Hydrogen inN-Methylacetamide:  Positions and Dynamics of the Hydrogen Atoms Using Neutron Scattering

Abstract

This work reports neutron diffraction and incoherent neutron scattering experiments on N-methylacetamide (NMA), which can be considered the model building block for the peptide linkage of polypeptides and proteins. Using the neutron data, we have been able to associate the onset of a striking negative thermal expansion (NTE) along the a-axis with a dynamical transition around 230 K, consistent with our calorimetric experiments. Observation of the NTE raises the question of possible proton transfer in NMA, which, from our data alone, still cannot be settled. We can only speculate that intermolecular repulsive forces increase as the O...H distance decreases upon cooling, and that around 230 K the lattice relaxes without observation of an actual proton transfer. However, the existence of a nonharmonic potential, reflected by the behavior of the phonon vibrations together with the observation of NTE, could be justified by the "vibrational" polaron theory in which a dynamic localization of the vibrational energy is created by coupling an internal molecular mode to a lattice phonon. More generally, this work shows that neutron powder diffraction techniques can be very powerful for investigating structural deformations in small peptide systems.

Polymorphism of Paracetamol: A New Understanding of Molecular Flexibility through Local Methyl Dynamics

“…Upon cooling, the interlayer compression in form II is larger than in form I, and the structure expands slightly in the crystallographic a -axis direction (i.e., along the direction close to the molecular axes and dominated by OH···O). In contrast, contraction is observed along the direction of the molecular layers (parallel to c axis) and in the softest direction of the NH···O ( b axis). ,, The observed anisotropy, which can also be described as a uniaxial negative thermal expansion (NTE), in the crystallographic a axis in form II has a marked temperature dependence, which is accentuated at temperatures below 160 K. Such thermal expansion has been previously reported for molecular crystals such as NMA and L-MET and has been interpreted as the result of anharmonicity of the interatomic potential. At the same time, experimental as well as computational results report an almost identical C–H bond distance of the methyl rotor in the monoclinic form I but significant bond length differences in the orthorhombic .…”

Proton Diffusivity in the Protic Ionic Liquid Triethylammonium Triflate Probed by Quasielastic Neutron Scattering

“…The gradual drop of the elastic intensity between 50 and 200 K resembles analogous dependencies of other compounds with end methyl groups, which undergo thermally activated rotation. [34][35][36] The temperature dependence of the relaxation time is then expressed in terms of Arrhenius' law,…”

Structural relaxation in the hydrogen-bonding liquids N-methylacetamide and water studied by optical Kerr effect spectroscopy

“…but lower than the crystalline unit cell volume, measured by neutron diffraction, of approximately 460 Å . 57 Simple liquids often display Debye relaxation modes but structured liquids like NMA and water are likely to be inhomogeneous. The higher structure of water is much debated, but there is a strong consensus that a range of cluster sizes exist and that an individual water molecule experiences a fluctuating range of environments with differing degrees of hydrogen bonding.…”