Theoretical analysis and computer simulation of fluorescence lifetime measurements. I. Kinetic regimes and experimental time scales

The configuration-controlled regime and the diffusion-controlled regime of conformation-modulated fluorescence emission are systematically studied for Markovian and non-Markovian dynamics of the reaction coordinate. A path integral simulation is used to model fluorescence quenching processes on a semiflexible chain. First-order inhomogeneous cumulant expansion in the configuration-controlled regime defines a lower bound for the survival probability, while the Wilemski-Fixman approximation in the diffusion-controlled regime defines an upper bound. Inclusion of the experimental time window of the fluorescence measurement adds another dimension to the two kinetic regimes and provides a unified perspective for theoretical analysis and experimental investigation. We derive a rigorous generalization of the Wilemski-Fixman approximation [G. Wilemski and M. Fixman, J. Chem. Phys. 60, 866 (1974)] and recover the 1/D expansion of the average lifetime derived by Weiss [G. H. Weiss, J. Chem. Phys. 80, 2880 (1984)].     

Theoretical study on the sound absorption of electrolytic solutions. I. Theoretical formulation

A theory is formulated that describes the sound absorption of electrolytic solutions due to the relative motion of ions, including the formation of ion pairs. The theory is based on the Kubo-Green formula for the bulk viscosity. The time correlation function of the pressure is projected onto the bilinear product of the density modes of ions. The time development of the product of density modes is described by the diffusive limit of the generalized Langevin equation, and approximate expressions for the three- and four-body correlation functions required are given with the hypernetted-chain integral equation theory. Calculations on the aqueous solutions of model electrolytes are performed. It is demonstrated that the theory describes both the activated barrier crossing between contact and solvent-separated ion pairs and the Coulombic correlation between ions.     

Theoretical and experimental investigation of the energetics of cis-trans proline isomerization in peptide models

The energetics of cis-trans proline isomerization in small peptide models have been investigated using the hybrid density functional theory method B3LYP with a 6-31+G* basis set. The molecules studied are models for the phospho-Ser/Thr-Pro substrate for Pin-1, a peptidyl-prolyl isomerase (PPIase) involved in cell division. Pin-1 requires phosphorylation of a Ser or Thr residue adjacent to a Pro residue in the substrate and catalyzes cis-trans isomerization about the proline amide bond. The dihedral angle that would correspond to the reaction coordinate for isomerization of the omega peptide bond was investigated for several small models. Relaxed potential energy scans for this dihedral angle in N-methylacetamide, 1, N,N-dimethylacetamide, 2, acetylpyrrolidine, 3 and acetylproline, 4, were carried out in 20 degrees steps using the B3LYP/6-31+G* level of theory. In addition, similar scans were carried out for 1-4 protonated on the acetylamide carbonyl oxygen. Optimized structures for 1-4 protonated on the amide nitrogen were also obtained at B3LYP/6-31+G*. Relative proton affinities were determined for each site at various angles along the reaction coordinate for isomerization. The relative proton affinities were anchored to experimental gas phase proton affinities, which were taken from the literature for 1 and 2, or determined in an electrospray ionization-quadrupole ion trap instrument using the extended kinetic method for 3 and 4. Proton affinities of 925 +/- 10 and 911 +/- 12 kJ/mol were determined for 3 and 4, respectively. These studies suggest that the nitrogen atom in these amides becomes the most basic site in the molecule at a dihedral angle of ca. 130 degrees . In addition, the nitrogen atoms in 2-4 are predicted to attain basicities in the range 920-950 kJ/mol, making them basic enough to be the preferred site for hydrogen bonding in the Pin-1 active site, in support of the proposed mechanism for PPIases.     

Intracule functional models: I. Angle-corrected correlation kernels

We explore the merits of applying a simple angle-dependent correction to the correlation kernel within the framework of Hartree-Fock-Wigner theory. Based on numerical results for the first eighteen atoms, we conclude that such a correction offers a significant improvement over the action kernel that we and others have explored previously.     

Transport and Helfand moments in the Lennard-Jones fluid. I. Shear viscosity

The authors propose a new method, the Helfand-moment method, to compute the shear viscosity by equilibrium molecular dynamics in periodic systems. In this method, the shear viscosity is written as an Einstein-type relation in terms of the variance of the so-called Helfand moment. This quantity is modified in order to satisfy systems with periodic boundary conditions usually considered in molecular dynamics. They calculate the shear viscosity in the Lennard-Jones fluid near the triple point thanks to this new technique. They show that the results of the Helfand-moment method are in excellent agreement with the results of the standard Green-Kubo method.     

