RHF Methods Research Articles

Aggregation Models of Potential Cyclical Trimethylsulfonium Dicyanamide Ionic Liquid Clusters

Raman and infrared spectra of trimethylsulfonium dicyanamide [(CH(3))(3)SN(CN)(2)] are reported and accurately reproduced by DFT methods (B3LYP and B3PW91), MP2, and MP3, and to a lesser extent by the RHF method. The (CH(3))(3)SN(CN)(2) ionic liquid forms two isomeric dimers that are of cyclic structure, one of which is 13 kcal/mol lower in energy than the other. Both isomeric cyclic pairs (versions 1 and 2), [(CH(3))(3)SN(CN)(2)](2), have the potential to further combine and form a common structure containing four pairs of (CH(3))(3)SN(CN)(2). This structure can then conceivably undergo a stacking procedure to form extended ionic liquid nanotubes of eight ionic liquids, [(CH(3))(3)SN(CN)(2)](8). The possible formation of gas phase ionic liquid clusters of two, four, and eight trimethylsulfonium dicyanamide ionic liquids is supported by highly exergonic free energy changes obtained from B3LYP/(6-311+G(d,p)) density functional calculations.

Molecular structure of 2-methylamino-5-[(5-methyl-2-benzoxazolinone-3-yl)methyl]-1,3,4-thiadiazole dihydrophosphate: a combined X-ray crystallographic and ab initio study

Crystal structure of the compound entitled 2-methylamino-5-[(5-methyl-2-benzoxazolinone-3-yl)methyl]-1,3,4-thiadiazole dihydrophosphate is determined using X-ray analysis and compared with the structure obtained from semiempirical and RHF methods at various levels of theory. RHF/6-31G(d) calculations offer the best conformity with X-ray results for bond lengths and bond angles. Moreover, at the result of the comparison of various combinations of basis sets and methods, it appears that there is not much gain in accuracy by using sophisticated methods.

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.

Overtone spectroscopy of chlorine substituted benzene derivatives

The absorption spectra of di-, tri- and tetra-derivatives of chlorobenzene have been studied in their pure form in the spectral range 400–20,000 cm −1. A large number of bands associated with the fundamental, the overtones and the combination frequencies of C H stretching mode have been observed. Vibrational frequencies, anharmonicity constants and dissociation energies, for the C H stretch vibrations for the six molecules have been determined using local mode model. The C H stretch frequencies obtained from experiments are compared with the corresponding frequency determined theoretically using RHF and DFT methods with same 6–31 + G* basis set. This information has been used for the assignment of several combination bands as well as some weak overtone bands. Effect of hydrogen atom substitution by chlorine atom has been studied by measuring changes in the vibrational frequency of the C H stretching mode and the C H bond length. Frequency changes have been well correlated with the change in charge density on carbon as well as chlorine atoms.

Theoretical and experimental study of molecular structure and vibrational spectra of N-(2-pyridylmethyl)-2-pyrazinecarboxamide

N-(2-Pyridylmethyl)-2-pyrazinecarboxamide was prepared and its crystal structure was investigated by X-ray analysis. The compound crystallizes in the triclinic space group \(P{\bar 1}\) with a = 4.262(3), b = 12.117(9), c = 20.840(18) A, α = 91.802(6), β = 89.834(7), γ = 91.845(6)°, V = 1075.2(16) A3, Z = 4, and D = 1.323 Mg/m3. The structure was solved by direct method and refined to R = 0.0699 and wR 2 = 0.1268 by full matrix anisotropic least-squares method. Using the Hartree-Fock and density functional method (B3LYP) with 6-31G(d) basis set, the molecular geometry and vibrational frequencies of the title compound has been investigated and compared with experimental ones from experimental studies. The optimized bond lengths obtained by RHF method and bond angles obtained by B3LYP method show better agreement with the experimental values. The vibrations computed of the title compound by the RHF and DFT methods are in good agreement with the observed IR spectra data.

