DOCSLIB.ORG

Electron Paramagnetic Resonance (EPR) Technique Is Useful to Quantify the Paramagnetic Species in Any Matrix

Total Page：16

File Type：pdf, Size：1020Kb

Download full-text PDF Read full-text
Electron Paramagnetic Resonance (EPR) Technique Is Useful to Quantify the Paramagnetic Species in Any Matrix EPR Dosimetry – An Update and Prospective Studies S. Murali1a, T. S. Sudheera, S. Thanamania, D. P. Ratha, J. A. Sapkala, V. Natarajanb, M. K. Bhideb, D.N. Sharmaa. a. Radiation Safety Systems Division, b. Radio Chemistry Division, Bhabha Atomic Research Centre, Trombay, Mumbai – 400 085, India. Abstract. Electron Paramagnetic Resonance (EPR) technique is useful to quantify the paramagnetic species in any matrix. The unpaired electrons present in paramagnetic materials have non – zero spin value, have an associated spin magnetic moment. When such a system is subjected to an external magnetic field, electronic Zeeman splitting of ground level state occurs. On application of suitable stimulant microwave energy, the electrons flip between the Zeeman levels of ground state, result in resonant absorption of the microwave energy. The intensity of resonant absorption signal is proportional to the concentration of the unpaired electrons in the irradiated material, could lead to possible use of such materials in EPR dosimetric applications. New materials were investigated for EPR dosimetry, wherein the radiation induced paramagnetic species retains the radiation signatures, lead to idea on radiation dose. Few of the materials have been identified as prospective EPR dosimeters. The radiation induced radical in Li2CO3 powder material being paramagnetic in nature (signals at g = 2.0036 and at g = 2.0006) and radical concentration varying as a function of irradiation dose, led to its identification for possible use in EPR dosimetric applications. Besides, during the neutron irradiations, 6 3 the reaction Li (n,α) H, led to the yield of radicals many folds higher compared to that of gamma irradiation. Thus, the commonly available Li2CO3 material has been assessed for the EPR dosimetric response in gamma and neutron environments. EPR investigation of Li2C2O4, Na2C2O4 mixtures was carried out to measure the radiation dose from γ - photons and thermal neutrons in a mixed radiation field. A single line spectrum of CO2 radical at g = 2.0045 ± 0.0005 was found on gamma and neutron irradiations. Of all the mixture combinations, the 2:1 - mixture was found more sensitive for gamma / thermal neutrons. Intensity of CO2 radical signal was found linear from 6 Gy – 11 kGy for gamma and 40 – 1500 kGy for thermal neutron flux. The radiation induced radical signal was found to be stable over a period of 300 days with marginal fading of < 1 %. The results of EPR dosimetry suggest that the Li2C2O4: Na2C2O4 mixture as the potential neutron dosimeter for high range dosimetry. The effect of gamma dose irradiation on sodium succinate was studied by EPR technique. It was − observed that the radiation induced CO3 radical (g = 2.00357) as linear in signal – dose response, in 35 − Gy – 4.4 kGy. Thus, sodium succinate powder samples could be used in EPR dosimetry, since CO3 radicals have been found stable for more than 6 months, post-irradiation. Solid State Nuclear Track Detector (SSNTD) material Tuffak polycarbonate film was studied for prospective EPR dosimetry, feasibility studies were carried out on gamma irradiated SSNTD film. The first derivative EPR spectra of irradiated Tuffak polycarbonate samples contained a singlet, signal at g = 3− 3− 2.00415. The signal was identified as CO3 from earlier reports. The EPR signal intensity of CO3 (g = 2.00415) was found linear in signal – dose response in 10 - 80 kGy. The present paper gives an update of newer EPR dosimetric materials that have been investigated, after a brief introduction to the basic principles of EPR. Further, prospective dosimetric materials with their suitability for applications in EPR dosimetry have been discussed. KEY WORDS: Electron Paramagnetic Resonance Dosimetry, Radicals, Signal – Dose, kGy. Author’s Email: [email protected] 1. Introduction Dosimetry is an area of interest to Health Physicists, Medical Physicists, accidental/high dose assessments during radiation incidences and during planned high irradiation for industrial products, pasteurisation of food materials, spices, radiation sterilisation