Chava Lifshitz

Kinetics of dissociation and thermionic emission in the C60 and C70 molecules

Abstract Kinetic energy release distributions are reported for the emission of C2 from the positive ions of C60 and C70. These are transformed into Arrhenius activation energies for dissociation of the neutral molecules. Microcanonical rate constants for dissociation the thermionic electron emission from these molecules are then presented. It is concluded that the latter process is parasitic to dissociation in each case.

C2 binding energy in C60

Abstract This article reviews, on the basis of mass spectrometric experiments, the current status concerning the C 2 binding energy or evaporation energy in C 60 , D (C 58 –C 2 ) = Δ E vap (C 60 ). Kinetic energy release distributions, time-resolved metastable fractions, breakdown curves, and thermionic emission rates all point to Δ E vap (C 60 + ) ≥ 9.5 eV and Δ E vap (C 60 ) ≥ 10 eV. These results are in agreement with high-level ab initio density functional theory calculations and with expectations from the known heats of formation of C 60 , C 70 , and C 2 . The C 2 evaporation is characterized by a very loose transition state with a pre-exponential factor close to the calculated upper limit.

Kinetic energy release distributions for C3H6O+- dissociations: A further test of the applicability of the energy-randomization hypothesis to unimolecular fragmentations

Abstract Kinetic energy release distributions (KERDs) have been determined experimentally from metastable peak-shapes for the reaction: C 3 H 6 O + − → C 2 H 3 O + + CH 3 . of the enol ion of acetone ( 1 ) and some of its deuterated analogues. The KERDs were observed to be bimodal and of different shapes for the losses of the two symmetrically placed methyl groups. Kinetic energy releases have been compared with data for the acetone ion ( 2 ). The results are explained by incomplete energy-randomization in the short-lived (0.5 picosecond) intermediate ion 2 .

Self‐consistent determination of fullerene binding energies BE (C+n–C2), n=58⋅ ⋅ ⋅44

Using recently measured accurate relative partial ionization cross section functions for production of the C60 fragment ions C+58 through C+44 by electron impact ionization, we have determined the respective binding energies BE(C+n–C2), with n=58,...,44, using a novel self‐consistent procedure. Appearance energies were determined from ionization efficiency curves. Binding energies were calculated from the corresponding appearance energies with the help of the finite heat bath theory. Then using these binding energies we calculated with transition state theory (TST), the corresponding breakdown curves, and compared these calculated ones with the ones derived from the measured cross sections. The good agreement between these breakdown curves proves the consistency of this multistep calculation scheme. As the only free parameter in this procedure is the binding energy C+58–C2, we studied the influence of different transition states chosen in the determination of this binding energy via TST theory and iterati...

Some new diatomic molecule containing endohedral fullerenes

Abstract Several new diatomic molecule-containing endohedral fullerenes were prepared by heating C 60 or C 70 under high pressures of the corresponding gases. The species prepared are N 2 @C 60 , N 2 @C 70 , 13 CO@C 60 , and 3 He 22 Ne@C 70 . Their existence was demonstrated through high sensitivity, wide dynamic range mass spectrometry. Out of two thousand C 60 molecules about one is observed to incorporate N 2 . The nitrogen molecule containing endohedral molecules are stable and the mass spectrometric signal is not lost even after several hours heating at 500 K. The corresponding endohedral ions undergo the Rice shrink-wrap mechanism; a mass-analyzed ion kinetic energy spectrum demonstrates the loss of a C 2 unit from the cage. The observation of 13 CO@C 60 by mass spectrometry opens up the possibility for future NMR studies of this molecule. The observation of 3 He 22 Ne@C 70 is in accordance with the “promoter” mechanism of Thiel and co-worker [J. Am. Chem. Soc. 118 (1996) 7164; Helv. Chim. Acta 80 (1997) 495], whereby the singly and doubly occupied fullerenes are in equilibrium but the empty and filled fullerenes are not.

