The low-lying bending vibrational levels of the CCH (<i>X</i>̃ 2Σ+) radical studied by laser-induced fluorescence

Hsu, Yen‐Chu; Lin, Jim J.; Papoušek, D.; Tsai, Jichiang

doi:10.1063/1.464761

Cited by 51 publications

(28 citation statements)

References 48 publications

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“…88 The non-trivial electronic spectrum [89][90][91] and hyperfine structure 92 have been explored extensively. One of the notable features of the ethynyl radical is that the A 2 Π excited electronic state lies only 3692 cm −1 (0.458 eV) above the ground X 2 Σ + state.…”

Section: Ethynyl Radical (·Cch)mentioning

confidence: 99%

High-Level ab Initio Studies of Hydrogen Abstraction from Prototype Hydrocarbon Systems

et al. 2006

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Symmetric and non-symmetric hydrogen abstraction reactions are studied using stateof-the-art ab initio electronic structure methods. Second-order Møller-Plesset perturbation theory (MP2) and the coupled-cluster singles, doubles and perturbative triples [CCSD(T)] methods with large correlation consistent basis sets (cc-pVXZ, where X = D,T,Q) are used in determining the transition-state geometries, activation barriers, and thermodynamic properties of several representative hydrogen abstraction reactions. The importance of basis set, electron correlation, and choice of zeroth-order reference wavefunction in the accurate prediction of activation barriers and reaction enthalpies are also investigated. The ethynyl radical (·CCH), which has a very high affinity for hydrogen atoms, is studied as a prototype hydrogen abstraction agent. Our high-level quantum mechanical computations indicate that hydrogen abstraction using the ethynyl radical has an activation energy of less than 3 kcal mol −1 for * To whom correspondence should be addressed. E-mail addresses: sherrill@gatech.edu; merkle@cc.gatech.edu; rfreitas@rfreitas.com 1 hydrogens bonded to an sp 2 or sp 3 carbon. These low activation barriers further corroborate previous studies suggesting that ethynyl-type radicals would make good tooltips for abstracting hydrogens from diamondoid surfaces during mechanosynthesis. Modeling the diamond C(111) surface with isobutane and treating the ethynyl radical as a tooltip, hydrogen abstraction in this reaction is predicted to be barrierless.

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Section: Ethynyl Radical (·Cch)mentioning

confidence: 99%

High-Level ab Initio Studies of Hydrogen Abstraction from Prototype Hydrocarbon Systems

et al. 2006

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“…This suggests that close to W 3(H 2 O), either the dust must be very hot or the conditions quite extreme (see also Helmich et al 1996). CCH has its lowest vibrational level at 370 cm −1 or 27 µm (Hsu et al 1993;Kanamori & Hirota 1988). At these wavelengths the infrared radiation is intense enough to pump this vibrational level.…”

Section: Vibrationally Excited Moleculesmentioning

confidence: 99%

Physical and chemical variations within the W3 star-forming region

Helmich

Jansen

Graauw³

et al. 1995

Lecture Notes in Physics

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Abstract. Results are presented of the 345 GHz spectral survey toward three sources in the W 3 Giant Molecular Cloud: W 3 IRS4, W 3 IRS5 and W 3(H 2 O). Nearly 90% of the atmospheric window between 334 and 365 GHz has been scanned using the James Clerk Maxwell Telescope (JCMT 1 ) down to a noise level of ∼ 80 mK per resolution element. These observations are complemented by a large amount of data in the 230 GHz atmospheric window. From this data set physical conditions and beam-averaged column densities are derived for more than 14 chemically different species (over 24 different isotopes). The physical parameters derived in Paper I (Helmich et al. 1994) are confirmed by the analysis of the excitation of other species, although there is evidence that the silicon-and sulfurbearing molecules exist in a somewhat denser and warmer environment. The densities are high, ≥ 10 6 cm −3 , in the three sources and the kinetic temperatures for the bulk of the gas range from 55 K for IRS4 to 220 K for W 3(H 2 O). The chemical differences between the three sources are very striking: silicon-and sulfur-bearing molecules such as SiO and SO 2 are prominent toward IRS5, whereas organic molecules like CH 3 OH, CH 3 OCH 3 and CH 3 OCHO are at least an order of magnitude more abundant toward W 3(H 2 O). Vibrationally excited molecules are also detected toward this source. Only simple molecules are found toward IRS4. The data provide constraints on the amount of deuterium fractionation and the ionization fraction in the observed regions as well. These chemical character- istics are discussed in the context of an evolutionary sequence, in which IRS5 is the youngest, W 3(H 2 O) somewhat older and IRS4, although still enigmatic, the oldest.

