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New transition metal complex of Pt(II) of the Schiff base ligand with (E)-2-((1-(thiophen-2-yl)ethylidene)amino)phenol HL have been synthesized using the ultrasonic irradiation and characterized by physicochemical and spectroscopic methods. The IR spectrum of Pt(II) complex showed that the coordination in metal ions via the imine N and S atoms. Magnetic and UV–Vis spectra showed that the geometrical structure of Pt(II) complex is square planar. The molar conductance data revealed that Pt(II) complex is non-electrolyte. The thermodynamic parameters for thermal decomposition of Pt(II) complex was calculated from analysis of the TG-DTG curves using Horowitz–Metzger method. The fluorescence of the ligand HL and its Pt(II) complex in different solvents and pH conditions have also been studied in detail. The quenching of the fluorescence intensity of the ligand can be noted obviously, in the presence of Pt(II) ion. Geometry optimization of the compounds was calculated by semiempirical molecular orbital theory PM3 method using Hyperchem 7.52 program.

© 2018 Elsevier B.V. Two newly synthetized imidazole derivatives (1-(4-methoxyphenyl)-4,5-dimethyl-1H-imidazole-2-yl acetate (MPDIA) and 1-(4-bromophenyl)-4,5-dimethyl-1H-imidazole-2-yl acetate (BPDIA)) have been prepared by solvent-free synthesis pathway and their specific spectroscopic and reactive properties have been discussed based on combined experimental and computational approaches. Aside of synthesis, experimental part of this work included measurements of IR, FT-Raman and NMR spectra. All of the aforementioned spectra were also obtained computationally, within the framework of density functional theory (DFT) approach. Additionally, DFT calculations have been used in order to investigate local reactivity properties based on molecular orbital theory, molecular electrostatic potential (MEP), average local ionization energy (ALIE), Fukui functions and bond dissociation energy (BDE). Molecular dynamics (MD) simulations have been used in order to obtain radial distribution functions (RDF), which were used for identification of the atoms with pronounced interactions with water molecules. MEP showed negative regions are mainly localized over N28, O29, O35 atoms, it is represent with red colour in rainbow color scheme for MPDIA and BPDIA (which are most reactive sites for electrophilic attack). The first order hyperpolarizabilities of MPDIA and BPDIA are 20.15 and 6.10 times that of the standard NLO material urea. Potential interaction with antihypertensive protein hydrolase (PDB ID: 4XX3) has been investigated by molecular docking procedure. The docked ligands form stable complexes with hydrolase inhibitor and the docking results suggest that the compounds might exhibit inhibitory activity against hydrolase inhibitor.

The electron donor–acceptor complexes formed between boron trifluoride and the nitrosyl halides ONF and ONCl have been studied by means of ab initio molecular orbital theory at the second order level of Moller–Plesset perturbation theory and with Dunning’s augmented correlation-consistent polarized valence triple-zeta basis set. The focus of the calculations was on the structures, interaction energies and vibrational spectra of the complexes. A variety of trial structures were examined, with electron donation occurring from the oxygen, nitrogen and halogen atoms, in an attempt to establish the most favorable site for interaction. In addition, a number of rotational isomers for each adduct were investigated. It was found that, in both cases, the halogen atom was the preferred donor atom. Several of the optimized structures suggested that the formation of ion pairs would lead to stable complexes, and four separate ion-pair structures were included among the possible associated species for each combination of interacting molecules. In those cases the computed spectra were more consistent with those of the NO + and BF 4 − or BClF 3 − ions than of the corresponding neutral components. The relative stabilities of the two families of complexes have been rationalized and some differences between the properties of the ONF and ONCl adducts have been observed and explained.

