Comparison of ab initio,CNDO/2 and experimental dipole moment derivatives for C2 hydrocarbons and formaldehyde

Dipole moment derivatives determined by ab initio and CNDO/2 calculations are compared with the corresponding data obtained from infrared intensities. For ethane, ethylene and formaldehyde, the recent results of quadratic force field calculations have been used to calculate experimental derivatives; for the latter two molecules, the individual intensities of certain overlapping bands were determined from the results of rovibrational analysis. The experimental dipole moment derivative with respect to the rocking symmetry coordinate, S₁₀, of ethylene has been found to be 0.03 D, as opposed to the value of ca. 0.4 D reported previously. CNDO results agree both in sign and magnitude with ab initio dipole moment derivatives.     

Hydrogen bond effect on the bonding and B2 molecular vibrations of pyrrole

The influence of the hydrogen bond on the infrared B₂ symmetry modes of pyrrole (C_2ν) has been studied. CNDO/2 calculations suggest that the most probable structure for the associated pyrrole involves the interaction NH ⋯ π. The shortest distance between the nitrogen atom and the ring of the nearest neighbor molecule is about 2.9 Å. The absorption bands of pyrrole in the gaseous state at 474, 626 and 720 and 826 cm⁻¹ have been assigned to the NH, ring and CH out-of-plane deformation modes.     

E´tude the´orique de la conformation de la 1,1-dime´thylhydrazine et du complexe (CH3)2N-NH2 · BF3

CNDO/2 calculations are performed in order to determine the preferred conformations of 1,1-dimethylhydrazine and 1,1-dimethylhydrazine-boron trifluoride complex. In each case, the calculations reveal the existence of an equilibrium between gauche and trans forms (25 % of trans form in the free molecule, 65 % in the complex). In agreement with experiment, the results confirm that BF₃ is attached to the nitrogen atom bearing the methyl groups.     

A CNDO/2 study of the ethylene-chlorine reaction

CNDO/2 calculations have been carried out of the energies of reaction and changes in charge distribution for some reactions involving the ethylene chlorine complex. The addition of chlorine to ethylene may involve the attack of a chloride ion on the ethylene chlorine complex.     

A theoretical study of biphenylene in its ground and excited states

Molecular orbital studies of biphenylene in its geometrically optimized ground and triplet states have been performed at the INDO level. These calculations, together with CNDO/S, specially parameterized CNDO and calculations involving empirical force constants, are used to estimate the relative energies and geometries of the T₁ and S₁ excited states of biphenylene. The relation of these geometry changes on excitation to the unusual photophysical properties of biphenylene are discussed.     

A molecular orbital study of the electronic structure and stereochemistry of dithiocyanate diamine and dicyanate diamine cupric complexes

CNDO/2—UHF molecular orbital calculations have been performed with the aim of studying the electronic structure and stereochemistry of the complexes Cu(NH₃)₂(NCX)₂ (X = O, S). Orbital energies, atomic charges, Wiberg indices, spin densities, total molecular energies and their partitioning into metal—ligand and ligand—ligand interactions have been calculated for three model structures of each complex. The role of axial interactions in stabilizing the planar arrangement of theCuN₄ chromophore is discussed. The harmonic force constants of some deformation vibrations of coordinated NCX groups have been estimated for the trans isomers and these estimates are compared with the values for the free ligands.     

Molecular structure of acetyldeyiethylphosphine,MeC(O)PMe2: II. CNDO/2 calculations

The sp, spd and spd' approximations of the CNDO/2 method have been applied to energy calculations for some models of the acetyldimethylphosphine molecule generated during treatment of the electron diffraction data. The results obtained for trial configurations with different angles of rotation of the acetyl group about the PC _ac bond predict two symmetric energy minima separated by a rather low barrier. This is in agreement with the electron diffraction data which are compatible with the suggestion of large amplitude torsional vibrations of the acetyl group. Energy calculations have also been performed for the final electron diffraction models. The calculations, however, fail to remove ambiguity from the question of a preferred model of the acetyldimethylphosphine molecule.     

