Ab Initio RHF Research Articles

Comparative study of semiempirical methods for calculating interactions between large molecules with an application to the actinomycin-Guanine complex

A comparison of semiempirical variational and perturbation calculations of intermolecular interactions is presented. The results are compared with ab initio rhf calculations, ab initio perturbation calculations, and with experiments for systems representing hydrogen bonding, electron donor-acceptor complexes, incipient chemical reactions, and π-π stacking interactions. The failure of CNDO supermolecule calculations is discussed. Various approaches to a perturbation scheme using CNDO wave functions are investigated and an optimal method is suggested. The calculation of the stacking interactions in the deoxyguanosine actinomycin D complex suggests that a dual charge-transfer complex is formed. The experimental observation of two inequivalent binding sites on the actinomycin chromophore is interpreted in terms of the calculated energetics of complex formation.

Conformational analysis of six-membered cyclic carbonates

The conformational isomerization of 2-oxo-1,3-dioxane and its methyl analogs was investigated by means of ab initio RHF//6-31G(d,p) and MP2//6-31G(d,p) quantum-chemical methods. It is shown that in comparison with 1,3-dioxanes the potential energy surface of the mentioned compounds has a fewer number of stationary points and includes two minima corresponding to conformers of sofa or distorted sofa configuration and one maximum corresponding to 2,5-twist form. The value of potential barrier of interconversion of cyclic carbonates is significantly lower than that of the analogously substituted 1,3-dioxanes.

Natural bond orbital (NBO) population analysis of para-substituted S-Nitroso-thiophenols

Theoretical study of several para-substituted S-Nitroso-thiophenols has been performed using quantum computational ab initio RHF and density functional B3LYP and B3PW91 methods with 6-31G(d,p) basis set. Geometries obtained from DFT calculations were used to perform NBO analysis. It is noted that the weakness in the S N sigma bond is due to n O 1 → σ S 3 N 2 ∗ delocalisation and is responsible for the longer S N bond length in para-substituted S-Nitroso-thiophenols. It is also noted that decreased occupancy of the localized σ SN orbital in the idealized Lewis structure, or increased occupancy of σ SN ∗ of the non-Lewis orbital, and their subsequent impact on molecular stability and geometry (bond lengths) are related with the resulting p character of the corresponding sulfur natural hybrid orbital (NHO) of σ SN bond orbital. In addition, the charge transfer energy decreases with the increasing of the Hammett constants of subsitutent groups and the partial charge distribution on the skeletal atoms shows that the electrostatic repulsion or attraction between atoms can give a significant contribution to the intra- and intermolecular interaction.

Study of the structure and stability of aqua ions La(H2O) n 3+ (n = 8, 9) by ab initio methods

The structural and energetic characteristics of aqua ions La(H2O)83+ and La(H2O)93+. have been calculated by the ab initio RHF and MP2 methods with the use of different quasi-relativistic effective core potentials for the lanthanum atom.

Nonlinear optical properties of pyridinium-betaines of squaric acid: Experimental and theoretical study

The linear and nonlinear optical properties of six pyridinium-betaines of squaric acid were characterized by means of electro-optical absorption measurements (EOAM) in dioxane solution. All chromophores studied exhibit intense absorption bands in the visible region within 372–441 nm, accompanied with decrease in dipole moment upon excitation. The static hyperpolarizabilities of the NLO-phores studied depend strongly on the substituent in the pyridinium ring and increase significantly going from donor to acceptor substituent. Specifically, the noncentrosymmetrically crystallizing 4-benzoyl compound showed the highest static hyperpolarizability. These empirical trends were supported and verified by quantum-chemical calculations (DFT and ab initio RHF). Bulk crystal properties such as decomposition points within the high temperature range 480–665 K and improved thermal stability in melt demonstrate the advantage of these materials for nonlinear optical applications.

