Dilithium
Dilithium is the diatomic molecule of lithium with the formula Li₂. It consists of two lithium atoms joined by a single covalent bond and is observed in the gas phase, where it behaves as a strongly nucleophilic species.[1] The ground electronic state of Li₂ is X¹Σ_g^+, with a bond order of 1, an equilibrium internuclear separation (r_e) of 2.6729 Å (267.29 pm), and a vibrational frequency (ω_e) of 351.43 cm⁻¹. The dissociation energy (D_0) from the ground state to two ^2S lithium atoms is 8473 cm⁻¹ (approximately 102 kJ/mol or 1.05 eV).[1] These parameters reflect its weak bonding compared to other alkali metal dimers, consistent with molecular orbital theory where the bond arises from σ_{2s} overlap.[2] Li₂ has been studied spectroscopically since the early 20th century, with applications in understanding alkali metal bonding and ultracold molecular physics. It is not stable under standard conditions but can be produced in vapor or matrix isolation.[1]Molecular Structure
Bonding Characteristics
The chemical bond in the dilithium (Li₂) molecule is a single covalent bond characterized by a bond order of 1, arising from the overlap of the 2s atomic orbitals from each lithium atom to form a σ bonding molecular orbital.[3][4] The electron configuration of Li₂ is (\sigma_{2s})^2, where the two valence electrons occupy the bonding σ_{2s} orbital, with the antibonding σ^*_{2s} orbital remaining empty.[3] There is no π bonding in Li₂, as the valence subshell is the filled 2s orbital, and the 2p orbitals are not involved in the ground-state bonding due to their higher energy and lack of valence electrons.[4] As an alkali metal, lithium's low first ionization energy of 5.39 eV contributes to the weak nature of the Li₂ bond compared to diatomic molecules of other periods, as this low energy results in diffuse 2s valence orbitals that provide limited overlap and stabilization in the molecular orbital.[5][6] Molecular orbital theory for homonuclear diatomic molecules of group 1 elements, such as Li₂ and Na₂, predicts a bond order of 1 based on the filling of the σ_{ns} bonding orbital with the two available ns valence electrons, consistent with the observed single-bond character and relative instability of these species.[4][3]Geometric Parameters
The dilithium molecule (Li₂) is a homonuclear diatomic species with a linear geometry, belonging to the D_{\infty h} point group in its ground electronic state due to its symmetric structure and inversion center.[7] The equilibrium internuclear separation in the ground state (X¹Σ_g⁺) is measured at 267.3 pm.[7] This value reflects the weak bonding interaction primarily arising from valence electron overlap, as determined experimentally through spectroscopic techniques. In contrast, the bond length increases significantly in excited electronic states; for example, in the A¹Σ_u⁺ state, the equilibrium distance extends to 310.8 pm, indicating a looser potential well.[7] Experimental geometric parameters are primarily derived from microwave spectroscopy, which probes rotational transitions to yield the equilibrium rotational constant B_e = 0.672 cm⁻¹ for the ground state.[7] This constant relates inversely to the square of the bond length via the reduced mass of the lithium nuclei, providing a precise empirical measure of the internuclear distance without relying on theoretical assumptions. Similar spectroscopic analyses for excited states confirm the variations in bond lengths across the electronic manifold.[7]Physical and Chemical Properties
Thermodynamic Data
The bond dissociation energy of dilithium (Li₂) in its ground state, D_e, is 102 kJ/mol or 1.06 eV, reflecting the weak single-bond character typical of alkali metal dimers. The well depth from the potential minimum, D_e, has been determined with high precision as 8518.6 cm⁻¹ through analysis of high-resolution spectroscopic data and potential energy curve fitting. The spectroscopic dissociation energy from the vibrational zero-point level, D_0, is 8473 cm⁻¹. These energetic parameters underscore the molecule's marginal stability, with the bond readily broken at modest temperatures, facilitating its observation primarily in gas-phase experiments.[8][7]| Thermodynamic Quantity | Value | Units | Notes |
|---|---|---|---|
| Bond dissociation energy (D_e) | 102 | kJ/mol | Ground state well depth |
| Bond dissociation energy (D_e) | 1.06 | eV | Equivalent electronic units |
| Bond dissociation energy (D_0) | 8473 | cm⁻¹ | From v=0 level, extrapolated |