4 research outputs found
QENS and NMR Study of Water Dynamics in SBA-15 with a Low Water Content
In this study, motions performed
by water molecules adsorbed on
the silica surface of SBA-15 material with 6.1% of water content (15%
of pore filling) were investigated using NMR and Quasielastic Neutron
Scattering (QENS) techniques. The results show no sign of translational
diffusion of water, but two types of stochastic localized motions
were identified, and both described using a model of proton jumps
between two sites. For both motions, the characteristic jump distances
and correlation times, as well as activation energies, have been extracted
and found to differ significantly. On this basis, the faster motion
was ascribed to jumps of water molecules between neighboring positions
(<i>d</i> = 2.5 Å, τ = 4 ps at 300 K, and <i>E</i><sub>a</sub> = 5.2 ± 0.2 kJ/mol from NMR data, and
5.6 ± 1.1 kJ/mol from QENS), while the slower one exhibits a
temperature dependent jump distance and was ascribed to jumps of water
molecules between more spatially separated positions (<i>d</i> = 2.9–4.3 Å, τ = 25 ps at 300 K, and <i>E</i><sub>a</sub> = 16.1 ± 0.3 kJ/mol from NMR, and 17.3 ± 0.3
kJ/mol from QENS data)
Computationally Assisted (Solid-State Density Functional Theory) Structural (X-ray) and Vibrational Spectroscopy (FT-IR, FT-RS, TDs-THz) Characterization of the Cardiovascular Drug Lacidipine
The
structural properties of a second-generation dihydropyridine calcium
antagonist, lacidipine, were explored by combining low-temperature
X-ray diffraction with optical vibrational spectroscopy and periodic
density functional theory (PBC DFT) calculations. Crystallographic
analysis cannot discriminate between two possible molecular symmetries
in crystals made of pure lacipidine: the space group <i>Ama</i>2, where the lacipidine molecule lies on mirror symmetry, or a <i>Cc</i> space group with distorted lacipidine molecules. Intermolecular
interactions analysis reveals an infinite net of moderate-strength
N–H···O hydrogen-bonds, which link the molecular
units toward the crystallographic <i>b</i>-axis. Weak interactions
were identified, revealing their role in stabilization of the crystal
structure. The vibrational dynamics of lacidipine was thoroughly explored
by combining infrared and Raman spectroscopy in the middle- and low-wavenumber
range. The given interpretation was fully supported by state-of-the-art
solid-state density functional theory calculations (plane-wave DFT),
giving deep insight into the vibrational response and providing a
complex assignment of spectral features. The vibrational analysis
was extended onto the lattice-phonon range by employing time-domain
terahertz spectroscopy. Analysis of the anisotropic displacement parameters
suggests noticeable dynamics of the terminal (<i>tert</i>-butoxycarbonyl)vinyl moiety. The terahertz study provides
direct experimental evidence of “crankshaft” type motions
in the terminal chain. By combining low-wavenumber vibrational spectroscopy
with the first-principles calculations, we were able to prove that
the quoted thermodynamically stable phase corresponds to the monoclinic <i>Cc</i> space group