419 research outputs found
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
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 grou
Molecular structure and benzene ring deformation of three ethynylbenzenes from gas-phase electron diffraction and quantum chemical calculations
No full textThe molecular structures of ethynylbenzene and s-triethynylbenzene have been accurately determined by
gas-phase electron diffraction and ab initio/DFT MO calculations and are compared to that of p-diethynylbenzene
from a previous study [Domenicano, A.; Arcadi, A.; Ramondo, F.; Campanelli, A. R.;
Portalone, G.; Schultz, G.; Hargittai, I. J. Phys. Chem. 1996, 100, 14625]. Although the equilibrium structures
of the three molecules have C2v, D3h, and D2h symmetry, respectively, the corresponding average structures
in the gaseous phase are best described by nonplanar models of Cs, C3v, and C2v symmetry, respectively. The
lowering of symmetry is due to the large-amplitude motions of the substituents out of the plane of the benzene
ring. The use of nonplanar models in the electron diffraction analysis yields ring angles consistent with those
from MO calculations. The molecular structure of ethynylbenzene reported from microwave spectroscopy
studies is shown to be inaccurate in the ipso region of the benzene ring. The variations of the ring C-C
bonds and C-C-C angles in p-diethynylbenzene and s-triethynylbenzene are well interpreted as arising from
the superposition of independent effects from each substituent. In particular, experiments and calculations
consistently show that the mean length of the ring C-C bonds increases by about 0.002 Å per ethynyl group.
MO calculations at different levels of theory indicate that though the length of the C≡C bond of the ethynyl
group is unaffected by the pattern of substitution, the Cipso-Cethynyl bonds in p-diethynylbenzene are 0.001-
0.002 Å shorter than the corresponding bonds in ethynylbenzene and s-triethynylbenzene. This small effect
is attributed to conjugation of the two substituents through the benzene ring. Comparison of experimental
and MO results shows that the differences between the lengths of the Cipso-Cethynyl and Cipso-Cortho bonds in
the three molecules, 0.023-0.027 Å, are correctly computed at the MP2 and B3LYP levels of theory but are
overestimated by a factor of 2 when calculated at the HF level
Is it Reasonable to Treat All Calcified Stenotic Valves With a Valve Stent? Probably Yes If We Get a Full Stent Expansion
No full textPolar Effects and Structural Variation in 4-Substituted 1-Phenylbicyclo[2.2.2]octane Derivatives: A Quantum Chemical Study
No full textMolecular Structure and Benzene Ring Deformation of Three Ethynylbenzenes from Gas-Phase Electron Diffration and Quantum Chemical Calculations
No full textThe molecular structures of ethyrylbenzene and s-triethynylbenzene have been accurately determined by gas-phase electron diffraction and ab initio/DFT MO calculations and are compared to that of p-diethynylbenzene from a previous study [Domenicano, A.; Arcadi, A.; Ramondo, F.; Campanelli, A. R.; Portalone, G.; Schultz, G.; Hargittai, I .J. Phys. Chem. 1996, 100, 14625]. Although the equilibrium structures of the three molecules have C(2v), D(3h), and D(2h) symmetry, respectively, the corresponding average structures in the gaseous phase are best described by nonplanar models of C(s), C(3v), and C(2v) symmetry, respectively. The lowering of symmetry is due to the large-amplitude motions of the substituents out of the plane of the benzene ring. The use of nonplanar models in the electron diffraction analysis yields ring angles consistent with those from MO calculations. The molecular structure of ethynylbenzene reported from microwave spectroscopy studies is shown to be inaccurate in the ipso region of the benzene ring. The variations of the ring C-C bonds and C-C-C angles in p-diethynylbenzene and s-triethynylbenzene are well interpreted as arising from the superposition of independent effects from each substituent. In particular, experiments and calculations consistently show that the mean length of the ring C-C bonds increases by about 0.002 angstrom per ethynyl group. MO calculations at different levels of theory indicate that though the length of the C equivalent to C bond of the ethynyl group is unaffected by the pattern of substitution, the C(ipso)-C(ethynyl) bonds in p-diethynylbenzene are 0.001-0.002 A shorter than the corresponding bonds in ethynylbenzene and s-triethynylbenzene. This small effect is attributed to conjugation of the two substituents through the benzene ring. Comparison of experimental and MO results shows that the differences between the lengths of the C(ipso)-C(ethynyl) and C(ipso)-C(ortho) bonds in the three molecules, 0.023-0.027 angstrom, are correctly computed at the MP2 and B3LYP levels of theory but are overestimated by a factor of 2 when calculated at the HF level
Ab initio determination of the equilibrium geometry and vibrational frequencies of borazine
No full textJournal of Molecular Structure: THEOCHEM
Volume 236, Issue 1-2, 1 November 1991, Pages 29-39
Ab initio determination of the equilibrium geometry and vibrational frequencies of borazine (Article)
Ramondo, F., Portalone, G., Bencivenni, L.
