Pyramidal Inversion in Ammonia

HOMO of aniline provides evidence for or against n bonding between the phenyl ring and the amino group. 

Repeat your analysis for the sequence of structures corresponding to inversion of trimethylamine. Is the inversion barrier smaller, larger or about the same as that in ammonia If significantly different, speculate on the origin of the difference. 

---

The distinction between atomic orbitals and basis functions in molecular calculations has been emphasized several times now. An illustrative example of why the two should not necessarily be thought of as equivalent is offered by ammonia, NH3. The inversion barrier for interconversion between equivalent pyramidal minima in ammonia has been measured to be 5.8 kcal mol However, a HF calculation with the equivalent of an infinite, atom-centered basis set of s and p functions predicts the planar geometry of ammonia to be a minimum-energy structure ... 

The repulsion between the hydrogen atoms could be still further reduced if the molecule became planar. The nitrogen atom would then have sp hybridization (Fig. 3.27c) with three electrons in orbitals which are (js + Ip) and two in the remaining 2p AO. The effective valence state would then be sp. To get here from the tetrahedral configuration would require further promotion of one-fourth of an electron from the 2s to the 2p AO apparently the hydrogen atoms are sufficiently content with their situation when nitrogen is tetrahedral for this further promotion not to be worthwhile. There is therefore a barrier to inversion in ammonia and in its alkyl derivatives of about 6 kcal/mole. This is the energy required to convert the pyramidal molecule into the planar intermediate. 

In stereochemical experiments, it cannot be decided whether the planar form, attained via pyramidal inversion of the chiral intermediates (Scheme 13, first line), is a transition state or an intermediate. By calculation (116, 117), however, it can be shown that 16-electron systems of the type C8H8Mn(CO)2, in contrast to the planar structures of such 18-electron systems as C8H8Co(CO)2, are pyramidal, like ammonia, with a barrier to inversion. This barrier should be higher for an acyl substituent than for an ester substituent, in agreement with the experimental results (117). 

The simpliest and most important molecule with a low barrier to inversion is ammonia, NH3. In its ground electronic state, NH3 has a pyramidal equilibrium configuration with the geometrical symmetry described by the point group C3V (Fig. 1). Configuration B which is obtained from A by the symmetry operation E is separated from A by an inversion barrier of about 2000 cm . A large amplitude... 

We shall discuss in this section large-amplitude motions which are either inversion as in ammonia, or are related to this type of motion. We shall concentrate on the so-called pyramidal inversion which occurs in molecules with tricoordinate atom whose stable position is not in the plane defined by the three atoms directly bounded to it. 

Phosphine, PH3, is the simpliest molecule next to ammonia, NH3, with pyramidal inversion. An inversion doubling has long been suspected in PH3. Costain and... 

The inversion barrier is found to be an example of a Case II energy change for the molecules NH3, PH3, HjO", and HjO (Table 6.5). Only the first and last of these molecules is discussed in detail. The A-H internuclear separations in ammonia and water are found to decrease when the pyramidal or bent geometry is transformed into its respective planar or linear form, by 0.0130 A in NH3 and by 0.0180 A in HjO. The approach of the protons towards the A nucleus, while resulting in a lowering of the electron-nuclear... 

The bonding in amines is similar to that in ammonia. The nitrogen atom is sp -hybridized, the three substituents are directed to three corners of a tetrahedron, and the lone pair of nonbonding electrons occupies the fourth corner of the tetrahedron. An interesting feature of this tetrahedral structure is that amines undergo a rapid pyramidal inversion, which interconverts mirror-image structures. 

The primary structural determinant is therefore seen to be the identity of the Group 15 element. Nitrogen is different from P and As in its tendency to pyramidalize, as measured by inversion barriers and by bond angles. The inversion barrier in ammonia is 5 kcal/mol, and it is 30 kcal/mol in phosphine and arsine. The inversion of the trans bent form (which goes through the planar form)... 

As a freely ionic, monomeric species, the silyl anion may undergo pyramidal inversion about the silicon center (equation 1). For the parent system H3Si , Nimlos and Ellison have obtained quantitative information about the inversion barrier from the photoelectron spectrum in the gas phase2. The photoelectron spectrum could be simulated by a model of the vibrational frequency as a linear oscillator perturbed by a Gaussian barrier. The out-of-plane angle (the deviation of one H from the plane defined by Si and the other two Hs) was found to be 32 2° and the barrier to inversion 9000 2000 cm 1 (26 6 kcal mol-1). This is the only experimental measurement to date of the barrier to inversion about trivalent, negative silicon. The anion was produced by reaction of silane (SiH4) with ammonia, and the photoelectron spectrum of the m/z 31 peak was then recorded. 

The inversion barrier of phosphine has not been determined experimentally. The best quantum chemical calculations yield a barrier of 141 kJ mol , i.e. more than five times higher than in ammonia [2]. Since the barrier to internal rotation in Si2H is smaller than in ethane, it may seem surprising that the inversion barrier of PH3 is higher than in NH3. The reason may be that the pyramidal structure of NH3 is significantly destabilized by repulsion between the H atoms. 

H3O+ is isoelectronic with the ammonia molecule NH3 and has a similar trigonal pyramidal shape. The HOH valence angle is slightly larger than the corresponding angle in NH3 (ZHNH = 107°) and the inversion barrier (11 kJ mol ) in less than half the barrier in ammonia, perhaps because 0-H bond distance is about 5 pm shorter than the N-H bond distance. 

For ammonia and amines the energy barrier for pyramidal inversion is around 25kJmol This precludes, for instance, the isolation of optically active vV-stereogenic amines, except in especial cases. 

As a result of pyramidal inversion, a chiral amine quite literally turns itself inside out, like an umbrella in a strong wind, and in the process becomes a racemic mixture. The activation energy for pyramidal inversion of simple amines is about 25 kj (6 kcal)/mol. For ammonia at room temperature, the rate of nitrogen inversion is approximately 2 X 10" s. For simple amines, the rate is less rapid but nonetheless sufficient to make resolution impossible. 

---