“… 3 Densities of the liquids studied as a function of temperature. The data for EHB and EHC are from refs and , respectively. The densities of DEHP have been reported in the literature, and they agree well with the experimental data.…”
Section: Resultsmentioning
confidence: 99%
“… 4 Viscosities of the liquids studied as a function of temperature. The data for EHB are from ref . Some of the data points for DEHP are from the literature .…”
Section: Resultsmentioning
confidence: 99%
“…Consequently, κ provides information about the degree of motional anisotropy. The validity of eq 6 has been justified in several complex, viscous liquids including EHB and EHC, , although the degree of rotational−translational coupling appeared to be affected by temperature or pressure if applied over an extended range of viscosity. ,, In all cases, the molecular shape has a significant effect on the rotational−translational coupling in neat liquids. ,,− …”
The motional behavior of complex phthalates and structurally
related liquids has been investigated in order
to study the structural effect of molecular framework on the
macroscopic and microscopic dynamic properties.
13C NMR spin−lattice relaxation times and nuclear
Overhauser enhancements (NOE) of individual carbon
nuclei of bis(2-ethylhexyl) cyclohexane-1,2-dicarboxylate (DEHHP),
bis(2-ethylhexyl) isophthalate (DEHIP),
bis(2-ethylhexyl) terephthalate (DEHTP), and
tris(2-ethylhexyl) trimellitate (TOTM) have been measured
as
a function of temperature from −40 to 90 °C. Individual
13C peaks were unambiguously assigned by using
2D hydrogen−carbon chemical shift correlation spectra. In
addition, the density and viscosity of these
compounds as a function of temperature have been measured. The
results were also compared with those of
2-ethylhexyl benzoate (EHB), 2-ethylhexyl cyclohexanecarboxylate (EHC),
and bis(2-ethylhexyl) phthalate
(DEHP), complex liquids studied earlier in our laboratory. Both
the macroscopic and microscopic dynamic
properties were significantly affected by the structure of the
molecular framework. The conjugation between
a phenyl ring and carboxyl groups was found to make the structural
framework stiffer and resulted in an
enhanced viscosity. The 13C NMR relaxation data were
interpreted in terms of a theoretical model assuming
a Cole−Davidson distribution of correlation times. The internal
motions of the common 2-ethylhexyl side
chains were found to be independent of the nature of the molecular
framework of the compounds. The
parameter β in the Cole−Davidson model, representing the
distribution width of the involved correlation
times of the molecular motions, contained useful information on the
detailed motional features of individual
carbon nuclei. The β values for the rigid phenyl ring carbons
reflected the reorientational anisotropy resulting
from the molecular shape of the structural framework and could be
successfully correlated with the parameter
κ introduced by McClung and Kivelson in the modified
Stokes−Einstein−Debye (SED) equation. On the
other hand, the β values for the flexible cyclohexane ring carbons
were affected by both the internal motion
of the cyclohexane ring and the anisotropy of the molecular
reorientation.
“… 3 Densities of the liquids studied as a function of temperature. The data for EHB and EHC are from refs and , respectively. The densities of DEHP have been reported in the literature, and they agree well with the experimental data.…”
Section: Resultsmentioning
confidence: 99%
“… 4 Viscosities of the liquids studied as a function of temperature. The data for EHB are from ref . Some of the data points for DEHP are from the literature .…”
Section: Resultsmentioning
confidence: 99%
“…Consequently, κ provides information about the degree of motional anisotropy. The validity of eq 6 has been justified in several complex, viscous liquids including EHB and EHC, , although the degree of rotational−translational coupling appeared to be affected by temperature or pressure if applied over an extended range of viscosity. ,, In all cases, the molecular shape has a significant effect on the rotational−translational coupling in neat liquids. ,,− …”
The motional behavior of complex phthalates and structurally
related liquids has been investigated in order
to study the structural effect of molecular framework on the
macroscopic and microscopic dynamic properties.
