Ab initio characterization of size dependence of electronic spectra for linear anionic carbon clusters C(n) (-) (n = 4-17)

Structures and Stabilities of Carbon Chain Clusters Influenced by Atomic Antimony

The C-C bond lengths of the linear magnetic neutral C_nSb, C_nSb⁺ cations and C_nSb^- anions are within 1.255-1.336 Å, which is typical for cumulene structures with moderately strong double-bonds. In this report, we found that the adiabatic ionization energy (IE) of C_nSb decreased with n. When comparing the IE~n relationship of C_nSb with that of pure C_n, we found that the latter exhibited a stair-step pattern (n ≥ 6), but the IE~n relationship of C_nSb chains took the shape of a flat curve. The IEs of C_nSb were lower than those of corresponding pure carbon chains. Different from pure carbon chains, the adiabatic electron affinity of C_nSb does not exhibit a parity effect. There is an even-odd alternation for the incremental binding energies of the open chain C_nSb (for n = 1-16) and C_nSb⁺ (n = 1-10, when n > 10, the incremental binding energies of odd (n) chain of C_nSb⁺ are larger than adjacent clusters). The difference in the incremental binding energies between the even and odd chains of both C_nSb and pure C_n diminishes with the increase in n. The incremental binding energies for C_nSb^- anions do not exhibit a parity effect. For carbon chain clusters, the most favorable binding site of atomic antimony is the terminal carbon of the carbon cluster because the terminal carbon with a large spin density bonds in an unsaturated way. The C-Sb bond is a double bond with Wiberg bond index (WBI) between 1.41 and 2.13, which is obviously stronger for a carbon chain cluster with odd-number carbon atoms. The WBI of all C-C bonds was determined to be between 1.63 and 2.01, indicating the cumulene character of the carbon chain. Generally, the alteration of WBI and, in particular, the carbon chain cluster is consistent with the bond length alteration. However, the shorter C-C distance did not indicate a larger WBI. Rather than relying on the empirical comparison of bond distance, the WBI is a meaningful quantitative indicator for predicting the bonding strength in the carbon chain.

Optical Spectroscopy of Small Carbon Clusters from Electron-Impact Fragmentation and Ionization of Fullerene-C60

A series of cationic molecular fragments (C _n⁺, n = 11, 12, 15, 16, 18, and 21), produced by electron-impact ionization of C₆₀ in the gas phase, were each mass-selected and accumulated in cryogenic Ne matrices. Optical absorption measurements in the UV-vis and IR spectral ranges reveal linear carbon chain structures. In particular, we have observed the known electronic transitions of linear C₁₁, C₁₅, and C₂₁. The NIR transitions of linear C₁₅^-, C₁₆^-, and C₁₈^- have also been detected, indicating that soft-landing of the corresponding cations can also involve charge-changing. Newly observed electronic absorptions at 410.3 and 429.9 nm have been assigned to linear C₁₈ absorptions at 438.2, 443.5, 422.3, and 433.7 nm, to linear C₁₅⁺, and absorption at 395.5 nm, to linear C₁₆. Increasing deposition energy leads to fragmentation upon impact. This is indicated by absorptions of C₁₀ (313, 316.3 nm), when depositing C _n⁺ ( n = 11, 15, 16) as well as C₁₂ (332 nm) or C₁₄ (347.4, 356.6 nm), when depositing C₁₅⁺ or C₁₆⁺, respectively. These were previously assigned to cyclic isomers. We reassign them to linear isomers here on the basis of plausibility arguments. The observations have been supported by time-dependent density functional theory calculations for ring and chain isomers of C _n^+/-/0, 10 ≤ n ≤ 20 up to the vacuum-UV range. The electronic absorptions of carbon chains are at least 1 order of magnitude stronger than all NIR electronic absorptions of C₆₀⁺, which have recently been attributed to several of the diffuse interstellar bands. Considering that fullerene multifragmentation yields long carbon chains that have very strong absorptions both in the UV-vis and IR spectral regions, these systems appear to be good candidates to be observed in regions of space containing fullerenes.

Ultraslow isomerization in photoexcited gas-phase carbon cluster [Formula: see text]

Isomerization and carbon chemistry in the gas phase are key processes in many scientific studies. Here we report on the isomerization process from linear [Formula: see text] to its monocyclic isomer. [Formula: see text] ions were trapped in an electrostatic ion beam trap and then excited with a laser pulse of precise energy. The neutral products formed upon photoexcitation were measured as a function of time after the laser pulse. It was found using a statistical model that, although the system is excited above its isomerization barrier energy, the actual isomerization from linear to monocyclic conformation takes place on a very long time scale of up to hundreds of microseconds. This finding may indicate a general phenomenon that can affect the interstellar medium chemistry of large molecule formation as well as other gas phase processes.

Low-lying electronic states and their nonradiative deactivation of thieno[3,4-b]pyrazine: an ab initio study

State-averaged complete active space self-consistent field (SA-CASSCF) calculations have been used to locate the four low-lying electronic states of thieno[3,4-b]pyrazine (TP), and their vertical excitation energies and emission energies have been determined by means of the multistate complete active space with second-order perturbation theory (MS-CASPT2) calculations. The present results indicate that the first weak (1)nπ∗ excited state has a C(s)-symmetry structure, unlike two bright (1)ππ∗ excited states in C(2v) symmetry. The predicted vertical excitation energies of the three low-lying excited states in the gas phase are 3.41, 3.92, and 4.13 eV at the restricted-spin coupled-cluster single-double plus perturbative triple excitation [RCCSD(T)] optimized geometry, respectively. On the basis of calculations, a new assignment to the observed spectra of TP was proposed, in which the (1)nπ∗ state should be responsible for the weak absorption centred at 3.54 eV and the two closely spaced (1)ππ∗ states account for the two adjacent absorption bands observed at 3.99 and 4.15 eV. The predicted vertical emission energies lend further support to our assignments. Surface hopping dynamics simulations performed at the SA-CASSCF level suggest that the plausible deactivation mechanism comprises an ultrafast relaxation of the (1)ππ∗ excited states to (1)nπ∗ excited state, followed by a slow conversion to the S(0) ground state via a conical intersection. This internal conversion is accessible, since the MS-CASPT2 predicted energy barrier is ∼0.55 eV, much lower than the Franck-Condon point populated initially under excitation. The dynamical simulations on the low-lying states for 500 fs reveal that the relatively high (1)ππ∗ excited states can be easily trapped in the (1)nπ∗ excited state, which will increase the lifetime of the excited thieno[3,4-b]pyrazine.