The absorption system originating around 1795 Aring; in isobutene has been studied in the parent compound and in three symmetrically deuterated isomers. Progressions are observed in theCdblbnd;Cstretch, sngbnd;CH3symmetric deformation,dblbnd;CH2deformation,dblbnd;CH2torsion, and dblbnd;CH3torsion vibrational modes. Sequences are observed in the dblbnd;CH3torsion and once in thedblbnd;CH2torsion. The groundhyphen; and upperhyphen;state methyl torsional potentials are calculated. The origin is tentatively identified as a false origin. The molecule is deduced to have undergone an extensive electronic change in the upper state. The transition is most probablyA23pylpar;sgr;*cc, b2rpar;larr;pgr;lpar;px, b1rpar;.
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