Ammonia charge distribution

The simplest model uses atom-atom pairwise potentials, as was done for clusters of molecules with rare gas atoms [45] and with other molecules [46]. The Haas group suggested such models for the anthracene- and perylene ammonia adducts, in which an electrostatic interaction [47] was added to account for the charge distribution in the molecules [40], and also as for the larger donors (dimethylaniline). The potential is written as... 

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 asymmetric shape of an ammonia molecule results in an unequal charge distribution and the molecule Is polar. 

The ET-sensitized photoamination of 1,1-diarylethylenes with ammonia and most primary amines yields the anti-Markovnikov adducts. Photoamination of unsymmetrically substituted stil-benes yields mixtures of regioisomers 15 and 16. Modest re-gioselectivity is observed for p-methyl or p-chloro substituents however, highly selective formation of adduct 15 is observed for the p-methoxy substituent (Table 5). Selective formation of 15 was attributed to the effect of the methoxy substituent on the charge distribution in the stilbene cation radical. This re-gioselectivity has been exploited in the synthesis of intermediates in the preparation of isoquinolines and other alkaloids." Photoamination of 1-phenyl-3,4-dihydronaphthalene yields a mixture of syn and anti adducts 17 and 18 (Scheme 5)." Use of bulky primary amines favors formation of the syn adduct (Table 5), presumably as a consequence of selective anti protonation of the intermediate carbanion. 

Another fundamental distinction between H and Li bonds is associated with the charge distributions occasioned by formation of the complex. Szczesniak et al. [191] explored these redistributions by way of spectroscopic atomic charges which act to mimic experimental quantities such as vibrational intensities. They noted that whereas most of the charge extracted from the ammonia in H3N HCl was picked up by the Cl atom, it is the Li atom in H3N LiCl that is the ultimate sink of electron density. The authors were able to discern a relationship also between the total charge transferred from NH3 to the electron acceptor and the calculated intensity of the intermolecular stretch. This finding conforms to the requirement of a changing molecular dipole moment in order to lend intensity to this vibration. 

Figure 8.6 (a) The structure and charge distribution of the ammonia molecule. The polarity of the N—H bonds occurs because nitrogen has a greater electronegativity than hydrogen, (b) The dipole moment of the ammonia molecule oriented In an electric field, (c) The electrostatic potential diagram for ammonia. 

The positive charge resulting from the addition of a proton on to an ammonia molecule is not associated with any particular hydrogen atom, once the bond is formed, and is distributed over the whole ion. 

Although the code is based on well-recognized models referenced in the literature, some of the underlying models are based on "older" theory which has since been improved. The code does not treat complex terrain or chemical reactivity other than ammonia and water. The chemical database in the code is a subset of the AIChE s DIPPR database. The user may not modify or supplement the database and a fee is charged for each chemical added to the standard database distributed with the code. The code costs 20,000 and requires a vendor supplied security key in the parallel port before use. 

---