Molecular Force Measurement in Liquids and Solids Using Vibrational Spectroscopy

Hutchinson, Erik J.; Ben‐Amotz, Dor

doi:10.1021/jp9730656

Cited by 20 publications

(36 citation statements)

References 45 publications

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“…10 The density dependence of the empirically determined attractive shift is essentially linear over the entire density range studied, in accord with the predictions of the simplest mean-field treatment, but not with other experiments and extensions of the theory, which would lead one to expect a higher order dependence at the higher densities. 5,6,17,26 A comparison of the equatorial and axial attractive shifts shows that the shift also scales with the polarizability derivative as predicted, at least when differences between equatorial and axial CH stretching are properly accounted for in the repulsive shift calculation. However, the theoretically predicted attractive shifts are a factor of 2 smaller than the empirical ones, consistent with what was found for cyclohexane-d 11 in liquid solvents.…”

Section: Discussionmentioning

confidence: 56%

“…For the liquid solvents with reduced density s s 3 Ϸ1, this treatment reduced the ratio of empirical to theoretical A ␣ values from about four to two, i.e., to roughly the same level of agreement obtained here for CO 2 at reduced densities р0. 6.…”

Section: B Frequency Shiftsmentioning

confidence: 99%

“…In particular, a number of studies in supercritical solvents have revealed a ''local solvent density enhancement'' or ''solvent clustering'' around certain solutes, a phenomenon which has often been detected spectroscopically. 13 The few previous studies of vibrational frequency shifts and vibrational dynamics in supercritical fluids have not always shown the signatures of this phenomenon, 14 but both Fayer and co-workers, 15 in work on the solute W(CO) 6 , and Schwarzer et al, 16 in work on azulene, have observed an unusual density dependence in vibrational shifts and vibrational relaxation in the region of the solvent's critical point which could be taken as evidence of a local density enhancement around the solute. Thus, an ancillary goal of our work was to look for evidence of similar behavior in the CH stretching spectra of cyclohexane-d 11 in supercritical CO 2 .…”

Section: Introductionmentioning

confidence: 99%

“…[1][2][3][4][5][6] The average solvent force along a vibrational coordinate in the solute will shift the vibrational frequency and corresponding band center in the vibrational spectrum, whereas fluctuations in the solvent forces lead to vibrational energy relaxation and dephasing, which determine the linewidth of the band. Thus, by examining the solute vibrational spectrum in different solvents and different solvent conditions, e.g., temperature and density, one can attempt to determine the nature of the underlying forces and their dynamics.…”

Section: Introductionmentioning

confidence: 99%

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Solvent–solute interactions and the Raman CH stretching spectrum of cyclohexane-d11. II. Density dependence in supercritical carbon dioxide

Pan

McDonald

MacPhail

1999

The Journal of Chemical Physics

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We have measured the isotropic Raman CH stretching spectrum of cyclohexane-d11 in supercritical CO2 at 49.7 °C and in liquid CO2 at room temperature over a range of densities from 0.2ρc to 2ρc, where the critical number density ρc for CO2 is 6.4 nm−3. The axial and equatorial CH stretching bands in the spectrum shift to lower frequencies and broaden with increasing density. As was the case in an earlier study of cyclohexane-d11 in liquid solvents [G. J. Remar and R. A. MacPhail, J. Chem. Phys. 103, 4381 (1995)], the “perturbed hard-fluid model” of Ben-Amotz and Herschbach provides a satisfyingly consistent description of the observed shifts in terms of competing contributions from repulsive and attractive solute–solvent forces along the CH bond. In particular, when the repulsive contribution to the shift is calculated according to the prescription developed in the liquid solution study, the attractive contribution is found to scale linearly with the density and with the polarizability derivative of the CH bond, as predicted by the model. The ratio of the equatorial to axial linewidths has a density-independent value of 1.2, nearly the same value found for the liquid solutions and numerically equivalent to the ratio of polarizability derivatives for the CH bonds. This equivalence is consistent with Schweizer and Chandler’s theoretical result for the width of a band that is inhomogeneously broadened by attractive force fluctuations, but the density dependence is not; their result would predict a nonlinear density dependence with a maximum near ρc, whereas the observed linewidths show a nearly linear dependence on density. Neither the frequency shifts nor the linewidths show any clear evidence for a “local solvent density enhancement” that would be predicted for this mixture near the critical point. In the accompanying paper, Frankland and Maroncelli describe molecular-dynamics simulations of cyclohexane in supercritical CO2 that reproduce the observed linewidths nearly quantitatively. They show convincing evidence that the linewidths are dominated by binary, collisional interactions between the hydrogen and the solvent, and they discuss the apparent absence of a density enhancement.

