Conformations of allyl amine: theory vs experiment

Area of Science:

• Computational chemistry
• Molecular modeling
• Quantum mechanics

Background:

• Allyl amine is a medium-sized aliphatic molecule with five distinct conformers.
• Understanding the relative stabilities of these conformers is essential for predicting molecular behavior.
• Previous studies may have lacked the computational power or advanced methods to accurately determine these energies.

Purpose of the Study:

• To estimate the relative stabilities of the five conformers of allyl amine.
• To compare theoretical predictions with experimental data.
• To assess the accuracy of various ab initio quantum mechanical methods and basis sets.

Main Methods:

• Application of ab initio quantum mechanical methods: MP2, QCISD, CCSD, and CCSD(T).
• Utilized Dunning correlation consistent basis sets up to aug-cc-pVQZ and cc-pV5Z.
• Calculated relative energies of allyl amine conformers.

Main Results:

• MP2 calculations approached the basis set limit for relative energies.
• Theoretical energies near the MP2 basis set limit were 135 cm-1 (cis-gauche), 103 cm-1 (gauche-trans), and 50 cm-1 (gauche-gauche).
• Experimental energies were 173 ± 12 cm-1 (cis-gauche), 92 ± 8 cm-1 (gauche-trans), and 122 ± 5 cm-1 (gauche-gauche).

Conclusions:

• Accurate predictions of allyl amine conformer energetics require methods incorporating correlation effects and large basis sets.
• Zero-point vibrational energy corrections are critical due to small energy differences.
• Discrepancies between theoretical and experimental results for the gauche-gauche conformer energy remain an area for further investigation.