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1,8,9,10,11,11-Hexachlorotetracyclo[6.2.1.13,6.02,7]dodec-9-en-4-exo-yl acetate, tetracyclic1; 1,2,2,3,10,11-hexachloropentacyclo[5.4.1.03,10.04,12.05,9]dodecan-8-exo-yl acetate, half-cage2; 1,2,2,3,10,11-hexachloropentacyclo[5.4.1.03,10.04,12.05,9]dodecan-8-endo-yl acetate, half-cage3 and 1,9,10,11,11,12-hexachlorohexacyclo[5.4.1.02,6.03,10.04,8.09,12]dodecane, birdcage4 were obtained from acid catalysed acetylation of isodrin. It was observed that the intramolecular rearrangement control is highly dependent on reaction time. The equilibria involved in these rearrangements were determined by gas chromatography. Semiempirical calculations at the PM3-MNDO, AM1 and MNDO levels have been performed to obtain the optimized geometry of the reagent, products, intermediates and transition states for the rearrangement mechanism. The results of the calculations are in good agreement with the experimental data. On the basis of the theoretical and experimental investigations we propose a revised mechanism which involves a new transition state and a new non-classical reaction intermediate.

Ab initio (Hartree Fock), hybrid density functional (B3LYP), and semiempirical SCF (MNDO and AM1) calculations on sumanene (2), trioxa-sumanene (3) and trithia-sumanene (4) show that the C3v-bowl structure is a minimum in all cases, but show dramatic variations in bowl depths and inversion barriers. Calculations on monosubstituted corannulenes C19XH10 (X = N+, B−, P+ and Si) at various levels predict that isoelectronic substituents possessing large atomic size increase the bowl-to-bowl inversion barrier at the hub position and decrease it at the rim position. Strain is a guiding factor, which accounts for the relative stability of positional isomers, curvature and bowl rigidity. The most stable positional isomer for a given substituent shows the minimum bowl-to-bowl inversion barrier in all cases. Calculations are performed on monosubstituted sumanenes derived by replacing skeletal C by isoelectronic atoms on sumanene (2), C20XH12 for X = N+ and Si. The general strategy of substituting larger atoms at rim positions flattens the bowl, and at the hub position it makes the bowl deeper. The strategy seems to work well. HF/3-21G and B3LYP/6-31G* computations are in very good agreement with each other, both qualitatively and quantitatively, and the central results are reproducible even at semiempirical levels. The performance of MNDO is consistently better than AM1 and becomes the method of choice when ab initio and DFT methods are not practical. Homodesmic equations, used to ascertain the thermodynamic stabilities of the monosubstitutions on corannulenes and sumanenes, show that substitution at appropriate sites imparts stability to the buckybowl framework. Linear correlation is obtained between the curvature, as estimated by the pyramidalization angle (Φ), and the inversion barrier. It is shown that bowl rigidity, curvature and the relative stabilities of positional isomers are controlled by the strain energy build up, which depends on the size of the substituent and the site of substitution.

MNDO-d and ab initio RHF and B3LYP energies and geometries are reported for the reactant and transition states for F–R extrusion reactions of RR′I–F iodine(III) involving dimeric, and novel trimeric and tetrameric structures. The trimer is found to have unique properties deriving from novel transition state coordination between the reacting fluorine and the three iodines, which includes a significant decrease in the barrier to reaction compared to the monomer, dimer and tetramer.

The conformational and dynamic behaviour of the 25,26,27,28-tetraalkoxycalix[4]arenes 1–7 (R = methyl, propyl) has been investigated by X-ray crystallography, dynamic 1D and 2D NMR spectroscopy, as well as with a series of force field (PIMM) and semiempirical (PM3, MNDO) calculations. The stereo-electronic effects of the upper rim substituents of tetramethoxycalix[4]arenes characteristically influence the preferred arrangement of the phenyl rings in the paco conformation. The in/out isomerisation (0001≈AAAA⇌0001≈AAAB) taking place in 4 is strongly hindered in 1 due to the smaller diameter of the upper rim. The ease of rotation of the methoxy group attached to the inverted ring in paco-4 can be altered by using a solvent (CD2Cl2), which is able to form a host–guest complex with the calixarene cavity. For 1–3 cone/paco and paco/paco equilibria have been observed by 2D EXSY spectroscopy. Gibbs activation energies for the cone(C2v)/cone(C2v) conversion were determined for 1, 2, 5 and 6. An NMR line shape analysis provided Arrhenius and Eyring activation parameters for the cone(C2v)/cone(C2v) conversion in 5 and 6.

Semi-empirical molecular orbital calculations (MNDO) have been carried out on isomers of C60X36 and C70X36 (X = H, F) in order to investigate the underlying cause of the parallel behaviour of the two addends towards [60]- and [70]-fullerenes. In both series the driving force for reaction (in addition to C–X bond formation) appears, on the MNDO model, to be relief of steric strain in the fullerene cage, rather than the electronic effects on the carbon framework of the addition of hydrogen and fluorine atoms. The formation of hydrogenated and fluorinated compounds with 36 added atoms from both [60]- and [70]-fullerene appears to be coincidental, arising essentially from the fact that in both cases this number of added atoms is sufficient to maximise the relief of strain in the appropriate cage. While a T point-group structure appears significantly more stable than others for C60F36, the differences amongst isomers of C60H36 are small. The MNDO predicted structures of C70, C70X40 (X = H, F) are analysed to determine the major contributions to strain energy.

