C NMR Spectroscopy

1 H and 13 C NMR compared: both give us information about the number of chemically nonequivalent nuclei (nonequivalent hydrogens or nonequivalent carbons) both give us information about the environment of the nuclei (hybridization state, attached atoms, etc.)

1 H and 13 C NMR compared: the signal for the NMR of a 13 C nucleus is times weaker than the signal for a hydrogen nucleus the signal for the NMR of a 13 C nucleus is times weaker than the signal for a hydrogen nucleus a signal for a 13 C nucleus is only about 1% as intense as that for 1 H because of the magnetic properties of the nuclei, and at the "natural abundance" level only 1.1% of all the C atoms in a sample are 13 C (most are 12 C)

1 H and 13 C NMR compared: 13 C signals are spread over a much wider range than 1 H signals making it easier to identify and count individual nuclei Check the spectra on the next slides: Figure (a) shows the 1 H NMR spectrum of 1- chloropentane; Figure (b) shows the 13 C spectrum. It is much easier to identify the compound as 1-chloropentane by its 13 C spectrum than by its 1 H spectrum.

Chemical shift ( , ppm) ClCH 2 Figure 13.20(a) (page 511) CH3CH3CH3CH3 ClCH 2 CH 2 CH 2 CH 2 CH 3 1H1H1H1H

Chemical shift ( , ppm) Figure 13.20(b) (page 511) Note: in these spectra the peak intensities are not exactly proportional to the number of carbon atoms. ClCH 2 CH 2 CH 2 CH 2 CH C CDCl 3 a separate, distinct peak appears for each of the 5 carbons

C Chemical Shifts are measured in ppm (  ) from the carbons of TMS

13 C Chemical shifts are most affected by: hybridization state of carbonhybridization state of carbon electronegativity of groups attached to carbonelectronegativity of groups attached to carbon

Examples (chemical shifts in ppm from TMS) sp 3 hybridized carbon is more shielded than sp 2

Examples (chemical shifts in ppm from TMS) OH O sp 3 hybridized carbon is more shielded than sp

Examples (chemical shifts in ppm from TMS) OHOHOHOH23 an electronegative atom deshields the carbon to which it is attached an electronegative atom deshields the carbon to which it is attached 61

Examples (chemical shifts in ppm from TMS) O138 an electronegative atom deshields the carbon to which it is attached 202

Table 13.3 (p 513) Type of carbon Chemical shift (  ), ppm RCH3RCH3RCH3RCH30-35 R2CH2R2CH2R2CH2R2CH R3CHR3CHR3CHR3CH25-50 R4CR4CR4CR4C30-40

Table 13.3 (p 513) Type of carbon Chemical shift (  ), ppm Type of carbon Chemical shift (  ), ppm RCH3RCH3RCH3RCH30-35 CR2CR2CR2CR2 R2CR2CR2CR2C65-90 CRCRCRCR RCRCRCRC R2CH2R2CH2R2CH2R2CH R3CHR3CHR3CHR3CH25-50 R4CR4CR4CR4C

Table 13.3 (p 513) Type of carbon Chemical shift (  ), ppm RCH 2 Br RCH 2 Cl RCH 2 NH RCH 2 OH RCH 2 OR 50-65

Table 13.3 (p 513) Type of carbon Chemical shift (  ), ppm Type of carbon Chemical shift (  ), ppm RCH 2 Br RCH 2 Cl RCH 2 NH RCH 2 OH RCH 2 OR RCOR O RCRRCRRCRRCRO