Structure elucidation involves identification of various parts of structure like its functional groups, double/triple bonds, cyclic groups, molecular weight, etc.

We need to let students know that all the individual techniques have their own specific application.

But, with advancements in Mass and IR techniques they have become more helpful in structure elucidation. Moreover, NMR would be one of the top preference for any organic molecule as it gives almost a clear picture of the structure along with any impurities in the compound.

SO, I WOULD SUGGEST NMR.

Structure elucidation involves identification of various parts of structure like its functional groups, double/triple bonds, cyclic groups, molecular weight, etc.

We need to let students know that all the individual techniques have their own specific application.

But, with advancements in Mass and IR techniques they have become more helpful in structure elucidation. Moreover, NMR would be one of the top preference for any organic molecule as it gives almost a clear picture of the structure along with any impurities in the compound.

SO, I WOULD SUGGEST NMR.

I think the answer will depend on the situation. if we have unknown compound we need to carry out all suggested spectroscopic methods to identify it. similarly when we end up with new compounds. in other cases, I think the most important thing is to let student and researcher chose the right techniques based on the information they required. for example: IR would be the right choice to distinguish between functional groups, but NMR is the most suitable for stereo-chemistry. add to that the cost and availability is a matter of interest some times.

There are certain questions that each technique answers. For instance you identify functional groups by IR, you get the molecular weight by MS and the configuration of the atoms which constitute the molecule by NMR. Students in my opinion should value equally each technique.

I think that generally NMR is best for organic molecule it gives idea about which compound is formed during reaction and other side products. We also get the idea about reaction is completed or not.

I would say that NMR hands-down gives the MOST information for small organic molecules if you can only use one method. This will work for the majority of molecules that you would use in the introductory organic chemistry courses. You can use it to see how things are connected AND infer most of the functional groups that you would need using a chart with shift values.

I agree with others that this will not work for EVERY molecule, and there are situations where the other techniques would also be needed.

However, NMR is the best starting place. Then you can expand from there. This is what I do. I find that students really appreciate the other techniques more and more as we go along and get to things that NMR cannot definitively answer. For example, IR is better for CN groups and MS is better if you might have a dimer.

The FTIR spectroscopy combined with NMR-data should give the general information on the structure of a compound. The next step is the mass molar of a compound determination ( mass spectrometry and physicochemical methods like cryoscopy, ebulliometry or using osmotic pressure measurements). Further, the presence of double or triple bonds and the quantities of the functional groups should be confirmed using the chemical methods.

Don't forget UV, especially for nitrogen heterocycles.

For most cases NMR+MS, all others are of course of relevance for special cases!

You may opt for HMBC NMR, HSQC

I agree with all the above comments. But in a simple sense, if someone provide basic proton NMR, ir , uv and mass spectrum of two isomers. Which of the above method is suitable for differentiate the two isomers without the help of advanced technologies.because in the mind of all the researcher and teachers working on spectroscopy have a doubt in their mind about the simplest known spectroscopic technique which provide the identity. In IR spectra, there is finger print region . That is unique for each compound. So in a simple sense, IR is best or NMR?

dear Preethanuj Preethalayam ,

NMR and in particular 13C NMR are the most suitable to provide information about stereoreqularity and isomerization. however, FTIR with a suitable technique such as 2D-Correlation spectroscopy could do the job in some cases. so the thing is relative to the special case you have.

In case of if there are diastereomers present it can be differentiated by NMR peaks. Major will have a higher peak intensity. You can refer to my paper of SynOpen for diastereomers

It is interesting how a relatively straightforward question will generate a wide ranging discussion. In general, I would agree that H1-NMR coupled with C13-NMR yields the most useful structural information. For a compound with a chiral center a chiral lanthanide resolving agent is useful in H- NMR. But I'm not too interested in looking at an infernally messy spectrum of a reaction mixture when TLC is more useful here. MS is useful also for a molecular weight but not purity. IR is less useful, as an IR spectrum indicates which functional groups are present and a pattern of aromatic substitution but little else. As I indicated before, UV is useful for nitrogen heterocycles. X-ray is wonderful as long as a usable crystal can be obtained.

However, in an undergraduate organic chem teaching lab it depends on the equipment available and the capabilities of said equipment which, depending on funding, may be limited.. MSs may be available only from the university analytical dept and may have a set charge which, for an undergraduate teaching lab cumulative charges, would not be cost effective.

A question for BD. To have diastereomers. at least two chiral centers are required. For a two chiral center compound of unknown structure, if one of the set of diastereomers is meso how do you determine by NMR that the structure is meso or not? Will a meso isomer have a simplified H-NMR? And, if so, how would you know that the structure may not have only one chiral center? And how do you differentiate the (+) and (-) diastereomers? A more complete citation to your paper would be useful.

NMR is the best technique to elucidate the structure of an organic compound. In the case of some crystalline structures X-Ray is the best.

Hi Preethanuj Preethalayam

NMR is the only technique out of the four you mentioned (NMR, IR, UV, Mass) to detect and differentiate the isomers of a compound. For example, if you have an aliphatic alpha-hydroxy-beta-ketone compound, then the carbinol proton with syn configuration will appear at around 3.75 ppm - 3.9 ppm where as the same proton with anti configuration will appear at 3.55 ppm - 3.75 ppm. That is the best way to identify the isomers and even their relative percentages in a mixture. Moreover, the carbinol carbon will appear at different positions in 13C-NMR.

