Photoejection of electrons from flavins in polar media

Riboflavin and 12 of its derivatives have been shown to form solvated electrons under ultraviolet irradiation (253.7 nanometers) in various water-methanol solvent mixtures. The highest quantum yield of solvated electrons (about 0.03) was obtained for flavins containing tyrosine on a side chain in the isoalloxazine N-3 or N-10 position. The splitting of hydrogen atoms from excited flavin molecules was also observed. From the results presented here, it can be determined that the semiquinone transients are formed not only by way of the flavin triplet, as usually suggested, but also by the attack of the electrons and hydrogen atoms on flavin molecules in the ground state. This is important, because the flavin radicals remaining after the electron-ejection or hydrogen-splitting processes must also be considered in the subsequent reaction mechanisms. The electron-ejection process from electronically excited flavins has important implications in the photobiology of these compounds.     

Chemiluminescence in the Agglomeration of Metal Clusters

The agglomeration of copper or silver atoms in a matrix of noble gas atoms to form small clusters may be accompanied by the emission of visible light. Spectral analysis reveals the intermediate formation of electronically excited atoms and dimers as the source of the chemiluminescence. A mechanism is proposed, according to which the gain in binding energy upon cluster formation may even lead to the ejection of excited fragments as a result of unstable intermediate configurations. A similar concept was introduced in the field of nuclear reactions by Niels Bohr 60 years ago.     

Laser manipulation of atoms and particles

A variety of powerful techniques to control the position and velocity of neutral particles has been developed. As examples of this new ability, lasers have been used to construct a variety of traps, to cool atoms to temperatures below 3 x 10(-6) kelvin, and to create atomic fountains that may give us a hundredfold increase in the accuracy of atomic clocks. Bacteria can be held with laser traps while they are being viewed in an optical microscope, and organelles within a cell can be manipulated without puncturing the cell wall. Single molecules of DNA can now be stretched out and pinned down in a water solution with optical traps. These new capabilities may soon be applied to a wide variety of scientific questions as diverse as precision measurements of fundamental symmetries in physics and the study of biochemistry on a single molecule basis.     

Theoretical evidence for a c60 "window" mechanism

On the basis of semiempirical and high-level ab initio calculations, theoretical evidence is presented of a "window" mechanism operable on the surface of C(60) and other fullerenes. Through this mechanism, large holes may be formed in fullerenes excited to their triplet state, openings through which atoms and small molecules can pass. This work provides a theoretical foundation for experiments that have prepared endohedral noble gas compounds of C(60) under thermal excitation. A method is proposed that could increase the efficiency of the process of noble gas insertion into C(60) and provide a more general means to create endohedral fullerene compounds.     

A homonuclear molecule with a permanent electric dipole moment

Permanent electric dipole moments in molecules require a breaking of parity symmetry. Conventionally, this symmetry breaking relies on the presence of heteronuclear constituents. We report the observation of a permanent electric dipole moment in a homonuclear molecule in which the binding is based on asymmetric electronic excitation between the atoms. These exotic molecules consist of a ground-state rubidium (Rb) atom bound inside a second Rb atom electronically excited to a high-lying Rydberg state. Detailed calculations predict appreciable dipole moments on the order of 1 Debye, in excellent agreement with the observations.     

