NPA Charges Research Articles

Reactions of trisilaallene and 2-germadisilaallene with various reagents

A number of the distinctive reactions of trisilaallene 1 and 2-germadisilaallene 4 with various reagents including water, alcohols, acetone, and haloalkanes were studied. The addition reactions of 1 to water and alcohols occur in a regiospecific manner to afford 1,3-dioxytrisilanes in high yields. The additions are explained by a stepwise mechanism involving the initial nucleophilic attack of a hydroxy oxygen to a terminal allenic silicon to give an intermediate unsymmetric disilene. The regiospecificity is rationalized by the shape of the frontier molecular orbitals and the NPA charge distribution of trisilaallene 1 and the intermediate disilene.

Interaction of Carboranes with Biomolecules: Formation of Dihydrogen Bonds

Noncovalent interactions of the polyhedral carborane 1-carba-closo-dodecaborane (CB(11)H(12))(-) with building blocks of biomolecules, modelled by glycine (GLY), serine (SER), phenylalanine (PHE), glutamic acid (GLU), lysine (LYS) and arginine (ARG), were investigated in vacuo by molecular dynamics simulations with the UFF empirical potential. Selected structures were further studied by accurate ab initio quantum chemical procedures. Interactions with a peptide bond (GLY-SER dipeptide) and a nucleic acid building block (guanine) were also considered. The RESP and NPA charges of carboranes and small model systems are compared and their use is discussed. The dominant interaction between carboranes and biomolecules is the formation of unconventional proton-hydride hydrogen bonds (dihydrogen bonds) characterized by a short distance between hydrogen atoms (as close as 1.8 A) and an average strength in the range of 4.2-5.8 kcal mol(-1). The total stabilization energy of complexes investigated is rather large, and the largest value (approximately 15 kcal mol(-1)) was found for the carborane complexes with ARG and the GLY-SER dipeptide. These interactions are ubiquitous under geometrical constraints influencing the strength of the interaction. The carborane forms dihydrogen bonds with biomolecules preferably with the hydrogen atoms of its lower hemisphere (i.e. the part of the cage opposite to the carbon atom). These two geometrical factors can be used to explain the specificity of inhibition of HIV protease by carboranes.

A Computational Study of [2.2]Cyclophanes

A computational study of isomeric [2.2]cyclophanes, namely [2.2]paracyclophane 1, [2.2]metacyclophane 2, and [2.2]metaparacyclophane 3, has been carried out. For 1, geometry optimizations performed by various methods at different basis sets showed that MP2/6-31+G(d,p) and B3PW91/6-31+G(d,p) provide the best results in comparison to the X-ray data. Compound 1 has D(2) symmetry with distorted bridges. A conformational search was performed for [2.2]cyclophanes 2 and 3. Each cyclophane exists in two conformations which have different energies in the case of 3 but are degenerate in the case of 2. Relative energies and strain energies at the bridges follow the same order, indicating that the relief of bridge tension and repulsion between pi clouds are determining factors for the stability of [2.2]cyclophanes. Through a decomposition of strain energy, it can be concluded that both the rings or the bridges can absorb strain, but it depends on the conformer of butane that is considered in the calculation of SE(br). Changes in aromaticity of these compounds were evaluated by NICS and HOMA and were compared with benzene and xylenes dimers as models. Despite distortions from planarity and shortening and lengthening of the C-C bonds relative to the mean, the phenyl rings are aromatic. NICS suggests a concentration of electronic density between the rings as a result of bridging process. Computed MK, NPA, and GAPT charges were compared for the isomeric cyclophanes. The GIAO chemical shifts were calculated and indicate that 1 has a larger diamagnetic anisotropy than the other isomers.

Study of effective hardness and condensed Fukui functions using AIM, ab initio, and DFT methods

The effective chemical hardness for some triatomic molecules has been studied using the ‘atom in a molecule’ approach (AIM). Molecular chemical hardness values calculated from the effective chemical hardness of individual atoms agree with experimental and theoretically calculated molecular hardness values. Condensed Fukui functions have been calculated using Mulliken, NPA and AIM charges calculated by HF and B3LYP methods using the 6-31+G** basis set. All population schemes have predicted few negative Fukui functions, which correlate well with effective chemical hardness values.

