Def.: Magnetism the property of a material to be attracted to (paramagnetic response) or repelled by (diamagnetic response) a magnetic field

Transcription

1 5.2 Magnetism: the basics Def.: Magnetism the property of a material to be attracted to (paramagnetic response) or repelled by (diamagnetic response) a magnetic field These effects arise mainly from electrons (WHY?) Source: Physical Methods for Chemists R. S. Drago, Surfside, Gainsville, FL, Diamagnetism arises from the circulation of paired electrons within the species to generate a weak magnetic dipole aligned parallel to the external magnetic field. Source: Ege, Organic Chemistry, 2 nd Ed., D. C. Heath and Co., 1989, Lexington, MA This is the same mechanism by which a ring current is induced in an aromatic molecule in an applied magnetic field observed as deshielding of peripheral protons in 1 H NMR spectra. Section 5.2-1

2 Since all atom-based matter has some paired electrons, all atom-based matter will exhibit a diamagnetic response to an applied magnetic field HOWEVER...that response is very small and will be overridden by any other magnetic properties that may arise if the matter also has unpaired electrons (i.e., is paramagnetic). The paramagnetic response of a material to a magnetic field is 2 to 3 orders of magnitude greater than the diamagnetic response from the underlying paired electrons. All other types of magnetism (ferromagnetism, antiferromagnetism, ferrimagnetism, superparamagnetism, etc.) are based on long range ordering of unpaired electrons and are critical phenomena i.e., at some high temperature they revert to paramagnets. Step-by-Step: Macroscopic (bulk) magnetic properties related to microscopic (atomic) principles I. All atom-based matter responds to an external applied magnetic field H i.e., internal magnetic dipole(s) are induced. II. The total magnetic induction B within a substance is proportional to the sum of the applied magnetic field H and the magnetization M of the substance: B = µ o (H + M) where µ 0 is the vacuum permeability µ o = 1 in the cgsemu system of units (Note: µ o = 4π10-7 newton/ampere 2 in the SI system of units, but this convention makes the equations more difficult to work with, so cgsemu units are typically favoured.) Another way of writing the above equation is:! B! H M! = µ 0 1 +! H The ratio of M/H is dependent on the material. It is basically a measure of how magnetizable a material is in the presence of an applied magnetic field in other words the ratio of the induced magnetization to the applied magnetic field. M/H is called magnetic susceptibility (χ):! Using the volume magnetization (M vol ) gives χ volume. M χ volume is unitless (i.e., dimensionless)! = χ H but is conventionally expressed in emu/cm 3 Thus the dimension of (electromagnetic units) emu is formally cm 3! Section 5.2-2

3 It is more useful to use the molar magnetic susceptibility (χ), related by dividing the volume magnetic susceptibility by the density of the material and multiplying by molecular weight. This is the magnetic susceptibility per mole: χ = χ MW ρ volume χ is expressed in units of cm 3 mol -1 or alternatively in emu/mol NOTE: From now on, we will consider the magnetization M to mean the molar magnetization and reference to magnetic susceptibility will imply molar magnetic susceptibility. It is worth noting that a more accurate representation of the relationship between molar magnetization and applied magnetic field is:! M! H = χ M is a vector, H is a vector, therefore χ is a second rank tensor. If the sample is magnetically isotropic, χ is a scalar. **If the magnetic field is weak enough, χ is independent of H and M = χh is valid.** III. A Note on Units (the cgsemu system)... Strictly speaking, the unit of magnetic field H is the Oersted (Oe). However, many authors express H in gauss (G) which is strictly speaking a unit of magnetic induction B created by the magnetic field. In a vacuum, B = µ o H where µ o = 1 so the two units are essentially interconvertible We will see that χt is often the quantity of interest. χt is expressed in emu K mol -1 Molar magnetization M is expressed in units of emu G mol -1 Alternatively, M may be expressed in units of Nβ N = Avogadro s number = x mol -1 β = electronic Bohr magneton (also µ B ) 1 Nβ = 5585 emu G mol -1 Section 5.2-3

