Dynamics of complex phthalate liquids. 2. Structural effects of side chains

Article abstract of DOI:10.1021/jp980365u

In this paper, the second of a series of two presenting the structural effects on the dynamic properties of complex liquids, we discuss the effect of structural changes in the alkyl side chain on the macroscopic viscosity and microscopic motional behavior of phthalates. ¹³C NMR spin-lattice relaxation times and nuclear Overhauser enhancements of individual carbon nuclei of di-n-octyl phthalate (DNOP), di-n-hexyl phthalate (DHP), and bis(6-methylheptyl) phthalate (DIOP) have been measured as a function of temperature from -40 to 70 °C. Individual ¹³C peaks were unambiguously assigned by using 2D ¹H-¹³C chemical shift correlation and specific relaxation of individual carbons in the alkyl chain of the molecule. In addition, the density and viscosity of these compounds as a function of temperature have been measured. The results were also compared with earlier experiments performed on bis(2-ethylhexyl) phthalate (DEHP). The optimized parameters of the Cole-Davidson distribution model obtained from the analysis of the ¹³C relaxation data provided the detailed information on the internal motions of the alkyl side chains. The NMR relaxation data and shear viscosity of the phthalate liquids were interpreted phenomenologically in terms of the alkyl side chain flexibility. The branching in the alkyl chain restricted the motional degree of freedom of the side chain, which made the molecule less flexible as a whole. The branching near the rigid part of the molecule was found to be more effective in restricting the internal motion of the alkyl chain.

Full text of DOI:10.1021/jp980365u

2778
J. Phys. Chem. A 1998, 102, 2778-2784
Dynamics of Complex Phthalate Liquids. 2. Structural Effects of Side Chains
Yoo Joong Kim and Jiri Jonas*
Department of Chemistry, School of Chemical Sciences and Beckman Institute for AdVanced Science and
Technology, UniVersity of Illinois Urbana, Illinois 61801
ReceiVed: December 1, 1997; In Final Form: February 17, 1998
In this paper, the second of a series of two presenting the structural effects on the dynamic properties of
complex liquids, we discuss the effect of structural changes in the alkyl side chain on the macroscopic viscosity
1
3
and microscopic motional behavior of phthalates. C NMR spin-lattice relaxation times and nuclear
Overhauser enhancements of individual carbon nuclei of di-n-octyl phthalate (DNOP), di-n-hexyl phthalate
(
-
DHP), and bis(6-methylheptyl) phthalate (DIOP) have been measured as a function of temperature from
40 to 70 °C. Individual 13C peaks were unambiguously assigned by using 2D H- C chemical shift
1
13
correlation and specific relaxation of individual carbons in the alkyl chain of the molecule. In addition, the
density and viscosity of these compounds as a function of temperature have been measured. The results
were also compared with earlier experiments performed on bis(2-ethylhexyl) phthalate (DEHP). The optimized
1
3
parameters of the Cole-Davidson distribution model obtained from the analysis of the C relaxation data
provided the detailed information on the internal motions of the alkyl side chains. The NMR relaxation data
and shear viscosity of the phthalate liquids were interpreted phenomenologically in terms of the alkyl side
chain flexibility. The branching in the alkyl chain restricted the motional degree of freedom of the side
chain, which made the molecule less flexible as a whole. The branching near the rigid part of the molecule
was found to be more effective in restricting the internal motion of the alkyl chain.
Introduction
1
In the preceding paper, hereafter referred to as I, the effect
of the structural changes in the molecular framework of
phthalates on their dynamic behavior has been investigated. In
this paper, we discuss the effect of structural changes in the
alkyl side chain of phthalate esters. As pointed out in I, the
importance of understanding the structure-property relation-
ships has motivated this systematic study of the effects of
structural modifications on dynamic properties in the model
lubricant system of complex phthalates, which are composed
of relatively rigid framework and flexible side chains. Since
the variation of chain length and the presence of the branching
in the alkyl chain considerably influence physicomechanical
properties of functional fluids, it is important to understand in
detail the effect of structural variation in the side chain on the
macroscopic and microscopic motional behavior.
