8662
J. Phys. Chem. B 1997, 101, 8662-8666
Studies of the Translational and Reorientational Motions of Planar Nickel Complex Ions Bruce A. Kowert,* Timothy L. Stemmler, Michael J. Fehr, Pamela J. Sheaff, Timothy J. Gillum, Nhan C. Dang, Angela M. Hughes, Bethany A. Staggemeier, and David V. Zavich Department of Chemistry, Saint Louis UniVersity, St. Louis, Missouri 63103 ReceiVed: April 14, 1997; In Final Form: June 2, 1997X
The translational diffusion constants, DT, of the bis(maleonitriledithiolato)nickel anion and dianion, Ni(mnt)2and Ni(mnt)22-, respectively, have been measured in acetone and ethyl alcohol solutions drawn through a microcapillary by reduced pressure at 25 °C. The translational radius, rt, obtained from DT is larger for Ni(mnt)22- than for Ni(mnt)2- in both solvents. The larger rt for Ni(mnt)22- is attributed to ion pairing with (n-Bu)4N+, while Ni(mnt)2- does not appear to be ion paired. rt for Ni(mnt)2- has been used to determine the rotational radius, ro, of this paramagnetic ion; ro has been used with ESR results to determine κ⊥, the solvent interaction parameter for Ni(mnt)2- in several solvents. ESR studies of Ni(mnt)2- in dimethyl phthalate (DMPT) and tris(2-ethylhexyl) phosphate (TEHP) have also been made. The analysis of the widths shows the reorientation of Ni(mnt)2- is fastest about its long in-plane axis by a factor of ≈3 in both solvents. The temperature dependence of the widths is discussed using the modified Stokes-Einstein-Debye (SED) model and the Vogel-Tammann-Fulcher (VTF) equation. The SED results indicate that Ni(mnt)2- has relatively weak interactions with both DMPT and TEHP. The ESR VTF parameters are consistent with those from viscosities and produce calculated values of the glass transition temperatures, Tg, that are also in general agreement with experiment.
Introduction The size of a solute in solution can be determined from its translational diffusion constant, DT, using the Stokes-Einstein relation1
DT ) kBT/(6πηrt)
(1)
where kB is Boltzmann’s constant, T is the absolute temperature, and η is the viscosity; rt, the radius of a spherical solute, is a length determined by the shape and size of nonspherical solutes. rt has also played an important role in the study of reorientational dynamics using electron spin resonance (ESR). The reorientational correlation time, τ2(0), that has been obtained for a number of radicals (S ) 1/2) has been found to follow the modified Stokes-Einstein-Debye (SED) model1-11
τ2(0) ) (4πro3ηκ⊥)/(3kBT) + τ0
(2)
where ro is the hydrodynamic rotational radius that can be obtained from rt, κ⊥ is a dimensionless parameter determined by the anisotropic interactions between the radical probe and the solvent molecules, and τ0 is the zero-viscosity intercept. The introduction of κ⊥ < 1 resulted from analyses of the ESR hyperfine line width variations of the vanadyl acetylacetonate (VOAA) radical1-5,12 for which τ2(0) is the correlation time for reorientation about the axes perpendicular to the unique V-O bond. As described in the Translational Diffusion Measurements section, consideration of the molecular shape showed that ro ) rt. The experimental τ2(0) were proportional to η/T and less than values calculated using rt3 and κ⊥ ) 1. κ⊥ was found to be independent of η and T in a given solvent but varied from solvent to solvent.1-5 A theoretical investigation of reorientation on the molecular level13 showed that κ⊥ (0 e κ⊥ e 1) is * Address correspondence to this author. X Abstract published in AdVance ACS Abstracts, September 15, 1997.
