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Organic molecular crystals that are extremely efficient at terahertz-pulse generation are in- vestigated. Terahertz pulses produced by optical rectification at 800 nm in (−)2-(α-methylbenzyl-amino)-5-nitropyridine have an order of magnitude higher power than those generated in the commonly used inorganic crystal ZnTe. The organic molecular crystals were also found to generate terahertz pulses when excited on resonance at 400 nm. This may pave the way for studying ultrafast charge-transport dynamics in three dimensions.

A single-crystal neutron diffraction study of the organic nonlinear optical material 3-(1,1-dicyanoethenyl)-1-phenyl-4,5-dihydro-1H-pyrazole (hereafter DCNP), space group Cc, is presented. The study was conducted in order to relate the structural characteristics of the compound to its physical properties. DCNP exhibits a very large second harmonic generation (SHG) output, an extremely large linear electrooptical effect and photoconductive and pyroelectric properties. The nature of the hydrogen-bonding revealed by the study, in part, accounts for the ®rst two of these phenomena. The neutron study also shows that some rather atypical atomic thermal motion is present in part of the molecule. With the aid of a variable-temperature single-crystal X-ray diffraction study, in conjunction with the neutron study, this thermal motion is attributed to libration and is fully characterized. As a result, suitable corrections to the bond geometry and the anisotropic displacement parameters of DCNP are made. The libration is also shown to enhance the SHG effect. The cell parameters from the variable-temperature X-ray study are also used in order to evaluate the thermal expansivity coef®cients of DCNP.

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Free Energy Surfaces and Transition State TheoryMany major chemistry textbooks contain diagrams exhibiting a function referred to vaguely as the "free energy" plotted against a quantity referred to by a phrase such as the "reaction co-ordinate." The diagrams usually have the general form indicated in Figure 1. The "free energy" curve goes through a maximum and this feature is interpreted as representing a "harrier" which the system must cross when reactants are converted to products.There are numerous variations-The term "free energy," (G), may sometimes be replaced by "free energy change," (AG); in some, rather rare, cases an attempt at qualifying the expression further to "standard free energy," (G°), or "standard free energy change," AG°is made and in some cases the same type of figure appears with the abscissa labelled "extent of reaction."As we shall see, the correct definition of such a free energy curve, or surface, requires highly sophisticated concepts (1-3) and involves the finest points of the definition of a transition state1 (4). It is the purpose of the present letter to try to establish that unless such diagrams are very precisely labelled and explained they are seriously misleading and often incorporate a major error of principle. Discussion of the points at issue can be found in some textbooks but rarely in those at undergraduate level (1,5,6). Extent of Reaction and Reaction Co-Ordinate The confusion probably begins with a tendency to regard the terms "extent of reaction" and "reaction co-ordinate" as interchangeable. A sharp distinction must be made between the two concepts involved in the definition of these terms. The usage in the present note is as follows.

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The organic crystal 4-nitro-4′-methylbenzylidene aniline (NMBA) was identified as a promising nonlinear material by the powder technique. The material gave a second harmonic intensity 16 times that of urea. Large single crystals of dimensions 5×3×1 cm3 were grown by the temperature lowering of a seeded supersaturated ethyl acetate solution. The principal dielectric axes were defined by orthoscopic examination. The dispersions of the refractive indices were determined to an accuracy of ±0.0015 using the minimum deviation technique and Maker fringe spacings. These dispersion curves were fitted to a Sellmeier equation which allowed the indices to be determined to ±0.0006. The nonlinear d coefficients d11, d33, d31, and d13 were evaluated at 1000, 1064, and 1300 nm using the Maker fringe technique. The coefficient d11 was over 200 times larger than potassium dihydrogen phosphate (KDP) d36. In addition, the nondiagonal coefficient d31 was similar to the phase-matching coefficient in the organic material 3-acetamido-4-dimethylamino-nitrobenzene (DAN). Critically phase-matched second harmonic signals were observed at all fundamental wavelengths. There was excellent agreement between the experimentally determined and theoretical phase-matched incidence angles. Noncritical phase-matched conditions have been calculated and are reported. Both angle and wavelength noncritical phase matching is possible with this crystal.