Niklas Meinander

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 ...

Two-dimensional vibrational potential energy surface for phthalan: The effect of large coupling on vibrational quantum states

The low-frequency analysis of the phthalan ring-puckering vibration shows a highly irregular pattern of energy levels which cannot be adequately described by a one-dimensional potential energy function. Because the ring-puckering mode interacts strongly with the ring-flapping vibration of the same symmetry, a two-dimensional analysis involving these two motions is required. Two-dimensional kinetic energy (reciprocal reduced mass) expansions were calculated for the puckering, flapping, and interaction terms and these were utilized in the calculation of the two-dimensional potential energy surface. This surface does an excellent job of reproducing the irregular pattern of observed energy spacings for the ring-puckering vibration in both the ground and excited flapping states. The potential-energy surface has a barrier to planarity of 35 cm−1 and energy minima at x1=0.09 A, x2=−0.03 A, and x1=−0.09 A, x2=0.03 A where x1=puckering and x2=flapping. Although the minima correspond to puckered conformations with ...

Evidence for resonant intermolecular coupling in liquid benzene and pyridine from Raman difference spectroscopy of isotopic mixtures

Raman difference spectroscopy (RDS) affords a powerful tool for measuring very small frequency differences between similar samples. This technique has been used to measure the changes upon isotopic dilution in band frequencies and bandwidths for a number of vibrations of liquid benzene and liquid pyridine. Both blue and red shifts of the vibrational frequencies were observed, and these were interpreted in terms of additive resonant intermolecular coupling and volume effects. Both effects may provide either positive or negative contributions to the frequency shifts. The frequency shifts observed in mixtures containing both benzene and pyridine have also been interpreted in terms of resonance and volume effects.

Computation of the energy levels of large-amplitude low-frequency vibrations. Comparison of the prediagonalized harmonic basis and the prediagonalized distributed Gaussian basis

Abstract The formalism involved in the solution of the Schrodinger equation for two-dimensional vibrational potential energy surfaces for large-amplitude low-frequency motions is reviewed. The performance of two different bases, the prediagonalized harmonic basis (PHB) and the prediagonalized distributed Gaussian basis (PDGB), is investigated. The calculated energy levels obtained with the two basis sets are in excellent agreement with one another.

A computer-controlled apparatus for laser-induced fluorescence spectroscopy in a supersonic jet

Abstract An experimental apparatus for laser-induced fluorescence excitation spectroscopy (FES) and for dispersed fluorescence studies in a supersonic jet has been constructed and optimized for the collection of spectra from weakly fluorescing samples. A detailed description of the hardware and software is presented here.

Fluorescence and ultraviolet absorption spectra and structure of coumaran and its ring-puckering potential energy function in the S1(π,π*) excited state

The fluorescence excitation (jet cooled), single vibrational level fluorescence, and the ultraviolet absorption spectra of coumaran associated with its S1(pi,pi*) electronic excited state have been recorded and analyzed. The assignment of more than 70 transitions has allowed a detailed energy map of both the S0 and S1 states of the ring-puckering (nu45) vibration to be determined in the excited states of nine other vibrations, including the ring-flapping (nu43) and ring-twisting (nu44) vibrations. Despite some interaction with nu43 and nu44, a one-dimensional potential energy function for the ring puckering very nicely predicts the experimentally determined energy level spacings. In the S1(pi,pi*) state coumaran is quasiplanar with a barrier to planarity of 34 cm(-1) and with energy minima at puckering angles of +/-14 degrees. The corresponding ground state (S0) values are 154 cm(-1) and +/-25 degrees . As is the case with the related molecules indan, phthalan, and 1,3-benzodioxole, the angle strain in the five-membered ring increases upon the pi-->pi* transition within the benzene ring and this increases the rigidity of the attached ring. Theoretical calculations predict the expected increases of the carbon-carbon bond lengths of the benzene ring in S1, and they predict a barrier of 21 cm(-1) for this state. The bond length increases at the bridgehead carbon-carbon bond upon electron excitation to the S1(pi,pi*) state give rise to angle changes which result in greater angle strain and a nearly planar molecule.

Vibrational Spectra, Theoretical Calculations, and Two-Dimensional Potential Energy Surface for the Ring-Puckering Vibrations of 2,4,7-Trioxa[3.3.0]octane

2,4,7-Trioxa[3.3.0]octane (247TOO) is an unusual bicyclic molecule which can exist in four different conformational forms which are determined by the directions of the two ring- puckering motions. The vibrational assignments of 247TOO have been made based on its infrared and Raman spectra and theoretical density functional theory (DFT) calculations. The two ring-puckering motions (in-phase and out-of-phase) were observed in the Raman spectra 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 was utilized to generate a two-dimensional potential energy surface (PES) for the two ring-puckering motions. The resulting quantum state energies for this PES were then calculated in order to better understand the patterns that are produced when the PES has four energy minima at different energy values. The wave functions corresponding to the different quantum states were also calculated. The NMR spectrum of 247TOO showed the presence of the two lowest energy conformations, consistent with the results of the ab initio calculations.

Infrared and Raman spectra, theoretical calculations, conformations, and two-dimensional potential energy surface of 2-cyclopenten-1-one ethylene ketal.

The infrared and Raman spectra of the bicyclic spiro molecule 2-cyclopenten-1-one ethylene ketal (CEK) have been recorded. Density functional theory (DFT) calculations were used to compute the theoretical spectra, and these agree well with the experimental spectra. The structures and conformational energies for the two pairs of conformational minima, which can be defined in terms of ring-bending (x) and ring-twisting (τ) vibrational coordinates, have also been calculated. Utilizing the results from ab initio MP2/cc-PVTZ computations, a two-dimensional potential energy surface (PES) was calculated. The energy levels and wave functions for this PES were then calculated, and the characteristics of these were analyzed. At lower energies, all of the quantum states are doubly degenerate and correspond to either the lower-energy conformation L or to conformation H, which is 154 cm(-1) higher in energy. At energies above the barrier to interconversion of 264 cm(-1), the wave functions show that the quantum levels have significant probabilities for both conformations.

Theoretical calculations and vibrational potential energy surface of 4-silaspiro(3,3)heptane.

Theoretical computations have been carried out on 4-silaspiro(3,3)heptane (SSH) in order to calculate its molecular structure and conformational energies. The molecule has two puckered four-membered rings with dihedral angles of 34.2° and a tilt angle of 9.4° between the two rings. Energy calculations were carried out for different conformations of SSH. These results allowed the generation of a two-dimensional ring-puckering potential energy surface (PES) of the form V = a(x1 (4) + x2 (4)) - b(x1 (2) + x2 (2)) + cx1 (2)x2 (2), where x1 and x2 are the ring-puckering coordinates for the two rings. The presence of sufficiently high potential energy barriers prevents the molecule from undergoing pseudorotation. The quantum states, wave functions, and predicted spectra resulting from the PESs were calculated.

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...