S1089-5647(97)01286-8 CCC: $14.00
Figure 1. Molecular structure of Ni(mnt)2- and the right-handed principal axes (x, y, z) of the anisotropic Zeeman interaction. The central Ni3+ is bonded to four S atoms.
proportional to the ratio of the mean-squared torques to the mean-squared forces experienced by the probe. Studies of both the reorientational and translational motions of a probe molecule are clearly important sources of information concerning solute-solvent interactions. However, τ2(0) data from ESR analyses have been used in conjunction with rt for only a small number of radicals, all of which are neutral (VOAA,1 ClO2,12 three nitroxides,14 and V(η5-C5H5)2Cl215). The motion of solute ions is of interest because it can be influenced by short-range interactions between the molecular charge and solvent. In this paper, we combine studies of the translational motion of the bis(maleonitriledithiolato)nickel anion, Ni(mnt)2-, with ESR reorientational results for this planar radical (Figure 1). We have recently shown Ni(mnt)2- to be a useful ESR probe;6,7 the principal axes for its nonaxially symmetric Zeeman interaction (gx > gy > gz) are shown in Figure 1 (there is no observable hyperfine splitting). Ni(mnt)2- was studied in several glass-forming liquids over temperature ranges of ≈100 K. In all solvents,6,7,16 the principal line associated with the isotropic g value in the motionally narrowed region and the intermediate g value in the glassy region has a well-defined first-derivative line shape for all temperatures between these © 1997 American Chemical Society
Motions of Planar Nickel Complex Ions
J. Phys. Chem. B, Vol. 101, No. 43, 1997 8663
Figure 2. ESR solvents used for Ni(mnt)2-: (a) DMPT; (b) TEHP.
two limits. The well-defined principal line was shown to be produced by axially symmetric Brownian rotational diffusion (BRD) with the long in-plane (y) axis as the symmetry axis of the rotational diffusion tensor. The reorientation is fastest about the y axis with D|/D⊥ ≈ 3.0; D| and D⊥ are the rotational diffusion constants for the reorientation about the parallel (y) and perpendicular (x, z) axes, respectively. This motional model was shown to produce agreement between the experimental and calculated principal line widths of Ni(mnt)2- when the anisotropic Zeeman interaction made the dominant contribution to the widths.6,7 The temperature dependence of the correlation time
τ2(0) ) (6D⊥)-1
(3)
obtained from a comparison of the experimental and calculated widths was discussed in terms of eq 2. Values of κ⊥ < 1 were found for Ni(mnt)2-, but the value of ro3 ) 97.0 Å3 used was an estimate based on X-ray structural data and van der Waals radii.6,7 We have now measured the translational diffusion constant of Ni(mnt)2- at 25 °C in solutions drawn through a microcapillary by reduced pressure.17 The retention time and width of the solute dispersion used to calculate D25 have been determined using ultraviolet detection in ethyl alcohol (EtOH) and acetone. The hydrodynamic radius rt, obtained from D25 for Ni(mnt)2can be used to calculate ro3 ) 78.0 Å3; this value, 20% smaller than our previous estimate, gives κ⊥ < 1 for Ni(mnt)2- in two new solvents, dimethyl phthalate (DMPT) and tris (2-ethylhexyl) phosphate (TEHP), as well as in our previous solvents. DMPT and TEHP are shown in Figure 2. The small values of κ⊥ in TEHP and DMPT are indicative of relatively weak interactions between Ni(mnt)2- and the solvent molecules. The temperature dependence of τ2(0) in DMPT and TEHP is also discussed in terms of the Vogel-Tamman-Fulcher (VTF) equation.18 The VTF analyses give parameters related to the glass transition temperature, Tg, and the strong or fragile nature of liquids. The parameters from the VTF fit are in reasonable overall agreement with analogous parameters obtained from VTF fits of the viscosities. A similar degree of agreement is found when values of Tg calculated from the ESR VTF parameters are compared with experiment. We have also measured D25 of the diamagnetic dianion, Ni(mnt)22-, in EtOH and acetone. In both solvents, rt for Ni(mnt)22- is larger than rt for Ni(mnt)2-; the larger rt for Ni(mnt)22- is attributed to ion pairing with the (n-Bu)4N+ counterion, while Ni(mnt)2- is considered not to be ion paired. This is consistent with data from electrochemical,19 conductivity,20 and vapor phase osmometry19 experiments in acetonitrile, which showed that Ni(mnt)22- but not Ni(mnt)2- is ion paired with Et4N+. As a control, we found that D25 for DL-phenylalanine in H2O is within 2% of the literature value.17 Experimental and Computational Procedures Ni(mnt)22- and Ni(mnt)2- were prepared as the (n-Bu)4N+ salts using the syntheses of Davison and Holm.21 For the ESR
