Improved 13C NMR Shift Prediction Program for Polysubstituted Benzenes and Sterically Defined Cyclohexane Derivatives
✍ Scribed by H. Hönig
- Publisher
- John Wiley and Sons
- Year
- 1996
- Tongue
- English
- Weight
- 840 KB
- Volume
- 34
- Category
- Article
- ISSN
- 0749-1581
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✦ Synopsis
By introduction of pair-induced chemical shift correction increments for "C NMR shifts of disubstituted aromatics, the average absolute difference between observed and calculated shifts can be lowered from 3 to less than 0.5 ppm. Thus the erroneous assignment of some polysubstituted aromatics in the literature could be corrected. Up to now, the following substituents have been included in a Windows program: Br, CH,OH, CH,, CHO, CI, CN, COCH,, COOCH,, NH,, NO,, OCH,, OCOCH, and OH. The same substituents have also been included for sterically defined polysubstituted cyclohexane derivatives via the introduction of separate axial and equatorial shift increments together with a population analysis of the possible conformers.
KEYWORDS NMR; 13C NMR; p0bsubstituted benzene derivatives; polysubstituted cyclohexane derivatives; Windows shift prediction program diction has proved to be helpful in the assignment of the ring carbon atoms.' As can be expected by the known electronic and steric interactions of two or more substitutents attached to a benzene ring (especially in ortho-substituted structural units), the differences between observed and calculated resonances sometimes become large, exceeding 5 ppm. This fact, together with the limited range of most of the common aromatic resonances from roughly 110-150 ppm, can lead to erroneous assignments as found in the literature. Some examples will be given later.
There are several theoretical approaches to this problem: but the most widely used shift prediction programs available nowadays, such as the "C NMR addendum to the chemical drawing program ChemWind~w,~ based on the work of Furst and Pretsch4 and CSPEC,5.6 do not account for this. There are other program systems using statistical methods such as SPECINF07 and CSEARCH; which, depending on the number of available single shifts, sometimes improve the results. A program with ortho correction for aromatic structures is knowng Recently we entered the field of catalytic hydrogenations of unsymmetrically substituted aromatics in order to obtain sterically defined cyclohexane derivatives with biological significance.'",' Therefore, we needed a simple tool to identify unambiguously both the aromatic starting material and the resulting cyclohexane product(s). We therefore developed a simple shift prediction program, HIPPO-CNMRS, for Windows with improved capabilities especially in this field. ' *