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

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

The role of symmetry and optical selection rules in revealing the molecular structure of the lowest Rydberg and ionic states of the 1,4-diazabicyclo[2.2.2]octane-Arn (n = 1,2,3) van der Waals complexes

The 1,4-diazabicyclo[2.2.2]octane–Arn (n = 1,2,3) van der Waals complexes (DABCO–Arn) have been investigated using a combination of (1 + 1[prime]) resonance enhanced multiphoton ionization (REMPI) and zero electron kinetic energy (ZEKE) spectroscopy. The additivity of the spectral shifts observed in both REMPI and ZEKE spectra, taken together with analysis of vibrational structure, suggest that in both DABCO–Ar and DABCO–Ar2 the argon atoms bind in equivalent equatorial (face) locations between two adjacent (CH2)2 bridges. However, the cumulative evidence from both REMPI and ZEKE spectra, together with ab initio results, suggests that the DABCO–Ar3 complex does not revert to D3h symmetry, but rather adopts a C2v structure in which all three argon atoms bind to one side of the DABCO framework. The exceptionally low wave-number vibrational structure observed in the REMPI spectra suggest that the van der Waals interaction in the excited state is extremely weak. However, ionization necessarily increases the strength of the interaction by virtue of the introduction of charge-induced dipole forces, as revealed by a consistent increase in vibrational wave numbers of the modes observed in the resultant ZEKE spectra

The weak hydrogen bond in the fluorobenzene-ammonia van der Waals complex: Insights into the effects of electron withdrawing substituents on p versus in-plane bonding

Author Institution: Department of Chemistry, University of York, Heslington, York YO10 5DD, UK.The fluorobenzene-ammonia van der Waals complex has been studied using a combination of two-colour resonance enhanced multiphoton ionisation spectroscopy (REMPI), high-level ab initio calculations of both ground and first excited electronic states and multidimensional Franck-Condon calculations. The experimental REMPI spectrum is characterised by a dominant, blue-shifted band origin, with weak activity in intermolecular vibrational modes. Second order approximate coupled cluster with singles and doubles (RICC2) geometry optimisations and numerical vibrational frequency calculations have been performed on a number of different structural isomers of the complex. Ground state counterpoise-corrected zero point binding energies show the in-plane complex, forming a pseudo-six membered ring connecting the fluorine atom and ortho hydrogen, to be consistently the most stable of all six conformations considered, at all levels of theory. Comparison of computed zero-point excitation energies for the most stable $\pi$ and in-plane conformers with fluorobenzene show that the in-plane complex is the only conformer predicted to exhibit a spectral blue-shift upon electronic excitation. The pattern of computed shifts in fluorobenzene S${_1}$ state intramolecular modes were also found to be unique for each conformer, thereby providing an additional aid to assignment of the experimental spectrum. The computed ground and first excited state geometries and frequencies were used to perform multidimensional Franck-Condon simulations of the S${_1}$-S${_0}$ vibronic spectrum for each of the most stable conformers. The simulations yielded a null spectrum for the $\pi$ complex but showed remarkable agreement with experiment for the in-plane complex, allowing an almost complete vibrational assignment. Molecular electrostatic potential calculations show that, in comparison to benzene, the effect of the fluorine substituent in fluorobenzene is to withdraw electron density from the aromatic $\pi$-system, thereby destabilising the $\pi$-bound conformer with respect to the in-plane conformer.} accepted for publication, 2007.

The role of symmetry and optical selection rules in revealing the molecular structure of the lowest Rydberg and ionic states of the 1,4-diazabicyclo[2.2.2]octane-Arn (n = 1,2,3) van der Waals complexes

The 1,4-diazabicyclo[2.2.2]octane–Arn (n = 1,2,3) van der Waals complexes (DABCO–Arn) have been investigated using a combination of (1 + 1[prime]) resonance enhanced multiphoton ionization (REMPI) and zero electron kinetic energy (ZEKE) spectroscopy. The additivity of the spectral shifts observed in both REMPI and ZEKE spectra, taken together with analysis of vibrational structure, suggest that in both DABCO–Ar and DABCO–Ar2 the argon atoms bind in equivalent equatorial (face) locations between two adjacent (CH2)2 bridges. However, the cumulative evidence from both REMPI and ZEKE spectra, together with ab initio results, suggests that the DABCO–Ar3 complex does not revert to D3h symmetry, but rather adopts a C2v structure in which all three argon atoms bind to one side of the DABCO framework. The exceptionally low wave-number vibrational structure observed in the REMPI spectra suggest that the van der Waals interaction in the excited state is extremely weak. However, ionization necessarily increases the strength of the interaction by virtue of the introduction of charge-induced dipole forces, as revealed by a consistent increase in vibrational wave numbers of the modes observed in the resultant ZEKE spectra