4 In the cgsemu system of units, energy is expressed in erg (1 erg = 10-7 J): Boltzmann constant k B = x erg K -1 Bohr magneton β = x erg T -1 But it is more common to express energy in cm -1, so: Boltzmann constant k B = cm -1 K -1 Bohr magneton β = x 10-5 cm -1 G -1 Using these common units, the constant Nβ 2 /3k B that appears in the Curie law (for future reference) is equal to emu/mol 1/8 Section 5.2-4

5 Section 5.2-5

6 IV. Types of Magnetism... There are many different types of magnetism. Each corresponds to a unique set of characteristics that can be observed by measuring the magnetic susceptibility χ of a bulk sample as a function of temperature and applied magnetic field. It is important to note that molar magnetic susceptibility χ is a bulk measurable quantity. The most common types of magnetism are classified based on the measured magnetic susceptibility χ as a function of two factors: 1) temperature (T) 2) applied magnetic field (H o ) Source: Drago T C (Curie Temperature) critical temperature for a ferromagnet T N (Néel Temperature) critical temperature of an antiferromagnet Section 5.2-6

7 Diamagnetism Diamagnetism is an underlying property of all atom-based matter. It is ALWAYS present, even when masked by paramagnetism, etc. In principle, the experimentally measured magnetic susceptibility (for any material above its critical temperature) is the algebraic sum: χ exp = χ dia + χ para + χ TIP Of these, χ para is typically the largest in magnitude and responsible for the majority of the magnetic character of a material. (NOTE: Most authors assume that χ para is implied when discussing χ.) In order to extract χ para from χ exp, we must first account for contributions from χ dia. There are a number of acceptable ways of doing this. 1) The magnetic susceptibility of the empty (and usually diamagnetic) sample holder is measured such that it can be subtracted from the experimental data. In addition... 2) The magnetic susceptibility of a structurally analogous diamagnetic material is measured and this is used to estimate the diamagnetic contribution in a paramagnetic sample. (e.g., replace paramagnetic transition metal dications with Zn 2+ ) 3) Use the rough approximation χ dia = k x m.w. x 10-6 cm -1 mol -1 where k is a weighting factor k = ( ) 4) Use Pascal s constants to calculate a better estimate of χ dia. Pascal s constants are tabulated literature values: χ dia = χ dia,atom + χ dia,bond (see table on following page) e.g. Using Pascal s constants to calculate χ dia for benzonitrile: χ dia,atom (x 10 6 cm 3 /mol) χ dia,bond (x 10 6 cm 3 /mol) C aromatic 6 x C N 1 x 0.8 H 5 x C 1 x N 1 x Total: χ dia = ( ) x 10 6 cm 3 /mol = x 10-6 cm 3 /mol N C Section 5.2-7

8 Source: Molecular Magnetism, Olivier Kahn, χ dia is always negative. The value of χ dia is substracted from χ exp leaving χ para + χ TIP. χ TIP may or may not be important. In many publications, it is assumed to be zero. Section 5.2-8

9 V. Temperature independent paramagnetism (χ TIP ) is usually small often the same order of magnitude as χ dia but opposite in sign (i.e., positive). What is the origin of temperature independent paramagnetism? If the only thermally populated state of a molecule is a spin singlet without first order angular momentum, then we expect that χ para = 0 and χ exp = χ dia. Therefore χ exp is negative. In some cases, the diamagnetic (i.e., non-degenerate) ground state may couple with a degenerate excited state (through a Zeeman perturbation...see Van Vleck equation ahead), as long as the energy gaps are not too large. THIS IS NOT A THERMAL POPULATION OF EXCITED STATES! This coupling of singlet ground state with degenerate excited state(s) generates a (+)ve magnetic susceptibility that is temperature independent (χ TIP ). χ TIP is not restricted to compounds with a diamagnetic ground state. The coupling between a magnetic ground state and non-thermally populated excited state(s) may also give χ TIP. VI. Another form of temperature independent paramagnetism, called Pauli paramagnetism (χ Pauli ) is associated with species that display metallic conductivity. Consider sodium metal: free electrons in the 3s band and positively charged, essentially diamagnetic ions with a [Ne] core make up the lattice points. When a magnetic field is applied, there is tendency for the magnetic dipoles associated with the intrinsic spin of the free electrons to align themselves parallel to the field, giving a positive magnetic moment. The electrons in a metal do not have a Boltzmann distribution (so we can t use the Langevin formula to calculate the magnetic susceptibility...see notes ahead). The Fermi-Dirac distribution function must be used instead...and since this varies little with temperature (Recall: it varies only with enormous changes in T), the susceptibility turns out to be practically independent of temperature!! Let s derive an expression for the Pauli paramagnetism using a model metal in which all electrons are spin paired (i.e., all energy levels are filled by two electrons of opposite spin) and the temperature is absolute zero (i.e., the Fermi surface is flat)... Section 5.2-9

