Specifics of first-order phase transitions in 1,22-docosanediol and 1,44-tetratetracontanediol

A comparative analysis of phase transitions in diols with different chain lengths, (CH₂)₄₄(OH)₂ and (CH₂)₂₂(OH)₂, was performed using differential scanning calorimetry. The use of temperature hysteresis made it possible to reveal a number of new effects associated with the specifics of first-order phase transitions. The parameters of transitions were quantitatively analyzed in terms of the self-consistent field theory for diffuse (A-shaped) first-order transitions.     

Collision-induced rotational transitions in nali(1Π)

Collision-induced rotational transitions in the electronically excited NaLi molecule have been studied using laser excited fluorescence. Due to the greater number of allowed transitions as compared with Na₂ and Li₂, more cases were found of transitions +ΔJ and ?ΔJ having greatly different cross sections (“+/ - asymmetry”). This observation is in agreement with recent predictions on the basis of Born's approximation. All observed transitions are classified according to the symmetry of the contributing potential terms. Data are presented which indicate the necessity for refinements of the theory.     

Phase transitions in molecular crystals of carboxylic acids

A comparative analysis of phase transitions in molecular crystals of carboxylic acids (n-alkano acids) with various chain lengths, (CH₃(CH₂) _n COOH) is performed with the use of DSC. A number of new effects related to first-order phase transitions are discovered. The temperature dependence of heat capacity is quantitatively analyzed in terms of the theory of blurred (Λ-shaped) phase transitions of the first order.     

Phase transitions in paraffins, <Emphasis Type="Italic">n</Emphasis>-alkane alcohols,and <Emphasis Type="Italic">α,ω</Emphasis>-alkane diols with different chain lengths

A comparative analysis of phase transitions in paraffins (C _n H_2n+2), n-alkane alcohols (C _n H_2n+1OH), and α,ω-alkane diols ?ubC _n H_2n (OH)₂?ub; has been performed via differential scanning calorimetry. The elimination of methodical errors makes it possible to calculate the true values of thermodynamic parameters of phase transitions and to gain insight into their nature. Methods of finding true heat capacity jumps in the region of low-temperature solid-phase transitions of the first order and high-temperature order-disorder transitions of the second-order have been proposed. The quantitative analysis of heat capacity jump profiles has been performed within the framework of the first-order diffuse transition theory and Landau’s theory of order-disorder transitions.     

Thermal stabilities of isoelectronic,isostructural nitrates,carbonates and borates

The effects of temperature changes on an isoelectronic, iso-structural group of compounds containing an iso-electronic, iso-structural complex of ions were studied. The stability of the nitrates is discussed on the basis of their temperature of fusion and solid state transitions; the carbonates from their decomposition temperatures and solid state transitions; and the borates on solid state transitions. The order of thermal stabilities is found to be ABO₃ > A′ CO₃ > A″ NO₃ .     

S 2p photoabsorption of the SF5CF3 molecule: Experiment,theory and comparison with SF6

The S 2p core excitation spectrum of the SF₅CF₃ molecule has been measured in the total ion yield mode. It resembles a lot the analogous spectrum of SF₆, also recorded in this study, displaying intense transitions to the empty molecular orbitals both below and above the S 2p ionization potential (IP) and weak transitions to the Rydberg orbitals. The S 2p photoabsorption spectra of SF₆ and SF₅CF₃ have been calculated using time-dependent density functional theory, whereby the spin–orbit coupling was included for the transitions below the S 2p IP. The agreement between experiment and theory is good for both molecules, which allows us to assign the main S 2p absorption features in SF₅CF₃.     

Transition de phases observee par measures optiques sur des monocristaux de SrAlF5 et de Pb3(CrF6)2

Optical measurements have been performed on SrAlF₅ and Pb₃(CrF₆)₂ crystals elaborated by a Bridgman technique. The thermal dependence of the birefringence shows anomalies which correspond to ferroelectric-paraelectric transitions. The transitions are of first order, the tetragonal crystal symmetry is not modified.     

