Bonding in singlet and triplet butalene: insights from spin-coupled theory.

Spin-coupled (SC, equivalent to full generalized valence bond) calculations for the (1)A(g) ground state of butalene at its optimal D(2h) planar geometry show that cross-ring Dewar-like modes of spin coupling are of comparable importance to the more usually considered Kekulé-like modes. There are marked similarities to the SC description of one of the isomers of benzo[1,2:4,5]dicyclobutadiene. A complication for both of these systems is the existence of SC solutions in which some of the orbitals resemble in- and out-of-phase combinations of semilocalized atom-centered orbitals.

Spin-coupled theory for 'N electrons in M orbitals' active spaces.

Spin-coupled (SC) theory, an ab initio valence bond (VB) approach which uses a compact and an easy-to-interpret single-orbital product wave function comparable in quality to a ‘N in N’ complete-active-space self-consistent field [CASSCF(N,N)] construction, is extended to ‘N in M’ (N ≠ M) active spaces. The SC(N,M) wave function retains the essential features of the original SC model: It involves just the products of nonorthogonal orbitals covering all distributions of N electrons between M orbitals in which as few orbitals as possible, |N – M|, are doubly occupied (for N > M) or missing (for N < M) and all other orbitals are singly occupied; each of these products is combined with a flexible spin function which allows any mode of coupling of the spins of the orbitals within the product. The SC(N,M) wave function remains much more compact than a CASSCF(N,M) construction; for example, the SC(6,7) wave function includes 35 configuration state functions (CSFs) as opposed to the 490 CSFs in the CASSCF case.

Quantum chemical study of the intermediate complex required for iron-mediated reactivity and antimalarial activity of dispiro-1,2,4-trioxolanes.

Quantum chemical methods were used to obtain a structure for the peroxide-iron intermediate complex required for the inner-sphere reduction of dispiro-1,2,4-trioxolane antimalarials. Investigation of this biologically important interaction with iron(II) allows further understanding of the mechanisms of action and clearance of this promising new class of fully synthetic peroxide antimalarials. UHF, B3LYP and B3LYP//MP2 calculations were undertaken to provide structural and energetic information about the coordination complex of iron(II) with five representative trioxolanes, ranging in both iron-mediated reactivity and antimalarial activity.

Role of Tyr(356(7.43)) and Ser(190(4.57)) in antagonist binding in the rat beta1-adrenergic receptor.

Site-directed mutagenesis and photoaffinity labeling experiments suggest the existence of at least two distinct binding orientations for aryloxypropanolamine competitive antagonists in the beta-adrenergic receptor (beta-AR), one where the aryloxy moiety is located near transmembrane alpha-helix 7 (tm 7) and another where it is near tm 5. To explore a hydrophobic pocket involving tms 1, 2, 3, and 7 for potential aryloxy interaction sites, we selected Tyr(356(7.43)) and Trp(134(3.

Can the bis(diboranyl) structure of B(4)H(10) be observed? The story continues.

Pathways for the conversion of the unknown bis(diboranyl) isomer of tetraborane(10) (B(4)H(10)) to the known arachno isomer have been determined for the first time with the use of an electron correlation ab initio quantum chemical method and without the use of constraints in determination of the transition structures. Two isomers of tetraborane(10), one new, with a pentacoordinated boron atom have been found on the theoretical potential energy surface. Several other pathways for molecular rearrangement of tetraborane(10) have also been characterized.