¹³C NMR Spectroscopy [Structure of croconic acid] [Structure of chlordane] [Structure of CCl₃F₂] HC≡C-C≡C-C≡C-C=CH₂ H₂C=CH₂ A polyacetylene when a lesser no. of H⁺ are present in the compound the structure cannot be determined with ¹H NMR Spectrum only. We need to further analyze ¹³C-NMR Spectrum. [Four different structures of substituted benzene rings are drawn, labeled I, II, III, IV] Principle involved in ¹³C NMR Spectroscopy Isotopes of carbon ¹²C ₆, ¹³C ₆, ¹⁴C ₆ 4A %: 98.9%, 1.1%, 0.006% I: 0, ½, 0 o, ✔, x ¹³C Nucleus which spins plus creates a magnetic moment μz=μ μz = -gN * μN * I_z angular momentum / 2M I=½ momentum of the Nucleus When External magnetic field is applied, the two spin states get separated by an energy. [Diagram showing two energy levels, E₁ and E₂, corresponding to spin up and spin down states in a magnetic field B] E₂ = μH₀ E₁ = -μH₀ ΔE = E₂ - E₁ = μH₀ - (-μH₀) = 2μH₀ Quantum theory Classical theory γ = μz / I_z = Magnetic momentum / angular momentum γ = 67.96 radians, Tesla. When the sample is placed at a magic angle parallel to the applied field, the frequency with which the nucleus spins is said to be precessional frequency. ω_L ∝ H₀ ω_L = γH₀ 1 x 10⁶ - 10¹⁰ H₀ [Diagram showing ¹³C nucleus opposing the less applied magnetic field, resulting in less H₀ and less frequency] The magnetic field produced by the spinning nuclei opposes the applied field. FT ¹³C NMR Spectroscopy Pulse - The Electromagnetic radiation in RF region containing all the frequencies matched with precessional frequency of all ¹³C nuclei. Pulses are of two types Hard pulse and soft pulse. Hard pulse: 10-50μs Soft pulse: 50-100μs In ¹³C Nuclei all the dipoles are randomly oriented, when it is exposed to external magnetic field, the two spin states of the proton gets differentiated by certain energy. [Diagram showing spin alignment with and against an external magnetic field B] according to Boltzmann distribution law most of the particles exist in α spin state. When both the spin states are equally populated then the particles which are present in β state relaxes to α state giving out some energy. The emitted energy can be recorded and amplified. [Diagram showing x,y,z axes with B₀ along the z-axis] when magnetic field is applied the dipole rotates in the direction of applied magnetic field. [Diagram showing the magnetic vector precessing around the z-axis] -> it interacts with magnetic field of RF region. when the RF region is applied in the x direction along with applied magnetic field it tilts towards y direction. Free Induction decay The magnetisation loud by the ¹³C nuclei is recorded. [Diagram showing a decaying sine wave labeled "Time domain" pointing to two sharp peaks labeled "ν" and "F.T"] -> ADC Similarities b/w ¹H NMR & ¹³C NMR Spectroscopy ① ¹H, ¹³C NMR nuclei are magnetic nuclei. I=½ ② Basic principle involved in the spectroscopy in same for both. ③ Reference is TMS for both. ④ Solvents are also same CDCl₃, D₂O, O=C-(CD₃)₂, D₃C-S-CD₃, CD₃OD, CF₃COOH ⑤ chemically & magnetically equivalent protons/carbons will give same signals. ⑥ chemically & magnetically non-equivalent protons/carbons will give diff signals. ⑦ Finally the spectrum gives the information regarding the no. & nature of protons/carbons. Diff b/w ¹H NMR & ¹³C NMR Spectroscopy ① Natural abundance ¹H 99.9% ¹³C 1.1% ② ¹H μ 2.7927 g 5.585 ¹³C γ 0.7022 1.405 2π 67.262x10⁶ radians/Tesla/sec ③ Sensitivity of magnetic nucleus (x): [ (μx / μH) ]³ * (Ix + 1 / IH + 1) * (Ix / IH) = (1/4)³ = 1/64. Overall sensitivity of ¹³C = [ (μ¹³C / μ¹H) ]³ * [ (N_A of ¹³C in % / N_A of ¹H in %) ] = (1/64) * (1.1 / 100) = 1/5800 Sensitivity of ¹³C is 64 times less than ¹H sensitivity overall " " " 6000 times " " Reasons ① The field experienced by ¹³C is more when compared to ¹H Nuclei The chemical shift values for ¹³C are very large, compared to chemical shift values of ¹H NMR Spectrum, this is due to the electronic environment around the ¹³C is diff from electronic environment around ¹H. The substituents is directly attached to the 'c' Therefore the substituents affect is directed on the 'c' atom compared to ¹H Nucleus. Therefore ¹³C nucleus feels more magnetic field to whereas ¹H will experience the effect of substituents via ¹³C Nuclei the field felt by the ¹H is less. ② In case of protons, electrons are present in 1s orbital which is spherically symmetrical around the ¹H Nucleus and generate large secondary magnetic field. Add the felt by ¹H is less. In case of ¹³C nuclei around ¹³C Nuclei the electrons are present either in p-orbital or hybridised orbital which are polarised and e⁻ density is not symmetrical around ¹³C nucleus and generates less secondary magnetic field. Therefore the field to felt by ¹³C is large, is called paramagnetic shift. ¹H undergoes diamagnetic shift because of this.