Conformational effects on optical rotation. 2-Substituted butanes.

Area of Science:

• Computational Chemistry
• Organic Chemistry
• Spectroscopy

Background:

• Understanding the relationship between molecular structure and optical activity is crucial in stereochemistry.
• Specific rotation, a key property, is influenced by various molecular factors including conformation and electronic properties.

Purpose of the Study:

• To computationally investigate the specific rotations of 2-substituted butanes as a function of their torsion angles.
• To experimentally determine the temperature dependence of specific rotations for 2-methylbutyronitrile and 2-chlorobutane.
• To compare computational results with experimental data and assess the influence of substituents and double bonds on optical rotation.

Main Methods:

• Density functional theory (DFT) calculations using the B3LYP/aug-cc-pVDZ level of theory.
• Analysis of specific rotations across a range of C-C-C-C torsion angles.
• Experimental measurement of specific rotations at varying temperatures.

Main Results:

• Calculated specific rotations for 2-fluoro-, 2-chloro-, 2-cyano-, and 2-ethynylbutane showed remarkable similarity despite differing electronic transition energies.
• Experimental temperature-dependent studies on 2-methylbutyronitrile and 2-chlorobutane aligned well with B3LYP/aug-cc-pVDZ computational predictions.
• Observed specific rotations for 2-chlorobutane were comparable to those of 3-chloro-1-butene, suggesting minimal impact of a double bond on optical rotation.

Conclusions:

• The specific rotation of 2-substituted butanes is largely independent of electronic transition energies and is primarily governed by conformational factors.
• Computational methods accurately predict the conformational dependence of specific rotation.
• The presence of a double bond has a limited effect on optical rotation values, although it can introduce discrepancies between calculated and observed rotations.