A vibrational action spectroscopic study of the Renner-Teller- and spin-orbit-affected cyanoacetylene radical cation HC3N. J Chem Phys 2023;158:084305. [PMID: 36859081 DOI: 10.1063/5.0135000]

The linear radical cation of cyanoacetylene, HC₃N⁺ (²Π), is not only of astrophysical interest, being the, so far undetected, cationic counterpart of the abundant cyanoaceteylene, but also of fundamental spectroscopic interest due to its strong spin-orbit and Renner-Teller interactions. Here, we present the first broadband vibrational action spectroscopic investigation of this ion through the infrared pre-dissociation (IRPD) method using a Ne tag. Experiments have been performed using the FELion cryogenic ion-trap instrument in combination with the FELIX free-electron lasers and a Laservision optical parametric oscillator/optical parametric amplifier system. The vibronic splitting patterns of the three interacting bending modes (ν₅, ν₆, ν₇), ranging from 180 to 1600 cm^-1, could be fully resolved revealing several bands that were previously unobserved. The associated Renner-Teller and intermode coupling constants have been determined by fitting an effective Hamiltonian to the experimental data, and the obtained spectroscopic constants are in reasonable agreement with previous photoelectron spectroscopy (PES) studies and ab initio calculations on the HC₃N⁺ ion. The influence of the attached Ne atom on the infrared spectrum has been investigated by ab initio calculations at the RCCSD(T)-F12a level of theory, which strongly indicates that the discrepancies between the IRPD and PES data are a result of the effects of the Ne attachment.

Direct IR Absorption Spectra of Propargyl Cation Isolated in Solid Argon

The direct infrared (IR) absorption spectra of propargyl cations were recorded. These cations were generated via the electron bombardment of a propyne/Ar matrix sample during matrix deposition. Secondary photolysis with selected ultraviolet (UV) light was used for grouping the observed bands of various products. The band assignment of the propargyl cation in solid Ar was performed according by referring to the previous infrared photodissociation (IRPD) and velocity-map imaging photoelectron (VMI-PE) data, and via theoretical predictions of the anharmonic vibrational wavenumbers, band intensities, and deuterium-substituted isotopic ratios. Almost all the IR active bands with an observable intensity were recorded and the ν₁₁ mode was reported for the first time.

Valence shell threshold photoelectron spectroscopy of C3Hx (x = 0–3)

We present the photoelectron spectra of C₃H_x (x = 0–3) formed in a microwave discharge flow-tube reactor by consecutive H abstractions from C₃H₄ (C₃H_x + F → C₃H_x−1 + HF (x = 1–4)), but also from F + CH₄ schemes by secondary reactions.

Accuracy of Frequencies Obtained with the Aid of Explicitly Correlated Wave Function Based Methods

We asses the basis set convergence of harmonic frequencies using different explicitly correlated wave function based methods. All commonly available CCSD(T) variants as well as MP2-F12 and MP4(F12*) are considered, and a hierarchy of the different approaches is established. As for reaction and atomization energies, CCSD(F12*)(T*) is a close approximation to CCSD(F12)(T*) and clearly superior to the other tested approximations. The used scaling for the triples correction enhances the accuracy relative to CCSD(F12*)(T) especially for small basis sets and is very attractive since no additional computational costs are added. However, this scaling slightly breaks size consistency, and therefore we additionally study the accuracy of CCSD(F12*)(T*) and CCSD(F12*)(T) in the context of calculating anharmonic frequencies to check if this causes problems in the generation of the potential energy surface (PES). We find a fast basis set convergence for harmonic and anharmonic frequencies. Already in the cc-pVDZ-F12 basis, the RMSD to the CBS limit is only around 4-5 cm^-1.

In situ flame chemistry tracing by imaging photoelectron photoion coincidence spectroscopy

Adaptation of a low-pressure flat flame burner with a flame-sampling interface to the imaging photoelectron photoion coincidence spectrometer (iPEPICO) of the VUV beamline at the Swiss Light Source is presented. The combination of molecular-beam mass spectrometry and iPEPICO provides a new powerful analytical tool for the detailed investigation of reaction networks in flames. First results demonstrate the applicability of the new instrument to comprehensive flame diagnostics and the potentially high impact for reaction mechanism development for conventional and alternative fuels. Isomer specific identification of stable and radical flame species is demonstrated with unrivaled precision. Radical detection and identification is achieved for the initial H-abstraction products of fuel molecules as well as for the reaction controlling H, O, and OH radicals. Furthermore, quantitative evaluation of changing species concentrations during the combustion process and the applicability of respective results for kinetic model validation are demonstrated. Utilization of mass-selected threshold photoelectron spectra is shown to ensure precise signal assignment and highly reliable spatial profiles.

Communication: A vibrational study of propargyl cation using the vacuum ultraviolet laser velocity-map imaging photoelectron method

By employing the vacuum ultraviolet (VUV) laser velocity-map imaging photoelectron (VUV-VMI-PE) method, we have obtained a vibrationally resolved photoelectron spectrum of gaseous propargyl radical [C(3)H(3)(X(2)B(1))] in the energy range of 0-4600 cm(-1) above its ionization energy. The cold C(3)H(3) radicals were produced from a supersonically cooled radical beam source based on 193 nm ArF photodissociation of C(3)H(3)Cl. The VUV-VMI-PE spectrum of C(3)H(3) thus obtained reveals a Franck-Condon factor (FCF) pattern with a highly dominant origin band along with weak vibrational progressions associated with excitations of the C-C ν(5)(+)(a(1)) and C≡C ν(3)(+)(a(1)) symmetric stretching modes and the CCH ν(7)(+)(b(1)) out-of-plane bending mode of C(3)H(3)(+)(X(1)A(1)). The ν(5)(+)(a(1)) vibrational frequency of 1120 cm(-1) determined in the present study is lower than the value deduced from the recent Ar-tagged infrared photodissociation study by 102 cm(-1), confirming the highly accurate vibrational frequency predictions obtained by the most recent state-of-the-art ab initio quantum calculations. The observation of the FCF disallowed ν(7)(+)(b(1)) mode is indicative of vibronic interactions. The discrepancy observed between the FCF pattern determined in the present study and that predicted by a recent high-level quantum theoretical investigation can be taken as evidence that the potential energy surfaces used in the latter theoretical study are in need of improvement in order to provide a reliable FCF prediction for the C(3)H(3)/C(3)H(3)(+) photoionization system.

Fulvenallenyl cation (C7H5(+)) and its complex with an argon atom: results of high-level quantum-chemical calculations

The fulvenallenyl cation (C(7)H(5)(+)) and its complex with an argon atom have been studied by explicitly correlated coupled cluster theory at the CCSD(T)-F12x(x = a, b) level and by the double-hybrid density functional B2PLYP-D. For the free cation, an accurate equilibrium structure has been established and ground-state rotational constants of A(0) = 8116.4 MHz, B(0) = 2004.3 MHz, and C(0) = 1606.9 MHz are predicted. The equilibrium dipole moment is calculated to be μ(e) = 1.305 D, with the positive end of the dipole at the acetylenic hydrogen site. Anharmonic wavenumbers of C(7)H(5)(+) were obtained by combination of harmonic CCSD(T*)-F12a values and B2PLYP-D anharmonic contributions. The most intense vibration is the pseudoantisymmetric CC stretching vibration at 2083 cm(-1). The potential energy surface of the complex C(7)H(5)(+)·Ar is characterized by two energy minima of C(s) symmetry which are separated by a very low energy barrier. The dissociation energy of the most stable structure is predicted to be D(0) = 530 ± 30 cm(-1).