Total Overlap Populations Research Articles

Electronic and geometric structure of the protonated forms of nickel β-diketonates

The effect of protonation on the electronic and spatial structure of nickel malonodialdehynate Ni(Mal)2 and acetylacetonate Ni(Acac)2 were studied by quantum chemical density functional theory (DFT) method. The metal ring geometry, the energies and the composition of molecular orbitals (MO), the effective charges on atoms, and the total overlap populations were determined, and the possible proton location sites were identified. The variation of the MO energy depending on the electron density distribution on the protonated and non-protonated ligands was considered. Proton addition to one of the oxygen atoms was shown to be most likely.

Quantum chemical calculations for understanding and predicting toxicity. II. The phosphorylation step in the inhibition of ache by organophosphorus anticholinesterases

The first step in the inhibition of acetylcholinesterase (AChE) by organophosphorus anticholinesterases is formation of the organophosphorous molecule-AChE complex. In the first paper of this series it was shown that the first step can be modeled well by the use of three-dimensional electrostatic molecular potential contour maps around the P compound. The second phosphorylation step of the mechanism by which organophosphorous anticholinesterases of the general formula inhibit AChE is related to the acidity of X and to the bond strength of the P—X bond. For a series of compounds, aryl N-methyl methyl phosphoramidates, in which the P—X bond is constant, this paper shows that the quantum chemically calculated P-O total overlap populations (TOPs) correlate the experimental log I50 for inhibition of AChE over four orders of magnitude with correlation coefficients of 0.96 [r2: TOP 0.9581; (TOP)1/2 0.9614].

Ab Initio MODPOT/VRDDO/MERGE calculations on energetic compounds. II. Nitroexplosives: RDX and α-, β- and δ-HMX

Ab initio MODPOT/VRDDO calculations have been carried out on RDX and α-,β-, and δ-HMX using our ab initio programs that incorporate several desirable options for calculations on large molecules. Among these are ab initio effective core model potentials (MODPOT) that permit calculations of valence electrons only explicitly, yet accurately, and a charge-conserving integral prescreening evaluation (which we named VRDDO—variable retention of diatomic differential overlap) that is especially effective for spatially extended molecules. The resultant theoretical indices are relevant to a number of properties and behaviorial characteristics of these molecules, both intra- and intermolecular. The charges on the atoms (from the gross atomic populations) are needed for calculation of the atomic multipole-atomic multipole electrostatic contributions (a major or dominant factor) to the intermolecular interaction energies. These electrostatic interaction energies are part of the input necessary for calculations on crystal packing and densities of these molecules. The total overlap populations between atoms are related to the inherent bond strengths, which are of interest in predicting and tracing initiating steps of explosive phenomena. The molecular orbital energies of the lowest unoccupied orbitals are of particular interest since these nitroexplosives have been implicated in testicular toxicity and the initial metabolic activation appears to proceed through a one-electron reduction of the nitroexplosive.

Molecular calculations with the MODPOT, VRDDO, and MODPOT/VRDDO procedures: I. HF, F2, HCl, Cl2, formamide, pyrrole, pyridine, and nitrobenzene

The nonempirical valence electron LCAO-MO-SCF model potential (MODPOT) procedure of Bonifacic and Huzinaga has been applied to the HF, F2, HCl, Cl2, formamide, pyrrole, pyridine, and nitrobenzene molecules. The results indicate that the method gives quite accurate valence orbital energies and potential energy curves for diatomics composed of first- and second-row atoms. It gives quite reliable valence orbital energies, gross atomic populations, total overlap populations, and dipole moment for moderately large organic molecules composed of first-row atoms and hydrogen. Use of the MODPOT approximation results in a substantial reduction in computer time and number of two-electron integrals that have to be processed. The nonempirical LCAO-MO-SCF charge-conserving variable retention of diatomic differential overlap (VRDDO) approximation of Wilhite and Euwema has been applied to the pyrrole, pyridine, and nitrobenzene molecules. The results indicate that the procedure gives accurate orbital energies, total energy, gross atomic populations, total overlap populations, and dipole moment for moderately large organic molecules. Use of the VRDDO approximation results in a substantial reduction in computer time. We also present pyrrole, pyridine, and nitrobenzene results obtained by introducing the VRDDO approximation into the MODPOT procedure. The accuracy of the VRDDO method remains unchanged if one considers only the valence electrons and there is an additional reduction in computer time.

