„ 31 P NMR spectroscopy – an experimentalists view on a powerful tool“ Lecture for B.Sc. and M.Sc. students Prof. R. Streubel Institut für Anorganische Chemie, Bonn, Germany http://anorganik.chemie.uni-bonn.de/akstreubel/Streubel_Home.html E-mail: [email protected]
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Lecture for B.Sc. and M.Sc. students Prof. R. Streubelanorganik.chemie.uni-bonn.de/akstreubel/POC_VL/POC-NMR-E-09.pdf · Lecture for B.Sc. and M.Sc. students Prof. R. Streubel ...
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„31P NMR spectroscopy – an experimentalists view on a powerful tool“
Relative receptivity of 31P: 0.0665 => less sensitive than 1H (1.00) !
Standard for 31P NMR measurements: H3PO4 (concentration 85%)
Resonances are given in δ [ppm] relative to the standard, „chemical shift=> sign convention: + (= resonance at lower field; „lowfield-shifted“)
Chemical shift (δ) is determined by • electronegativity (χ) - π-electron overlap (nπ)• bond angle (θ)differences occur upon change in the bonding environment(J. R. van Wazer):
2. Phosphanes (phosphines)
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Structural features (I): RmPHm-n(σ3λ3-P, PIII, pyramidal, between p3 and sp3 hybridisation)
Strong downfield-shift upon increasing degree of C substitution at P
2. Phosphanes (phosphines)
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Steric effects !?
2. Phosphanes (phosphines)
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Examples of phosphanes and their chloro derivatives of RmPClm-n(σ3λ3-P, PIII, pyramidal)
Strong downfield-shift upon increasing degree of Cl substitution at P
Introducing an increment system based on:1) Electronegativity of directly bound atoms2) Electronegativity of atoms bonded to the α, β, etc. atom
Electronegative elements such as chlorine lead to a deshielded P center
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2. Estimating chemical shifts: increments systems
2. Phosphanes and chloro derivatives
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Examples of phosphanes and their chloro derivatives of RmPClm-n(σ3λ3-P, PIII, pyramidal)
∆δ values: 1) PMe2Cl vs PMeCl2:: 98.82) P(t-Bu2)Cl vs P(t-Bu)Cl2: 53.3 (!?)
Attention:In addition, the increment system has to take steric and electronic effects into account !
3. Phosphane oxides vs. phosphate esters
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Examples of phosphane oxides and their alkoxy derivatives RmP(O)ORm-n
(σ4λ5-P, PV, tetrahedral; ~ sp3 hybridisation with some d-orbital contribution)
Upfield-shift upon increasing degree of OR/OAr substitution at P
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Examples of phosphane oxides and their SR and NR derivatives RmP(E) (σ4λ5-P, PV, tetrahedral; ~ sp3 hybridisation with some d-orbital contrib
Note: OR and NR derivatives resonate upfield from respective SR deriv.=> SR show weaker π-backbonding
3. Phosphane oxides vs. sulfides and imines
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1) σ4λ5-P, PV, tetrahedral; 2) σ5λ5-P, PV, trigonal bipyramidal; 3) σ6λ5-P, PV, octahedral; ~ sp3 hybridisation with some d-orbital contribution
4. Phosphane oxides vs. phosphoranes vs. phosphates
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Why is their a shielding or deshielding effect in complexes?
5. MLn complexes of phosphanes and derivatives (I)
Why are Cr complexes more deshielding than Mo or W complexes ?
Coordination chemical shift values (∆δ): Cr > Mo > W (~ 20-30 ppm)
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5. MLn complexes of phosphanes and derivatives (II)
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5. BH3 complexes of phosphanes and derivatives
Borane complexation has a deshielding effect – unless β-atoms can deliever electron density !