Showing papers in "Journal of Computational Chemistry in 1980"

A systematic preparation of new contracted Gaussian‐type orbital sets. III. Second‐row atoms from Li through ne

Abstract: Four minimal Gaussian basis sets are generated for the second-row atoms Li through Ne. The first one, MINI-1, consists of a 3-term contraction of primitive Gaussian-type orbitals for 1s, 2s, and 2p atomic orbitals. The convenient shorthand notation would be (3,3) for LiBe and (3,3/3) for BNe. The second one, MINI-2, can be represented by (3,3/4) for BNe. In the same way, MINI-3 is described as (4,3) for LiBe, and MINI-3 and MINI-4 are represented by (4,3/3) and (4,3/4) for BNe, respectively. Although the four basis sets are the minimal type, they give the valence shell orbital energies which are close to those of DZ. These four and other sets derived from them are tested for the hetero- and homodiatomic molecules and some organic molecules. They are found to give the orbital energies that agree well with those given by extended calculations. Atomization energies and other spectroscopic constants are also calculated and compared with those of extended calculations. The results clearly indicate that the present basis sets can be used very effectively in the molecular calculations.

The application of molecular mechanics to the structures of carbohydrates

Abstract: A study is reported of the accuracy with which the geometries of pyranose and methyl pyranoside molecules are predicted by molecular mechanics. Calculations of the conformational energies of the model compounds dihydroxymethane, methoxymethanol, and dimethoxymethane, made with the program MMI, produced results that compare well with previous ab initio molecular orbital calculations. This indicates that MMI gives a satisfactory account of the energetic and conformational aspects of the anomeric effect, a conclusion further supported by calculations on 2‐methoxytetrahydropyran. The prediction of the observed preferred conformations of the primary alcohol group in aldohexopyranoses appears to be less satisfactory. MMI‐CARB, a version of MMI with changes in some of the equilibrium CO bond lengths of the program, has been used to calculate the geometries of 13 pyranose and methyl pyranoside molecules, the crystal structures of which have been studied by neutron diffraction. When the CCOH torsion angles are constrained to approximately the values observed in the crystal structures, good agreement is obtained between the theoretical and experimental molecular geometries. The rms deviation for CC and CO bonds, excluding those significantly affected by thermal motion in the crystal structure determinations, is 0.005 Å. Corresponding figures for the valence angles that do not involve hydrogen atoms and for the ring torsion angles are 1.2° and 2.0°, respectively. The Cremer and Pople puckering parameters for the pyranose rings are reproduced within 0.026 Å in Q and 5.4° in θ.

Random walks and their diagnostic value for characterization of atomic environment

Abstract: A characterization of atomic environments based on counting random walks in a molecular skeleton is outlined. To each atom in a molecule a sequence of integers w1, w2, w3,…, wn is assigned, where wi represents the number of self‐returning walks of length k, the length being defined by the number of bonds traversed. Properties of the derived atom codes are discussed. The codes display an impressive diversity and are superior to atomic codes based on enumeration of self‐avoiding walks (or paths) in discriminating atomic environments. In certain cases the codes of individual atoms are not unique and the same codes appear in different molecules or even within the same molecule. The occurrence of the nonunique codes can be related to special structural situations, associated with the occurrence of isospectral graphs. These isospectral graphs which have atoms with identical codes can generate additional isospectral structures by attaching any arbitrary group to such points. If nonequivalent atoms of a single molecule have identical random walk codes, substitution at the singular points alternatively will produce isospectral graphs. Examples of such situations are given.

Integrated spatial electron populations in molecules: Application to simple molecules

Abstract: The electron projection function P(x, z) = ∫ ρ(x, y, z) dy is used to evaluate charge transfer and covalency in two series of molecules, LiX and CH3X (X = Li, BeH, BH2, CH3, NH2, OH, and F), with wavefunctions derived from STO-3G, 4-31G, and, in some cases, 6-31* ab initio calculations. The precision of the method and comparison with Mulliken populations analysis are described. Particular attention is given to CH3Li which by our criteria is wholly ionic.

