Molecular structures,electronic spectra,and aromaticity of 1,6- and 1,8-dioxa analogues of pyrene and benzo[a]pyrene. Semiempirical molecular orbital calculations

The molecular structures of [1]benzopyrano[6,5,4-def][1]benzopyran, [2]benzopyrano[7,8,1,def]-1-benzopyran, 1-benzopyrano[6,5,4-mna]xanthene, and 2-benzopyrano[7,8,1-mna]xanthene have been optimized by using the PM3 semiemperical MO method. These calculated molecules have been shown to be planar. Their aromaticity has been investigated by use of the HOMA index. The molecules are found to be less aromatic than the correspolding parednt hydrocarbons, pyrene and benzo[a]pyrene. The CNDO/S MO method has been used to interpret th experimental uv-vis specrtoscopic data. The result of the PM3, CNDO/S and HOMA-index clculations are in good arteement with the experimental data.     

Calculation of excited-state properties of some small molecules: Comparative study of the CNDO/S and CNDO/2-vn−1 potential methods

The transition energy and geometry of the lowest excited (nπ*) singlet and triplet states of CO, CS, HNO, H₂CO, HFCO, and F₂CO molecules are calculated by CNDO /S and CNDO /2-V_N?1 potential methods, and the results are compared with those of experimental and ab initio theoretical studies, wherever available. In the calculation of the vertical transition energy, the performance of the CNDO /S method is seen to be generally more satisfactory than that of the CNDO /2-V_N?1 potential method, while the reverse is true for the excited-state geometry. The CNDO /S method as such fails to describe the geometry of the excited state, but a combined version (CNDO /S-2) of CNDO /S and CNDO /2, as well as the CNDO /2-V_N?1 potential method is fairly successful in this regard.     

AM1/CI, CNDO/S and ZINDO/S computations of absorption bands and their intensities in the UV spectra of some 4(3H)-quinazolinones

A detailed analysis of both frontier MOs and electronic transitions in UV spectra of 16 4-quinazolinone derivatives has been carried out in MO terms, by semiempirical methods AM1/CI, CNDO/S and ZINDO/S. On the basis of experimental and theoretical investigations by the ZINDO/S and CNDO/S methods the long-wavelength bands of 4(3H)-quinazolinone and its derivatives have been assigned to n-->pi(*) transition of the CO fragment and to the transition caused by intramolecular charge transfer from Ph and NCN fragments to CO group. It was shown that theoretically obtained electronic transitions applying method AM1/CI are not in agreement with experimental data observed for the 4(3H)-quinazolinone and 2,4(1H,3H)-quinazolinedione. Good correlation of theoretical and experimental data has been obtained by the method ZINDO/S for the wavelengths and the molar extinction coefficients of the compounds studied. Satisfactory correlation of theoretical and experimental data has also been obtained by the method CNDO/S with singly and doubly excited configurations, for the wavelengths only. Such correlations on experimental and theoretical wavelength and molar absorption coefficients of 4-quinazolinone derivatives are carried out for the first time.     

The fragments-in-molecules method. II. Fim method for σ-bonded systems

A semiempirical SCF MO method has been developed in which the wave function of a composite molecule is written as a linear combination of localized fragment orbitals and which is formulated such that strictly transferable empirical data for the fragments may be introduced into the calculation. Results of FIM calculations in the CNDO /2 approximation for a number of R? X molecules with R = alkyl and X = F, OH, NH₂, and CH₃ are presented and used to illustrate the possibilities and limitations of the method.     

Deorthogonalization of atomic originals in the CNDO approach

The procedure for deorthogonalization (D) of atomic orbitals in the semiempirical CNDO approach is reviewed. For comparative studies, CNDO/2, CNDO/2D, and STO -3G calculations of molecular dipole moments and Mulliken populations are carried out on 35 prototype molecules containing H, C, N, O, and F atoms. The calculated values are assessed on the basis of how well they agree with experimental trends, chemical bonding theories, and ab initio molecular orbital (MO) values. Results of analyses indicate that the CNDO/2D values for dipole moments are in reasonable agreement with experimental values, and those for net atomic charges and electron populations bear greater resemblance to the ab initio (STO -3G and 6-31G**) values than the original CNDO/2 values. These findings, together with those of previous investigators, demonstrate unequivocally the advantages of incorporating deorthogonalization into routine CNDO/2 or INDO calculations as a means to obtain reasonable estimates of charge distributions.     

