Pressure-induced Spectral Changes Research Articles

H2O and D2 mixtures under pressure: Spectroscopy and proton exchange kinetics

We have investigated the pressure-induced spectral changes and the proton exchange reactions of D(2)-H(2)O mixtures to 64 GPa using micro-Raman spectroscopy. The results show the profound difference in the rotational and vibrational Raman spectra of hydrogen isotopes from those of the pure samples, showing the vibrational modes at higher frequencies and continuing to increase with pressure without apparent turnover. This indicates the repulsive nature of D(2)-H(2)O interaction without hydrogen bonds between the two and, thus, interstitial fillings of D(2) molecules into the bcc-like ice lattice. The spectral analysis using the Morse potential yields a hydrogen bond distance of 0.734 Å at 6 GPa--slightly shorter than that in pure--attributed to the repulsive interaction. The pressure-dependent spectral changes suggest that the proton-ordering transition in the ice lattice occurs over a large pressure range between 28 and 50 GPa, which is substantially lower than that of pure ice (40-80 GPa). This again indicates the presence of high internal pressure arising from the repulsive interaction. The Raman spectra show evidences that the proton exchange occurs in various phases including in solid D(2) and H(2)O mixtures. Based on the time-dependent spectral changes, we obtained the proton exchange rates of k ~ 0.085 h(-1) at 0.2 GPa in fluid D(2) and water mixtures, k ~ 0.03 h(-1) and 0.003 h(-1) at 2 GPa and 4 GPa, respectively, in fluid D(2)-ice mixtures, and k ~ 10(-3) h(-1) at 8 GPa in solid D(2) and ice mixtures.

Effects of High Pressure on the Luminescent Properties of Nanocrystalline and Bulk Y2O3:Eu3+

High pressure-induced spectral changes in a 20-nm cubic nanocrystalline yttria doped with europium and its corresponding bulk were studied in the range of 550-750 nm, corresponding to the 5D0 --> 7Fd (J = 0-4) transitions. The results demonstrate that the bulk Y2O3 underwent phase transition from the cubic phase to the monoclinic phase as the pressure increased to 15 GPa, while the 20-nm nanocrystals did not. This can be concluded from the fact that the 5D0 --> 7F0 line and the three 5D0 --> 7F1 sublines originating from the cubic phase disappeared, while another group of 5D0 --> 7F0 and 5D0 --> 7F1 lines appeared. In addition, the relative intensity of the peak around 630 nm to that around 611 nm varied obviously as the pressure surpassed 15 GPa. The variations in the nanocrystals were more sluggish in comparison to those in the bulk, indicating that the nanocrystalline yttria had improved compressibility, which is attributed to an increased surface energy in nanocrystals. The local environment surrounding luminescent Eu3+ in the nanocrystals and the bulk both became more disordered with the increase of the pressure. The phase transition from the cubic to the monoclinic is irreversible.

Electronic Phase Transitions in F-electron Metals at High Pressures: Synchrotron X-ray Spectroscopic Studies on GD to 100 GPa

AbstractUnusual phase transitions driven by electron correlation effects occur in many f-electron metals (lanthanides and actinides alike) from localized phases to itinerant phases at high pressures. The dramatic changes in atomic volumes and crystal structures associated with some of these transitions signify equally important changes in the underlying electronic structure of these correlated f-electron metals. Yet, the relationships among the crystal structure, electronic correlation and electronic structure in f-electron metals have not been well understood. In this study, utilizing recent advances in third-generation synchrotron x-ray spectroscopies and high-pressure diamond-anvil cell technologies, we describe the pressure-induced spectral changes across the volume collapse transition in Gd at 60 GPa and well above. The spectral results suggest that the f-electrons of high-pressure Gd phases are highly correlated even at 100 GPa – consistent with the Kondo volume collapse model and the recent experimental evidence of strong electron correlation of α-Ce.

