Cyclobutane , ring-puckering

C4H8 (cyclobutane) ring-puckering, 191 CgHg (benzene)... 

Figure 6.42 (a) Cyclobutane, its ring-puckering vibration and dihedral angle, (b) Cyclopentene... 

In the anti head-to-head dimer of thiocoumarin 70, the cyclobutane ring is puckered and the thiopyran rings adopt conformations between half-chair and twist-boat. Bond lengths and angles have been reported <1994AXC1922, 1995HCA1079>. 

In cyclobutane and cyclope- ntane the angle strain is minimised by ring puckering, while for cyclohexane there is no angle strain., ... 

The cyclobutane ring exists in a non-planar, open-book conformation, which may flex further open or shut. The cyclopentane ring exists in two puckered conformations, the half chair and the envelope, both of which are non-planar. The position of the puckering may migrate around the ring, and this is called pseudorotation. 

Fig. 2.3. Combination and difference band progressions involving the ring puckering vibration and a CH2 scissoring mode in the mid-infrared spectrum of cyclobutane.

Fig. 2.6. Variation of the A rotational constants (in MHz) with ring-puckering vibrational state for cyclobutanone and methylene-cyclobutane.

Fig. 4.11. Raman spectra of gaseous cyclobutane and cyclobutane-ds- The observed Q-branch transitions are the Av = 2 ring-puckering transitions.

One obvious way to attack the problem is simply to reduce the value of the bending force constant sufficiently so that the heat of formation can be accurately reproduced. While the heat of formation can be taken care of in this way, the cyclobutane ring by such a calculation will be planar, or nearly so, rather than puckered. In order to get the pucker and the energy both right, we need a smaller value for the curve at 90 degrees than we would get from the spectroscopic force constants, and we also need a smaller slope to the... 

In addition, the puckered cyclobutane ring may become more planar in the transition structure. The mechansim of this and similar reactions will be considered in Chapter 11. 

Whereas the cyclobutane rings in (8 X = O or S) are planar, that in (9) is puckered, and two short Br Br distances of 3.55=A are observed in the crystals. Planarity is also seen in ds-cyclobutene-3,4-dicarboxylic acid and in substituted cyclopentadi-enyl cobalt complexes of tetraphenylcyclobutadiene, in which each metal atom is sandwiched between parallel, planar fo ir- and five-membered rings. Crystal structures have been obtained for a variety of polycyclic compounds containing cyclobutane rings. ... 

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