Electronic structures of cephalosporins and penicillins. 2. Disulfide and beta-lactam chromophores. Structure-activity relationships.
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Cephalosporins↗
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Electronic structure, antimalarial activity, and phototoxicity of selected quinolinemethanol derivatives and analogs.
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Animals↗
Electronic structures of cephalosporins and penicillins. 15. Inductive effect of the 3-position side chain in cephalosporins.
Induction appears to be the primary means by which the side chain at position 3 of the cephem nucleus influences the chemical reactivity of the beta-lactam ring. In vitro antibacterial activity data suggest that when the cephalosporin is in the active site of the target bacterial enzymes, the presence of a leaving group in the side chain can promote inhibition.
Cephalosporins↗
Conformational studies and electronic structures of tamoxifen and toremifene and their allylic carbocations proposed as reactive intermediates leading to DNA adduct formation.
Toremifene, a compound which differs from tamoxifen by the substitution of a chlorine atom for a hydrogen atom in the ethyl group, is significantly less potent than tamoxifen in causing DNA adduct formation in rats. To examine the relationship of the DNA adduct-forming ability of these compounds with their physicochemical properties such as stable conformation and chemical reactivity, we carried out molecular mechanics, molecular dynamics, and quantum mechanics calculations for the two compounds. For tamoxifen, six stable conformers were identified by conformational search with CFF91 force field. Molecular dynamics simulations showed that these were often interconverted within 1.0 ns. On the other hand, although the conformation of stable conformers and dynamical behavior of toremifene were almost the same as those of tamoxifen, a few conformations were slightly different from those of tamoxifen owing to the effect of the chlorine atom at chloroethyl group. In addition, the stability of the allylic carbocation, which had been proposed as the reactive intermediate leading to DNA adduct formation, was calculated with both semiempirical and density functional methods. Results showed that the carbocation intermediate of toremifene was less stable than that of tamoxifen by 4-5 kcal/mol, suggesting that toremifene was less frequently activated to the intermediate than tamoxifen. Furthermore, the carbocation intermediates of two other tamoxifen derivatives, 4-iodotamoxifen and droxifene, which show no DNA adduct-forming ability, were also less stable compared with that of tamoxifen. These calculated results suggest a close relation between the stability of the proposed carbocation intermediate and DNA adduct-forming ability.
Antineoplastic Agents, Hormonal↗
Electronic structure of terpenoids.
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Molecular Conformation↗
N-tert-Butoxy-1-aminopyrenyl Radicals. Isolation, Electronic Structure, and Magnetic Characterization.
N-tert-Butoxy-2,7-di-tert-butyl-1-pyrenylaminyl (4), N-tert-butoxy-2-tert-butyl-1-pyrenylaminyl (5), and N-tert-butoxy-7-tert-butyl-1-pyrenylaminyl (6) free radicals were generated by the reaction of the lithium amides of the corresponding 1-aminopyrenes with tert-butyl peroxybenzoate in THF at -78 degrees C. Although 6 could not be isolated due to the gradual decomposition in solution, 4 and 5 were quite persistent and could be isolated as monomeric radical crystals. The X-ray crystallographic analyses for the isolated free radicals were successfully carried out, indicating that the N and O atoms are almost coplanar with the pyrene ring. The ESR spectra of 4 and 5 were very complex due to the presence of many magnetically unequivalent protons. Therefore, the proton hyperfine coupling (hfc) constants were determined by (1)H ENDOR/TRIPLE resonance spectroscopy. To assign the hfc constants for the pyrene ring protons, a partially deuterated radical, 4-d(4), was prepared and the ENDOR and ESR spectra were measured. To discuss the spin density distribution for 4 and 5 ab initio molecular orbital calculations were performed by the DFT UBecke 3LYP method, using the STO 6-31G basis set. Magnetic susceptibility measurements were carried out for 4 and 5 with a SQUID magnetometer. For 4 a weak antiferromagnetic interaction was observed, and for 5 a very strong antiferromagnetic interaction was observed. The antiferromagnetic interactions were explained by their crystal structures.
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Quantifying the reactivity of a remarkably long-lived difluorinated enol in acidic methanol via solution kinetics and electronic structure calculations.
A simple enol acetal underwent rapid cleavage in acidic solution to generate a difluorinated enol, which was sufficiently long-lived to be characterized by 2D NMR in a protic solvent at ambient temperature. Density functional theory calculations on a model reaction suggest that there are significant differences in protonation transition state timing between the fluorinated and nonfluorinated enols.
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Electronic Structure and Unimolecular Reactions of Cyclopropenone Carbonyl Oxide. A Theoretical Study.
A theoretical investigation on the lowest singlet and triplet potential energy surfaces of cyclopropenone carbonyl oxide 1 is presented. The calculated equilibrium geometry and dipole moment suggest that 1 possesses a strong zwitterionic character. The energy barrier for the isomerization of 1 to the cyclic isomer dioxirane 2 is computed to be 9.4 kcal/mol. The singlet state of 1 dissociates into cyclopropenone and excited singlet oxygen atom ((1)D), while the triplet state of 1 dissociates yielding oxygen atom in its ground state ((3)P). The dissociation process is endothermic for the singlet state but highly exothermic for the triplet.
