Iowa State University Capstones, Theses and Retrospective Theses and Dissertations Dissertations 1978 Determination of bond energies by mass spectrometry: some transition metal carbonyls Gary Dean Michels Iowa State University Follow this and additional works at: https://lib.dr.iastate.edu/rtd Part of the Physical Chemistry Commons Recommended Citation Michels, Gary Dean, "Determination of bond energies by mass spectrometry: some transition metal carbonyls " (1978). Retrospective Theses and Dissertations. 6576. https://lib.dr.iastate.edu/rtd/6576 This Dissertation is brought to you for free and open access by the Iowa State University Capstones, Theses and Dissertations at Iowa State University Digital Repository. It has been accepted for inclusion in Retrospective Theses and Dissertations by an authorized administrator of Iowa State University Digital Repository. For more information, please contact [email protected]. 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University Microfilms International 300 North Zeeb Road Ann Arbor, Michigan 48106 USA St. John's Road, Tyler's Green High Wycombe, Bucks, England HP10 8HR 7904003 I MICHELS, GARY DEAN DETERMINATION DF BOND ENERGIES BY MASS SPECTROMETRY. SOME TRANSITION METAL CARaOMYLS. lOKA STATE UNIVERSITY, PH.D., 1976 UniversiW MicrOTilms Intemationcil 300 n. zeeb road, anw arbor, mi 48106 Determination of bond energies by mass spectrometry. Some transition metal carbonyls by Gary Dean Michels A Dissertation Submitted to the Graduate Faculty in Partial Fulfillment of The Requirements for the Degree of DOCTOR OF PHILOSOPHY Department; Chemistry Major: Physical Chemistry Approved: Signature was redacted for privacy. In Charge I^or Work Signature was redacted for privacy. For the Majqj^ Depart&nt Signature was redacted for privacy. For the Graj^te College Iowa State University Ames, Iowa 1978 ii TABLE OF CONTENTS Page I. INTRODUCTION 1 II. BACKGROUND 3 A. Bond Energies 3 1. Defined quantities 3 2. Approximate quantities 21 3. Determination by mass spectrometry 29 B, Appearance Potential Measurements 38 1. Positive ions 38 2. Negative ions 43 III. INSTRUMENTAL 45 A, Mass Spectrometer 45 B, Computer 47 C, Interface 49 1. Hardware 49 2, Software 55 a. Description of the system 56 1). Acquire 61 2). Background subtract 64 3). Chain 65 4), Delta-least-squares 65 5). Eliminate 67 6), File save 67 7). IE print 67 8), Mass print 67 9). Normalization parameters 67 10), Optimize 68 11). Print table 68 12), Read data 70 13). Set electron energy 70 14). Time 70 15). Warren's plots 70 iii Page b. Operation of the system 74 c. Limitation of the system 76 3. Tests of the interface 77 IV. EXPERIMENTAL 85 A, Preparation and Synthesis 87 1. Pentacarbonyl bromides of Mn, Tc, and Re 87 a. Mn(C0)5Br 88 b. Tc(C0)5Br 88 c. Re(C0)5Br 88 2. Mixed-metal decacarbonyls 89 a. MhRe(CO)^Q 90 b. MnTc(CO)iQ 91 c. TcRe(CO)io 91 B. Purification and Identification 91 1. Group VIIB pentacarbonyl bromides 91 2. Group VIIB mixed-metal decacarbonyls 92 V. RESULTS AND DISCUSSION 100 A. Group VIB Carbonyls and Thiocarbonyls 108 1. Mass spectra 110 2. Ionization potentials 114 3. Ionic bond dissociation energies 117 4. Neutral dissociation energies and heats 132 of formation 5. Conclusions 135 B. Group VIIB Metal and Mixed-Metal Decacarbonyls 137 1. Mass spectra 141 2. Ionization potentials 141 3. Ionic bond dissociation energies 144 iv Page 4. Neutral dissociation energies and heats 158 of formation 5, Conclusions 159 VI. SUGGESTIONS FOR FURTHER RESEARCH 160 VII. BIBLIOGRAPHY 164 VIII. ACKNOWLEDGMENTS 169 IX. APPENDIX A: DIAGNOSTIC PROGRAMS 171 A. D-A Test Number One (DATSTl) 172 B, D-A Test Number Two (DATST2) 179 X. APPENDIX B: IONIZATION EFFICIENCY 182 ACQUISITION SYSTEM 1 I. INTRODUCTION A molecule can be thought of as a storehouse of energy. This energy is proportioned among the bonds of the molecule. Since most chemical reactions involve the breaking of old bonds and the making of new ones, the chemistry of the molecule is