Molecular calculations with the MODPOT, VRDDO, and MODPOT/VRDDO procedures. V. Ab initio and MODPOT LCAO-MO-SCF calculations on the chlorofluoromethanes

Ab initio LCAO-MO-SCF calculations were carried out on the chlorofluoromethanes CF4, CF3Cl, CF2Cl2, CFCl3, and CCl4 both with a minimum STO 4–3G basis set and with a larger, extended s. p basis set. Ab initio effective core model potential (31G/MODPOT) calculations were then carried out on this same series of chlorofluoromethanes. Valence orbital energies, core orbital energies, total energies, gross atomic populations, total overlap populations, and dipole moments are presented for comparison among the various computational results as well as to measured experimental properties. The results of the 31G/MODPOT calculations compared favorably with those from the ab initio larger extended s. p basis set, which themselves were in better agreement with experiment than those from the ab initio minimum 4–3G set.

Comparison for pyrrole and pyrazole of orbital energies and population analyses from ab-initio SCF, CNDO/2, INDO, extended hückel, and ARCANA calculations

The results of our recent ab-initio SCF calculations on pyrrole and pyrazole using large Gaussian basis sets are compared to those using less rigorous methods: CNDO/2, INDO, PCILO extended Huckel, and ARCANA (a type of iterative extended Huckel calculation). The ab-initio orbital energies are higher in energy than all those resulting from less rigorous methods. For the two highest occupied orbitals, both of π type, (pyrrole— 1a2 and 2b1; pyrazole—3a″ and 2a″) there was excellent agreement between the measured photoelectron spectroscopic ionization potentials and the Koopman's theorem values from the ab-initio results. Thus the orbital energies calculated by the less rigorous methods all lie too low in energy. The CNDO/2 and INDO orbital energies also span too large a width for the valence band compared to the ab-initio results. The gross atomic populations from the CNDO/2, INDO and PCILO methods show discrepancies from the ab-initio results for both the heteroatoms and the hydrogens. Total overlap populations calculated from the ab-initio SCF wave functions show large negative overlap populations between nonbonded ring atoms. Using the off-diagonal density matrix elements from CNDO/2 and INDO wave functions as if they were true off-diagonal elements from non-ZDO calculations, scaling them (shown previously to lead to reasonable values for TOPS between bonded atoms) led to TOPS between nonbonded ring atoms that were essentially zero. Wiberg bond indices, by definition only capable of giving positive numbers, also cannot reproduce these negative overlap populations. The extended Huckel and ARCANA methods, which are non-ZDO methods, both gave negative tops between nonbonded ring atoms, although not as large as those which resulted from ab-initio calculations.

Nb xTa 7− xS 2 ( x=2.73), a structurally distinct (Nb,Ta)-rich sulfide obtaining its stability from the dissimilar cohesive energy of the two metals

Nb x Ta 7− x S 2 ( x=2.73) has been prepared by application of high temperature synthesis techniques. The structure was determined by single crystal X-ray diffraction at ambient temperature to be orthorhombic, space group Pnma ( Z=4) with a=547.08(5), b=1971.33(9), c=1030.25(7) pm. Nb x Ta 7− x S 2 represents the metal-richest member of a series of phases with composition (Nb,Ta) n S 4. It is the only phase of its class that has no counterpart in one of the adjacent binary systems and is structurally related with the Ta-rich rather than the Nb-rich sulfides. Icosahedral (Nb,Ta) 13 clusters fused into pentagonal antiprismatic metal columns are the characteristic motif of the structure. The trend of the fractional site occupancies of the non-randomly distributed metal atoms can be conclusively rationalized on the basis of calculated total Mulliken overlap populations. Enrichment of the metal with the higher cohesive energy, namely Ta, is highly correlated with the strength of M–M interactions which differ significantly at the various crystallographic sites in this structurally distinguished metal-rich sulfide.

Ab initio MRD–CI calculations on cubane (neutral, carbocation, carboanion) and dissociation of nitrocubanes based on localized orbitals