The addition of photochemically generated phosphoryl radicals [˙P(O)(OPri)2] to [76]fullerene has been studied by EPR and seven out of the nineteen possible RC76˙ regioisomers are obtained, six of which are stable. This compares with the previously observed addition of phosphoryl radicals to [70]fullerene which afforded three of the five expected RC70˙ regioisomers (only one being stable). Semi-empirical MNDO/PM3 calculations of the energies of all nineteen HC76˙ models and the unpaired electron distributions within them, were carried out in order to aid the assignment of the observed spectra to specific P(O)(OPri)2C76˙ regioisomers.

We have conducted a theoretical linear solvation energy relationship (TLSER) investigation of gas chromatographic (GC) retention indices for a series of 37 organosulfur compounds on three different columns, deriving regression equations based on descriptors obtained using the MNDO, AM1 and PM3 Hamiltonians. In all cases, satisfactory regressions based on two or three descriptors result, with molecular volume being the most important descriptor. Our results are qualitatively similar to those of Woloszyn and Jurs who, considering the same sulfur vesicant GC retention data set, applied an objective feature selection procedure to winnow descriptors from an initial set of 100, but were not able to treat any compounds containing sulfur–sulfur bonds. Our approach was free of this restriction, allowing for consideration of all 37 compounds. Only relatively small differences were obtained in the statistical quality of the regressions derived from the three Hamiltonians considered, and among the three GC columns.

Logarithmic rates for nucleophilic substitution towards ethyl iodide in acetonitrile correlate well with the specific interaction enthalpies for the relevant nucleophile, ΔHt,SIAN → MeOH(Nu–). According to the correlation, the bromide ion reaction falls into the imidide ion reaction series, while the chloride ion reaction falls into the carboxylate ion reaction series. The partial desolvation accompanying activation is crucial in determining the reactivity behaviour of the nucleophile anions. Semi-empirical quantum mechanical calculations (MNDO/PM3 and MNDO/PM3/COSMO) give reasonable estimates for the enthalpies of activation as well as for reaction enthalpies in acetonitrile. The relative location of the transition state, estimated on the basis of empirical analysis, is in accord with the variation of electronic charges for the exposed atoms, which has been evaluated through semi-empirical quantum mechanical procedures.

The absorption spectrum of 4-hydroxy-7-nitrobenzofurazan (NBDOH) and its conjugate anion has been recorded in 23 solvents. The data has been analysed according to the Taft and Kamlet treatment and compared with that of some parent compounds: 4-methoxy-, 4-propylamino- and 4-diethylamine-7-nitrobenzofurazan. The phenolate anion NBDO– has been shown to exhibit the solvatochromic behaviour characteristic of this NBD series in aprotic media, although in protic media hydrogen bonding had a drastic effect on the UV–VIS spectrum. Such a sensitivity to erotogenic solvents was not encountered for other phenoxide anions such as picrate. This difference of behaviour is discussed on the basis of electrostatic potentials obtained by MNDO calculations. Evidence is given of strong negative charges located on the C(5) and C(7) atoms of NBDO–, indicating that strong hydrogen bonding may take place with solvent molecules. However, this stabilization of the anion by hydrogen bonding does not seem to influence the dissociation equilibrium, the acidity of NBDOH being perfectly in line with that of other nitrophenols whatever the solvent.

Structural optimisations at the MNDO level for all nine isomers compatible with the published eight-line 19F NMR spectrum of C60F48 favour a chiral structure of D3 symmetry and its isoenergetic meso form by a margin of at least 175 kJ mol–1; both low-energy isomers would have 10-line 13C NMR signatures if fully 19F-decoupled, and their IR spectra are predicted to be closely similar.

The structure of 3-(4-tolyl)sydnone has been calculated using the semiempirical MNDO, AM1 and PM3 methods and at the ab initio STO-3G, 3–21 G and 6–31 G** levels and the resulting geometries compared and contrasted with the well resolved crystal structure. While the geometry of the phenyl ring is well reproduced particularly by the ab initio methods, none of the methods give a satisfactory account of the geometry of the sydnone ring. The calculated charge distributions of atoms in the heterocyclic ring vary considerably depending on both the molecular orbital method used and the analysis technique adopted. The calculated dipole moments are generally overestimated with the best results obtained with the MNDO method for the semiempirical procedures and the STO-3G basis set for the ab initio methods.

