Two C3‐Symmetric Dy3III Complexes with Triple Di‐μ‐methoxo‐μ‐phenoxo Bridges,Magnetic Ground State,and Single‐Molecule Magnetic Behavior

Two series of isostructural C₃‐symmetric Ln₃ complexes Ln₃ ? [BPh₄] and Ln₃ ? 0.33[Ln(NO₃)₆] (in which Ln^III=Gd and Dy) have been prepared from an amino‐bis(phenol) ligand. X‐ray studies reveal that Ln^III ions are connected by one μ₂‐phenoxo and two μ₃‐methoxo bridges, thus leading to a hexagonal bipyramidal Ln₃O₅ bridging core in which Ln^III ions exhibit a biaugmented trigonal‐prismatic geometry. Magnetic susceptibility studies and ab initio complete active space self‐consistent field (CASSCF) calculations indicate that the magnetic coupling between the Dy^III ions, which possess a high axial anisotropy in the ground state, is very weakly antiferromagnetic and mainly dipolar in nature. To reduce the electronic repulsion from the coordinating oxygen atom with the shortest Dy?O distance, the local magnetic moments are oriented almost perpendicular to the Dy₃ plane, thus leading to a paramagnetic ground state. CASSCF plus restricted active space state interaction (RASSI) calculations also show that the ground and first excited state of the Dy^III ions are separated by approximately 150 and 177 cm^?1, for Dy₃ ? [BPh₄] and Dy₃ ? 0.33[Dy(NO₃)₆], respectively. As expected for these large energy gaps, Dy₃ ? [BPh₄] and Dy₃ ? 0.33[Dy(NO₃)₆] exhibit, under zero direct‐current (dc) field, thermally activated slow relaxation of the magnetization, which overlap with a quantum tunneling relaxation process. Under an applied H_dc field of 1000 Oe, Dy₃ ? [BPh₄] exhibits two thermally activated processes with U_eff values of 34.7 and 19.5 cm^?1, whereas Dy₃ ? 0.33[Dy(NO₃)₆] shows only one activated process with U_eff=19.5 cm^?1.     

12‐ to 22‐Membered Bridged β‐Lactams as Potential Penicillin‐Binding Protein Inhibitors

As potential inhibitors of penicillin‐binding proteins (PBPs), we focused our research on the synthesis of non‐traditional 1,3‐bridged β‐lactams embedded into macrocycles. We synthesized 12‐ to 22‐membered bicyclic β‐lactams by the ring‐closing metathesis (RCM) of bis‐ω‐alkenyl‐3(S)‐aminoazetidinone precursors. The reactivity of 1,3‐bridged β‐lactams was estimated by the determination of the energy barrier of a concerted nucleophilic attack and lactam ring‐opening process by using ab initio calculations. The results predicted that 16‐membered cycles should be more reactive. Biochemical evaluations against R39 DD‐peptidase and two resistant PBPs, namely, PBP2a and PBP5, revealed the inhibition effect of compound 4d , which featured a 16‐membered bridge and the N‐tert‐butyloxycarbonyl chain at the C3 position of the β‐lactam ring. Surprisingly, the corresponding bicycle, 12d , with the PhOCH₂CO side chain at C3 was inactive. Reaction models of the R39 active site gave a new insight into the geometric requirements of the conformation of potential ligands and their steric hindrance; this could help in the design of new compounds.     

Ab initio and semiempirical computational studies on 1‐{(2E)‐2‐[(aminocarbonothioyl)hydrazono]‐2‐(3‐mesityl‐3‐methylcyclobutyl)ethyl}‐pyrrolidine‐2,5‐dione

