An account is given of the origins, in the molecular electronic properties and crystal structure, of the large quadratic optical nonlinearities found in some organic crystals. Particular cases, in which powerful second-harmonic generation or large Pockels effects have been found, chosen to illustrate the influence of different aspects of molecular or crystal structure, are introduced. Three of these, MBANP, NMBA and DAN are described in greater detail in the light of recent experimental and theoretical work.
The CNDO/VS sum-over-states (SOS) method has been used to calculate the scalar components of the second-order hyperpolarizability tensors appropriate to d.c.-field-induced second-harmonic generation (DC-SHG) at 1.064 nm and third-harmonic generation (THG) at 1.064 and 1.9 nm. The molecules considered are benzene, aniline, nitrobenzene and p-nitroaniline (PNA).The above calculations have been carried out using both the singly excited configuration interaction (SCI), and the singly and doubly excited configuration interaction (SDCI) models. Their suitability for evaluating third-order effects has been assessed, by extensively testing the convergence of the perturbation sum for the above molecules, although it must be noted that the results obtained are only semiquantitative. Furthermore, the importance of the charge-transfer state of PNA has also been assessed and the frequency dependence of its contribution investigated.
846A MULTISOLUTION METHOD OF PHASE DETERMINATION. V which should lead to a speed up by a factor of five. We expect this will bring the phase-extension capabilities of MICE up to those of SAYTAN; the superiority of likelihood over other figures of merit should then confer a definite advantage to the maximum-entropy approach over other direct methods. We will put these expectations to the test by attempting the direct structure determination of APP as soon as possible.We are grateful to Professor Michael Woolfson and Dr Yao for providing the SAYTAN phase sets and much useful discussion. This research was supported by an SERC grant. GB acknowledges the unstinting support of Trinity College, Cambridge.References BRICOGNE, G. (1984). Acta Cryst. A40, 410-445. BRICOGNE, G. (1991). Acta Cryst. A47, 803-829. BRICOGNE, G. & GILMORE, C. J. (1990
AbstractThe numerical method for calculating the elastic constants of molecular crystals, using the crystal packing program PCK 83, developed and applied to the model case of CO2 in previous work is here extended to benzene (C6H6) and urea [(NH2)2CO] crystals. Interatomic potential-energy functions coupled with rigidmolecule constraints are used, taking the entire angular dependence into account. Correlation with other calculations and experimental data is good.
The version of time-dependent perturbation theory commonly used in the calculation of the hyperpolarizabilities of organic molecules through an expansion over electronic excited states is briefly rederived by a direct method, simplified for application to the nonresonant case. The application of the method within the framework of semiempirical quantum theory and singly and doubly excited configuration interaction is described. A particular parametrization of the cnoo/s method (CNDO/VS) that has been successful in estimating molecular p values for polar aromatic molecules is reviewed. Applications of the method to the interpretation of the molecular and crystal second-order properties of 2-(a-methylbenzylamino)-5-nitropyridine (MBANP) and to the calculation of the y tensor for various types of conjugated compounds are given.
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