Anomeric and tautomeric equilibria in d-2-glucosamine Schiff bases

The structure of some glucosamine Schiff bases has been studied by means of ab initio RHF and DFT calculation and CP/MAS 13C and 15N NMR measurements. The anomeric and tautomeric equilibria in a DMSO solution have been studied by 1H, 13C and 15N NMR spectroscopy. The anomeric composition of d-2-glucosamine Schiff bases in the solid state and in DMSO solution has been shown to depends on the tautomeric form of Schiff bases and electronic properties of substituents on the aromatic ring.

Natural bond orbital (NBO) population analysis of cyclic thionylphosphazenes, [NSOX (NPCl 2) 2]; X = F ( 1), X = Cl ( 2)

Theoretical study of cyclic thionyl phosphazenes, [NSOX(NPCl 2) 2]; X = F ( 1), X = Cl ( 2) have been performed using quantum computational ab initio RHF and density functional B3LYP and B3PW91 methods (DFT) with the 6-31G(d,p) basis set implemented in the Gaussian-98 program suite. Geometries obtained from DFT calculations were used to perform NBO analysis by the NBO 3.1 program as implemented in Gaussian-98. The results obtained through these theoretical calculations revealed that decreasing occupancy of the localized σ SX orbital in the idealized Lewis structure, or increasing occupancy of σ SX ∗ of the non-Lewis orbital, and their subsequent impact on molecular stability and geometry (bond lengths) are related with the resulting p character of the corresponding sulfur natural hybrid orbital (NHO) of σ SX bond orbital. The natural population analysis (NPA) results reveal strong partial charges separation on the skeletal atoms of molecules 1 and 2 which can be contributed in the stabilization energies of the molecules using the most likely phosphazene bonding alternatives, ionic bonding and its complementary negative hyperconjugation due to interactions of filled NBOs with antibonding NBOs, determined by standard perturbative approach referred as E(2) energies. This approach approves anticipating that the electrostatic repulsion or attraction between atoms gives a significant contribution to the intra-and inter-molecular interactions.

On the Lack of Ring‐Current Aromaticity of (Heteroatom) [N]Radialenes and their Dianions

Current-density maps, calculated at the ab initio RHF//6-31G**/CTOCD-DZ level, show no significant pi ring current in planar equilateral geometries of neutral and dianionic [N]radialenes, oxocarbons and thiocarbons C(N)Y(N) (q-) (Y=CH(2), O, S; N=4, 5, 6; q=0 (1 a-12 a), 2 (1 b-12 b)). Only the N=3 deltate dianions C(3)Y(3) (2-) (Y=CH(2), O, S (1 b, 5 b and 9 b)) have discernible pi ring current, and then with at most 20-25 % of the strength of the standard benzene current. On the magnetic criterion, lack of current is definitive evidence against aromaticity. Pictorial molecular-orbital analysis within the ipsocentric approach shows this to be an inevitable consequence of the nodal structure of the pi and pi* orbitals of [N]radialene-like systems. On grounds of angular-momentum symmetry, spatial distribution, or both, the HOMO-LUMO excitation does not contribute a significant central diamagnetic ring current.

Effect of Polarity and Structural Design on Molecular Photorefractive Properties of Heteroaromatic‐Based Push–Pull Dyes

A combined experimental (optical and electro-optical absorption measurements) and computational (ab initio RHF and DFT) approach has been used to investigate the molecular low- and high-T(g) photorefractive (PR) performances of neutral and zwitterionic heteroaromatic dipolar chromophores in terms of structural and solvent-polarity effects. We have found that the nature of the building units (donor, acceptor, and spacer) and the polarity of the surrounding medium strongly affect all the relevant ground-state and nonlinear optical properties involved in the PR activity, that is, the dipole moment, the polarizability anisotropy, and first hyperpolarizability of the electronic ground-state. The variation of these properties is in turn transferred to molecular low- and high-T(g) PR figures of merit. It is shown that PR molecular performance not only relies on a proper choice of structural components but varies by orders of magnitude as a function of the medium polarity, and this suggests that a combination of molecular design and host-matrix engineering is required for optimized performances of PR materials.