The geometry and inversion barrier of the ring substituted nitrogen in a quisqualic acid model compound 5, and the corresponding carbon (hydantoin) and nitrogen (dioxotriazolidine) analogues (6 and 7) has been investigated using the three MOPAC hamiltonians, PM3, AM1 and MNDO. In the case of the quisqualic acid and the nitrogen analogue model compounds, all three methods predict essentially pyramidal nitrogen configurations in agreement with X-ray crystallographic data for related compounds (1 and 8, respectively). For the carbon analogue model compound (6), only MNDO predicts a planar nitrogen geometry, in agreement with that found in the crystal structure of 3-(2,4-dioxo-2,3,4,5-tetrahydroimidazol-1-yl)alanine 3, with the other two methods suggesting a considerably more pyramidal structure. A search of the Cambridge crystallographic database indicates that nitrogen planarity is a general feature of this carbon containing hydantoin ring system, further emphasising the value of the MNDO hamiltonian when studying this type of heterocyclic structure. Our results indicate that different potential energy surfaces may be obtained from identical reaction coordinate calculations on different computer platforms, when PRECISE convergence criteria are used.

MNDO and PM3 calculations of the most favoured conformations of antitumour N-hydroxymethylmelamines including N-(hydroxymethyl)pentamethylmelamine (2) and trimelamol indicate that the O–H group is orientated towards the closest ring nitrogen but is too distant for hydrogen bonding. Modelling of decomposition pathways of 2 using MNDO predicts that concerted loss of formaldehyde is not favoured. Loss of [H2COH]+ from oxygen-protonated 2 is also a high energy process, but loss of formaldehyde from deprotonated 2 is exothermic with a low activation barrier; this is the most likely decomposition mechanism under basic or neutral conditions. Under acidic conditions loss of water from protonated 2 to give an iminium ion is exothermic from O-protonated 2, but the initial O-protonation is disfavoured over protonation on nitrogen, particularly ring nitrogens. Reaction of 2 with nucleophiles is difficult by SN2 routes but proceeds exothermically with low activation energies by the SN1 mechanism from protonated 2 to give products of similar thermodynamic stability to ring-protonated 2. Reaction of O-protonated 2 with amines gives aminals that may decompose with low activation energies to pentamethylmelamine and a methylene iminium ion.

A backpropagation neural network was trained with the molecular electrostatic potentials (MEPs) of a series of substituted imidazoles to predict their corresponding pKa. Using MEPs determined with a variety of semiempirical and ab initio methods, the predictive power of the trained network was found to be sensitive to the quality of the basis set. The network was also trained to predict the proton affinity (Epa) and pKa, both individually and combined, for a series of pyrazole MNDO MEPs.

The reactivity of the butoxy and 3-methylcyclohexyl radicals, as representatives of alkoxy radicals, was studied by MNDO-PM3 semiempirical MO method. In both cases it was found that the oxy radical underwent easily 1,5-H migration giving a δ-hydroxyalkyl radical. The hydrogen abstraction reaction goes through two transition states. The first transition state (TS) with short O–H distance has an energy very similar to, or lower than, the TS for 1,5-H migration. The second TS is of higher energy and is much product-like. The overall cyclisation reaction has a high activation energy (over 40 kcal mol –1), and is therefore unlikely to occur spontaneously. Most likely, the cyclisation step needs assistance of some other surrounding molecules to speed up the reaction.

The thermal decomposition of thiirane and 2-methylthiirane was investigated in a flow system by following the changes in their photoelectron spectra at various temperatures. Similar decomposition patterns were observed in both cases. Thus at 600 °C, formation of sulfur and an alkene was detected. At higher temperatures, conversion of thiirane into ethenethiol and thioacetaldehyde, and of 2-methylthiirane into cis- and trans-prop-1-ene-1-thiol was observed. Hydrogen sulfide, alkynes, carbon disulfide and thiophene were detected as products of secondary reactions. The experimental data pertaining to the rearrangement processes are compared with potential energy profiles, calculated by using the MNDO/CHAIN procedure. A mechanism involving initial C–S bond breaking, followed by 1,2-hydrogen shift, is suggested by the results.

Molecular mechanics (MM2) calculations have been performed to estimate the strain energies of hydrocarbons derived from the fragments of C70 which retain C5 or C2 symmetry. There are two ways of dissecting C70 retaining C2 symmetry, C2(a) and C2(b). The key fragment along the C2(a) route is pinakene. C28H14(12). It exists in a bowl shape. Its strain energy is close to that of the corresponding C28 fragment of C70. The bowl depth was calculated to be 2.03 A at the MNDO level, and the ‘bowl-to-bowl’ inversion barrier calculated to be about 48.6 kcal mol–1, implying that this is a very rigid bowl. The strain energy build-up from the sequential placement of methylene bridges in going from dibenzo[fg,op] naphthacene (26) to pinakene was estimated. Methyl derivatives of dibenzo[fg,op]naphthacene were taken as the key precursors in this study. Strain energies of various positional isomers were also evaluated. Synthetic strategies towards pinakene are delineated.

A series of singly annellated semibullvalenes has been studied by semiempirical (MNDO, AM1, PM3) calculations. The Cope rearrangement is examined in each case. In the ethano-annellated system 3 the groundstate for the molecule is predicted to be the symmetrical homoaromatic species 3c. This provides another example of the elusive class of neutral homoaromatics.

