Ab Initio SCF MO Calculations Research Articles

Ab initio SCF MO study on the force field of amides

Ab initio SCF MO calculations of the force fields, normal vibrations, and dipole moment derivatives of acetamide and N-methylacetamide were carried out using the 4–31 G basis set for in-plane models and the 4–31 G and 4–31 G ∗ basis sets for out-of-plane modes. The absolute values of the calculated normal frequencies were 10–15% higher than the observed fundamental frequencies, which is the common trend of such a calculation with the 4–31 G basis set under the assumption of harmonic vibration. However, vibrational modes, isotope shifts of vibrational frequencies, and infrared intensities were reproduced quite well. Comparing the force constants of a series of amides, i.e., formamide [Y. Sugawara, Y. Hamada, A. Y. Hirakawa, M. Tsuboi, S. Kato, and K. Morokuma, Chem. Phys. 50, 105 (1980)], N-methylformamide [Y. Sugawara, A. Y. Hirakawa, M. Tsuboi, S. Kato, and K. Morokuma, Chem. Phys. 62, 339 (1981)], acetamide, and N-methylacetamide, systematic differences were found in the CONH moieties. On the other hand, the force constants of the CCH 3 and NCH 3 moieties were shown to be transferable among these amides.

A D 4D structure for [8]-prismane

[n]-Prismanes, n > 7,need not belong to D nh point groups. Ab initio SCFMO Calculations on the model C 2H 6, and molecular mechanics calculations on [n]-prismanes indicate that [8]-prismane may adopt a D 4D, structure, 1.

MNDO semiempirical and 4-31G ab initio SCF-MO calculations of heteroaromatic compounds

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTMNDO semiempirical and 4-31G ab initio SCF-MO calculations of heteroaromatic compoundsEmpar Fos, Jaume Vilarrasa, and Javier FernandezCite this: J. Org. Chem. 1985, 50, 24, 4894–4899Publication Date (Print):November 1, 1985Publication History Published online1 May 2002Published inissue 1 November 1985https://doi.org/10.1021/jo00224a049RIGHTS & PERMISSIONSArticle Views86Altmetric-Citations55LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (811 KB) Get e-AlertsSupporting Info (1)»Supporting Information Supporting Information Get e-Alerts

A quantitative analysis of the factors controlling the inversion barriers in AH 3 molecules

The importance of the energy effects associated with the HOMO-LUMO interaction in a series of AH 3 molecules has been investigated quantitatively in the framework of ab initio SCF-MO computations in terms of total energies and in terms of a PMO approach. The computations have been performed at the STO-3G and 4-31G levels, with the addition of s, p diffuse and d polarization functions and the effects associated with the various types of function have been discussed. In all cases, potential energy curves with and without the various types of energy effects have been constructed. A theoretical model which rationalizes the trend of the inversion barriers in the AH 3 molecules has been derived, consistent with the results obtained using an STO-3G basis set plus diffuse functions. This computational level seems, in fact, to provide accurate values of the inversion barriers.

A bond-length-bond-order relationship for intermolecular interactions based on the topological properties of molecular charge distributions

Ab initio SCF MO calculations for the hydrogen-bonded complexes between nitriles and hydrogen fluoride suggest a strong linear relationship between the charge density at the hydrogen-bond critical point and the hydrogen-bond energy. Further investigation of the topological properties of the charge density indicates that the generalization of the bond-length-bond-order relationship of CC bonds due to Bader et al. may be extended to intermolecular hydrogen bonding. Calculations at the 6–31G ** level, including complete geometry optimization, are reported for the ▪ complexes, where R  H, Li, F, Cl, HO, LiO, NC, HCC, CH 3 and CH 3O.

