Electronic structure of carbon trioxide and vibronic interactions involving Jahn-Teller states

The electronic structure of CO3 is characterized by equation-of-motion and coupled-cluster methods. C(2v) and D(3h) isomers are considered. Vertical excitation energies, transition dipoles, and the molecular orbital character of the excited states are presented for singlet and triplet manifolds. Ground-state equilibrium structures and frequencies are strongly affected by vibronic interactions with low-lying excited states. At D(3h) geometries, the vibronic interactions are enhanced by the Jahn-Teller character of the excited states. The curvature of the potential energy surface and the existence of the D(3h) minimum are very sensitive to the correlation treatment and the basis set. The correlation effects are stronger at D(3h), in agreement with a smaller HOMO-LUMO gap.     

Generation and stereoselective transformations of 3-phenylcyclopropene

A convenient and inexpensive approach to the generation of 3-phenylcyclopropenes is described. Reaction of these compounds with a range of dienophiles and dipolarophiles led to the stereoselective formation of [4+2]- and [3+2]-cycloadducts, which were exclusively exo-3-phenyl-cis-1,2-disubstituted cyclopropanes. Efficient trapping of 1-lithio-3-phenylcyclopropene with different electrophiles is also discussed. Ab initio calculations suggest that the lowest energy conformation of 3-phenylcyclopropene has the plane of the benzene ring perpendicular to the cyclopropene π-bond but with a low rotation barrier.     

Microscale separation methods for enzyme kinetics assays

Miniaturization continues to be one of the leading trends in analytical chemistry and one that brings advantages that can be particularly beneficial in biochemical research. Use of a miniaturized scale enables efficient analysis in a short time and requires very small amounts of samples, solvents, and reagents. This can result in a remarkable decrease in costs of enzyme kinetics studies, especially when expensive or rare enzymes and/or substrates are involved. Free zone electrophoresis is without a doubt the most common microscale separation technique for capillary and on-chip enzyme assays. Progress and applications in this field are reviewed frequently whereas other modes of separation, although successfully applied, receive only marginal interest in such publications. This review summarizes applications of less common modes of separation in capillary or chip formats, namely micellar electrokinetic chromatography, liquid chromatography, gel electrophoresis, isoelectric focusing, and isotachophoresis. Because these techniques are based on separation mechanisms different from those of free zone electrophoresis, they can be, and have been, successfully used in cases where zone electrophoresis fails. Advantages and drawbacks of these alternative separation techniques are discussed, as also are the difficulties encountered most often and solutions proposed by different research groups.     

The preparation,crystal structure and electrochemistry of (5Z,5′Z)-2,2′-(alkane-α,ω-diylsulfanyldiyl)bis(5-(3-pyridylmethylene)-3,5-dihydro-4H-imidazol-4-ones) and their complexes with cobalt(II) chloride

A series of S-alkylated derivatives of 2-thiohydantoins, containing two 5-(3-pyridylmethylene)-3,5-dihydro-4H-imiazole-4-one fragments joined by polymethylene bridges with different numbers of carbon atoms, was synthesized. The title (5Z,5′Z)-2,2′-(alkane-α,ω-diylsulfanyldiyl)bis(5-(3-pyridylmethylene)-3,5-dihydro-4H-imidazol-4-ones) (L) were obtained by alkylation of 5-(3-pyridylmethylene)-3-phenyl-2-thiohydantoine by 1,2-dibromoethane, 1,6-dibromohexane or 1,10-dibromodecane in DMF in the presence of potassium carbonate. The complexes of ligands L with CoCl₂ · 6H₂O have been synthesized. It is shown that, regardless of the L:CoCl₂ ratio, complexes with LCoCl₂ composition are obtained in all cases. The structure of the cobalt(II) chloride complex with (5Z,5′Z)-2,2′-(ethane-1,2-diyldisulfanyldiyl)bis(5-(3-pyridylmethylene)-3-phenyl-3,5-dihydro-4H-imidazol-4-one) was determined by means of the RSA method. The cobalt atom in this complex has a tetrahedral ligand environment and it is coordinated by two chloride anions and two nitrogen atoms of pyridine fragments, forming a 19-membered metallocycle. Electrochemical investigations of the synthesized ligands and complexes have been made by CVA and RDE methods. It is established that the first stage of reduction for the complexes takes place on the metal, whereas the first stage of oxidation takes place on the coordinated chloride-anions.     

Structural diversity in native cyclodextrins/folic acid complexes--from [2]-rotaxane to exclusion compound

The formation of different complexes of folic acid depending on the size of the host cyclodextrin resulting in either an exclusion compound (with the smallest α-cyclodextrin) or 2-rotaxane, where cyclodextrin is threaded over folic acid (with β- and γ-cyclodextrins), is presented. The formation is carried out in water which allows both possible application in pharmaceutical sciences and usage of environmentally friendly "green chemistry". The obtained compounds are thoroughly characterized using one and two dimensional NMR, mass spectrometry, differential scanning calorimetry and thermogravimetric analysis.     