Graph Theoretical Characterization and Computer Generation of Certain Carcinogenic Benzenoid Hydrocarbons and Identification of Bay Regions

Abstract: An algorithm is developed for generating and characterizing carcinogenic catacondensed benzenoid hydrocarbons. The bay regions in these structures are identified by a technique that we developed at Johns Hopkins. Using the three-digit code proposed by Balaban, and the concept of ring adjacency matrix expounded here, we generate catacondensed benzenoid hydrocarbons in the computer and identify the number of potentially carcinogenic bay regions in each of them. The results of computer generation agree with the combinatorial enumeration of Harary and Read. All structures containing up to five rings and some with six rings and the number of bay regions in these are presented. Computer results for the structures and bay regions of all seven-, eight-, and nine-membered unbranched catacondensed benzenoid hydrocarbons and the number of bays are available from the authors.

A method for constrained energy minimization of macromolecules

Abstract: Two algorithms for the local energy minimization of the structure of macromolecules in the presence of constraints are proposed. They are a combination of the method of steepest descents and the method of conjugate gradients with the procedure SHAKE, by which distance constraints can be satisfied. The two algorithms are tested by applying them to a small protein, the bovine pancreatic trypsin inhibitor (BPTI), and compared with the penalty function method for conserving constraints. The efficiency of the proposed methods depends on the level of interdependence of the constraints. For bond‐length constraints, the use of SHAKE is superior to the penalty function method. However, when bond‐angle constraints are included, SHAKE is more efficient only if the curvature of the penalty function is considerably greater than that of the potential function being minimized. The results indicate that with bond‐length constraints the minimization behavior is similar to that without constraints. However, the simultaneous application of bond‐length and bond‐angle constraints appears to confine the molecule to a very limited part of configuration space, very different from the part covered by an unconstrained minimization. This conclusion calls into question energy minimizations of protein systems in which only the dihedral angles are allowed to vary.

Small elemental clusters. I. The structures of Be2, Be3, Be4, and Be5

Abstract: The geometries and energies of beryllium clusters up to Be5 are examined using ab initio molecular orbital theory. Allowances are made for electron correlation with Moller—Plesset perturbation theory to fourth order. Correlation is found to have a dramatic effect on the relative energies of the several structures examined for Be4 and Be5. Furthermore, the effect of d-type basis functions on the correlation energy results in an increased binding energy for the clusters. Be2 is only weakly bound. For Be3, the best estimate of the binding energy is 6 kcal/mole for the singlet equilateral triangle. Be4 is tetrahedral in its ground state and the estimated binding is 56 kcal/mole. The best structure for Be5 is a singlet trigonal bipyramid, and the binding energy is 88 kcal/mole at the highest level of theory used.

Conformational analysis of proteins: Algorithms and data structures for array processing

Abstract: Current efforts to determine the nature of the interactions that influence protein folding involve, among other things, minimization of an appropriate empirical conformational energy function (ECEPP, Emprical Conformational Energy Program for Peptides) to obtain the native structure. Because of the prohibitive cost of such a massive computational project, either on a conventional large-scale machine at a self-supporting installation or on a dedicated minicomputer, an alternative computer hardware system has been developed to aid in the conformational analysis of proteins. It consists of a Floating Point Systems AP-120B array processor and a Prime 350 minicomputer host. A version of ECEPP has been adapted to run on the AP-120B. The data structures and algorithms chosen for this version reflect the highly unusual parallel architecture of this machine. Benchmark comparisons with BPTI (Bovine Pancreatic Trypsin Inhibitor), a protein of 58 residues and a known structure, have been carried out on this system as well as on an IBM 370/168. They show a significant advantage in speed for the AP-120B/Prime 350 system as well as a substantially lower cost. An energy minimization of BPTI with 154 variable dihedral angles is reported, an effort heretofore prohibited by the computer costs involved.

The torsional potential function for n-butane

Abstract: The energies of four different conformations for n-butane were calculated by the ab initio method using an STO-3G basis set. Fully relaxed molecular geometries obtained from molecular mechanics (MM2) were used. The two energy minima [anti (C2h), gauche (C2)] and the two maxima (C2, C2v) had the following relative energies: 0.0, 0.88, 3.56, 5.99 kcal/mole. These are approximate Hartree–Fock numbers. It is estimated that inclusion of electron correlation in the calculation would lower the last number to about 5.1 kcal/mole while leaving the first three values essentially unchanged.

