High resolution electronic spectroscopy of 9-fluorenemethanol in the gas phase: New insights into the properties of Π-hydrogen bonds

Rotationally resolved S(1)←S(0) fluorescence excitation spectra of 9-fluorenemethanol (9FM) and deuterated 9-fluorenemethanol (9FMD) have been observed and assigned. Two conformers were detected; sym-9FM and unsym-9FM. The sym conformer has the -OH group symmetrically placed above the fluorene short axis, with its hydrogen atom pointing towards the top of an aromatic ring, whereas the unsym conformer has the -OH group tilted away from this axis, with its hydrogen atom pointing towards the side of an aromatic ring. Only the sym conformer shows a tunneling splitting associated with the torsional motion of the -OH group; the unsym conformer is "rigid." Additionally, a third subband was observed in the spectrum of sym-9FMD, evidencing secondary minima on the potential energy surfaces of the ground and excited electronic states. Studies of these surfaces along the -OH torsional coordinate provide new insights into the properties of π-hydrogen bonds.