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The infrared and ultraviolet spectroscopy of o-, m-, and p-ethynylstyrene isomers (oES, mES, and pES) were studied by a combination of methods, including resonance-enhanced two-photon ionization (R2PI), UV-UV hole-burning spectroscopy (UVHB), resonant ion-dip infrared spectroscopy (RIDIRS), and rotationally resolved fluorescence excitation spectroscopy. In addition, the newly developed method of stimulated emission pumping-population transfer spectroscopy (SEP-PTS) was used to determine the energy threshold to conformational isomerization in m-ethynylstyrene. The S(1) <-- S(0) origin transitions of oES and pES occur at 32 369 and 33 407 cm(-1), respectively. In mES, the cis and trans conformations are calculated to be close in energy. In the R2PI spectrum of mES, the two most prominent peaks (32672 and 32926 cm(-1)) were confirmed by UVHB spectroscopy to be S(1) <-- S(0) origins of these two conformers. The red-shifted conformer was identified as the cis structure by least-squares fitting of the rotationally resolved fluorescence excitation spectrum of the origin band. There are also two possible conformations in oES, but transitions due to only one were observed experimentally, as confirmed by UVHB spectroscopy. Density functional theory calculations (B3LYP/6-31+G) predict that the cis-ortho conformer, in which the substituents point toward each other, is about 8 kJ/mol higher in energy than the trans-ortho isomer, and should only be about 5% of the room temperature population of oES. Ground-state infrared spectra in the C-H stretch region (3000-3300 cm(-1)) of each isomer were obtained with RIDIRS. In all three structural isomers, the acetylenic C-H stretch fundamental was split by Fermi resonance. Infrared spectra were also recorded in the excited electronic state, using a UV-IR-UV version of RIDIR spectroscopy. In all three isomers the acetylenic C-H stretch fundamental was unshifted from the ground state, but no Fermi resonance was seen. The first observed and last unobserved transitions in the SEP-PT spectrum were used to place lower and upper bounds on the barrier to cis --> trans isomerization in m-ethynylstyrene of 990-1070 cm(-1). Arguments are given for the lack of a kinetic shift in the measurement. The analogous trans --> cis barrier is in the same range (989-1065 cm(-1)), indicating that the relative energies of the zero-point levels of the two isomers are (E(ZPL)(cis) - E(ZPL)(trans))= -75 to +81 cm(-1). Both the barrier heights and relative energies of the minima are close to those determined by DFT (Becke3LYP/6-31+G) calculations.

Near-pure samples of (E)-phenylvinylacetylene ((E)-PVA) and (Z)-phenylvinylacetylene ((Z)-PVA) were synthesized, and their ultraviolet spectroscopy was studied under jet-cooled conditions. The fluorescence excitation and UV-UV holeburning (UVHB) spectra of both isomers were recorded. The S0-S1 origin of (E)-PVA occurs at 33,578 cm(-1), whereas that for (Z)-PVA occurs at 33,838 cm(-1), 260 cm(-1) above that for (E)-PVA. The present study focuses primary attention on the vibronic spectroscopy of (E)-PVA. Single vibronic level fluorescence spectra of many prominent bands in the first 1200 cm(-1) of the S0-S1 excitation spectrum of (E)-PVA were recorded, including several hot bands involving low-frequency out-of-plane vibrations. Much of the ground-state vibronic structure observed in these spectra was assigned by comparison with styrene and trans-beta-methylstyrene, assisted by calculations at the DFT B3LYP/6-311++G(d,p) level of theory. Both S0 and S1 states of (E)-PVA are shown to be planar, with intensity appearing only in even overtones of out-of-plane vibrations. Due to its longer conjugated side chain compared with that of its parent styrene, (E)-PVA supports extensive Duschinsky mixing among the four lowest-frequency out-of-plane modes (nu45-nu48), increasing the complexity of this mixing relative to that of styrene. Identification of the v'' = 0-3 levels of nu48, the lowest frequency torsion, provided a means of determining the 1D torsional potential for hindered rotation about the C(ph)-C(vinyl) bond. Vibronic transitions due to (Z)-PVA were first identified as small vibronic bands that did not appear in the UVHB spectrum recorded with the hole-burn laser fixed on the S0-S1 origin of (E)-PVA. The LIF and UVHB spectra of a synthesized sample of (Z)-PVA confirmed this assignment.

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