For this purpose, several liquid phthalates with different side
chains have been selected: di-n-octyl phthalate (DNOP), di-
2
Figure 1. Structural formulas of the compounds studied. Carbon atoms
are labeled considering chemical equivalence.
isooctyl phthalate (DIOP), and di-n-hexyl phthalate (DHP). As
in I, 13C NMR relaxation data for individual carbon nuclei in
these compounds, as well as their density and viscosity, have
the effect of varying the chain length can be examined. The
structural formulas of these compounds are shown in Figure 1.
been measured as a function of temperature from -40 to 70
C. The 13C NMR relaxation data have been analyzed in terms
°
of the Cole-Davidson distribution model. The information of
Experimental Section
motional behavior of these compounds are also compared with
1,3
those of bis(2-ethylhexyl) phthalate (DEHP), which represents
the model lubricant liquid. A comparison of the motional
behavior of three isomers, DIOP, DEHP, and DNOP, provides
information as to the influence of branching on the physical
properties of these liquids. By comparing DNOP and DHP,
The DNOP (98%) and DHP (98%) samples were purchased
from Chem Service and used without further purification. All
of the commercially available samples of “diisooctyl phthalate”
were mixtures of various C-8 isomers, which were unsuitable
13
for the C NMR relaxation study. DIOP (bis(6-methylheptyl)
phthalate) was synthesized by esterification of phthalic anhy-
dride with 6-methyl-1-heptanol with phosphotungstic acid (H3-
*
E-Mail: j-jonas@beckman.uiuc.edu. Fax: (217) 244-3993.
S1089-5639(98)00365-X CCC: $15.00 © 1998 American Chemical Society
Published on Web 04/02/1998

Dynamics of Complex Phthalate Liquids. 2
J. Phys. Chem. A, Vol. 102, No. 17, 1998 2779
TABLE 1: Carbon-13 Chemical Shift Assignments for the
Phthalate Liquids Studied (Chemical Shifts (ppm) Are
Relative to TMS)
carbona
DEHPb
DIOP
DNOP
DHP
A
B
C
D
1
2
3
4
5
6
7
167.0
133.0
129.0
130.9
67.6
39.1
30.7
29.2
23.2
14.2
24.0
11.1
167.0
133.0
129.1
131.0
65.5
29.0
26.6
27.5
39.3
28.2
22.9
166.9
132.8
128.9
130.8
65.4
28.8
26.2
29.52
29.49
32.1
22.8
14.1
167.1
132.9
129.0
131.0
65.5
28.8
26.0
31.8
22.8
14.1
8
a
See Figure 1 for the carbon nuclei labels. b From I.
PO4‚12WO3‚xH2O, Aldrich) as a catalyst.4 6-Methyl-1-heptanol
5
was also prepared according to the literature using hydrobo-
ration of 6-methyl-1-heptene (Wiley Organics). The purity of
the final DIOP product was determined by gas chromatography
to be 97%.
Figure 2. Densities of the phthalate liquids studied as a function of
temperature.
The experimental details, including density measurements,
1
13
viscosity measurements, two-dimensional H- C correlation
1
3
(
HETCOR) experiments for C NMR peak assignments, and
unambiguously assigned. Taking advantage of the tendency for
T1 values to increase in the motional narrowing regime as one
moves away from the ring system along a flexible side chain,
1
3C NMR relaxation measurements are the same as described
in I.
13
the alkyl chain C peaks that have similar chemical shifts were
clearly identified.
Results and Discussion
(
A) 13C Peak Assignments. All carbon-13 nuclei of DIOP,
The 13C chemical shift assignments, relative to tetramethyl-
silane (TMS), are listed in Table 1 for all of the compounds
13
DNOP, and DHP were resolved in the C NMR spectra well
enough for the relaxation measurements under the experimental
conditions, except for carbon nuclei 4 and 5 of DNOP. The
two 13C resonance signals in DNOP have very similar chemical
shifts and could not be resolved completely for the relaxation
measurements, even in the high magnetic field of 9.3 T.