Pressure-induced spectral changes for the special-pair radical cation of the bacterial photosynthetic reaction center

The effect of pressure up to 6 kbars on the near to mid infrared absorption spectrum (7500-14,300 cm(-1) or 1333-700 nm) of the oxidized reaction center of Rhodobacter sphaeroides is measured and interpreted using density-functional B3LYP, INDO, and PM5 calculations. Two weak electronic transition origins at approximately 8010 and approximately 10,210 cm(-1) are unambiguously identified. The first transition is assigned to a Qy tripdoublet band that involves, in the localized description of the excitation, a triplet absorption on one of the bacteriochlorophyll molecules (PM) in the reaction center's special pair intensified by the presence of a radical cation on the other (PL). While most chlorophyll transition energies decrease significantly with increasing pressure, the tripdoublet band is found to be almost pressure insensitive. This difference is attributed to the additional increase in the tripdoublet-band energy accompanying compression of the pi-stacked special pair. The second band could either be the anticipated second Qy tripdoublet state, a Qx tripdoublet state, or a state involving excitation from a low-lying doubly occupied orbital to the half-occupied cationic orbital. A variety of absorption bands that are also resolved in the 8300-9600 cm(-1) region are assigned as vibrational structure associated with the first tripdoublet absorption. These sidebands are composites that are shown by the calculations to comprise many unresolved individual modes; while the calculated pressure sensitivity of each individual mode is small, the calculated pressure dependence of the combined sideband structure is qualitatively similar to the observed pressure dependence, preventing the positive identification of possible additional electronic transitions in this spectral region.

High-pressure Raman investigation of the Pb-free relaxorBaTi0.65Zr0.35O3

We report high-pressure Raman measurements (up to 27 GPa) on the perovskite-type relaxor ferroelectric ${\mathrm{BaTi}}_{0.65}{\mathrm{Zr}}_{0.35}{\mathrm{O}}_{3}({\mathrm{BTZ}}_{35}).$ Distinct changes of the Raman spectra with increasing pressure illustrate that ${\mathrm{BTZ}}_{35}$ presents important pressure instabilities leading to at least two phase transitions around 5.7 GPa and 15.1 GPa, respectively. The observed pressure-induced spectral changes for ${\mathrm{BTZ}}_{35}$ are reminiscent of features that have been observed for pure ${\mathrm{BaTiO}}_{3},$ thus for a classic ferroelectric. The latter is surprising since earlier reports indicate a relaxor-characteristic Raman signature as a function of temperature, thus illustrating that ${\mathrm{BTZ}}_{35}$ might well occupy a peculiar niche in perovskite-type ferroelectrics.

High-pressure FTIR study of the stability of horseradish peroxidase. Effect of heme substitution, ligand binding, Ca++ removal, and reduction of the disulfide bonds.

The pressure stability of horseradish peroxidase isoenzyme C and the identification of possible stabilizing factors are presented. The effect of heme substitution, removal of Ca(2+), binding of a small substrate molecule (benzohydroxamic acid), and reduction of the disulfide bonds on the pressure stability were investigated by FTIR spectroscopy. HRP was found to be extremely stable under high pressure with an unfolding midpoint of 12.0 +/- 0.1 kbar. While substitution of the heme for metal-free mesoporphyrin did not change the unfolding pressure, Ca(2+) removal and substrate binding reduced the midpoint of the unfolding by 2.0 and 1.2 kbar, respectively. The apoprotein showed a transition as high as 10.4 kbar. However, the amount of folded structure present at the atmospheric pressure was considerably lower than that in all the other forms of HRP. Reduction of the disulfide bonds led to the least pressure stable form, with an unfolding midpoint at 9.5 kbar. This, however, is still well above the average pressure stability of proteins. The high-pressure stability and the analysis of the pressure-induced spectral changes indicate that the protein has a rigid core, which is responsible for the high stability, while there are regions with less stability and more conformational mobility.