A theoretical study of paths for decomposition and rearrangement of dihydroxycarbene

Abstract: The transition states for fragmentation of dihydroxycarbene [C(OH)2] to H2 and CO2 and for the rearrangement of this carbene to formic acid were located by ab initio calculations. The relative energies of the transition states were determined at several levels of theory and the basis set dependence of the energies is discussed. At the best level of theory; using a basis set of double-zeta quality augmented by polarization functions and with the inclusion of extensive CI, we found that the transition state for fragmentation was considerably higher in energy than that for rearrangement. This finding is at variance with the predictions of the Woodward--Hoffmann rules because fragmentation represents an “allowed” reaction, whereas rearrangement is “forbidden.” In conformity with the Woodward–Hoffman rules, the transition state for rearrangement was found to be close in energy to H· + ·CO2H. The even higher energy of the transition state for concerted fragmentation to H2 and CO2 is attributed to the need for the latter fragment to remain substantially bent in order to permit H2 formation while maintaining a modicum of OH bonding. Difficulties in locating the transition state for concerted fragmentation are discussed and a new method for finding transition states is proposed.

The structure and energies of some unsaturated four‐membered ring carbocycles

Abstract: A number of cyclobutane derivatives containing one or more double bonds in endo- or exocyclic positions have been studied by the molecular mechanics method within the context of the MM2 force field. Generally speaking, the structures and energies of these compounds are well calculated in cases in which they are known experimentally and are predicted in others. Examples are shown of the use of the moments of inertia of molecules, which are known from microwave studies, in conjunction with molecular mechanics calculations to yield better structures than could be obtained by either method alone. Compounds examined include cyclobutene, methylenecyclobutane, Dewar benzene, and related compounds.

The structure of carbon trioxide

Abstract: Open (1) and cyclic (2) singlet forms of CO3 were investigated by means of ab initio calculations. At the highest level of theory employed, MP2/6-31G* (which includes the effects of electron correlation), 2 was indicated to be much more stable than 1 and thermodynamically stable toward dissociation into CO2 and O(3P). The open form 1 has a long OO bond and can be regarded as a weak dative complex between CO2 and a singlet oxygen atom.

INDO‐Type calculations on the ground state and various ionic states of transition metal tricarbonyls

Abstract: By means of the ΔSCF and transition operator (TO) methods based on a recently developed INDO extension to the first transition metal series, the first ionization potentials of benzene—chromium tricarbonyl (I), cyclopentadienyl manganese tricarbonyl (II), the iron—tricarbonyl complexes with trimethylenemethane (III), and cyclobutadiene (IV) have been calculated and compared with experimental data. It is shown that the electronic structure of I to IV can be rationalized by Hoffmann's fragment approach in both the ground state and the cationic hole states. Within the series I—IV there are remarkable energy differences in the ground state for MOs derived from the 1a1 and 1e orbitals of the M(CO)3 fragment. The observation that only one band is associated with the ionization events from MOs predominantly localized at the metal site is traced back to large relaxation effects. In the cationic hole states the split of the M(CO)3 fragment orbitals 1a1 and 1e is minute in all four compounds.

Ab initio calculations of the rotational potential functions for propylamine and ethylmethylamine

Abstract: Calculations at the STO‐3G and 4–31G levels have been carried out on propylamine and ethylmethylamine, using geometries determined by molecular mechanics by allowing complete molecular relaxation in all degrees of freedom except for torsion about the central bond, and at 30° increments for the latter. It was found that a butanelike potential exists in each case. From 0° (cis) to 360° in order, the 4–31G values for the energy extrema are 5.92, 0.12, 3.88, 0.00, 3.94, and 0.51 kcal/mole for propylamine (with the nitrogen lone pair gauche to carbon), and 7.06, 1.45, 3.44, 0.0, 2.87, and 1.44 kcal/mole for ethylmethylamine.

On the gearing of methyl groups in hexamethylbenzene

Abstract: Empirical force field calculations were performed on hexamethylbenzene to elucidate the internal motions of the methyl groups. When the benzene ring is constrained to be planar (as in solid-phase studies), the methyl groups undergo a geared, disrotatory motion. When this constraint is relaxed, results are force field dependent. Calculated barriers are in good agreement with experimentally determined values.

Properties of strong hydrogen‐bonded systems. II. Ab initioSCF‐MO study of the hydrogen bond between nitric acid and ammonia

Abstract: The hydrogen-bonded complex between nitric acid and ammonia molecules has been studied by the ab initio molecular orbital method using the 4-31G basis set. The calculated interaction energy for the complex (ΔE = −91.4 kJ mole−1) indicates that one is dealing with the strongest “nonionic” H-bonded complex considered hitherto by theoretical methods. Other properties of the hydrogen-bonded complex such as geometrical parameters, dipole moment, amount of charge transfer, and stretching force constants of the OH and (OH)… N bonds are calculated and discussed.

