Quantum chemical calculation of phosphorescence microwave double resonance spectra

Lifetimes of individual spin sublevels, vibronic intensities of phosphorescence, zero field splitting, hyperfine and nuclear quadruple tensors for the lowest triplet state, g factors, and dipole nature of microwave transitions have been calculated on the bases of CNDO and INDO methods taking into account spin–orbit, spin–spin, vibronic, and hyperfine interactions by perturbation theory. The results are in qualitative agreement with phosphorescence microwave double resonance data. Influence of intermolecular interaction on the zero field splitting are also investigated.     

An inertial model of atoms: atomic spectra as a resonance problem

A new model of atoms based on the theory of material waves is proposed. Atoms are characterized by an inertial aggregation of negative charge within the atomic shell, the calculation yields, for hydrogen atoms, an atomic radius of 0.470 nm. Atomic spectra result from radial material waves, quantization being the consequence of boundary conditions imposed on the fundamental wave equation. Stark effects and Zeeman effects are treated in detail, they are referred to deformations and rotations of the atomic shell. An analysis of Stern--Gerlach experiments bases subsequent deflections of single atoms on rotations and interactions of magnetic fields.     

Integrals of gaussian and continuum functions for polyatomic molecules. An addition theorem for solid harmonic gaussians

An addition theorem for solid harmonic gaussian functions in terms of surface spherical harmonics is derived and presented in a form particularly useful for evaluating molecular integrals involving gaussian and continuum functions on array processors.     

Absorption,fluorescence and resonance Raman spectra of adriamycin and its complex with DNA

Absorption, fluorescence and resonance Raman spectra of adriamycin and of its complex with DNA have been measured. These results together with the analysis of the excitation profiles of the solid drug were used to interpret the complex pattern of the absorption spectra as due to the vibronic structure of a single electronic state. In addition a surplus band, depending on the concentration, was attributed to self-association.Further support was given for the intercalation nature of the drug-DNA complex. In particular it was found that the monomer form of adriamycin interacts with the nucleic acid and that interaction involves the whole chromophore framework.     

Absorption spectra and energy levels of Sm3+:Y3Al5O12

Absorption spectra between 0.2 and 6.7 μm are reported for trivalent samarium in single-crystal yttrium aluminum garnet, Y₃Al₅O₁₂, at liquid-helium, liquid-nitrogen, and room temperatures. Energy levels (4f⁵[SL]Jμ) associated with sextet, quartet, and doublet states of Sm³⁺ are established from data between 1500 and 46000 cm⁻¹. Most samarium ions occupy yttrium ion sites, which have D₂ point-group symmetry in the lattice. A free-ion wavefunction calculation predicts the assignment of 35 isolated [SL]J manifolds with an rms deviation of 32 cm⁻¹ between calculated and observed centers of gravity. A hamiltonian consisting of coulombic, spin—orbit, and crystalline electric field (D₂ symmetry) terms is diagonalized for the two lowest sextet states [6H]J and [6F]J. The calculated Stark levels based on a single set of B_km crystal field parameters are compared to the observed levels (54) yielding an rms deviation of 4.9 cm⁻¹.     

Study on the resonance Rayleigh scattering spectra and resonance non-linear spectra of congo red-amikacin system and its analytical application

The interaction between congo red (CR) and amikacin (AMK) was studied by resonance Rayleigh scattering (RRS), frequency doubling scattering (FDS) and second-order scattering (SOS) combining with absorption spectrum. In a weak acidic medium, CR combined with AMK to form an ion association complex with the composition ratio of 1∶1 by electrostatic interaction, hydrophobicity and charge transferring effect. As a result, the new spectra of RRS, FDS, and SOS appeared and their intensities were enhanced greatly. The maximum wavelengths of RRS, FDS and SOS were located at 563 nm, 475 nm and 940 nm, and the scattering intensities were proportional to the concentration of AMK. These three methods have very high sensitivities, and the detection limits were 4.0 ng·mL?1 for RRS, 3.6 ng·mL?1 for FDS and 1.9 ng·mL?1 for SOS, respectively. At the same time, the methods have better selectivity. A new method for the determination of trace amounts of AMK with congo red by resonance scattering technique has been developed. The recovery for the determination of AMK in blood serum and urine sample was between 95.5% and 105.5%. In this study, the properties, such as enthalpy of formation, charge distribution and mean polarizability, were calculated by AM1 quantum chemistry method. In addition, the reaction mechanism and the reasons for the enhancement of scattering spectra were discussed.     

Ab initio calculations of vibronic activity in phosphorescence microwave double resonance spectra of p-dichlorobenzene

The phosphorescence spectrum of p-dichlorobenzene has been calculated using multiconfiguration self-consistent-field wave functions and the quadratic response technique. Attention has been paid to the intensity distribution of the singlet–triplet (³B_1u¹A_g) transition through a number of vibronic subbands. The second order spin–orbit coupling (SOC) contribution to the spin splitting of the ³B_1u (^3*) state is found to be almost negligible, and the calculations therefore provide a good estimate for the zero-field splitting (ZFS) parameters based only on the electron spin–spin coupling expectation values. Nuclear quadrupole resonance constants for the different Cl isotopes are also calculated to accomplish the ZFS assignment. The electric dipole activity of the spin sublevels in the triplet–singlet transitions to the ground-state vibrational levels is estimated by calculations of derivatives using distorted geometries which are shifted from the equilibrium position along different vibrational modes. A vibrational analysis of the phosphorescence spectrum, based on the SOC-induced mixing of the singlet and triplet states calculated along different vibrational modes, provides reasonable agreement with experimental data.Acknowledgment O. R.-P. would like to thank the European MOLPROP network for support. The authors thank Alexander Baev for fruitful discussions. This work was supported by the Swedish Royal Academy of Science (KVA).     