Cresols—An investigation into their electronic structure and spectra

The three isomeric cresols were subjected to the all-valence-electron CNDO/2 andPPP-CI calculations. Results from this study were used:

1. to compare the electronic structures of these isomers vis-à-vis parent compounds—phenol and toluene,
2. to obtain a quantitative picture of their chemical reactivities and electronic absorption spectra.

Using the σ-core charges derived from CNDO/2 calculations and subsequently revising the valence-state ionisation potential and one-center-two-electron repulsion integrals, thePPP-CI calculations were performed on the title compounds according toNishimoto andForster scheme. In these calculations the pseudo-unsaturated nature of the methyl group has been given due consideration. In spectral assignment, compared to the conventionalPPP approach, the CNDO/2-basedPPP-CI method gave a better agreement with the experimental data.     

Electron diffraction study and CNDO/2 calculations on the complex of aluminium trichloride with ammonia,Cl3Al.NH3

The molecular geometry of the complex of aluminium trichloride with ammonia, Cl₃Al.NH₃, has been studied by electron diffraction. The most important internuclear distances in terms of r_a parameters are as follows: r(Al-Cl) = 2.100±0.005 Å, r(Al-N) = 1.996±0.019 Å, r(Cl·Cl) = 3.569±0.011 Å and r(Cl·N) = 3.165±0.012 Å. The Cl-Al-Cl bond angle in terms of an approximate r_a structure is 116.9°. The assumptions of a staggered model in the structure analysis was justified by CNDO/2 calculations. The experimental data indicate strong linkage between the donor and acceptor parts. The flat pyramidal average configuration of the AlCl₃ part of the complex suggests planar equilibrium structure for free AlCl₃. Variations in the bond configurations of the donor and acceptor parts, as compared with those of the respective free molecules, are discussed.     

All-valence-electron scf-mo calculations on the electronic structure and reactivities of some halogenated benzene and azabenzene derivatives

All-valence-electron CNDO/2 SCF-MO calculations have been performed on some fluoro, chloro and methoxy derivatives of benzene, pyridine and the diazines. The computed charge distributions and the dipole moments are discussed and compared with available experimental data. The relative basicities of the derivatives of pyridine and the diazines have been investigated and an attempt made to correlate the results with known pK_a values. Localisation energies for the nucleophilic substitution of halogen by methoxide ion have also been calculated and the predicted relative reactivities compared with experimental observations, where these were available.     

Infrared study of some solid local anesthetics

The infrared spectra of some local anesthetics in the solid state are reported. A temperature study is also realized, especially on the p-amino group. The CNDO/2 method has been applied to calculate the V₂ torsional barrier, these values being compared with those obtained from other conventional methods. The origin of the tilt angle in these compounds is discussed.     

Molecular structure and electronic spectrum of ellagic acid: Semiempirical molecular orbital calculations

The molecular structure of ellagic acid has been optimized by using PM3 semiempirical MO method. Ellagic acid has been calculated to be planar with the molecular symmetry of C_2h. The lactone carbonyl groups are not tilted from the molecular plane. CNDO/S MO method has been used to interpret the experimental uv-vis spectroscopic data. The results of the PM3 and CNDO/S calculations have been in good agreement with the experimental data.     

Calculation of molecular crystals within the PCILOCC framework

Stabilization energies and equilibrium distances of one-dimensional (HF)_n and (H₂O)_n chains have been calculated by means of the PCILO method for finite chain length and by the PCILOCC method for infinite chain length. Both types of calculation are compared with corresponding CNDO /2-MO and CNDO /2-CO calculations. Further we have performed an analysis of the individual contributions of the stabilization energies per monomer of the PCILO and PCILOCC calculations. The results show that the PCILOCC method is well suited for the calculation of molecular associations with translational symmetry.     