Using both the H- C chemical shift correlation from the
D HETCOR spectra and the spin-lattice relaxation time
comparison, all of the carbon-13 spectral peaks could be
3
studied, including DEHP. The chemical shifts for the carbonyl
and the phenyl ring carbons (A-D), which are the common
structural framework in these molecules, are practically the same
for all of the phthalates.
(B) Density and Viscosity Measurements. The temperature
dependencies of the density for the compounds studied are
shown in Figure 2. The density for each of these liquids are
very well represented as a linear function of temperature, and
1
13
2
TABLE 2: Temperature Dependence of 13C Spin-Lattice Relaxation Times (seconds) for the Proton-Bearing Carbon Nuclei of
DIOP Measured at 100.6 MHz
carbon
66.2 °C
51.4 °C
38.5 °C
29.5 °C
19.9 °C
6.7 °C
-3.8 °C
-15.1 °C
-26.5 °C
-36.2 °C
C
D
1
2
3
4
5
6
7
0.412
0.384
0.522
0.670
0.878
1.09
1.31
2.85
2.52
0.300
0.277
0.362
0.462
0.602
0.740
0.972
1.98
0.239
0.220
0.271
0.351
0.438
0.555
0.688
1.46
0.208
0.191
0.229
0.287
0.362
0.444
0.550
1.12
0.192
0.178
0.200
0.240
0.297
0.364
0.438
0.838
0.891
0.207
0.200
0.188
0.200
0.239
0.289
0.331
0.634
0.629
0.272
0.277
0.194
0.189
0.210
0.253
0.279
0.516
0.479
0.466
0.535
0.220
0.192
0.208
0.242
0.251
0.457
0.347
0.903
1.06
0.273
0.234
0.225
0.243
0.248
0.448
0.267
0.374
0.306
0.269
0.302
0.255
1.89
1.49
1.19
0.212
TABLE 3: Temperature Dependence of 13C Spin-Lattice Relaxation Times (seconds) for the Proton-Bearing Carbon Nuclei of
DNOP Measured at 100.6 MHz
carbon
66.2 °C
51.4 °C
38.5 °C
29.5 °C
19.9 °C
6.7 °C
-3.8 °C
-15.1 °C
-26.5 °C
-36.2 °C
C
D
1
0.456
0.407
0.560
0.700
0.946
1.24
2.01
2.90
4.99
0.331
0.294
0.390
0.507
0.659
0.890
1.41
0.259
0.235
0.294
0.380
0.496
0.659
1.03
0.220
0.202
0.247
0.310
0.396
0.532
0.808
1.15
0.196
0.180
0.208
0.252
0.321
0.435
0.632
0.898
1.88
0.194
0.190
0.187
0.208
0.257
0.340
0.482
0.641
1.38
0.230
0.230
0.188
0.189
0.223
0.286
0.392
0.501
1.07
0.349
0.373
0.203
0.184
0.207
0.257
0.331
0.397
0.832
0.601
0.699
0.239
0.201
0.206
0.246
0.297
0.342
0.649
1.10
1.30
0.302
0.240
0.223
0.252
0.284
0.311
0.520
2
3
4
6
7
8
/5a
1.90
3.89
1.47
2.83
2.35
a
The peaks for carbons 4 and 5 overlap due to similar chemical shifts. The T
may be considered as an average of the T ’s for carbons 4 and 5.