Investigation of secondary structure elements by IR/UV double resonance spectroscopy: analysis of an isolated beta-sheet model system

An isolated beta-sheet model system is investigated in a molecular beam experiment by means of mass- and isomer-selective IR/R2PI double resonance spectroscopy as well as ab initio and DFT calculations. As the exclusive intermolecular assembly, a beta-sheet motif is formed by spontaneous dimerization of two isolated peptide molecules. This secondary structure is produced from the tripeptide model Ac-Val-Tyr(Me)-NHMe without any further environment to form the binding motif which is analyzed by both the characteristic amide A and I vibrations. The experimental and theoretical investigations yield the assignment to an antiparallel beta-sheet model. The result of this detailed spectroscopic analysis on an isolated beta-sheet model indicates that there are intrinsic properties of a beta-sheet structure which can be formed without a solvent or a peptidic environment.     

Absorption spectra related to heterogeneous electron transfer reactions: the perylene TiO2 system

Linear absorption spectra of dye-semiconductor systems (perylene attached to nanostructured TiO2) are studied theoretically and experimentally. The systems show ultrafast photoinduced heterogeneous electron transfer (HET). By applying a time-dependent formulation of the absorbance, the theoretical analysis of the measured data is carried out. The respective electron-vibrational wave packet propagation fully accounts for the electronic coupling to the conduction band continuum of TiO2 and is based on a single-reaction-coordinate model (corresponding to a perylene in-plane C-C stretching vibration with a quantum energy of 1370 cm(-1)). By the insertion of different bridge-anchor groups, the electronic coupling responsible for HET is varied. The dye absorbance in a solvent and the trends in the line broadening of the vibrational progression due to the coupling to the conduction band continuum are reproduced for all investigated types of bridge-anchor groups. HET rates deduced from the calculations on the absorbance displaying line broadenings follow the qualitative trend obtained from transient absorption spectra.     

Pepto: An expert system for automatic peak assignment of two-dimensional nuclear magnetic resonance spectra of proteins

Two-dimensional Nuclear Magnetic Resonance (2DFTNMR) is presently the most powerful tool to determine protein structures in solution. Peak assignment (an interpretation of the two-dimensional spectra that leads to the individuation of pairs of Hydrogen atoms that are involved in an NOE peak) is a cornerstone of such use of 2DFTNMR. Manual peak assignment of a protein often requires months of work by a specialized equipe. An automation of this task could speed up the protein study process, or alternatively allow to study previously unmanageable proteins. This article describes PEPTO, an expert system for the interpretation of sets of 2DFTNMR spectra on proteins. The present version of the program deals with spectra obtained from NOESY and COSY experiments. Tests of PEPTO on simulated spectra of five proteins with known assignments are also described and dicussed.     

ABSINTH: a new continuum solvation model for simulations of polypeptides in aqueous solutions

A new implicit solvation model for use in Monte Carlo simulations of polypeptides is introduced. The model is termed ABSINTH for self-Assembly of Biomolecules Studied by an Implicit, Novel, and Tunable Hamiltonian. It is designed primarily for simulating conformational equilibria and oligomerization reactions of intrinsically disordered proteins in aqueous solutions. The paradigm for ABSINTH is conceptually similar to the EEF1 model of Lazaridis and Karplus (Proteins 1999, 35, 133). In ABSINTH, the transfer of a polypeptide solute from the gas phase into a continuum solvent is the sum of a direct mean field interaction (DMFI), and a term to model the screening of polar interactions. Polypeptide solutes are decomposed into a set of distinct solvation groups. The DMFI is a sum of contributions from each of the solvation groups, which are analogs of model compounds. Continuum-mediated screening of electrostatic interactions is achieved using a framework similar to the one used for the DMFI. Promising results are shown for a set of test cases. These include the calculation of NMR coupling constants for short peptides, the assessment of the thermal stability of two small proteins, reversible folding of both an alpha-helix and a beta-hairpin forming peptide, and the polymeric properties of intrinsically disordered polyglutamine peptides of varying lengths. The tests reveal that the computational expense for simulations with the ABSINTH implicit solvation model increase by a factor that is in the range of 2.5-5.0 with respect to gas-phase calculations.     

Microwave-optical double resonance in thioformaldehyde: Further studies of high rovibronic levels of the ground state

Microwave-optical double resonance experiments have been carried out on the J_O,J rotational levels of the 4¹ ùA₂ excited state of thioformaldehyde (H₂CS). The number of microwave transitions to high rovibronic levels of the ground state observed in a given frequency range increases in a roughly linear manner with J, supporting the proposition that K_a is not a good quantum number for these high rovibronic levels.     

Theory of electron localization at dielectric-metal interfaces: a continuum model

Localization of electrons at dielectric-metal interfaces is studied in the framework of a continuum model. The layer of thickness L, with a negative electron affinity, is characterized by the static dielectric constant epsilons and by the optical dielectric constant epsiloninfinity. It is found that the electron localization along the plane of the interface occurs if the layer thickness exceeds a critical value Lc. In the case of a high polar layer, the electron energy of the localized ground state shows a nonmonotonic dependence on the layer thickness. A strong correlation between low-lying excitations and the spread of the localized state has been established. The magnitude of the correlation parameter is close to the analogous correlation for the solvated electron in the bulk. The localization dynamics is discussed in terms of relaxation along a polarization coordinate, which is directly connected to the polarization energy of the layer.     

SpecTool: a knowledge-based hypermedia system for interpreting molecular spectra

Summary A hypermedia environment was designed to set up a collection of reference data, reference spectra and computational tools. The system may serve as an aid for the combined interpretation of molecular spectra. This paper describes the context dependent presentation of the pieces of information.