Contribution of Benchtop NMR Spectroscopy to the Identification of Unknown Liquids Separated by Fractional Distillation

Abstract

Fractional distillation is part of the essential experimental techniques taught to first-year chemistry students, especially during practical organic chemistry work. The characterization of the distillate generally involves measuring the boiling temperature, the refractive index or the infrared spectrum. In this paper, an unknown mixture composed of two organic solvents is given to the students in order to be separated by fractional distillation and characterized. To perform this task, the benchtop 1 H NMR spectroscopy was introduced to complement other analytical methods and to familiarize students with this technique while implementing their reasoning abilities.

A CASE (Computer-Assisted Structure Elucidation) for Bench-Top NMR Systems in the Undergraduate Laboratory for De Novo Structure Determination: How Well Can We Do?

“…Magnetic field strengths of less than 1.8 T (80 MHz) may encounter difficulties in de novo structure determination for some large molecular weight compounds. Nevertheless, their potential applications in the undergraduate lab, ranging from reaction monitoring to mixture quantification, advocate and encourage their incorporation to the chemistry curriculum. ,,− For the 1.8 T (80 MHz) magnetic field strength, instructors can expect to study a much wider breadth of organic compounds with varying molecular weights. Based on previous studies, compounds with a high degree of symmetry can provide additional benefits due to the reduction in the number of resonances, which leads to better spectral resolution .…”

“…Second, these systems are simple to operate in that no special training is required for its operation, making it an ideal beginner instrument for the undergraduate lab. Knowing that most undergraduates are intimidated by the operational complexity of NMR, such as shimming, tuning and matching, and pulse calibration of a high magnetic field NMR instrument, these BT NMR instruments offer a unique opportunity for students to gain confidence in operating an NMR instrument before moving on to other high magnetic field NMR systems. ,,, In large part, shimming, matching and tuning, and pulse calibration are not required on the BT systems, allowing students to put in their sample and immediately start acquisition. , Lastly, these systems are highly accessible for all students compared to most high field NMR instruments where space restrictions often exist for safety purposes due to stray magnetic fields. ,,, Therefore, this further improves inclusivity within the chemistry classroom.…”

“…4 This is particularly exacerbated in the 1 H NMR spectra where, especially for Quinidine, the complex multiplet patterns obscure most of the 1 H NMR spectral width at 43 and 80 MHz compared to that seen for 500 MHz. 4 While the 1 H NMR spectra are highly congested at low magnetic field strength, 13 C spectra are highly resolved owing to its greater chemical shift dispersion and the lack of 13 C− 13 C couplings at natural abundance, as shown in Figure 2 for the case of Nicotine and Quinidine across all magnetic field strengths. Many of the 13 C signals are resolved with spectral characteristics similar to those at 500 MHz.…”

“…Second, these compounds contain aromatic moieties that are commonly found in chemicals used in the undergraduate lab to illustrate various aromatic substitution reactions and are regions that are often plagued with spectral overlaps due to higher order couplings, which can be an issue for BT NMR systems. Third, their respective 1 H and 13 C resonances span the entire 1 H and 13 C chemical shift range, allowing students to see the effect of magnetic field strengths on spectral resolution, particularly for which is a stereoisomer of Quinidine, was verified using a 80 MHz NMR spectrometer, illustrating the potential of BT NMR system in de novo structure determination. 9 ) Therefore, these compounds can serve as excellent test cases to illustrate the effect of magnetic field strength on the ability to derive long-range correlation for structure determination, allowing for a better understanding of the limitations for a BT NMR system.…”

“…CASE offers a systematic approach that allows for quantitation of errors; therefore, this offers a measure of confidence in the proposed structural candidates. The quantitation of errors is based on the deviation between calculated and experimental chemical shifts in both 1 H and 13 C dimensions. Thus, this provides a measure of confidence of each structural candidate provided through CASE.…”