Two effect

Our conclusion is that the AA conformation of POE itself, as with the POE model, is slightly more stable than the other conformations in the gas phase, in terms of energy. The AG conformation, however, becomes more stable in solutions, particularly at higher dielectric constant. Since these conformations are relatively close in energy, the entropy effect will be overwhelming so that POE itself will be a serious conformational mixture. 

But for a-D-mannose, with an axial hydroxyl at C-2, the equilibrium yielded 67% of the a epimer [Reaction (4)]  

It seemed that there was something about the particular kind of conformational structure of P-mannose that destabilized it by about l.Okcal/mol. In other words, there was another effect, which Reeves called the delta-two effect, which was about the same size but in the opposite direction (in this case) from the normal anomeric effect. The conformations about the C-l-C-2 bond for mannose (or other compounds with a p-hydroxyl at C-2) can be written in Newman projections as in Structure 4, looking from C-2 toward C-1  

Note that in the P confonner in Structure 4, the oxygen on C2 is between two other oxygens on Ci. This particular conformation is destabilized by the delta-two effect. 

We have previously discussed the anomeric effect and the gauche effect, where we have geminal (1,1) or vicinal (1,2) hydroxyl groups. Note that in the delta-two effect we have both of these arrangements at the same time (1,1,2). The delta-two effect is, if you will, a measure of the lack of additivity of anomeric and gauche effects. 

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The secondary reactions are series with respect to the chloromethane but parallel with respect to chlorine. A very large excess of methane (mole ratio of methane to chlorine on the order of 10 1) is used to suppress selectivity losses. The excess of methane has two effects. First, because it is only involved in the primary reaction, it encourages the primary reaction. Second, by diluting the product, chloromethane, it discourages the secondary reactions, which prefer a high concentration of chloromethane. 

It thus appears that the flow rate of the nonkey components may account for the diflerences between sequences. Essentially, nonkey components have two effects on a separation. They cause... 

Pump-probe absorption experiments on the femtosecond time scale generally fall into two effective types, depending on the duration and spectral width of the pump pulse. If tlie pump spectrum is significantly narrower in width than the electronic absorption line shape, transient hole-burning spectroscopy [101. 102. 103. 104. 105. 106. 107. 108. 109. 110. 111. 112 and 113] can be perfomied. The second type of experiment, dynamic absorption spectroscopy [57, 114. 115. 116. 117. 118. 119. 120. 121 and 122], can be perfomied if the pump and probe pulses are short compared to tlie period of the vibrational modes that are coupled to the electronic transition. 

The balance of these two effects was found to depend delicately on the stoichiometry, pressure, and temperature. The results were used to develop a more comprehensive C0/H20/02/N0 reaction mechanism, incorporating the explicit fall-off behaviour of recombination reactions [46, 47],... 

For many applications, especially studies on enzyme reaction mechanisms, we do not need to treat the entire system quantum mechanically. It is often sufficient to treat the center of interest (e.g., the active site and the reacting molecules) quantum mechanically. The rest of the molecule can be treated using classical molecular mechanics (MM see Section 7.2). The quantum mechanical technique can be ab-initio, DFT or semi-empirical. Many such techniques have been proposed and have been reviewed and classified by Thiel and co-workers [50] Two effects of the MM environment must be incorporated into the quantum mechanical system. 

Appreciating the beneficial influences of water and Lewis acids on the Diels-Alder reaction and understanding their origin, one may ask what would be the result of a combination of these two effects. If they would be additive, huge accelerations can be envisaged. But may one really expect this How does water influence the Lewis-acid catalysed reaction, and what is the influence of the Lewis acid on the enforced hydrophobic interaction and the hydrogen bonding effect These are the questions that are addressed in this chapter. 

First of all, given the well recognised promoting effects of Lewis-acids and of aqueous solvents on Diels-Alder reactions, we wanted to know if these two effects could be combined. If this would be possible, dramatic improvements of rate and endo-exo selectivity were envisaged Studies on the Diels-Alder reaction of a dienophile, specifically designed for this purpose are described in Chapter 2. It is demonstrated that Lewis-acid catalysis in an aqueous medium is indeed feasible and, as anticipated, can result in impressive enhancements of both rate and endo-exo selectivity. However, the influences of the Lewis-acid catalyst and the aqueous medium are not fully additive. It seems as if water diminishes the catalytic potential of Lewis acids just as coordination of a Lewis acid diminishes the beneficial effects of water. Still, overall, the rate of the catalysed reaction... 

This thesis describes a study of catalysis of Diels-Alder reactions in water. No studies in this field had been reported at the start of the research, despite the well known beneficial effects of acpieous solvents as well as of Lewis-add catalysts on rate and endo-exo selectivity of Diels-Alder reactions in organic solvents. We envisaged that a combination of these two effects might well result in extremely large rate enhancements and improvements of the endo-exo selectivity. 

The rate of the Lewis-acid catalysed Diels-Alder reaction in water has been compared to that in other solvents. The results demonstrate that the expected beneficial effect of water on the Lewis-acid catalysed reaction is indeed present. However, the water-induced acceleration of the Lewis-add catalysed reaction is not as pronounced as the corresponding effect on the uncatalysed reaction. The two effects that underlie the beneficial influence of water on the uncatalysed Diels-Alder reaction, enforced hydrophobic interactions and enhanced hydrogen bonding of water to the carbonyl moiety of 1 in the activated complex, are likely to be diminished in the Lewis-acid catalysed process. Upon coordination of the Lewis-acid catalyst to the carbonyl group of the dienophile, the catalyst takes over from the hydrogen bonds an important part of the activating influence. Also the influence of enforced hydrophobic interactions is expected to be significantly reduced in the Lewis-acid catalysed Diels-Alder reaction. Obviously, the presence of the hydrophilic Lewis-acid diminished the nonpolar character of 1 in the initial state. 

