RHF Levels Research Articles

π-Extended o-quinoidal tropone derivatives: experimental and theoretical studies of naphtho[2,3-c]tropone and anthro[2,3-c]tropone

pi-Extended o-quinoidal tropone derivatives, naphtho[2,3-c]tropone (3) and anthro[2,3-c]tropone (4), have been investigated theoretically as well as experimentally. The geometrical optimization of 3,4 and related compounds at the B3LYP level employing 6-31G* basis set as well as the GIAO calculations at the RHF level employing the 6-31+G* basis set have been performed to evaluate the contributions of the polarized resonance forms to these molecules. The GIAO calculated NICS(1) values indicate that the aromaticities of the tropone rings of o-quinoidal 3 and 4 are significantly increased as compared with that of parent tropone (1) at the expense of the fused benzenoid rings, consistent with the significant electronic polarization of these molecules in the ground state. On the other hand, the fusion of a benzene or naphthalene ring to the 2,3- or 4,5-position of tropone leads to diminution of aromaticity in the resulting tropone moiety. Experimentally, irradiation of 6,7-(2',3'-naphtho)bicyclo[3.2.0]hepta-3,6-dien-2-one (10) in a rigid glass at -196 degrees C leads to the formation of, which exhibits a characteristic UV-Vis absorption extending to 700 nm and undergoes rapid [pi 12 + pi 14] dimerization upon thawing the glass. In contrast, 6,7-(2',3'-anthro)bicyclo[3.2.0]hepta-3,6-dien-2-one (11) showed no sign of isomerization to 4 under the same reaction conditions.

Hydrolysis of cyclic phosphates by ribonuclease A: a computational study using a simplified ab initio quantum model.

The second step in the enzyme-catalyzed hydrolysis of phosphate esters by ribonuclease A (RNase A) was studied using an ab initio quantum-based model of the active site including constrained parts of three critical residues, His-12, His-119, and Lys-41, and a small substrate. The competition between release of the cyclic phosphate intermediate and subsequent hydrolysis following transphosphorylation was explored to determine the electronic factors that contribute to preferential intermediate product release observed experimentally. The structural and energetic results obtained at both the RHF and MP2 levels reveal several contributing factors consistent with experimental observation. Although the intrinsic electronic effects tend to favor hydrolysis slightly with an overall activation free energy of approximately 70 kJ mol(-1), entropic and environmental effects favor release of the cyclic phosphate intermediate over hydrolysis. Exploration of the second, hydrolysis step also revealed interesting similarity with the transphosphorylation step, including the observation of autocatalysis by the substrate. Moreover, both steps of the overall RNase A reaction reveal multiple pathways involving proton transfers to sites of similar proton affinities. The anionic phosphate in both steps can act as a stable proton binding site as protons are moved around the active site throughout the progress of the reaction. These results suggest autocatalysis may be representative of more general behavior in enzymes containing highly charged substrates, especially phosphates.

Basis set effects on the intermolecular interaction of the F 2–F 2 system

Intermolecular potential energy surface (IPS) for F 2–F 2 system has been examined using RHF, MP2 and DFT-B3LYP methods. A number of basis sets from the double-zeta and triple-zeta family were used in order to evaluate the basis set effects. These effects vary with the level of theory used. Counterpoise (CP) correction has been used to show the extent of the basis set superposition error (BSSE) on the potential energy curves obtained for F 2–F 2 system. CP corrections revealed that B3LYP and RHF levels of theory predict a totally repulsive interaction between the two monomers of F 2–F 2 system. The deepest BSSE-corrected potential well have been obtained at MP2 level of theory with 6-31G* basis set. At RHF and B3LYP levels of theory the least repulsive BSSE-corrected potential have been obtained with 6-31G* basis set. Effects of basis set on the characteristics of the calculated IPS have also been analyzed based on the position of the potential minimum and its well depth as well as its corresponding hard sphere collision diameter.

