Knowledge Bank

The spin-lattice relaxation rates of protonated and deuterated n-dodecane are measured neat and in solutions of benzene, carbon tetrachloride and carbon disulfide. An isotopic dilution technique provides a means of evaluating the intramolecular relaxation of protonated n-dodecane. It was found that the ratio of the deuteron to proton intramolecular relaxation rates is approximately 8.5 for the solutions studied. The observed molecular motions are to a first approximation consistent with the Stokes-Einstein hydrodynamic $model1$ corrected for the particulate nature of the solvent by a microviscosity $coefficient.2$ For pure n-dodecane and n-dodecane in carbon tetrachloride the temperature dependence of the relaxation rate is the same as the temperature dependence of the viscosity divided by the absolute temperature. In pure deuterated n-dodecane n-dodecane the activation energy obtained by the spinlattice relaxation measurements is $4,066\pm 0.009$ kcal mole$\mathrm{<mrow\; data-mjx-texclass="ORD">-1</mrow>}$ in the temperature range from $264\circ K$ to $295\circ K$ and $3.515\pm 0.015$ kcal mole$\mathrm{<mrow\; data-mjx-texclass="ORD">-1</mrow>}$ between $295\circ K$ and $370\circ K$. In the temperature range between $264\circ K$ and $297\circ K$ the activation energy of n-$C12H26$ is $3.76\pm 0.04$ kcal mole$\mathrm{<mrow\; data-mjx-texclass="ORD">-1</mrow>}$. From $297\circ K$ to $373\circ K$ this energy is $3.64\pm 0.04$ kcal mole$\mathrm{<mrow\; data-mjx-texclass="ORD">-1</mrow>}$. In a solution of 25% n-dodecane and 75% carbon tetrachloride by volume, the activation energies of the deuterated and protonated compounds are essentially the same. Between $264\circ K$ and $348\circ K$ they are observed to be approximately 3.1 kcal mole$\mathrm{<mrow\; data-mjx-texclass="ORD">-1</mrow>}$.