عنوان
Is single-reference calculation adequate to address kinetics of CH(X2Π) reaction with N2(1Σg)?
نویسنده (گان)
Keshavarz,Fatemeh;Mousavipour,Seyed hosein
چکیده مقاله
CH (X2Π) radical (methylidyne) is an omnipresent radical that plays an important role in atmospheric, interstellar and combustive systems [1]. This radical can interact with N2 and result into prompt NO formation and ozone layer depletion through production of HCN+N [2]. Though there are several experimental studies reported about this reaction, theoretical data are limited. Therefore, this study explores detailed mechanism of this reaction using single-reference calculations at CCSD(T)/aug-cc-pVTZ//M06/aug-cc-pVTZ level of theory. The obtained mechanism is shown in Fig. 1. According to the calculated reaction enthalpies, the results of CCSD(T)/aug-cc-pVTZ, G2M(RCC) [2] and multi-reference CASPT2/aug-cc-pVTZ [2] calculations are about 8.0 to 23.6, 0.5 to 9.2 and 38.1 kJ mol-1 different from the corresponding experimental values. Furthermore, there are three transition states that have been reported by G2M(RCC) and/or CASPT2 studies but cannot be found at CCSD(T) level of theory. With respect to these observations, T1 diagnostic values were extracted, which suggested that, except TSn4, all transition states have strong multi-reference characters and should be treated with multi-reference calculations. The strong multi-reference character is due to the presence of CH radical with T1 diagnostic value of 0.108. Meantime, the apparent activation energy of the overall reaction, NCN+H formation and HCN+N production were calculated as 90.2 (81.6), 90.2 (73.3) and 56.9 kJ mol-1 at CCSD(T) (CASPT2 [2]) level of theory, which can be compared with the experimental values of 92.7, 71.2 and 53.1 to 58.2 kJ mol-1, respectively. Therefore, both single and multi-reference calculations of this reaction might involve some discrepancies with respect to experimental results.

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