A new indo-type procedure for conjugated non-rigid molecules. I. Ground-state conformations and barriers to internal rotation

Conformational dependence of electronic spectra and zero-field splitting parameters in 2-naphthylphenylcarbene

“…Note that, in this represeritation, the atomic functions describing the two unpaired electrons on C 1 are the minus (n type) and plus (a type) combinations of the two sp3-type orbitals pointing in the plane orthogonal to the C2C 1C3 plane; the conventional description in terms of n and n , orbitals can be restored by simply going back from the hybridized orbital basis to the (s, p,, p,, p,) primitive basis set. The one-center integrals were computed by the Ohno-Klopman formula, a very suitable recipe for CI treatments including polyexcited configurations (22). It has been shown, in particular, that combining polyexcited CI treatments with ~h n &~l o~r n a n Coulomb integrals provides good predictions of the triplet state energies (22,23).…”

“…The one-center integrals were computed by the Ohno-Klopman formula, a very suitable recipe for CI treatments including polyexcited configurations (22). It has been shown, in particular, that combining polyexcited CI treatments with ~h n &~l o~r n a n Coulomb integrals provides good predictions of the triplet state energies (22,23). All the computations were carried out with our CS-INDO-CIPSI program running on an IBM RISC-SYSTEM16000 workstation.…”

syn–anti Isomerism in the 1,3‐dipolar cycloaddition to cis‐3,4‐disubstituted cyclobutenes. 4. Intramolecular interactions

“…This feature is better emphasized by model calculations on 'distorted' molecules where different values are imposed and only a values are varied to satisfy energy optimization. As an example, Figure 3c reports the comparison between dimethylcyclobutene (11) and 'deformed' dimethylcyclobutenes (I and 111). It does not seem easy to explain this relationship by ubond-u-bond repulsions.…”

Conformational behavior of azabiphenyls

“…Conformational problems of the present type cannot be satisfactorily investigated, however, by semiempirical methods of the NDO type (no matter which parametrization is used) since they are often unrealistic in this connection, due to a strong prefer-ence for conformations in which the 7~ systems of the two rings are in perpendicular, or nearly perpendicular, planes [ 15,221. Since other semiempirical all-valence methods also show significant shortcomings [25,26], an alternative approach is provided by nonempirical quantum-mechanical computations that should not suffer from these difficulties, and the only obstacle to their systematic use is the prohibitive amount of computer time required if extended basis sets and possibly configuration interaction were to be employed even for an analysis of general trends. Several studies [12, 15,27-341 have, however, shown that minimal (e.g., sTo-3G) or moderately large basis set (e.g., 4-31G or 3-21G) Hartree-Fock computations provide a very good compromise between the reliability of the results and the necessary amount of computer time, and the same applies for the geometric structure, relative stabilities of isomers, and electron distributions.…”

“…The comformational behavior of heteroaromatic systems formed by two cycles linked together by essentially single bonds has been the subject of several experimental [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15] and semiempirical [15][16][17][18][19][20][21][22][23][24][25][26] quantum mechanical investigations. In fact the elucidation of the electronic structure and molecular conformation of biphenyl and its mono-and polyaza analogs can provide a valuable guideline for rationalizing their use and role in several fields of physical and chemical technology, including, for instance, homogeneous catalysis or liquid crystals and dye lasers technology.…”