High resolution infrared spectra of the linear carbon cluster C7: The ν4 stretching fundamental band and associated hot bands

Infrared bands of neutral gas-phase carbon clusters in a broad spectral range

The identification of species in the interstellar medium requires precise and molecule-specific spectroscopic information in the laboratory framework, in broad spectral ranges and under conditions relevant to interstellar environments. In this work, we measure the gas-phase infrared spectra of neutral carbon clusters, CN (N = 6-11), in a molecular beam. The CN distribution is formed by photofragmentation of C60 molecules, concurrently showing a top-down formation mechanism. A broad spectral range in the infrared between 500-3200 cm-1 (20-3.125 μm) is investigated. We observe strong bands between 5 and 6 μm, in conjunction with novel features in the 3 μm region. Density functional theory calculations reveal that these short wavelength modes correspond to combination bands with significant infrared intensity. Moreover, we identify the N ≤ 10 clusters as linear, while C11 adopts a ring configuration, placing the linear-to-ring transition at N = 11 under our molecular beam conditions. The linearity of C10 is discussed based on the formation pathway from larger clusters in energetic conditions. Given the vast and very precise infrared information already been released from the James Webb Space Telescope mission, this infrared spectroscopic data set in conjunction with information on formation mechanisms is of major relevance for identifying neutral carbon clusters in astronomical environments.

Infrared Spectroscopy of Disilicon-Carbide, Si2C: The ν3 Fundamental Band

The ν₃ antisymmetric stretching mode of disilicon-carbide, Si₂C, was studied using a narrow line width infrared quantum cascade laser spectrometer operating at 8.3 μm. The Si₂C molecules were produced in an Nd:YAG laser ablation source from a pure silicon sample with the addition of a few percent methane diluted in a helium buffer gas. Subsequent adiabatic expansion was used to cool the gas down to rotational temperatures of a few tens of kelvin. A total of 183 infrared transitions recorded in the spectral range between 1200 and 1220 cm^-1 were assigned to the fundamental ν₃ mode of Si₂C. In addition, pure rotational transitions of K_a = 1 and 2 between 278 and 375 GHz were recorded using a supersonic jet spectrometer for submillimeter wavelengths. Molecular parameters for the ( v₁ v₂ v₃) = (001) vibrationally excited state were derived and improved molecular parameters for the vibrational ground-state (000) were obtained from a global fit data analysis, which includes our new laboratory data and millimeter wavelength data from the literature. We found the rotational levels K_a = 0 and K_a = 2 in the vibrationally excited (001) state being perturbed by a Coriolis-type interaction with energetically close lying levels of the symmetric stretching and triple-excited bending mode (130). The data analysis was supported by quantum chemical calculations performed at the coupled-cluster level of theory. All experimental results were found to be in excellent agreement with the theory.

Theoretical characterization of C7, C7-, and C7+

We present a theoretical investigation of neutral and ionic C7 molecules. Since carbon chains present isomerism and the number of possible structures increases fast with the number of carbon atoms, a B3LYP∕aug-cc-pVTZ search of stationary points has been achieved. For C7, we found twelve minimal structures. Among these forms, eleven C7 isomers are located into the lowest singlet hyper potential energy surface. The most stable form of C7 is linear and possesses a (1)Σg(+) symmetry species. For C7(-), we characterized fifteen stable forms, where twelve are of doublet spin-multiplicity. The global minimum of C7(-) is a (2)Πg doubly degenerate Renner-Teller structure. For C7(+) cation, we found eleven doublet and three quartet isomers with a 7-atom cycle, C7(+) (X(2)A1) ground state. For the most stable forms, explicitly correlated (R)CCSD(T)-F12 calculations have been performed for the determination of equilibrium geometries and for the spectroscopic characterization of C7, C7(-), and C7(+), providing accurate rotational constants and harmonic frequencies. Vertical excitation energies to the lowest electronic states have been computed at the CASSCF∕MRCI∕aug-cc-pVTZ level. Thirty five electronic states of C7, suitable of being involved in reactive processes, lie below 7 eV. Fourteen metastable electronic states of C7(-) have been found below 3.5 eV. For linear-C7, we compute the electron affinity and the ionization energy to be 3.38 eV and 10.42 eV, respectively.

Isotopic study of new fundamentals and combination bands of linear C5, C7, and C9 in solid ar

A high yield of carbon chains has been produced by the laser ablation of carbon rods having (13)C enrichment. FTIR spectroscopy of these molecules trapped in solid Ar has resulted in the identification of two new combination bands for linear C(5) and C(9). The (ν(1) + ν(4)) combination band of linear C(5) has been observed at 3388.8 cm(-1), and comparison of (13)C isotopic shift measurements with the predictions of density functional theory calculations (DFT) at the B3LYP/cc-pVDZ level makes possible the assignment of the ν(1)(σ(g)(+)) stretching fundamental at 1946 cm(-1). Similarly, the observation of the (ν(2) + ν(7)) combination band of linear C(9) at 3471.8 cm(-1) enables the assignment of the ν(2)(σ(g)(+)) stretching fundamental at 1871 cm(-1). The third and weakest of the infrared stretching fundamentals of linear C(7), the ν(6)(σ(u)(+)) fundamental at 1100.1 cm(-1), has also been assigned.

Energetics of linear carbon chains in one-dimensional restricted environment

The energetics of even and odd linear C(n) carbon chain clusters are investigated by hybrid density functional theory (DFT) calculations. These molecular species are especially interesting due to their recent observation inside carbon nanotubes by polarized resonant Raman spectroscopy and high-resolution transmission electron microscopy (HRTEM) by different research groups. Neutral, anionic and dianionic carbon chains were studied with sizes up to n=75, although most presented calculations are limited to n<or= 24. Aggregation into longer chains is favored for neutral and anionic chains of any size. The barrier to aggregation of 2C(n)<-->C(2n) is of the order of 40-20 kcal mol(-1), which gradually decreases with increasing chain size, n. These barriers can be overcome during the high temperature synthesis or annealing conditions, but not when cooled down for the HRTEM and Raman experiments. Therefore, in addition to the already observed long chains also shorter chains should be observable under appropriate conditions inside carbon nanotubes.