Raman Scattering Studies on n-Heptane under High Pressure

Pressure and Composition Effects on a Common Nanoparticle Ligand-Solvent Pair

The effect of pressure on the properties of nanoparticles is a growing area of investigation. These measurements are typically performed in a colloidal suspension; however, pressure-induced changes in the interactions between the nanoparticle surface and the solvent are often neglected. Here, we report vibrational spectroscopy of a common nanoparticle ligand, 1-dodecanethiol, and a common solvent, toluene, under pressure. We find that the pressure-induced phase change of the 1-dodecanethiol is altered by the presence of toluene and that change depends on the concentration of the free ligand in the solution. At near-equal concentrations, phase segregation is observed and the dodecanethiol crystallizes independently from the toluene. On the other hand, at unequal concentrations, concerted phase transitions are observed in the dodecanethiol and toluene, and a disordered conformation of dodecanethiol is maintained under much higher pressures. These results shed light on the pressure-induced changes in intermolecular interactions between nanoparticle ligands and solvents, which must be considered in the design of high-pressure investigations of colloidal nanoparticles.

High-Pressure Raman Study of n-Octane up to 15 GPa

The high-pressure (HP) phase transition and conformational change of n-octane (hereafter abbreviation as n-C₈) up to 15.3 GPa were studied using Raman spectroscopy to investigate the relationship between the HP phase state and the alkyl chain length of n-alkanes. The Raman spectral analysis of n-C₈ indicated that the liquid-solid transition (solidification) occurs at ∼0.9 GPa and that the corresponding transition pressure of n-alkanes depends on their density. Further pressurization at ∼4 GPa increased the population of the gauche conformer, while the solid (order)-amorphous transition occurred at ∼6 GPa along with a change in the full width at half maximum of the ruby R₁ fluorescence line. The comparison of our findings with previously reported results suggested that the even-odd effect in the HP phase transition after solidification of n-alkanes appears between n-C₇ and n-C₈ as their HP phase transition up to ∼15 GPa was different.

Raman spectroscopy of solid-phase n-dodecane and methyl oleate under high pressure

Room-temperature Raman spectra of n-dodecane and methyl oleate were acquired at pressures up to 4 GPa. Both have monoalkyl chains with characteristic Raman rotamer profiles (trans and gauche conformers) along the skeletal CC bonds. The trans conformer was dominant in the high-pressure solid phase of n-dodecane. In solid methyl oleate, most of the gauche conformers transformed to the trans conformers. The all-trans dominated in the high-pressure solid phase and a triclinic subcell structure was indicated on the basis of previous vibrational spectra of n-alkanes and lipids. The solidification pressure of methyl oleate was determined against temperature up to 413 K.

Remarkable pressure-induced emission enhancement based on intermolecular charge transfer in halogen bond-driven dual-component co-crystals

A series of two-component co-crystals driven by IN interactions based on the bipyridine (BIPY) chromophore with one among three different co-former building blocks, iodopentafluorobenzene (IPFB), 1,4-diiodotetrafluorobenzene (DITFB) and 1,3,5-trifluoro-2,4,6-triiodobenzene (IFB), were prepared and analysed via infared spectroscopy and single-crystal X-ray diffraction. By comparing the IN distances in the co-crystal structures, we found that the higher the -F ratio in the building blocks the closer the contact of the IN bond, enhancing the intermolecular interactions in these co-crystals as well. That is, the positive electrostatic potential on the iodine atom(s) in the co-formers was enhanced by the presence of strong electron-withdrawing groups. The distinct spectroscopic behaviours (fluorescence and Raman spectra) among the two-component BIPY co-crystal systems in response to hydrostatic pressure were also investigated. Interestingly, the fluorescence of BIPY-DITFB presented intriguing abnormal evolution from dark to bright, suggesting a new charge transfer state due to the decreased intermolecular distance and the enhanced IN interactions. Theoretical simulations by Materials Studio also showed the shortened IN distance and the increased angle of C-IN, evidencing the enhanced IN interactions. In contrast, BIPY-IFB showed only slightly enhanced fluorescence intensity at 550 nm consistent with BIPY-DITFB. Once the pressure was relieved, both the Raman and fluorescence spectra for BIPY co-crystal systems entirely self-recovered. Remarkable emission enhancement in a solid-state co-crystal has been rarely reported in previous publications and in fact, this study paves a unique way for designing and developing novel stimuli-responsive photo-functional materials.

Raman and IR Spectroscopy Studies on Propane at Pressures of Up to 40 GPa

Raman and IR spectroscopy studies on propane were performed at pressures of up to 40 GPa at ambient temperatures using the diamond anvil cell technique. Propane undergoes three phase transitions at 6.4(5), 14.5(5), and 26.5(5) GPa in Raman spectroscopy and at 7.0(5), 14.0(5), and 27.0(5) GPa in IR spectroscopy. The phase transitions were identified using the Raman and IR splitting modes and the appearance or disappearance of peaks, which clearly corresponded to the changes in the frequencies of the modes as the pressure changed. Our results demonstrate the complex high-pressure behavior of solid propane.

