J. Phys. Chem. 1994,98, 6684-6687
6684
Temperature Dependence of the Deuteron and Oxygen Quadrupole Coupling Constants of Water in the System WatedDimethyl Sulfoxide R. Ludwig' and T.C. Farrar Department of Chemistry, University of Wisconsin-Madison, Madison, Wisconsin 53706
M.D. Zeidler Institut j3r Physikalische Chemie der R WTH Aachen, Templergraben 59, 0-52056 Aachen, Germany Received: March 2, 1994; In Final Form: May 5, 1994'
NMR deuteron and oxygen-17 relaxation rates of water in a mixture of 68 mol Iwater and 32 mol Idimethyl sulfoxide (DMSO)were measured for temperatures between 308 and 183 K. Using these data and the rotational correlation times obtained from the oxygen- 17-induced proton relaxation rate in a former study, the quadrupole coupling constants for deuterium and oxygen-17 were derived. Both quadrupole coupling constants show a slight temperture dependence. Comparison of deuteron and oxygen- 17 correlation times indicates that the reorientation of the water molecule in the H20/DMSO mixture is anisotropic. Introduction A direct measurement of quadrupole coupling constants in the liquid phase is not possible. Values for thegas phase are available from microwave spectroscopy, while those for the solid phase can be measured by nuclear quadrupole or magnetic resonance. The deuteron quadrupole coupling constant (DQCC) in water varies from 213-226 kHz in ice14 to 308-318 kH+ in vapor. The oxygen- 17 quadrupole coupling constant (OQCC) changes from 6.41-7.14 MHz3.4*e13to 10.175 MHzlC16between the two states. The different ice values depend on the crystal lattice and site within the lattice. The large variation between the ice and the vapor phase makes it important to discover what happens in the liquid. Several years ago b y t e and co-workers opened a new path for obtaining the DQQC in liquid water.17-19 From the oxygen-17induced proton relaxation rate they determined the first reliable value of the rotational correlation time of water. With this correlation time, TOH, and the measured deuteron relaxation rate they calculated the DQCC in liquid water (see eq 1 below). This procedure is valid because for the deuteron relaxation rate the relevant vector is also along the OH bond so that the problem of a possible anisotropy in the rotational motions does not arise. This is due to the fact that the principal axis of the deuterium electric field gradient tensor lies almost along the 0-H bond and its asymmetryparameter is sma1l.m Leyte's experimentalDQCC values of 253 f 6 and 254 & 8kHz at 298 K1*.19are in good agreement with a recent molecular dynamics study combined with ab inirio SCF calculations giving about 256 f 5 kHz at 300 K.21 For the temperature range of 263-336 K, Leyte found a slight increase of the coupling from 237 to 254 kI-Iz.19 In recent theoretical work, Huber and -workers observed a small variation of about 8 kHz in the DQCC with change in temperature from 260 to 359 K.21 The evaluation of the oxygen quadrupole coupling constant (OQCC) in water using the same procedure as for the deuterons is much more problematic because the direction of the principal axis of the oxygen field gradient tensor is perpendicular to rather than along the OH bond and the asymmetry parameter is rather large, 7 = 0.75.22 If, however, the rotational motion of the water molecule is isotropic, then the correlation times for the deuterium and oxygen field gradient tensors are equal. Leyte and co-workers arrived at the conclusion (see page 9) that the rotational motion Abstract published in Aduance ACS Absrracrs, June 15. 1994.
0022-3654/94/2098-6684S04.50/0
of pure water is isotropic. This implies a value for the OQCC of 7.9 f 0.3 MHz.18J9 This value is substantially smaller than the al, initio value of 8.9 f 0.3 MHZ.~' The excellent agreement between theory and experiment in the deuteron case and the remarkable difference in the oxygen case could be due to the invalidity of the conclusion made about the isotropic nature of the rotational motions. This could also be responsiblefor the lack of agreement found for the temperature dependence of the OQCC. The theoretical work of Huber et al. indicates that there is no significant temperature dependence of the OQCC (they calculate a value of 9.0 MHz at 360 K and a value of 8.9 MHz at 300 K). Leyte and co-workers measured a variation of about 0.6 MHz for the temperature range between 263 and 326 K. It is interesting to note that the values for both quadrupole coupling constantsin thecomputer simulation study lie well within the solid and the gas limits, but their asymmetry parameters do not. For the deuteron asymmetry parameter Huber and coworkers obtained a value of 0.164 f 0.003.21 The experimental values are 0.1-0.134 for the solid and 0.135 for thegas. For the oxygen asymmetry parameter they calculated 0.72 f 0.04:' and thecorrespondingexperimentalvalues are0.93 for thesolid phase and 0.75 for the gas phase. Several years ago Gordalla and Zeidler published correlation times and values for the DQCC for water in the system waterdimethyl sulfoxide (water-DMSO),?3 following the procedure byLeyteandco-~orkers.'~J*Theyfoundastrongvariation (206300 kHz) over the measured composition range (100-30 mol % water) for the DQCC. These experiments were restricted to a temperature region in which the molecular mobility is high and thus the NMR relaxation is frequency independent (in theextreme narrowing region where W T
