Electronic spectra of the C2n+1H (n=2–4) radicals in the gas phase

Si2C5H2 isomers - search algorithms versus chemical intuition

The pros and cons of using search algorithms alone in identifying new geometries have been discussed by using the Si₂C₅H₂ elemental composition as an example. Within 30 kcal mol^-1 at the CCSD(T)/def2-TZVP//PBE0/def2-TZVP level of theory, the coalescence kick and cuckoo methods postulate merely four isomers (1, 3, 6, and 7) for Si₂C₅H₂ (O. Yañez et. al., Chem. Commun., 2017, 53, 12112). On the contrary, chemical intuition yields fourteen (2, 4, 5, and 8-18) new isomers within the same energy range at the B3LYP/6-311++G(2d,2p) level of theory. Based on the relative energies of the first eleven isomers of Si₂C₅H₂ (1, C_2v, 0.00; 2, C_s, 21.39; 3, C_s, 21.95; 4, C_s, 22.76; 5, C_s, 24.74; 6, C_s, 25.34; 7, C_s, 25.64; 8, C_s, 25.79; 9, C_s, 27.20; 10, C_2v, 28.59; and 11, C_2v, 29.16 kcal mol^-1) calculated at the CCSD(T)/cc-pVTZ level of theory, it is evident that the search algorithms had missed at least seven isomers in the same energy range. The relative energy gaps of isomers 12-18 fall in the range of 30-40 kcal mol^-1 at the latter level of theory. Consequentially, this scenario triggers a speculation going forward with search algorithms alone in the search of all new isomers. While one cannot underestimate the power of these algorithms, the role of chemical intuition may not be completely neglected. Retrospectively, the fourteen new isomers found by chemical intuition may help in writing better search algorithms. All eighteen isomers - including the most stable isomer with a planar tetracoordinate carbon atom 1- remain elusive in the laboratory to date. Thus, structural and spectroscopic parameters have been presented here, which may possibly aid the future experimental studies.

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.

Optical spectra of the silicon-terminated carbon chain radicals SiCnH (n = 3,4,5)

The gas-phase optical spectra of three silicon-terminated carbon chain radicals, SiCnH (n = 3 - 5), formed in a jet-cooled discharge of silane and acetylene, have been investigated by resonant two-color two-photon ionization and laser-induced fluorescence/dispersed fluorescence. Analysis of the spectra was facilitated by calculations performed using equation-of-motion coupled cluster methods. For SiC3H and SiC5H, the observed transitions are well-described as excitations from a (2)Π ground state to a (2)Σ state, in which vibronic coupling, likely involving a higher-lying Π state with a very large predicted f-value (close to unity), is persistent. The lowest (2)Σ states of both species are characterized by a rare silicon triple bond, which was identified previously [T. C. Smith, H. Y. Li, D. J. Clouthier, C. T. Kingston, and A. J. Merer, J. Chem. Phys. 112, 3662 (2000)] in the lowest (2)Σ state of SiCH. Although a strong Π - Π transition is predicted for SiC4H, the observed spectrum near 505 nm more likely corresponds to excitation to a relatively dark Σ state which is vibronically coupled to a nearby Π state. In contrast to the chains with an odd number of carbon atoms, which exhibit relatively sharp spectral features and lifetimes in the 10-100 ns range, SiC4H shows intrinsically broadened spectral features consistent with a ∼100 fs lifetime, and a subsequent long-lived decay (>50 μs) which we ascribe to mixing with a nearby quartet state arising from the same electronic configuration. The spin-orbit coupling constants for both SiC3H and SiC5H radicals were determined to be approximately 64 cm(-1), similar to that of SiCH (69.8 cm(-1)), suggesting that the unpaired electron in these species is localized on the silicon atom. Motivated by the new optical work, the rotational spectrum of linear SiC3H was detected by cavity Fourier-transform microwave spectroscopy in the 13-34 GHz range. Each rotational transition from the [Formula: see text] ground state exhibits well-resolved Λ-doubling and hyperfine structure; the derived rotational constant of B = 2.605 GHz is in excellent agreement with our calculations.

Theoretical studies on structures and electronic spectra of linear free radicals CnH (n=5-12)

The B3LYP, CAM-B3LYP, and coupled cluster CCSD(T) calculations have been utilized to determine the equilibrium structures of linear carbon radicals CnH (n=5-12) in their ground states, as well as the CASSCF method used to optimize the ground and selected low-lying excited states. DFT-calculations show that even-n radicals C2nH have polyacetylene-like structures with significant single-triple bond length alternation, whereas the odd-numbered analogues C2n+1H exhibit a trend from polyacetylene-like characters into cumulenic-like arrangement towards C ends along the carbon chains. The stabilities of the system under study have been evaluated by analyses of the vibrational frequencies and incremental binding energies. For the whole CnH (n=5-12) series, the vertical excitation energies and oscillator strengths have been calculated at the CASPT2/cc-pVTZ level of theory. At the B3LYP optimized geometries, the lowest 1(2)Δ←X2Π transitions for C5H and C7H occur at 2.36 and 2.14 eV, respectively, comparing well with the observed values of 2.33 and 2.09 eV. Moreover, the strongest 2(2)Π←X2Π transitions for C2nH (n=3-6) are predicted to be at 2.39, 2.00, 1.80, and 1.64 eV, respectively, which are in agreement with the experimental observations. Additionally, the possible dissociation channels and the fragmentation energies of CnH (n=5-12) series are discussed in the paper.

