A theoretical study on NH bond dissociation enthalpies of oxo, thio and seleno carbamates and their N-protonated and N-deprotonated species

The effect of N-protonation and N-deprotonation on structure, NH bond dissociation enthalpies (BDEs) and stabilities of radicals formed on H-abstraction from nitrogen atom of carbamates and their thio- and seleno-analogs have been investigated. For those molecules where experimental results are available for comparison, the ROB3LYP/6-311++G(d,p)//B3LYP/6-31+G* theoretical level is in agreement within the estimated experimental uncertainty. The NH BDE of carbamates H₂NC(=X)YCH₃ [X = O; Y = O, S, Se] are higher but lower when X = S, Se and Y = O, S, Se in comparison to NH BDE of NH₃. DFT calculations indicate that the NH bond dissociation enthalpies are decreased by protonation and deprotonation at nitrogen atom; but the effect of deprotonation is rather smaller than the protonation. The variations are analyzed in terms of stabilities of molecules, their protonated and deprotonated species along with their respective radicals. The electron delocalization from nitrogen, X and Y atoms, electrostatic interactions, conjugative interactions and spin delocalization are the important factors affecting the stability. The spin delocalization and shift of radical center to chalcogen X (X = S, Se) are the main determinants for radical stability.     

Homogeneous and supported copper(II) acetate as catalyst for CO coupling reactions

The reactivity of copper(II) acetate as catalyst in a standard CO coupling reaction has been systematically evaluated. Optimization of the reaction conditions resulted in a protocol involving stoichiometric amounts of reagents, a substoichiometric amount of base and 20 mol% catalyst, at 50 °C in 1,2-dichloroethane and under 1 atm O₂. Next, the reactivity of polymer-supported copper(II) acetate was evaluated. Although it is found that, in contrast to previous results obtained in related CN coupling reactions, the polymer-supported catalyst is in this case less efficient than the corresponding homogeneous one, the catalyst turns out to be conveniently recovered from the reaction mixture by simple filtration.     

Blue-shifting intramolecular CH O(S) contacts in sterically strained systems

A group of model systems which may form chelate-type structures with intramolecular CH  Y (Y = O, S) contact is investigated computationally. The existence of several conformers permits to identify a reference molecule without the CH  Y intramolecular contact and to establish the blue-shifting character of this interaction. The CH stretching frequency in chelate forms is found to increase with respect to its value in the reference system. A parallel decrease of the CH bond distance is also established. The blue-shifting character of the intramolecular CH  Y contact is interpreted in terms of the sterically enforced repulsion between the hydrogen atom in CH and the electron donor Y. This interpretation is supported by the negative (repulsive) estimates of the energy contribution due to CH  Y contacts.     

Evaluation of the origin of rotational barrier in NH2X (X = NO, NS)

The origin of rotational barrier in N-thionitrosamine NH₂NS (TNA) has been examined with the aid of topological theory of atoms in molecules (AIM) and natural bond orbital (NBO) analyses and has been compared with N-nitrosamine NH₂NO (NA). Frequency calculations show that the rotational barrier for TNA is greater than NA. This can be attributed to the more charge transfer from nitrogen of amino group to sulfur in TNA than from nitrogen to oxygen in NA. NBO analysis reveals pyramidalization of nitrogen atom of NH₂ group leads to decrease of delocalization energy contribution and increase of Lewis energy contribution on total energy.     

The structure of p-chlorophenol and barrier to internal OH rotation in the S1-state

The structure and barrier to internal rotation of 4-chlorophenol in the ground state and the electronically excited S₁-state has been examined by rotationally resolved laser induced fluorescence spectroscopy of 4-³⁵Cl-phenol, 4-³⁷Cl-phenol, 4-³⁵Cl-phenol-d₁, and 4-³⁷Cl-phenol-d₁. The overlapping spectra have been assigned simultaneously using a genetic algorithm approach. The rotationally resolved spectrum of the electronic origin of 4-chlorophenol is comprised of two subbands, which are split by 60 MHz due to the internal rotation of the hydroxy group. The torsional barrier in the electronically excited state could be estimated to be 1400 cm⁻¹, only about 250 cm⁻¹ higher than in the ground state. The CCl bond lengths decreases by approximately 6 pm upon electronic excitation and the aromatic ring is distorted quinoidally.     