The spectroscopic and molecular structure of 2-{(E)-[(6-methylpyridin-2-yl)imino]methyl}phenol (MPMP) has been characterized by FT-IR, UV–Vis and X-ray diffraction. The ground state geometry of MPMP has been optimized by using density functional theory at DFT/B3LYP/6-31G** and DFT/B3LYP/6-31 + G* level of theories. The DFT/B3LYP/6-31 + G* level of theory is good to reproduce the geometrical parameters. We shed light on the frontier molecular orbitals; highest occupied molecular orbitals (HOMOs) and lowest unoccupied molecular orbitals (LUMOs) which play important role in charge transfer properties. The absorption spectrum was calculated by using time dependent density functional theory at TD-B3LYP/6-31 + G* level of theory. The absorption wavelengths were computed in different solvents by using polarizable continuum model (PCM). The solvent polarity effect was studied on the absorption wavelengths. The structure–property relationship has been discussed intensively.

The molecular structure of trimethylsilyl trifluoroacetate, CF 3 C(O)OSi(CH 3 ) 3 , has been determined in the gas phase from electron-diffraction data supplemented by ab initio (MP2) and DFT calculations using 6-31G(d), 6-311G(d,p), 6-311++G(d,p) and 6-311++G(3df,3pd) basis sets. Experimental data indicate that only one conformer, with C s symmetry [dihedral angle ϕ (CCOSi) = 180°, and all groups staggered], is observed in the gas phase. Theoretical data indicate that both this anti conformer and a gauche conformer, created by rotating about the C(O) O bond, are possible, although the preferred conformation is the staggered anti one. The torsional energies for different values of the CCOSi and COSiC dihedral angles have been calculated using the RHF, MP2 and B3LYP methods with the 6-311++G(d,p) basis set. For rotation around the CC OSi bond, a sixfold decomposition of the rotational barrier has been performed in terms of a Fourier-type expansion, enabling us to analyze the nature of the potential function, showing that the coefficients related to electrostatic interactions and steric effects are the dominant terms. The preference for the anti conformation was studied using the total-energy scheme, comparison of dipole moments, and the natural bond orbital partition scheme. The infrared spectra for the liquid and gas phases and the Raman spectrum for the liquid phase have also been recorded and the observed bands assigned to the vibrational normal modes. The experimental vibrational data, along with calculated theoretical force constants, were used to define a scaled quantum mechanical force field for the target system that enabled us to estimate the measured frequencies with a final root-mean-square deviation of 9.7 cm −1 .

The hydrogen-bonded molecular complexes formed between methanol and dimethylamine, as proton donors, and the series of proton acceptors trimethylamine, dimethyl ether, methyl fluoride, trimethyl phosphine, dimethyl sulphide and methyl chloride, have been studied by means of ab initio molecular orbital theory, at the second order level of Moller–Plesset perturbation theory, and using the 6-311++G(d,p) basis set. The properties determined were the molecular structures, the interaction energies and the vibrational spectra. The results show that, based on the trends in the values of these properties, the order of strength of interaction per proton donor is methanol > dimethylamine and the order per proton acceptor is trimethylamine > dimethyl ether > methyl fluoride for the first row bases and trimethyl phosphine > dimethyl sulphide > methyl chloride for the second row. The first row proton acceptors consistently form stronger hydrogen-bonded complexes than their counterparts from the second row.

The full multicentre molecular Hamiltonian in local density approximation for a mononuclear transition metal complex is transformed into a single-centre Hamiltonian explicitly including overlap, covalency and ligand-field effects. The orbital interactions of the metal d-orbitals with the ligand orbitals appear as a repulsive pseudopotential yielding the dominant contribution to the ligand-field splitting. This repulsive pseudopotential exhibits the same angular dependence as the electrostatic potential from the ligands entering the Hamiltonian of ligand-field theory. For this reason, ligand-field theory very often yields the correct splitting pattern of the d-orbitals. The radial part, however, is considerably different from the simple expression of ligand-field theory. In particular, there is no general theoretical justification for a R − 5 -dependence of the ligand-field splitting even for complexes of cubic symmetry. The reliability and capability of this new approach is demonstrated by calculating the d-orbital splitting pattern for a number of selected systems.

