HCCS Radical Research Articles

A theoretical investigation on the HCCS radical and its ions

The thioketenyl (HCCS) radical and its related cation, HCCS +, and anion, HCCS −, have been investigated at a high level of accuracy. The singles and doubles coupled-cluster method including a perturbational correction for connected triple excitations, CCSD(T), in conjunction with correlation consistent basis sets ranging in size from quadruple to sextuple zeta have been employed. Extrapolation to the complete basis set limit has been used with accurate treatments of core-valence correlation effects to accurately predict equilibrium structures, dipole moments as well as ionization potential and electron affinity. For all the species studied, harmonic vibrational frequencies have also been evaluated to obtain zero-point corrections to energies.

An ab initio model for handling the Renner–Teller effect in tetra-atomic molecules. II. Study of the crossing of potential surfaces

In the present paper, we show that the Renner–Teller effect in tetra-atomic molecules is always accompanied by (avoided) crossings of potential surfaces corresponding to the electronic states that correlate with a spatially degenerate species of linear molecule. We restrict ourselves to Π electronic states and the lowest-order (harmonic) approximation. It is demonstrated that the simple model we employ, implicitly involves a diabatic transformation of the electronic states, which makes possible to remove the difficulties caused by the breakdown of the Born–Oppenheimer approximation at nonlinear as well as linear nuclear arrangements. The reliability of the model is illustrated on example of the X 2Π electronic state of the HCCS radical. Numerical computations are carried out employing the data obtained in extensive ab initio calculations.

An ab initio study of the hyperfine structure in the X2 Π electronic state of HCCS–calculation of vibronically averaged components of the anizotropic hyperfine tensor

The results of ab initio calculations of the vibronically averaged components of the anisotropic magnetic hyperfine tensor in the low-lying vibronic species of the X2Π electronic state of the HCCS radical are reported. The electronically averaged hyperfine coupling constants for hydrogen, deuterium, 13C and 33S are obtained as functions of two bending vibrational modes by the density functional theory method. The vibronic wave functions are calculated with the help of a variational approach which takes into account the Renner–Teller effect and spin-orbit coupling. The results of ab initio calculations are compared to the corresponding experimental findings.

Is the HCCS radical linear in the excited state?

The A (2)Pi-X (2)Pi 415 nm band system of the linear HCCS radical has been known since 1978, but the vibronic structure in this complex spectrum, which has both spin-orbit and Renner-Teller complications, has never been satisfactorily assigned, despite serious experimental and theoretical efforts. In a further attempt to understand the spectrum, we have studied the laser-induced fluorescence spectra of jet-cooled HCCS and DCCS, produced from thiophene precursors using the discharge jet technique. The 0(0) (0) bands of HCCS and DCCS have been rotationally analyzed, providing precise ground and excited state spin-orbit splittings. The energy levels of the v(')=0 (2)Pi(3/2) component of DCCS are found to be perturbed by a very low-lying (2)Sigma vibronic level, indicating that the HCC bending mode Renner-Teller effect is much larger than predicted by ab initio calculations with a linear excited state geometry. With this observation, the vibronic bands in the spectra of both isotopomers have been consistently assigned for the first time. Model calculations show that the large Renner-Teller effect and substantially different HCCS and DCCS excited state zero-point spin-orbit splittings can be explained with the assumption of a quasilinear excited state geometry.

Ab initio study of the A 2Π–X 2Π electronic transition in HCCS

Potential energy surfaces for the electronic states of the HCCS radical correlating at linear nuclear arrangement with the A 2Π state are calculated by means of an extensive ab initio approach. They are used to compute the vibronic and spin-orbit structure of the A 2Π–X 2Π electronic transition. These calculations are carried out by means of a new variational approach based on the use of normal bending coordinates. The results of calculations question various interpretations of the available experimental data; on the other hand they do not offer reliable explanation of all features observed, pointing in this way at the shortages of the present, as well as of previous theoretical handling of the problem in question.

Hyperfine Interaction Constants of the HCCS and DCCS Radicals Studied by Fourier Transform Millimeter-Wave Spectroscopy

The rotational spectral lines of HCCS and DCCS have been observed with a Fourier transform millimeter-wave spectrometer in combination with a pulsed discharge nozzle. The HCCS radical is produced by discharging a mixture of C2H2 and CS2 diluted in Ar. The DCCS radical is produced by using C2D2 instead of C2H2. The spectral lines of HCCS and DCCS in both the 2Π3/2 and 2Π1/2 states are measured in the frequency range from 16 to 48 GHz, and the molecular constants are determined accurately from a joint least-squares analysis with the reported millimeter- and submillimeter-wave data. The hyperfine interaction constants of the hydrogen and deuterium nuclei are determined for the first time, and are discussed in relation to the Renner–Teller effect on this molecule.

