Introduction molecular structure determination

Whereas two methyl N-substituents are inefficient in transmitting the chiral information encoded at the rear side of the heterocyclic ligand (Entry 1 in Table 1), the stereoselectivity is improved by the introduction of N-benzyl substituents (Entry 2). The steric repulsion between the ferf-butyl and benzyl groups leads to a C2 symmetric arrangement of the latter with respect to the carbene donor function as is apparent in the molecular structure determined by X-ray diffraction for the silver(I) complex 20 [51]. In this way the chirality in the heterocycle is transmitted towards the reaction center. Introduction of methoxy groups in the meta-position of the phenyl rings of the benzyl substituents slightly increases the selectivity of the catalyst (Entry 3). 

GeH2 13 C and 73 Ge NMR. Chair conformation as determined from spectroscopy, in accordance with molecular structure calculations, similarly to the analogous carbocyclic compound. Introduction of bulky substituents results in a preferred boat conformation. 60... 

Introduction of two very important physical techniques in polymer science, viz. Light Scattering for molecular weight determination (by Debye) and Infra-red Spectroscopy for structural analysis (by Thompson)... 

Among the variety of compounds emerging from prototype structures 10-28 mentioned in the Introduction (Table I), X-ray structural determinations have been performed on six representative examples the mesomeric betaines 55, 116, 117, and 120 the higher homologue 119 and the novel aza analogue of sesquifulvalene with a betaine character 118 (Scheme 9 and Table VIII). As mentioned earlier, the experimental dipole moments for molecules 55,116-119 were found to be in the range 9 to 13 D (III,B, Scheme 8 and Table Vll). Comparison of the experimental molecular geometries and dipole moment values with those obtained from semiempir-ical molecular orbital calculations is discussed below (III,D). 

An introduction to the phenomena of NLO will be given first (Section 2), followed by the evaluation of molecular second-order polarizabilities by theoretical models that both allow their rationalization and the design of promising molecular structures (Section 3). It will be necessary to develop different models for molecular symmetries, but the approach will remain the same. NLO effects and experiments used for the determination of molecular (hyper)polarizabilities will be dealt with in Section 4. Finally, experimental investigations will be dealt with in Section 5, followed by some concluding remarks. 

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