Unique Magnetic Signature of Transition Metal Atoms on Organic Template Anil K. Kandalam* Department of Physics Michigan Technological University In collaboration with: Prof. Ravi Pandey, Prof. B. K. Rao*, Prof. P. Jena* *Physics Department, Virginia Commonwealth University, Richmond, VA

Overview of this Presentation Introduction Background Previous Studies by our research group Computational Method Results Present Work Computational Procedure Results from Present work Summary

Interaction of clusters with metallic/organic supports is important Introduction Stable nano-clusters can act as building blocks for novel materials Problem Bare clusters interact with each other and finally coalesce at short distances and hence are not stable in proximity of each other Solution Weakly interacting clusters Isolating the clusters from each other Inserting them in zeolites Depositing on substrates Coating with organic materials Interaction of clusters with metallic/organic supports is important

Background What happens when transition metal atoms are supported on organic molecular surfaces ??

Questions to be Answered Effect of organic substrates on the magnetic moments of transition metal atoms Equilibrium geometries of transition metal organic complexes in gas phase Energetics like Electron Affinity, Ionization Potential and Dissociation Energies Solutions Experimental Studies (Laser Vaporization techniques) Theoretical Calculations

Previous Studies Experimental Studies: Theoretical Studies: K. Judai, M. Hirano, H. Kawamata, S. Yabushita, A. Nakajima, K. Kaya, Chem. Phys. Lett. 270 (1997) 23 P. Weis, P. R. Kemper, M. T. Bowers, J. Phys. Chem. A. 101 (1997) 8207 (Expt and theory) T. Kurikawa, H. Takeda, M. Hirano, K. Judai, T. Arita, S. Nagao, A. Nakajima, K. Kaya, Organometallics 18 (1999) 1430 D. van Heijnsbergen, G. von Helden, G. Meijer, P. Maitre, M. A. Duncan, J. Am. Chem. Soc. 124 (2002) 1562 Theoretical Studies: S. M. Mattar, W. Hamilton, J. Phys. Chem. 93 (1989) 2997 C. W. Bauschlicher, H. Partridge, S. R. Langhoff, J. Phys. Chem. 96 (1992) 3273

Previous Theoretical Studies by our group M – (Benzene) systems Chem. Phys. Lett. 321 (2000) 142-150 Mn – (Benzene)m (n = 1; m = 1, 2) systems J. Am. Chem. Soc. 123 (2001) 3799-3808 M = 3d Transition metal atoms

Computational Method DMol program : “Density functional theory calculations for Molecules” Density Functional Theory (DFT) based calculations Exchange-Correlation functional form: BPW91 Basis set : Double Numeric supplemented with polarization functions (DNP) Geometry Optimization: C6v symmetry constraint for M-(Benzene) complexes D6h symmetry constraint for M-(Benzene)2 complexes

M-(Benzene) Complex M Neutral Anion Sc 2.00 2.08 Ti 1.97 1.59 V 1.61 Cr 1.85 Mn 1.52 1.68 Fe 1.50 1.63 Co 1.46 1.34 Ni 1.45 Distances are given in Angstroms

Distances are given in Angstroms M-(Benzene)2 Complex M Neutral Anion Sc 1.97 1.95 Ti 1.78 1.75 V 1.67 1.68 Cr 1.60 1.62 Mn 1.69 Fe 1.72 Co 1.83 1.76 Ni 1.91 1.86 Distances are given in Angstroms

Variations in Spin Multiplicity

De[M(Bz)2] = -{E[M(Bz)2] – E[MBz] – E[Bz]} Dissociation Energy De(MBz) (eV) De[M(Bz)2] (eV) M Theo. Expt. Sc 1.78 2.04 Ti 1.71 0.96 3.32 3.20 V 0.81 0.79 3.57 3.19 Cr 0.09 0.12 2.78 2.70 Mn 0.37 1.18 Fe > 0.7 1.09 Co 1.83 0.34 0.42 Ni 1.70 0.87-1.30 0.02 De[M(Bz)2] = -{E[M(Bz)2] – E[MBz] – E[Bz]}

Brief Summary The structure of M(Benzene) complexes have C6v symmetry M-(Benzene)2 complexes prefer sandwich structures M-(Benzene) complexes: Magnetic moments of Sc, Ti, and V atoms are increased Mn, Fe, Co and Ni are decreased from their free – atom values M-(Benzene)2 complexes: Magnetic moments are quenched to their lowest possible values Addition of extra benzene significantly increases the binding energy of M-(Benzene)2 complexes (Sc to Mn)