The molecular structure of 2,2,2-trifluoroethyl trifluoroacetate, CF3CO2CH2CF3, has been determined in the gas-phase from electron-diffraction data supplemented by ab initio (MP2) and DFT calculations using basis sets up to 6-311++G(d,p). Both experimental and theoretical data indicate that, although both structures with anti, anti (Cs) and anti, gauche (C1) conformations exist by rotating about the O C(H2) bond, the anti, anti structure is preferred. The difference in free energy was calculated to be 2.1 kJ mol−1 (Cs conformer lower in energy) and as the C1 conformer has a double multiplicity relative to the Cs conformer, the ratio of C1–Cs conformer was predicted to be 0.41: 0.59. This conformational preference was studied using the total energy scheme and the natural bond orbital partition scheme. Additionally, the total potential energy has been deconvoluted using six-fold decomposition in terms of a Fourier-type expansion. Infrared spectra of CF3CO2CH2CF3 have been obtained for the gaseous, liquid and solid phases and the Raman spectrum for the liquid phase. Harmonic vibrational frequencies and a scaled force field have been calculated, leading to a final root-mean-square deviation of 7.3 cm−1.

A new type of macrocycle containing 2-thio-4-aminopyrimidine and 6-methyluracil moieties has been studied by IR and UV spectroscopy. The vibrational spectra, conformations and amine–imine tautomerism of the aminopyrimidine fragments have been analysed within the framework of ab initio MO theory (HF/6-31G**, MP2/6-31G**) and density functional theory (B3LYP/6-31G**). According to experimental and theoretical results, the amino form is preferred in these compounds. Both intra- and intermolecular hydrogen bonds of the type NH⋯N are present in crystals, whereas in solutions, conformational equilibria with free NH groups are also obtained.

A semi-phenomenological model describing wideband dielectric and far-infrared spectra of liquid water was proposed recently by the same authors [J. Mol. Struct. 606 (2002) 9], where a small dipole-moment component changing harmonically with time determines a weak absorption band (termed here the R-band) centred at the wavenumber ν∼200 cm−1. In the present work, a rough molecular theory of the R-band based on the concept of elastic interactions is given. Stretching and bending of hydrogen bonds cause restricted rotation (RR) of a polar water molecule in terms of a dimer comprising the H-bonded molecules. Analytical expression for the RR frequency νstr is derived as a function of the RR amplitude, geometrical parameters and force constants. The density g(νstr) of frequency distribution is shown to be centred in the R-band. The spectrum of the dipolar auto-correlation function calculated for this structural-dynamical model is found. A composite model comprising two intermolecular potentials is proposed, which yields for water a good description of the experimental wideband (from 0 to 1000 cm−1) spectra of complex permittivity and of absorption coefficient. The presented interpretation of these spectra is based on a concept that water presents a two-component solution, with components differing by the types of molecular rotation.

The concept of multicenter bond is paradoxically counter intuitive in chemistry because most of chemistry and molecular physics is thought in terms of two body interactions. Moreover there is a technical difficulty to account for multicenter bonding in the framework of standard approaches such as the MO theory. The topological approach of the chemical bonding enables a position space partition of the electron density in terms of basins of attractor whose chemical significance is given by their location with respect to the nuclei. There are core and valence basins. Core basins correspond to the inner atomic shell density whereas valence basin density is organized around and between the core basins. The valence basins are characterized by the number of core basin with which they have a boundary. This number is called the synaptic order. There are therefore monosynaptic basins corresponding to electron lone pairs, disynaptic basins to conventional two-center bonds, trisynaptic basins to 3c-2e bonds, etc. The usefulness of the concept is illustrated by examples belonging to both molecular and solid state chemistry.

We have obtained two initial complexes (linear HPHCl and HPClH), and studied the reaction mechanism of singlet radical phosphinidene with polar hydrogen chloride molecule by ab initio molecular orbital theory at HF/6-31G(d), MP2(fc)/6-311+G(d,p) and G2 levels. Moreover, we have also calculated the thermodynamic and kinetic functions from 100 to 1100 K in steps of 200 K making use of the statistical thermodynamics and Eyring transition state theory with Wigner correction. It is concluded that there are two parallel reaction paths: Reaction A, an addition reaction which has the transition state (TS) formed by three-membered ring, has thermodynamic favor but does not have kinetic favor at low temperatures. However, it has both thermodynamic favor and kinetic favor at high temperatures comparing with Reaction B, a dehydrogenation reaction which has the TS formed by four-membered ring.

