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A New Exploration of the Torsional Energy Surface of n-Pentane Using Molecular Models and Molecular Modeling Software

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Sérgio E. Galembeck , Giovanni F. Caramori and José Ricardo Romero

Departamento de Química, Universidade de São Paulo, 14040-901 Ribeirão Preto, São Paulo, Brazil

J. Chem. Educ., 2005, 82 (12), p 1800

DOI: 10.1021/ed082p1800

Publication Date (Web): December 1, 2005

Section:

History, Education, and Documentation

Abstract

This article presents a computational experiment about the conformational analysis of n-pentane that can be performed by advanced undergraduate or beginning graduate students. Initially, the four main conformers were determined using molecular models, and their relative stabilities were estimated by the steric repulsions. The structures of these conformers were drawn in molecular graphics software and were optimized utilizing a force field. Vibrational frequencies were also calculated. The results obtained in this study were compared to benchmark calculations and to other force fields. Conformations were also located by conformational search software. From an estimation of the entropy, it was possible to calculate the Gibbs free energy, the relative population of each conformer, and average ³J_HH coupling constants. This experiment exposes the students to several important concepts and techniques such as exploration of the conformational potential energy surface by molecular models or computational techniques and the use of literature to obtain the best available results, experimental or theoretical. It is also able to show that some experimental values represent averages between values for several conformations.

Keywords (Audience):

Upper-Division Undergraduate

Keywords (Domain):

Physical Chemistry

Keywords (Pedagogy):

Computer-Based Learning

Keywords (Subject):

Computational Chemistry

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