Molecular structure and puckering potential function of cyclobutane studied by gas electron diffraction and infrared spectroscopy

摘要

The electron diffraction intensity of cyclobutane was measured and analyzed conjointly with the rotational constant,B0=0.355thinsp;82(11) cmminus;1, determined by an analysis of FTIR spectra of the ngr;14(CH2scissoring,B1g) and ngr;16(CH2rocking,A2u) bands. Therzstructure was determined to berz(Cndash;C)=1.552plusmn;0.001 Aring;,rz(Cndash;H)=1.093plusmn;0.003 Aring;, agr;z(angupr;HCH)=106.4plusmn;1.3deg;, and the ring dihedral angle, thgr;z=27.9plusmn;1.6deg;; thergdistances of the Cndash;C and Cndash;H bonds are 1.554plusmn;0.001 and 1.109plusmn;0.003 Aring;, respectively. The uncertainties represent estimated limits of error. The rocking angle bgr;zbetween the bisectors of the adjacent Hndash;Cndash;H and Cndash;Cndash;C angles was found to be 6.2plusmn;1.2deg;, the axial Cndash;H bonds in the 1,3 positions being tilted towards each other. The coefficient of coupling of the ringhyphen;puckering and CH2hyphen;rocking motions was estimated to be bgr;z/thgr;z=0.22plusmn;0.05. The combination and difference sideband structures appearing in the ngr;14band due to the puckering mode ngr;6were analyzed. The puckering energy levels thus obtained were consistent with the existing data. A doublehyphen;minimum potential function for the puckering mode was determined by taking into account the coupling of the puckering motion with the CH2hyphen;rocking motion. The potential gave the zerohyphen;point vibrational average value of the ring dihedral angle, 27.5plusmn;1.1deg;, which agreed with that determined in the EDndash;IR joint analysis mentioned above. The puckering potential for C4D8was found to be consistent with that for C4H8when the coupling between the puckering and the rocking motions was considered.