The molecular geometry, vibrational frequencies, and gauge including atomic orbital (GIAO) ¹H‐ and ¹³C NMR chemical shift values of the title compound in the ground state have been calculated using the Hartree‐Fock (HF) and density functional theory (DFT) methods with 6‐31G(d) basis sets, and compared with the experimental data. The calculated results show that the optimized geometries can well reproduce the crystal structural parameters and the theoretical vibrational frequencies, and ¹H‐ and ¹³C NMR chemical shift values show good agreement with experimental data. To determine conformational flexibility, the molecular energy profile of the title compound was obtained by semiempirical (AM1) calculations with respect to the selected torsion angle, which was varied from ?180° to +180° in steps of 10°. The energetic behavior of the title compound in solvent media was examined using the B3LYP method with the 6‐31G(d) basis set by applying the Onsager and the polarizable continuum model (PCM). The results obtained with these methods reveal that the PCM method provided more stable structure than Qnsager's method. By using TD‐DFT method, electronic absorption spectra of the title compound have been predicted and a good agreement with the TD‐DFT method and the experimental one is determined. The predicted nonlinear optical properties of the title compound are much greater than ones of urea. In addition, the molecular electrostatic potential (MEP), frontier molecular orbitals (FMO) analysis, NBO analysis and thermodynamic properties of the title compound were investigated using theoretical calculations. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2012     

On the Reaction of Glycerol Dehydratase with But‐3‐ene‐1,2‐diol

Intriguing inactivation : Calculations suggest that the ability of relatively high‐energy radical intermediates to inactivate glycerol dehydratase (GDH) may reflect a general and hitherto unidentified inactivation mechanism in the reaction of coenzyme B₁₂‐dependent enzymes and 3‐unsaturated 1,2‐diols (see scheme; AdoCbl: adenosylcobalamin or coenzyme B₁₂).

     

A New Approach to the Design of Neutral 10‐C‐5 Trigonal‐Bipyramidal Carbon Compounds: A “π‐Electron Cap” Effect

DFT (B3LYP, M06‐2X) and MP2 methods are applied to the design of a wide series of the potentially 10‐C‐5 neutral compounds based on 6‐azabicyclotetradecanes: XC¹(YCH₂CH₂CH₂)₃N 1 – 3 , XC¹(YC₆H₄CH₂)₃N 4 – 6 , XC¹[Y(tBuC₆H₃)CH₂]₃N 7 – 9 and carbatranophanes 10 – 25 (X=Me, F, Cl; Y=O, NH, CH₂, SiH₂; Z=O, CH₂, (CH₂)₂, (CH₂)₃). Carbatranophanes 10 – 25 are characterized by a sterical compression of their axial 3c–4e XC¹←N fragment with respect to that in the parent molecules 4 – 6 . A magnitude of the revealed effect depends on a valence surrounding of the central carbon atom C¹, the size and the nature of the side chains (Z) that link the “π‐electron cap” with a tetradecane backbone. This circumstance allowed us to obtain 10‐C‐5 structures with the configuration of the bonds around the C¹ atom, which corresponds to practically an ideal trigonal bipyramid. In these compounds, the values of the covalence ratio χ of approximately 0.6 for the coordination C¹←N contacts with a covalent contribution (atoms in molecules (AIM) and natural bond orbital (NBO)) are record in magnitude. These values lie close to a low limit of the interval of the χ_Si←D change (0.6–0.9) being characteristic of the dative and ionic‐covalent (by nature) Si←D bond (D=N, O) in the known 10‐Si‐5 silicon compounds.     

The Stability of α‐Hydroperoxyalkyl Radicals

High‐level ab initio and Born–Oppenheimer molecular dynamic calculations have been carried out on a series of hydroperoxyalkyl (α‐QOOH) radicals with the aim of investigating the stability and unimolecular decomposition mechanism into QO+OH of these species. Dissociation was shown to take place through rotation of the C?O(OH) bond rather than through elongation of the CO?OH bond. Through the C?O(OH) rotation, the unpaired electron of the radical overlaps with the electron density on the O?OH bond, and from this overlap the C=O π bond forms and the O?OH bond breaks spontaneously. The CH₂OOH, CH(CH₃)OOH, CH(OH)OOH, and α‐hydroperoxycycloheptadienyl radical were found to decompose spontaneously, but the CH(CHO)OOH has a decomposition energy barrier of 5.95 kcal mol^?1 owing to its steric and electronic features. The systems studied in this work provide the first insights into how structural and electronic effects govern the stabilizing influence on elusive α‐QOOH radicals.     

