Disruption of the hexagonal networks of trimesic acid (benzene-1,3,5-tricarboxylic acid, TMA) by acetic acid

The single crystal X-ray structure of the acetic acid solvate of trimesic acid shows the complete disruption of the hydrogen-bonded hexagonal networks of TMA by bonding to an acetic acid molecule.     

Vibrationally mediated photodissociation of ethene isotopic variants preexcited to the fourth C-H stretch overtone

H and D photofragments produced via vibrationally mediated photodissociation of jet-cooled normal ethene (C2H4), 1,2-trans-d2-ethene (HDCCDH), and 1,1-d2-ethene (CH2CD2), initially excited to the fourth C-H stretch overtone region, were studied for the first time. H and D vibrational action spectra and Doppler profiles were measured. The action spectra include partially resolved features due to rotational cooling, while the monitored room temperature photoacoustic spectra exhibit only a very broad feature in each species. Simulation of the spectral contours allowed determination of the band types and origins, limited precision rotational constants, and linewidths, providing time scales for energy redistribution. The H and D Doppler profiles correspond to low average translational energies and show slight preferential C-H over C-D bond cleavage in the deuterated variants. The propensities toward H photofragments emerge even though the energy flow out of the initially prepared C-H stretch is on a picosecond time scale and the photodissociation occurs following internal conversion, indicating a more effective release of the light H atoms.     

Perturbation of embedded eigenvalues in the generalizedN-body problem

We discuss the perturbation of continuum eigenvalues without analyticity assumptions. Among our results, we show that generally a small perturbation removes these eigenvalues in accordance with Fermi's Golden Rule. Thus, generically (in a Baire category sense), the Schrödinger operator has no embedded non-threshold eigenvalues.Supported in part by NSF Grant DMS 8602826     

Generating reaction coordinates by the Pauling relation

The Pauling relation of bond order and bond length together with the BEBO postulate are utilized to generation reaction coordinates on potential energy surfaces of simple exchange reactions. A generalization of the Pauling relation where the constant is dependent on the equilibrium separation is proposed.     

Vibrational structure and methyl C-H dynamics in propyne

Our previous study [J. Chem. Phys. 122, 224316 (2005)] presented the photoacoustic and action spectra of the V=2, 3, 4, and 5 manifolds of the C-H methyl stretching vibrations of propyne and their analysis in terms of a simplified joint local mode/normal mode model. In the current paper the C-H transition intensities were calculated using B3LYP6-311++G(d,p) level of theory to obtain the dipole moment functions. The diagonalization of the vibrational Hamiltonian revealed new model parameters obtained by least square fitting of the eigenvalues to the action spectra band origins, while examining the correspondence between the calculated intensities and simulated band areas. The newly derived parameters predict well the band positions and the observed intensities, allowing new assignment of the features. The derived Hamiltonian was also used to obtain the overall temporal behavior of the C-H stretches as a result of the Fermi couplings and interactions with the bath states. These results indicate that any specificity attained by suitable excitation of the methyl C-H stretches is lost on picosecond time scale, primarily due to strong interactions with doorway states in the lower overtone and coupling with bath states in the region of the higher ones.     

Mode-dependent enhancement of photodissociation and photoionization in a seven atom molecule

We report the first experimental demonstration of vibrational mode-dependent enhancement in photodissociation and photoionization of a seven atom molecule, methylamine (CH(3)NH(2)). The fundamental C-H stretches and the overtones or combinations of CH(3) bends were prepared via stimulated Raman excitation (SRE) prior to their 243.135 nm one-photon dissociation or two-photon ionization. The photodissociation or photoionization of the vibrationally excited molecules was achieved via 10 ns delayed or temporally overlapping SRE and UV pulses, respectively. It is shown that bending modes are more effective than stretches in promoting photodissociation and photoionization, since their UV excitation is favored by larger Franck Condon factors. This behavior provides clear evidence for vibrational mode-dependence in a relatively large molecule with a torsional degree of freedom, indicating that these modes survive intramolecular vibrational redistribution on a time scale considerably longer than hitherto inferred from previous studies.     

In situ Generation of Superoxide Anion Radical in Aqueous Medium under Ambient Conditions

In the recent decades superoxide [O^2?.] has become the subject of considerable interest. Nonetheless, generation of superoxide compounds is still a substantial challenge. The standard methods for synthesis of superoxide derivatives are either through the oxidation of molten alkali metals with hot air or by using electrolytic reduction of oxygen in aprotic solvent such as dimethylformamide. No methodology is available for the generation of superoxides in protic solutions and particularly not in water. We propose a new in situ method for alkali superoxide preparation by using sodium hydroxide and hydrogen peroxide at room temperature and in aqueous solution.