Threshold Photoionization of Fluorenyl, Benzhydryl, Diphenylmethylene, and Their Dimers

Two π-conjugated radicals, fluorenyl (C₁₃H₉) and benzhydryl (C₁₃H₁₁), as well as the carbene diphenylmethylene (C₁₃H₁₀) were studied by imaging photoelectron–photoion coincidence spectroscopy using VUV synchrotron radiation. The reactive intermediates were generated by flash pyrolysis from 9-bromofluorene and α-aminodiphenylmethane (adpm), respectively. Adiabatic ionization energies (IE_ad) for all three species were extracted. Values of 7.01 ± 0.02 eV for fluorenyl and 6.7 ± 0.1 eV for benzhydryl are reported. For the triplet diphenylmethylene, an IE_ad of 6.8 ± 0.1 eV is found. The dissociative photoionization of 9-bromofluorene, the precursor for fluorenyl, was also studied and modeled with an SSACM approach, yielding an appearance energy AE_0K(C₁₃H₉⁺/C₁₃H₉Br) of 9.4 eV. All experimental values are in very good agreement with computations. For fluorenyl, the IE_ad agrees well with earlier values, while for the benzhydryl radical, we report a value that is more than 0.6 eV lower than the one previously reported. The geometry change upon ionization is small for all three species. Although individual vibrational bands cannot be resolved, some vibrational transitions in the threshold photoelectron spectrum of fluorenyl are tentatively assigned based on a Franck–Condon simulation. In addition, the dimerization products of fluorenyl and the benzhydryl radical were detected. Ionization energies of (7.69 ± 0.04) and (8.11 ± 0.04) eV were determined for C₂₆H₁₈ and C₂₆H₂₂, respectively. On the basis of the ionization energies, we identified both molecules to be the direct dimerization products, formed in the pyrolysis without further rearrangement. Both dimers might be expected to play a role in soot formation because the radical monomers do appear in flames

Partitioning Behavior of Silica-Coated Nanoparticles in Aqueous Micellar Two-Phase Systems: Evidence for an Adsorption-Driven Mechanism from QCM‑D and ATR-FTIR Measurements

Quartz crystal microbalance with dissipation (QCM-D), attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR), and total organic carbon detection (TOC) are employed to examine the cause of the differences in the partitioning of silica-coated nanoparticles in an aqueous micellar two-phase system based on nonionic surfactant Eumulgin ES. The particles partition into the micelle-rich phase at pH 3 and into the micelle-poor phase at pH 7. Our results clearly show that the nonionic surfactants are adsorbed to the silica surface at pH 3. Above the critical temperature, a stable surfactant bilayer forms on the silica surface. At pH 7, the surfactants do not adsorb to the particle surface; a surfactant-loaded particle is therefore drawn to the micelle-rich phase but otherwise repelled from it. These results suggest that the partitioning in aqueous micellar two-phase systems is mainly driven by hydrogen bonds formed between the surfactants and the component to be partitioned

The B ¹B₁ State of Cyclopropenylidene, <i>c</i>-C₃H₂

The B ¹B₁ ← X ¹A₁ transition of isolated cyclopropenylidene, <i>c</i>-C₃H₂, has been studied by multiphoton ionization and H-atom photofragment Doppler spectroscopy. The carbene is produced by flash pyrolysis of 1-chlorocycloprop-2-ene. Three bands are observed at 271.0, 266.9, and 264.6 nm. The 271 nm band is assumed to be the origin of the transition, in agreement with TD-DFT computations that yield a vertical excitation energy of 4.74 eV (262 nm). The appearance of H-atom photofragments indicates that <i>c</i>-C₃H + H is an important reaction channel at UV excitation energies

Decomposition of Diazomeldrum’s Acid: A Threshold Photoelectron Spectroscopy Study

