Optical activity computational studies

Most of the zwitterionic compounds studied so far are chiral, with a chiral A5S/-silicate skeleton. Most of them have been isolated as racemic mixtures and in some cases as enantiomerically pure compounds, some of the optically active compounds being configurationally stable in solution. With these experimental investigations, in combination with computational studies, a new research area concerning the stereochemistry of molecular pentacoordinate silicon compounds has been developed. 

Computational efforts to describe the conformational preferences of (R,R)-tartaric acid and its derivatives - mainly for isolated molecules - were made recently [18-25]. The conformations of these molecules also attracted attention from experimental chemists [22-40]. (/ ,/ [-tartaric acid and its dimethyl diester were observed in crystals, in conformations with extended carbon chain and planar a-hydroxy-carboxylic moieties (T.v.v and Tas for the acid and the ester, respectively) [25-28] (see Figure 2). The predominance ofthe T-structure was also shown by studies of optical rotation [31], vibrational circular dichroism (VCD) [23], Raman optical activity [32, 35], and nuclear magnetic resonance (NMR) [22, 33, 34]. The results of ab-initio and semiempirical calculations indicated that for the isolated molecules the Tsv and T as conformers were those of lowest energy [22, 21, 23, 25]. It should be noted, however, that early interpretations of NMR and VCD studies indicated that for the dimethyl diester of (/ ,/ [-tartaric acid the G+ conformation is favored [36-38]. 

The structure of this contribution is as follows. After a brief summary of the theory of optical activity, with particular emphasis on the computational challenges induced by the presence of the magnetic dipole operator, we will focus on theoretical studies of solvent effects on these properties, which to a large extent has been done using various polarizable dielectric continuum models. Our purpose is not to give an exhaustive review of all theoretical studies of solvent effects on natural optical activity but rather to focus on a few representative studies in order to illustrate the importance of the solvent effects and the accuracy that can be expected from different theoretical methods. 

Due to recurring problems with the treatment of methyl rotations in calculations of vibrational averages of optical rotations, Mort and Autschbach [149] have proposed a computational protocol for a separate treatment of hindered rotations. In a nutshell, in this approach a quasi-free rotation of a functional group is treated separately from the vibrational average, with numerically calculated hindered-rotor wavefunctions. With regards to optical activity, the authors studied their protocol on three molecules (R)-methyloxirane, (lS)-norbomanone, and... 

Crawford highlights in his review article three particular studies that demonstrate the varied results one might expect to attain with computed optical activities. In particular, he notes molecules where the CC approach performs much better than DFT, where DFT performs better than CC, but for the wrong reason, and a case where DFT appears to perform better than CC. 

The first reports of a reaction of an amine with an aldehyde by Schiff [584] led to the establishment of a large class of ligands called Schiff bases. Among the most important of the Schiff bases are the tetradentate salen ligands (N,N -bis(salicy-laldehydo)ethylenediamine), which were studied extensively by Kochi and coworkers, who observed their high potential in chemoselective catalytic epoxidation reactions [585]. The best known method to epoxidize unfunctionalized olefins enantioselectively is the Jacobsen-Katsuki epoxidation reported independently by these researchers in 1990 [220,221]. In this method [515,586-589], optically active Mn salen) compounds are used as catalysts, with usually PhlO or NaOCl as the terminal oxygen sources, and with a O=Mn (salen) species as the active [590,591] oxidant [586-594]. Despite the undisputed synthetic value of this method, the mechanism by which the reaction occurs is still the subject of considerable research [514,586,591]. The subject has been covered in a recent extensive review [595], which also discusses the less-studied Cr (salen) complexes, which can display different, and thus useful selectivity [596]. Computational and H NMR studies have related observed epoxide enantioselectivities... 

The chiroptical properties of optically active thiazoUdines derived from aldoses and natural mercapto aminoacids was studied [92]. PMR parameters for thiazoHdine-4(R)-carboxylate derivatives were obtained by computer-assisted analysis of their spectra. The polyacetoxy-alkyl side chains have planar zig-zag conformations. The configurations at C-2 in the di-astereoisomers were ascertained on the basis of the Jnh.ch coupling constants [93]. The conformation and stereochemistry of diastereomeric sulfoxides of methyl 3-acetyl-5,5-dimethyl-2-(D-galactopentaacetoxypentyl)-1,3-thiazolidine-4-carboxylate 1-oxides were performed by H- and C-NMR spectral analysis [94]. 

The method and accuracy of proving the presence of a chiral stmcture in a polymer vary depending on the types of study and the stmcture of the polymer. Stmctural questions can be addressed by (1) various methods based on computer calculations or observations of molecular models, (2) achiral spectroscopic evidence (nuclear magnetic resonance (NMR) spectra, absorption spectra, fluorescence spectra, Raman spectra, X-ray diffraction (XRD), and so on), (3) viscosity or light scattering data giving information on the shape and size of an entire molecule, (4) chiroptical properties (optical activity, optical rotatory dispersion, electronic circular dichroism... 

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