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Near edge x-ray absorption fine structure (NEXAFS) spectra at the boron K-edge were measured for aqueous boric acid, borate, and polyborate ions, using liquid microjet technology, and compared with simulated spectra calculated from first principles density functional theory in the excited electron and core hole (XCH) approximation. Thermal motion in both hydrated and isolated molecules was incorporated into the calculations by sampling trajectories from quantum mechanics/molecular mechanics simulations at the experimental temperature. The boron oxide molecules exhibit little spectral change upon hydration, relative to mineral samples. Simulations reveal that water is arranged nearly isotropically around boric acid and sodium borate, but the calculations also indicate that the boron K-edge NEXAFS spectra are insensitive to hydrogen bonding, molecular environment, or salt interactions. [ABSTRACT FROM AUTHOR]

Near edge x-ray absorption fine structure (NEXAFS) spectroscopy at the nitrogen and carbon K-edges was used to study the hydration of adenosine triphosphate in liquid microjets. The total electron yield spectra were recorded as a function of concentration, pH, and the presence of sodium, magnesium, and copper ions (Na+/Mg2+/Cu2+). Significant spectral changes were observed upon protonation of the adenine ring, but not under conditions that promote π-stacking, such as high concentration or presence of Mg2+, indicating that NEXAFS is insensitive to the phenomenon. Intramolecular inner-sphere association of Cu2+ did create observable broadening of the nitrogen spectrum, whereas outer-sphere association with Mg2+ did not. [ABSTRACT FROM AUTHOR]

Near edge x-ray absorption fine structure spectra have been measured at the carbon and nitrogen K-edges of the prototypical aromatic molecule, pyrrole, both in the gas phase and when solvated in water, and compared with spectra simulated using a combination of classical molecular dynamics and first principles density functional theory in the excited state core hole approximation. The excellent agreement enabled detailed assignments. Pyrrole is highly reactive, particularly in water, and reaction products formed by the auto-oligomerization of pyrrole are identified. The solvated spectra have been measured at two different temperatures, indicating that the final states remain largely unaffected by both hydration and temperature. This is somewhat unexpected, since the nitrogen in pyrrole can donate a hydrogen bond to water. [ABSTRACT FROM AUTHOR]

Relaxation of highly vibrationally excited pyrimidine (C4N2H4) by collisions with carbon dioxide has been investigated using diode laser transient absorption spectroscopy. Vibrationally hot pyrimidine (E′=40 635 cm-1) was prepared by 248-nm excimer laser excitation, followed by rapid radiationless relaxation to the ground electronic state. The nascent rotational population distribution (J=58–80) of the 0000 ground state of CO2 resulting from collisions with hot pyrimidine was probed at short times following the excimer laser pulse. Doppler spectroscopy was used to measure the CO2 recoil velocity distribution for J=58–80 of the 0000 state. Rate constants and probabilities for collisions populating these CO2 rotational states were determined. The measured energy transfer probabilities, indexed by final bath state, were resorted as a function of ΔE to create the energy transfer distribution function, P(E,E′), from E′-E∼1300–7000 cm-1. P(E,E′) is fitted to a single exponential and a biexponential function to determine the average energy transferred in a single collision between pyrimidine and CO2 and parameters that can be compared to previously studied systems using this technique, pyrazine/CO2, C6F6/CO2, and methylpyrazine/CO2. P(E,E′) parameters for these four systems are also compared to various molecular properties of the donor molecules. Finally, P(E,E′) is analyzed in the context of two models, one which suggests that the shape of P(E,E′) is primarily determined by the low-frequency out-of-plane donor vibrational modes and one which suggests that the shape of P(E,E′) can be determined by how the donor molecule final density of states changes with ΔE. [ABSTRACT FROM AUTHOR]

Relaxation of highly vibrationally excited pyrimidine (C4N2H4) by collisions with carbon dioxide has been investigated using diode laser transient absorption spectroscopy. Vibrationally hot pyrimidine (E′=40 635 cm-1) was prepared by 248-nm excimer laser excitation, followed by rapid radiationless relaxation to the ground electronic state. The nascent rotational population distribution (J=58–80) of the 0000 ground state of CO2 resulting from collisions with hot pyrimidine was probed at short times following the excimer laser pulse. Doppler spectroscopy was used to measure the CO2 recoil velocity distribution for J=58–80 of the 0000 state. Rate constants and probabilities for collisions populating these CO2 rotational states were determined. The measured energy transfer probabilities, indexed by final bath state, were resorted as a function of ΔE to create the energy transfer distribution function, P(E,E′), from E′-E∼1300–7000 cm-1. P(E,E′) is fitted to a single exponential and a biexponential function to determine the average energy transferred in a single collision between pyrimidine and CO2 and parameters that can be compared to previously studied systems using this technique, pyrazine/CO2, C6F6/CO2, and methylpyrazine/CO2. P(E,E′) parameters for these four systems are also compared to various molecular properties of the donor molecules. Finally, P(E,E′) is analyzed in the context of two models, one which suggests that the shape of P(E,E′) is primarily determined by the low-frequency out-of-plane donor vibrational modes and one which suggests that the shape of P(E,E′) can be determined by how the donor molecule final density of states changes with ΔE. [ABSTRACT FROM AUTHOR]

The diode laser probing of the HCN photoproduct is used for measuring the quantum yield for HCN formation through the photodissociation of 2,3-, 2,5-, and 2,6-dimethylpyrazine (DMP). The supercollisions are linked to low-frequency out-of-plane motion, suggesting that the molecular motions leading to efficient energy transfer are the same motions involved in photodissociation.

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