A new laboratory source of ozone and its potential atmospheric implications

Although 248-nanometer radiation falls 0.12 electron volt short of the energy needed to dissociate O(2) large densities of ozone (O(3)) can be produced from unfocused 248-nanometer KrF excimer laser irradiation of pure O(2). The process is initiated in some undefined manner, possibly through weak two-photon O(2) dissociation, which results in a small amount of O(3) being generated. As soon as any O(3) is present, it strongly absorbs the 248-nanometer radiation and dissociates to vibrationally excited ground state O(2) (among other products), with a quantum yield of 0.1 to 0.15. During the laser pulse, a portion of these molecules absorb a photon and dissociate, which results in the production of three oxygen atoms for one O(3) molecule destroyed. Recombination then converts these atoms to O(3), and thus O(3) production in the system is autocatalytic. A deficiency exists in current models of O(3) photochemistry in the upper stratosphere and mesosphere, in that more O(3) iS found than can be explained. A detailed analysis of the system as it applies to the upper atmosphere is not yet possible, but with reasonable assumptions about O(2) vibrational distributions resulting from O(3) photodissociation and about relaxation rates of vibrationally excited O(2) a case can be made for the importance of incuding this mechanism in the models.     

Synthesis and Single-Crystal X-ray Structure of a Highly Symmetrical C60 Derivative, C60Br24

C(60) and liquid bromine react to form C(60)Br(24), a crystalline compound isolated as a bromine solvate, C(60)Br(24)(Br(2))(x), The x-ray crystal structure defines a new pattern of addition to the carbon skeleton that imparts a rare high symmetry. The parent C(60) framework is recognizable in C(60)Br(24), but sp(3) carbons at sites of bromination distort the surface, affecting conformations of all of the hexagonal and pentagonal rings. Twenty-four bromine atoms envelop the carbon core, shielding the 18 remaining double bonds from addition. At 150 degrees to 200 degrees C there is effectively quantitative reversion of C(60)Br(24) to C(60) and Br(2).     

Ice nucleation by coprecipitated silver iodide and silver bromide

The ability of silver iodide to cause freezing of supercooled water is improved if up to 30 percent of the iodine atoms in the crystal are replaced with bromine atoms.     

Ti8C12+-Metallo-Carbohedrenes: A New Class of Molecular Clusters?

During the course of studying the dehydrogenation reactions of hydrocarbons by titanium atoms, ions, and clusters, an exceptionally stable and abundant cluster which contains 8 titaniums and 12 carbons was discovered. "Titration" reactions with ND(3) reveal the uptake of eight molecules, pointing to the fact that the titanium atoms are at exposed positions of similar coordination. A dodecahedral structure of T(h) point group symmetry is proposed to account for the unusual stability of this molecular cluster. The Ti(8)C(12)(+) dodecahedron has 12 pentagonal rings and each of the rings is formed by two titanium and three carbon atoms, where each titanium is bound to three carbons. Based on the model, it is expected that neutral Ti(8)C(12) would be a stable metallo-carbododecahedral molecule and may comprise one member of a new class of molecules, namely metallo-carbohedrenes.     

Multiple-quantum nuclear magnetic resonance spectroscopy

A nuclear magnetic resonance (NMR) event is popularly viewed as the flip of a single spin in a magnetc field, stimulated by the absorption or emission of only one quantum of radio-frequency energy. Nevertheless, resonances between nuclear spin states that differ by more than one unit in the Zeeman quantum number also can be induced in systems of coupled spins by suitably designed sequences of radio-frequency pulses. Pairs of states excited in this way oscillate coherently at the frequencies of the corresponding multiple-quantum transitions and produce a response that may be monitored indirectly in a two-dimensional time-domain experiment. The pattern of multiple-quantum excitation and response, influenced largely by the concerted interactions of groups of coupled nuclei, simplifies the NMR spectrum in some instances and provides significant new information in others. Applications of multiple-quantum NMR extend to problems in many different areas, ranging from studies of the structure and function of proteins and nucleic acids in solution to investigations of the arrangements of atoms in amorphous semiconductors. The specific spectroscopic techniques are varied as well and include methods designed, for example, to simplify spectral analysis for liquids and liquid crystals, eliminate inhomogeneous broadening, study interatomic connectivity in liquid-state molecules, identify clusters of atoms in solids, enhance the spatial resolution in solid-state imaging experiments, and probe correlated molecular motions.     

