction in the adiabatic BO vibronic wavefunction, the spin-free hamiltonian is diagonalized to generate eigenfunctions and eigen-energies. It is found that the electronic transtion moments based on these eigenfunctions show dependences upon the vibrational modes which are not directly involved in vibronic coupling. This leads to interpretations of the deuterium isotope effects in T₁ → S₀ radiative transitions of aromatic hydrocarbons and S₀ → S₁ absorption in pyrazine which are not based on the conventional Herzberg—Teller or non-BO coupling.     

New polymorphic types of diamond

The results of calculations by PM3 and LDA-DFT methods of the structure and properties of six new polymorphic types of diamond, in which all atomic sites are crystallographically equivalent, are presented. The structures of LA5 (Cmca), LA7 (Cmcm), and LA8 (I4₁/amd) phases are obtained as a result of stitching graphene layers and of CA9 \((Fd\bar 3m)\) , CA10 \((R\bar 3m)\) , and CA11 (P6₃/mmc) phases by stitching fullerenelike clusters. For these phases the geometrically optimized structures are calculated and the structural parameters, density, sublimation energy, bulk modulus, electron density of states, and X-ray diffraction pattern are measured. It is found that the properties of polymorphic types of diamond depend on the degree of their structure deformation in comparison with the cubic diamond structure.     

Cooling of melts: kinetic stabilization and polymorphic transitions in the KInSnSe 4 system

Through a flame-melting/rapid-cooling process, metastable forms of solid state compounds can be discovered. We describe here an example where both slow and rapid crystallizations of a stoichiometric "KInSnSe(4)" melt give rise to kinetic forms of KInSnSe(4). These forms (alpha- and beta-) convert to the thermodynamically stable gamma-form upon heating below the melting point.     

Pressurization effects on the polymorphic forms of famotidine

The effects of high static pressure on the polymorphic modifications A and B of famotidine were examined by differential scanning calorimetry, infrared and Raman spectroscopy, and X-ray powder diffractometry. The obtained effects appeared to differ significantly depending on whether they were monitored by DSC or any of the other above techniques. In particular, DSC measurements tend to deceptively amplify a tendency of the pure modification B to turn into the more stable form A under pressurization, while the spectroscopic methods and XRPD exhibit no essential change in the crystal structure of the metastable form B. The apparent morphological transformation in the pressed samples stems from the nature of the DSC method itself.     

Magnetodielectric effects at magnetic ordering transitions

The dielectric constant in many insulating magnets exhibits pronounced changes at magnetic ordering temperatures or with the application of external magnetic fields. As the dielectric response is sensitive to materials properties at non-zero wavevectors, dielectric spectroscopy provides additional insight into spin structures in these insulators beyond what can be determined using bulk magnetization measurements alone. We present illustrative examples of intrinsic magnetodielectric coupling in non-polar ferro- and antiferromagnets, SeCuO₃ and TeCuO₃ respectively, and of magnetodielectric coupling in a polar system, specifically Ni₃V₂O₈. We also show an example of how spurious magnetocapacitive effects can significantly modify the magnetodielectric response of a system. We include with some general comments on how the magnetodielectrics may be useful for device applications and how the coupling may be enhanced by modifying the materials.     

Relativistic effects in transitions of highly ionized heavy atoms

Wavelenghts, oscillator strengths, and lifetimes of KL shell heavy ions such as Fe²⁰⁺, Br³⁰⁺, etc. are predicted, including both relativistic and correlation effects. In such ions the distinction between “normal” electric dipole (El) and spin—orbit allowed electric dipole transitions (SOAEl) disappears and the Z-dependence of oscillator strengths is very different from the predictions of non-relativistic orbital theories. The feasibility of observing these ions in heavy ion accelerators is indicated. The results have implications for relativistic many-electron theory, ESCA, X-ray spectra, and molecular total energies.     

