Internal energy dependence of the fragmentation patterns of C60 and C60+

Gas phase ion chemistry of titanium-oxofullerene with ligated solvents

We investigated the gas phase fragmentation events of highly symmetric fullerene-like (FN-like) titanium oxo-cluster anions, [H₁₂Ti₄₂O₆₀(OCH₃)₄₂(HOCH₃)₁₀(H₂O)₂]^2- (1) and [H₇Ti₄₂O₆₀(OCH₃)₄₂(HOCH₃)₁₀(H₂O)₃]^1- (2). These oxo-clusters contain a closed cage Ti₄₂O₆₀ core, protected by a specific number of methoxy, methanol, and water molecules acting as ligands. These dianionic and monoanionic species were generated in the gas phase by electrospray ionization of the H₆[Ti₄₂(μ³-O)₆₀(OⁱPr)₄₂(OH)₁₂] (TOF) cluster in methanol. Collision induced dissociation studies of 1 revealed that upon increasing the collision energy, the protecting ligands were stripped off first, and [Ti₄₁O₅₈]^2- was formed as the first fragment from the Ti₄₂O₆₀ core. Thereafter, systematic TiO₂ losses were observed giving rise to subsequent fragments like [Ti₄₀O₅₆]^2-, [Ti₃₉O₅₄]^2-, [Ti₃₈O₅₂]^2-, etc. Similar fragments were also observed for monoanionic species 2 as well. Systematic 23 TiO₂ losses were observed, which were followed by complete shattering of the cage. We also carried out computational studies using density functional theory (DFT) to investigate the structures and fragmentation mechanism. The fragmentation of TOF was comparable to the fragmentation of C₆₀ ions, where systematic C₂ losses were observed. We believe that this is a consequence of topological similarity. The present study provides valuable insights into the structural constitution of TOF clusters and stability of the parent as well as the resulting cage-fragments in the gas phase.

Thermodynamic energetics underlying genomic instability and whole-genome doubling in cancer

Genomic instability contributes to tumorigenesis through the amplification and deletion of cancer driver genes. DNA copy number (CN) profiling of ensembles of tumors allows a thermodynamic analysis of the profile for each tumor. The free energy of the distribution of CNs is found to be a monotonically increasing function of the average chromosomal ploidy. The dependence is universal across several cancer types. Surprisal analysis distinguishes two main known subgroups: tumors with cells that have or have not undergone whole-genome duplication (WGD). The analysis uncovers that CN states having a narrower distribution are energetically more favorable toward the WGD transition. Surprisal analysis also determines the deviations from a fully stable-state distribution. These deviations reflect constraints imposed by tumor fitness selection pressures. The results point to CN changes that are more common in high-ploidy tumors and thus support altered selection pressures upon WGD.

Photo-ionization and fragmentation of Sc3N@C80 following excitation above the Sc K-edge

We have investigated the ionization and fragmentation of a metallo-endohedral fullerene, Sc₃N@C₈₀, using ultrashort (10 fs) x-ray pulses. Following selective ionization of a Sc (1s) electron (hν = 4.55 keV), an Auger cascade leads predominantly to either a vibrationally cold multiply charged parent molecule or multifragmentation of the carbon cage following a phase transition. In contrast to previous studies, no intermediate regime of C₂ evaporation from the carbon cage is observed. A time-delayed, hard x-ray pulse (hν = 5.0 keV) was used to attempt to probe the electron transfer dynamics between the encapsulated Sc species and the carbon cage. A small but significant change in the intensity of Sc-containing fragment ions and coincidence counts for a delay of 100 fs compared to 0 fs, as well as an increase in the yield of small carbon fragment ions, may be indicative of incomplete charge transfer from the carbon cage on the sub-100 fs time scale.

Addressing the Challenge of Molecular Change: An Interim Report

Invited by the editorial committee of the Annual Review of Physical Chemistry to "contribute my autobiography," I present it here, as I understand the term. It is about my parents, my mentors, my coworkers, and my friends in learning and the scientific problems that we tried to address. Courtesy of the editorial assistance of Annual Reviews, some of the science is in the figure captions and sidebars. I am by no means done: I am currently trying to fuse the quantitative rigor of physical chemistry with systems biology while also dealing with a post-Born-Oppenheimer regime in electronic dynamics and am attempting to instruct molecules to perform advanced logic.