Comparison of experimental DPR and viscosity cross sections

It has previously been shown that the shape of the depolarized Rayleigh line in gases is governed by a correlation function exhibiting a spread in correlation times. Here it is shown that effective cross sections obtained from data on flow birefringence and the field dependence of the viscosity are in agreement with the DPR results. Moreover, the non-lorentzian behaviour of the H/p dependence of the viscosity, is fully interpretable in terms of the non-exponential behaviour of the DPR correlation function.     

High-performance pulse and differential pulse polarography: Part I. Theoretical considerations

A theoretical approach is given for complete elimination of the charging current caused by drop growth in modified normal and differential pulse polarography (n.p.p. and d.p.p.). Maximum sensitivity can be obtained in modified n.p.p., but the resolution of waves is better in modified d.p.p. or can be increased in differential modified n.p.p. The best overall performance can be achieved with modified d.p.p., considering sensitivity and wave resolution together. Preliminary results for the determination of chloramphenicol are described.     

Theoretical investigation on hydroformylation reactions : I. Structures and reactivities of cobalt carbonyls and hydrocarbonyls

Extended Hückel Theory calculations have been carried out in a study of the most important cobalt carbonyls and hydrocarbonyls involved in the hydroformylation reaction. The geometries of the stable isomers of Co₂(CO)₈, Co₂(CO)₇, Co(CO)₄, Co(CO)₃ have been calculated and used to interpret the changes in the IR spectrum of Co₂(CO)₈ observed on varying the temperature. The reaction paths for the interconversions of the stable isomers have also been investigated. The optimized geometry of HCo(CO)₄ agrees well with the experimental structure. The C_s symmetry found for the most stable isomer of HCo(CO)₃ is of much interest, serves to explain the formation of the complex with olefins.     

Spinning drop experiments on interfacial phenomena: Theoretical background and experimental evidence

Interfacial phenomena contribute strongly to structure formation in multiphase polymer blends. Spinning drop experiments allow not only to analyze the magnitude of the interfacial energy, but also to determine dynamic features such as coarsening or the break-up of elongated threads. This contribution reports the theoretical background of static and dynamic spinning drop experiments; it describes experimental evidence on the break-up dynamics of such elongated threads and it finally sketches a spinning drop apparatus designed for investigations of viscous polymer blends, even at elevated temperatures.Dedicated to Prof. H. H. Kausch on the occasion of his 60th birthday.     

Temperature effects on positronium formation and inhibition: a contribution to the elucidation of early spur processes. I. Glycerol/water mixtures

In order to elucidate the mechanism of positronium (Ps) formation in liquids, the effect of temperature, T, on the inhibiting properties of various solutes has been investigated in glycerol/water mixtures. Whereas the inhibition constants of Cl^? and I^? are found to increase markedly with T, that of CH₃NO₂ is T insensitive and that of NO₃^? diminishes with T. These findings are consistent with our previous model, according to which Ps would be formed via two pathways, either through the quasi-free entities or by the reaction of localized, not yet fully solvated, electrons and positrons. The increase with T of the Ps yield is found to be due to the fraction arising from the latter reaction. The results confirm Cl^? and I^? react with e_loc⁺, while CH₃NO₂ and NO₃^? scavenge quasi-free electrons. Regarding the behaviour of the inhibition constants of these latter solutes, a provisional explanation is given: CH₃NO₂ would scavenge epithermal electrons while NO₃^? would react with electrons at a lower energy state, in competition with the localization process. Hot Ps atoms are not likely to be involved.     

Studies on the synthesis of furanosteroids. I. Viridin models

Alkyne oxazoles of general structure I are transformed directly to furo[2,3-b]phenol derivatives II by a sequence involving intramolecular Diels-Alder/retro-Diels-Alder reaction followed by tautomerization. Suitably functionalized phenols II undergo an intramolecular phenol-dienone-aldol condensation, generating the A,B,E-ring skeleton III characteristic of the viridin (1) class of furanosteroids.     

