Ammonia electron repulsion

The steric number identifies how many groups of electrons must be widely separated in three-dimensional space. In ammonia, for example, the nitrogen atom bonds to three hydrogen atoms, and it has one lone pair of electrons. How are the three hydrogen atoms and the lone pair oriented in space Just as in methane, the four groups of electrons are positioned as far apart as possible, thus minimizing electron-electron repulsion. 

Also, we can explain on the basis of electron repulsions why the bond angle in phosphine, PH3 (93°), is less than that in ammonia, NH3 (107.3°),... 

The inversion barrier of ammonia is repulsive dominant Fee and Fnn increase more in the TS than the attractions Vne, the largest variation being that of Fee- Thus NH3 is pyramidal in part because the lone-pair repels the N—H bonding electrons. 

Inversion, Non-equivalence, and Configuration.—Analysis of SCF-LCAO calculations indicates that whereas the inversion of ammonia is dominated by electronic repulsions, the inversion of phosphine is controlled by nuclear repulsion. The phosphorus f/-orbital functions markedly affected the properties of both the pyramidal and planar states. ... 

The three molecules of interest are methane (4A), ammonia (6A), and water (7A), shown first in the Lewis electron dot representations. Using the VSEPR model, these three molecules are drawn again using the wedge-dashed line notation. Methane (CH4, 4B) has no unshared electrons on carbon but there are electrons in the C-H covalent bonds. Assume that repulsion of the electrons in the bonds leads to a tetrahedral arrangement to minimize electronic repulsion. Ammonia (H3N, 6B) has a tetrahedral array around nitrogen if the electron pair is taken into account. If only the atoms are viewed, however, 6B has the pyramidal shape shown. Water (HOH, 7B) has two electron pairs that occupy the corners of a tetrahedral shape, as shown. 

Figure 1-20 Bonding and electron repulsion in ammonia and water. The arcs indicate increased electron repulsion by the lone pairs located close to the central nucleus.

One final example worth mentioning is the reductive alkylation/arylation with lithium and alkyl/aryl halides in liquid ammonia. This is a two-step process in which negatively charged nanotubes are formed via electron transfer from the metal. This step is relatively easy and fast due to the CNTs electron sink properties, and it enables exfoliation of the tubes through electrostatic repulsion in the second stage, the alkyl/aryl halides react with the charged tubes to form a radical anion which can dissociate into the alkyl radical and the halide anion, with the former species undergoing addition to the CNT sidewalls [42]. 

The difference in repulsive forces between electron pairs means that when lone pairs are present the geometries change. Let s examine two common substances to see how the presence of lone pairs affects the geometries of molecules. Ammonia, NH3, contains three bonding pairs and one lone pair surrounding the nitrogen atom ... 

Fig. 5. The pseudo-Jahn-Teller effect in ammonia (NH3). (a) CCSD(T) ground state potential energy curve breakdown of energy into expectation value of electronic Hamiltonian (He), and nuclear-nuclear repulsion VNN. (b) CASSCF frequency analysis of pseudo-Jahn-Teller effect showing the effect of including CSFs of B2 symmetry is to couple the ground and 1(ncr ) states to give a negative curvature to the adiabatic ground state potential energy surface for the inversion mode.

---