¹³C-NMR Spectroscopy

Chemical Shift.

Range for carbon: 0-200 ppm; all positive values are less shielded than TMS (that is, less electron density around the carbon). Negative values are possible, but rare.

Chemical shift data can be calculated from any type of ¹³C experiment.

Generalizations:

In otherwise equivalent environments, the fewer the H's on the carbon, the greater the d.

Electron withdrawal by electronegative atoms (inductive effects) causes deshielding (larger d ).

These inductive effects are somewhat additive: CH₂Cl ₂ ( d 50 ppm) absorbs downfield of CH₃Cl ( d 40 ppm), and upfield of CHCl3 ( d 77 ppm).

Hybridization has a strong affect: sp³ carbons absorb at 0-100 ppm, sp² carbons at 100-220 ppm, sp carbons at 70-100 ppm.

Carbonyl carbons are particularly characteristic, absorbing at about 170-220 ppm.

Chemical Equivalence

Carbons with identical environments in a molecule have the same chemical shift (the same d value).

Examples: a) Any molecule with a plane of symmetry; carbons that are reflections of each other are in the same environment. b) Any portion of a molecule that is interconverted with another by rotation about a single bond.

¹³C-NMR Conditions

Proton Noise Decoupled (PND). No effect from neighboring hydrogens. Carbon signals are single lines for each environment (see below). Optimized to determine chemical shift.

Gated Decoupling. No effect from neighboring hydrogens. Carbon signals are single lines for each environment, but the area is proportional to the number of carbons. Optimized to determine integration.

Off-Resonance Decoupling ("spin coupling"). The conditions are set to give a split signal: one line for each hydrogen directly bonded to the carbon being observed, plus one more (n + 1). Optimized to determine bonded hydrogens.

Intensity in ¹³C-NMR.

Intensities, or areas, of peaks only have meaning in spectra obtained under Gated Decoupled conditions. For other conditions, peaks will have unreliable intensity.

When appropriate conditions are used, peak areas will be proportional to the number of carbons in that environment. Since these values are not absolute, one can only get ratios: for example 2:1 or 3:2 for peaks. Usually, there is a single carbon somewhere to use as a base.

Spin-spin coupling (splitting of signals).

This is seen in off-resonance decoupling experiments only. Each directly bonded H adds or subtracts slightly from the chemical shift of the attached carbon. Thus, for each hydrogen, each signal due to the carbon is split in half. Since the width of the splitting is the same for each H, there is overlap of interior lines, leading to one additional line for each H.

Carbon-carbon splittings are not seen: since ¹³C abundance is 1.1%, the chance of 2 neighboring is (1.1%)² or about 0.01%.