Linearization, hydrogen bridges

Due to the perfect linear arrangement of the N-H -N hydrogen bridges in 13, extremely fast cooperative proton transfers occur in solution which... 

Recently-reported pyrimido[4,5-, pyrimidine 168, prepared as in Equation (90), has been reported as a Janus-AT heterocycle, where the molecule is designed to have the hydrogen-bonding characteristics of both a diaminopurine and uracil/thymine <2007JOC466>. Crystallographic studies show that 168 forms a linear formate-bridged supra-molecular array. 

In our case a similar explanation can be advanced. Preliminary IR data (15) indicate that CO can be chemisorbed on catalyst C (RhSnfC g)2/Si02) in a linear and bridged manner, suggesting that the dialkyl tin fragment and CO could be adsorbed on rhodium in a close vicinity. It is also reasonnable to assume that tin is in the divalent oxidation state. In this case, the mechanism of the citral hydrogenation could be the following ... 

Reduction at Pt electrodes and Pt group metals of Ir and Rh in aqueous solution has led to CO production [56-58]. Intermediate-adsorbed species have been studied by Giner in acidic solution at Pt [59], while similar spectroscopic experiments at Pt electrodes have also been conducted by Nicolic et al. [60], It appears that the reduction of C02 occurs by reaction with adsorbed hydrogen atoms to produce both linear and bridge-bonded CO. The reduced C02 species at Pt electrodes, however, caused a rapid poisoning of the electrodes to further C02 reduction [61]. 

Some polymers, such as cellulose, although linear in structure, have such a strong molecular interaction, mostly due to hydrogen bridges and polar groups that they do not soften or melt. Consequently, the transition temperatures as such are less important to this class of polymers. Normally they are highly crystalline, with a crystalline melting point (far) above the decomposition temperature. Their physical behaviour - except for... 

FTIR model experiments were performed to reveal the nature of catalyst deactivation in C02. The spectrum taken at 15 bar in a C02/H2 mixture is shown in Fig. 1. The bands at 2060 and 1870 cm 1 indicate considerable coverage of Pt by linearly and bridge-bonded CO [12], formed by the reduction of C02 on Pt (reverse water gas shift reaction). The three characteristic bands at 1660, 1440 and 1235 cm 1 are attributed to C02 adsorption on A1203, likely as carbonate species [13, 14], It is well known [15] that CO is a strong poison for the hydrogenation of carbonyl compounds on Pt, but can improve the selectivity of the acetylene — olefin type transformations. Based on the above FTIR experiments it cannot be excluded that there are other strongly adsorbed species on Pt formed in small amounts. It is possible that the reduction of C02 provides also -COOH and triply bonded COH, as proposed earlier [16]. 

It had been found that removal of hydrogen from nickel at 350°C, instead of at room temperature, produced a profound difference in the properties of the nickel with respect to ethylene chemisorption (4). Therefore the chemisorption of carbon monoxide was repeated using nickel which had been degassed at 350°C. This produced a startling difference compared to the results shown in Fig. I now most of the carbon monoxide was chemisorbed in the linear structure (5). Similar experiments have not been made with palladium but it is reasonable to predict that the ratio of linear to bridged carbon monoxide would be increased by a more thorough removal of hydrogen from this metal. 

The 17e metal-based radical Ta 3(CO)4(dppe) was formed via hydrogen atom extraction from TaIH(CO)4(dppe).659 In solution, this radical abstacted halogen atoms from many organic halides RX to give TaIX(CO)4(dppe). Ta°(CO)4(dppe) existed in solution as an equilibrium mixture of monomer and dimer [(dppe)(CO)3Ta](/i2-CO)2. The latter is the form stable in the solid state. While no crystal structures are available, DFT calculations indicated a stable pseudo-octahedral stereochemistry for the monomer and several possible (/i2-CO)2 structures for the dimer.656 Each of the latter featured linear, semi-bridging carbonyls supporting a weak, delocalized Ta—Ta interaction. 

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