The (not so) fast free radical reaction R + HX (X=Br, I) - RH + X revisited : Implications for free radical thermochemical parameters

• Research unit: COST
• Project number: C11.0052

Key words (English)

Free radical reactions; chemical kinetics; tempera-ture dependence; single photon photoionization detection of free radicals; REMPI detection; syn-chrotron radiation; thermochemical parameters; ab initio electronic structure calculations

Research programs (English)

COST-Action CM0901 - Detailed chemical kinetic models for cleaner combustion

Short description (English)

See abstract

Further information (English)

Full name of research-institution/enterprise: Paul Scherrer Institut (PSI) Laboratorium für Atmosphärenchemie (LAC)

Partners and International Organizations (English)

AT, BG, CH, CZ, DE, DK, EL, ES, FI, FR, HU, IE, IT, LT, PL, PT, RO, SE, UK

Abstract (English)

A new apparatus using VUV single photon ionization mass spectrometry (VUV SPIMS) of an effusive molecular beam emanating from a Knudsen flow reactor is described. It was designed to study free radical-molecule kinetics over a significant temperature range (300 to 630 K). Its salient features are: (1) external free radical source, (2) counterpropagating molecular beam and diffuse VUV photon beam meeting in a cross-beam ion source of a quadrupole MS with perpendicular ion extraction, (3) analog detection of the photocurrent of the free radical molecular cation, and (4) possibility of detecting both free radicals and closed shell species in the same apparatus and under identical reaction conditions. The absolute rate constants of the metathesis reactions C2H5o + HBr ? C2H6 + Bro (1) and C2H5o + HI ? C2H6 + Io (2) have been measured and led to the following Arrhenius expressions: k1 = 3.69(±0.95) 10-11 exp(-10.62(±0.66)/RT), k2 = 1.20(±0.38) 10-11 exp(-7.12(± 1.059)/RT) in the temperature range 293-623 K (A/cm3 molec-1 s-1, R = 8.314 J/K mol). DELTAH(C2H5o) was 117.3 ± 3.1 kJ/mol resulting from a 'Third Law' evaluation using S(C2H5o) = 242.9 ± 4.6 J/K mol. The present work is in satisfactory agreement with (DELTAH(C2H5o) = 119 ± 2 kJ/mol) obtained by W. Tsang. The absolute rate constants of the metathesis reactions i-C3H7o + HI ? C3H8 + Io (3) and n-C3H7o + HI ? C3H8 + Io (4) have been measured and led to the following Arrhenius expressions: k3 = 1.94(±0.42) 10-11 exp(-6.190(±0.855)/RT) and k4 = 6.49 (±1.86) 10-11 exp(-11.084(±0.913)/RT) over the temperature range 293-623 K. Results point towards a slightly lower value for the standard heat of formation of i-C3H7o compared to the currently accepted value whereas no significant adjustment for n-C3H7o is necessary. The recommended values are DELTAH(i-C3H7o) = 86.6 ± 2.1 and DELTAH(n-C3H7o) = 101.1 ± 2.1 kJ mol-1 resulting from a 'Third Law' evaluation using S(i-C3H7o) = 289.9 and S(n-C3H7o) = 290.3 J/K mol. Agreement with results obtained by W. Tsang, namely 88 +- 2 and 100 +- 2 kJ/mol for i- and n-propyl, respectively, is excellent. The absolute rate constants of the metathesis reactions t-C4H9o + HBr ? i-C4H10 + Bro (5) and t-C4H9o + HI ? i-C4H10 + Io (6) have been measured and led to the following Arrhenius expressions: k5 = 5.62(±1.41) x 10-12 exp(-6.76(±0.94)/RT) and k6 = 2.02(±0.58) x 10-11 exp(-8.48(±0.94)/RT) over the range 293 to 623 K. The standard heat of formation DELTAH (t-C4H9o) = 44.3 ± 1.7 kJ/mol was obtained when combined with the kinetics of the inverse halogenation reaction taken from the literature using S (t-C4H9o) = 322.2 J/K mol following a 'Third Law' evaluation. For ethyl and t-butyl radical identical metathesis rate constants were also measured from changes in the recombination products upon HX addition. Radical wall losses were small, thus insignificant over the used temperature range in all cases studied.

References in databases (English)

Swiss Database: COST-DB of the State Secretariat for Education and Research Hallwylstrasse 4 CH-3003 Berne, Switzerland Tel. [phone] Swiss Project-Number: C11.0052