If a structure is already reported somewhere in the literature then NMR is sufficient enough to prove the formation of that compound just by looking at major signals. If not reported, then NMR can be easily predicted using CHEMDRAW and therefore can be used to match with the analyzed one.

SO AGAIN NMR IS THE BEST TECHNIQUE SO FAR OUT OF THE FOUR. @

Yes I do agree but preduction by chemdraw is not recommended

I agree NMR is the best spectroscopic technique. I always prefer NMR and mass for characterisation. But the graduate students not aware of these facts and advanced technologies. I just quated the question asked by them. I can tell NMR is better. In NMR there is no finger print region for each molecule. If a monomer and dimer almost show similar spectrum. In that case they have to depend higher version of NMR and mass spectrometry. In this context, it is difficult to explain. But every molecule have their own IR finger print region. Which makes unique from other and similar molecule. If they have better idea on all the techniques, surely they will prefer NMR. But they know only basics of spectroscopy. That is the problem. Thank you all for your comments and explanation.

Raman spectroscopy is a complementary IR technique. Raman approach the polarization changes

NMR mainly is the best and the most important and widely applicable. Some firms dealing with bioactivity screening demand only NMR spectra as absolutely necessary characteristic of new compound synthesized. But apparent exclusions are too complex mixtures, insoluble or solvalitically unstable compounds, paramagnetic substances and many others.

I agree with all above answers .

I do agree.... Thank you all for your suggestions and comments.

Though all the above recommended NMR as the first choice for structure identification, I still think that NMR cannot solve all the problem. Definitely, the combination of NMR, IR and MS, sometimes including element analysis (EA) for determination of the chemical formula, is the most reliable choice. In fact, I often suggest that IR act as the first method because the measurement is very simple and cheap, and the spectra can provide a lot of structure information. The NMR spectra are relatively easy to interprate since one special proton (or carbon) can only generate one chemical shift. But for IR or MS, one group always yield several bands (or peaks), which might help to interprate the structure more exactly.

In addition, for polymers, especially the non-dissolved polymers, NMR cannot give more useful structure information.

If we want to write the order of priority, what will be the order...

Nmr, IR, Mass, UV...or

NMR, Mass, IR, UV?

I think the 2nd one will be better

I agree with professors .

Nowadays most of the reputed international journal not asking IR spectra of molecule. But it is essential for functional group identification, all these factors will make confusions about the accuracy of the values obtained from IR.

This NMR.

Mass spectrometry is used as a supporting method to confirm the structure,which are already confirmed by NMR. Also mass is not spectroscopy, it is a spectrometric method.

mass spectroscopy

I agree with all answers .

i recommended NMR and LCMS and XRD

all these

Dear friend, as other friends has mentioned every technique has its advantages and disadvantages but what determine direction for you is your situation and target! Are you going to clarify the structure of a compound that you don’t have any viewpoint about its structure or may be a new or novel compound? Or, are you going to specify the structure of a compound which probably you know that?

If you have no speculation about the structure of your compound you should prepare spectra one by one until the sufficient data be supplied to determine your compound structure. In this way Mass spectra specify the fragments of compound and finally the molecular weight of molecule. The IR defines functional groups of your molecule. And NMR spectra considering 1D and 2D will help you to define the arrangement of carbons, protons and stereochemistry of compound. Of course in interpreting the NMR spectra, the data have been extracted from Mass and IR spectra are so beneficial.

If your structure is reported for the first time usually you are supposed to present all spectrometric data including 1H-NMR, 13C-NMR, Mass spectrum and IR as well as specific rotation.

But, if your compound has been reported previously, you should prove the structure by using to direction; the first NMR and the second Mass spectrum which corresponds the NMR spectrum. It should be added I personally assume in this way both 1H-NMR and 13C-NMR are needed.

mass spectroscopy

Two comment:

• MS+NMR is standard, IR/Raman hardly ever neede.
• But: none of the methods mentioned (except for special cases with the Mosher method and NMR) can discriminate Enantiomers! Therefore (and out of other reasons) for essentially no practical case a DE-NOVO structure determination by a single method will be possible.

Kind regards

Alfred

The best spectroscopic method for the structure elucidation of organic molecule without the help of other method Is NMR , 2DNMR & CMR.

Raman micro-analysis is one of the best regarding materials in the solid state, from my experience. It can discriminate the molecularly spatial structure of the compound as presented as it is of a molecue in the solid state. Raman spectroscopy is associated directly with the molecular bonding structures. Having said that, it is neither easy nor strightforward to interpret the Raman peaks correlated with each bond of a molecular structure. So far, it is definitely capable to determine triple/double bonds from single bond, C-H or N-H from O-H bond. It can discriminate bonded water/solvent molecules, i.e. hydrate/solvate forms from surface water/solvents. It can also confirm API salt or co-crystal, etc. formation, showing advantage over liquid NMR and mass spec with respect of some types of, e.g. sodium salts, etc..

The best spectroscopic method for the structure elucidation of organic molecule without the help of other method is NMR with the help of which you can find out the nature of hydrogen, nature of carbon by 13NMR and stereochemistry by COSY, NOSY, Het-COR etc.

Actually all spectroscopic methods are useful for structure determination. Functional group identification by IR, Nature of hydrogen by PMR, Nature of Carbon by CMR, Moleculr wt. by Mass along with the structural elements present.Then you can arrive at the structure of compound.