Charge distributions in polyacetylene chains containing a positively charged defect

AbstractWe present a comparative study of the AIM, CHELPG, GAPT, MK, Mulliken, NPA, and RESP charge distributions associated with a positively charged soliton on increasingly large trans‐polyacetylene chains, at HF, MP2, and DFT levels of theory. The charge storage in the soliton‐bearing systems is explored in detail, including charge magnitude, charge separation, charge alternation, and chain length effects. The grouping of the charge distributions at a given level of theory, as well as the sensitivity of a given charge distribution to the inclusion of electron correlation in its computation, are investigated using similarity analysis. Several of the charge definitions have been applied for the first time for charged soliton‐bearing systems, and there are substantial differences between the charge distributions for the charged and neutral systems. Thus, AIM charges are no longer one of the largest charge values, the AIM charges can be in counterphase with other definitions, and the GAPT charges for neutral systems are quite different from the GAPT charges for charged systems, e.g., the magnitudes of the GAPT charges are anomalously large and increase with the size of the charged system. © 2005 Wiley Periodicals, Inc. Int J Quantum Chem, 2005

Substituent effect of large conjugate groups on the DNA base pair derivatives: Density functional study

AbstractThe substituent effects caused by 2‐nitro‐naphthalen (aNO2), 2‐hydroxyl‐naphthalen (aOH), 1‐nitro‐4‐vinyl‐benzene (bNO2), and 1‐hydroxyl‐4‐vinyl‐benzene (bOH) have been investigated in this report. The geometries of various substituted base pairs have been optimized using the B3LYP method at the 6‐31G* level with no constraints. The vertical ionization potential energy has been calculated. Natural bond orbital (NBO) and CHelp analyses have also been carried out on both the neutral and cationic systems at the same level. The outcomes show that the hydrogen bond lengths of these substituted base pairs are similar to unsubstituted bases, while the geometries of the substituted bases have an interesting aspect; i.e., the substituent and the substituted base are nonplanar when the substituted base is pyrimide, while it is coplanar when the substituted base is purine. The stabilization energies changed slightly as compared with the corresponding A:T or G:C base pairs. NBO analysis shows that charge transfer is the largest in aOH‐T:A, while it is small in the other derivatives. Population analysis shows that the NPA and Mulliken charge of H has a relationship with the H‐bond lengths, while that of Chelp does not. © 2005 Wiley Periodicals, Inc. Int J Quantum Chem, 2005

Regioselectivity in iron-catalyzed [2+2+1] cycloadditions: a DFT investigation of substituent effects in 1,4-diazabutadienes.

The transition-metal-catalyzed [2+2+1] cycloaddition reaction of 1,4-diazabutadienes, in which the imine-carbon atoms are part of an oxazine ring system, with ethylene and carbon monoxide leads to the regioselective formation of pyrrolidinone derivatives. To explain this regioselectivity, the transition states and intermediates of the rate-determining step of the catalysis are determined by high-level DFT calculations. The experimentally observed regioselectivity is consistent with the lower activation energy of the addition of ethylene towards the carbon atom next to the oxazine oxygen atom. Furthermore, the activation barrier of a conceivable back reaction is higher for the intermediates with the experimentally observed regioselectivity. These thermodynamic and kinetic arguments at first sight appear to be confirmed by the calculated NPA charges in the transition states, which reveal that the differences in these charges are greatest for those transition states that lead to the formation of the energetically favored transition structures. Nevertheless, calculations of analogous transition structures and reaction products starting from 1,4-diazabutadienes with a 2-fluoro, 2-hydroxo or 2-amino substituent revealed that the regioselectivity is not determined by the electronegativity of the heteroatom and thus by the differences in the NPA charges or the resulting Coulombic interactions in the transition structures. The main reason for the observed regioselectivities is the pi-donor ability of the substituent to contribute to a delocalized pi system incorporating the adjacent imine moiety. The increasing pi-donor capability results in decreased reactivity of this moiety and increases the (relative) reactivity of the second imine group. This effect can even overcompensate for strong intramolecular Coulombic attractions in the transition structures.

