HCO radical

Yeom and Frei [96] showed that irradiation at 266 nm of TS-1 loaded with CO and CH3OH gas at 173 K gave methyl formate as the main product. The photoreaction was monitored in situ by FT-IR spectroscopy and was attributed to reduction of CO at LMCT-excited framework Ti centers (see Sect. 3.2) under concurrent oxidation of methanol. Infrared product analysis based on experiments with isotopically labeled molecules revealed that carbon monoxide is incorporated into the ester as a carbonyl moiety. The authors proposed that CO is photoreduced by transient Ti + to HCO radical in the primary redox step. This finding opens up the possibility for synthetic chemistry of carbon monoxide in transition metal materials by photoactivation of framework metal centers. 

In recent wrork particular emphasis has been given to studies of flame spectra and the evidence as to the formation and reaction of excited species such as C2, CH, OH, and HCO from acetylene and oxygen (17, 29, 31, 41, 43, 54). The occurrence of excited hydrocarbon flame bands attributable to HCO radicals led Herman, Hombeck, and Laidler (31) to suggest the reaction... 

The lifetimes and kinetic behavior of many triplet states of molecules, particularly of aromatic compounds, have been studied by Porter and his coworkers. The identification of the absorption spectra of methyl radicals, of HCO radicals and of NH2 radicals, mainly in Herzberg s laboratory at Ottawa, has permitted flash photolysis to be used directly to measure the rates of radical reactions. 

The probability of the radiationless transition leading to predissociation is governed by the Franck Condon principle and by a group of selection rules first given by Kronig (1930). One reason why predissociation is so widespread is that, especially for non-linear molecules, the selection rules are extremely permissive. A striking example of a forbidden predissociation from the linear 2H (2I I) Renner state of the HCO radical has been described by Ramsay (1959). 

The H and HCO radicals react with each other and with atmospheric 02, resulting in the final production of CO and H2 (Figure 9.8). 

Both the H and HCO radical fragment can in turn react with 02 to yield hydroperoxyl radical as follows ... 

In addition, the HCO radical fragment is also produced by the reaction of HCHO and hydroxyl radical... 

Although the HCO radical has been identified by ESR in a photochemical matrix, few analogous aliphatic RCO radicals have been reported in photolysis systems. 

A recent study has shown that this reaction enriches deuterium in formaldehyde relative to methoxy [142]. Moore and co-workers have reviewed the considerable literature concerning the photodissociation dynamics of formaldehyde [143-145]. In addition the so-called roaming atom pathway has recently been discovered by Bowman and co-workers. This exciting theoretical and experimental effort has demonstrated a new photodissociation path in which one hydrogen atom takes a long trajectory around the HCO radical before abstracting the other H atom [146-151]. Recently Troe has analysed the photodissociation and thermal dissociation yields of formaldehyde [152-154]. 

The experimental activation energy calculated on the basis of a %-order reaction is found to be 46 Kcal. If we use the simple scheme, this should be equal [Eq. (XIII.14.6)] to E2 + HiEi — E ). The value of E2 has been measured at 7.5 Kcal (Table XII.6), while Eb = 0. If the bond energy of the H in the HCO radical is assigned the value 15 Kcal, current thermal data give a value of 82.4 Kcal to the enthalpy change of reaction 1, so that if there is no activation energy for the back reaction, this can be taken as the minimum value of Ei and the calculated value of the chain decomposition is then 48.7 Kcal — 22T/2 48 Kcal at 800°K, which may be considered to be in excellent agreement with the data. 

Figure 12. ESR spectrum of HCO radical produced by addition of H atoms to CO at 4.2 K.

Table 1.32 summarizes the data for these reactions, where X = H, OH, O, HO2 and CH3. It is important to note (Table 1.33) the high rate constant for reaction (79) which effectively means that all HCO radicals are converted very rapidly into CO. Reaction (80) becomes more important at elevated temperature, low [O2] and high total pressures. 

The FTIR spectra of CO-CH4 mixed thin solid films, below 50 K, gave evidence for the formation of a CO.CH4 complex (vCO 2136 cm-1).338 An ab initio calculation has been made of the vibrational wavenumbers for the formyl (HCO) radical.339 A high-resolution IR study of v3 (vCC) of the a1 A electronic state of the CCO radical showed that the band origin was at 1082.97894(94) cm-1.340 For the ground state the corresponding value was 1066.62407(54) cm-1.341... 

There are many other approaches to obtain resonance energies and widths, many are reviewed in this volnme. One that we consider in the next two sections is the distorted wave Born approximation (DWBA). In the following section the DWBA is tested against accurate complex coordinate calcnlations reported previously for a collinear model van der Waals system(l). The DWBA is then used to obtain the resonance energies and widths for the HCO radical. A scattering path hamiltonian is developed for that system and a 2ND approximation to it is given for the J>0 state. 

These results confirm and complement the earlier work of Beswick and Jortner who compared DWBA widths with those from collinear coupled-channel scattering calculations(33) where, as here, good agreement was observed. In the next section the DWBA widths are calculated for the chemically bonded HCO radical to give H+CO. Based on the present comparisons with exact results we are optimistic that the DWBA will provide realistic widths for this system. 

For formaldehyde, the OH reaction yields the formyl (HCO) radical, which subsequently reacts rapidly with Oz to form H02 and CO [reactions (34) and (36)]. For the higher aldehydes, the acyl (RCO) radical initially formed,... 

Other tropospheric sources of OH radicals are those that yield H atoms or HO2 radicals either directly or Indirectly, as, for example, the photolysis of HCHO to yield the HCO radical, which Is rapidly converted to HO2 (60-64) ... 

Reduction of metal ions by these radicals then regenerates acetone. In the case of formate ions, the exact photochemical reactions are still relatively unknown—the supposed main products are the HCO radical and 0, deprotonated form of OH (Petersen et al. 2006, Talu and Diyamandoglu 2004). Similarly, even the counter-ions of metal salts can be the source of reactive species (nitrates NO3 produceOH radicals in their complex photochemistry (Mack and Bolton 1999, Marie et al. 1996), whereas halide ions X form X radicals and Ca, (former et al. 1964). 

The gas phase photolysis of glyoxal produces two HCO radicals as the most important pathway under atmospheric conditions. Tadic et al. (2006) estimated 7obs = 1.0 lO" s F Although glyoxal has a very low effective quantum yield (< eff = 0.035 0.007), photolysis remains an important removal path in the atmosphere. 

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