of medical products etc., at industrial irradiators. Varieties of techniques are presently available for the dosimetry depending upon the: requirements of dose measurement ranges, convenience of dose measurements, dosimetric material characteristics and reproducibility of the dose measurements. The interaction of radiation with matter is best understood from the study of the end products generated. A large number of radiation dosimetric processes have a focused interest in the unpaired electrons / free radicals generated as radiation damages in such substances. Thus the study of changes in the concentrations of unpaired electrons / free radical as the radiation induced damages is found to be useful in the dosimetric studies[III]. Hence the technique wherein the concentration of the unpaired electrons could be easily quantified using the associated paramagnetic property may have applications in the field of dosimetry. 2. Electron Paramagnetic Resonance (EPR) technique Electron Paramagnetic Resonance (EPR) is the study of direct transitions between the electronic Zeeman levels of ground state. EPR is a method of magnetic resonance under suitable microwave stimulation, enabling to detect the unpaired electrons by their magnetic moment[II]. In the absence of an externally applied magnetic field, the spin magnetic moments have no preferred direction, are randomly oriented. If a uniform magnetic field is imposed, the components of spin angular momentum Ms get resolved due to the interaction of spin magnetic moment (of unpaired electrons) with the external magnetic field. Each unpaired electron has a spin angular momentum associated with it, due to the spin quantum number s = ½. The magnitude of the spin momentum for the unpaired electron would be Ms = + ½ or Ms = - ½, depending upon the orientation of the spin angular momentum. In an external magnetic field the electrons have a resolved component of either Ms = + ½ or Ms = - ½ in the direction of the applied field. The application of external magnetic field divides the unpaired electrons into two groups, those electrons with magnetic moments lined up parallel to the magnetic field have the energy reduced by an amount ½ gβH while those with magnetic moments lined up anti-parallel to the magnetic field have the energy increased by the same amount ½ gβH[IX]. The basic principle of the electron resonance technique is to apply the external electromagnetic radiation and arrange the frequency υ, to be such that hυ is equal to the energy difference between the two groups of electrons. The transition of electron from an energy level to another energy level can be induced by the external electromagnetic perturbation of suitable frequency and polarization. A photon of energy hυ = gβH is absorbed, it results in the transition by satisfying ∆ms = ± 1 and ∆mI = 0. EPR spectral absorption indicates that the unpaired electron in the lower energy level is more numerous than in the higher level. The population between the two energy levels is given by −−∆∆∆∆ΕΕΕΕ / kT N2 = N1 exp k – Boltzmann constant, T – Temperature in Kelvin[IX]. Since resonant absorption takes place at γγγ, in presence of the external electromagnetic radiation either of the states will be equally populated. The microwave power is generally kept low to avoid the saturation of the signal. The difference between N1, N2 can be made as large as possible applying large magnetic field H. The energy level population explains the necessity of the values of the operational parameters. With reference to the equation on population of states the parameters that can vary is ‘H’ the externally applied magnetic field. Thus, the value of the magnetic field at resonance determines the value of the g factor. 