On the Rotational Intergrowth of Hierarchical FAU/EMT Zeolites

A structural study of a hierarchical zeolite X, which is similar to the one first synthesized by Inayat et al., 12 was performed using transmission electron microscopy imaging and diffraction. Evidence is provided, by comparison to simulations, that this material is an intergrowth of FAU and EMT and a conceptual model is presented for the growth of the FAU material with a small fraction of EMT in an atypical morphology of assembled sheets with well‐defined intersection angles.     

Symmetry calculation for molecules and transition states

The symmetry of molecules and transition states of elementary reactions is an essential property with important implications for computational chemistry. The automated identification of symmetry by computers is a very useful tool for many applications, but often relies on the availability of three‐dimensional coordinates of the atoms in the molecule and hence becomes less useful when these coordinates are a priori unavailable. This article presents a new algorithm that identifies symmetry of molecules and transition states based on an augmented graph representation of the corresponding structures, in which both topology and the presence of stereocenters are accounted for. The automorphism group order of the graph associated with the molecule or transition state is used as a starting point. A novel concept of label‐stereoisomers, that is, stereoisomers that arise after labeling homomorph substituents in the original molecule so that they become distinguishable, is introduced and used to obtain the symmetry number. The algorithm is characterized by its generic nature and avoids the use of heuristic rules that would limit the applicability. The calculated symmetry numbers are in agreement with expected values for a large and diverse set of structures, ranging from asymmetric, small molecules such as fluorochlorobromomethane to highly symmetric structures found in drug discovery assays. The new algorithm opens up new possibilities for the fast screening of the degree of symmetry of large sets of molecules. © 2014 Wiley Periodicals, Inc.     

Optimization of critical medium components for the maximal production of gentamicin by Micromonospora echinospora ATCC 15838 using response surface methodology

Optimization of the fermentation medium components for maximum gentamicin production by Micromonospora echinospora ATCC 15838 was carried out. Response surface methodology was applied to optimize the medium constituents. A 2⁴full-factorial central composite design was chosen to explain the combined effects of the four medium constituents, viz. starch, soyabean meal, K₂HPO₄, and CoCl₂ and to design a minimum number of experiments. A second order model was developed and fitted using least square method. The R ² value of the model was 0.9723, which shows that model is best fit for the present studies. The results of analysis of variance and regression of a second order model showed that the linear effects of starch (p<0.001697) and CoCl₂(p<7.99E-13), and cross product effects of starch and soyabean meal (p<0.029876) and soyabean meal and CoCl₂ (p<0.008909) were more significant, suggesting that these were critical variables having the greatest effect on the production of gentamicin in the production medium. The optimized medium consisting of 9 g/L starch, 3 g/L soyabean meal, 0.9 g/L K₂HPO₄, and 0.01 g/L CoCL₂ predicted 850 mg/L of gentamicin which was almost 110% higher than that of the unoptimized medium. The amounts of starch, soyabean meal, and K₂HPO₄ required were also reduced with RSM.     

Conformational Panorama and Chirality Controlled Structure–Energy Relationship in a Chiral Carboxylic Acid Dimer

Chirality recognition in dimers of tetrahydro‐2‐furoic acid (THFA) was studied in a conformer‐specific manner using rotational spectroscopy and theoretical approaches. THFA shows a strong preference for the trans‐ over the cis‐COOH configuration. Two drastically different scenarios are possible for the detectable (THFA)₂: a kinetically preferred dimer bound by feeble interactions between two trans THFAs or a thermodynamically favored dimer with a double hydrogen‐bonded ring structure between two cis subunits. To identify the conformers responsible for the extremely dense rotational spectra observed, it was essential not only to locate several hundred homo/heterochiral (THFA)₂ minima in ab initio calculations but also to evaluate the energetic connectivities among the minima. The study further reveals an interesting chirality dependent structure–energy ordering relationship. A method for enantiomeric excess (ee) determination of THFA is presented using a recently proposed chiral self‐tag approach.     

