Bonding, aromaticity, and structure of trigonal dianion metal clusters.

Various isomers of the trigonal dianion metal clusters, X(3)(2-), X = Be, Mg, Ca, and their mono- and disodium complexes are optimized at the B3LYP/6-311+G(d) level. Different conceptual density functional theory based reactivity descriptors as well as the induced magnetic field values are calculated to understand the stability and aromaticity of these systems. Possibility of bond stretch isomerism is explored. Genetic algorithm results lend additional insights into the structures of these isomers.

Quantum chemistry of C(3)H(6)O molecules: structure and stability, isomerization pathways, and chirality changing mechanisms.

Electronic structure calculations were carried out to study the various isomers of formula C(3)H(6)O, as a part of our current quantum chemical and dynamical approaches to intra- and intermolecular kinetics for the C(n)H(2n)O (n = 1, 2, 3) molecules. The usefulness of the GRRM (global reaction route mapping) program developed by Ohno and Maeda in predicting the structure of all isomers and of the transition states connecting them is fully exploited. All the isomers are identified as local minima on the MP2/CC-PVDZ potential energy surface. Acetone is the most stable isomer. In increasing order of stability the others are propanal, 2-propenol, 1-propenol, allyl alcohol, methyl vinyl ether, cyclopropanol, propylene oxide, and oxetane. Various isomerization paths connecting them are identified. All the transition states are fully characterized using intrinsic reaction coordinate calculations. The isomerization reactions may proceed through a single step or involve an intermediate species which is either a carbene or a diradical. Special attention is devoted to propylene oxide, a favorite molecule in current photochemical and stereodynamical studies because of its chiral nature. It is a rigid molecule, and chirality switching is found to be supported by its isomers. Two different chirality switching mechanisms which are assisted by propanal and allyl alcohol are presented.

Synthesis of unsymmetrical substituted 1,4-dihydropyridines through thermal and microwave assisted [4+2] cycloadditions of 1-azadienes and allenic esters.

Thermal and microwave assisted [4+2] cycloadditions of 1,4-diaryl-1-aza-1,3-butadienes with allenic esters lead to cycloadducts, which after a 1,3-H shift afford variedly substituted unsymmetrical 2-alkyl-1,4-diaryl-3-ethoxycarbonyl-1,4-dihydropyridines in high yields. Reactions carried out under microwave irradiation are cleaner and give higher yields with much shortened reaction times. Density functional theory (DFT) at the B3LYP/6-31G* level has been used to calculate geometric features of the reactants, barrier for s-trans to s-cis and reverse isomerization of azadienes (5a-d, 10a-e), dihedral angles between N(1), C(2), C(3), and C(4) atoms of azadienes along with various indices such as chemical hardness (eta), chemical potential (micro), global electrophilicity (omega), and the difference in global electrophilicity (Deltaomega) between the reacting pairs and Fukui functions (f (+) and f(-)). The results revealed that s-trans is the predominant conformation of azadienes at ambient temperature and the barrier for conversion of the s-trans rotamer of 1-azadienes to s-cis may be the major factor influencing the chemoselectivity, i.e., [4+2] verses [2+2] cycloaddition. The regiochemistry of the observed cycloadditions is collated with the obtained local electrophilicity indices (Fukui functions). Transition states for the formation of both [4+2] and [2+2] cycloadducts as located at the PM3 level indicate that the transition state for the formation of [4+2] cycloadducts has lower energy, again supporting the earlier conclusion that preferred formation of [4+2] cycloaaducts at higher temperature may be a consequence of barrier for s-trans to s-cis transformation of 1-azadienes.