Are water–aromatic complexes always stabilized due to π–H interactions? LMP2 study

Eight complexes of various aromatic molecules with water have been studied theoretically at the local Møller–Plesset 2nd order theory (LMP2)/aug‐cc‐pVTZ(‐f)//LMP2/6‐31+G* level of theory. Two types of complexes can be formed, depending on the electronic structure of aromatic molecules. Donor hydrocarbons form A‐type complexes, while aromatics bearing electron‐withdrawing substituents form B‐type complexes. A‐type complexes are stabilized due to π–H interactions with the OH bond pointing to the aromatic molecule plane, while B‐type complexes have geometry with the oxygen atom pointing to the aromatic molecule plane stabilized by the interaction of highest occupied molecular orbital (HOMO) of water molecule with π* orbitals of the aromatics. It has been found that a (? HOMO–lowest unoccupied molecular orbital (LUMO)/2 value of aromatic molecule, which can be called “molecular electronegativity,” is useful to predict the type of complex formed by aromatic molecule and water. Aromatic hydrocarbons with “molecular electronegativity” of <0.15 tend to form A‐type complexes, while aromatic molecules with “molecular electronegativity” of <0.15 a.u. form B‐type complexes. The binding energy of water–aromatic complexes undergoes a minimum in the area of switching from A‐type to B type complexes, which can be rationalize in terms of frontier orbital interactions. © 2005 Wiley Periodicals, Inc. Int J Quantum Chem, 2005     

Generalized Polar Decompositions for Closed Operators in Hilbert Spaces and Some Applications

We study generalized polar decompositions of densely defined closed linear operators in Hilbert spaces and provide some applications to relatively (form) bounded and relatively (form) compact perturbations of self-adjoint, normal, and m-sectorial operators. Based upon work partially supported by the US National Science Foundation under Grant Nos. DMS-0400639 and FRG-0456306, and the Austrian Science Fund (FWF) under Grant No. Y330.     

Ultraviolet thermal lensing detection of amino acids

Thermal lensing (TL) permits ultra-sensitive measurements of optical absorption of analytes in very small liquid volumes. We report the construction and use of a TL detector based on pulsed ultraviolet (UV) laser excitation (266 nm). We applied this detector to quantitate amino acids using capillary electrophoresis (CE) as a means of separation. Sixteen individual amino acids are readily detected, but the signal has a complex dependence on intensity caused by the combination of (1) one-photon absorption; (2) two-photon absorption (TPA); and (3) photodestruction of amino acid molecules in the focus of the laser beam. An aqueous solution containing tyrosine, tryptophan, and cysteine is electrophoretically separated and the individual amino acids are detected by UV TL. The estimated limit of detection is 7 microM for tyrosine, 2.5 microM for tryptophan and 33 microM for cysterine, which translates into 0.35 fmol for tyrosine, 0.125 fmol for tryptophan, and 1.65 fmol for cysteine in the 140pL detection volume. It is found that two-photon absorption of water and the formation of color centers in the fused silica walls of the flowcell can contribute a significant, drifting background signal, but this interference can be minimized by selecting an appropriate focus condition and excitation-detection geometry. We suggest that as UV laser sources become available, UV TL may become a method of choice for measuring the concentrations of many analytes in different separation formats in which the volume is highly limited.     

Total Flooding Time and Rumor Propagation on Graphs

We study the discrete time version of the flooding time problem as a model of rumor propagation where each site in the graph has initially a distinct piece of information; we are interested in the number of “conversations” before the entire graph knows all pieces of information. For the complete graph we compare the ratio between the expected propagation time for all pieces of information and the corresponding time for a single piece of information, obtaining the asymptotic ratio 3 / 2 between them.