Derivatives of meldrum’s acid are known precursors for a number of reactive intermediates. Therefore, we investigate diazomeldrum’s acid (DMA) and its pyrolysis products by photoionization using vacuum ultraviolet (VUV) synchrotron radiation. The threshold photoelectron spectrum of DMA yields an ionization energy (IE) of 9.68 eV. Several channels for dissociative photoionization are observed. The first one is associated with loss of CH₃, leading to a daughter ion with <i>m</i>/<i>z</i> = 155. Its appearance energy AE_0K was determined to be 10.65 eV by fitting the experimental data using statistical theory. A second parallel channel leads to <i>m</i>/<i>z</i> = 69, corresponding to N₂CHCO, with an AE_0K of 10.72 eV. Several other channels open up at higher energy, among them the formation of acetone cation, a channel expected to be the result of a Wolff-rearrangement (WR) in the cation. When diazomeldrum’s acid is heated in a pyrolysis reactor, three thermal decomposition pathways are observed. The major one is well-known and yields acetone, N₂ and CO as consequence of the WR. However, two further channels were identified: The formation of 2-diazoethenone, NNCCO, together with acetone and CO₂ as the second channel and E-formylketene (OCCHCHCO), propyne, N₂ and O₂ as a third one. 2-Diazoethenone and E-formylketene were identified based on their threshold photoelectron spectra and accurate ionization energies could be determined. Ionization energies for several isomers of both molecules were also computed. One of the key findings of this study is that acetone is observed upon decomposition of DMA in the neutral as well as in the ion and both point to a Wolff rearrangement to occur. However, the ion is subject to other decomposition channels favored at lower internal energies

Threshold Photoelectron Spectra of Combustion Relevant C₄H₅ and C₄H₇ Isomers

Threshold photoelectron spectra of combustion relevant C₄H₅ isomers, 2-butyn-1-yl and 1-butyn-3-yl, and C₄H₇ isomers, 1-methylallyl and 2-methylallyl, have been recorded using vacuum ultraviolet synchrotron radiation. Adiabatic ionization energies (IE_ad) have been determined by assigning spectroscopic transitions in mass-selected threshold photoelectron spectra aided by Franck–Condon simulations. The following values were obtained: (7.97 ± 0.02) eV (1-butyn-3-yl), (7.94 ± 0.02) eV (2-butyn-1-yl), (7.48 ± 0.01) eV (1-E-methylallyl), (7.59 ± 0.01) eV (1-Z-methylallyl), and (7.88 ± 0.01) eV (2-methylallyl). Good agreement with CBS-QB3 calculations and simulations could be achieved

Phenylpropargyl Radicals and Their Dimerization Products: An IR/UV Double Resonance Study

Two C₉H₇ isomers, 1-phenylpropargyl and 3-phenylpropargyl, have been studied by IR/UV double resonance spectroscopy in a free jet. The species are possible intermediates in the formation of soot and polycyclic aromatic hydrocarbons (PAH). The radicals are generated by flash pyrolysis from the corresponding bromides and ionized at 255–297 nm in a one-color, two-photon process. Mid-infrared radiation between 500 and 1800 cm^–1 is provided by a free electron laser (FEL). It is shown that the two radicals can be distinguished by their infrared spectra. In addition, we studied the dimerization products originating from the phenylpropargyl self-reaction. We utilize the fact that the pyrolysis tube can be considered to be a flow reactor permitting us to investigate the chemistry in such a thermal reactor. Dimerization of phenylpropargyl produces predominately species with <i>m</i>/<i>z</i> = 228 and 230. A surprisingly high selectivity has been found: The species with <i>m</i>/<i>z</i> = 230 is identified to be <i>para</i>-terphenyl, whereas <i>m</i>/<i>z</i> = 228 can be assigned to 1-phenylethynyl-naphthalene. The results allow to derive a mechanism for the dimerization of phenylpropargyl and suggest hitherto unexplored pathways to the formation of soot and PAH

Pyrolysis of 3‑Methoxypyridine. Detection and Characterization of the Pyrrolyl Radical by Threshold Photoelectron Spectroscopy

Pyrolysis of 3-methoxypyridine in a heated pyrolysis reactor was found to be an efficient way to generate the pyrrolyl radical, <i>c</i>-C₄H₄N, in the gas phase. The threshold photoelectron (TPE) spectrum of this radical was recorded using vacuum ultraviolet synchrotron radiation. The spectrum revealed a singlet ground state at 9.11 ± 0.02 eV (X̃^+ 1A) and an excited triplet state (ã^+ 3A) at 9.43 ± 0.05 eV. Vibrational structure was observed for both cationic states and could be assigned to ring deformation modes. Furthermore, (<i>E</i>)- and (<i>Z</i>)-1-cyanoallyl radicals were found to contribute to the TPE spectrum below 8.9 eV. In addition, we have identified two parallel decomposition channels of the pyrrolyl radical, yielding either hydrogen cyanide and propargyl radical or acetylene and cyanomethyl radical. The reaction energy profiles have also been calculated for these reactions. In addition, the dissociative photoionization of the precursor 3-methoxypyridine is reported