13C NMR spin−lattice relaxation times and nuclear
Overhauser enhancements (NOE) of individual carbon
nuclei of bis(2-ethylhexyl) cyclohexane-1,2-dicarboxylate (DEHHP),
bis(2-ethylhexyl) isophthalate (DEHIP),
bis(2-ethylhexyl) terephthalate (DEHTP), and
tris(2-ethylhexyl) trimellitate (TOTM) have been measured
as
a function of temperature from −40 to 90 °C. Individual
13C peaks were unambiguously assigned by using
2D hydrogen−carbon chemical shift correlation spectra. In
addition, the density and viscosity of these
compounds as a function of temperature have been measured. The
results were also compared with those of
2-ethylhexyl benzoate (EHB), 2-ethylhexyl cyclohexanecarboxylate (EHC),
and bis(2-ethylhexyl) phthalate
(DEHP), complex liquids studied earlier in our laboratory. Both
the macroscopic and microscopic dynamic
properties were significantly affected by the structure of the
molecular framework. The conjugation between
a phenyl ring and carboxyl groups was found to make the structural
framework stiffer and resulted in an
enhanced viscosity. The 13C NMR relaxation data were
interpreted in terms of a theoretical model assuming
a Cole−Davidson distribution of correlation times. The internal
motions of the common 2-ethylhexyl side
chains were found to be independent of the nature of the molecular
framework of the compounds. The
parameter β in the Cole−Davidson model, representing the
distribution width of the involved correlation
times of the molecular motions, contained useful information on the
detailed motional features of individual
carbon nuclei. The β values for the rigid phenyl ring carbons
reflected the reorientational anisotropy resulting
from the molecular shape of the structural framework and could be
successfully correlated with the parameter
κ introduced by McClung and Kivelson in the modified
Stokes−Einstein−Debye (SED) equation. On the
other hand, the β values for the flexible cyclohexane ring carbons
were affected by both the internal motion
of the cyclohexane ring and the anisotropy of the molecular
reorientation.
“…[351] Intermolecular cross relaxation rates , the differences between rates in the neat EHB liquid and corresponding intramolecular rates, were investigated , and related to the inverse of the rate of translational diffusion through Torrey's translational model for relaxation. [345,351,352].…”
The nuclear Overhauser effect (NOE) is a consequence of cross-relaxation between nuclear spins mediated by dipolar coupling. Its sensitivity to internuclear distances has made it an increasingly important tool for the determination of throughspace atom proximity relationships within molecules of sizes ranging from the smallest systems to large biopolymers. With the support of sophisticated FT-NMR techniques, the NOE plays an essential role in structure elucidation, conformational and dynamic investigations in liquid-state NMR. The efficiency of magnetization transfer by the NOE depends on the molecular rotational correlation time, whose value depends on solution viscosity. The magnitude of the NOE between 1 H nuclei varies from +50 % when molecular tumbling is fast to -100% when it is slow, the latter case corresponding to the spin diffusion limit.In an intermediate tumbling regime, the NOE may be vanishingly small. Increasing the viscosity of the solution increases the motional correlation time, and as a result, otherwise unobservable NOEs may be revealed and brought close to the spin diffusion limit. The goal of this review is to report the resolution of structural problems that benefited from the manipulation of the negative NOE by means of viscous solvents, including examples of molecular structure determination, conformation elucidation and mixture analysis (the ViscY method).3.7. Overview of advantages/disadvantages of viscous solvents in molecule mixture analysis 68
Conclusion 71Acknowledgements References 71Glossary 82
“…2-Ethylhexyl benzoate (EHB) has been studied extensively using NMR, − infrared, , and Raman spectroscopies as a model compound for elastohydrodynamic lubrication. A previous experiment in our laboratory 26 using 2D NOESY found cross peaks appearing at the elevated pressure, indicating possible space dipolar coupling between the aromatic protons and the methyl protons on the ends of the alkyl chains in EHB.…”
The Raman noncoincidence effect has been used to
investigate the changes in intermolecular interactions
induced by applying high pressure to neat liquid alkyl benzoates.
The noncoincidence of the carbonyl band
of a homologous series of straight chain alkyl benzoates (methyl,
ethyl, propyl, butyl, and hexyl benzoate)
and a branched chain alkyl benzoate (2-ethylhexyl benzoate) was
measured at 20 and 40 °C over the pressure
range 1−5000 bar. The density was measured as a function of
pressure for all molecules. A transition point
dividing the noncoincidence behavior as a function of density into two
regions was found, for all molecules
except methyl benzoate. Below the transition point, at lower
density, the noncoincidence value was relatively
insensitive to changes in density, while above the transition point,
the noncoincidence value dropped sharply
with increasing density. The decrease in the value of
noncoincidence above the transition point was interpreted
as the change in intermolecular interactions resulting from the
conformational change in favor of a “folded”
form of the alkyl side chain shielding the carbonyl group. To
examine the plausibility of the presence of
energetically allowable conformations with a folded alkyl side chain,
conformational searches based on
molecular mechanics calculations have also been performed. The
strain energy of some alkyl benzoate
conformers with folded side chain was calculated to be within a few
kcal/mol of the global minimum
conformation. Both the experimental results and the conformational
analysis suggest that the population of
the folded conformer increases under high-pressure conditions owing to
its compactness.
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