One of the questions that we always get at tradeshows and conferences is how our instrument compares to high-field data. There are significant inherent differences between low-field and high-field instruments, but the most important from a chemistry point of view are sensitivity (S/N) and resonance dispersion (signal separation).Read More
We have recurring interest from industrial customers (e.g., pharma, flavour/fragrances) to acquire spectra using only small amounts of analyte in our benchtop NMR instruments – considerably less than our recommended 0.1 M concentration (link). Why is our recommended concentration so high? Well, it’s reflective of the consequences of a low-field instrument. That is, lower-field results in inherently lower sensitivity and accordingly a decrease in observed signal to noise ratio (S/N).Read More
Before the early 1950’s lithium was an ingredient found in Seven-up®; however, after concerns by the FDA it was removed from this popular beverage. Interestingly, about the same time it was found that lithium had antipsychotic properties, but it was only approved for human consumption in the 1970’s. Despite being such a simple molecule lithium carbonate is still the most effective drug for the treatment of bipolar affective disorder! We don’t hear this side of the lithium history very often. In recent years this element has become very popular and its demand has increased considerably due to its widespread use in batteries for small electronic devices such as smart phones and laptops.Read More
When monitoring reaction progress for determination of reaction kinetic parameters, NMR spectroscopy has increasingly become the method of choice. The ease in which one can calculate the concentration changes of a substrate being consumed or a product being formed over time, directly from peak integration are the reason behind this.Read More
Sugar substitutes are gaining more and more relevance due to the heath problems associated with the consumption of high amounts of sugar. Specifically, excess sugar consumption has been associated with obesity, type II diabetes, cardiovascular disease, certain cancers, and tooth decay. In the United States, it is estimated that the average person consumes more than 126 grams of sugar per day, in Canada around 89 grams per day.Read More
PAT is an extremely powerful and useful tool for analyzing, optimizing and controlling chemical processes. Chemical, food and pharmaceutical industries could especially benefit from this technique.Read More
Last month (which you can see here), we learned about how an acidic proton behaves in a 1H NMR experiment, particularly when it’s surrounded by D2O. For example, when an H+ leaves CH3COOH to join an accommodating D2O molecule, the resulting acetate (H3CCOO–) segment is reasonably comfortable bearing that negative charge. This phenomenon is the reason the solution is “acidic” in the first place. But why is acetate so capable of dealing with this negative electronic charge?Read More
Acidity is something that you encounter on a daily basis, probably without even realizing it. The tangy taste of an orange (citric acid), that Vitamin C tablet you took this morning (ascorbic acid), those terrible jeans from the 80’s that you still wear (acid wash). My favourite acid is acetic acid.Read More
Wishing you all a happy holiday season!
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. 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 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.
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.