The __first way__ is to
input as much information as you can and then hope that the
search yields just a few possible trial structures. Because many
substituent constants are similar, hundreds of different
substitutent patterns will give the same chemical shift.
Therefore, one chemical shift will match many possible chemical
environments. This approach is most successful if you know
the formula and the functional groups that are present. You are
most likely to get just a few possibilities if your chemical shift
is large.

The __second way__ to use this application is to say
"what-if". "What-if" a carbonyl is near? What structures would
match my chemical shift? "What-if" a carboxylic acid group is
near? Or, if you can't tell if your -CH2- protons are equivalent or
not, "what-if" they are inequivalent? Or "what-if" my -CH3 group is part
of an isolated ethyl group?

Whichever approach you take, using this program will help to spark some ideas for possible structures. One of the biggest pitfalls in spectral interpretation is falling into the trap of not considering all possibilities. Using this program will help to free up your decision making by helping you to look at multiple possibilities.

More importantly, the exact formula allows the calculation of the number of double bonds and rings, dbr. Some authors call the number of double bonds and rings the degree of unsaturation. If the trial formula is CcHhOoNn then the number of double bonds and rings is:

dbr = (2c-h+n+2)/2

For example, the number of double bonds and rings for benzene is 4. This parameter helps to narrow down the functional groups that are possible and helps in screening possible trial strucutres. For example, if the dbr for your compound is 3, then you can't have an aromatic ring present. The dbr is also helpful in prediciting the spin-spin multiplicity of a trial structure. For example, if your compound has dbr=2 and you have two unsaturated function groups, then there can't be any rings. If there are no rings or chiral centers then -CH2- geminal protons will be equivalent.

minimum multiplicity = n+1 , where n=total number of coupled spins

The expected multiplicity assumes some rough values for the J coupling constants, and groups equivalent spins by their J's. The maximum multiplicity assumes all the groups of equivalent protons are distinct. These three guesses are printed in the order minimum < expected < maximum. If the program is assuming a J4 W coupling for the maximum multiplicity, a W will be printed. More information on the scoring is available, below.

expected multiplicity = (n2+1)*(n6+1)*(ngem+1)

where n2=number of spins with J=2Hz, n6=number of spins with J=6 Hz, and
ngem = 1 for inequivalent -CH2- groups.

The program checks the "equivalent/inequivalent" selection and the
number of double bonds and rings (dbr) for the trial structure to
determine if geminal coupling is possible.

The maximum multiplicity assumes all the groups of equivalent protons are distinct. In other words, the assumption is that all the vinyl protons are inequivalent, all methine protons are inequivalent, all the methylene protons are inequivalent if rings or chiral centers are possible, and all methyl groups not on the same carbon are inequivalent:

maximum multiplicity = (nMe+1)*2^{n'}

where nMe is the number of vicinal methyl group protons and n'=n-nMe.

- Sometimes expected couplings vanish. For the minimum multiplicity, if a ring is possible, one vanishing J3 coupling is subtracted for each adjacent -CH2-.
- Sometimes long range couplings appear. For the maximum multiplicity, if a ring is possible, a W coupling is added if appropriate.
- Only one W coupling is allowed (but that's pretty reasonable).
- No J5 couplings are used.
- Sometimes geminal couplings vanish, but this is not taken into account.

if the input multiplicity = expected : score = 4

if the input multiplicity is close to the expected : score = 3

if the input multiplicity > expected*2 : score = 2

if the input multiplicity < expected/2 ) : score = 1

List of Substituents and Literature References

*Colby College Chemistry, T. W. Shattuck, 2000*