Benzene-water complex

A few experimental measurements of the interaction energy of the benzene-water complex (Eo) have been reported [78-80]. The most recently reported experimental Eo value is - 2.44 0.09 kcal/mol. The calculated AZPE value is 1.0 kcal/mol [81]. The Eq and AZPE values lead to an Ee value of - 3.4 kcal/mol. The OH/tt interaction is substantially weaker than the hydrogen bond of the water dimer (- 5 kcal/mol). 

Recently reported CCSD(T)-level interaction energies of a few orientations of the benzene-water complex show that the MP2 calculations slightly... 

Fig. 14 Orientation dependence of the interaction energy for the benzene-water complex. Emp2 is the total interaction energy. es is the electrostatic energy...

Supercomputers become more and more useful, and the Insights they can generate become more and more unique, as the complexity of the system modelled Is Increased. Thus Interfaclal phenomena are a very natural field for supercomputation. In addition to the examples In this volume It may be useful to mention the work of Llnse on liquid-liquid benzene-water interfaces, which he studied with 504 H2O molecules, 144 CgHg molecules, and 3700 Interaction sites. He generated over 50 million configurations In 56 hours on a Cray-lA, and he was able to quantitatively assess the sharpness of the Interfaclal density gradient, which Is very hard to probe experimentally. Similarly Spohr and Helnzlnger have studied orientational polarization of H2O molecules at a metallic Interface, which is also hard to probe experimentally. 

The DTA results for the benzene-water-hydrogen chloride system are summarized in Table II and appear to be much less complex than the butanediol results. 

There are, however, examples indicating that in ion molecule reactions between a protonated species (AH+) and benzene (B), two isomeric forms of the intermediate complex may exist (AH+)(B) and (A)(BH+) [74,286]. In the cases of water [287] and propene [74], quantum chemical calculations clearly indicate that the former corresponds to a n complex where A-H acts as a hydrogen bond donor towards the centre of the benzene ring, while the latter is a hydrogen bonded complex between the benzenium ion and A. In neither case has a barrier been located, but is probably rather low in both cases. The role of the n complex has still not been clarified, since direct downhill routes from the reactants to the a complex exist. It has been pointed out that n complex formation between a pro electrophile and the substrate may be important in solution and in biological systems for molecular recognition purposes. In such cases the proelectrophile is activated to form the actual electrophile subsequent to n complexation, thereupon giving rise to the a complex. This has been shown by quantum chemistry to provide a reasonable scenario for the reaction between HF and benzene, in which BF3 is ultimately required to promote ion formation of the HF/benzene tt complex [288]. 

In several cases, the in situ formation of hydrogen peroxide is the first step of the process. Thus, phenol can be obtained from benzene, carbon monoxide (5 atm) and oxygen (65 atm) at 70 °C in a benzene-water-methyl isobutyl ketone mixture, with TS-1 and a palladium complex as catalysts [26]. Despite a 91% selectivity to phenol, benzene conversion (3.2%) and productivity are still too low for industrial application. The palladium complex is required to promote hydrogen peroxide formation upon reaction of oxygen, carbon monoxide and water [27[. 

Herriott and Picker also note that the crown ether dicyclohexyl-18-crown-6 functions as an efficient phase-transfer catalyst in the benzene-water system. They suggest that some of the catalytic effects reported for this crown ether may result from phase-transfer catalysis rather than complexation of cations. 

For our second nonideal system, we look at a process that has extremely nonideal VLB behavior and has a complex flowsheet. The components involved are ethanol, water, and benzene. Ethanol and water at atmospheric pressure form a minimum-boiling homogeneous azeotrope at 351K of composition 90mol% ethanol. Much more complexity is introduced by the benzene/water system, which forms two liquid phases with partial miscibility. The flowsheet contains two distillation columns and a decanter. There are two recycle streams, which create very difficult convergence problems as we will see. So this complex example is a challenging simulation case. 

Tsionski and coworkers extended the study to the electron transfer through a lipid monolayer at a benzene-water interface [67]. The electron transfer reactions between the oxidized form of zinc porphyrin (ZnPor+) in a benzene phase and the reduced form of a metal complex (R,... 

Navas-Diaz et al. (1993) quantified low nanogram levels of thiamine, riboflavin, and niacin by fiber-optic fluorodensitometry after silica gel HPTLC with methanol-water (7 3) mobile phase. Perisic-Janjic et al. (1995) used TLC for the analysis of the B-complex vitamins in some commercial pharmaceutical B-complex vitamin products. The stationary phase used was a newly synthesized carbamide formaldehyde polymer (aminoplast) with mobile systems 1-butanol-water-acetone (25 9 5) and 1-butanol-methanol-benzene-water (20 10 10 8). Comparative studies examined the B-complex vitamins on cellulose thin layers. 

The effects of oxygen-surface groups, on this graphite, on the enthalpies of immersion using benzene, water and methanol were also studied by Barton et al. (1972,1975). In addition, Rodriguez-Reinoso et al. (1997) studied carbons from olive stones, activated to 37 wt% bum-off in steam at 730 °C, and finally oxidized to several extents with nitric acid (6N) to place surface oxygen complexes on the surfaces. This series of carbons was then heated in the range 100-900 °C (10 samples in all). 

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