Nucleophilic attack on ketene, 6‐methylene‐2,4‐cyclohexadienylideneketene, 6‐oxo‐2,4‐cyclohexadienylideneketene and 4‐oxo‐2,5‐cyclohexadienylideneketene: an ab initio study

AbstractNucleophilic attack of cyanide anion at the carbonyl carbon was studied computationally at the MP2/6–311+G** level for ketene and at the MP2/6–31+G* and MP2/6–31+G*//HF/6–31+G* levels for all four title compounds. Heats of addition were computed for formation of in‐plane adducts and perpendicular adducts where the terms ‘in‐plane’ and ‘perpendicular’ indicate the direction of nucleophilic attack relative to the ketene plane. Heats of activation were also computed for the two modes of attack. For ketene (1) the in‐plane adduct is favored by 37.6 kcal mol−1 and the in‐plane transition state by 18.8 kcal mol−1. For the 6‐methylene compound (2) the in‐plane adduct is favored by 20.1 kcal mol−1 and the in‐plane transition state by 7.3 kcal mol−1. For the 6‐oxo compound (3) the in‐plane adduct is more stable by 11.2 kcal mol−1 whereas the in‐plane transition state is preferred by 8.8 kcal mol−1. For the 4‐oxo compound (4) the in‐plane adduct is favored by 19.3 kcal mol−1 but its transition state by only 2.8 kcal mol−1. The in‐plane adduct for ketene is the enolate of acetyl cyanide while the perpendicular adduct is a pyramidalized carbanion attached to a cyanocarbonyl group. Calculated NPA charges support this conclusion. The perpendicular adduct is actually the transition state for rotation about the CC bond of the ketene moiety in the in‐plane adduct, and the 37.6 kcal mol−1 enthalpy difference is closely matched by the calculated activation enthalpy for rotation about the CC bond of acetaldehyde enolate: ΔH‡ = 33.6 kcal mol−1. The smaller differences found for in‐plane vs perpendicular adduct formation for the 6‐methylene, 6‐oxo and 4‐oxo compounds are correlated with decreases in bond length alternation within the six‐membered rings which occur upon addition of the nucleophile. Less alternation may be associated with an approach to aromaticity, hence an increase in stability. Both modes of attack lead to reduced bond length alternation, but the amount of alternation is lowered more for the perpendicular adducts than for the in‐plane adducts. Changes in the CO bond lengths are also diagnostic. For all four ketenes the ketenyl CO length in the in‐plane adducts is greater than in the perpendicular adducts reflecting more single bond (enolate) character in the former and more double bond (carbonyl) character in the latter. On the other hand the lengths of the 6‐oxo and 4‐oxo CO bonds in their in‐plane adducts are smaller than in the perpendicular adducts, consistent with increased conjugation with the oxo substituent in the latter. The greater NPA charge on the 6‐oxo and 4‐oxo oxygens of the perpendicular adducts supports this conclusion. Calculated enthalpies of activation are in the order ketene > 6‐methyleneketene > 6‐oxoketene, in qualitative agreement with experimentally determined rates of hydration. Copyright © 2004 John Wiley & Sons, Ltd.

1,3-Disila-2-gallata- and -indataallenic Anions [>SiMSi

The first compounds containing Si=Ga or Si=In double bonds, 1,3-disila-2-gallata- (2-) and -indataallenic anions (3-), were obtained in the form of lithium salts as dark red crystals by the reaction of bis(di-tert-butylmethylsilyl)dilithiosilane 1 with GaCl3 or InCl3. X-ray crystallographic analysis of 2-.[Li(thf)4]+ and 3-.[Li(thf)4]+ showed that the >SiMSi< frameworks (M = Ga, In) are not linear and the two Si=M bond lengths are about 9% shorter than typical Si-M single bonds. In the 29Si NMR spectra, the signals of the terminal Si atoms in 2- and 3- are greatly shifted upfield (2-: -79.9; 3-: -77.6 ppm). The NPA charge distribution for the model compound, [(Me3Si)2SiGaSi(SiMe3)2]- (4-) showed a large part of the negative charge localizing on the two terminal silicon atoms (-0.736 and -0.729). In addition, 2-.[Li(thf)4]+ reacted with MeI to give the corresponding iodogallane 5 nearly quantitatively. This result confirms that the allenic [>SiMSi<]- fragments are highly polarized.