Dynamics of Isolated 1,8-Naphthalimide and <i>N</i>‑Methyl-1,8-naphthalimide: An Experimental and Computational Study

In this work we investigate the excited-state structure and dynamics of the two molecules 1,8-naphthalimide (NI) and <i>N</i>-methyl-1,8-naphthalimide (Me-NI) in the gas phase by picosecond time- and frequency-resolved multiphoton ionization spectroscopy. The energies of several electronically excited singlet and triplet states and the S₁ vibrational wavenumbers were calculated. Nonadiabatic dynamics simulations support the analysis of the radiationless deactivation processes. The origin of the S₁ ← S₀ (ππ*) transition was found at 30 082 cm^–1 for NI and at 29 920 cm^–1 for Me-NI. Furthermore, a couple of low-lying vibrational bands were resolved in the spectra of both molecules. In the time-resolved scans a biexponential decay was apparent for both Me-NI and NI. The fast time constant is in the range of 10–20 ps, whereas the second one is in the nanosecond range. In accordance with the dynamics simulations, intersystem crossing to the fourth triplet state S₁ (ππ*) → T₄ (nπ*) is the main deactivation process for Me-NI due to a large spin–orbit coupling between these states. Only for significant vibrational excitation internal conversion via a conical intersection becomes a relevant deactivation pathway

Electronic Spectroscopy of 1‑(Phenylethynyl)naphthalene

Recently 1-(phenylethynyl)­naphthalene (1-PEN) was suggested to be the primary dimerization product of phenylpropargyl radicals and therefore an important polycyclic hydrocarbon in combustion processes. Here we describe a spectroscopic investigation of a genuine 1-PEN sample by several complementary techniques, infrared spectroscopy, multiphoton ionization (MPI), and threshold photoelectron spectroscopy. The infrared spectrum recorded in a gas cell confirms that 1-PEN is indeed the previously observed dimerization product of phenylpropargyl. The origin of the transition into the electronically excited S₁ state lies at 30823 cm^–1, as found by MPI. Considerable vibrational activity is observed, and a number of low-wavenumber bands are assigned to a progression in the torsional motion. Values of 6 cm^–1 (S₀) and 17 cm^–1 (S₁) were derived for the fundamental of the torsion. In the investigated energy range the excited state lifetimes are in the nanosecond range. Spectra of the 1-PEN/Ar cluster exhibit a red shift of the electronic origin of 22 cm^–1, in good agreement with other aromatic molecules. A threshold photoelectron spectrum recorded using synchrotron radiation yields an ionization energy of 7.58 eV for 1-PEN. An excited electronic state of the cation is found at 7.76 eV, and dissociative photoionization does not set in below 15 eV

Time-Resolved Study of 1,8-Naphthalic Anhydride and 1,4,5,8-Naphthalene-tetracarboxylic Dianhydride

We investigate the excited electronic states of 1,8-naphthalic anhydride (NDCA) and 1,4,5,8-naphthalene-tetracarboxylic dianhydride (NTCDA) by time- and frequency-resolved electronic spectroscopy in the gas phase using picosecond lasers and by femtosecond time-resolved transient absorption in cyclohexane. The experiments are accompanied by calculations that yield the energy of the excited singlet and triplet states as well as by surface hopping dynamics simulations and calculations of spin–orbit couplings that give insight into the photochemistry. The origin of the A ¹A₁ ← X ¹A₁ (ππ*) transition in isolated NDCA was found at 30 260 cm^–1, and several low-lying vibrational bands were observed. The lifetime drops sharply from 1.2 ns at the origin to around 30 ps at an excess energy of 800 cm^–1. Both internal conversion (IC) and intersystem crossing (ISC) are possible deactivation pathways as found in coupled electron–nuclear dynamics simulations. In cyclohexane solution, two time constants were observed. Deactivation of the initially excited state by ISC seems to dominate as supported by computations. For NTCDA we observed a gas phase lifetime of 16 ps upon excitation at 351 nm