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Structural imaging of transient excited-state species is a key goal of molecular physics, promising to unveil rich information about the dynamics underpinning photochemical transformations. However, separating the electronic and nuclear contributions to the spectroscopic observables is challenging, and typically requires the application of high-level theory. Here, we employ site-selective ionisation via ultrashort soft X-ray pulses and time-resolved Coulomb explosion imaging to interrogate structural dynamics of the ultraviolet photochemistry of carbon disulfide. This prototypical system exhibits the complex motifs of polyatomic photochemistry, including strong non-adiabatic couplings, vibrational mode couplings, and intersystem crossing. Immediately following photoexcitation, we observe Coulomb explosion signatures of highly bent and stretched excited-state geometries involved in the photodissociation. Aided by a model to interpret such changes, we build a comprehensive picture of the photoinduced nuclear dynamics that follows initial bending and stretching motions, as the reaction proceeds towards photodissociation. Coulomb Explosion imaging is a promising technique to study the ultrafast nuclear dynamics which underpin molecular photochemistry. By initiating Coulomb explosion through soft X-ray ionization, the authors are able to image ultrafast nuclear dynamics of a prototypical photoreaction. [ABSTRACT FROM AUTHOR]

By employing the vacuum ultraviolet (VUV) laser velocity-map imaging (VMI) photoelectron scheme to discriminate energetic photoelectrons, we have measured the VUV-VMI-threshold photoelectrons (VUV-VMI-TPE) spectra of propargyl radical [C3H3(X2B1)] near its ionization threshold at photoelectron energy bandwidths of 3 and 7 cm-1 (full-width at half-maximum, FWHM). The simulation of the VUV-VMI-TPE spectra thus obtained, along with the Stark shift correction, has allowed the determination of a precise value 70 156 ± 4 cm-1 (8.6982 ± 0.0005 eV) for the ionization energy (IE) of C3H3. In the present VMI-TPE experiment, the Stark shift correction is determined by comparing the VUV-VMI-TPE and VUV laser pulsed field ionization-photoelectron (VUV-PFI-PE) spectra for the origin band of the photoelectron spectrum of the X+-X transition of chlorobenzene. The fact that the FWHMs for this origin band observed using the VUV-VMI-TPE and VUV-PFI-PE methods are nearly the same indicates that the energy resolutions achieved in the VUV-VMI-TPE and VUV-PFI-PE measurements are comparable. The IE(C3H3) value obtained based on the VUV-VMI-TPE measurement is consistent with the value determined by the VUV laser PIE spectrum of supersonically cooled C3H3(X2B1) radicals, which is also reported in this article. [ABSTRACT FROM AUTHOR]

Vacuum ultraviolet (VUV) laser pulsed field ionization-photoelectron (PFI-PE) spectroscopy has been applied to the study of the sulfur monoxide radical (SO) prepared by using a supersonically cooled radical beam source based on the 193 nm excimer laser photodissociation of SO2. The vibronic VUV-PFI-PE bands for the photoionization transitions SO+(X2Π1/2; v+ = 0) ← SO(X3Σ-; v = 0); and SO+(2Π3/2; v+ = 0) ← SO(X3Σ-; v = 0) have been recorded. On the basis of the semiempirical simulation of rotational branch contours observed in these PFI-PE bands, we have obtained highly precise ionization energies (IEs) of 83 034.2 ± 1.7 cm-1 (10.2949 ± 0.0002 eV) and 83 400.4 ± 1.7 cm-1 (10.3403 ± 0.0002 eV) for the formation of SO+(X2Π1/2; v+ = 0) and SO+(2Π3/2; v+ = 0), respectively. The present VUV-PFI-PE measurement has enabled the direct determination of the spin-orbit coupling constant (A0) for SO+(X2Π1/2,3/2) to be 365.36 ± 0.12 cm-1. We have also performed high-level ab initio quantum chemical calculations at the coupled-cluster level up to full quadruple excitations and complete basis set (CBS) extrapolation. The zero-point vibrational energy correction, the core-valence electronic correction, the spin-orbit coupling, and the high-level correction are included in the calculation. The IE[SO+(X2Π1/2,3/2)] and A0 predictions thus obtained are found to be in remarkable agreement with the experimental determinations. [ABSTRACT FROM AUTHOR]

The ionisation energy (IE) of ZrO, the bond dissociation energies (D 0s) and the heats of formation at 0 K (ΔH ○ 0K f ) and 298 K (ΔH ○ 298K f ) for ZrO/ZrO+ are predicted by the coupled cluster methods utilizing upto single, double, triple and quadruple excitations and complete basis set limit approximation (CCSDTQ/CBS). The CCSDTQ/CBS approach also includes the zero-point vibrational energy, high-order correlation, core-valence (CV) electronic and spin-orbit coupling corrections while the scalar relativistic contribution is handled by employing pseudopotential basis sets. The present calculations yield IE(ZrO) = 6.794 eV and D 0(Zr+-O) − D 0(Zr-O) = −0.193 eV which are in good agreement with the respective experimental values of 6.81272(10) eV and −0.1788(1) eV determined in a two-colour laser-pulsed field ionisation-photoelectron study. The CCSD(T) and multireference configuration interaction (MRCI) methods with CV correlations included give equally remarkable predictions of the harmonic frequencies and the bond lengths for ZrO/ZrO+. This study together with the previous investigations has demonstrated that, with pseudopotential basis sets, the CCSDTQ/CBS protocol can be readily extended to investigate 4d-transition metal-containing diatomic molecules to yield comparable accuracy (±20 meV) to that achieved in the IE and D 0 predictions of 3d-transition metal-containing species. [ABSTRACT FROM AUTHOR]

