Reactions of fluorocarbon ions in C2F6. Implications for the radiolysis

Abstract

Reacti ons of the fragment ions formed in the photoionization of C,F., Xe-C,F., and Kr-C 2F. mixtures have been investigated in th e NBS photoion ization mass spectrometer usin g both helium (21.2 eV) and neo n (16_66-16_84 eV) resonance radiation_ Contrary to previously held views , it is shown that CF i ions havin g no internal excitation energy undergo the F -tran sfer reaction :A rate constant of 4 ± 1 X lO -" cm 3/molecule -s is determined for reaction (a) at press ures below 10 -2 torr_ On the basis of th e collision rate for these reactants_ it ca n be estimated that, on th e average, each CF t ion undergoes 16 unreactive collisions before undergoin g reaction (a)_ Therefore , from the facts that (1) in pure C 2F 6, all CFt ions undergo reaction (a) at high pressures_ and (2) CF; ions formed by charge tran sfe r from Xe + ions with a maximum of 8_4 kl/Mol (2 kcal/mol) excess e nergy und ergo reaction (a), one must co nclude that reaction (a) is either th ermoneutral or exothermic for ground state CF; ions _ Therefore, the earlier estimate for t1H j (C2F ; ) of -33 kl/mol (8 kcal/m ol) must be revised downward: t1Hj (C 2F;) ";;; 3_8 kl/mol (0_9 kcal/m ol)_ The C2F ; ion is unreactive towards C 2F., but does react with alkanes through the H -tran sfer reaction: (b) It is suggested that the CF 4 observed in previou s gas and li quid phase rad iolys is studies of C 2F 6-0 2 mixtures can be entirely ascribed to reaction (a)_ Small concentrations of im purities or accumulated products will react with the C2Ft ions under normal low dose rate radiolysis conditions_

Chemistry of CFn+ (n = 1-3) ions with halocarbons

“…Using a highpressure mass spectrometer, Pabst et al28 measured a rate constant of 3.3 X 10™m cm3 s'1 at 323 K for reaction 1 and noted evidence for internally excited CF+, produced by electron impact on CF4.Richter and Lindinger29 measured a rate constant upper limit of <5 X 10™12 cm3 s'1 for reaction 1 at 0.05-eV center-of-mass collision energy in a drift tube experiment at 300 K, in agreement with the present results for the slow-reacting state of CF*. Rate constants of ~1.9 X 10"10 and 3.94 X 10™" cm3 s™1 were measured for reaction 2 by Sieck et al32 and Marcotte and Tiernan,33 respectively. The value of Sieck et al is reported as an estimate due to poorly known ion residence time.…”

Structure and Reactivity of Protonated α,α,α-Trifluorotoluene in the Gas Phase. A Combined FT-ICR, Radiolytic, and ab Initio MO Study

“…The finding of an appreciable efficiency for the deprotonation reaction of CF 3 C 6 H 6 + ions by benzene (efficiency = 32%) under ICR conditions prompted us to use the trifluoromethylation reaction by radiolytically formed CF 3 + as a convenient way to point out and study the proton transfer between truly ground state arenium ions and benzene. The trifluoromethylation of benzene by CF 3 + ions under radiolytic conditions is a well-characterized ionic process, 15a,n where the ionic reactant and intermediates may attain thermal equilibration by unreactive collisions with the bulk gas before undergoing reaction.…”

“…However, it should not be taken for granted that the high-pressure radiolytic medium does ensure fast thermal equilibration of reactive intermediates. For instance, this condition is not met in the trifluoromethylated arenium ions from CF 3 + attack to C 6 D 5 CH 2 CH 2 C 6 H 5 undergoing fast multiple H + /D + transfer between the trifluoromethylated and the spectator ring 15d. In this system the proximity of the second phenyl ring promotes H + /D + transfer from the CF 3 -substituted arenium ring before the latter had a chance to dissipate its excess energy by unreactive collisions, initiating an extensive sequence of interannular H/D exchange.…”

“…The major product channel from the CF 3 + reaction with benzene under mass spectrometric conditions is reported to give C 6 H 5 CF 2 + as ionic product, suggested to arise from CF 3 + addition to a ring carbon followed by HF elimination (eq 1a).The same reaction evolves into neutral substitution products when allowed to occur in CF 4 at 1 atm, 15a,b,d,n using the radiolytic technique . Such diverse behavior is explained by the fact that the high exothermicity of the CF 3 + addition process, estimated to be of the order of 60 kcal mol -1 ,15a yields excited adduct ions. Their excess energy content may be dispersed either by the unimolecular fragmentation of HF occurring under mass spectrometric conditions (eq 1a) or by efficient collisional quenching with CF 4 at atmospheric pressure.…”

“…The gas phase reactivity of fluorinated ions and neutrals is directed by the strong electronic effects exerted by fluorine as substituent and by the distinct behavior of fluorine itself when acting as reaction site. , Several studies have reported on the protonation of fluoro- and poly(fluorobenzene)s, where the number and relative position of fluorine atoms modulates the relative proton affinities (PA). − Recent detailed studies have focussed on unimolecular fragmentation processes involving loss of HF from protonated fluorobenzene. − To date, no information is available concerning the protonation of a closely related molecule, α,α,α-trifluorotoluene, apart from computational data on the proton affinity of m- and p-positions by semiempirical MNDO and STO-3G ab initio methods. , Our interest in the protonation of CF 3 C 6 H 5 stems from previous studies on the reactivity of gaseous CF 3 + ions, powerful electrophilic species yielding ion−molecule adduct ions destabilized by the electronic effect exerted by the CF 3 group and prone to undergo, whenever possible, HF elimination processes. The major product channel from the CF 3 + reaction with benzene under mass spectrometric conditions is reported to give C 6 H 5 CF 2 + as ionic product, suggested to arise from CF 3 + addition to a ring carbon followed by HF elimination (eq 1a).The same reaction evolves into neutral substitution products when allowed to occur in CF 4 at 1 atm, 15a,b,d,n using the radiolytic technique . Such diverse behavior is explained by the fact that the high exothermicity of the CF 3 + addition process, estimated to be of the order of 60 kcal mol -1 ,15a yields excited adduct ions.…”