Study of the structure and stability of aqua complexes Y(H2O) n 3+ (n = 1–10) by Ab initio methods

The structural and energetic characteristics of aqua complexes Y(H2O) n 3+ (n = 1–10) have been calculated by the ab initio RHF and MP2 methods.

Theoretical studies on vibrational spectra of some mixed carbonyl-halide complexes of Osmium(II)

The vibrational spectra of Os(CO)62+ and some of its mixed carbonyl-halide complexes, cis-Os(CO)2X42−, fac-Os(CO)3X3− and Os(CO)5X+ (X=F, Cl, Br and I), have been systematically investigated by ab initio RHF and density functional B3LYP methods with LanL2DZ and SDD basis sets. The calculated vibrational frequencies of complexes Os(CO)62+, cis-Os(CO)2X42− and fac-Os(CO)3X3− are evaluated via comparison with the experimental values. In infrared frequency region, the C–O stretching vibrational frequencies calculated at B3LYP level with two basis sets are in good agreement with the observed values with deviations less than 5%. In the far-infrared region, the B3LYP/SDD method achieved the best results with deviations less than 9% for Os–X stretching and less than 8% for Os–C stretching vibrational frequencies. The vibrational frequencies for Os(CO)5X+ that have not been experimentally reported were predicted.

Theoretical studies on vibrational spectra of some halides of group IVB elements

The vibrational spectra of group IVB elements halides MX 4 (M = Ti(IV), Zr(IV), Hf(II); X = F, Cl, Br and I), have been investigated by ab initio RHF, MP2 and density functional theory B3LYP method with LanL2DZ basis sets. The optimized geometries, calculated vibrational frequencies and Far-IR intensities of MX 4 are evaluated via comparison with experimental data. The vibrational frequencies, calculated by these methods, are compared to each other. The results indicate that B3LYP method is more reliable than RHF and MP2 methods for the frequencies calculations for these compounds. With this method, some vibrational frequencies of M 2X 6 2+(M = Ti(IV), Zr(IV) and Hf(II); X = F, Cl, Br and I) are also predicted.

Interaction of pyridine on Nb 2O 5

We have studied the interaction of pyridine on oxide surface in order to characterize the Lewis acid sites. Ab initio RHF method was employed at 3-21G* all electron basis set level. The LanL2DZ effective core potential was also used and compared with all electron basis set results. Cluster model of Nb 2O 5 was built using the crystallographic data. The optimized pyridine molecule was compared to the experimental free molecule showing small variation of ionization energies. The vibrational frequencies of adsorbed pyridine were calculated at RHF/3-21G* level. Population analysis was carried out using Mulliken, ChelpG and NBO schemes. The adsorbed pyridine geometry shows a small deviation of optimized bond angles and interatomic distances relative to free pyridine molecule. The Nb–N interatomic distance is almost 2.41 Å.

FTIR, Raman spectra and ab initio calculations of 2-mercaptobenzothiazole

FTIR and Raman spectra of a rubber vulcanization accelerator, 2-mercaptobenzothiazole (MBT), were recorded in the solid phase. The harmonic vibrational wavenumbers, for both the toutomeric forms of MBT, as well as for its dimeric complex, have been calculated, using ab initio RHF and density functional B3LYP methods invoking different basis sets upto RHF/6-31G** and B3LYP/6-31G** and the results were compared with the experimental values. Conformational studies have been also carried out regarding its toutomeric monomer forms and its dimer form. With all the basis sets the thione form of MBT (II) is predicted to be more stable than thiol form (I) and dimeric conformation (III) is predicted to be more stable with monomeric conformations (I) and (II). Vibrational assignments have been made, and it has been found that the calculated normal mode frequencies of dimeric conformation (III) are required for the analysis of IR and Raman bands of the MBT. The predicted shift in NH− stretching vibration towards the lower wave number side with the B3LYP/6-31G** calculations for the most stable dimer form (III), is in better agreement with experimental results. The intermolecular sulfur-nitrogen distance in N H···S hydrogen bond was found to be 3.35 Å from these calculations, is also in agreement to the experimental value.

