Nuclear spin relaxation in benzyl fluoride. I. 2H relaxation for internal rotational barrier determination and 1H and 19F intra- and intermolecular relaxation in pure benzyl fluoride

Abstract

2H relaxation times of pure benzyl fluoride- d 7, and of the pure monodeuterated 2 d 1-, 3 d 1-, and 4 d 1-benzyl fluoride were measured. The deuteron relaxes only through the quadrupolar mechanism. The aromatic deuteron relaxation times for the specifically deuterated benzyl fluorides show that the motion of benzyl fluoride is anisotropic: a quantitative description with a symmetric rotor model is given. The aromatic deuteron relaxation times for the perdeuterated molecule treated in the spherical rotor model show that the molecular motion is diffusional. The internal rotational barrier of the CD 2F group around the aromatic -benzylic bond can be measured (11.3 kJ mole −1) from the benzylic deuteron relaxation times using Woessner's treatment. Comparisons are made with rotational barriers of related products determined by different methods. 1H and 19F relaxation times of benzyl fluoride were measured in bulk and in benzyl fluoride- d 7 solution. The proton and the fluorine nuclei relax through intramolecular dipole-dipole, intermolecular dipole-dipole, and spin-rotation mechanisms. This intramolecular dipole-dipole contribution can also be calculated from the quadrupolar relaxation times from 2H measurements on pure benzyl fluoride through the appropriate geometrical, magnetogyric, and quadrupolar coupling constants for the aromatic protons and through the appropriate combination of motions (overall and internal rotations) for the benzylic protons and fluorine. The mean activation energy of these intramolecular dipole-dipole relaxation times is 12 kJ mole −1. The intermolecular dipole-dipole contribution can be evaluated from measurements at several concentrations. It has an activation energy of 14.7 kJ mole- −1. These values and the plots of the relaxation rates versus the molar fraction suggest an intermolecular self-association. The spin-rotation interaction is observed only on benzylic protons and fluorine at high temperature and is smaller than that for toluene and benzotrifluoride.