Antibonding Sigma Research Articles

Trifluoromethyl-stabilized 2D nitrogen clusters: a theoretical study

The stability of 2D all nitrogen clusters containing from 6 to 96 nitrogen atoms, terminated with CF3 groups, has been explored using two computational models: dispersion corrected B3LYP functional and scaled opposite spin Møller-Plesset perturbation theory (SOS-MP2). Single point domain-based local pair natural orbital coupled-cluster theory calculations (DLPNO-CCSD(T)) was used for further energy refinement. All systems were found to be minima, and their stability increases with CF3/N ratio. Larger clusters and anion radicals were not dynamically stable, while some of the cation radicals were found to be minima on potential energy surface. The mechanism of cluster stabilization by CF3 groups is related with interaction of orbitals holding lone electron pairs and antibonding sigma orbitals.

Density functional theory on ionic liquid as carbonate scale dissolver in petroleum pipelines

The carbonate scale is the common oilfield scale found in the petroleum pipeline, which will cause flow assurance problems. In the purpose to conquer this problem, the alternative approach is to use ionic liquids (ILs) particularly in dissolving carbonate scale. The main objective of this study is to determine the effective ILs to dissolve carbonate scale in petroleum pipelines, and to evaluate the effect of intermolecular interaction in ILs toward their properties such as dissolution capabilities. The density functional theory (DFT) calculation is used to investigate the intermolecular interaction of a series of different ion pair, namely cations and anions-based of ILs with carbonate ion. All the calculations are completed by Gaussian software at the hybrid Becke 3-Lee-Yang-Parr (B3LYP) level using the 6-311++G(d, p) basis set. The optimized structure of carbonate ion and ion pairs of ILs have been obtained from the calculation which was further complemented by harmonic vibrational frequency, binding energies, bond type, bond order, and natural bond orbital (NBO). The binding energy for the cation and anion-based ILs are obtained where the ammonium ion is −302.2627 kJ/mol and tetrafluoroborate ion is −562.9194 kJ/mol which depict to have the highest value compare to others. Both of them are made up of the single bond and are characterized as the polar covalent bond. The NBO analysis pointed out the higher second-order perturbation energy, E (2) for ammonium ion is at a lone pair, LP (1) N 9 to antibonding sigma, BD*(1) O 3H 6 while BF4 ion is at lone pair LP (3) F 2 to antibonding lone pair, LP*(1) Ca 6. The selective ILs, ammonium tetrafluoroborate shown the higher negative binding energy which is −371.93 kJ/mol having the same nature of bond type and bond order from the findings in cation and anion-based ILs.

Investigation of the role of stereoelectronic effects in the conformation of piperidones by NMR spectroscopy and X-ray diffraction.

This paper reports the synthesis of a series of piperidones 1–8 by the Mannich reaction and analysis of their structures and conformations in solution by NMR and mass spectrometry. The six-membered rings in 2,4,6,8-tetraphenyl-3,7-diazabicyclo[3.3.1]nonan-9-ones, compounds 1 and 2, adopt a chair–boat conformation, while those in 2,4-diphenyl-3-azabicyclo[3.3.1]nonan-9-ones, compounds 3–8, adopt a chair–chair conformation because of stereoelectronic effects. These stereoelectronic effects were analyzed by the 1JC–H coupling constants, which were measured in the 13C satellites of the 1H NMR spectra obtained with the hetero-dqf pulse sequence. In the solid state, these stereoelectronic effects were investigated by measurement of X-ray diffraction data, the molecular geometry (torsional bond angles and bond distances), and inter- and intramolecular interactions, and by natural bond orbital analysis, which was performed using density functional theory at the ωB97XD/6311++G(d,p) level. We found that one of the main factors influencing the conformational stability of 3–8 is the interaction between the lone-pair electrons of nitrogen and the antibonding sigma orbital of C(7)–Heq (nN→σ*C–H(7)eq), a type of hyperconjugative interaction.

