To Do s Read Chapters 2, 3 & 4. Complete the end-of-chapter problems, 2-1, 2-2, 2-3 and 2-4 Complete the end-of-chapter problems, 3-1, 3-3, 3-4, 3-6 and 3-7 Complete the end-of-chapter problems, 4-1, 4-2, 4-3. 4-4, 4-8, 4-10, 4-11, 4-13, 4-16, 4-17, 4-18, 4-20 and 4-21 Answer Keys are available in CB204
NMR (Nuclear Magnetic Resonance) - NMR looks at magnetically active nuclei that is nuclei with non-zero nuclear spin. - We will learn 1 - and 13 C-NMR
NMR Gives Detailed Structural Information Molecular structure Peak assignment Structure elucidation NMR spectra Recording NMR is equivalent of seeing the molecule (almost).
Magnetically Active Nuclei (a) Organic molecules usually contain many hydrogens (b) More sensitive than the others (c) istorically, it s done first. ~ 6000 times less sensitive than 1 -NMR. Friebolin: Ein und zweidimensionale NMR-Spektroskopie VC (1988) - The relative sensitivity is given at constant magnetic field and equal number of nuclei. - The absolute sensitivity is the product of relative sensitivity multiplied by natural abundance. - Another common NMR nuclei is 19 F. 100% natural abundance and its γ is nearly as large as 1.
What Do We Look for in 1 -NMR? # of unique protons Peak positions (chemical shift) Shielding by bonding electrons Magnetic anisotropy Peak splitting (spin-spin coupling) Integrals Peak area are proportional to # of protons
Ethyl p-ydroxybenzoate, 1 -NMR 3 1, singlet E (triplet) 2 2 A C B 2 D(quartet) Chemical shift scale
What do we look for in 13 C-NMR? # of unique carbons Peak positions 13 C-NMR chemical shifts are spread in a much wider range than 1 -NMR, due to the presence of p-electrons.
Ethyl p-ydroxybenzoate, 13 C-NMR symmetry 5 3 1 2 4 6 7 A wider chemical shift range
Nuclear Spin and Magnetic Moment
Nuclear Spin and Spin States 1 and 13 C have a spin quantum number of 1/2. They can have two different spin states - ± 1/2 or α/β or The different spin states have the same energy unless they are put in an external magnetic field
Small Magnet in an External Magnetic Field or ΔE = B 0 γh 2π h : Plank s constant γ : gyromagnetic ratio B 0 : external magnetic field
Energy Differences ΔE = B 0 γh 2π
Population Difference and spin state β ΔE 100,000 CW-NMR The energy gap between the two spin states creates a small population difference. When the system is irradiated with electromagnetic radiation, energy is absorbed at the frequency that matches to the energy gap. E α 100,001 in 200 Mz instrument E = hν hν = B γh 0 2π ν = γb 0 2π ν frequency
FT-NMR Sample Magnetization Response Data Spectrum Magnet Perturbation (Rf pulse) Detection Fourier Transformation Storage
Precession of Magnetic Moment and Net Magnetization ω 0 = γb 0 (rad/sec) ω 0 : Larmor frequency (angular velocity) B 0 ω 0 animation M : net magnetization http://mutuslab.cs.uwindsor.ca/schurko/nmrcourse/animations/precess/precess.htm
Pulsed Experiments B 1 sum of + and - vectors + : clockwise rotation - : counter clockwise rotation ν = γb 0 2π ω 0 = 2πν Energy is absorbed, and M will be tilted around the x-axis θ = γb 1 t p t p : pulse duration (usually ~ msec)
Relaxation and Free Induction Decay (FID) z M time x y Animation Magnetization on x-y plane http://mutuslab.cs.uwindsor.ca/schurko/nmrcourse/animations/eth_anim/bloch_normal.gif
Rf Pulse and FID Animation Tilt and recovery Animation x-y plane magnetization http://mutuslab.cs.uwindsor.ca/schurko/nmrcourse/animations/eth_anim/puls_evol.gif http://mutuslab.cs.uwindsor.ca/schurko/nmrcourse/animations/animated_gifs/fid_one_line.gif
FID Signals O 3 C C 3 O 3 C O a C 3 b O O a b c d O
Fourier Transform of FID Single FT-NMR experiment takes ~ 5 sec or less ( 1 -NMR) Many FID signals can be accumulated and averaged Better S/N ratio
Chemical Shift Scale Different NMR instruments have different magnets, and operate at different frequencies. The same proton would resonate at different frequencies. Chemical shift (δ) = ν obs - ν ref ν instrument x 10 6 ppm
1 -NMR of Menthol
NMR Reference Compounds 3 C C 3 Si C 3 C 3 Chemically stable Can be removed easily (b.p. = 27 C) Most protons appear to the left of TMS Tetramethylsilane (TMS) Positive chemical shifts C 3 3 C Si SO 3 For D 2 O C 3 4,4-dimethyl-4-silapentane-1-sulfonic acid(dss)
Common NMR Solvents Do not use them as references
Chemical Shift Range for 1 - NMR TMS (δ=0)
Why Different Chemical Shifts Are Observed? Shielding by bonding electrons Magnetic anisotropy This will be discussed later in details. First, we will learn how to identify different protons in a given structure.
