Magnetic and High-Field EPR Studies of New Spin-Frustrated Systems Saritha Nellutla
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Florida State University Libraries Electronic Theses, Treatises and Dissertations The Graduate School 2006 Magnetic and High-Field EPR Studies of New Spin-Frustrated Systems Saritha Nellutla Follow this and additional works at the FSU Digital Library. For more information, please contact [email protected] THE FLORIDA STATE UNIVERSITY COLLEGE OF ARTS AND SCIENCES Magnetic and High-Field EPR Studies of New Spin-Frustrated Systems By SARITHA NELLUTLA A dissertation submitted to the Department of Chemistry and Biochemistry in partial fulfillment of the requirements for the degree of Doctorate of Philosophy Degree Awarded: Spring Semester, 2006 The members of the Committee approve the dissertation of Saritha Nellutla defended on January 9, 2006. Naresh Dalal Professor Directing Dissertation James Brooks Outside Committee Member Johan van Tol Committee Member Oliver Steinbock Committee Member Igor Alabugin Committee Member Approved: Naresh Dalal, Chair, Department of Chemistry and Biochemistry The office of Graduate Studies has verified and approved the above named committee members. ii ACKNOWLEDGEMENTS I dedicate this manuscript to my beloved parents Kishan Rao and Rama Devi Nellutla and sisters Swetha Nellutla and Shobitha Nellutla, whose never-ending love and support have been my joy and stabilizing force not only these past five years but also through out my life. Though an adequate list of all my friends to whom I owe my gratitude is long, I would like to acknowledge Shivdas, Stephanie, Vijay and Seshu for their friendship, hospitality, knowledge, and wisdom. Their constant encouragement has helped me to keep my goals in perspective. I would also like to thank the Dalal group members who have provided a great amount of assistance over the past few years. I would like to acknowledge all my high school and undergraduate teachers for influencing the outcome of my education. I would not have been in Chemistry were it not for my high school teacher Mr. Naresh Ankaraju, whose patience allowed a young mind to mature. I express my deepest admiration to my research advisor, mentor and friend, Prof. Naresh Dalal, whose optimistic attitude and scientific talent has been a continuous inspiration through out my graduate school and will remain as the high standard by which I will judge my professional development. Further acknowledgement goes to Dr. Ulrich Körtz’s group at International University of Bremen, Germany for his synthetic expertise and Dr. Johan van Tol at National High Magnetic Field Laboratory, Florida, for helping with the High-Field EPR instruments. Last but not least, I would like to thank all the staff members of Department of Chemistry and Biochemistry at Florida State University for their help with all aspects of technical assistance. iii TABLE OF CONTENTS List of Tables……………………………………………………………………viii List of Figures……………………………………………………………………..x Abstract………………………………………………………………………….xiv 1. INTRODUCTION…………………………………………………………….1 1.1 Introduction……………………………………………………………….1 1.2 Overview………………………………………………………………….5 1.3 Organization……………………………………………………………...19 2. MAGNETIZATION AND EPR CHARACTERIZATION OF A TRI- COPPER(II) COMPLEX: A MODEL SPIN-FRUSTRATED TRIANGULAR SYSTEM…………………………………………………………………………20 2.1. Introduction……………………………………………………………...21 2.2. Synthesis and Experimental Details……………………………………..22 2.2.1. Synthesis………………………………………………………….22 2.2.2. X-ray Crystallography……………………………………………22 2.2.3. dc Magnetic Susceptibility measurements……………………….23 2.1.1. Powder and Single crystal EPR measurements…………………..23 2.3. Molecular Structure……………………………………………………...24 2.4. Magnetic Susceptibility………………………………………………….27 2.4.1. Theoretical Model………………………………………………...27 2.4.2. Experimental Data Analysis……………………………………...31 2.5. EPR Spectroscopy……………………………………………………….33 2.6. Summary………………………………………………………………...38 2+ 3. EPR AND MAGNETISM OF A (Cu )4 SUBSTITUTED GERMANO TUNGSTATE: A TRIPLET GROUND STATE…..………………………...40 3.1. Introduction……………………………………………………………...41 3.2. Experimental Details…………………………………………………….42 iv 3.2.1. Synthesis………………………………………………………….42 3.2.2. X-ray Crystallography……………………………………………42 3.2.3. Magnetic and EPR Measurements……………………………….43 3.3. Crystal and Molecular Structure………………………………………...44 3.4. Magnetic Studies…………………………………..