Related Experiment Video
Updated: Oct 8, 2025
09:37
Imine Metathesis by Silica-Supported Catalysts Using the Methodology of Surface Organometallic Chemistry
Published on: October 18, 2019
9.8K
Alkanes versus Oligosilanes: Conformational Effects on σ-Electron Delocalization
Milena Jovanovic1, Josef Michl1,2
1Department of Chemistry, University of Colorado, Boulder, Colorado 80309-0215, United States.
Journal of the American Chemical Society
|December 29, 2021
Summary
Alkanes and oligosilanes exhibit different sigma-electron delocalization due to electronegativity. Silicon
Area of Science:
- Computational Chemistry
- Materials Science
- Organic Chemistry
Background:
- Understanding sigma-electron delocalization in saturated chains is crucial for predicting molecular properties.
- Alkanes (CnH2n) and oligosilanes (SinnH2n+2) are isoelectronic and isosteric analogues, offering a comparative study platform.
- Conformational dependence of electronic structure influences chemical reactivity and physical characteristics.
Purpose of the Study:
- To investigate and compare the conformational dependence of sigma-electron delocalization in alkanes and oligosilanes.
- To elucidate the underlying electronic reasons for the observed differences in delocalization behavior.
- To develop an intuitive model for understanding these conformational effects.
Main Methods:
- Analysis of approximate algebraic solutions using a Hückel-level extended ladder Hamiltonian model.
- Utilized the effective mass of a hole as a quantitative measure of sigma-electron delocalization.
- Model parametrization involved fitting to results from density functional theory (DFT) and Hartree-Fock (HF) calculations.
Main Results:
- Significant differences in the extent and conformational dependence of sigma-electron delocalization were observed between alkanes and oligosilanes.
- Two delocalization modes identified: conformation-sensitive skeletal delocalization (X-X bonds) and less sensitive lateral delocalization (X-H bonds).
- Alkanes show delocalization in all conformations due to complementary modes; oligosilanes primarily exhibit skeletal delocalization of holes, strongly dependent on conformation.
Conclusions:
- The differing electronegativity between silicon and hydrogen, compared to carbon and hydrogen, is the primary reason for the distinct delocalization patterns.
- A simplified ladder model adequately describes the strong conformational dependence in oligosilanes.
- This study provides fundamental insights into the electronic structure of silicon-based materials and their conformational flexibility.
Related Concept Videos
02:18Conformations of Ethane and Propane
15.5K
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...
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...
15.5K
02:29Conformations of Cycloalkanes
13.0K
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...
13.0K
02:18Constitutional Isomers of Alkanes
20.4K
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...
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...
20.4K
02:47Structure and Bonding of Alkenes
18.3K
Olefins, which are unsaturated hydrocarbons containing one or more carbon–carbon double bonds, are broadly divided into alkenes and cycloalkenes. The general chemical formula of an alkene is CnH2n.
Doubly bonded carbons are sp2 hybridized and have a trigonal planar geometry. The double bond is composed of a σ bond formed by the overlap of hybrid orbitals and a π bond produced by the lateral overlap of unhybridized 2p orbitals on both the carbons. Each carbon atom is...
Doubly bonded carbons are sp2 hybridized and have a trigonal planar geometry. The double bond is composed of a σ bond formed by the overlap of hybrid orbitals and a π bond produced by the lateral overlap of unhybridized 2p orbitals on both the carbons. Each carbon atom is...
18.3K
02:33Physical Properties of Alkanes
12.8K
Alkanes are nonpolar molecules due to the presence of only carbon and hydrogen atoms. The electronegativity difference between carbon and hydrogen is minimal, and hence alkanes have a zero dipole moment. This leads to the presence of only dispersion forces between the molecules. The strength of dispersion forces is dependent on the surface area of the molecules on which they act. Since the surface area increases with the molecular length for straight-chain alkanes, the dispersion forces also...
12.8K
02:01Isomerism in Alkenes
13.2K
Alkenes like 1-butene and 2-butene exhibit constitutional isomerism, as they differ in the position of the double bond. Further, 2-butene exhibits stereoisomerism and exists as two distinct compounds differing in spatial arrangement.
An isomer is called cis-2-butene when the methyl groups are on the same side of the double bond, and the other stereoisomer, in which methyl groups are on the opposite side of the double bond, is called trans-2-butene. The cis and trans stereoisomers are not...
An isomer is called cis-2-butene when the methyl groups are on the same side of the double bond, and the other stereoisomer, in which methyl groups are on the opposite side of the double bond, is called trans-2-butene. The cis and trans stereoisomers are not...
13.2K