Paul R. Kemper

Structures of carbon cluster ions from 3 to 60 atoms: Linears to rings to fullerenes

A new method for identifying and characterizing cluster ion isomers is presented. Results indicate that most carbon clusters have more than one stable form, with 29≤n≤45 indicating three or four different structures. Fullerenes first appear at C+30 and begin to dominate above C+45. From small to large the isomeric progression is from linear to rings to fullerenes.

Gas-Phase Ion Chromatography: Transition Metal State Selection and Carbon Cluster Formation

Gas-phase ion chromatography can separate ions that have the same mass but differ in isomeric structure or electronic configuration. The main features of this technique are briefly outlined, and applications to a series of problems in transition metal chemistry and carbon cluster chemistry are described. Examples in transition metal chemistry include state-selective reactivity, excited state deactivation, and state-selective ligand binding energies. For clusters, ion chromatography was used to determine the structure of pure carbon cluster ions as a function of size from C4 to C84. The results indicate that carbon grows first in linear chains, transforms to monocyclic planar rings at about C10, and forms new families of planar bi-, tri-, and tetracyclic rings at C20, C30, and C40, respectively. Fullerenes, which mysteriously appear at C30 and dominate by C50, are generated by heating the planar ring systems above an isomerization barrier rather than by growth of graphite precursors.

Design of a new electrospray ion mobility mass spectrometer

Abstract The design of a new ion mobility mass spectrometer is presented. The design features an electrospray ion source; an ion funnel to transmit ions efficiently from the source to the mobility cell and to accumulate ions in the pulsed ion mode; a mobility cell, and a quadrupole mass analyzer. Each part of the instrument is described in detail. Preliminary results obtained with the new instrument are presented to demonstrate its capabilities. Equilibrium experiments showed that the Δ G °(300 K) values for the addition of the first water molecule to the doubly protonated peptides bradykinin, angiotensin II, and LHRH are in the range from −3.5 to −2.5 kcal/mol. The corresponding values for the singly protonated ions are >−0.5 kcal/mol for angiotensin II and LHRH, but equal to −2.6 kcal/mol for bradykinin. The stronger bonding in bradykinin may be due to the presence of a salt bridge structure. Ion arrival time distributions showed that singly protonated peptides can form aggregates of the form ( n M + n H) n + . The mobilities of these ions indicated that they are near spherical. Heating the drift cell to ∼450 K caused dissociation of the (2M + 2H) 2+ ion into two (M + H) + units on the 1 ms experimental time scale. A theoretical fit to the experimental data yielded rate constants and a barrier for dissociation of 30 ± 2 kcal/mol for bradykinin and 39 ± 3 kcal/mol for LHRH.

A new method for studying carbon clusters in the gas phase: Observation of size specific neutral fragment loss from metastable reactions of mass selected C+n, n≤60

A new method for studying the generation and structure of carbon cluster ions is described. A rotating, translating carbon rod is subjected to the focused output of an excimer laser operating at 308 nm. The resulting plume is directly sampled by a high resolution reverse geometry mass spectrometer operating between 2 and 8 kV. In this paper we report initial results for the positive cluster ions up to n=60. The mass spectra are bimodal with a minimum occurring between C+26 and C+31 in agreement with findings of other workers using related techniques. Metastable neutral loss is observed for all cluster ions with n≥5. Below C+30 the most intense neutral loss is C3 but intense C1 and C5 loss is observed for a significant number of systems and C10 and C14 for a few systems. Above C+30 the metastable loss spectra dramatically change and only C2, and a minor amount of C4 loss, is observed. The C+60 ion is slightly enhanced in the mass spectrum but its metastable reactivity is no different than any of the other ...

On the structure, reactivity and relative stability of the large carbon cluster ions C+62, C+60 and C+58

Abstract The exclusive metastable loss of C 2 from C + 62 , C + 60 and C + 58 generated by laser ablation from graphite is studied using a high resolution reverse geometry mass spectrometer. Kinetic energy release distributions are peaked at low energies (≈0.2 eV) and fall off quasi-exponentially at higher energies. Successful modeling by statistical phase space theory indicates no reverse activation energy for C 2 -loss. A molten droplet of spheroidal geometry for the structure of these ions best fits the data. The modeling also indicates C 2 is bound to C + 62 by 3.0 eV and to C + 60 and C + 58 by ≈4.6 eV. Electronic rather than structural effects are suggested as the cause of the instability of C + 62 relative to C + 60 and C + 58 . Kinetic effects that reflect the instability of C + 62 and to a lesser extent the stability of C + 60 are suggested as the principal reasons why C + 60 can be a dominant feature in the mass spectrum under favorable experimental conditions.

