Dynamics of dilute water in carbon tetrachloride

A dilute solution of water in a hydrophobic solvent, such as carbon tetrachloride (CCl ₄), presents an opportunity to study the rotational properties of water without the complicating effects of hydrogen bonds. We report here the results of theoretical, experimental, and semiempirical studies of a 0.03 mole percent solution of water in CCl ₄. It is shown that for this solution there are negligible water-water interactions or water-CCl ₄ interactions; theoretical and experimental values for proton NMR chemical shifts (δ _H) are used to confirm the minimal interactions between water and the CCl ₄. Calculated ab initio values and semiempirical values for oxygen-17 and deuterium quadrupole coupling constants (χ) of water/CCl ₄ clusters are reported. Experimental values for the ¹⁷O, ²H, and ¹H NMR spin-lattice relaxation times, T ₁, of 0.03 mole percent water in dilute CCl ₄ solution at 291 K are 94 ±3 ms, 7.0 ± 0.2 s, and 12.6 ± 0.4 s, respectively. These T ₁ values for bulk water are also referenced. "Experimental" values for the quadrupole coupling constants and relaxation times are used to obtain accurate, experimental values for the rotational correlation times for two orthogonal vectors in the water molecule. The average correlation time, τ _c, for the position vector of ¹⁷O (orthogonal to the plane of the molecule) in monomer water, H ₂ ¹⁷O, is 91 fs. The average value for the deuterium correlation time for the deuterium vector in ²H ₂O is 104 fs; this vector is along the OD bond. These values indicate that the motion of monomer water in CCl ₄ is anisotropic. At 291 K, the oxygen rotational correlation time in bulk ²H ₂ ¹⁷O is 2.4 ps, the deuterium rotational correlation time in the same molecule is 3.25 ps. (Ropp, J.; Lawrence, C.; Farrar, T. C.; Skinner, J. L. J. Am. Chem. Soc. 2001, 123, 8047.) These values are a factor of about 20 longer than the τ _c value for dilute monomer water in CCl ₄.