A Combined Experimental and Computational Study on the Ionization Energies of the Cyclic and Linear C3H Isomers

Gas-phase synthesis of corannulene – a molecular building block of fullerenes

Corannulene can be formed through molecular mass growth processes in circumstellar envelopes.

Reactivity of the Indenyl Radical (C9 H7 ) with Acetylene (C2 H2 ) and Vinylacetylene (C4 H4 )

The reactions of the indenyl radicals with acetylene (C₂ H₂ ) and vinylacetylene (C₄ H₄ ) is studied in a hot chemical reactor coupled to synchrotron based vacuum ultraviolet ionization mass spectrometry. These experimental results are combined with theory to reveal that the resonantly stabilized and thermodynamically most stable 1-indenyl radical (C₉ H₇ ^. ) is always formed in the pyrolysis of 1-, 2-, 6-, and 7-bromoindenes at 1500 K. The 1-indenyl radical reacts with acetylene yielding 1-ethynylindene plus atomic hydrogen, rather than adding a second acetylene molecule and leading to ring closure and formation of fluorene as observed in other reaction mechanisms such as the hydrogen abstraction acetylene addition or hydrogen abstraction vinylacetylene addition pathways. While this reaction mechanism is analogous to the bimolecular reaction between the phenyl radical (C₆ H₅ ^. ) and acetylene forming phenylacetylene (C₆ H₅ CCH), the 1-indenyl+acetylene→1-ethynylindene+hydrogen reaction is highly endoergic (114 kJ mol^-1 ) and slow, contrary to the exoergic (-38 kJ mol^-1 ) and faster phenyl+acetylene→phenylacetylene+hydrogen reaction. In a similar manner, no ring closure leading to fluorene formation was observed in the reaction of 1-indenyl radical with vinylacetylene. These experimental results are explained through rate constant calculations based on theoretically derived potential energy surfaces.

A vacuum ultraviolet photoionization study on the formation of methanimine (CH2NH) and ethylenediamine (NH2CH2CH2NH2) in low temperature interstellar model ices exposed to ionizing radiation

Methylamine (CH3NH2) and methanimine (CH2NH) represent essential building blocks in the formation of amino acids in interstellar and cometary ices. In our study, by exploiting isomer selective detection of the reaction products via photoionization coupled with reflectron time of flight mass spectrometry (Re-TOF-MS), we elucidate the formation of methanimine and ethylenediamine (NH2CH2CH2NH2) in methylamine ices exposed to energetic electrons as a proxy for secondary electrons generated by energetic cosmic rays penetrating interstellar and cometary ices. Interestingly, the two products methanimine and ethylenediamine are isoelectronic to formaldehyde (H2CO) and ethylene glycol (HOCH2CH2OH), respectively. Their formation has been confirmed in interstellar ice analogs consisting of methanol (CH3OH) which is ioselectronic to methylamine. Both oxygen-bearing species formed in methanol have been detected in the interstellar medium (ISM), while for methanimine and ethylenediamine only methanimine has been identified so far. In comparison with the methanol ice products and our experimental findings, we predict that ethylenediamine should be detectable in these astronomical sources, where methylamine and methanimine are present.

Computational Study on the Unimolecular Decomposition of JP-8 Jet Fuel Surrogates III: Butylbenzene Isomers ( n-, s-, and t-C14H10)