Conformational adjustment for high-pressure glass formation of 1-alkyl-3-methylimidazolium tetrafluoroborate

The role of the alkyl-chain length (the conformational adjustment effect) in high pressure glass formation of 1-alkyl-3-methylimidazolum tetrafluoroborate.

Comparative high pressure Raman studies on perfluorohexane and perfluoroheptane

High pressure Raman spectroscopic studies on perfluorohexane and perfluoroheptane have performed up to 12 GPa. Perfluorohexane under goes two pressure induced transitions: (1) liquid-solid transition at 1.6 GPa and (2) solid-solid transition at 8.2 GPa. On the contrary, perfluoroheptane under goes three phase transitions, they are as follows: (1) liquid-solid transition at 1.3 GPa, (2) intermediate solid I transition at 3 GPa, (3) solid II transition at 7 GPa. The change in slope (dω/dP) shows that the solid I transition at 3.0 GPa could be the conversion of mid-gauche defect into trans conformers for perfluoroheptane. The pressure induced Raman spectra and the behavior of individual band with pressure shows that the solid phase comprises more than one conformer beyond crystallization. The intensity ratio for both the compounds shows that the high pressure phase beyond 8.2 and 7.0 GPa tends to have close packing with distorted all-trans conformers.

In situ high-pressure and high-temperature experiments on n-heptane

The Raman spectroscopy of n-heptane was investigated in a moissanite anvil cell at ambient temperatures and a diamond anvil cell under pressures of up to ~2000 MPa and at temperature range from 298 to 588 K. The results show that at room temperature the vibration modes, assigned to the symmetric and antisymmetric stretching of CH(3) and CH(2) stretching, shifted to higher frequency according to quasi-linearity with increasing pressure, and a liquid-solid phase transition occurred at near 1150 MPa. The high-temperature solidus line of n-heptane follows a quadratic function of P = 0.00737T(2) + 5.27977T - 1195.76556. Upon phase change, fitting the experimental data obtained in the temperature range of 183∼412 K to the Clausius-Clapeyron equation allows one to define the thermodynamic parameters of n-heptane of dP/dT = 0.01474T + 5.27977.

SERS study of rotational isomerization of cysteamine induced by magnetic pulling force

The effects of unidirectional pulling forces on covalently bridged cysteamine between superparamagnetic particles and Ag nanoparticles (NPs) were studied with SERS spectroscopy. With an increase in the pulling force from 0 to 100 pN per magnetic particle, the nu(C-S)(Trans)/nu(C-S)(Gauche) intensity ratio was increased from 0.6 to 1.08, the Raman frequency of nu(C-S)(Trans) was shifted from 716 to 719 cm(-1), and the Raman bands associated with the amide groups were diminished. From these observations, it was concluded that the magnetic forces induced the extension of distance between the magnetic particle surface and the Ag NP surface and the rotational isomerization equilibrium of SC-CN was shifted from the gauche to the trans conformation with the longer molecular length.

Response of small-scale, methyl rotors to protein-ligand association: a simulation analysis of calmodulin-peptide binding

Changes in the free energy barrier (DeltaE), entropy, and motional parameters associated with the rotation of methyl groups in a protein (calmodulin (CaM)) on binding a ligand (the calmodulin-binding domain of smooth-muscle myosin (smMLCKp)) are investigated using molecular dynamics simulation. In both the bound and uncomplexed forms of CaM, the methyl rotational free energy barriers follow skewed-Gaussian distributions that are not altered significantly upon ligand binding. However, site-specific perturbations are found. Around 11% of the methyl groups in CaM exhibit changes in DeltaE greater than 0.7 kcal/mol on binding. The rotational entropies of the methyl groups exhibit a nonlinear dependence on DeltaE. The relations are examined between motional parameters (the methyl rotational NMR order parameter and the relaxation time) and DeltaE. Low-barrier methyl group rotational order parameters deviate from ideal tetrahedrality by up to approximately 20%. There is a correlation between rotational barrier changes and proximity to the protein-peptide binding interface. Methyl groups that exhibit large changes in DeltaE are found to report on elements in the protein undergoing structural change on binding.

Pressure-Induced Structural Transition in n-Pentane: A Raman Study

Pressure-induced Raman spectroscopy studies on n-pentane have been carried out up to 17 GPa at ambient temperature. n-Pentane undergoes a liquid-solid transition around 3.0 GPa and a solid-solid transition around 12.3 GPa. The intensity ratio of the Raman modes related to all-trans conformation (1130 cm-1 and 2850 cm-1) to that of gauche conformation (1090 cm-1 and 2922 cm-1) suggests an increase in the gauche population conformers above 12.3 GPa. This is accompanied with broadening of Raman modes above 12.3 GPa. The high-pressure phase of n-pentane above 12.3 GPa is a disordered phase where the carbon chains are kinked. The pressure-induced order-disorder phase transition is different from the behavior of higher hydrocarbon like n-heptane.