Estimation Of The 79Br Nqr Frequencies Of Bromo-Containing Molecules Using Ab Initio Calculations At Different Levels

Ab initio calculations of bromo-containing molecules on the RHF, B3LYP and MP2 levels and 6−31G(d), 6−31+G(d), 6−311G(d) and 6311+G(d) basis sets were executed. They were used to estimate the 79Br NQR frequencies of these molecules. A satisfactory agreement between experimental and estimated NQR frequencies is obtained for the sum of populations of 13p- and 14p-components of the Br atom valence p-orbitals obtained from the RHF, B3LYP and MP2 calculations (particularly from RHF calculations) with the split valence basis sets 6−311G(d) and 6−311+G(d). The agreement between the experimental and estimated NQR frequencies is worse for the populations of the 9p-components of the Br atom valence p-orbitals obtained from these calculations with the basis sets 6−31G(d) and 6−31+G(d). An analogous conformity was not obtained using the populations of other components of the Br atom valence p-orbitals or their total populations obtained from all above-mentioned calculations.

Structural parameters, barriers to internal rotation, normal coordinate analysis and quantum mechanics calculations of 1,1,1-trimethyldisilane

The molecular structure, vibrational frequencies and barriers to internal rotation of the staggered and eclipsed conformations of 1,1,1-trimethyldisilane, H 3SiSi(CH 3) 3, have been studied using ab initio molecular orbital calculations. In these calculations, up to 6-311+G(d,p) basis set at the levels of RHF and MP2 with full electron correlation were employed as well as the hybrid density functionals at the theoretical level of B3LYP. From these data, the energy difference between the staggered and eclipsed conformations has been determined with an average of 318 cm −1 (0.91 kcal/mol) in favor of the staggered rotamer, while, the eclipsed form has been barred due to the presence of an imaginary torsion frequency. The –SiH 3 and –CH 3 barriers of 1.05 and 1.98 kcal/mol, respectively, have been predicted from potential function scans at MP2/6-31(d) basis set with full electron correlations. The r o Si–H bond distance of 1.4885 Å has been obtained from the observed Q-branch in the gaseous infrared spectrum. The calculated infrared and Raman spectra were obtained using harmonic vibrational frequencies, infrared intensities and Raman activities from RHF/3-21G(d) in addition to MP2 and B3LYP levels at 6-31G(d) basis set. The current study provides a complete vibrational assignment for all the vibrational modes, which is supported by normal coordinate analysis, force constants and potential energy distributions. The results are discussed and theoretical values are compared to experimental values whenever possible.

Rearrangement of the Cyclohexadiene Derivatives of C60 to Bis(fulleroid) and Bis(methano)fullerene: Structure, Stability, and Mechanism.

The cyclohexadiene derivative of C60 rearranges photochemically to bis(fulleroid) (two [6,5] open structure) and bis(methano)fullerene (two [6,6] closed structure). During this process, a [6,5] open/[6,6] closed intermediate is observed. The isolated intermediate undergoes photochemical rearrangement to bis(fulleroid) and bis(methano)fullerene. On the other side, it undergoes retrorearrangement to the starting material in the dark. The structure and energetics of these C60 derivatives have been studied at the AM1, PM3, RHF, and B3LYP levels of theory. It is found that bis(fulleroid) bearing four tert-butoxycarbonyl substituents is 5.8 kcal/mol (B3LYP) more stable than the corresponding bis(methano)fullerene. The isolated intermediate having the [6,5] open/[6,6] closed structure is 6.7 kcal/mol more favorable than the previously proposed two [6,5] closed intermediate, and the formation of this compound is well explained by the di-π-methane rearrangement. 13C NMR calculation at the B3LYP level reproduced th...

Experimental and Theoretical Studies of Acid−Base Equilibria of Substituted 4-NitropyridineN-Oxides

By using the potentiometric titration method, acidity constants in the polar aprotic solvent acetonitrile (in the form of values) of cations obtained by protonation of 13 substituted 4-nitropyridine N-oxides and cationic homoconjugation constants (KBHB+) of the cationic acids conjugated with the N-oxides studied have been determined. A correlation has been established between the tendency toward cationic homoconjugation (expressed as log KBHB+) and the basicity of the N-oxides in acetonitrile ( ). Further, by using ab initio methods at the RHF and MP2 levels utilizing the Gaussian 6-31G* basis set, energies and Gibbs free energies have been determined of protonation and formation of homocomplexed cations stabilized by O···H···O bridges in the gas phase. The calculated protonation energies, ΔEprot, and Gibbs free enthalpies, ΔGprot, in vacuo have been found to correlate well with the acid dissociation constants (expressed as values) of protonated N-oxides, whereas the calculated energies, ΔEBHB+, and Gibbs free energies, ΔGBHB+, of homoconjugation do not correlate with the cationic homoconjugation constant values determined in acetonitrile.