Time-resolved dynamics in N2O4 probed using high harmonic generation

The attosecond time-scale electron-recollision process that underlies high harmonic generation has uncovered extremely rapid electronic dynamics in atoms and diatomics. We showed that high harmonic generation can reveal coupled electronic and nuclear dynamics in polyatomic molecules. By exciting large amplitude vibrations in dinitrogen tetraoxide, we showed that tunnel ionization accesses the ground state of the ion at the outer turning point of the vibration but populates the first excited state at the inner turning point. This state-switching mechanism is manifested as bursts of high harmonic light that is emitted mostly at the outer turning point. Theoretical calculations attribute the large modulation to suppressed emission from the first excited state of the ion. More broadly, these results show that high harmonic generation and strong-field ionization in polyatomic molecules undergoing bonding or configurational changes involve the participation of multiple molecular orbitals.     

The role of Br2 and BrCl in surface ozone destruction at polar sunrise

Bromine atoms are believed to play a central role in the depletion of surface-level ozone in the Arctic at polar sunrise. Br2, BrCl, and HOBr have been hypothesized as bromine atom precursors, and there is evidence for chlorine atom precursors as well, but these species have not been measured directly. We report here measurements of Br2, BrCl, and Cl2 made using atmospheric pressure chemical ionization-mass spectrometry at Alert, Nunavut, Canada. In addition to Br2 at mixing ratios up to approximately 25 parts per trillion, BrCl was found at levels as high as approximately 35 parts per trillion. Molecular chlorine was not observed, implying that BrCl is the dominant source of chlorine atoms during polar sunrise, consistent with recent modeling studies. Similar formation of bromine compounds and tropospheric ozone destruction may also occur at mid-latitudes but may not be as apparent owing to more efficient mixing in the boundary layer.     

Delocalization of protons in liquid water

We find that the vibrational potential of the O-H stretch vibrations of liquid water shows extreme anharmonicity that arises from the O-H O hydrogen bond interaction. We observe that already in the second excited state of the O-H stretch vibration, the hydrogen atom becomes delocalized between the oxygen atoms of two neighboring water molecules. The energy required for this delocalization is unexpectedly low and corresponds to less than 20% of the dissociation energy of the O-H bond of the water molecule in the gas phase.     

Solution of a protein crystal structure with a model obtained from NMR interproton distance restraints

Model calculations were performed to test the possibility of solving crystal structures of proteins by Patterson search techniques with three-dimensional structures obtained from nuclear magnetic resonance (NMR) interproton distance restraints. Structures for crambin obtained from simulated NMR data were used as the test system; the root-mean-square deviations of the NMR structures from the x-ray structure were 1.5 to 2.2 A for backbone atoms and 2.0 to 2.8 A for side-chain atoms. Patterson searches were made to determine the orientation and position of the NMR structures in the unit cell. The correct solution was obtained by comparing the rotation function results of several of the NMR structures and the average structure derived from them. Conventional refinement techniques reduced the R factor from 0.43 at 4 A resolution to 0.27 at 2 A resolution without inclusion of water molecules. The partially refined structure has root-mean-square backbone and side-chain atom deviations from the x-ray structure of 0.5 and 1.3 A, respectively.     

