Infrared studies of conformational isomerism in some gem-chloronitrosoalkanes

The IR spectra of 2-chloro-2-nitrosopropane, 2-chloro-2-nitrosobutane and 2-chloro-3-methyl-2-nitrosobutane have been studied in the gas phase and solid phase, and in solutions of varying polarity. The spectra reveal clear evidence for the hindered internal rotation of the NO functional group about the CN bond in a splitting of the NO stretching band. The spectra can be interpreted as arising from energetically favoured cis conformational isomers, exhibiting the higher of the observed NO stretching frequencies. In each case, assignment of the lower NO stretching frequency is uncertain on the basis of experimental evidence. The results of semi-empirical molecular orbital calculations, however, support the assignment of this feature to the gauche conformational isomers. Temperature dependent IR spectroscopy has been employed in the determination of conformational enthalpy differences in both the gas phase and non-polar solution, which are then compared to the results of semi-empirical molecular orbital calculations.