Physical organic chemistry has produced a lot of arguments in favour of the flexibility of the skeleton of the phenanthrene dianion, 1 2-. Circumstantial evidence for the non-planarity of dianions derived from phenanthrene substituted at the 'bay' position has been published.'S2 It has also been demonstrated that 1 2is a highly paratropic species and is hence regarded as a so-called antiaromatic molecule.3i4 It was suggested that in order to minimize paratropicity the molecular skeleton may undergo a deformation so that the efficiency of the 71conjugation is reduced and antiaromaticity is less pronounced. At the same time semiempirical calculations predict that 12is planar, and only a sophisticated treatment including the counter-ion enabled the non-planarity of 12-/2 Li' to be achieved computationally.2*6 We find that ab initio calculations in minimal basis set (STO3G) as well as semiempirical methods (MIND0/3, MNDO, AMl) describe the ground state of 12as a planar structure. However, when the basis set for ab initio calculations was extended to STO 3-2 1 G and further to STO 4-3 1 G, the planar structure of 12proved to be not a minimum, but a transition state (vim = 5 3 cm-' on HF/3-21G level) between two nonplanar minima with a dihedral angle C(4)

Molecular energetics of the N-(3-halobenzylidene)-3-haloanilines have been studied, using semi-empirical molecular orbital calculations, and served as a basis for comparison the results from a gas-phase electron diffraction (GED) experiment. As the molecule contains a π-electronic system—a phenyl ring linked through a single bond to a double bond—which has proved problematic for semi-empirical methods, a number of these have been tested: AM1, MINDO/3, PM3 and MNDO in addition to molecular mechanics [CAChe-MM2 and MM2(91)], and this study serves as a critical comparison and evaluation of these methods for treating such a classic molecular electronic linkage. The computational methods that were found to be suitable for the N-benzylideneaniline molecule were AM1 and MM2(91). For two of the title derivatives there is a significant difference between the conformation found in the crystal structure and that determined by GED in concert with the calculations based on the AM1 method.

The preparation, by azido-transfer reaction, of 2-azido-1 and 3-azido-selenophene 2 using the appropriate heteroarylithium derivatives and tosyl azide, followed by fragmentation of the intermediate triazene lithium salts is reported here. Different chemical reactivity and kinetic behaviour were observed for azides 1 and 2 in either 1,3-cycloaddition reactions, with (trimethylsilyl)acetylene and trimethyl(vinyl)silane, or thermal decomposition. Compound 1 gives cyclo-ANAE adducts (silylated triazole or triazoline)ca. three times faster than compound 2. Both the elusive triazoline adducts undergo rapid ring-contraction, with extrusion of nitrogen, to give rise to the corresponding 1-(selenophenyl)-2-(trimethylsilyl)aziridine (1b, 2b). Kinetic measurements of the unimolecular thermal decompositions afford distinct activation parameters: Ea= 21.5 and 30.4 kcal mol–1, ΔS‡=–10.7 and –0.9 cal mol–1 K–1 for 1 and 2. respectively. Experimental data, as a result of geometric and electronic disturbances exerted by the azido-group located at α- or β-positions of the selenophene ring, are qualitatively supported by measurement of 1H, 13C and 77Se NMR chemical shifts. The present experimental evidence and those previously obtained with related 2-azido- and 3-azido-thiophenes are corroborated with the determination of the structures and comparison between 13C substituent chemical shift (SCS) and charge distributions by using a semiempirical computational MNDO/PM3 method.

The Hel photoelectron spectra of nine isomeric [b,d]-annellated dithienopyridines (5–13) are reported and discussed including those of some closely related compounds such as phenanthridine (2), dithieno[3,4-a: 3′,4′-c]benzene (4) and its tetrahydro derivative 3. The ionization potentials are assigned to molecular orbitals using the results of MNDO, AM1 and PM3 calculations. The ability of PE spectroscopy as an analytical method in the analysis of the isomers 5–13 is investigated. From the spectra as well as from the theoretical results, the isomers can be divided into two groups, depending on the mode of annellation. The crystal and molecular structures of one member of each group (isomers 9 and 12) are determined by X-ray diffraction. The mode of annellation is also reflected in the bond lengths of the pyridine and thiophene rings.

The ionization potentials and electron affinities of a variety of organic radicals and closed-shell molecules have been calculated with the outer valence Green's function (OVGF) technique, coupled with semiempirical MNDO, AM1 and PM3 calculations. The calculations cover a wide range of energies > 11 eV in a variety of radicals and molecules. It is found that the OVGF method gives significantly better agreement with the experimental data than do results obtained with semiempirical calculations using Koopmans' theorem. Of the three semiempirical methods tested (MNDO, AM1, PM3) the OVGF(AM1) method gives the best agreement with experiment (e.g., for the vertical ionization potentials of 38 different organic radicals the mean deviation between experimental data and theoretical values is only 0.35 eV), and thereby can serve as a routine method for the calculation of the above-mentioned molecular properties of a variety of species.

