Jaebum Choo

Spectroscopic determination of the two-dimensional vibrational potential energy surfaces for the ring-puckering and ring-flapping modes of indan in its S0 and S1(π,π*) electronic states

The vapor-phase far-infrared, mid-infrared, ultraviolet, Raman, and laser-induced fluorescence spectra of indan have been recorded and analyzed. The far-infrared spectra, which are very similar to those previously reported, together with the Raman and dispersed fluorescence (SVLF) spectra of the jet-cooled molecules were used to reassign the ring-puckering and ring-flapping energy levels for the S0 ground state. These were then utilized to calculate a two-dimensional vibrational potential energy surface (PES) which nicely fits all of the assigned puckering and flapping levels. The PES has a barrier of 488 cm−1 as compared to a previously reported value of 1979 cm−1, which was based on a one-dimensional analysis and earlier assignments. The dihedral angle of puckering is ±30°. Fluorescence excitation spectra of jet-cooled indan together with ultraviolet absorption spectra were used to assign the flapping and puckering levels in the S1(π,π*) electronic excited state. The PES for this state has a barrier of ...

Laser-induced fluorescence, electronic absorption, infrared and Raman spectra, and ab initio calculations of 1,2-dihydronaphthalene: Investigation of the out-of-plane ring modes for the ground and S1(π,π*) excited states

The laser-induced fluorescence spectra and dispersed fluorescence spectra of jet-cooled 1,2-dihydronaphthalene have been analyzed to investigate the ring inversion process in both the S0 and S1(π,π*) excited states. Ultraviolet absorption, infrared, and Raman spectra were also recorded to complement the analyses. Ab initio calculations predict the inversion process to involve four out-of-plane ring motions, and linear combinations of these were made to model the inversion process. The data show the barrier to inversion in the ground state to be 1363±100u2009cm−1 (the triple-zeta ab initio value is 1524u2009cm−1). The experimental data indicate that the barrier increases substantially in the excited state, for which the calculated barrier is 1526u2009cm−1 with a CIS/6-311+G(d) basis set.

Far-infrared spectra and two-dimensional potential energy surfaces for the out-of-plane ring vibrations of tetrahydrofuran-3-one in its S0 and S1(n,π*) electronic states

The far-infrared spectra of tetrahydrofuran-3-one show ring-bending (100–120u2009cm−1), ring-twisting (227–237u2009cm−1), difference (115–137u2009cm−1), and overtone (200–225u2009cm−1) bands. Calculations for the two-dimensional potential energy surface governing the twisting and bending motions were carried out. The minima of the potential surface correspond to twisted conformations with twist angles of approximately ±35°. The barrier to planarity is in the 1500 to 2000u2009cm−1 range. No saddle points corresponding to bent structures are present for the surface. A potential energy surface for the S1(n,π*) excited state, based on previous fluorescence excitation data, was also calculated. The latter has an assumed barrier to planarity of 1400u2009cm−1 and a barrier to pseudorotation of 880u2009cm−1.

Infrared and Raman spectra and molecular mechanics calculations of 4H-pyran and related molecules

Abstract The infrared and Raman spectra of 4H-pyran have been analyzed and assigned in terms of C2v symmetry. Molecular mechanics calculations (MM3) have been carried out for this molecule and also for 1,4-cyclohexadiene and 1,4-dioxin in order to compare the calculated vibrational frequencies with those observed. In most cases the MM3 calculations do a reasonable job of estimating the frequencies. Band type calculations were carried out to aid with the assignments.

Far‐infrared spectra and two‐dimensional potential energy surface for the ring‐bending and ring‐twisting vibrations of 5,6‐dihydro‐4H‐thiopyran

5,6‐Dihydro‐4H‐thiopyran has been synthesized and its far‐infrared spectrum has been recorded. Eleven ring‐bending bands originating at 120.7 cm−1 and four ring‐twisting bands originating at 274.5 cm−1 were observed. Twelve sum and difference bands in the 383–397 and 148–166 cm−1 regions were also observed and these facilitated the construction of a detailed energy map including numerous excited vibrational states of the two coupled vibrations. The two‐dimensional potential energy surface, which satisfactorily fits the observed data, was determined to be V=9.48 ×104x4−4.13×104x2+1.37×104τ4−1.82×104τ2+1.10 ×105x2τ2, where x and τ are the bending and twisting coordinates, respectively. The minima on the potential energy surface correspond to twisting angles of ±48° (half‐chair conformation). The lowest energy bent (boat) conformation corresponds to a saddle point 1500 cm−1 above the twisted conformation on the potential energy surfaces, and the barrier to planarity was estimated to be 6000 cm−1. Both of the...

Far-infrared spectra and ring-puckering potential energy functions of two oxygen-containing ring molecules with unusual bonding interactions

Over the past quarter century we have carried out a large number of studies of the ring- puckering vibrations of small ring-molecules. The far-infrared and Raman spectra of these large amplitude motions have been used to determine the molecular conformation and 1-D potential energy functions. The forces which are usually the principal contributors to the potential functions are the ring-angle strain and torsional forces, which often result when CH2 groups are next to each other. In the present study we have examined the far-infrared and Raman spectra of two oxygen-containing molecules which have turned out to have unusual potential energy functions. 1,3-dioxole and 4H-pyran.

FAR-INFRARED SPECTRA AND POTENTIAL ENERGY SURFACE FOR 5,6-DIHYDRO-4H-THIOPYRAN

The far infrared spectrum of 5,6-dihydro-4H-thiopyran shows eleven ring bending bands (near 120 cm−1), four ring-twisting bands (near 275 cm−1), and twelve sum and difference bands in the 383–397 cm−1 and 148–166 cm−1 regions. From these frequency data a detailed energy map was constructed for many of the excited vibrational states of the two coupled out-of-plane ring vibrations. A two-dimensional potential energy surface, which satisfactorily fits the observed data, was determined in terms of the bending and twisting coordinates respectively. The minima on the potential energy surface correspond to twisting angles of ±48° (half-chair conformation). The lowest-energy bent (boat) conformation correspnds to a saddle point 1500 cm−1 above the twisted conformation on the potential energy surfaces, and the barrier to planarity was estimated to be 6000 cm−1.