1
’s reported here are the values for the overlapping peaks which
1

2780 J. Phys. Chem. A, Vol. 102, No. 17, 1998
Kim and Jonas
TABLE 4: Temperature Dependence of 13C Spin-Lattice Relaxation Times (seconds) for the Proton-Bearing Carbon Nuclei of
DNOP Measured at 45.2 MHz
carbon
68.7 °C
49.8 °C
30.8 °C
20.0 °C
9.1 °C
-0.6 °C
-14.0 °C
-23.6 °C
-32.8 °C
-42.7 °C
C
D
1
0.480
0.458
0.518
0.741
0.985
1.27
2.07
3.09
5.68
0.281
0.259
0.303
0.428
0.567
0.733
1.21
0.160
0.148
0.172
0.253
0.323
0.419
0.679
1.03
0.131
0.119
0.135
0.181
0.234
0.300
0.495
0.742
1.79
0.106
0.103
0.107
0.133
0.170
0.228
0.346
0.502
1.27
0.101
0.0993
0.0895
0.109
0.137
0.182
0.272
0.378
0.961
0.127
0.131
0.0901
0.0897
0.112
0.144
0.202
0.263
0.676
0.171
0.180
0.0943
0.0888
0.0985
0.128
0.172
0.218
0.555
0.304
0.316
0.108
0.0963
0.0978
0.125
0.159
0.187
0.462
0.124
2
3
0.102
0.123
0.146
0.166
0.399
4
6
7
8
/5a
1.82
3.80
2.33
a
The peaks for carbons 4 and 5 overlap due to close chemical shifts. The T
be considered as an average of the T ’s for carbons 4 and 5.
1
’s reported here are the values for the overlapping peaks which may
1
TABLE 5: Temperature Dependence of 13C Spin-Lattice Relaxation Times (seconds) for the Proton-Bearing Carbon Nuclei of
DHP
carbon
68.4 °C
49.7 °C
30.8 °C
20.0 °C
9.8 °C
-1.3 °C
-12.8 °C
-24.5 °C
-33.0 °C
-43.5 °C
C
D
1
2
3
4
5
6
0.602
0.563
0.719
1.10
1.62
2.27
0.365
0.325
0.409
0.627
0.955
1.37
0.203
0.186
0.231
0.352
0.517
0.728
1.04
0.151
0.135
0.169
0.258
0.359
0.520
0.724
1.78
0.119
0.109
0.128
0.174
0.250
0.350
0.515
1.30
0.101
0.0954
0.100
0.128
0.174
0.247
0.354
0.923
0.102
0.105
0.0898
0.0995
0.135
0.186
0.253
0.693
0.146
0.151
0.0905
0.0892
0.110
0.149
0.191
0.527
0.241
0.264
0.0976
0.0882
0.105
0.140
0.164
0.441
0.122
0.100
0.112
0.139
0.158
0.359
3.27
5.43
1.94
3.77
2.32
Figure 3. Viscosities of the phthalate liquids studied as a function of
temperature. Data for DEHP was copied from I. DNOP solidified at
Figure 4. Temperature coefficient of viscosity, -(1/η)(dη/dT), of the
phthalate liquids studied. The curves were obtained by numerical
differentiation of the analytical function of the VFT form fitted by the
experimental viscosity data.
-
35 °C.
the temperature-density coefficients of these liquids are nearly
-
4
3
the same ((7.6 ( 0.2) × 10 g/cm ‚°C).
smallest possible temperature coefficient of viscosity (or lowest
viscosity-temperature slope) is usually desirable for a wide
working range of temperature. The temperature coefficients of
viscosity (-(1/η)(dη/dT)) of the phthalate liquids are shown in
Figure 4. The curves in the figure were obtained by fitting the
experimental viscosity data to generate an analytical function
of VFT form and then numerically differentiating the resultant
The viscosities of the phthalate liquids as a function of
temperature are shown in Figure 3, where the temperature-
viscosity dependencies of each of the phthalate liquids are well
6
described by the Vogel-Fulcher-Tammann (VFT) equation
over the experimental temperature range. By comparing the
viscosity of the three C-8 phthalate isomers, it is obvious that
the presence of a branch in the alkyl chain makes the fluid more
viscous. The branch also seems to be more effective in
increasing viscosity if it is located near the rigid ring part of
the molecule (DEHP >DIOP).
