Conformation near-planar

Figure 4.6 The conformation of cyclopentane. Carbons 1, 2, 3, and 4 are nearly planar, but carbon 5 is out of the plane. Part (c) is a Newman projection along the C1-C2 bond, showing that neighboring C-H bonds are nearly staggered.

The above-mentioned [Hg(P—Z-Bu2)2]2 has been obtained in a phosphide-transfer reaction between HgMe2 and Ga(P-z-Bu2)3. If CdMe2 and Ga(P—/-Bu2)3 are reacted, the product is the trimeric [MeCd(P-/-Bu2)2]3 with a six-membered Cd3P3 ring in twist-boat conformation and exocyclic Cd—Me bonds thus, Cd achieves near-planar, three-coordinate geometry. The Cd—P bond lengths in the ring are all similar (rav(Cd—P) 258.5 pm) in solution these Cd—P bonds... 

A number of fluorescent dyes with internal charge transfer mechanism allow the molecule to twist (rotate) between the electron donor and electron acceptor moieties of the fluorescent dipole. In most cases, the twisted conformation is energetically preferred in the excited Si state, whereas the molecule prefers a planar or near-planar conformation in the ground state. For this reason, photoexcitation induces a twisting motion, whereas relaxation to the ground state returns the molecule to the planar conformation. Moreover, the Si — So energy gap is generally smaller in the twisted conformation, and relaxation from the twisted state causes either a... 

The number of X-ray structures published since the publication of CHEC-II(1996) has increased, underlining the importance of this technique in structure elucidation. The structure of a number of 1,2,4-thiadiazoles and 1,2,4-thiadiazolidines has been determined by X-ray techniques and they are listed in Table 1. The first preparation of an A7-oxide derivative of a 1,2,4-thiadiazole 2 has been reported. The X-ray structure of compound 2 shows that it has a nearly planar ring this conformation is stabilized by hydrogen bonding with the carboxamide group <1999J(P1)2243>. 

The six-membered heterocyclic ring has twisted boat conformation for Y = S and is nearly planar for Y = O. 

With acyclic dienes, the quantum yield for cyclobutene formation (4>cb) rarely exceeds ca 0.1, the expected result of the fact that the planar s-trans conformer normally comprises the bulk (96-99%) of the conformer distribution at room temperature. However, 4>cb is often significantly larger than the mole fraction of s-cis form estimated to be present in solution. For example, 1,3-butadiene, whose near-planar (dihedral angle 10-15°105 106) s-cis conformer comprises ca 1% of the mixture at 25 °C, yields cyclobutene with < >cb = 0.04140, along with very small amounts of bicyclo[1.1.0]butane141. A second well-known example is that of 2,3-dimethyl-l,3-butadiene (23 ca 4% gauche s-cis at 25 °C107), which yields 1,2-dimethylcyclobutene (25) with < >cb = 0-12 (equation 16)111. Most likely, these apparent anomalies can be explained as due to selective excitation of the s-cis conformed under the experimental conditions employed, since it is well established that s /raw.v... 

Figure 5.2. Grabowski s model of TICT formation in DMABN the locally excited (LE) state with near-planar conformation is a precursor for the TICT state with near perpendicular geometry. The reaction coordinate involves charge transfer from donor D to acceptor A. intramolecular twisting between these subunits, and solvent relaxation around the newly created strong dipole. Decay kinetics of LE and rise kinetics of the TICT state can be followed separately by observing the two bands of the dual fluorescence. For medium polar solvents, well-behaved first-order kinetics are observed, with the rise-time of the product equal to the decay time of the precursor, but for the more complex alcohol solvents, kinetics can strongly deviate from exponentiality, interpretable by time-dependent rate constants. 52 ...

Although no crystal structures of cyclotriphosph(III)azanes have yet been reported, it is reasonable to assume that the ring will adopt a chair conformation and that the nitrogen atoms will have a planar or near-planar distribution of bonds. This is consistent with the structures of the six-membered ring fragments found in the cage... 

The structures of these molecules show the effects of intramolecular electrostatic interactions. Two examples are the lone pair—lone pair repulsion that is an important determinant of hydroxylamine and oxime conformations, and the intramolecular hydrogen bonding in hydroxamic acids that promotes the near-planarities of their —C(=0)—NO frameworks. 

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