Structural Phase Transitions of a Molecular Metal Oxide

The structural phase of a metal oxide changes with temperature and pressure. During phase transitions, component ions move in multidimensional metal–oxygen networks. Such macroscopic structural events are robust to changes in particle size, even at scales of around 10 nm, and size effects limiting these transitions are particularly important in, for example, high-density memory applications of ferroelectrics. In this study, we examined structural transitions of the molecular metal oxide [Na@(SO₃)₂(n-BuPO₃)₄Mo^V₄Mo^VI₁₄O₄₉]⁵⁻ (Molecule 1 ) at approximately 2 nm by using single-crystal X-ray diffraction analysis. The Na⁺ encapsulated in the discrete metal-oxide anion exhibited a reversible order–disorder transition with distortion of the Mo–O molecular framework induced by temperature. Similar order–disorder transitions were also triggered by chemical pressure induced by removing crystalline solvent molecules in the single-crystal state or by substituting the countercation to change the molecular packing.     

On the role of intramolecular vibrations in the nonradiative decay of excited charge-transfer states of molecular complexes

An attempt for a theoretical treatment of radiationless transitions from excited charge-transfer states in molecular complexes is made within the framework of the statistical limit of radiationless transitions theory. This work deals with the S₁ → S₀ internal conversion in charge-transfer complexes of tetracyanoethylene (an electron acceptor) with benzene and toluene and their perdeuterated analogues. A dominant role of the high-frequency totally symmetric intramolecular vibrational modes in the nonradiative decay of excited charge-transfer states is assumed (this was inferred from the experimentally observed deuterium isotope effect on radiationless S₁ → S₀ transitions). Calculated absolute rate constants for internal conversion are found to be in good agreement with experimental ones. The results of our calculations reflect very well the observed moderate deuterium isotope effect.     

The heat capacity of Cu2Te(c), CuTe(c), Ag2Te(c) And Ag1.64 Te(c)

The heat capacities and enthalpies of Cu₂Te(c), CuTe(c), Ag₂Te(c) and Ag_1.64Te(c) have been measured for the temperature range (280–8OO K) using differential scanning calorimetry. Several transitions were observed in these substances and the enthalpies associated with these transitions have been determined.     

Effect of semi-empirical parameters on the triplet energy levels and triplet–triplet transition in benzene

Triplet energy levels and triplet–triplet transition in benzene have been calculated semi-empirically by considering CI up to and including doubly excited configurations and using various values of the core resonance and electron repulsion integrals. The usual method of calibrating β (core resonance integral) from one of the observed transitions in the semi-empirical methods is critically examined with respect to the triplet levels of this molecule. Semi-empirical parameters are found to have a pronounced effect on the energy of the triplets and the triplet–triplet transitions; but the intensities of such transitions are quite insensitive to the choice of such parameters. Numerical results show several ³B_1u → ³E_2g transitions of various intensities. Out of these, the transitions which correspond energetically to the observed triplet–triplet bands are found to have low intensity. Some strong triplet–triplet bands are predicted in the far ultra violet region.     

Calculation of the ultraviolet and visible absorption spectra of the complex of nickel (ii) with 1, 5-diphenylformazan

The ultraviolet and visible absorption spectra of 1,5-diphenylf ormazan and its nickel complex in acetone, ethanol, and carbon tetrachloride have been calculated. For a qualitative interpretation of the spectrum, the calculations for the molecules of the complex and the ligand were carried out by the MO LCAO method in the Mulliken-Helmholz-Wolf sberg approximation. According to the results obtained the oscillator strengths of the transitions in the spectrum of the complex decrease as the interaction of the solvents with Ni increases in the order CCl₄, C₂H₅OH, C₃H₆O. This leads to a decrease in the negative charges on the nitrogen atoms and a decrease in the oscillator strengths of the transitions between the MO including the AO of nitrogen. All the transitions are interpreted as —* transitions.     

Crystal field theory treatment of S2 ion impurity centres with Oh, D4h, C4v and C2v symmetry in alkali halides

The effect of impurity anions on the electron transitions in alkali halide phosphors has been studied. The energies of electron transitions have been calculated for O_h, D_4h, C_4v and C_2v symmetries in the cases of strong and weak fields. The theoretical model has been shown to agree satisfactorily with experimental data assuming the formation of higher symmetry impurity centres to be more probable.     

