A study of molecular vibrational relaxation mechanism in condensed phase based upon mixed quantum-classical molecular dynamics. I. A test of IBC model for the relaxation of a nonpolar solute in nonpolar solvent at high density

In order to investigate vibrational relaxation mechanism in condensed phase, a series of mixed quantum-classical molecular dynamics calculations have been executed for nonpolar solute in nonpolar solvent and polar solute in polar solvent. In the first paper (Paper I), relaxation mechanism of I2 in Ar, where Lennard-Jones force is predominant in the interaction, is investigated as a function of density and temperature, focusing our attention on the isolated binary collision (IBC) model. The model was originally established for the relaxation in gas phase. A key question, here, is "can we apply the IBC model to the relaxation in the high-density fluid?" Analyzing the trajectory of solvent molecule as well as its interaction with the solute, we found that collisions between them may be defined clearly even in the high-density fluid. Change of the survival probability of the vibrationally first excited state on collision was traced. The change caused by collisions with a particular solvent molecule was also traced together with the interaction between them. Each collision makes a contribution to the relaxation by a stepwise change in the probability. The analysis clearly shows that the relaxation is caused by collisions even in the high-density fluid. The difference between stepwise relaxation and the continuous one found for the total relaxation in the low-density fluid and in the high-density one, respectively, was clarified to come from just the difference in frequency of the collision. The stronger the intensity of the collision is, the greater the relaxation caused by the collision is. Further, the shorter the collision time is, the greater the resultant relaxation is. The discussion is followed by the succeeding paper (Paper II), where we report that molecular mechanism of the relaxation of a polar molecule in supercritical water is significantly different from that assumed in the IBC model despite that the density dependence of the relaxation rate showed a linear correlation with the local density of water around the solute, the linear correlation being apparently in good accordance with the IBC model. The puzzle will be solved in Paper II.     

Synthesis and properties of the first stable silylene-isocyanide complexes

The first stable silylene-isocyanide complexes, [Tbt(Mes)SiCNAr] (5 c: Ar=Tip, 5 d: Ar=Tbt, 5 e: Ar=Mes*; Tbt=2,4,6-tris[bis(trimethylsilyl)methyl]phenyl, Mes=mesityl, Tip=2,4,6-triisopropylphenyl, Mes*=2,4,6-tri-tert-butylphenyl) were successfully synthesized by the reaction of a kinetically stabilized disilene, [Tbt(Mes)Si=Si(Mes)Tbt] (1), with bulky isocyanides, ArNC (3c-e). The spectroscopic data of 5 c-e and theoretical calculations for a model molecule indicated that 5 c-e are not classical cumulative compounds but the first stable silylene-Lewis base complexes. The reactions of 5 c-e with triethylsilane and 1,3-dienes gave the corresponding silylene adducts, and they underwent isocyanide-exchange reactions in the presence of another isocyanide at room temperature. These results indicate dissociation of complexes 5 c-e to the corresponding silylene 2 and isocyanides 3 c-e under very mild conditions. The reaction of 5 c with methanol gave the MeOH adduct 16, [Tbt(Mes)SiHC(OMe)NTip], which has a hydrogen atom on the silicon atom. This regioselectivity can be explained in terms of the contribution of zwitterionic resonance structures D and E, which have an anion on the silicon atom. This result indicates that 5 c is not a classical cumulene having Si=C double bonds that should react with methanol to give adducts bearing a methoxyl group on the silicon atom. Although the reactions of 5 c-e with electrophilic reagents such as methanol, hydrogen chloride, and methyl iodide gave the formal silylene adducts, the studies on the reaction mechanism by trapping experiments and the observation of the intermediate suggested that the reaction mainly or partially proceeds by initial nucleophilic attack of the silicon atom, as is the case in the formation of 16 in the reaction of 5 c with methanol. It was revealed that 5 c-e show the nucleophilicity of the silicon atom, most likely resulting from the contribution of the zwitterionic resonance structures D and E.     