The electronic spectrum of the C(s)-C11H3 radical

The electronic gas-phase absorption spectrum of the bent carbon-chain radical, HC(4)CHC(6)H with C(s) symmetry, is recorded in the 595 nm region by cavity ring-down spectroscopy through an expanding hydrogen plasma. An unambiguous spectroscopic identification becomes possible from a systematic deuterium labeling experiment. A comparison of the results with recently reported spectra of the nonlinear HC(4)CHC(4)H and HC(4)C(C(2)H)C(4)H radicals with C(2v) symmetry provides a more comprehensive understanding of the molecular behavior of π-conjugated bent carbon-chain systems upon electronic excitation. We find that the electronic excitation in the bent carbon-chain HC(4)CHC(2n)H (n = 1-4) series exhibits a similar trend as in the linear HC(2n+1)H (n = 3-6) series, shifting optical absorptions towards longer wavelengths for increasing overall bent chain lengths. The π-conjugation in bent HC(4)CHC(2n)H (n = 1-4) chains is found to be generally smaller than in the linear HC(2n+1)H (n = 3-6) case for equivalent numbers of C-atoms. The addition of an electron-donating group to the bent chain causes a slight decrease of the effective conjugation.

Structure determination of the nonlinear hydrocarbon chains C9H3 and C11H3 by deuterium labeling

A systematic deuterium labeling experiment is presented that aims at an unambiguous determination of the geometrical ground state structure of the C(9)H(3) and C(11)H(3) hydrocarbon chains. Cavity ring-down spectroscopy and special plasma expansions constituting C/H, C/D, and C/H/D are used to record optical transitions of both species and their (partially) deuterated equivalents in the 19,000 cm(-1) region. The number of observed bands, the quantitative determination of isotopic shifts, and supporting calculations show that the observed C(9)H(3) and C(11)H(3) spectra originate from HC(4)(CH)C(4)H and HC(4)[C(C(2)H)]C(4)H species with C(2v) symmetry. This result illustrates the potential of deuterium labeling as a useful approach to characterize the molecular structure of nonlinear hydrocarbon chains.

Electronic Spectroscopy of Carbon Chains

Investigators have recorded the electronic spectra of assorted carbon-chain systems in the gas phase using a variety of methods, ranging from direct cavity ringdown absorption spectroscopy to photofragmentation techniques that utilize the cooling capabilities of an ion trap. We summarize the results from these studies and compare them with astronomical measurements of the diffuse interstellar band (DIB) absorptions. Although carbon chains comprising up to a handful of carbon atoms cannot be the carrier species, we explore which chains remain viable. In particular, the 1Σu+–X1Σg+ transitions of the odd-numbered carbon chains (n = 17,19,…) possess large oscillator strengths and lie in the 400–900-nm DIB range. The origin bands of larger bare carbon rings, such as C18, have also been observed, with striking similarities to some DIB measurements at high resolution, although at other wavelengths. Finally, we consider recently obtained electronic spectra of metal-containing carbon chains.

The Reaction of Tricarbon with Acetylene: An Ab Initio/RRKM Study of the Potential Energy Surface and Product Branching Ratios

Ab initio calculations of the potential energy surface for the C3(1Sigmag+)+C2H2(1Sigmag+) reaction have been performed at the RCCSD(T)/cc-pVQZ//B3LYP/6-311G(d,p) + ZPE[B3LYP/6-311G(d,p)] level with extrapolation to the complete basis set limit for key intermediates and products. These calculations have been followed by statistical calculations of reaction rate constants and product branching ratios. The results show the reaction to begin with the formation of the 3-(didehydrovinylidene)cyclopropene intermediate i1 or five-member ring isomer i7 with the entrance barriers of 7.6 and 13.8 kcal/mol, respectively. i1 rearranges to the other C5H2 isomers, including ethynylpropadienylidene i2, singlet pentadiynylidene i3, pentatetraenylidene i4, ethynylcyclopropenylidene i5, and four- and five-member ring structures i6, i7, and i8 by ring-closure and ring-opening processes and hydrogen migrations. i2, i3, and i4 lose a hydrogen atom to produce the most stable linear isomer of C5H with the overall reaction endothermicity of approximately 24 kcal/mol. H elimination from i5 leads to the formation of the cyclic C5H isomer, HC2C3, +H, 27 kcal/ mol above C3+C2H2. 1,1-H2 loss from i4 results in the linear pentacarbon C5+H2 products endothermic by 4 kcal/mol. The H elimination pathways occur without exit barriers, whereas the H2 loss from i4 proceeds via a tight transition state 26.4 kcal/mol above the reactants. The characteristic energy threshold for the reaction under single collision conditions is predicted be in the range of approximately 24 kcal/mol. Product branching ratios obtained by solving kinetic equations with individual rate constants calculated using RRKM and VTST theories for collision energies between 25 and 35 kcal/mol show that l-C5H+H are the dominant reaction products, whereas HC2C3+H and l-C5+H2 are minor products with branching ratios not exceeding 2.5% and 0.7%, respectively. The ethynylcyclopropenylidene isomer i5 is calculated to be the most stable C5H2 species, more favorable than triplet pentadiynylidene i3t by approximately 2 kcal/mol.