work, DMPT (Fisher Purified) was placed over 4 Å molecular sieves in a vacuum desiccator and TEHP was vacuum distilled twice (collection over 4 Å molecule sieves) before both solvents were stored over 4 Å molecular sieves on the vacuum line used to prepare the samples (≈10-3 M). Other aspects of the ESR experiments (spectrometer, temperature control, etc.) have been given previously.7,22 The widths of the principal line of Ni(mnt)2- have been calculated (from Freed’s23 theory and the procedure of refs 6 and 7) using the g factors in each solvent and the motional model described in the Introduction; we have not used an orientation-independent half width, T2-1. The regression analyses used to determine the parameters for the SED model and the VTF equation as well as the associated root mean square errors are discussed in ref 7. The viscosities used for DMPT are from ref 24, while those for TEHP are from refs 25 and 26. For the translational diffusion experiments, absolute EtOH (Aaper Alcohol & Chemical) and acetone (Aldrich HPLC grade and Mallinckrodt Analytical Reagent) were used as received. The weight percent compositions for the EtOH solutions were 0.075% Ni(mnt)2N(n-Bu)4 and 0.073% Ni(mnt)2[N(n-Bu)4]2; in acetone, the solutions of both salts were 0.016%. DL-Phenylalanine (Aldrich) was 0.27 wt % in H2O, which had been distilled and passed through a Barnstead Nanopore deionizer. To record the elution profile of a particular solute in a given solvent, the pure solvent was first drawn through a fused silica microcapillary (Polymicro Technology, 76.8 µm i.d.) for several hours by reduced pressure (≈50 Torr less than atmospheric pressure) due to an aspirator and controlled by a Gast regulator. The reduced pressure was then broken, and the capillary was dipped in the solute-containing solution for 2 s before the capillary was returned to the pure solvent, the reduced pressure was reset, and the data acquisition was simultaneously started. Ideally, a very narrow (“δ function”) plug of solution should be loaded into the capillary. Several widths and retention times were also measured for a 10 s load time. A linear extrapolation was used to determine “zero load time” parameters; the translational diffusion constants calculated with the extrapolated parameters were 3% larger than those calculated for 2 s load time (and the corresponding correction was applied to all of the 2 s data). The solution flow through the capillary was monitored with a Thermo Separation Products Model SC100 variable wavelength detector. The detection wavelengths for Ni(mnt)2- were 314 (EtOH) and 333 nm (acetone), while those for Ni(mnt)22were 316 (EtOH) and 333 nm (acetone); 214 nm was employed for DL-phenylalanine in H2O. The distance between the variable-wavelength detector and the end of the capillary in the pure solvent or solution was 50 cm. The detector was interfaced with a Gateway 2000 PC (4DX2-66 hard drive), and the data were collected, stored, and analyzed using the Chrom Perfect software package (Justice Innovations). Direct temperature control of the solutions, capillary, and detector was not possible. We did, however, measure room temperature during each run (determination of an elution profile). The temperature was constant during a given run (2-4 min) and varied from 24 to 28 °C; the result for a given run was adjusted to 25 °C using eq 1. The literature values of the viscosities were used for EtOH,26 acetone,26 and H2O.27 Translational Diffusion Measurements Background. The method by which DT, the molecular translational diffusion constant of a solute at temperature T, can be determined from its elution profile in a microcapillary using Taylor-Aris dispersion theory is discussed in refs 17, 28, and
8664 J. Phys. Chem. B, Vol. 101, No. 43, 1997
Kowert et al.