Reaction of Paramagnetic Synthon,Lithiated 4,4,5,5‐Tetramethyl‐4,5‐dihydro‐1H‐imidazol‐1‐oxyl 3‐oxide,with Cyclic Aldonitrones of the Imidazole Series

It was shown that dipole‐stabilized paramagnetic carbanion lithiated 4,4,5,5‐tetramethyl‐4,5‐dihydro‐1H‐imidazol‐1‐oxyl 3‐oxide can be attached in a nucleophilic manner to either isolated or conjugated aldonitrones of the 2,5‐dihydroimidazole 3‐oxide and 2H‐imidazole 1‐oxide series to afford adducts the subsequent oxidation of which leads to polyfunctional mono‐ and diradicals. According to XRD, at least two polymorphic modifications can be formed during crystallization of the resulting paramagnetic compounds, and for each of them, geometric parameters of the molecules are similar. An EPR spectrum of the diradical in frozen toluene has a complicated lineshape, which can be fairly well reproduced by using X‐ray diffraction structural analysis and the following set of parameters: D=14.9 mT, E=1.7 mT; tensor a(¹⁴N)=[0.260 0.260 1.625] mT, two equivalent tensors for the nitronyl nitroxide moiety a(¹⁴N)=[0.198 0.198 0.700] mT, and g≈2.007. According to our DFT and ab initio calculations, the intramolecular exchange in the diradical is very weak and most likely ferromagnetic.     

Defective α‐Fe2O3(0001): An ab Initio Study

By using density functional theory calculations at the PBE+U level, we investigated the properties of hematite (0001) surfaces decorated with adatoms/vacancies/substituents. For the most stable surface termination over a large range of oxygen chemical potentials (${\mu _{\rm{O}} }$ ), the vacancy formation and adsorption energies were determined as a function of ${\mu _{\rm{O}} }$ . Under oxygen‐rich conditions, all defects are metastable with respect to the ideal surface. Under oxygen‐poor conditions, O vacancies and Fe adatoms become stable. Under ambient conditions, all defects are metastable; in the bulk, O vacancies form more easily than Fe vacancies, whereas at the surface the opposite is true. All defects, that is, O and Fe vacancies, Fe and Al adatoms, and Al substituents, induce important modifications to the geometry of the surface in their vicinity. Dissociative adsorption of molecular oxygen is likely to be exothermic on surfaces with Fe/Al adatoms or O vacancies.     

Sensitizing DNA Towards Low‐Energy Electrons with 2‐Fluoroadenine

2‐Fluoroadenine (^2FA) is a therapeutic agent, which is suggested for application in cancer radiotherapy. The molecular mechanism of DNA radiation damage can be ascribed to a significant extent to the action of low‐energy (<20 eV) electrons (LEEs), which damage DNA by dissociative electron attachment. LEE induced reactions in ^2FA are characterized both isolated in the gas phase and in the condensed phase when it is incorporated into DNA. Information about negative ion resonances and anion‐mediated fragmentation reactions is combined with an absolute quantification of DNA strand breaks in ^2FA‐containing oligonucleotides upon irradiation with LEEs. The incorporation of ^2FA into DNA results in an enhanced strand breakage. The strand‐break cross sections are clearly energy dependent, whereas the strand‐break enhancements by ^2FA at 5.5, 10, and 15 eV are very similar. Thus, ^2FA can be considered an effective radiosensitizer operative at a wide range of electron energies.     

Dissociative Electron Attachment to β‐Alanine

A detailed study on dissociative electron attachment (DEA) to β‐alanine (βA) in the gas phase is presented. Ion yields as a function of the incident electron energy from about 0 to 15 eV have been measured for most of the fragments. As for all α‐amino acids, the main reaction corresponds to the loss of a hydrogen atom, although many other fragments have been observed that involved more complex bond cleavages. Threshold energies have been calculated by using the G4(MP2) method for various decomposition reactions. Fragmentation pathways were also investigated to measure metastable decays of the intermediate fragment anion (βA?H)^? by using the mass‐analyzed ion kinetic energy (MIKE) scan technique. Comparisons with α‐alanine and other amino acids are made when relevant.     