Role of Symmetry in Coupled Localized Surface Plasmon Resonance of a Nanosphere Pair

We investigated localized surface mode resonance of two nanospheres immersed in a continuum dielectric medium within the framework of quasistatic approximation. We solved the Laplace equation in the bispherical coordinates subjected to the boundary conditions on both spheres. By using the matrix formulation, new method for surface mode resonance calculation is derived in terms of a nonlinear eigenvalue problem. This allows us to discuss the effects of symmetry breaking due to the difference in particle materials and particle sizes. We calculated the surface plasmon resonance (LSPR) energy for pairs of Drude metallic spheres. The solutions show that the LSPR coupling in an asymmetric pair can be interpreted as the coupling between symmetric pairs of its constituent particles. we also analyzed the LSPR excitation sequence to identify the modes of peaks from optical measurement. We found that the ambiguous dipole antibonding sigma mode of the asymmetric Au-Ag pair in the experimental work (Sheikholeslami et al., Nano Lett 10(7):2655–2660, 2010) must be the quadrupole bonding sigma mode.

Generalized anomeric effect of α-chloro-O-oxime ethers; influence of various substitutions by DFT, NBO and AIM studies

α-Halo-O-oxime ethers have been reported to be the key intermediates in the synthesis of O-oxime ethers. Their stability has a very important role in the synthesis possibility and the product yield, and the anomeric effect is the main factor representing their stability. The density functional theory (DFT) has been used to investigate the effect of various substitutions (R1/R2=CH3, CF3, t-But, OMe, N(Me)2) with different induction or resonance influences on the anomeric effect of α-chloro-O-oxime ethers. The results from the natural bond orbital (NBO) and atoms in molecules (AIM) analyses have been discussed and useful comparisons have been carried out. All calculations at B3LYP level with 6-31G(d,p) and 6-311G(d,p) basis sets, also at correlation method CCSD/6-31G(d,p)//B3LYP/6-311G(d,p), show that in all compounds the gauche conformers are the most stable conformation, and the reasons have been attributed to the hyperconjugation and steric effects. The NBO analyses show that increasing the electron-donating strength of the substitutions in R1 and/or R2 positions leads to more charge transfer from the lone pairs of the oxygen atom to the C–Cl anti-bonding sigma orbital, which results in more amount of the anomeric effect. It has been shown that the LPO→πN–C∗ interaction could be considered as a lowering factor for the anomeric effect in these compounds. The influences of substituting in different R1 and R2 positions on the anomeric effect of the studied compounds have been also investigated. The AIM studies on the topological properties of the bond critical point (BCP) of the C–Cl and C–O bonds in the studied compounds confirm the results of the NBO and structural analyses.

Effect of Sulfur Oxidation on the Transmission Mechanism of 4JHH NMR Coupling Constants in 1,3-Dithiane

Long-range 4J(HH) couplings in 1,3-dithiane derivatives are rationalized in terms of the effects of hyperconjugative interactions involving the S=O group. Theoretical and experimental studies of 4J(HH) couplings were carried out in 1,3-dithiane-1-oxide (2), cis-1,3-dithiane-1,3-dioxide (3), 1,3-dithiane-1,1,3-trioxide (4), and 1,3-dithiane-1,1,3,3-tetraoxide (5) compounds. Hyperconjugative interactions were studied with the natural bond orbital, NBO, method. Hyperconjugative interactions involving the LP(O), oxygen lone pair and sigma*(C2-S1) and sigma*(S1-C6) antibonding orbitals yield an increase of 4J(H(eq)-H(eq)) couplings. Long-range 4J(H(ax)-H(ax)) couplings were also observed between hydrogen atoms in axial orientation, which are rationalized as originating in hyperconjugative interactions involving the bonding sigma(C6-H(ax)) and antibonding sigma*(S=O) orbitals. The symmetry for orbital interactions is possible only when the S=O group is in the axial orientation.

C-Hax···YaxContacts in Cyclohexane Derivatives Revisited−Identification of Improper Hydrogen-Bonded Contacts

The structure of 111 cyclohexane derivatives bearing the axial substitution Y(ax)-C was optimized at the B3LYP/6-31+G(d,p) level. The natural bond orbital analysis revealed the presence of overlap interactions between the axial substituent and the antibonding sigma*(C-Hax) orbitals; these calculated hyperconjugative interactions suggest the presence of improper H-bonded contacts. The addition of an appropriate bridging fragment between the axial substituent and cyclohexane carbon strengthens significantly the hydrogen-bonding component of the contact and several structures of axially substituted cyclohexane derivatives including such hydrogen-bonded C-H(ax)...Y(ax)-C contacts were retrieved from the Cambridge Crystallographic Database. Overall, the calculations predicted that the C-H(ax)...Y(ax)-C contacts in common cyclohexane derivatives that are generally thought to be steric in nature (Pauli repulsive forces) include an improper hydrogen-bonding component.

