Theoretical Investigations On Mechanisms For Several Important Radical-Molecule Reactions

Reactions of radicals play a significant role in diverse areas such as organic chemistry, photochemistry, biochemistry, environmental chemistry, combustion chemistry, interstellar chemistry, and atmospheric chemistry. In this thesis, quantum chemical investigations on the potential energy surfaces of a series of important radical-molecule reactions as well as radical-molecule reactions catalyzed by interstellar water ice have been carried out. Important information of potential energy surfaces such as structures and energies of intermediate isomers and transition states, possible reaction channels, reaction mechanisms and major products are obtained. The results obtained in the present thesis may lay a strong foundation for building important radical-molecule models in organic chemistry, interstellar chemistry and combustion chemistry, and provide theoretical supports and warranty for future experimental study and detection of interstellar molecules in space. The main results are summarized as follows:1. Theoretical investigations are performed on mechanism of PH3CH reaction with a series ofσ-bonded molecule HX (X=CH3, NH2, OH,F, CH2F, CHF2, CF3, NHCH3 / CH2NH2, OCH3/CH2OH, the central atom belongs to elements of the second line.) andπ-bonded molecules such as C2H4/C2H2 and H2CO. The main results are as follows:(1) For PH3CH+ HX, main reaction pathways are as follows: R PH3CH+HX→PH3XCH2→→products (X= NH2,OH,F, NHCH3 and OCH3) R PH3CH+HX→PH3CH2+X(X= CH3, CH2F, CHF2, CF3, CH2NH2, CH2OH) So, PH3CH toward HX(X= CH3, CH2F, CHF2, CF3, CH2NH2, CH2OH) behaves H-abstraction mechanism; however, for X= NH2,OH,F, NHCH3 and OCH3, reactions prefer a nucleophilic addition and elimination mechanism. The latter is a new type of neutral radical-molecule reaction, in contrast to previous known atom/group abstraction and carbenoid insertion mechanisms.(2) For PH3CH+C2H4/C2H2 reaction, the main reaction pathways can be indicated as: R PH3CH+C2H4→PH3CHCH2CH2 ? cPH3CHCH2CH2→PH3-cCHCH2CH2→cCHCH2CH2 +PH3 R PH3CH+C2H4→PH3CHCH2CH2 ? cPH3CHCH2CH2→PH2CHCH2CH3→PHCH2CH2CH3→PHCH2+C2H5 R PH3CH+C2H4→PH3CHCH2CH2?cPH3CHCH2CH2→PH2CHCH2CH3→→PH2CHCH2 +CH3 R PH3CH+C2H2→PH3CHCHCH→cPH3CHCHCH→PH2CHCHCH2→PHCHCHCH3→PHCHCHCH2+H.For both reactions, the four-membered ring species (cPH3CHCH2CH2 and cPH3CHCHCH, respectively) are important entrance intermediates, whereas they are of negligible importance for normal unsaturated radical reactions with alkenes and alkynes. For both the PH3CH+C2H4/C2H2 reactions, theλ5-P bonding within PH3CH is effectively transformed into theλ3-P bonding within the products, also in contrast to normal reactions of unsaturated radicals.(3) For PH3CH+H2CO reaction, the main reaction pathways are presented as: Path 1 R PH3CH+H2CO→cPH3CHCH2O→P2 PH3CHCHO+H Path 2 R PH3CH+H2CO→cPH3CHCH2O→PH3-cCHCH2O→P4 PH3 + cCH2CHOFor the entrance channels, [2+2] cycloaddition to form oxaphosphetane radical cPH3CHCH2O is more favorable, which is an analogical process to Wittig reaction of P-Ylide. So PH3CH shows P-Ylide character in entrance channel, However, in subsequent transformation processes, the reaction behaves radical-like character. So, we conclude that PH3CH has duplicate reactivity.2. A combined quantum chemical and master equation rate constant calculational study is performed on the mechanism of the NCO+C2H2 reaction. The main results are as follows: Path 1: R NCO+C2H2→HCCHNCO L1→cCHCHNC-O r4→NCHCHCO L5→P2 HCN+HCCO, Path 2: R NCO+C2H2→HCCHNCO L1→cCHCHNC-O r4→NCHCHCO L5→P5 NCCHCO+H.The most favorably reaction channel is a previously ignored four-membered ring channel that will eventually generate the products P2 HCN+HCCO and P5 OCCHCN+H. The previously proposed"H-abstraction,""C2H-abstraction,"and"five-membered ring"channels are thermodynamically and kinetically much less competitive. With the new mechanism, the calculated rate constants of the title reaction have a positive temperature effect and no distinct pressure dependence effect. Our calculated results are in qualitative agreement with available experimental data. Also, it seems unlikely to synthesize the five-membered ring heterocycle oxazoles via the title reaction.3. A detailed mechanistic study on potential energy surfaces was reported for reaction C2H3+H2CO. the main results are as follows: Path 1i: R C2H3+H2CO→H2CCHCH2O L10→P1 H2CCHCHO+H Path 1ii: R C2H3+H2CO→H2CCHCH2O L10→H2CCH2CHO L3→P2 C2H4+HCO→P9 C2H4+CO+H Path 2: R C2H3+H2CO→P2 C2H4+HCO→P9 C2H4+CO+H G3B3 and CBS-QB3 computational methods both predict that the addition-elimination process Path 1 is more competitive, in contrast to the previously suggested H-abstraction mechanism.4. A detailed mechanistic study on potential energy surfaces was reported for reaction HCCO+C2H2, most feasible channel is shown as: R HCCO+C2H2→HCCHCHCO L1→OC-cCHCHCH r3→P6 cCHCHCH+COIt is demonstrated that cC3H3 should be responsible for the observed C3H3+ ion in one early experiment. Moreover, the predominance of P6 from the HCCO+C2H2 reaction suggests that this reaction could be a useful precursor for generating the long-sought cyclopropyl radical (c-C3H3).5. Detailed theoretical investigations are performed on reactions of 1,3O+H3CN, C2n+1N+H2O and C3H+H2O catalyzed by interstellar water ice. Applying this gas-grain model, we successfully account for the formation mechanism of interesting interstellar propynal and cyanomethylene, which to date, have not been resolved by a variety of gas-reaction models. Moreover, chemical behavior of C2n+1N on water ice should have a prominent influence on the concentration of the polycyanoacetylene radicals C2n+1N (n=1-4), as well as their parents HC2n+1N. Our conclusions also enrich radical-molecule or atom-molecule reaction chemistry catalyzed interstellar water ice.