New insight into power-law behavior of fragment size distributions in the C₆₀ multifragmentation regime

Previous experimental work has shown that a phase transition in C60 multifragmentation induced by nanosecond laser occurs at almost constant temperature covering a wide range of laser fluency. Here the relative yields of ionic fragments (IFs) C(n)(+) (n = 1-20) resulting from the multifragmentation are measured within the phase transition region. By excluding two small IFs and magic IFs due to their abnormal behavior, the data for residual IFs are used to estimate the size distributions of primary intermediate-mass IFs in the multifragmentation regime. The distributions are found to obey power laws n(-τ). Furthermore, the exponent τ values have sensitive dependence on lower laser fluency and converge to a constant of about 2.4 ± 0.2 for larger fluencies. These observations are in good agreement with an explanation based on the Fisher droplet model, offering the tantalizing possibility of a liquid-to-gas phase transition in C60 systems.

Intensity-resolved IR multiple photon ionization and fragmentation of C60

The sequential absorption of multiple infrared (IR) photons by isolated gas-phase species can lead to their dissociation and/or ionization. Using the newly constructed "Free-Electron Laser for IntraCavity Experiments" (FELICE) beam line at the FELIX facility, neutral C(60) molecules have been exposed to an extremely high number (approximately 10(23)) of photons/cm(2) for a total time duration of up to 5 micros. At wavelengths around 20 microm, resonant with allowed IR transitions of C(60), ionization and extensive fragmentation of the fullerenes are observed. The resulting photofragment distributions are attributed to absorption in fragmentation products formed once C(60) is excited to internal energies at which fragmentation or ionization takes place within the duration of the laser pulse. The high IR intensities available combined with the large interaction volume permit spatially resolved detection of the ions inside the laser beam, thereby disentangling the contributions from different IR intensities. The use of spatial imaging reveals intensity dependent mass distributions that are substantially narrower than what has been observed previously, indicating rather narrow energy distributions. A simple rate-equation modeling of the excitation process supports the experimental observations.

Infrared multiphoton ionization of superhot C60: experiment and model calculations

We address, both experimentally and theoretically, the issue of infrared (IR) resonance enhanced multiphoton ionization (IR-REMPI) and thermally induced redshifts of IR absorption lines in a very large and highly vibrationally excited molecular system. Isolated superhot C60 molecules with well defined and variable average vibrational energy in the range of 9-19 eV, effusing out of a constant flux thermal source, are excited and ionized after the absorption of multiple (500-800) infrared photons in the 450-1800 cm(-1) spectral energy range. Recording the mass-selected ion signal as a function of IR wavelength gives well resolved IR-REMPI spectra, with zero off-resonance background signal. An enhancement of the ion signal of about a factor of 10 is observed when the temperature is increased from 1200 to 1800 K under otherwise identical conditions. A pronounced temperature dependent redshift of some of the IR absorption lines is observed. The observations are found to be in good agreement with a model which is based on the sequential absorption of single photons, always followed by instantaneous vibrational energy redistribution. The mass spectra (C60(+) fragmentation pattern) are found to be strongly excitation wavelength dependent. Extensive fragmentation down to C32(+) is observed following the absorption of 1350-1400 cm(-1) as well as 1500-1530 cm(-1) photons while negligible fragmentation is observed when exciting around 520 cm(-1).

Mechanical Simulation of the Pressure and the Relaxation to Thermal Equilibrium of a Hot and Dense Rare Gas Cluster

A cold atomic cluster can be very rapidly heated and compressed by a hypersonic impact at a hard surface. The impact can be simulated by computing a classical trajectory for the motion of the atoms. By suddenly confining the hot and dense cluster within a rigid container, it is possible to monitor the time evolution of the force acting on the faces of the container. It is found that the pressure computed this way very rapidly decays to a time-independent value. After a somewhat longer time, this value reproduces the value for the pressure computed as the sum of the kinetic and internal pressures. This agreement is expected for a system in equilibrium. These observations support the conclusion that there is a fast relaxation to thermal equilibrium in these essentially hard-sphere systems. The deviation from equilibrium is primarily due to the propagation of shock waves within the cluster. The equilibrium pressure can reach up to the megabar range.