Ambidentate coordination in hydrogen bonded dimethyl sulfoxide, (CH3)2SO⋯H3O+, and in dichlorobis(dimethyl sulfoxide) palladium(ii) and platinum(ii) solid solvates, by vibrational and sulfur K-edge X-ray absorption spectroscopy

The strongly hydrogen bonded species (CH3)2SO...H3O+ formed in concentrated hydrochloric acid displays a new low energy feature in its sulfur K-edge X-ray absorption near edge structure (XANES) spectrum. Density Functional Theory-Transition Potential (DFT-TP) calculations reveal that the strong hydrogen bonding decreases the energy of the transition S(1s) --> LUMO, which has antibonding sigma*(S-O) character, with about 0.8 eV. Normal coordinate force field analyses of the vibrational spectra show that the SO stretching force constant decreases from 4.72 N cm(-1) in neat liquid dimethyl sulfoxide to 3.73 N cm(-1) for the hydrogen bonded (CH3)2SO...H3O+ species. The effects of sulfur coordination on the ambidentate dimethyl sulfoxide molecule were investigated for the trans-Pd((CH3)2SO)2Cl2, trans-Pd((CD3)2SO)2Cl2 and cis-Pt((CH3)2SO)2Cl2 complexes with square planar coordination of the chlorine and sulfur atoms. The XANES spectra again showed shifts toward low energy for the transition S(1 s) --> LUMO, now with antibonding sigma*(M-Cl, M-S) character, with a larger shift for M = Pt than Pd. DFT-TP calculations indicated that the differences between the XANES spectra of the geometrical cis and trans isomers of the M((CH3)2SO)2Cl2 complexes are expected to be too small to allow experimental distinction. The vibrational spectra of the palladium(II) and platinum(II) complexes were recorded and complete assignments of the fundamentals were achieved. Even though the M-S bond distances are quite similar the high covalency especially of the Pt-S bonds induces significant increases in the S-O stretching force constants, 6.79 and 7.18 N cm(-1), respectively.

Theoretical and experimental sulfur K-edge X-ray absorption spectroscopic study of cysteine, cystine, homocysteine, penicillamine, methionine and methionine sulfoxide

The experimental sulfur K-edge X-ray absorption near-edge structure (XANES) spectra of the amino acids cysteine, homocysteine, penicillamine, methionine, including the oxidation products methionine sulfoxide and the disulfide cystine, have been analyzed by transition potential DFT calculations. The absolute energies and intensities of the main pre-edge sulfur 1s electron transitions have been computed to determine the character of the receiving unoccupied molecular orbitals (MO), and to investigate the influence of external interactions, especially by introducing water molecules hydrogen-bonded to the ionic species present in different pH ranges. When the thiol group deprotonates for cysteine, homocysteine and penicillamine and also for the cysteine residue in glutathione the energy of the main transition, to an MO with antibonding sigma*(S-H) character, reduces by approximately 1.1 eV and the receiving MO obtains sigma*(S-C) character. The changes in transition energy due to hydrogen-bonding were in most cases found to be relatively small, although the transition intensities could vary significantly due to the changes induced in the molecular charge distribution, thereby affecting the shapes of the spectral features. For the cysteine and penicillamine zwitterions deconvolution of the experimental spectra allowed the microscopic acid dissociation constants to be extracted separately for the thiol and the protonated amine groups, pK(a)(S) = 8.5 +/- 0.1 and 8.2 +/- 0.1, and pK(a)(N) = 8.9 +/- 0.1 and 8.8 +/- 0.1, respectively, with the thiol group in both cases being the more acidic. Coordination of cysteine to nickel(II) or mercury(II) introduced a new low energy transition involving metal ion orbitals in the receiving LUMO. The small experimentally observed energy differences between the similar main absorption features of the cysteine and methionine zwitterions, 0.2-0.3 eV in comparable surrounding, as well as a minor difference in their intensities, are reflected in the calculated transitions. The S K-edge XANES spectrum of the disulfide cystine displays a characteristic double peak with the lower energy transition (2469.9 eV) into the antibonding sigma*(S-S) MO. The second peak, at 2471.5 eV in aqueous solution, contains several transitions into MOs with sigma*(S-C) character involving also charge transfer to the water molecules hydrating the protonated amine groups (NH(3)(+)) of cystine. For solid cystine without hydrogen bonding the experimental energy difference between the two peaks is 0.2 eV larger, while no such increase occurs for the oxidized disulfide of glutathione, with a similar -S-S- bond between its cysteine residues as in cystine, because the amine groups are engaged in peptide bonds. This study shows that externally induced changes in the intramolecular bonding, e.g., by coordination, conformation geometry or hydrogen-bonding, can significantly influence the S K-edge spectra, and emphasizes the importance of a similar chemical surrounding when choosing the model compounds for standard spectra of sulfur functional groups, used to deconvolute composite experimental spectra.