A theoretical study of the ligand effect of thiobisphenoxytitanium complex catalyst on the catalytic activity for ethylene polymerization

The effect of ortho-substituents (R=methyl, tert-butyl and triisopropylsilyl) of phenoxy groups in thiobisphenoxytitanium complex catalyst on the catalytic activity of ethylene polymerization was analyzed by applying ab initio molecular orbital theory and density functional theory. The calculation results revealed that the activation energy values of ethylene insertion reaction into a methyltitanium bond of thiobisphenoxytitanium complexes with substituents on the ortho position of phenoxy ligands at the RHF level are in good correspondence with the tendency of ethylene polymerization activities. The activation energy values at the B3LYP/DZ, RHF/LANLL2DZ and B3LYP/LANL2DZ//RHF/DZ levels also show similar tendency. While the differences of the molecular weight of polyethylene prepared with these bisphenoxy complexes could not be explained from the activation energy values of β-H elimination reaction and the chain transfer reaction to monomer. The activation energy of ethylene insertion reaction is supposed to control the molecular weight of polyethylene.

Rearrangement of the cyclohexadiene derivatives of C60 to bis(fulleroid) and bis(methano)fullerene: structure, stability, and mechanism.

The cyclohexadiene derivative of C(60) rearranges photochemically to bis(fulleroid) (two [6,5] open structure) and bis(methano)fullerene (two [6,6] closed structure). During this process, a [6,5] open/[6,6] closed intermediate is observed. The isolated intermediate undergoes photochemical rearrangement to bis(fulleroid) and bis(methano)fullerene. On the other side, it undergoes retrorearrangement to the starting material in the dark. The structure and energetics of these C(60) derivatives have been studied at the AM1, PM3, RHF, and B3LYP levels of theory. It is found that bis(fulleroid) bearing four tert-butoxycarbonyl substituents is 5.8 kcal/mol (B3LYP) more stable than the corresponding bis(methano)fullerene. The isolated intermediate having the [6,5] open/[6,6] closed structure is 6.7 kcal/mol more favorable than the previously proposed two [6,5] closed intermediate, and the formation of this compound is well explained by the di-pi-methane rearrangement. (13)C NMR calculation at the B3LYP level reproduced the experimental chemical shifts with very good accuracy for each molecular system. Theoretical studies mainly at the unrestricted B3LYP level on singlet and triplet state potential energy surfaces on fullerene derivatives support the di-pi-methane rearrangement mechanism. The previously proposed symmetrical [4+4]/[2+2+2] and the novel proposed unsymmetrical di-pi-methane pathways may coexist during the reaction.

Ab initio and DFT study of carbon monoxide cyclic oligomers, (CO) 2 to (CO) 6

Structural, thermochemical stability, electrical and molecular orbital properties of (CO) n cyclic oligomers of CO (oxocarbons) have been calculated using RHF and DFT-B3LYP levels of theory with 6-31G* basis set. Contributions of these compounds in the macroscopic properties of solid CO have been investigated. Atoms in molecule (AIM) and NBO analyses have been carried out to study in detail the nature of the C–C bonds in these molecules. Population analysis has been used to search for any possible aromaticity in the rings of these cyclic oxocarbons. AIM and population analysis showed that ring bonds in the (CO) 3 molecule have an aromatic character significantly more than those in other molecules do. The results of this study showed that (CO) n cyclic oligomers do not contribute significantly to the physicochemical properties of CO in the solid phase due to being thermodynamically instable at standard pressure and temperature.