Ionisation of fullerenes and fullerene clusters using ultrashort laser pulses

We give a brief review of the literature concerning the ultra-short pulse ionisation of fullerenes in the gas phase. Emphasis is placed on the excitation time dependence of different ionisation regimes as manifested by photoelectron spectroscopy. The ionisation rates are modelled for the intermediate situation where the excitation energy is equilibrated between electronic degrees of freedom but not yet coupled to vibrational degrees of freedom. The model is shown to describe many aspects of the experiments. New results are presented on the intra-cluster molecular fusion of fullerene molecules when van der Waals bound clusters of fullerenes are exposed to ultra-short laser pulses. Pump-probe measurements give a decay time constant for the intra-cluster fusion reaction of 520 +/- 55 fs. A comparison with monomer ionisation results suggests that the time window for the fusion reaction is influenced by the coupling of the electronic excitation energy to vibrational degrees of freedom of the molecules in the cluster.

Fragmentation dynamics of fullerenes in intense femtosecond-laser fields: Loss of small neutral fragments on a picosecond time scale

The fragmentation dynamics of C60 irradiated with intense femtosecond laser pulses is studied with one-color pump-probe spectroscopy. Small neutral fragments (C, C2, and C3) are formed by an 800-nm pump pulse which are then postionized by a delayed probe pulse. The respective ion signals detected by the time-of-flight mass spectrometry dramatically increase on a time scale of 10-20 ps.

Activation of large ions in FT-ICR mass spectrometry

The advent of soft ionization techniques, notably electrospray and laser desorption ionization methods, has enabled the extension of mass spectrometric methods to large molecules and molecular complexes. This both greatly extends the applications of mass spectrometry and makes the activation and dissociation of complex ions an integral part of these applications. This review emphasizes the most promising methods for activation and dissociation of complex ions and presents this discussion in the context of general knowledge of reaction kinetics and dynamics largely established for small ions. We then introduce the characteristic differences associated with the higher number of internal degrees of freedom and high density of states associated with molecular complexity. This is reflected primarily in the kinetics of unimolecular dissociation of complex ions, particularly their slow decay and the higher energy content required to induce decomposition--the kinetic shift (KS). The longer trapping time of Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) significantly reduces the KS, which presents several advantages over other methods for the investigation of dissociation of complex molecules. After discussing general principles of reaction dynamics related to collisional activation of ions, we describe conventional ways to achieve single- and multiple-collision activation in FT-ICR MS. Sustained off-resonance irradiation (SORI)--the simplest and most robust means of introducing the multiple collision activation process--is discussed in greatest detail. Details of implementation of this technique, required control of experimental parameters, limitations, and examples of very successful application of SORI-CID are described. The advantages of high mass resolving power and the ability to carry out several stages of mass selection and activation intrinsic to FT-ICR MS are demonstrated in several examples. Photodissociation of ions from small molecules can be effected using IR or UV/vis lasers and generally requires tuning lasers to specific wavelengths and/or utilizing high flux, multiphoton excitation to match energy levels in the ion. Photodissociation of complex ions is much easier to accomplish from the basic physics perspective. The quasi-continuum of vibrational states at room temperature makes it very easy to pump relatively large amounts of energy into complex ions and infrared multiphoton dissociation (IRMPD) is a powerful technique for characterizing large ions, particularly biologically relevant molecules. Since both SORI-CID and IRMPD are slow activation methods they have many common characteristics. They are also distinctly different because SORI-CID is intrinsically selective (only ions that have a cyclotron frequency close to the frequency of the excitation field are excited), whereas IRMPD is not (all ions that reside on the optical path of the laser are excited). There are advantages and disadvantages to each technique and in many applications they complement each other. In contrast with these slow activation methods, the less widely appreciated activation method of surface induced dissociation (SID) appears to offer unique advantages because excitation in SID occurs on a sub-picosecond time scale, instantaneously relative to the observation time of any mass spectrometer. Internal energy deposition is quite efficient and readily adjusted by altering the kinetic energy of the impacting ion. The shattering transition--instantaneous decomposition of the ion on the surface--observed at high collision energies enables access to dissociation channels that are not accessible using SORI-CID or IRMPD. Finally, we discuss some approaches for tailoring the surface to achieve particular aims in SID.

