Synthesis of a Radical Trap

Transcription

1 Chemistry Catalyzed oxidation with hydrogen peroxide Trapping of a free radical (spin trapping) Technique Acquisition and interpretation of ESR spectra

2 Radical trap molecule that reacts with short-lived radical to form a persistent radical + R R

3 Radical trap molecule that reacts with short-lived radical to form a persistent radical.. + R R R CA Index: 2-Propanamine, 2-methyl--(phenylmethylene)-, -oxide itrone, -tert-butyl-α-phenyl- 2-Methyl--(phenylmethylene)-2-propanamine -oxide 2-Phenyl--tert-butylnitrone Benzylidene-tert-butylamine -oxide Benzylidene-tert-butylamine oxide C-Phenyl--tert-butylnitrone -Benzylidene-tert-butylamine -oxide -Benzylidene-tert-butylamine oxide -tert-butyl-2-phenylnitrone -tert-butyl-c-phenylnitrone -tert-butyl-α-phenylnitrone tert-butyl(benzylidene)amine -oxide α-phenyl--tert-butylnitrone aminoxyl radical rather than nitroxyl radical is IUPAC prefered name: REVISED MECLATURE FR RADICALS, IS, RADICAL IS AD RELATED SPECIES Pure Appl. Chem. 1993, 65, ,

4 PhC 2 C(C 3 ) W 4 2- Ph PhC 2 C(C 3 ) 3 "" Ph W W Ph Ph C 3 Re C 3 Re C 3 Re Ph Ph "" - 2 Ph Ph

5 Radical trap molecule that reacts with short-lived radical to form a persistent radical.. + R R R C C C 3 C 3 C C C 3 C3 benzoylperoxide azobisisobutyronitrile (AIB)

6 Electron Spin Resonance Spectroscopy MR ESR Sample is placed in a strong magnetic field and exposed to an orthogonal low-amplitude high-frequency field Radiowave frequency (Mz) Transitions between two nuclear spin states, Δm I = ±1 Frequency varied, magnetic field held constant Spectrum recorded as absorption ΔE = hν = g μ B o ; for a 10,000 gauss (1 T) field ν = Mz A 300 Mz spectrometer requires a 70 kg (7 T) magnet Microwave frequency (Gz) Transitions between two states of unpaired electron, Δm s = ±1 Frequency held constant, magnetic field varied Spectrum recorded as derivative of absorption ΔE = hν = g e μ B B o ; for a 10,000 gauss (1 T) field ν = 28,026 Mz An x-band spectrometer operates at 9.5 Gz and a field of 3400 G (0.34 T)

7 Electron Spin Resonance Spectroscopy MR ESR Sample is placed in a strong magnetic field and exposed to an orthogonal low-amplitude high-frequency field Radiowave frequency (Mz) Transitions between two nuclear spin states, Δm I = ±1 Frequency varied, magnetic field held constant Spectrum recorded as absorption Microwave frequency (Gz) Transitions between two states of unpaired electron, Δm s = ±1 Frequency held constant, magnetic field varied Spectrum recorded as derivative of absorption

8 Region Radio Microwave Infrared Visible Ultraviolet X-Rays Gamma Rays Spectrum of Electromagnetic Radiation Wavelength (Angstroms) > < 0.1 Wavelength (centimeters) > x x x x < 10-9 Frequency (z) < 3 x x x x x x x x x x x > 3 x Energy (ev) < > 10 5

9 Electron Spin Resonance Spectroscopy E ms = gμ B B o m s μ B = eh/4πm e = x J/T g e = If the electron has nonzero orbital angular moment, L, the g-value becomes g = 1 + S(S + 1) - L(L + 1) + J(J + 1) 2J(J + 1) rganic radicals have L ~ 0 and J ~ S so that g is about 2. Situation is very different for transition metals

10 Electron Spin Resonance Spectroscopy When molecule contains nuclei with magnetic moments (nuclear spins) these augment the external applied field and lead to multiple transitions, Δm s = ± 1, Δm I = 0 μ = g μ [I(I + 1)] 1/2 μ = eh/ 4πm p = x J/T Electron-nuclear interaction will depend on the projections of both electron and nuclear spins: E electron-nuclear = Am I m s A is hyperfine coupling constant/interaction (hfi), value depends on: the g-values for the electron and the nucleus distance between them orientation of magnetic dipoles with respect to the external field (dipoledipole interaction). Fast molecular rotation averages out anisotropic part; the remaining isotropic part is given by the Fermi contact interaction; A is proportional to the unpaired electron density at the nucleus, i.e., A = (8π/3)g μ g e μ B ρ(0) where ρ(0) = ψ(0) 2 is the unpaired electron density at the nucleus yperfine coupling constants, a = A/g e μ B, are expressed in gauss or millitelsa (mt), 1 G = 0.1 mt= Mz

11 Electron Spin Resonance Spectroscopy m I = +1/2 (α ) m I = -1/2 (β ) m I = -1/2 (β ) E = g e μ B m s (B o + Σ a i m Ii ) - g μ B o m I m I = +1/2 (α ) Signs of two terms are different because of the opposite charges of the electron and the proton, so α spin of the electron is higher in energy than β, but β spin of the proton is higher in energy than α Effective magnetic field experienced by the electron differs from B o by an integer times a i and the sign of this term is opposite for α e and β e

12 Electron Spin Resonance Spectroscopy Coupling to four equivalent hydrogens Coupling to (I=1) and two equivalent hydrogens 2I+1 lines of equal intensity 2S+1 lines of 1:2:1 intensity

13 Electron Spin Resonance Spectroscopy toluene a = 1.53 mt, a = 0.73 mt J. Chem. Soc., Perkin Trans. 2, 2000, , a =1.669 mt, a = mt, g = Angew. Chem., Int. Ed., 2000, 39, 2436 C 2 Cl 2 a = 1.507, a = mt J. Chem. Soc., Perkin Trans. 2, 2000, 2436

14 ESR spectrum of dibenzyl aminoxy radical a a g J. Chem Soc. B 1970, 96.

15 Electron Spin Resonance Spectroscopy i i Ph 2 P PPh 2

16 Acquiring a good spectrum Fast scan rates lead to poorly resolved and distorted spectra Sample size (points) must be large enough (>2000) Spectral line widths (and resolution) are sensitive to: concentration of paramagnetic species including analyte and molecular oxygen sparging sample with gaseous nitrogen or argon to remove dissolved oxygen is crucial to obtaining a good spectrum instrument settings Power (in range of 6.3 x 10-2 mw) modulation frequency (100 Kz) modulation amplitude (0.5 G)

17 The spectrum below of A is properly resolved and displayed the one in lower right is not. + - aminoxyl radical rather than nitroxyl radical is IUPAC prefered name