Theoretical studies on vibrational spectra of mixed cyanide–halide complexes of platinum(IV) and palladium(IV)

The vibrational spectra of mixed cyanide–halide complexes, M(CN) 4X 2 2− and M(CN) 5X 2− (M=Pt and Pd; X=F, Cl, Br and I), have been systematically investigated by ab initio RHF, B3LYP and MP2 methods with LanL2DZ and SDD basis sets. The calculated vibrational frequencies of platinum complexes are evaluated via comparison with the experimental values. In the infrared frequency region, the CN stretching vibrational frequencies calculated at B3LYP level with two basis sets are in good agreement with the observed values with deviations, −16–4 cm −1 for Pt(CN) 4X 2 2− and −18 to −2 cm −1 for Pt(CN) 5X 2−. However, in far-infrared region, the results obtained at RHF level are better than those calculated at B3LYP and MP2 levels. For RHF/SDD method, the deviations for PtX and PtC stretching vibrational frequencies are −14–1 and −12 to −2 cm −1 in the complex Pt(CN) 4X 2 2−, −19 to −11 and −15–14 cm −1 in the Pt(CN) 5X 2− complex, respectively. The vibrational frequencies of palladium(IV) and some platinum(IV) complexes that have not been experimentally reported are predicted.

Estimation of the 35Cl NQR Frequencies of Some Organic and Organometallic Molecules Using ab initio Calculations at Different Levels and Basis Sets

Ab initio calculations of organic and organometallic molecules at RHF, B3LYP and MP2 levels and 6-31G(d), 6-31+G(d), 6-311G(d) and 6-311+G(d) basis sets were executed. They were used to estimate the 35Cl NQR frequencies of these molecules. A satisfactory agreement between experimental and estimated NQR frequencies was obtained for the populations of the less diffuse 3p-components of the Cl atom valence p-orbitals obtained from the RHF, B3LYP and MP2 calculations with the split valence basis sets 6-31G(d) and 6-31+G(d). An analogous conformity was not obtained using the 6-311G(d) and 6-311+G(d) basis sets.

Synthesis and characterization of the SO(2)N(3)(-), (SO(2))(2)N(3)(-), and SO(3)N(3)(-) anions.

SO(2) solutions of azide anions are bright yellow, and their Raman spectra indicate the presence of covalently bound azide. Removal of the solvent at -64 degrees C from CsN(3) or N(CH(3))(4)N(3) solutions produces yellow (SO(2))(2)N(3)(-) salts. Above -64 degrees C, these salts lose 1 mol of SO(2), resulting in white SO(2)N(3)(-) salts that are marginally stable at room temperature and thermally decompose to the corresponding azides and SO(2). These anions were characterized by vibrational and (14)N NMR spectroscopy and theoretical calculations. Slow loss of the solvent by diffusion through the walls of a sealed Teflon tube containing a sample of CsSO(2)N(3) in SO(2) resulted in white and yellowish single crystals that were identified by X-ray diffraction as CsSO(2)N(3).CsSO(3)N(3) with a = 9.542(2) A, b = 6.2189(14) A, c = 10.342(2) A, and beta = 114.958(4) degrees in the monoclinic space group P2(1)/m, Z = 2, and Cs(2)S(2)O(5).Cs(2)S(2)O(7).SO(2), respectively. Pure CsSO(3)N(3) was also prepared and characterized by vibrational spectroscopy. The S-N bond in SO(2)N(3)(-) is much weaker than that in SO(3)N(3)(-), resulting in decreased thermal stability, an increase in the S-N bond distance by 0.23 A, and an increased tendency to undergo rotational disorder. This marked difference is due to SO(3) being a much stronger Lewis acid (pF(-) value of 7.83) than SO(2) (pF(-) value of 3.99), thus forming a stronger S-N bond with the Lewis base N(3)(-). The geometry of the free gaseous SO(2)N(3)(-) anion was calculated at the RHF, MP2, B3LYP, and CCSD(T) levels. The results show that only the correlated methods correctly reproduce the experimentally observed orientation of the SO(2) group.