Strain energies of the hydrocarbons derived from the fragments of C60 along the C5, C3 and C2 symmetry pathways indicate that the strain energy per carbon (Es) increases gradually along the C3 and C2 paths, but has a maximum at C40H10 along the C5 path. C21-Sumanene, 10, has been recognised as a leading fragment along C3 route with an inversion barrier of 24.2 kcal mol–1 and a bowl depth of 1.15 A at MNDO level. This contrasts with C20-corannulene, 6, which is a more shallow bowl with a lower inversion barrier. The bond alternation in the central six membered ring of sumanene resembles that of C60. The higher fragments en route to C60 are deep bowls with higher barriers for inversion. An approach to sumanene via a triphenylene derivative, 26, has been probed and an increase in strain through sequential placement of methylene bridges has been estimated, using MNDO and MM2 calculations. Synthetic routes to a doubly bridged triphenylene, 28, and schemes for further elaboration of 26a and sumanene derivative, 29, towards C60 are considered.

MNDO calculations of the most favoured conformations of antitumour 3-(hydroxymethyl)-3-methyl-1-(4-X-phenyl)triazenes (2) indicate N–CH2OH bond lengths of over 1.49 A(lengthened by electron-withdrawing groups), correlated with high lability under physiological conditions (t1/2 under 1 s). Hydrolysis rates of 2o(X = NO2) in methanol/acetonitrile containing water were determined using high performance liquid chromatography (HPLC) and extrapolated to give a t1/2 of 43 s at 100% water. Modelling of decomposition pathways of 2d(X = CH3) using MNDO predicts that concerted loss of formaldehyde is not favoured. Loss of [H2COH]+ from nitrogen protonated 2d is also a high energy process, but loss of formaldehyde from deprotonated 2d seems feasible under basic conditions. Under, acidic conditions the energetically preferred process is loss of water from protonated 2d to give an iminium ion, even though the required initial O-protonation is disfavoured over protonation on nitrogen.

Dibenzo[b,g]chrysene dianion (32–) shows in its NMR spectrum a partitioning of charge. In the dianion most of the charge is located on the ‘anthracene’ moiety. These observations have been confirmed by MNDO calculations. In the calculations, the location of the lithium cation is taken into account. This unusual phenomenon is connected with the system's paratropicity and with its charge alternation. The NMR assignments were assisted by 2D techniques.

Helicene dianions, e.g. phananthrene derivatives, once believed to be non-helical, maintain their chirality as 4nπ dianions. Phenanthrene 5, as well as substituted phenanthrene derivatives, undergo a two-electron reduction to form the respective 4nπ dianion e.g., 52–/2Li+. Phenanthrene derivatives substituted at the 4- and 5-positions (bay substituents)e.g.6–11, which are helical, afford stable dianions. These dianions are also prepared by a two-electron reduction of the (4n+ 2)π electron hydrocarbons and show, in their 1H NMR spectra, a quench of the paratropicity compared to 52– as well as a line shape dependence on their twist angle. The quench of paratropicity was also observed in the closely related charged helicenes derived from the benzo[g]chrysene system, i.e. anions 122– and 132–. The twist angles were calculated by MMX and MNDO calculations for the neutral systems, i.e., 5–11, and by MNDO for the dianions. MNDO calculations also included the preferred location of the counter cation. A dynamic NMR spectroscopic study proves experimentally the helicity of anion 112– thus shedding light on the behaviour of this novel class of dianions.

To obtain new efficient electron acceptors, three bromine-substituted benzocycloheptene-1,4,7-triones, 14, 15, 16, and 7-dicyanomethylene-7H-benzocycloheptene-1,4-dione 36 have been synthesized. The electron affinities of the bromine-substituted quinones were found to be raised considerably as the number of bromine atoms increased compared with the parent trione 3. A linear correlation between E1redox and ELUMO(MNDO) has been shown. The dicyanomethylene substitution (36) of the tropone carbonyl of 3 caused an increase in the electron affinity by E1+ 0.2 V. To obtain an acceptor carrying electron-withdrawing substituents on C6 and C8 of 3, the 6,8-dimethoxycarbonyl derivative 18 was oxidized and gave a ring-contracted product 22 in fair yield. The formation is discussed in terms of electron distribution in the molecule of the expected product 19.

The radical cations of bicyclo[3.2.0]hepta-2,6-diene and bicyclo[3.2.0]hept-2-ene have been obtained by γ-irradiation of the parent compounds in Freon matrices and their structures investigated by EPR spectroscopy and MNDO and INDO calculations. In a CFCl3 matrix, ring opening to the isomeric cycloheptatriene and cycloheptadiene radical cations occurred, whereas in a CF2ClCFCl2 matrix the prevalent reaction was deprotonation to the neutral bicyclo[3.2.0]hepta-2,6-dien-4-yl and bicyclo[3.2.0]hept-2-en-4-yl radicals, respectively.