The NMR chemical shift of a three-dimensional polyacetylene crystal in the orthorhombic form was calculated by a combination of ab initio tight-binding MO theory and the sum-over-states method of the chemical shift theory within the STO-3G minimal basis set, in order to elucidate the inter-molecular interaction effect of the 13 C NMR chemical shift and electronic structure, as associated with the crystallographic form. The effects of inter- and intra-chain on the 13 C NMR chemical shift and the electronic structure were reasonably evaluated.

Formulae for calculating the NMR chemical shift of a three-dimensional (3D) polymer crystal were derived by a combination of ab initio tight-binding MO theory and the sum-over-states method of the chemical shift theory. This formalism was applied to the calculation of the 13C NMR chemical shift of 3D polyethylene chains in the orthorhombic and the monoclinic crystallographic forms by using the STO-3G minimal basis set. The effects of inter- and intra-chain interactions on the 13C NMR chemical shift and the band structure were evaluated with a change by means of the a and c axes and the b axis in the lattice constants. The calculated results explain reasonably the experimental data.

The structure, energetics, and vibrational properties of complexes formed between NH 3 and CO have been investigated by matrix isolation FTIR spectroscopy and ab initio molecular orbital theory. Two stable computational minima were found representing nearly linear hydrogen bonds between the subunits. The NH 3 CO and NH 3 OC species were calculated to be bound by − 3.5 and − 1.7 kJ mol −1 , respectively. Experimentally, both weak complexes could be identified in low temperature argon matrix with the oxygen-attached complex dominating, quite surprisingly, in photolytic complex formation from formamide.

The structure, energetics, and vibrational properties of complexes formed between H 2 S and CO have been investigated by matrix isolation FTIR spectroscopy and ab initio molecular orbital theory. Two stable computational minima were found representing nearly linear hydrogen bonds between the subunits. The H 2 SCO and H 2 SOC species were calculated to be bound by 5.22 and 1.54 kJ mol −1 , respectively. The computational results were reproduced by experimental assignments for the carbon attached complex. The stretching vibrations of the complex subunits were found to be similarly perturbed upon complexation both experimentally and computationally.

Extensive Brownian dynamics simulations have been performed on a dipolar cubic lattice to study the dynamics of orientational relaxation in the presence of dipolar interactions. In particular, detailed calculations are carried out to obtain the single-particle orientational correlations $C_l(t), C_l(t) = [4\pi/(2l+ 1)] \Sigma^l_{m = - l}$ $\langle Y^*_{lm} [\Omega(O)] Y_{lm}[\Omega(t)]\rangle$ where $Y_{lm}(\Omega)$ are the usual spherical harmonics and the dielectric friction on a rotating dipole. It is found that the orientational correlation functions become progressively non-exponential as the polarity of the system is increased. At the highest polarity studied (with a dielectric constant of 19.0), the $C_1 (t)$ factor is strongly non-exponential, indicating a considerable effect of dipolar interactions on orientational relaxation. The correlation functions obtained from simulations are compared with the molecular theory of Madden and Kivelson and the continuum-model based theories of Nee and Zwanzig and Hubbard and Wolynes. At low polarity the molecular-based theory of Madden and Kivelson fares better than the continuum theories. At intermediate and higher polarity, however, agreement between the simulation and the theories becomes unsatisfactory. The important prediction of Hubbard and Wolynes on the rank dependence of dielectric friction and its effect on the higher correlation functions also has been examined through simulations and compared with the theories mentioned above. The dielectric friction was found to decrease rapidly with increasing I, in qualitative agreement with the predictions of Hubbard and Wolynes. The observed effect however is much stronger than the predictions of the existing theories. The present simulations provide unambiguous proof of the important role of dipolar interactions on orientational relaxation in dipolar systems.