Brittle-tough transition temperatures in impact tests on rubber-toughened plastics

— In notched impact tests, rubber-toughened plastics show abrupt transitions in toughness with increasing temperature. Typically, there are two such transitions, one just above the T_g of the rubber phase, and the second some tens of degrees higher. The factors affecting the positions of these transitions are discussed. The relaxation behaviour of the rubber phase is a key factor, but rubber content and particle size are also important. Examples of impact transitions in high-impact polystyrene (HIPS) and toughened nylon are discussed.     

Etudes par spectroscopie dans l'ultra-violet et le visible, du fullerène C60 en solution dans le n-hexane ou adsorbé sur des solides diélectriques

Ultraviolet and visible spectroscopy studies of the fullerene C₆₀ melted in n-hexane or adsorbed on dielectric solids. Experiments of spectroscopy have been carried out, between the far ultraviolet and the near infrared, with pure C₆₀ or containing endohedral complexes rare gas atom-C₆₀, melted in n-hexane, and also pure C₆₀ adsorbed on surfaces of solids.Several phenomena have been observed: 1. a charge transfer due to the influence of interactions between the molecules of C₆₀ and surfaces of solids, yielding new transitions observed in the far ultraviolet ; and interpreted as super excited states of Rydberg type; 1.1. red and blue shifts for some transitions of adsorbed C₆₀, interpreted on the basis of attractive, or turned repulsive, dispersion forces; 1.1.1. emission processes for several transitions of adsorbed C₆₀, explained on the basis of a resonant coupling between these transitions and modes of the surface plasmon induced by the collective excitations of the C₆₀ molecule; this kind of process might be drawn to the Faraday instability in acoustics.     

Single-crystal absorption spectra of CsVX3 (X = Cl,Br, I)

Polarized single-crystal absorption spectra of CsVCl₃, CsVBr₃ and CsVI₃ have been measured between 5000 and 30000 cm^?1 at temperatures ranging from 6 to 273 K. Spin-allowed transitions arise through a vibronic single-ion mechanism. Spin-forbidden transitions are strongly enhanced through an exchange intensity mechanism.     

Infrared—infrared double resonance using an acousto—optic modulator

The use of infrared—infrared double resonance to identify near coincidences between vibration—rotation transitions of CH₂NOH and ¹³CO₂ laser lines is described. The dipole moments derived from these data show that all the transitions belong to the ν₈ A′ band of formaldoxime.     

Hydrogen bonding between solvation shells around gd3+ from cooperative vibronic spectra

Cooperative vibronic spectra involving Gd³⁺ electronic transitions and the vibrational transitions of nearby water molecules are used to determine the stretching spectrum for isotopically dilute OH in a solution of GdCl₃ in D₂O. The OH stretching spectrum of water molecules in the first hydration sphere is shifted to lower energy than that of the bulk liquid.     

Isotope shifts and hyperfine structures investigation of doubly excited levels in SrI

We measured isotope shifts and hyperfine structure of visible transitions of stable strontium isotopes by means of Doppler-free saturated absorption spectroscopy. In particular, we investigated transitions between excited states where the upper level involves two excited electrons. We report hyperfine coupling constants for the levels 5p4d¹D₂, 5p4d³F₂, 5s4d³D₁, 5s6s³S₁, 5s5p³P₁ and, for some of the studied transitions, we separate the specific mass and volume contribution to the isotope shifts.     

Magnetic circular dichroism and molecular orbital studies on bicyclo[6,2,0] decapentaene

The magnetic circular dichroism (MCD) spectrum of bicyclo[6,2,0] decapentaene has revealed four skeletal π → π^* electronic transitions in the visible and ultraviolet region. The four MCD bands are assigned to the B₂ ← A₁, A₁ ← A₁. B₂ ← A₁ and A₁ ← A₁ electronic transitions in increasing order of energy.