Raman spectroscopic study of structural changes in calcium aluminate (CaAl 2O 4) glass at high pressure and high temperature

The Raman spectrum of calcium aluminate (CaAl 20 4) glass has been obtained as a function of pressure at ambient temperature to ∼ 15 GPa, and as a function of temperature at room pressure to 1650 K. On increasing pressure at 300 K, the Raman spectrum changes drastically between 8 and 10 GPa. The intensity of the overall spectrum weakens, the intensity of the 580-cm −1 band decreases markedly, and a new broad band centered near 750 cm −1 begins to dominate the spectrum. We interpret these spectral changes as due to the formation of highly coordinated A1 species through the involvement of bridging oxygens (i.e. formation of [3]O species). Upon decompression, the pressure-induced spectral changes are mostly reversible. However, the decompressed samples do not relax completely to their normal ambient state, and the spectra reflect important changes in the inter-tetrahedral AlOA1 linkages. These changes are consistent with some high-coordinate species being retained in the decompressed sample. On increasing temperature through the glass transformation range ( T g = 1178 K) at room pressure, our Raman spectra show evidence for important configurational changes, after vibrational effects are removed. The major spectral change consists of growth of a broad band between 600 and 1100 cm −1 appearing immediately above T g. We suggest that these configurational changes concern the removal of O 2− ions from the fully polymerized aluminate framework, with the resulting formation at high temperature of [3]O species (“triclusters”), coordinated by three tetrahedral aluminate groups.

High-pressure laser spectroscopy of Cr3+:Gd3Sc2Ga3O12 and Cr3+:Gd3Ga5O12.

We report on the effect of high pressure on the room-temperature emission spectra and lifetimes of ${\mathrm{Cr}}^{3+}$:GSGG (${\mathrm{Gd}}_{3}$${\mathrm{Sc}}_{2}$${\mathrm{Ga}}_{3}$${\mathrm{O}}_{12}$) and ${\mathrm{Cr}}^{3+}$:GGG (${\mathrm{Gd}}_{3}$${\mathrm{Ga}}_{5}$${\mathrm{O}}_{12}$). In both systems we observed a dramatic change of the overall emission band shape upon increasing pressure, from a nearly structureless broadband ${(}^{4}$${\mathit{T}}_{2}$${\ensuremath{\rightarrow}}^{4}$${\mathit{A}}_{2}$) to a highly structured narrow band ${(}^{2}$E${\ensuremath{\rightarrow}}^{4}$${\mathit{A}}_{2}$). From the peak energy of the broadband emission, we estimated the pressure-induced blueshift of the $^{4}$${\mathit{T}}_{2}$${\ensuremath{\rightarrow}}^{4}$${\mathit{A}}_{2}$ transition to be 10 (\ifmmode\pm\else\textpm\fi{}2) ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$/kbar. High-resolution measurements in the R-line region (\ensuremath{\sim}700 nm) revealed that the $^{2}$E${\ensuremath{\rightarrow}}^{4}$${\mathit{A}}_{2}$ transition hardly shifts at low pressures (40 kbar), whereas at higher pressures (>60 kbar) a nearly linear redshift of 0.65 (\ifmmode\pm\else\textpm\fi{}0.05) ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$/kbar is observed. Besides pressure-induced spectral changes, an enormous increase in the emission lifetime with increasing pressure was found for both systems. In the case of ${\mathrm{Cr}}^{3+}$:GSGG, the lifetime changed from 110 \ensuremath{\mu}s at ambient pressure to 4.4 ms at 125 kbar. For ${\mathrm{Cr}}^{3+}$:GGG, the lifetime increased from 168 \ensuremath{\mu}s to 7.3 ms for the same pressure range. The pressure-induced spectral and lifetime changes are described by a single configurational coordinate model that considers the effect of pressure on the thermal and spin-orbit coupling of the $^{2}$E and $^{4}$${\mathit{T}}_{2}$ states. A previously reported pressure-induced R-line-shift reversal in ${\mathrm{Cr}}^{3+}$:GSGG and the effect of high pressure on the lifetime in ${\mathrm{Cr}}^{3+}$:YAG are also discussed within the same framework.

