Microwave spectroscopy 6- membered rings

The geometric parameters of the three-membered ring sulfones and sulfoxides have been determined via X-ray diffraction techniques and gas-phase microwave spectroscopy. The accumulated data for some selected thiirane and thiirene oxides and dioxides (16-19) as well as for the corresponding thiirane (20) and the acyclic dimethyl sulfone (for the sake of comparison) are given in Table 3, together with the calculated values. 

The special types of bonding in three-membered ethyleneimine rings (41—43) have been studied using microwave spectroscopy (44—47), electron diffraction (48), and photoelectron spectroscopy (49—51), and have occupied theoretical chemists up to the present day (52). These studies reveal that ethyleneimine has a distincdy shortened C—C bond of 0.148 nm (as compared to 0.154 nm in open-chain compounds) and a noticeably lengthened C—N bond of 0.149 nm (compared to 0.146 nm). Because of the high s character of the free electron pair on the nitrogen, ethyleneimine also shows a lower basicity (p Ka = 7.98) than noncyclic aliphatic amines such as dimethyl amine (p Ka = 10.7) (53). 

The effect of introducing /j -hybridized atoms into acyclic molecules was discussed in Section 2.2.1, and it was noted that torsional barriers in 1-alkenes and aldehydes are somewhat smaller than in alkanes. Similar effects are seen when sp centers are incorporated into six-membered rings. Whereas the energy barrier for ring inversion in cyclohexane is 10.3 kcal/mol, it is reduced to 7.7 kcal/mol in methylenecy-clohexane ° and to 4.9 kcal/mol in cyclohexanone. The conformation of cyclohexene is described as a half-chair. Structural parameters determined on the basis of electron diffraction and microwave spectroscopy reveal that the double bond can be accommodated into the ring without serious distortion. The C(l)—C(2) bond length is 1.335 A, and the C(l)-C(2)-C(3) bond angle is 123°. The substituents at C(3) and C(6) are tilted from the usual axial and equatorial directions and are referred to as pseudoaxial and pseudoequatorial. 

SThe structure of tetrahydropyran in the gaseous state has been determined by electron diffraction and microwave spectroscopy. These show that the saturated oxygen six-membered ring exists in a chair conformation with Q symmetry (see Fig. 6.5), which is somewhat flattened when compared with cyclohexane ... 

Formula (II) is accepted because sulfur dioxide does not behave like a peroxide, and microwave spectroscopy gives the O-S-0 angle as 119.5° and the S-0 distance as 1.43 A. The latter is similar to the distance in molecules that have to be written with a double SO bond, e.g. 1.45 A in SOCI2. Three-membered rings, with angles of 60°, are unusual in molecules. 

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