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First principles investigation of the electronic structure of the iron carbide cation, FeC+.
We have studied 40 states of the diatomic iron carbide cation FeC(+) by multireference methods coupled with relatively large basis sets. For most of the states, we have constructed complete potential energy curves, reporting dissociation energies, usual spectroscopic parameters, and bonding mechanisms for the lowest of the studied states. The ground state is of (2)Delta symmetry, with the first excited state (a(4)Sigma(-)) lying 18 kcal/mol higher. The X(2)Delta state displays a triple-bond character, with an estimated D(0) value of 104 kcal/mol with respect to the adiabatic products or 87 kcal/mol with respect to the ground-state fragments.
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Evolution of the crystal and electronic structures of magnetic RuSr(2-x)La(x)GdCu2O8 superconductors upon La substitution.
We have investigated systematically the effects of La substitution on the chemical bonding nature and physical properties of magnetic RuSr(2-x)La(x)GdCu2O8 superconductors. X-ray diffraction and energy dispersive spectroscopic microprobe analyses reveal that a fraction of Sr ions can be successfully replaced by La ions with the contraction of unit cell volume. According to electrical resistance and dc magnetization measurements, the La substitution gives rise to a significant reduction of superconducting transition temperature (T(c)) but to an increase of magnetic ordering temperature with depressed remanent magnetization. Ru K- and Cu K-edge X-ray absorption spectroscopic results clarify that average Ru and Cu oxidation states decrease upon the La substitution. On the basis of the spectroscopic evidences presented here, we are able to attribute the T(c) reduction upon the La substitution to the depletion of the hole density in CuO2 layers and the accompanying variation of magnetic coupling behavior to the change of Ru oxidation state.
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Infrared spectra and electronic structure calculations for the group 2 metal M(OH)2 dihydroxide molecules.
Reactions of laser-ablated Mg, Ca, Sr, and Ba atoms with O2 and H2 in excess argon give new absorptions in the O-H and O-M-O stretching regions, which increase together upon UV photolysis and are due to the M(OH)2 molecules (M = Mg, Ca, Sr, and Ba). The same product absorptions are observed in the metal atom reactions with H2O2. The M(OH)2 identifications are supported by isotopic substitution and theoretical calculations (B3LYP and MP2). The O-H stretching frequencies of the alkaline earth metal dihydroxide molecules decrease from 3829.8 to 3784.6 to 3760.6 to 3724.2 cm(-1) in the family series in solid argon, while the base strength of the solid compounds increases. Calculations show that Sr(OH)2 and Ba(OH)2 are bent at the metal center, owing to d orbital involvement in the bonding. Although these molecules are predominantly ionic, the O-H stretching frequencies do not reach the ionic limit of gaseous OH- going down the family group because of cation-anion polarization and p(pi) --> d(pi) interactions.
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Electronic structure of mercury oligomers and exciplexes: models for long-range/multicenter bonding in phosphorescent transition-metal compounds.
Spectroscopic and bonding properties of Hg(n) oligomers and *Hg(n) exciplexes are determined by rigorous theoretical treatments. Reliable values that agree well with experimental data have been computed for the luminescence energies and other molecular spectroscopic parameters by making a careful selection of theoretical methods and basis sets. The calculations clarified the assignments for several phosphorescence bands in the mercury vapor based on calculated energies and other parameters that quantify the large excited-state distortion in the emitting states. Both the weak ground-state mercurophilic bonding and the stronger covalent bonding in the triplet and quintet excited states studied are found to be cooperative, which is important for fundamental and applied research for luminescent and magnetic materials that have spectral behavior similar to that of Hg(n) systems.
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Electronic structure of the nucleobases.
We present a comparison between experimental and calculated soft X-ray spectra of DNA's nucleobases, adenine (A), guanine (G), cytosine (C), and thymine (T) using X-ray absorption spectroscopy (XAS) and soft X-ray emission spectroscopy (XES). Spectra of the 1s thresholds of carbon, nitrogen, and oxygen give a complete picture of the occupied and unoccupied partial density of states of the nucleobases. A combination of both Hartree-Fock and density functional theory calculations are used in the comparison to experimental results. Most experimental results agree well with our theoretical calculations for the XAS and XES of all bases. All spectral features are assigned. A comparison of the experimental highest occupied molecular orbital-lowest unoccupied molecular orbital energy gaps is made to the diverse values predicted in the literature.
Adenine↗
X-ray absorption spectroscopy of liquid methanol microjets: bulk electronic structure and hydrogen bonding network.