Ab initio SCF MO and modified electron gas studies of electron deficient anions and ion pairs in mineral structures

Ab initio SCF MO calculations on small molecular cluster models of solids have recently yielded equilibrium bond distances and angles with an accuracy rivaling that of analogous calculations on small gas phase molecules. Modified electron gas (MEG) calculations also yield accurate bond distances although they give much larger errors in cohesive energy than do the ab initio calculations. The MEG method has been mainly used to calculate the interaction energies of closed shell atoms and ions. However, recent work suggests that non-closed shell electron-deficient anions such as CO30 may occur in minerals. Similarly, IR studies on CO2 dissolved in silicates suggests the presence of ion pairs such as MgO2CO. Such species are here studied using both ab initio SCF and MEG methods.

Anab initioSCF MO study of conformational stability in ethyl methyl sulphide

The results of ab initio SCF MO calculations on ethyl methylsulphide using various basis sets are presented. The geometrical parameters were reproduced very well by the 3-21 G + d(0·8C + 0·6S) set. Using this basis set, the MP2 perturbation procedure was applied to include electron correlation. This procedure explained very reasonably the relative conformational stability of the trans and gauche forms, which has been studied for a long time with difficulty because of their small energy difference. From these calculations, the contradictions between the results proposed by spectroscopists and theoretical chemists were clarified.

ChemInform Abstract: AB INITIO SCF MO CONFORMATIONAL ANALYSIS OF 2,3‐BUTADIEN‐1‐OL

Abstract Ab initio SCF MO calculations were performed on 2,3-butadien-1-ol and the optimized geometries obtained by the use of the STO-3G and 3-21G basis sets. The single-point calculation using the 4-31G basis was also applied to the molecule. The optimized geometry is in good agreement with the observed one which was estimated from analysis of the microwave spectra. The relative stability of the several rotational isomers of the molecule, if they exist, is discussed in the light of the results of the present calculations. The main factor to stabilize the form was explained on the basis of the electronic structures. The variation of the bond angle α(CCO) due to the rotational isomerism is analyzed.

Ab initio SCF MO conformational analysis of 2,3-butadien-1-OL

Ab initio SCF MO calculations were performed on 2,3-butadien-1-ol and the optimized geometries obtained by the use of the STO-3G and 3-21G basis sets. The single-point calculation using the 4-31G basis was also applied to the molecule. The optimized geometry is in good agreement with the observed one which was estimated from analysis of the microwave spectra. The relative stability of the several rotational isomers of the molecule, if they exist, is discussed in the light of the results of the present calculations. The main factor to stabilize the form was explained on the basis of the electronic structures. The variation of the bond angle α(CCO) due to the rotational isomerism is analyzed.

In-plane force constants of the peptide group: Least-squares adjustment starting from ab initio values of N-methylacetamide

Least-squares adjustment of the force constants of N-methylacetamide belonging to the A′ symmetry block was carried out starting from the force constant values obtained by an ab initio SCF MO calculation with the 4–31G basis set. Fourteen diagonal force constants were adjusted by the use of 122 vibrational frequencies of 9 isotope derivatives in the liquid state observed in the region 2000-100 cm −1. Here, all the off-diagonal elements were fixed at the initial values calculated with the 4–31G basis set. The greatest difference between the observed and calculated frequencies was 30 cm −1, and the differences for 94 modes were less than 10 cm −1. Most of the force constatns of bond stretching modes were reduced by 5–10%, and those of angle deformation modes by 20–30% in the course of the least-squares adjustment. The CO stretching force constant was lowered by 30% in the adjustment, and this is understandable because the adjustment involves the effect of bringing the peptide group from a free to a hydrogen-bonded state.

Electronic and structural properties of borazine and related molecules

Ab initio SCF MO calculations are reported for benzene, s-triazine, borazine and boroxine. The Laplacian of the charge density and the Mulliken population analysis procedure demonstrate that the delocalization of the π electrons decreases and the polarity of the ring bonds increases substantially as the atoms in the ring become more dissimilar. Several other properties, including distortion of the ring angles, puckering of the ring and nuclear quadrupolar coupling constants, emphasize the different chemical properties within the isoelectronic series.

