Capacitive-discharge-heated anisotropic pyrolytic graphite furnace used for atomic absorption spectrometry Part. 1 Thoeretical Modelling of the Heating Characteristics of the Furnace Surface and of the Gas Phase

A model of the temperature distribution in the tube wall and in the gas phase of an anisotropic pyrolytic graphite furnace heated by capacitive discharge technique is proposed. The heat loss from the graphite tube by conduction via the contact electrodes to the water-cooled electrode supports and by radiation to its surroundings are the main processes condidered. In calculating the gas temperature, heat transfer by conduction from the tube wall to the gas phase is the only mode taken into account. The instantaneous temperature of a graphite furnace is expressed in the form of a differential equation. A finite-difference form of the differential equation is used in a computer program to calculate the temperature at each time step. The computer simulation offers the capability of studying the factors affecting the characteristics of temperature/time profiles and the distribution of surface and gas phase temperatures of the furnace. The results of simulation studies of the effect of the capacitance and the initial voltage of the capacitor bank on the heating characteristics of the furnaces show a reasonable agreement with those obtained experimentally.     

Conformal gauge fixing in Minkowski space

Conformal gauge fixing on simply connected parts of world sheets in Minkowski space is examined in detail. It is proved that conformal gauge fixing can be imposed, including the usual boundary conditions.     

Charge-transfer-type fluorescence of 4-(1H-pyrrol-1-yl)benzonitrile (PBN) and N-phenylpyrrole (PP) in cryogenic matrixes: evidence for direct excitation of the CT band

The fluorescence spectrum of PBN in a neat argon matrix is excitation-wavelength-dependent: at short excitation wavelengths, it consists of dual emission assigned to a charge-transfer (CT) state and a much weaker band assigned to the locally excited (LE) state. The CT emission is broad and almost completely devoid of vibrational structure, whereas the LE band is characterized by vibrationally resolved emission. At long excitation wavelengths, only CT emission is observed, indicating that the CT state is populated directly by light absorption and not via the LE state. Comparison with jet-cooled spectra of the bare molecule allows the unambiguous assignment of the LE spectrum and the location of the 0,0 band. The matrix LE emission spectrum is blue-shifted with respect to that of the gas phase, showing that the dipole moment of the LE state is smaller than that of the ground state. The fluorescence spectrum of PBN in an argon matrix does not change appreciably when acetonitrile (AN) is added to the matrix, in contrast to the case of N-phenylpyrrol (PP) (Schweke, D.; Haas, Y. J. Phys. Chem. A 2003, 107, 9554), for which addition of AN results in the appearance of two well separated emission bands. The different photophysical behaviors of PP and PBN in an argon matrix (and in supersonic jets) are analyzed by a simple model that considers the restriction of large-amplitude motions in the matrix. The implications of these low-temperature studies for understanding the properties of these systems in liquid solution are discussed.     

A Reversible DNA Logic Gate Platform Operated by One‐ and Two‐Photon Excitations

We demonstrate the use of two different wavelength ranges of excitation light as inputs to remotely trigger the responses of the self‐assembled DNA devices (D‐OR). As an important feature of this device, the dependence of the readout fluorescent signals on the two external inputs, UV excitation for 1 min and/or near infrared irradiation (NIR) at 800 nm fs laser pulses, can mimic function of signal communication in OR logic gates. Their operations could be reset easily to its initial state. Furthermore, these DNA devices exhibit efficient cellular uptake, low cytotoxicity, and high bio‐stability in different cell lines. They are considered as the first example of a photo‐responsive DNA logic gate system, as well as a biocompatible, multi‐wavelength excited system in response to UV and NIR. This is an important step to explore the concept of photo‐responsive DNA‐based systems as versatile tools in DNA computing, display devices, optical communication, and biology.     

Theoretical determination of molecular structure and conformation : Part 12. Puckering of 1,3,5-cycloheptatriene, 1H-azepine, oxepine and their norcaradienic valence tautomers

Restricted Hartree—Fock calculations have been carried out for 1,3,5-cycloheptatriene, norcaradiene, 1H-azepine, benzenimine (azanorcardiene), oxepine and benzene oxide (oxanorcaradiene) employing the STO—3G, 4—31G and 6—31G^* basis sets. Theoretical geometries, conformations and barriers to ring inversion have been obtained and compared with the available experimental structural data. It has been found that the cyclotrienes possess a boat conformation with a constant admixture of 22% chair character leading to a flattening of the triene part and π-electron delocalization typical for a planar polyene. The structural data obtained in this work suggest that cyclic delocalization of either 6π (homoaromaticity) or 8π electrons (antiaromaticity) is not present in the three cyclotrienes. The former possibility, however, cannot be excluded in the case of the three norcaradienes.     

Photophysics of phenylpyrrole derivatives and their acetonitrile clusters in the gas phase and in argon matrixes: simulations of structure and reactivity

Recent experiments on the dual fluorescence of phenylpyrrole (PP) and pyrrolobenzonitrile (PBN) in supersonic jets and in cryogenic matrixes are analyzed. The structures of the 1:1 clusters are calculated using ab initio, density functional theory (DFT) and molecular mechanics (MM) methods. In these calculations, the structures of PP and PBN in the ground state and in two possible minima on the charge-transfer excited state are taken from a recent theoretical analysis. The structures of PP and PBN clusters with a larger number of acetonitrile molecules are also calculated using the molecular mechanics method. It is shown that the fact that small PP:AN and PBN:AN clusters do not exhibit any charge-transfer (CT) type emission, whereas for PBN:AN(n) clusters (n > or = 4) CT emission is observed, can be understood on the basis of the calculated structures. The trapping of PP and of PBN in an argon matrix (neat and doped with acetonitrile) is simulated by a molecular dynamics procedure. The observation of locally excited (LE) fluorescence only from PP in neat argon, whereas from argon-trapped PBN both CT and LE emission bands are observed, is readily understood on the basis of these simulations. Moreover, the appearance of CT emission from PP-doped argon matrixes when acetonitrile is added is also explained, as well as the relatively small spectral shift observed upon addition of acetonitrile to PBN-doped argon matrixes.     

Fluorescence quenching of flavins by reductive agents

The fluorescence behaviour of the flavins riboflavin, flavin mononucleotide (FMN), flavin adenine dinucleotide (FAD), and lumiflavin in aqueous solution at pH 8 in the presence of the reducing agents β-mercaptoethanol (β-ME), dithiothreitol (DTT), and sodium nitrite (NaNO₂) is studied under aerobic conditions. The fluorescence quantum yields and fluorescence lifetimes are determined as a function of the reducing agent concentration. For all three reducing agents diffusion controlled dynamic fluorescence quenching is observed which is thought to be due to photo-induced reductive electron transfer. For DTT additionally static fluorescence quenching occurs.