Above the surface multifragmentation of surface scattered fullerenes

C(60) (-) ions were scattered from a gold surface at impact energies of 80-900 eV. The C(n) (-) fragments abundance distribution (odd and even) and the sharp fragmentation threshold observed, point at a prompt shattering event. The measured angle and energy distributions of the C(n) (-) fragments (n=2-12) provide clear evidence for a multifragmentation process where the superheated fullerenes leave the surface "intact" and disintegrate away from the surface.

Surface-induced dissociation of peptide ions: kinetics and dynamics

Kinetics and dynamics studies have been carried out for the surface-induced dissociation (SID) of a set of model peptides utilizing a specially designed electrospray ionization Fourier Transform ion cyclotron resonance mass spectrometer in which mass-selected and vibrationally relaxed ions are collided on a orthogonally-mounted fluorinated self-assembled monolayer on Au [111] crystal. The sampling time in this apparatus can be varied from hundreds of microseconds to tens of seconds, enabling the investigation of kinetics of ion decomposition over an extended range of decomposition rates. RRKM-based modeling of these reactions for a set of polyalanines demonstrates that SID kinetics of these simple peptides is very similar to slow, multiple-collision activation and that the distribution of internal energies following collisional activation is indistinguishable from a thermal distribution. For more complex peptides comprised of several amino acids and with internal degrees of freedom (DOF) of the order of 350 there is a dramatic change in kinetics in which RRKM kinetics is no longer capable of describing the decomposition of these complex ions. A combination of RRKM kinetics and the "sudden death" approximation, according to which decomposition occurs instantaneously, is a satisfactory description. This implies that a population of ions-which is dependant on the nature of the peptide, kinetic energy and sampling time-decomposes on or very near the surface. The shattering transition is described quantitatively for the limited set of molecules examined to date.

Collisional activation of peptide ions in FT-ICR mass spectrometry

In the last decade, the characterization of complex molecules, particularly biomolecules, became a focus of fundamental and applied research in mass spectrometry. Most of these studies utilize tandem mass spectrometry (MS/MS) to obtain structural information for complex molecules. Tandem mass spectrometry (MS/MS) typically involves the mass selection of a primary ion, its activation by collision or photon excitation, unimolecular decay into fragment ions characteristic of the ion structure and its internal excitation, and mass analysis of the fragment ions. Although the fundamental principles of tandem mass spectrometry of relatively small molecules are fairly well-understood, our understanding of the activation and fragmentation of large molecules is much more primitive. For small ions, a single energetic collision is sufficient to dissociate the ion; however, this is not the case for complex molecules. For large ions, two fundamental limits severely constrain fragmentation in tandem mass spectrometry. First, the center-of-mass collision energy-the absolute upper limit of energy transfer in a collision process-decreases with increasing mass of the projectile ion for fixed ion kinetic energy and neutral mass. Secondly, the dramatic increase in density of states with increasing internal degrees of freedom of the ion decreases the rate of dissociation by many orders of magnitude at a given internal energy. Consequently, most practical MS/MS experiments with complex ions involve multiple-collision activation (MCA-CID), multi-photon activation, or surface-induced dissociation (SID). This review is focused on what has been learned in recent research studies concerned with fundamental aspects of MCA-CID and SID of model peptides, with an emphasis on experiments carried out with Fourier transform ion cyclotron resonance mass spectrometers (FT-ICR MS). These studies provide the first quantitative comparison of gas-phase multiple-collision activation and SID of peptide ions. Combining collisional energy-resolved data with RRKM-based modeling revealed the effect of peptide size and identity on energy transfer in collisions-very important characteristics of ion activation from fundamental and the analytical perspectives. Finally, the combination of FT-ICR with SID was utilized to carry out the first time-resolved experiments that examine the kinetics of peptide fragmentation. This has lead to the discovery that the time-dependence of ion dissociation varies smoothly up to a certain collision energy, and then shifts dramatically to a time-independent, extensive dissociation. This near-instantaneous "shattering" of the ion generates a large number of relatively small fragment ions. Shattering of ions on surfaces opens up a variety of dissociation pathways that are not accessible with multiple-collision and multiphoton excitation.