Radical cations of cis and trans isomers of 1,3-di- and 1,3,5-tri-methylcyclohexane, stabilized in various γ-irradiated solute–halocarbon matrices at 77 K and above, have been investigated by means of EPR spectroscopy. The hfs constants have been compared with those calculated using semiempirical methods, INDO (spin density) and MNDO (geometry optimizations). The SOMOs of the methyl-substituted cyclohexane cations are all similar to either of the orbitals, ag or bg, following a Jahn–Teller split of the eg orbital (HOMO) of cyclohexane. The selection of SOMO depends on matrix and isomer. The trans isomers have one axial methyl group and the cations were found to take an ag-like SOMO in all matrices used, with large spin density on the equatorial hydrogen attached to the axially substituted ring-carbon. Using CF3CCl3 as matrix the EPR signal due to the trans-1,3,5-trimethylcyclohexane cation contains (in addition) a spectrum of the bg-like SOMO superimposed. The cation of cis-1,3-dimethylcyclohexane shows contributions from both SOMOs in the matrices CF3-cC6F11, CF2ClCFCl2 and CF3CCl3, but exclusively a bg-like SOMO in cC6F12. A bg-like SOMO is observed for the cis-1,3,5-trimethylcyclohexane cation in all matrices. The abundance ratios, [ag]/[bg], for the cases where the matrix-stabilized cation shows contribution from both bg- and ag-like SOMOs simultaneously, are found to be ca. 1/4 from line shape simulations. No prominent temperature dependence of this ratio is detected.

The reaction between L-ascorbic acid (AAH2) and OH radical has been investigated theoretically. The addition site on AAH2 and AAH– is found to be the olefinic carbon adjacent to the carbonyl group, with much more distinct selectivity for AAH–. Dehydration is found to occur readily not from the neutral ˙OH adduct AAH2OH˙, but from the anion adduct, AAHOH˙–, which leads to formation of the most probable key intermediate radical 17(ascorbate anion radical). The parent molecule of AAH2, triose reductone TRH2(2,3-dihydroxprop-2-enal), undergoes ˙OH addition and dehydration in a similar manner to AAH2. The addition site has been confirmed by comparison of MO energies for four possible transition states of the TRH2(ab initio and MNDO) and AAH2(MNDO) systems. The importance of the conjugate bases, AAH– and TRH–, for yielding the key radicals is suggested.

Based on 1H, 7Li, and 13C NMR chemical shifts of phenalenyl ions, it is argued that the observed diamagnetism of the systems is due to the diatropic benzene and naphthalene subcircuits. MNDO and graph theoretical calculations support this observation.

To confirm the range of application of the dipole–dipole interaction approximation in the coupled oscillator theory, the rotational strength of several p-nitrophenylglycosides (glucopyranoside, galactopyranoside, mannopyranoside, xylopyranoside, and fucopyranoside) and phenylglycoside (glucopyranoside) have been calculated with the Kirkwood–Tinoco equation. The orientations of the aromatic groups estimated by the above equation turn out to be in good agreement with those obtained from the semiempirical molecular orbital calculation (MNDO method) and molecular mechanics calculation (CHARMm). It is confirmed, therefore, that the Kirkwood–Tinoco equation is sufficiently applicable even when a chromophore is located relatively near a saturated group.

The molecular structure and strain energy of [5]metacyclophane (1) have been calculated with the ab initio STO-3G minimal basis set. The results are compared with previously reported theoretical data of (1) obtained with molecular-mechanics calculations (MM) and the semi-empirical MNDO method, and the X-ray structure determination of 8,11-dichloro[5] metacyclophane (2). Despite quantitative differences the STO-3G results confirm the MNDO results qualitatively. In comparison with the available experimental data the calculated STO-3G structure of the bent benzene ring shows better agreement. The bent benzene ring in the STO-3G structure possesses essentially delocalized aromatic carbon–carbon bond lengths. Although the calculated STO-3G strain energy [Es(tot)] is larger than either the MM or MNDO value, the distribution of Es(tot) over the bent benzene ring [Es(bb)] and the oligomethylene bridge [Es(br)] is similar.

MNDO and ab initio calculations, in conjunction with the ‘sphere-charge’ technique of charge partition, consistently account for stereochemical preferences in the generations and alkylations of allyl and heteroallyl anions. The conventional localised orbital treatments of these phenomena are shown to be inadequate.

Semiempirical SCF-MO calculations for the four-centre elimination of molecular hydrogen from substituted cyclohexadienes and cyclopentenes reveal the MNDO and AM1 enthalpic barriers to be too high by ca. 50 and 20 kcal mol–1 respectively compared with experiment, whereas the calculated entropies of activation are in good agreement with experiment. The alternative stepwise mechanism involving intermediate formation of a hydrogen atom and an alkenyl radical by C–H bond homolysis is significantly lower in energy than the concerted path at the MNDO level, but very similar at the AM1 level. Bis-α(oxy-anion) substituents are calculated to reduce the concerted barrier to hydrogen elimination by 22–26 kcal mol–1 compared to the neutral diol. Kinetic deuterium isotope effects calculated for the concerted reaction of cyclohexa-1,4-diene or cyclopentene agree very well with experimentally reported values. The importance of tunnelling contributions is discussed. The corresponding isotope effects for the two-centre stepwise reaction are calculated to be significantly greater.

