Thermal broadening of the J-band in disordered linear molecular aggregates: a theoretical study

We theoretically study the temperature dependence of the J-band width in disordered linear molecular aggregates, caused by dephasing of the exciton states due to scattering on vibrations of the host matrix. In particular, we consider inelastic one- and two-phonon scatterings between different exciton states (energy-relaxation-induced dephasing), as well as the elastic two-phonon scattering of the excitons (pure dephasing). The exciton states follow from numerical diagonalization of a Frenkel exciton Hamiltonian with diagonal disorder; the scattering rates between them are obtained using the Fermi golden rule. A Debye-type model for the one- and two-phonon spectral densities is used in the calculations. We find that, owing to the disorder, the dephasing rates of the individual exciton states are distributed over a wide range of values. We also demonstrate that the dominant channel of two-phonon scattering is not the elastic one, as is often tacitly assumed, but rather comes from a similar two-phonon inelastic scattering process. In order to study the temperature dependence of the J-band width, we simulate the absorption spectrum, accounting for the dephasing-induced broadening of the exciton states. We find a power-law (T(p)) temperature scaling of the effective homogeneous width, with an exponent p that depends on the shape of the spectral density of the host vibrations. In particular, for a Debye model of vibrations, we find p approximately 4, which is in good agreement with the experimental data on J aggregates of pseudoisocyanine [I. Renge and U. P. Wild, J. Phys. Chem. A, 101, 7977 (1997)].     

Effect of anharmonicity and vibrations resonance interaction in the vibrational spectra of H-bonded crystals

On the basis of the absorption coefficient calculation it is shown that strong optical and mechanical anharmonicity of the OH group vibrations and the interaction between the main tone and the overtone of the other vibration of the same OH group and the same symmetry (Fermi-resonance) leads to the formation of two-phonon bound states (biphonons) seen in spectra as “additional” sharp bands. The IR and Raman bands half-width and shape of the main tone and the bound state is measured as a function of temperature and a possible theoretical treatment is proposed. The anharmonicity parameters and low-frequency vibrations responsible for the temperature behavour of the main tone and the biphonons are determined.     

Size Dependence of Nanoscale Confinement on Chiral Transformation

Molecular dynamic simulations of the chiral transition of a difluorobenzo[c]phenanthrene molecule (C₁₈H₁₂F₂, D molecule) in single‐walled boron‐nitride nanotubes (SWBNNTs) revealed remarkable effects of the nanoscale confinement. The critical temperature, above which the chiral transition occurs, increases considerably with the nanotube diameter, and the chiral transition frequency decreases almost exponentially with respect to the reciprocal of temperature. The chiral transitions correlate closely with the orientational transformations of the D molecule. Furthermore, the interaction energy barriers between the D molecule and the nanotube for different orientational states can characterize the chiral transition. This implies that the temperature threshold of a chiral transition can be controlled by a suitable nanotube. These findings provide new insights to the effect of nanoscale confinement on molecular chirality.     

Superpositions of chiral states in the presence of electric dipole, magnetic dipole, and electric quadrupole interactions

This paper is a straightforward generalization of Maierle-Harris proposal regarding parity implications on the superpositions of chiral states of a molecule. It is shown that the inclusion of electric quadrupole and magnetic dipole interactions removes several of restrictions on the preparation of superpositions of mid R:L and mid R:R states of a chiral molecule. It is also found that the dephasing of mid R:L and mid R:R superpositions, due to the spontaneous emission from the chiral molecule, has opposing contributions from electric quadrupole-magnetic dipole and electric dipole interactions.     

A physico-chemical analysis of soluble and immobilized enzyme stabilization

The literature and our experimental data on the effect of chemical modification and immobilization on the thermostability of enzymes are analyzed. The effect of various factors causing changes in the stability of enzymes after their modification or immobilization is demonstrated. It is shown that changes in the temperature dependence of the inactivation rate constant are associated with the change in the effective values of thermodynamic activation parameters for the inactivation processes. An increase in the activation energy of thermoinactivation, E_a, leads to the stabilization of a modified or immobilized enzyme at temperatures below the iso-kinetic temperature (“low-temperature” stabilization) and a decrease inE _a entails a “high-temperature” stabilization of enzymes. It is shown that with immobilized enzymes the high-temperature stabilization is invariably observed.     

