Wydział Chemii

L. Latanowicz

Wojciech Medycki

Ryszard Jakubas

2005

Journal of Physical Chemistry A

109

3097-3104

10.1021/jp045080y

Naukowa

Angielski

Artykuł

This paper reports the temperature dependence of the relaxation time T₁ (55.2 and 90 MHz) and the second moment of the NMR line for protons in a polycrystalline sample of [NH₂(CH₃)₂]₃Sb₂Cl₉ (DMACA). The fundamental aspects of molecular dynamics from quantum tunneling at low temperatures to thermally activated reorientation at elevated temperatures have been studied. The experimentally observed spin−lattice relaxation rate is a consequence of dipolar interactions between the spin pairs inside the methyl group (1/T_1AE contribution) as well as the spins belonging to neighboring methyl groups and pairs, methyl spin−outer methyl spin (1/T_1EE contribution). These contributions are considered separately. Two methyl groups in the dimethylammonium (DMA) cations are dynamically inequivalent. The values of the tunnel splitting of separate methyl groups are obtained from the T₁ (55.2 MHz) experiment. The tunneling dynamics taking place below the characteristic temperatures 74 and 42 K for separate methyl groups are discussed in terms of the Schrödinger equation. These temperatures point to the one at which thermal energy C_pT and potential barrier take the same value. It is established that the second moment of the proton NMR line below 74 K up to liquid helium temperature is much lower than the rigid lattice value, which is due to a tunneling stochastic process of the methyl groups.

https://doi.org/10.1021/jp045080y

https://www.acs.org/content/acs/en.html