Ab initio study of the vibronic spectrum in the X 2Π electronic state of HCCS

Potential energy surfaces for the electronic states of the HCCS radical correlating at linear nuclear arrangement with the X 2Π state are calculated by means of an extensive ab initio approach. Particular attention is paid to calculating accurate three-dimensional potential surfaces involving variations of two bending and torsional coordinates, which play the central role in vibronic interactions (Renner–Teller effect), determining the structure of spectra of this radical. In the second part of this paper we use these potential surfaces and the ab initio computed spin–orbit coupling constant to calculate vibronic spectra of HCCS and DCCS in the framework of a theoretical model developed in our laboratory. The results of the present study are in excellent agreement with those derived by Tang and Saito [J. Chem. Phys. 105, 8020 (1996)] and thus strongly support the interpretation of their experimental findings.

Theoretical prediction of the spin-orbit splitting in the NCO, NCS, HCCO and HCCS radicals

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Structure and spectra of the thioketenyl (HCCS) radical in its ground and first excited states obtained by ab initio coupled-cluster methods

There is recent interest in finding the HCCS radical in interstellar sources and a detailed investigation of its UV spectrum has also been reported. By using state of the art coupled-cluster (CC) based ab initio methods, such as coupled-cluster singles and doubles (CCSD) with approximate triple excitation correction (CCSD(T)), the equation-of-motion CC method for excited states (EOMEE-CC) and for ionization potential (EOMIP-CC) together with large basis sets, we present the structure, harmonic vibrational frequencies and dipole moment of the ground and first excited 2Π states. We confirm the experimental rotational constant and most of the assignment made on the vibrational structure of the UV spectrum, but we also suggest reassignments in some cases. Special attention is paid to the Renner–Teller splitting present for such 2Π states.

Laser-induced fluorescence spectra and the observation of quantum beats in the [formula omitted] transition of the HCCS radical

LIF spectra of the A ̃ 2Π i − X ̃ 2Π i transition of the HCCS radical were observed in a supersonic jet generated by using a pulsed-discharge nozzle. Adiabatic cooling yielded definitive rotational assignments for ≈ 40 vibronic bands. The fluorescence from single rovibronic levels exhibited a quantum beat in most of the observed bands. The number of beat components increased gradually with internal energy, approaching the limiting case for a ‘large molecule’. The observed quantum interference is ascribed, based on the estimated densities of background states, to the internal conversion coupling with highly excited vibrational levels in the X ̃ 2Π i state.

A computational study of the HCCO and HCCS radicals

The equilibrium geometries of the lowest electronic states of the HCCO and HCCS radicals have been determined by ab initio SCF and Møller-Plesset perturbation theory calculations using split-valence plus polarization basis sets. When optimized at a correlated level the ground state, 2A″, of HCCO is bent with a linear 2Π( 2A′) excited state forming the other half of a Renner-Teller pair. In contrast, ground state HCCS is predicted to be linear at all levels of calculation considered. The UMP3/6-31 G* predictions for the geometries of HCCO are: 2A″, r e(CH) = 1.074 Å, r e(CC) = 1.301 Å, r e(CO) = 1.170 Å, α e(CCH) = 133.23°, α e(CCO) = 169.96°; and 2Π( 2A′), r e(CH) = 1.064 Å, r e(CC) = 1.264 Å and r e(CO) = 1.181 Å. The UMP3/6-3 IG* geometry optimization for HCCS predicted: 2Π, r e(CH) = 1.066 Å, r e(CC) = 1.212 Å, and r e(CS) = 1.634 Å.

Emission spectrum of the HCCS radical

Emission bands obtained in the region 3900–4500 Å by a radio-frequency discharge in thiophene are studied; they are assigned to the HCCS transient species whose absorption spectrum was recently observed. A rotational analysis is carried out for the strong bands, which are assigned to a 2Πi–2Πi transition in which both states are close to Hund's case (a). Vibrational assignments are discussed.

Theoretical study of the HCCS radical

We report a preliminary ab initio study of the ground state of the HCCS radical. The results are discussed in relation to a transient absorption spectrum observed in the flash photolysis of thiophene.

The 4114 Å absorption system of the HCCS radical

The transient absorption spectrum observed in the region 3770–4170 A during the flash photolysis of thiophene is assigned to the HCCS free radical. Isotope shifts are observed when deuterated thiophene is used and establish the presence of a single H (or D) atom in the carrier. Under high resolution the bands show doublet P and R branches. The bands are tentatively assigned to a 2Π(b)–2Π(b) transition of a linear molecule, though the possibility that the molecule is slightly nonlinear in one of the combining states is not excluded. Rotational assignments are given for the two strongest bands of HCCS, including the O–O band near 4114 A. The rotational constants for this band are B″0= 0.188 41(6) cm–1 and B′0–B″0=–0.014 399(6) cm–1, where the error limits are 3σ. (See, however, note added in proof.)