Localized molecular orbital studies in momentum space. I. compton profiles of 1s core electrons and hydrocarbons

The calculations of momentum space properties for the polyatomic molecules CH₄, C₂H₄ and C₂H₆ using localized molecular orbitals of double zeta quality basis sets are presented. The LMO analysis shows that localized and canonical core electrons have different momentum space properties, and that in agreement with the experimental data of Eisenberger and Marra one can distinguish the momentum properties of the CC single and double bonds. The effect of environment on a bond is seen by comparing the CH bond in these three molecules.The concept of electron pair size is introduced as a quantitative guide for interpreting momentun space properties.     

An application of the method of conjugate gradients to the calculation of minimal basis set localized orbitals

Using an STO -3G basis set, energy localized molecular orbitals (LMO ) were determined for the ten electron series HF, H₂O, NH₃, and CH₄ as well as for CH₃OH and C₂H₂F₂. The method of conjugate gradients is shown to be a viable alternative to other non-eigenvalue methods. The characterization of the LMO in terms of first and second moment measures indicates that the STO -3G basis set LMO may be accurately correlated to larger sp basis set LMO . Also, it is shown that the first and second moment measures display a good linear correlation with the classical concept of electronegativity.     

Calculation and properties of non-orthogonal,strictly local molecular orbitals

A general procedure to calculate non-orthogonal, strictly local molecular orbitals (NOLMOs) expanded using only a subset of the total basis set is presented. The energy of a single determinant wave function is minimised using a Newton-Raphson approach. Total energies and barriers to internal rotation for CH₄, NH₃, H₂O, CH₃CH₃, CH₃NH₂, CH₃OH, NH₂NH₂, NH₂OH and HOOH, and certain properties of the NOLMOs present in these molecules, are investigated using the 4-31G basis set.     

Dependence of approximate ab initio molecular loge sizes on the quality of basis functions

Size and shape parameters for the core, bonding, and lone electron pairs of the ten-electron hydrides (CH₄, NH₃, H₂O, HF) were determined from ab initio MO wave functions using various Gaussian basis sets. The fundamental features of approximate electron pair loge representation are somewhat more sensitive to the quality of the basis functions than the molecular total energy. The total size of the molecular electron distribution is less affected by basis set variations than its components: the core, bonding, and lone pair sizes. There is an apparent tendency to “preserve” the total size of molecular distribution.     

Correlation balance in basis sets for electronic structure calculations

The balanced addition of polarization functions to the 6–31G and 6–311G basis sets for correlated wave functions is evaluated using bond energy predictions at the MP 2 and full MP 4 levels as a measure of correlation-balanced basis sets. The homolytic dissociations of the XH bonds in H₂, CH₄, NH₃, H₂O, and HF and the XY bonds in C₂H₆, NH₂NH₂, HOOH, and CH₃OH are used as the basis for the evaluation. It is found that correlation balance is achieved for HH, XH, and XY bonds, particularly at the MP 2 level, only if at least as many polarization sets, and sometimes more, are added to the hydrogens as are added to the heavy atoms.     

K-shell ionization energies in molecules by SCF GF calculations

Energies of CH₄, NH₃, H₂O and C₂H₄ K-ionized molecules are calculated by means of a Group Function method using minimal or near minimal basis sets of STO's. Further results from very large basis sets are reported for CH₄, NH₃, and H₂O. Results seemingly do not suffer the shortcomings of a previous SCF MO treatment.     

Minimal basis sets in calculations of intermolecular interaction energies

Several minimal (7, 3/3) Gaussian basis sets have been used to calculate the energies and some other properties of CH₄ and H₂O. Improved basis sets developed for these molecules have been extended to NH₃ and HF and employed to H₂CO and CH₃OH. Interaction energies between XH_n molecules have been calculated using the old and the new minimal basis sets. The results obtained with the new basis sets are comparable in accuracy to those calculated with significantly more extended basis sets involving polarization functions. Binding energies calculated using the counterpoise method are not much different for the new and the old minimal basis sets, and are likely to be more accurate than the results of much more extended calculations.     

Geometry optimization in ab initio SCF calculations. VII. Proton affinities and ion structures calculated with floating orbital geometry optimization (FOGO)

The FOGO method is used to calculate absolute proton affinities of the molecules H₂, HF, NH₃, H₂O, CH₃OH, C₂H₅OH, H₂O₂, CH₂O, CO, and CH₂CO. Comparison with experimental values demonstrates that the geometrical and energetical data resulting from this type of ab initio calculation are of chemical accuracy. Predictive data for higher energy isomers, such as hydroxymethylene and ethynol are given as possible aid for the identification of these species.     