The electrostatic potential of yeast tRNAPhe. II. The potentials of the phosphate groups in their various conformational states

Abstract: The range of conformational states of the phosphate groups observed in the published crystal structure of tRNAPhe is used as the basis for theoretical studies on the effect of conformation on the electrostatic potentials of these moieties. Deductions concerning the influence of these effects on the potential of a complete tRNAPhe macromolecule are presented.

Graphite: A molecular mechanics treatment

Abstract: Graphite is the limiting case of an infinite aromatic hydrocarbon, and as such is an important benchmark in force-field calculations. Using experimentally determined physical properties for graphite, a force-field parameter set was developed for molecular mechanics calculations with unsaturated hydrocarbons and is described and discussed.

An ab initio study of the geometry and energy of six planar conformers of β‐hydroxyacrolein

Abstract: Ab initio calculations with full geometry optimization have been carried out on the planar cCc, cTc, tTc, tCt, tTt, and cCt conformers of β-hydroxyacrolein using the 4-21G basis set, and on the cCc and cCt conformers using the 4-31G basis set. The hydrogen-bonded cCc conformer is the most stable and the cCt conformer the least stable, with the other conformers following the above sequence. β-Hydroxy substitution has scarcely any influence on the geometry of the trans-acrolein structure, whereas the geometry of the cis-acrolein structure shows significant changes which depend on whether the OH group is cis or trans with respect to the CHO group about the CC bond. The ΔET values for cis → trans isomerization about the CC bond in cCt and cTc support the hypothesis that these changes in geometry are the result of a destabilizing interaction in cCt and a stabilizing interaction in cTc. The geometry of the hydrogen-bonded structure cCc sets it apart from all the other conformers: it has by far the longest CC, the longest CO, the longest OH, the shortest CC, and the shortest CO. Its formation from cCt involves a lengthening of CC, CO, and OH and a shortening of CC and CO, indicating a delocalization of charge within the ring. 4-21G calculations have also been made for a distorted cCt structure that has the same bond lengths and angles as the equilibrium cCc structure, and the distortion energy, cCt (equm. geom.) → cCt (distorted geom.), is found to be +13.1 kJ mole−1. Taking the energy of this distorted cCt structure as the baseline, the hydrogen-bonding energy in cCc is found to be —80.3 kJ mole−1.

Ab initio calculations of the rotational potential functions for propanol and ethyl methyl ether

Abstract: Calculations at the STO-3G and 4–31G levels have been carried out on propanol and ethyl methyl ether, with geometries obtained from molecular mechanics calcualations. Full relaxation was allowed in all degrees of freedom except for the torsion about the central bond, which was varied at 30° increments. A butane-type potential was found, the maximum and minimum values of energy are from 0° to 180° 5.55, 0.00, 4.02, and 0.00 kcal/mole for propanol, and 8.35, 2.74, 3.31, and 0.00 kcal/mole for ethyl methyl ether (4–31G).

A theoretical approach to substituent effects. Interactions between directly bonded groups in the neutrals XNH2, XOH, and XF and the anions XNH− and XO−

Abstract: Ab initio molecular orbital calculations have been carried out for the neutrals XNH2, XOH, and XF and the anions XNH− and XO− with substituents X = Li, BeH, BH2, CH3, NH2, OH, and F. All structures have been fully optimized with the 4-31G basis set which is found to perform considerably better than the minimal STO-3G basis in predicting the lengths of strongly polar bonds. A quantitative analysis of interactions between the directly bonded groups, utilizing energy changes in hydrogenation reactions, is presented and rationalized with the aid of perturbation molecular orbital theory. Favorable interactions occur when electron-donor groups bond to electron-acceptor groups. This applies to both σ and π interactions, the relative importance of which depends on the particular substituents.