Semi-empirical molecular orbital theory was used to investigate the structures and energies of species involved in the Stevens' rearrangement of 12 different alkylammonium ylides. By starting with simple prototypes and systematically building up the molecules, we were able to elucidate some of the effects that determine the geometries and barrier heights in this reaction. Complete geometry optimisations were successfully carried out for all species, including our largest system, which corresponds to the original experimental reaction reported by Stevens in 1928. We also investigated the use of a bromine atom to mimic the steric effects of a phenyl substituent, and found that this gives satisfactory results. Finally, the results presented in this paper confirm that at the MNDO level of theory the gas phase Stevens' rearrangement proceeds via a stepwise rather than a concerted mechanism.

The optimized structures, and the infrared and Raman spectra of the dimers of nitrosyl fluoride and nitrosyl chloride have been predicted by means of ab initio molecular orbital theory, using the Gaussian 86 and Gaussian 88 computer programs. The calculations were carried out at both the restricted Hartree—Fock and the second order Moller—Plesset levels of theory, using the 6-31G * split-valence polarized basis set. The results have been compared with a similar set of calculations on the respective monomers and, in the case of nitrosyl chloride, with the Raman spectrum of the pure solid and with the infrared spectra of the solid and of nitrosyl chloride isolated in argon matrices. The energies of dimerization, internal valence force constants and atomic polar tensors, and their invariants have also been determined, and have been rationalized on the basis of the derived structures and the proposed mode of interaction.

The molecular structures, interaction energies and infrared spectra of seven H 2 O·CO and five H 2 O·N 2 complexes have been computed using ab initio molecular orbital theory. The most probable structures predicted by these calculations agree with the evidence obtained from gas-phase molecular spectroscopy. The infrared spectra of the various complex species are very similar to one another and to the spectra calculated for the free monomers. A limited number of relationships has been established between the calculated OH-stretching band ratios and the CO- and NN-stretching wavenumber shifts, and the computed interaction energies of the hydrogen-bonded adducts. Comparisons have been made between the wavenumbers calculated for the most probable structure in each case and those determined experimentally in cryogenic matrices. Agreement is very favourable, averaging 5–7% too high for the water submolecules and slightly too low for the CO and N 2 units.

Tight-binding MO calculations on the 13 C NMR chemical shifts for n -alkane chains in the rotator phase have been carried out within the INDO/S sum-over-states framework, taking into account the interchain interactions. The calculations show that the interchain interactions play an important role in determining chemical shift behaviour in the rotator phase. The relationship between the structure and 13 C NMR chemical shift was further clarified through the understanding of the electronic state of the n -alkane chains in the rotator phase.

17 O NMR spectra (natural abundance) for selected acylsilanes show high-frequency chemical shifts with respect to the corresponding ketones. An MO qualitative analysis clarifies the origin of the above experimental finding as being due to other well-known chemical and physical differences between the two types of compound. The importance of a relatively small HOMO—LUMO gap in acylsilanes is stressed. The alkyl and silyl substituents promote different electron distributions over the CO σ and π fragment orbitals which can ultimately contribute to the shift of the NMR resonances. The MO analysis addresses new viewpoints on acylsilanes: (i) the silyl groups, rather than strong σ-donors should be better defined as weak σ-acceptors; (ii) the carbonyl carbon atom is negatively charged, but the electron density is accumulated in the σ orbitals and not in the p π one; (iii) the IR data are rationalized in terms of a weakened in plane component of the CO bond, while the π ⊥ bond is unaffected; (iv) the carbonyl carbon atom keeps its electrophilic character in acylsilanes and it can exert nucleophilicity only upon departure of SiR + 3 , a good leaving group.