reflected by the energy of its bonds. Individual bond energies can be used to predict chemical reactivity and to calculate heats of formation [l, pp. 158- 170]. They are important in the consideration of molecular structure. Several methods have been developed to measure bond energies [2, Chapters 3-5]. One of them is mass spectrometry. The purpose of this research was to utilize a mass spectrometer to determine bond energies. The practicality of using a mass spectrometer to measure the energies of chemical bonds was demonstrated by Stevenson [3, 4], but the method was not developed to its fullest potential. The early mass apectroscopists were primarily instrumentalists whose major concern was to develop consistent methods for obtaining physical data from a mass spectrometer. There was considerable debate over whose method was superior [5, pp. 26-37], The calculation of bond energies was of secondary importance. From the onset it was apparent that the physical measurements were inherently inaccurate. The errors were difficult if not impossible to correct. Calculated values of bond en­ ergies were often 20-30% higher than the corresponding calorimetric values [6]. The value of the mass spectroscopic method was uncertain. The question of how to obtain useful information from such an inaccurate 2 method was evaded. The challenge of this research was to answer that question. If useful information is to be obtained by the mass spectroscopic method, the data must be obtained as precisely as possible and any conclusions must be formulated by comparative studies. For precision a computerized method for acquiring and interpreting the data was developed. For comparison, uhe mass spectrometry of the Group VIB carbonyls and thiocarbonyls and the Group VIB metal and mixed-metal decacarbonyls was studied. Through these studies a deeper insight into the concept of a bond energy and into the nature of the bonding in transition metal carbonyls was attained. A new molecular quantity was defined and utilized as the measure of a bond energy. 3 II. BACKGROUND A. Bond Energies The terra "bond energy" is one of the most misused terms in science [7]. The reason for this is not obvious because the phrase is correct conceptually. Chemical bonds are the binding forces between the atoms in a molecule. The energies of these interactions are rightfully referred to as bond energies. But there is no unique, measurable quantity known as a bond energy. Rather, there are three quantities which are collectively known as bond energies. They are bond dissociation energy, bond energy term, and intrinsic bond energy [8-16, 17, pp. 153-167], 1. Defined quantities The idea of a boTid dissociation energy originated from the principle that the strength of a chemical bond could be given by the energy required to break it. The concepts of a bond energy term and an intrinsic bond energy developed from the approximation that the heat of formation of a compound consists of several independent terms. For an ideal gas these terms are the vibrational zero point energy; thermal energy of vibration, rotation, and translation; and the chemical binding energy. Thus, the heat of atomization of a molecule in the ideal gas state can be related to the strength of the bonds comprising the molecule. Although the concepts are founded upon different principles, each quantity can be represented by the energy of a reaction. 4 The energies are enthalpy changes measured at 25° C and referenced to the ideal gas state. The reactions are the defining processes for each of the three quantities. Because these processes can be either ionic or molecular, a distinction must be made between bond energies in the cation, molecule, or aniori^. 2 The dissociation energy of a bond R-S in the positive ion, molecule, or negative ion, S), is defined as the enthalpy change for the dissociation: '-4:)'-4:)' +'(g)' A reactant molecule (ion) in its ground state dissociates into two fragments by the cleavage of a chemical bond.