Stellar and 0ther high-temperature molecules

Optical spectroscopy of stellar molecules trapped at 4 degrees K is particularly valuable when the data can be used to complement the corresponding gas data. The ground state is then directly established by measurement of the absorption spectrum at the low temperature, since all transitions beginning from excited states are eliminated. Isotopic substitution establishes the (0,0) bands of transitions to excited electronic states, and when these data are combined with infrared and fluorescence measurements at 4 degrees K, most of the vibrational properties of the ground and excited states can be obtained. Of the many examples cited and discussed here, C(3) is perhaps the most outstanding. Because the various molecules trapped in matrices are usually identified through prior mass spectrometric work, the optical observations often lead to the discovery of band systems of molecules whose spectra have not previously been observed-for example, those of Si(2)C(3), TaO(2), and WO(2). In these cases the location of the spectral regions in which molecular transitions appear may also serve to encourage the gas spectroscopist to further exploration with high-dispersion spectrographs. I share Ramsay's view (4, p. 204) that further investigation of f-number determinations from matrix spectra should be encouraged, particularly because of the lack of such data for molecules in stars. The principal source of error probably lies in the estimation of the number of molecules per square centimeter in the matrix, but no real test of this has been made. The only existing f values from matrix spectra are those for the C(3) (43, 44) and C(2) (51) molecules, and these are not ideal for test purposes. Because of the anomalous nature of the matrix results discussed above, the rather good agreement between f values for the solid and gas phases of C(2) (51) cannot be considered as support for the matrix determinations. What is needed is a matrix determination of several f(v'v") values (that is, determinations for specific bands) for molecules such as CN and NO or, preferably, for a gas vaporized at high temperature, for which these f values are relatively well established in the gas phase. In this connection the possibility of determining other molecular properties in matrices comes to mind. However, it has been clearly shown that the shape of the potential energy function in the electronic states of molecules can be affected when molecules are trapped in matrices. Brewer, Brabson, and Meyer, in work on S(2) (55), and Schnepp and Dressler, in work on O(2) (56), have examined the anharmonicity in matrices over many vibrational levels. Distortion of the gas potential energy curves occurs in the heavier matrices and sometimes at high vibrational levels in the light ones. Recently work has been begun on comparing the Franck-Condon factors connecting the ground state and two excited states of ScF trapped in a neon matrix (57) with factors calculated from the gas-spectrum data of R. F. Barrow et al. (58) (Deltar(e), the change in interatomic distance upon excitation, has a relatively large value of ~ 0.1 A in these systems). As is well known, such factors are particularly sensitive to the value of Deltar(e), but differences in anharmonicity do not, however, have as significant an effect upon the Franck-Condon factors. Hence a comparison of the matrix and gas factors will lead to further information about matrix effects and will indicate whether Franck-Condon factors can be obtained from matrix spectra. One of the important problems in the study of stellar molecules is the determination of the low-lying electronically excited states, similar to the (1)Delta <--> X(3)Delta difference (~ 580 cm(-1)) in TiO measured by Phillips. Most of the transition-metal oxides have such low-lying levels, and they must be taken into consideration in any calculation of thermodynamic effects at high temperature. It appears that the study of emission spectra in the infrared at 4 degrees K may be one approach to this problem, and an attempt is now being made to confirm the TiO value in order to test the method. Perhaps the greatest advantage of matrix-isolation is the fact that it allows study of these molecules-or of any molecules difficult to produce in a microwave cavity-by electron-paramagnetic-resonance spectroscopy. Study of molecules by this means can provide information about ground state wave functions that is not obtainable by optical spectroscopy, as illustrated by the investigation of ScO, YO, and LaO. Also, it seems likely that the preferential orientation of molecules in matrices, which is probably achievable in most cases, will be a valuable asset in the study of the magnetic properties of molecules by electron-paramagneticresonance spectroscopy, regardless of whether the molecules are "hot" or "cold" (60).     

Ion binding by synthetic macrocyclic compounds

The existence of synthetic macrocyclic molecules with hydrophilic cavities containing multiple binding atoms and with hydrophobic exteriors gives rise to extraordinary possibilities with respect to the design and synthesis of molecules with specific cation and anion binding properties. The preparation of many new macrocyclic compounds has recently been reported, but few practical applications for them have been suggested. From the information available, it is becoming clear that it should be possible to synthesize macrocycles that will have specified, or selected, ion binding properties. Cavity size can be varied to accommodate only those cations or anions within a specified narrow band of sizes. Numbers and types of coordinating atoms can be chosen to give essentially electrostatic or covalent bonding or a combination of the two in a metalmacrocycle complex. The metal ligand bond appears to be predominantly ionic in the case of the cyclic polyethers but the covalent character increases on substitution of sulfur or nitrogen for oxygen donor atoms. The essential hydrophobic exteriors of the macrocycles can be modified by the addition of side chains and groups to facilitate the solution of anions and cations in organic solvents. The structures of many macrocycles can be made to approximate naturally occurring molecules, that is, cyclic polyethers similar to macrocyclic antibiotics of the valinomycin and nonactin types and cyclic polyamines similar to porphyrins. Macrocycles are also useful as model compounds for the study of metal interactions with biological systems. The synthetic macrocycles thus represent an intriguing new area of coordination chemistry, the systematic study of which should lead to many interesting and useful chemical applications in the field of metal complexation in solution.     