The electronic structure and optical activity of conjugated dienes: 1,3‐Butadiene and α‐ and β‐phellandrene

Abstract: The optical activity of conjugated dienes is investigated by means of ab initio SCF–CI calculations. The computed electronic spectrum of trans‐1,3‐butadiene is shown to be in good agreement with the results of more rigorous calculations of the valence transitions and in satisfactory agreement with experiment. The optical rotatory strengths of the lower electronic transitions of twisted 1,3‐butadiene as a function of dihedral angle are presented and simulated CD spectra are produced. The N → V1 (π2 → π3*) transition is predicted to have a positive rotational strength for all dihedral angles that correspond to a right‐handed twist of the chromophore, in accord with the empirically deduced “diene rule” although for a twist angle of 60°, the rotatory strength is calculated to be almost zero. The role of the orientation of allylic bonds is investigated in the model system 1‐butene in which the rotational strength of the π → π* transition as a function of rotation about the 2,3 bond is determined. The effect of allylic bond disposition in dienes on the optical activity of the long‐wavelength π2 → π3* transition is simulated by use of the exciton coupling model of Harada and Nakanishi in which two 1‐butene molecules with suitable geometries are coupled via interactions of the electric dipole transition moments of their π → π* transitions. The model systems 1,3‐butadiene and 1‐butene are used to rationalize the apparently anomalous optical activity of (−)‐α‐phellandrene and (−)‐β‐phellandrene, both of which should have a diene chromophore with a right‐handed twist in their most stable conformers and so should be dextrorotatory. The experimental CD spectrum of α‐phellandrene is determined at several temperatures down to −180°C. The observed variation of the apparent rotational strength of the N → V1 transition is in good agreement with that predicted by use of the exciton coupling model.

A theoretical approach to substituent effects. Structural consequences of electrostatic and orbital interactions in model mono‐ and disubstituted methanes

Abstract: Ab initio molecular orbital theory is used to examine the effect of substituents on bond lengths in mono‐ and disubstituted methanes. The relative importance of electrostatic and orbital interaction terms are assessed. The results suggest that for substituents (X) which show powerful σ effects and weak π interactions (e.g., F), the changes in bond length are due primarily to the electrostatic component except in some disubstituted methanes in which case the change in the hyperconjugative ability of the C—X bond is also important. On the other hand, substituents X which show weak σ effects but powerful π interactions (e.g., NH2) affect bond lengths primarily through hyperconjugative interaction of a filled or vacant π‐type orbital on X with the adjacent bonds.

Isomeric structures of protonated ozone: A theoretical study

Abstract: Ab initio molecular orbital calculations have been used to determine the structure of protonated ozone. Four stable minima were found on the O3H+ singlet potential energy surface. Three forms correspond to ozone protonated at the central oxygen (C2v) or at the terminal oxygen (two Cs isomers, E and Z). The fourth isomer (Cs) is a derivative of trioxirane. The most stable structure is the planar E form I. The proton affinity of ozone (to give I) is given as 123.6 kcal/mole (MP2/6-31G*//4-31G). The energy difference between I and protonated trioxirane VI is greater than that between ozone and trioxirane.

Molecular charge distribution and chemical binding in five‐membered heterocycles. I

Abstract: Electron density maps for the heterocycles thiophene, furan, and pyrrole are determined from ab initio 4‐31G wavefunctions. The charge distributions in these molecules are analyzed in terms of the total molecular density and difference density maps and their profiles. The atomiclike core, especially the L core of sulfur, is found to play an important role, via its polarization and interaction, in determining the extent and direction of valence density transfer from the carbon to the heteroatom. The changes in the charge distributions that occur in the immediate vicinity of the heteroatoms and the relation of density quantities to binding and antibinding characteristics are discussed. The quantum topological features of the molecular charge distributions of the three heterocycles are analyzed and discussed and the different bonding situations, e.g., ring strain, ionic and covalent binding, etc., are compared in a model‐independent way.

Conformational energy surfaces of triplet‐state isomeric methyloxiranes

Abstract: A conformational study was carried out on the three ring-opened structures of triplet methyloxirane with a minimal Gaussian basis set, within the unrestricted Hartree–Fock framework. For the two structures energy surfaces E(θ1, θ2) were generated, where θ1 measures the methyl rotation and θ2 is associated with the torsion about the other CC bond. For the third structure an energy hypersurface E(θ1, θ2, θ3) was generated, where energy was a function of methyl rotation θ1 and two nonequivalent CO rotations θ2 and θ3. Analysis of the surfaces revealed the locations and relative energies of the critical points (minima, saddle points, and maxima). The overall stereochemical finding was that these ring-opened triplet C3H6O species possessed rather flexible structures.

