Kaoru Yamazaki

Communication: Two-step explosion processes of highly charged fullerene cations C60q+ (q = 20–60)

To establish the fundamental understanding of the fragmentation dynamics of highly positive charged nano- and bio-materials, we carried out on-the-fly classical trajectory calculations on the fragmentation dynamics of C60(q+) (q = 20-60). We used the UB3LYP/3-21G level of density functional theory and the self-consistent charge density-functional based tight-binding theory. For q ≥ 20, we found that a two-step explosion mechanism governs the fragmentation dynamics: C60(q+) first ejects singly and multiply charged fast atomic cations C(z+) (z ≥ 1) via Coulomb explosions on a timescale of 10 fs to stabilize the remaining core cluster. Thermal evaporations of slow atomic and molecular fragments from the core cluster subsequently occur on a timescale of 100 fs to 1 ps. Increasing the charge q makes the fragments smaller. This two-step mechanism governs the fragmentation dynamics in the most likely case that the initial kinetic energy accumulated upon ionization to C60(q+) by ion impact or X-ray free electron laser is larger than 100 eV.

Electronic excited state paths of Stone-Wales rearrangement in pyrene: roles of conical intersections.

We investigated the reaction paths of Stone-Wales rearrangement (SWR), i.e., π/2 rotation of two carbon atoms with respect to the midpoint of the bond, in graphene and carbon nanotube quantum chemically. Our particular attention is focused on the roles of electronic excitations and conical intersections (CIs) in the reaction mechanism. We used pyrene as a model system. The reaction paths were determined by constructing potential energy surfaces at the MS-CASPT2//SA-CASSCF level of theory. We found that there are no CIs involved in SWR when both of C-C bond cleavage and formation occur simultaneously (concerted mechanism). In contrast, for the reaction path with stepwise cleavage and formation of C-C bonds, C-C bond breaking and making processes proceed through two CIs. When SWR starts from the ground (S(0)) state, the concerted and stepwise paths have an equivalent reaction barrier ΔE(‡) (9.5-9.6 eV). For the reaction path starting from excited states, only the stepwise mechanism is energetically preferable. This path contains a nonadabatic transition between the S(1) and S(0) states via a CI associated with the first stage of C-C bond cleavage and has ΔE(‡) as large as in the S(0) paths. We confirmed that the main active molecular orbitals and electron configurations for the low-lying electronic states of larger nanocarbons are the same as those in pyrene. This result suggests the importance of the nonadiabatic transitions through CIs in the photochemical reactions in large nanocarbons.

Fragmentation network of doubly charged methionine: Interpretation using graph theory

The fragmentation of doubly charged gas-phase methionine (HO2CCH(NH2)CH2CH2SCH3) is systematically studied using the self-consistent charge density functional tight-binding molecular dynamics (MD) simulation method. We applied graph theory to analyze the large number of the calculated MD trajectories, which appears to be a highly effective and convenient means of extracting versatile information from the large data. The present theoretical results strongly concur with the earlier studied experimental ones. Essentially, the dication dissociates into acidic group CO2H and basic group C4NSH10. The former may carry a single or no charge and stays intact in most cases, whereas the latter may hold either a single or a double charge and tends to dissociate into smaller fragments. The decay of the basic group is observed to follow the Arrhenius law. The dissociation pathways to CO2H and C4NSH10 and subsequent fragmentations are also supported by ab initio calculations.

Simulation of Nuclear Dynamics of C60: From Vibrational Excitation by Near-IR Femtosecond Laser Pulses to Subsequent Nanosecond Rearrangement and Fragmentation

Impulsive Raman excitation of C60 by single or double near-IR femtosecond pulses of λ = 1,800 nm was investigated by using a time-dependent adiabatic state approach combined with the density functional theory method. We confirmed that the vibrational energy stored in a Raman active mode of C60 is maximized when T p ∼ T vib/2 in the case of a single pulse, where T p is the pulse length and T vib is the vibrational period of the mode. In the case of a double pulse, mode selective excitation can be achieved by adjusting the pulse interval τ. The energy of a Raman active mode is maximized if τ is chosen to equal an integer multiple of T vib, and it is minimized if τ is equal to a half-integer multiple of T vib. The energy stored can be larger than the barrier heights for rearrangement or fragmentation processes. The picosecond or nanosecond dynamics of resulting Stone-Wales rearrangement (SWR) and fragmentation are also investigated by using the density functional-based tight-binding semiempirical method. We present how SWRs are caused by the flow of vibrational kinetic energy on the carbon network of C60. In the case where the hg(1) prolate-oblate mode is initially excited, the number of SWRs prior to fragmentation is larger than in the case of ag(1) mode excitation for the same excess vibrational energy. Fragmentation by C2-ejection is found to occur from strained, fused pentagon/pentagon defects produced by a preceding SWR, which confirms the earliest mechanistic speculations of Smalley et al. (J. Chem. Phys. 88, 220, 1988). The fragmentation rate of C60 → C58 + C2 in the case of hg(1) prolate-oblate mode excitation does not follow a statistical description as employed for instance in the Rice-Ramsperger-Kassel (RRK) theory, whereas the rate for ag(1) mode excitation does follow predictions made by RRK. We also found for the hg(1) mode excitation that the nonstatistical nature still remains in the distribution of barycentric velocities of fragments C58 and C2. This result suggests that it is possible to control rearrangement and subsequent bond breaking in a “nonstatistical” way by initial selective mode excitation.