Ab initio studies of excited states and reactions of organic molecules

• Main Author: Harding, Lawrence Brook
• Format: Others
• Language: en
• Published: 1979
• Online Access: https://thesis.library.caltech.edu/5389/1/Harding_lb_1979.pdf; Harding, Lawrence Brook (1979) Ab initio studies of excited states and reactions of organic molecules. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/19z3-cp09. https://resolver.caltech.edu/CaltechTHESIS:11192009-132537342 <https://resolver.caltech.edu/CaltechTHESIS:11192009-132537342>

Part A: Extensiveab initio calculations (double zeta, plus polarization function basis with correlated wave functions) on the addition of ^1O_2 ethylene are combined with standard thermochemical methods of estimating substituent effects to predict the energetics of the addition of ^1O_2 to substituted olefins. The results include estimates for peroxy biradical, open 1,4-zwitterion and perepoxide intermediates. It is concluded that only the first two play a role in this reaction. Detailed comparisons of the theoretical predictions with experimental results are also reported.It is shown that many aspects of the stereospecificity and regiospecificity can be understood assuming a biradical intermediate or transition state. Part B: Generalized valence bond (GVB) and configuration interaction (CI) calculations using an extensive basis [double zeta plus polarization functions (DZd)] have been carried out on peroxymethylene (H_2COO) often referred to as carbonyl oxide or as the Criegee intermediate), dioxirane, and dioxymethylene (OCH_2O). The ab initio thermochemical results are combined with existing thermochemical data to analyze possible modes of ozonolysis. The predicted heat of formation of peroxymethylene is 29.1 kcal, indicating that the dissociation of the primary ozonide to form peroxymethylene biradical and formaldehyde is 9 kcal endothermic. The ring state, dioxirane, is predicted to be 36 kcal below peroxymethylene with dioxymethylene lying 15 kcal above the ring state. Gas phase experimental results are shown to be consistent with the predicted thermochemistry. In addition, solution phase results on the stereospecificity of ozonolysis are shown to ae consistent with a biradical intermediate. Part C: Large basis set configuration interaction, bending potential curves for three states (^3B_1, ^1A_1, and ^B_1) of netural CH_2 and one state (^2B_1) of CH_2^- are reported. Vibronic calculations using these potential curves are found to lead to excellent agreement with the observed ^1B_1 - ^1A_1 spectrum. Similar calculations on the ^3B_1 - ^2B_1 and ^1A_1 - ^2B_1 photoelectron spectra indicate the presence of hot bands in the observed negative ion spectrum. Reassignment of the observed spectrum based on these calculations leads to the prediction of ^1A_1 - ^3B_1 splitting of O. 38 ± 0.05 eV. Part D: The ground and valence excited states of ketene (H_2CCO) were studied using ab initio generalized valence bond (GVB) and configuration interaction (GVB-CI) wavefunctions. The character and properties of the states are analyzed in terms of the GVB wave- functions. The calculated vertical excitation energies (in eV) are 3.62 ^3(n → π*) or ^3A_2, 3.69 ^1(n → π*) or ^1A_2, 5.39 and 3(n → π*) or 1 ^3A_1, and 7.37 ^3(π → π*) or 2^3A_1. (Here π indicates a π-like orbital in the plane of the molecule.) These results are in excellent agreement with the observed electron impact excitation energies, 3.8 (^1A_2) and 5.35 ev (^3A_1). Note in particular the small separation (0.07 eV) of the ^3A_2 and ^1A_2 states (0.5 eV for H_2CO) and the 2-eV separation in the π π* triplet states in the two planes. The calculated ground state dipole moment, 1.62 D, is in fair agreement with the experimental value of 1.41 D. The calculated dipole moments of the ^3A_2, ^1A_2, 1 ^3A_1, and 2 ^3A_1 excited states are 2.76, 3.43, 2.43 and 0.27 D respectively. Part E: Ab inito configuration interaction (GVB-CI) methods are used to study the excited Rydberg states of formaldehyde formed by exciting out of either the n or π orbital into the various 3s, 3p, and 3d-like Rydberg orbitals. The resulting excitation energies are in good agreement (within ~ 0.1 eV) with the available experimental results. Calculated oscillator strengths are in fair agreement with experiment. Two states ^1(π→π*) and ^1(π→3s) are calculated to lie between 10.7 and 10.8 eV, corresponding closely to a broad unassigned peak in the electron impact spectrum (10.5-11.0 eV). We have assigned other peaks in the electron impact spectrum at 11.4-19.n eV and 12.5-12.8 eV as resulting from (π→3p) and (π→3d) transitions, respectively.