2.1 EPR dosimetry Electron Paramagnetic Resonance (EPR) technique finds application in the field of dosimetry i. due to paramagnetic species formed by irradiation of the material, with concentration of paramagnetic defect centers is proportional to the dose delivered to the specimen and ii. due to the EPR signal intensity of the resonance peak is proportional to the unpaired electrons / cc of the specimen. Hence, the signal intensity is used as a function of the radiation dose delivered to the sample in EPR dosimetry. Besides, EPR is a non-destructive, non-invasive method, highly sensitive technique (responsive for spin concentration ≥ 109 -1011/ cc)[III]. The dose measurements can be repetitive with dosimetric signal accumulated and can be used at wide range of temperatures. EPR read-out technique is based on the quantitative measure of the absorption signal of irradiated dosimeter material. Free radicals in organic materials correspond to trapped electrons or rupture of covalent bonds. The advantage of EPR dosimetry is its ability to formulate dosimeters equivalent to mammalian tissues, water and products processed by irradiation. Besides EPR spectroscopy has proven potential for dosimetry of high-level radiation. New materials / common lab reagents were investigated for EPR dosimetry, wherein the radiation induced paramagnetic species retains the radiation signatures, lead to idea on radiation dose. 3. Experimental The prospective dosimetric samples for EPR dosimetry were chosen such that the radiation signatures in the form of paramagnetic radicals are available in the commonly available reagents / chemicals in the laboratory. Powder samples about 100 mg, heated for moisture removal and in select
Load more

Recommended publications
  • Magnetism, Magnetic Properties, Magnetochemistry
    Magnetism, Magnetic Properties, Magnetochemistry 1 Magnetism All matter is electronic Positive/negative charges - bound by Coulombic forces Result of electric field E between charges, electric dipole Electric and magnetic fields = the electromagnetic interaction (Oersted, Maxwell) Electric field = electric +/ charges, electric dipole Magnetic field ??No source?? No magnetic charges, N-S No magnetic monopole Magnetic field = motion of electric charges (electric current, atomic motions) Magnetic dipole – magnetic moment = i A [A m2] 2 Electromagnetic Fields 3 Magnetism Magnetic field = motion of electric charges • Macro - electric current • Micro - spin + orbital momentum Ampère 1822 Poisson model Magnetic dipole – magnetic (dipole) moment [A m2] i A 4 Ampere model Magnetism Microscopic explanation of source of magnetism = Fundamental quantum magnets Unpaired electrons = spins (Bohr 1913) Atomic building blocks (protons, neutrons and electrons = fermions) possess an intrinsic magnetic moment Relativistic quantum theory (P. Dirac 1928) SPIN (quantum property ~ rotation of charged particles) Spin (½ for all fermions) gives rise to a magnetic moment 5 Atomic Motions of Electric Charges The origins for the magnetic moment of a free atom Motions of Electric Charges: 1) The spins of the electrons S. Unpaired spins give a paramagnetic contribution. Paired spins give a diamagnetic contribution. 2) The orbital angular momentum L of the electrons about the nucleus, degenerate orbitals, paramagnetic contribution. The change in the orbital moment
    [Show full text]
  • Electromagnetic Radiation and Quantum Theory Questions KEY
    Electromagnetic Radiation and Quantum Theory Questions KEY What you will be given for the test: A periodic table (you cannot use the blank one where you filled in the electron configuration) c=λv c=3.0x108 m/s E=hv h=6.626x10-34 J*s For the test, you should be able to answer these questions…. 1. What does ROY G. BV represent? The colors of the visible light portion of the electromagnetic spectrum. 2. How does the energy of a red photon compare to that of a blue photon? Explain. A blue photon has higher energy than a red photon. Blue photons have a shorter wavelength and therefore a higher frequency and higher energy. 3. According to the Bohr model of the hydrogen atom, how does the hydrogen atom emit light? When an electron absorbs energy, it jumps to a higher energy level. When the electron falls back down, energy is released from the atom in the form of light. 4. What is the difference between an electron at ground state and an excited electron? A ground state electron is the lowest possible energy for that electron. An excited electron is one that has absorbed energy and is in a higher energy level. 5. What accounts for different color lines (red, blue-green, blue, and violet) in the emission spectrum of the hydrogen atom? The electron releases different amounts of energy has it drops to different energy levels. Each of these drops in energies corresponds to a specific frequency and color of light. 6. Can two different elements produce the same identical emissions spectrum? No two elements can produce the same emission spectrum, it is similar to a fingerprint for an atom.
