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Ab initio molecular orbital calculations using an 8 s , 3 p ; 3 s Gaussian basis set, with contraction, have been used to study a series of primary amines XNH2, where X = H, CH3, OH, F, CN, CHO, and NO2. The geometries of the corresponding ammonium ions have been optimised and the energy differences have been used to estimate relative proton affinities. The 1 s orbital energies for both the amines and ammonium ions, when corrected for the effects of charges on the other atoms in the molecule by use of an ESCA equation, give a good correlation with the computed charge on the nitrogen atom.

The ionization potential of the closed shell H2CO, H2COH+, H2COH22+ as well as the proton affinity of H2CO and H2COH+ have been studied at a variable CO bond length in the ground singlet and some of the low lying excited triplet and ionized doublet states.

A conformational ab initio MO study has been carried out for the thiathiophthene molecule (TTP) and two related model compounds, thiomalonaldehyde (TMA) and its conjugate base (TMA(-)). The conformational energy surfaces for TMA, TMA(-) and TTP were generated using a least squares fit to the calculated data and plotted on a CALCOMP plotter. The results of the calculations showed that the cis-cis planar conformation of TTP is the most stable in agreement with experimental findings. For TMA and TMA(-) the cis-cis planar conformation is not the most stable. Contour plots of the five occupied π-MO's of TTP show great similarity to those of naphthalene.

Molecular energy is a function of two main types of variables, namely orbital exponents {αi} and geometrical parameters {qj} $$E = E(\alpha ,{\alpha _2} \ldots {\alpha _N},{q_1},{q_2}, \ldots {q_{3n - 6}}) = E(\underset{\raise0.3em\hbox{$\smash{\scriptscriptstyle-}$}}\to {\alpha } ,\underset{\raise0.3em\hbox{$\smash{\scriptscriptstyle-}$}}\to {q} )$$ The overall problem is often subdivided into two smaller problems $${\text{E }} = {\text{ E}}(\underset{\raise0.3em\hbox{$\smash{\scriptscriptstyle-}$}}\to {\alpha } )$$ $${\text{E }} = {\text{ E}}(\underset{\raise0.3em\hbox{$\smash{\scriptscriptstyle-}$}}\to {q} )$$ The first of these leads to exponent optimization and the latter one is important in molecular geometry optimization.

A series of model compounds capable of intramolecular sulfur(II)-oxygen interaction of the 1,5-type was investigated by the ab initio SCF-MO method. Compounds of the type X-S-A=B-Y(Z)=O with cis configuration about A=B, containing X = F, OH, NH/sub 2/, CH/sub 3/, and SH; A = CH; B = CH and N; and Z = H, O, as well as lone pair, were studied. This results in four families containing a total of twenty compounds. The geometries of two basic planar conformations of these compounds, s-cis/s-trans (CT) and s-trans/s-trans (TT), obtained by internal rotations about the B-Y and S-C bonds, respectively, were optimized by using three different basis sets. These energy minimum conformations represent optimum geometries with and without S...O interactions, respectively. Four quantities were used to measure the extent of S...O interactions. These included the lengthening of the Y-O bond on going from TT to CT (..delta..R(Y-O)), the shortening of the S...O distance (..delta..R(S...O)), the energy change between TT and CT (..delta..E(S...O)), and the Mayer type bond order in the CT conformations (B(S...O)). The linear interdependence of these four quantities indicated that each of these four parameters was about equally good measure of the extent of S...O interaction. With the aidmore » of these numerical measures earlier qualitative principles may now be evaluated quantitatively.« less

Since a conformational process involves the rearrangement of the atomic nuclei coupled with the redistribution of electron density, conformational potential energy surfaces are offered as prototypes for reactive potential energy surfaces.

A total of nine molecular Hartree‐Fock wavefunctions have been computed for CH3O− (I), CH3–OH (IIA–IId), and CH3–OH2+ (IIIa–IIId). The lowest energies obtained, in hartree atomic units, were: − 114.34285 (CH3O−), − 115.01162 (CH3OH), and − 115.32787 (CH3OH2+). The two proton affinity values for the successive protonations (CH3O−→ CH3OH→ CH3OH2+) were calculated to be − 419.4 and − 198.9 kcal/mole, respectively. For CH3–OH, the barriers to rotation and to in‐plane inversion were computed to be 1.44 and 32.5 kcal/mole, respectively; CH3–OH2+ showed no barrier to either rotation or inversion. Some excitation energies, dipole moments, and charge distributions have also been computed.

Non-empirical SCF–MO computations have been performed on [CH2CHO]−, the simplest enolate anion. Following a preliminary search of the conformational hypersurface using an STO-3G basis set to determ...

Although the title of this NATO Advanced Study Institute (ASI) specified the subject matter as “Computational Theoretical Organic Chemistry”, certain fundamentals used explicitely or im-plicitely are valid regardless whether the molecule is classified as organic, inorganic or biological. For this reason the term “Organic” is omitted from the title of this chapter.

Multidimensional theoretical stereochemistry and conformational potential energy surface topology.- Some practical suggestions for optimizing geometries and locating transition states.- Reaction topology and quantum chemical molecular design on potential energy surfaces.- Topology of molecular shape and chirality.- Adiabatic and diabatic surfaces in the treatment of chemical reactivity. I. Theory.- Adiabatic and diabatic surfaces in the treatment of chemical reactivity. II. An illustrative application to the Diels Alder reaction.- A qualitative valence bond model for organic reactions.- Solvent effects on potential energy surfaces and chemical kinetics.- Modifications of potential energy surfaces by solvation and catalysis.- Computational tests of potential energy surfaces from dynamical properties.- Dynamical formulation of transition state theory: variational transition states and semiclassical tunneling.- Theoretical models for reaction dynamics in polyatomic molecular systems.- Practical applications of new theoretical concepts in organic chemistry.