Studies of molecular orbital momentum distributions by (e, 2e) spectroscopy

Binary electron impact ionization or (e, 2e) spectroscopy has recently been employed to measure molecular orbital momentum distributions in a variety of molecules. Interpretation of the momentum distributions ρ(q) has been facilitated by the use of the wave-function autocorrelation function B(r). In some cases differences in the autocorrelation function of two orbitals may be semiquantitatively interpreted in terms of contour plots of the average and difference of their position-space orbital amplitudes. The σg and σu orbitals of a homonuclear diatomic provide a simple illustration of this point. Calculated and experimental spherically averaged ρ(q) for the predominantly N 2p lone-pair HOMOs of NH3 and CH3NH2 are shown using a ΔB(r) analysis to indicate significant participation of the CH3 hydrogens in the lone-pair orbital of CH3NH2. Although the difference in ρ(q) between the lone pairs of NH3 and CH3NH2 is adequately reproduced by split valence (SV) level ab initio SCFMO calculations the experimental ρ(q) have their maxima at considerably smaller absolute values of q then that obtained from either SV or near Hartree–Fock SCFMO calculations. Experimental and calculated momentum distributions are also presented for the 2t2g HOMO of Cr(CO)6. A ΔB(r) analysis of ρ(q) for this orbital confirms that it has mixed Cr 3d and CO 2π character.

Parallelism in quantum chemistry: Hydrogen bond study in DNA base pairs as an example

We present results on the energetics of the hydrogen bridges for the Guanine–Cytosine pair obtained in a DNA fragment consisting of three stacked base pairs in the B-DNA conformation. The wave function computations on the 87 atom system are all-electron ab initio SCF-MO computations obtained with a basis set of 1032 primitive gaussian functions contracted to 315. Even if the results are only preliminary, one can tentatively advance conclusions relative to the molecular field effect of stacked base-pairs on the potential energy surface for a hydrogen bond. These computations have been performed with a modified version of our molecular program, which uses an IBM 4341 host and six to ten attached array processors (FPS-164) in parallel. The strategy to convert the program from sequential to parallel is briefly outlined and comparisons with our parallel system are made with a present-day “vector” super-computer. From these studies, we conclude that if one adopts the “parallel” approach presented and tested here, much larger chemical systems than previously are now amenable to all-electron ab initio computations.

Computer experiments on aqueous solutions. III. Monte Carlo calculation on the hydration of tertiary butyl alcohol in an infinitely dilute aqueous solution with a new water–butanol pair potential

Monte Carlo calculations have been carried out for an infinitely dilute aqueous solution of tertiary butylalcohol (TBA) at 298.15 K and ordinary density by the Metropolis scheme in an NVT ensemble. The total number of molecules is 216, one of which is TBA. For water–water interaction, the MCY (Matsuoka–Clementi–Yoshimine) potential is used, whereas a new pair potential is determined for water–TBA interaction from ab initio LCAO SCF MO calculations for more than 380 different dimeric configurations with an STO-3G basis set and subsequent multiparameter fitting of MO data thus obtained to an appropriate potential function with a nonlinear optimization method. In the Monte Carlo run, 3 600 000 configurations have been generated and final 2 000 000 configurations have been used to obtain energetic and structural information on the hydration structure of TBA. As in the case of the methanol solution studied previously, the potential energy and structure of water tend to be stabilized by the introduction of one TBA molecule, and two strong hydrogen bonds between TBA and the surrounding water molecules favor the formation of a bulky and stable hydration shell which includes cooperatively connected hydrophobic hydration structures. This is clearly shown in density diagrams for water around TBA which visualize remarkable structuration of water. The iceberglike structure around TBA is more pronounced than that of methanol.

X-Ray and SCF–MO model study of the complex formed between N-bromosuccinimide and 1,4-diazabicyclo[2.2.2]octane

X-Ray analysis of the structure of the complex (1) formed between N-bromosuccinimide and the tertiary base 1,4-diazabicyclo[2.2.2]octane(DABCO)shows the compound to contain a nearly linear [175.1(2)°]N–Br–N bond with an exceptionally long bromine to DABCO distance [2.332(4)A]and a bromine to succinimido distance of 1.945(4)A; MNDO and ab initio SCF-MO calculations on the model system N-chlorosuccinimide + NH3show charge transfer to occur from both the halogen and the amine to the succinimido moiety upon complex formation.

