Large amplitude motions within molecules trapped in solid parahydrogen.

Molecules of the β-diketone and β-dialdehyde families were trapped in solid parahydrogen (pH2) to investigate the vibrational behavior of systems containing an intramolecular hydrogen bond (IHB). In the simplest β-diketone, acetylacetone (AcAc), H transfer related to the IHB is coupled with methyl torsions. In pH2, the study of nuclear spin conversion (NSC) in methyl groups allows the characterisation of the influence of these large amplitude motions on the vibrational modes.

Molecular Recognition by a Short Partial Peptide of the Adrenergic Receptor: A Bottom-Up Approach.

Receptor-neurotransmitter molecular recognition is key for neurotransmission. Although crystal structures of the receptors are known, the mechanism for recognition is not clear. Reported here is the ultraviolet (UV) and infrared (IR) spectra of complexes between a partial peptide (SIVSF), mimicking the binding motif of a catechol ring in the adrenergic receptor, and various ligands.

Nuclear spin conversion to probe the methyl rotation effect on hydrogen-bond and vibrational dynamics.

A noteworthy example of a molecule with coupled large-amplitude motions is provided by acetylacetone (methyl group torsions and intramolecular hydrogen bonds). The molecule was trapped in solid parahydrogen to investigate the complex proton tunneling processes. Nuclear spin conversion in methyl groups is observed and, combined with IR spectra, documents the coupling between high frequency modes and large amplitude motions.

Photochemistry of acetylacetone isolated in parahydrogen matrices upon 266 nm irradiation.

The photochemistry of the chelated enol form of acetylacetone (AcAc) was investigated by UV excitation of the S(2) state at 266 nm in parahydrogen matrices, complemented by experiments in neon and normal hydrogen matrices. Infrared (IR) spectroscopy, combined with theoretical calculations, was used to identify the photoproducts. Isomerization towards various non-chelated forms (no intramolecular H-bond) of AcAc is the dominant channel whereas fragmentation is very minor.