The molecular geometries of twenty-one monosubstituted acetylenes and their anions have been optimized using the LCAO-SCF-MO method at the STO-3G level. Single 4-31G calculations at the STO-3G geometries permit comparison of the different basis set results to test the reliability of intrinsic acidities calculated from the differences in molecular energy of the neutral acetylenes and the corresponding anions [E(RCC)E(RCCH)] The size and centroid of charge have been computed for the acetylenic CH bond and lone electron pairs and were found to be substituent dependent. The calculated intrinsic acidities correlate well with tritium isotope exchange rates reported elsewhere, indicating that the anion solvation effects are relatively uniform throughout the series of monosubstituted acetylenes, presumably due to the large intramolecular distance from the substituent to the reaction center.

Interconversion of the three isomeric or valence tautomeric structures of C 2 H 4 Br + Br is hindered by the apparent high barrier to 1,2 hydride shift (III → II) in spite of the fact that the final (I) and initial (III) states have about the same stability.

It is suggested that in addition to the size of an electron pair, defined as the expectation value of the spherical quadratic operator (r 2) over the LMO basis evaluated at the centroid of charge 〈r〉0 $$ \langle {{r}^{2}}\rangle = \langle {{r}^{2}}\rangle 0 - \langle r\rangle {{0}^{2}}, $$ , the components of this expectation value $$ \langle {{r}^{2}}\rangle = \langle {{x}^{2}}\rangle + \langle {{y}^{2}}\rangle + \langle {{z}^{2}}\rangle $$ may be used to characterize the shape of the electron pair. This definition of shape is particularly useful if the 〈x 2〉, 〈y 2〉 and 〈z 2〉 components are in alignment with the major and two minor axes of the elipsoid.

Non-empirical double zeta quality molecular orbital calculations on −CH2OH as a function of the C—O bond length (r), the rotational angle about the C—O bond (θ), and the pyramidal angle at carbon a...

The rotational barrier height E rot in the lowest triplet state oxirane molecule was calculated to be 26.3 kcal/mole using a double zeta basis set with partial geometry optimization. This suggest ldrelaxedrd rotation and the computed e (T 1 - E (S o ) + E rot value is commensurate with the enthalpy change for the oxirane-forming O( 3 P) + C 2 H 4 reaction, thus providing a rationale for the stereochemical features of the reaction.

The results of a nonempirical SCF–MO calculation on the linear and cyclic structures of the ion C2H3S+ indicate that the cyclic form is 65·9 kcal/mole more stable than the linear form.

Barriers to pyramidal inversion of directly bonded three-co-ordinate atoms exhibit an interdependence which is determined principally by the differences in the electronegativities of these atoms; thus the phosphorus inversion barrier in H2NPH2 is higher than that in PH3.

Systematic non-empirical SCF-MO calculations were carried out on the two limiting structures of C2H4SH+. In both of these cases two different atomic orbital basis sets were used. The results indicate that the open structure (β-thioethyl cation) lacks thermodynamic stability and spontaneously interconverts into the bridged structure (thiiranium ion). The pyramidal inversion at sulphur in the thiiranium ion was computed to be 78.11 kcal mol–1.

Ab initio SCF molecular orbital calculations were carried out on the amide H2P(O)NH2(IV) and its N-protonated H2P(O)NH3+(V), and O-protonated H2P(OH)NH2+(VI) forms using optimized geometries. The total energies as well as bond lengths and angles were obtained for each individual structure. Conjugate acid (VI) was found more stable than (V). The preferred conformations of (VI) involve the syn- and anti-periplanar arrangement of the nitrogen lone pair and the POH group. In order to investigate the possibility of the intramolecular hydrogen bonding in (VI) a potential energy surface involving OH torsion and inversion at nitrogen was generated.The results obtained are related to the reactivity of phosphoric amides under acidic conditions.

Non-empirical SCF-MO calculations, using two different basis sets, were carried out on a cyclic sulphurane and a thiiranium–chloride ion-pair. The calculated energies indicated that in the gas phase the covalent cyclic sulphurane is favoured over the ionic species. The activation energy for the internal collapse of the cyclic sulphurane to product was calculated to be 42 kcal mol–1, well below the energy of the thiiranium–chloride ion-pair. These results support the suggestion that thiiranium ions prevail in polar solvents but tetracovalent sulphur intermediates are favoured in solvents of low polarity.

Non-empirical SCF-MO wave functions were computed for the two limiting structures of C2H3S+ with full geometry optimization using three different atomic orbital bases. It was found that the bridged structure (thiirenium ion) was somewhat more stable (1–14 kcal mol–1 depending on the basis set) than the open structure (β-thiovinyl cation). The barrier of interconversion between these two valence tautomers was calculated to be 12·8 kcal mol–1 without geometry optimization. The pyramidal inversion at sulphur in the thiirenium ion was computed to be 72·9 kcal mol–1.

Sulphur 3d-orbitals are not necessary to explain the structure of dimethyl sulphide dicarbanion.