Gas-phase electronic spectra of C18 and C22 rings

The electronic spectra of C(18) and C(22) in the 15 150-36 900 cm(-1) range have been detected in the gas phase by a mass-selective resonant two-color two-photon ionization technique coupled to a laser ablation source. The spectra were assigned to several electronic systems of monocyclic cumulenic isomers with a D(9 h) symmetry for C(18) and D(11 h) for C(22), based on time-dependent-density-functional calculations and reactivity with respect to H(2). The best cooling conditions were achieved with Kr as the buffer gas, and the origin of the A(1)A(2) (")<--X(1)A(1) (') transition of C(18) at 592.89 nm shows a pair of 1 cm(-1) broadbands spaced by 1.5 cm(-1). The next electronic transitions exhibited much broader, approximately 30 (in the visible) to 200 cm(-1) (in ultraviolet range), features. The spectrum of C(22) exhibits an absorption pattern similar to C(18), except that the narrow features to the red are missing; the oscillator strength of the A<--X transition is predicted to be low.

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

By means of Fourier transform microwave spectroscopy of a supersonic molecular beam, we have detected the singly substituted carbon-13 isotopic species of C(5)H, C(6)H, and C(7)H. Hyperfine structure in the rotational transitions of the lowest-energy fine structure component ((2)Pi(12) for C(5)H and C(7)H, and (2)Pi(32) for C(6)H) of each species was measured between 6 and 22 GHz, and precise rotational, centrifugal distortion, Lambda-doubling, and (13)C hyperfine coupling constants were determined. In addition, resolved hyperfine structure in the lowest rotational transition (J = 32-->12) of the three (13)C isotopic species of C(3)H was measured by the same technique. By combining the centimeter-wave measurements here with previous millimeter-wave data, a complete set of (13)C hyperfine coupling constants were derived to high precision for each isotopic species. Experimental structures (r(0)) have been determined for C(5)H and the two longer carbon-chain radicals, and these are found to be in good agreement with the predictions of high-level coupled-cluster calculations. C(3)H, C(5)H, and C(7)H exhibit a clear alternation in the magnitude and sign of the (13)C hyperfine coupling constants along the carbon-chain backbone. Because the electron spin density is nominally zero at the central carbon atom of C(3)H, C(5)H, and C(7)H, and at alternating sets of carbon atoms of C(5)H and C(7)H, owing to spin polarization, almost all of the (13)C coupling constants at these atoms are small in magnitude and negative in sign. Spin-polarization effects are known to be important for the Fermi-contact (b(F)) term, but prior to the work here they have generally been neglected for the hyperfine terms a, c, and d.

The Optical Spectroscopy of Extraterrestrial Molecules

The ongoing quest to identify molecules in the interstellar medium by their electronic spectra in the visible region is reviewed. Identification of molecular absorption is described in the context of the elucidation of the carriers of the unidentified Diffuse Interstellar Bands, and molecular emission is discussed with reference to the unidentified Red Rectangle bands. The experimental techniques employed in undertaking studies on the optical spectroscopy of extraterrestrial molecules are described and critiqued in the context of their application.

Isomeric Structures and Visible Electronic Spectrum of the C7H3 Radicals

The 2-(buta-1,3-diynyl)cycloprop-2-yl-1-ylidene radical, a new three-membered ring chain with Cs symmetry, has been detected by electronic spectroscopy in the gas phase. The experimental investigation used a mass selective resonant two color two photon ionization technique coupled to a supersonic plasma source. Structures and relative stability energies of eight isomers of the C7H3 radical have been calculated. Based on the rotational analysis and the theoretical calculations, the observed spectrum is assigned as an 2A" <-- X2A' electronic transition of this exotic chemical species. This result shows that such a plasma source is a powerful tool to investigate intermediates involved in hydrocarbon chemistry as in flames.