8
function. Interestingly, two phthalates with unbranched side
chains, DNOP and DHP, have very similar temperature coef-
ficients of viscosity, although the absolute magnitude of
viscosity of DNOP is larger (in Figure 3) owing to its greater
molecular weight. Considering that the temperature coefficient
Besides viscosity, the temperature coefficient of viscosity is
another important bulk property in determining the performance
9
of viscosity depends on the molecular flexibility, this result
7
characteristics of functional fluids including lubricants. The
suggests that the alkyl side chains of DNOP and DHP have a

Dynamics of Complex Phthalate Liquids. 2
J. Phys. Chem. A, Vol. 102, No. 17, 1998 2781
Figure 5. 13C N‚T
open circles at 45.2 MHz. The lines represent the fitting results using the optimized parameters.
1
1
’s of DNOP fitted by the Cole-Davidson distribution model. Solid circles indicate the experimental T ’s at 100.6 MHz and
similar degree of flexibility. The temperature coefficients of
the phthalate esters with branched side chains are higher than
those with unbranched side chains, which is also understandable
because the branching reduces the flexibility of the alkyl chain
by restricting internal rotation of the chain. The flexibility of
the alkyl side chain seems to be reduced more by branching
near the rigid ring/carboxyl group (in DEHP) than by branching
near the end of chain (in DIOP).
as mentioned above, and the T1’s reported here are the data for
the overlapping peaks. These T1 values were assumed to be
the average of the T1’s for the two C nuclei.
1
3
The experimental relaxation data for the individual carbon
nuclei, except for the methyl carbons, were analyzed in terms
of a model assuming a Cole-Davidson distribution of relaxation
times. The inadequacy of the analytical methods for the
quaternary (A and B) and methyl carbons has been discussed
(
C) 13C NMR Relaxation Data Analysis. The low-temper-
in the earlier papers. For the relationship between temperature
1,3
ature limit of the spin-lattice relaxation times (T1) and nuclear
and correlation time, the limiting correlation time τ0 in the
Cole-Davidson distribution model was assumed to have a VFT
temperature dependence, and T0 for each compound was
determined by fitting the temperature-dependent viscosity data
in Figure 3 as in I. The experimental relaxation data for DIOP,
DNOP, and DHP were very well represented by the Cole-
Davidson distribution model. The optimized set of parameters
of the model simultaneously represented the experimental T1
and NOE data at a single frequency, as well as the T1 data at
Overhauser enhancement (NOE) measurements for individual
1
3
C nuclei was again determined by a low signal-to-noise ratio
or inability to resolve the individual peak at that temperature.
The experimental T1 data as a function of temperature are
presented in Tables 2-5 for proton-bearing carbon nuclei in
DIOP (measured at 100.6 MHz), DNOP (both at 100.6 and 45.2
MHz), and DHP (45.2 MHz). Two 13C peaks with similar
intensity for carbons 4 and 5 in DNOP were not resolved well

2782 J. Phys. Chem. A, Vol. 102, No. 17, 1998
Kim and Jonas
Figure 6. Comparison of the â values in the Cole-Davidson
distribution model for the analyzed carbon nuclei in the phthalates
studied. The â values for DEHP are from I.
Figure 8. Average correlation times ( τj ) âτ
0
) for the alkyl carbons
of the phthalate liquids studied. Asterisks indicate the branching point,
which is carbon 2 in DEHP and carbon 6 in DIOP.
framework (the phenyl ring and carboxyl group) in units of
carbon atoms. In this figure, â values for the methyl carbons,
which are very low (e0.02) and physically meaningless, are
indicated as a dotted line only for the purpose of avoiding
confusion in locating the end of the alkyl chain. The â values
of alkyl carbons generally decrease as one moves away from
the relatively rigid part of the molecule to the end of the chain,
indicating mobility gradients along the alkyl chain. Comparing
the mobility gradients of DNOP and DHP, both of which have
unbranched side chains with different lengths, the total decreases
of the â values along each alkyl chain are nearly the same (from
0
.22 for C-1 to 0.08 for C-(n - 1)) due to the steeper gradient
1
3
of the shorter chain of DHP. That is, C nuclei in both chains
take a similar range of â values. This result implies that the
flexibilities of both side chains are similar, which can be related
to the similar temperature coefficients of viscosity of these two
compounds in the previous section (Figure 4).
