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

3-Benzyl-1,5-hexadiyne (BHD) was studied by a combination of methods, including resonance-enhanced-two-photon ionization, UV−UV hole-burning spectroscopy, resonant ion-dip infrared spectroscopy, and rotational band contour analysis. There are five conformations of BHD observed in the expansion with their S₁ ← S₀ origins occurring at 37520, 37565, 37599, 37605, and 37631 cm^-1. DFT calculations predict six low energy conformations. Conformational assignments have been made by comparison of the experimental infrared spectra in the alkyl and acetylenic CH stretch region to DFT vibrational frequency and infrared intensity calculations. Rotational band contours provided further confirmation of these assignments. The electronic origin shifts of BHD compare favorably to the electronic origin shifts of 5-phenyl-1-pentyne with the exception of one conformation. This conformation is unique in that it is the only structure with both acetylenic groups in the gauche position over the ring. This gauche−gauche conformation exhibits a perpendicular (b-type) transition and produces extensive vibronic coupling reminiscent of symmetric monosubstituted benzenes.