Experimental and actual values of self-diffusion coefficients of liquids in porous media

Self-diffusion coefficients D* of liquids in porous media, as measured by the pulsed-field gradient NMR method, have been investigated as functions of the interval t ₁ between the first and the second radio-frequency pulses (the time of spin dephasing) in the “stimulated echo” pulsed procedure. Experimental D*(t ₁) dependences have been shown to be adequately described by the relations of the Fatkullin theory for moderate and short correlation times of spin motion in an internal random Gaussian magnetic field. Actual self-diffusion coefficients and some parameters of the porous media have been estimated.     

Self‐Assembly of 1,1′‐Biphenyl‐2,2′,6,6′‐tetracarboxylic Acid: Formation of an Achiral Grid with Chiral Compartments

D ₄ ‐symmetric chiral hydrogen‐bonded cyclotetramers (see the structure in the picture) are present in the self‐assembled achiral title compound in the solid state. The unilayered network set up from the chiral “square” blocks is achiral as a consequence of the crystal symmetry.     

A stationary‐wave model of enzyme catalysis

An expression for the external force driving a system of two coupled oscillators in the condensed phase was derived in the frame of the Debye theory of solids. The time dependence and amplitude of the force is determined by the size of the cell embedding the coupled oscillators and its Debye temperature (θ_D). The dynamics of the driven system of oscillators were followed in the two regimes of (a) low θ_D and cell diameter, as a model of liquid water, and (b) large θ_D and cell diameter, as a model of the core of a protein. The response in potential energy of the reference oscillator was computed for all possible values of the internal parameters of the system under investigation. For protein cores, the region in the parameter space of high maximum potential energy of the reference oscillator is considerably extended with respect to the corresponding simulation for water. © 2009 Wiley Periodicals, Inc. J Comput Chem, 2010     

Biphonon dynamics in crystals: Nitrous oxide

Coherent picosecond excitation and probe techniques are employed to measure the dephasing times of the very weak 2v₂ two-phonon resonance in solid N₂O as a function of temperature. T₂ times are also obtained for the v₁ one-phonon transition which is in feeble Fermi interaction with 2v₂.     

Chameleonic Behavior of the α-Methylcyclopropyl Group and Its Through-Space Interactions: A Route to Stabilized Three Redox States in Diarylnitroxides

The α-methylcyclopropyl (MCP) group with conformationally dependent electronic properties is suggested as an additional structural “instrument” for stabilization of both open-shell and ionic states of diarylnitroxides, to be used as “ambipolar” redox active materials. New MCP-substituted diphenylnitroxides (fully characterized by electrochemical, spectral, and X-ray data) are the most stable nitroxides of the diaryl type known to date [τ_1/2 in benzene exceeds three months (2310 h)]. The radicals are capable to reversible oxidation and reduction, yielding stable oxoammonium cations and aminoxyl anions. DFT investigation of the electronic structure and geometry of the compounds confirmed the conformational switching of the cyclopropyl orientation relative to the adjacent aromatic π system is dependent on the nitroxide's redox state. Additional through-space stabilizing interaction between the π-acceptor orbital of the NO⁺ moiety and the cyclopropyl “banana” bond orbital was also detected, highlighting its good hyperconjugative ability. The estimated σ(para)_MCP value (−0.32) confirms its strong electron-donating properties.     

Molecular orbital theory of the hydrogen bond. 24. Ground-state water–uracil complexes

Ab initio SCF calculations with the STO -3G basis set have been performed to investigate the structural, energetic, and electronic properties of mixed water–uracil dimers formed at the six hydrogen-bonding sites in the uracil molecular plane. Hydrogen-bond formation at three of the carbonyl oxygen sites leads to cyclic structures in which a water molecule bridges N₁? H and O₂, N₃? H and O₂, and N₃? H and O₄. Open structures form at O₄, N₁? H, and N₃? H. The two most stable structures, with energies of 9.9 and 9.7 kcal/mole, respectively, are the open structure at N₁? H and the cyclic one at N₁? H and O₂. These two are easily interconverted, and may be regarded as corresponding to just one “wobble” dimer. At 1 kcal/mole higher in energy is another “wobble” dimer consisting of an open structure at N₃? H and a cyclic structure at N₃? H and O₄. The third cyclic structure at N₃? H and O₂ collapses to the “wobble” dimer at N₃? H and O₄. The two “wobble” dimers are significantly more stable than the open dimer formed at O₄, which has a stabilization energy of 5.4 kcal/mole. Uracil is a stronger proton donor to water through N₁? H than N₃? H, owing to a more favorable molecular dipole moment alignment when association occurs through H₁. Hydration of uracil by additional water molecules has also been investigated. Dimer stabilization energies and hydrogen-bond energies are nearly additive in most 2:1 water:uracil structures. There are three stable “wobble” trimers, which have stabilization energies that vary from 7 to 9 kcal/mole per water molecule. Hydrogen-bond strengths are slightly enhanced in 3:1 water:uracil structures, but the cooperative effect in hydrogen bonding is still relatively small. The single stable water–uracil tetramer is a “wobble” tetramer, with two water molecules which are relatively free to move between adjacent hydrogen-bonding sites, and a stabilization energy of approximately 8 kcal/mole per water molecule. Within the rigid dimer approximation, successive hydration of uracil is limited to the addition of one, two, or three water molecules.     

