Assessment of Density Functional Methods for the Calculation of Ionization Potentials of Unsaturated Molecules

Isoselenocyanates versus Isothiocyanates and Isocyanates

Alkyl and aryl isoselenocyanates are well known intermediates in the synthesis of various organoselenium compounds, but the knowledge of the physicochemical properties of simple unsaturated derivatives is still fragmentary. Vinyl-, 2-propenyl-, and cyclopropyl isoselenocyanates have been prepared by reaction of selenium in powder with the corresponding isocyanides. The isoselenocyanates of this series, with a variable distance between the N═C═Se group and the unsaturated or pseudounsaturated group, have been studied by UV-photoelectron spectroscopy and quantum-chemical calculations. For each of these three isoselenocyanates, the exploration of conformers and geometrical optimization always converge toward only one local minimum. The vinyl and cyclopropyl derivatives are characterized by similar order of magnitude of interactions between the NCSe group and the substituent, while for allylic compound two noninteracting moieties should be considered. The same conclusions were obtained for vinylic and cyclopropylic sulfur and oxygen derivatives. Thus the type and extent of interactions between the N═C═X (X = O, S, Se) group and an unsaturated (vinyl, allyl, or cyclopropyl) moiety are now clarified.

Electronic structure of BN-aromatics: Choice of reliable computational tools

The importance of having reliable calculation tools to interpret and predict the electronic properties of BN-aromatics is directly linked to the growing interest for these very promising new systems in the field of materials science, biomedical research, or energy sustainability. Ionization energy (IE) is one of the most important parameters to approach the electronic structure of molecules. It can be theoretically estimated, but in order to evaluate their persistence and propose the most reliable tools for the evaluation of different electronic properties of existent or only imagined BN-containing compounds, we took as reference experimental values of ionization energies provided by ultra-violet photoelectron spectroscopy (UV-PES) in gas phase-the only technique giving access to the energy levels of filled molecular orbitals. Thus, a set of 21 aromatic molecules containing B-N bonds and B-N-B patterns has been merged for a comparison between experimental IEs obtained by UV-PES and various theoretical approaches for their estimation. Time-Dependent Density Functional Theory (TD-DFT) methods using B3LYP and long-range corrected CAM-B3LYP functionals are used, combined with the ΔSCF approach, and compared with electron propagator theory such as outer valence Green's function (OVGF, P3) and symmetry adapted cluster-configuration interaction ab initio methods. Direct Kohn-Sham estimation and "corrected" Kohn-Sham estimation are also given. The deviation between experimental and theoretical values is computed for each molecule, and a statistical study is performed over the average and the root mean square for the whole set and sub-sets of molecules. It is shown that (i) ΔSCF+TDDFT(CAM-B3LYP), OVGF, and P3 are the most efficient way for a good agreement with UV-PES values, (ii) a CAM-B3LYP range-separated hybrid functional is significantly better than B3LYP for the purpose, especially for extended conjugated systems, and (iii) the "corrected" Kohn-Sham result is a fast and simple way to predict IEs.

UV-photoelectron spectroscopy of 1,2- and 1,3-azaborines: a combined experimental and computational electronic structure analysis

We present a comprehensive electronic structure analysis of structurally simple BN heterocycles using a combined UV-photoelectron spectroscopy (UV-PES)/computational chemistry approach. Gas-phase He I photoelectron spectra of 1,2-dihydro-1,2-azaborine 1, N-Me-1,2-BN-toluene 2, and N-Me-1,3-BN-toluene 3 have been recorded, assessed by density functional theory calculations, and compared with their corresponding carbonaceous analogues benzene and toluene. The first ionization energies of these BN heterocycles are in the order N-Me-1,3-BN-toluene 3 (8.0 eV) < N-Me-1,2-BN-toluene 2 (8.45 eV) < 1,2-dihydro-1,2-azaborine 1 (8.6 eV) < toluene (8.83 eV) < benzene (9.25 eV). The computationally determined molecular dipole moments are in the order 3 (4.577 D) > 2 (2.209 D) > 1 (2.154 D) > toluene (0.349 D) > benzene (0 D) and are consistent with experimental observations. The λ(max) in the UV-vis absorption spectra are in the order 3 (297 nm) > 2 (278 nm) > 1 (269 nm) > toluene (262 nm) > benzene (255 nm). We also establish that the measured anodic peak potentials and electrophilic aromatic substitution (EAS) reactivity of BN heterocycles 1-3 are consistent with the electronic structure description determined by the combined UV-PES/computational chemistry approach.

