Resonant two-photon ionization and ab initio conformational analysis of haloethyl benzenes (PhCH2CH2X,X=Cl,F)

• Published in: The Journal of chemical physics 2007-10, Vol.127 (13)
• Main Authors: Martin, Danielle E., Robertson, Evan G., Morrison, Richard J. S., Dobney, Bruce
• Format: Article
• Language: English
• ISSN: 0021-9606; 1089-7690
• DOI: 10.1063/1.2772612

The S1←S0 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 37673cm−1 is redshifted 50cm−1 relative to the corresponding anti origin, while CEB origin bands overlap at 37646cm−1. Relative conformational stability and populations in the jet have been estimated for both molecules, based on the intensity ratio of S1←S0 band origin transitions. These are compared with a range of related molecules with the structural motif PhCH2CH2X (X=CH3,CH2CH3,NH2,OH,COOH,CCH,CN). Theory and experimental results for FEB and CEB show repulsive interactions between the halogen substituents and the π 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 a∕c hybrid in keeping with theoretically computed TM “rotations” (θelec) of −7° and −56°, respectively. This difference is attributed largely to rotation of the side chain in opposite directions about the C1Cα bond. Spectra of FEB(H2O) and CEB(H2O) single water clusters show evidence of an anti conformation in the host molecule.