Nuclear Magnetic Resonance

Information Gained:• Different chemical environments of nuclei being

analyzed (1H nuclei): chemical shift• The number of nuclei with different chemical

environments: number of signals in spectrum• The numbers of protons with the same chemical environment:

integration• Determine how many protons are bonded to the same carbon:

integration• Determine the number of protons that are adjacent to

one another: splitting patterns• Determine which protons are adjacent to one another: coupling

constants

Lecture 6

Chemical Shifts

• Shielded protons appear more upfield (lower ppm value).• Deshielded protons appear more downfield (higher ppm value).

Correlation Chart

Chemical Shifts

TMS - Tetramethylsilane (Me4Si) is the internal reference used in manyexamples. TMS’s chemical shift is set at zero since most peaks appear more downfield from it. (Note: You will use your NMR solvent, CDCl3, as your reference peak. Why use CDCl3 to make up your sample for NMR?)The Delta () Scale

• An arbitrary scale

• 1 = 1 part per million (ppm) of the spectrometer operating frequency. For example, if using an 80 MHz instrument to run a 1H NMR spectrum, 1 would be 1 ppm of 80,000,000 Hz, or 80 MHz.

• Since the radiofrequency absorption of a nuclei depends on the magnetic field strength, chemical shift in Hz would vary from instrument to instrument. The stronger the field, the greater the E (magnetic transition).

• Thus, report the nuclei absorption in relative terms () as opposed to absolute terms (Hz). This way, the chemical shifts will be the same for nuclei of a sample despite what instrument you use - leads to correlation charts!

1H NMR Spectrum of Ethanol

ppm

CH3CH2OH

TMS

Three signals - three different types of H’s

a ab

b

c

c

downfield upfield

Equivalent & Non-Equivalent Hydrogens

As seen in the 1H NMR spectrum of ethanol, the number of signals equals the number of different types of protons in a compound.

General rules: • Protons attached to the same sp3 carbon are equivalent (if there are no chiral centers in the molecule; if there are, could be equivalent or non-equivalent).• If there is symmetry in the molecule, protons that are symmetrical will have the same signal, the same chemical shift.

Note: Protons attached to the same sp2 carbon (in alkenes) need to be evaluated for equivalency.

Equivalent & Non-Equivalent Hydrogens

Consider the following molecules. Determine which protons are equivalent and non-equivalent. Predict the number of signals that would appear in the 1H NMR spectra of these compounds.

1H NMR Spectrum of Ethanol: Spin-Spin

Splitting

ppm ()

CH3CH2OH

TMS

a - tripletb - quartetc - singlet

a ab

b

c

c

downfield upfield

Spin-Spin Splitting

CH3CH2OH

a - tripletb - quartetc - singlet

a b c General rules:• Neighboring, non-equivalent protons split each other’s signals• Equivalent protons do not split each other’s signals• Use the n + 1 rule to predict the splitting pattern of a proton’s signal

n + 1 ruleThe signal of a proton with n equivalent neighboring protons is split into amultiplet of n + 1 peaks.

In ethanol, a neighbors b; they split each other’s peaks. Note that b neighbors c and no splitting occurs between the two; b is only affected by a.In general, protons that reside on heteroatoms (O, N) do not get involvedwith spin-spin splitting with neighboring protons. Thus, c appears as a singlet.

Spin-spin Splitting