Recent experimental and theoretical results have suggested that organic acids such as pyruvic acid, can be photolyzed in the ground electronic state by the excitation of the OH stretch vibrational overtone. These overtones absorb in the near-infrared and visible regions of the spectrum where the solar photons are plentiful and could provide a reaction pathway for the organic acids and alcohols that are abundant in the earth's atmosphere. In this paper the overtone initiated photochemistry of aqueous pyruvic acid is investigated by monitoring the evolution of carbon dioxide. In these experiments CO(2) is being produced by excitation in the near-infrared, between 850 nm and ∼1150 nm (11,765-8696 cm(-1)), where the second OH vibrational overtone (Δν = 3) of pyruvic acid is expected to absorb. These findings show not only that the overtone initiated photochemical decarboxylation reaction occurs but also that in the aqueous phase it occurs at a lower energy than was predicted for the overtone initiated reaction of pyruvic acid in the gas phase (13,380 cm(-1)). A quantum yield of (3.5 ± 1.0) × 10(-4) is estimated, suggesting that although this process does occur, it does so with a very low efficiency.
It was recently predicted by simulations and confirmed by neutron diffraction experiments that the structure of liquid tetrahydrofuran (THF) contains cavities. The cavities can be quite large and have a net positive electrostatic potential, so they can serve as pre-existing traps for excess electrons created via photodetachment from various solutes. In this paper, we use electron photodetachment via charge-transfer-to-solvent (CTTS) excitation of sodide (Na(-)) to probe for the presence of pre-existing cavities in a series of ether solvents: THF, diethyl ether, 1,2-dimethoxyethane (DME), and diglyme (DG). We find that electrons photodetached from sodide appear after a time delay with their equilibrium spectrum in all of these solvents, suggesting that the entire series of ethers contains pre-existing solvent cavities. We then use the variation in electron recombination dynamics with CTTS excitation wavelength to probe the nature of the cavities in the different ethers. We find that the cavities that form the deepest electron traps turn on at about the same energy in all four ether solvents investigated, but that the density of cavities is lower in DG and DME than in THF. We also examine the dynamics of the neutral sodium species that remains following CTTS photodetachment of an electron from sodide. We find that the reaction of the initially created gas-phase-like Na atom to form a (Na(+),e(-)) tight-contact pair occurs at essentially the same rate in all four ether solvents, indicating that only local solvent motions and not bulk solvent rearrangements are what is responsible for driving the partial ejection of the remaining Na valence electron.
An experiment is described where students determine the binary phase diagram of mixed monolayer films of pentadecanoic acid (C15H30O2) and isopalmitic acid (C16H32O2). They do this by using Langmuir–Blodgett troughs to measure isotherms, surface pressure (π) measurements as a function of surface area, of the mixed films above an aqueous surface. The experiment is appropriate for third-year physical chemistry students. Students determine the surface pressure of the start of the phase transition between the liquid expanded (LE) phase and the tilted condensed (TC) phase from their isotherms and use it to obtain a boundary line between the LE and coexistence region. They also calculate the mole fraction of the solution in the tilted condensed phase at the pressure where the beginning of the LE to TC transition occurs to obtain the boundary line between the coexistence region and the TC region, allowing the students to create a binary phase diagram for the LE to TC transition. Students also integrate their isotherms to determine the free energy of mixing of their solutions at two surface pressures. This experiment allows for several variations with different types of phase diagrams, and students seemed to enjoy the more modern take on the classic binary phase diagram experiment.
A low-cost, time-resolved spectroscopy experiment appropriate for third year physical chemistry students is presented. Students excite o-methyl red in basic solutions with a laser pointer and use a modular spectrometer with a CCD array detector to monitor the transient spectra as the higher-energy cis conformer of the molecule converts back to the thermodynamically more stable trans form. The transient absorption dynamics are monitored as a function of time averaged between 340 and 360 nm, and the transient bleach dynamics are monitored as a function of time averaged between 440 and 460 nm. The bleach dynamics are monitored in basic solutions at several values of pH, and the observed rate constants are used to extract the rate constant for the isomerization of the protonated form and the deprotonated form of the molecule.
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