Chapter 13: Molecular Spectroscopy Electromagnetic Radiation E = hν h = Planck s Constant (6.63 x 10-34 J. s) ν = frequency (s -1 ) c = νλ λ = wavelength (nm) Energy is proportional to frequency Spectrum Wavelength (nm) Frequency (s - 1 )
Energy is quantized E E2 ΔE = E 2 E 1 = hν Molecules absorb electromagnetic radiation at the energy difference between 2 states. Nuclear Magnetic Resonance (NMR) = nuclear spin states (radio) Infrared = vibrational states UV-Vis = electronic states E1
The nuclei that are most useful to organic chemists are: 1 and 13 C both have spin = ±1/2 1 is 99% at natural abundance 13 C is 1.1% at natural abundance
Nuclear Spin + + Nuclei ( 1 or 13 C), generate a magnetic field. The magnetic field generated by a nucleus of spin +1/2 is opposite in direction from that generated by a nucleus of spin 1/2.
The distribution of nuclear spins is random in the absence of an external magnetic field. + + + + +
An external magnetic field causes nuclear magnetic moments to align parallel and antiparallel to applied field. + + 0 + + +
There is a slight excess of nuclear magnetic moments aligned parallel to the applied field. + + 0 + + +
NMR Spectroscopy 1 Nuclei have intrinsic spin. Overall spin = spin quantum number. Only certain isotopes can be studied by NMR. (For example, 1 = ½ and 13 C = ½) but ( 12 C) cannot. In the absence of a magnelc field, spin states are degenerate. Therefore, NMR is performed in the presence of a large magnelc field.
NMR Spectroscopy 2 EnergeLc difference between nuclear spin states is very small. Large magnets are used (4.7 22 Τesla). OUen referred to by 1 frequency (200-900 Mz). Different isotopes have different resonance frequencies. Can look at different isotopes separately (for example, 1 and 13C). See Figure 13.5 (for modern NMR spectrometer) PracLcal ConsideraLons: 1-100 mg of compound is needed Compound most be dissolved in solvent (deuterated). CDCl3 is the most common.
Important relationships in NMR The frequency of absorbed electromagnetic radiation is proportional to the energy difference between two nuclear spin states which is proportional to Units z kj/mol (kcal/mol) the applied magnetic field tesla (T)
Important relationships in NMR The frequency of absorbed electromagnetic radiation is different for different elements, and for different isotopes of the same element. For a field strength of 4.7 T: 1 absorbs radiation having a frequency of 200 Mz (200 x 10 6 s -1 ) 13 C absorbs radiation having a frequency of 50.4 Mz (50.4 x 10 6 s -1 )
Important relationships in NMR The frequency of absorbed electromagnetic radiation for a particular nucleus (such as 1 ) depends on its molecular environment. This is why NMR is such a useful tool for structure determination.
