Conformational Analysis. XLVI. The Conformational Energies of the Simple Alkyl Groups1-3

1966 ◽  
Vol 31 (3) ◽  
pp. 894-897 ◽  
Author(s):  
Norman L. Allinger ◽  
Leslie A. Freiberg
Keyword(s):  
Conformational Analysis ◽  
Conformational Energies

A Caveat Concerning the Use of the AM1 and PM3 Semiempirical Methods in Calculating Conformational Preferences in Acyclic Amines and Saturated Azaheterocycles

1993 ◽  
Vol 46 (4) ◽  
pp. 547 ◽  
Author(s):  
MJ Shephard ◽  
MN Paddonrow
Keyword(s):  
Conformational Analysis ◽  
Lone Pair ◽  
Conformational Energy ◽  
Semiempirical Methods ◽  
Poor Agreement ◽  
Isopropyl Group ◽  
Butyl Group ◽  
Conformational Preferences ◽  
Nitrogen Lone Pair ◽  
Conformational Energies

A theoretical investigation of the conformational analysis of several acyclic amines, N- alkylated saturated azaheterocycles and alkylcyclohexanes (alkyl = Me, Et, isopropyl, t-butyl) has been carried out by using the MNDO, AM1 and PM3 semiempirical methods. It is found that all three methods correctly predict, qualitatively, the conformational preferences in alkylcyclohexanes, although the PM3 method underestimates the conformational energy for the t-butyl group by as much as 13 kJ/mol. Both AM1 and PM3 overestimate the conformational energy of the nitrogen lone pair to the extent that it exceeds that of the isopropyl group and (in the case of PM3) the t-butyl group, thereby leading to erroneous predictions of favoured conformations in amines and azaheterocycles . Thus, axial N-alkylpiperidines are predicted to be more stable than the equatorial conformers. Thus, application of the AM1 and PM3 methods to the conformational analysis of molecules containing amine functionalities is not recommended. Although the MNDO method gives a qualitative account of the conformational analysis of such molecules, with the exception of hexahydropyridazines and hexahydropyrimidines, the resulting conformational energies and molecular geometries are in poor agreement with experimental data.

Conformational Analysis of C60 Ball-and-Chain Molecules: a Molecular Orbital Study

1996 ◽  
Vol 49 (3) ◽  
pp. 395 ◽  
Author(s):  
MJ Shephard ◽  
MN Paddonrow
Keyword(s):  
Conformational Analysis ◽  
Computational Study ◽  
Hydrocarbon Chain ◽  
Am1 Method ◽  
Methyl Groups ◽  
Chain Molecules ◽  
Molecular Orbital Study ◽  
C 30 ◽  
Bichromophoric Molecules ◽  
Conformational Energies

A computational study of the conformational analysis of a series of C60 ball-and-chain bichromophoric molecules (1)-(6) has been carried out by means of the AM1 and HF/3-21G//AM1 theoretical methods. It is found that the AM1 method underestimates both the relative conformational energies and the magnitude of the energy barriers folded and extended conformers. In contrast, the HF/3-21G//AM1 method gives reasonable values for these quantities. The HF/3-21G//AM1 calculated energy differences between the folded and extended conformers of systems (2)-(5) are less than 5 kJ/mol and the barriers to their interconversion are c. 30 kJ/mol. The conformational bias in these systems may be modified by the judicious placement of methyl groups in the region of the hydrocarbon chain closest to the C60 cage. It is found that the length of the chain and the nature of the non-C60. chromophore have little effect on the conformational energetics.

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