ACD/HNMR Predictor

Technical Information

ACD/HNMR has an internal database (.DAT) of over 1,578,000 experimental chemical shifts and over 568,000 coupling constants. Each δ_H value is pre-assigned to the specific structural fragment. Each coupling constant is pre-assigned to a pair of structural fragments and the connecting fragmental skeleton. The internal database of pre-assigned shifts and coupling constants has been generated from over 193,000 experimental HNMR spectra.

ACD/HNMR Predictor uses many different subalgorithms for estimating δ_H and J for those fragments that are not in the internal database of experimental values. Each separate algorithm predicts δ_H and J for a specific class of structures. Both the algorithms and their derivation are proprietary to ACD/Labs, but the following should help you understand what we do in general.

When you draw a new structure in ACD/HNMR Predictor, the program automatically "splits" it into a set of unique fragments which are then compared to the fragments from the internal database:

• If a fragment from your structure coincides with a fragment from the database, the program uses its experimental δ_H to compose the final set of chemical shifts for your structure. For such δ_Hs the program does not show confidence intervals in the Table of Chemical Shifts.
• If a pair of fragments with the connecting fragmental skeleton from your structure is found in the database the program acts as in the case above.
• If some fragments from your structure are not found in the internal database, the program looks for the most similar fragments in the database. First, the program composes sets of fragments from the database which are structurally very similar to the fragments generated from your structure. Second, the program estimates δ_H values of fragments from your structure using secondary algorithms and compares them to the estimated δ_H values of fragments selected from the database. This allows the program to narrow down the set of similar fragments from the database. Third, the program calculates the average values (δ_Av) of the experimental and estimated δ_Hs left after applying the second criterion. Then the resulting δ_H value is calculated using both the estimated δ_H value of your fragment and the average δ_Av values. The obtained d_Hs are used to compose the final set of chemical shifts for your structure.
• If some pairs of fragments with the connecting fragmental skeleton from your structure are not found in the database, then the program acts as in the above case.

The sets of fragments and their incremental values that are used to predict each δ_H can be viewed in the Calculation Protocol Window and modified as a "patch" entry.

After composing final lists of chemical shifts and coupling constants, ACD/HNMR Predictor composes and diagonalizes a spin Hamiltonian matrix to generate the exact number, location, intensities, and assignment of spectral lines of your structure.

When you create a User Database with your own structures, the program automatically "splits" all of them into a set of unique fragments. As you assign new chemical shifts and coupling constants to atoms, the program treats these data as an update to its internal database. If you draw a new structure and keep the User Database open, the program performs all the same actions as described above, but it pays primary attention to the data which you have entered in your User Database.