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“…A similar conclusion appears to hold for the C 2 H (v 2 ϭ4) product energy disposal. The rotational term values of the v 2 ϭ4, lϭ0 ( 2 ⌺ ϩ ), and lϭ2 ( 2 ⌬) levels are known, 56 but, as Fig. 3͑b͒ shows, these are insufficient to provide an adequate explanation for the observed structure in the P(E T ) spectrum.…”

Section: Resultsmentioning

confidence: 95%

“…The ability to excite the parent molecule to a small number of low J rovibronic states, combined with a long TOF path, results in unprecedented resolution for these P(E T ) spectra. Consequently direct assignment of the various rovibronic energy levels of C 2 H is relatively straightforward, but confirmation of the conclusions implied by the assignments is aided by the precise term values that are now available from high resolution absorption 54,55 and dispersed LIF emission [56][57][58] spectroscopy. Knowing the photon energy, the bond strength D 0 ͑H-CCH͒ϭ46 074Ϯ8 cm Ϫ1 follows directly from the measured TKER of HϩC 2 H(X, vЉϭ0, NЉ ϭ0) fragments after applying the small (Ϫ9 cm Ϫ1 ) lab.…”

Section: Methodsmentioning

confidence: 99%

Near threshold photodissociation of acetylene

Mordaunt

Ashfold

Dixon

et al. 1998

The Journal of Chemical Physics

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The photodissociation of jet-cooled HCCH molecules following excitation to their S 1 state has been investigated further, at a number of wavelengths in the range 205-220 nm, using the H atom photofragment translational spectroscopy ͑PTS͒ technique. Analysis of the rovibrational structure evident in the total kinetic energy release ͑TKER͒ spectra so obtained confirms previous reports that the resulting C 2 H(X) fragments are formed in most ͑if not all͒ of the v 2 bending vibrational levels permitted by energy conservation, and that there is a clear preference for populating those states in which the axial projection of this vibrational angular momentum is maximized ͑i.e., states with l ϭv 2 ͒. The distribution of H atom recoil velocity vectors resulting from photolyses at the shorter excitation wavelengths ͑e.g., phot ϭ205.54 nm͒ shows bimodal rotational distributions, and a marked anisotropy-especially in the case of those H atoms that are formed in association with C 2 H(X) fragments carrying little rotational excitation. Two competing dissociations mechanisms have been identified. Our discussion of these observations is guided by the recent ab initio calculations of Cui and Morokuma ͓Chem. Phys. Lett. 272, 319 ͑1997͔͒. Channel I conforms to their proposal that the S 1 molecules reach the HϩC 2 H(X) asymptote as a result of sequential nonadiabatic couplings via the T 3 , T 2 , and T 1 potential energy surfaces. The product energy disposal at the longest excitation wavelengths is rationalized in terms of the forces acting as the dissociating molecule traverses a late barrier in the C-H exit channel on the T 1 surface, while the propensity for populating states with lϭv 2 reflects the importance of parent torsional motion in promoting the S 1 →T 3 , T 3 →T 2 , and T 2 →T 1 surface couplings. The population of low rotational states with high recoil anisotropy at shorter excitation wavelengths is ascribed to channel II, involving a direct nonadiabatic transition from S 1 to T 1 for a structure with one near linear CCH angle. In contrast to channel I, there is no extensive torsional motion and the anisotropy of the initial excitation is retained through to fragmentation. Excitation of the 1 Ј mode of HCCH enhances the branching to channel II.

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The low-lying bending vibrational levels of the CCH (X̃ 2Σ+) radical studied by laser-induced fluorescence

Cited by 51 publications

References 48 publications

High-Level ab Initio Studies of Hydrogen Abstraction from Prototype Hydrocarbon Systems

High-Level ab Initio Studies of Hydrogen Abstraction from Prototype Hydrocarbon Systems

Physical and chemical variations within the W3 star-forming region

Near threshold photodissociation of acetylene

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