Report No xxxx - Instytut Fizyki JÄ drowej PAN

DEUTERON SPIN-LATTICE RELAXATION STUDY OF D 2 SECLUDED
IN THE SUPERCAGES OF ZEOLITE NAY
Jerzy S. Blicharski 1 , Aleksander Gutsze 2 , Agnieszka M. Korzeniowska,
Zdzisław T. Lalowicz, Zbigniew Olejniczak
H. Niewodniczański Institute of Nuclear Physics, Kraków, Poland; 1 Institute of Physics, Jagiellonian
University, Kraków,Poland; 2 Biophysics Department, Medical Academy, Bydgoszcz, Poland
Nuclear magnetic resonance provides means to study molecular dynamics in every
state of matter. When going from solid state over liquids to gases, besides of molecular
reorientations, also translational diffusion appears. Additionally, molecular rotation becomes
much less quenched by intermolecular interactions. Single molecules of D 2 or CD 4 inserted
into zeolite supercages provide new specific model system for studies of rotational and
translational dynamics. One may have cages of different dimensions, with different wall
features and different filling factors.
At high temperatures molecules fly freely across cages, with the life-time τ J depending
on temperature and cage dimensions. Molecules behave as free quantum rotors and the main
contribution to the relaxation comes from the quadrupole interaction. On decreasing
temperature molecules become adsorbed on cage walls and undergo thermally activated
reorientations characterized by a correlation time τ C = τ 0 exp(E/kT). This process can be
described as surface diffusion taking place under the condition of a distribution of potential
well depth E [1]. One may also introduce the time of residence of a molecule on the surface,
τ ads . An adsorbed molecule may perform many translational jumps before it leaves the
surface.
Main contributions to the spin-lattice relaxation rate 1/T 1 come from quadrupole and
spin-rotational terms [2] for very short τ J at high temperatures:
1/T 1SR = (8π 2 /3) C 2 SR τ J + (3π 2 /2) C 2 Q τ J ,
and the quadrupole relaxation below about 110K:
1/T 1Q = (3 π 2 /10) C 2 Q [J(ω 0 )+4J(2ω 0 )], where: is the mean rotational quantum
number, and J(ω 0 ) = τ C /(1 + ω 2 0 τ 2 C ) is the spectral density function. The coupling constants
C were obtained from molecular beam studies: C SR = 8.25 kHz, C Q = 225.04 kHz [3].
The observed spin-lattice relaxation rate at high temperature indicates that the lifetime
of the free rotor state, appears to be τ J = n τ f , where τ f is the time of a single flight across
the cage. Scattering on the walls is elastic and introduces only a weak perturbation. Factor n
exhibits a linear temperature dependence. On the other hand, when considering the
quadrupole relaxation below 110 K, we get highly reduced value of the effective coupling
ef
constant C Q = 48.3kHz. This can be explained by a very fast reorientation of D 2 molecules
on a cone with an opening angle close to 90 o . Translational diffusion on the cage surface
interrupts such motion by steps involving molecular turns, characterized by the time τ C .
Measurements of the NMR line intensity and line-width provide also interesting new
observations. The spin-conversion between ortho and para spin species of the quantum rotor,
as well as the adsorption on cage walls, are involved processes to be studied.
These results are just highlights of the new research project dedicated to the analysis
of rotational effects in deuteron spin relaxation for D 2 and CD 4 molecules secluded in
zeolites.
This project is supported, during 2002-2005, by the State Committee for Scientific
Research grant No 2 P03B 135 23.
1. H.A. Resing, Advan. Mol. Relaxation Processes, 1, 109 (1967-68)
2. J.S.Blicharski, Acta Phys. Polon. 24, 817 (1963)
3. R.F. Code, N.F. Ramsey, Phys. Rev. A4, 1945 (1971)
14