Damir Kovaček

Mills-nixon effect in benzocyclobutenes

Abstract The structural features of benzocyclobutenes were studied using several semiempirical and ab initio techniques. Qualitative hybridization arguments and actual 6–31 G Hartree-Fock calculations show conclusively that benzocyclobutenes exhibit a typical Mills-Nixon effect which is most pronounced in benzo[1,2:3,4:5,6]tricyclobutene. It is concluded that the experimental X-ray structure of the perfluoro derivative of the latter compound is seriously in error.

Strong Mills-Nixon effect in biphenylene

Abstract Structural features of biphenylene are studied by semiempirical and ab initio SCF methods employing STO-3G, 3-21G and 6-31G basis sets. The latter gives results in very good agreement with the X-ray data. The distribution of bond distances reveals the presence of a strong Mills—Nixon effect which has been questioned many times in similar systems. The origin of the highly pronounced Mills—Nixon effect is analyzed. It is found that it arises due to concerted and synergistic action of σ- and Π-electrons which leads to Mills—Nixon type of bond fixation.

Strong mills-nixon effect in benzocyclobutadienes

Abstract Structural properties of benzocyclobutadienes are examined by the scaled 3–21G SCF procedure. Resulting geometries are compared with the corresponding benzocyclobutene systems. It appears that the former series exhibits much stronger Mills-Nixon type of bond fixation within the benzene ring. The origin is found in a more pronounced rehybridization at the junction atoms and higher π electron localization. They both act cooperatively in the same direction.

THEORETICAL MODEL CALCULATIONS OF THE ABSOLUTE PROTON AFFINITIES OF BENZONITRILE, NITROSO- AND NITROBENZENE

Abstract Protonation in benzonitrile ( 1 ), nitrosobenzene ( 2 ) and nitrobenzene ( 3 ) is examined by using the MP2(fc)/6-31G ∗∗ //HF/6-31G ∗ + ZPE(HF/6-31G ∗ ) theoretical model. It is shown that the studied systems are protonated at heteroatoms: nitrogen in 1 and 2 and oxygen in 3 . The absolute proton affinities (PAs) are in good accordance with the available measured data. However, PAs of nitrogen and oxygen atoms in 2 are relatively close implying that much more accurate calculations are necessary for an ultimate assignment. Analysis of the descriptors of covalent bonding reveals that the heteroatomic protonation increases the resonance interaction between substituent groups and the benzene moiety. The ring protonation, on the other hand, is energetically less favorable. It yields lower PAs than in the parent benzene molecule. Deactivation of the aromatic fragment toward electrophilic (proton) attack is rationalized by the antagonism between the two π-bond localization patterns: one caused by the electron withdrawing substituents CN, NO and NO 2 and the other arising from protonation and the subsequent formation of a sp 3 center. The ipso protonation is the least favorable in all three cases since the attacked C(1) atom has a depleted electron density due to the highly electronegative nearest neighbor (inductive effect). In addition, the resonance interaction between the benzene ring and the substituent are practically switched off, since protonation at the ipso position bends the substituents covalent bond out of the molecular plane leading to substantial ring puckering of the aromatic moiety at the same time.

Semiempirical AM 1 study of the structural properties in some large fused molecular systems

Abstract A number of fused molecules exhibiting the Mills-Nixon (MN) effect are examined by the semiempirical AM1 method. The studied compounds include systems involving a naphthalene “aromatic” moiety fused to four- and five-membered rings, annelated biphenylene fragments consisting of alternating aromatic and anti-aromatic rings, and an eight-membered central ring (cyclooctatetraene or cyclooctatetradiyne) flanked by benzene or by benzo-cyclobutane and biphenylene fragments. The structural parameters are in good qualitative agreement with available experimental data. Most of the considered systems exhibit MN distortions which are discussed in terms of σ-rehybridization and π-bond orders. The importance of rehybridization is illustrated by the difference in C-C bond distances within the central ring in all- cis -tris(benzocyclobuta)cyclohexane (7)- a molecule with flat cyclohexane where π-electrons are absent in the central six-membered fragment. A difference in stability in angular and linear [3]phenylene is interpreted by synergistic and counteractive interaction of σ- and π-electrons, respectively. The concept of concertedness of σ- and π-electrons resolves the paradox that bent [3]phenylene has a lower total energy in spite of the fact that it is a more localized system than the linear isomer. The primitive graph-theoretical approach based on the “conjugate circuits” model fails completely in this respect.

Diamagnetic susceptibilities in some linear van der waals complexes using the promolecule model

Abstract The second moments and diamagnetic susceptibilities of some linear van der Waals compounds may be calculated by the simple promolecule model utilizing an atomic monopole approximation for the electronic charge distribution. The results are in good agreement with experimental data. The estimated values for a large number of van der Waals systems can aid in interpreting measured Zeeman effects in rotational spectra.

Influence of substituents on the Mills-Nixon effect in some naphthodicyclobutenes and naphthodicyclobutadienes

Abstract Structural features of substituted naphthalenedicyclobutenes and naphthalenedicyclobutadienes were studied using the SCF formalism and the 3-21 G basis set. It is shown that substitution of fluorine and lithium atoms at the naphthalene positions leads to structural changes which strongly depend on the electronegativity of the substituents. The deformations induced are more pronounced in naphthalenedicyclobutadienes. The dominant effect which causes variation in the structural parameters is rehybridization of the local hybrid orbitals. Additional π-electrons of the terminal double bonds in naphthalenedicyclobutadienes substantially contribute to increased bond fixation within the aromatic moiety.

Second moments and diamagnetic susceptibilities in van der waals complexes by the promolecule model

Abstract The promolecule model has been shown to give reasonable estimates of the second moments and diamagnetic susceptibilities of small Van der Waals complexes. The model is here extended to larger Van der Waals complexes. Results for medium-size complexes are found to be in good agreement with experimental data and for ab initio values.