Figure 7. â values of the side chain carbons versus the distance from
the rigid framework (the phenyl ring and carboxyl group) in units of
carbon atoms. â values for the methyl carbons are indicated as dotted
lines only for the purpose of avoiding confusion in locating the end of
the alkyl chains. Asterisks indicate the branching point, which is carbon
The most conspicuous feature in Figure 7 is that the â value
of the carbon atom at the branching point (carbon 2 in DEHP
and carbon 6 in DIOP, which are indicated in the figure) is
large compared to those of the other alkyl carbons. Also the â
value for the carbon atom “before” the branching point (carbon
2
in DEHP and carbon 6 in DIOP.
two different frequencies as in the case of DNOP. As an
illustration, the resultant fits of N‚T1’s as a function of
temperature for the individual 13C nuclei in DNOP using the
Cole-Davidson distribution model are shown in Figure 5.
Interpretation of the optimized parameters of the Cole-
Davidson distribution model provides useful information on the
motional behavior of these molecules. Among the parameters,
the distribution width, â, contains information on the detailed
motional features of each carbon atom of the molecule. The
optimized values of â for the individual carbon nuclei of the
compounds are shown in Figure 6. In all of the molecules, the
â value for carbon D is higher than that of carbon C, indicating
the prevailing reorientation along the axis perpendicular to the
D-D′ bond.3
1
in DEHP and carbon 5 in DIOP) appears to have increased.
The higher â value in the Cole-Davidson distribution model
indicates a narrower distribution of correlation times in the
spectral density function, which implies in this case that the
number of modes of internal motion is smaller. In other words,
the internal motion of an alkyl chain is restricted by the presence
of the branch, especially at the branching point. The branching
also seems to affect the motional freedom of the C-H
internuclear vector adjacent to the branching point, especially
nearer to the relatively rigid part of the molecule. Obviously a
restriction of the internal motion reduces the flexibility of the
alkyl chain, and macroscopically results in enhanced viscosity
and temperature coefficients (Figures 3 and 4).
Comparing the rise in the â value at the branching point in
DEHP with that in DIOP, the position of branching seems to
be also important, and the extent of motional restriction appears
to be affected by the proximity of the branching point to the
relatively rigid framework of phthalate. In fact, the larger â
The main points of interest in this investigation are the â
values for the alkyl carbons in the side chains of the molecules.
The â values of the side chain carbons are plotted in Figure 7,
where the x-axis is the distance from the relatively rigid

Dynamics of Complex Phthalate Liquids. 2
J. Phys. Chem. A, Vol. 102, No. 17, 1998 2783
Figure 9. Energy contour plot of the conformational space composed of φ
chain in DEHP. The figures on the contour represent strain energy in kcal/mol relative to the global minimum in the conformational space. The
contours up to 15 kcal/mol above the minimum are shown here, beyond which the conformation is considered to be inaccessible.
1
-φ
2
(a) for an unbranched alkyl chain in DNOP and (b) for a branched
value at the branching point does not necessarily mean that the
correlation time for the C-H internuclear vector at the branching
point is greater than that of the other carbons. The average
correlation times ( τj ) âτ0) for the alkyl carbons are plotted in
Figure 8. The limiting correlation time (τ0), which represents
the slowest process (or overall motion) of the molecular motion,
decreases along the alkyl side chain away from the rigid
framework. As the distance from the rigid part of the molecule
increases, the effect of the overall motion of the ring at the site
of the side chain carbon diminishes more effectively due to the
extensive segmental motion of the alkyl chain. This is also
accompanied by an increase in motional freedom indicated by
a decrease in the â values (mobility gradient). In a branched
side chain, the internal motion of the alkyl chain is restricted at
the available conformational space is more restricted compared
to the unbranched alkyl chain, and the energy wells in the
conformational space are steeper implying higher energy barriers
to rotation about these torsional angles. Consequently the
internal motions of the alkyl chain are restricted by the presence
of the branch, which reduces the flexibility of the molecule.
