Ab Initio SCF MO Calculations Research Articles

Induced infrared absoption bands of ethylene adsorbed on mordenites: Experimental and theoretical studies of electron-donor-acceptor interactions

Infrared spectra of ethylene adsorbed on ion-exchanged mordenites (metallic cation = Li +, Na +, Ca 2+, Zn 2+ and Cd 2+) were measured. Weak bands were found at 1612–1614 and at 1338–1340 cm −1 in Li +, Na + and Ca 2+ mordenites. They are assigned to the totally symmetric ν(CC) stretching and symmetric δ(CH 2) bending modes of the ethylene monomer. In Zn 2+ and Cd 2+ mordenites the corresponding bands shift to lower energy and increase in intensity. These forbidden bands become IR active through the electron-donor-acceptor (charge-transfer) interaction between ethylene and the metallic cation. To estimate the IR intensity induced by the charge-transfer interaction, ab initio SCF MO calculations were carried out for ethylene-chlorine monofluoride (C 2H 4-CIF) complex selected as a model complex. The calculations show that only a small amount of charge-transfer modulation of the CC stretching mode can induce its IR activity enough to observe it in the spectra, and that the direction of the transition dipole is perpendicular to the CC bond.

The CH3CH2⊖ internal rotation in thiopropionic acid as studied by ab initio SCF-MO method

The CH3CH2⊖internal rotation in thiopropionic acid was studied by ab initio SCF-MO calculations using 3–21G and 3–21G+d(0.65S) basis sets. Energies and structures of several conformations of the two tautomeric forms of this molecule (the thiol and thion forms) determined by gradient geometry refinement are reported, and some conformationally dependent local geometry trends are discussed. The results are compared with available data on propionic and dithiopropionic acids to assess the effects of substitution of oxygen by sulphur on molecular properties. While the CH3-CH2-CO axis adopts preferentially the syn conformation (C-C-CO dihedral angle equal to 0°), the lowest-energy conformation of the CH3-CH2-CS axis occurs for a C-C-CS dihedral angle in the 100–120° region. However, both axes possess energy maxima near 60° and at 180°. The calculated molecular atomic charges, dipole moments and ionization potentials are reported and compared with available experimental data.

Rotational isomerism of allylamine as studied by gas electron diffraction: a joint analysis with rotational constants and SCF MO calculation

The geometrical parameters and the relative abundances of the three main conformers of allylamine, CT, ST, and S + G +, have been determined by the electron diffraction intensity measured in the present study. A joint analysis of the diffraction data and the rotational constants of the three conformers has been performed using the optimized geometries and the quadratic force field derived from an ab initio SCF MO calculation. For the most abundant S + G + conformer, the bond lengths ( r g) and bond angles ( r z ) are r(C'H)=1.106(2) Å, r(CC)=1.336(7) Å, ∠CCC=125.6(6)°, and the dihedral angle ( r z ∠CCCN=−124.7(12)°. The numbers in parentheses represent the estimated limits error (2.5σ). The relative abundance for cis ( CT and CG): skew ( ST, S + G +, and S + G −) is 25±6:75∓6.

Theoretical analysis of torsional potential functions. application to the CXS-CXS (X = H, F) series of molecules

A detailed investigation of the general behaviour and properties of potential functions hindering the internal rotation of one-rotor molecules is presented. An interpolated three-parameter Fourier representation of torsional potential functions is shown to be adequate to describe rotational isomerization processes. Simple analytic expressions to explore quantitatively and qualitatively the main characteristics of transition states are formulated. It is shown that the barrier to internal rotation is given by a two-term expression involving an intrinsic barrier associated with the shape of the potential wells, and a contribution due to the energy difference between the stable isomers. Conformational functions are introduced and used together with the Brønsted coefficient to get a quantitative version of the Hammond postulate. These concepts are applied to the analysis of ab-initio SCF-MO calculations on the rotational isomerism in a series of dithioglyoxal molecules.

Molecular structure and properties of thioacetic acid

Ab initio SCF-MO calculations have been carried out for thioacetic acid using the 3–21G and 4–31G basis sets. Energies and structures of several conformations of the two tautomeric forms of this molecule (the thiol and thion forms) determined by gradient geometry refinement are reported, and some conformationally dependent local geometry trends discussed. The results are compared with available data on acetic and dithioacetic acids in order to assess the effects of the substitution of oxygen by sulphur on molecular properties. It is shown that the O(H)→S(H) substitution originates important changes in the molecular properties, which can be correlated with the greater conjugating ability of the oxygen atom. On the other hand, the calculations indicate that the O=→S=substitution causes minor changes in the relative conformational energies, although other properties may be significantly affected. Charges on atoms, dipole moments and ionization potentials are also presented and compared with the available experimental values.

