ION-MOLECULE REACTIONS OF ARN2-ENERGY( WITH BUTANE AND ISOBUTANE AT THERMAL)

Product ion distributions and rate constants have been determined for thermal energy reactions of ArN2+ with n-C4H10 and i-C4H10 by using an ion-beam apparatus. C4H9+, C3Hn+ (n=5-7), and C2Hn+ (n=4,5) are produ ced from n-C4H10 with branching ratios of 6, 57, and 37%, while C4H9and C3Hn+ (n=5-7) are formed from i-C4H10 with branching ratios of 13 and 87%, respectively. A comparison of the product ion distribution in the ArN2+/n-C4H10 reaction with that predicted from the fragmentation pattern of n-C4H10+ suggests that most of all fragment ions are forme d through (pre)dissociation of precursor n-C4H10+ states at ca. 13.2 e V. Since this energy is close to the effective recombination energy of ArN2+ (ca. 13.5 eV), it is concluded that the ArN2+/n-C4H10 dissociat ive charge-transfer reaction proceeds through near-resonant n-C4H10+, states. The total rate constants are (6.9+/-2.3)x10(-10) cm3 s-1 for n -C4H10 and (9.0+/-2.6)x10(-10) CM3 S-1 for i-C4H10, which amount to 58 and 75% of the collision rate constants estimated from Langevin theor y, respectively.