DNA nucleosides and their radical anions: molecular structures and electron affinities.

The deoxyribonucleosides have been studied to determine the properties of combinations of 2-deoxyribose with each of the isolated DNA bases for both neutral and anionic species. We have used a carefully calibrated theoretical method [Chem. Rev. 2002, 102, 231], employing the B3LYP hybrid Hartree-Fock/DFT functional with the DZP++ basis set. Predictions are made of the geometric parameters, adiabatic electron affinities, charge distributions based on natural population analysis, and decomposition enthalpy for the neutral and anionic forms of the four 2'-deoxyribonucleosides in DNA: 2'-deoxyriboadenosine (dA), 2'-deoxyribocytidine (dC), 2'-deoxyriboguanosine (dG), and 2'-deoxyribothymidine (dT). Geometric changes in the anions show that the glycosidic bond exhibits little change with excess charge for the guanosine but significant shortening for the adenosine and for the pyrimidines. The zero-point corrected adiabatic electron affinities in eV for each of the 2'-deoxyribonucleosides are as follows: 0.06, dA; 0.09, dG; 0.33, dC; and 0.44, dT. These values are uniformly greater than those of the corresponding isolated bases (-0.28, A; -0.07, G; 0.03, C; 0.20, T) and the isolated 2-deoxyribose (-0.38) at the same level of theory. The vertical detachment energies of dT and dC are substantial, 0.72 and 0.94 eV, and these anions should be observable. A high VDE, 0.91 eV, is also found for dA but its anion is unlikely to be stable due to the small AEA of 0.06 eV. The high VDE reflects the fact that the molecular structures of the anions and the corresponding neutral species are quite different. Valence character is displayed for the SOMOs of dA, dC, and dT, while some component of diffuse character is visible in the SOMO of dG. Further analysis of electronic changes upon electron attachment include an examination of the NPA charges, which show that in the neutral 2'-deoxyribonucleosides the sum of NPA charges for every base is the same, -0.28 with the sum of 2-deoxyribose charges being positive, +0.28. In the anions, the trend in charge division varies based on the nature of the excess electron in the anions. Thermodynamically, the overall enthalpy change for the reaction of water with the neutral nucleosides to give bases and ribose is approximately zero. The analogous decomposition is exothermic by 8 to 11 kcal mol-1 for the anions, indicating possible challenges for anionic gas-phase nucleoside exploration in the presence of water.

Rotational Barriers, Charges, and Spin Densities in Alkyl Aryl Sulfide Radical Cations: A Density Functional Study

The structures and the torsional potentials for the rotation around the aryl−SR bond of a series of neutral alkyl aryl sulfides and their corresponding radical cations were studied by applying density functional theory at the B3LYP/6-311G** level. In all radical cations the most stable conformer corresponds to the structure in which the S-alkyl group is in the planar conformation, with sulfur and alkyl atoms coplanar with the aromatic ring. The rotational energy barriers were relatively high, ranging from 12.307 to 19.496 kcal/mol. In all cases substantial geometrical changes with respect to the parent sulfide were observed, the radical cation presenting a quinoidic structure, with quasi-single bonds alternating to quasi-double bonds, and a shorter CAr−S bond. These results illustrate the importance, in the radical cation, of the conjugation between the sulfur atom and the aromatic π system. Such a conjugation is not so important in the neutral molecules and in fact the energy minimum for alkyl phenyl sulfides is located in correspondence of the perpendicular form, with the exception of thioanisole for which the two effects seem to have a similar role. Calculation of Mulliken spin populations, MK and NPA charges showed that PhSR•+ and 4-NCC6H4SR•+ are mainly sulfur-centered radical cations, whereas 4-Me2NC6H4SMe•+ and 4-MeOC6H4SMe•+ are essentially aromatic radical cations with some charge and spin density at the sulfur atom. Ionization energies were also calculated for both the planar and perpendicular forms; for the PhSR series (particularly in the former case), they resulted in good agreement with the experimental ones.

