HOCO Radical Research Articles

High level ab initio studies on the excited states of HOCO radical

The HOCO radical has a ground (X 2A′) and two lowest A″2 excited states that are located using the CCSD(T) level of theory with the cc-pVDZ and cc-pVTZ basis sets. The harmonic frequencies are calculated at the CCSD(T) level of theory with the cc-pVDZ basis set. The vertical excitation energies for the 2 2A′, 3 2A′, 1 2A″, and 2 2A″ states of HOCO are obtained at the MRCI level of theory with the cc-pVTZ and aug-cc-pVTZ basis sets. The first excited state (1 2A″) is calculated to be 70.7 kcal mol−1 above the ground state for trans-HOCO. Comparisons are made between the excited states of HOCO and HCO. It is demonstrated that the HOCO states are not similar to those of HCO.

Rate constants for the reactions of Cl atoms with HCOOH and with HOCO radicals

Rate constants have been measured at room temperature for the reactions of Cl atoms with formic acid and with the HOCO radical: Cl + HCOOH → HCl + HOCO (R1) Cl + HOCO → HCl + CO2 (R2)Cl atoms were generated by flash photolysis of Cl2 and the progress of reaction was followed by time-resolved infrared absorption measurements using tunable diode lasers on the CO2 that was formed either in the pair of reactions (R1) plus (R2), or in reaction (R1) followed by O2 + HOCO → HO2 + CO2 (R3)In a separate series of experiments, conditions were chosen so that the kinetics of CO2 formation were dominated either by the rate of reaction (R1) or by that of reactions (R1) and (R2) combined. The results of our analysis of these experiments yielded: k1 = (1.83 ± 0.12) × 10−13 cm3 molecule−1 s−1 k2 = (4.8 ± 1.0) × 10−11 cm3 molecule−1 s−1 © 2000 John Wiley & Sons, Inc. Int J Chem Kinet 32: 85–91, 2000

Detection and reactions of the HOCO radical in gas phase

The HOCO radical has been successfully detected by a photoionization mass spectrometry in gas phase. The HOCO radical produced by the reaction of Cl+HCOOH (with small excess energy; 13.0 kcal mol−1) was found to be stable and its lifetime was confirmed to be very long (≳10 ms) at room temperature. On the other hand, the HOCO radical formed by the 193 nm photolysis of C2H3COOH (with large excess energy; 55.8 kcal mol−1) exhibited fast, nonexponential, and total pressure-dependent decay. This behavior was well interpreted by a competition of dissociation and relaxation of ‘‘hot’’ radicals. Further, the decay of ‘‘hot’’ radicals showed an apparent isotope effect (DOCO/HOCO). This fact together with an energetic consideration suggests that the observed decay is dominated by a tunneling dissociation to H+CO2, which implies the importance of the tunneling in the OH+CO reaction. The vibrational relaxation rate of the HOCO radical was roughly evaluated from the pressure dependence of the decay profile of ‘‘hot’’ HOCO radicals. The rate of reaction of HOCO radical with molecular oxygen has also been determined, and appeared pressure-independent.

B-dipole transitions in trans-HOCO observed by far infrared laser magnetic resonance

Far infrared laser magnetic resonance spectroscopy is used to measure components of 12 rotational transitions in the ground state of the HOCO radical. The transitions are all b-dipole in character in contrast to the a-dipole rotational spectrum previously reported [Radford, Wei, and Sears, J. Chem. Phys. 97, 3989 (1992)]. The new data determine the A rotational constant to high precision and allow the determination of several centrifugal distortion constants for the first time. The hyperfine coupling in the radical leads to observable splittings in several of the observed transitions and these are used to estimate two of the four expected nonzero hyperfine parameters in the radical.

Product study of the reaction of Cl atoms with HCOOH

Fourier transform infrared spectroscopy has been used to investigate the products of the reaction of Cl atoms with HCOOH in 700 Torr of either air or N 2 diluent at 295±2 K. In all cases CO 2 was the sole carbon-containing product observed. The CO 2 yield was measured to be 96±5%, quoted errors represent 2Σ. An upper limit of 2% was established for any CO product. These results are discussed with respect to the interpretation of product information from environment chamber experiments and to the atmospheric fate and thermochemistry of the HOCO radical.

Correlation effects on cis—trans energy differences in open-shell systems: The hoco radical as an example

Ab initio SCF and Cl calculations have shown that the most important factor in determining the relative energies of conformational isomers in free radicals is the movement of electron density into the singly occupied orbital. This is most favorable when an electron pair is situated trans to the radical site (trans correlation effect). In the HOCO radical we find that the trans isomer is more stable than the cis by 3.3 kcal/mol while SCF calculations yield virtually no energy difference between the two isomers.

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

AbstractThe 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 H2O2 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 16k/18k = 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 HOCO† → CO + OH being greater than that for the forward reaction HOCO† → CO2 + H. The derived isotopic equilibrium constant for HOCO ⇄CO favors the enrichment of 13C in the more strongly bound HOCO.

Thermochemical properties of HOCO and HOCO † formed from OH + CO

Thermochemical properties from 100 to 300 K in the ideal gas state are reported for the HOCO radical and for excited HOCO † containing the vibrational energy corresponding to HOCO formation from thermalized reagents OH and CO.

The mechanism of the OH + CO reaction and the stability of the HOCO radical

The mechanism of the reaction between OH radicals and CO is discussed in relation to recent experiments which indicate that the rate constant, k = −(dln[OH]/d t)/[CO], depends on total pressure. It is shown that this observation is quite consistent with the known spectroscopic and thermodynamic properties of the HOCO radical, as long as the dissociation of HOCO to H + CO 2 is no faster than that to OH + CO.