Behavior of the Fermi Contact term of NMR Spin-Spin coupling constants in X-C-CHO systems (X = lone pair bearing atom)

G. GOTELLI; S. SUAREZ; M.C.PÉREZ SCHMIT; E. BENTZ; R. GLISONI; M. POMATA; C. QUEVEDO; A. BULIGOVICH; L. ORELLI; R.H. CONTRERAS

Florianópolis, Brazil

Conferencia; Clafqo: 10th Latin American Conference on Physical Organic Chemistry; 2009

In a previous paper it was observed that in 5-X-furfurals the spin-spin coupling constant, SSCC,  through the carbonyl group, 2JCa,Hf presents an unusual dependence with the CHO group conformation, i.e. for a syn conformation between both oxygen atoms it is close to  43 Hz while for the anti conformation it is close to 21 Hz. Such results suggest that similar effect can be expected in other type of compounds with a carbonyl group with a b lone-pair bearing atom. In the literature it is found that in methyl chlorinated acetaldehydes, O=CHC(ClnH3-n) (n = 1, 2, 3), 2JCaHf = 32.5; 35.8 and 46.3 Hz, respectively. Such trend suggests that a similar effect to that studied in furfurals is playing an important role in defining the SSCC trend. In this communication will be presented a detailed study of transmission mechanisms of the Fermi Contact, FC, term, of such SSCCs for different conformations of the chlorinated methyl group. This theoretical analysis is based on the  DFT-B3LYP/6-311G**//B3LYP-X, with X = TZVP, EPR-III, calculation of SSCCs, including the NBO analysis of molecular orbital interactions. The latter basis set could be employed only in non-chlorine atoms, where in such case the former basis set was used.  Similar calculations were also carried out on O=CHCFH2. In this latter compound it was possible to change at will the Mulliken bond-order between the F and O atoms by introducing a point charge in the vicinity of the former.  For a fixed F‑-- distance a very good correlation between bond-order and 2JCaHf is found, although it departs somehow from linearity.