Synergism of Catalysis and Reaction Center Rehybridization. A Novel Mode of Catalysis in the Hydrolysis of Carbon Dioxide

A quantum mechanical study is presented of the activation barrier of the one-, two-, and three-water hydrolyses of CO2. Geometries were optimized, frequencies were calculated, and NPA charges were determined at MP2(full)/6-31G* and MP2(full)/6-311G**, and energies were also determined at MP4(full,SDTQ)/6-311G**//MP2(full)/6-311G** and QCISD(T)/6-31G**//MP2(full)/6-31G**. The activation barriers are ΔH0 = 223.0 kJ/mol and ΔG298 = 235.6 kJ/mol for the one-water hydrolysis and they are ΔH0 = 149.8 kJ/mol and ΔG298 = 164.4 kJ/mol for the two-water hydrolysis at MP4(full,SDTQ)/6-311G**. The catalytic effect of the second water molecule is due to the alleviation of ring strain in the proton-transfer transition state. The placement of the third water molecule in the proton-transfer ring causes only an insignificant catalytic effect with respect to the two-water hydrolysis. The placement of the third water molecule opposite the site of proton transfer is explored here and it leads to activation barriers of ΔH0 = 122.6 kJ/mol and ΔG298 = 143.1 kJ/mol. The catalytic effect of the third water molecule is approximately 20 kJ/mol with respect to the two-water hydrolysis, and this is attributed to charge relaxation and rehybridization in the transition state. The results are compared to studies on the three-water hydrolysis of carbodiimide to discern the effects of atomic polarizability on the activation barriers of the hydrolyses of heterocumulenes. The conceptional insights predict that the catalytic effect should increase with more polarizable heteroatoms.

Bond energy M-C/H-C correlations: dual theoretical and experimental approach to the sensitivity of M-C bond strength to substituents.

DFT methods are used to quantify the relationship between M-C and H-C bond energies; for MLn = Re(eta5-C5H5)(CO)2H and fluorinated aryl ligands, theoretical and experimental investigations of ortho-fluorine substitution indicate a much larger increase in the M-C than in the H-C bond energy, so stabilising C-H activation products.

The Electronegativity Equalization Method II: Applicability of Different Atomic Charge Schemes

The amenability of different schemes for the calculation of atomic charges in the electronegativity equalization method (EEM) is investigated. To that end, a large training set of molecules was composed, comprising H, C, N, O, and F, covering a wide range of medicinal chemistry. Geometries are calculated at the B3LYP/6-31G* level. Atomic charges are calculated using five different methods, belonging to different types of population analysis. Effective electronegativities and hardness values are calibrated from the different quantum chemically calculated atomic charges. The resulting quality of EEM charges is investigated for the different types of atomic charge calculation methods. EEM-derived Mulliken and NPA charges are in good agreement with the ab initio values, electrostatic potential derived, and Hirshfeld charges show no good agreement.

Persistent oxidation dications from twisted fluoranthenes, benzo[k]fluoranthene and dimethyldibenzo[j.l]fluoranthene: charge delocalization mode, tropicity, and formation of novel 8,8'-bifluoranthenyls. An NMR and theoretical study.

