Structures and energies of small carbon clusters: what experiment and theory have to say about C+8, C+9 and C+10

An improved descriptor of cluster stability: application to small carbon clusters

The mass spectra of gas-phase clusters in cluster beams have a rich structure where the relative heights of the peaks compared to peaks corresponding to the clusters of neighboring sizes reveal the stability of the clusters as a function of size N. In an analysis of the published mass spectrum of carbon cluster cations CN+ with N ≤ 16 we have employed the most common descriptor of cluster stability, which is based on the comparison of the total energy of the cluster of size N with the averaged energies of clusters with sizes N + 1 and N - 1. These energies have been obtained from density functional calculations. The comparison between the stability function and the mass spectrum leaves some experimental features unexplained; in particular, the correlation with the detailed variation of the height of the mass peaks as a function of size N is not satisfactory. We then propose a novel stability descriptor which improves the features substantially, in particular the correlation with the detailed variation of the height of the mass peaks. The new stability index is based on the comparison of the atom-evaporation energy of the cluster of size N with the averaged atom-evaporation energies of clusters with sizes N + 1 and N - 1. The substantial improvement achieved is attributed to the fact that evaporation energies are quantities directly connected with the processes controlling the cluster abundances in the beam.

Development of an ion mobility spectrometer using radio-frequency electric field

This paper describes the development of a new ion mobility spectrometer (IMS) using the radio-frequency (RF) electric field. The proposed IMS has high ion transmission efficiency. Seven connected IMS devices, in which the RF and DC electric fields are created by separate electrodes, are constructed. The ions are confined by the RF electric field and drifted by the DC electric field. The electrodes in each IMS device include short quadrupole electrodes and segmented vane electrodes. The uniform electric field in the IMS is verified by simulated results obtained using SIMION. To measure the exact value of reduced mobility K ₀ at low Td (1 Td = 10^-17 V cm²), two ion gates are installed in the IMS. By installing the ion gates at suitable positions for eliminating the effect of gas flow, the exact ion velocity through the IMS can be measured. The K ₀ values of O₂ ⁺ and C₆H₆ ⁺ ions are measured as a function of Td. In addition, the K ₀ of CH₃OCH₂ ⁺ fragment ions is measured. These K ₀ measurement results are consistent with previous results obtained using electrostatic drift tube apparatus. In summary, as our IMS can measure K ₀ under low Td conditions, it can be used to better understand the structure of small molecular or fragment ions.

Even-odd product variation of the C(n)(+) + D(2) (n = 4-9) reaction: complexity of the linear carbon cation electronic states

We have studied reactions between linear Cn(+) (n = 4-9) and D2, using ion mobility mass spectrometry techniques and quantum chemical calculations in order to understand the complex reactivity of the linear cluster cations. Only linear CnD(+) products were observed for the odd (n = 5, 7, 9) linear clusters, while CnD2(+) was the main product for the even clusters. For the reaction rate constants determined for these two channels, we obtained the following two features: (1) the rate constant decreases with the size n, and (2) even-sized clusters have lower rate constants than neighboring odd-sized clusters. In the theoretical calculations using the CCSD(T) and B3LYP methods with the cc-pVTZ basis, we found that a low lying (2)Σ state in odd clusters may play an important role in these reactions. This opposes the previous interpretation that the (2)Πg/u state is the dominant electronic state for linear Cn(+) (n = 4-9) clusters. We showed that a barrierless radical abstraction forming CnD(+) occurs through a direct head on approach for the (2)Σ state Cn(+). In contrast, a carbene-like insertion forming CnD2(+) occurs through a sideways approach for the (2)Πg/u state Cn(+). We have concluded that the higher rate constants for the odd clusters come from the existence of symmetry broken (2)Σ states which are absent in even linear clusters.

Electronic absorption spectra of linear and cyclic C(n)(+) n=7-9 in a neon matrix

The C(n)(+) n=7-9 cations were produced by electron-impact ionization of perchloronaphthalene, mass selected, and their electronic absorption spectra in 6 K neon matrices recorded. The linear and cyclic isomers of C7(+) and C8(+) are detected. Three systems of linear C7(+) are observed with origin bands near 770, 332, and 309 nm. The cyclic C7(+) shows two transitions near 676 and 448 nm. One system of linear C9(+) is observed commencing at 371 nm. Linear C8(+) shows five dipole-allowed electronic transitions from the X 2pi(g) ground state, and the strongest ones have the origin bands at 890.8 and 308.1 nm. Five electronic transitions of cyclic C8(+) are also discernible.

Electronic and infrared absorption spectra of linear and cyclic C6+ in a neon matrix

Electronic and infrared absorption spectra of mass-selected C6+, generated by dissociative electron impact ionization of C6Cl6 and C6Br6, have been recorded in 6 K neon matrices. Linear and cyclic forms of C6+ have been observed. The 2Pig<--Chi2Piu electronic transition of linear C6+ has its origin band at 646 nm whereas for the (2) 2B2<--Chi2A1 system of the cyclic isomer it lies at 570 nm. An infrared active fundamental mode in the ground electronic state of C6+ is observed at 2092 and 1972 cm(-1) for the linear and cyclic isomer, respectively.

Gas-phase conformations of cationized poly(styrene) oligomers

The gas-phase conformations of poly(styrene) oligomers cationized by Li+, Na+, Cu+, and Ag+ (M+ PSn) were examined using ion mobility experiments and molecular mechanics/ dynamics calculations. M+PSn ions were formed by MALDI and their ion-He collision cross-sections were measured by ion mobility methods. The experimental collision cross-sections of each M+PS n-mer were similar for all four metal cations and increased linearly with n. Molecular modeling of selected M+ PS oligomers cationized by Li+ and Na+ yielded quasi-linear structures with the metal cation sandwiched between two phenyl groups. The relative energies of the structures were approximately 2-3 kcal/mol more stable when the metal cation was sandwiched near the middle of the oligomer chain than when it was near the ends of the oligomer. The cross-sections of these theoretical structures agree well with the experimental values with deviations typically around 1-2%. The calculations also show that the metal cation tends to align the phenyl groups on the same side of the -CH2-CH- backbone. Calculations on neutral poly(styrene), on the other hand, showed structures in which the phenyl groups were more randomly positioned about the oligomer backbone. The conformations and metal-oligomer binding energies of M+PS are also used to help explain CID product distributions and fragmentation mechanisms of cationized PS oligomers. etry