1. PBE-GGA Calculations on Hydroxyl Substituted Mn12O12(COOH)16 (H2O)4
Javaria Batool1,2 and Mark R Pederson1
1Department of Chemistry, The Johns Hopkins University, Baltimore, MD USA
2Department of Physics, Government College University, Faisalabad, Pakistan
We investigated a dehydrogenated Mn12-Acetate compound (hydroxyl substituted for water). This magnetic molecule has an electronic and geometric structure that is similar to the oxygen-evolving complex (OEC) found in photosystem II (PSII). In contrast to the OEC, Mn12-Acetate is easy to synthesize and non-destructive measurements of the magnetic properties of this molecule, from both theory and experiment, are in good agreement. Here, we concentrate on identifying relationships between the magnetic properties and the structural properties for two different purposes. First, from the standpoint of molecular magnetism, a long-term goal has been to determine whether it is possible to increase the magnetic anisotropy by modifying the ligands surrounding the Mn atom. For example there has been a report that replacing the water molecule with methanol can enhance the magnetic strength.  In addition to considering the possibility of strengthening the magnetism by water-methanol substitution, we have considered many other ligand-transition metal substitutions. We have determined that the original molecular magnetic properties are already optimal and difficult to improve. However, we have found that certain ligand leads to a significant chemical re-arrangement , which brings us to the second purpose for these studies. Understanding how OEC catalyze chemical rearrangements is a problem of significant interest and one, which is likely to grow as the research community attempts to understand how water is converted into oxygen and/or hydrogen.  Conventional mechanisms suggest that 4-electron / 4- proton interactions are important for making the reaction proceed and assume that these interactions are driven by multiple solar. Such multiple-electron mechanisms allow for total spin conservation due the simultaneous appearance of multiple oxygen dimers. Because the Mn12-Acetate molecule naturally provides four reaction centers and also has four nearly degenerate LUMO levels for storage of four electrons and because the computed magnetic structure of Mn12-Acetate, within DFT, agrees so well with experiment, this molecule can be viewed as a “model reactor” with many features that are similar to the oxygen evolving complex. Since it is well characterized experimentally and theoretically it may provide a molecular-scale “lab-experiment” for understanding important mechanisms that are relevant to catalysis. Here show that the magnetic anisotropy, a physical characteristic that can be non-destructively measured, can be used to Hydrogen generation
ELECTRONIC & ABSORPTION PROPERTIES
RESEARCH QUESTIONS
MAGNETIC STRUCTURE
The maximum absorption probability of molecule is found in region 3.01 to 4.73 eV. As it is experimentally found the incident photon energy should equal to 3.3 eV water splitting. This shows that the molecular magnet has optical properties similar to those that might be used for practical water splitting purposes and is therefore a good model reactor.
Investigation of effect of substitution on the magnetic, physical and absorption properties of Mn12 O12 -acetate?
Investigation of relationship between physical properties and chemical reaction ?
Investigation of properties of Mn12O12 _OH for possible hydrogen production from water?
Molecule
Local magnetic moment
Total magnetic moment ( µB)
Gap (eV)
Anisotropy (K)
Mn12O12_acetate
Mn001:+3.6 20
0.5
58.83
Mn002: 2.6
Mn003:+3.6
Mn12O12_OH
Mn001:+2.7 16
34.64
Mn002:-2.6
THEORY AND COMPUTATIONAL APPROACH
For the optimization of geometry of molecules DFT based Naval Research Laboratory Molecular Orbital Library (NRLMOL) computational code has been used. NRLMOL performs massively parallel electronic structure calculations by employing Gaussian orbital methods[3]. Further spin–polarized calculations are performed with the inclusion of spin orbit coupling in order to check the temperature dependence of magnetic properties.
Electron detachment spectra from the anion have been calculated using PBE-GGA, NRLMOL and very large Gaussian-orbital basis sets and there is relatively good agreement between theory and experiment.
Synopsis: The magnetic signature associated with conversion of water to hydrogen molecules and a hydroxyl substituted Mn12-Acetate molecule is shown in the table. A sharp reduction in the magnetic anisotropy results when the spin on the active crown Mn atoms decreases from S=2 (m=3.6) to S=3/2 (m=2.7)
We investigated the electronic properties of molecule in term of electronic DOS. Total electronic density of states (DOS) for hydroxyl substituted Mn12O12are presented along with Mn(3d) and O(2p) partial DOS in Fig. 3. The Fermi level is at eV is indicated by drawing a line. The energy gap between highest occupied molecular orbital (HOMO) to the lowest unoccupied(LUMO) molecular orbital is found to 0.5 eV. It can be easily seen that the major contribution of Mn(3d) below and above the Fermi level to total DOS. While O (2p) has minor contribution to total DOS. Therefore we predicted that Mn atom is main cause of light absorption in Mn12O12 _OH and it also mainly contributed to the magnetic properties of system. Also the HOMO–LUMO gap for majority spin and minority spins are found about 0.47 eV and 1.93 eV approximately. Fig. 4 is representing the spin polarized (Majority and Minority)DOS for outer and inner Mn atom. Mn001 and Mn003 are representing the Mn atoms in outer ring of molecule while Mn002 is representing Mn in inner ring.
MAGNETIC PROPERTIES
As found in experiments the Mn12O12-Acetate has a ferrimagnetic structure with inner core of Mn4O4 and while in the outer ring has eight Mn. Four Mn of outer ring has collaboration with water . Therefore we replaced Mn-H2O unit with Mn- in the outer ring and find following molecular structure )4.It is found that the substitution of OH- strongly effect magnetic properties of material. We designed molecule is found to a ferromagnetic structure in which inner Cubane of Mn4O4 and outer core of Mn contributed to minority and majority spin respectively. The calculated locall magnetic moment for inner core Mn 002 (RMT=2.23) is found about -2.6 µB. while the four Mn001 (RMT=2.23) which is connected to water molecules have local magnetic moment of µB for each Mn, and the remaining four Mn003 (RMT=2.23) in outer ring has local magnetic moment about 3.6 µB for each Mn atom. The difference of magnetic moment refers a system which has spin polarized electronic states. The total magnetic moment of system is found approximately 16 µB. Since spin flip energy is an important Physical parameter to find the stability of any magnetic system as it gives temperature dependence of magnetism of any magnet [4]. Therefore by inclusion of spin orbit coupling (SOC), we find the value of magnetic anisotropy for Mn 12O12 _OH about 34K.
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