Why Is Re−Re Bond Formation/Cleavage in [Re(bpy)(CO)3]2 Different from That in [Re(CO)5]2? Experimental and Theoretical Studies on the Dimers and Fragments

The Re(NN)(CO)3(THF) (NN=bpy=2,2'-bipyridine or dmb=4,4'-dimethyl-2,2'-bipyridine) radical, produced by homolysis of [Re(NN)(CO)3]2 in THF solution by visible irradiation, dimerizes with rate constants kd=20 +/- 3 and 11 +/- 4 M(-1) s(-1) for NN=dmb and bpy, respectively. The dimerization processes are strikingly slow compared to those of typical metal radicals including Re(CO)5 (kd approximately 10(9) M(-1) s(-1)). In order to explain such slow reactions, we have performed B3LYP hybrid DFT and fully ab initio RHF and MP2 calculations on several conformations of [Re(bpy)(CO)3]2 (cis, trans, skewed cis, skewed trans) and [Re(CO)5]2 (staggered) and on their constituent monomer radicals and anions. The calculations show that the most stable geometry of [Re(bpy)(CO)3]2 is skewed cis, and the experimental infrared spectrum and photochemical properties of the [Re(bpy)(CO)3]2 dimer are best described by the calculated properties of the skewed cis conformer in which there is no low-lying unoccupied orbital that is predominantly sigma(MM) in character. The Re(bpy)(CO)3(THF) ligand radical is more stable than the 5-coordinate "17-electron" metal radical, Re(bpy)(CO)3, suggesting that the extremely slow dimerization rate most likely arises from the solvent blocking the binding site (i.e., the estimated fraction of the five-coordinate monomer is 1.6 x 10(-2)). Theoretical results are consistent with our experimental results that the dimerization process proceeds via the Re centered radical, which is involved in a pre-equilibrium favoring the ligand-centered radical. Furthermore, time-dependent DFT calculations on [Re(bpy)(CO)3]2 and [Re(bpy)(CO)3]- identify the origin of UV-vis absorption in THF.

Weak C–H⋯O and C–H⋯π hydrogen bonds in crystal 1-indanone. An structural and spectroscopic analysis

The structure of 1-indanone has been studied by X-ray crystallography, vibrational analysis and theoretical calculations. The crystal structure (determined at 120 K) is stabilised by an extensive network of weak hydrogen bonds of the C–H⋯O and C–H⋯π type. The molecular structure has been optimised in the frame of the isolated molecule approximation using ab initio RHF and DFT/B3LYP and BLYP methods with an assortment of basis sets, i.e. 6-31G(d), 6-31G(d,p), 6-31++G(2d,p), 6-311G(2d,p) and 6-311++G(2d,p). This allows to compare the theoretical molecular structure and that obtained through X-ray diffraction in such a way that conclusions about the magnitude of the intermolecular interactions can follow. Furthermore, a proposal of assignment of the vibrational spectrum is given since the only early studies available so far are non-conclusive and incomplete. This proposal is assessed by a molecular force field which has been performed according to the SQM methodology starting from the theoretical B3LYP/6-31G(d) and BLYP/6-31G(d) force fields and using a set of experimental IR and Raman spectra.

Origin of the Endo/Exo Stereoselectivity and Syn/Anti Face-Selectivity in Diels–Alder Reactions, Determined by Transition State Energy Partitioning