    [Show full text]
  • Structure Controlled Magnetism in Mg-Ti-O Thin Films
    In Quest of a Ferromagnetic Insulator - Structure Controlled Magnetism in Mg-Ti-O Thin Films Johannes Frantti*,1, Yukari Fujioka1, Christopher Rouleau2, Alexandra Steffen3, Alexander Puretzky2, Nickolay Lavrik2, Ilia N. Ivanov2 and Harry M. Meyer2 1Finnish Research and Engineering, Helsinki 00180, Finland 2Center for Nanophase Materials Sciences, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA 3Neutron Scattering Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA *Corresponding author: e-mail johannes.frantti@fre.fi 1 Abstract Ferromagnetic insulator thin films can convey information by spin waves, avoiding charge displacement and Eddy current losses. The sparsity of high-temperature insulating ferromagnetic materials hinders the development of spin wave based devices. Stoichiometric magnesium titanate, MgTiO3, has an electronic- energy-band structure in which all bands are either full or empty, being a paramagnetic insulator. The MgTiO3 ilmenite consists of ordered octahedra and cation network in which one third of the octahedra are vacant, one third host magnesium and one third titanium. By giving up these characteristics, a rich variety of different magnetic structures can be formed. Our experiments and electronic-energy-band-structure computations show that the magnetic and electric properties of Mg-Ti-O films can drastically be changed and controlled by Mg- and Ti-cation arrangement and abundancy in the octahedra. Insulating titanium- and semiconducting magnesium-rich films were ferromagnetic up to elevated temperatures. The presence and origin of ferromagnetic insulating phase in the films is not apparent - the expectation, based on the well- established rules set by Goodenough and Kanamori, is paramagnetic or antiferromagnetic ordering. We show that ferro- and paramagnetic phases, possessing the same stoichiometry, can be obtained by merely rearranging the cations, thus allowing defect-free interfaces in multilayer structures.
    [Show full text]
  • Nitric Oxide: a Brief Overview of Chemical and Physical Properties Relevant to Therapeutic Applications
    SPECIAL FOCUS y Nitric oxide-releasing materials Special Report For reprint orders, please contact [email protected] 1 Special Report 2015/06/30 Nitric oxide: a brief overview of chemical and physical properties relevant to therapeutic applications Future Sci. OA Nitric oxide (nitrogen monoxide, •NO) has been intensively studied by chemists and Jack R Lancaster Jr physicists for over 200 years and thus there is an extensive database of information Departments of Pharmacology & that determines its biological actions. This is a very brief overview of the chemical and Chemical Biology, Medicine, & Surgery, School of Medicine, University of physical properties of •NO that are most relevant to Biology in general and to the Pittsburgh, 4200 Fifth Ave, Pittsburgh, 1(1), FSO59 development of •NO releasing materials in particular. PA 15260, USA [email protected] Keywords: diffusion • oxidation-reduction • radical • transition metals In 1987 the startling discovery was made electron. This is due to the fact that it has that the small reactive radical molecule an odd total number of electrons (15). The nitric oxide (•NO; nitrogen monoxide) is property of possessing an unpaired electron specifically produced and functions as a is called paramagnetism and it is this prop­ major messenger in the mammalian cardio­ erty that most determines its chemical reac­ vascular system, for which the 1998 Nobel tivity. However, the small uncharged nature Prize in Physiology or Medicine was awarded of •NO is especially important in determin­ to Robert Furchgott, Louis Ignarro and Ferid ing mechanisms for its selective delivery, the Murad [1] . This discovery represented a classi­ topic of this monograph.
