Experimental Structural Methods

Various spectroscopic methods have been used to determine the structure and conformation of cyclic sulfites and cyclic sulfates. 

The 100-MHz H NMR spectra of several methyl- and phenyl-substituted ethylene sulfites have also been recorded and discussed in detail. The data were used to calculate the chemical shift difference between the ring protons by considering the shielding or deshielding effect. Thus, for ethylene sulfite, the chemical shifts of the protons cis (fi) and trans (a) to the S = 0 bond were given by 

From these studies, it has been concluded that ethylene sulfite exists in two twist-envelope forms that are interconvertible by rapid pseudo-rotation not involving inversion at the sulfur center [Eq. (1)]. 

Similar H NMR studies of several substituted cyclic sulfamidites have been conducted to determine their conformations. 

The H NMR spectra of 3-aryl-4-methyl-l,2,3-oxathiazolidine 2-oxides 10 recorded at 400 MHz in CDCI3 indicates that the ratio of cis and trans isomers depends on the nature of substituents on the aromatic ring (90JHC195) (Table II). From these studies it is clear that in 3-aryl-4-methyl-l,2,3-oxathiazolidine 2-oxide, the trans form, is predominant. The conformation of cis and trans forms also depends on the substituents. 

5-Diaza-l,6-dioxa-6 -thiapentalene has been extensively studied by various diffraction methods X-ray, neutron and electron spectroscopy 1996CHEC-II(4)409 . There has been substantial progress in the spectral investigation of disubstituted 1,2,3-oxathiazolidine 2-oxides. Infrared (IR) and mass spectra were less employed for dithiazoles and oxathiazoles than other methods. 

While there are very many reports of crystal structures containing pyridines and their derivatives, the material covered in this section is limited to those cases where the information is either unusual or of specific interest, giving insight into an inherent structural property related to the pyridine component of the molecule. 

The crystal structures of the nicotinamide derivatives 14 and 15 have been used to demonstrate the importance of interactions between the pyridinium ring and either carbonyl or thiocarbonyl moieties through intramolecular 

While structures in the solid state do not necessarily parallel those in solution, they have been used to explain solution-phase behavior where other evidence is not available. For example, the conformation of the isopropyl groups in the highly substituted pyridine 16 in the solid state is as shown, as a result of the substituents adjacent on the ring 2004JOC536 . This was used to account for the greatly reduced nucleophilicity of this species. 

Several H and 13C NMR data of azetidines and azetidin-3-ones have been reported. A significant variation is observed in geminal coupling constants of the methylene group attached to nitrogen of different classes of azetidines. The substituents at the nitrogen atom and the conformations of the compounds appear to affect the J values. 

Selected H- and 13C-Chemical Shifts (in Parts Per Million) of 1V-(1-Haloalkyl)Hetarylium Halides 7, 33, and 43 

The CH carbon shift ranges between S 62.4 and 95.1 ppm and is typical of carbons carrying two electronegative substituents. This could be a hint for the observed overall ease of substitution reaction at this center. However, electronic and structural properties of these substituents cause no characteristic differences in the S 13C values. 

In order to obtain further insight into the structural properties of such compounds, the calculations were extended to include the cations 40 (R = MeO-CH2-), 41 (R = Me3SiO-CH2-), 42 (R = X = H), and 43 (R = Cl-CH(CC13)-) (99JOC3113). 

6 RB3LYP/6-31 + G //RB3LYP/6-31 + G, cf. (99JOC 3113) A/7r includes the zero-point energy correction scaling factor, 0.98. 

A Hg NMR study of 1-phenylpyrazole and l-phenyl-3,4,5-trimethylpyrazole organomercuric complexes (138) and (139) (see Section 3.01.5.4.7) has been published 93AJC1323 . 

---