Auxiliary basis sets for density fitting second-order Møller-Plesset perturbation theory: correlation consistent basis sets for the 5d elements Hf-Pt

Auxiliary basis sets specifically matched to the correlation consistent cc-pVnZ-PP, cc-pwCVnZ-PP, aug-cc-pVnZ-PP, and aug-cc-pwCVnZ-PP orbital basis sets (used in conjunction with pseudopotentials) for the 5d transition metal elements Hf-Pt have been optimized for use in density fitting second-order M?ller-Plesset perturbation theory and other correlated ab initio methods. Calculations of the second-order M?ller-Plesset perturbation theory correlation energy, for a test set of small to medium sized molecules, indicate that the density fitting error when utilizing these sets is negligible at three to four orders of magnitude smaller than the orbital basis set incompleteness error.     

Optimized effective potential method: is it possible to obtain an accurate representation of the response function for finite orbital basis sets?

The optimized effective potential (OEP) equations are solved in a matrix representation using the orbital products of occupied and virtual orbitals for the representation of both the local potential and the response function. This results in a direct relationship between the matrix elements of local and nonlocal operators for the exchange-correlation potential. The effect of the truncation of the number of such products in the case of finite orbital basis sets on the OEP orbital and total energies and on the spectrum of eigenvalues of the response function is examined. Test calculations for Ar and Ne show that rather large AO basis sets are needed to obtain an accurate representation of the response function.     

Electrocyclization of phosphahexatrienes: an approach to λ5-phosphinines

We experimentally verified an assumption that the substitution of a carbon atom with a pentavalent phosphorus atom in 1-alkoxy (dialkylamino) hexatrienes will not hamper its ability to electrocyclize. A series of 1-, 3-, and 5-phosphahexatrienes were synthesized. It was shown that parent λ(5)-phosphinines could be synthesized by electrocyclization of the 3- and 5-phosphahexatrienes. The resultant electrocyclization is a convenient method for the synthesis of parent λ(5)-phosphinines bearing different substituents on the phosphorus atom.     

Stacking of nucleic acid bases: optimization of the computational approach—the case of adenine dimers

Structural Chemistry - Stacking interactions play an important role in stabilizing DNA and RNA secondary structure. To select a computational level to study the stacking interactions, both energy...     

Reaction of adenine with an α-chlorooxetane: An approach to the synthesis of oxetane nucleosides

The reaction of adenine with 3,5-anhydro-5R-chloro-1,2-O- isopropylidenexylofuranose, a stable α-chlorooxetane, gives a mixture of the two epimers of 5-[9-adenyl]-3,5-anhydro- 1,2-O-isopropylidenexylofuranose; the structure of 5R-[9- adenyl)-3,5-anhydro-1,2-O-isopropylidenexylofuranose was established by X-ray crystallography.     

Requirements of the glow discharge techniques to the fundamentals – an exemplary approach

The importance of gaining knowledge on the fundamental processes in glow discharges in the field of the signal generation, the sputtering phenomena taking place in the case of solids and the analyte breakdown in the excitation of gases is treated. For gaining more knowledge on the plasma processes diagnostics including temperature and electron number density as well as gas and analyte atom and ion densities are required. For the sputtering process, it is shown at the hand of measurements with gas-jet enhanced sputtering and magnetically enhanced sputtering that selective sputtering may occur and that the influence of the analyte loading of the glow discharge plasma needs further study. For the case of the introduction of gaseous samples, the analyte distribution as well as the break-down mechanisms and kinetics in the case of molecular species ask for further study as shown by experiments with gas-sampling glow discharges.     

Supercritical CO2: an effective medium for the chemo-enzymatic synthesis of block copolymers?

In this review, we describe the combination of enzymatic polymerisation and controlled free radical polymerisation in supercritical carbon dioxide. This combination facilitates the preparation of a range of block and graft copolymers, some of which cannot easily be obtained by conventional polymer synthesis. Biocatalysis in polymer science provides significant new opportunities and will open up a very broad range of new polymeric materials.     

Correlation consistent, Douglas–Kroll–Hess relativistic basis sets for the 5p and 6p elements

The diatomic carbon molecule has a complex electronic structure with a large number of low-lying electronic excited states. In this work, the potential energy curves (PECs) of the four lowest lying singlet states ( $X^{1} \Sigma^{ + }_{g}$ , $A^{1} \Pi_{u}$ , $B^{1} \Delta_{g}$ , and $B^{\prime1} \Sigma^{ + }_{g}$ ) were obtained by high-level ab initio calculations. Valence electron correlation was accounted for by the correlation energy extrapolation by intrinsic scaling (CEEIS) method. Additional corrections to the PECs included core–valence correlation and relativistic effects. Spin–orbit corrections were found to be insignificant. The impact of using dynamically weighted reference wave functions in conjunction with CEEIS was examined and found to give indistinguishable results from the even weighted method. The PECs showed multiple curve crossings due to the $B^{1} \Delta_{g}$ state as well as an avoided crossing between the two $^{1} \Sigma^{ + }_{g}$ states. Vibrational energy levels were computed for each of the four electronic states, as well as rotational constants and spectroscopic parameters. Comparison between the theoretical and experimental results showed excellent agreement overall. Equilibrium bond distances are reproduced to within 0.05 %. The dissociation energies of the states agree with experiment to within ~0.5 kcal/mol, achieving “chemical accuracy.” Vibrational energy levels show average deviations of ~20 cm^?1 or less. The $B^{1} \Delta_{g}$ state shows the best agreement with a mean absolute deviation of 2.41 cm^?1. Calculated rotational constants exhibit very good agreement with experiment, as do the spectroscopic constants.     

