Probing the [CoC₃H₈]⁺Potential Energy Surface:  A Detailed Guided-Ion Beam Study

Abstract

The potential energy surface for the CoC 3 H 8 + system is examined in studies using a guided-ion beam tandem mass spectrometer in conjunction with a dc discharge/flow tube ion source. Bimolecular reactions of Co + + C 3 H 8 , Co + (C 3 H 6 ) + H 2 (D 2 ), Co + (C 2 H 4 ) + CH 4 (CD 4 ), and CoCH 2 + + C 2 H 6 are examined, as well as threshold collisional activation of Co + (C 3 H 8 ). The results allow details of the [CoC 3 H 8 ] + potential energy surface to be elucidated. A key feature of the potential energy surface is a rate limiting transition state for C-H bond activation processes, measured as 0.29 ( 0.10 eV below the Co + + C 3 H 8 asymptote. For C-C bond activation, experiments are interpreted to yield two rate-limiting transition states lying 0.08 ( 0.08 eV below and 0.05 ( 0.04 eV above this asymptote. From the threshold collision activation study, we determine a 0 K bond dissociation energy for D 0 (Co + -C 3 H 8 ) of 1.34 ( 0.06 eV. We also determine a lower limit for D 0 (Co + -C 2 H 5 ) of 1.76 ( 0.09 eV (estimated as 2.00 ( 0.11 eV), and D 0 (DCo + -C 2 H 4 ) ) 0.95 ( 0.18 eV. On the basis of recent theoretical studies, we assign the rate-limiting TSs to multicenter elimination structures and show that this reaction scheme can explain both the present and previous experimental observations.

A Theoretical Study of the Reaction Paths for Cobalt Cation + Propane

“…Overall, the energetics predicted in this work appear to be largely in good agreement with experiment, 3 although some discrepancies are observed. Some of the experimental energetics quoted in Table 2 were deduced from figures in that paper, and therefore may not be accurate.…”

Crossed-Beam Study of Co+(3F4)+Propane: Experiment and Density Functional Theory

“…The corresponding value of À 29.0 kcal mol À1 is close to the estimate of 30.9 kcal mol À1 for Co ÀC 3 H 8 binding energy from a threshold collisional activation study. [37] However, interpretation of those experiments is presumably subject to much the same modeling uncertainty that we confront here.…”

“…[36] This agrees well with the value 8 % at 0.2 eV from earlier ion beamgas experiments by Armentrout and co-workers. [37] The simple TOF-MS with t ext 8 ms in effect samples the decay kinetics of collision complexes over a uniform distribution of times in the window t 2 ± 10 ms. The resulting prompt product branching fractions including adducts are summarized for two collision energies in Table 1.…”

“…[48] This functional was combined with the standard D95** polarized, double-z basis set for carbon and hydrogen. For cobalt, the (14s9p5d) primitive set of Wachters [49] is supplemented with one diffuse p-function (a 0.1219) and one diffuse d-function according [38], [39] % 0.5 IB G 2 5 7 5 [40] < 0.2 IB G 1 9 8 1 [37] < 0.1 IB G 3 3 6 7 [21] 0.05 IB G 2 3 7 3 [41] TE [b] ICR 31 69 [20] TE FR 24 76 [42] TE TMS 4 96 [41] TE TMS [d] 18 82 [18] TE TMS [e] 53 47 [18] [a] CB crossed beams; IB G ion beam plus gas cell; ICR ion cyclotron resonance; FR flow reactor at 0. to Hay, [50,51] resulting in a (6211111 j 33121 j 411) 3 [8s5p3d] contraction. The transition states reported here were located by unconstrained geometry optimization using analytical gradients.…”

“…This lies in good agreement with collision-induced dissociation experiments that find D 0 (Co ÀC 3 H 8 ) 30.9 AE 1.4 kcal mol À1 . [37] For comparison, a similar level of theory finds the Ni(C 3 H 8 ) complex at À 35.6 kcal mol À1 . [7] The relative energies of the MCTSs along the lowest energy pathways to the three observed elimination products lie in qualitative accord with the experimental branching ratios.…”