Analytic gradients for the multicentred integrated QM:QM method for weakly bound clusters: efficient and accurate 2-body:many-body geometry optimizations

In this work we present analytic gradients for the multicentred integrated QM:QM method for weakly bound clusters that was recently introduced [Chem. Phys. Lett. 427,185 (2006)]. By treating pairwise interactions as overlapping model systems, this integrated QM:QM approach essentially reduces the computation on a cluster into a 2-body:many-body problem. Consequently for a cluster composed of n fragments (or monomers), the high-level computations only need to be performed on at most n monomers and n(n − 1) / 2 pairs of fragments (or ‘dimers’). The analytic gradients introduced here have been implemented in a stand-alone interface to the MPQC ab initio software package and subsequently applied to the geometry optimization of 15 different hydrogen bonded clusters of hydrogen fluoride, water and methanol. Structures were optimized with the RHF and MP2 methods as well as the MP2:RHF multicentred integrated QM:QM method. Geometrical parameters and energetics obtained with the MP2:RHF procedure are nearly identical to those from conventional MP2 computations despite the fact that the RHF results differ substantially from the MP2 target values. For example, the MP2:RHF dissociation energies are within 0.51kJmol− 1 (or 0.4%) of the MP2 results on average, while RHF always underestimates the MP2 dissociation energy by at least 12.86kJmol− 1 (or 20.4%). Similarly, the RHF hydrogen bond lengths are typically about 0.14Å too long while the MP2:RHF hydrogen bond lengths deviate from the MP2 values by only 0.0028Å on average.

Local spins: An alternative treatment for single determinant wave functions

The standard formula for the expectation value 〈 S ˆ 2 〉 of the total spin operator for single determinant wave functions is transformed to a form in which it appears a physically reasonable sum of atomic and diatomic contributions. The result obtained is much similar to that of previous treatments of local spins, but is free from their difficulty: it does not assign any spins to the atoms in molecules treated by the closed shell RHF method.

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.

Popular Theoretical Methods Predict Benzene and Arenes To Be Nonplanar

Planar (D6h) benzene has one (1181i cm-1, b2g) and three (1844i cm-1, b2g; 462i cm-1, e2u) imaginary vibrational frequencies at the MP2/6-311++G(d,p) and MP2/6-311++G levels of theory, respectively! The spurious frequencies correspond to D3d chair (b2g) and C2v boat (e2u) out-of-plane distortions. Numerous, similar examples where planar benzene is not a minimum are documented at the MP2, MP3, and CISD levels with popular Pople-type basis sets, while the RHF, B3LYP, and BLYP methods exhibit no such problems. We show that, in the sp and spd atomic-orbital limits of MP2 theory, benzene is nonplanar. The observed failure of electron correlation methods with unbalanced basis sets to predict planar minima is not unique to benzene but is also found for other pi-delocalized molecules, including pyridine, naphthalene, anthracene, the cyclopentadienyl and indenyl anions, and the tropylium cation. Detailed mathematical analysis reveals that an insidious, geometry-dependent, two-electron basis set incompleteness error (BSIE) is responsible for the problem and that balanced, correlation-consistent constructions of basis sets are generally required to ensure reliable predictions for arenes with correlated wave functions.

Anharmonic analysis of the vibrational spectra of some cyanides and related molecules of astrophysical importance

A detailed analysis of the vibrational spectra of carbonyl cyanide, diethynyl ketone and acetyl cyanide has been conducted in harmonic and anharmonic approximations. RHF, MP2 and density functional theory (DFT) methods with 6-311++G(2df,2p) basis sets and B3LYP functionals have been employed. Spectroscopic constants such as anharmonicity constants, rotational and centrifugal distortion constants, rotation–vibration coupling constants and Coriolis coupling coefficients have been calculated for each molecule and compared with the experimental data, where available. A close agreement between the calculated and experimental values of the spectroscopic constants has been obtained. Complete assignments have been provided to the fundamental bands, overtones and combination tones of the molecules. Density functional theory based anharmonic frequencies compare well with the experimental frequencies within ±18cm−1 on an average. RHF and MP2 methods, however, give much higher values for the frequencies that need scaling even in the anharmonic approximation.

