3971using high power pulsed lasers3394 is suitable for more conventional mass spectrometers in which ion-residence times arelod s. ciation of isomers often yields fragment ions in ratios that vary only slightly with the structure of the mecursor ion.z935 ~ -* Thus infrared photodissociation can be developed as a complementary technique to collisional activation for ion structure analysis. In particular, infrared excitation may be better suited to distinguish isomeric species because collision-induced disso-Acknowledgment. This work was supported by the United States Department Of Energy and the President's Fund Of the California Institute of Technology. We are grateful to Ms. Jocelyn Schultz for providing the photoelectron spectrum of c-C3F6. (34) Rosenfield, R. N.; Jasinski, J. M.; Brauman, J. I.Abstract: Vibration-induced detachment of electrons from molecular anions has recently been experimentally observed. In this paper, the coupling of the anion's vibration-rotation motion to its electronic degrees of freedom is analyzed in an attempt to predict under what circumstances efficient detachment would be expected. A set of propensity rules based upon the shapes of the anion and neutral potential-energy surfaces is put forth and applied to several prototypical situations. The development is not aimed at permitting ab initio calculations of electron detachment rates. Rather, it is designed to provide a useful tool both for interpreting experimental data and for designing especially interesting experiments to carry out.Recently it has been obsd.' that electrons can be detached from Abstract: Statistical mechanics simulations have been carried out for liquid dimethyl ether (DME) and methyl ethyl ether (MEE) at their boiling points and for liquid diethyl ether (DEE) at 25 OC. The intermolecular interactions were described by Coulomb and Lennard-Jones terms in the TIPS format. The internal rotations about the central CO bonds in the MEE and DEE monomers were also included, using potential functions derived from ab initio molecular orbital calculations. Optimization led to the interesting result that the dihedral angle for the gauche conformer in MEE occurs at ca. 85O in accord with electron diffraction data. A key finding is that the conformational equilibria for MEE and DEE are essentially unaffected by transfer from the gas phase to the pure liquids at the normal densities. The thermodynamic and detailed structural results for the liquids are also thoroughly analyzed. Excellent agreement is found with experiment for the computed heats of vaporization of all three liquids. A simulation of liquid DME under constant pressure conditions yielded a density within 4% of the experimental value which provides further support for the validity of the TIPS model. The liquids are disordered with high coordination numbers and no obvious repeating polymeric units. Trends in the structures and properties as a function of monomer size are discussed.

No abstract

No abstract

We report for the first time an atomic force microscopy (AFM) investigation of the dependence of unbinding force on temperature at controlled loading rate. AFM force measurements of biotin-avidin interactions were acquired at various temperatures ranging from 13 to 37°C using force loading rates that are slow enough to ignore dissipative friction and to assume thermal equilibrium. With our established Poisson statistical analysis method, the unbinding force at a fixed loading rate between an individual biotin-avidin pair was extracted and was found to decrease by ∼5-fold with increasing temperature over this range. On the basis of a thermodynamic model similar to that of Bell, but developed here with additional insights, the bond stiffness, effective unbinding length, and the critical unbinding energy per biotin-avidin complex were estimated. This work takes quantitative AFM studies to the next level by allowing energies to be determined from force measurements.