A. Chemical Shifts: Hydrogen Bonding

polarization of the solvent due to a permanent dipole moment in the molecule,

σEX represents the effective short-range exchange interactions, and σS is the

contribution from specific interactions such as hydrogen bonding.

For the molecules investigated, the (CH3 )3 , CH3 , and OH groups will each

experience their own shielding environment as seen in Eq. (1). One assumes that

changes in pressure or temperature affect the nonspecific contributions to the

nuclear shielding in a similar manner for all the different groups’ resonances.

Thus, the difference between the shielding of the groups can be related to the

specific interactions in solution, σS , which is due to hydrogen bonding of the

OH group. The use of the chemical shift difference [ δ (ppm)] between either

the OH and (CH3 )3 groups or the OH and CH3 groups for tert-butanol and

methanol, respectively, eliminates the nonspecific contributions that augment the

nuclear shielding as a function of pressure and temperature. Therefore, through

the investigation of the chemical shifts of the (CH3 )3 , CH3 , and OH groups

of pure tert-butanol and methanol over an extended temperature and pressure

range, δ can be used to qualitatively estimate changes in the hydrogen bond

network in solution as a function of pressure and temperature.

Figure 1 is a plot of δ (ppm) vs. pressure at various temperatures for

tert-butanol and methanol. Focusing on tert-butanol, at constant temperature, δ

increases as pressure increases. At constant pressure, δ decreases with increasing temperature. The slope [(∂ δ/∂P )T] for the three temperatures shown in

Figure 1 for tert-butanol (50, 100, and 150◦ C) increases with increasing tempera-



Figure 1 Plot of the chemical shift difference δ (ppm) vs. pressure for tert-butanol

and methanol. tert-Butanol: (᭺, 50◦ C), (᭹, 100◦ C), (ᮀ, 150◦ C), pressure to 0.5 kbar.

Methanol: (᭺, 50◦ C), (᭹, 100◦ C), (ᮀ, 150◦ C), (᭿, 200◦ C), (᭝, 250◦ C), (᭡, 300◦ C),

(᭛, 350◦ C), (᭜, 400◦ C), ( , 450◦ C), ( , 500◦ C), pressure to 2.0 kbar.



Copyright 2002 by Marcel Dekker. All Rights Reserved.



ture. These observations can be explained within the framework of the hydrogen

bonding occurring in solution. Hydrogen bonding removes electron density from

the vicinity of the 1 H nucleus contributing to the deshielding of the proton. Qualitatively, an increase in δ correlates with an increase in the deshielding of the

OH proton relative to that of the (CH3 )3 group and thus an increase in hydrogen

bonding in solution. The results in Figure 1 demonstrate that increasing temperature at constant pressure tends to decrease the extent of hydrogen bonding

in tert-butanol, whereas increasing pressure at constant temperature increases

hydrogen bonding in solution. One would anticipate that increasing temperature

would more readily disrupt hydrogen bonds in solution. Increasing pressure at

high temperatures should have a large effect on the solutions’ hydrogen bond

network, contributing to the larger slope [(∂ δ/∂P )T ] seen at the higher temperatures. This behavior is more readily apparent for methanol as discussed in

the following section.

For methanol, the δ data were obtained over a much wider range of

pressure and temperature (50–500◦ C and 2 kbar). Similar behavior is seen for

δ in methanol as a function of pressure and temperature when compared to

tert-butanol. A dramatic change of δ in the vicinity of the methanol critical

point (methanol Tc is 239.4◦ C) at low pressure is observed (note the 250◦ C

to 350◦ C isotherm). This is related to the large changes in density and thus

hydrogen bonding of solution in this region. As pressure is increased through

this temperature region, hydrogen bonding increases, which contributes to a

change in shielding of the nucleus and thus to a change in δ. If one focuses on

the pressure region of 0.5–2.0 kbar, it is more readily discernible that the slope

[(∂ δ/∂P )T ] increases at higher temperatures. This could be due to a change in

both the extent and strength of the hydrogen bond network at high temperatures

as one changes pressure as compared to a change in hydrogen bond strength

alone at low (50◦ C) temperatures with increasing pressure (22). However, the

NMR chemical shift data presented in Figure 1 clearly indicate that significant

hydrogen bond interactions exist for methanol at high temperatures and pressures

and in the critical region. The results reported here using the capillary highpressure NMR cell are in good agreement with earlier measurements (18–20) at

comparable temperatures and pressures.

Further support for hydrogen bonding interactions remaining important in

methanol in the near-critical region is provided by a comparison with gaseous

methanol. Ultimately, at the limit of infinitely high temperatures and low densities any substance approaches the ideal gas limit where there are no intermolecular interactions and hence no hydrogen bonding. Hoffmann and Conradi

measured the hydroxyl proton chemical shifts of ethanol, methanol (19), and water (24) over pressure and temperature ranges that cover all three phases: vapor,

liquid, and supercritical. These measurements established for all three solvents

the hydroxyl proton chemical shift value for the low-density, high-temperature



Copyright 2002 by Marcel Dekker. All Rights Reserved.



