NMR spectroscopy of keto-enol equilibrium

nuclear spin

protons have intrinsic magnetic moment from nuclear spin; typically I = ½, but dependent on atom

nuclear magnetic resonance (NMR) spectroscopy

interaction of the nuclear magnetic moment with external magnetic field; radio waves induce transitions between spin states

Energy of NMR interaction

E = -μ•B or E= -μ_z•B (when B is directed along Z axis)

(negative of magnetic moment (as a vector) multiplied by external magnetic field (as a vector))

E = hν

equivalent energies

magnetic moment of a proton

μ=γI

(magnetogyric ratio multiplied with spin angular momentum)

spin angular momentum

I_z= m_Ih*

discrete values

magnetogyric ratio

dependent on particle identity

γh* = g_Nμ_N^.

(nuclear g-factor multiplied by nuclear magneton)

nuclear magneton

e = elementary charge

m_{p =} mass of proton

resonance frequency

dependent on chemical environment

B_loc=B(1-σ)

σ = shielding constant

shielding constant (σ)

electrons in vicinity of proton “shield” it from the full effect of external magnetic field

NMR signal

provides:

1. Chemical shift (δ)

2. Integral

3. Coupling/multiplicity

Chemical shift (δ)

location of signal on x-axis; difference btwn resonance frequency and standard; measured in ppm; indepedent of external chemical field and frequency of spectrometer.

integral

determine relative number of hydrogens giving rise to the signal; area under the peak

Coupling/multiplicity

number of peaks in signal; spin of surrounding protons affects splitting of signals; distance between lines = coupling constant J (Hz)

tetramethylsilane/TMS/(CH₃)₄Si

standard for NMR against which chemical shifts are measured; most other protons are not shielded as well

typical chemical shifts

aromatic ~ 7-8 ppm

olefin ~ 4-6 ppm

porphyrin can have negative sshift - enhanced shielding

Spin-spin splitting

neighboring protons affect resonance field; resonance frequency depends on spin orientation of neighbors; transmitted through bonding electrons

n+1 rules

how many lines from how many neighbouring protons that affect splitting

keto-enol equilibrium

spontaneous tautomerisation of ketones and alkene alcohols

Keq = [enol] / [keto]

continuous wave NMR

radiofrequency held constant; mag. field scanned slowly; absorption as function of field strength

FT-NMR

stroong ext. mag. field that aligns protons along z-axis; strong radio pulse that rotaes magnetization into xy-plane (90°); produces oscillating RF signal, detected by receiver coil

Larmor frequency (ν)

how fast the proton precesses about the applied mag. field

free induction decay

dampening of sinal over time; minimised via shimming (reduces heterogenity of applied mag. field) → slower dephasing + longer FID → better signal; Fourier transformed into peaks