Large amplitude motions within molecules trapped in solid parahydrogen.

Area of Science:

• Physical Chemistry
• Molecular Spectroscopy
• Quantum Chemistry

Background:

• Intramolecular hydrogen bonds (IHB) significantly influence molecular properties.
• Solid parahydrogen (pH2) offers a unique environment for studying molecular vibrations and large-amplitude motions.
• Nuclear spin conversion (NSC) in methyl groups can impact vibrational spectra.

Purpose of the Study:

• To investigate the vibrational behavior of β-diketones and β-dialdehydes with IHBs in solid parahydrogen.
• To understand the influence of large-amplitude motions, such as methyl torsions, on vibrational modes.
• To explore the effects of deuteration and substitution on IHB dynamics and NSC.

Main Methods:

• Trapping of β-diketone and β-dialdehyde molecules in solid parahydrogen (pH2).
• Infrared (IR) spectroscopy to analyze vibrational modes and nuclear spin conversion (NSC).
• Studying deuterated and chlorinated derivatives to probe specific molecular interactions.

Main Results:

• In acetylacetone (AcAc), H transfer coupled with methyl torsions was observed, with NSC influencing vibrational modes.
• Deuteration of the IHB proton in AcAc complicated spectral analysis and obscured NSC evidence.
• In 3-chloroacetylacetone, H transfer was decoupled from methyl torsion, and NSC effects were undetectable.
• Hydrogen tunneling splitting was observed in a few modes of 2-chloromalonaldehyde, assigned to tunneling levels.

Conclusions:

• Solid parahydrogen is a valuable medium for studying vibrational dynamics and large-amplitude motions in molecules with IHBs.
• The coupling between H transfer and methyl torsions significantly affects molecular vibrational spectra.
• Hydrogen tunneling phenomena can be detected in β-dialdehydes under these conditions, though some effects remain puzzling.