Electronic structure of the two-leg spin ladder (C5H12N)2CuBr4

A density functional theory study of the two-leg spin ladder (C(5)H(12)N)(2)CuBr(4) reveals antiferromagnetic interactions through the rungs and legs, although the latter are significantly smaller. Our work suggests interest in manipulating the physical behavior of this prototypical system by doping or chemical modifications.     

Coordination complexes exhibiting anion...pi interactions: synthesis, structure, and theoretical studies

The polydentate ligand 2,4,6-tris(dipyridin-2-ylamino)-1,3,5-triazine (dpyatriz) in combination with the Cu(ClO 4) 2/CuX 2 salt mixtures (X (-) = Cl (-), Br (-), or N 3 (-)) leads to the formation of molecular coordination aggregates with formulas [Cu 3Cl 3(dpyatriz) 2](ClO 4) 3 ( 2), [Cu 3Br 3(dpyatriz) 2](ClO 4) 3 ( 3), and [Cu 4(N 3) 4(dpyatriz) 2(DMF) 4(ClO 4) 2](ClO 4) 2 ( 4). These complexes consist of two dpyatriz ligands bridged via coordination to Cu (II) and disposed either face-to-face in an eclipsed manner ( 2 and 3) or parallel and mutually shifted in one direction. The copper ions complete their coordination positions with Cl (-) ( 2), Br (-) ( 3), or N 3 (-), ClO 4 (-), and N, N-dimethylformamide (DMF) ( 4) ligands. All complexes crystallize together with noncoordinate ClO 4 (-) groups that display anion...pi interactions with the triazine rings. These interactions have been studied by means of high level ab initio calculations and the MIPp partition scheme. These calculations have proven the ClO 4 (-)...[C 3N 3] interactions to be favorable and have revealed a synergistic effect from the combined occurrence of pi-pi stacking of triazine rings and the interaction of these moieties with perchlorate ions, as observed in the experimental systems.     

Theoretical study of the structure and vibrational spectra of the (H2O)2…HF and H2O…(HF)2 molecular complexes

Using the Hartree–Fock and MP 2 methods with bases of up to 6-31++G (2d, 2p) quality, the optimum geometry of the 1:2 and 2:1 (H₂O)_n… (HF)_m complexes of water and hydrogen fluoride is searched in a systematic way. Two minimum-energy conformations are found for the 1:2 complex connected through a low-energy transition state. For the 2:1 complex, only one minimum-energy structure is obtained. The analysis of the geometries of the minima and their vibrational frequencies shows that none of them can be used to explain the existence of the H …F? H reverse complex detected experimentally. © 1994 John Wiley & Sons, Inc.     

Engineered L‐Serine Hydroxymethyltransferase from Streptococcus thermophilus for the Synthesis of α,α‐Dialkyl‐α‐Amino Acids

α,α‐Disubstituted α‐amino acids are central to biotechnological and biomedical chemical processes for their own sake and as substructures of biologically active molecules for diverse biomedical applications. Structurally, these compounds contain a quaternary stereocenter, which is particularly challenging for stereoselective synthesis. The pyridoxal‐5′‐phosphate (PLP)‐dependent L ‐serine hydroxymethyltransferase from Streptococcus thermophilus (SHMT_Sth; EC 2.1.2.1) was engineered to achieve the stereoselective synthesis of a broad structural variety of α,α‐dialkyl‐α‐amino acids. This was accomplished by the formation of quaternary stereocenters through aldol addition of the amino acids D ‐Ala and D ‐Ser to a wide acceptor scope catalyzed by the minimalist SHMT_Sth Y55T variant overcoming the limitation of the native enzyme for Gly. The SHMT_Sth Y55T variant tolerates aromatic and aliphatic aldehydes as well as hydroxy‐ and nitrogen‐containing aldehydes as acceptors.     

Neighbor games and the leximax solution

Neighbor games arise from certain matching or sequencing situations in which only some specific pairs of players can obtain a positive gain. As a consequence, the class of neighbor games is the intersection of the class of assignment games (Shapley and Shubik (1972)) and the class of component additive games (Curiel et al. (1994)). We first present some elementary features of neighbor games. After that we provide a polynomially bounded algorithm of order p ³ for calculating the leximax solution (cf. Arin and Iñarra (1997)) of neighbor games, where p is the number of players. This authors work has been supported by CentER and the Department of Econometrics, Tilburg University and by the Foundation for the Hungarian Higher Education and Research (AMFK).     

Internal rotation in squaramide and related compounds. A theoretical ab initio study

The structural and energetic changes associated with C–N bond rotation in a squaric acid derivative as well as in formamide, 3-aminoacrolein and vinylamine have been studied theoretically using ab initio molecular orbital methods. Geometry optimizations at the MP2(full)/6-31+G* level confirmed an increase in the C–N bond length and a smaller decrease in the C=O length on going from the equilibrium geometry to the twisted transition state. Other geometrical changes are also discussed. Energies calculated at the QCISD(T)/6-311+G** level, including zero-point-energy correction, show barrier heights decreasing in the order formamide, squaric acid derivative, 3-aminoacrolein and vinylamine. The origin of the barriers were examined using the atoms-in-molecules approach of Bader and the natural bond orbital population analysis. The calculations agree with Pauling's resonance model, and the main contributing factor of the barrier is assigned to the loss of conjugation on rotating the C–N bond. Finally, molecular interaction potential calculations were used to study the changes in the nucleophilicity of N and O (carbonyl) atoms upon C–N rotation, and to obtain a picture of the abilities of the molecules to act in nonbonded interactions, in particular hydrogen bonds. The molecular interaction potential results confirm the suitability of squaramide units for acting as binding units in host–guest chemistry. Received: 13 March 2002 / Accepted: 23 June 2002 / Published online: 21 August 2002     

Squaramido-based receptors: applicability of molecular interaction potential to molecular recognition of polyalkylammonium compounds

 A computational study of the mechanism of host–guest complexation between quaternary ammonium compounds and squaramido-based tripodal receptors has been carried out. Semiempirical molecular orbital calculations, which are in qualitative agreement with experimental results have been performed using the PM3 Hamiltonian. Molecular interaction potential (MIP) maps were used to analyze the suitability of both host and guest binding units for a high-affinity recognition process. MIP calculations were computed from PM3 wavefunctions of the corresponding ammonium cations and dimethyl squaramide as a model compound for the hydrogen-bond-acceptor unit of the receptors. MIP analyses are helpful for understanding the host–guest process from the point of view of the double-complementarity principle. Received: 23 June 1999 / Accepted: 22 September 1999 / Published online: 17 January 2000     

Morita Equivalence for Rings with Involution

We develop the theory of Morita equivalence for rings with involution, and we show the corresponding fundamental representation theorem. In order to allow applications to operator algebras, we work within the class of idempotent nondegenerate rings. We also prove that two commutative rings with involution are Morita *-equivalent if and only if they are *-isomorphic.