13C One- and Two-Dimensional

Benchtop NMR Spectroscopy
Bertram Manza, Federico Casanovab, Jonathan Singhc, Hemi Cummingc, Andrew Coya, Juan Perlob(a) Magritek Ltd, 32 Salamanca Road, Wellington, New Zealand(b) Magritek GmbH, Pauwelsstr. 19 - D-52074, Aachen, Germany(c) School of Chemical and Physical Sciences, Victoria University, Wellington, New Zealand

The benchtop NMR system

Quinine

13C NMR spectroscopy is an important tool in chemical structure elucidation, as it detects

the carbon backbone of organic molecules. While 13C NMR is routinely used at high field,
it has until now not been utilised in benchtop NMR. One of the main reasons for this is
the low sensitivity of the 13C nucleus. Overall, the receptivity is reduced by about 4 ordersof magnitude compared to 1H NMR. The acquisition of meaningful 13C NMR spectra ofdilute samples is only possible if the spectrometer provides high stability to enable signal
averaging and sufficient sensitivity.
In this work we demonstrate the performance of 13C NMR on a 1 Tesla permanent magnet
system using a dilute, non-enriched organic molecule. The results presented here showthat benchtop 13C NMR methods can reveal information which is not available with 1H
NMR and that the findings are consistent with high field NMR.

(b)

(a)

Quinine ((8α,9R)-6'methoxycinchonan-9-ol) is a naturalwhite
crystalline
alkaloidhavingantipyretic (fever-reducing), antimalarial,analgesic(painkilling),andantiinflammatory properties and a bitter
taste. The molecular formula is
C20H24N2O2, with a mass of 324.4 Da. The
sample for the experiments reported
here was a solution of non-enriched
quinine in CDCl3 with a concentration of2 M in a 5 mm NMR tube. Figure 2 shows
the molecular structure.

Figure 2: Chemical structure of quinine [1].

CH3Figure 1: (a) The SpinsolveCarbon benchtop NMR system, (b) carbon-13 spectra of neat
ethylbenzene. The spectrum at the top is the full 1D spectrum showing all carbons in themolecule. Spectral editing such as DEPT enable the measurement of subspectra
originating from CH, CH2 and CH3 groups.

Comparing high and low field 1D NMR spectroscopy
One of the difficulties encountered when dealing with low field proton NMR spectra isthat there is often a strong peak overlap due to the homonuclear spin couplings being
similar in size as the chemical shift separation between inequivalent spins. These strong
coupling effects decrease as the frequency difference between multiplets increases, sothat high-field (i.e. high-frequency) NMR spectra display less distortion than lower
frequency spectra. Figure 3 (a) shows 1H spectra of the quinine sample at 43 and 500
MHz. At low field, the spectrum shows severe peak overlap due to strong coupling effects,
whilst at high field all peaks are well separated.
Due to the low natural abundance of the 13C nucleus, there is no significant homonuclear
spin coupling in 13C spectra. As a consequence, the peak structure of 13C spectra at low
field is identical to high field, as shown in Figure 3 (b).

2D proton-carbon correlation spectroscopy
Heteronuclear 2D correlation spectroscopy experiments are particularly useful in helping
assign crowded spectra with overlapping peaks, as they detect coupling partners of different
nuclei. The introduction of the second (f1) dimension in these experiments provides valuable
structural information to organic chemists.
The HSQC (Heteronuclear Single Quantum Correlation) type experiments correlate thechemical shift of protons with the chemical shift of the directly bonded carbon. Cross-peaks inthese spectra are between carbons and directly attached protons.
The HMBC (Heteronuclear Multiple Bond Correlation) experiment differs from the HSQC in
that multiple-bond couplings over two or three bonds (J = 2-15 Hz) are utilised. Direct onebond correlations are suppressed, and cross-peaks in HMBC spectra are between protons andcarbons that are two or three bonds away.
Figures 4 and 5 show the HSQC (a) and HMBC spectra (b) of quinine at 1 T and 11.7 T,respectively. While it is possible to observe all available correlations in the HSQC experimentat high-field (500 MHz), some of the more diagnostic correlations do stand out at low-field(43 MHz). For example, the oxymethyl CH3-25 can be observed, as well as oxymethine CH-21
and protonated alkene carbon CH2-19. Likewise in the HMBC experiment, the characteristic
one-bond leakthrough from CH3-25 is observed, along with its correlation to C-3. The lower
left quadrant of the 2D spectrum shows correlations that are consistent with three-bondconnectivity in the heteroaromatic region of quinine.

(a)

(b)

(a)

Figure 4: HSQC (a) and HMBC (b) spectrum of quinine at 1 T.

(a)

(b)

(b)

Figure 5: HSQC (a) and HMBC (b) spectrum of quinine at 11.7 T.
Figure 3: Proton (a) and carbon (b) spectra of the quinine sample measured at 11.7 T(blue) and 1 T (brown). Note that the benchtop proton spectrum shows significant peakoverlap due to strong coupling, whilst the benchtop carbon spectrum is essentially
identical to the one at high field allowing unambiguous peak assignment [2].

References[1] http://www.chemspider.com/Chemical-Structure.84989.html[2] http://www.chem.wisc.edu/~cic/nmr/NMRdatab/res_cmpd/pdfs/
quin_assignment_example-for-pdf.pdf