Spectroscopy and photophysics of structural isomers of naphthalene: Z-phenylvinylacetylene.

The fluorescence spectroscopy of Z-phenylvinylacetylene (Z-PVA) has been studied under jet-cooled conditions. The laser-induced fluorescence (LIF) spectrum shows vibronic activity up to 600 cm(-1) above the pi pi* electronic origin at 33 838 cm(-1). In contrast, the single vibronic level fluorescence spectrum of the electronic origin shows strong intensity in transitions ending in ground state levels at least 1200 cm(-1) above the ground state zero-point level. The double-resonance technique of ultraviolet depletion (UVD) spectroscopy was used to show that there are strong absorptions in Z-PVA that are not observed in the LIF spectrum due to the turn of a nonradiative process in this electronic state. The LIF and UVD spectra were compared quantitatively to calculate the relative single vibronic level fluorescence quantum yields. Upon inspection, there are some indications of state specific effects; however, the nature of these effects is unclear. Ab initio and density functional theory calculations of the ground and excited states were used to map the first two excited states of Z-PVA along the C[triple bond]CH bending coordinate, determining them to be pi pi* and pi sigma*, respectively, in character. The crossing of these two states is postulated to be the underlying reason for the observed loss in fluorescence intensity 600 cm(-1) above the pi pi* origin. The spectroscopy of Z-PVA has been compared to the previously characterized E isomer of phenylvinylacetylene [Liu, C. P., Newby, J. J., Muller, C. W., Lee, H. D. and Zwier, T. S. J. Phys. Chem. A 2008, 112 (39), 9454.].

Spectroscopic characterization of structural isomers of naphthalene: (E)- and (Z)-phenylvinylacetylene.

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.

Isomer-specific spectroscopy and conformational isomerization energetics of o-, m-, and p-ethynylstyrenes.

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.

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 1<-- 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.

Discovery of 3-methyl-2-buten-1-yl acetate, a new alarm component in the sting apparatus of Africanized honeybees.

We analyzed the alarm pheromone components from five colonies of Africanized honeybees and three colonies of European honeybees collected in Mexico. Analyses revealed a novel alarm pheromone component that was only present in appreciable quantities in the Africanized bee samples. Analysis of the mass spectrum and subsequent synthesis confirmed that this compound is 3-methyl-2-buten-1-yl acetate (3M2BA), an unsaturated derivative of IPA. In Africanized honeybees, sampling from stings of guards showed that 3M2BA was present at levels of 0-38% the amount of isoamyl acetate (IPA). Behavioral assays from three colonies each of Africanized and European bees showed that 3M2BA recruited worker bees from hives of both Africanized bees and European bees at least as efficiently as isopentyl acetate IPA, a compound widely reported to have the highest activity for releasing alarm and stinging behavior in honeybees. However, a mixture of of 3M2BA and IPA (1:2) recruited bees more efficiently than either of the compounds alone. None of the compounds differed in their efficacy for inducing bees to pursue the observers.