Theoretical Studies On Several Unimolecular Reactions And Stability Of Molecular Cluster

Unimolecular reaction divides into two different processes, that is dissociation andisomerization. Theoretical study on unimolecular reaction plays an important role tounderstand the mechanism of chemical reaction of unimolecule. On the other hand,investigating the characteristics of the electronic properties and structural stabilitiesfor neutral and protonated molecular clusters, which is very important to understandthe formation mechanism of the molecular clusters composed of unimolecules, clusterdynamics, proton transfer process and solvation effect in-depth.Quantum chemistry, based on the principles of quantum mechanics, can obtainmicroscopic information which is difficult to observe in experiment, for example,predicting molecular structure and its energy, chemical reaction path, structure andenergy of the transition state, electronic structure of the excited state and so on. So itis an efficient way to understand the reaction mechanism and explain the experimentalphenomena. Nowadays, the quantum chemical calculation methods are widely used tostudy the relationships between molecular structure and its nature, molecular structureand its reactivity, and determine the structure of clusters, etc.In this thesis, we have studied the processes of the dissociation and isomerizationreactions of several unimolecules, and the structures and stabilities of molecularclusters by quantum chemistry calculational method. The main contents aresummarized as follows: （1） Study the structure and dissociation mechanism of doubly charged molecularions, which plays an important role in understanding the chemical and physicalprocess of plasma, ionospheric and interstellar space. NCO radical and its chargedions are the smallest chemical compounds including nitrogen, carbon, oxygen whichare the most commonly found elements in organic chemistry and biology, it plays animportant role in astrophysical and environmental chemistry. It was chosen as theresearch object. The potential energy surfaces of the low-lying states of NCO²⁺havebeen explored by CASSCF and MRCI methods, and its stability and dissociationprocess have been discussed. Given the dominant electron configurations of theelectronic states of NCO²⁺in the energy range of0¹2eV with respect to the groundelectronic state of NCO²⁺, they were consisted by exciting one or two electrons fromthe valence orbital of6,7or1to2or3orbital. The N⁺–CO⁺and NC⁺–O⁺collinear dissociation paths were mapped and energy variations with bendingcoordinate have been explored for the lowest excited states of NCO²⁺. The groundstate X2and the lowest excited state a4are metastable with a major collineardissociation channel into N⁺（X³Pg）+CO⁺（X²ï¼‰. It showed the significant effect ofnonadiabatic transition in the formation and decomposition processes of NCO²⁺. Forthe excited states with energies of3⁸eV the pre-dissociation processes are dominant,while those states with energies above8eV are repulsive. Several avoided crossingsand conical intersections for NCO²⁺have also been assigned. The obtainedinformation is useful for understanding the reactions and basic processes of NCOradicals, involved in various extreme environments.（2） α-CHD molecule is an important structural unit of six-membered ringsystems, which has important applications in the study of biology and syntheticscience. It is found that some fragments can be obtained through isomerization anddissociation, and the α-dissociation can be done in triplet state in the vacuumultraviolet absorption spectrum and induction photolysis experiment of α-CHD. Inorder to understand the molecular isomerization of α-CHD and reveal the resource ofthose fragments, the potential energy surfaces of the isomerization and dissociation reaction for α-CHD molecule in singlet and triplet states were studied by B3LYP andCCSD（T） methods. It obtained the reaction paths of the products, such asP1(^1,3c-C₅H₈O+¹CO), P2(^1,3C₂H₄+¹C₂H₄+2¹CO), P3/P3’(^1,3CH₂CHCH₂CH₂CHO+¹CO), P4(^1,3C₂H₂O+¹C₂H₂O+¹C₂H₄), P5（¹CH₃CHCO⁺1CH₂CHCHO） andP5’（³CH₃CHCO+¹C₂H₄+¹CO）. The reaction mechanism can be summarized as theisomerization and dissociation processes, and the processes mainly involve thehydrogen atom transfer, ring-opening and C-C bond cleavages. The triplet state ofα-CHD molecule can complete α-dissociation directly, verified by the TDDFT results,but the entrance energy is very