Infrared Absorption Spectroscopy of Small Carbon−Sulfur Clusters Isolated in Solid Ar

Photochemistry of XCH2CN (X = -Cl, -SH) in Argon Matrices

We report infrared spectra and photochemical behavior of the potentially astrochemically significant species, mercaptoacetonitrile (HS-CH₂C≡N) and, for comparison purposes, chloroacetonitrile (Cl-CH₂C≡N), both suspended in an argon matrix at 6 K. Photolytic formation of the isocyano products HS-CH₂-NC and Cl-CH₂-NC were observed as well as CH₃NSC and CH₃SCN (in HS-CH₂CN photolysis). While no dissociation products were observed for Cl-CH₂-CN, photolysis of HS-CH₂-CN produced compounds necessitating the loss of the CN group to form CH₂═S, the SH group to form H₂C-CN and HC-CN, or both CN and SH to form CH₃ and CH₄. Observation of emission spectra upon annealing indicates the presence of free sulfur atom in matrices of photolyzed HS-CH₂-CN.

Photoelectron spectroscopy and density functional calculations of C(n)S(m)(-) (n = 2-7; m = 1, 2) clusters

CnSm(-) (n = 2-7; m = 1, 2) clusters were investigated by using photoelectron spectroscopy combined with density functional theory calculations. We found that the vertical detachment energies of both CnS(-) and CnS2(-) (n = 2-7) clusters exhibit a strong odd-even alternation with an increasing number of carbon atoms: the VDEs of even-n clusters are higher than those of adjacent odd-n clusters. The most stable structures of the anionic and neutral CnS (n = 2-7) clusters are linear with the S atom locating at one end of the carbon chain except that the structure of C3S(-) is slightly bent. The ground state isomers of the anionic and neutral CnS2 (n = 2-7) clusters are all linear structures with two S atoms locating at two ends of the carbon chain. The electron affinities of the neutral CnS (n = 2, 4-7) and CnS2 (n = 2-7) clusters are determined based on the experimental adiabatic detachment energies of the corresponding anion species, because the most stable structures of the neutral clusters are similar to those of the corresponding anions.

Electronic spectra of linear isoelectronic clusters C2n+1S and C2n+1Cl+ (n = 0-4): an ab initio study

Structures and stabilities of linear carbon chains C2n+1S and C2n+1Cl+ (n=0-4) in their ground states have been investigated by the CCSD and B3LYP approaches. The CASSCF calculations have been used to determine geometries of selected excited states of both isoelectronic series. Linear C2n+1S cluster has a cumulenic carbon framework, whereas its isoelectronic C2n+1Cl+ has a dominant character of acetylenic structure in the vicinity of terminal Cl. The vertical excitation energies of low-lying excited states have been calculated by the CASPT2 method. Calculations show that the excitation energies have nonlinear size dependence. The 2(1)Sigma+<--X1Sigma+ transition energy in C2n+1S has a limit of 1.78 eV, as the chain size is long enough. The predicted vertical excitation energies for relatively strong 1(1)Pi<--X1Sigma+ and 2(1)Sigma+<--X1Sigma+ transitions are in reasonable agreement with available experimental values. The spin-orbit effect on the spin-forbidden transition in both series is generally small, and the enhancement of the spin-forbidden transition by spin-orbit coupling exhibits geometrical and electronic structural dependence.

Electronic spectra of heteroatom-containing isoelectronic carbon chains C2nS and C2nCl+ (n=1–5)

Structures and stabilities of carbon chains C(2n)S and C2(n)Cl+ (n=1-5) in their ground states have been investigated by the density functional theory and the coupled cluster approach using single and double substitutions. The complete active space self-consistent-field method has been used for geometry optimization of selected excited states in both series. Calculations show that both C(2n)S (n=1-5) and C2(n)Cl+ (n=3-5) have linear structures in the triplet ground state 3Sigma-, while C2Cl+ and C4Cl+ have nonlinear structures in the ground state 3A". The vertical transition energies and emission energies by the multiconfigurational second-order perturbation theory in linear clusters C(2n)S and C2(n)Cl+ exhibit similar size dependences. In comparison with the available experimental observations, the predicted excitation energies for the allowed 2 3Sigma- <--X 3Sigma- transitions have an accuracy of no more than 0.24 eV. Spin-orbit coupling configuration interaction calculations indicate that the spin-forbidden 2 1Sigma+<--X 3Sigma- transition in these species has an oscillator strength with the magnitude of 10(-4)-10(-5), and they may be observable experimentally.

Ring Opening of 2,5-Didehydrothiophene: Structures and Rearrangements of C4H2S Isomers

Electronic structures and rearrangement pathways of several C4H2S isomers are computationally investigated by methods based on coupled cluster theory and density functional theory. Six singlet C4H2S isomers lie within ca. 30 kcal/mol above butatrienethione (6), the apparent global minimum. Ethynylthioketene (7) lies only 2 kcal/mol higher in energy than cumulene 6. Two open-chain isomers, butadiynylthiol (8) and diethynyl sulfide (9), reside ca. 9 and 24 kcal/mol above 6, respectively. Lying 30 kcal/mol above 6, two cyclic singlet isomers, ethynylthiirene (10) and cyclopropenylidenemethanthione (11), are nearly degenerate in energy. Thiophene-2,5-diyl (12) lies substantially higher in energy than 6 (ca. 45 kcal/mol) and is predicted to rearrange preferentially by C-S bond cleavage, leading to thioketene 7, rather than by C-C bond cleavage, leading to diethynyl sulfide (9; retro-Bergman cyclization). Accurate spectroscopic properties of these C4H2S isomers, as well as an understanding of their rearrangement pathways, should facilitate the detection and characterization of these isomers in the laboratory and the interstellar medium.