A model study for coatings containing hexamethoxymethylmelamine

Using a model reaction we have studied the crosslinking chemistry of hydroxy-functional polymers and hexamethoxymethylmelamine. The transetherification of optically active monofunctional alcohols and hexamethoxymethylmelamine was monitored with polarimetry and ¹H-NMR. The reaction rate constants for both the forward (k₁) and the backward (k_?1) reaction of the sulphonic-acid-catalyzed alcoholysis were determined. Primary and secondary alcohols showed the same reaction rate and activation energy (E_a = 96 kJ/mol) for the forward reaction. However, the backward reaction in the equilibrium is considerably slower for primary alcohols than for secondary alcohols, with activation energies of E_a = 96 and 79 kJ/mol, respectively. When amine salts of sulphonic acids are used as catalysts, the E_a is increased from 97 to 116 kJ/mol in the case of primary alcohols. In concentrated aprotic solutions the reaction order in acid is 2.5. The same order in acid is found for the alcoholysis of acetaldehyde diethyl acetal. All the results strongly support the statement that the crosslinking reaction proceeds by an Sn-1 mechanism. The results of this model study are compared with results obtained in network-forming reactions. The important role of the evaporation of the condensation product methanol is discussed.     

Supramolecular organization of alpha,alpha'-disubstituted sexithiophenes

The self-assembly of alpha,alpha'-linked sexithiophenes with chiral and achiral penta(ethylene glycol) chains attached at the alpha-positions of the terminal rings, that is, 2,2':5',2':5',2':5',2':5',2'-sexithiophene-5,5'-dicarboxylic acid-2S)-2-methyl-3,6,9,12,15-pentaoxahexadecyl ester (1) and 2,2':5',2':5',2':5',2':5',2'-sexithiophene-5,5'-dicarboxylic acid-3,6,9,12,15-pentaoxahexadecyl ester (2), respectively is described. Analysis of the UV/vis, fluorescence, circular dichroism, and circular polarization of luminescence spectroscopic data shows that these compounds form chiral aggregates in polar solvents and in the solid state. In n-butanol aggregation occurs at temperatures below 30 degrees C, while above this threshold temperature the aggregates break up without an intermediate disordered state of aggregation, and the compounds are molecularly dissolved. The "melting temperature" of the aggregates depends on the concentration of sexithiophene, indicating that the optical changes observed are a result of intermolecular processes. Mass spectrometric measurements reveal that 1 and 2 can form mixed aggregates. Analysis of the optical spectra reveals that in these mixed aggregates, chiral 1 molecules act as "sergeants" to direct the packing of the "soldiers" 2, illustrating cooperativity within the columns. In water, the same type of chiral aggregates are formed as in n-butanol below 30 degrees C; however, these aggregates are still present, but the chirality is lost above 30 degrees C. In spin-coated films of 1 chiral aggregates are present. AFM studies show that 1 self-organizes into chiral fiberlike structures in the solid state. Furthermore both 1 and 2 display thermotropic liquid crystalline behavior between 180 and 200 degrees C.     

Density functional theory based molecular-dynamics study of aqueous iodide solvation

We study the solvation of iodide in water using density functional theory based molecular-dynamics simulations. Detailed analysis of the structural and dynamical properties of the first solvation shell is presented, showing a disruptive influence of the ion on the local water structure. Iodide-water hydrogen bonding is weak, compared to water-water hydrogen bonds. This effective repulsive ion-water interaction leads to the formation of a quite unstructured solvation shell. The dynamics of water molecules surrounding the iodide is relatively fast. The intramolecular structural and electronical properties of water molecules around the ion are not affected.     

Infrared spectra of gas-phase V(+)-(benzene) and V(+)-(benzene)(2) complexes

The organometallic ions V+-(benzene) and V+-(benzene)2 are produced by laser vaporization in a pulsed nozzle source. They are trapped and mass selected in an ion-trap/time-of-flight mass spectrometer, and their infrared spectra are measured with resonance-enhanced multiphoton photodissociation (IR-REMPD) spectroscopy with a tunable free-electron laser. Vibrational bands in the 600-1800 cm-1 region are characteristic of the benzene molecular moiety perturbed by the metal cation bonding. Experimental data are compared to the IR spectra derived from density functional calculations. Vibrational patterns in V+-(C6H6) indicate that the metal is bound in an eta6 pi-bonding configuration, while V+-(C6H6)2 is a sandwich. Trapped-ion IR-REMPD is a general method to access the vibrational spectroscopy of organometallic ions and their clusters.     

Synthesis of π‐conjugated oligomer that can form metallo polymers

An oligo(p‐phenylene vinylene) that contains terpyridine ligands has been synthesized. Upon addition of metal ions, a π‐conjugated metallo polymer is formed in which the well‐defined character of oligomers and the material properties of polymers are combined. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 4020–4023, 2002