What to expect from the tert-butanol 1D and 2D 13C NMR analysis?

¹H and ¹³C NMR analyses are routinely used in organic laboratories. In the ¹H spectrum interpretation, it is common to explore the concept of homonuclear coupling constant (¹H-¹H), and the heteronuclear (¹H-¹³C) couplings are referred to as the ¹³C satellites.

When acquiring the ¹³C spectrum, we usually acquire it with ¹H decoupling taking advantage of NOE enhancement, and little is explored about the ¹³C-¹H coupling pattern in the ¹³C spectrum. However, this information obtained from 1D and/or 2D experiments can be crucial to confirm the chemical structure of unknown compounds.¹

As ¹H, ¹³C also has spin ½, so the signal multiplicity is dictated by the N+1 rule and the intensity of the multiplet by Pascal’s triangle. For extra reading about homo and heteronuclear coupling constants, do not hesitate to read some of our previous blogs: (1) Heteronuclear J-Coupling (Part 1 and Part 2), (2) J-Coupling & Tin Can Phones, (3) Heteronuclear Spin-Spin Coupling, and (4) Spin-Spin Coupling – Beyond Multiplicity.

This blog aims to give insights and reflections on what can be observed in the coupled ¹³C spectrum of a simple molecule such as tert-butanol (Figure 1), supported by 2D analysis.

Figure 1: Representation of tert-butanol chemical structure.

What multiplicity is expected for C1 in the 1D ¹³C spectrum? The correct answer is a dectet because there are nine chemically equivalent protons nearby (H2, H3 and H4).

What multiplicity is expected for each carbon of the methyl groups (C2, C3 and C4) in the 1D ¹³C spectrum? The correct answer is a quartet of septets (Figure 2), as each ¹³C shows one-bond coupling (¹J_CH) with three protons and three-bond coupling (³J_CH) with six protons.

Figure 2: ¹³C NMR spectrum of tert-butanol in CDCl₃ at 25 MHz. The expansion inset shows the quartet of septet, highlighting the ¹J_CH and the ³J_CH. In the tert-butanol molecular structure, C3 was used as a reference to show the pathway for ¹J_CH (green bonds) and ³J_CH (purple bonds).

The understanding of the tert-butanol ¹³C multiplicity is critical to interpreting and taking conclusive information from the 2D’s spectra. The HSQC (Figure 3a) shows one bond proton-carbon correlations, so the carbon resonances corresponding to C2, C3 and C4 will have a correlation with their respective protons (H2, H3, and H4). From the HMBC spectrum (Figure 3b), we expect to see two (²J_CH) and three bond (³J_CH) correlations. So, the quaternary carbon (C1) correlates with nine hydrogens (²J_CH), and the methyl carbon resonance for C2, C3 and C4 couples with their six vicinal protons (³J_CH).

An important characteristic of the HMBC data is that it is acquired without decoupling, so any residual one-bond coupling (¹J_CH) (not removed by the low pass J filter) will appear as two correlations centered in the corresponding ¹H signal, so it can be easy differentiated from ²J_CH, and ³J_CH, which are strong correlations aligned with the corresponding signal.

To illustrate how ¹J_CH looks like in the HMBC spectrum the HMBC acquisition parameter, average value for ¹J_CH, was set to 200 Hz, instead of the usual value of 145 Hz. The proper setting of this parameter leads to HMBC data without the residual ¹J_CH.

Figure 3: a) ¹H-¹³C HSQC spectrum of tert-butanol in CDCl₃ at 100 MHz, the correlation represents ¹J_CH. b) ¹H-¹³C HMBC spectrum of tert-butanol in CDCl₃ at 100 MHz, the correlations represent ²J_CH and ³J_CH. To illustrate our point in this blog the HMBC acquisition parameter, average value for ¹J_CH, was set to 200 Hz (usually 145 Hz is used) to allow the visualization of the residual ¹J_CH.

This analysis can be extrapolated for any molecule containing geminal methyl groups and can be used to assign ¹³C spectrum and confirm the suggested molecule structure. The key information is when two or three methyl groups show HMBC correlations to their “own” ¹³C signal, it defines much of the skeletal structure of the molecule since it is strong evidence that these are geminal carbons.

[1] Burns, D. C., Reynolds, W. F. Magn Reson Chem. 2021, 59, 5, 500-533.