Choosing the Right Tool

If you have successfully read this far, it might have occurred to you that some problems could well be solved by either an NOE-based approach, or by an HMBC approach and you might be wondering which technique would be preferable under such circumstances. In truth, there may not be a right or wrong answer to this question and each problem should be considered on its merits. The selection of experiment may even be down to personal preference or to the hardware available to you. Questions of positional isomerism can often be resolved by either approach. We have seen how our naphthalene problem could be resolved by using an NOE technique (Structure 9.3). 

This problem could also have been resolved by an HMBC approach - you would expect to see a correlation from the protons of either the -CH2-, or the -CH3 to one of the quaternary carbons at the junction of the two rings. This same carbon should also show correlations to at least two, and ideally three of the protons on the unsubstituted aromatic ring and one of the protons on the substituted ring. 

It is when questions of stereochemistry arise that the NOE techniques come into their own. For example, consider the compounds in Structure 9.4. There would be no chance of resolving these two structures by HMBC, but an NOE technique might well prove successful. (The methyl group would be expected to give an enhancement to either of the -CH2-OH protons in one isomer or to the CH-CH20H in the other, depending on which face of the ring the two substituents lie relative to each other.) 

As in the case of all NMR problem-solving, the issue is always one of using the most appropriate tool for the job. The two techniques are in no way mutually exclusive. Too much data is not a bad thing if the instrument time is available but taking a chance on insufficient data can be a costly mistake in the long run. 

Type of carbonyl Typical shift Type of carbonyl 

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The availability of several alternative methodologies is a toolbox that facilitates the job of the engineer. It is the responsibility of the engineer to choose the right tool from this toolbox for a given situation. 

Choose the right tool for the job. Also, make sure it is not only the right tool, but it is the correct size for the job. 

Enzymes used in DNA analysis come under a range of headings, depending on their type of activity. Some are highly specific in their action, some less so. It is therefore important to choose the right tool for the investigation being carried out. 

Choose the right sampling tool. Examine the tool and look for violations of the principle of correct sampling. The tool must be capable of taking the sample that is defined and taking it correctly. 

Choosing the right type of equipment is only part of the solution, as well as the best resin. Good tool-... 

RSL is a decision-making tool developed by NIOSH to help employers choose the right kind of respirators for their workplaces by taking them through a series of questions on how and in what environments the respirator will be used, and with what types of contaminants. The RSL takes into account the Assigned Protection Factors (APF) developed by OSHA to indicate the level of protection that a respirator or a class of respirators is expected to provide. 

Computer-related expert tools appear to be the right approach to the problem for two reasons. The difficulty of unclear etiological factor or the determination of acting chemical agent can be overcome by additional reliability criteria, and the selection of a definite clinical syndrome allows to choose between other types of nosological entities and the one having characteristic clinical symptoms. 

FIGURE 2.5 These screenshots from the CatApp (http //suncat.slac.stanford.edu/catapp/) show examples of elementary reaction PEDs that can be obtained from this tool. Left and center. splitting on close-packed and stepped Ru(OOOl), respectively. Right select view of the CatApp. Here, the user can choose the reaction and surface parameters from drop-down menus. Taken from Hummelshpj et al. (2012) with permission from Wiley. 

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