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We have studied the reactions of the gas-phase first transition series metal ions Sc+ through Zn+ with CH4, C2H6, and C3Hg in the multicollision environment of He buffer gas at 0.75 Torr. Laser vaporization of pure metal targets creates reactant metal ions M+ in a fast flow reactor. For each primary M+ + alkane reaction step we obtain quantitative effective bimolecular rate constants and product branching fractions. The multicollision rate constants provide a strikingly different view of M+-alkane interactions than previous single-collision studies. At 0.75 Torr, all 10 M+ species react with all 3 alkanes with the single exception of the Mn+ + CH4 pair. Stabilization of long-lived adduct ions of stoichiometry M(alkane)+ by third-body collisions with He is the dominant process in most reactions, although H2 and CH4 elimination channels do occur for the same M+-alkane pairs that undergo single-collision elimination reactions. The rate constants vary widely and nonmonotonically across the transition series for a given alkane in a pattern that is remarkably similar for CH4, C2H6, and C3Hg. This similarity argues against direct insertion of M+ into C-C bonds as the initial step of the M+-alkane interaction, even for those reactant pairs that yield CH4 elimination products. From a simple collisional quenching model we obtain estimates of the time scale for the decay of long-lived hot adduct ions both backwards to reactants and forwards to elimination products. The experimental results suggest that the M+-alkane attractive forces that control the lifetime of the hot adduct ion involve either a donor-acceptor interaction or C-H bond insertion of the metal ion, i.e., strong chemical interactions rather than simple electrostatic forces. For the relatively inert ions Mn+ and Zn+, the dissociation energies Z)°(M+-alkane) exceed 10 kcal/mol. A qualitative model including the interplay of M+ size effects on long-range repulsive forces, of potential surface intersections, and of orbital symmetry and electron spin conservation effects can explain much of the existing M+-alkane reaction data in both multicollision and single-collision environments.

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We have recently developed vetted methods for obtaining quantitative infrared directional-hemispherical reflectance spectra using a commercial integrating sphere. In this paper, the effects of particle size on the spectral properties are analyzed for several samples such as ammonium sulfate, calcium carbonate, and sodium sulfate as well as one organic compound, lactose. We prepared multiple size fractions for each sample and confirmed the mean sizes using optical microscopy. Most species displayed a wide range of spectral behavior depending on the mean particle size. General trends of reflectance versus particle size are observed such as increased albedo for smaller particles: for most wavelengths, the reflectivity drops with increased size, sometimes displaying a factor of 4 or more drop in reflectivity along with a loss of spectral contrast. In the longwave infrared, several species with symmetric anions or cations exhibited reststrahlen features whose amplitude was nearly invariant with particle size, at least for intermediate and large size sample fractions: that is, ≳150  μm. Trends of other types of bands (Christiansen minima, transparency features) are also investigated as well as quantitative analysis of the observed relationship between reflectance versus particle diameter.