Theoretical Studies On The Mechanisms Of Two Important Intermediates HCNO And HCCCO In The Combustion And Atmospheric Chemistry

It is known, as an important research topic in the combustion and atmospheric chemistry, that species HCNO and HCCCO are investigated as important intermediates in NO-reburning processes for the reduction of NO_x pollutants from fossil-fuel combustion emissions. In this article, by means of the current quantum chemistry theory and calculation method, the potential energy surfaces (PES) are built up for the following five reactions: a. The reaction of HCNO + OH b. The reaction of HCNO + O c. The reaction of HCNO + H d. The reaction of HCNO + NH e. The reaction of HCCCO + NO where the reactions a, b, c and d have been noticed by some experimental investigators and then to show, by experiments, some useful information of the reaction products.For the five reactions, all calculations are carried out by using Gaussian 98 program packages. The geometries of the reactants, products, intermediates, and transition states are optimized by using hybrid density functional B3LYP method with the 6-311G(d,p) basis set. To confirm that the transition states connects designated intermediates, we also perform intrinsic reaction coordinate (IRC) calculations at the B3LYP/6-311G(d,p) level. In addition, single-point calculations are performed at the CCSD(T)/6-311G(d,p) level for the B3LYP/6-311G(d,p) optimized geometries of all species. B3LYP/6-311G(d, p) zero-point vibrational energies (ZPVE) are included.The main conclusions for the five reactions are given as follows:1. The mechanism of reaction HCNO + OH:For the reaction HCNO+OH, the potential energy surfaces (PES), in which there are seven pathways from P₁ to P7 involved, is built up to show the product channels whether to proceed in the manner such as energetically favorable or less or unfavorable. It is shown that both pathways P₆(CO+H₂NO) and P₃(HNO+HCO) are energetically favorable and they are of, by experiments, the major channels; the pathway P₅(NO+H₂CO) is the less favorable and it is of, by experiments, the minor one; the two pathways P₁(H₂O+NCO) and P7(CO+H₂+NO) are the more less favorable and, by experiments, they are at most the very minor channels; the pathways P₄(HCN+HO₂) and P₂(NH₂+CO₂), which possess high barriers needed to overcome, proceeded uneasily, and they are energetically unfavorable.The above theoretical results are in harmony with the first and direct experimental observations, in the study of reaction OH+HCNO, by means of laser-induced fluorescence and IR diode absorption spectroscopy.Since different mechanisms of the reaction OH+HCNO have been proposed by the previous investigators, for this, a correct mechanism has been worked out by the present author and published in the"THEORETICAL CHEMISTRY ACCOUNTS".2. The mechanism of reaction HCNO + O:It is shown that the singlet and triplet PES are of non-overlap to give that the six singlet- and triplet- pathways are in dependent. Notice that the former and the latter pathways possess the same six products such that P₁(HCO+NO), P₂(HNO+CO), P₃(NCO+OH), P₄(NH+CO₂), P₅(H+CO+NO) and P₆(CNO+ OH). (1) For the singlet and triplet, P₁(HCO+NO) is the major product; (2) for the singlet and triplet, both P₂(HNO+CO) and P₃(NCO+OH) are the minor products, but for the singlet, P₂ is easily obtaind than P₃, whereas for the triplet, P₃ is easily obtaind than P₂; (3) for the singlet and triplet, P₄(NH+CO₂) and P₅(H+CO+NO) are at most the very minor products; (4) for the singlet and triplet, P₆(CNO+OH), which possesses high barriers needed to overcome, proceeded uneasily, and it is energetically unfavorable.For the six products, only the products P₁, P₂ and P₃ are observed by experiments.3. The mechanism of reaction HCNO + H:The doublet PES showns that there are four pathways, respectively, leading to the four products P₁(HCN+OH), P₂(CO+NH₂), P₃(CH₂O+N) and P₄(CH₂+NO). The product P₁(HCN+OH) is major one, and the products P₂(CO+NH₂), P₃(CH₂O+N) and P₄(CH₂+NO) are the minor species. It is known that the products P₂(CO+NH₂), P₃(CH₂O+N) and P₄(CH₂+NO) are identified by experiments.4. The mechanism of reaction HCNO + NH:The singlet PES describes three pathways, by writingP₁(CH₂O+N₂), P₂(NH₂+CNO), P₃(HCN+HNO), where both P₁(CH₂O+N₂) and P₃(HCN+HNO) are the major products, and, alternatively, P₂(NH₂+CNO) is the minor one. The theoretical results are in accordance with experiments.5. The mechanism of reaction HCCCO + NO:The singlet PES shows that there are four products, respectively, characterized by the major, the minor and the less favorable, i.e., P₁(HCCO+NCO)>P₃(H(CNC)O+CO)≈P₄(HCN+2CO)>P₂(HCCNO+CO).In the study of the four reactions HCNO+OH, HCNO+O, HCNO+H and HCNO+NH, the theoretical results are in harmony with the first and direct experimental observations, by means of laser-induced fluorescence and IR diode absorption spectroscopy. Furthermore, the theoretical mechanisms of five reactions HCNO+OH, HCNO+O, HCNO+H, HCNO+NH and HCCCO+ NO, will provide some useful information for the experimental studies.