Electronic Absorption Spectra of CnCl Radicals (n = 5, 6) and Their Cations in Neon Matrices

A THEORETICAL STUDY OF CARBON CHAINS OF TYPE CNF(N = 1–7) IN NEUTRAL AND IONIC FORMS

The structures, binding energies, and molecular parameters of C n F clusters (n = 1–7) have been investigated theoretically on neutral and ionic forms with the RCCSD(T)//B3LYP/aug-cc-pVTZ level of theory. It can be concluded that for the ground states the linear structures are stable only for CF , C 6 F , and C 7 F . Other neutral species of C n F (n = 2–5) are slightly bent. The variation in stability with growth of the carbon chain is flat but the ionization potential and electron affinity exhibit clear variations with the size of the clusters. Cation clusters with an even n value have triplet ground states whereas singlet ground states exist when n is odd. All anions except CF -, have singlet ground states. The alternation in stability is found in both cationic and anionic forms, but the trends are reversed. Predictions of several molecular properties may be useful in the identification of these species in the laboratory and in space.

Pure rotational spectra of the CCCF radical

Pure rotational transitions of a new carbon-chain radical CCCF in a supersonic jet have been observed for the first time using a Fourier-transform microwave spectrometer with a pulsed-discharge nozzle. The radical was produced by a pulsed electric discharge in a C(2)H(2) and CF(4) mixture diluted to 0.1% and 0.1% with Ne, respectively. Rotational transitions with spin and hyperfine splittings have been observed in the region from 9.1 GHz for N(K(a)K(c)) = 1(01)-0(00) to 27.3 GHz for N(K(a)K(c)) = 3(03)-2(02). The rotational constant, the spin-rotation interaction constant, and the hyperfine coupling constants due to the F nucleus have been precisely determined from the least-squares analysis, yielding B = 4555.8043(44), gamma(eff) = -7.105(16), b(F,eff) = 368(19), and c(eff) = -284.832(61) MHz. The determined molecular constants were compared with those obtained from high-level ab initio calculations and concluded that the CCCF radical has a bent ground state X(2)A(').

Fourier-transform microwave spectroscopy of the CCCCl radical

Pure rotational spectra of the CCCCl radical in a supersonic jet have been observed for the first time by Fourier-transform microwave spectroscopy. The radical was produced by a pulsed electric discharge in a C(2)H(2) and CCl(4) mixture diluted to 0.3% and 0.2% with Ne, respectively. Transitions with spin and hyperfine splittings were observed for two isotopologs, CCC(35)Cl and CCC(37)Cl, in the region from 11.4 GHz for N=2-1 to 34.2 GHz for N=6-5. The molecular constants including the hyperfine coupling constants due to the Cl nucleus have been determined precisely. From the rotational analyses and high-level ab initio calculations, the molecular structure of the CCCCl radical is concluded to be bent in the ground electronic state.

C4Cl: Bent or linear?

The ground state structure for the CCCCCl radical was computed by using symmetry-adapted cluster configuration-interaction (SAC-CI) theory along with density functional theory to overcome the differences raised in the recently published paper [Y. Sumiyoshi et al., Chem. Phys. Lett. 414, 82 (2005)] between the theory and the experiment. SAC-CI results clearly support the earlier experimental conclusion that the radical has the bent ground state structure corresponding to 2Pi symmetry. Contrarily, probably due to spin contamination, mixing of a bent doublet ground state with the quartet components of a linear structure, coupled-cluster singles and doubles (CCSD) calculations were unable to provide reliable results. Results obtained using density functional theory also show that the radical has a bent structure. Some low-lying doublet excited states were also studied using the SAC-CI theory. The energy difference between the ground Pi state and the nearby Sigma state is around 0.2 eV. The excitation energy for the transition with the largest oscillator strength agrees with the strongest absorption peak.

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.

Electronic absorption spectra of linear and cyclic C(n)(+) n=7-9 in a neon matrix

The C(n)(+) n=7-9 cations were produced by electron-impact ionization of perchloronaphthalene, mass selected, and their electronic absorption spectra in 6 K neon matrices recorded. The linear and cyclic isomers of C7(+) and C8(+) are detected. Three systems of linear C7(+) are observed with origin bands near 770, 332, and 309 nm. The cyclic C7(+) shows two transitions near 676 and 448 nm. One system of linear C9(+) is observed commencing at 371 nm. Linear C8(+) shows five dipole-allowed electronic transitions from the X 2pi(g) ground state, and the strongest ones have the origin bands at 890.8 and 308.1 nm. Five electronic transitions of cyclic C8(+) are also discernible.

Ab initioprediction of the infrared-absorption spectrum of the C2Cl radical

The three lowest (1(2)A', 2(2)A', and 1(2)A") potential-energy surfaces of the C2Cl radical, correlating at linear geometries with 2Sigma+ and 2Pi states, have been studied ab initio using a large basis set and multireference configuration-interaction techniques. The electronic ground state is confirmed to be bent with a very low barrier to linearity, due to the strong nonadiabatic electronic interactions taking place in this system. The rovibronic energy levels of the 12C12C35Cl isotopomer and the absolute absorption intensities at a temperature of 5 K have been calculated, to an upper limit of 2000 cm(-1), using diabatic potential-energy and dipole moment surfaces and a recently developed variational method. The resulting vibronic states arise from a strong mixture of all the three electronic components and their assignments are intrinsically ambiguous.

Electronic absorption spectra of C3Cl, C4Cl, and their ions in neon matrices

Electronic absorption spectra of C3Cl, C3Cl+, C3Cl-, C4Cl, and C4Cl+ have been recorded in 6 K neon matrices following mass selection. Ab initio calculations were performed (CCSD(T) and CASSCF) to identify the ground and accessible excited states of each molecule. The estimated excitation energies and transition moments aid the assignment. The absorptions observed for C3Cl are the 5(2)A' <-- X(2)A' and 3(2)A'' <-- X(2)A' transitions of the bent isomer and the (2)A1 <-- X(2)B2 transition of the cyclic form in the UV (336.1 nm), visible (428.7 nm), and near-IR (1047 nm) regions, respectively. The band systems for bent C3Cl- (435.2 nm) and linear C3Cl+ (413.2 nm) are both in the visible region and correspond to 2(1)A'' <-- X(1)A' and (1)pi <-- X(1)sigma+ type transitions. The C4Cl and C4Cl+ chains are linear, and the band origins of the 2(2)pi <-- X(2)pi and 2(3)pi <-- X(3)pi electronic transitions are at 427.0 and 405.7 nm. The spectral assignments are supported by analysis of the vibrational structure associated with each electronic transition.