high. In addition, as same as the singlet state, thetriplet state can also isomerize first, and then dissociate. In such case, the requiredenergy is much smaller and the channel can be opened more easily. Based on the UVphotolysis experiment of α-CHD with the wavelength of253.7nm （112.7kcal/mol）and the theoretical calculations of the singlet and triplet potential energy surfaces, werecognize that Path1is the most possible channel, Path3is the second one, Path1（2）,Path3（2）, Path2（3） and Path2（4） are unlikely ones, and Path5is difficult to beachieved. So P1is the major product, P2and P3are subsidiary products, maybe aminor distribution of¹CH₃CHCO, but the products P4, P3’ and P5’ are difficult to beobtained. It agrees well with the analysis results of mass spectrometry in experiment.The studied results clarify the microcosmic reaction mechanism of the isomerizationand dissociation for α-CHD molecule in the ground state and the first excited state. Itwill provide important reference for realizing its spectrum in-depth.（3） Butanone is the organic compound with volatile typical, as the solvent iswidely used in industry, and it plays an important role in the photochemistry andenvironmental science. It is found that some fragments such as C2H3+and HCO⁺could not be produced directly from the bond-cleavage of butanone by analyzing themass spectra, implying that an isomerization may occur prior to decomposition ofbutanone. The isomerization processes of butanone molecule were studied by B3LYPand QCISD（T） methods. Six main reaction pathways are confrmed using the IRCmethod, and the corresponding isomerization products are1-buten-2-ol,2-buten-2-ol, butanal or1-buten-1-ol, methyl1-propenyl ether, methyl allyl ether, and ethyl vinylether, respectively. Among them, there exist three pathways passing through butyleneoxide, indicating that butylene oxide is important intermediate product duringbutanone isomerization. These isomerizations are mainly caused by ring opening,hydrogen transfer, and formation/breakdown of the bonds. Since butylene oxide is atypical point chiral molecule, we also discussed the chirality switching mechanism ofbutylene oxide taking butanal and butanone as a starting point respectively. Thecalculated vertical ionization energies of the reactant and its products are in a goodagreement with the available experimental values. Considering the relative energies oftransition states and the number of high-energy barriers, we infer that the reactionpathway butanoneâ†'1-buten-2-olâ†'2-buten-2-ol is the most competitive one. Ourresults give a relative comprehensive view for understanding the mechanism ofbutanone isomerization, and provide useful information for future studies on theketone molecular isomerization. Additionally, we have observed some clusterfragment ions and protonated cluster of butanone by the mass spectrometrymeasurement of photoionization/photodissociation for butanone molecular clusters,under the irradiation of355nm and118nm laser light. We optimized the stablegeometric structures of neutral butanone clusters （CH₃COC₂H₅）n, cationic clusters（CH₃COC₂H₅）n+and protonated cluster （CH₃COC₂H₅）nH+（n=2¹0） atB3LYP/6-31G（d） level with density functional theory. The structural characteristicsand stabilities of various size of clusters for butanone clusters, inculding the averagebinding energy, first-order difference energy, second-order difference energy andHOMO-LUMO gap, were analyzed and discussed systematically, and we alsocompared these results with acetone clusters. It is found that single ring structure isthe most stable for these clusters from the beginning of n=3. With cluster sizeincreasing, the stability of double ring structure rises ceaselessly. The most stablestructure of acetone clusters （n=7） is double ring which appeared earlier thanbutanone clusters （including the neutral and cationic clusters, n=8）. The stability ofneutral butanone cluster in n=3is the best, it corresponds to the strong peaks inexperiment, and the stability of cationic clusters in n=6is better. It can be seen that the growth of structures of protonated clusters （CH₃COC₂H₅）nH+（n=2¹0） hasregularity, which can be the structure of n=2as solvation shell, a new moleculeattacks the solvation shell from the different active site （the carbon attached to oxygenatom, ethyl and methyl of those butanone molecules） separately. The structures andstability of butanone cluster are analyzed and discussed systematically.