NEGATIVE HYPERCONJUGATION IN METHYL AND SILYL ANIONS AND ITS EFFECT ON THE ACIDITIES OF CH(4-N)FN, CH(4-N)CLN, SIH(4-N)FN AND SIH(4-N)CLN

Group separation reactions calculated using an ab intio molecular orbi tal calculation at the MP4/6-31 + +G(d,p) level of theory, show the ne gative hyperconjugation between fluorine atoms to be larger in methane s than in silanes. Stabilisation due to negative hyperconjugation is l arger in anions than in identically substituted neutral molecules, e.g . 43.1 kcal mol-1 in CF3- compared with 26.7 kcal mol-1 in CHF3. By co ntrast, in chloro-substituted methanes, silanes, methyl anions and sil yl anions, group separation energies are approximately zero, indicatin g no appreciable negative hyperconjugation. An alpha-chloro substituen t is more effective than an alpha-fluoro one at delocalising the negat ive charge of an anion and, as a consequence, the chloromethanes and c hlorosilanes are all more acidic than the identically substitued fluor omethanes and fluorosilanes. For chloro-substituted molecules the acid ity is linearly dependent on the number Of chlorine atoms; for fluoro- substituted molecules stabilisation by negative hyperconjugation resul ts in each additional fluorine atom increasing the acidity by larger i ncrements.