Molecular structure and vibrational treatment of dimethyl sulphide and ethyl methyl sulphide in complexes with hcl: a theoretical study

The results of ab initio SCF MO calculations on dimethyl and ethylmethyl sulphides in 1:1 complexes with HCl are presented. The calculated results are consistent with the observed molecular geometry and the relative conformational stabilities estimated by a variety of spectroscopic techniques. The vibrational analyses of the parent molecules and those of the HClcomplexes are performed based on these ab initio calculations. The experimental infrared frequency shifts of dimethyl sulphide due to the complexation with HClare explained. The behaviour of the proton in the HCl molecule due to the hydrogen-bond formation on the sulphur lone-pair orbitals is discussed.

Conformational analysis of Be(II) and Mg(II) complexes of N-acetyl alanine methyl ester

Conformational changes of N-acetyl alanine methyl ester upon binding of Be(II) and Mg(II) have been investigated by means of ab initio MO-SCF calculations with minimal basis set. Results obtained with single step optimized geometries are compared with those computed with full energy gradient optimization. The latter proved to be of far superior quality, but both methods lead to the same prediction towards the most stable complex structure, which are chelate complexes with simultaneous ion binding to peptide and ester group. This conformation involves drastic structural changes in the ligand molecule, hence indicating the ions' ability to induce some conformational changes also in biological macromolecules with similar functional groups.

UV photoelectron and quantum mechanical characterization of DNA and RNA bases: valence electronic structures of adenine, 1,9-dimethyl-guanine, 1-methylcytosine, thymine and uracil

HeI UV photoelectron spectra of adenine, 1,9-dimethylguanine, 1-methylcytosine, thymine and uracil have been interpreted with the aid of results from ab initio SCF-MO calculations employing a double zeta 4–31G basis set, and HAM/3 semiempirical calculations. Ionization potentials from results of the SCF-MO calculations have been obtained through the application of Koopmans' theorem. For cytosine and guanine, problems associated with the occurrence of more than one tautomeric form in the gas phase have been circumvented by examining methyl substituted derivatives of these bases. For adenine, 1,9-dimethylguanine, thymine and uracil the current assignments, based on the results of the 4–31G SCF and HAM/3 calculations, are essentially the same as those provided in earlier studies. For 1-methylcytosine, the results of the current calculations and recent findings concerning the tautomerism of cytosine in nonpolar environments has led to new assignments. The vertical ionization potentials of the first (π 1), second (π 2), third (π 3) and fourth (π 4) highest occupied π orbitals in adenine are 8.48, ≈9.6, ≈10.5, and 12.10 eV, respectively. The ionization potentials of the first through third (n 1-n 3) lone-pair orbitals are ≈9.6, ≈10.5, and 11.39 eV. In 1,9-dimethylguanine, the π 1, π 4 and π 5 orbitals have ionization potentials of 8.09, ≈11.0, and 12.31 eV, while the π 2 and π 3 orbitals have ionization potentials in the range between 9.3 and 10.2 eV. Values for the ionization potentials of the n 1 and n 2 orbitals lie in the range between 9.3 and 10.2 eV, and the value for the n 3 orbital is ≈11.0 eV. In 1-methylcytosine, the ionization potentials of the π 1 through π 4 orbitals are 8.65, ≈9.7, 11.53, and 12.40 eV; the values for the n 1 and n 2 orbitals lie in the range 9.5 to 10.0 eV. For uracil, the ordering of the ionization potentials is π 1 (9.59 eV), n 1 (10.11 eV), π 2 (10.56 eV), n 2 (11.16 eV), and π 3 (12.63 eV). In thymine, the energetic ordering of orbitals is the same as in uracil and the corresponding ionization potentials have values of 9.18, 10.03, 10.39, 10.82 and 12.27 eV. An examination of DNA and RNA alkylation patterns occurring in reactions with dimethyl-sulfate, methylmethanesulfonate and N-methyl- N-nitrosourea indicates that the reactivity of nucleotide bases towards methylating reagents increases as the base ionization potentials decrease.

Dithiocarbimates from sulfonamides Part 3. The crystal structure of bis(tetraphenylarsonium)- N-( p-chlorphenylsulfonyl)-dithiocarbimatodisulfide hydrate and an ab initio MO study on the characteristic SS dihedral angle in the dinegative model system [HNC(S)CS(S)CNH] 2−

The N-( p-chlorphenylsulfonyl)-dithiocarbimatodisulfide [S(S)CNS(O 2)C 6H 4Cl] 2 2− has been prepared from the corresponding dithiocarbimate [S 2CNS(O 2)C 6H 4Cl] 2− by oxidation with I 2. A complete X-ray structure has been carried out on the bis(tetraphenylarsonium) salt. The bond length of the central disulfide group SS is 2.047 Å, the dihedral angle CSSC is 80.8°. The characteristics of the disulfide group concerning the SS dihedral angle are investigated by ab initio SCF MO calculations using different basis-sets. A point-charge model has been introduced to simulate crystal field effects.