Time-dependent mass spectra and breakdown graphs. The kinetic shift in aniline

Abstract Time-resolved appearance energies and metastable peak shapes were determined by trapped-ion mass spectrometry (TIMS) for the unimolecular dissociation of aniline cations. The long-time (milliseconds) appearance energy limit, AE(C5H+6) = 11.26 = 0.2 eV. suggests the formation at threshold energies of the cyclopentadienyl cation with neutral hydrogen isocyanide. HNC.

Appearance and ionization energies of multiply-charged C70 parent ions produced by electron impact ionization

Abstract Using an improved crossed beams/mass spectrometer apparatus we have extended previous electron impact ionization studies concerning ionization cross sections and appearance energies of up to quadruply-charged ions of C 70 to ions with charge states up to 6. A novel refined data procedure, involving a simultaneous non-linear weighted least-squares fit of two functions, allows the extraction of the cross section threshold. The ionization energies obtained depend linearly on the precursor charge state and compare well with theoretical predictions and the molecular capacitor scaling law. The corresponding maximum cross sections of C 70 5+ and C 70 6+ are about 1.5 × 10 −18 and 1 × 10 −20 cm 2 , respectively.

Breakdown Curves for Polyatomic Negative Ions

Negative‐ion breakdown curves were obtained at low (0–1 eV) energies for SF6 and a number of fluorocarbons. The dissociation rate constants, as a function of internal energy of the parent negative ions, were computed for the processes observed and used to estimate kinetic shifts and thermochemical appearance potentials, at 0°K, for the fragments. The results (e.g., metastable curve widths and temperature effects) were found to be in agreement with the quasiequilibrium theory (QET) of mass spectra. It is predicted that most of the parent negative ions studied (e.g., C7F14− from perfluoromethylcyclohexane) would not fragment if formed by the capture of a thermal electron by the vibrationally cold molecule.

Elimination of water from the carboxyl group of GlyGlyH

The elimination of water from the carboxyl group of protonated diglycine has been investigated by density functional theory calculations. The resulting structure is identical to the b2 ion formed in the mass spectrometric fragmentation of protonated peptides (therefore named “b2” in this study). The most stable geometry of the fragment ion (“b2”) is an O-protonated diketopiperazine. However, its formation is kinetically disfavored as it requires a free energy of 58.2 kcal/mol. The experimentally observed N-protonated oxazolone is 3.0 kcal/mol less stable. The lowest energy pathway for the formation of the “b2” ion requires a free energy of 37.5 kcal/mol and involves the proton transfer from the amide oxygen of protonated diglycine to the hydroxyl oxygen. Fragmentation initiated by proton transfer from the terminal nitrogen has also a comparable free energy of activation (39.4 kcal/mol). Proton transfer initiating the fragmentation, from the highly basic terminal nitrogen or amide oxygen to the less basic hydroxyl oxygen is feasible at energies reached in usual mass spectrometric experiments. Amide N-protonated diglycine structures are precursors of mainly y1 ions rather than “b2” ions. In the lowest energy fragmentation channels, proton transfer to the hydroxylic oxygen, bond breaking and formation of an oxazolone ring occur concertedly but asynchronously. Proton transfer to hydroxyl oxygen and cleavage of the corresponding C-O bond take place at the early stages of the fragmentation step, while ring closure to form an oxazolone geometry occurs at the later stages of the transition. The experimentally observed low kinetic energy release is expected to be due to the existence of a strongly hydrogen bonded protonated oxazolone-water complex in the exit channel. Whereas the threshold energy for “b2” ion formation (37.1 kcal/mol) is lower than for the y1 ion (38.4 kcal/mol), the former requires a tight transition state with an activation entropy, ΔS‡ = −1.2 cal/mol.K and the latter has a loose transition state with ΔS‡ = +8.8 cal/mol.K. This leads to y1 being the major fragment ion over a wide energy range.

An artificial molecule of Ne2 inside C70

Abstract C 70 was heated with 22 Ne at high pressure. Analysis by mass spectrometry unexpectedly showed the formation of Ne 2 @C 70 in addition to Ne@C 70 . The ratio of empty C 70 to mono-occupied to di-occupied was 1000:1:0.02. A mechanism where a small fraction of the fullerene breaks open and reaches equilibrium with the neon and then closes is suggested to explain these results.