The infrared spectra of the stable hydrogen bonded homodimers and heterodimers formed from among water, ammonia and hydroxylamine have been predicted using ab initio molecular orbital theory, at the level of second order Moller-Plesset perturbation theory and using the split valence polarized 6–31G** basis set. The intermolecular modes may be classified generally as hydrogen bond stretching and in-plane and out-of-plane bending, etc. Depending on the structure and symmetry of the complex, however, the forms of the intermolecular normal modes can vary widely, and we recognize three separate situations: (1) linear hydrogen bonded complexes of Cs symmetry, (2) symmetrical cyclic hydrogen bonded complexes with a plane of symmetry (C2h), and (3) unsymmetrical cyclic hydrogen bonded complexes with no plane of symmetry (C1). The forms of the normal modes of the complexes within each group are discussed, in the light of their structures, and their computer wavenumbers and intensities.

A variational least-squares technique is developed for calculating general non-spherical continuum orbitals for Auger electrons. The use of the continuum orbitals in obtaining Auger transition rates and the special cases of one-center and two-center systems are discussed.

The structure of isopropenyl radical has been calculated using ab initio MO theory. It was found that CC bond is longer and CC bond is shorter than those in propene. Vibrational frequencies of isopropenyl radical (CC stretching, HCH bending, CH 2 out of plane wagging, and CCC bending) as determined by the normal coordinate calculation have the lower values than in propene IR spectrum.

The HeI photoelectron spectrum of trans -1,2-dibromocyclohexane has been recorded. From an analysis of the spectrum and the application of molecular orbital theory we obtain information about the interactions between the lone pair orbitais of the two bromine atoms in the molecule.

The geometries of the trans and cis forms of 3-chlorobenzaldehyde (I and II, respectively) were optimized using standard single determinant MO theory with the STO—3G basis. The calculated energy difference between I and II was 90 cal mole −1 . Mean amplitudes of vibration were calculated for the system by vibrational analysis. Computed molecular properties — amplitude differences and ab initio differences between some geometrical parameters — were combined with the gas electron diffraction data for the compound to constrain least-squares data analysis. The model which was in best agreement with experiment represented a conformational equilibrium with a composition of 64(16)% trans and 36% cis for the vapor of 3-chlorobenzaldehyde at 120°C. This result contradicts a previous vapor phase far infrared investigation of the compound and the torsional potential derived from it. Potentially misleading factors may, therefore, generally have to be taken into account whenever torsional potentials are derived from vibrational data. Beyond the scope of the structural problem considered, the paper demonstrates the advantages of molecular orbital constrained electron diffraction (MOCED) studies.

A MO theory of chemisorption is presented that relates the structure of the adsorption site to the group orbital local d. of states at the Fermi level projected out from the surface MOs by the adsorbate orbitals. Preferential adsorption of CO to \"on-top\" sites of the faces of noble metals such as Ni, Pt, and Rh is ascribed to the subtle balance between on-top directing interactions with the metal valence d-electrons and bridge coordination directing interactions with the metal valence s-electrons. The difference in hydrogenolysis behavior between Ni and Pt is explained

Internal rotation in 1,4-disubstituted but-2-ynes XCH 2 CCCH 2 Y (X, Y = H, CH 3 , Li, F) has been studied with the aid of ab initio molecular orbital theory. When either or both of X and Y are approximately electroneutral (H or CH 3 ), the barrier to internal rotation is very small. On the other hand, when X and Y are both electropositive (LiCH 2 CCCH 2 Li) or both electronegative (FCH 2 CCCH 2 F), there are pronounced preferences for conformations in which the dihedral angle XC·CY is slightly greater than 90°. When X is electropositive and Y electronegative (LiCH 2 CCCH 2 F), the stable rotational isomers are cis and trans with the former being preferred. For all molecules examined here the threefold component of the torsional potential function is very small.

Theoretical structures for vinyl alcohol are obtained using ab initio molecular orbital theory and the minimal STO-3G and split-valence 4-31G basis sets. The preferred conformation has HOCC syn . Correction for the systematic deficiencies of both basis sets and use of experimental rotational constants leads to a prediction of the complete r o structure for vinyl alcohol.