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.

Conformational, structural analysis and vibrational spectra of a new carbacylamidophosphate compound: Experimental and theoretical study

N, N′-bis(pyrolidino), N″-trichloroacetyl phosphoric triamide, the new carbacyclamidophospate, was synthesized and its crystal structure has been determined by X-ray single crystal diffraction analysis. The crystal structure showed two independent conformers A and B. This compound produced dimmeric aggregate due to hydrogen bonding. The compound crystallizes in orthorhombic space group Pbca with a=12.033(2) Å; b=20.148(3) Å; c=24.876(3) Å; V=6030.7(13) (Å) 3, Z=8; and D=1.538 mg/m 3. The structure was solved by direct methods and refined to R=0.0657 and wR 2=0.1415 by full matrix anisotropic least-squares methods. The molecular geometry and vibrational frequencies of the compound in the ground state have been calculated using the Hartree-Fock with HF/6-31+G* and HF/6-311++G** and density functional method (B3LYP) with B3LYP/6-31+G* and B3LYP/6-311++G** basis set. The optimized bond lengths and bond angles are show better agreement with the experimental values by RHF/6-311++G** method. The conformer A is approximately 6 kcal/mol more stable than conformer B. The energy profile was calculated for more stable structure, conformer A, by RHF/6-31+G* method as a function of θ° [O(1)–P(1)–N(3)–C(9)]. The harmonic vibrations computed of this compound by the RHF and DFT methods are in a good agreement with the observed IR spectral data.

Infrared, Raman and electronic spectra of alanine: A comparison with ab intio calculation

The infrared, Raman and electronic spectra of alanine molecule have been studied in solid as well as in aqueous solution. The vibrational frequencies for the fundamental modes and the energies of low lying electronic states of alanine in neutral and its zwitterionic form have been calculated using AM1, RHF and DFT method with different basis sets. RHF/6-31G, DFT/6-31G, 6-31+G* and 6-311++G** calculations for vibrational frequencies of both l and d-alanine and zwitterionic alanine(zala) have been performed in both gas phase and in aqueous solution. It is concluded that while there is no significant difference between the corresponding frequencies of l- and d-alanine in gas phase but frequencies are changed for zala and alanine in water. A solvation model(PCM) for neutral alanine and zala at DFT/6-31+G* and 6-311++G** level has also been performed. Gas phase and solvation (PCM) model calculations for alanine and zala reveal that neutral alanine is more stable in gas phase while the reverse is true in aqueous medium. A comparison between the experimentally observed IR spectra of alanine in solid and water solution does not show much variation in corresponding frequencies but theoretically some changes are predicted. The rotational constants and dipole moments have also been calculated.

B3LYP, RHF and PM5 theoretical studies on phosphorescent cyclometalated Ir(III) complexes

PM5, RHF/3-21G and B3LYP/cc-pVDZ ground sate geometries are reported for two Ir(III) complexes with cyclometalated and β-diketonate ligands. Subsequently, electronic spectra were predicted using time-dependent density functional (TD-DFT) at PM5 and B3LYP/cc-pVDZ geometries. Despite the fact that PM5 predicts reasonable interatomic distances in the coordination polyhedron, this method presents large deviations in the N 1–C 2–C 3–C 4 dihedral angle, in cyclometalated ligands, when compared to X-ray geometries. B3LYP and RHF methods thus seem to be more suitable to predict ground state geometries for both complexes, whereas TD-DFT calculations, using B3LYP/cc-pVDZ geometries, yield excited states in good agreement to experimental ones.

