Proton spin-lattice relaxation and methyl group rotation

Abstract

Proton spin-lattice relaxation times have been measured at 16, 31, and 59 MHz in 4-methyl-2,6ditertiarybutyl phenol between 80 K and its melting point, 340 K. The variation of T1 with temperature shows too distinct minima. The lower-temperature minimum has been analyzed in terms of relaxation by reorientation of four of the six t-butyl methyl groups with an average apparent activation energy of about 2.4 kcal mole −1 (104 meV molecule −1). The highertemperature minimum has been analyzed in terms of relaxation by reorientation of the t-butyl groups about their C3 axes with four of the six t-butyl methyl groups reorienting very rapidly, and the remaining two reorienting with correlation time similar to that of the t-butyl group. The activation energy for the higher-temperature minimum is 5.76 kcal mole −1 (250 meV molecule −1). Steric potential calculations are used to add weight to these assignments, and a number of peculiarities displayed by the lower-temperature minimum are discussed.

Methyl and t-butyl group reorientation in planar aromatic solids: Low-frequency nuclear magnetic resonance relaxometry and x-ray diffraction

“…These assumptions and their rationale are carefully laid out elsewhere. 22,31 If the reorientation of the jth methyl group in the ith t-butyl group is characterized by the correlation time i j and the reorientation of the ith t-butyl group is characterized by the correlation time i , R i j intra is given by 5,7,8 …”

Electron spin relaxation times and internal motions of radicals in the solid state investigated by ENDOR and pulsed EPR

“…In particular the rotational motion of the 4-methyl group in para position with respect to the oxygen atom was studied by several authors, both in the undamaged molecule [5] and in the radical in single crystal [6,7]. Moreover, Beckmann and coworkers [8,9] studied in the undamaged molecule the different motions of the t-butyl groups, i.e., methyl groups rotations and rotation of the t-butyl group as a whole, by investigating the proton spin-lattice relaxation at different temperatures and at different Larmor frequencies. As far as the t-butyl groups dynamics is concerned, they observed two minima in the proton T\ results at 125 and 300 K, by using a frequency of 31 MHz [8].…”

“…Moreover, Beckmann and coworkers [8,9] studied in the undamaged molecule the different motions of the t-butyl groups, i.e., methyl groups rotations and rotation of the t-butyl group as a whole, by investigating the proton spin-lattice relaxation at different temperatures and at different Larmor frequencies. As far as the t-butyl groups dynamics is concerned, they observed two minima in the proton T\ results at 125 and 300 K, by using a frequency of 31 MHz [8]. They attributed the low temperature minimum to the condition ' Z"c~COo \, where COo is the proton Larmor frequency, for the correlation time of the rotational motion of four of the six methyl groups belonging to the t-butyl groups, and the high temperature minimum to the same condition for both the rotational motion of the other two methyl groups and the t-butyl group as a whole.…”

The relationship between crystal structure and methyl and t-butyl group dynamics in van der Waals organic solids

“…[5][6][7] Rapid spinspin ͑or transverse͒ relaxation ensures a common spin temperature and the effect of spin-spin interactions that are not modulated by the motion is to slow the exponential relaxation process. The proton spin-lattice relaxation rate is given by 5 [5][6][7] of A ␣ involves approximations since the spin-spin vectors are changing both in length and direction as a t-butyl group and its resident methyl groups reorient, unlike methyl group reorientation where only the direction of the spin-spin vectors is changing. The experiments are performed on polycrystalline samples and the calculations of A ␣ and A ␤ properly account for the averaging of the angles between the rotation axes and the magnetic field.…”