Shattering of Peptide ions on self-assembled monolayer surfaces

Time- and collision energy-resolved surface-induced dissociation (SID) of des-Arg(1)- and des-Arg(9)-bradykinin on a fluorinated self-assembled monolayer (SAM) surface was studied by use of a novel Fourier transform ion cyclotron resonance mass spectrometer (FT-ICR MS) specially equipped to perform SID experiments. Time-resolved fragmentation efficiency curves (TFECs) were modeled by an RRKM-based approach developed in our laboratory that utilizes a very flexible analytical expression for the internal energy deposition function capable of reproducing both single- and multiple-collision activation in the gas phase and excitation by collisions with a surface. Both experimental observations and modeling establish a very sharp transition in the dynamics of ion-surface interaction: the shattering transition. The experimental signature for this transition is the appearance of prompt (time-independent) fragmentation, which becomes dominant at high collision energies. Shattering opens a variety of dissociation pathways that are not accessible to slow collisional and thermal ion activation. This results in much better sequence coverage for the singly protonated peptides than dissociation patterns obtained with any of the slow activation methods. Modeling demonstrated that, for short reaction delays, dissociation of these peptides is solely determined by shattering. Internal energies required for shattering transition are approximately the same for des-Arg(1) and des-Arg(9)-bradykinin, resulting in the overlap of fragmentation efficiency curves obtained at short reaction delays. At longer delay times, parent ions depletion is mainly determined by a slow decay rate and fragmentation efficiency curves for des-Arg(1) and des-Arg(9)-bradykinin diverge. Dissociation thresholds of 1.17 and 1.09 eV and activation entropies of -22.2 and -23.3 cal/(mol K) were obtained for des-Arg(1) and des-Arg(9)-bradykinin from RRKM modeling of time-resolved data. Dissociation parameters for des-Arg(1)-bradykinin are in good agreement with parameters derived from thermal experiments. However, there is a significant discrepancy between the thermal data and dissociation parameters for des-Arg(9)-bradykinin obtained in this study. The difference is attributed to the differences in conformations that undergo thermal activation and activation by ion-surface collisions prior to dissociation.

Excitation and fragmentation mechanisms in ion-fullerene collisions

Electronic and vibronic excitations as well as fragmentation mechanisms in high energy ion ( H+, C+, Ar+) fullerene collisions are investigated within a fully microscopic approach, called nonadiabatic quantum molecular dynamics. The total kinetic energy loss of the projectile depends dramatically on ion mass, but, surprisingly, does not depend on the impact velocity for all ions in a certain range. This is in striking contrast to the predictions of the "stopping power" concept of solids, but explains apparently contradicting experimental observations. Signatures for nonstatistical fragmentation mechanisms are predicted.

High-energy collision-induced dissociation of small polycyclic aromatic hydrocarbons

High-energy collision-induced dissociation (CID) experiments on polycyclic aromatic hydrocarbons (PAHs) having 2-6 rings, naphthalene, anthracene, phenanthrene, fluoranthene, pyrene and coronene, were performed, and the relative abundances of their fragment ions were investigated as a function of collision energy. The results revealed that the PAHs except naphthalene showed a bimodal-type distribution of positive fragmentation ions, which is closely similar to the fragment-ion distribution reported for the CID of three-dimensional fullerene, C(60)(+) and C(70)(+). The three-ring isomers of anthracene and phenanthrene and the four-ring isomers of fluoranthene and pyrene can be distinguishable in their spectra under an electron ionization energy of 70 eV, but the high-energy CID spectra of the three- and four-ring isomers were almost identical. The fragmentation corresponding to fragment ions in the low-mass region of the bimodal CID spectra could be interpreted by the simple statistical model that fragment ions are formed by random evaporation from the molecular ions after a considerable structural rearrangement, 'phase transition', occurring at some high-energy state.