Calculated lifetimes and linewidth temperature dependence (1.5–150 K) of four internal vibrations of naphthalene crystal

The linewidths of four totally symmetric vibrations of naphthalene have been calculated as a function of temperature between 1.5 and 150 K on the basis of a population relaxation mechanism. The anharmonic coefficients coupling internal and lattice phonons are obtained from an intermolecular potential that includes atom-atom and quadrupole-quadrupole contributions. The agreement with the experimental data is good for two of the modes considered, while it is unsatisfactory for the others. The results show that the population relaxation (T₁ decay) can contribute substantially to the observed dephasing also at relatively high temperature. The internal and external modes responsible for the vibrational decay are individualized, showing that several distinct two-phonon processes with different weights are involved in the relaxation of the optical modes.     

The “Inverted Bonds” Revisited: Analysis of “In Silico” Models and of [1.1.1]Propellane by Using Orbital Forces

This article dwells on the nature of “inverted bonds”, which refer to the σ interaction between two sp hybrids by their smaller lobes, and their presence in [1.1.1]propellane. Firstly, we study H₃C−C models of C−C bonds with frozen H-C-C angles reproducing the constraints of various degrees of “inversion”. Secondly, the molecular orbital (MO) properties of [1.1.1]propellane and [1.1.1]bicyclopentane are analyzed with the help of orbital forces as a criterion of bonding/antibonding character and as a basis to evaluate bond energies. Triplet and cationic states of [1.1.1]propellane species are also considered to confirm the bonding/antibonding character of MOs in the parent molecule. These approaches show an essentially non-bonding character of the σ central C−C interaction in propellane. Within the MO theory, this bonding is thus only due to π-type MOs (also called “banana” MOs or “bridge” MOs) and its total energy is evaluated to approximately 50 kcal mol⁻¹. In bicyclopentane, despite a strong σ-type repulsion, a weak bonding (15–20 kcal mol⁻¹) exists between both central C−C bonds, also due to π-type interactions, though no bond is present in the Lewis structure. Overall, the so-called “inverted” bond, as resulting from a σ overlap of the two sp hybrids by their smaller lobes, appears highly questionable.     

Isotropic and anisotropic Raman scattering study in liquid p-dioxane

The isotropic and anisotropic profiles of the 835 and 2965 cm⁻¹ Raman lines of p-dioxane in the neat liquid and in solution have been studied as a function of temperature and concentration. From the correlation functions obtained by Fourier inversion of band contours, the possible interaction responsible for the vibrational dephasing of the oscillators and their reorientational relaxation are considered. It is shown that the p-dioxane molecule tumbling about the C₂ axis in the molecular plane perpendicular to the oxygen-oxygen direction proceeds by small-step Brownian diffusion associated with an Arrhenius activation energy of 9.0 kJ mol⁻¹. The vibrational relaxation mechanism of the two modes is interpreted in terms of pure dephasing due to weak collisions.     

