The conformational landscape of 2-(4-fluoro-phenyl)-ethylamine: Consequences of fluorine substitution at the para position

  Afik Shachar  ,  Nitzan Mayorkas  ,  Hanan Sachs  ,  Ilana Bar  
Ben-Gurion University of the Negev

Fluorination is considered as a possible means for alteration of conformational landscapes in molecules. The effect of fluorine substitution was studied here by measuring the vibronic and vibrational spectra of gas phase 2-(4-fluoro-phenyl)-ethylamine (4-FPEA) by resonant two-photon ionization (R2PI) and by ionization-loss stimulated Raman spectroscopy (ILSRS). The measurement of survey ILSR spectra of 4-FPEA in the amino group region allowed to associate the bands in the R2PI spectrum to origin and vibronic transitions of the ground (S0) to the excited (S1) electronic states of three different conformers. The presence of these conformers of 4-FPEA in the molecular beam was confirmed by measurement of distinctive ILSRS spectral signatures in the 400 - 1700 and 2750 - 3500 cm-1 broad spectral range. The interpretation of their structures was assisted by the results of quantum chemical calculations, including a torsional potential energy surface, energies of the fully optimized geometries in the S0 state and harmonic Raman spectra as well as natural bond orbital and atoms in molecule analyses. Comparison of the spectra with the results of scaled harmonic Raman spectra revealed two gauche configurations with folded ethylamino side chains and amino groups aiming to different directions together with a symmetric anti structure with an outward extended chain. The site-specific para-fluorination, at a distant position from the side chain modified the energetic ordering of the conformers, relative to the structure with the fluorine atom at the ortho position, but kept it close to that of PEA.