We have measured the X-ray absorption (XA) spectrum of liquid (298 K) methanol at the oxygen and carbon K edges. The 4a(1) orbital at the O K edge exhibits a pronounced sensitivity to the formation of intermolecular hydrogen bonds, with significant differences observed between the vapor and bulk spectra, whereas the C K edge reveals only subtle corresponding spectral changes. Comparison with DFT computed spectra of model methanol clusters indicates that the bulk liquid comprises long chains (n > 6) and rings of hydrogen-bonded monomers.
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Angle-resolved photoemission spectroscopy study of adsorption process and electronic structure of silver on ZnO(1010).
The adsorption process and valence band structure of Ag on ZnO(1010) have been investigated by angle-resolved photoelectron spectroscopy utilizing synchrotron radiation. The coverage-dependent measurements of the Ag 4d band structure reveal that the Ag bands with a dispersing feature are formed even at low coverages and that the basic structure of the bands is essentially the same throughout the submonolayer region. These results indicate that the Ag atoms aggregate to form islands with an atomically ordered structure from the low coverages. Upon annealing the Ag-covered surface at 900 K, the Ag 4d band undergoes only a minor change, suggesting that the ordered structure within the Ag islands is persistent against mild annealing. From the dispersive feature of the Ag 4d states, we propose that the atomic structure has locally rectangular symmetry with a good lattice matching with the ZnO(1010) surface.
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Intramolecular nitro-assisted proton transfer in photoirradiated 2-(2',4'-dinitrobenzyl)pyridine: polarized optical spectroscopic study and electronic structure calculations.
The nitro-assisted proton transfer (NAPT), responsible for the photoactivity of ortho-nitrobenzylpyridines and a model for the nitro-based caged compounds, is studied along with the parent compound 2-(2',4'-dinitrobenzyl)pyridine (DNBP) with polarized optical spectroscopy and theoretical calculations. The transition dipole moments of a DNBP single-crystal identified oriented molecules of the long-lived enamine tautomer (NH), rather than of the aci-nitro tautomer (OH), as carriers of the photoinduced blue coloration. It is clarified that the blue second singlet transition owes to intramolecular charge transfer from the allyl-pyridinium part to the dinitrophenyl fragment of NH. The theoretical modeling of the ground-state potential energy surface showed that while NH and OH can interconvert by means of direct proton transfer, such a process between the initial form CH and either OH and NH would require significant rotation of the aromatic rings. In the ground state, OH is less stable but the kinetically preferred product over NH. Once created, regardless of whether via ground-state or excited-state routes, the aci-nitro group of OH undergoes energetically inexpensive rotation to deliver the proton to the nitrogen acceptor. The "softening" of the energy surface around OH due to its structural flexibility, that is, mediation of the proton transfer by the nitro group, is crucial to overcome the high barrier for a direct proton jump from CH to NH, even in cases of unfavorable donor-acceptor geometry. The very small structural change experienced by the surrounding of a molecule undergoing NAPT is promising for the design of photoactive systems which retain their crystallinity during a prolonged operation.
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A new family of pi-conjugated delocalized biradicals: Electronic structures of 1,4-bis(2,5-diphenylimidazol-4-ylidene)cyclohexa-2,5-diene.
A new family of pi-conjugated delocalized biradical compound is developed. The solution of 1,4-bis(2,5-diphenylimidazol-4-ylidene)cyclohexa-2,5-diene shows the ESR signal consisting of a moderately broad unresolved line, Delta H(pp) approximately 1 mT, at room temperature. The presence of the thermal equilibrium between a triplet biradical state and a singlet state is confirmed by the ESR measurements, and the spin concentration is determined as 7.90 x 10(21) spin/mol at 300 K. The spin concentration can also be controlled by modifying the molecular planarity. Moreover, the unrestricted DFT/B3LYP calculations suggest the biradical character of the singlet ground state, and the modulation of the energy gap between the singlet state and the triplet state is investigated from the theoretical point of view. Controlling the equilibrium between a diamagnetic state and a paramagnetic state will provide significant progress in the field of biradical chemistry, and the materials with the biradical character in a ground state will lead to a novel development of molecular-based organic magnets.
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Comparison of levels of electronic structure theory in direct dynamics simulations of C2H5F --> HF + C2H4 product energy partitioning.
Direct dynamics simulations at the MP2/6-311++G** level of theory were performed to study C(2)H(5)F --> HF + C(2)H(4) product energy partitioning. The simulation results are compared with experiment and a previous MP2/6-31G* simulation. The current simulation with the larger basis set releases more energy to HF vibration and less to HF + C(2)H(4) relative translation as compared to the previous simulation with the 6-31G* basis set. The HF rotation and vibration energy distributions determined from the current simulation are in overall very good agreement with previous experimental studies of C(2)H(5)F dissociation by chemical activation and IRMPA. A comparison of the simulations with experiments suggests there may be important mass effects for energy partitioning in HX elimination from haloalkanes. The transition state (TS) structures and energies calculated with MP2 and the 6-31G* and 6-311++G** basis sets are compared with those calculated using CCD, CCSD, CCSD(T), and the 6-311++G** basis set.
Journal Article↗