Isomers of small carbon cluster anions : linear chains with up to 20 atoms

The structure of small carbon cluster anions, Cn- (4 ≤ n ≤ 20), was investigated with the use of ion chromatography. With this technique, both the existence and the relative amounts of possible structural isomers can be determined. More than 99% of the ions C5- to C9- were found to be linear under these experimental conditions. Starting with C10-, a monocyclic isomer was observed, and linear and moncyclic structures coexisted from C10- through at least C20-. This result is in contrast to previous work on positive ions, which showed the existence of linear isomers from C5+ to C10+, with linear and cyclic isomers coexisting only from C7+ to C10+. Above C10+, no linear clusters were observed.

A hybrid double-focusing mass spectrometer—high-pressure drift reaction cell to study thermal energy reactions of mass-selected ions

A new instrument is described that couples a reverse-geometry mass spectrometer operating at 5 kV to a high-pressure, temperature-variable drift reactor that operates at thermal energies. The first-stage mass spectrometer is a home-built instrument with the same dimensions and ion optics as a V.G. Instruments ZAB-2F. Ions exiting this instrument are decelerated to between 0 and 10 eV and focused on the entrance hole of the drift cell. The drift cell is 4.0 cm long and 1.52 cm in diameter and has a uniform drift field provided by stepped voltages applied to eight field guard rings. Pressure in the cell can be varied from 0 to 2 torr, and temperatures from 100 to 500 K. Ions exiting the cell are accelerated, passed through a quadrupole mass filter, and detected by an electron multiplier. A description is given of the ion optics used to focus and decelerate the beam from the first-stage mass spectrometer. Substantial analysis and discussion are given to ion energies in the cell and methods of pressure measurement. Experimental results obtained compare well with data in the literature when available. A new result on mobilities of Co+ ions in helium is presented. The data indicate that ions are present in both the ground state and metastable excited electronic state, exhibiting substantially different mobilities.

Do small fullerenes exist only on the computer? Experimental results on C=/−20 and C+/−24

Abstract Experimental results on the structure of carbon cluster anions and cations with 20 and 24 atoms are presented. Recent quantum chemical calculations predict either cage or graphitic structures to be lowest in energy for C20 and C24. However experimentally we find C+20 and C−20 are dominated by monocyclic ring structures, with C−20 exhibiting minor amounts of planar bicyclic structures and a linear structure. For C+24 and C−24 both monocyclic rings and planar bicycle rings are observed. No fullerene or “graphitic” structures are observed for these clusters. A number of possible bicyclic structures for C24 are discussed.

Landing of size-selected Agn+ clusters on single crystal TiO2 (110)-(1×1) surfaces at room temperature

Mass-selected Ag(n) (+) (n=1,2,3) clusters with impact energy less than 2 eV per atom were deposited from the gas phase onto rutile titania (110)-(1x1) single crystal surfaces at room temperature and imaged using ultra-high vacuum scanning tunneling microscopy. Upon reaching the surface, Ag monomers sintered to form three-dimensional islands of approximately 50 atoms in size, with an average measured height of 7.5 A and diameter of 42 A. This suggests that the monomers are highly mobile on the titania surface at room temperature. Dimers also sintered to form large clusters upon deposition, approximately 30 atoms in size, with an average height of 6.2 A and diameter of 33 A. Clusters formed from monomer deposition appeared approximately three times more frequently at step edges than clusters formed from dimer deposition, indicating that the surface mobility of deposited monomers is higher than that of deposited dimers. In sharp contrast to the deposition of monomers and dimers, the deposition of trimers resulted in a high density of very small clusters on the order of a few atoms in size, indicative of intact trimers on the surface, implying that deposited trimers have very limited mobility on the surface at room temperature.

The formation and reactivity of HOC+: Interstellar implications

CHO+ ions are made by electron impact on CD3OH in the source (ICR1) of a tandem ion cyclotron resonance spectrometer. These ions are injected into a differentially pumped analysis cell (ICR2) where they are reacted with a number of small molecules. The internal energy distribution in the CHO+ ions is obtained using total reactivity studies with neutral molecules of varying proton affinities. About 40% of the CHO+ ions react with D2 either by proton transfer to form D2H+ or isotopic exchange to form CDO+ ions. A series of experiments are performed that conclusively show these ions are the HOC+ isomer and the exchange is due to the catalytic isomerization reaction HOC++D2→DCO++HD which is approximately 37 kcal/mol exothermic. The product DCO+ ions are vibrationally cool indicating the reaction releases most of its energy as kinetic energy. Absolute rate constants for reactions of CHO+ ions with the neutrals 13CO, 15N2, CO2, O2, D2, and Ar are reported. HOC+ reacts with D2 at about 30% of the collision rate....