Effects of substituents and n-donors on the energies of Si←N,Si←O,and Si=C bonds in hypervalent silenes

The effect of the nature of substituents at sp²-hybridized silicon atom in the R₂Si=CH₂ (R = SiH₃, H, Me, OH, Cl, F) molecules on the structure and energy characteristics of complexes of these molecules with ammonia, trimethylamine, and tetrahydrofuran was studied by the ab initio (MP4/6-311G(d)//MP2/6-31G(d)+ZPE) method. As the electronegativity, χ, of the substituent R increases, the coordination bond energies, D(Si← N(O)), increase from 4.7 to 25.9 kcal mol⁻¹ for the complexes of R₂Si=CH₂ with NH₃, from 10.6 to 37.1 kcal mol⁻¹ for the complexes with Me₃N, and from 5.0 to 22.2 kcal mol⁻¹ for the complexes with THF. The n-donor ability changes as follows: THF ≤ NH₃ < Me₃N. The calculated barrier to hindered internal rotation about the silicon—carbon double bond was used as a measure of the Si=C π-bond energy. As χ increases, the rotational barriers decrease from 18.9 to 5.2 kcal mol⁻¹ for the complexes with NH₃ and from 16.9 to 5.7 kcal mol⁻¹ for the complexes with Me₃N. The lowering of rotational barriers occurs in parallel to the decrease in D _π(Si=C) we have established earlier for free silenes. On the average, the D _π(Si=C) energy decreases by ∼25 kcal mol⁻¹ for NH₃· R₂Si=CH₂ and Me₃N·R₂Si=CH₂. The D(Si←N) values for the R₂Si=CH₂· 2Me₃N complexes are 11.4 (R = H) and 24.3 kcal mol⁻¹ (R = F). sp²-Hybridized silicon atom can form transannular coordination bonds in 1,1-bis[N-(dimethylamino)acetimidato]silene (6). The open form (I) of molecule 6 is 35.1 and 43.5 kcal mol⁻¹ less stable than the cyclic (II, one transannular Si←N bond) and bicyclic (III, two transannular Si←N bonds) forms of this molecule, respectively. The D(Si←N) energy for structure III was estimated at 21.8 kcal mol⁻¹. Dedicated to Academician N. S. Zefirov on the occasion of his 70th birthday. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 9, pp. 1952–1961, September, 2005.     

Rotational superposition and least squares: the SVD and quaternions approaches yield identical results. Reply to the preceding comment by G. Kneller

We clarify once again that Kabsch's method and the Quaternion method are mathematically equivalent methods, that is, that they contain identical information and, when properly understood and applied, lead to identical answers to any questions regarding least-squares rotational superposition, either by proper rotations or by rotation-reflections. We also provide the correct bounds for the eigenvalue spectrum.     

Further Study of A-Related Quantum Interference of П-State Diatomic on Collision-Induced Rotational Energy Transfer

In our previous theoretical studies [Meng-Tao Sun, Yong-Qing Lee, and Feng-Cai Ma, Chem. Phys.Lett. 371 (2003) 342], we have reported the quantum interference on collision-induced rotational energy transfer on CO (A1П, v = 3) with inert gases, which originates from the difference between the two A-related collision potential energy surfaces. The interference angle, which measures the degree of coherence, is presented in this paper. Based on the time-dependent first order Born approximation, taking into account the anisotropic Lennard-Jones interaction potentials, the relation of the interference angle with the factors, including experimental temperature, partner, and rotational quantum number, are obtained. The changing tendencies with them are discussed. This theoretical model is important to understanding and performing this kind of experiment.     

Relative Thermodynamic Stabilities of Isomeric Vinyl Ethers Derived from 2-Acetyland 2-Isobutyryltetrahydrofurans. An Experimental and DFT Study

The relative thermodynamic stabilities of four isomeric vinyl ethers obtained by acid-catalyzed cleavage of MeOH from the dimethyl acetals of both 2-acetyltetrahydrofuran and 2-isobutyryltetrahydrofuran have been determined by chemical equilibration in cyclohexane solution. In addition, the structures and relative energies of many of these compounds have also been studied theoretically by DFT calculations. In each series of compounds, the endocyclic isomers, 5-(1-methoxyethyl)-2,3-dihydrofuran and 5-(1-methoxy-2-methylpropyl)-2,3-dihydrofuran, respectively, show the highest thermodynamic stabilities. This is particularly prominent for the vinyl ethers derived from 2-isobutyryltetrahydrofuran in which the equilibrium mixture consists of 90% of the endocyclic isomer at room temperature. The geometrical (exocyclic) isomers of the latter series of compounds show unexpected relative thermodynamic stabilities, but on the basis of the geometry-optimized structures provided by the DFT calculations, a reasonable explanation can be found.