An ab initio study of the geometry, harmonic and anharmonic force fields, and fundamental vibrational frequencies of cis‐ and trans‐ thiolformic acid

Abstract: The geometry, harmonic and anharmonic force fields, and fundamental vibrational frequencies of cis- and trans-thiolformic acid are studied ab initio in the 4-31G basis set. An extensive comparison is made between changes in diagonal and off-diagonal quadratic and cubic force constants and diagonal stretching quartic constants in going from the chain to the ring structure in thiolformic acid and formic acid. The changes in the force constants are indicative of a much weaker interaction in the trans conformer between SH and OC, compared with O–H and OC, in keeping with the weaker hydrogenbonding property of the SH group in general.

The C4H4CO potential surface. Reactions involving bicyclo[2.1.0]pentenone

Abstract: MNDO and MINDO/3 calculations were used to study the photochemical formation, thermal rearrangements, and dissociation of bicyclo[2.1.0]pentenone. The “forbidden” thermal conversion to cyclopentadienone requires little activation, which accounts for the low kinetic stability of bicyclo[2.1.0]pentenone. The theoretical results seem to be compatible with the available experimental evidence for the tri-tert-butyl-substituted systems and suggest an explanation for observed differences in reactivity.

A Pseudopotential SCF-MO Study of Te$+

Abstract: A recently developed ab initio pseudopotential molecular orbital approach was applied to the Teq’ ion, a system outside the practical reach of conventional all-electron treatments. Computations were carried out with a minimal STO-4G basis set. Results account reasonably well for the observed optical absorption spectrum and suggest the origin of a hitherto unassigned weak band. Ground-state properties, which included the structure, force field, and vibrational frequencies, were also investigated. Treated as a free, gas-phase ion, tetratellurium (11) yielded a bond length 0.05 A shorter than the experimental value for the ion in a crystal lattice. Placement of static, point-charge counterions in the Te:’ coordination sphere increased the bond length to a value 0.005 A longer than derived by experiment. Calculations on neutral, cyclic Te4 provided a theoretical single-bond reference length 0.09 A longer than that obtained for the ion in a counterion environment. Comparisons between observed and calculated frequencies suggest an assignment of the vibrational spectrum different from the provisional assignment in the literature. The enormous impact of ab initio molecular orbital calculations on the field of chemistry during the last decade is well known. Nevertheless, practical considerations have severely limited the region of the periodic table that receives attention. A recent development that promises to reduce this restriction drastically is the successful procedure described elsewhere for incorporating the Phillips-Kleinman pseudopotential operator into polyatomic, molecular orbital calc~lations.~-3 Calculations based on this procedure, which requires an explicit accounting only of the valence electrons, appear to yield orbital energies, structures, and force fields in close agreement with those of ab initio all-electron calculations without introducing empirical parameters. Therefore, calculations are virtually as simple to carry out with systems of heavy atoms as with those of light atoms, although, to be sure, relativistic corrections may become important for the former cases. One interesting class of compound not amenable to treatment by standard ab initio procedures constitutes the heavy-atom homopolyatomic cations principally studied by Gille~pie,”~ Corbett,10-12 Bjerrum,13-15 and Stephens.16J7 An attractive candidate for study, whose “beautiful crimson red color,” noted two centuries ago,18 was interpreted only recently, is Te;’. Because its structure is known accurately from x-ray analyses,12 and some information is available about its vibrational spectrum,8>9J1J2 it is a useful trial system for the pseudopotential method. As discussed below, the method was found not only to account for established properties of the ion but also to provide an alternative interpretation of the vibrational spectrum and to corroborate the recent assignment of the crimson color. PROCEDURE Pseudopotential Method

Orbital topology. II. Orbital mapping of unsymmetrical molecules. A survey of the thermal isomerizations of dewar isomers of isoelectronically substituted benzenes, cyclopentadienes, and cyclopentadienyl ions

Abstract: Orbital mapping analysis, based on EHT and CNDO/2 semiempirical molecular orbitals, has been used to survey the thermal, disrotatory, ring-opening isomerizations of bicyclo[2.2.0]hexa-2,5-dienes (Dewar benzenes), bicyclo[2.1.0]pent-2-enes, and bicyclo[2.1.0]pent-2-en-5-yl ions to their planar isomers. Results indicate that isoelectronic substitution (CH replaced by C−, O+, N, NH+, etc.) in the molecular framework may favor allowed thermal reactions in some cases, in contrast to the disallowed reaction predicted for the parent hydrocarbons.