CHEM 344 Spectroscopy of Organic Compounds Lecture 2 6 th and 10 th September 2007

Review of Lecture 1 Spectroscopy: the study of molecular structure by the interaction of electromagnetic radiation with matter Energy levels in molecules quantized (ΔE = hv) NMR uses magnetic fields and radio-waves to flip the spin-state of a nucleus (e.g. 1 H, 13 C, 29 Si etc.) Different local magnetic fields within the molecule give rise to different signals in the NMR spectrum Local magnetic field influenced by local structure of molecule (e.g. electron withdrawing groups) Equivalent hydrogen atoms = same chemical shift

Spin-Spin Splitting Non-equivalent hydrogen atoms will (almost) always have different chemical shifts. When non-equivalent hydrogens are on adjacent carbon atoms spin-spin splitting will occur due to the hydrogens on one carbon feeling the magnetic field from hydrogens on the adjacent carbon. This is the origin of signal multiplicity The size of the splitting between two hydrogen atoms (measured in Hz) is the coupling constant, J.

Spin-Spin Splitting - Origin of the Doublet

Spin-Spin Splitting - Origin of the Triplet

Spin-Spin Splitting - Origin of the Quartet

Pascal’s Triangle # eq. protons Multiplicity Relative Intensity 0Singlet1 1 Doublet 1:1 2Triplet 1:2:1 3 Quartet 1:3:3:1 4Quintet 1:4:6:4:1 5Sextet 1:5:10:10:5:1 6Septet1:6:15:20:15:6:1

The n+1 rule If Ha is a set of equivalent hydrogen atoms and Hx is an adjacent set of equivalent hydrogen atoms which are not equivalent to Ha: (i.e. Ha ≠ Hx) The NMR signal of Ha will be split into n+1 peaks by Hx. (where n = # of hydrogen atoms in the Hx set.) The NMR signal of Hx will be split into n+1 peaks by Ha. (where n = # of hydrogen atoms in the Ha set.) If there are n equivalent protons on an adjacent atom(s), they will split a signal into n+1 peaks.

1 H-NMR spectrum of bromoethane

Formula: C 3 H 7 I 1 H-NMR δ : 1.90 (d, 6H), 4.33 (sept., 1H)

Formula: C 2 H 4 Cl 2 1 H-NMR δ : 2.03 (d, 3H), 5.92 (quartet, 1H)

Formula: C 3 H 6 Cl 2 1 H-NMR δ : 2.20 (pent., 2H), 3.62 (triplet, 4H)

Infrared Spectroscopy Energy of photons in the IR region corresponds to differences in vibrational energy levels within molecules (~10 kcal/mol = ~40 kJ/mol). Vibrational energy levels are dependent on bond types and bond strengths, and are quantized. IR is useful to determine if certain types of bonds (functional groups) are present in the molecule.

IR Spectrum of Ethanol

IR Correlation Table

Key Functional Groups by Region of the IR Spectrum

IR Spectrum of Benzaldehyde

IR Spectrum of Cyclohexanone

IR Spectrum of Propanoic Acid