Protonated oxirane structure

Single- and double-selective relaxation-rates, together with n.O.e. experiments, have been used to examine the configuration and conformation of asperlin (1) in benzene solution." " Comparing experimental distances for the proton pairs H-4,H-7 and H-5,H-7 with those obtained from molecular models, it was possible to confirm earlier evidence that the oxirane ring is trans, and also to show that, of the two possible diastereoisomeric forms (49a and 49b), the data are more fully compatible with structure 49a, the... 

Detailed structural calculations have been carried out for this system. This is because the neutral isomer, C2HsO, which is implicated in the thermochemistry of ethanol, is of interest in pollution control, atmospheric chemistry, and combustion. Also, there is new information available from photoionization experiments with which to compare theoretical calculations. For details of these comparisons, see Curtiss et al.73 In the earlier theoretical studies of Nobes et al.,74 calculations were performed at the MP2 and MP3 levels with basis sets of double plus polarization (6-13G ) with electron correlation. These studies revealed four stable minima for the system protonated acetaldehyde, CHj-C H-OH <-> CH3-CH=0+H the methoxymethyl cation, CH3OCH2 protonated oxirane, (CH2)2OH+ and vinylox-... 

The reaction of alkenes with peroxycarboxylic acids to produce epoxides was discovered by Prilezhaev over 80 years ago.14 It is still the most widely used method for epoxidation, and considerable work has been carried out to elucidate the mechanism. The commonly accepted explanation for oxirane formation involves a cyclic polar process where the proton is transferred intramolecularly to the carbonyl oxygen, with simultaneous attack by the alkene rc-bond. This concerted process was suggested by Bartlett,15 and because of the unique planar transition structure it is referred to as the butterfly mechanism (Figure 3.2). 

The exact geometry of oxonium salts is not known. Calculations using the CNDO-2 method indicate the following structures for protonated and methylated oxirane ... 

Tabulated compilations of the chemical shifts of the protons in oxiranes are frequently found in the literature, " but structural conclusions are rarely drawn from the chemical shifts as the coupling constant is a more exact means of establishing the configuration. Configurations and conformations of oxiranes derived from prostaglandin intermediates have been determined from H shifts with the aid of the ring current model. ... 

For molecules dissolved in a nematic thermotropic liquid-crystal, the direct coupling constants can be determined and from these the molecular geometry can be calculated. If the satellites are determined, not only the proton structure but the carbon skeleton of the molecule can be established. Oxirane has been measured in two laboratories. It proved possible to determine the orientation, the sign of the indirect coupling constant, and the geometry. Enantiomers can readily be determined by recording measurements in optically active liquid-crystals as solvents. ... 

Spectra and Theoretical Chemistry of Oxirans.—Electrophilic additions to a series of norcarene derivatives to yield the oxirans (84 R = H, = Me), (84 R = Me, R = H), and (85) have been described. Proton n.m.r, spectra and A -ray structures for these oxirans are given, and their conformations have been established. The chemical shifts and coupling constants for the H and n.m.r. spectra of a series of cis- and tran5-epoxystilbenes have been reported. ... 

Carbon-carbon bonds in oxirans and aziridines are strengthened by protonation or co-ordination of the heteroatom lone pair, although JIT-ray crystal structures of several aziridinium salts show no consistent C—C bond shortening. 

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