Shielding An external magnetic field affects the motion of the electrons in a molecule, inducing a magnetic field within the molecule. The direction of the induced magnetic field is opposite to that of the applied field. C 0
Shielding and Chemical ShiU Bo = external magnelc field B = induced magnelc field generated by electrons
Shielding The induced field shields the nuclei (in this case, C and ) from the applied field. A stronger external field is needed in order for energy difference between spin states to match energy of rf radiation. C 0
Chemical Shift Chemical shift is a measure of the degree to which a nucleus in a molecule is shielded. Protons in different environments are shielded to greater or lesser degrees; they have different chemical shifts. C 0
Chemical Shift Chemical shifts (δ) are measured relative to the protons in tetramethylsilane (TMS) as a standard. 3 C C 3 Si C 3 C 3 δ (ppm) = position of signal - position of TMS peak spectrometer frequency x 10 6
Downfield Decreased shielding Upfield Increased shielding (C 3 ) 4 Si (TMS) 10.0 9.0 8.0 7.0 6.0 5.0 4.0 3.0 2.0 1.0 0 Chemical shift (δ, ppm) measured relative to TMS
Chemical Shift Example: The signal for the proton in chloroform (CCl 3 ) appears 1452 z downfield from TMS at a spectrometer frequency of 200 Mz. δ = position of signal - position of TMS peak spectrometer frequency x 10 6 δ = 1452 z - 0 z 200 x 10 6 x x 10 6 δ = 7.26
δ 7.26 ppm Cl C Cl Cl 10.0 9.0 8.0 7.0 6.0 5.0 4.0 3.0 2.0 1.0 0 Chemical shift (δ, ppm)
Effects of Molecular Structure on 1 Chemical Shifts protons in different environments experience different degrees of shielding and have different chemical shifts
Electronegative substituents decrease the shielding of methyl groups least shielded most shielded C 3 F" C 3 OC 3" (C 3 ) 3 N" C 3 C 3 " (C 3 ) 4 Si" δ 4.3 δ 3.2 δ 2.2 δ 0.9 δ 0.0
Electronegative substituents decrease shielding δ 0.9 δ 1.3 δ 0.9 3 C C 2 C 3 δ 4.3 δ 2.0 δ 1.0 O 2 N C 2 C 2 C 3
Effect is cumulative CCl 3 δ 7.3 C 2 Cl 2 δ 5.3 C 3 Cl δ 3.1
Protons attached to sp 2 hybridized carbon are less shielded than those attached to sp 3 hybridized carbon C C C 3 C 3 δ 7.3 δ 5.3 δ 0.9
But protons attached to sp hybridized carbon are more shielded than those attached to sp 2 hybridized carbon δ 5.3 C C δ 2.4 C C C 2 OC 3
Protons attached to benzylic and allylic carbons are somewhat less shielded than usual 3 C C 3 δ 1.5 δ 0.8 δ 0.9 δ 1.3 δ 0.9 δ 1.2 δ 2.6 3 C C 2 3 C C 2 C 3
Proton attached to C=O of aldehyde is very deshielded C δ 2.4 O 3 C C C δ 9.7 C 3 δ 1.1
Type of proton Chemical shift (δ), ppm Type of proton Chemical shift (δ), ppm C NR 2.2-2.9 C Cl 3.1-4.1 C C 4.5-6.5 C Br 2.7-4.1 Ar O 6.5-8.5 C O 3.3-3.7 C 9-10
Type of proton Chemical shift (δ), ppm NR 1-3 OR 0.5-5 O OAr O C 6-8 10-13
Information contained in an NMR spectrum includes: 1. number of signals 2. their intensity (as measured by area under peak) 3. splitting pattern (multiplicity)
Number of Signals protons that have different chemical shifts are chemically nonequivalent exist in different molecular environment
N CC 2 OC 3 OC 3 NCC 2 O 10.0 9.0 8.0 7.0 6.0 5.0 4.0 3.0 2.0 1.0 0 Chemical shift (δ, ppm)
Chemically equivalent protons are in identical environments have same chemical shift replacement test: replacement by some arbitrary "test group" generates same compoun 3 CC 2 C 3 chemically equivalent
Chemically equivalent protons Replacing protons at C-1 and C-3 gives same compound (1-chloropropane) C-1 and C-3 protons are chemically equivalent and have the same chemical shift ClC 2 C 2 C 3 C 3 C 2 C 2 Cl 3 CC 2 C 3 chemically equivalent
Diastereotopic protons replacement by some arbitrary test group generates diastereomers diastereotopic protons can have different chemical shifts Br δ 5.3 ppm C C 3 C δ 5.5 ppm
13.6 Interpreting Proton NMR Spectra
13.7 Spin-Spin Splitting in NMR Spectroscopy not all peaks are singlets signals can be split by coupling of nuclear spins
Figure 13.12 (page 536) Cl 2 CC 3 4 lines; quartet 2 lines; doublet C C 3 10.0 9.0 8.0 7.0 6.0 5.0 4.0 3.0 2.0 1.0 0 Chemical shift (δ, ppm)