Infrared chemiluminescence: Evidence for adduct formation in the H + CH2XI reaction and studies of the N + CH2X (X = Cl/F/I/H) reactions

Abstract

Infrared chemiluminescence from a flow reactor has been used to study the H þ CH 2 XI and N þ CH 2 X (X ¼ Cl, F, I, H) reactions at 300 K. Both the HI þ CH 2 Cl and HCl þ CH 2 I channels were identified for the H þ CH 2 ClI reaction. The HCl channel involves adduct, HICH 2 Cl, formation as confirmed by the D þ CH 2 ClI reaction, which gave both HCl and DCl products. The nascent HCl(v) distribution from the H þ CH 2 ClI reaction was P 1-P 5 ¼ 25 : 29 : 26 : 13 : 7. The rate constant for the HCl(v) formation channel is estimated to be 4 times smaller than that for the H þ Cl 2 reaction. The highest HCl(v) level observed from the H þ CH 2 ClI reaction implies that the C-Cl bond energy is 50.2 kJ mol À1 lower than that of the Cl-CH 3 bond, which is in modest agreement with recent theoretical estimates. The H þ CH 2 FI reaction gave a HF(v) distribution of P 1-P 3 ¼ 77 : 15 : 8. The C-F bond energy in CH 2 FI is estimated to be 4 460.2 kJ mol À1 , based on the highest HF(v) level observed, the upper bound being the same as that of F-CH 3. When N atoms are added to the flow reactor, the HCl(v) emission intensities from H þ CH 2 ClI increased by up to 2-fold, which is attributed to the N þ CH 2 Cl ! HCl þ HCN reaction. Concomitant weak emission from HCN and HNC could also be observed; however, the main product channel is thought to be NCH 2 þ Cl. Strong visible CN(A-X) emission was also observed when H=N=CH 2 XI were present in the reactor. If the CH 2 X radicals were produced by the F þ CH 3 X reaction in the presence of N atoms, similar results were obtained. The N þ CH 2 N reaction is proposed as the first step that leads to CN(A) formation with NCN as an intermediate.

Dissociative Photoionization and Thermochemistry of Dihalomethane Compounds Studied by Threshold Photoelectron Photoion Coincidence Spectroscopy

“…As a final observation, we point out that the value of 301.2 kJ/mol reported as an upper limit for the Cl−CH 2 I bond energy by Arunan et al, using the infrared chemiluminescence technique, and used as reference by Luo, has been confirmed neither by our present result (328.2 kJ/mol) nor by theoretical calculations (326.4 kJ/mol) . It may suggest that the infrared chemiluminescence is not the most adequate technique for deriving reliable bond dissociation energies of dihalomethanes.…”

“…Among the mixed dihalomethanes, not much information is found in the literature. The BDE values of 206.6 ( 0.8 kJ/mol for I-CH 2 -Cl and 253.4 ( 1.8 for Br-CH 2 Cl have been estimated by Skorobogatov et al 47,45 These values differ from our present 59 As a final observation, we point out that the value of 301.2 kJ/mol reported as an upper limit for the Cl-CH 2 I bond energy by Arunan et al, 60 using the infrared chemiluminescence technique, and used as reference by Luo, 56 has been confirmed neither by our present result (328.2 kJ/mol) nor by theoretical calculations (326.4 kJ/mol). 59 It may suggest that the infrared chemiluminescence is not the most adequate technique for deriving reliable bond dissociation energies of dihalomethanes.…”

“…The interpretation of the infrared chemiluminescence data usually requires accurate thermochemical information, which particularly was not available for the CH 2 ICl molecule. Other important sources of error in the C−Cl bond energy reported by Arunan et al are related to the activation energy ( E a ) for the reaction H + CH 2 ICl → HCl + CH 2 I, and thermal energy ( T th ) for the CH 2 ICl molecule, required for the bond energy determination, which have not been measured in their work as well. Their estimated E a value, which was based only on the comparison of the HCl(υ) IR emission intensities from reaction H + CH 2 ICl with the ones from reaction H + Cl 2 , and also the estimation for the thermal energy (at 300 K), may explain the discrepancy observed for their estimated Cl−CH 2 I bond energy value, as compared to the theoretical value 59 and to our present result.…”