UV spectroscopic analysis of substituted 1,2,4-trioxanes. Theoretical and experimental study of cis-6-phenyl-5,6-(2-phenylpropylidene)-3,3-tetramethylene-1,2,4-trioxacyclohexane, cis-6-(4-fluorophenyl)-5,6-(2-(4-fluorophenyl)-propylidene)-3,3-tetramethylene-1,2,4-trioxacyclohexane and cis-6-(-4-meth