Ion-pair dissociation of N2O in the 16.25–16.41eV: Dynamics and electronic structure

The ion-pair dissociation dynamics of N(2)O -->(XUV) N(2)(+)(X (2)Sigma(g)(+), v) + O(-)((2)P(j)) at 16.248, 16.271, 16.389, and 16.411 eV have been studied using the velocity map imaging method and tunable XUV laser. The electronic structures of the ion-pair states have been studied by employing the ab initio quantum chemical calculation. The translational energy distributions and the angular distributions of the photofragments have been measured. The results show that about 40% of available energies are transformed into the translational energies, and the first excited vibrational states are populated most strongly for all four excitation energies. The anisotropy parameters beta are approximately 1. The ab initio calculations at the level of CASSCF6-311++g(3df) show that the equilibrium geometries of the ion-pair states are nonlinear with bond lengths R(N-N) = 1.10 A, R(N-O) = 2.15 A, and bond angle N-N-O = 103 degrees, respectively. The ion-pair states are formed by electron migration from the bonding sigma orbital of N[triple bond]N to the antibonding sigma orbital localized primarily on the O atom. Combining the experimental and theoretical results, it is concluded that the ion-pair dissociation occurs via predissociation of Rydberg states with (1)Sigma(+) symmetry, which converges to the ion-core N(2)O(+)(A (2)Sigma(+)).

Formation of a Sandwich-Structure Assisted, Relatively Long-Lived Sulfur-Centered Three-Electron Bonded Radical Anion in the Reduction of a Bis(1-substituted-uracilyl) Disulfide in Aqueous Solution

The one-electron reduction of bis[1-(2',3',5'-tri-O-acetylribosyl)uracil-4-yl] disulfide, initiated by hydrated electrons in a radiation chemical study, has been shown to yield 1-(2',3',5'-tri-O-acetylribosyl)-4-thiouracil as a stable molecular product. The reduction reaction leads, in the first instance, to a transient, albeit remarkably stable disulfide radical anion. This is characterized by a 2-center-3-electron bond with two bonding sigma-electrons and an antibonding sigma*-electron in the sulfur-sulfur bridge, (-S therefore S-)(-). It receives its stability from a sandwich-structure with the two uracilyl moieties facing each other (possibly further assisted by the 2',3',5'-tri-O-acetylribosyl substituents). A considerable lengthening of the original disulfide bridge from 2.02 to 2.73 A in the radical anion seems to facilitate the interaction of the heterocycles and leads to a gain in stabilization energy of 24 and 33 kcal/mol (100 and 140 kJ/mol) as evaluated by UMP2/cc-pVTZ and UMP2/cc-pVDZ calculations, respectively. The (-S therefore S-)(-) bonded radical anion shows a broad optical absorption band with lambdamax=450 nm, epsilonmax=6000 M(-1) cm(-1), and a half-width of 1.0 eV. It exists in equilibrium with the conjugated 1-(2',3',5'-tri- O-acetylribosyl)uracil-4-yl thiyl radical -S(*), and the corresponding thiolate, -S(-). The rate determining step for the disappearance of the disulfide radical anion appears to be protonation of both the radical anion and the free thiolate by reaction with H(+)aq. Absolute rate constants have been measured for these protonation processes, for the formation of the stable thiouridine product, and for the electron transfer from the disulfide radical anion to molecular oxygen. With the (-S therefore S-)(-) <--> -S(*) + -S(-) equilibrium lying very much on the left-hand side, the reduced disulfide system exhibits predominantly reducing properties whereas any oxidizing property of the conjugated thiyl radical has only little if any chance to materialize. Besides attaching directly to the disulfide bridge, the hydrated electrons react also, with about equal efficiency, with the uracil moiety of the investigated compound. This leads to a structurally totally different and electronically distinguishable species than that with the reduced disulfide bridge. In particular, there is no face-to-face interaction between the two heterocyclic moieties and no increased electron density in the S-S bond. The C-centered radicals resulting from the reduction of the uracil and possibly also generated from the ribosyl moieties initiate further cleavage of the S-S bond and thus contribute to the formation of thiouridine. The overall yield of the latter, as determined from steady-state gamma-radiolysis, indicates a small chain process (G=1.54 micromol/J). Possible mechanisms are discussed.