Ab initio G3(CCSD,MP2)//B3LYP/6-311G(d,p) calculations of potential energy surfaces have been carried out to unravel the mechanism of the initial stages of pyrolysis of three C₁₀H₁₄ isomers: n-, s-, and t-butylbenzenes. The computed energy and molecular parameters have been utilized in RRKM-master equation calculations to predict temperature- and pressure-dependent rate constants and product branching ratios for the primary unimolecular decomposition of these molecules and for the secondary decomposition of their radical fragments. The results showed that the primary dissociation of n-butylbenzene produces mostly benzyl (C₇H₇) + propyl (C₃H₇) and 1-phenyl-2-ethyl (C₆H₅C₂H₄) + ethyl (C₂H₅), with their relative yields strongly dependent on temperature and pressure, together with a minor amount of 1-phenyl-prop-3-yl (C₉H₁₁) + methyl (CH₃). Secondary decomposition reactions that are anticipated to occur on a nanosecond scale under typical combustion conditions split propyl (C₃H₇) into ethylene (C₂H₄) + methyl (CH₃), ethyl (C₂H₅) into ethylene (C₂H₄) + hydrogen (H), 1-phenyl-2-ethyl (C₆H₅C₂H₄) into mostly styrene (C₈H₈) + hydrogen (H) and to a lesser extent phenyl (C₆H₅) + ethylene (C₂H₄), and 1-phenyl-prop-3-yl (C₉H₁₁) into predominantly benzyl (C₇H₇) + ethylene (C₂H₄). The primary decomposition of s-butylbenzene is predicted to produce 1-phenyl-1-ethyl (C₆H₅CHCH₃) + ethyl (C₂H₅) and a minor amount of 1-phenyl-prop-1-yl (C₉H₁₁) + methyl (CH₃), and then 1-phenyl-1-ethyl (C₆H₅CHCH₃) and 1-phenyl-prop-1-yl (C₉H₁₁) rapidly dissociate to styrene (C₈H₈) + hydrogen (H) and styrene (C₈H₈) + methyl (CH₃), respectively. t-Butylbenzene decomposes nearly exclusively to 2-phenyl-prop-2-yl (C₉H₁₁) + methyl (CH₃), and further, 2-phenyl-prop-2-yl (C₉H₁₁) rapidly eliminates a hydrogen atom to form 2-phenylpropene (C₉H₁₀). If hydrogen atoms or other reactive radicals are available to make a direct hydrogen-atom abstraction from butylbenzenes possible, the C₁₀H₁₃ radicals (1-phenyl-but-1-yl, 2-phenyl-but-2-yl, and t-phenyl-isobutyl) can be formed as the primary products from n-, s-, and t-butylbenzene, respectively. The secondary decomposition of 1-phenyl-but-1-yl leads to styrene (C₈H₈) + ethyl (C₂H₅), whereas 2-phenyl-but-2-yl and t-phenyl-isobutyl dissociate to 2-phenylpropene (C₉H₁₀) + methyl (CH₃). Thus, the three butylbenzene isomers produce distinct but overlapping nascent pyrolysis fragments, which likely affect the successive oxidation mechanism and combustion kinetics of these JP-8 fuel components. Temperature- and pressure-dependent rate constants generated for the initial stages of pyrolysis of butylbenzenes are recommended for kinetic modeling.

Valence shell threshold photoelectron spectroscopy of C3Hx (x = 0–3)

We present the photoelectron spectra of C₃H_x (x = 0–3) formed in a microwave discharge flow-tube reactor by consecutive H abstractions from C₃H₄ (C₃H_x + F → C₃H_x−1 + HF (x = 1–4)), but also from F + CH₄ schemes by secondary reactions.

Synthesis of the Smallest Member of the Silylketene Family: H3 SiC(H)=C=O

Exploiting photoionization reflectron time-of-flight mass spectrometry (PI-ReTOF-MS) combined with electronic structure calculations, it is shown that the hitherto elusive silylketene molecule (H₃ SiC(H)=C=O)-the isovalent counterpart of the well-known methylketene molecule-is forming via interaction of energetic electrons with low-temperature silane-carbon monoxide ices. In combination with the infrared spectroscopically detected triplet dicarbon monoxide reactant, electronic structure calculations suggest that dicarbon monoxide reacts with silane via a de facto insertion of the terminal carbon atom into a silicon-hydrogen single bond. This is followed by non-adiabatic reaction dynamics triggered by the heavy silicon atom intersystem crossing from the triplet to the singlet manifold, eventually leading to the formation of silylketene. The non-equilibrium nature of the elementary reactions within the exposed ices results in an exciting and novel chemistry which cannot be explored via traditional preparative chemistry. Since the replacement of hydrogen in silane can introduce side groups such as silyl or alkyl, the reaction of triplet dicarbon monoxide with silane represents the parent system for a previously disregarded reaction class revealing an elegant path to access the largely reactive group of silylketenes.

