Interpretation of photoelectron spectra in the AM1 semiempirical method. 3. 2-Substituted benzimidazoles

Analysis of photoelectron spectra of 2-substituted benzimidazoles showed that their orbital structure is determined by the form and energy position of the fragment orbital (-symmetry) of the substituent.For previous communication, see [1].Irkutsk Institute of Organic Chemistry, Siberian Branch, Russian Academy of Sciences, Irkutsk 664033. Translated from Izvestiya Akademii Nauk, Seriya Khimicheskaya, No. 4, pp. 880–884, April, 1992.     

A theoretical study of the rotational isomers of nitrosomethanol by semiempirical (AM1) and ab initio methods

The conformational potential energy surface as a function of the two internal torsion angles in C-nitrosomethanol has been obtained using the semiempirical AM1 method. Optimized geometries are reported for the local minima on this surface and also for the corresponding points on the HF/6-31G, 6-31G*, and 6-31G** surfaces. All methods predict cis and trans minima which occur in degenerate pairs, each pair being connected by a transition state of C_s symmetry. The AM1 structures are found to compare well with the corresponding ab initio structures. Ab initio HF/6-31G and HF/6-31G* harmonic vibrational frequencies are reported for the cis and trans forms of nitrosomethanol. When scaled appropriately the calculated frequencies are found to compare well with experimental frequencies. The ab initio calculations predict the energy barrier for cis → trans isomerization to be between 5.8 and 6.5 kcal/mol with the trans → cis isomerization barrier lying between 2.3 and 6.5 kcal/mol. The corresponding AM1 energy barriers are around 1 kcal/mol lower in energy. The ab initio calculations predict the barrier to conversion between the two cis rotamers to be very small with the AM1 value being around 1 kcal/mol. Both AM1 and ab initio calculations predict interconversion between trans rotamers to require between 1.2 and 1.4 kcal/mol.     

AM1 semiempirical computational analysis of the homopolymerization of spiroorthocarbonate

Spiroorthocarbonates (SOCs) are monomers that have been shown to expand when homopolymerized. SOCs are potential monomer systems that can be combined with other monomers such as epoxy resin to produce a non-shrinking dental matrix for dental composites. The purpose of this study was to use a computer model (AM1) to study possible homopolymerization pathways for several SOC monomers. The gas phase transition states of three feasible reaction mechanisms for the homopolymerization of four spiroorthocarbonate 1,5,7,11-tetraoxaspiro[5,5]undecane (TOSU) systems have been examined using the AM1 semiempirical quantum mechanical model. In addition to the base TOSU noted above, the 2,8-dimethyl, 2,4,8,10-tetramethyl, and the 3,3,9,9-tetramethyl analogs were used in this study. The results of these calculations produced the heats of reaction, activation enthalpies and transition state structures. Our calculations indicate stabilization of the transition states by electron-donating and resonance-stabilizing substituent groups. The energies of activation of all of these systems were between 24 and 38 kcal/mol and all reactions were endothermic. Further, we found that there was a significant intermolecular attraction between TOSU monomers (≈3.5 kcal/mol). When compared with experimental studies of methylated TOSU by Sakai and co-workers, our calculations agree with the preferred site of nucleophilic attack, but not with the experimental rate results. It was concluded that the homopolymerization of the unsubstituted TOSU and its derivatives studied was endothermic and that the rate of homopolymerization of TOSU depends on an intermolecular pre-association of TOSU monomer in the condensed phase.     

Study of the thermal decomposition of 3-cyclopentenone by using the AM1 semiempirical method

Summary RHF/AM1 and UHF/AM1 low-dimensionality surfaces were calculated by applying the reaction-coordinate technique to the thermal decomposition of 3-cyclopentenone. Several stationary points were accurately located within the entire coordinate space. Although the two formalisms used predicted asynchronous reaction pathways for the process, our results illustrate the limitations of single reference treatments of two-bond reactions while the nature of the reaction path remains uncertain.     