The structure and spectra of 1,1-bis(trimethylsilylethynyl)-cyclopropane: a priori calculated force field and vibrational effects

The molecular structure of 1,1-bis(trimethylsilylethynyl)cyclopropane has been studied by the gas electron diffraction method, by vibrational spectroscopic methods and by ab initio calculations at the RHF and MP2 levels. A scaled quantum-chemical force field was used for band assignment in the experimental IR (4000–100 cm −1) and Raman (4000–200 cm −1) spectra. The root-mean-square vibrational amplitudes and harmonic shrinkage corrections were calculated taking into account non-linear relations between Cartesian and internal vibrational coordinates at the level of first-order perturbation theory ( h1) and with the use of the traditional scheme ( h0).

Ab initio study of the mechanism of singlet-dioxygen addition to hydroxyaromatic compounds: negative evidence for the involvement of peroxa and endoperoxide intermediates.

In this article we report our study of two possible mechanisms of photooxidation of hydroxyaromatic compounds, involving the intermediacy of zwitterionic peroxa intermediates or 1,4-endoperoxides. To study the pathway of the first of them, as yet unexplored by theoretical methods, a simpler system composed of 1,3-butadiene-1-ol and singlet ((1)Delta(g)) dioxygen was considered first, for which calculations were carried out at the CASSCF/MCQDPT2 ab initio level, mostly with the 6-31G* basis set. The cumulative activation barrier to this reaction was found to be 20 kcal/mol and corresponded to a proton transfer (from the hydroxy oxygen atom to the attached oxygen molecule) in the cyclic zwitterionic peroxacyclopenta-3-ene-2-ol intermediate. This intermediate and the proton-transfer transition state were found to have a closed-shell character, which enabled us to estimate the corresponding activation barrier for the phenol-dioxygen system by carrying out optimization at the RHF level and single-point calculations at the MP2, CASSCF, and MCQDPT2 levels of theory. The energy barrier to the reaction was estimated to at least about 40 kcal/mol, rendering this mechanism for the phenol-oxygen system unlikely for nonpolar solvents. Similarly, calculations of the barrier to proton transfer from the 1,4-endoperoxide of phenol to its hydroperoxide were found to exceed 60 kcal/mol, eliminating such a mechanism too, which leaves only the earlier postulated mechanisms involving an initial charge or hydrogen-atom transfer to dioxygen as probable.

High Level ab Initio Study of Thermal 1,3-Sigmatropic Shift in CH2CHCH2X with X = BH2, NH2, and CH3

The thermal 1,3-sigmatropic migrations in X-CH2−CHCH2 with X = BH2, NH2, and CH3 have been investigated at the RHF, MP2, B3LYP, G3, and CBS-APNO levels using basis sets with added diffuse and polarization functions. In all three cases, the suprafacial allowed path proceeds through a TS in which the CC π electron is delocalized into the 2p AO in the migrating X. For X = BH2 with a vacant 2p orbital, the activation enthalpy is less than 1.0 kcal mol-1 (CBS-APNO) so that the 1,3-shift is almost barrierless. For X = NH2 there are two suprafacial pathways, one of which (higher energy path a) proceeds by participation of the lone pair on N. The lower energy pathway has a barrier height (ΔH⧧ = 68.9 kcal mol-1) of only 1.0 kcal mol-1 below the bond dissociation energy of the C1−N bond, so that a diradical pathway can compete with the concerted sigmatropic shift. For X = CH3, the activation barrier (ΔH⧧ = 76.1 kcal mol-1) is higher (by ca. 1 kcal mol-1 at the CBS-APNO level) than the bond dissociation energy of the C1−X(CH3) bond, so that the diradical reaction path is favored. The antrafacial 1,3-shifts are forbidden in all cases as evidenced by the second-order saddle-points (with two imaginary frequencies) and much higher barrier heights than those for the corresponding suprafacial shifts.