29. DT is obtained using
DT ) m1R2/(24m2)
(4)
where R is the capillary radius and m1 and m2 are the first and second moments of the profile, respectively. Equation 4 follows from eqs 2-6 of ref 17 and is valid when 4m2/m12 , 1 and 3(Um2/m1)2 . R2 (U is the mean solution velocity); these inequalities hold for all of our systems. In the absence of specific interactions between the solute and the capillary walls, the elution profile has a Gaussian shape28 for which
m1 ) tR
(5)
m2 ) (w1/2)2/5.545
(6)
TABLE 1: Translational Diffusion Constants and Radii
where tR is the elution (retention) time for the maximum of the profile and w1/2 is the full width at half-height. Relation of rt to r0. Analytical expressions for the rt of ellipsoids (with semiaxes a * b ) c) have been given by Perrin.30 Solutes with no two equal semiaxes (a > b > c) have31,32
rt ) (a sin φ)/F(θ,φ)
(7)
where F(θ,φ) is an elliptic integral of the first kind and
al.1
Figure 3. Elution profile for Ni(mnt)2- in EtOH at 24.5 °C and a profile calculated assuming a Gaussian shape (b).
θ ) sin-1[(a2 - b2)/(a2 - c2)]1/2
(8a)
φ ) sin-1[1 - (c2/a2)]1/2
(8b)
Hwang et used the value of rt from D25 to determine ro for VOAA, a procedure directly related to its “size” in solution. Consideration of structural data suggested that VOAA could be approximated as an oblate ellipsoid with a < b ) c. When the Perrin expressions were used,1,30 calculated values of (ro/ rt)3 varied only between 0.98 and 1.06 for 1.41 e b/a e 2.69. The structural data gave b/a ) 2.2, for which (ro/rt)3 ) 1.00 and Hwang et al.1 set ro ) rt. The shape of Ni(mnt)2- is closer to that of a prolate ellipsoid (a > b ) c). Calculations for this shape show that (ro/rt)3 is not a weak function of a/b; (ro/rt)3 varies from 1.11 to 1.63 as a/b varies from 1.41 to 2.83. Use of the dimensions estimated previously6 for Ni(mnt)2- (ry ) 6.70 Å, rx ) 3.64 Å, rz ) 1.80 Å) with a ) ry, b ) (rxrz)1/2 ) 2.56 Å (a/b ) 2.62) gives (ro/ rt)3 ) 1.56. This result is not appreciably altered if we remove the assumption of b ) c. Budo et al.33 give numerical data for reorientational friction constants when a * b * c; their results and our rx/ry ) 0.543, rz/ry ) 0.269 can be used with eqs 7 and 8 for rt to calculate (ro/rt)3 ) 1.59, which will be used in the following discussion. Results. Figure 3 shows an elution profile for Ni(mnt)2- in EtOH at 24.5 °C; its shape is in good agreement with a Gaussian shape calculated using w1/2 ) 10.2 s measured from the profile (tR ) 167.8 s). Equations 4-6 have been used to determine D25 for Ni(mnt)2-, Ni(mnt)22-, and DL-phenylalanine. Our 106 D25 ) 6.92 ( 0.15 cm2 s-1 for DL-phenylalanine is within 2% of values obtained by separate determinations using the Rayleigh interference method3,4 and microcapillary flow measurements17 similar to our own. D25 for Ni(mnt)2- and Ni(mnt)22- in each solvent are given in Table 1. The rt obtained from D25 for our solutes are also given in Table 1. In both solvents, the rt for Ni(mnt)22- are the same (within experimental error) and are greater than the rt for Ni(mnt)2- (which are also equal, within experimental error). Lingane19 found similar results in electrochemical studies of
