The Effect of Low-Temperature Dynamics of the Dimethylammonium Group in [(CH₃)₂NH₂]₃Sb₂Cl₉ on Proton Spin−Lattice Relaxation and Narrowing of the Proton NMR Line

Abstract

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(p)T 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.

Magnetic excitation and readout of methyl group tunnel coherence

“…Neither can these signals arise from any magnetic field-dependent level mixing. Modulation depth decreased above 30 K, and the signals vanished at about 80 K. We speculated that they stem from tunnel splitting of the methyl groups (9). Tunneling sidebands have been observed in EPR spectra as early as 1972 (10), and the temperature dependence of the frequencies of our signals is akin to the one of methyl tunnel splitting observed by EPR (11) and electron nuclear double resonance (ENDOR) (12).…”

1H NMR study of internal motions and quantum rotational tunneling in (CH3)4NGeCl3

“…[8] have shown that at room temperature (293 K), (CH 3 ) 4 NGeCl 3 , here after abbreviated as TMAC-germanate, crystallizes in an orthorhombic modification (Pna2 1 ) with a = 13.096 Å, b = 8.911 Å, c = 9.128 Å and z = 4. The TMACgermanate has been reported [9] to reveal five phases in the temperature region 463-130 K, in which highest temperature phase being cubic perovskite structure above 424 K. Further, to understand the mechanism of phase transitions, the compound has been studied using alternating current (AC) conductivity, temperature dependent single crystal XRD and 35 Cl NQR/NMR techniques. [7] The title compound was also investigated by Winkler et al [10] using quasielastic and inelastic neutron scattering and Raman spectroscopic techniques and they observed the molecular reorientation of the whole tetramethylammonium (TMA) and methyl groups in the temperature region 446-4 K. Further, they have also observed that the structural phase transitions at 170, 200 and 424 K are unrelated to the temperature-induced changes of the molecular dynamics.…”

Complex dynamics of 1.3.5-trimethylbenzene-2.4.6-D3 studied by proton spin–lattice NMR relaxation and second moment of NMR line

“…(21)) can be written as [46][47][48][49] 3 is caused by the over the barrier jumps. Because cos 2 Θ 3 ¼cos 2 (120 0 )¼ 1/4, the first reduction, is higher than the second reduction.…”