Effects of substitution on the effective molarity (EM) for five membered ring-closure reactions – A computational approach

DFT molecular orbital calculations of kinetic parameters for ring closing reactions of substituted 4-bromobutyl alcohols 1–5 and 4-bromobutyl amines 6–10 (Brown’s system) indicate that accelerations in ring closing rate in both systems are largely the result of strain effects as opposed to the currently advanced proximity orientation. Furthermore, the calculated effective molarity (EM) values derived from the DFT data reveal that replacing the nitrogen (6–10, Brown’s system) with an oxygen (1–5, alcoholic analog) results in a decrease in demands on directional flexibility to form 5-membered ring, thus enhancing the rate of the cyclization reaction. In the absence of experimental data the DFT approach could be utilized to predict effective molarities (EM) for intramolecular processes.     

What are the limits to the size of effective dynamic combinatorial libraries?

Using simple computer simulations of model dynamic combinatorial libraries, we show that the best binders can be amplified to useful concentrations in libraries containing 10-10(6) compounds. [structure: see text]     

Is the dynamical polarization a significant part of the contribution of the triples to the correlation energy?

One may call dynamical polarization of doubly excited configurations the energy lowering of these configurations under the response of the other electrons to the so-created fluctuation of the electric field. This contribution of triply excited configurations may be identified and calculated through a computation that only requires a computation time proportional to the sixth power of the number of molecular orbitals (MOs), instead of the seventh power for the total contribution of the triples. Its amplitude depends on the choice of the MOs and becomes important when localized MOs are used.     

A computational study on the stability–aromaticity correlation of triply N-confused porphyrins: CMMSE-09

The B3LYP density functional theory methodology in conjunction with the 6-31G(d,p) basis set has been used to characterize triply N-confused meso-tetraphenylporphyrins. According to our computations, there is no a direct correlation between stability and aromaticity as already found for non-substituted confused porphyrins. The inclusion of these substituents in the calculations provokes a decrease of the planarity and aromaticity of these macrocycles along with a notable rise of their relative stability with respect to the non-substituted case. Steric repulsions, both among phenyl rings and β atoms in the pyrrolic rings, and among H atoms in the core of the macrocycles, dominate over aromaticity in the establishment of the most stable conformation of each isomer.     

Can extrapolation to the basis set limit be an alternative to the counterpoise correction? A study on the helium dimer

Configuration interaction and coupled cluster calculations are reported for He₂ using various orbital basis sets of the d-aug-AVXZ type, with the results being extrapolated to the one electron basis set limit both with counterpoise and without counterpoise correction. A generalized uniform singlet- and triplet-pair extrapolation scheme has been utilized for such a purpose. Using appropriate corrections to mimic full configuration interaction, the energies were predicted in excellent agreement with the best available estimates. The results also suggest that extrapolation to the complete basis set limit may be a general alternative to the counterpoise correction that yields a more accurate potential energy while being more economical.     

Ir-catalyzed chemoselective reduction of β-amido esters: A versatile approach to β-enamino esters

The conversion of β-amido esters to β-enamino esters is an indispensable step for some synthetic approaches to alkaloids and related medicines. Known methods for such transformation are not only stepwise, but also proceed with low atom-efficiency. Herein, we report a direct and versatile approach that features the Ir-catalyzed chemoselective reduction of β-amido esters with 1,1,3,3-tetramethyldisiloxane (TMDS). In addition, a lack of some signals was observed in the ¹³C NMR spectra of some alicyclic β-enamino esters. This revealed a longstanding existing but being ignored phenomenon in the literature.     

A computational approach towards predicting π-facial selectivity in sterically unbiased olefins: an evaluation of the relative importance of electrostatic and orbital effects

Computational studies are presented to show that electrostatic interactions significantly impact the stereochemical outcome in electrophilic addition to a number of sterically unbiased alkenes. Transition states have been located for the reaction of different electrophiles with all the sterically unbiased alkenes studied here and the calculations effectively include interactions involving the σ and σ^∗ orbitals of the newly formed bond. Electrostatic interaction between the substrates and electrophiles was modelled by removing the electrophiles from the transition state geometry and placing the calculated charge at a distance from a selected atom as observed in TS structures. Electrostatic interactions between the electrophiles and the substrate seem to effectively determine the face selectivities in the systems studied and our model calculations indicate that it may not be important to invoke Cieplak type orbital interactions to rationalize the observed face selectivities. The face selectivities predicted for these alkenes and electrophiles with DFT B3LYP/6-31G^∗ and ab initio MP2/6-31G^∗ levels are generally in good agreement.     

The microbial reductive splitting of the NO bond of dihydrooxazines; an alternative to the chemical reduction

In the presence of artificial mediators many bacteria reductively split the NO bond of dihydrooxazines in a chemoselective way if hydrogen is used as an electron source. The yields as determined by HPLC are almost quantitative i.e. considerably better than by chemical reductions.