Vibrational spectra and molecular structure of methyl vinyl ether

The IR and Raman spectra of methyl-d₃ vinyl and methyl-d₃ vinyl-d₃ ethers are reported (along with earlier published spectra of the undeuterated molecule). The normal coordinate analysis, based upon CNDO/2 force constant calculations, confirms the absence of significant changes in CH₃ group dynamical properties in methyl vinyl ether compared to saturated ethers. Normal coordinate calculations of possible rotamers (cis structure force field with certain assumptions has been used) favours the planar trans (or close to it) structure of the second isomer.     

Zur quantenchemischen Behandlung von Kation-Amid-Solvatkomplexen

A comparison ofab initio calculations employing different basis sets with corresponding CNDO/2 results for the Li⁺/HCONH₂ complex shows that these methods lead to completely different energy surfaces for this system. Reduction of the basis set, even to the minimal size, does not bring about serious changes in the results of theab initio calculations, whereas in the semiempirical treatment some methodical errors seem to occur. When using however, theab initio minimum geometry the CNDO/2 calculations also give a qualitatively correct picture for the influence of the cation on the amide modecule.     

Electronic structure of ferrocence derivatives studied by He(I) photoelectron spectroscopy and CNDO /2 method

The effect of substituents in the Cp ligands on the electronic structure has been studied for the 1,1′-disubstituted ferrocenes Fe(CpX)₂, with X = C₂H₅, OCH₃, CN, COCH₃, COOCH₃, OOCCH₃, CH₂C₆H₅, or C₆H₅, by UV photoelectron spectroscopy and by CNDO /2 calculations. The energy gap between the ²E_2g and ²AT_1g ion states, 0.36 eV in the parent ferrocene, is affected only by the COCH₃ and COOCH₃ substituents, which lower it to 0.22 and 0.28 eV, respectively. Splitting of e_1u(π) level due to the lowering of the symmetry is the only effect observed in the photoelectron spectra. There is a strong conjugation between the phenyl and cyclopentadienyl β-orbitals in 1,1′-diphenylferrocene. The changes in the a_1g(d) ionization energy calculated by the ΔSCF method using CNDO /2 total energies are in a good agreement with the experimental data.     

Zur struktur eines donor-akzeptor-substituierten [4]-annulens: das photoelektronenspektrum des bis(diäthylamino)-bis(äthoxy-carbonyl)-cyclobutadiens

He(I) photoelectron (PE) spectra of donor-acceptor-substituted cyclobutadiene (1), cyclooctatetraene (4), ethylene (2) and acetylene (5) have been recorded and are assigned by means of CNDO/S calculations. According to PE, ESCA, NMR and UV data as well as semiempirical calculations the push-pull substituted cyclobutadiene (1) is the first well established example of a delocalized [4]-annulene.     

Electronic structures and spectra of halogenated purines and pyrimidines III.σ-Electronic structure and some comments on the mechanism of the xanthine oxidase reaction of 2-halopurines and related analogs

The extended HMO (EHMO) and Pople-Segal SCF-MO-CNDO /2 calculations on purine, 8-oxopurine, 2-oxopurine, 2-fluoropurine, and 2-chloropurine indicate significant polarizations of the σ cores. It is shown that the polarizations of both σ and π frameworks are mutually opposing in some cases. The results from the two methods are compared for these complex biomolecules. It is found that the EHMO calculations tend to over-polarize the σ and π frameworks. However, the CNDO shows anomalous π-electron densities on N₍₇₎ and C₍₈₎ of the purine rings, and the reason for this anomaly is not certain. The application of the results to the xanthine oxidase system indicates that the substrate molecules are subject to a specific orientation on the enzyme surface to counteract the electronic reactivity, in support of the previous prediction based on the π-electron calculations. The CNDO results appear to be more satisfactory than the EHMO in this respect.