Chemical Shifts and Chemical Equivalence Chemically equivalent protons have the same chemical shift. ow do we find chemically equivalent(or non-equivalent) protons? Z substitution Z Z Z Z Z Z Same compound Chemically equivalent (homotopic)
Toluene and p-xylene b c a d b c b b a a b b
13 C-NMR of Toluene and p- Xylene
Methylacetate
alogenated Methanes a a a a C a a a C Cl a a C Cl Cl Enantiotopic protons a a C Cl Br Z C Cl Br Z C Cl Br A pair of enatiomers Enantiotopic protons have the same chemical shift although they are not chemically equivalent.
alogenated Ethanes a a C a a C a a Cl a C a b C b b Cl a C a a C a Cl Enantiotopic protons omotopic protons Diastereotopic protons Cl b C c a C Br Cl Cl Z C C Br Cl Cl C Z C Br Cl A pair of diastereomers Diastereotopic protons have different chemical shift.
Trityl-serine lactone
Typical 1 Chemical Shift Ranges
13 C-NMR Chemical Shift Range Both s- and p-electrons are responsible for shielding
Spin-spin Coupling A C C X Spin-spin coupling is commonly observed between protons that are separated by 3 bonds or less.
1,1,2-Trichloroethane Cl Cl C C Cl
1,1,2-Trichloroethane a b a Cl Cl C C Cl a b A doublet 2 1 1 A triplet
1,1,2-Trichloroethane a Cl 2 a Cl C C Cl a b Spin-spin coupling 1 b TMS a : doublet (d) b : triplet (t)
Diethyl Ether a b O C C b a b a 3 C C 2 O C 2 C 3 b Jab a 3 3 Less shielded More shielded 1 1 A quartet
N+1 Rule and Pascal s Triangle singlet doublet triplet quartet quintet sextet septet etc A proton with N adjacent neighbors will split into N+1 lines. omotopic or enatiotropic protons do NOT split each other (there are exceptions) The intensity of the lines may be determined using Pascal s triangle. Spin-spin couplings are commonly observed between protons that are separated by three bonds or less.
Commonly Observed Splitting Patterns
When two J values are different J ac a Cl c Cl Cl C C Cl C C < J ac a b a b b b J ac J ac 2 1 1 1 1 1 1 J ac J ac Triplets are a special case of dd. A doublet of doublets (dd)
Vinyl Acetate dd singlet dd dd Important to recognize splitting patterns
Typical Coupling Constants
Cis and Trans Alkenes
Ethyl-trans-Crotonate 3 C O O C 2 C 3
Alkenic Protons a 3 C c b b O O C 2 C 3 c = 7 z J ac = 1 z J bc = 16z a doublet of quartets
Allylic Protons a 3 C c b O O C 2 C 3 = 7 z J ac = 1 z J bc = 16z a doublet of doublets (dd)
Trityl-serine lactone a : doublet b : ddd c : doublet of doublets (dd) d : dd
Trityl-serine lactone
ydrogen Bonded Protons Usually appear as a broad peak Their chemical shifts depend on concentration, temperature, solvent etc. Generally do not show spin-spin coupling. They are exchangeable, and can be identified by D 2 O shake. * R-O + D 2 O R-OD + DO + D 2 O (excess) DO
Ethanol
N-Methyl Aniline N C 3 N
Problem 2.4(a)
Problem 2.4(a) Br ID = 0 quintet sextet Br triplet triplet 3 Br 2 2 2 Br Don t be afraid of writing wrong structures!
Problem 2.4 (d)
ID = 5 Problem 2.4 (d) X Y 2 2 2 2 Z C 2 C 3 3 N 2
Problem 2.4 (d) X Y N 2 Z C 2 C 3 C 9 12 NO 2 CO 2 missing 2 N O C 2 C 3 O 2 N O O C 2 C 3