8619sufficient to remove much of the excess rotational and vibrational energy present due to the ionization process. Approach to equilibrium was followed from endoergic and exoergic directions. Prior to reaction, one of the parent ions was ejected from the reaction cell and the population change of both parent ions was monitored at set time intervals. Equilibrium was deemed to have been achieved when the ratio of the two parent ion populations reached a constant value within experimental error.Partial pressures of the various neutrals were determined by using an ion gauge calibrated with an MKS baratron capacitance manometer (in the Torr range) extrapolated to experimental conditions. In order to approach dynamic pressure equilibrium throughout the vacuum chamber, the 300 L SKI pumping speed of the diffusion pump connected to the high-vacuum chamber was reduced to ca. 75 L s-l. Neutral gas pressures were calibrated for all reactants in open (75 L s-I) and closed (no pumping) systems. It has been shown that partial pressure is independent of neutral gas leak rate.49 A calibrated ion gauge connected to a Granville-Phillips controller was positioned at the site of the reaction cell with the magnetic field off, thus providing a field free vacuum system. The pressure measured at the middle of the vacuum chamber where the reaction cell is located, was equivalent to metallocene pressures determined at the remote ion gauge following pressure ~alibrations.4~ Temperature Dependence. The temperature dependencies of electron-transfer equilibria were investigated by using a customized cell heater designed to heat a 1" X 1" X 13/qll analyzer cell. The heater consisted of two Macor plates (l1/T X 2" X l/P) (Astro Met Inc.) attached to the long sides of the reaction cell. Ni-Cu wire (0.015'' diameter, Omega) was wrapped around the external Macor plates and was resistively heated by using an Omega digital temperature controller (up to temperatures of up to 520 K). Cell temperatures were measured by using an Omega RTD thin film detector fastened to the analyzer cell. Additionally, the entire high-vacuum chamber was heated by using the (49) (a) Bruce, J. E.; Ryan, M. F. Unpublished Results. (b) Bruce, J. E.; Eyler, J. R. J. Am. SOC. Mass Specrrom.. in press.vacuum bake-out system in order to minimize radiative temperature loss to the vacuum chamber walls.Following the measurement of Kq at a set temperature, the cell heater and bake-out were allowed to cool to a lower temperature and the entire system was allowed to equilibrate at the new temperature for 30 min. Experimental reproducibility was then tested by following the temperature dependence of Kq as the reaction cell temperature was increased from 350 to 500 K. Cell temperature was measured before and after each reaction and usually fluctuated f 2 K during a single experiment. Typically, reactions were repeated three times at a single temperature. Linear regression and statistical analyses of the all measured equilibrium constants provided error limits at the 95% confidence ...
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