Theoretical Investigations on Heteronuclear Chalcogen−Chalcogen Interactions: On the Nature of Weak Bonds between Chalcogen Centers

To understand the intermolecular interactions between chalcogen centers (O, S, Se, Te), quantum chemical calculations on model systems were carried out. These model systems were pairs of monomers of the composition (CH3)2X1 (X1 = O, S, Se, Te) as the donors and CH3X2Z (with X2 = O, S, Se, Te and Z = Me, CN) as the acceptors. The variation of X1, X2, and Z leads to 32 pairs with 8 homonuclear cases (X1 = X2 = O, S, Se, Te) and 24 heteronuclear cases (X1 not equal X2). The MP2/SDB-cc-pVTZ, 6-311G* level of theory was used to derive the geometrical parameters and the interaction energies of the model systems. The pairs with Z = CN (17-32) show a considerably higher interaction energy than the pairs with CH3 groups only (1-16). Natural bond orbital (NBO) analysis revealed that the interaction of the dimers 1, 2, 5, 6, 9, 10, 13, 14, 17, 21, 25, and 29 is mainly due to weak hydrogen bonding between methyl groups and chalcogen centers. These systems all contain hard chalcogen atoms as acceptors. For all other systems, the chalcogen-chalcogen interaction dominates. The one-electron picture of an interaction between the lone pair of the donor chalcogen atom and the chalcogen-carbon antibonding sigma* orbital serves as a model to qualitatively rationalize trends found in many of these systems. However, it has to be applied with some amount of skepticism. A detailed analysis based on symmetry-adapted perturbation theory (SAPT) reveals that induction and dispersion forces dominate and contribute to the bonding in each case. Hydrogen-bonded compounds involve bonding electrostatic contributions. Compounds dominated by chalcogen-chalcogen interactions exhibit bonding due to electrostatic interactions only if one of the chalcogen atoms involved is sulfur or oxygen.

Experimental Determination of the nN → σ*P-O Interaction Energy of O-Equatorial C-Apical Phosphoranes Bearing a Primary Amino Group

The reaction of a chlorophosphorane (9-Cl) with primary amines produced anti-apicophilic spirophosphoranes (5, O-equatorial phosphoranes), which violate the apicophilicity concept, having an apical carbon-equatorial oxygen configuration, along with the ordinarily expected O-apical stereoisomers (6) with the apical oxygen-equatorial carbon configuration. Although the amino group is electronegative in nature, the O-equatorial phosphoranes were found to be stable at room temperature and could still be converted to their more stable O-apical pseudorotamers (6) when they were heated in solution. X-ray analysis implied that this remarkable stability of the O-equatorial isomers could be attributed to the orbital interaction between the lone-pair electrons of the nitrogen atom (n(N)) and the antibonding sigma(P-O) orbital in the equatorial plane. A kinetic study of the isomerization of 5 to 6 and that between diastereomeric O-apical phosphoranes 13b-exo and 13b-endo revealed that 5b bearing an n-propylamino substituent at the central phosphorus atom was found to be less stable than the corresponding isomeric 6b by ca. 7.5 kcal mol(-1). This value was smaller than the difference in energy (11.9 kcal mol(-1)) between the O-equatorial (1b) and the O-apical n-butylphosphorane (2b) by 4.4 kcal mol(-1). This value of 4.4 kcal mol(-1) can be regarded as the stabilization energy induced by the n(N) --> sigma(P-O) interaction. The experimentally determined value was in excellent agreement with that derived from density functional theory (DFT) calculations at the B3PW91 level (4.0 kcal mol(-1)) between the nonsubstituted aminophosphoranes (5g is less stable than 6g by 10.1 kcal mol(-1)) and their P-methyl-substituted counterparts (1a is less stable than 2a by 14.1 kcal mol(-1)).