Prediction of the photoelectron spectrum for XPY2(X = H, F, Cl; Y = O, S) type of molecules by ab initio and DFT methods

The present study reports the theoretical vertical ionization potentials (IPs) for all the valence electrons in six XPY2molecules by utilizing the corrected orbital energies calculated with three theoretical methods, namely, the ab initio Hartree–Fock (HF), and both the pure and hybrid density functional theory (DFT) methods at, respectively, the BLYP/6-311+G* and B3lYP/6-311+G* levels of theory. Evaluation of the numerical corrections to the orbital energies was achieved by comparisons with the IP values obtained via explicit computation of the energy differences between the neutral molecules and the corresponding radical cations (the ΔSCF method) and shows values from –0.9 to –1.9 eV for the HF, and positive values from 2.9 to 3.9 eV and from 1.8 to 2.4 eV for the pure and hybrid DFT methods, respectively. In contrast to the orbital energies, the ΔSCF method is shown to give consistent values among the three methods, as well as reasonable agreement with the experimental IP values.Key words: ionization potential, phosphorane, Koopmans' theorem, Janak's theorem.

Calculation of Ionization Potentials of Small Molecules: A Comparative Study of Different Methods

With the help of various theoretical methods, ionization potentials (IPs) have been computed for a panel of small molecules containing atoms of group 14, 15, or 16 and representing different singly, doubly, or triply bonded systems with or without an interacting heteroatom lone pair. Comparison of experimental IP values to theoretical results indicates that (i) the standard outer valence green function (OVGF), density functional theory (DFT), and DeltaSCF methods lead to rather accurate values, (ii) the CASPT2 method systematically underestimates IPs, (iii) the method of deducing IPs from a shift of some standard DFT eigenvalue spectrum is a straightforward approach leading to rather accurate IPs, (iv) the eigenvalue spectrum obtained with the so-called statistical average of different orbital model potential (SAOP) exchange-correlation model potential is an efficient approach leading directly to quite accurate IPs, and (v) a good prediction of the IP spectrum can be obtained from the shifted excitation spectra of the system calculated by the time-dependent DFT (TD-DFT) method. It is also shown that the TD-DFT calculations of the ionized species bring a significant improvement over the calculations of the neutral molecules, indicating that a great part of the electronic relaxation is already taken into account (in a similar way for all ionizations). Finally, in the case of TD-DFT calculations of neutral molecules, the statistical average of different orbital model potential (SAOP) functional does not lead to significantly better results than the B3LYP functional.

Photochemistry of the CpNiNO complex. A theoretical study using density functional theory

The photochemistry of the CpNiNO complex has been investigated using density functional theory. The whole potential energy curve along the NiNO angle coordinate is presented for the first time with both ground and metastable states, and transition states connecting the minima. The excited states of the GS, MS(I), and MS(II) species have been calculated using time-dependent density functional theory. Furthermore, the structure of the excited states pertaining to the photochemistry of CpNiNO has been optimized. From these results it is shown that the backward GS <-- MS(II) <-- MS(I) reaction is more efficient than the forward GS --> MS(II) --> MS(I) scheme.

Gas-Phase Generation and Photoelectron Spectra of Reactive Unsubstituted Cycloalkenethiones

The flash vacuum thermolysis of cycloalkenyl allyl sulfides, potential precursors of unsubstituted cycloalkenethiones, has been investigated by photoelectron spectroscopy. This technique allowed us to characterize in the gas phase very reactive species of this series, the conjugated and nonconjugated cyclopentene and cyclohexenethiones, and to compare the reactivity of these compounds. The assignment of the PE spectra is supported by electronic structure calculations, particularly by the HF method at the MP2 level and the density functional theory (DFT) using the B3LYP and BP86 functionals with the 6-311G(d, p) basis set. Copyright 2001 Academic Press.