Shortening Attributes

On ornamental plants CCC is appHed to a2aleas, geraniums, and hibiscus (Hibiscus sp] to make compact plants, and to poinsettias to reduce stem height and increase the red color of the bracts. A considerable amount of work has been carried out on cereals with CCC to reduce stem length and inhibit lodging. In Europe, the effect of CCC on shortening the culms of cereals is dependent upon the genotype. It has been demonstrated that the effect is as follows wheat > triticale > durum wheat > rye > oats > barley > corn = millet = rice (37). In barley, culms are initially inhibited but later the plant overcomes the inhibition (37). This has been attributed to poor assimilation, translocation, and rapid breakdown in wheat (38). 

The free t-butyl cation [7" ] in the gas phase is nothing more than a species detectable by the electron impact method (Yeo and Williams, 1970). However, it is not only an observable species by nmr studies in SbFs/FSOsH (Olah et al., 1964), but can be isolated from the solution in the form of its SbF or Sb2Ffi salt (Olah and Lukas, 1967a,b Olah et al., 1973 Yannoni et al., 1989). The crystal structure shows that this ion is planar and its carbon-carbon bonds are shortened to 144.2 pm (Hollenstein and Laube, 1993). Its particular electronic stabilization among aliphatic carbocations is attributed by physical organic chemists to the operation of both inductive and hyperconjugative effects in the cr bond system. 

The authors attributed the 3.2eV band in the neutral film to the n +n transition of n electrons in the HOMO and the 2.7 eV band was assigned to this transition in the fully oxidised form based on, for example, the systematic shift of this absorption to lower energy as the conjugation length shortens in pyrrole oligomers. 

The effective ionic radii of Shannon and Prewitt (1969) can frequently be used to predict average interatomic distances and to correlate unit cell volumes of series of isostructural oxides and fluorides. However, some systematic discrepancies were recently found in tetrahedral oxy-anion distances and in the unit cell volumes of certain series of fluoride compounds. It was pointed out by Banks, Greenblatt, and Post (1970) that the observed V—0 distances in Ca2VC>4Cl are smaller than those predicted by the effective ionic radii. Subsequently, the discrepancies in Ca2VC>4Cl and other tetrahedral oxy-anion distances were attributed to covalency effects (Shannon, 1971, and Shannon and Cairo, 1972) in which bonds exhibiting a greater degree of covalency were assumed to shorten. 

The previous literature on the effects of partial covalence on interatomic distances is contradictory. Pauling (1960) cites the examples of CuF, BeO, AIN, and SiC where observed bond lengths are shorter than the sum of the covalent radii. He attributes these differences to partial ionic character and thus implies that partial ionic character shortens covalent bonds. This conclusion is in accord with the Schoemaker— Stevenson (1941) rule Dab = a + pb—C nx— b where > interatomic distance between A and B, rx and r = covalent radii of A and B, a and xb = electronegativity of A and B and C = constant. 

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