solute
solvent
106D25, cm2 s-1
rt, Å
profiles
Ni(mnt)2Ni(mnt)22Ni(mnt)2Ni(mnt)22-
EtOH EtOH acetone acetone
5.73 ( 0.36 4.28 ( 0.30 19.0 ( 1.0 15.7 ( 0.5
3.55 ( 0.22 4.76 ( 0.31 3.77 ( 0.20 4.54 ( 0.15
16 24 16 7
TABLE 2: Solvent Interaction Parameters for Ni(mnt)2solvent
106C⊥, s K R-1 a
κ⊥b
0c
EtOH eugenol DMPT BuOH TBP TEHP acetone
2.01 1.38 1.18 1.13 0.899 0.583
0.85 0.58 0.50 0.48 0.38 0.25
24.325d 9.720e 8.524f 17.125d 8.125g ≈8.0h 21.320d
a From ref 7 except for DMPT and TEHP (this work). b Calculated using ro3 ) 78.0 Å3. c Subscript is °C. d Reference 37. e Reference 38. f Reference 27. g Reference 39. h Estimated using for TBP39 and 0 tricresyl phosphate.27
these two ions in acetonitrile; D25 for Ni(mnt)2- was larger than that for Ni(mnt)22-. The electrochemical D25 give rt ) 3.41 ( 0.12 Å for Ni(mnt)2- and rt ) 4.31 ( 0.12 Å for Ni(mnt)22-; these are in agreement, within experimental error, with our average values of rt in EtOH and acetone (3.66 ( 0.30 Å for Ni(mnt)2- and 4.65 ( 0.34 Å for Ni(mnt)22-). Additionally, Lingane19 carried out vapor pressure osmometry experiments which showed that, in agreement with conductivity20 results, Ni(mnt)22- but not Ni(mnt)2- was ion paired with its Et4N+ counterion; the ion pairing accounts for the larger radius of the dianion. The agreement of our rt with those in acetonitrile suggests that Ni(mnt)22- is appreciably ion paired with (nBu)4N+ and that Ni(mnt)2- is not in both acetone and EtOH. Our average rt ) 3.66 Å for Ni(mnt)2- can be used with (ro/rt)3 ) 1.59 to obtain ro3 ) 78.0 Å3. This is 20% less than our previous estimate6 of 97.0 Å3, but the difference is not considered serious. The κ⊥ ) 0.68 in EtOH, becomes κ⊥ ) 0.85 when the new ro3 is used. Table 2 gives the C⊥ ) (4πro3κ⊥)/(3kB) (see eq 2) obtained from the ESR line widths of Ni(mnt)2- and the κ⊥ calculated using ro3 ) 78.0 Å3. In addition to EtOH, the solvents listed in Table 2 are 4-allyl-2methoxyphenol (eugenol), n-butyl alcohol (BuOH), and tributyl phosphate (TBP) as well as DMPT and TEHP (which are discussed in the next section). The possibility of Ni(mnt)2- being ion paired in solvents such as TBP, TEHP, and DMPT with low dielectric constants, 0, deserves mention (the 0 are given in Table 2). The calculation of κ⊥ assumes a common ro3 can be used. The same motional model is found for the ESR-active species in all solvents (the reorientation is fastest about the long in-plane axis by a factor of ≈3 for axially symmetric BRD); this speaks to a common shape similar to that of a near-prolate ellipsoid, but it would
Motions of Planar Nickel Complex Ions
J. Phys. Chem. B, Vol. 101, No. 43, 1997 8665
TABLE 3: Glass Spectrum gi for Ni(mnt)2-
b
solvent
DMPT
TEHP
gx gy gz g0,calc g0,exptb
2.1425 2.0423 1.9951 2.0600 2.0618
2.1440 2.0408 1.9935 2.0594 2.0614
a Calculated using the glassy g and g ) (1/3)(g + g + g ). i 0 x y z Determined from the motionally narrowed singlet.
