Ammonia molecular representation

About the same time Beutier and Renon (11) also proposed a similar model for the representation of the equilibria in aqueous solutions of weak electrolytes. The vapor was assumed to be an ideal gas and < >a was set equal to unity. Pitzer s method was used for the estimation of the activity coefficients, but, in contrast to Edwards et al. (j)), two ternary parameters in the activity coefficient expression were employed. These were obtained from data on the two-solute systems It was found that the equilibria in the systems NH3+ H2S+H20, NH3+C02+H20 and NH3+S02+H20 could be represented very well up to high concentrations of the ionic species. However, the model was unreliable at high concentrations of undissociated ammonia. Edwards et al. (1 2) have recently proposed a new expression for the representation of the activity coefficients in the NH3+H20 system, over the complete concentration range from pure water to pure NH3. it appears that this area will assume increasing importance and that one must be able to represent activity coefficients in the region of high concentrations of molecular species as well as in dilute solutions. Cruz and Renon (13) have proposed an expression which combines the equations for electrolytes with the non-random two-liquid (NRTL) model for non-electrolytes in order to represent the complete composition range. In a later publication, Cruz and Renon (J4J, this model was applied to the acetic acid-water system. 

Another way to obtain an analytical expression for V (r) is by a point charge representation of the molecular charge distribution. Such a procedure can be of practical use only if the number of point charges is reasonably limited. In our experience, it is quite difficult to get a sufficiently accurate representation of the charge distribution for mediumsized molecules, whereas for small molecules, like water and ammonia, it is relatively easy to do so. 

The bonding electrons in ammonia are displaced toward the more electronegative nitrogen atom. The bonds do not cancel in the asymmetrical pyramidal shape, so the molecule is polar. The three-dimensional representation, which attempts to show the molecular shape, better suggests the charge displacement toward the nitrogen atom. 

We will see later that the irreducible representations found here provide descriptions of the molecular vibrations that involve the N—H stretching modes of the ammonia molecule. The vibration of the molecule is a collective motion of all the atoms that make it up, and these irreducible representations are describing these collective motions. The vibrations fall into three patterns two are a doubly degenerate pair ( ) and the third a separate single vibration (Ai). 

NH3 dipole moment in the molecular bond representation. The first global description of the dipole moments of ammonia was reported by Marquardt et al. [66] who computed ab initio DMSs of NH3 using the MP2/ aug-cc-pVQZ and MCSCF/aug-cc-pVQZ levels of theory. This was also the first work where the MB-representation was used for describing DMSs of an XY3 molecule, formulated as follows ... 

Fig. 1. Three-dimensional representation of effects of varying electron allocation coefficient (EAC) and uptake hydrogenase activity (Hup) on respiratory costs of ammonia synthesis fixation of molecular nitrogen (see text for assumptions used and basis of calculation of costs).

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