Low-temperature Fourier transform infrared spectra and hydrogen bonding in polycrystalline L-alanine

Area of Science:

• Solid-state chemistry
• Spectroscopy
• Crystallography

Background:

• L-alanine is a fundamental amino acid with a well-defined crystal structure.
• Hydrogen bonding plays a crucial role in the structural and dynamic properties of crystalline amino acids.
• Infrared (IR) spectroscopy is a powerful tool for probing molecular vibrations and interactions.

Purpose of the Study:

• To investigate the vibrational properties of L-alanine using FTIR spectroscopy.
• To elucidate the nature of hydrogen bonds in crystalline L-alanine through temperature and isotopic substitution studies.
• To revise existing literature assignments of the IR spectra of L-alanine.

Main Methods:

• Fourier-transform infrared (FTIR) spectroscopy was employed to record spectra of pure and isotopically substituted (deuterated) L-alanine.
• Measurements were performed over a temperature range of 10–300 K.
• Analysis focused on temperature dependence and isotopic shifts of spectral bands.

Main Results:

• Three distinct bands associated with NH/ND stretching vibrations and hydrogen bonds were identified in doped L-alanine crystals.
• Observed red shifts correlated with hydrogen bond distances, providing an indirect estimation of hydrogen bond enthalpies.
• Low-frequency bands were assigned to the NH3+ group, and band splitting at low temperatures indicated structural reorganization due to proton position changes.

Conclusions:

• The study successfully identified and characterized NH/ND stretching vibrations related to different hydrogen bonds in L-alanine.
• FTIR spectral analysis offers an indirect method for estimating hydrogen bond enthalpies.
• Revised spectral assignments for crystalline L-alanine were proposed based on temperature and deuteration effects.