The role of C-H and C-C stretching modes in the intrinsic non-radiative decay of triplet states in a Pt-containing conjugated phenylene ethynylene

The intrinsic non-radiative decay (internal conversion) from the triplet excited state in phosphorescent dyes can be described by a multi-phonon emission process. Since non-radiative decay of triplet excitons can be a significant process in organic light-emitting diodes, a detailed understanding of this decay mechanism is important if the overall device efficiency is to be controlled. We compare a deuterated Pt(II)-containing phenylene ethynylene with its non-deuterated counterpart in order to investigate which phonon modes control to the non-radiative decay path. We observe that deuteration does not decrease the non-radiative decay rate. A Franck-Condon analysis of the phosphorescence spectra shows that the electronic excitation is coupled strongly to the breathing mode of the phenyl ring and the C≡C carbon stretching modes, while high-energy C-H or C-D stretching modes play an insignificant role. We, therefore, associate the internal conversion process with the carbon-carbon stretching vibrations.     

Electronic Structure and Redox Properties of Metal Nitride Endohedral Fullerenes M3N@C2n (M=Sc,Y, La,and Gd; 2n=80, 84, 88, 92, 96)

An extensive study of the redox properties of metal nitride endohedral fullerenes (MNEFs) based on DFT computational calculations has been performed. The electronic structure of the singly oxidized and reduced MNEFs has been thoroughly analyzed and the first anodic and cathodic potentials, as well as the electrochemical gaps, have been predicted for a large number of M₃N@C_2n systems (M=Sc, Y, La, and Gd; 2n=80, 84, 88, 92, and 96). In particular, calculations that include thermal and entropic effects correctly predict the different anodic behavior of the two isomers (I_h and D_5h) of Sc₃N@C₈₀, which is the basis for their electrochemical separation. Important differences were found in the electronic structure of reduced M₃N@C₈₀ when M=Sc or when M is a more electropositive metal, such as Y or Gd. Moreover, the changes in the electrochemical gaps within the Gd₃N@C_2n series (2n=80, 84, and 88) have been rationalized and the use of Y‐based computational models to study the Gd‐based systems has been justified. The redox properties of the largest MNEFs characterized so far, La₃N@C_2n (2n=92 and 96), were also correctly predicted. Finally, the quality of these predictions and their usefulness in distinguishing the carbon cages for MNEFs with unknown structures is discussed.     

Accessing the structural diversity of pyridone alkaloids: concise total synthesis of rac-citridone A

A unique route to the structural diversity of pyridone alkaloids is described based on the concept of a common synthetic strategy. Three different core structure analogues corresponding to akanthomycin, septoriamycin A, and citridone A have been prepared by using a highly selective and novel carbocyclization reaction.     

A Process to Produce Penicillin G Acylase by Surface-Adhesion Fermentation Using <Emphasis Type="Italic">Mucor griseocyanus</Emphasis> to Obtain 6-Aminopenicillanic Acid by Penicillin G Hydrolysis

The production of extracellular and mycelia-associated penicillin G acylase (maPGA) with Mucor griseocyanus H/55.1.1 by surface-adhesion fermentation using Opuntia imbricata, a cactus, as a natural immobilization support was studied. Enzyme activity to form 6-aminopencillanic acid (6-APA) from penicillin G was assayed spectrophotometrically. The penicillin G hydrolysis to 6-APA was evaluated at six different times using PGA samples recovered from the skim milk medium at five different incubation times. Additionally, the effect of varying the penicillin G substrate concentration level on the PGA enzyme activity was also studied. The maximum reaction rate, V _max, and the Michaelis constant, K _M, were determined using the Michaelis–Menten model. The maximum levels for maPGA and extracellular activity were found to be 2,126.50 international unit per liter (IU/l; equal to 997.83 IU/g of support) at 48 h and 755.33 IU/l at 60 h, respectively. Kinetics of biomass production for total biomass showed a maximum growth at 60 h of 3.36 and 2.55 g/l (equal to 0.012 g of biomass per gram of support) for the immobilized M. griseocyanus biomass. The maPGA was employed for the hydrolysis of penicillin G to obtain 6-APA in a batch reactor. The highest quantity of 6-APA obtained was 226.16 mg/l after 40-min reaction. The effect of substrate concentration on maPGA activity was evaluated at different concentrations of penicillin G (0–10 mM). K _M and V _max were determined to be 3.0 × 10⁻³ M and 4.4 × 10⁻³ mM/min, respectively.     

The dielectric response of interfacial water—from the ordered structures to the single hydrated shell

Water is the universal solvent in nature. Does this imply, however, that its interaction with its environment is also a universal feature? While this question maybe too fundamental to be answered by one method only, we present evidence that the broadening of the dielectric spectra of water presents universal features of dipolar interactions with different types of matrixes. If in aqueous solutions the starting point of water’s state can be considered as bulk, with only partial interactions with the solute, then the state of water adsorbed in heterogeneous materials is determined by various hydration centers of the inhomogeneous material (the matrix) and it is significantly different from the bulk. In both cases, the dielectric spectrum of water is symmetrical and can be described by the Cole–Cole (CC) function. The phenomenological model that describes a physical mechanism of the dipole–matrix interaction in complex systems underlying the CC behavior has been applied to water adsorbed in porous glasses. It was then extended to analyses of the dynamic and structural behavior of water in nonionic and ionic aqueous solutions. The same model is then used to analyze the CC relaxation processes observed in clays, aqueous solutions of nucleotides, and amino acids.