Accurate heats of atomization and accurate bond lengths.: I. Benzenoid hydrocarbons

1968 ◽  
Vol 46 (12) ◽  
pp. 2027-2040 ◽  
Author(s):  
Donald H. Lo ◽  
M. A. Whitehead
Keyword(s):  
Bond Energies ◽  
Benzenoid Hydrocarbons ◽  
Bond Lengths ◽  
Consistent Field ◽  
Resonance Energies ◽  
Self Consistent Field ◽  
Individual Bond ◽  
Semi Empirical ◽  
Practical Accuracy ◽  
Sigma Bond

A semi-empirical P.P.P. type s.c.f.m.o. method has been developed to calculate, with practical accuracy, heats of atomization (at 25 °C), bond lengths, and resonance energies of benzenoid hydrocarbons. Sigma bond energies Eσb are estimated simultaneously with the pi bond energies Eπb in this method. Self-consistent field bond lengths are obtained, by minimization of the total bond energy, Eσb + Eπb, with respect to each individual bond length. The empirical resonance energies are calculated and a new term, the stabilization energy per CC bond, is introduced.

Theoretical Studies of Nonbenzenoid Hydrocarbons: 16 π-Electron Systems

1971 ◽  
Vol 49 (17) ◽  
pp. 2840-2849 ◽  
Author(s):  
F. W. Birss ◽  
N. K. Das Gupta
Keyword(s):  
Molecular Orbital ◽  
Molecular Orbital Calculations ◽  
Theoretical Studies ◽  
Electron Systems ◽  
Consistent Field ◽  
Resonance Energies ◽  
Self Consistent ◽  
Self Consistent Field ◽  
Semi Empirical

Hückel and self-consistent-field semi-empirical π-electron molecular orbital calculations on pentalenoheptalene are reported and the results compared to those of pyrene, acepleiadylene, napth[cde]azulene, and cyclohept[bc]acenaphthylene which contain the same number of π electrons. Resonance energies of all molecules calculated by the methods of Dewar and Whitehead are reported. It is suggested that pentalenoheptalene can be treated as two fused azulene nuclei.

Charge-transfer complexes of purines and pyrimidines

1968 ◽  
Vol 21 (2) ◽  
pp. 419 ◽  
Author(s):  
A Fulton ◽  
LE Lyons
Keyword(s):  
Charge Transfer ◽  
Field Method ◽  
Charge Transfer Complexes ◽  
Absorption Bands ◽  
Consistent Field ◽  
Highest Occupied Molecular Orbital ◽  
Self Consistent Field ◽  
Semi Empirical ◽  
Purines And Pyrimidines ◽  
Good Agreement

The spectra of 20 purines and pyrimidines with chloranil, bromanil, and p-benzoquinone in dimethyl sulphoxide were studied. Most of the systems exhibited absorption bands which were concluded to be charge transfer in nature. The ionization energies of the molecules, derived from the positions of the bands, correlated well with the highest occupied molecular orbital energies calculated using the simple H�ckel method and were also in good agreement with ionization energy values calculated by a semi-empirical self-consistent field method.

Analysis of diborane X-ray diffraction data utilizing structure factors calculated from molecular wave functions

1970 ◽  
Vol 26 (2) ◽  
pp. 196-207 ◽  
Author(s):  
D. S. Jones ◽  
W. N. Lipscomb
Keyword(s):  
Cell Structure ◽  
Wave Functions ◽  
X Ray Diffraction ◽  
X Ray ◽  
Bond Lengths ◽  
Consistent Field ◽  
Crystal Unit Cell ◽  
Structure Factors ◽  
Self Consistent Field ◽  
Molecular Wavefunctions

Crystal unit-cell structure factors for diborane, B2H6, have been calculated for four possible molecular geometries, using densities obtained from self-consistent field molecular wavefunctions. These structure factors were fitted for various B–H distances to the experimental X-ray data for B2H6 by varying the parameters of several thermal vibration models. B–H bond lengths so determined have values about 0.05 Å longer than those determined by the usual spherical atom analysis of the X-ray data. Consideration of additional factors, such as the X-ray B–H bond shortening due to rigid rotation of the molecule in the crystal, leads to the conclusion that the bond length correction given by this treatment accounts for about two-thirds of the observed discrepancy between X-ray and electron diffraction values for the B–H bond lengths in diborane.

scholarly journals Effects of fluoro substitution on 4-bromodiphenyl ether (PBDE 3)

2008 ◽  
Vol 64 (1) ◽  
pp. 108-119 ◽  
Author(s):  
J. Klösener ◽  
D. C. Swenson ◽  
L. W. Robertson ◽  
G. Luthe
Keyword(s):  
Biological Effects ◽  
Diphenyl Ether ◽  
Environmental Pollutants ◽  
Torsion Angles ◽  
Bond Lengths ◽  
Structure Activity ◽  
Self Consistent Field ◽  
Semi Empirical ◽  
Van Der Waals Radii ◽  
Persistent Environmental Pollutants

It is our hypothesis that fluoro substitution provides a powerful tool to modulate the desired characteristics and to increase the specificity of studies of structure–activity relationships. 4-Bromodiphenyl ether (PBDE 3) and its five corresponding monofluorinated analogues (F-PBDEs 3) have been synthesized and fully characterized (using 1H, 13C and 19F NMR spectroscopy, and mass spectrometry). The accurate structure from X-ray crystal analysis was compared with iterative calculations using semi-empirical self-consistent field molecular-orbital (SCF-MO) models. The compounds studied were 4-bromodiphenyl ether (PBDE 3), the 13C6-isotopically labeled PBDE 3 (13C6-PBDE 3) and 2-fluoro-4-bromodiphenyl ether (3-2F), 2′-fluoro-4-bromodiphenyl ether (3-2′F), 3-fluoro-4-bromodiphenyl ether (3-3F), 3′-fluoro-4-bromodiphenyl ether (3-3′F), and 4′-fluoro-4-bromodiphenyl ether (3-4′F). Solid-state intermolecular interactions for PBDE 3 and the F-PBDEs 3 isomers are dominated by weak C—H(F,Br)...π and C—H...F interactions. The C—F bond lengths varied between 1.347 (2) and 1.362 (2) Å, and the C4—Br bond length between 1.880 (3) and 1.904 (2) Å. These bond lengths are correlated with electron-density differences, as determined by 13C shifts, but not with the strength of the C—F couplings. The interior ring angles of ipso-fluoro substitution increased (121.9–124.0°) as a result of hyperconjugation, a phenomenon also predicted by the calculation models. An attraction between the vicinal fluoro and halo substituents (observed in fluoro substituted chlorobiphenyls) was not observed for the bromo substituted F-PBDEs. The influence of a fluoro substituent on the conformation was only observable in PBDEs with di-ortho substitution. Calculated and observed torsion angles showed a positive correlation with increasing van der Waals radii and/or the degree of substitution for mono- to tetra-fluoro, chloro, bromo and methyl substitutions in the ortho positions of diphenyl ether. These findings utilizing F-tagged analogues presented here may prove fundamental to the interpretation of the biological effects and toxicities of these persistent environmental pollutants.

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