14 research outputs found
Threshold Photoionization of Fluorenyl, Benzhydryl, Diphenylmethylene, and Their Dimers
Two Ï-conjugated radicals,
fluorenyl (C<sub>13</sub>H<sub>9</sub>) and benzhydryl (C<sub>13</sub>H<sub>11</sub>), as well as
the carbene diphenylmethylene (C<sub>13</sub>H<sub>10</sub>) were
studied by imaging photoelectronâphotoion 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<sub>ad</sub>) 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<sub>ad</sub> 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<sub>0K</sub>(C<sub>13</sub>H<sub>9</sub><sup>+</sup>/C<sub>13</sub>H<sub>9</sub>Br)
of 9.4 eV. All experimental values are in very good agreement with
computations. For fluorenyl, the IE<sub>ad</sub> 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 FranckâCondon 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 C<sub>26</sub>H<sub>18</sub> and C<sub>26</sub>H<sub>22</sub>, 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 QCMâD 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 <sup>1</sup>B<sub>1</sub> State of Cyclopropenylidene, <i>c</i>-C<sub>3</sub>H<sub>2</sub>
The B <sup>1</sup>B<sub>1</sub> â X <sup>1</sup>A<sub>1</sub> transition of isolated cyclopropenylidene, <i>c</i>-C<sub>3</sub>H<sub>2</sub>, 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>-C<sub>3</sub>H + H is an important reaction channel at UV excitation energies
Decomposition of Diazomeldrumâs Acid: A Threshold Photoelectron Spectroscopy Study
Derivatives of meldrumâs acid
are known precursors for a
number of reactive intermediates. Therefore, we investigate diazomeldrumâs
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 CH<sub>3</sub>, leading to a daughter
ion with <i>m</i>/<i>z</i> = 155. Its appearance
energy AE<sub>0K</sub> 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 N<sub>2</sub>CHCO, with an AE<sub>0K</sub> 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 diazomeldrumâs acid is heated in a
pyrolysis reactor, three thermal decomposition pathways are observed.
The major one is well-known and yields acetone, N<sub>2</sub> and
CO as consequence of the WR. However, two further channels were identified:
The formation of 2-diazoethenone, NNCCO, together with acetone and
CO<sub>2</sub> as the second channel and E-formylketene (OCCHCHCO),
propyne, N<sub>2</sub> and O<sub>2</sub> 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 C<sub>4</sub>H<sub>5</sub> and C<sub>4</sub>H<sub>7</sub> Isomers
Threshold photoelectron spectra of
combustion relevant C<sub>4</sub>H<sub>5</sub> isomers, 2-butyn-1-yl
and 1-butyn-3-yl, and C<sub>4</sub>H<sub>7</sub> isomers, 1-methylallyl
and 2-methylallyl, have been
recorded using vacuum ultraviolet synchrotron radiation. Adiabatic
ionization energies (IE<sub>ad</sub>) have been determined by assigning
spectroscopic transitions in mass-selected threshold photoelectron
spectra aided by FranckâCondon 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 C<sub>9</sub>H<sub>7</sub> 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
255â297 nm in a one-color, two-photon process. Mid-infrared
radiation between 500 and 1800 cm<sup>â1</sup> 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 3âMethoxypyridine. 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>-C<sub>4</sub>H<sub>4</sub>N, 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 (XÌ<sup>+ 1</sup>A)
and an excited triplet state (aÌ<sup>+Â 3</sup>A) 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>âMethyl-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 S<sub>1</sub> vibrational wavenumbers were calculated.
Nonadiabatic dynamics simulations support the analysis of the radiationless
deactivation processes. The origin of the S<sub>1</sub> â S<sub>0</sub> (ÏÏ*) transition was found at 30âŻ082 cm<sup>â1</sup> for NI and at 29âŻ920 cm<sup>â1</sup> 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 10â20 ps, whereas the
second one is in the nanosecond range. In accordance with the dynamics
simulations, intersystem crossing to the fourth triplet state S<sub>1</sub> (ÏÏ*) â T<sub>4</sub> (nÏ*) is the
main deactivation process for Me-NI due to a large spinâorbit
coupling between these states. Only for significant vibrational excitation
internal conversion via a conical intersection becomes a relevant
deactivation pathway
Electronic Spectroscopy of 1â(Phenylethynyl)naphthalene
Recently 1-(phenylethynyl)Ânaphthalene
(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 S<sub>1</sub> state lies at 30823 cm<sup>â1</sup>, 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<sup>â1</sup> (S<sub>0</sub>) and 17 cm<sup>â1</sup> (S<sub>1</sub>) 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<sup>â1</sup>, 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 spinâorbit couplings that give insight into the photochemistry.
The origin of the A <sup>1</sup>A<sub>1</sub> â X <sup>1</sup>A<sub>1</sub> (ÏÏ*) transition in isolated NDCA was found
at 30 260 cm<sup>â1</sup>, 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<sup>â1</sup>. Both internal conversion (IC) and intersystem crossing (ISC) are
possible deactivation pathways as found in coupled electronânuclear
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