| Summary: | 碩士 === 國立成功大學 === 化學系碩博士班 === 93 === The Weinhold’s natutal bond orbital ( NBO ) has been widely and rapidly employed on
conformational studies, especially for conformations that can be stabilized by σ􀃆σ* or σ􀃆π*orbital interaction. This σ orbital delocalization due to orbital interaction is well known as hyperconjugation. Similarly the one-center lone-pair orbital, n, and the low-lying two-center σ/σ* bond orbitals, e.g. the nF and σCF/σ*CF 4-electron 3-orbital system, can be mutually stabilized and it is ascribed to the concept known as negative hyperconjugation (NHC ). In this research the ClONO2 molecule is selected as a parent compound in which the donor( filled BO )-acceptor( vacant BO ) orbital interaction n􀃆σ*, has been quantified by the values of second-order perturbation energy, E(2). The magnitude of E(2), also called delocalization energy, is derived from the Fock matrix elements within the LCBO expansion of the weinhold’s NBO basis.It is found that the donor-acceptor interaction between the terminal oxygen’s lone-pair, n, and the adjacent σ*NO gives the largest delocalization energy ( 306 kcal/mol ).
This is similar to the NHC describing the interaction between the terminal oxygen’s lone pair orbital, nO, and the CF antibonding orbital, σ*CF, either in fluorinated methanols or in Ph-C-(CF3)2. The second important delocalization in ClONO2 is ascribed to the bridged oxygen nO􀃆π*NO DA interaction
( 18 kcal/mol ), which is analogous to the NHC published in literature for the allyl anion system,-C-C=C. Although NHC is generally unlike for second-row elements such as sulfur, the
nCl􀃆σ*NO interaction is verified by monitoring values of E(2) calculated for various Cl-O-N-O
dihedral angles. The relative degree of chlorine NHC to oxygen NHC is found near 8 : 50.
Replacement of the chlorine atom with either fluorine or bromine reveals the same observation.
For the –OONO2 anion, the negative charge is found to enhance the NHC effect as expected.
Further examination made on HOONO2, as well, supports the conclusion that Cl participate NHC
in the ClONO2 molecule and the acceptor is.
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