The conformational preferences and structural trends for a series of fluorine- and chlorine-substituted methyl and ethyl radicals are rationalized. The methodology used to analyse the pyramidality of substituted methyl radicals is based upon a quantitative decomposition of the molecular orbitals for the pyramidal structure in terms of the molecular orbitals for the planar form. A second-order perturbation treatment is also applied in order to have a simple approximation to the aforementioned decomposition. Using these methods it is possible to evaluate quantitatively the relative contributions of electronegativity and conjugation effects in determining the extent of pyramidality of the radicals. It is shown that the electronegativity of the substituents plays the major role. The conformational preferences for β-halogenoethyl radicals are also discussed. The calculations are of the MNDO type with UHF and half-electron formalisms

Chemical ionization (c.i.) mass spectrometry has been used to study stereochemical effects in the isomeric norbornenols (1)–(5). Protonation and subsequent water abstraction ractions can be used to probe the stereochemistry of these alkenols. The nature of the protonation sites, and the possibilities of interaction between them or of proton transfer, seem to play a very important role in determining the relative stabilities of the ions MH+ and (MH – H2O)+. The interpretation of the results is supported by MNDO quantum chemical calculations executed on the protonation sites of the norbornen-7-ols (4). The evidence for gas-phase anchimeric assistance in the water-abstraction reactions of some norbornenols is discussed.

Factors determining the geometry of arylamino groups were investigated using MNDO semi-empirical molecular orbital calculations with full geometry optimization. The species studied were aniline, ring-protonated anilines, aminopyridines, and aminopyridines protonated at the ring nitrogen. The amino group is calculated to be pyramidal with its nitrogen atom displaced from the plane of the aromatic ring unless the nitrogen lone pair is conjugated with a positive centre, when the amino group becomes coplanar with the ring.

The observed differences in selectivity between salicylaldoximes and 2-hydroxyacetophenone and 2-hydroxybenzophenone oximes as reagents for the extraction of copper from aqueous acids has been examined in terms of electronic and structural differences within the ligands. Geometry optimisation studies are reported on salicylaldoxime at the CNDO/2, MINDO/3, MNDO, AMI, STO-3G, STO-6G and 3-21G levels and the results compared with the known crystallographic data. Geometries for the other oximes with unknown structures have been generated using the STO-3G method. The results obtained from the calculations suggest that steric factors are responsible for the different extraction behaviour.

The unimolecular reactions of methyl acetate and its sulphur analogues, S-methyl and O-methyl monothioacetate and methyl dithioacetate, were investigated using the semi-empirical MNDO method with respect to ketene–thioketene formation and 1,3-methyl migration. For methyl acetate, 1,3-methyl migration of the methyl ester was calculated to be more favourable than ketene formation. S-Methyl monothioacetate undergoes ketene formation preferentially while O-methyl monothioacetate rearranges via 1,3-methyl migration to the more thermodynamically stable S-methyl isomer. Methyl dithioacetate also prefers methyl migration; however, unlike the other three compounds, the theoretical results condtradict earlier reported experimental results.

Mechanisms for the benzilic acid and related rearrangements have been studied using the MNDO SCF-MO method. The barriers to concerted closed shell [1,2] migration of a substituent R in the initially formed intermediate (2) were found to display a much smaller range of values than is commonly found in rearrangements involving cations or radicals. Both the formation of (2) and its subsequent rearrangement are predicted to be enhanced if either R or the non-migrating group X bears electron-withdrawing substituents. The calculated hydrogen isotope effect and the Bell tunnelling correction for the rearrangement of glyoxal are both significantly larger than for the analogous hydride transfer in the Cannizzaro reaction of formaldehyde. Alternative open shell pathways involving intramolecular single-electron-transfers (SET) to the carbonyl group are relatively high in energy if the non-migrating substituent X is not capable of stabilising an adjacent radical centre (i.e. X = H), but are much more favourable for e.g. the case of X = phenyl. When R bears an electron-withdrawing substituent, electron-transfer to this group is favoured. We suggest on this basis that an SET mechanism may explain the migration of hydrogen rather phenyl in the reaction of phenylglyoxal. Electron-transfer is also favoured for the rearrangement of benzoin to benzilic acid itself, and is predicted to result in the formation of low-energy cyclic intermediates. Intervention of a counter-ion was modelled with Li(H2O)2+ co-ordinating to two oxygen atoms during migration. The migratory barriers were actually increased as a result, and it is suggested that the role of the counter-ion in non-polar solvents is to alter the pre-equilibrium in favour of the intermediate (2).

An assignment of the constitution of the compound obtained in the reaction of Ph2PSiMe3 with BrCH2COCH3(1) is made. Subsequent dehydrobromination is reported to give 2-oxopropyl-2-oxopropylidenediphenylphosphorane (4) instead of diphenylphosphinoacetone (2). Lithiation of (4) yields the corresponding enolate (6). The crystal and molecular structure of (6) has been resolved by X-ray diffraction analysis, and spectroscopic properties of (4) are interpreted on the basis of MNDO calculations.

2,5-Diacetyl-3,4-diazahexa-2,4-diene consists of two essentially planar halves that are twisted about each other by 102.7°. Bond lengths and orbital energies indicate strong conjugation between the electron lone-pair of one nitrogen atom and the double bonds of the other half of the molecule. The molecular structure and torsion around the N–N and C–C bonds have been studied by MNDO calculations. While the structure parameters are in reasonable agreement with experimental values, conformational properties are not treated adequately. According to these calculations orbital energies are nearly invariant to conformational changes.

