Spectroscopic and theoretical investigation of selected cyclic and bicyclic molecules in their ground and excited electronic states



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The structures, vibrational frequencies, and potential energy functions of several molecules in their ground and excited electronic states were determined using various spectroscopic and theoretical methods. High-level ab initio and density functional theory (DFT) calculations were utilized to investigate the previously reported structures and vibrational spectra of 1,3- disilacyclobutane (13DSCB) and its 1,1,3,3-d4 (13DSCB-d4) isotopomer. These calculations confirmed the finding from earlier microwave work that the CSiC angles of the 13DSCB ring are unexpectedly larger than the SiCSi angles. The calculated vibrational spectra using density functional theory agreed well with the experimental data and showed CH2 modes to have unusually low values. The calculations also confirmed that the individual molecules in the vapor phase are puckered whereas in the solid they become planar. The one-dimensional potential energy surfaces (PESs) for the ring inversion vibration of 2-cyclohexen-1-one and its 2,6,6-d3 isotopomer in its ground and singlet S1(n,?) electronic states were determined using ultraviolet cavity ringdown spectroscopy (CRDS). The CRDS data allowed several of the quantum states of the ring inversion vibration to be determined for both the ground and excited electronic states, and the data were fit very well with PESs with high barriers to inversion. The infrared and Raman spectra and DFT calculations were utilized to complete a vibrational assignment of 2CHO and 2CHO-d3. A remarkable agreement was seen between the experimental and calculated spectra. The fluorescence excitation spectra (FES) and the single-vibronic level fluorescence (SVLF) spectra of jet-cooled 1,4-dihydronaphthalene (14DHN) were acquired to determine its ring-puckering potential energy function for the ground and singlet S1(?,?) electronic states. Ultraviolet, infrared, and Raman spectra were also recorded to complement the analysis. The potential energy functions showed that the molecule is planar in both the ground and S1(?,?*) states. A complete vibrational assignment was carried out for 14DHN using the infrared and Raman data and aided by DFT calculations. The ab intio calculations carried out on 2-methyl-2-cyclopenten-1-one (2MCP) showed that the molecule can have 3 different conformers. Infrared and Raman spectra of the liquid-phase molecule were recorded and analyzed to complement the theoretical calculations.