The ionisation energy (IE) of ZrO, the bond dissociation energies (D 0s) and the heats of formation at 0 K (ΔH ○ 0K f ) and 298 K (ΔH ○ 298K f ) for ZrO/ZrO+ are predicted by the coupled cluster methods utilizing upto single, double, triple and quadruple excitations and complete basis set limit approximation (CCSDTQ/CBS). The CCSDTQ/CBS approach also includes the zero-point vibrational energy, high-order correlation, core-valence (CV) electronic and spin-orbit coupling corrections while the scalar relativistic contribution is handled by employing pseudopotential basis sets. The present calculations yield IE(ZrO) = 6.794 eV and D 0(Zr+-O) − D 0(Zr-O) = −0.193 eV which are in good agreement with the respective experimental values of 6.81272(10) eV and −0.1788(1) eV determined in a two-colour laser-pulsed field ionisation-photoelectron study. The CCSD(T) and multireference configuration interaction (MRCI) methods with CV correlations included give equally remarkable predictions of the harmonic frequencies and the bond lengths for ZrO/ZrO+. This study together with the previous investigations has demonstrated that, with pseudopotential basis sets, the CCSDTQ/CBS protocol can be readily extended to investigate 4d-transition metal-containing diatomic molecules to yield comparable accuracy (±20 meV) to that achieved in the IE and D 0 predictions of 3d-transition metal-containing species. [ABSTRACT FROM AUTHOR]

The ferrate(vi) ion, [FeO4]2−, has attracted much interest in recent years because of its potential use as a green oxidant in organic synthesis and water treatment. Although there have been several reports on the use of ferrate(vi) for the oxidation of alcohols to the corresponding carbonyl compounds, the mechanism remains unclear. In this work, the kinetics of the oxidation of a series of alcohols with α-C–H bond dissociation energies ranging from 81 to 95 kcal mol−1 have been studied by UV/Vis spectrophotometry. The reactions are first-order in both [FeO4]2− and [alcohol]. The deuterium isotope effects for the oxidation of methanol/d4-methanol, ethanol/d6-ethanol and benzyl alcohol/d7-benzyl alcohol are 18.0 ± 0.1, 4.1 ± 0.1 and 11.2 ± 0.1, respectively. A linear correlation is found between the second-order rate constants and the α-C–H bond dissociation energies (BDEs) of the alcohols, consistent with a hydrogen atom transfer (HAT) mechanism. The proposed HAT mechanism is supported by DFT calculations. [ABSTRACT FROM AUTHOR]

Wereport a study of the unimolecular dissociation of BrCH2CH2O radicals produced from the photodissociation of BrCH2CH2ONO at 351/355 nm. Using both a crossed laser-molecularbeam scattering apparatus with electron bombardment detection anda velocity map imaging apparatus with tunable VUV photoionizationdetection, we investigate the initial photodissociation channels ofthe BrCH2CH2ONO precursor and the subsequentdissociation of the vibrationally excited BrCH2CH2O radicals. The only photodissociation channel of the precursor wedetected upon photodissociation at 351 nm was O–NO bond fission.C–Br photofission and HBr photoelimination do not compete significantlywith O–NO photofission at this excitation wavelength. The measuredO–NO photofission recoil kinetic energy distribution peaksnear 14 kcal/mol and extends from 5 to 24 kcal/mol. There is alsoa small signal from lower kinetic energy NO product (it would be 6%of the total if it were also from O–NO photofission). We usethe O–NO photofission P(ET) peaking near 14 kcal/mol to help characterize the internalenergy distribution in the nascent ground electronic state BrCH2CH2O radicals. At 351 nm, some but not all of theBrCH2CH2O radicals are formed with enough internalenergy to unimolecularly dissociate to CH2Br + H2CO. Although the signal at m/e=93 (CH2Br+) obtained with electron bombardmentdetection includes signal both from the CH2Br productand from dissociative ionization of the energetically stable BrCH2CH2O radicals, we were able to isolate the signalfrom CH2Br product alone using tunable VUV photoionizationdetection at 8.78 eV. We also sought to investigate the source ofvinoxy radicals detected in spectroscopic experiments by Miller andco-workers (J. Phys. Chem.A2012, 116, 12032) from the photodissociation of BrCH2CH2ONO at 351 nm. Using velocity map imaging and photodissociatingthe precursor at 355 nm, we detected a tiny signal at m/e= 43 and a larger signal at m/e= 15 that we tentatively assign to vinoxy. Anunderlying signal in the time-of-flight spectra at m/e= 29 and m/e= 42, the two strongest peaks in the literature electron bombardmentmass spectrum of vinoxy, is also apparent. Comparison of those signalstrengths with the signal at HBr+, however, shows thatthe vinoxy product does not have HBr as a cofragment, so the priorsuggestion by Miller and co-workers that the vinoxy might result froma roaming mechanism is contraindicated. [ABSTRACT FROM AUTHOR]

