Theoretical Prediction of Electronically Excited States and Vibrational Frequencies of Interstellar and Planetary Radicals, Anions, and Cations
Fortenberry, Ryan Clifton
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In the search for molecular species in the interstellar medium and extraterrestrial planetary atmospheres, theoretical methods continue to be an invaluable tool to astronomically minded chemists. Using state-of-the art methods, this doctoral work characterizes the electronically excited states of interstellar radicals, cations, and even rare anions and also predicts the gas phase fundamental vibrational frequencies of the cis and trans-HOCO radicals, as well as the cis-HOCO anion. First, open-shell coupled cluster methods of singles and doubles (CCSD) and singles and doubles with triples-inclusion (CC3) are tested on the C2H and C4H radicals. The significant double-excitation character, as well as the quartet multiplicity of some states yields inaccurate excitation energies and large spin contamination with CCSD. CC3 somewhat improves this for select states, but discrepancies between CC and multiref- erence results for certain states exist and likely arise from the lack of spin adaptation in conventional spin-orbital CC. Next, coupled-cluster methods predict the presence of an ex- cited state of the closed-shell allyl cation and its related H2CCCHCH2+ cousin at 443 nm near an unidentified laboratory peak at 442.9 nm which is also close to one of the largest unattributed interstellar absorption features. Additionally, the dipole moments, electron binding energies, and excited states of neutral radicals and corresponding closed-shell anions of interstellar interest are also computed. These are calibrated against experimental data for CH2CNâ and CH2CHOâ . Since coupled cluster theory closely reproduces the known experimental data, dipole-bound excited states for eight previously unknown anions are pre- dicted: CH2SiNâ , SiH2CNâ , CH2SiHOâ , SiNâ , CCOHâ , HCCOâ , SiCCNâ , and SiNCâ . In addition, we predict the existence of one rare valence-bound excited state of CH2SiNâ and iii also SiCCNâ as well as even rarer two valence-bound states of CCSiNâ . Lastly, the reaction of CO + OH and its transient potential intermediate, the HOCO radical, may be responsible for the regeneration of CO2 in the Martian atmosphere, but past spectroscopic observations have not produced a full gas-phase set of the fundamental vibrational frequencies of the HOCO radical. Using established, highly-accurate quantum chemical coupled cluster tech- niques and quartic force fields, all six fundamental vibrational frequencies for 1 2Aâ ² cis and trans-HOCO and 1 1Aâ ² cis-HOCOâ are computed in the gas phase.
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