Effects of internal and external pressure on the structure and dynamics of micelles: A FTIR study of sodium and potassium decanoates in D 2O 1

Infrared spectra of aqueous micellar solutions of sodium and potassium decanoates were measured as a function of external pressure up to and exceeding 50 kbar. Counterion interactions also generate an internal pressure in these micellar systems which strongly affects the features and shape of the infrared spectra. The internal pressure-induced spectral changes are countercation dependent and are discussed in terms of countercation effects on the structural and dynamic properties of these aqueous surfactants. External pressure, on the other hand, induces a structural phase transition from the micellar state to a coagel state, which is also affected by the nature of the countercation. The critical coagelization pressure is 21.2 kbar for sodium decanoate and 22.5 kbar for potassium decanoate. The decanoate molecules in the micellar state are conformationally and orientationally highly disordered. External pressure cannot induce conformational ordering of the methylene chains in the micellar phase; on the contrary, the conformational disorder in this phase increases with increasing pressure. This is the first demonstration that disordered structures can exist in a methylene chain system at pressures as high as 20 kbar. The alkyl chains become fully extended and the reorientational fluctuation of the decanoate molecules are completely damped at the critical coagelization pressures. The structure of the pressure-induced coagel phases is different from that of the corresponding temperature-induced coagel phases due to the different magnitude of the counterion binding force under extreme conditions of temperature or pressure.

Raman intensity measurements for determining conformer populations as a function of pressure

Raman spectroscopy of fluids under pressure has been increasingly studied1,2 and applied3,4 over the past few years. Essential to the extension of these investigations is the reliable interpretation of the pressure-induced spectral changes of model systems in terms of variations in conformation and molecular dynamics. Conformational investigations have been made on systems that were suspected of undergoing conformational change at high pressures, for example, n-alkanes5–9, haloalkanes10–13 and substituted cyclo-hexanes14–19. Raman investigations reported here reveal significant pressure-induced spectral changes in both conformationally sensitive and insensitive systems. We suggest that these arise from a breakdown in the tenet of the pressure invariance of a direct proportionality between Raman scattering intensity and concentration. This is most apparent from the differential behaviour of symmetrical and antisymmetrical vibrations. We propose a re-evaluation of some previous interpretations of the pressure-induced spectral changes.

High pressure NMR studies of hemoproteins. The effect of pressure on the quaternary structure of hemoglobin

Proton NMR spectra for nitrosyl-, aquomet- and deoxy des-Arg(α141)-hemoglobin in H 2O were studied at high pressures up to 1400 atm with attention to the exchangeable proton resonances due to the intra- and intersubunit hydrogen bonds. For aquomethemoglboin, the T state marker signal at 6.4 ppm is insensitive to pressure while the R state marker signal at 6.0 ppm exhibits progressive upfield shift upon pressurization. For nitrosylhemoglobin, the T state signals at 9.6 and 6.5 ppm decrease their intensities upon pressurization while the R state marker signal at 6.0ppm remains unchanged. Pressure-induced spectral changes for some of exchangeable resonances are also encountered for deoxy des-Arg(α141)-hemoglobin while the R and T quaternary structural indicators at 6.0 and 9.4 ppm are insensitive to pressure. These pressure-induced spectral changes for these hemoglobin derivatives are significantly distinguished from those associated with the R-T transition induced by addition of IHP or by variatiuon of pH. It is therefore concluded that pressure induces subtle quaternary structural changes in these hemoglobin derivatives without causing the R-T transition.

Effects of pressure on visible spectra of complexes of myoglobin, hemoglobin, cytochrome c, and horse radish peroxidase.

The spectra of the ferric form of most heme proteins [metmyoglobin, methemoglobin, horse radish peroxidase (EC 1.11.1.7), and ferricytochrome c at pH 1.5] are converted from high-spin (open crevice) structure to low-spin (closed crevice) form under pressure. Pressures up to 8000 kg/cm2 (780 MPa) have no effect on the spectra of high-spin ferro- and ferricytochrome c, which have a closed crevice structure at pH 7.0. Spectra of deoxy-ferromyoglobin and deoxy-ferrohemoglobin are reduced in intensity, but pressure does not change the positions of the absorption maxima. Cyanide ion prevents pressure-induced spectral changes in metmyoglobin and methemoglobin up to 8000 kg/cm2. Carbon monoxide (with a high affinity for the ferro heme iron) has a similar effect on ferromyoglobin and ferrohemoglobin. The pressure required to cause spectral changes in the heme proteins falls in the order, cytochrome c (pH 7.0) greater than horse radish peroxidase greater than myoglobin greater than hemoglobin. We have calculated a volume change of --50 cm3/mol associated with the configurational change accompanying the reformation of the iron-methionine bond in cytochrome c at low pH.