Orientation and dynamics of water molecules in beryl Vojt ech Chlan1aMartin Adamec1 2Helena St ep ankov a1Victor G. Thomas3and Filip Kadlec2

2025-04-29 0 0 5.64MB 27 页 10玖币
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Orientation and dynamics of water molecules in beryl
Vojtˇech Chlan,1, a) Martin Adamec,1, 2 Helena ˇ
Stˇep´ankov´a,1Victor G. Thomas,3and
Filip Kadlec2
1)Charles University, Faculty of Mathematics and Physics,
Department of Low Temperature Physics, V Holeˇsoviˇck´ach 2, 180 00 Prague 8,
Czech Republic
2)Institute of Physics of the Czech Academy of Sciences, Na Slovance 2,
182 00 Prague 8, Czech Republic
3)V.S. Sobolev Institute of Geology and Mineralogy SB RAS, 630090 Novosibirsk,
Russia
Behavior of individual molecules of normal and heavy water in beryl single crystals
was studied by 1H and 2H nuclear magnetic resonance spectroscopy. From tempera-
ture dependences of the spectra we deduce that type-I water molecules embedded in
the beryl voids are oriented quite differently from the view established in the litera-
ture. Namely, contrary to earlier assumptions, their H-H lines deviate by about 18◦
from the hexagonal axis. We suggest that this is due to the molecules attaching to the
oxygen atoms forming the beryl structural voids by a hydrogen bond. Our analysis
shows that the molecules perform two types of movement: (i) rapid librations around
the axis of the hydrogen bond, and (ii) less frequent orientational jumps among the
twelve possible binding sites in the beryl voids. The frequencies of the librational
motions are evaluated from a simple thermodynamic model, providing a good quanti-
tative agreement with the frequencies of librations from optical experiments reported
earlier.
a)Electronic mail: vojtech.chlan@mff.cuni.cz
1
arXiv:2210.11160v1 [physics.chem-ph] 20 Oct 2022
I. INTRODUCTION
A. Water confined in beryl
Water confined in nanoscale volumes manifests many unusual properties and has recently
drawn a considerable attention. Among the known corresponding structures, hydrated beryl
Be3Al2Si6O18 is a system attracting a broad interest where tendencies to low-temperature
ferroelectric ordering of the crystal water were clearly demonstrated.1,2 In fact, the water
molecules can occupy regularly spaced crystal sites which are enclosed by oxygen atoms
(see Fig. 1). These sites define well the molecules’ positions; in contrast, their angular ori-
entations are generally variable. Note also that, as a rule, only a partial beryl hydration
is achieved. Thus, in this system, unlike in common condensed phases of water, hydrogen
bonding among the water molecules is suppressed, and they interact predominantly via elec-
tric dipole–dipole interactions involving their dipole moments of 1.85 D (6.17 ×10−30 C m).
Incipient ferroelectricity has been documented especially by observations of collective vibra-
tions of the water molecules, producing a ferroelectric soft phonon mode. This soft mode was
observed in the THz-range spectra of dielectric permittivity, obeying the usual Curie-Weiss
and Cochran temperature dependences.2In the reported case, a negative Curie temperature
of TC≈−20 K was determined, so no ferroelectric state could be achieved. Additionally, at
temperatures below about 20 K, the phonon no more softened; instead, its frequency was
leveling off, which has been attributed to quantum tunneling.3,4 For the above reasons, hy-
drated beryl has been one of the most studied crystal systems featuring confined water.1–7
Owing to its well defined geometry and interesting observed phenomena, hydrated beryl can
serve as a model structure for more detailed spectroscopic and theoretical studies, with the
aim of improving the current knowledge of the underlying phenomena which may favor or
suppress the ordering of confined water molecules.
The crystal structure of beryl is hexagonal (space group P6/mcc), and it contains channels
of voids running along its hexagonal axis. Each of these voids may accommodate one water
molecule. For more than fifty years, the water molecules have been supposed to take up
two possible types of orientations within the voids, as hypothesized first by Wood and
Nassau based on their infrared spectra analysis.10 They concluded that the molecules will
orient themselves with the H–H lines oriented either parallel to the hexagonal axis (“type-I
2
FIG. 1. View of the crystal structure of beryl along the hexagonal axis c. The thin lines denote
the unit cell whereas the yellow circles indicate the possible positions of the water molecules within
the structural voids. The structural data8was visualized using the Vesta software.9
water”), or perpendicular to it (“type-II water”); the latter type should be present especially
in crystals with an additional doping, as the oxygen may bind to an impurity atom located
within the channel. The earlier studies dealing with the interactions among the water
molecules and their collective dynamics assumed the type-I molecules to rotate around
the hexagonal axis of beryl, so the molecules’ planes remained parallel to the hexagonal
axis. At the same time, it was supposed that the molecules are subjected, in their angular
orientations, to a local potential exhibiting six equivalent minima separated by angles of
60◦.3–6,11 Whereas the properties of the molecular ensemble were studied quite extensively—
see the above references—, the orientations and dynamics of the individual molecules in
the voids have been still less explored, despite being crucial for the bulk properties. Such
