Isotope effects on the structural transformation and relaxation of deeply supercooled water.

We have examined the structure of supercooled liquid D2O as a function of temperature between 185 and 255 K using pulsed laser heating to rapidly heat and cool the sample on a nanosecond timescale. The liquid structure can be represented as a linear combination of two structural motifs, with a transition between them described by a logistic function centered at 218 K with a width of 10 K. The relaxation to a metastable state, which occurred prior to crystallization, exhibited nonexponential kinetics with a rate that was dependent on the initial structural configuration. When the temperature is scaled by the temperature of maximum density, which is an isostructural point of the isotopologues, the structural transition and the non-equilibrium relaxation kinetics of D2O agree remarkably well with those for H2O.

Structural relaxation and crystallization in supercooled water from 170 to 260 K.

The origin of water's anomalous properties has been debated for decades. Resolution of the problem is hindered by a lack of experimental data in a crucial region of temperatures, T, and pressures where supercooled water rapidly crystallizes-a region often referred to as "no man's land." A recently developed technique where water is heated and cooled at rates greater than 109 K/s now enables experiments in this region. Here, it is used to investigate the structural relaxation and crystallization of deeply supercooled water for 170 K < T < 260 K. Water's relaxation toward a new equilibrium structure depends on its initial structure with hyperquenched glassy water (HQW) typically relaxing more quickly than low-density amorphous solid water (LDA). For HQW and T > 230 K, simple exponential relaxation kinetics is observed. For HQW at lower temperatures, increasingly nonexponential relaxation is observed, which is consistent with the dynamics expected on a rough potential energy landscape. For LDA, approximately exponential relaxation is observed for T > 230 K and T < 200 K, with nonexponential relaxation only at intermediate temperatures. At all temperatures, water's structure can be reproduced by a linear combination of two, local structural motifs, and we show that a simple model accounts for the complex kinetics within this context. The relaxation time, τ rel , is always shorter than the crystallization time, τ xtal For HQW, the ratio, τ xtal /τ rel , goes through a minimum at ∼198 K where the ratio is about 60.

Reversible structural transformations in supercooled liquid water from 135 to 245 K.

A fundamental understanding of the unusual properties of water remains elusive because of the limited data at the temperatures and pressures needed to decide among competing theories. We investigated the structural transformations of transiently heated supercooled water films, which evolved for several nanoseconds per pulse during fast laser heating before quenching to 70 kelvin (K). Water's structure relaxed from its initial configuration to a steady-state configuration before appreciable crystallization. Over the full temperature range investigated, all structural changes were reversible and reproducible by a linear combination of high- and low-temperature structural motifs. The fraction of the liquid with the high-temperature motif decreased rapidly as the temperature decreased from 245 to 190 K, consistent with the predictions of two-state "mixture" models for supercooled water in the supercritical regime.

Photodynamics of [FeFe]-Hydrogenase Model Compounds with Bidentate Heterocyclic Ligands.

Two asymmetrically structured model compounds for the hydrogen-generating [Fe-Fe]-hydrogenase active site were investigated to determine the ultrafast photodynamics, structural intermediates, and photoproducts compared to more common symmetric di-iron species. The bidentate-ligand-containing compounds studied were Fe2(µ-S2C3H6)(CO)4(bipy), 1, and Fe2(µ-S2C3H6)(CO)4(phen), 2, in dilute room temperature acetonitrile solution and low-temperature 2Me-THF matrix isolation using static FTIR difference and time-resolved infrared spectroscopic methods (TRIR). Ultraviolet-visible spectra were also compared to time-dependent density functional theory (TD-DFT) to ascertain the orbital origins of long wavelength electronic absorption features. The spectroscopic evidence supports the conclusions that only a propyl-bridge flip occurs in low-temperature matrix, while early time CO ejection leads to the formation of solvated isomeric species on the 25 ps time scale in room temperature solution.

Intramolecular CH Activation and Metallacycle Aromaticity in the Photochemistry of [FeFe]-Hydrogenase Model Compounds in Low-Temperature Frozen Matrices.

The [FeFe]-hydrogenase model complexes [(µ-pdt){Fe(CO)3 }2 ], [(µ-edt){Fe(CO)3 }2 ], and [(µ-mdt){Fe(CO)3 }2 ], where pdt=1,3-propanedithiolate, edt=1,2-ethanedithiolate, and mdt=methanedithiolate, undergo wavelength dependent photodecarbonylation in hydrocarbon matrices at 85 K resulting in multiple decarbonylation isomers. As previously reported in time-resolved solution photolysis experiments, the major photoproduct is attributed to a basal carbonyl-loss species. Apical carbonyl-loss isomers are also generated and may undergo secondary photolysis, resulting in ß-hydride activation of the alkyldithiolate bridge, as well as formation of bridging carbonyl isomers. For [(µ-bdt){Fe(CO)3 }2 ], (bdt=1,2-benzenedithiolate), apical photodecarbonylation results in generation of a 10 π-electron aromatic FeS2 C6 H4 metallacycle that coordinates the remaining iron through an η(5) mode.

Rotameric transformations in the photochemistry of TpM(CO)2(η(3)-C3H4R), where Tp = tris(pyrazolyl)borate, M = Mo or W, and R = H or Me.

Low energy photolysis of TpM(CO)2(η(3)-C3H4R), where Tp = tris(pyrazolyl)borate, M = Mo or W, and R = 2-H or 2-Me in PVC matrices at 85 K results in exo/gauche isomerism of the allyl ligand. This transformation comes in contrast to the behaviour observed in cyclopentadienyl analogues which undergo exo/endo isomerism. DFT computations reveal an η(3) → η(1)* → η(3) mechanism for the allyl rotameric interconversion where the η(1)*-allyl intermediate is generated upon MLCT excitation.

Photochemistry of the permanganate ion in low-temperature frozen matrices.

Photolysis of the permanganate anion, MnO4(-), in tetralkylammonium tetrafluoroborate matrices at 85 K results in formation of a single product, the metastable manganese(V) peroxo complex MnO2(η(2)-O2)(-). Although previously unobserved, this peroxo species has been postulated to be an intermediate in the photodecomposition of permanganate, yielding O2 and MnO2(-). Results from variable-temperature and intensity-dependence photolysis experiments in solution, however, suggest that MnO2(η(2)-O2)(-) does not lose O2 thermally or photochemically and is not an intermediate in the photodecomposition reaction. A mechanism is proposed in which MnO2(η(2)-O2)(-) is formed through vibrational relaxation of an excited [MnO4(-)]* species, which may also follow an alternative relaxation pathway that results in the formation of MnO2(-) and O2 photodecomposition products.

Photochemically induced intramolecular six-electron reductive elimination and oxidative addition of nitric oxide by the nitridoosmate(VIII) anion.

UV photolysis of the nitridoosmate(VIII) anion, OsO3 N(-) , in low-temperature frozen matrices results in nitrogen-oxygen bond formation to give the Os(II) nitrosyl complex OsO2 (NO)(-) . Photolysis of the Os(II) nitrosyl product with visible wavelengths results in reversion to the parent Os(VIII) complex. Formally a six-electron reductive elimination and oxidative addition, respectively, this represents the first reported example of such an intramolecular transformation. DFT modelling of this reaction proceeds through a step-wise mechanism taking place through a side-on nitroxyl Os(VI) intermediate, OsO2 (η(2) -NO)(-) .