Fe + (acetylene) n ion−molecule complexes are produced in a supersonic molecular beam with pulsed laser vaporization. These ions are mass selected and studied with infrared photodissociation spectroscopy in the C−H stretching region, complemented by computational chemistry calculations. All C−H stretch vibrations are shifted to frequencies lower than the vibrations of isolated acetylene because of the charge transfer that occurs between the metal ion and the molecules. Complexes in the size range of n = 1−4 are found to have structures with individual acetylene molecules bound to the core metal ion via cation−π interactions. The coordination is completed with four ligands in a structure close to a distorted tetrahedron. Larger complexes in the range of n = 5−8 have external acetylene molecules solvating this n = 4 core ion via CH−π bonding to inner-shell ligands. DFT computations predict that quartet spin states are more stable for all complex sizes, but infrared spectra for quartet and doublet spin states are quite similar, precluding definitive determination of the spin states. There is no evidence for any of these complexes having acetylenes coupled into reacted structures. This is consistent with computed thermochemistry, which finds significant activation barriers to such reactions.

Laser photochemistry of pressed-pellet samples of polycyclic aromatic hydrocarbons (PAHs) produces covalently bonded dimers and some higher polymers. This chemistry was discovered initially via laser desorption time-of-flight mass spectrometry experiments, which produced masses (m/z) of 2M-2 and 2M-4 (where M is the monomer parent mass). Dimers are believed to be formed from photochemical dehydrogenation and radical polymerization chemistry in the desorption plume. Replication of these ablation conditions at higher throughput allowed PAH dimers of pyrene, perylene, and coronene to be produced and collected in milligram quantities. Differential sublimation provided purification of the dimers and elimination of residual monomers. The purified dimers were investigated with UV–visible, IR, and Raman spectroscopy, complemented by computational studies using density functional theory at the CAM-B3LYP/def2-TZV level. Calculations and predicted spectra were calibrated by comparison with the corresponding monomers and used to determine the lowest energy dimer structures. Infrared and Raman spectroscopy provided few distinctive signatures, but UV–visible spectra detected new transitions for each dimer. The comparison of simulated and experimental spectra allows determination of the most prevalent structures for the PAH dimers. The work presented here provides interesting insights into the spectroscopy of extended aromatic systems and a new strategy for the photochemical synthesis of large PAH dimers.

Laser desorption time-of-flight mass spectrometry (LD-ToF-MS) experiments on pressed-pellet samples of polycyclic aromatic hydrocarbons (PAHs) exhibit the formation of covalently-bonded dimers at masses (m/z) of 2M-2 and 2M-4 (where M is the parent mass). Through replication of these LD-ToF-MS conditions at higher throughput, PAH dimers have been produced and collected in milligram quantities. For collected samples of pyrene, perylene, and coronene, differential sublimation has isolated the dimer sample from residual monomers. After confirmation of the purification using LD-ToF-MS, the samples are analyzed using a variety of spectroscopic methods, including UV-Vis, IR, and Raman spectroscopy. Theoretical calculations of dimer samples have been done using density functional theory with the CAM-B3LYP method at the def2TZV level of theory. Theory calculations were calibrated and checked by comparison with the monomer samples, and used to determine the lowest energy dimer structures. The simulated spectra were compared with collected spectra of isolated dimers to determine the actual structures of the dimerized PAHs.

Laser desorption time-of-flight mass spectrometry (LD-ToF-MS) experiments on pressed-pellet samples of polycyclic aromatic hydrocarbons (PAHs) exhibit the formation of covalently-bonded dimers at masses (m/z) of 2M-2 and 2M-4 (where M is the parent mass). Through replication of these LD-ToF-MS conditions at higher throughput, PAH dimers have been produced and collected in milligram quantities. The formation of the covalently-bonded dimers was confirmed under similar LD-ToF-MS conditions and through HPLC separation with a UV-Vis detector. For collected samples of pyrene, differential sublimation has removed the residual monomer to leave a mixture of dimerized pyrene. Decrease of the monomer mass peak was confirmed using LD-ToF-MS, and the samples are analyzed using several spectroscopic methods, including UV-Vis, IR, and Raman spectroscopy. Theoretical studies of possible dimer structures have been calculated using density functional theory with the CAM-B3LYP method at the def2TZV level of theory using the Gaussian 09 program. Theory calculations were calibrated and checked by comparison with the monomer samples, and used to determine the lowest energy dimer structures. The simulated spectra were compared with collected spectra of isolated dimers to determine the contributing structures of the dimerized PAHs.