Self-organized lipid-porphyrin bilayer membranes in vesicular form: nanostructure,photophysical properties,and dioxygen coordination

An amphiphilic tetraphenylporphyrin and its iron complex bearing four phospholipid substituents, in which a trimethylolethane residue connects the two acyl chains (lipid-porphyrins), have been synthesized. The free-base lipid-porphyrin 6a self-organizes in aqueous media to form spherical unilamellar vesicles with a diameter of 100 nm and a uniform thickness of 10 nm, which corresponds to twice the length of the molecule. In the visible absorption spectrum, the porphyrin Soret band was significantly red-shifted (12 nm) relative to that of the monomer in benzene/MeOH solution due to the excitonic interaction of the porphyrin chromophores. The [symbol: see text]-A isotherm of 6a gave an area per molecule of 2.2 nm2, which allowed the estimation of the number of molecules in a single vesicle (2.3 x 10(4)). Double-layered Langmuir-Blodgett (LB) films of 6a on a glass surface exhibited an absorption spectrum identical to that of the 6a vesicles in bulk aqueous solution, and this suggests that they contain similar geometric arrangements of the porphyrin moieties. Exciton calculations on the basis of our structural model reproduced the bathochromic shift of the Soret band well. In the photophysical properties of the 6a vesicles, the characteristics of J-aggregated porphyrins substantially predominate: strong fluorescence and extremely short triplet lifetime. The iron complex 6b with a small molar excess of 1-dodecylimidazole (DIm) also formed spherical unilamellar vesicles (100 nm phi). Scanning force microscopy after evaporation on a graphite surface revealed 6b/DIm vesicles with a vertical height of 19.8 nm, which coincided with the thickness of the double bilayer membranes. The ferrous 6c formed a bis(DIm)-coordinated low-spin FeII complex under an N2 atmosphere. Upon addition of O2 to this solution, a kinetically stable O2 adduct was formed at 37 degrees C with a half-life of 17 h. Distinct gel-phase (liquid-crystal) transitions of the lipid-porphyrin membranes were clearly observed; the free base 6a displayed a higher transition temperature (56 degrees C) than the iron complex. Magnetic circular dichroism and infrared spectroscopic studies proved that molecular O2 coordinates to the self-organized lipid-porphyrinatoiron(II) vesicles in aqueous media.     

Studies on the biosynthesis of corrinoids and porphyrinoids. II. The origin of nitrogen of vitamin B12

To clarify the origin of nitrogen of vitamin B12, 15N-labeled aminolevulinic acid (ALA) was prepared and administered to Propionibacterium shermanii. Vitamin B12 thus isolated showed four signals in the nitrogen-15 nuclear magnetic resonance (15N-NMR) spectrum. The nitrogen of [5-15N]riboflavine was incorporated into the benzimidazole part of vitamin B12. Hydroxycobalamin was transformed into cyanocobalamin by treatment with [15N]potassium cyanide, and the 15N-NMR spectrum was measured. The results of these experiments revealed the origin of the nitrogen atoms of vitamin B12, and allowed the 15N-NMR signals to be assigned.     

Spin-probe ESR study on the dynamics of liquid molecules in the MCM-41 nanochannel: temperature dependence on 2-propanol and water

A spin-probe ESR study has been made on the dynamics of 2-propanol and water molecules in the nanochannel of MCM-41 at various temperatures. In the former system, 2-propanol is separated into two phases: one with molecules immobilized in the ESR time scale and the other with mobile ones, even at temperatures more than 40 degrees higher than the bulk melting point. In the case of water, on the other hand, only the "immobilized" water was detected at a temperature as high as 313 K. At higher temperature, spin-probe molecule undergoes anisotropic rotational diffusion to reduce resistance from the solvent molecules in the nanochannel. These results are explained in relation to the intermolecular network intensified in the nanochannel. Static as well as dynamic structures of these solutions have been discussed.     

The First Rotational Isomers of Stable Selenobenzaldehydes and their η1-Tungsten Complexes

The first rotational isomers of stable selenoaldehydes are synthesized by deselenation of cyclic polyselenides having an efficient steric protection group, 2, 4, 6-tris[bis(trimetylsilyl)methyl]phenyl (Tbt). Reactions of these selenoaldehydes with W(CO)₅.THF gives the corresponding η¹-selenoaldehyde tungsten complexes, the structures of which are established by X-ray crystallography.     

Synthesis and application of head-to-head-type styrene dimers bearing two fluoroalkyl end-groups

Head-to-head-type styrene and substituted styrene dimers bearing two fluoroalkyl end-groups have been efficiently synthesized by a simple reaction of perfluoroalkyl iodide with styrene under radical conditions as a mixture of meso and racemic forms. The meso form obtained from the mixture by recrystallization gave a crystal suitable for X-ray diffraction study and the crystal structure was found to be based on π-stacking of benzene rings and aggregation of fluoroalkyl chains. Dynamic light scattering measurements showed that meso-styrene dimers bearing two fluoroalkyl end-groups can form the nanometer size-controlled self-assemblies through the intermolecular π-stacking of benzene rings and aggregation of end-capped fluoroalkyl groups in methanol.