Extension and re-examination Process M – (Benzene)2 geometries: Restricted to sandwich structures (D6h symmetry) Co – (Benzene)2 and Ni – (Benzene)2: Not in agreement with experimental results No experimental or theoretical works on negatively charged Mn – (Benzene)m (n >1; m > 2) complexes Anionic complexes are useful in studying the photo-detachment spectroscopy

Present Work System under Study: Neutral and Anionic Vn – (Benzene)m (m = n +1, n = 1-3) complexes Aim: To identify the equilibrium geometries and ground state spin multiplicities Calculate the Electron Affinity (EA) Ionization Potential (IP) Dissociation Energies (De)

Computational Procedure Gaussian98 program suite More flexibility in the basis sets Density Functional Theory (DFT) based calculations Gradient Corrected Density Functional: BPW91 Exchange functional: Becke88 Correlation functional: Perdew-Wang91 Two different basis sets are used Lanl2dz (frozen core) 6-311G** (all electron basis)

Geometry and Spin Optimization A two-step Optimization Approach BPW91/Lanl2dz: Geometry optimization for all the possible spin states BPW91/6-311G**: For the lowest energy spin state, geometry re-optimization Geometry optimization was done without any symmetry constraints

VBz Complex

V(Benzene)2 Complexes

Rice Ball to Sandwich

Results for V(Benzene)2 Neutral Anion Lanl2dz 6-311G** A 1.44 1.43 B 1.09 C 1.69 1.67 1.68 1.66 All the distances are in Angstroms Staggered sandwich is 0.05 eV higher in energy Anionic V(Benzene)2 is unstable against auto-detachment

V2(Benzene)3 Complexes

Results for V2(Benzene)3 Distances are given in Angstroms Neutral Anion Lanl2dz 6-311G** A 1.45 1.43 B 1.46 1.47 C 1.09 D 1.67 1.65 1.68 1.64 E 1.75 1.73 1.71 1.70 Staggered Sandwich: 0.09 eV and Rice-ball: 0.68 eV higher in energy

V3(Benzene)4 Complexes Rice-ball structure is not considered Only Lanl2dz basis set is used for the calculations

Results for V3(Benzene)4 Distance (Å) Neutral Anion A 1.45 B 1.47 C 1.09 D 1.66 1.65 E 1.76 1.78 F 1.72 1.68

Variation of bond distances in Benzene

Variation in Spin Multiplicities Lanl2dz based results

Vertical Ionization Potential

Vertical Ionization Potential (eV) System Lanl2dz 6-311G** Expt. VBz 5.53 5.71 5.11 ± 0.04 VBz2 5.87 5.96 5.75 ± 0.03 V2(Bz)3 4.73 4.82 4.70 ± 0.04 V3(Bz)4 4.07 ---- 4.14 ± 0.05

Electron Affinity (eV) System Vertical Adiabatic Expt. (Adiabatic) VBz 0.52 0.44 0.62 ± 0.07 VBz2 -0.48 -0.50 Negative V2(Bz)3 0.16 0.13 ---- V3(Bz)4 0.56 Calculations are performed using Lanl2dz basis set

Dissociation Energy System BPW91/Lanl2dz (eV) Expt. (eV) VBz 0.67 0.79 De (VBz) = - [E (VBz) – E (V) – E (Bz)] De [V(Bz)2] = - [E (VBz2) – E (VBz) – E (Bz)] De [V2(Bz)3] = - [E (V2Bz3) – E (VBz2) – E (VBz)] De [V3(Bz)4] = - [E (V3Bz4) – E (V2Bz3) – E (VBz)] System BPW91/Lanl2dz (eV) Expt. (eV) VBz 0.67 0.79 VBz2 3.13 3.19 V2(Bz)3 2.32 ---- V3(Bz)4 2.29

Summary Vn – (Benzene)m systems prefer sandwich structures to rice-ball structures C – C and C – H distances are independent of the size of the system Magnetic moment of Vn – (Benzene)n+1 complexes increases linearly with the size of the system (i.e., n) Negligible geometrical changes upon addition of an electron V – (Benzene)2 anion is unstable against auto-detachment of extra electron Ionization Potential decreased with an increase in the size of the system