The rotational potential surfaces of formic anhydride and divinyl ether have been investigated using ab initio molecular orbital theory with minimal and extended basis sets and a flexible rotor geometric model. For formic anhydride, the favoured conformation is planar cis-trans (IB), with additional minima corresponding to planar cis-cis (IA) and trans-trans (IC) structures; these lie. respectively, about 1–3 and 3–7 kcal mol−1 above the cis-trans structure. The preferred conformation for divinyl ether is non-planar with C1 symmetry, corresponding to a distortion of the cis-trans structure (IIB). Other minima are found for the planar C2v cis-trans structure (IIC) and for a non-planar structure with C2 symmetry near to cis-cis (IIA). These are predicted to have energies 0.7 and 2.4 kcal mol−1, respectively, higher than the preferred C1 structure.

A consistent set of numerical information on the electron organisation in the amide unit has been obtained for a number of small amides, using the readily available Gaussian-70 computer package and a modest amount of computer time. There is good agreement with experimental facts concerning bond lengths, bond angles, rotation barriers and cis/trans energy differences. The results have been analysed in terms of simple molecular orbital theory and other simple valence ideas to make the results intelligible in a physical sense, so that they may be used in larger molecules. The rotation barrier about the CN bond seems to be a simple π-electron conjugation effect, whose magnitude is influenced in the main by the internal geometry of the CNO fragment rather than by substituents.

The molecular orbital (MO) energy levels of S 8 , Se 4 S 4 and Se 8 have been computed by the CNDO/2 method. The extravalent d orbitals of sulphur and selenium have also been included in the basis set orbitals. The calculated and observed transition energies are found to agree very well in the case of S 8 and Se 4 S 4 and a qualitative explanation has been given for the different mechanism of transport and the widely differing mobilities of electrons in orthorhombic sulphur and monoclinic selenium. The low-lying virtual orbitals of all three molecules are found to predominantly consist of d orbitals. As a result of the substitution of four selenium atoms into the eight-membered sulphur ring the upper occupied orbitals of Se 4 S 4 are dis- placed relative to those of S 8 and Se 8 . The properties of Se 4 S 4 are therefore not expected to be intermediate between those of orthorhombic sulphur and monoclinic selenium.

A simple graphical method is presented for determining the non-bonding molecular orbitals in conjugated hydrocarbons. Graph transformations which leave the number of non-bonding MO's unchanged are also shown and some examples discussed.

The conformational energies of 1-fluoro-2-propanol have been studied using ab initio molecular orbital theory employing minimal (STO-3G) and extended (4-31G) basis sets. The calculations favor the internally H-bonded conformations (I and III). A recent microwave study of the molecule has only detected conformation I, suggesting that conformation III is at least 0.75 kcal/mol higher in energy. Partial geometry optimizations at both levels of theory suggest that the two gauche conformations (I and III) are rather similar energetically and should be experimentally detectable. The conformational energies are analyzed in terms of a Fourier-type expansion of the potential function. The barrier to rotation of the methyl group has also been computed.

The application of ab initio molecular orbital theory to the study of molecular conformational analysis is discussed. Examples presented include methyl rotational barriers, internal rotation in 1,2-dihalogenoethanes, cis-trans isomerism in 1,2-dihalogenoethylenes, rotational barriers in substituted acetones and conformational preferences in substituted hydrazines.

Ab initio molecular orbital theory with minimal and extended basis sets and a flexible rotor geometric model has been used to investigate the rotational potential surfaces of methyl formate and methyl vinyl ether. For both molecules, the most stable structures (IA and IIA, respectively) are planar cis; additional potential minima are found which correspond to planar trans structures (IB and IIB). The latter lie respectively about 4—8 and 1—2 kcal mol−1 above the corresponding cis rotational isomers. Methyl rotational barriers have been determined for cis and trans structures of each molecule. For trans methyl formate, there is a slight but unexpected preference for an eclipsed arrangement of the methyl group.

