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Conformations of Butane

Conformations of Butane

Unlike ethane and propane that have only two major conformations, butane has more than two conformers. The staggered form of butane in which the bulky methyl groups on the two carbons are placed on opposite sides, that is, at a dihedral angle of 180°, is the lowest energy, most stable form — called the anti conformer. This conformation is stabilized due to the absence of steric repulsion between the largely spaced out methyl groups. The other two staggered conformations are...

Conformations of Ethane and Propane

Conformations of Ethane and Propane

In an organic molecule, free rotation about the carbon-carbon single bond results in energetically different conformers of the molecule. Due to this rotation, called the internal rotation, ethane has two major conformations — staggered and eclipsed.
Staggered conformation is a low energy and more stable conformation with the C-H bonds on the front carbon placed at 60°dihedral angles relative to the C-H bonds on the back carbon, leading to a reduced torsional strain. In staggered...

Stability of Conjugated Dienes

Stability of Conjugated Dienes

Introduction
A comparison of the enthalpies of hydrogenation of dienes reveals that conjugated dienes release less heat on hydrogenation, rendering them more stable than their nonconjugated analogs.

Constitutional Isomers of Alkanes

Constitutional Isomers of Alkanes

Organic compounds of the same molecular formula can have different structural formulas called constitutional isomers, and the phenomenon is known as constitutional isomerism. Alkanes with four or more carbons showing multiple structures with the same molecular formula thereby exhibit constitutional isomerism.
The linear isomer of an alkane is prefixed by the term “n”; hence a linear isomer of pentane is known as n-pentane. Based on the type of branching, some of the...

Conformations of Cycloalkanes

Conformations of Cycloalkanes

Adolf von Baeyer attempted to explain the instabilities of small and large cycloalkane rings using the concept of angle strain — the strain caused by the deviation of bond angles from the ideal 109.5° tetrahedral value for sp3 hybridized carbons. However, while cyclopropane and cyclobutane are strained, as expected from their highly compressed bond angles, cyclopentane is more strained than predicted, and cyclohexane is virtually strain-free. Hence, Baeyer’s theory that...

Stability of Substituted Cyclohexanes

Stability of Substituted Cyclohexanes

This lesson discusses the stability of substituted cyclohexanes with a focus on energies of various conformers and the effect of 1,3-diaxial interactions.
The two chair conformations of cyclohexanes undergo rapid interconversion at room temperature. Both forms have identical energies and stabilities, each comprising equal amounts of the equilibrium mixture. Replacing a hydrogen atom with a functional group makes the two conformations energetically non-equivalent.
For example, in...

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Related Experiment Video

Updated: Mar 22, 2026

Computation of Atmospheric Concentrations of Molecular Clusters from ab initio Thermochemistry

Computation of Atmospheric Concentrations of Molecular Clusters from ab initio Thermochemistry

Published on: April 8, 2020

Enthalpy Differences of the n-Pentane Conformers.

József Csontos¹, Balázs Nagy^1,2, László Gyevi-Nagy³

¹MTA-BME Lendület Quantum Chemistry Research Group, Department of Physical Chemistry and Materials Science, Budapest University of Technology and Economics , H-1521 Budapest, P.O. Box 91, Hungary.

Journal of Chemical Theory and Computation

|April 21, 2016

Summary

This study re-evaluates experimental enthalpy differences for pentane conformers using advanced statistical models and high-level quantum chemistry calculations. Results provide benchmark data and suggest experimental biases may exist.

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Area of Science:

Computational Chemistry
Spectroscopy
Chemical Thermodynamics

Background:

Conformational analysis of alkanes is crucial for understanding molecular behavior.
Previous studies highlighted potential biases in experimental data due to statistical modeling.
Accurate determination of energy and enthalpy differences is essential for various applications.

Purpose of the Study:

To re-evaluate experimental enthalpy differences for pentane conformers with an appropriate statistical model.
To perform high-level quantum chemical calculations for accurate energy and enthalpy differences of pentane conformers.
To provide benchmark quality data for pentane conformers across different temperatures.

Main Methods:

Re-evaluation of experimental data using advanced statistical methods.
Coupled-cluster-based focal-point analysis (FPA) for high-accuracy electronic structure calculations.
Inclusion of corrections beyond Born-Oppenheimer and nonrelativistic approximations.

Main Results:

Revised experimental enthalpy differences for pentane conformers with improved uncertainty quantification.
Benchmark energy and enthalpy differences for pentane conformers at 0 K and 298.15 K.
Prediction of a positive shift in experimental enthalpy differences due to a new Raman band.

Conclusions:

The study provides highly accurate computational data for pentane conformers, serving as benchmarks.
Re-evaluation highlights the importance of appropriate statistical models in interpreting experimental spectroscopic data.
Quantum chemical methods can achieve accuracy comparable to or exceeding experimental techniques.