CNDO-MO calculation of NnFm molecules

The s-p separation model CNDO -MO method has been introduced and developed for both the open- and closed-shell systems since 1975. This method has two chief advantages over most of the other CNDO methods. (1) The ns and np bases of the same atom may be considered independent to infer different bonding behavior. (2) Pariser–Parr and Nishimoto–Mataga approximations are applied to the Coulombic repulsion integrals, which not only simplifies the calculation and saves computer time but also gives reliable computational results. A series of nitrogen fluoride molecules such as NF, NF₂, NF₃, cis-N₂F₂, trans-N₂F₂, and N₂F₂ have been selected for this MO calculation. In each case, both ionization potential and dipole moment were calculated. The results are closer to the observed values than those reported in other works.     

Evaluation of EFG parameters in XO−3 ions (XCl,Br,I) by CNDO/2 and indo methods

One can not apply the conventional point charge model to evaluate the EFG parameters at the halogen site in chlorates, bromates and iodates since, in XO^?₃ ions (XCl,Br,I), the halogen atom is covalently bonded to the oxygens of the group and the effect of the cation on EFG is negligible. Therefore molecular orbital calculations by both CNDO/2 and INDO methods have been attempted to evaluate the EFG parameters. The currently available program (ref.1) for CNDO/2 and INDO methods has been extended to III and IV row elements using Hase and Schweig (ref.2) and Deb and Coulson (ref.3) parameters. The EFG parameters obtained from population density matrix (ref.4) from these two methods have been compared with the experimental data wherever available. It has been concluded that CNDO/2 method is better suited for the determination of EFG parameters in XO^?₃ ions compared to INDO method. The reason for this is discussed.     

Structure moléculaire et propriétés moléculaires de quatre isomères de formule C8H10N4

The molecular structures of the three Meyer isomers [3-methyl-3-(5′-amino-3′-methyl-l-pyrazolyl)acrylonitrile; acetylacetonitrile azine; 2,5-dimethyl-7-aminopyrazolo[1,5-a]pyrimidine] have been compared with that of the fourth isomer, 2,7-dimethyl-5-amino-pyrazolo[1,5-a]pyrimidine. The CNDO/2 and CNDO/S calculations utilizing these geometries have been accomplished. These include electronic transitions, dipole moments, ionisation potentials, charge densities, bond ordres and total energies. The calculated values have been compared to some experimental data. Uv spectra, ¹³C chemical shifts, ¹H-¹H coupling constants and relative stability of the four isomers are included.     

Semiempirical quantum-chemical calculations on the conformations of methyl formate and methyl thiolformate

The cis, trans, and gauche conformations of the methyl esters of formic and thiolformic acids have been investigated by different semiempirical methods. Total geometry optimization (CNDO/2, MINDO/3) and bond angle optimization (PCILO, NDDO) have been performed for the O-alkyl ester. The S-alkyl ester has been studied by the MINDO/3 method at the total geometry optimization level and by CNDO/2 and PCILO methods at the bond angle optimization level. The influence of sulphur d orbitals on the optimized molecular geometry as well as on the magnitude and direction of the dipole moment vector has been investigated in the CNDO/2 framework. The total energy differences of the conformers are compared to the experimental and ab initio results. CNDO/2 and NDDO energy partitioning have been performed to obtain information on the origin of the cis—trans energy difference and of the rotation barrier. The extent of the lone-pair delocalization has been studied in the different conformations using localized molecular orbitals. Calculations have been performed on the staggered and eclipsed positions of the methyl group in the planar conformations of both esters.     