10 Note: In this diagram, W is energy, so dn/dw is the density of states (DOS). Here, the DOS is explicitly divided into electrons with spins parallel (+) to an applied field and spins anti-parallel (-) to the applied field. Source: Electricity and Magnetism Bleaney and Bleaney, Clarendon Press, Oxford, When a magnetic field H is applied, an electron can only flip its spin magnetic dipole from anti-parallel to parallel if the decrease in its magnetic energy (2βH) is sufficient to supply the extra kinetic energy required to raise it to an empty translational energy level. (RECALL: β is the Bohr magneton...the intrinsic magnetic moment of an electron) Another way of saying this is that the application of H lowers the energy of the spin parallel states by an amount 2βH. The total magnetic moment of the system is 2xβ, where x is the number of electrons transferred from the anti-parallel to the parallel orientation......note that it is 2x times the Bohr magneton because flipping a spin creates both one more spin parallel AND one less spin anti-parallel...so net effect is 2x. The volume magnetic susceptibility is then χ = 2xβ H µ o So how do we find the value of x? It must be related to 2βH. We assume that the energy difference w between successive energy levels at the top of the Fermi distribution is approximately constant. To flip an electron s spin from antiparallel to parallel requires that its kinetic energy be increased by w, since we may take an electron from the topmost filled level and put it in the next highest level, which is vacant. Section

11 To flip the second electron requires an additional kinetic energy of 3w, since the next two levels with parallel orientation are already filled. The third electron must be given extra energy equal to 5w, and so forth. For the x th electron, the excess kinetic energy will be (2x-1)w. If x is very large (x >> 1), we can use 2xw as an approximation...therefore at equilibrium 2xw = 2βH. Substituting this into the above equation, we get 2 2β χ = µ o w This still doesn t help us much until we can estimate the value of w. Since two electrons with spins anti-parallel to one another can occupy each kinetic energy level, the number of such levels in the range W to W + dw is ½(dn), where dn is the number of electrons in this range. Hence the energy separation w between successive levels at the top of the Fermi distribution is dw / ½(dn) = 2 (dn / dw) F -1 (where W F is the Fermi energy) Therefore 2 β χ = µ o dn dw F Conveniently, the number dn of electrons per unit volume which have a kinetic energy in the range W to W + dw has been worked out to be: 1 2 3nW dw dn = (see a solid state physics text if you really want to 3 2 2W F know how this was derived!) Thus χ = 2 3nβ 2µ ow F Pauli was the first to derive this expression. Notice that temperature doesn t factor into the equation, hence the temperature independence of Pauli paramagnetism! Section

12 It was later shown by Landau that the effect of the magnetic field on the translational motion of the free electrons gives a diamagnetic contribution to the susceptibility, whose value is just -1/3 of the Pauli paramagnetism originally calculated as above. Therefore the net volume Pauli paramagnetism effect is χ Pauli 2 nβ = µ W o F Note: Eq. (20.16) in this table is the above equation. Source: Bleaney and Bleaney. So now we have dealt with the diamagnetic contribution χ dia and the possible types of temperature independent contributions, χ Pauli and χ TIP (which we will look at in more detail when we derive the van Vleck equation)......now we get to heart of most molecular systems...the paramagnetic susceptibility χ para Section