High-Pressure Ion–Molecule Reactions in Carbon Monoxide and Carbon Monoxide–Methane Mixtures

Abstract

In carbon monoxide at pressures above 0.2 torr, the principal product ion is C20 2+ formed by a threebody process. The rate constant for the reaction is 1.43X1O--'8 cm 6 molecule-2 ·secl • Above about 0.8 torr, the reaction appears to approach equilibrium with an equilibrium constant of 1482 referred to 1 atm as standard. At pressures of methane above 0.2 torr and small additions of CO, the only reaction observed in the protonation of CO by CH.+ with a rate constant of 5.54X1o--10 cm s molecule-I·sec-I . All other reactions are endothermic and are not observed. At pressures of carbon monoxide above 0.2 torr, the addition of small amounts of methane results in an H-atom transfer reaction to CO+ with a rate constant of 13.7X1O--I0 cm s molecule-I·sec l • In addition, C20 2 + reacts to form HCO+, C2HsO+, and CsHsO+ with rate constants of 9.44,4.37, and 0.76X10-IO cm S molecule-I'sec l , respectively.The origin of chemical binding is analyzed with the help of variational reasoning for the ground state of the hydrogen molecule-ion. The bond-parallel component of the kinetic-energy integral is shown to be the critical term. The effect of electron sharing on this term is such that the variation process yields, at all internuclear distances, a lower energy for the molecule than for the separated atoms. This appears to be paradoxical in as much as, at the equilibrium distance, the potential part of the binding energy is negative and the kinetic part is positive. The paradox is resolved by showing that the actual values of the potential and kinetic binding energy are the result of a scale variation which is similar to that found in the isolated atom and contracts the wavefunction towards the two nuclei. The variational origin of this redistribution of the kinetic-and potential-energy integrals, which establishes the virial relationship, is analyzed. It is concluded that the variational facts are inadequately described by the view that the chemical bond is formed "because the potential energy can be lowered through accumulation of charge in the bond." Rather, the accumulation of charge in the bond is essential for chemical binding because it profoundly influences the behavior of the kinetic-energy integral under scale variations.

State-Selected Reactivity of Carbon Dioxide Cations (CO2+) With Methane

“…Rate constant and product branching ratio measurements were made using drift techniques, either flow drift tubes (FDT) (Rakshit and Warneck, 1980; Durup-Ferguson et al, 1983) or selected ion flow tube (SIFT) (Smith et al, 1978; Copp et al, 1982), ion cyclotron resonance (ICR) techniques (Huntress et al, 1980) and ion beam methods (Tsuji et al, 1994). Earlier determinations where done using high-pressure mass spectrometry (HPMS) (Harrison and Myher, 1967; Chong and Franklin, 1971; Kasper and Franklin, 1972) and electron space charge traps (SCT) (Ryan and Harland, 1974). The most relevant results are summarized in Table 1.…”

Molecular Beam Photoionization and Photoelectron-Photoion Coincidence Studies of High Temperature Molecules, Transient Species, and Clusters

“…128 The PEPICO technique is more sensitive than PIE and PE measurements in determining IEs because background ionization events due to scattered light are greatly reduced in coincidence studies. This upper bound value corresponds to a lower limit 231,232 However, this value is a factor of two lower than the theoretical prediction of » 60 kcal/mol obtained by the ab initio calculation of Blair et al 224 The value of 14.50 ± 0.08 eV for IEfCN^J determined from the (N 2 ) 2 + PEPICO data is only slightly lower than estimates based on the PIE 128 (Table VI), the binding energies for …”

Temperature-Dependent Kinetics of Charge Transfer, Hydrogen-Atom Transfer, and Hydrogen-Atom Expulsion in the Reaction of CO+ with CH4 and CD4

“…The collision rate constants, as calculated using quasi-classical trajectory calculations parametrized by Su and Chesnavich, are 1.18 × 10 –9 and 1.10 × 10 –9 cm 3 /s for CH 4 and CD 4 reactions, respectively. Previous rate constant measurements have only been made at 300 K, ,,,− and we tabulate these values alongside our data for the methane and deuterated methane reactions in Tables and , respectively. We find good agreement with most all of the previous rate constant measurements for the methane reaction; recent ICR experiments find a slightly lower value (∼80% of the collision rate constant), which they attribute to neutral CO diffusion from the source and potentially different thermalization conditions.…”

“…Second, CO + represents the only cationic system that can facilitate HAT at two sites, resulting in two isomeric HAT products (HCO + and COH + ) . The structure and isomerization energy of these formyl isomers have received significant attention, mainly due to their importance in interstellar chemistry. − In this light, the reaction of CO + with methane has been studied by a handful of groups in the gas phase, as early as the late 1960s. − Rate constant and product branching measurements have been made using selected ion flow tube (SIFT) or ion cyclotron resonance (ICR) techniques. , It has been shown that the reaction proceeds at the collision rate, and in addition to the HAT pathway, charge-transfer (CH 4 + + CO) and H-atom expulsion (CH 3 CO + + H) products are found. The mechanistic details of the HAT pathway in this reaction have only very recently been tied to experimental results, utilizing ICR methods coupled with quantum chemical calculations .…”