Simulation of temperature-modulated DSC of transitions represented by abrupt changes in heat capacity

Computer simulations have been applied to elucidate the response of a sample to temperaturemodulated differential scanning calorimetry (tm-DSC) during transitions. Two cases have been simulated; a latent heat without supercooling (represented by an abrupt heat capacity pulse with perfect reversibility) and a latent heat with perfect supercooling or large hysteresis (an abrupt heat capacity change without reversibility, i.e. the change in heat capacity is seen on heating, but not on cooling). Because the simulation was applied to these well-characterized phenomena, the results are useful to reveal actual sample thermal responses during transitions. The non-reversible component was observed in both cases and has no distinct difference. Higher harmonics due to non-linearity of the transitions were also observed. Furthermore, by inspecting thermal response of the sample and the essential feature of tm-DSC, a new method of data analysis has been devised.     

Molecular-orbital calculations on transition-metal complexes: Charge-transfer transitions and the interpretation of photoelectron spectra

The results of some molecular-orbital calculations on some transition-metal complexes according to a novel all-valence electron method are summarized. A comparison is made between the calculated and experimental charge-transfer spectra. In addition, attention is drawn to the remarkable sequence of the energy levels and the resulting consequences for the interpretation of photoelectron spectra.     

New polymorphic modification of the crystal structure of bromopeganol

The crystal structure of 6-bromopeganol in a new polymorphic modification is found and determined by single crystal X-ray diffraction analysis. For the crystal structure of the new polymorph the formation of a centrosymmetric closed dimer (associate) is also characteristic. It consists of two bromopeganol molecules bound by centrosymmetric reciprocal hydrogen bonds O-H...N(1). This polymorph differs from the known one in the mutual arrangement and interaction of these associates. Original Russian Text Copyright ? 2009 by R. Ya. Okmanov, A. G. Tozhiboev, K. K. Turgunov, B. Tashkhodzhaev, N. I. Mukarramov, and Kh. M. Shakhidoyatov __________ Translated from Zhurnal Strukturnoi Khimii, Vol. 50, No. 2, pp. 396–398, March–April, 2009.     

Magnetic phase transitions and specific heat of single crystalline cupric oxide

The antiferromagnetic phase transitions in a CuO single crystal are studied by specific heat in magnetic fields up to 6T. The magnetic field dependence of the incommensurate-to-commensurate-antiferromagnetic transition atT _L is found to be highly anisotropic.T _L is observed to increase nonlinearly for B_a c-axis, whereas, a linear reduction is observed forB _a b-axis. The magnetic field dependence ofT _L and the jumps in magnetic susceptibility atT _L are explained thermodynamically using the Clausius-Clapeyron equation.We thank Dr. T. Chattopadhyay for providing the CuO crystal. We thank K. Ripka and the Röntgen service of the institute for technical assistance.     

Size and surface effects on phase transitions

Size effects on liquid crystal phase transitions are investigated by scanning calorimetry of samples absorbed in porous silica. For pore diameter of order 100 Å, the liquid crystal transition depressions are of order 0.1°C and the solid melting points of order 10°C.     

New interpretation of the AgL-series x-ray spectrum

Based on the de-excitation model of atomicL-shell vacancies, recent put forward by us, a new interpretation of Paratt's AgL x-ray spectrum has been successfully carried out, by using experimental and semiempiricalL ₁-hole decay data reported in recent years. An initialL-vacancy distributionN ₁:N ₂:N ₃=1:1.44:3.14 has been calculated from the silver spectrum. Present value of AgM-electron shakeoff probability due to anL-shell hole isP _M =0.034, which is much larger than theoretical calculations and semiempirical data.     

Rotational effects on intramolecular radiationless transitions in a large molecule

We report the observation of a pronounced rotational-state dependence for the fluorescence quantum yield and fluorescence decay from the S₁(0,0+1160 cm⁻¹) state of jet-cooled 9-cyanoanthracene. We attribute these results to the effects of Coriolis rotational-vibrational interaction on intramolecular vibrational energy redistribution, which influence electronic relaxation.     

Novel spectroscopic effects in molecular transitions involving degenerate electronic states

We discuss the intricate Jahn—Teller-like and non-Jahn—Teller spectroscopic patterns which result via the recently revealed cross-quadratic terms involves modes of different symmetry which arise in the nuclear potential of degenerate electronic states of non-linear molecules.