Product translational energy release distributions are used to investigate the potential energy surfaces for elimination of H2 and small hydrocarbons from ionic cobalt and nickel complexes with alkanes. The amount of energy appearing as product translation can be used to infer details of the potential energy surfaces in the region of the exit channel and has implications for the ease with which the reverse reactions may occur. The potential energy surfaces for hydrogen and alkane elimination reactions are discussed in view of the very different kinetic energy release distributions observed for these processes. For dehydrogenation reactions, both the shape of the distribution and the maximum kinetic energy release are correlated with the reaction mechanism. For example, the amount of energy appearing in product translation is quite distinctive between reactions known to involve metal-induced 1,2-and 1,4-hydrogen elimination. The selective dehydrogenations of 2-methylpropane-2-dl by Cot and b~tane-1,1,1,4,4,4-~~ by Ni+ serve, respectively, as models for these processes. A comparison of these translational energy distributions with those observed for loss of H2, HD, and D2 from the dehydrogenation of butane-1,1,1,4,4,4-d6 by Cot suggests that 1,4-elimination is dominant for the cobalt system and that the observation of different isotopic products results from scrambling processes. All the dehydrogenation processes examined were characterized by kinetic energy release distributions which could not be described by statistical theories. For these reactions, the maximum kinetic energy release approaches the estimated reaction exothermicity. In contrast, the more exothermic alkane eliminations have maximum kinetic energy releases which are less than half the reaction exothermicity, and the distributions can be fit with statistical models. For these processes the excess energy in the activated complex is approximately equal to the reaction exothermicity, suggesting a loose transition state for the disruption of a complex in which the intact alkane to be eliminated is interacting strongly with the metal center. Comparison of experiment with theory yields a Co+-propene bond strength of 48 f 3 kcal/mol, a Cot-CO bond strength of 34 f 3 kcal/mol, and a sum of the first and second metal bond strengths in Co(CDJ2+ of 110 f 3 kcal/mol at 298 K. The latter two values are derived from statistical kinetic energy release distributions observed for the loss of C2D6 and CO, respectively, in the reaction of Co+ with acetone-& (7) There is some indication from theoretical studies that the d orbitals on the metal may facilitate concerted, multicenter reaction mechanisms: Steigerwald, M. L.; Goddard, W. A.

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Ab initio calculations were performed at several levels for neutral C02, C02 dimer, and the cations (C02)2+ and (C02)3+. At the highest level, a C02 neutral dimer of C2h symmetry (staggered side-by-side) is bound by 1.3 kcal/mol, which is 0.2 kcal/mol more stable than a T-shaped dimer (C^symmetry). The dissociation energy of a (C02)2+ complex of C2* symmetry is calculated to be 16.2 kcal/mol which is 4.4 kcal/mol greater than the dissociation energy of the T-shaped (C02)2+ complex. Due to a smaller amount of spin contamination (spin polarization), the stability of this complex which is characterized by a partial bond between oxygens is overestimated at the [MP4SDQ/6-31G*] level relative to the T-shaped ion complex and C02+ (plus C02) which have significant spin contamination. Projecting out the largest spin contaminant at the PMP2/6-31G* level and at the PMP4/3-21G level leads to an estimated decrease in the energy separation between the C2A symmetry complex and the T-shaped complex of 6.3 kcal/mol (relative to the [MP4SDQ/6-31G*] method). Two trimers of (C02)3+ were considered, a cyclic complex (C3A) and a cross-shaped complex (Dlh). The more stable (C02)3+ complex (D2h) is bound

An experimental and computational study of the ion-molecule reactions initiated by the reaction of SC2H4"* 1" with SC2H4 has been carried out. The dimer ion is observed in a hybrid drift tube/ion source under chemical ionization conditions at low temperatures and high pressures. However, the ion intensity is much lower than expected for a product containing a stable two-center three-electron (2c-3e) S/.S bond. Above room temperature, ion intensities for m/z 92 S2C2H4'"1", m/z 124 S3C2H4"1", m/z 156 S4C2H4'"1", and m/z 184 S4C4H8•"1" dominate. The first three ions result from sequential association of SC2H4'"1" with SC2H4 followed by elimination of ethylene, while the latter ion (m/z 184) is due to the association of S3C2H4•"1" with SC2H4 without loss of ethylene. As the temperature is lowered, clustering is promoted relative to reaction channels involving loss of ethylene. The experimental results are not in full agreement with a published ab initio study which found that the product channel was 33-kJ/mol endothermic with respect to reactants. The present ab initio calculations indicate that another reaction channel is possible, resulting in a fourmembered-ring C-S2C2H44", which is exothermic by 43.0 kJ/mol. The potential energy surface for this channel, including transition states, was computationally studied in detail. An interesting aspect of this mechanism is the stepwise addition of sulfur atoms forming successively larger rings which continues until the six-membered-ring S4C2H44, is formed.

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The results of a study of the photodissociation of the carbon dioxide ion-molecule cluster, (C02)2+, from 458 to 650 nm are presented. The experiments were performed with a high-energy ion beam crossed with a laser beam. C02+ was the only product observed. Photodissociation kinetic energy release distributions and angular distributions were measured. The kinetic energy release distributions are bimodal indicating that C02+ is generated by two mechanisms. The product peak shapes were simulated by the measured product velocity distributions and two product angular distributions of the form 1 + ß 2(cos ) with ß = 1.0 for the high-energy component in the kinetic energy release distribution and ß = 0.25 for the low-energy component. The results suggest that the mechanisms leading to C02+ are rapid fragmentation on a repulsive surface on a time scale which is short compared to a rotational period and statistical vibrational predissociation (statistical unimolecular dissociation) of highly vibrationally excited ground electronic state (C02)2+ on a time scale which may be longer than a rotational period. Phase space calculations of the statistical vibrational predissociation component compare well with our results. The analysis suggests that 11% of the C02+ product is formed by statistical vibrational predissociation at 650 nm and 22% at 458 nm; the remainder is formed by direct dissociation from a repulsive surface. The average product relative kinetic energy increases with increasing photon energy and varies in a linear fashion with the available energy.