A Computational and Experimental Study on the Relative Stabilities of Cis and Trans Isomers ofN-Alkylamides in Gas Phase and in Solution

The cis/trans ratio of N-methylformamide (NMF), N-methylacetamide (NMA), and N-(2-endo-norbornyl)formamide (NNF) in dilute solutions in chloroform, methanol, and water have been experimentally measured by 1H NMR. Unlike NMA and NMF, the cis/trans ratio of NNF is sensitive to the solvent environment. To understand the anomalous behavior of NNF, we have performed calculations of ΔG and other observables between the cis and trans isomers of NMA, NMF, and NNF both in gas phase and in solution using several ab initio and DFT methods and SCRF models. The strikingly predominance of the trans isomers of N-alkyl substituted formamides can be rationalized in terms of close contact interactions between the formyl group and the C−H bond of the N-alkyl group. The molecular geometries and dipole moments of the studied amides using the RHF and B3LYP theoretical levels and split valence sets are in good accordance with the experimental values. The rotational barriers (ca. 7.0 kcal/mol) around the C2−N bond of cis- and tr...

A New Family of Supramolecular Complexes with 3D Cationic Hg/Z Frameworks, SnX3− Guest Anions (Z = P, As, Sb; X = Cl, Br, I): Crystal Structures and Host−Guest Interactions

Five new inorganic supramolecular complexes, [Hg6P4Cl3](SnCl3) (I), [Hg6As4Cl3](SnCl3)Hg0.13 (II), [Hg6As4Br3](SnBr3) (III), [Hg6Sb4I3](SnI3)Hg0.16 (IV), and [Hg7P4Br3](SnBr3) (V), have been prepared and their structures determined by X-ray single crystal experiments. They all crystallize in the cubic space group P213 (No 198) with Z = 4, and unit cell parameters a = 11.865(1) (I), 12.233(1) (II), 12.383(1) (III), 13.285(2) (IV), and 12.490(1) A (V). The crystal structures of these compounds are composed of the 3D positively charged frameworks [Hg6Z4X3]+ (I−IV) or [Hg7P4Br3]+ (V), with SnX3− anions trapped in the larger cavities of the frameworks (X = Cl, Br, I; Z = P, As, Sb). In the structures of II and IV the smaller cavities are partly filled by mercury atoms. The structures of I−V and of previously reported [Hg7As4I3](SnI3) (VI) are discussed with respect to the matching and mutual adjusting of the host frameworks and guest anions. A comparison of the observed and calculated (ab initio, RHF level) geometry of the SnX3− anions is used to analyze the weak host−guest interactions.

Electronic structures and properties of some charge transfer complexes: usefulness of hybridization displacement charge

Geometries of the molecules HCN, CH3CN, NH3, SO3 and those of the complexes HCN–SO3, CH3CN–SO3 and NH3–SO3 were optimized at the RHF/6-31G∗∗ and MP2/6-31G∗∗ levels. Partial geometry optimization calculations were also carried out in each case at the RHF level for different fixed N–S distances. Hybridization displacement charges (HDC) and molecular electrostatic potential-derived CHelpG charges at the atomic sites were obtained at the RHF and MP2 levels. Dipole moments and values of charge transfer (CT) (from each of HCN, CH3CN and NH3 to SO3) obtained using HDC point charges vary with N–S distance in the same way as those obtained using CHelpG point charges at the RHF level. Further, the calculated values of CT as obtained using both the CHelpG charges and HDC decrease with increasing N–S distance. The experimental and the present calculated values of CT in NH3–SO3 at the optimized and experimental N–S distances agree quite well while the calculated values of CT in HCN–SO3 and CH3CN–SO3 help resolve an existing ambiguity in this context. The locations of the different HDC components with respect to the corresponding atoms have been obtained. The calculated results indicate that the complexes are bound largely by CT.

Peptide Models XXXVIII. Proline conformers from X-ray crystallographic database and from ab initio computations

A systematic comparison is made between experimental and computational data gained on l-proline in peptides. Data on 214 XPY type amino acid sequence units taken from the Cambridge Structural Database as well as ab initio calculations on the model peptide HCO-l-Pro-NH2 at the 3-21G and 6-31+G(d) RHF levels of theory are presented. Cis–trans isomerism, backbone conformation and ring puckering are studied. Ring puckers are characterized by pseudorotational coordinates. A convenient nomenclature of conformers is introduced. Population data gained directly from statistics and derived from energy calculations are also discussed.