Looking at orbitals in the laboratory: The experimental investigation of molecular wavefunctions and binding energies by electron momentum spectroscopy

The experimental technique of electron momentum spectroscopy (EMS ) (i.e., binary (e, 2e) spectroscopy) is discussed together with typical examples of its applications over the past decade in the area of experimental quantum chemistry. Results interpreted within the framework of the plane wave impulse and the target Hartree—Fock approximations provide direct measurements of, spherically averaged, orbital electron momentum distributions. Results for a variety of atoms and small molecules are compared with calculations using a range of Fourier transformed SCF position space wavefunctions of varying sophistication. Measured momentum distributions (MD ) provide a “direct” view of orbitals. In addition to offering a sensitive experimental diagnostic for semiempirical molecular wavefunctions, the MD's provide a chemically significant, additional experimental constraint to the usual variational optimization of wavefunctions. The measured MD's clearly reflect well known characteristics of various chemical and physical properties. It appears that EMS and momentum space chemistry offer the promise of supplementary perspectives and new vistas in quantum chemistry, as suggested by Coulson more than 40 years ago. Binding energy spectra in the inner valence region reveal, in many cases, a major breakdown of the simple MO model for ionization in accord with the predictions of many-body calculations. Results are considered for atomic targets, including H and the noble gases. The measured momentum distribution for H₂ is also compared with results from Compton scattering. Results for H₂ and H are combined to provide a direct experimental assessment of the bond density in H₂, which is compared with calculations. The behavior of the outer valence MD ''s for small row two and row three hydride molecules such as H₂O and H₂S, NH₃, HF, and HCl are consistent with well known differences in chemical and physical behavior such as ligand-donor activity and hydrogen bonding. MD measurements for the outermost valence orbitals of HF, H₂O and NH₃ show significant differences from those calculated using even very high-quality wavefunctions. Measurements of MD's for outer σ_g orbitals of small polyatomic molecules such as CO₂, COS, CS₂, and CF₄ show clear evidence of mixed s and p character. It is apparent that EMS is a sensitive probe of details of electronic structure and electron motion in atoms and molecules.     

Asymmetry and Non‐Adiabaticity in Fragmentation of Disulfide Bonds upon Electron Capture

Although it has been generally assumed that electron attachment to disulfide derivatives leads to a systematic and significant activation of the S? S bond, we show, by using [CH₃SSX] (X=CH₃, NH₂, OH, F) derivatives as model compounds, that this is the case only when the X substituents have low electronegativity. Through the use of MP2, QCI and CASPT2 molecular orbital (MO) methods, we elucidate, for the first time, the mechanisms that lead to unimolecular fragmentation of disulfide derivatives after electron attachment. Our theoretical scrutiny indicates that these mechanisms are more intricate than assumed in previous studies. The most stable products, from a thermodynamic viewpoint, correspond to the release of neutral molecules; CH₄, NH₃, H₂O, and HF. However, the barriers to reach these products depend strongly on the electronegativity of the X substituents. Only for very electronegative substituents, such as OH or F, the loss of H₂O or HF is the most favorable process, and likely the only one observed. This is possible because of two concomitant factors, 1) the extra electron for [CH₃SSX]^? (X=OH, F) occupies a σ*(S? X) MO, which favors the cleavage of the S? X bond, and 2) the activation barriers associated with the hydrogen transfer process to produce H₂O and HF are rather low. Only when the substituents are less electronegative (X=H, CH₃, NH₂) the extra electron is located in a σ*(S? S) orbital and the cleavage of the disulfide bridge becomes the most favorable process. The intimate mechanism associated with the S? S bond dissociation process also depends strongly on the nature of the substituent. For X=H or CH₃ the process is strictly adiabatic, while for X=NH₂ it proceeds through a conical intersection ( CI ) associated with the charge reorganization necessary to obtain, from a molecular anion with the extra electron delocalized in a σ*(S? S) antibonding orbital, two fragments with the proper charge localization.     

