An ab initio and experimental study of vibrational effects in low energy O++C2H2 charge-transfer collisions

Theoretical and experimental studies are performed to elucidate the low ene rgy charge-transfer dynamics of the reaction, O+(S-4)+C2H2(X (1)Sigma (+)(g ))-->O+C2H2+. In particular, the role of the low-frequency acetylene bendin g modes (612 and 730 cm(-1)) in promoting charge transfer was examined. Hig h-temperature guided-ion beam measurements are carried out over the energy range from near-thermal to 3 eV at 310 and 610 K. The charge-transfer cross sections are found to decrease up to 0.5 eV, to have a constant value at i ntermediate energies between 0.5 and 1.5 eV, and then to dramatically incre ase above a threshold of a spin-allowed process determined to be at 1.7 eV. A bending vibrational enhancement of similar to8 is observed at intermedia te energies. Thermal energy rate co-efficients are measured in a variable t emperature-selected ion flow drift tube apparatus from 193 to 500 K. At eac h temperature, a negative energy dependence is observed. In order to elucid ate the reaction mechanism in detail, high level ab initio calculations usi ng Complete Active Space Self-Consistent Field and Multi-Reference Single- and Double-excitation Configuration Interaction methods have been performed . The results indicate that the charge transfer reaction occurs at an early stage via nonadiabatic transition between quartet and doublet states. Ther e is a weak van der Waals minimum at the entrance channel between O+(S-4) a nd C2H2 with the relative energy of -1.51 kcal/mol. The minimum of the quar tet/doublet crossing seam (Q/D MSX), where the spin-forbidden nonadiabatic transition is most likely to take place, lies very near this minimum at R-C O=4.06 Angstrom, R-CC=1.20 Angstrom, and angle CCH=166.6 degrees with a rel ative energy of -1.48 kcal/mol. After the nonadiabatic transition, the syst em propagates on the doublet surface to reach the exothermic O(D-1)+C2H2+(( X) over tilde (2)Pi (u)) products. No energy barrier exists on the reaction pathway, strongly suggesting that the reaction should occur at low energy with a negative energy dependence, which is consistent with the experiment. The Q/D MSX has a bent acetylene moiety, which suggests that the excitatio n in bending modes will enhance the reaction, in agreement with the experim ent. (C) 2001 American Institute of Physics.