Unmarked carbon atoms

Figure 3. The arrows indicate how the local dipoles of the H-bonds might interact with the local dipoles of the p- and m-nitroanilines to give head to tail and head to head interactions. The + and - signs indicate centers of charge. Unmarked carbon atoms are neutral.

Today acetic acid enriched in C would probably be used, which would not require this lengthy degradation. One would look for stronger signals in the NMR spectrum of the product compound for the marked carbon atoms and normal intensity for the unmarked carbon atoms, and also coupling (see later). 

In some n molecular orbitals the phase at a given carbon atom may be zero and hence the p orbital in such a case is left unmarked. 

An important attribute of benzenoids is next noted by differentiating between carbon atoms having one direction of electron spin (referred to as "marked") and those with opposed spin ("unmarked"). One measure of the... 

The syi ms are called alternant if their carl3on atoms can he conditionally divided into two groups so that each atom of one group finds itself directly bonded only to the atoms of the other group. Essentially, atoms are subdivided as follows in graphic formulas carbon atoms are alternately marked with asterisks. If then the marked atoms become bonded only to the unmarked ones, the system is alternant. For example ... 

The sum of the coefficients on all the unmarked atoms joined to any one of the marked atoms must be zero. Thus we can start at the para position and call the coefficient there a. The coefficients on the ortho positions must both be —a, in order that the second rule may be obeyed when applied to the (marked) meta positions. Now we look at the ring carbon which has the exocyclic carbon atom joined to it. It has a total of three unmarked atoms next to it, two of which, we have deduced, have coefficients of —a. The third atom, the exocyclic one, must therefore have a coefficient of la, in order that the second rule may be obeyed. Thus the coefficients are those shown in Fig. 3-9a. 

Tlie carbon atoms and nitrogen atoms marked widi an asterisk (C and N ) are sp hybridized unmarked... 

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