Structural imaging of transient excited-state species is a key goal of molecular physics, promising to unveil rich information about the dynamics underpinning photochemical transformations. However, separating the electronic and nuclear contributions to the spectroscopic observables is challenging, and typically requires the application of high-level theory. Here, we employ site-selective ionisation via ultrashort soft X-ray pulses and time-resolved Coulomb explosion imaging to interrogate structural dynamics of the ultraviolet photochemistry of carbon disulfide. This prototypical system exhibits the complex motifs of polyatomic photochemistry, including strong non-adiabatic couplings, vibrational mode couplings, and intersystem crossing. Immediately following photoexcitation, we observe Coulomb explosion signatures of highly bent and stretched excited-state geometries involved in the photodissociation. Aided by a model to interpret such changes, we build a comprehensive picture of the photoinduced nuclear dynamics that follows initial bending and stretching motions, as the reaction proceeds towards photodissociation. Coulomb Explosion imaging is a promising technique to study the ultrafast nuclear dynamics which underpin molecular photochemistry. By initiating Coulomb explosion through soft X-ray ionization, the authors are able to image ultrafast nuclear dynamics of a prototypical photoreaction. [ABSTRACT FROM AUTHOR]

We present a composite procedure based on explicitly correlated CCSD(T)-F12 calculations for accurate energetic predictions for carbon chain molecules HC n H encompassing both the even (HC 2 k H) and odd series (HC 2 k -1 H), with the shorter members playing a key role in the evolution of cosmic carbon compounds in both circumstellar envelopes and interstellar medium. This approach considers the contributions of core-valence correlation, scalar relativistic effect, spin-orbit coupling, and zero-point vibrational energy in an additive manner. The computed ionization energies demonstrate outstanding agreement (±0.07 eV) up to a chain size of k = 6 and the literature heats of formation for k ≤ 2 are reproduced with "chemical accuracy" of 1 kcal mol -1 . Among the various corrections included, the importance of core-valence correlation effect has been highlighted in the thermochemical calculations for carbon chain growth. The thermochemical trend toward infinite length is also highlighted by extrapolation of ionization energy and triplet-singlet splitting at the CCSD(T) level for k up to 15. The correlation between the end-group effect and the even-odd parity effect observed for HC n H chains has been established with the aid of intrinsic bond orbital localization.

The ionization energies of VCH 2 and VCH 3 , the various 0 K bond dissociation energies ( D 0 s) in their neutrals and cations, and their respective heats of formation at 0 and 298 K are computed by the single-reference, wave function-based CCSDTQ/CBS procedure. The core of the composite method is the approximation to the complete basis set (CBS) limit at the coupled cluster (CC) level which includes up to full quadruple excitations. The zero-point vibrational energy, core-valence correlation, spin-orbit coupling, and scalar relativistic effects have their contributions incorporated in an additive manner. For the species in the current study, this protocol requires geometry optimizations and harmonic frequency calculations practically no higher than the CCSD(T)/aug-cc-pwCVTZ and CCSD(T)/aug-cc-pVTZ levels, respectively. The present calculations successfully predict D 0 (V + -CH 3 ) = 2.126 eV and D 0 (V + -CH 2 ) = 3.298 eV in remarkable agreement with the data recently measured by a spin-orbit state selected V + + CH 4 collision experiment ( Phys. Chem. Chem. Phys . 2021 , 23 , 273-286). The good accord encourages the use of CCSDTQ/CBS protocol in thermochemical predictions of various feasible product channels identified in methane activation by transition metal species.

The ferrate(vi) ion, [FeO 4 ] 2- , has attracted much interest in recent years because of its potential use as a green oxidant in organic synthesis and water treatment. Although there have been several reports on the use of ferrate(vi) for the oxidation of alcohols to the corresponding carbonyl compounds, the mechanism remains unclear. In this work, the kinetics of the oxidation of a series of alcohols with α-C-H bond dissociation energies ranging from 81 to 95 kcal mol -1 have been studied by UV/Vis spectrophotometry. The reactions are first-order in both [FeO 4 ] 2- and [alcohol]. The deuterium isotope effects for the oxidation of methanol/d 4 -methanol, ethanol/d 6 -ethanol and benzyl alcohol/d 7 -benzyl alcohol are 18.0 ± 0.1, 4.1 ± 0.1 and 11.2 ± 0.1, respectively. A linear correlation is found between the second-order rate constants and the α-C-H bond dissociation energies (BDEs) of the alcohols, consistent with a hydrogen atom transfer (HAT) mechanism. The proposed HAT mechanism is supported by DFT calculations.