NMR and molecular modeling in environmental chemistry: prediction of 13C chemical shifts in selected C10-chloroterpenes employing DFT/GIAO theory.

Accurate predictions of 13C NMR chemical shifts (standard error approximately 1.7 ppm) are achieved for a subset of chlorinated bornanes by empirical scaling of shifts from GIAO calculations with geometries obtained from HF/6-31G* calculations. The optimized molecular geometries were compared with X-ray structures for three of the toxaphene components most frequently detected in environmental samples (Parlar nos. 26, 50 and 62), and the concordance between the experimental and calculated values was found to be satisfactory. Taken overall, the results indicate that theoretical methods hold great promise for rationalizing 13C NMR chemical shifts in organohalogen compounds. However, it appeared that the DFT/GIAO shifts need to be empirically scaled to achieve good numerical agreement with experimental shifts in chlorinated bornanes. Obviously, there is a need to develop new computational methods to describe the large deshielding effects of chlorine atoms properly.