Data bank

30.4-nm He(II) photoelectron spectra of organic molecules: Part V. Hetero-compounds containing first-row elements (C, H, B, N, O, F)☆

Synchrontron radiation enabled recent measurement of the complete photoemission spectrum of acrylamide reported by Evangelisti et al. in Photochem. 2 (2022) 463. It seems to be a pity that calculation with the usually reliable ab initio method SAC-CI did not produce good agreement with experiment. In the present study, we use density functional theory methods in order to improve the agreement between experiment and theory. Such agreement helps to validate the DFT methods used. Our DFT results also suggest that two peaks in the N1s X-ray absorptiom spectrum have been incorrectly assigned.

A cost-free modification of second-order electron propagator theory (ex-SCS-EP2) to improve the description of ionization energies based on spin-component-scaled approximation is proposed. The relaxation part (PRX) and correlation part (PRM) in second-order self-energy are partitioned into parallel- and anti-parallel-spin component then scaled separately. The scaling factors are optimized with 23 valence orbital energies and results shown that ex-SCS-EP2 performs significantly better than regular EP2, SCS-EP2 and OVGF with 0.22 eV of mean absolute error (MAE) and 0.11 eV of mean absolute deviation (MAD). The effects of SCS parameters on self-energy are also examined to ensure better description in total correlation energy.

A VUV photoionization study of acetamide was carried out over the 8–24 eV photon energy range using synchrotron radiation and photoelectron/photoion coincidence (PEPICO) spectroscopy. Threshold photoelectron photoion coincidence (TPEPICO) measurements were also made. Photoion yield curves and branching ratios were measured for the parent ion and six fragment ions. The adiabatic ionization energy of acetamide was determined as I.E. (1²A′) = (9.71 ± 0.02) eV, in agreement with an earlier reported photoionization mass spectrometry (PIMS) value. The adiabatic energy of the first excited state of the ion, 1²A″, was determined to be ≈10.1 eV. Assignments of the fragment ions and the pathways of their formation by dissociative photoionization were made. The neutral species lost in the principal dissociative photoionization processes are CH₃, NH₂, NH₃, CO, HCCO and NH₂CO. Heats of formation are derived for all ions detected and are compared with literature values. Some astrophysical implications of these results are discussed.

Gas phase adiabatic ionization energies (AIEs) and standard state (298.15 K, 1 atm) enthalpies of formation $\left({\mathrm{\Delta }}_{\text{f}}{\text{H}}_{(\text{g})}^{°}\right)$ were calculated at the Gaussian-4 (G4) level of theory for a suite of 319 and 398 main group compounds, respectively, containing the elements hydrogen through bromine. National Institute of Standards and Technology (NIST) evaluated AIEs were available for comparison with 44 compounds, yielding good agreement when the significant variability among the individual experimental reports was taken into consideration. AIEs were estimated at the G4 level of theory for a further 126 main group compounds having at least one experimental datapoint in the NIST database, but not a NIST evaluated AIE, as well as 149 main group compounds without any experimental data in the NIST database. Experimental ${\mathrm{\Delta }}_{\text{f}}{\text{H}}_{(\text{g})}^{°}$ were available for comparison with 144 compounds, yielding good agreement, particularly against compounds with evaluated ${\mathrm{\Delta }}_{\text{f}}{\text{H}}_{(\text{g})}^{°}$. ${\mathrm{\Delta }}_{\text{f}}{\text{H}}_{(\text{g})}^{°}$ were estimated at the G4 level of theory for a further 254 main group compounds without any experimental data. The datasets comprise a benchmark set of theoretical data against which to assess future experimental and theoretical results.

Static and dynamic aspects of the Jahn–Teller (JT) and pseudo-Jahn–Teller (PJT) interactions between the ground and first excited electronic states of the methyl cyanide radical cation are theoretically investigated here. The latter involves construction of a theoretical model by ab initio computation of electronic potential energy surfaces and their coupling surfaces and simulation of the nuclear dynamics employing time-independent and time-dependent quantum mechanical methods. The present system represents yet another example belonging to the (E + A) ⊗ e JT–PJT family, with common JT and PJT active degenerate (e) vibrational modes. The theoretical results are found to be in very good accord with the recent experimental data revealing that the JT interactions are particularly weak in the ground ${\stackrel{\sim }{X}}^{2}E$ electronic manifold of methyl cyanide radical cation, On the other hand, the PJT interactions of this ground electronic manifold with the first excited ${\stackrel{\sim }{A}}^2{A}_1$ electronic state of the radical cation are stronger which cause an increase of the spectral line density. The effect of deuteration on the JT–PJT dynamics of the methyl cyanide radical cation is also discussed.

Valence shell angle resolved photoelectron spectra of CH₃CN and CF₃CN have been recorded in the photon energy range 14–120 eV, thereby allowing asymmetry parameters and branching ratios to be derived. The carbon and nitrogen K-shell photoabsorption spectra of these two molecules exhibit features ascribed to shape resonantly enhanced transitions. Energy dependent variations observed in the asymmetry parameters and branching ratios provide evidence of shape resonances influencing the valence shell photoionisation dynamics. In addition to the main lines associated with single-hole states, complex satellite structure appears in the inner valence region of the photoelectron spectrum due to many-electron effects. The experimental spectra have been interpreted using previously reported ionisation energies and spectral intensities obtained from Green's function calculations.