An analysis of the pathways for magnesium ion in the field of a macrocyclic ligand

An abinitio MO-SCF calculations have been performed with a minimal Gaussian lobe basis set for numerous Mg(II) 1,4,7,10-tetraazacyclododecane (cyclen) configurations. Mg(II) shows a strong interaction with all functional groups of cyclen. The strongest interaction occurs when magnesium points to the cavity and perpendicular to the ligand plane. However, in the complex formation processes, the ion does not prefer to move along this direction because of the repulsive interaction with the NH functional groups. The favourable path for entering the cavity of the ligand encloses an angle of 30° with the z axis and lies in the plane between the opposite NH groups. Keywords: cyclen, macrocyclic effect, complex formation, favourable path, abinitio calculations.

The quantitative prediction and interpretation of the vibrational spectra of organophosphorus compounds: Part II. Methylphosphonic difluoride CH 3(PO)F 2, methylphosphonothioic difluoride CH 3(PS)F 2 and methylphosphonofluoridic acid CH 3(PO)FOH

The results of ab initio SCF MO calculations made using the 3-21G* and 6-31G* basis sets for three related methylphosphono difluoride compounds are presented. The optimized geometries from these calculations are compared with the experimental bond distances and bond angles obtained from studies of their microwave spectra. The vibrational spectra [frequencies and infrared (IR) intensities] for these three molecules have been calculated using the 3-21G* basis set. The predicted spectra have been compared with the experimental IR spectra of the molecules in the gas phase. The calculated potential-energy distributions provide a basis for assigning the experimental spectra, which is in general agreement with the assignment made previously on the basis of spectra of model compounds and characteristic frequencies. In a few cases the calculations suggest some minor adjustments to the previous assignment. The agreement between the previous assignment and that based on the calculations for CH 3(PO)F 2 and for CH 3(PS)F 2 provides support for our assignment of the IR spectrum of CH 3(PO)F(OH). The intensity sums are examined, and the calculated values of the “mass-weighted squared effective charges” (one of the invariants of the atomic polar tensors) for all the atoms in all of these molecules are presented.

Bonding in selected normal (octet), hypervalent (decet) and super-hypervalent (duodecet) sulfur compounds

A series of sulfur compounds with eight valence electrons (octet) such as SF 2 and S(CH 3) 2, 10 valence electrons (decet) such as SF 4, SF 2(CH 3) 2 and S(CH 3) 4 and 12 valence electrons (duodecet) such as SF 6, SF 4(CH 3) 2, SF 2(CH 3) 4 and S(CH 3) 6 were subjected to ab initio SCF-MO computations. Conclusions were drawn from the numerical results with respect to the relative stabilities and ease of syntheses for the super-hypervalent (duodecet) sulfur compounds.

S-cis and s-trans Conformers of formic, thioformic and dithioformic acids. An ab initio study

Ab initio SCF-MO calculations have been carried out for formic, thioformic and dithioformic acids using the 6–31G* basis set. Fully optimized geometries, atomic charges, relative stabilities and harmonic force fields for s-cis and s-trans conformers of these molecules have been determined and the effects of oxygen-by-sulphur substitution analysed. A realistic description of the molecular charge distribution can be reached by introducing a quantum-mechanical correction to the Mulliken atomic charges, derived from the ‘charge’-‘charge flux’-‘overlap’(CCFO) model. Unlike reported theoretical results, the present ab initio calculations yield relative stabilities of the thioformic acid conformers in agreement with experiment [s-cis(thiol) > s-trans(thiol) > s-cis(thione) > s-trans(thione)]. The success of these ab initio calculations should be partially ascribed to the inclusion of polarization functions on all non-hydrogen atoms.

Nitrogen-14 quadrupole double resonance in some substituted hydroxamic acids

/sup 14/N quadrupole coupline constants and asymmetry parameters have been measured in a number of hydroxamic acids by the double-resonance field-cycling techniques based on either irradiation in zero magnetic field or cross relaxation. The compounds all display high asymmetry parameters. Those in which this quantity is greater than 0.9 show remarkable line shapes for the two lower /sup 14/N frequencies (/gamma//sub y/, /gamma//sub z/) in their irradiation spectra. They are explained in terms of a thermal-mixing mechanism, which generates polarization of the /sup 1/H dipolar levels when these nearly degenerate frequencies are strongly irradiated in zero field, and then subsequently modified by level crossing when the sample is returned to high field to measure the remaining /sup 1/H signal. Ab initio SCF-MO calculations of the /sup 14/N quadrupole tensor in a group of molecules at the orientation found in crystals of acetohydroxamic acid hemihydrate and oxalodihydroxamic acid are in reasonable agreement with experiment and predict that in all the hydroxamic acids studied in the maximum principal component is negative and closely parallel to the direction of the 2p/sub pi/ orbital. 53 references, 6 figures, 6 tables.

