Empirical conformational energy study of the diastereomeric dipeptides MeCO-L-Ala-L-Ala-NHMe and MeCO-L-Ala-D-Ala-NHMe: an approach to the conformational analysis of stereocopolypeptides

1983 ◽  
Vol 16 (3) ◽  
pp. 476-482 ◽  
Author(s):  
Vincenzo Rizzo ◽  
Gian Paolo Lorenzi
Keyword(s):  
Conformational Analysis ◽  
Conformational Energy

Conformational analysis of coordination compounds: R-N,N,N',N'-Tetrakis-(2'-aminoethyl)-1,2-diaminopropanecobalt(III)

1967 ◽  
Vol 20 (11) ◽  
pp. 2395 ◽  
Author(s):  
JR Gollogly ◽  
CJ Hawkins
Keyword(s):  
Conformational Analysis ◽  
Metal Complexes ◽  
Coordination Compounds ◽  
A Priori ◽  
Van Der Waals ◽  
Energy Difference ◽  
Conformational Energy ◽  
Van Der Waals Interactions ◽  
Large Energy

The stereospecificity of the ligand, R-N,N,N?,N?-tetrakis(2?- aminoethyl)-1,2-diaminopropane, when coordinated as a sexadentate chelate to cobalt(III), has been investigated by an a priori calculation of the conformational energy difference between the various possible absolute configurations of the complex. It has been shown that the L isomer is more stable than the D isomer by an extremely large energy difference which is due mainly to van der Waals interactions. Some of the terms which contribute to conformational energy differences between metal complexes have not been considered previously.

Conformational effect in the stannous chloride debromination of sym-tetrabromoethane

1967 ◽  
Vol 45 (10) ◽  
pp. 1161-1164 ◽  
Author(s):  
W. K. Kwok ◽  
Sidney I. Miller
Keyword(s):  
Transition State ◽  
Conformational Analysis ◽  
State Energy ◽  
Energy Analysis ◽  
Stannous Chloride ◽  
Conformational Energy ◽  
Present System ◽  
Transition State Energy ◽  
Conformational Effect ◽  
Conformational Energy Analysis

The initial ratios of cis- to trans-dibromoethene product in the stannous chloride reduction of sym-tetrabromoethane in dimethylformamide were 1.51 ± 0.04 at 25°, 1.36 ± 0.03 at 50°, and 1.20 ± 0.03 at 75°. Transition state energy differences in the trans–gauche tetrabromoethane rotamers are estimated as (Ht≠ – Hg≠) = 480 and (Ft≠ – Fg≠) = 244 cal/mole at 25°. A conformational energy analysis was performed. We have shown how a rate-equilibrium parallelism can be adapted to a conformational analysis for systems of this type, and how α, a constant between zero and unity, can be used to characterize the transition state relative to reactants and products (eq. [8]). In the present system, however, there is no rate-equilibrium parallelism and α cannot be defined.

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.

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