    [Show full text]
  • CHEM1102 2014-J-2 June 2014 • Compounds of D-Block Elements
    CHEM1102 2014-J-2 June 2014 Marks Compounds of d-block elements are frequently paramagnetic. Using the box notation • 2 to represent atomic orbitals, account for this property in compounds of Co2+. 2+ 7 Co has a 3d configuration: ↑↓ ↑↓ ↑ ↑ ↑ Co2+ is a d7 system, so must have at least 1 unpaired electron. Consequently it must be paramagnetic. CHEM1102 2014-J-5 June 2014 Marks • Name the complex [CoCl2(en)2]. en = ethylenediamine = NH2CH2CH2NH2 4 dichloridobis(ethylenediamine)cobalt(II) Draw all possible isomers of this complex. THE REMAINDER OF THIS PAGE IS FOR ROUGH WORKING ONLY. CHEM1102 2014-N-7 November 2014 Marks • The structure below represents the active site in carbonic anhydrase, which features a 7 Zn2+ ion bonded to 3 histidine residues and a water molecule. The pKa of uncoordinated water is 15.7, but the pKa of the water ligand in carbonic anhydrase is around 7. Suggest an explanation for this large change. The high charge on the Zn2+ ion draws electron density out of the O–H bonds in the water molecule. This weakens the O–H so the H+ is more likely to leave. The water in carbonic anhydrase is therefore more acidic, as shown by the large decrease in pKa. When studying zinc-containing metalloenzymes, chemists often replace Zn2+ with Co2+. Using the box notation to represent atomic orbitals, work out how many unpaired electrons are present in the Zn2+ and Co2+ ions. 2+ 10 Zn , 3d ↑↓ ↑↓ ↑↓ ↑↓ ↑↓ 2+ 7 Co , 3d ↑↓ ↑↓ ↑ ↑ ↑ Zn2+ has 0 unpaired d electrons, Co2+ has 3 unpaired d electrons. Co2+ is therefore paramagnetic and will be attracted by a magnetic field.
    [Show full text]
  • Assignment 9 Solutions Chapter 8, #8.32, 36, 38, 42, 54, 56, 72, 100
    Chemistry 08 Spring 2010 Assignment 9 Solutions Chapter 8, #8.32, 36, 38, 42, 54, 56, 72, 100, 102, Chapter 10, #10.24, 40, 55, 63. 8.32. Collect and Organize Of B3+, I–, Ca2+, and Pb2+ we are to identify which have a complete valence-shell octet. Analyze Electron Number of e– in Ion Configuration Valence Shell B3+ [He] 0 I– [Kr]4d105s25p6 8 Ca2+ [Ar] 0 Pb2+ [Xe]4f145d106s2 2 Solve The ions I– and Ca2+ have complete valence-shell octets. B3+ does not have an octet but rather the duet of the He atom. Think About It Notice that cations are formed by loss of electrons to achieve a core noble gas configuration, which leaves no electrons in the valence shell, and anions are formed by gain of electrons to give 8 electrons in the valence shell. 8.36. Collect and Organize For the diatomic species BN, HF, OH–, and CN–, we are to determine the total number of valence electrons. Analyze For each species we need to add the valence electrons for each atom. If the species is charged, we need to reduce or increase the number of electrons as necessary to form cations or anions, respectively. Solve (a) 3 valence e– (B) + 5 valence e– (N) = 8 valence e– (b) 1 valence e– (H) + 7 valence e– (F) = 8 valence e– (c) 6 valence e– (O) + 1 valence e– (H) + 1 e– (negative charge) = 8 valence e– (d) 4 valence e– (C) + 5 valence e– (N) + 1 e– (negative charge ) = 10 valence e– 8.38.
