So why am I bringing this up? Well, as weвЂ™ve previously discussed, 60 MHz spectrometers have inherently lower resolution in terms of chemical shift dispersion.

For the two doublet situation described above, with pronounced tenting, it is referred to as an вЂAB quartetвЂ™ (as opposed to вЂtwo doubletsвЂ™ as would be the case in first order spectra). For magnetically inequivalent, but chemically equivalent, nuclei they get denoted with a вЂprimeвЂ™ (e.g., o-dichlorobenzeneВ will have a spin system of AAвЂ™BBвЂ™). Although every magnetically equivalent nucleus gets the same letter, you denote the number of nuclei with a subscript of the number (i.e.,В CH = вЂAвЂ™, CH2 = вЂA2вЂ™, CH3 = вЂA3вЂ™). First order systems are assigned separated letters (e.g.,В AX, or AMX), whereas second order systems get allocated consecutive letters (e.g.,В AB or ABC). Whether the assigned letters are separated or consecutive is dependent on the spin system. The most downfield resonance is assigned the latest letter in the alphabet and the increasingly upfield resonances, the preceding letters, ending in вЂAвЂ™ for the most upfield resonance. Using the Pople NotationВ can help simplify this вЂ“ you assign letters to the nuclei of a chemical shift with magnetic equivalence. вЂњSpectrometer Identification of Organic CompoundsвЂќ 7th Ed. See this figure taken from Silverstein, for example.ДЇrom:В Silverstein, R. As the ratio decreases, the signal looks more and more like a quartet instead of two doublets. Depending on the degree that this occurs, it may actually be helpful as it gives you added information of what is bonded together. That is, the outside doublet lines decrease in intensity relative to the inside peaks, creating a sloping effect. Two doublets, for example, instead of being 1:1, will slope towards each other creating a tenting or leaning effect. So what can you expect to see as О”Оґ/J gets smaller? Well, depending on your spectrum, one thing that you will observe is differences in the predicted line intensities. Okay, that may be a few too many вЂgreater thansвЂ™ but, like I said, there is no magic ratio, but the chemical shift difference should be more than approximately 10x the coupling constant. IвЂ™ve read many NMR textbooks/papers over the course of my chemistry career and IвЂ™m fairly certain that no two had the same value AND even more annoyingly, IвЂ™ve seen the range from 4 to about 20. Okay вЂ“ so what value of О”Оґ/J are we talking about here? When will we observe a second order spectrum? Honestly, thereвЂ™s no clear answer to this. In strongly coupled systems the lines of a multiplet can even disappear! As О”Оґ/J decreases, the:В (i)В multiplets approach each other В (ii)В do not follow the aforementioned rules В (iii)В may give rise to inaccurate information and (iv)В may be impossible to solve through simple inspection.

Basically what weвЂ™re referring to here is that a spectrum will become increasingly complicated as this ratio of chemical shift difference and coupling constant (О”Оґ/J) gets smaller. The physical basis of strong coupling is subtle, having to do with complex quantum-mechanical phenomena that I wonвЂ™t go into here.ДЄlthough second-order sounds bad вЂ“ it isnвЂ™t necessarily. Otherwise, this means youвЂ™re observing strong couplingВ and youвЂ™ve entered second order spin system land. This condition is satisfied if the difference between two chemical shifts (О”Оґ) is much larger than the coupling constant (J) between them. First order spectra are awesome because they can be solved through visual inspection (i.e.,В chemical shifts and coupling constants can be extracted). There are a number of criteria that define a truly first order system, but basically the designation вЂfirst orderвЂ™ is given when multiplicity follows the вЂn + 1 ruleвЂ™ and these multiplets have intensities defined by PascalвЂ™s Triangle. So the coupling that I previously describedВ applies to first order spin systems.

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