local, molecular-level information can be provided namely by Nuclear Magnetic Resonance
(NMR) spectroscopy.
3
B. NMR spectroscopy of water in beryl
Water molecules are expected to produce a single peak in the 1H NMR spectrum, be-
cause the spin of the 1H nucleus (proton) is ½and both hydrogen atoms are equivalent
due to symmetry. The peak may become split and/or shifted when anisotropic interactions
are involved, e.g., dipolar interactions or anisotropy of chemical shielding. In cases when
the water molecules are highly mobile, such as in liquid or gas phases, the effect of these
anisotropic interactions on the NMR spectrum gets averaged by the fast molecular reorien-
tations (motional narrowing). Then, the 1H nuclei in water molecules are exposed to the
same, averaged local fields and the 1H NMR spectrum of water consists of a single, usually
very narrow peak.12
Water molecules in solids have their dynamics significantly restricted. When the
molecules are static or their reorientations are very slow (e.g., in ice or as a water of
crystallization), the anisotropic interactions are not fully averaged and the NMR spectrum
comprises contributions from all arrangements of the molecule present in the sample. For
single crystals, the NMR spectra depend on the crystal orientation with respect to B0,
while the NMR spectra of powders with randomly oriented grains display powder-pattern
features.12 In terms of magnitude, the anisotropy of chemical shielding of 1H in water
molecule ranges from 19 to 35 ppm for various phases of water,13 which for B0∼10 T cor-
responds to shifts or splittings of peaks in the 1H spectrum of about 10 kHz. The effect
of dipolar interactions between 1H nuclei within the water molecule may be an order of
magnitude larger, the dipolar interaction is thus often the dominant source of anisotropy in
1H NMR spectra of solids.12
The character of the 1H NMR spectrum of water molecules enclosed in the crystal voids is
different from the two cases presented above. The confined molecules are not static as in ice,
and the molecular reorientations are not isotropic as in liquid water. Thus, the anisotropic
interactions are not fully averaged and the 1H NMR spectrum of water consists of more
than one peak. This is the case of water molecules confined in the voids of beryl crystal,
which was first studied using NMR by Par´e and Ducros14 and by Sugitani et al.15 already
in 1960s. In both these works, the 1H NMR spectra comprise doublet of peaks arising due
to the nuclear dipolar interaction between the 1H nuclei within the water molecules. The
signal was attributed to type-I water. Moreover, the observed dipolar splitting increased
4
linearly with decreasing temperature and saturated below 100 K. The natural interpretation
then was that the water molecules oscillate around their equilibrium positions when H–H
line is parallel to the hexagonal axis, and that the amplitudes of oscillations increase with
temperature.14
The idea that the type-I water molecules are simply oscillating around the hexagonal axis
is, however, not entirely correct, since even at the lowest temperatures the observed dipolar
splitting does not reach the expected value which is precisely given by the distance between
the 1H nuclei within the water molecule. As we show in this paper, the movements of water
molecules must be more complex. This deduction is based on measuring and analyzing the
dipolar splitting in 1H NMR quantitatively, and especially by measuring and analyzing 2H
NMR in beryl hydrated by heavy water.
The spin quantum number of the 2H nucleus equals 1 and so, in the presence of an external
magnetic field, two transitions between the nuclear energy levels of the Zeeman multiplet
are observable. Thus, the 2H NMR spectrum of heavy water in beryl is expected to comprise
two doublets of peaks, one for each 2H nucleus. In contrast to the 1H case, the 2H dipolar
interaction is small and a quadrupolar splitting occurs due to an interaction between the
electric quadrupole moment of the 2H nucleus and the gradient of surrounding electric fields.
The quadrupole splitting depends on the direction of the external magnetic field vector B0
with respect to the axes of the electric field gradient (EFG) tensor. Whereas in the 1H case
it is the orientation of the H–H line that determines the magnitude of dipolar splitting, in
case of 2H the orientation of the O–H bond becomes important instead, since in the H2O
molecule the principal axis of the EFG tensor at the hydrogen site points approximately
along the O–H bond. This is an essential difference between the NMR interactions in H2O
and D2O that allows for extracting more complete information about the orientation of
water molecule in the beryl voids: the isotopes 1H and 2H serve as two completely different
probes. Since the values of dipolar splitting in 1H and quadrupole splitting in the 2H NMR
spectra depend significantly on the orientations of water molecules, we can evaluate the
orientation and dynamics of the water molecules by analyzing the temperature dependences
of the measured dipolar and quadrupolar splittings.
5
摘要：

OrientationanddynamicsofwatermoleculesinberylVojtechChlan,1,a)MartinAdamec,1,2HelenaStepankova,1VictorG.Thomas,3andFilipKadlec21)CharlesUniversity,FacultyofMathematicsandPhysics,DepartmentofLowTemperaturePhysics,VHolesovickach2,18000Prague8,CzechRepublic2)InstituteofPhysicsoftheCzechAcademyo...

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