A vacuum ultraviolet photoionization study on high-temperature decomposition of JP-10 (exo-tetrahydrodicyclopentadiene)

High-temperature pyrolysis of JP-10 in flow reactors were performed both experimentally and theoretically. Dozens of products were detected and the decomposition pathways of JP-10 were discussed.

Vacuum ultraviolet photoionization mass spectrometric study of cyclohexene

In this work, photoionization and dissociation of cyclohexene have been studied by means of coupling a reflectron time-of-flight mass spectrometer with the tunable vacuum ultraviolet (VUV) synchrotron radiation. The adiabatic ionization energy of cyclohexene as well as the appearance energies of its fragment ions C6 H9 (+) , C6 H7 (+) , C5 H7 (+) , C5 H5 (+) , C4 H6 (+) , C4 H5 (+) , C3 H5 (+) and C3 H3 (+) were derived from the onset of the photoionization efficiency (PIE) curves. The optimized structures for the transition states and intermediates on the ground state potential energy surfaces related to photodissociation of cyclohexene were characterized at the ωB97X-D/6-31+g(d,p) level. The coupled cluster method, CCSD(T)/cc-pVTZ, was employed to calculate the corresponding energies with the zero-point energy corrections by the ωB97X-D/6-31+g(d,p) approach. Combining experimental and theoretical results, possible formation pathways of the fragment ions were proposed and discussed in detail. The retro-Cope rearrangement was found to play a crucial role in the formation of C4 H6 (+) , C4 H5 (+) and C3 H5 (+) . Intramolecular hydrogen migrations were observed as dominant processes in most of the fragmentation pathways of cyclohexene. The present research provides a clear picture of the photoionization and dissociation processes of cyclohexene in the 8- to 15.5-eV photon energy region.

On the absolute photoionization cross section and dissociative photoionization of cyclopropenylidene

We report the determination of the absolute photoionization cross section of cyclopropenylidene, c-C₃H₂, and the heat of formation of the C₃H radical and ion derived by the dissociative ionization of the carbene.

Chemical dynamics, molecular energetics, and kinetics at the synchrotron

Scientists at the Chemical Dynamics Beamline of the Advanced Light Source in Berkeley are continuously reinventing synchrotron investigations of physical chemistry and chemical physics with vacuum ultraviolet light. One of the unique aspects of a synchrotron for chemical physics research is the widely tunable vacuum ultraviolet light that permits threshold ionization of large molecules with minimal fragmentation. This provides novel opportunities to assess molecular energetics and reaction mechanisms, even beyond simple gas phase molecules. In this perspective, significant new directions utilizing the capabilities at the Chemical Dynamics Beamline are presented, along with an outlook for future synchrotron and free electron laser science in chemical dynamics. Among the established and emerging fields of investigations are cluster and biological molecule spectroscopy and structure, combustion flame chemistry mechanisms, radical kinetics and product isomer dynamics, aerosol heterogeneous chemistry, planetary and interstellar chemistry, and secondary neutral ion-beam desorption imaging of biological matter and materials chemistry.

Chemistry of energetically activated cumulenes - from allene (H2CCCH2) to hexapentaene (H2CCCCCCH2)

During the last decade, experimental and theoretical studies on the unimolecular decomposition of cumulenes (H(2)C(n)H(2)) from propadiene (H(2)CCCH(2)) to hexapentaene (H(2)CCCCCCH(2)) have received considerable attention due to the importance of these carbon-bearing molecules in combustion flames, chemical vapor deposition processes, atmospheric chemistry, and the chemistry of the interstellar medium. Cumulenes and their substituted counterparts also have significant technical potential as elements for molecular machines (nanomechanics), molecular wires (nano-electronics), nonlinear optics, and molecular sensors. In this review, we present a systematic overview of the stability, formation, and unimolecular decomposition of chemically, photo-chemically, and thermally activated small to medium-sized cumulenes in extreme environments. By concentrating on reactions under gas phase thermal conditions (pyrolysis) and on molecular beam experiments conducted under single-collision conditions (crossed beam and photodissociation studies), a comprehensive picture on the unimolecular decomposition dynamics of cumulenes transpires.