Vibrational spectra of imidazolium tetrafluoroborate ionic liquids

The Raman and infrared spectra of a series of 1-alkyl-3-methylimidazolium tetrafluoroborate ionic liquids ([C2-4MIM][BF 4]) are reported and analyzed using Density Functional Theory (DFT) and RHF methods at the 6-311+G(2d,p) computational level. The B3LYP (DFT) and RHF calculations reproduce the vibrational spectra of 1-ethyl-3-methyl imidazolium tetrafluoroborate [EMIM][BF 4], 1-propyl-3-methyl imidazolium tetrafluoroborate [PMIM][BF 4] and 1-butyl-3-methyl imidazolium tetrafluoroborate [BMIM][BF 4] using correction factors of 0.963–0.967 (DFT) and 0.913–0.916 (RHF) with correlation coefficients R 2 of 0.999 using the fully optimized structures. Theoretical (DFT) alkyl side chain conformational changes in the 1-alkyl-3-methyl tetrafluoroborate ionic liquids have only a limited effect on the theoretical gas phase vibrations. The gas phase molecular structures of the [C 2–4MIM][BF 4] ion pairs suggest hydrogen bonding interactions between the fluorine atoms of the BF 4 − anion and the C2 hydrogen on the imidazolium ring. Additional interactions are observed between [BF 4] and H atoms on the adjacent alkyl side chains in all polymorphic structures.

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.

Dependence of the Electronic Spectra of the Chlorin Molecule on the Degree of Alternation of the Macrocycle Bond Lengths

The geometric structure of the chlorin molecule (H2Ch) has been calculated by the restricted and unrestricted Hartree-Fock (RHF and UHF) methods with an AM1 Hamiltonian. Transformations of this molecule into excited electronic states have been calculated by the CNDO/S method. The RHF method without symmetry restrictions gives a plane structure for the chlorin macrocycle with an alternation of the lengths of the bonds along the 18-member azacyclopolyene and a C1h symmetry for the molecule as a whole. The level of the first excited state Q1 of this structure is substantially shifted (δ $$E_{Q_1 }$$ ≅ +4000 cm−1) relative to the Qx level of porphin (H2P) toward shorter waves, which is in contradiction with the experimental data, according to which this shift is long-wave and is equal to δ $$E_{Q_x }$$ = −400 to −550 cm−1. The optimization of the geometry of H2Ch by the UHF method has shown that it has a structure with an 18-member azacyclopolyene with bonds of equal lengths and a D2h symmetry. For this geometry of H2Ch the calculated shift of the Qx level, equal to δ $$E_{Q_x }$$ = −70 cm−1, is bathochromic and the position of the Qy level is practically exactly coincident with the experimental one. For the geometry calculated by the RHF method with restrictions on the D2h symmetry of the 18-member azacyclopolyene δ $$E_{Q_x }$$ = +180 cm−1, and for the geometry calculated with restrictions on the highest C2v symmetry of the H2Ch molecule δ $$E_{Q_x }$$ = +620 cm−1. The latter result shows that the “natural” requirement for the C2v symmetry of the H2Ch macrocycle, which is frequently used in various calculations, is inadequate to achieve a quantitative agreement between the calculation and experimental data and, in this case, the lengths of the bonds along the 18-member azacyclopolyene are not equal.

A Study on the Intermolecular Hydrogen Bonds of α-Glycylglycine in Its Actual Crystalline Phase Using ab Initio Calculated 14N and 2H Nuclear Quadrupole Coupling Constants

alpha-Glycylglycine in its actual crystalline phase is studied by ab initio calculated nuclear quadrupole coupling constants. These physical quantities are computed for 2H and 14N in the hydrogen bonds. The type of hydrogen bond is the N-H...O type. The computations are performed with the RHF and B3LYP methods and 6-31++G** and 6-311++G** basis sets using the Gaussian 98 program. Values of the calculated nuclear quadrupole coupling constants are shown in Tables 1-3. The aim of this work is the study of 2H and 14N quadrupole coupling constants which contribute in the CON2H...O=CN2H type of hydrogen bond. The computed nuclear quadrupole coupling constants of 2H nuclei meet the related experimental values. In addition, the computed chi value of 14N belonging to the -CO-14NH- group agrees well with values obtained experimentally. However, there are some discrepancies between calculated 14N chi values of the N+H3 residue and experiments. Also, the values of these physical parameters are calculated for >C2H2 of alpha-glycylglycine in its crystalline phase. Calculations for these parameters are carried out in a single molecule using X-ray diffraction coordinates, too.