Conclusion
The effect of structural changes in the side chains of
phthalates on their motional characteristics could be investigated
13
by the C NMR relaxation data analyzed in terms of the Cole-
Davidson distribution model. The presence of a branch in the
alkyl side chain of phthalate liquids was found to restrict the
internal motion of the alkyl chain and consequently reduce the
flexibility of the molecule, which was also confirmed by using
molecular modeling. The branching near the rigid-ring portion
of the molecule was more effective in restraining the motional
degree of freedom of the alkyl chain in the molecule. The
microscopic motional features of the alkyl chain could be related
to the macroscopic viscosity and its temperature dependence,
which is the most significant bulk property in deciding the
performance characteristics of functional fluids including lu-
bricants.
(or near) the branching point. If the branching point is far from
the phenyl ring, however, the effect of the restriction should
decrease due to the internal motions of the preceding part of
the alkyl chain. In this respect, branching near the rigid ring
portion of the molecule would appear to be more effective in
reducing the flexibility of the alkyl chain in the molecule, which
also agrees with the macroscopic observation that the temper-
ature dependence of viscosity in DEHP is larger than that of
DIOP.
The effect of branching on the restriction of the internal
motion can be illustrated by examining the conformational space
for the flexible alkyl chain. Figure 9 compares the contour of
strain energy calculated by the UFF force field in the
conformational space composed of two torsional angles, φ1 and
φ2, around the branching point in DEHP (a) and for an
unbranched alkyl chain in DNOP (b). For the branched chain,
Acknowledgment. This work was supported in part by the
Air Force Office for Scientific Research under Grant AFOSR
F49620-93-1-0241 and the National Science Foundation under
Grant NSF CHE 95-26237.
1
0
References and Notes
(1) Kim, Y. J.; Jonas, J. J. Phys. Chem. A 1998, 102, 2767.

2784 J. Phys. Chem. A, Vol. 102, No. 17, 1998
Kim and Jonas
(
2) In fact, the common name “diisooctyl phthalate” is ambiguous. In
(7) Briant, J.; Denis, J.; Parc, G. Rheological Properties of Lubricants;
EÄ ditions Technip: Paris, 1989.
this context, it indicates bis(6-methylheptyl) phthalate.
(
(
(
3) Kim, Y. J.; Jonas, J. J. Phys. Chem. 1995, 99, 6777.
4) Schwegler, M. A.; van Bekkum, H. Appl. Catal. 1991, 74, 191.
5) Markgraf, J. H.; Lusskin, S. I.; McDonald, E. C.; Volpp, B. D. J.
(8) Koonin, S. E.; Meredith, D. C. Computational Physics (Fortran
Version); Addison-Wesley: New York, 1990; Chapter 1.
Chem. Ecol. 1983, 9, 211.
6) (a) Fulcher, G. S. J. Am. Ceram. Soc. 1923, 8, 339. (b) Tammann,
G.; Hesse, G. Z. Anorg. Allg. Chem. 1926, 156, 245. (c) Harrison, G. The
Dynamic Properties of Supercooled Liquids; Academic Press: New York,
(
9) Bondi, A. Physical Properties of Lubricating Oils; Reinhold: New
(
York, 1951; Chapter 2.
(10) Rapp e´ , A. K.; Casewit, C. J.; Colwell, K. S.; Goddard, W. A., III.;
1
976.
Skiff, W. M. J. Am. Chem. Soc. 1992, 114, 10024.