CN bond rotation and E–Z isomerism in some N-benzyl-N-methylcarbamoyl chlorides: A DFT study

The electronic and geometrical structures of thiosulfinic acids, RSSOH, 1, (1a, R = H; 1b, R = CH₃; 1c, R = t-C₄H₉; 1d, R = C₆H₅; 1e, R = F) and their anions have been investigated by the ab initio and density functional methods. The calculations show that the stability of the thiolo-tautomers of 1, RS(O)SH, and the thiono-tautomers, RS(S)OH, is almost the same in the gas phase, the energy of the thiolo-tautomers being slightly lower. The tautomers of 1 having the thiosulfone structure, RS(O)(S)H, were found to be the least stable. The thiosulfinate anions show ambident character with the negative charge dispersed over terminal O and S atoms and are stabilized by electronegative substituents.     

Ion transfer across liquidliquid interfaces from transition-state theory and stochastic molecular dynamics simulations

We investigate the kinetics and dynamics of ion transfer across liquidliquid interfaces. We calculate the potential of mean force (pmf) of ion transfer from Monte Carlo simulations of a lattice–gas model, assuming independent chemical and electrostatic contributions. The shape of the pmf justifies considering the transfer as activated. The kinetics are obtained from transition-state theory and independently from stochastic molecular dynamics simulations. Both methods yield consistent results, with straight Tafel plots and friction effects in line with Kramers’ theory, but stronger than for a diffusing particle. A higher friction makes barrier recrossing more likely.     

Theoretical investigation of C–HM interactions in organometallic complexes: A natural bond orbital (NBO) study

The nature of C–HM agostic interactions in model metal complexes [M²⁺(CH₂CH₃)(PH₃)_nCl] (where M = Sc, Ti, V, Ti, Cr, Mn, Fe, Co, Ni, Cu, Zn; n = 1, 2, 3, 4) was studied with the natural bond orbital analysis (NBO) approach using density functional theory (DFT) optimized geometries at the B3LYP/6-31G(d,p) level of theory. The effect of nature of metal, coordination number, oxidation state and ligand field effects on the agostic interaction is examined. A set of 20 crystal structures of organometallic complexes taken from the Cambridge Structural Database (CSD) was studied computationally employing AIM theory and NBO analysis, and the applicability of these methods was critically accessed in demarcating the two types of interaction.     

-mononitroalkyl derivatives of indium

Dealkylation of trialkylindium by -mononitroalkane leads to the formation of the previously unknown -nitroalkyl derivatives of indium. An investigation of the chemical properties and of the ¹¹⁵In nuclear quadrupole resonance spectra of these compounds has been undertaken. An analysis of the data obtained from these investigations suggests the existence of intramolecular O→In coordination in the -nitroalkyl derivatives of indium.     

,-Difluoroketone inhibitors of HMG CoA reductase

The rationale for a new class of HMG CoA reductase inhibitors, represented by ,ga-difluoroketones 3 and 4, is described. The syntheses of 3, 4, and their nonfluorinated analogue 5 are presented.     

Boron trifluoride-catalyzed degradation of poly--caprolactone at ambient temperature

Poly--caprolactone (PCL) can be accelerated to degrade in the presence of boron trifluoride at ambient temperature. The degradation behaviors were studied by using the inherent viscosity measurement, gel permeation chromatography (GPC), infrared analysis (FTIR), nuclear magnetic resonance analysis (NMR), and thermal analysis (DSC). With increasing the addition amount of boron trifluoride, the molecular weight of PCL decreases; the molecular weight distribution is broadened; and the degree of crystallinity of PCL increases at first at low BF₃ level, then decreases when BF₃ content exceeds to 2.64 wt%. The results of IR, ¹HNMR and GPC reveal that -caprolactone monomer does not occur and the main degradation products are the oligomers of PCL with low molecular weight. The mechanism for boron trifluoride-catalyzed degradation of PCL is discussed.     