A study of photoelectron spectra in the framework of the semiempirical AM1 method

The characteristics of photoelectron (PE) spectra of 2-phenylpyrroles and biphenyls have been correlated with the UV spectroscopy data for their radical cations. The geometric and electronic structures of the compounds and radical cations have been calculated using the AM1 method. The absorption bands of the radical cations observed in the visible region of spectrum have been found to pertain to the quasi-Koopmans type,i.e., they are realized between the doubly and partially occupied -MO, but their energies differ strongly from those calculated on the basis of PE spectra. This is explained by the distinction between the spatial structure of a molecule and that of the respective radical cation. The second cause of the discrepancy between the data of the two spectral methods consists in orbital — interactions, which are most conspicuous in the case of 4(4)-halogen-substituted compounds.For Part 4 see ref. 1.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 5, pp. 869–874, May, 1993.     

Comparison of semiempirical MO methods applied to large molecules

The heats of formation for 19 molecules have been calculated with PM3 and AM1 semiempirical methods. The values obtained have been compared with experimental heats of formation. With PM3 and AM1 the average differences between calculated and experimental heats of formation are 8.45 and 12.34 kcal mol^?1 respectively. There are significant differences when large molecules are considered: this suggests that the parameterization should be done including larger molecules.     

A semiempirical SCF–LCAO–MO treatment of some oxygen- and nitrogen-containing heterocyclic molecules

The π-electronic properties of furan, oxazole, benzofuran, benzoxazole, anthranil, and dibenzofuran are calculated by the semiempirical self-consistent-field molecular orbital method. A single set of parameters is found which satisfactorily reproduces the π → π* electronic transition energies and other π-electronic properties.     

Designing molecules with optimal properties using the linear combination of atomic potentials approach in an AM1 semiempirical framework

The linear combination of atomic potentials (LCAP) approach is implemented in the AM1 semiempirical framework and is used to design molecular structures with optimized properties. The optimization procedure uses property derivative information to search molecular space and thus avoid direct enumeration and evaluation of each molecule in a library. Two tests are described: the optimization of first hyperpolarizabilities of substituted aromatics and the optimization of a figure of merit for n-type organic semiconductors.     

Electronic and structural properties of the metalloporphyrin structural isomers: semiempirical AM1 and PM3 calculations

Semiempirical calculations using AM1 and PM3 have been performed on the zinc(II) and magnesium(II) complexes of nine structural isomers of tetrapyrrole macrocycles such as porphyrin, porphycene, corrphycene and hemiporphycene, N-confused porphyrin and other isomers that have not been synthesized. The optimized geometry and the bond parameters obtained compare favorably with results obtained from X-ray and spectral studies. Heats of formation, ionization potentials, HOMO-LUMO energy differences, dipole moments, and the splitting of HOMOs and LUMOs of the metal complexes of each of these isomers are also reported and compared with experimental results. The “four-orbital model” of Gouterman remains valid for the investigated structural isomers. The present study represents an unusually appropriate opportunity to study, via molecular orbital methods, the interaction between various metal ions, and the electronic and geometrical environment of the central cavity of the closely related isomeric macrocycles. The major outcome of this study is the verification of the expected differential behavior of metal ions employed in the present study as a sophisticated probe of cavity properties which also suggests that this procedure can be extended to other metal complexes. This study also serves as an interesting prototype for more elaborate ab initio calculations. However, such calculations on the presently investigated macrocyclic systems may have to be performed at a higher level than MP2 or DFT to account for the unusual delocalization, as suggested by a recent study by Schaefer and co-workers on delocalized [10]annulene (H.M. Sulzbach, H.F. Schaefer, W. Klopper and H.P. Luthi, J. Am. Chem. Soc., 118 (1996) 3519).     

