Vinylamine geometry

Vinylamine (R1 = R2 = R3 = H) is the simplest primary enamine. It was prepared by gas-phase pyrolysis of ethylamine and characterized by its microwave spectrum which led to a crude determination of its molecular geometry56. The interesting question refers to the extent of planarity or tetrahedral geometry of the amino group and this is discussed in the section on calculations of molecular structures. 

FIGURE 2. Calculated PRDDO geometry of vinylamine (115) type 118 shown in projection onto the CCN plane and dihedral angles shown along the N—C1 and C1=C2 bonds (distances in A, angles in degrees). (Reproduced with permission from Reference 201)... 

TABLE 4. Basis set dependence of calculated geometries of planar vinylamine (type 116) and related molecules. Distances in A and angles in deg... 

Tesseyre and coworkers21 used the CNDO/2 method with optimized geometries to elucidate the protonation pathway for vinylamine and concluded that protonation occurred only by attack on the nitrogen atom they could not envisage the attack on the carbon atom even by considering the solvation effect via Jano s model39. Consequently, they stated that conversion of the enammonium into the iminium form could not take place via an intramolecular process. 

In conclusion, theoretical calculations suggest that, in the equilibrium non-planar geometry vinylamine features a single protonation pathway on the nitrogen the situation can be changed only if the molecular geometry is adequately distorted, presumably by another molecule. 

FIGURE 6. Relative energies (kcal mol -1) and the geometries of the complexes of vinylamine with LiH. Reprinted with permission from J. Am. Chem. Soc., 110, 8360 (1988). Copyright (1988) American Chemical Society... 

Full ab initio optimizations of molecular geometries of enamines (and of any other kind of molecules) depend strongly on the kind of applied basis sets application of STO-3G 2 3, 3-21G 3-2lG 4-3lG 6-3lG 6-31G " and 6-31G basis sets leads to optimizations for the coplanar framework of all atoms of vinylamine, but it was not stated in these references whether coplanarity was assumed by input constraint or not. Contrary to that, the use of a double-zeta basis set with heavy atom polarization functions as well as 6-31 - -G ° based optimization yielded a non-planar amino group for 115. 

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