Rotational spectrum and carbon-13 hyperfine structure of the C3H, C5H, C6H, and C7H radicals

Vibrational satellites of C2S, C3S, and C4S: microwave spectral taxonomy as a stepping stone to the millimeter-wave band

We present a microwave spectral taxonomy study of several hydrocarbon/CS₂ discharge mixtures, in which more than 60 distinct species/vibrational states were detected and analyzed.

Symmetry breaking and spectral considerations of the surprisingly floppy c-C3H radical and the related dipole-bound excited state of c-C3H. J Chem Phys 2017;146:224303. [PMID: 29166048 DOI: 10.1063/1.4985095]

The C₃H radical is believed to be prevalent throughout the interstellar medium and may be involved in the formation of polycyclic aromatic hydrocarbons. C₃H exists as both a linear and a cyclic isomer. The C_2v cyclopropenylidenyl radical isomer was detected in the dark molecular cloud TMC-1, and the linear propenylidenyl radical isomer has been observed in various dark molecular clouds. Even though the c-C₃H radical has been classified rotationally, the vibrational frequencies of this seemingly important interstellar molecule have never been directly observed. Established, highly accurate quartic force field methodologies are employed here to compute useful geometrical data, spectroscopic constants, and vibrational frequencies. The computed rotational constants are consistent with the experimental results. Consequently, the three a₁ (ν₁, ν₂, and ν₃) and one b₁ (ν₆) anharmonic vibrational frequencies at 3117.7 cm^-1, 1564.3 cm^-1, 1198.5 cm^-1, and 826.7 cm^-1, respectively, are reliable predictions for these, as of yet unseen, observables. Unfortunately, the two b₂ fundamentals (ν₄ and ν₅) cannot be treated adequately in the current approach due to a flat and possible double-well potential described in detail herein. The dipole-bound excited state of the anion suffers from the same issues and may not even be bound. However, the trusted fundamental vibrational frequencies described for the neutral radical should not be affected by this deformity and are the first robustly produced for c-C₃H. The insights gained here will also be applicable to other structures containing three-membered bare and exposed carbon rings that are surprisingly floppy in nature.

Electronic transitions of C5H(+) and C5H: neon matrix and CASPT2 studies

Two electronic transitions at 512.3 and 250 nm of linear-C5H(+) are detected following mass-selective deposition of m/z = 61 cations into a 6 K neon matrix and assigned to the 1 (1)Π←X (1)Σ(+) and 1 (1)Σ(+)←X (1)Σ(+) systems. Five absorption systems of l-C5H with origin bands at 528,7, 482.6, 429.0, 368.5, and 326.8 nm are observed after neutralization of the cations in the matrix and identified as transitions from the X (2)Π to 1 (2)Δ, 1 (2)Σ (-), 1 (2)Σ(+), 2 (2)Π, and 3 (2)Π electronic states. The assignment to specific structures is based on calculated excitation energies, vibrational frequencies in the electronic states, along with simulated Franck-Condon profiles.

A crossed molecular beam study on the synthesis of the interstellar 2,4-pentadiynylidyne radical (HCCCCC)

Crossed molecular beam experiments are performed to elucidate the synthesis of the 2,4-penta-diynylidyne [HCCCCC(X (2)Pi)] radical under single collision conditions--a crucial reaction intermediate to form polycyclic aromatic hydrocarbons and carbonaceous nanostructures in the interstellar medium and in combustion flames. The experiments demonstrate that the chemical dynamics of ground state carbon reacting with diacetylene [HCCCCH(X (1)Sigma(g)(+))] are indirect and proceed via addition of the electrophilic carbon atom to the pi electron density of the diacetylene molecule yielding ultimately the carbenelike HCCCCCH(X (3)Sigma(g)(-)) molecule. This intermediate fragments via hydrogen atom emission to yield the 2,4-pentadiynylidyne [HCCCCC(X (2)Pi)] radical. The chemical dynamics elucidated also allows us to predict that reaction of carbon atoms with polyynes of the generic formula H(C[triple bond]C)(n)H leads to the formation of hydrogen-terminated carbon clusters of the generic form HC(2n+1) in extreme environments. The acetylene-related reactivity and electronic structure of the diacetylene molecule also allow us to project that reactions of the diacetylene molecule with cyano and ethynyl radicals result in a stepwise extension of the carbon skeleton forming cyanodiacetylene (HCCCCCN) and triacetylene (HCCCCCCH) plus atomic hydrogen. These predictions open the door to extensive laboratory studies involving hitherto poorly understood reactions of the diacetylene molecule under single collision conditions.

Rotational spectra of the carbon-chain radicals HC5O, HC6O, and HC7O

Three new free carbon-chain radicals, HC5O, HC6O, and HC7O, and their deuterated isotopic species have been observed by Fourier transform microwave spectroscopy of a supersonic molecular beam. In contrast to the shorter HCnO radicals, these all have linear heavy-atom backbones and 2Pi electronic ground states. Like the isovalent HCnS radicals, the ground states of the HCnO radicals alternate with odd and even numbers of carbon atoms: those of HC5O and HC7O are 2Pi1/2 and that of HC6O is 2Pi3/2. From frequency measurements between 6 and 26 GHz, the rotational constant B, the centrifugal distortion constant D, and the lambda-type doubling and magnetic hyperfine constants have been determined to high precision for each chain. Predicted properties from coupled-cluster calculations are also reported for chains up to HC9O. The production of HCnO radicals for n even was highly favored when O2 was used as the source of oxygen, but those with n odd were best produced with CO.