The relative weakness of the two effects, and the adoption of the kinetic form of the catalysis to the linear law only when the concentration of the additive was greater than c. 0-2 mol 1 , results from the equilibria existing in anhydrous nitric acid. In the absence of catalyst,... 

The halogens F Cl Br and I do not differ much in their preference for the equatorial position As the atomic radius increases in the order F < Cl < Br < I so does the carbon-halogen bond dis tance and the two effects tend to cancel... 

It IS not possible to tell by inspection whether the a or p pyranose form of a par ticular carbohydrate predominates at equilibrium As just described the p pyranose form IS the major species present m an aqueous solution of d glucose whereas the a pyranose form predominates m a solution of d mannose (Problem 25 8) The relative abundance of a and p pyranose forms m solution depends on two factors The first is solvation of the anomeric hydroxyl group An equatorial OH is less crowded and better solvated by water than an axial one This effect stabilizes the p pyranose form m aqueous solution The other factor called the anomeric effect, involves an electronic interaction between the nng oxygen and the anomeric substituent and preferentially stabilizes the axial OH of the a pyranose form Because the two effects operate m different directions but are com parable m magnitude m aqueous solution the a pyranose form is more abundant for some carbohydrates and the p pyranose form for others... 

As ions and neutrals evaporate from a heated filament surface, the amount of sample decreases and the surface densities (C, Cq) must decrease. Therefore, Equation 7.1 covers two effects. The first was discussed above and concerns the changing value for the ratio n+/n° as the temperature of the filament is varied, and the other concerns the change in the total number of ions desorbing as the sample is used up. The two separate effects are shown in Figure 7.8a,b. Combining the two effects (Figure 7.8c) reveals that if the temperature is increased to maintain the flow of ions, which drops naturally as the sample is used up (time), then eventually the flow of ions and neutrals becomes zero whatever the temperature of the filament because the sample has disappeared from the filament surface. 

Next we consider the net field at the molecule. This turns out to be the sum of two effects the macroscopic field given by Eq. (10.12) plus a local field that is associated with the charge on the surface of the cavity surrounding the molecule of interest. The latter may be shown to equal (l/3)(aj j/eo). Hence the net field at the molecule is... 

It might be thought that the small number of molecules in a typical supersonic jet or beam would seriously limit the sensitivity of observation of the spectra. Flowever, the severe rotational cooling which may be produced results in a collapsing of the overall intensity of a band into many fewer rotational transitions. Vibrational cooling, which greatly increases the population of the zero-point level, concentrates the intensity in few vibrational transitions, and these two effects tend to compensate for the small number of molecules. 

Electrostatic and Magnetic Effects. These two effects are generally small but may be significant in laboratory weighing. 

Recombination reactions are highly exothermic and are inefficient at low pressures because the molecule, as initially formed, contains all of the vibrational energy required for redissociation. Addition of an inert gas increases chemiluminescence by removing excess vibrational energy by coUision (192,193). Thus in the nitrogen afterglow chemiluminescence efficiency increases proportionally with nitrogen pressure at low pressures up to about 33 Pa (0.25 torr) (194). However, inert gas also quenches the excited product and above about 66 Pa (0.5 torr) the two effects offset each other, so that chemiluminescence intensity becomes independent of pressure (192,195). 

Decoupling has two effects all multiplets are coUapsed into singlets, one for each nonequivalent atom and the transfer of NOE from the excited H to... 

The two effects almost cancel one another to yield an approximation for the minimum work potential used in a distillation (3,4). 

Physical and chemical properties of the three most important forms of sodium sulfate are summarized ia Table 3. The solubiUty of sodium sulfate ia water from 0 to 360°C is shown ia Figure 1 (5). The solubiUty of the NaClNa2S04-H2 0-saturated system is also shown. The aqueous solubiUty of sodium sulfate changes rapidly from 0 to 40°C, and addition of NaCl to a saturated solution of Na2S04 dramatically suppresses this solubiUty. These two effects are exploited by all manufacturers of sodium sulfate. 

Dyes and Indicators. The effects of bromine ia dye or iadicator molecules, ia place of hydrogen, iaclude a shift of light absorption to longer wavelengths, iacreased dissociation of phenoHc hydroxyl groups, and lower solubiHty (see Dyes and dye intermediates). The first two effects probably result from iacreased polarizatioa caused by bromine s electroaegativity compared to that of hydrogea. 

In the analytical chromatographic process, mixtures are separated either as individual components or as classes of similar materials. The mixture to be separated is first placed in solution, then transferred to the mobile phase to move through the chromatographic system. In some cases, irreversible interaction with the column leaves material permanently attached to the stationary phase. This process has two effects because the material is permanently attached to the stationary phase, it is never detected as leaving the column and the analysis of the mixture is incomplete additionally, the adsorption of material on the stationary phase alters the abiHty of that phase to be used in future experiments. Thus it is extremely important to determine the ultimate fate of known materials when used in a chromatographic system and to develop a feeling for the kinds of materials in an unknown mixture before use of a chromatograph. 

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