Transport Processes at α‐Quartz–Water Interfaces: Insights from First‐Principles Molecular Dynamics Simulations

Car–Parrinello molecular dynamics (CP–MD) simulations are performed at high temperature and pressure to investigate chemical interactions and transport processes at the α‐quartz–water interface. The model system initially consists of a periodically repeated quartz slab with O‐terminated and Si‐terminated (1000) surfaces sandwiching a film of liquid water. At a temperature of 1000 K and a pressure of 0.3 GPa, dissociation of H₂O molecules into H⁺ and OH^? is observed at the Si‐terminated surface. The OH^? fragments immediately bind chemically to the Si‐terminated surface while Grotthus‐type proton diffusion through the water film leads to protonation of the O‐terminated surface. Eventually, both surfaces are fully hydroxylated and no further chemical reactions are observed. Due to the confinement between the two hydroxylated quartz surfaces, water diffusion is reduced by about one third in comparison to bulk water. Diffusion properties of dissolved SiO₂ present as Si(OH)₄ in the water film are also studied. We do not observe strong interactions between the hydroxylated quartz surfaces and the Si(OH)₄ molecule as would have been indicated by a substantial lowering of the Si(OH)₄ diffusion coefficient along the surface. No spontaneous dissolution of quartz is observed. To study the mechanism of dissolution, constrained CP–MD simulations are done. The associated free energy profile is calculated by thermodynamic integration along the reaction coordinate. Dissolution is a stepwise process in which two Si? O bonds are successively broken. Each bond breaking between a silicon atom at the surface and an oxygen atom belonging to the quartz lattice is accompanied by the formation of a new Si? O bond between the silicon atom and a water molecule. The latter loses a proton in the process which eventually leads to protonation of the oxygen atom in the cleaved quartz Si? O bond. The final solute species is Si(OH)₄.     

The Low‐Lying Excited States of 2,2′‐Bithiophene: A Theoretical Analysis

The low‐energy regions of the singlet→singlet, singlet→triplet, and triplet→triplet electronic spectra of 2,2′‐bithiophene are studied using multiconfigurational second‐order perturbation theory (CASPT2) and extended atomic natural orbitals (ANO) basis sets. The computed vertical, adiabatic, and emission transition energies are in agreement with the available experimental data. The two lowest singlet excited states, 1¹B_u and 2¹B_u, are computed to be degenerate, a novel feature of the system to be borne in mind during the rationalization of its photophysics. As regards the observed high triplet quantum yield of the molecule, it is concluded that the triplet states 2³A_g and 2³B_u, separated about 0.4 eV from the two lowest singlet excited states, can be populated by intersystem crossing from nonplanar singlet states.     

Interaction Modes and Absolute Affinities of α‐Amino Acids for Mn2+: A Comprehensive Picture

Manganese is involved as a cofactor in the activation of numerous enzymes as well as the oxygen‐evolving complex of photosystem II. Full understanding of the role played by the Mn²⁺ ion requires detailed knowledge of the interaction modes and energies of manganese with its various environments, a knowledge that is far from complete. To bring detailed insight into the local interactions of Mn in metallopeptides and proteins, theoretical studies employing first‐principles quantum mechanical calculations are carried out on [Mn‐amino acid]²⁺ complexes involving all 20 natural α‐amino acids (AAs). Detailed investigation of [Mn‐serine]²⁺_, [Mn‐cysteine]²⁺, [Mn‐phenylalanine]²⁺, [Mn‐tyrosine]²⁺, and [Mn‐tryptophan]²⁺ indicates that with an electron‐rich side chain, the most stable species involves interaction of Mn²⁺ with carbonyl oxygen, amino nitrogen, and an electron‐rich section of the side chain of the AA in its canonical form. This is in sharp contrast with aliphatic side chains for which a salt bridge is formed. For aromatic AAs, complexation to manganese leads to partial oxidation as well as aromaticity reduction. Despite multisite binding, AAs do not generate strong enough ligand fields to switch the metal to a low‐ or even intermediate‐spin ground state. The affinities of Mn²⁺ for all AAs are reported at the B3LYP and CCSD(T) levels of theory, thereby providing the first complete series of affinities for a divalent metal ion. The trends are compared with those of other cations for which affinities of all AAs have been previously obtained.     