Ab initio SCF-MO calculations on AlO4 centres in alpha-quartz. I.

Ab initio SCF-MO calculations were done on nine-atom and fifteen-atom clusters designed to simulate alpha-quartz, i.e., a [SiO4]0 unit, and aluminum impurity centres [AlO4]−, [AlO4]0, and [AlO4]+ in quartz, terminated with protons or SiH3 groups. The geometry of the small model of each of these centres was optimized. Using these models and the enlarged versions thereof, calculated values for relative total energies, various static structural parameters, electron spin densities, net atomic charges, and orbital energies are compared with experimental data from spectroscopic and other techniques, especially high resolution electron paramagnetic resonance. The calculated results agree remarkably well with experiment. Thus it is evident that small clusters of atoms can be used successfully to simulate the local surroundings of a substitutional Al ion in quartz, i.e., the short-range effects are the most important.

Ionization energies and hydrogen-bond strength of the water clusters

The first vertical ionization energies of the water clusters (H 2O) n for n = 1–8 have been evaluated from ab initio SCF MO calculations. The values obtained for n = 5 and 8 are in reasonable agreement with the experimental values of ice. The stability of the clusters is examined in terms of the hydrogen-bond strength per bond ( B h).

Structure of Liquid N,N-Dimethylformamide Studied by Means of X-Ray Diffraction

Abstract The structure of N,N-dimethylformamide (DMF) was determined at 25 °C by means of X-ray diffraction using a θ-θ type X-ray diffractometer. The least-squares analysis of the reduced intensities results that a DMF molecule has a practically planar skeletal structure in the liquid state. Intramolecular structural parameters within a DMF molecule were determined as follows: C=O; 1.24 Å, N–C(CHO); 1.35 Å, and N–C(CH3); 1.45 Å. The bond lengths are not appreciably different from those found in the gas phase, except for the C=O length, which is slightly longer by 0.04 Å in the liquid state than in the gaseous one. The radial distribution curve of DMF shows that the intermolecular arrangement of DMF molecules is practically fully disordered and no evidence has been found for the cluster formation of DMF molecules in the liquid state. The experimental result for the weak intermolecular interactions between DMF molecules is supported by ab initio MO-SCF calculations.

Self-association of oxime: electronic and vibrational structures of formaldoxime monomer, dimer, and trimer

The self-association of oxime is investigated from the aspect of intermolecular hydrogen bonding through ab initio SCF MO calculations on formaldoxime monomer, dimer, and trimer. It is understood from population analysis that formaldoxime monomer has inherently a suitable electronic distribution for constructing OH⋯3N hydrogen-bonded cyclic dimer and trimer. The formaldoxime trimer is characterized as the depressed nine membered ring. It is also confirmed to be more stable and preferable as a predominantly existing species than the dimer, which accounts for the experimental value of average association number for oximes. Vibrational analysis reveals that upon the self-association the OH stretching vibration is red-shifted, while both the OH in-plane and out-of-plane bending ones are blue-shifted, the results of which lead to a global feature of motion that ring wagging and ring puckering are activated. Largely enhanced intensities of IR-active OH stretching vibrations are well rationalized with more prominent charge polarization centered around =NOH sites in the formaldoxime dimer and trimer.

Ab initio MO calculation of force constants and dipole derivatives for the formamide dimer. An estimation of hydrogen-bond force constants

Ab initio SCF MO calculations (with the 4-31G basis set) have been carried out to determine the equilibrium geometry, vibrational frequencies, dipole-moment derivatives, and force constants for intermolecular modes of the formamide dimer and its d 4 and d 6 derivatives. The results are correlated with monomer calculations and experimental data for crystalline formamide.