Conformational studies of (S)idazoxan and (S)methoxyidazoxan using AM1 and PM3 semiempirical molecular orbital methods

Semiempirical calculations of the α-2 receptor antagonist, Idazoxan, its methoxy derivative, and their respective cations and anions revealed the most stable conformers were significantly affected by electrostatic interactions. These studies showed the importance of the intramolecular electrostatic interactions between the oxygen of the benzodioxane ring and the nitrogens of the imidazoline ring in determining the global minima of these molecules. The NH bond of the imidazoline ring was attracted to the dioxane oxygen and caused the imidazoline ring to rotate to a position allowing close proximity of these groups. Favorable interactions between the oxygens and the imidazoline stabilized the axial conformer, whereas unfavorable interactions increased the stability of the equatorial isomer. AM 1 and PM 3 calculations sometimes predicted different global minimum conformers, indicating the need for caution in reliance on either method, particularly when hydrogen bonds play an important role in determining the structure. © 1992 John Wiley & Sons, Inc.     

Reactivity of biologically important reduced pyridines. VIII. A semiempirical (AM1) study of the oxidation of 3-substituted-1-methyl-1,4-dihydropyridines

Significant linear correlations were observed between AM1-derived adiabatic, but not vertical, ionization potentials and the log of ferricyanide-mediated oxidation of 3-substituted-1-methyl-1,4-dihydropyridines. This result is consistent with a rate-determining electron loss in the reaction sequence and suggests molecular relaxation subsequent to the initial electron transfer. In addition, useful relationships were generated between oxidation rate and absolute electronegativity, a parameter derived from hard-soft acid-base theory.     

Comparison of AM1 and PM3 semiempirical to ab initio methods in the study of Diels-Alder reactions of butadiene and cyclopentadiene with cyanoethylenes

Assuming a concerted synchronous mechanism with one transition state of the Diels-Alder reactions, the structures of the transition states and the activation energies for the reactions of butadiene and cyclopentadiene with cyanoethylenes were calculated by AM1 and PM3 semiempirical methods. The structural parameters were compared with those obtained by high level Gaussian calculations, whereas the activation energies were compared both with the ab initio calculations and those obtained experimentally. The structural properties calculated with PM3 methods are in general in better agreement with the ab initio calculations. The low level ab initio calculations are in many cases worse than the semiempirical methods. All predicted activation energies with both semiempirical methods are up to 300% higher than the experimental values. The predicted reactivity is also opposite to the experimental data. Only the very high level Gaussian calculations are in good correlation with experimental results. The predicted selectivity of the reaction is also opposite to the experimental facts. Two explanations are offered for this discrepancy: AM1 and PM3 methods cannot handle the calculation of the concerted Diels-Alder transition states and are not recommended to be used for that purpose, or this Diels-Alder reaction is not concerted but is stepwise.     

Interpretation of photoelectron spectra within the framework of the AM1 semiempirical method. 2. Pyridines

Analysis of the orbital structure of isomeric pyridines carried out using photoelectron spectroscopy and the AM1 (GF) method showed that this structure is independent of the nature of the substituent at C ²⁽⁶⁾ and C ³⁽⁵⁾ : <n<. All the possible sequences of the highest occupied MO of different symmetry (, ) are realized for 4-substituted pyridines.For previous communication, see [1].Irkutsk Institute of Organic Chemistry, Siberian Branch, Russian Academy of Sciences, Irkutsk 664033. Translated from Izvestiya Akademii Nauk, Seriya Khimicheskaya, No. 4, pp. 872–880, April, 1992.     

Dimer form of 1,3-dimethyluracil studied by the AM1 semiempirical method

A study of the dimer form of 1,3-dimethyluracil was carried out. The optimum geometric parameters in the monomeric and dimeric forms were obtained using the AM 1 semiempirical method. The results were compared with those reported by X-ray diffraction. The energies in the dimer formation and in the intramolecular C? H···O interaction were calculated. Electronic density maps in two and three dimensions were drawn. Several calculated thermodynamic parameters are discussed. © 1994 John Wiley & Sons, Inc.     

