An experimental and ab initio investigation of the low-frequency vibrations of coumaran

Abstract

Coumaran (2,3-dihydrobenzofuran) has been studied using a combination of (1+1′) resonantly enhanced multiphoton ionization (REMPI) and zero electron kinetic energy (ZEKE) studies, supported by ab initio molecular orbital calculations, in order to characterize the low wave number vibrational structure of the S1 neutral excited and D0 ionic ground states. These studies focus primarily on the modifying effects of electronic excitation and ionization on the balance of forces driving the S1 and D0 equilibrium structures toward or away from planarity. The results suggest that coumaran retains a puckered structure in the S1 state, having a barrier significantly smaller than that in the electronic ground state, but is apparently pseudo-planar or weakly puckered in the cation ground state. In each state the drive towards or away from planarity results from a competition between decreasing bond order in the aromatic system which increases torsional interactions thereby favoring a higher barrier and an increase in bond order in the furan ring which has the opposite effect. The lack of symmetry in coumaran lifts any restrictions on which out-of-plane modes can couple, resulting in a rich combination band structure in REMPI and ZEKE spectra, principally involving the ring twisting (44) and the ring pucker (45) vibrational modes. The butterfly mode (43) on the other hand shows surprisingly little activity.

S0 Ring-Puckering Potential Energy Function for Coumaran

“…We are also in the process of further analyzing the laser induced fluorescence (LIF) spectra and ultraviolet absorption spectra of coumaran to determine the potential energy surface for the S 1 (π,π*) state. Watkins and co-workers 6 have studied the REMPI and ZEKE spectra of this molecule and proposed assignments for the S 1 (π,π*) excited states. Even at this stage it is evident that their assignments need revision, and we will be publishing our data for this state soon.…”

Laser-induced fluorescence and single vibronic level emission spectroscopy of chiral (R)-1-aminoindan and some of its clusters in a supersonic jet

“…Spectroscopic determination of the structures and potential energy surfaces for a number of five-member ring compounds has been investigated by different groups. [7][8][9][10] Hollas et al have investigated the puckering vibration of pseudo five-member rings and constructed a 1-D symmetric double well potential for indan and related molecules. [11][12][13] They considered a quadratic double well potential of the form V(x) ¼ Ax 2 þ Bx 4 for the puckering vibration in indan and reported barriers of 1979 cm À1 and 1820 cm À1 for the S 0 and S 1 states, respectively.…”

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

“…This knowledge allowed us to reassess the far-infrared assignments and determine the one-dimensional ring-puckering potential function for coumaran in its S 0 state. Watkins et al 11 have studied the resonantly enhanced multiphoton ionization ͑REMPI͒ and zero electron kinetic energy ͑ZEKE͒ spectra of this molecule and proposed assignments and a potential function for the S 1 ͑ , * ͒ state. This barrier arises from the single torsional interaction between the two CH 2 groups.…”