Abstract The origins of stereo- and face-selective Diels–Alder reactions of 5-substituted 1,3-cyclopentadienes (CP) with some dienophiles were elucidated by means of semiempirical PM5, DFT(B3LYP), and ab initio RHF methods. The activation energies by the improved PM5 and B3LYP/6-31+G(d) methods were in good agreement with the experimental results. We partitioned the activation energy (ΔEact) into diene deformation (ΔEdf-dien) and dienophile deformation (ΔEdf-dphil), and the diene–dienophile interaction (Eint) energies, in addition to the new intrinsic reaction coordinate (IRC) energy (ΔEirc) partitioning. Such analysis revealed the major factors in the determining endo/exo and syn/anti selectivities of the reactions of 5-substituted CP. The syn-selectivity of the 5-F-CP is explained by the lower ΔEact (2–3 kcal/mol) caused by relatively smaller ΔEdf-dien, which leads to slighter repulsion between the reactants. This low activation energy is inferred to be caused by the better second orbital interaction or intramolecular orbital interactions between the diene π and fluoro-substituent n orbital moieties, as well as by better intermolecular π–π orbital interactions between the diene-π and dienenophile-π moieties. On the other hand, the anti-selectivity of the methyl substituted CP mainly appears from bigger Edf-dien in the syn-isomer. A novel economical method of IRC energy partitioning to the reactants, TS intermediates and products is also introduced in order to verify the energy balance of other calculation methods and other reactions.

Ab initio and DFT studies on vibrational spectra of some mixed carbonyl-halide complexes of ruthenium(II)

The vibrational spectra of Ru(CO) 6 2+ and some of its mixed carbonyl-halide complexes, cis-Ru(CO) 2X 4 2−, fac-Ru(CO) 3X 3 − and Ru(CO) 5X + (X = F, Cl, Br and I), have been systematically investigated by ab initio RHF and density functional B3LYP methods with LanL2DZ and SDD basis sets. The calculated vibrational frequencies of complexes Ru(CO) 6 2+, cis-Ru(CO) 2X 4 2− and fac-Ru(CO) 3X 3 − are evaluated via comparison with the experimental values. In the infrared frequency region, the C–O stretching vibrational frequencies calculated at B3LYP level with two basis sets are in good agreement with the observed values with deviations less than 5%. In the far-infrared region, the B3LYP/SDD method achieved the best results with deviations less than 8% for Ru–X stretching and less than 2% for Ru–C stretching vibrational frequencies. The vibrational frequencies for Ru(CO) 5X + that have not been experimentally reported were predicted.

Density functional theory and ab initio studies of geometry, electronic structure and vibrational spectra of novel benzothiazole and benzotriazole herbicides

The ground-state geometries, electronic structures and vibrational wavenumbers of S-1,3-benzothiazolyl-4-bromobenzenecarbothioate and S-(5,7-dimethyl-3H-4λ-5-[1,2,4]triazolo[1,5- a]pyridinyl)4-chlorobenzenecarbothioate were studied by DFT-B3LYP, BLYP and ab initio RHF method with different basis sets. The comparison was performed for optimized geometries, thermodynamic parameters and electronic structures at different levels of theory. Because of the larger repulsion effect in triazole ring, the vibrational wavenumbers of skeleton vibration of triazole ring are significantly lower than that of thiazole ring.

DFT and HF Studies of the Geometry, Electronic Structure, and Vibrational Spectra of 2-Nitrotetraphenylporphyrin and Zinc 2-Nitrotetraphenylporphyrin

The ground-state geometries and electronic structures of 2-nitrotetraphenylporphyrin (H2-2-NO2-TPP) and zinc 2-nitrotetraphenylporphyrin (Zn-2-NO2-TPP) with Cs and C1 symmetry have been determined from density functional theory, using the Becke−Lee−Yang−Parr composite exchange-correlation functional (B3-LYP) and ab initio RHF method and 6-31G(d) basis set. The optimized geometries are then compared with the crystallographic data of related compounds. The energy and electronic structures of different conformers are analyzed and compared with each other. The conformers with C1 symmetry are found to be more stable than that of Cs symmetry. The relative order of the highest occupied a2u and a1u orbitals determined by B3LYP (a2u > a1u) is reversed by RHF (a1u > a2u). The vibrational wavenumber, IR, and Raman intensities are also calculated at B3LYP/6-31G(d) optimized geometries. The calculated wavenumbers are scaled by a uniform scaling factor and compared with the experimental one. Most of the scaled modes are found to be in good agreement with the observed fundamentals. A single β-NO2 substitution slightly changes the geometries, the vibrational wavenumbers, and the frontier orbitals energy level.