VUV and photoelectronic spectra of methylstannane. Theoretical and experimental approach

The vacuum ultraviolet (VUV) and photoelectron spectra of SnH₃CH₃ were recorded between 6.20 and 11.28 eV and between 8 and 17 eV, respectively. Spectra were interpreted using ab initio CI calculations. The photoelectron spectrum confirmed the low SnC bond energy. The first two ionization potentials (IP) observed were attributed to the ionization of the a₁ (10.65 eV) and e orbitals (11.15 and 11.60 eV, split by the Jahn-Teller effect), thereby showing an inversion of IPs compared with ethane. Similarly, the first two bands of the VUV spectrum (at 7.04 and 7.72–8.16 eV) were attributed to a₁ and e transitions towards the Rydberg s orbital. A splitting of the same order of magnitude as that of the photoelectron spectrum could be noted in the E state. Observed transitions between 8.65 and 10 eV showed a strong interaction between the Rydberg p MO and the σ^*_SnC antibonding orbital. Primarilyvalence transitions were encountered beyond 10 eV.     

Theoretical models for diffusion in glassy polymers. II

The author refines and generalizes a model for diffusion in glassy polymers which he previously introduced. The model unifies many diverse observations by explicity formulating the common property of a glassy polymer in all its various modes, namely the finite relaxation time due to its slow response to changing conditions. An integral approximation method is used to study the motion of the penetrant front and the glass-gel interface and a useful polynomial approximation method is introduced for use in special simple situations.     

Theoretical studies on structure-property relationships of cellulose and nitrocellulose: I. Conformation,stability and conductivity

The banding and electronic structures of a series of long-chain macromolecules of cellulose, 2-, 3-, 6-mononitrocellulose, 2,3-,2,6-, 3,6-dinitrocellulose and trinitrocellulose as well as their structural units (i.e. single-, double- or three-ring systems) have been calculated by both the EH and CNDO/2 methods. The increase of molecular total energies is consistent with the decrease of their stabilities at the three conformations of gg, gt and tg. The Mulliken bond order of O—NO₂ bond is the smallest in each molecule at any of the three conformations, which indicates that this bond is the weakest, and supports the view of initial homolytic cleavage of O—NO₂ bond on slow thermal decomposition. The band gap at the edge of the first Brillouin zone far surpasses 5 eV for cellulose, and is less than 3 eV for mono-, di-, and trinitrocellulose. The results show that cellulose is a typical insulator, as we know, and it can be predicted that nitrocellulose has electric conductivity similar to that of semiconductor.     

Theoretical and experimental studies on formation of diethylzinc-triphenylphosphine complex

This work is mainly focused on understanding the complex-forming behavior of diethylzinc with neutral phosphine ligands such as triphenylphosphine (PPh₃) to yield [Zn(PPh₃)₂Et₂]. The complex formation in solution is observed in the presence of a large excess of diethylzinc, but leaving an insoluble solid on aging. The product formed in solution was analyzed by spectroscopic data. ³¹P-NMR was also used as a tool to observe this behavior, i.e., the disappearance of the chemical shift of PPh₃ (δ -5.45) requires 14-fold excess of ZnEt₂ in solution. The alkyl chains reduce the Lewis acidity on Zn and thereby the formation of phosphine adducts is restricted. Results obtained from orbital analyses calculations reveal that the LUMO appears to be asymmetrically distributed, and localized on one of the PPh₃ ligands. The length of alkyl chains also influence the stability of [Zn(PPh₃)₂R₂] and the longer chains on Zn impart less stability.     

Theoretical study of electronically excited radical cations of naphthalene and anthracene as archetypal models for astrophysical observations. Part I. Static aspects

Motivated by the recent discovery of new diffuse interstellar bands and results from laboratory experiments, ab initio quantum chemistry calculations are carried out for the lowest six electronic states of naphthalene and anthracene radical cations. The calculated adiabatic electronic energies are utilized to construct suitable diabatic electronic Hamiltonians in order to perform nuclear dynamics studies in Part II. Complex entanglement of the electronic states is established for both the radical cations and the coupling surfaces among them are also derived in accordance with the symmetry selection rules. Critical examination of the coupling parameters of the Hamiltonian suggests that 29 (out of 48) and 31 (out of 66) vibrational modes are relevant in the nuclear dynamics in the six lowest electronic states of naphthalene and anthracene radical cations, respectively.