-Fluoro decalones as chiral epoxidation catalysts: fluorine effect

Three rigid monofluorinated trans-decalones 4a, 5e, and 6e (90% ee) have been synthesized from commercially available (−)-(R)-methyl naphthalenone (90% ee). Their structures have been fully characterized (NMR, X-ray): ketones 4a and 5e are Me,F-disubstituted to the carbonyl with the fluorine axial and equatorial, respectively, while ketone 6e is F-monosubstituted to the carbonyl with the fluorine equatorial. The use of these ketones as chiral catalysts for the epoxidation of trans-olefins (such as stilbene, β-methylstyrene and p-methoxy cinnamate) through the formation of dioxiranes shows (i) that dioxiranes with an equatorial fluorine to the dioxirane ring are less reactive and provide lower ee’s than dioxiranes with an axial fluorine and having the same chirality and (ii) that an axial methyl to the dioxirane ring is significantly less efficient than a fluorine. The results corroborate Armstrong and Houk’s theoretical model and our first hypothesis to rationalize the inverted enantioselectivities observed using -fluorinated cyclohexanones having the same chirality, i.e.: rapid ring inversion (Curtin–Hammett principle) allows the dioxirane conformation to have the fluorine axial (even if less populated than the other) to contribute significantly to the epoxidation reaction.     

-Halogenoketones—XIII Chromone epoxides; their synthesis and acid-catalysed rearrangement

The chromone epoxide ring system has been synthesised. Base-catalysed cyclization and dehydrobromination of -bromo-o-acyl (aroyl) oxyacetopheones or 2-bromo-1, 3-diones yielded 3-substituted chromone epoxides. Acid-catalysed rearrangement of a 2-methylchromone epoxide in an aprotic solvent gave a 2-methylenech-romanonol, while a 3-methoxymethylchromone epoxide fragmented to chromonol under these conditions; in alchols, chromone epoxides gave 2-alkoxychromanonols.     

Electronic structure and luminescence of [AuS2PPh(OCH2CHCH2)]2 complex

The complex [AuS₂PPh(OCH₂CHCH₂)]₂ (1) presents an Au(I)–Au(I) intramolecular and intermolecular bonding with luminescence properties. To understand the nature of these features, fully optimized geometries were obtained by three computational methods, DFT/B3LYP, MPW1B95 and MP2. An Au(I)–Au(I) intramolecular bond was found in the ground state, at the three levels of theory, exhibiting an aurophilic interaction between the two gold atoms. Two molecules of the complex were optimized using DFT/B3LYP, in order to analyze the intermolecular interaction between them. The resulting intermolecular bonding distance between the two adjacent gold atoms on each molecule is 3.16 Å, indicating a strong aurophilic attraction. Time dependent calculations indicate that the first excited state with nonzero oscillator strength is a singlet, with an excitation energy equal to 3.16 eV. This should correspond to the absorption band seen experimentally at 3.10 eV. The lowest energy emission of (1) was obtained at 2.73 eV, which corresponds to the emission peak resulting from phosphorescence and located at 2.53 eV. This transition comes from an excited electron on the p orbitals of the ligands that is transferred to the d orbitals of the gold atoms on the HOMO. This interaction may be attributed to Ligand to Ligand–Metal Charge Transfer (LL–MCT).     

Distribution of Sorbitan Monooleate in poly(l-lactide-co--caprolactone) nanofibers from emulsion electrospinning

The aim of this study was to investigate the distribution of Sorbitan Monooleate (Span80) in poly(l-lactide-co--caprolactone) (PLLACL) nanofibers from emulsion electrospinning. The hypothesis was that PLLACL/Span80 nanofibrous mats would have some Span80 on the surface of the composite nanofibers. To test the hypothesis, the electrospinning of emulsions made of PLLACL, chloroform, Span80, and distilled water to prepare PLLACL/Span80 nanofibers was systematically investigated. The morphology of PLLACL/Span80 nanofibers was investigated by atomic force microscopy. The surface hydrophilicity of the nanofibrous mats were examined by water contact angle test. The distribution of Span80 on the surface of nanofibrous mats was also confirmed by the performance of pig iliac endothelium cells on the nanofibrous mats.     