Theoretical influence of third molecule on reaction channels of weakly bound complex CO2… HF systems

In this investigation, reaction channels of weakly bound complexes CO₂…HF, CO₂…HF…NH₃, CO₂…HF…H₂O and CO₂…HF…CH₃OH systems were established at the B3LYP/6‐311++G(3df,2pd) level, using the Gaussian 98 program. The conformers of syn‐fluoroformic acid or syn‐fluoroformic acid plus a third molecule (NH₃, H₂O, or CH₃OH) were found to be more stable than the conformers of the related anti‐fluoroformic acid or anti‐fluoroformic acid plus a third molecule (NH₃, H₂O, or CH₃OH). However, the weakly bound complexes were found to be more stable than either the related syn‐ and anti‐type fluoroformic acid or the acid plus third molecule (NH₃, H₂O, or CH₃OH) conformers. They decomposed into CO₂ + HF, CO₂ + NH₄F, CO₂ + H₃OF or CO₂ + (CH₃)OH₂F combined molecular systems. The weakly bound complexes have four reaction channels, each of which includes weakly bound complexes and related systems. Moreover, each reaction channel includes two transition state structures. The transition state between the weakly bound complex and anti‐fluoroformic acid type structure (T₁₃) is significantly larger than that of internal rotation (T₂₃) between the syn‐ and anti‐FCO₂H (or FCO₂H…NH₃, FCO₂H…H₂O, or FCO₂H…CH₃OH) structures. However, adding the third molecule NH₃, H₂O, or CH₃OH can significantly reduce the activation energy of T₁₃. The catalytic strengths of the third molecules are predicted to follow the order H₂O < NH₃ < CH₃OH. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2006     

Limits on the localized interpretation of molecular orbital wavefunctions

A new measure of orbital localizability is proposed. The actual improvement in classical electrostatic interpretations of electronic energy contributions on localization of CH₄, NH₃, H₂O, HF and Ne LCAO wavefunctions is computed to be small. Substantial valence LMO spatial overlapping remains.     

Doubly excited configurations in modified CNDO and INDO methods

The effect of different CI basis sets, including doubly excited configurations, on transition energies calculated by modified CNDO and INDO methods is examined for H₂O, NH₃, CH₄, and H₂CO.     

A theoretical study of the condensation reactions of methyl cation with ammonia,water, hydrogen fluoride and hydrogen sulphide

Ab initio molecular orbital calculations have been used to study the condensation reactions of CH₃^? with NH₃, H₂O, HF and H₂S. Geometry optimization has been carried out at the Hartree—Fock (HF) level with the split-valence plus d-polarization 6-31G* basis set and improved relative energies obtained from calculations which employ the split-valence plus dp-polarization 6-31G** basis set with electron correlation incorporated via Moller—Plesset perturbation theory terminated at third order (MP3). Zero-point vibrational energies have also been determined and taken into account in deriving relative energies. The structures of the intermediates CH₃XH^? (X = NH₂, OH, F and SH) have been obtained and dissociation of these intermediates into CH₂X⁺ + H₂ on the one hand, and CH₃^? + HX on the other, has been examined. It is found that for those species for which the methyl condensation reaction is observed to have an appreciable rate (X = NH₂ and SH), the transition structure for hydrogen elimination from CH₃XH^? lies significantly lower in energy than the reactants CH₃^? + HX (by 75 and 70 kJ mol^?1 respectively). On the other hand, for those species for which the methyl condensation reaction is not observed (X = OH and F), the transition structure for H₂ elimination lies higher in energy than CH₃^? + HX (by 6 and 87 kJ mol^?1 respectively).     

A novel approach to molecular similarity

Summary We review briefly the general problem of assessing the similarity between one molecule and another. We propose a novel approach to the quantitative estimation of the similarity of two electron distributions. The procedure is based on momentum space concepts, and avoids many of the difficulties associated with the usual position space definitions. Results are presented for the model systems CH₃CH₂CH₃, CH₃OCH₃, CH₃SCH₃, H₂O and H₂S.     