    [Show full text]
  • Magnetochemistry
    Magnetochemistry (12.7.06) H.J. Deiseroth, SS 2006 Magnetochemistry The magnetic moment of a single atom (µ) (µ is a vector !) μ µ μ = i F [Am2], circular current i, aerea F F -27 2 μB = eh/4πme = 0,9274 10 Am (h: Planck constant, me: electron mass) μB: „Bohr magneton“ (smallest quantity of a magnetic moment) → for one unpaired electron in an atom („spin only“): s μ = 1,73 μB Magnetochemistry → The magnetic moment of an atom has two components a spin component („spin moment“) and an orbital component („orbital moment“). →Frequently the orbital moment is supressed („spin-only- magnetism“, e.g. coordination compounds of 3d elements) Magnetisation M and susceptibility χ M = (∑ μ)/V ∑ μ: sum of all magnetic moments μ in a given volume V, dimension: [Am2/m3 = A/m] The actual magnetization of a given sample is composed of the „intrinsic“ magnetization (susceptibility χ) and an external field H: M = H χ (χ: suszeptibility) Magnetochemistry There are three types of susceptibilities: χV: dimensionless (volume susceptibility) 3 χg:[cm/g] (gramm susceptibility) 3 χm: [cm /mol] (molar susceptibility) !!!!! χm is used normally in chemistry !!!! Frequently: χ = f(H) → complications !! Magnetochemistry Diamagnetism - external field is weakened - atoms/ions/molecules with closed shells -4 -2 3 -10 < χm < -10 cm /mol (negative sign) Paramagnetism (van Vleck) - external field is strengthened - atoms/ions/molecules with open shells/unpaired electrons -4 -1 3 +10 < χm < 10 cm /mol → diamagnetism (core electrons) + paramagnetism (valence electrons) Magnetism
    [Show full text]
  • 7 Breaking Octet Rule.Pdf
    11/6/2020 Breaking the octet rule Odd electron Some compounds have atoms with more or less than 8 electrons Although rare, it is possible to have compounds that have an odd Lewis dot structures with atoms that break the around them (breaking the octet rule) number of electrons. octet rule There are 3 types Obviously it is not possible to satisfy the octet rule with an odd 1. odd numbered electrons number of electrons, ONE CANNOT BE A PAIR! 2. electron deficient compounds Two compounds are NO and NO2 3. expanded valence shells They will have a free electron on N. Lewis dot Electron deficient compounds This is rare. It is only found Boron, Beryllium or Aluminum The formal charge is appropriate for each atom and compounds. All are highly reactive. the molecule Beryllium, aluminum are a metals, however, since its valence shell Compounds with a free electron, unpaired electron, is so close to the nucleus it can form covalent bonds (rarely). are called free radicals. All atoms are electron deficient (have less than 4 electrons), and They are highly reactive. therefore when they form bonds they can’t make it all the way up These two compounds are used in nitrous oxide to 8 electrons systems in cars Expanded valance shells Examples Examples One atom will have more than 8 valence electrons. This requires a highly electronegative atoms (fluorine, oxygen, or Bl3 PF5 chlorine) to bond with a nonmetal with its outer shell 3 p or higher. The BeH XeF 2 extra electrons can use the unused d orbital.
    [Show full text]
  • Electron Orbitals/ Quantum Numbers
    Orbitals: What are they and how do they work? While it is common for electrons to be displayed as concentric rings around the nucleus, they are actually arrayed in a three-dimensional series of orbitals that are either type s, p, d, or f Electrons can be thought of as “filling in” the smallest orbitals towards the outermost, with the final electrons to be filled in being the valence electrons. These orbitals effectively stack on top of one another in layers as more electrons are added. Hydrogen has a single electron in the 1s orbital (1s1), whereas Helium has two electrons in the 1s orbital (1s2). The pattern that this process follows is shown below. Both images are effectively displaying the same information, but in different patterns. Hint hint: The diagram on the right is easy to draw from memory for a test S Orbitals can carry 2 electrons, p can carry 6, d can carry 10, and f can carry 14. Quantum Numbers Each and every electron that is found in any orbital has a unique quantum number, like the electron's home address. It lists the layer of the orbital group, or how many orbital groups from the nucleus it is; it lists the type of orbital the electron is found in; it lists the location within the orbital that the electron can share with another electron, and finally it lists the spin of the electron, which denotes it from the electron it is paired with. Principal Quantum Number (n) - describes the size/layer of the orbital. They are represented by discrete integers, where the larger value represents a larger orbital.
