Browsing by Subject "molecular structure"
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Item Spectroscopic investigations of the vibrational potential energy surfaces in electronic ground and excited states(Texas A&M University, 2007-09-17) Yang, JuanThe vibrational potential energy surfaces in electronic ground and excited states of several ring molecules were investigated using several different spectroscopic methods, including far-infrared (IR), Raman, ultraviolet (UV) absorption, fluorescence excitation (FES), and single vibronic level fluorescence (SVLF) spectroscopies. Based on new information obtained from SVLF and millimeter wave spectra, the far-IR spectra of coumaran were reassigned and the one-dimensional ring-puckering potential energy functions for several vibrational states in the S0 ground state were determined. The barrier was found to be 154 cm-1 and the puckering angles to be ???? 25????, in good agreement with the millimeter wave barrier of 152 cm-1 and puckering angles of ???? 23????. Moreover, the UV absorption and FES spectra of coumaran allowed the one-dimensional ring-puckering potential energy functions in the S1 excited state to be determined. The puckering barrier is 34 cm-1 for the excited state and the puckering angles are ???? 14????. Several calculations with different basis sets have been carried out to better understand the unusual vibrational frequencies of cyclopropenone. It was shown that there is strong interaction between the C=O and symmetric C-C stretching vibrations. These results differ quantitatively from a previous normal coordinate calculation and interpretation. The vapor-phase Raman spectrum of 3,7-dioxabicyclo[3.3.0]oct-1,5-ene was analyzed and compared to the predicted spectrum from DFT calculations. The spectrum further shows it has D2h symmetry, in which the skeletons of both rings are planar. The infrared and Raman spectra of vapor-phase and liquid-phase 1,4-benzodioxan and 1,2,3,4-tetrahydronaphthalene were collected and the complete vibrational assignments for both molecules were made. Theoretical calculations predicted the barriers to planarity to be 4809 cm-1 for 1,2,3,4-tetrahydonaphthalene and 4095 cm-1 for 1,4-benzodioxan. The UV absorption, FES, and SVLF spectra of both molecules were recorded and assigned. Both one and two-dimensional potential energy functions of 1,4-benzodioxan for the ring-twisting and ring-bending vibrations were carried out for the S0 and S1(????,????*) states, and these were consistent with the high barriers calculated for both states. The low-frequency spectra of 1,2,3,4-tetrahydronaphthalene in both S0 and S1(????,????*) states were also analyzed.Item Vibrational Spectra, Theoretical Calculations, and Structures of Cyclic Silanes, 2,4,7-Trioxa(3.3.0)Octane and Botryococcenes(2014-12-08) Chun, Hye JinThe vibrational spectra and structures of several cyclic silanes and a bicyclic molecule have been investigated with high-level ab initio and density function theory (DFT) calculations. In addition, the Raman spectra of botryococcene hydrocarbons have been studied to help with their identification. Infrared and Raman spectra and ab initio and DFT calculations have been utilized to study 1,3-disilacyclopent-3-ene, 1,3-disilacyclopentane, 1-silacyclopent-3-ene, silacyclopentane and their derivatives. In each case the agreement between observed and calculated infrared and Raman spectra was very good. Theoretical computations have also been used to calculate the potential energy surfaces (PES) for four cyclic silanes. The calculated ring-puckering potential energy functions of 1-silacyclopent-3-ene and 1,3-disilacyclopent-3-ene had barriers of 3.8 cm^-1 and 0 cm^-1, respectively, in good agreement with experimental results. The calculated results for and 1,3-disilacyclopentane predicted ring-twisting barriers of 2493 cm^-1 (vs. 2110 cm^-1 observed) and 1395 cm^-1, respectively. The conformational energies for the bent forms were calculated to be 1467 cm^-1 (vs. 1509 cm^-1 observed) for the former and 878 cm^-1 for the latter relative to the energy of the twist minima. The vibrational assignments of 2,4,7-trioxa(3.3.0)octane have been made based on its infrared and Raman spectra and theoretical DFT calculations. The two ring-puckering motions (in-phase and out-of-phase) were observed in the Raman spectrum of the liquid at 249 and 205 cm^-1 and these values correspond well to the DFT values of 247 and 198 cm^-1. Ab initio calculations were utilized to calculate the structures and conformational energies for the four energy minima and the barriers to interconversion and the data were utilized to generate a two-dimensional PES for the two ring-puckering motions. The Raman and infrared spectra of liquid squalene, which is a building block molecule for the production of essential cellular molecules, have been collected and assigned using DFT calculations. This was helpful for analyzing the Raman spectra of botryococcus braunii. DFT calculations also assisted in understanding the Raman spectra of the botryococcenes. The spectral region from 1600-1700 cm^-1 shows C=C stretching bands specific for botryococcenes, and this is of great value for identifying the specific molecules.