Ab Initio Molecular Dynamics of Liquid 1,3-Dimethylimidazolium Chloride

Density-functional-based Car-Parrinello molecular dynamics (CPMD) simulations have been performed for the ionic liquid 1,3-dimethylimidazolium chloride, [dmim]Cl, at 438 K. The local structure of the liquid is described in terms of various partial radial distribution functions and anisotropic spatial distributions, which reveal a significant extent of hydrogen bonding. The cation-anion distribution simulated with the BP86 functional is in qualitative agreement with the structural model derived from neutron diffraction data for the liquid, whereas the theoretical cation-cation distribution shows less satisfactory accord. Population analyses indicate noticeable charge transfer from anions to cations, and specific CH...Cl hydrogen bonds are characterized in terms of donor-acceptor interactions between lone pairs on Cl and antibonding sigma(CH) orbitals.

Electron Delocalization in Nickel Metallic Wires: A DFT Investigation of Ni3(dpa)4Cl2 and [Ni3(dpa)4]3+ (dpa = Dipyridylamide) and Extension to Higher Nuclearity Chains

The electronic structure of Ni(3)(dpa)(4)Cl(2) (1) has been investigated within the framework of the density functional theory (DFT), using two types of exchange-correlation functionals and various basis sets. The "broken-symmetry" approach proposed by Noodleman for the characterization of electronic states displaying an antiferromagnetic coupling has been applied to 1. All calculations lead to the conclusion that the ground state results from an antiferromagnetic coupling between the terminal Ni atoms, both displaying a high-spin electronic configuration. The central Ni atom is in a low-spin configuration, but is involved in a superexchange interaction connecting the two magnetic centers. These results are in agreement with the assignments recently proposed by the group of F. A. Cotton on the basis of magnetic measurements. It is shown that the ground state electronic configuration calculated for 1 provides the trinickel framework with some delocalized sigma bonding character. The observed geometry of 1 is accurately reproduced by the broken-symmetry solution. The doublet ground state assigned to the oxidized species [Ni(3)(dpa)(4)](3+) (2) and the dramatic contraction of the coordination sphere of the terminal metals observed upon oxidation are also confirmed by the calculations. However, the formal Ni-Ni bond order is not expected to increase in the oxidized species. The contraction of the Ni-Ni distance in 2 is shown to result in part from the vanishing of the important trans influence originating in the axial ligands, and for the rest from a more efficient shielding of the metal nuclear charge along the Ni-Ni-Ni axis. The conclusions deduced from the analysis of the bonding in 1 and 2 can be extended to their homologues with higher nuclearity. More specifically, it is predicted that the single occupancy of the most antibonding sigma orbital, extending over the whole metal framework, will provide the (Ni(p))(2)(p)(/(2)(p)(+1)+) chains with some delocalized bonding character and, possibly, with electrical conduction properties.

XANES study on Ruddlesden-Popper phase, La(n+1)Ni(n)O(3n+1) (n=1,2 and infinity).

Ruddlesden-Popper phase, La(n+1)Ni(n)O(3n+1 (n=1, 2, and infinity) compounds were prepared by citrate sol-gel method. We revealed the origin of the variation of the electrical conductivities in La(n+1)Ni(n)O(3n+1 (n=1, 2, and infinity) using resistivity measurements, Rietveld analysis, and X-ray absorption spectroscopy. According to the XANES spectra, it is found that the degree of 4p(pi) - 4psigma energy splitting between 8345 eV and 8350 eV is qualitatively proportional to the elongation of the out-of-plane Ni-O bond length. With the decrease of 4p(pi) - 4psigma splitting, the strong hybridization of the sigma-bonding between Ni-3d and O-2p orbitals creates narrow antibonding sigma* bands, which finally results in the lower electrical resistivity.

A novel class of antitumor prodrug, 1-(2'-oxopropyl)-5-fluorouracil (OFU001), that releases 5-fluorouracil upon hypoxic irradiation.