    [Show full text]
  • Chapter 7. Atomic Physics
    Chapter 7. Atomic Physics Notes: • Most of the material in this chapter is taken from Thornton and Rex, Chapter 8. 7.1 The Pauli Exclusion Principle We saw in the previous chapter that the hydrogen atom could be precisely understood by considering the Schrödinger equation, including the electrostatic potential energy that accounts for the interaction between the nucleus and the electron. We then discovered that the family of stationary states, which are the solutions of the Schrödinger equation can be completely characterized by three quantum numbers . However, we n, , and m also found that the state of the electron is further characterized by its intrinsic spin, which acts in a way akin to the angular momentum in that it provides an extra magnetic moment and is quantized similarly. But comparisons between the Sˆ and Lˆ operators should probably not be pushed too far, as for the electron the spin is half-integer with s = 1 2 independent of any other parameter, while 0 ≤ ≤ n −1 and always an integer. As a result, the states of the hydrogen can only be fully specified by combining the previous three quantum numbers for the solutions of the Schrödinger equation and the magnetic spin quantum number m , forming the n,,m,m foursome. As we will now discover s ( s ) there is another very important aspect of the spin that is absolutely essential for understanding the structure of many-electron atoms. Although it would be in principle possible to solve the Schrödinger equation for more complicated atoms, the presence of several interaction terms between the different electrons makes the problem analytically intractable and basically impossible to solve.
    [Show full text]
  • Molecular Orbitals and Bonding: What Do Unpaired Electrons Do?
    Molecular orbitals and bonding: What do unpaired electrons do? Anna I. Krylov and Pavel Pokhilko Department of Chemistry, University of Southern California, Los Angeles, California 90089-0482 Description: Students learn how to compute and visualize molecular orbitals using IQmol and review the key concepts of the MO-LCAO picture of bonding. They also learn how to connect bond orders with experimentally observable properties (bond lengths and frequencies). Suggested level: Beginning graduate. It is recommended to precede this activity by the \Bonding and Molecular Orbitals" lab. Learning objectives: Learn how to interpret molecular orbitals in polyatomic molecules in terms of bonding patterns; understand the connection between bond orders and molecular properties. I. BRIEF BACKGROUND TABLE I: Molecular orbitals of the CH3 radical, ordered by energy (1 is the lowest) orbital occupation picture 1, core 2 2, valence 2 3, valence 2 4, valence 2 5, valence 1 Lets us analyze molecular orbitals (MOs) of the CH3 radical | this will be useful for understanding bonding patterns in other species studied in this lab (it is often convenient to analyze the character of complicated molecules in terms on the orbitals of the constituent fragments). CH3 radical has 9 electrons. Table I shows occupied MOs, ordered by energy. In the ground state, CH3 has four doubly occupied MOs and a one singly occupied MO. Important observations: 2 • The core orbital (1sC ) is very compact and it does not participate in bonding; only the valence orbitals contribute to bonding. • As the orbitals' energies increase, the orbitals develop more and more nodes. • The canonical MOs are always delocalized.
    [Show full text]
  • Building Molecules
    BUILDING MOLECULES INTRODUCTION Atoms, the smallest (and invisible) free particles of an element, combine to form molecules. For example, O2, CO2, CH4 are all molecules. Atoms use electrons located in the outer shell in this process. These electrons are called valence electrons. For "A" family elements, the number of valence electrons equals the Group number. For example, aluminum is found in group 3A, and therefore it has three valence electrons and its electron dot symbol is: Al This procedure can be done for any "A" family element. There is a complicating feature, however. After four electrons have been added to the valence shell, the next electron(s) must be paired with one of the previous four electrons. For example, oxygen would look like: O Notice that oxygen now has two lone pairs (nonbonding pairs) of electrons and two electrons remain unpaired. These unpaired electrons remain available to form bonds (bonding pairs) with other atoms. The elements found in column 8A have eight electrons in their valence shell and are particularly stable and unreactive toward other atoms. Atoms having fewer than eight electrons in the outer shell tend to react with other similar atoms to achieve the stability exhibited by the elements in column 8A (also known as the noble gases). The tendency for atoms to form bonds to acquire eight electrons is known as the "octet rule." The octet rule is only an approximation to aid in understanding the bonding and molecular shape of molecules. The octet rule can be illustrated with hydrochloric acid. Atoms, such as hydrogen and chlorine (each having one unpaired electron), combine their unpaired electrons to form a single bonding pair.
    [Show full text]