…………………….46 3.4.1. Theoretical Model………………………………………………..46 3.4.2. Magnetic Susceptibility and Magnetization……………………...48 3.5. EPR Spectroscopy……………………………………………………….53 3.6. Summary………………………………………………………………...57 4. MAGNETIC AND EPR PROBING OF THE SPIN GROUND STATE OF A COPPER(II) PENTAMER………………………………………………...58 4.1. Introduction……………………………………………………………...59 4.2. Synthesis and Experimental Details……………………………………..61 4.2.1. Synthesis………………………………………………………….61 4.2.2. X-ray Crystallography……………………………………………61 4.2.3. Magnetic Measurements………………………………………….62 4.2.4. Powder EPR measurements………………………………………62 4.3. Molecular Structural Details……………………………………………..62 4.4. dc Magnetic Susceptibility and Magnetization Studies………………….65 4.5. High Frequency EPR Studies……………………………………………72 4.6. Summary………………………………………………………………...77 5. MAGNETIZATION AND EPR STUDIES OF AN IRON(III) HEXAMER: DIAMAGNETIC GROUND STATE WITH LOW LYING EXCITED STATES…………………………………………………………………………..79 5.1. Introduction……………………………………………………………...80 5.2. Synthesis and Experimental Details……………………………………..81 5.2.1. Synthesis………………………………………………………….81 5.2.2. X-ray Crystallography……………………………………………82 5.2.3. dc Magnetic Susceptibility Measurements……………………….82 v 5.2.4. EPR measurements………………………………………………83 5.3. Molecular Structure……………………………………………………..83 5.4. Magnetochemistry………………………………………………………87 5.4.1. Theoretical Model………………………………………………..87 5.4.2. Magnetic Susceptibility Data Analysis…………………………..89 5.5. EPR Spectroscopic Studies……………………………………………...92 5.6. Summary………………………………………………………………...97 2+ 6. MAGNETIC AND HIGH-FIELD CHARACTERIZATION OF A (Co )15 CLUSTER: A TRIANGLE OF TRIANGLES COUPLED SYSTEM…………100 6.1. Introduction……………………………………………………………101 6.2. Synthesis and Experimental Details…………………………………...103 6.2.1. Synthesis………………………………………………………..103 6.2.2. X-ray Crystallography………………………………………….103 6.2.3. dc Magnetic Susceptibility and EPR Measurements…………...103 6.3. Structural Details………………………………………………………104 6.4. Magnetic Studies………………………………………………………108 6.4.1. Theoretical Model………………………………………………108 6.4.2. dc Magnetic Susceptibility Data Analysis…………………...…112 6.5. High-Frequency EPR Studies….………………………………………116 6.6. Summary………………………………………………………………116 7. CONCLUSIONS AND CRITIQUE………………………………………..120 APPENDIX A INSTRUMENTAL DETAILS OF THE HIGH FREQUENCY SET UP………………………………………………………...123 APPENDIX B BASIS FUNCTIONS, EXCHANGE MATRIX AND 2+ EIGENVALUES FOR THE (Cu )5 PENTAMER OF SiCu5…………………125 B.1. Basis Functions…….…………………………………………………..125 B.2. Exchange Matrix and Eigenvalues……………..………………………125 APPENDIX C COMPUTER CODES TO CALCULATE THE SPIN 3+ 2+ EXCHANGE ENERGIES OF (Fe )6 AND (Co )9 CLUSTERS……………..131 vi 3+ C.1. (Fe )6 Hexamer………………………………………………………..131 C.1.1. Review of spin-exchange model………………………………..131 3+ C.1.2. Program Code for (Fe )6 hexamer of GeFe6…………………..133 2+ C.2. (Co )9 Nonamer……………………………………………………….135 C.2.1. Review of spin-exchange model………………………………..135 2+ C.2.2. Computer code for (Co )9 cluster of SiCo15…………………...137 BIBLIOGRAPHY………………………………………………………………139 BIOGRAPHICAL SKETCH…...……………………………………………….151 vii LIST OF TABLES Table 2.1 Crystal Structure Data for AsCu3……………………………………..23 Table 2.2 Axial and Equatorial Bond Distances and Angles of Copper(II) Triangle…………………………………………………………………………..24 Table 2.3 Possible Spin states and their Energies for Equatorial and Isosceles Exchange Models………………………………………………………………...29 Table 3.1 Crystal Structure Data for GeCu4……………………………………..43 Table 3.2 Selected Bond Distances and Angles for GeCu4……………………...44 Table 3.3 Spin state Energies of Copper Tetramer………………………………48 Table 4.1 Crystal Structure Data for SiCu5……………………………………...61 2+ Table 4.2 Bond Distances and Angles for the (Cu )5 core of SiCu5..…………..65 Table 4.3 Spin Exchange Hamiltonian Eigenvalues for SiCu5………………….69 Table 5.1 Crystal Structure Data for GeFe6……………………………………..82 3+ Table 5.2 Bond Distances and Angles for the (Fe )6 core of GeFe6……………85 Table 5.3 Intra- and inter-trimer Fe···Fe Distances in GeFe6…………………....88 3+ Table 5.4 Various Spin state Energies and their Degeneracies for (Fe )6 cluster…………………………………………………………………………….90 Table 6.1 Crystal Structure Data for SiCo15……………………………………104 2+ Table 6.2 Bond Distances and Angles of Co ions in SiCo15…………………107 Table 6.3 Intra- and inter-trimer Co···Co Distances in SiCo15………………...107 2+ Table 6.4 Various Spin state Degeneracies for (Co )9 cluster………………...110 2+ Table 6.5 Spin Exchange Energy levels for (Co )9 cluster……………………111 2+ Table B.1 Eigenfunctions set for (Cu )5 pentamer of SiCu5…………………..126 viii 2+ Table B.2 Exchange Eigenvalues of (Cu )5 pentamer of SiCu5……………....130 3+ Table C.1 Sample output file of (Fe )6 hexamer of GeFe6……………………134 Table C.2 Sample list of (Co2+)9 exchange energies generated by the code…...138 ix LIST OF FIGURES Figure 1.1 Diagram of a Spin-frustrated Equilateral Triangle………………….....3 Figure 1.2 Structures of Complete and Tri-lacunary Keggin Moieties…………...4 Figure 1.3 Structure of {Na3Cu3(H2O)9O12} unit of AsCu3………………………7 Figure 1.4 Combined Polyhedral and Ball and Stick representation of AsCu3 molecule…………………………………………………………………...8 Figure 1.5 Polyhedral representation of GeCu4 molecule………………………...9 Figure 1.6 Ball and Stick representation of {Cu4(H2O)4O4} moiety of GeCu4….10 Figure 1.7 Polyhedral