The geometry of the R1(X=)C(sp2)-N(sp3)R2R3molecular fragment has been studied in 90 X-ray crystallographically determined molecules containing such a group (amides, thioamides, enamines, amidines, anilines, etc.). This tends to be planar owing to the partial double bond character of the C-N bond. In a crystal environment, however, it is often deformed from planarity, the deformation being described by the torsion angle ϒ around C-N (0 ≤ |ϒ| ≤ 90°) and the out-of-plane bending of the amino nitrogen, Xn, ranging from 0° for planar sp2 to 60° for tetrahedral sp3 nitrogen. It is shown that out-of-plane deformations are consistent with two different processes: (a) the simple out-of-plane bending of sp2 nitrogen (a vibrational coordinate) and (b) a more complex motion combining the twisting around the C-N bond and the nitrogen bending (which, according to the structural correlation method, can be considered to map the geometrical changes of the group along the “reaction” path of cis,trans-isomerization of the group itself). An energetic model of the isomerization process, based on molecular mechanics considerations, has been proposed. Proper choice of the parameters in the force field gives rise to an energy map, E(ϒ,Xn), where the experimentally determined deformations are shown to be nicely located along the energy valley leading from the planar conformation to the rotated and pyramidalized transition state. © 1986, American Chemical Society. All rights reserved.

Stereochemistry of the R1(X=)C(sp2)-N(sp3)R2R3 Fragment. Mapping of the Cis-Trans Isomerization Path by Rotation Around the C-N Bond from Crystallographic Structural Data

GILLI, Gastone;BERTOLASI, Valerio;FERRETTI, Valeria
1986

Abstract

The geometry of the R1(X=)C(sp2)-N(sp3)R2R3molecular fragment has been studied in 90 X-ray crystallographically determined molecules containing such a group (amides, thioamides, enamines, amidines, anilines, etc.). This tends to be planar owing to the partial double bond character of the C-N bond. In a crystal environment, however, it is often deformed from planarity, the deformation being described by the torsion angle ϒ around C-N (0 ≤ |ϒ| ≤ 90°) and the out-of-plane bending of the amino nitrogen, Xn, ranging from 0° for planar sp2 to 60° for tetrahedral sp3 nitrogen. It is shown that out-of-plane deformations are consistent with two different processes: (a) the simple out-of-plane bending of sp2 nitrogen (a vibrational coordinate) and (b) a more complex motion combining the twisting around the C-N bond and the nitrogen bending (which, according to the structural correlation method, can be considered to map the geometrical changes of the group along the “reaction” path of cis,trans-isomerization of the group itself). An energetic model of the isomerization process, based on molecular mechanics considerations, has been proposed. Proper choice of the parameters in the force field gives rise to an energy map, E(ϒ,Xn), where the experimentally determined deformations are shown to be nicely located along the energy valley leading from the planar conformation to the rotated and pyramidalized transition state. © 1986, American Chemical Society. All rights reserved.
1986
Gilli, Gastone; Bertolasi, Valerio; Bellucci, F; Ferretti, Valeria
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Utilizza questo identificativo per citare o creare un link a questo documento: https://hdl.handle.net/11392/470625
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