Molecular orbitals of the vanadyl pentaqua ion - Java version
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Molecular structure
The vanadyl pentaqua ion, [VO(H2O)5]2+ is an inorganic molecule (a transition metal ion complex) consisting of a vanadium (IV) ion strongly bound to an oxygen (-II) ion (oxyanion) and five water ligands.
The initital view in the Jmol-frame above shows the bond lengths and angles of the complex as obtained from a geometry optimization (see Computational details below). The structure is close to octahedral but with significant tetragonal distortions. The oxyanion and the water molecule on the opposite side of the vanadium atom are called axial ligands while the remaining four water molecules are called equatorial ligands. In addition one can see that the electron density is delocalized over the whole molecule.
Molecular orbitals
The vanadium 3d orbitals form bonding and antibonding molecular orbitals (MO) with oxygen 2p orbitals of both the oxyanion and the water molecules. The shape and energy of the MO:s have been calculated (see details below) and some of them can be viewed by selecting a radio button above. The MO:s have been ordered and numbered according to their energy. At the bottom are the low-energy orbitals which are occupied by electrons in the ground state of the molecule. These MO:s are generally bonding or non-bonding (with respect to the vanadium-ligand bonds). At the top are listed a few unoccupied high-energy orbitals. Those shown here are all anti-bonding. The energies and assignments of the MO:s is shown in this diagram »
There is an unpaired electron in MO 39, which mainly consists of a 3dxy vanadium orbital. This MO is called the SOMO (semi-occupied MO). The electron spin density is mainly localized on the vanadium atom, but there is also significant spin density (with opposite sign) on the oxyanion (select "Electron spin density" above). The spin density makes the vanadyl ion paramagnetic and suitable for studies with EPR (electron paramagnetic resonance) spectroscopy.
Below the SOMO (on an energy scale) is the HOMO (highest occupied MO, MO 38) which is fully occupied by two electrons, as are the other lower orbitals. Immediately above the SOMO is the LUMO (lowest unoccupied MO, MO 40) which doesn't contain any electrons, as is the case for the orbitals at higher energies.
Spectroscopic properties
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Computational details
The results reported here have been computed by Örjan Hansson with density functional theory (DFT) using the quamtum-chemical program package ORCA (version 2.9.1) [Neese, 2012].
Initital coordinates were taken from [Ballhausen & Gray, 1962] and the geometry was optimized with ORCA in three steps: First, vibrational frequencies were calculated with a semiempiric method (NDDO/ZNDDO_1). Secondly, the resulting Hessian was used in an initital optimization with DFT using UKS wave functions and the BP86 functional with grid 2 and the RI approximation. The basis used was SVP (with SV_J for RI). Finally, this was followed by a refined optimization with DFT using UKS wave functions and the BP86 functional with grid 4 and the RI approximation. The basis used was TZVP (with TZV_J for RI). A relativistic DKH2 treatment was included in the third step as well as a COSMO model of water solvent.
Molecular orbitals, Löwdin atomic charges, spin populations, bond orders and molecular properties (dipole moment, optical absorption spectrum and EPR parameters g and A) were then calculated as for the third step above, but using the B3LYP hybrid functional (no RI or TZV_J) and CP basis functions for the vanadium atom.
Surfaces depicting electron and spin densities and selected molecular orbitals were saved to files on a Gaussian CUBE format. These files were then converted to the JVXL format using Jmol, resulting in a reduction of file sizes from approximately 3 MB to 7 kB and a drastically faster loading of the figure above.
The computations were done on a 2.3 GHz Intel Core i5 PC under Windows 7. The three geometry optimization steps required 1.1, 5.5 and 48 min, respectively, while the property calculation required 72.5 min.
References
- Ballhausen, C.J. & Gray, H.B. (1962) Inorg. Chem. 1, 111-122.
- Neese, F. ORCA - An ab initio, DFT and semiempirical SCF-MO package, Version 2.9; Max Planck Institute for Bioinorganic Chemistry, Muelheim/Ruhr, Germany, 2012.