Conformational information on the title compounds obtained from low temperature 13C and 1H n.m.r. studies is in agreement with the geometry calculated by the MNDO SCF molecular orbital procedure. The barriers to rotation about the exocyclic P–N bond have been measured and the rotational transition state has been investigated by MNDO. Some u.v. photoelectron and 15N n.m.r. data are reported and the possibility of enhanced π-bonding in the exocyclic PN bond is discussed.

The MNDO molecular orbital method has been used to calculate the preferred conformation and values of electronic indices for some antitumour triazenes. The computed rotation barrier about the N(2)–N(3) bond shows a good correlation with the Hammett σ substituent constant, and increases as the ability of the substituent to withdraw electrons increases. Analysis of the stretching force constant for the N(3)–Me bond indicates that the demethylation of triazenes observed in vitro should occur more readily in the planar conformation, which is the most stable. On the other hand, the antitumour activity shows a high correlation with the LUMO orbital energy, suggesting a charge-transfer mechanism with electron-acceptor triazenes. Finally, with respect to toxicity, no important correlation was found with the electronic indices considered.

Hydrogenation of acridine (1) using a commercial Pd–Al2O3 catalyst was kinetically and stereochemically studied under variable conditions. A consecutive pathway, (1)→ 9,10-dihydroacridine (2)→ 1,2,3,4,4a,9,9a,10-octahydroacridine (5), which competes with hydrogenative isomerization of (2) to 1,2,3,4,5,6,7,8-octahydroacridine (4)via 1,2,3,4-tetrahydroacridine (3) or (5) was suggested from the observed product distributions over the time course of the reactions and their kinetic simulation. Thus, selectivity for the hydrogenated products is found to be controlled by either kinetics or thermodynamics according to the reaction conditions. Thermodynamic stabilities of the products are discussed based on quantum chemical (MNDO) and molecular mechanics (MM2) calculations to rationalize the reaction scheme. The stereoisomers of (5) and perhydroacridine (6) were identified and quantified by detail analyses using g.c.–i.r. and 13C n.m.r. The stereoselectivity is governed by the preference in adsorption of the intermediates on the catalyst surface at a lower reaction temperature (150 °C); however, a higher temperature (250 °C) produces the thermodynamically stable products through equilibrium control.

The cycloaddition reaction between tetracyanoethylene and vinyl alcohol has been studied by means of the MNDO method with CI. Catalytic and solvent effects have been simulated. It has been found that in the gas phase the mechanism is non-concerted with a biradical intermediate. In protic solvents, the intermediate has zwitterionic character and leads to the formation of an acetal.

Semi-empirical SCF-MO calculations, mainly using the MNDO program, have been used to determine the smallest possible change that can be made to the bicyclic ring system of thromboxane A2 in order to increase its stability. The bicyclic system 2,6-dioxabicyclo[3.1.1]heptane is predicted to be more stable with respect to its hydrolysis product when one or more oxygen atoms are replaced by sulphur, the order of stability being 2,6-dithiabicyclo[3.1.1]heptane > 2-oxa-6-thia analogue > 6-oxa-2-thia-analogue > 2,6-dioxabicyclo [3.1.1]heptane. These predictions are qualitatively in agreement with observations on the stability of synthetic monothia- and dithia-analogues of thromboxane A2. The optimised geometries for these bicyclic systems imply a considerable flattening of the end of the six-membered ring that is remote from the bridge, but a consideration of n.m.r. data for a monothia-analogue suggests that the optimisation overestimated the extent of flattening of the six-membered ring.

Geometries, torsional barriers, and dipole moments have been computed for mono-, di-, tri-, and tetra-substituted five-membered cyclic carbonates and for some substituted cyclopentanones, oxolanes, 4-butyrolactones, and 1,3-dioxolanes. With respect to cyclic carbonates it was deduced that the potential curve increases moderately with T3456(torsion angle) and for most compounds a 1 kcal difference between 4T5 or 5T4 conformers and planar form corresponds to a torsional angle of ±14–16°. The minimum is, in general, flattened and close to planar. Additional modes of ring, puckering such as 5E, E5, 4E, E4 and 2E, E2 were also examined. Between the enantiomeric structures with opposite distortions, the quasi equatorial conformation of the alkyl group is preferred. endo and exo anomeric effects in carbonate systems have been studied by introducing strong electron-withdrawing substituents, in the carbonate 4-position. The exo anomeric effect was observed with many of the polar substituents, but the role of the anomeric effect in ring puckering (endo effect) was found to be small.For other five-membered ring systems MNDO predicts that the energy barrier to interconversion of planar ↔ twist is in the following order: carbonates > 1,3-dioxolanes > butyrolactones > oxolanes > cyclopentanones.N.m.r data are also included as well as the single-crystal X-ray diffraction study of 4-chlorophenyloxymethyl-1,3-dioxolan-2-one.