A new statistical thermodynamic theory for substoichiometric fluorite structure compounds and its application III. Application of the model theory based on “tetrahedral” defects to some substoichiometric fluorite-structure compounds: Phase diagrams and “residual structures” at high temperature

In previous papers, a theory was proposed to explain the thermodynamic functions of substoichiometric fluorite structure compounds, on the basis of a new defect, which was called the tetrahedral defect, constituted by an oxygen vacancy which is bound with two reduced cations in its coordination tetrahedron. In this paper, the relevant energy parameters are discussed, and their number reduced to a “bonding energy” δ of the tetrahedral defect, a “strain energy” Δ measuring in first approximation, an isotopic strain introduced by packing tetrahedral defects in the fluorite lattice, a long-range electrostatic interaction u between “free” charged point defects and a “dipolar interaction energy” ϵ between tetrahedral defects: the latter two parameters can be in principle evaluated. By varying suitably the energy parameters, and comparing the results with the phase diagrams of oxides of the lanthanide and actinide elements, the difference between PuO_2−x (and AmO_2−x) (for which no low-temperature subphase is known) and all other fluorite-structure oxide systems may be explained.Variations in the slope of experimental oxygen potential vs stoichiometry curves which are found at high temperature for all these system, are explained (and fairly reproduced) in terms of residual structures.     

Spectroscopic studies of the ν2 mode of methyl iodide by Raman spectroscopy

Polarized Raman studies of the ν₂ mode in methyl iodide at 1 and 1600 bar have confirmed, through an independent route, the results previously found by isotopic-dilution studies. By using a cubic-close-packed approximation of the liquid state and a 1/r⁶ interaction potential for dipolar dephasing between molecules, the data may be reconciled. This result is interpreted to be strong evidence for the dominance of the intermolecular transition-dipole dephasing mechanism.     

The potential energy surface of the jahn-teller-distorted 2E' ground state of copper trimer

It is shown that the configuration interaction calculations of Walch and Laskowski for the ground ²E' state of Cu₃ are consistent with truncating the potential energy at second order in the distance from the D_3h conical intersection, which leads to a diabatic representation (the “quadratic coupling model”) containing three parameters. Fitting these parameters to three of the vibronic spacings of Rohlfing and Valentini leads to a reasonable explanation of their spectrum. The resulting potential energy surfaces show a Jahn-Teller stabilization energy of 221 cm⁻¹ and a pseudorotation barrier of 95 cm⁻¹.     

Next‐Generation D2‐Symmetric Chiral Porphyrins for Cobalt(II)‐Based Metalloradical Catalysis: Catalyst Engineering by Distal Bridging

Novel D₂‐symmetric chiral amidoporphyrins with alkyl bridges across two chiral amide units on both sides of the porphyrin plane (designated “HuPhyrin”) have been effectively constructed in a modular fashion to permit variation of the bridge length. The Co^II complexes of HuPhyrin, [Co(HuPhyrin)], represent new‐generation metalloradical catalysts where the metal‐centered d‐radical is situated inside a cavity‐like ligand with a more rigid chiral environment and enhanced hydrogen‐bonding capability. As demonstrated with cyclopropanation and aziridination as model reactions, the bridged [Co(HuPhyrin)] functions notably different from the open catalysts, exhibiting significant enhancement in both reactivity and stereoselectivity. Furthermore, the length of the distal alkyl bridge can have a remarkable influence on the catalytic properties.     

Temperature dependence of the vibrational phase relaxation in gases: Application to H2-rare gas mixtures

The vibrational dephasing rate is calculated for H₂-rare gas mixtures between 85 and 300 K. The semiclassical calculation which only considers unbounded trajectories is based on binary interaction of particles through a Lennard—Jones potential. A comparison is made with three-dimensional and with collinear results obtained when a purely repulsive potential is considered. It is shown that the temperature dependence of the vibrational dephasing rate increases monotonically as T rises. Experimental investigation of Q₁ (1) Raman lines for n-H₂ perturbed by neon, argon and krypton between 100 and 300 K has been realized. The comparison between calculated and measured linewidths suggests that bounded states have to be taken into account at the lowest temperatures considered here.     

Nanographene Imides Featuring Dual‐Core Sixfold [5]Helicenes

A novel kind of nanographene imide, namely pentaperylene decaimides (PPD) featuring dual‐core sixfold [5]helicenes and ten imide groups, was efficiently obtained. Among the possible 28 stereoisomers, which include 14 pairs of enantiomers, only one pair of enantiomers was obtained selectively which could be separated by chiral HPLC. Single‐crystal X‐ray diffraction analyses revealed that it exhibits a D₂‐symmetric “four‐bladed propeller” conformation composed of conjoined double “three‐bladed propeller”, which is very stable and could not convert into other conformations even when heated up to 200 °C. Meanwhile, enantiomerically pure PPD also exhibits an excellent resistance to thermally induced racemization, which makes it a promising candidate for various applications in chiral material science.