AbstractAb initio MRD–CI calculations based on localized orbitals were carried out for cubane (neutral, carbocation, carboanion) both in our customary MODPOT basis set and in an all‐electron 4–31G basis set. The calculated MRD–CI charge distributions on C1 (the skeletal atom from which the H− or H+ was removed) (ab initio MODPOT neutral 4.221, carbocation 3.796, carboanion 4.282; all‐electron 4–31G neutral 6.171, carbocation 5.717, carboanion 6.078) indicate that the + or ‐ charge does not remain localized on C1 but redistributes itself. This has significant implications for preparative reactions of energetically substituted cubanes. The MRD–CI population analyses differ somewhat from the SCF population analyses, especially in the calculated total overlap populations. To investigate this effect on electrostatic molecular potential contour (EMPC) maps generated from SCF or MRD–CI wave functions, we wrote additional routines to calculate EMPC maps from MRD–CI wave functions. The EMPC maps generated from SCF or MRD–CI wave functions are different if the molecule needs an MRD–CI multideterminant wave function to describe it adequately. The EMPC map is a one‐electron property. One‐electron properties are derived from the 1‐matrix. The 1‐matrix is different for SCF or MRD–CI wave functions. Thus, all the one‐electron properties (EMPC maps, population analyses, bond deviation indices, etc.) are different when calculated from SCF or MRD–CI wave functions if MRD–CI wave functions are necessary to describe a system properly. We calculate these one‐electron properties from the 1‐matrix from the final natural orbitals. Our preliminary calculations for the dissociation pathway indicate it takes more energy to dissociate a bond in 1‐nitrocubane than in octanitrocubane. Even in their ground electronic states at equilibrium geometry, both 1‐nitrocubane and octanitrocubane require MRD–CI wave functions to describe them properly. The c2 of the single determinant SCF wave function is only 0.8401 for 1‐nitrocubane and 0.8300 for octanitrocubane. There are contributions from skeletal excitations as there are for cubane itself as well as excitations involving the nitrogroup. As the bond in nitrocubane is dissociated to 8.00 bohrs, the c2 of the SCF contribution drops to only 0.4606 (1‐nitrocubane) and 0.4445 (octanitrocubane). At this C1N1 intermolecular distance, the largest excitations are in the C1N1 bond: (C1N1)2 → (C1N1*)2, (C1N1) → (C1N1*). We also calculated the first electronically excited state for the dissociation pathway for selected points for both 1‐nitrocubane and octanitrocubane.

Hydrogen bonding in salicylaldehyde: A molecular orbital study

Calculations were done at the 6–31G level on hydrogen-bonded salicylaldehyde and the rotamer in which the OH group is rotated 180° about the CO bond axis, assuming that the structures are planar but otherwise with full geometry optimization. The parameters for the hydrogen bridge, the six-membered hydrogen-bonded ring, and the changes in converting the rotamer into the hydrogen-bonded structure are compared with values obtained for the aliphatic analog, β-hydroxyacrolein, calculated using the 4–31G basis set. The concomitant changes in bond length in the benzene ring are similar in magnitude, showing that the perturbation of the OH group spreads throughout the entire molecule. Mulliken population analysis indicates that the changes in total atomic charge and overlap populations are due primarily to π-electron and σ-electron effects, respectively. The hydrogen bond energy is evaluated using this rotamer as the reference state, the rotamer in which the HCO group is rotated 180° about the CC bond axis, and also benzene, benzaldehyde and phenol as a composite reference state. The procedures by which intramolecular hydrogen bond energies are derived, in part or entirely, from experimental measurements are compared and contrasted with the methodology involved in molecular orbital calculations.

A molecular orbital study of ethylene and the all- trans conjugated polyenes: C 4H 6, C 6H 8, C 8H 10 and C 10H 12

We have carried out calculations on ethylene, trans 1,3-butadiene and on all- trans 1,3,5-hexatriene, 1,3,5,7-octatetraene and 1,3,5,7,9-decapentaene at the 6-31G level, on 1,3-butadiene and 1,3,5-hexatriene at the 6-31G* level, and on 1,3-butadiene at the 6-311G** level, with full geometry optimization. As the chain length is increased, the terminal CC bond shows an increase in length, the adjacent CC bond a decrease, the next CC bond along the chain an increase, and the next CC bond a decrease, the change in each case tending towards a limiting value. The terminal CC bond is shorter than the others, and the CC bond next to the end is longer than the others. The HC bonds in the middle of the longer chains also show a progressive increase towards a limiting value. The terminal C-atoms serve as both σ- and π-charge acceptors, whereas the other C-atoms are σ-charge acceptors but π-charge donors. The σ-charge transfer tends to level off at a value of about −0.182 in the middle of the chain, while the π-charge transfer drops almost to zero. The total overlap populations between the C-atoms of the CC and CC bonds are larger the shorter the bond length, and vice-versa, following straight line relationships. The breakdown of the total overlap population into contributions from atom centers, proximal and distal bonded atoms, and antibonded atoms, finds significant antibonded atom contributions amounting to 10–14% of that from proximal bonded atoms. Isodesmic bond separation energies and homodesmotic group separation energies are evaluated to assess the conjugation energy (stabilization energy) in the polyenes, and comparisons are made with the corresponding energies for benzene.