ideal gas limit. In each case, an estimate of the extent of hydrogen bonding near

the critical point was obtained by applying a simple two-state model, where

each site is either bonded or nonbonded. The chemical shift measurement is

an average value over all sites, which leads to a linear relationship between the

chemical shift σ and the extent of hydrogen bonding θ. Setting θ = 0 at the ideal

gas limit and θ = 1 at ambient conditions, the extent of hydrogen bonding at

the critical point of water, ethanol, and methanol compared to room temperature

was 0.25, 0.3, and 0.4, respectively. This analysis further supports the premise

that hydrogen bonding remains important at the critical point for these three solvents. Hoffmann and Conradi also provide a comparison between the solvents on

the basis of reduced thermodynamic variables (19). The density-reduced extent

of hydrogen bonding (θ/ρ∗ ) was calculated for the alcohols where the density

ρ∗ was scaled to ambient conditions. The density dependence of this quantity

reveals a near constancy at liquid-like densities but a strong density dependence

at gas-like densities. The observed strong increase of the θ/ρ∗ isotherms with

density at gas-like densities was related to the formation of hydrogen bonded

oligomers with indications that dimer formation is limited in favor of trimers

and possibly higher ordered oligomers. The NMR evidence for the formation of

hydrogen-bonded oligomers in water and the alcohols at low, gas-like densities

is in keeping with a large body of experimental gas phase results, as reviewed

in reference (25).

B. Chemical Shifts: Solution Behavior

High-pressure NMR has been used to study the solution behavior of polymers in supercritical CO2 . The polymers investigated by Dardin et al., using

a folded capillary design for the high-pressure NMR cell, were poly(1,1-dihydroperfluorooctyl acrylate) and poly(1,1-dihydroperfluorooctyl acrylate-blockstryene) (26). The proton chemical shifts of the polymers were determined as

a function of temperature and pressure. The upper critical solution temperature

and the upper critical solution density were determined from a transition in the

chemical shift of the proton resonances of the polymers with CO2 density. A coexistence region was determined at intermediate densities of CO2 , in which the

polymers existed in two distinct solution environments. In the two-phase region,

the chemical shift of the polymer-rich phase was used to estimate the amount of

CO2 in that phase. The block copolymer is known to form micelles in supercritical CO2 . From the NMR studies it was inferred that at high CO2 densities the

styrene units close to the core–shell interface were highly solubilized in CO2 .

In a second series of experiments by Dardin et al. (27), using a foldedcapillary high-pressure NMR cell, the authors investigated the 1 H and 19 F chemical shifts of hexane, perfluorohexane, and 1,1-dihydroperfluorooctylpropionate

in supercritical CO2 as a function of pressure and temperature. The nuclear



Copyright 2002 by Marcel Dekker. All Rights Reserved.



shielding for a molecule as described by Eq. (1) is related to specific and nonspecific intermolecular interactions. These interactions will all contribute to the

experimental chemical shift of the nuclei. The 1 H chemical shifts of hexane in

solution were determined to be solely governed by the contribution to the nuclear

shielding due to the bulk susceptibility, σB , which is a function of CO2 density.

For the 19 F chemical shift, the van der Waals dispersion interaction term, σW ,

was also needed to explain the experimental chemical shifts of perfluorohexane

as a function of temperature and pressure. These findings were interpreted as

indicating nonspecific van der Waals interactions between the fluorinated sites

on the solute molecule and CO2 .

Overall, high-pressure capillary NMR investigations are proving to be very

useful for studying solute–solvent interactions in solution and other solvent effects (i.e., hydrogen bonding) as a function of solution pressure and temperature.

C. Relaxation Time (T1 ) Measurements

NMR relaxation measurements provide information about the rotational reorientation and spatial reorientation (translational motion) of molecules in solution. A recent review of the density dependence on rotational and translational

molecular dynamics was published in 1993 (28). Using high-pressure capillary

NMR spectroscopy, the determination of 19 F, 1 H, and 2 H relaxation times (T1 )

of perfluorobenzene, benzene, and perdeuterobenzene were measured in carbon

dioxide as a function of pressure and temperature to address the role of potential CO2 /F intermolecular interactions in solution (29). The pressure range for

the relaxation time measurement was between 0.4 and 2.33 kbar over the temperature limits of 25–150◦ C. The density of the solvent, carbon dioxide, over

these conditions was between 0.55 and 1.27 g/cm3 . Over these conditions, the

contributions to the molecular relaxation processes for both 1 H and 19 F in CO2

could be determined. From the comparison of the relaxation processes for 19 F

and 1 H in CO2 , especially at high densities, the occurrence of specific molecular

interactions between CO2 and fluorine could be addressed.

The spin-lattice relaxation of a molecule is governed by its interactions

with the surrounding solvent bath through complex processes, some of which

are composed of internal reorientation, spatial translation, and changes in angular momentum. The spin-lattice relaxation mechanism can be described as

being composed of dipole–dipole interactions (DD), spin-rotation interactions

(SR), quadrupolar interactions (Q), chemical shift anisotropy (CSA), and scalar

coupling (SC). These represent the more common nuclear relaxation processes

encountered in liquids and gases. These magnetic interactions DD, SR, CSA, SC

and magnetic/electric field interactions (Q), will contribute to different degrees

in the reequilibration of the nuclei after excitation by the radiofrequency (rf)

field pulse.



Copyright 2002 by Marcel Dekker. All Rights Reserved.