Mechanistic insights into the substrate-controlled stereochemistry of glycals in one-pot rhodium-catalyzed aziridination and aziridine ring opening.

We carried out a principle study on the reaction mechanism of rhodium-catalyzed intramolecular aziridination and aziridine ring opening at a sugar template. A sulfamate ester group was introduced at different positions of glycal to act as a nitrene source and, moreover, to allow the study of the relative reactivity of the nitrene transfer from different sites of the glycal molecule. The structural optimization of each intermediate along the reaction pathway was extensively done by using BPW91 functional. The crucial step in the reaction is the Rh-catalyzed nitrene transfer to the double bond of the glycal. We found that the reaction could proceed in a stepwise manner, whereby the N atom initially induced a single-bond formation with C1 on the triplet surface or in a single step through intersystem crossing (ISC) of the triplet excited state of the rhodium-nitrene transition state to the singlet ground state of the aziridine complexes. The relative reactivity for the conversion of the nitrene species to the aziridine obtained from the computed potential energy surface (PES) agrees well with the reaction time gained from experimental observation. The aziridine ring opening is a spontaneous process because the energy barrier for the formation of the transition state is very small and disappears in the solution calculations. The regio- and stereoselectivity of the reaction product is controlled by the electronic property of the anomeric carbon as well as the facial preference for the nitrene insertion, and the nucleophilic addition.

Structure and vibrational spectra of H(+)(HF)(n) (n=2-9) clusters: An ab initio study.

The morphological development of the hydrogen bond network in the protonated hydrogen fluoride clusters, H(+)(HF)(n) (n=2-9), is investigated in detail by ab initio methods. We find a dominance of the linear morphology, which is energetically well separated from the other minimum energy morphologies of the clusters. The geometry for these clusters shows a pattern due to the cooperativity effect prevalent in the hydrogen bonds, as a result of the difference in electronegativities of hydrogen and fluorine atom in the HF molecule. The variations in the covalent HF and hydrogen bond distances in the clusters are in turn reflected in the vibrational spectra. Distinct HF stretching modes for the linear and ring with tail structures were identified. We have discussed the signature peaks for the two possible ion-core morphologies present in the clusters. The highly corrosive nature of HF makes it difficult to study using experiments. We, thus, believe that our structure and vibrational spectra calculations would be useful in understanding the key features in these systems.

Highly stereoselective synthesis of aminoglycosides via rhodium-catalyzed and substrate-controlled aziridination of glycals.

The flexible installations of a sulfamate ester on a glycal scaffold at C3, C4, or C6 approaching alpha- or beta-aminoglycosides is communicated. A variety of glycal acceptors (O, S, and N) were applied, enhancing the utility of this method as an operationally simple protocol for the stereoselective synthesis of polyfunctionalized alpha- or beta- aminosaccharides.

Noniterative density functional response approach: application to nonlinear optical properties of p-nitroaniline and its methyl-substituted derivatives.

We report the effect of substitution, position of the substituent, and the symmetry on the nonlinear optical properties of p-nitroanline (PNA) and its derivatives using our implementation of the noniterative approximation of couple-perturbed Kohn-Sham (CPKS) equation in the deMon2k. Dipole moment, static polarizability, and first hyperpolarizability of these pi-conjugated donor-acceptor organic derivatives of PNA and its methyl-substituted analogs are calculated using our method at different exchange correlation functionals, namely, BP86, BPW91, and BLYP, using 6-31++G** basis set. A comparison of results obtained by our method with those obtained by MP2 (finite-field perturbation) method is presented in this paper. The effect of optical gap on charge transfer and subsequently on polarizabilities has been illustrated.

Density Functional Static Dipole Polarizability and First-Hyperpolarizability Calculations of Nan (n = 2, 4, 6, 8) Clusters Using an Approximate CPKS Method and its Comparison with MP2 Calculations.

We report the static dipole polarizability and first-hyperpolarizability of the sodium atom clusters, Nan, n = 2, 4, 6 and 8, using our recent implementation of a numerical-analytical approach to the coupled-perturbed Kohn-Sham (CPKS) equations in deMon2k. The calculations are reported for VWN and BP86 exchange-correlation functionals using Sadlej and TZVP-FIP1 basis sets which have been previously optimized for polarizability calculations. All-electron calculations were performed for the optimizations at the VWN/DZVP/A2 and PW86/DZVP/A2 levels. Comparisons are made with Hartree-Fock (HF) and MP2 benchmark calculations.