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We recently observed that the activation barriers of O-atom abstraction reactions between metal atoms and N2O, in which both reactants are in their ground electronic states and the atoms contain no valence p electrons, vary systematically with the sums of the metal atom ionization potential and the energy required to promote a valence s electron to the lowest p orbital. It is shown here that this observation can be explained by the assumption that the activated complex results from the resonance interactions of ionic and covalent structures. Activation barriers for 43 reactions are calculated and where experimental measurements are available, are shown to be in good agreement with those. New interpretations are offered for literature data on the Ca and Cr reactions. The resonance treatment leads to a more general relationship in which activation barriers depend simultaneously on ionization potentials, electron affinities, promotion energies, and bond energies of the reactants. A number of further series of reactions, where activation barrier or rate coefficient trends with some of these parameters have been observed, are discussed and seen to be special cases of this formalism. Good agreement is found between the height of the barrier for the N2O+H→N2+OH reaction calculated by the present resonance treatment and an ab initio method.

The H+N2O reaction has been investigated using the high-temperature photochemistry (HTP) technique. H(1 2S) atoms were generated by flash photolysis of NH3 and monitored by time-resolved atomic resonance fluorescence with pulse counting. The bimolecular rate coefficient for H-atom consumption, leading essentially to N2+OH, from 390 to 1310 K is found to be given by k1(T)=5.5×10−14 exp(−2380 K/T)+7.3×10−10 exp(−9690 K/T) cm3 molecule−1 s−1; the accuracy is assessed as approximately 25% at the 2σ confidence level. Above 750 K, k1 closely follows the Arrhenius behavior of the second term alone. Distinct curvature is evident below 750 K. k1 is compared to theoretical BAC-MP4 predictions and good agreement is found for a model involving rearrangement of an HNNO intermediate coupled with tunneling through an Eckart potential barrier, which dominates at the lower temperatures. The branching ratio for the channel leading to NH+NO is discussed in the context of recent thermochemical information and a maximum rate coefficient of <1×10−9 exp(−15800 K/T) cm3 molecule−1 s−1 is set for temperatures up to 2000 K.

Rate coefficients for the O + C6H6 reaction have been measured by using the HTP (high-temperature photochemistry) technique with 193-nm laser photolysis of S02 to generate ground-state oxygen atoms and atomic resonance fluorescence to monitor relative concentrations of the O atoms. The data cover the 600-1310 K range and are best represented by k(T) = 5.35 X 10"11 exp(-261 \K/T) cm3 molecule"1 s'1 with a precision of better than ±10% and corresponding accuracy of ±23%, both at the 2 statistical confidence level. The present data are in excellent agreement with those from several other investigations; together, they can be represented by k(T) = 4.0 X 10~n exp(-2349A"/7) cm3 molecule"1 s"1 for the 300-1450 K range, again with an estimated accuracy of about ±23%. The results indicate that the O atom addition mechanism dominates over this entire temperature range.

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The temperature dependence of the rate coefficients for the H + NH3 -+ NH2 + H, reaction is measured by the high-temperature photochemistry (HTP) technique. Technique improvements are discussed. For the 490-960 K range, we find k , ( T ) = 1.21 X exp(-6920 K I T ) cm3 molecule-l s-'. When combined with two other sets of direct measurements, a best fit gives k,(n = 9.0 X (T/K)2.40 exp(-4991 K / T ) cm3 molecule-' s-' for the 490-1780 K range. It is shown that these data can be more closely fitted by a calculation based on conventional transition-state theory plus an Eckart tunneling model than with other current transition-state models considered. Using the potential energy surface for the H + NH3 reaction, several tunneling models fit the results for the D + ND3 reaction. The kinetic isotope effect for these two reactions is compared to results from different theoretical models, with the Eckart model giving the best approximation. IntroductionRate coefficients for the elementary reaction

The spectral distribution of the chemiluminescent reactions O+NO→NO2+hv has been determined over the wavelength region 3875–14 000 Å. The absolute rate constant was determined by a method of chemical actinometry which required only relative intensity measurements and which is virtually free from geometry errors. The rate constant over this spectral region was found to be 6.4×10—17 cm3 molecule—1 sec—1 within an accuracy of 30%. This reaction can be used as a standard from which the rate constants for other chemiluminescent reactions can be readily obtained without requiring detector calibration or geometry corrections.

The reaction of ground state NH with H2 has been studied in a high-temperature photochemistry (HTP) reactor. The NH (X 3 Σ) radicals were generated by the 2-photon 193 nm photolysis of NH3, following the decay of the originally produced NH(A 3 Π) radicals. Laserinduced fluorescence on the NH(A 3 Π-X 3 Σ 0,0) transition at 336 nm was used to monitor the progress of the reaction. We obtained k (833-1432 K) = 3. showing the barriers leading to these products to be too high compared to the measured activation energies. The calculations indicate the likelihood of formation of adducts, of low stability. These then may undergo further reactions. The NH + H2O reaction is briefly discussed and it is similarly argued that HNO + H2 cannot be the products, as had been previously suggested.3