Cooperative atomic migrations in the associates of formic acid molecule with H2O...X systems (X=CH3OH,NH2OH,H2O2, HOF,and H2O)

The pathways and activation barriers of cooperative biproton migrations in the associates of the formic acid molecule with H₂O and X molecules (X=CH₃OH, NH₂OH, H₂O₂, FOH, and H₂O) are calculated by an ab initio method (3-21G and 6-31G^** basis sets). A cooperative triproton transfer occurs in the system with X=H₂O. The activation barriers of this transfer calculated in the 3-21G and 6-31G^** basis sets are 6.94 and 27.29 (through the structure of C₂ symmetry) or 7.99 and 26.08 kcal/mole (through the structure of C_s symmetry), respectively. In the systems with X=H₃COH, HOOH, and FOH, the biproton transfer is accompanied by synchronous shifts of two hydroxyl groups and overcomes high activation barriers (>40 kcal/mole), which is accounted for by poor stereochemical similarity for the low-barrier cooperative processes in the given molecular associates. Scientific Research Institute of Physical and Organic Chemistry, Rostov State University. Translated fromZhurnal Strukturnoi Khimii, Vol. 37, No. 5, pp. 845–858, September–October, 1996. Translated by I. Izvekova     

Can STO basis sets do a good job in evaluating molecular electromagnetic properties? II: Second hyperpolarizability of H<Subscript>2</Subscript>O,CH<Subscript>4</Subscript> and NH<Subscript>3</Subscript> according to the TDHF theory

The assessment of the performance of STO basis sets for the ab initio estimation of nonlinear electromagnetic response properties of molecules, using a Time Dependent Hartree-Fock procedure, has been extended from the first to the second dynamic hyperpolarizability of three bench polyatomics (H₂O, CH₄, NH₃). Calculations based on extended basis sets are reported and briefly discussed in order to provide a wider perspective on the existing comparisons.     

An ab initio molecular orbital study of the deprotonation of amines

The geometries of the amines NH₂X and amido anions NHX^?, where X = H, CH₃, NH₂, OH, F, C₂H, CHO, and CN have been optimized using ab initio molecular orbital calculations with a 4-31G basis set. The profiles to rotation about the N? X bonds in CH₃NH^?, NH₂NH^?, and HONH^? are very similar to those for the isoprotic and isoelectronic neutral compounds CH₃OH, NH₂OH, and HOOH. The amines with unsaturated bonds adjacent to the nitrogen atoms undergo considerable skeletal rearrangement on deprotonation such that most of the negative charge of the anion is on the substituent. The computed order of acidity for the amines NH₂X is X = CN > HCO > F ≈ C₂H > OH > NH₂ > CH₃ > H and for the reaction NHX^? + H⁺ → NH₂X the computed energies vary over the range 373–438 kcal/mol.     

An ab initio study of atomic interactions in monosubstituted benzenes

Molecules of monosubstituted benzenes XC₆H₅ (X = F, Cl, Br, OH, NH₂, CH₃, CH₂CH₃) were studied by the RHF/6-311G(d) method with full geometry optimization. Analysis of the molecular orbitals and contributions made to them by atomic orbitals, and also of the populations of the valence p orbitals of atoms in substituents X directly bonded to the aromatic ring showed that the features of the electron distribution in such molecules should not be attributed to the capability of the lone electron pairs of the heteroatoms in these substituents for p,π conjugation with the π-electron system of the molecule.     

NLO‐X (X = I–III): New Gaussian basis sets for prediction of linear and nonlinear electric properties

New adjusted Gaussian basis sets are proposed for first and second rows elements (H, B, C, N, O, F, Si, P, S, and Cl) with the purpose of calculating linear and mainly nonlinear optical (L–NLO) properties for molecules. These basis sets are new generation of Thakkar‐DZ basis sets, which were recontracted and augmented with diffuse and polarization extrabasis functions. Atomic energy and polarizability were used as reference data for fitting the basis sets, which were further applied for prediction of L–NLO properties of diatomic, H₂, N₂, F₂, Cl₂, BH, BF, BCl, HF, HCl, CO, CS, SiO, PN, and polyatomic, CH₄, SiH₄, H₂O, H₂S, NH₃, PH₃, OCS, NNO, and HCN molecules. The results are satisfactory for all electric properties tested; dipole moment (µ), polarizability (α), and first hyperpolarizability (β), with an affordable computational cost. Three new basis sets are presented and called as NLO‐I (ADZP), NLO‐II (DZP), and NLO‐III (VDZP). The NLO‐III is the best choice to predict L–NLO properties of large molecular systems, because it presents a balance between computational cost and accuracy. The average errors for β at B3LYP/NLO‐III level were of 8% for diatomic molecules and 14% for polyatomic molecules that are within the experimental uncertainty. © 2014 Wiley Periodicals, Inc.