Hydration of small anions: Calculations by the AM1 semiempirical method

The AM1 semiempirical molecular orbital method has been used to calculate successive heats of hydration of small anions, including hydride, hydroxide, and the halogen ions, for cluster sizes up to 11 water molecules surrounding the central anion. Heats of hydration agree with available experimental data to within a few kcal/mol. Structures, however, do not always agree well with available ab initio calculations on clusters with one or two water molecules. The results indicate that the AM1 semiempirical technique applied to finite-sized clusters must be used with caution in understanding how hydration affects the chemical reactions of anions.     

Electron distribution on an intramolecular hydrogen bond: A semiempirical AM1 study

Dipole moments of seven molecules were studied by AM1, each containing an intramolecular hydrogen bond between a hydroxyl group and a carbonyl or nitro group as hydrogen acceptors. The hydrogen bond causes two electron shifts: from H to O within the hydroxyl group and from C to O within the carbonyl group. The latter is accompanied by withdrawal of electrons from even more distant atoms. If the total electron density change is expressed as a vector, its direction is close to the direction of the O-H bond. This electron redistribution is in agreement with the previous, somewhat, puzzling experimental results. However, it differs from the commonly accepted picture according to which electron density changes on the hydrogen acceptor moiety are less important than those on the O-H bond.Dedicated to Professor Viktor Gutmann on the occasion of his 70th birthday.     

Vibrational Stark effect: Theoretical determination through the semiempirical AM1 method

The vibrational Stark effect of a series of small molecules has been calculated by means of the semiempirical AM1 method through addition of the electron–field interaction term in the one-electron Hamiltonian. Optimized geometrical parameters along with harmonic frequencies and line intensities are determined for different strengths of the applied uniform electric field. The perturbed spectra are compared with theoretical studies carried out at the ab initio level and with experimental results. The vibrational Stark effect of the retinal molecule is also computed, showing that this kind of study is feasible in systems of biochemical interest. © 1992 by John Wiley & Sons, Inc.     

An AM1 semiempirical study of host-guest complexation in hemicarcerand complexes

Semiempirical AM1 calculations have been carried out on host-guest complexes of model hemicarcerands 1a and 2a. The justification for the choice of the AM1 Hamiltonian was based on a comparison between reported X-ray data for the smaller tetrabromocavitand 4a and computational results obtained using several different Hamiltonians. The complexation behavior of hemicarcerands 1a and 2a have been compared with experimental results reported by Cram et al. for the related hemicarcerands 1b and 2b. Based on this comparison, a criterion for predicting guest encapsulation was developed, E(complexation), which relies on the calculation of AM1 heats of formation for host, guest, and hemicarceplex. If E(complexation) is lower than 10 kcal/mol, then a guest will be encapsulated, while if it is greater than 30 kcal/mol, a guest will not be encapsulated. The use of constrained-path AM1 optimizations to determine the energy barriers to guest entry and exit from the host was found to be a useful tool for examining suitable host-guest combinations when the E(complexation) criteria does not hold. We have computed the barriers to exit of N, N-dimethylformamide (dmf) and furan from the hemicarcerand 1a, the former has been compared with the experiment and shows excellent agreement. Based on the success of the above computational methods in predicting which host-guest combinations will form stable hemicarceplexes we have synthesized a new target hemicarceplex 1b.furan.     

A study by Raman spectroscopy and the semiempirical AM1 method on several 1,2-dihydroxybenzene solutions

The Raman spectra of 1,2-dihydroxybenzene (catechol) dissolved in six different compounds have been obtained and, on the basis of frequencies and depolarization ratios, the relevant structure of the solute has been inferred in every case. In addition, rotational barrier calculations using the AMI semiempirical method have been performed on several catechol—solvent systems. The theoretical results were in accord with experimental data and were related to structural properties of the solvents.