Conformation-Specific Spectroscopy of 4-Phenyl-1-butyne and 5-Phenyl-1-pentyne

Abstract

4-Phenyl-1-butyne and 5-phenyl-1-pentyne were studied by a combination of methods including resonance-enhanced-two-photon ionization, UV-UV hole-burning spectroscopy, and rotational band contour studies. There are two conformations of 4-phenyl-1-butyne observed in the expansion with their S1<-- S0 origins occurring at 37617 and 37620 cm(-1). MP2 and DFT calculations identify these two low energy conformations (with the acetylenic group anti or gauche with respect to the ring) and confirm that these are the only two low energy conformations anticipated to have population in them. The experimental rotational band contours of the origin bands were compared to simulations based on transition moment directions and rotational constants predicted by CIS calculations. This comparison leads to definitive assignments for the bands, with the gauche and anti conformations assigned to the red and blue-shifted conformers, respectively. Three conformations of 5-phenyl-1-pentyne were observed in the expansion with their S1<-- S0 origins occurring at 37538, 37578, and 37601 cm(-1). MP2 and DFT calculations predict four low energy structures arising from gauche or anti conformations about each of the Calpha-Cbeta and Cbeta-Cgamma bonds. Rotational band contour analysis was used to assign the above transitions to gauche-anti (ga), gauche-gauche (gg), and anti-gauche (ag) structures, respectively.