Anisotropic reorientation of polyaromatic rings in solution. A 13C NMR methodology for planar asymmetric rotors

Abstract

A methodology is described to discuss the anisotropic reorientation of planar asymmetric molecules from T 1 and NOE 13C NMR data, using Huntress's equations. A parabolic equation τ c = a + bx + cx 2 of positive curvature should be obtained when plotting overall correlation times τ c vs x = cos 2 φ, where φ is the polar angle of the internuclear 13CH vector with the principal axis of inertia. Ratios A = c b and B = b a should take values between well-defined limits represented by the equations B = 2 A/(5 − 2 A 2) and B = 4 A/(1 + 6 | A | − 4 A 2) restricted to the range A = ± 3 2 and B = ±6. The determination of A, B, and C allows one to compute the rotational iffusion coefficients D x , D y , D z . The method is applied to a polyaromatic compound, chrysene in C 2D 2Cl 4, where D x , D y , D z = 0.36, 0.065, and 0.52 × 10 10s −1. The reorientation of this planar asymmetric rotor is found to be anisotropic about in-plane axes x, y as expected from unequal moments of inertia and is faster about the out-of-plane axis z, showing strongly decreased frictional forces for rotation in the molecular plane.