Resonant two-photon ionization and ab initio conformational analysis of haloethyl benzenes (PhCH(2)CH(2)X,X=Cl,F)

The S(1) <-- S(0) transitions of the gaseous (2-fluoroethyl)-benzene (FEB) and (2-chloroethyl)-benzene (CEB) have been investigated using a combination of two-color resonant two-photon ionization and UV-UV hole burning spectroscopy. Both anti and gauche conformers have been identified on the basis of rotational band contour analysis supported by ab initio calculations on the ground and electronically excited states. The gauche origin band of FEB at 37,673 cm(-1) is redshifted 50 cm(-1) relative to the corresponding anti origin, while CEB origin bands overlap at 37,646 cm(-1). Relative conformational stability and populations in the jet have been estimated for both molecules, based on the intensity ratio of S(1) <-- S(0) band origin transitions. These are compared with a range of related molecules with the structural motif PhCH(2)CH(2)X (X=CH(3),CH(2)CH(3),NH(2),OH,COOH,CCH,CN). Theory and experimental results for FEB and CEB show repulsive interactions between the halogen substituents and the pi cloud of the phenyl rings destabilizing the gauche conformers, but the preference for the anti conformers is relatively modest. The gauche conformer origins show very different hybrid character: FEB is largely b type, while CEB is an ac hybrid in keeping with theoretically computed TM "rotations" (theta(elec)) of -7 degrees and -56 degrees , respectively. This difference is attributed largely to rotation of the side chain in opposite directions about the C(1)C(alpha) bond. Spectra of FEB(H(2)O) and CEB(H(2)O) single water clusters show evidence of an anti conformation in the host molecule.