Molecular orbital and DFT studies of the alimemazine radical cation

Semiempirical molecular orbital methods including CNDO, MNDO, AM1 and PM3, and density function theory method B3LYP/3-21G(d) were employed in the study of the alimemazine radical cation. It was found that PM3 was much better than CNDO, MNDO and AM1 in the structural optimization. The bond lengths and bond angles by PM3 were close to the experimental data, and comparable with the results by the density function theory method.     

Photosensitivity,substituent and solvent-induced shifts in UV-visible absorption bands of naphthyl-ester liquid crystals: a comparative theoretical approach

The substituent- and solvent-induced shifts in UV-visible absorption bands of naphthyl-ester nematic liquid crystals, viz., 4-octylphenyl-6-octyloxy-2-naphthoate (NAPHE1) and 6-octyloxy-2-naphthylyl-4-octyloxybenzoate (NAPHE2), have been investigated using the DFT, CNDO/S and INDO/S methods. A correlation has been made between molecular charge distribution and phase stability based on Mulliken, Loewdin, AM1, PM3, MNDO, CNDO/S and INDO/S methods. The observed π→π* and n→π* electronic transitions have been reported. The substituent- and solvent-induced shifts in absorption bands, transition energies and energy gaps have been discussed. The photosensitivity of the molecules has been analysed based on these shifts. It has been observed that the substituent has a dominant role on both absorption maxima and energy band gap, whereas the solvent has a dominant role only on absorption maxima, and no effect has been observed on the energy gap. These shifts may provide beneficial consequences in determining the end use of compounds.     

Assignments in photoelectron spectroscopy using the CNDO/2 method

In the photoelectron spectra of molecules vibrational fine structure often accompanies the observed ionization potentials (IPs). For non-diatomic molecules it is difficult to make vibrational assignments because the frequencies of the ion often differ greatly from those of the neutral molecule. In order to make both the vibrational assignments and the assignments of the IPs to molecular orbitals (MOs) using Koopmans' theorem CNDO/2 MO calculations have been carried out. Within the framework of this method a quantity E _AB has been calculated which is the change in potential energy between an atom-pair A-B for a particular MO on ionization. Application to some diatomic molecules has shown that this quantity reflects the changes in vibrational frequencies on ionization. The IP-MO assignments are made such that removal of an electron from a particular MO is in accord with the observed vibrational fine-structure; attempting to make these assignments on the basis of the calculated MO ordering alone has been shown to be very unsatisfactory.

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Zusammenfassung Photoelektronenspektren von Molekülen zeigen neben den Ionisierungspotentialen oft noch eine Schwingungsfeinstruktur, deren Zuordnung bei nicht-zweiatomigen Molekülen wegen der starken Frequenzverschiebungen vom Molekül zum Ion Schwierigkeiten macht. Um sowohl Schwingungsais auch Ionisierungs-Zuordnungen vornehmen zu können, wurden Rechnungen mit dem CNDO/2-Verfahren durchgeführt. Dabei stützt man sich auf die Änderung der potentiellen Energie zwischen dem Atompaar AB in bezug auf ein bestimmtes MO bei Ionisierung (E _AB). Die Anwendung auf zweiatomige Moleküle zeigt, daß diese Größe tatsächlich die Änderung der Schwingungsfrequenzen wiedergibt, und die Zuweisungen des IP werden so vorgenommen, daß die Entfernung eines Elektrons aus einem MO im Einklang mit der Schwingungsfeinstruktur ist. Dagegen erweist sich die einfache Zuweisung auf Grund der MO-Reihenfolge als schlechter.

     

Influence of charge transfer on parameters of band structure of organic superconductor (TMTSF)2PF6

The semiempirical method SCF MO LCAO in the CNDO/S valence approximation has been used to calculate tetramethyltetraselenafulvalene dimers (TMTSF)₂. Parameters of the band structure of (TMTSF)₂ ⁺PF₆ ^– have been calculated with an accounting for charge transfer.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 28, No, 2, pp. 140–143, March–April, 1992.     