First examples of persistent oxidation dications from fluoranthene-PAHs namely 1,3,4,6,7,10-hexamethyl- 2 and 3,4-dichloro-1,6,7,10-tetramethylfluoranthene 3, benzo[k]fluoranthene 6, and 3,6-dimethyldibenzo[j,l]fluoranthene 9 are reported. Charge delocalization mode and tropicity in the resulting nonalternant dications are examined. Quenching of the superacid solutions of the dications resulted in the formation of novel 8,8'-bifluoranthenyls 2a-4a. AM1 was used as an initial guide for dication generation (DeltaDeltaH(f) degrees and ionization potentials) and for probing the structures of the crowded fluoranthene substrates and their bifluoranthenyls. In selected cases, the dications and their neutral precursors were calculated at the B3LYP/6-31G(d,p) level. Charge delocalization mode (difference in NPA charges) and DFT/GIAO-derived NMR chemical shifts were obtained for comparison with experiment; aromaticity was assessed via nucleus independent chemical shift (NICS) calculations.

Stable ion study of protonated cyclopenta[a]phenanthrenes. Structure–reactivity relationships and charge delocalization in the carbocations †

Protonation studies are reported for a series of cyclopenta[a]phenanthrenes Cp[a]P in superacid media. Hydrocarbons 1, 4, 7, are ring protonated in FSO3H–SO2ClF to form monoarenium ions. The Δcompounds 3, 6 are protonated at the D-ring double-bond to form stable α-phenanthrene-substituted carbocations. The 17-keto derivatives 2, 5, 8, 9, 19, 20 are CO-protonated in FSO3H–SO2ClF to form carboxonium ions. Carboxonium ions derived from 8 and 20 undergo ring fluorosulfonation in the biologically important A-ring under thermodynamic control (higher temperatures and prolonged reaction times). Low temperature protonation of 8 and 9 with FSO3H·SbF5 (4∶1)–SO2ClF gives their corresponding carboxonium-arenium dications (protonation of 2 with FSO3H·SbF5 (1∶1)–SO2ClF gave a mixture of mono- and dications), where ring protonation sites are controlled by the position of the methyl group and occur in the A-ring for the A-ring methylated derivatives (8, 9). Whereas the 11-methoxy derivative (16) forms a carboxonium ion in FSO3H–SO2ClF analogous to the 11-Me derivative (5), the 11-phenol derivative (15), the ethoxy (17) and propoxy (18) derivatives are more reactive, forming a mixture of mono- and dication (with 15 and 17) or give mostly a carboxonium-arenium dication (with 18). Substituent effects observed under stable ion conditions emphasize relative carbocation stability and relief of peri-strain. Under thermodynamic control, carboxonium ions undergo fluorosulfonation in the biologically important A-ring. Charge delocalizations in the resulting mono- and dications (deduced primarily based on magnitude of Δδ13C) are discussed and compared. In an effort to further enhance the NMR assignments and for comparison, mono-arenium ions 1H++, 4H++, 6H++, 7H++ and their neutral precursors were calculated at the B3LYP/6-31G(d,p) level of ab initio theory; their 1H and 13C NMR chemical shifts were computed by the GIAO method and their overall charge delocalization paths were deduced via differences in the NPA charges (cation minus neutral). The results are compared and discussed. Stable ion studies of CP[a]P provide useful insights into the contrasting regioselectivities observed in chemical and biological activiation. p

A Quantum Chemical and Electrostatic Study of Alkyl and Substituted Benzenonium Cations and Related Molecules: The Effect of Atomic Charge Distribution on Carbocation Energy and Geometry

Geometries, NMR chemical shifts, atomic charges, and bond orders were calculated at the MP2/6-31G* level of theory for the isopropyl, sec-butyl, tert-butyl, benzenonium, and six methyl- and trifluoromethyl-substituted benzenonium ions. The NPA charges correlate better than the CHELPG, MK, and AIM charges with the experimental and calculated carbon chemical shifts. The electrostatic energies of isomeric ions, calculated by treatment of the NPA charges as point charges, are comparable to their MP2 energy differences. The electrostatic calculations, although classical approximations, provide a qualitative understanding of the coulombic basis of the methyl and trifluoromethyl substituent effects. It is apparent that these species are stabilized by having adjacent, oppositely charged atoms (for example, having the negatively charged carbon of a methyl group bonded to a positive carbon). It is proposed that a methyl group be termed a nucleophilic (rather than electron-donating) substituent and that a trifluoromethyl group be termed an electrophilic substituent. In the benzenonium ions and in toluene, fluoromethylbenzene, difluoromethylbenzene, and benzotrifluoride, the endocyclic angle at the substituent increases with increasing electrophilicity of the substituent. An electrostatic calculation again provides some insight into the coulombic basis of these changes.

