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Bulletin of the Korean Chemical Society (BKCS)

ISSN 0253-2964(Print)
ISSN 1229-5949(Online)
Volume 18, Number 10
BKCSDE 18(10)
October 20, 1997 

Ab-initio and NMR Studies on the Rotational Barrier for Thioacetamide and Acetamide
Yoong-Kee Choe, Gun-Il Song, Young-Sang Choi, Chang-Ju Yoon*
The rotational barriers of thioacetamide (TA) and acetamide (AA) were studied using the ab-initio molecular orbital theory and NMR spectroscopy. The calculated rotational barriers using MP2/6-31G**//MP2/6-31G** for TA was 72.26 kJ/mol and 58.19 kJ/mol for AA, respectively. These results are good agreement with the experimental data. The tendency for the change of structural parameters is consistent with the result of formamide. In both amides, the rotational barrier arises from the pyramidalization of nitrogen. The chemical shifts of both amides are shifted upfield when temperature is raised, which confirms pyramidalization of nitrogen. The lineshape of 1H-NMR spectra of TA shows quintet which is contributed from two triplet spectra. This means that the distribution of electrons around the nitrogen is rather symmetric. Ab-initio calculations of electric field gradient for both amides confirm the above results. The above experimental results are well understood by Keith's view on thioamides, which excludes the contribution of resonance structure and considers the origin of rotational barrier to be the same in both thioamides and in corresponding amides.
1094 - 1099
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