Electronic structures of large,extended, nonperiodic systems by using the elongation method: Model calculations for the cluster series of a polymer and the molecular stacking on a surface

The elongation method based on the molecular orbital (MO ) theory, which enables us to extend a polymer with any molecular fragments theoretically, has recently been developed by our group. As the next step, we introduced an approach based on the crystal orbital (CO ) theory into above treatment. In the present work, the elongation method was developed at the Hartree–Fock level with CNDO /2 parameters and applied to model systems composed of the cluster series of a polymer and the molecular stacking on a surface. In the cluster-series calculations, the hydrogen molecule [(H₂)_n], hydrogen fluoride [(HF)_n], polyethylene, and polyacetylene were created successively to approximate their one-dimensional periodic polymers by using the MO -based elongation method. In the molecular-stacking models, we described the hypothetical surface of crystal as periodically arranged hydrogen molecules by the CO s, and several hydrogen molecules were stacked up on the surface one after another with the elongation procedure. Furthermore, the lattice defect on surface in which a part of stacked layer is lacking was dealt with by our approach. We also treated carbon monoxide chemisorption on a periodic magnesium chain as a more realistic model. Results for these systems support the applicability of our method for nonperiodic interactions in one- and two-dimensional large systems. © 1995 John Wiley & Sons, Inc.     

A molecular orbital study of the difluorodiazine (N2F2) system

A semiempirical MO method (CNDO/2) with empirical constants adjusted to give agreement with ground state geometries has been used to study isomerization mechanism in the N₂F₂ system. Calculated transition states do not correspond to the simple postulated transition states.     

Applications of ESCA to polymer chemistry: Studies of some nitroso rubbers

Molecular core binding energies of the polymers formed by copolymerization of CF₃NO with CF₂?CF₂, CF₂?CFCl, and CF₂?CFH, respectively, have been studied by means of ESCA. The results are interpreted in terms of CNDO/2 SCF MO calculations on some model systems. Some evidence for structural irregularity is found for the copolymers with CF₂?CFCl and with CF₂?CFH. The reaction mechanism for the polymerization is also discussed in terms of the experimental results and INDO SCF MO calculations.     

On the OCS2 Singlet potential energy surface

The reaction between atomic oxygen and carbon disulfide is predicted to lead to at least two primary products, which are the dithiiranone ( 1 ) and the oxathiirane-thione ( 2 ) and/or the carbon disulfide S-oxide ( 4 ). The possible intramolecular equilibria 1 ? 2, 1 ? 3, 2 ? 4 , and 2 ? 5 as well as the fragmentations of the possible intermediates 1 – 5 have been studied theoretically within the semiempirical CNDO /B framework as conceivable ground-state reactions. On the basis of MO correlations and potential energy changes along the reaction paths, supplementary with previously reported experimental data, the single molecular transformations and the eventual product formations are discussed.     

The Low-Temperature UV./VIS. Absorption Spectrum of [18]Annulene

The UV./VIS. absorption spectrum of [18]annulene has been remeasured in 3-rnethylpentane at room and at liquid nitrogen (glass) temperature and interpreted by the CNDO/S-CI-method. The confrontation of the experimental electronic transitions with the CNDO/S-CI-calculated ones favors a structure with D_6h-symmetry, i.e, a structure with delocalized π-bonds.     

Hydrogen bonding: A molecular orbital treatment by the EHT and the CNDO/2 methods of methanol and of formic acid

By both the EHT and the CNDO/2 calculations, the linear dimer of methanol is found to be more stable than the cyclic dimer. The hydrogen bonds in the trimer are stronger than those in linear dimers. The proton potential function, charge densities, and overlap populations in the linear dimer of methanol have been obtained. The CNDO/2 calculations show that the cis-form of formic acid is more stable than the trans-form, in agreement with experimental data. The cyclic dimer of formic acid is more stable than the open dimer. The -form of formic acid trimer is more stable than the -form. The proton potential function and the charge densities in the cyclic dimer of formic acid have been obtained. The CNDO/2 method gives more realistic proton potential functions for the dimers of methanol and formic acid. The O ... O stretching force constant in the dimers of methanol and formic acid have been estimated to be 0.13 × 10⁵ dynes/cm and 0.27 × 10⁵ dynes/cm, respectively, in agreement with experimental data.