Abstract
The ZINDO-computed quadratic hyperpolarizability (β) of p-nitroaniline (PNA) is compared with that of its carbomer in its molecular mechanics (MM) optimized geometry. Complete carbomerization of PNA leads to a new molecule with a static β0 value of 941.3 × 10-30 cm5 esu-1 which is 2 orders of magnitude better than that of PNA. Within the model of push−pull molecular topography, an empirical five-parameter formula is proposed in order to estimate the β value of 32 partial carbomer structures of p-nitroaniline. A compromise between high β values and a priori synthetic feasability is achieved for structures with three carbomerized domains only: the carbon skeleton of the p-phenylene ring, the nitro group, and the phenylene−nitro bond. The results are discussed within the framework of the two-level model on the basis of an orbital analysis. A relevant measure of the molecular NLO response is proposed to be the “specific quadratic hyperpolarizability”: [β] = β/N, where N is the number of non-hydrogen atoms i...
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