Structure and dynamics of methylene iodide in solution from nuclear spin–lattice relaxation studies

Abstract

The nuclear spin–lattice relaxation of carbon-13 enriched methylene iodide (diiodomethane) dissolved in benzene-d6 has been studied both with and without proton decoupling and using various pulse techniques to perturb the AX2(13CH2) spin system from thermal equilibrium. The return of the spin system to steady state was monitored using carbon-13 Fourier transform nuclear magnetic resonance techniques. It is shown that the equation of motion of the spin density matrix reduces in general to the master equation for populations in terms of interlevel transition rates, and that a linear transformation based on the complete set of irreducible spherical tensor operators which span the spin space further simplifies the equation of motion. The relaxation was modeled as intramolecular dipole–dipole interactions modulated by rotational reorientation of the molecule plus other mechanisms which can be treated collectively as external random magnetic fields interacting with the nuclear spins of interest. Extreme narrowing is assumed. The model was parameterized in terms of the spectral densities of fluctuations in the molecular rotational reorientation and in the external random field along with other necessary parameters characterizing the operation of the NMR spectrometer. A best least squares fit of this model to all of the relaxation data was calculated. From the values of the spectral densities thus obtained all of the interlevel transition rates were calculated. Assuming a rotational diffusion model for the molecular rotational reorientation, the following structural and dynamical parameters were calculated from the dipole–dipole spectral densities: &HCH=104°±2°, Dxx/Dzz =0.205±0.0033, and Dyy/Dzz=0.079±0.057, where Dαα, α=x,y,z, are the elements of the diagonalized rotational diffusion tensor.