Experimental and DFT studies on the transmission mechanisms of analogous NMR JCH and JCC couplings in 1-X- and 1-X-3-methylbicyclo[1.1.1]-pentanes

The main aim of this work is to compare the transmission mechanisms for the Fermi contact term of spin–spin couplings, SSCCs, in series 1‐X‐bicyclo[1.1.1]‐pentane, (1), and 1‐X‐3‐methylbicyclo[1.1.1]pentane, (2), and from that comparison to gain insight into some subtle aspects of the FC transmission. To this end, 18 members of the latter series were isotopically enriched in 13C at the methyl position and the following couplings were measured; 1J C 3C Me, 3J C 1C Me and 4J C XC Me. These three types of SSCCs in (2) are compared, respectively, with 1J C 3H 3, 3J C 1H 3 and 4J C XH in (1); these latter values were taken from previous works. Since electron delocalization plays an important role in the transmission of the FC interaction, the natural bond orbital (NBO) method is employed to quantify electron delocalization interactions within selected members of series (1) and (2). It is found that 1J C 3H 3 SSCCs in (1) is more efficiently transmitted than 1J C 3C Me SSCCs in (2). On the other hand, 3J C 1H 3 and 4J C XH SSCCs in (1) are notably less efficiently transmitted than 3J C 1C Me and 4J C XC Me SSCCs in (2), although substituent effects on these two SSCCs show the opposite trends. These different efficiencies are rationalized in terms of different σ‐hyperconjugative interactions in both series of compounds. Copyright © 2007 John Wiley & Sons, Ltd.