The kinetic isotope effect for carbon and oxygen in the reaction CO + OH

Abstract

The kinetic isotope effect (KIE) for carbon and oxygen in the reaction CO + OH has been measured over a range of pressures of air and at 0.2 and 1.0 atm of oxygen, argon, and helium. The reaction was carried out with 21-86% conversion under static conditions, utilizing the photolysis of HzOz as a source of OH radicals. The value of the KIE for carbon varies with pressure and the kind of ambient gas; for air the ratio of the reaction rates 12k/13k has the value 1.007 at 1.00 atm and decreases to 0.997 at 0.2 atm; for oxygen and argon over the same pressure range the values are 1.002-0.994 and 1.000-0.991, respectively. The value of the KIE for the CO oxygen atom is l6k/l*k = 0.990 over the pressure range 0.2-1.0 atm and is independent of the kind of ambient gas. No exchange of the oxygen atoms in the activated complex, followed by decomposition to the starting molecules, was observed. From the mechanistic standpoint the normal KIE observed for carbon at the high pressure is attributed to the initial formation of the activated HOCO radical, whereas the inverse KIE observed at low pressures is a result of the KIE for the reverse reaction HOCOt -CO + OH being greater than that for the forward reaction HOCOt -+ COz'+ H. The derived isotopic equilibrium constant for HOCO s CO favors the enrichment of 13C in the more strongly bound HOCO.

Using <sup>14</sup>C, <sup>13</sup>C, <sup>18</sup>O and <sup>17</sup>O isotopic variations to provide insights into the high northern latitude surface CO inventory

“…This reaction involves an inverse isotope effect of about −10‰ for 18 O, i.e. here C 18 O is removed 10‰ faster by OH than C 16 O Stevens et al, 1980). Accordingly, after the seasonal maximum near 11‰ in early spring, δ 18 O declines quickly concomitant with the strong CO decline (Fig.…”

“…3.1.3 δ 13 C Due to the kinetic isotope effect in the reaction CO + OH, the OH sink induces a variation in δ 13 C: OH preferentially removes 12 CO over 13 CO, and the remaining CO becomes enriched in 13 C. The kinetic fractionation is strongly pressure dependent and has an average strength of 4 to 5‰ in the lower troposphere Smit et al, 1982;Stevens et al, 1980). Consequently in spring, when CO levels decrease after the seasonal maximum, δ 13 C rapidly rises from values of around −27‰ up to −23‰ within three months (Fig.…”

On the theory of the CO+OH reaction, including H and C kinetic isotope effects

“…1 Reflecting its importance and the unusual temperature and pressure dependence of its rate constant, the CO+ OH→ CO 2 + H reaction has been examined extensively in many experimental [3][4][5][6][7][8][9][10][11][12][13][14][15][16][17] and theoretical studies, [17][18][19][20][21][22][23][24][25][26][27][28] including experimental studies over a very wide range of temperatures and pressures. [4][5][6][7][8][9][10][11][12][13]17 Nevertheless, carbon and oxygen isotope effects have been observed [29][30][31][32][33] and the anomalous effects for these heavy-atom isotopes have not yet been treated in the literature. Again, Simons and co-workers [63][64][65] observed in their molecular-beam study of the reverse reacti...…”

An estimation of the <sup>18</sup>O / <sup>16</sup>O ratio of UT/LMS ozone based on artefact CO in air sampled during CARIBIC flights

“…This is seen from the B96 data at [CO] below 50 nmol/mol that lineup in a near linear relationship towards the end members with lowest 18 O / 16 O ratios. These result from the largest share of the 18 O-depleted photochemical component and extra depletion caused by the preferential removal of C 18 O in reaction with OH (fractionation about +11 ‰ at pressures below 300 hPa, Stevens et al, 1980;Röckmann et al, 1998b).…”

“…In the stratosphere, O 3 number concentrations are high, but the remoteness of the sampling domain is a problem. In the troposphere, low O 3 number densities are the main obstacle, as indicated by few experiments performed to date (Krankowsky et al, 1995;Johnston and Thiemens, 1997;Vicars and Savarino, 2014). Nevertheless, recent analytical improvements, namely the use of an indirect method of reacting atmospheric O 3 with a substrate that can be analysed for the isotopic composition of the O 3 -derived oxygen (Vicars et al, 2012), has greatly improved our ability to obtain information on the O 3 isotopic composition.…”