obviously be desirable to measure rt for Ni(mnt)2- in one of the solvents with small values of κ⊥ and 0. It is noteworthy that Ni(mnt)2- has the largest C⊥ in EtOH, which also has the largest 0. Should ion pairing occur in a solvent with small 0, one might expect a C⊥ possibly as large as those found in solvents with relatively large 0; the use of (ro/rt)3 ) 1.59 and rt ) 4.65 Å for ion-paired Ni(mnt)22-, gives ro3 ) 160 Å3. However, the C⊥ in TBP, TEHP, and DMPT are smaller than C⊥ in EtOH by a factor of ≈2; if ro is larger in the solvents with small 0, the corresponding κ⊥ would obviously be less than the values in Table 2. Preliminary experiments involving Ni(mnt)2- in DMPT revealed an asymmetric profile shape caused by “tailing” of the solute; consequently eqs 5 and 6 could not be used to obtain D25. The high viscosity of DMPT at 25 °C (0.140 P versus 0.0107 P for EtOH and 0.00305 P for acetone) may be a factor. The use of computer simulations to obtain m1 and m2 from the overall profile shape is being considered. Conductivity experiments in our solvents are also planned. Electron Spin Resonance Results Glassy Spectra. The glassy gi of Ni(mnt)2 in DMPT and TEHP were determined by the computer simulations described previously7 and are given in Table 3. Also given in Table 3 are the isotropic g factors, g0, which, as for all other solvents,6,7,16 are larger than the values calculated using g0 ) (1/3)(gx + gy + gz). As before,6,7 we have used the glassy gi for all of our line width calculations because they give generally good agreement between the experimental and calculated widths when the anisotropic Zeeman interaction makes the dominant contribution to the widths and because the single parameter g0 gives no prescription for varying the individual gi. SED Analyses. The widths calculated for Ni(mnt)2- in DMPT with D|/D⊥ ) 2.85 and the glassy gi are compared with the experimental widths using the SED model in Figure 4. The overall agreement is good. The C⊥, τ0, and root mean square errors for the fits in both DMPT and TEHP are given in Table 4. For TEHP, two sets of experimental viscosities were found in the literature; those determined by Barlow, Erginsav, and Lamb (BEL)25 were higher than those given by Viswanath and Natarajan (VN)26 by 3-19% for 223.7 e T e 292.2 K (the maximum difference was at 246.7 K where BEL25 found η ) 2.35 P). The C⊥ and τ0 given in Table 4 were determined using the average of the two viscosities; use of the viscosities from the individual sets only changed C⊥ by 10% and the root mean square error by 3-4%. The C⊥ are relatively small in both DMPT and TEHP and are in agreement with qualitative interpretations given in ref 7. C⊥ for DMPT is similar to eugenol (see Figure 2 of ref 7) which also has a central benzene ring with two oxygen-containing substituents ortho to each other. C⊥ in TEHP is the smallest we have found. In ref 7, we suggested that Ni(mnt)2- would be weakly solvated by the butyl groups and electronegative oxygens of TBP; the interactions between Ni(mnt)2- and TEHP appear to be even weaker because of the larger 2-ethylhexyl groups of TEHP.
Figure 4. Principal line widths of Ni(mnt)2- in DMPT. The SED model was used for the experimental widths (2); D|/D⊥ ) 2.85 was used for the calculated widths (b).
TABLE 4: SED Parameters for Ni(mnt)2solvent
D|/D⊥a
1011τ0, s
106C⊥, s K P-1
rms errorb
DMPT TEHP
2.85 3.00
5.74 13.5
1.18 0.583
0.071 0.258
a
Value used for calculated widths. b rms ) room mean square.
TABLE 5: VTF Parameters for Ni(mnt)2- a solvent
Trange, K
1012A, s
D
T0, K
rms errorb
Tg,calcc
DMPT TEHP
248.7-358.7 217.7-336.7
12.3 1.73
1.66 6.75
209.5 134.0
0.086 0.104
218.4 157.2
-
a Values of D and D used for the calculated widths are given in | ⊥ Table 4. b rms ) root mean square. c Calculated using D, T0, and eq 10; the experimental Tg are given in Table 6.
