Pseudopotential and multiple scattering xα calculations of nuclear quadrupole coupling constants and other properties of diatomic halogen molecules and their monoanions and monocations

Relativistic four-component MRCISD+Q calculations of the six lowest valence states of molecular I 2 − anion including breit interactions

CONTEXT AND RESULTS

This study aimed to obtain potential energy curves within a multireference 4-component relativistic method and to present spectroscopic constants (Re ,ω e ,ω e xe ,ω e ye , De , D0 , Be ,α e ,β e ,γ e ), accurate extended Rydberg analytical form, and rovibrational levels for the 6 low-lying states of the I2 − anion. For these states, some spectroscopic constants, rovibrational levels, and an accurate analytical form are presented for the first time in literature, and they are of interest for femtosecond and dynamics experiments of I2 − as well as for electron attachment of I2 . This study suggests that the inclusion of relativistic and correlation effects treated at the MRCISD+Q level is needed to obtain reliable results, specially for De .

COMPUTATIONAL AND THEORETICAL TECHNIQUES

The potential energy curves of the ground and the excited states of the molecular iodine anion (I2 − ) were investigated at multireference configuration interaction (MRCISD) with Davidson size-extensivity correction (denoted as +Q) within a fully relativistic four-component relativistic framework including Breit interaction.

Nature of the Three-Electron Bond

We analyze the properties of 15 3-electron bonds, which include σ-3-electron-bonds, such as dihalide radical anions and di-noble gas radical cations, π-3-electron-bonds as in hydrazine radical cations, and doubly-π-(3e)-bonded species such as O₂, FeO⁺, S₂, etc. The primary analytical tool is the breathing-orbital valence-bond (BOVB) method, which enables us to quantify the charge shift resonance energy (RE_CS) of the three electrons, and the bond dissociation energies (D_e). BOVB is tested reliable against MRCI calculations. Our findings show that in all 3-electron bonds, none of the VB structures have by themselves any bonding. In fact, in each VB structure, the three electrons maintain Pauli repulsion, while the entire bonding energy arises from resonance due to the charge shift between the two or more constituent VB structures. Hence, 3e-bonds are charge shift bonds (CSBs). The CSB character is probed by calculating the Laplacian (L) of the 3e-bond. Thus, much like the CSBs in electron-pair bonds, such as F₂ or the central bond in [1.1.1]propellane, here too L is positive, thus showing the excess kinetic energy of the shared density due to the Pauli repulsion in the 3-electron VB structures. The RE_CS values for 3-electron bonds are invariably larger than the corresponding bond energies. For the doubly-π-(3e)-bonded species, RE_CS is very large, exceeding 100 kcal mol^-1. As such, it is fitting to conclude that σ- and π-3-electron-bonds find their natural place in the CSB family along with two-electron CSBs, with which they share identical energetic and topological characteristics. Experimental manifestations/tests of 3e-CSBs are proposed.

Methyl substituent effects in [H(n)X...XH(n)](+) three-electron-bonded radical cations (X = F, O, N, Cl, S, P; n = 1 - 3). An ab initio theoretical study

The effects of methyl substitution on the geometries and bonding energies of a systematic series of three-electron-bonded radical cations of the type [H(n)X...XH(n)](+), covering all possible symmetrical three-electron bonds that may take place between atoms of the second and third rows of the periodic table, have been investigated at the level of Møller-Plesset perturbation theory. Methyl substitution leads to significant weakening and lengthening of the X...X bond when X is a second-row atom. The effects increase with the number of substitutions and are more and more important in the series X = N, O, F. By contrast, methyl substitution leaves the bonding energies between third-row atoms practically unchanged but leads to a surprising bond shortening in the S...S and P...P cases. These seemingly contradictory effects are rationalized through a qualitative analysis based on an elementary molecular orbital description of three-electron bonding.