We have been developing prodrugs of anticancer agents such as 5‐fluorouracil (5‐FU) that are activated by irradiation under hypoxic conditions via one‐electron reduction. Among them, OFU001 [1‐(2′‐oxopropyl)‐5‐fluorouracil] is a prototype radiation‐activated prodrug. In this study, we investigated the radiation chemical reactivity and the biological effects of OFU001. This prodrug is presumed to release 5‐FU through incorporation of hydrated electrons into the antibonding σ* orbital of the C(1′)‐N(1) bond. Hydrated electrons are active species derived from radiolysis of water, but are readily deactivated by O2 into superoxide anion radicals () under conditions of aerobic irradiation. Therefore, 5‐FU release occurs highly specifically upon irradiation under hypoxic conditions. OFU001 dissolved in phosphate buffer released 5‐FU with a G‐value (mol number of molecules that are decomposed or produced by 1 J of absorbed radiation energy) of 1.9×10−7 mol/ J following hypoxic irradiation, while the G‐value for 5‐FU release was 1.0×10−8 mol/J following aerobic irradiation. However, the G‐values for decomposition of OFU001 were almost the same, i.e., 3.4×10−7 mol/J following hypoxic irradiation and 2.5×10−7 mol/J following aerobic irradiation. When hypoxically irradiated (7.5–30 Gy) OFU001 was added to murine SCCVII cells for 1–24 h, a significant cell‐killing effect was observed. The degree of this cytotoxicity was consistent with that of authentic 5‐FU at the corresponding concentrations. On the other hand, cytotoxicity was minimal when the cells were treated with aerobically irradiated or unirradiated OFU001. This compound had no radiosensitizing effect against SCCVII cells under either aerobic or hypoxic conditions when the drug was removed immediately after irradiation. Since hypoxia is generally most marked in tumors and irradiation is applied at the tumor site, this concept of prodrug design appears to be potentially useful for selective tumor treatment with minimal adverse effects of anticancer agents.

Structure and dynamics of excited electronic states at the adsorbate/metal interface: C6F6/Cu(111).

Excited state electron transfer at the adsorbate/metal interface represents a key step in molecular electronic devices. The dynamics of such processes are governed by ultrafast energy relaxation which can be probed directly by time-resolved two-photon photoemission (2PPE). Using 2PPE spectroscopy we investigate the energetics and lifetimes of the unoccupied electronic states of C6F6 adsorbed on Cu(111) as a model system for electron transfer at organic/metal interfaces. With increasing C6F6 layer thickness we find a pronounced decrease in the energetic position of the lowest unoccupied state, which is accompanied by a strong increase in its lifetime as well as a decrease in the effective electron mass. The frequently employed dielectric continuum model which describes delocalized (quantum well) states within adsorbate layers does not give a consistent explanation of these findings. By adsorption of Xe overlayers onto C6F6/Cu(111) we can show that, even for one monolayer of C6F6, the excited state must be localized predominantly inside the C6F6 layer and thus originates from a molecular state (presumably an antibonding sigma* orbital). With increasing coverage this state becomes more delocalized within the adsorbate layer, which reduces the coupling to the metal substrate and thus enhances the excited state lifetime.

Decay channels of core-excited and ionized HCl studied by time-of-flight mass spectroscopy

The decay channels of HCl after excitation of Cl 2p electrons to different resonance states below the ionization threshold have been studied by measuring the ionic fragments using time-of-flight mass spectrometry. The fragmentation at resonances is found to deviate significantly from that obtained above the ionization threshold. The fragmentation is also observed to depend strongly on whether the 2pCl electron is excited to the antibonding sigma * orbital or to the Rydberg orbitals. The interpretation is based on theoretical calculations.

Optical luminescence excitation spectra of molecular oxygen in the soft x-ray region

The observation of an anomaly in the optical luminescence excitation spectrum of oxygen in the region of the oxygen K edge is reported. Dispersed luminescence spectra were obtained for x-ray excitation at the pi and sigma resonances, at the anomaly, and in the continuum. These spectra indicate enhanced production of O2+2 ions at both the sigma resonance and at the anomaly. The anomaly thus is attributed to a shake-up or shake-off state associated with an antibonding sigma molecular orbital of oxygen. This work also demonstrates that optical luminesence spectra provide state-specific information about the products of core hole excitation and relaxation.