1,721,034 research outputs found
Transmission of electronic substituent effects through a chain of conjugated double bonds: a quantum chemical study
No full textElectronegativity, resonance, and steric effects and the structure of monosubstituted benzene rings: an ab initio MO study.
No full textThe deformation of the carbon skeleton of the benzene ring under substituent impact has been analyzed from
the structures of 74 monosubstituted derivatives, as determined by ab initio MO calculations. The geometry
of the substituted ring is shown to contain valuable information on the electronegativity, resonance, and
steric effects of the substituent, and also on other, more subtle effects, affecting primarily the length of the
Cipso-Cortho bonds. The results obtained substantially augment previous knowledge from the analysis of
experimental geometries (Domenicano, A.; Murray-Rust, P.; Vaciago, A. Acta Crystallogr., Sect. B 1983,
39, 457). Varying the electronegativity of the substituent causes a concerted change of the ring angles at the
ipso, ortho, and para positions, coupled with a change in the Cipso-Cortho bond length. The values of the ipso
angle span a remarkably wide range, 113-126°. Enhancing the resonance interaction between a substituent
and the ring causes a complex pattern of angular distortions, arising from the superposition of two separate
effects. The first originates from the decreased length of the C-X bond, and consists primarily in a concerted
change of the ipso and ortho angles. It occurs irrespective of whether the substituent is a π donor or a π acceptor. The second effect is associated with π-charge alternation on the ring carbons. It involves all the
internal ring angles, and depends on the substituent being a π donor or a π acceptor. These angular changes
are generally accompanied by changes in all C-C bond lengths, as expected from an enhanced contribution
of polar canonical forms to the electronic structure of the molecule. By using symmetry coordinates, we have
derived two orthogonal linear combinations of the internal ring angles, SE and SR, measuring the electronegativity
and resonance effects of a substituent, respectively, as seen from their impact on the ring geometry. SE and
SR values are affected in a typical way by steric effects
Group Electronegativities from Benzene Ring Deformations: A Quantum Chemical Study
No full textWe propose a new scale of group electronegativities, derived from benzene ring deformations in Ph-X
molecules. A recent analysis of such deformations (Campanelli, A. R.; Domenicano, A.; Ramondo, F. J.
Phys. Chem. A 2003, 107, 6429) has shown that two orthogonal linear combinations of the internal ring
angles, termed SE and SR, are directly related to the electronegativity and resonance effects of the substituent,
respectively. In the present paper, we show that SE increases linearly with the electronegativity of X within
each of the first two rows of the periodic table, acting as a sensitive indicator of the polarity of the Ph-X
bond. By using SE values from ab initio quantum chemical calculations, we have derived the electronegativities
of 100 organic and inorganic groups. Nonbonded interactions with the ortho hydrogens and carbons may
fictitiously alter the electronegativity of a group; in most cases, however, they are easily eluded by changing
the conformation of the substituent with respect to the benzene ring. Although the atom directly linked to the
ring tends to dominate the electronegativity of a group, the role of its adjacent atoms is also important. Their
effect depends markedly on the nature of chemical bonding and electron density distribution within the group
Molecular structure of trimethylsilylbenzene: a study by electron diffraction and molecular mechanics calculations
No full textTransmission of electronic substituent effects across the 1,12-dicarba-closo-dodecaborane cage: a computational study based on structural variation, atomic charges, and 13C NMR chemical shifts
No full textThe ability of the 1,12-dicarba-closo-dodecaborane cage to transmit long-range substituent effects has been investigated by analyzing the structural variation of a phenyl probe bonded to C1, as caused by a remote substituent X at C12. The geometries of 41 Ph–CB10H10C–X molecules, including 11 charged species, have been determined by MO calculations at the B3LYP/6-311++G** level of theory. The structural variation of the phenyl probe is best represented by a linear combination of the internal ring angles, termed SFCARB. Multiple regression analysis of SFCARB, using appropriate explanatory variables, reveals the presence of resonance effects, superimposed onto the field effect of the remote substituent. The ability of the para-carborane cage to transmit resonance effects is, on average, about one-half of that of the para-phenylene frame in coplanar para-substituted biphenyls. Analysis of the π-charge variation of the phenyl probe confirms that the para-carborane frame is less capable than the coplanar para-phenylene frame of transmitting π-electrons from the remote substituent to the phenyl probe, or vice versa. The para-carborane cage is a better π-acceptor than π-donor; this makes π-donor substituents less effective than π-acceptors in exchanging π-electrons with the phenyl probe across the cage. When the remote substituent is an uncharged group, the para-carborane cage acts as a very weak π-acceptor toward the phenyl probe. The structural variation of the para-carborane cage has also been investigated. It consists primarily of a change of the C1···C12 nonbonded separation, coupled with a change of the five B–C–B angles at C12. This concerted geometrical change is controlled by the electronegativity of the substituent and the resonance interactions occurring between substituent and cage. These, however, appear to be important only when π-donor substituents are involved. The 13C NMR chemical shifts of the para-carbon of the phenyl probe correlate nicely with SFCARB, pointing to the reliability of these quantities as measures of long-range substituent effects. On the contrary, the 11B and 13C chemical shifts of the cage atoms do not convey information on electronic substituent effects
”Structural Studies of Benzene Derivatives. VII. The Structure of p-Aminobenzoic Acid Hydrochloride”.
No full textField and resonance effects in biphenyl derivatives: a quantum chemical study based on structural variation
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