Bonding trends in MCH2 systems: Simple orbital interpretation and evidence for double bonds

MCH₂ systems, where M is a metal from 4th up to 7th period, are studied at DFT level using B3LYP functional and small-core quasirelativistic pseudopotential or fully relativistic four-component methodology. We obtained structural data for 44 elements, M, and our results can be used to infer double-bond lengths for these elements. Our results also suggest that the bonding of these MCH₂ systems can be understood by a simple pictorial approach, even when spin–orbit effects are present.     

Patterning of biomolecules on a poly(-caprolactone) film surface functionalized by ion implantation

Biomolecule patterning is important due to its potential applications in biodevices, tissue engineering, and drug delivery. In this study, we developed a new method for a biomolecular patterning on poly(-caprolactone) (PCL) films based on ion implantation. Ion implantation on a PCL film surface resulted in the formation of carboxylic acid groups. The generated carboxylic acid groups were used for the covalent immobilization of amine-functionalized p-DNA, followed by hybridization with fluorescently tagged c-DNA. Biotin-amine was also covalently immobilized on the carboxylic acid generated PCL surfaces. Successful biotin-specific binding of streptavidin further confirmed the potential of this strategy for patterning of various biomolecules.     

Theoretical study of the influence of para- and meta-substituents on X-pyridineHF hydrogen bonding

The effects of O⁻, N (CH₃)₂, NH (CH₃), NH₂, C₂H₅, CH₃, OH, F, Cl, OF, Br, NO₂ and substituents in para- and meta-positions on X-pyridineHF hydrogen bond has been studied by HF, B3LYP and MP2 methods using 6-311++G(d,p) basis set. The relationship between hydrogen bond formation energy ΔE and electron donating (or withdrawing) of substituents has been investigated. In this respect, population analysis has been performed by atoms in molecules (AIM) and natural bond orbital (NBO) theories. The results of AIM and NBO analyses are in good agreement with calculated energy values. The relationship between Hammett coefficient and complexation energy has been established and the ρ constant has been calculated for hydrogen bonding. There is a relationship between σ and ΔE with a correlation coefficient equal to 0.94.     

A two-plasmid system for the glycosylation of polyketide antibiotics: bioconversion of -rhodomycinone to rhodomycin D

Background: The biological activity of many microbial products requires the presence of one or more deoxysugar molecules attached to agylcone. This is especially prevalent among polyketides and is an important reason that the antitumor anthracycline antibiotics are avid DNA-binding drugs. The ability to make different deoxyaminosugars and attach them to the same or different aglycones in vivo would facilitate the synthesis of new anthracyclines and the quest for antitumor drugs. This is feasible using the numerous bacterial genes for deoxysugar biosynthesis that are now available. Results: Production of thymidine diphospho (TDP)- -daunosamine (dnm), the aminodeoxysugar present in the anthracycline antitumor drugs daunorubicin (DNR) and doxorubicin (DXR), and its attachment to -rhodomycinone to generate rhodomycin D has been achieved by bioconversion with a strain of Streptomyces lividans that bears two plasmids. One contained the Streptomyces peucetius dnmJVUZTQS genes plus dnmW (previously named dpsH and considered to be a polyketide cyclase gene), dnrH, which is not required for the formation of rhodomycin D, and dnrl, a regulatory gene required for expression of the dnm and drr genes. The other plasmid had genes encoding glucose-1-phosphate thymidylyltransferase and TDP-glucose-4,6-dehydratase (dnmL and dnmM, respectively, or mtmDE, their homologs from Streptomyces agrillaceus) plus the drrAB DNR/DXR resistance genes. Conclusions: The high-yielding glycosylation of the aromatic polyketide -rhodomycinone using plasmid-borne deoxysugar biosynthesis genes proves that the minimal information for -daunosamine biosynthesis and attachment in the heterologous host is encoded by the dnmLMJVUTS genes. This is a general approach to making both known and new glycosides of anthracyclines, several of which have medically important antitumor activity.