Ab initioMODPOT/VRDDO/MERGE calculations on energetic compounds. III. Nitroexplosives: Polyaminopolynitrobenzenes (including DATB, TATB, and tetryl)

AbstractQuantum chemical ab initio MODPOT/VRDDO calculations have been carried out on the following aminonitrobenzenes for which crystal structures had been determined experimentally: 4‐nitroaniline; N,N‐dimethyl‐p‐nitroaniline; 2,4,6‐trinitroaniline; 1,3‐diamino‐2,4,6‐trinitrobenzene (DATB—Form I); 1,3,5‐triamino‐2,4,6‐trinitrobenzene (TATB); 2,3,4,6‐tetranitroaniline; N‐methyl‐N,2,4,6‐tetranitroaniline (Tetryl); and N‐(β,β,β‐trifluoroethyl)‐N,2,4,6‐tetranitroaniline. These quantum chemical calculations were performed on the molecules in their conformations as found in their crystal structures. The calculations were carried out with our own ab initio programs which also incorporate as options several desirable features for calculations on large molecules: ab initio effective core model potentials (MODPOT) which enable calculations of valence electrons only explicitly, yet accurately, and a charge conserving integral prescreening evaluation (which we named VRDDO‐variable retention of diatomic differential overlap) especially effective for spatially extended molecules. Aminonitrobenzenes are especially interesting since there are inherent intramolecular ring distortions and deviations from planarity and intramolecular hydrogen bonds as well as intermolecular hydrogen bonds causing further deviations from planarity. The theoretical indices resulting from the quantum chemical calculations are relevant to a number of properties and behavioral characteristics of these molecules, both intramolecular and intermolecular. The charges on the atoms [from the gross atomic populations (GAP's)] are needed for calculation of the atomic multipole–atomic multipole electrostatic contributions (a dominant factor) to the intermolecular interaction energies. These electrostatic interaction energies are part of the input necessary for calculations on the crystal packing and densities of these molecules. These GAP's are also of value in interpreting the experimental photoelectron and ESCA spectra of these molecules. The total overlap populations (TOP's) between atoms are related to the inherent bond strengths and can serve as a quantitative replacement for the old empirical bond length‐bond order‐bond energy relationship still used by explosives chemists to identify the “target bonds” (the weakest bonds). The TOP's are of considerable value in predicting and tracing initiation and subsequent steps of explosive phenomena. The molecular orbital energies of the lowest unoccupied orbitals are of interest since nitroexplosives have been implicated in testicular toxicity and the initial metabolic activation appears to proceed through a one‐electron reduction of the nitroexplosive.

Estimation of bond energies from mulliken overlap populations

Estimations of the bond energies from Mulliken overlap populations are taken. The strength of a bond is undoubtedly connected with the density of electrons constituting this bond. In the present paper the electron densitiesϱ i bond belonging to the particular bonds are represented by the total overlap populations defined byMulliken. The heat of atomization of the molecule is subdivide in the ratio of these electron densitiesϱ i bond belonging to the individual bond.

Application of molecular orbital calculations to the bicyclo[3.2.2]nonatrienyl anion and some related ions

Abstract The stabilities and the electronic structures of (CH)9− and (CH)9+ systems were investigated using INDO MO calculation. The total overlap populations of the C-C bonds of bicyclo[3.2.2]nonatrienyl anion indicated its high degree of conjugation, in contrast to less conjugated bicyclo[3.2.2]nonatrienyl cation. On the other hand, the (CH)9− intermediate of the D3h symmetrical structure was shown to be much less conjugated than the D3h symmetrical (CH)9+ ion. It was suggested from the calculated results that the sum of the total overlap populations of all C-C bonds involved in a (CH)n molecule could be a quantitative measure of aromaticity for the (CH)n systems in the three-dimensional structures.

Similarity in the pattern per atom‐pair in benzene of contributions to total energy or total overlap populations

Similarity in the pattern per atom‐pair in benzene of contributions to total energy or total overlap populations

Infrared spectra and normal vibrations of cobalt(II), nickel(II) and palladium(II) complexes with N, N'-ethylenebis(acetylacetoneimine)

The i.r. spectra of Co(II) −, Ni(II) −, and Pd(II)-BAE (where BAE = N,N'-ethylene-bis(acetylacetoneiminato)) have been investigated between 4000 and 200 cm −1. A normal coordinate analysis has been performed using an Urey-Bradley force field (with suitable general-valence-force-field terms) in order to check the empirical assignments and to provide an approximate description of the vibrational mode for most of the observed frequencies. In particular, the results indicate that the metal-nitrogen and metal-oxygen stretching vibrations are coupled with other skeletal stretching and deformation modes. The force constants of the metal-nitrogen and metal-oxygen bonds vary in the order Co(II) < Ni(II) < Pd(II), in agreement with the total bond overlap populations calculated for these bonds.