The structure and energetics of the complexes formed between methanol and N 2 or CO have been investigated by ab initio molecular orbital theory. The most stable complexes are formed when the diatomic molecules form a nearly linear hydrogen bond with methanol. The large effect of electron correlation calculated at the MP2 level is especially notable for complexes with CO due to correlation-induced sign reversal of the dipole moment. Energy decomposition analysis shows the important role of electrostatic interaction for the structure and interaction energy. Although the interaction in studied complexes is rather weak, substantial perturbation of the vibrational spectrum of methanol, especially a shift of the torsion band of OH is found for hydrogen-bonded structures. This fact is in quantitative agreement with experimental observation.

The conformational energies of 1-amino-2-propanol, 2-amino-1-propanol and 1,2-diaminopropane are studied using ab initio molecular orbital theory employing minimal (STO-3G) and extended (4-31G) basis sets. Calculations at both levels of theory generally favor conformations stabilized by internal H-bonding for all molecules considered. Results are first presented for conformations employing assumed geometries. Since the conformational energy differences as found by the initial set of calculations are in some cases rather small it then becomes necessary to introduce geometry optimizations into the study at the minimal STO-3G level. In addition, to get a better estimate of the energy differences of the various conformations 4-31G calculations are performed on the STO-3G optimized structures. These latter results indicate the following, (a) For 1-amino-2-propanol only one conformation that is stabilized by intramolecular H-bonding is low in energy; this has the methyl and amino groups anti . The other H-bonded conformer, where the methyl and amino groups are gauche , is predicted to be ca. 1.2 kcal mol −1 less stable. Similar findings for this molecule have recently been provided by micro-wave spectroscopy. (b) For 2-amino-1-propanol the two H-bonded conformers are only separated by about 0.5 kcal mol −1 , with the anti conformer being more stable. Micro-wave spectroscopy again supports these calculations. (c) For 1,2-diaminopropane the gauche conformer is predicted to be of rather high energy (ca. 2.5 kcal mol −1 ) compared to the corresponding anti H-bonded conformer. The value of 2.5 kcal mol −1 should be taken as an upper limit, since the geometry optimization of the gauche conformer of 1,2-diaminopropane is incomplete compared to the optimization carried out for the anti conformer.

A simple method has been devised to define the profile and size of the lone pair orbitals in CH 2− , NH 3− and H 2 O − like systems. In addition, the results derived in the present work and those obtained within the framework of molecular orbital theory are compared.

MNDO calculations, involving the precise location of all stationary points, predict the cyclic ethylene fluoronium ion to exist in a shallow (3.8 kcal mol −1 ) minimum, 18.7 kcal mol −1 above the classical 2-fluoroethyl cation. In the stable conformation of the latter, the CF bond is perpendicular to the empty p orbital of the methylene group, rotation of which is hindered by a barrier of 7.4 kcal mol −1 . A three-dimensional energy surface for these interconversions is generated and discussed.

A bicentricly rescaled energy partitioned version of CNDO/2 molecular orbital theory is employed in a study of the atomization energies of the following pnicogen fluoride molecules, their cations and anions: NF, NF 2 , NF 3 , N 2 F 2 , N 2 F 4 , PF, PF 2 , PF 3 , PF 4 , PF 5 and P 2 F 4 . Trends predicted for ionization energies and electron affinities are discussed in terms of orbital structure and compared with available experimental data. Results indicate that normal CNDO/2 parameterization incorrectly weights relative bicentric contributions to the total molecular energy.

The electronic spectra of some hydroxyazo dyes, derivatives of azobenzene and benzeneazonaphthalene, are investigated and electronic transitions in the UV and visible regions assigned. The difference in spectral assignment of o - and p -hydroxyazobenzenes is explained using MO theory. Observations are made about possible azo—quinonehydrazone tautomerism in o -, m - and p -hydroxy derivatives of 4-phenylazo-1-naphthol and 1-phenylazo-2-naphthol and the quantitative value for the contribution of each tautomeric form is calculated. Calculations using the SCl—Cl method, within the PPP approximation, are performed for some hydroxyazo derivatives. The results of the calculations agreed significantly with the experimental spectral curves. Spectral differences between o - and p -hydroxyazobenzenes are established. It is shown that there is charge transfer from the aromatic rings to the azo group in the first excited singlet state.