Electronegativity and Chemical Shift
FIGURE 2: STACKED 1H NMR SPECTRA SHOWING CHANGE IN CHEMICAL SHIFT OF ALPHA METHYLENE.
Electronegativity is a concept that we have to study and understand early in our science or engineering degrees. In chemistry this concept stays with us and we commonly invoke it in order to explain multiple phenomena what we observe or study…and of course, NMR is not the exception!
Electronegativity is defined as the ability of an atom in a molecule to attract electrons to itself.[1] The first, and probably most commonly used, scale to measure electronegativity was developed by Linus Pauling. In this scale fluorine is the most electronegative element with an electronegativity of 4.0 and cesium is the least electronegative element with an electronegativity of 0.7. In the periodic table the electronegativity generally increases from left to right across a period and decreases from top to bottom down a group
(Figure 1).
In proton NMR spectroscopy (this also applies for other nuclei) the chemical shift of a particular C-H system depends on the electronic environment around the proton. Electrons in the neighboring atoms could reduce the field experienced by the proton, shielding it from the external magnetic field and moving the signal to lower ppm (or upfield). On the contrary, electronegative groups close to the C-H system reduce the surrounding electron density, deshielding the proton from the external magnetic field and moving the signal to higher ppm (downfield). Do you get confused when somebody uses the downfield and upfield terminology?...you are not alone! They are confusing terms because they seem opposite of what we would expect. This terminology is not strictly correct in pulse Fourier transform (FT) NMR instruments because the magnetic field remains unchanged, but somehow it has been passed from the old days of NMR. When Fred Flintstone and Barney Rubble used the original NMR instruments (continuous wave), the magnetic field strength was increased from left to right at constant frequency. Thus, the resonances on the right were referred as upfield signals and on the left as downfield signals. Technically both terms are obsolete and have been replaced by shielding and deshielding respectively.
Hahahaha that was me trying to make a joke….Use this sentence “In the continuous wave (CW) method of measuring NMR spectra, the magnetic field strength was increased from left to right at constant frequency” instead of the Fred Flintstone one.
In a very easy to do experiment reported by Everest and Vargason[3] we can experimentally observe these electronegativity trends. In Figure 2A we see how the protons closer to X (in red) are sequentially shielded as we move down the group due to the decrease in electronegativity from fluorine to iodine. On the other hand, as we move across the period from carbon to fluorine, the protons in red are deshielded (Figure 2B) due to the increase in electronegativity. Keep in mind that the signal of interest in 1-fluoropentane appears a distorted doublet of triplets centered at 4.4 ppm due to coupling with the fluorine center. As you can see in these spectra, at 60 MHz the resolution of the spectra is lower compared to a high-field instrument. However, you can also see that the results that we got in this experiments are perfectly fine at 60 MHz. If you navigate through our website you will discover that in most cases the NMReady will give you the information that you are looking for and there is no need to need to use an expensive high-field instrument!
The beauty of this experiment is that it can be conducted in the first semester of General Chemistry or in second year Organic Chemistry. In the first case, the experiment can be used to emphasize the concepts of electronegativity and electron clouds in a guided-inquiry approach without the need of previous knowledge about NMR spectroscopy. Furthermore, the use of the NMReady allows the students to just touch the screen to get the spectra without having to worry about selecting parameters or shimming samples. In the second case, this experiment can be used as one of the first experiments in an NMR spectroscopy lab in order to experimentally discover and reinforce some of the important concepts in NMR spectroscopy.
References
1 - Modern Physical Organic Chemistry University Science Books, 2007.
2 - Carr, D. (2016, January 26). Periodicity (1) Ionisation energy and electronegativity of the elements. Retrieved from https://derekcarrsavvy-chemist.blogspot.ca/2016/01/periodicity-1-ionisation-energy-and.html
3 - Everest, M. A.; Vargason J. M. Journal of Chemical Education 2013, 90, 926.