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.

Isomer specific spectroscopy of C10Hn, n = 8–12: Exploring pathways to naphthalene in Titan's atmosphere

“…The ''head-to-head'' recombination of benzyl and propargyl radicals would produce 4-phenyl-1-butyne, a C 10 H 10 isomer with several conformational isomers that has been studied recently by conformation-specific methods (Scheme 3). 10 The analogous ''head-to-head'' recombination of benzyl and allyl radicals will produce 4-phenyl-1-butene (C 10 H 12 ), whose single-conformation spectroscopy is reported here (Scheme 4).…”

“…One of the interesting aspects of the spectroscopy of 4-phenyl-1-butene is that the alkenyl tail is sufficiently long that it can form spectroscopically distinguishable conformational isomers, much as occurs in 4-phenyl-1-butyne. 10 Thus we are faced with the spectroscopic challenge of observing and assigning the single-conformation ultraviolet spectra of the low-lying conformational isomers (often referred to via the contraction 'conformers') of 4PB. In addition to providing the spectroscopic signatures of these conformational isomers, they highlight the fact that in molecules this size, conformational isomerization may need to precede structural isomerization or intermolecular reaction.…”

Fermi Resonance Effects in the Vibrational Spectroscopy of Methyl and Methoxy Groups

“…The experimental spectra reported herein are obtained via jet cooled, fluorescence dip infrared (FDIR) spectroscopy . The ability of this technique to isolate the contributions of individual conformers ,,− to the overall vibrational spectrum makes it a particularly valuable tool for developing theoretical models for exploring the complexities of the alkyl CH stretch spectral region.…”

Conformation-Specific Spectroscopy of 3-Benzyl-1,5-hexadiyne

“…BHD is a branched chain alkynylbenzene in which the two branches fuse together butyne and pentyne straight chains (Figure ). The conformational spectroscopy of the two corresponding phenylalkynes, 5-phenyl-1-pentyne and 4-phenyl-1-butyne, was the subject of the preceding paper . By comparison, BHD is a natural next step toward more conformationally complex phenylalkynes.…”