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A high multiplicity state is therefore the same as a high-spin state. The lowest-energy state with maximum multiplicity usually has unpaired electrons all with parallel spin. Since the spin of each electron is 1/2, the total spin is one-half the number of unpaired electrons, and the multiplicity is the number of unpaired electrons + 1.
Each has two electrons of opposite spin in the π* level so that S = 0 and the multiplicity is 2S + 1 = 1 in consequence. In the first excited state, the two π* electrons are paired in the same orbital, so that there are no unpaired electrons. In the second excited state, however, the two π* electrons occupy different orbitals with opposite spin.
A hydrogen on the α position of a carbonyl compound is weakly acidic and can be removed by a strong base to yield an enolate ion. In comparing acetone (pK a = 19.3) with ethane (pK a = 60), for instance, the presence of a neighboring carbonyl group increases the acidity of the ketone over the alkane by a factor of 10 40.
In molecules which have resonance or nonclassical bonding, bond order may not be an integer.In benzene, the delocalized molecular orbitals contain 6 pi electrons over six carbons, essentially yielding half a pi bond together with the sigma bond for each pair of carbon atoms, giving a calculated bond order of 1.5 (one and a half bond).
Using this model, one sidesteps the need to invoke hypervalent bonding considerations at the central atom, since the bonding orbital effectively consists of two 2-center-1-electron bonds (which together do not violate the octet rule), and the other two electrons occupy the non-bonding orbital.
Relatively more stable entities with unpaired electrons do exist, e.g. the nitric oxide molecule has one. According to Hund's rule, the spins of unpaired electrons are aligned parallel and this gives these molecules paramagnetic properties. The most stable examples of unpaired electrons are found on the atoms and ions of lanthanides and ...
“By allowing the two electrons independent ‘movement’ in a three-centre system, the three-centre bond allows the electrons a fairly considerable chance of being near one another”. Similarly, the resonance forms shown above also increase the degree of inter-electronic repulsions as the electrons are paired up in the boron-hydrogen bonds.
Pi bonds occur when two orbitals overlap when they are parallel. [9] For example, a bond between two s-orbital electrons is a sigma bond, because two spheres are always coaxial. In terms of bond order, single bonds have one sigma bond, double bonds consist of one sigma bond and one pi bond, and triple bonds contain one sigma bond and two pi bonds.