Quadrupolar solvents

J. Jeon and H. J. Kim, A continuum theory of solvation in quadrupolar solvents. I. Formulation, J. Chem. Phys., 119 (2003) 8606-25 J. Jeon and H. J. Kim, A continuum theory of solvation in quadrupolar solvents. II. Solvation free energetics, dynamics, and solvatochromism, J. Chem. Phys., 119 (2003) 8626-35. 

Solution of a dipolar liquid in a quadrupolar solvent. Let us assume the solvent (component 1) to be non-dipolar, and the solute (component 2) as consisting of dipolar molecules. Moreover, for the sake of simplicity, let the molecides of both components be axially symmetric. Equation (227) now yields, on ne ecting fluctuations and non-linear polarizability ... 

Clearly, neither the dielectric constant nor the dipole moment is an adequate means to define polarity. The reason is that there are liquids whose constituent molecules have no net dipole moment, for symmetry reasons, but nevertheless have local polar bonds. This class of solvents, aheady mentioned above, comprises just the notorious troublemakers in solvent reactivity correlations, namely the aromatic and chlorinated solvents. These solvents, called nondipolar in the literature, stabilize charge due to higher solvent multipoles (in addition to dispersive forces) like benzene ( quadrupolar ) and carbon tetrachloride ( octupolar ). Of this class, the quadrupolar solvents are of primary importance. 

Kirby, C.F. McHugh, M.A. Phase Behavior of Polymers in Supercritical Fluid Solvents. Chem. Rev. 1999, 99, 565 Kauffman, J.F. Quadrupolar Solvent Effects on Solvation and Reactivity of Solutes Dissolved in Supercritical CO2 J. Phys. Chem. A 2001, 105, 3433. 

The signal from H(3) is broadened because N-H(3) spin coupling is not completely washed out by quadrupolar relaxation of the nitrogen atom (68MI41701>. This broadening niay be useful in assigning signals in the spectrum and it is usually reduced in solvents of... 

A variety of relaxation time studies have been performed on toluene. The choice of deuterated toluene avoids certain complicating factors which affect proton NMR studies, such as, dipolar or spin-spin couplings. The dominant relaxation mechanism is quadrupolar and the relaxation times are determined by the reorientation of the C-D bond vector. Relaxation times such as T, are sensitive to the motions of the solvent around the larmor frequency, which is on the order of 14 MHz in this study. T2 measurements may probe slower motions if the molecule undergoes slow and/or anisotropic motion. The relaxation time results presented in Figure 3 are significantly shorter than those found in bulk toluene solutions (18.19). In bulk toluene, the T and T2 values are equal above the melting temperature (1.2.). In this polymer system T2 < T indicative of slow and/or anisotropic reorientation. 

From these studies of aryllithium complexes, it can further be concluded that the quadrupolar interaction is a very sensitive indicator of the type of aggregation and solvation state. In certain cases the /-value ranges overlap, but additional investigations can often easily clarify the ambiguity, such as the ligand/metal ratio. The latter can, for example, be determined by a solution H NMR spectrum of the solid material in a suitable solvent. 

Numerous X-ray investigations have unravelled the solid state structure of contact and solvent-separated ion pairs. It was therefore considered to be of interest to evaluate also the potential of solid state NMR as a tool for the investigation of this structural problem. In addition to the study of chemical shifts discussed above (Section II.B), the quadrupole coupling constant of the nuclide Li, x( Li), was expected to be an ideal sensor for the bonding situation around the lithium cation because, due to its dependence on the electric field gradient, the quadrupolar interaction for this spin-3/2 nucleus is strongly influenced by local symmetry, as exemplified in Section II.C.3. This is also shown with some model calculations in Section ILF. 

A function closely related to QCC is the Li quadrupolar splitting constant (QSC), defined as QSC = (1 -h/7 /3) x( Li), where r is the asymmetry parameter. The Li QSC values can be estimated from the Li and C(para) spin-lattice relaxation times. The QSC values are correlated with the effects of structure, solvent and temperature on association in solution for aryllithium compounds (155, 171, 172). Conclusions can be drawn about the structure of the associated species in cases where no supporting XRD evidence is available. ... 

Supercritical fluids possess several advantages from the NMR point of view [249]. The very low viscosity of these solvents produces a beneficial line-narrowing effect on quadrupolar nuclei such as Co, Cr, Zr, Mo, Mn and due to... 

An even more useful property of supercritical fluids involves the near temperature-independence of the solvent viscosity and, consequently, of the line-widths of quadrupolar nuclei. In conventional solvents the line-widths of e. g. Co decrease with increasing temperature, due to the strong temperature-dependence of the viscosity of the liquid. These line-width variations often obscure chemical exchange processes. In supercritical fluids, chemical exchange processes are easily identified and measured [249]. As an example. Figure 1.45 shows Co line-widths of Co2(CO)g in SCCO2 for different temperatures. Above 160 °C, the line-broadening due to the dissociation of Co2(CO)g to Co(CO)4 can be easily discerned [249]. 

Table 3 2PA properties of quadrupolar molecules with various types of jr-bridges. The solvent in which the measurement was performed is indicated in parenthesis after the molecular structure ( )... 

Due, in the main, to quadrupolar line-broadening effects, together with the poor sensitivity, there has been relatively little general interest in 14N magnetic resonance studies and, although the quadrupolar broadening has been shown to be minimized by the use of low viscosity solvents,465 the 14N chemical shifts of only four pyrroles have been reported.465,466 The 14N chemical shift of pyrrole has been reported as either 230465 or 227466 ppm relative to the N02 ion 14N resonance, and values of 227, 230, and 225 ppm have been recorded for 1-methylpyrrole, 2,5-dimethylpyrrole, and l-methyl-2-methoxy-carbonylpyrrole, respectively.465 The associated effect of electric... 

The syntheses of the precursors were carried out following procedures reported in the literature. All methods used distilled solvents, an argon filled glove box or Schlenk type glassware connected to a vacuum line. Thermoanalytical studies were carried out on a Setaram TG-DTA 92 analyzer in line with a Leybold Heraeus QX 2000 quadrupolar mass spectrometer. 

More realistic approaches should, of course, comprise solvent models that give rise to electrostatic interactions. Shelley and Patey [273] used grand canonical MC simulations to investigate the demixing transition in model ionic solutions where the solvent is explicitly included. Charged hard-sphere ions in neutral, dipolar, and quadrupolar hard-sphere solvents were consi-... 

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