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Section: Discussionmentioning

confidence: 56%

Section: B Frequency Shiftsmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Solvent–solute interactions and the Raman CH stretching spectrum of cyclohexane-d11. II. Density dependence in supercritical carbon dioxide

Pan

McDonald

MacPhail

1999

The Journal of Chemical Physics

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“…The force along a dangling OH bond was estimated by using the following expression for the solvent mean force along a pseudodiatomic bond (28,29,36,37): F Ϸ [Ϫ2f 2 /(3g)](⌬/). The ratio ⌬/ represents the frequency shift of the dangling OH relative to that at the air-water interface, whereas f Ϸ 918 nN/nm and g Ϸ Ϫ19,600 nN/nm 2 are the harmonic and anharmonic force constants, respectively, of a localized (isolated) OH stretching vibration.…”

Section: Methodsmentioning

confidence: 99%

Observation of water dangling OH bonds around dissolved nonpolar groups

Perera

Fega

Lawrence

et al. 2009

Proc. Natl. Acad. Sci. U.S.A.

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164

186

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We report the experimental observation of water dangling OH bonds in the hydration shells around dissolved nonpolar (hydrocarbon) groups. The results are obtained by combining vibrational (Raman) spectroscopy and multivariate curve resolution (MCR), to reveal a high-frequency OH stretch peak arising from the hydration shell around nonpolar (hydrocarbon) solute groups. The frequency and width of the observed peak is similar to that of dangling OH bonds previously detected at macroscopic air-water and oil-water interfaces. The area of the observed peak is used to quantify the number of water dangling bonds around hydrocarbon chains of different length. Molecular dynamics simulation of the vibrational spectra of water molecules in the hydration shell around neopentane and benzene reveals high-frequency OH features that closely resemble the experimentally observed dangling OH vibrational bands around neopentyl alcohol and benzyl alcohol. The red-shift of Ϸ50 cm ؊1 induced by aromatic solutes is similar to that previously observed upon formation of a -H bond (in low-temperature benzene-water clusters).hydrophobic ͉ interface ͉ vibration ͉ Raman C hanges in the structure and dynamics of water induced by nonpolar groups have long been considered to play a key role in protein folding, ligand binding, and the formation of biological cell membranes (1). Early thermodynamic evidence suggested that water may form an ''iceberg'' or clathrate-like structure around nonpolar molecules (2). Although no such rigid structures are currently thought to form (3), recent experimental (4, 5) and theoretical (6) results indicate that the rotational mobility of water molecules is reduced around nonpolar solutes (relative to bulk water). Moreover, fundamental theoretical arguments (and simulation measurements) imply that the size of a hydrophobic group may play a critical role in dictating water structure (7). This has led to the provocative suggestion that the structure of water around hydrophobic groups of nanometer (or greater) size may bear some resemblance to that at a macroscopic air-water interface (8). Although cohesive interactions between water and nonpolar groups tend to suppress the formation of an interfacial vapor layer (8-11), recent experimental (and molecular dynamics) studies indicate that the nonpolar binding cavity in bovine ␤-lactoglobulin is completely dehydrated in liquid water (12). Moreover, both experimental and simulation evidence suggests that water at nonpolar interfaces experiences significantly larger fluctuations than either bulk water or water at hydrophilic interfaces (13,14). Here, we present experimental evidence that reveals a similarity between the structure of water around dissolved hydrocarbon groups and that at macroscopic oil-water interfaces (15-18), in the sense that both interfaces induce the formation of dangling OH bonds.We have detected water dangling OH bonds by combining vibrational Raman spectroscopy with multivariate curve resolution (MCR). This procedure is used to decompose solution sp...

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