The weak fundamental NH-stretching transition in amines
Sidsel D Schrøder1, Anne S Hansen1, Jens H Wallberg1
1Department of Chemistry, University of Copenhagen, Universitetsparken 5, Copenhagen Ø DK-2100, Denmark.
Spectrochimica Acta. Part A, Molecular and Biomolecular Spectroscopy
|November 5, 2016
Summary
Researchers measured NH-stretching transition intensities in various amines using spectroscopy. Aniline showed typical intensity decreases with excitation, while other amines had weak fundamental transitions, unlike those with conjugated double bonds.
Area of Science:
- Molecular Spectroscopy
- Computational Chemistry
- Physical Chemistry
Background:
- Understanding the vibrational properties of amine functional groups is crucial in chemistry.
- Previous studies have explored NH-stretching vibrations, but detailed intensity analysis across various amine structures is less common.
Purpose of the Study:
- To experimentally measure and theoretically calculate the absolute intensities of NH-stretching transitions in gas-phase amines.
- To investigate the relationship between molecular structure, particularly conjugation, and the intensity of NH-stretching vibrations.
- To support spectral assignments using advanced computational methods.
Main Methods:
- Employed long path length conventional absorption spectroscopy for gas-phase amine samples.
- Utilized the local mode model for theoretical calculations of transition frequencies and intensities.
- Performed ab initio calculations at the CCSD(T)-F12a/VDZ-F12 level to obtain local mode parameters and dipole moment functions.
Main Results:
- Aniline exhibited a typical intensity decrease (order of magnitude) for successive NH-stretching vibrational excitations.
- Methylamine, ethylamine, cyclopropylamine, methylethylamine, diethylamine, and pyrrolidine showed weak fundamental NH-stretching transition intensities, comparable to or weaker than their first overtones.
- A correlation was observed: amines with a normal fundamental intensity often possess a conjugated double bond adjacent to the amine group.
Conclusions:
- The study provides a detailed spectroscopic and computational analysis of NH-stretching transitions in diverse amines.
- Structural factors, especially conjugation, significantly influence the intensity patterns of NH-stretching vibrations.
- The findings offer insights into the vibrational dynamics and electronic structure of amines.
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