Superacid protonation of dihydrocyclobuta[e]pyrene and its C60–o-quinodimethane adduct. An NMR, ab initio/GIAO and AM1/PM3 study

Dihydrocyclobuta[e]pyrene 1 is protonated in FSO3H–SO2ClF to give a ca. 2∶1 mixture of two arenium ions of α-protonation 1H+ and 1aH+. Ab initio/GIAO theory at the B3LYP/6-31G(d,p) level was utilized to calculate the NPA charges, relative energies and to predict the NMR chemical shifts for 1H+ and 1aH+ as a means to enhance the solution NMR-based assignments. The resulting regioisomeric arenium ions are highly delocalized and exhibit a uniform charge alternation path within a pyrenium moiety. Reasonable predictions of their relative energies and the overall charge delocalization mode can also be obtained at the AM1/PM3 levels. In an attempt to determine the influence of C60 on tropicity in an anchored-pyrenium ion, the C60–o-quinodimethane adduct 2 was protonated in FSO3H·SbF5–SO2ClF; formation of regioisomeric bound-arenium ions (2H+, 2aH+) which are very poorly soluble in superacid media was inferred from 1H NMR and AM1/PM3 calculations were used to model these cations. Further model studies using a fragment of C60 bound to regioisomeric pyrenium ions were realized at the LSDA/DN level of theory with electrostatic potential derived charges at the B3LYP/6-31G(d,p) level.

(Hyper)polarizability of chalcogenophenes C 4H 4X ( X = O, S, Se, Te) Conventional ab initio and density functional theory study

Equilibrium geometries, APT and NPA charge densities, dipole moments, and linear and quadratic polarizabilities of the five-membered chalcogenophenes C 4H 4X ( X = O, S, Se, Te) were calculated by means of the all-electron conventional ab initio and density functional theory methods (S-null, B3-null, S-VWN and B3-LYP) using the specifically optimized Dougherty-Spackman and Sadlej basis sets. Very good agreement is found between theoretical and available experimental data. The results highlight the heteroatom and electron correlation effects on the molecular properties. Both linear and quadratic dipole polarizabilities increase steadily as the atomic number increases, the trend along the series being dominated by the heteroatom property and by the stability of the molecule. Possible correlations between dipole (hyper)polarizability and aromaticity, hardness, and electronic structure were investigated and briefly discussed. The exaltation polarizability parameter τ, a possible new aromaticity index, together with atomization energy evaluations, suggest that, in addition to molecular size effects, thermodynamic stability factors determine the average dipole polarizability values.

The Phosphonium Dibenzylide Anion as a Ligand in Organobarium Chemistry

The structural analysis of [Ph2P(4-methylbenzylide)2]2Ba represents that of the first organobarium compound not belonging to the cyclopentadienyl series. The structure consists of two C2-symmetric phosphonium ligands (of opposite chirality) encapsulating the Ba2+ ion. The Ba2+ ion is in bonding contact with the ylidic and benzylic ring carbon atoms. The benzylidic carbon adopts a planar sp2 bonding geometry, in the solid state as well as in solution. In solution, a dynamic process exchanges the two antipodes of the chiral dibenzylide ligands. Ab initio calculations and NPA charge analyses show that the di-ylide ligands in [Ph2P(4-methylbenzylide)2]2Ba should be described as species with highly polar P+−C- bonds. Calculations and detailed analysis of the benzyl ring geometries in the crystal structure show that the negative charge on the benzyl group is mainly localized on the ylidic carbon. The bonding geometries at P are distorted from tetrahedral geometry such as to increase the effect of C(lone pair) →...