Excited‐State Dipole Moments and Transition Dipole Orientations of Rotamers of 1,2‐, 1,3‐, and 1,4‐Dimethoxybenzene

Abstract

Rotationally resolved electronic Stark spectra of rotamers of 1,2-, 1,3-, and 1,4-dimethoxybenzene have been recorded and analyzed using evolutionary strategies. The experimentally determined dipole moments as well as the transition dipole moments are compared to the results of ab initio calculations. For the electronic ground states of the experimentally observed dimethoxybenzenes, the permanent dipole moments can be obtained from vectorial addition of the monomethoxybenzene dipole moment. However, this is not the case for the electronically excited states. This behavior can be traced back to a state mixing of the lowest electronically excited singlet states for the asymmetric rotamers. For the symmetric rotamers however, this is not valid. We discuss several possible reasons for the non-additivity of the dipole moments in the excited states of the symmetric rotamers.

Conformational landscape and inertial defect of methoxyphenol isomers studied by mm-wave spectroscopy and quantum chemistry calculations

“…The main contribution to the non-planarity and to the negative ∆I of MP comes from the out of plane hydrogen atoms in the methoxy [36][37][38] and anisole. 39 The ∆I values for MP/cresol and methylanisole/dimethoxybenzene [24][25][26]40 indicate a stronger contribution on the negative inertial defects for the methoxy group compared to the methyl group, probably explained by the contribution of the OCH 3 out of plane contribution (ν l = ν 45 for MP) leading to a slight negative zero point inertial defect. 44 Now, when we focus our attention on the isomeric dependence on the inertial defects, the most negative value is always obtained for the para isomer and the ∆I value of the ortho isomer appears to be slightly lower than that of the meta isomer.…”