ENUMERATION OF THE CONFORMERS OF UNBRANCHED ALIPHATIC ALKANES

An effective one-electron quantum chemical method was applied to enume rate the conformers of unbranched aliphatic alkanes. The results obtai ned for butane, pentane, hexane, and heptane were utilized to derive f our rules with which the number and sequences of the existing conforme rs up to undecane could be reproduced. The validity of the rules was c onfirmed at Hartree-Fock and second-order Moeller-Plesset levels too. Full ab initio conformational analyses were performed for the butane, pentane, hexane, heptane; and octane molecules. The rules demonstrate that the most important factors governing the conformational behavior of unbranched aliphatic alkanes are the nonbonded repulsive-attractive (van der Waals) interactions between the hydrogen atoms attached to t he carbon atoms at positions 1,4; 1,5; 1,6; and 1,7. The calculated ga sphase standard heats of formation of the unbranched aliphatic alkanes closely matched the experimental values.