水分胁迫下黄瓜叶片光系统_电子传递及其含铁组分变化

黄瓜叶片在水分胁控下随着胁迫程度的加深，细胞膜透性增加。表现出PSⅡ荧光激发光谱特征峰峰值的降低。同时其PSⅡ电子传递速率及PSⅡ氧化则DCIP光还原活性下降。通过对PSⅡ的^57Fe穆斯堡尔谱及其参数进行分析显示；水分胁迫使PSⅡ的^57FeMossbauer谱的Ⅱ，Ⅵ子谱2套双峰消失；铁醌复合物（Fe-Q)、细胞色素b559(Cytb559)由还原态向氧化态转化。以上结果表明：铁醌复合体、Cytb559因光能在PSⅡ色素分子之间共振效率降低及PSⅡ氧化侧的电子受到抑制、不能及时得珐其氧化侧传来的电子，而以高自旋Fe^3 -Q复合物及低自旋氧化型Cytb559的形式存在。致使PSⅡ电子由于含铁复合物的Fe^2 /Fe^3 氧还电热的变化而受到阻抑。     

苦马豆素-BSA的合成研究

用溴乙酸制备的活性酯与苦马豆素反应 ,产物真空干燥后得到白色粉末状物 ,产率为 77%。经MP、IR鉴定和元素分析的结果判定 ,白色粉末状物为苦马豆素 -活性酯结合的季铵盐。季铵盐再与 BSA反应 ,产物经透析、冻干后呈白色针状结晶 ,按 BSA计算 ,产率为 91% ,经测定 ,平均每个 BSA分子上结合了 18.17个季铵盐分子。     

Scandium clusters in fullerene cages

The production and spectroscopic characterization of fullerene-encapsulated metal-atom clusters is reported. In particular, both solution and solid-state electron paramagnetic resonance (EPR) spectra of Sc(3)C(82) have been obtained. ScC(82) also gives an EPR spectrum, but Sc2Cn species-the most abundant metallofullerenes in the mass spectrum-are EPR-silent even though Sc(2) is EPR-active in a rare-gas matrix at 4.2 K. The results suggest that the three scandium atoms in Sc(3)C(82) form an equilateral triangle-as was previously suggested for Sc(3) molecules isolated in a cryogenic rare-gas matrix. The spectrum of ScC(82) has features similar to those found earlier for LaC(82) and YC(82), suggesting that it can also be described as a +3 metal cation within a -3 fullerene radical anion. An implication of this work is that production of macroscopic quantities of clustercontaining fullerenes may make possible the fabrication of exotic new structures with regular arrays of metal-atom clusters isolated in fullerene molecules, resulting in a new type of host/guest nanostructured material.     

Excitation spectra of circular, few-electron quantum dots

Studies of the ground and excited states in semiconductor quantum dots containing 1 to 12 electrons showed that the quantum numbers of the states in the excitation spectra can be identified and compared with exact calculations. A magnetic field induces transitions between the ground and excited states. These transitions were analyzed in terms of crossings between single-particle states, singlet-triplet transitions, spin polarization, and Hund's rule. These impurity-free quantum dots allow "atomic physics" experiments to be performed in magnetic field regimes not accessible for atoms.