VTF Analyses. The VTF equation18
τ2(0) ) A exp[DT0/(T - T0)]
(9)
was also used for the experimental widths in DMPT and TEHP. In eq 9, D characterizes the “strong” or “fragile” nature of the liquid, while T0 is less than Tg and has been shown to equal the Kauzmann temperature TK.18,35 Tg is given in terms of D, and T0 by18,35
Tg/T0 ) 1 + D[2.303 log(ηg/η0)]-1
(10)
which is based on the VTF relation for viscosities with log(ηg/ η0) ≈ 17. The A, D, and T0 for the ESR fits are given in Table 5. The relatively small root mean square errors (also in Table 5) show that use of the VTF equation for the experimental widths gives good agreement with the calculated widths. These ESR VTF parameters can be directly compared with the analogous parameters from VTF fits of the solvent viscosities (given in Table 6). For DMPT, the agreement is reasonably good although Barlow et al.24 did not carry out a single VTF fit for the η data; high-temperature (273.2-369.5 K) and lowtemperature (243.6-263.2 K) VTF fits were made. The low values of D from ESR as well as the two η fits indicate that DMPT is a fragile liquid. The ESR D (1.66) is closest to that from the high-temperature fit (2.38); the low-temperature fit gives D ) 3.57. The ESR value of T0 is 6 and 10% higher than the values from the high- and low-temperature fits, respectively. For TEHP, the small D from ESR and both η fits25,26 are again indicative of a fragile liquid. Our ESR D and
8666 J. Phys. Chem. B, Vol. 101, No. 43, 1997
Kowert et al.
TABLE 6: VTF Parameters from Viscosities solvent
Trange, K
D
T 0, K
Tg,exp, K
DMPTa DMPTa TEHPc TEHPf
273.2-369.5 243.6-263.2 e297d 213.2-348.7
2.38 3.57 8.64 5.06
197.8 188.4 131.0 151.2
195b 195b 160e 172e
a Reference 24. b Reference 36. c Reference 25. d Reference 25 gives the VTF parameters for TEHP but does not give the lowest T at which η was measured. e Temperature at which η ) 1013 P. f Reference 26.
T0 are in good agreement with those from the η VTF fit of BEL25 (Table 6) and are not unduly far from those of VN.26 The values of Tg calculated using the ESR parameters and eq 14 (Table 5) can also be compared with the experimental values (Table 6). For DMPT, the ESR Tg is 12% higher than the experimental value determined by differential scanning calorimetry.36 For TEHP, the experimental Tg is the temperature at which η ) 1013 P; the ESR Tg is in good agreement with that of BEL25 and within 10% of VN’s value.26 Overall, the situation for the VTF fits in DMPT and TEHP parallels that found previously for EtOH, eugenol, TBP, and BuOH;7 although some exceptions are found, the agreement between the D and T0 obtained from ESR and viscosities is generally good as is the agreement between the experimental Tg and those calculated from the ESR parameters using eq 10. Acknowledgment. We thank Dr. Barry Hogan for helpful discussions and assistance during the initial stages of the translational diffusion experiments; the equipment for those studies was purchased with grants to Dr. Hogan from the Research Corp. and the donors of the Petroleum Research Fund, administered by the American Chemical Society. The variabletemperature unit used in the ESR work was purchased with funds provided by the Beaumont Faculty Development Fund and the Department of Chemistry, Saint Louis University. References and Notes (1) Hwang, J.; Kivelson, D.; Plachy, W. J. Chem. Phys. 1973, 58, 3173. (2) Wilson, R.; Kivelson, D. J. Chem. Phys. 1966, 44, 154. (3) Hoel, D.; Kivelson, D. J. Chem. Phys. 1975, 62, 1323, 4535. (4) Kowert, B.; Kivelson, D. J. Chem. Phys. 1976, 64, 5206. (5) Patron, M.; Kivelson, D.; Schwartz, R. N. J. Phys. Chem. 1982, 86, 518. (6) Kowert, B. A.; Broeker, G. K.; Gentemann, S. J.; Fehr, M. J. J. Magn. Reson. 1992, 98, 362.
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