cobalt-59 nuclear magnetic resonance studies correlated to ... · cobalt-59 nuclear magnetic...

Dept. of Chemistry and Physics

Will Lynch, George Gergely, Patrick Sisco, Jeremy Olsen, Brian Helmly, Joyce Chow, Pat Liola, Brian Jacobs

Dept. of Chemistry & PhysicsArmstrong Atlantic State UniversitySavannah, Georgia 31419

Cobalt-59 Nuclear Magnetic Resonance Studies Correlated to Electronic Transitions - an Inorganic Experiment


Why this study / experiment?

New Instrument: 300 MHz with multinuclear capability

Multinuclear NMR: Students exposure to nuclei besides 1H and 13C is limited

Spectrochemical Series: Links theory with experiment via UV-Vis / NMR correlation

Introduction


Properties of Cobalt NMR

Abundance: Cobalt – 59 is 100 % abundantFrequency: 47.45 MHzNatural Receptivity: 1538 vs Carbon – 13

0.277 vs Hydrogen-1Nuclear Spin: I = 7/2 (quadrapolar nuclei)Quadrapole Moment: 0.40 X 10-28 cm2

Chemical Shift Range: 18,000 ppm [Co(CO3)3]3- highest – [Co(PF3)4]1- lowest

Line Widths: 30 kHz – 40 Hz

Introduction


Properties of Cobalt NMR

Cobalt(III): low spin, d6, diamagneticCompounds: All pseudo octahedral (N,O donors)

Intermediate quadrapole makes Co linewidths sensitive to symmetry about the cobalt atom. Octahedral geometry results in lower electric field gradients (sharper lines) than lower symmetry.

Introduction


Vibrational Spectroscopy – Spring / Force ConstantElectronic Spectroscopy – 1 electron “jump”

between orbitalsNMR –The more negative the charge density on the nuclei, the higher the shielding, the more negative the resonance

Shielding increases as the electronegativity of a liganddecreases, more negative shift, this periodic relationship generally holds across a row in p blockCo-O 12-14 kppm, Co-N 5-10 kppm, [Co(CN)6]3- 0 ppm

Mechanical Image of NMR?


NMR – Transition metals – valence level circulations change the shielding on nucleus

Chemical Shift – result is a mixing of magnetic field states (dipole allowed) and are influenced by electronic circulations or currents that modify the applied field

Deshielding – More effective if complex has 1) lower excitation energy (spectrochemical dependence) or 2) closer circulation to nucleus (nephelauxetic dependence)

Mechanical Image of NMR?


•I > ½ requires small quadrapole moment (limits broadening)•Natural abundance must be sufficiently large•Intrinsic NMR receptivity increases as γ3AI(I+1)

•Thus from I = ½ to 7/2 increases by factor of 21•Line Broadening – Fast relaxation leads to uncertainty in peak position—line broadening•Quadrapolar relaxation and line width ∝

•(2I + 3) / I2(2I –1)•Thus from I = 1 to 7/2 increases by factor of 29

Limitations of Transition Metal NMR


•The value of the ligand field splitting parameter, ie. the amount by which the degeneracy of the d-orbitalsis disturbed by the effect of the electrostatic field generated by the ligands, depends upon the identity of the ligands. •F- < OH- < C2O4

2- < H2O < NCS- < CH3CN < NH3< en < bipy < phen < NO2

-

•Sigma only – increased donor ability, increase splitting of ligand field•More sigma donation, more e- density on metal, more shielded, more negative resonance

Spectrochemical Series


Co(III) Low Spin Tanabe Sugano Diagram d6


NMR Range Compared to Electron Radial Functions


Ground state is t2g6

Low Spin – High Field (favored by Co(III) Oh complexes)

Chemical Shift σ = σd + σpParamagnetic Screening Constant (σp)

σp – Arises from Nephelauxetic and Spectrochemical Effect

Chemical Shifts vs ∆


Chemical Shifts vs ∆

)(

2

11

11

3

320

11

11

8

gTA

gzgdBpiso E

TLAr

−

−

∆−=

πµµσ

2

11

11

gzg TLA

dr

3

3−

Orbital Angular Momentum Integral

Radial Factor

20 Bµµ Vacuum permeability, Bohr magneton


Experiment

•Series of eleven N,O Co(III) octahedral complexes are synthesized

•Reduce nephelauxetic effect•Characterize – FTIR, UV-Vis, Mag. Susc., 1H and 13C NMR

•Make students defend inclusion•Run Co-59 NMR and correlate to lowest energy transition

•As ∆E increases, chemical shift decreases


Results

3.28 X 10 -1960613530Co(C2O4) 3

3.33 X 10 -1959712511Co(AcAc) 3

3.37 X 10 –1959112634Co(TFA)3

3.74 X 10 -195328881[Co(NH3)5Cl]2+

3.81 X 10 -195228396c-[Co(en)2(N3)2 ]2+

4.04 X 10 -194929040[Co(NH3)5(H2O)]3+

4.05 X 10 -194917630[Co(NH3)5(ONO)]2+

4.19 X 10 -194758111[Co(NH3)6]3+

4.27 X 10 -194667131[Co(en)3]3+

4.33 X 10 –194597556[Co(NH3)5(NO2)]2+

4.60 X 10-194326310t-[Co(en)2(NO2)2]1+

Energy (J)λmax (nm)δ (ppm)Compound


IR Spectra of cis-[Co(en)2(N3)2]NO3

20

50

80

450145024503450Wavenumber (cm-1)

2070, 2013 cm-1, νa(NNN) cis1751 cm-1, ν1 + ν4 (symm + in plane bend)


UV-Vis of [Co(NH3)6]Cl3

Absorbance (AU) vs. Wavelength (nm)

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

250 300 350 400 450 500 550 600 650 700 750


NMR Spectrum of [Co(en)3]Cl3

X : parts per Thousand : 59Co

7.4 7.3 7.2 7.1 7.0 6.9 6.8 6.7 6.6

(B

illi

ons)

01.

02.

03.

04.

05.

06.

07.

08.

09.

010

.011

.012

.013

.0

59Co(en)33+.5


NMR Spectrum of cis-[Co(en)2(N3)2]NO3


NMR Spectrum of trans-[Co(en)2(NO2)2]NO3


δ vs Energy

Transition Energy vs. Chemical Shift

y = -5E+22x + 29828R2 = 0.89

0

2000

4000

6000

8000

10000

12000

14000

16000

3.2E-19 3.6E-19 4.0E-19 4.4E-19 4.8E-19

Transition Energy, J

ppm


Conclusion• New laboratory project for inorganic students to pool

data• Syntheses are straightforward• Characterization by IR yields is varied (azido, nitrito,

oxalato)• Co-59 NMR characterization to give students exposure

to inorganic nuclei• Correlation of UV-Vis lowest energy transition and

NMR is indirect


Acknowledgements• National Science Foundation Division of

Undergraduate Education: 9952343 for NMR• Chip Detmer – JEOL• Richard Wallace- AASU

cobalt-59 nuclear magnetic resonance studies correlated to ... · cobalt-59 nuclear magnetic...

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