Metals in biology: Low-lying states of iron-oxygen and iron-sulfur complexes modeling electron transfer in redox reactions

Energy surfaces of low-lying states of planar complexes [Fe(C 2H 2X 2) 2] n with X=O and S and total charges n varying between + 3 and − 2 have been investigated by quantum chemical ab initio MO-SCF calculations of double zeta quality. Through studies of such properties as geometry, electron density distribution and molecular orbital energies it has been concluded that some of the states can be referred to as Fe (III) and others as Fe (II) states. The lowest states are found to be those with n = − 1 and 0 for X = O and n = − 1 and − 2 for X = S. For both the oxygen and the sulfur complexes Fe (III) and Fe (II) states are very close in energy. Supposing other electron acceptors and donors to be available, possible schemes for redox reactions between metal and ligand are suggested.

Ab initio calculations on tetramethoxymethane

Minimal basis set ab initio SCF-MO calculations were performed on the 21-atom system of tetramethoxymethane (tetramethyl orthocarbonate). The geometric configuration of this model was optimized in two conformations, one having quasi-S4 symmetry and the other D2d symmetry. The S4 conformation was found to be 8 kJ mol−1 lower in energy than the D2d conformation, at the STO-3G level. The calculated energy difference is consistent with the recently measured geometric configuration of crystalline tetrabenzyl orthocarbonate. The calculated values of the bond lengths and angles were compared to the results of an electron diffraction study of the methyl species, and agree well with experiment. The theoretical electric dipole moment was calculated to be 0.01 D.

Ab initio structural and conformational studies of thioacetone and thiopropionaldehyde

Ab initio SCF-MO calculations have been carried out for thioacetone and thiopropionaldehyde using STO-3G and 3-21G basis sets. Structures and energies of several conformations of these molecules have been determined by the gradient geometry refinement, and some conformationally dependent local geometry trends are discussed. The results are compared with available data on acetone and proprionaldehyde to assess the effects of oxygen-by-sulphur substitution on the molecular properties. It was found that thiopropionaldehyde possesses energy minima at CCCS dihedral angles of 0° and near 120°, and energy maxima near 60° and at 180°. The calculated molecular atomic charges, dipole moments and ionization potentials are reported and compared with available experimental data.

ChemInform Abstract: Low‐Temperature X‐Ray Crystal‐Structure Analysis of the Thermally Unstable Lithiated 2‐Butenyl tert‐Butyl Sulfide: A Comparison with Model ab initio MO Calculations

Single crystals of the N,N,N′,N′-tetramethylethylenediamine (TMEDA) complex 6 of the title compound have been isolated. Compound 6 decomposes in the crystalline state above −20°. From the bond lengths and angles obtained by X-ray crystal-structure analysis (data collected at −70°), compound 6 is best described as a (E)-1-(tert-butylthio)-1-lithio-2-butene with the double bond acting as an additional ligand on lithium (unsymmetrical allylic group). The S-atom is in a cisoid arrangement in a common plane with the four C-atoms of the butenyl system. The t-Bu group and the Li-atom are located above and below this plane. The structure is discussed with respect to the reactivity of 6 (α/γ reactivity). The gross structure is reproduced surprisingly well by an ab initio SCF MO calculation of the model lithiopropene-1-thiol 7(HS instead of t-BuS, CH2 instead of CHCH3, no solvation of Li). The prominent difference is the symmetry of the allyllic moiety in the calculated structure.

Overtone spectral investigation of structurally and conformationally non-equivalent CH bonds in tricarbonyl(η 5-methylcyclopentadienyl)manganese I

CH-stretching overtone spectra of tricarbonyl(η 5-methylcyclopentadienyl)manganese I are measured in the region of Δν CH=2–5 in the liquid phase and Δν CH=2 in the gas phase. The spectra are interpreted within the local mode model with the help of ab initio SCF MO calculations on the methylcyclopentadienyl anion. Evidence is given for conformationally non-equivalent CH bonds in the methyl group and structurally non-equivalent CH bonds in the cyclopentadienyl ring.

Ab initio molecular orbital calculation of the methyl nitrite syn-anti isomerization potential

Ab initio SCF MO calculations were performed for the CONO torsional potential for the syn-anti isomerization of methyl nitrite. Full geometry optimization was used. The energy minima occur for the dihedral angles equal to zero and 180°. The barrier height is calculated to be 9.84 kcal/mol and is located at 87°. The syn conformation is predicted to be more stable than the anti by 1.09 kcal/mol. The potential values were fit to a six-term cosine series.