Chiral Fluorinated α‐Sulfonyl Carbanions: Enantioselective Synthesis and Electrophilic Capture,Racemization Dynamics,and Structure

Enantiomerically pure triflones R¹CH(R²)SO₂CF₃ have been synthesized starting from the corresponding chiral alcohols via thiols and trifluoromethylsulfanes. Key steps of the syntheses of the sulfanes are the photochemical trifluoromethylation of the thiols with CF₃Hal (Hal=halide) or substitution of alkoxyphosphinediamines with CF₃SSCF₃. The deprotonation of RCH(Me)SO₂CF₃ (R=CH₂Ph, iHex) with nBuLi with the formation of salts [RC(Me)? SO₂CF₃]Li and their electrophilic capture both occurred with high enantioselectivities. Displacement of the SO₂CF₃ group of (S)‐MeOCH₂C(Me)(CH₂Ph)SO₂CF₃ (95 % ee) by an ethyl group through the reaction with AlEt₃ gave alkane MeOCH₂C(Me)(CH₂Ph)Et of 96 % ee. Racemization of salts [R¹C(R²)SO₂CF₃]Li follows first‐order kinetics and is mainly an enthalpic process with small negative activation entropy as revealed by polarimetry and dynamic NMR (DNMR) spectroscopy. This is in accordance with a C_α? S bond rotation as the rate‐determining step. Lithium α‐(S)‐trifluoromethyl‐ and α‐(S)‐nonafluorobutylsulfonyl carbanion salts have a much higher racemization barrier than the corresponding α‐(S)‐tert‐butylsulfonyl carbanion salts. Whereas [PhCH₂C(Me)SO₂tBu]Li/DMPU (DMPU = dimethylpropylurea) has a half‐life of racemization at ?105 °C of 2.4 h, that of [PhCH₂C(Me)SO₂CF₃]Li at ?78 °C is 30 d. DNMR spectroscopy of amides (PhCH₂)₂NSO₂CF₃ and (PhCH₂)N(Ph)SO₂CF₃ gave N? S rotational barriers that seem to be distinctly higher than those of nonfluorinated sulfonamides. NMR spectroscopy of [PhCH₂C(Ph)SO₂R]M (M=Li, K, NBu₄; R=CF₃, tBu) shows for both salts a confinement of the negative charge mainly to the C_α atom and a significant benzylic stabilization that is weaker in the trifluoromethylsulfonyl carbanion. According to crystal structure analyses, the carbanions of salts {[PhCH₂C(Ph)SO₂CF₃]Li? L }₂ ( L =2 THF, tetramethylethylenediamine (TMEDA)) and [PhCH₂C(Ph)SO₂CF₃]NBu₄ have the typical chiral C_α? S conformation of α‐sulfonyl carbanions, planar C_α atoms, and short C_α? S bonds. Ab initio calculations of [MeC(Ph)SO₂tBu]^? and [MeC(Ph)SO₂CF₃]^? showed for the fluorinated carbanion stronger n_C→σ* and n_O→σ* interactions and a weaker benzylic stabilization. According to natural bond orbital (NBO) calculations of [R¹C(R²)SO₂R]^? (R=tBu, CF₃) the n_C→σ*_{S? R} interaction is much stronger for R=CF₃. Ab initio calculations gave for [MeC(Ph)SO₂tBu]Li ? 2 Me₂O an O,Li,C_α contact ion pair (CIP) and for [MeC(Ph)SO₂CF₃]Li ? 2 Me₂O an O,Li,O CIP. According to cryoscopy, [PhCH₂C(Ph)SO₂CF₃]Li, [iHexC(Me)SO₂CF₃]Li, and [PhCH₂C(Ph)SO₂CF₃]NBu₄ predominantly form monomers in tetrahydrofuran (THF) at ?108 °C. The NMR spectroscopic data of salts [R¹(R²)SO₂R³]Li (R³=tBu, CF₃) indicate that the dominating monomeric CIPs are devoid of C_α? Li bonds.     

Retro‐Diels–Alder Approach to the Synthesis of π‐Expanded Azuliporphyrins and Their Porphyrinoid Aromaticity

Bicyclo[2.2.2]octadiene (BCOD) fused azuliporphyrins were synthesized by 3+1 porphyrin synthesis of azulitripyrranes with diformylpyrroles. Subsequent retro‐Diels–Alder reaction of the BCOD‐fused azuliporphyrins afforded azulibenzo‐, azulidibenzo‐, and azulitribenzoporphyrins 1 – 5 . NMR and UV/Vis spectra, as well as nucleus‐independent chemical shift (NICS) calculations revealed that 1 – 5 and their diprotonated dications exhibit relatively low porphyrinoid aromaticity, which was dependent on the position and number of fused benzene rings present.