Dipartimento di Chimica, Universitá degli Studi di Roma La Sapienza, P. le A. Moro 5, 00185 RomaItaly
View references (20)
Abstract
The equilibrium structure of borazine, B3N3H6, has been optimized at the HF-SCF level using the 6-31G, 6-31G*, 6-31 + G*, 6-31 + + G**, 6-311G**, DZP and TZP basis sets and at the secondorder Moeller-Plesset perturbation theory, with all orbitals active, with the 6-31G* basis set (MP2/ 6-31G*). The D3h symmetry conformation is established to be, at all the adopted levels of theory, the global minimum of the molecule. The planar structure has been studied employing several basis sets and the effects of basis set extension and of polarization d-functions on the molecular geometry are discussed. The bond distances and the ring angles of borazine determined at the MP2/6-31G* level are, (BN) 1.430 Å, (BH) 1.197 Å, (NH) 1.012 Å, (BNB) 122.9° and (NBN) 117.1°. HF-SCF and MP2/6-31G* vibrational frequencies have been calculated in the harmonic approximation and the vibrational spectrum of borazine is reassigned accordingly. © 199
Molecular structure and benzene ring deformation of three cyanobenzenes from gas-phase electron diffraction and quantum chemical calculations
No full textTransmission of Electronic Substituent Effects through a Benzene Framework: A Computational Study of 4-Substituted Biphenyls Based on Structural Variation
No full textPolar effects and structural variation in 4-substituted 1-phenylbicyclo[2.2.2]octane derivatives: a quantum chemical study
No full textThe transmission of polar effects through the bicyclo[2.2.2]octane framework has been investigated by
ascertaining how the geometry of a phenyl group at a bridgehead position is affected by a variable substituent
at the opposite bridgehead position. We have determined the molecular structure of several Ph-C(CH2-CH2)3C-X molecules (where X is a charged or dipolar substituent) from HF/6-31G* and B3LYP/6-311++G**
molecular orbital calculations and have progressively replaced each of the three -CH2-CH2- bridges by a
pair of hydrogen atoms. Thus the bicyclo[2.2.2]octane derivatives were changed first into cyclohexane
derivatives in the boat conformation, then into n-butane derivatives in the anti-syn-anti conformation, and
eventually into assemblies of two molecules, Ph-CH3 and CH3-X, appropriately oriented and kept at a
fixed distance. For each variable substituent the deformation of the benzene ring relative to X = H remains
substantially the same even when the substituent and the phenyl group are no longer connected by covalent
bonds. This provides unequivocal evidence that long-range polar effects in bicyclo[2.2.2]octane derivatives
are actually field effects, being transmitted through space rather than through bonds. Varying the substituent
X in a series of Ph-C(CH2-CH2)3C-X molecules gives rise to geometrical variation (relative to X = H) not
only in the benzene ring but also in the bicyclo[2.2.2]octane cage. The two deformations are poorly correlated.
The rather small deformation of the benzene ring correlates well with traditional measures of long-range
polar effects in bicyclo[2.2.2]octane derivatives, such as σF or σI values. The much larger deformation of the
bicyclo[2.2.2]octane cage is controlled primarily by the electronegativity of X, similar to deformation of the
benzene ring in Ph-X molecules. Thus the field and electronegativity effects of the substituent are well
separated and can be studied simultaneously, as they act on different parts of the molecular skeleton
- …
