Evaluation of MINDO/3 calculated structures. III. Saturated acyclic compounds with chlorine,nitrogen, oxygen,or sulfur

Fifty-one structures have been calculated by the MINDO /3 method to evaluate the errors introduced by branching and by the presence of the heteroatom. The structures are evaluated by comparisons which reveal that the calculated ΔH_f values reflect a bias because of the presence of the heteroatom. With two carbons or more in a chain attached to a heteroatom group, a linear relationship exists which makes possible the calculation of reasonably accurate ΔH_f values for unbranched alcohols, primary or secondary amines, ethers, thioethers, thiols, and alkyl chlorides. Branching errors do not seem to be linearly related among the systems. Some errors in calculations of geometries are also discussed.     

Hydrocarbon acidities calculated with MINDO/3, MNDO,and AM1

Acidities of 32 hydrocarbons have been calculated using MINDO/3, MNDO, and AM1. All three semiempirical procedures have systematic errors and reproduce experimental acidities poorly. A linear correlation, however, does exist between the calculated and experimental results. Correction of the AM1 or MNDO acidities leads to good agreement with literature values even for acids, such as methane and ethylene, whose conjugate bases are small localized anions. Predictions for several hydrocarbons are given.     

The calculation of monocyclic molecular structures using MINDO/3

A comparison is made of the experimentally determined structures for some 4, 5, and 6 membered monocyclic rings and the MINDO/3 calculated structures. Among several systematic deficiencies the calculated ring structures are found to be too flat. This is attributed to the fact that MINDO/3 underestimates 1,4-eclipsing interactions.     

The first one-pot amidation of alkanes and cycloalkanes

Alkanes (or cycloalkanes) and CO in the presence of superelectrophilic systems CX₄·2AlBr₃ (X = Cl, Br) have been applied for the first time as equivalents of acylium salts in one-pot selective syntheses of amides from amines.     

Doubly charged benzene and isomeric dications. Fragmentation energetics and charge distributions calculated by MINDO/3 molecular orbital theory

MINDO/3 calculations for singlet and triplet doubly charged benzene [C₆H₆]²⁺ are in satisfactory agreement with the experimentally determined values of the vertical double ionization energy of benzene; calculations for straight chain isomeric structures are consistent with the observed kinetic energy release on fragmentation to [C₅H₃]⁺ and [CH₃]⁺. Symmetrical doubly charged benzene ions relax to a less symmetrical cyclic structure having sufficient internal energy to fragment by ring opening and hydrogen transfer towards the ends of the carbon chain. Fragmentation of [CH₃C₄CH₃]²⁺ to [CH₃C₄]⁺ and [CH₃]⁺ is a relatively high energy process (A), whereas both (B): [CH₃CHC₃CH₂]²⁺ to [CHC₃CH₂]⁺ and [CH₃]⁺ and (C): [CH₃CHCCHCCH]²⁺ to [CHCCHCCH]⁺ and [CH₃]⁺ may be exothermic processes from doubly charged benzene. Furthermore, the calculated energy for the reverse of process (A) is less than the experimentally observed kinetic energy released, whereas larger energies for the reverse of processes B and C are predicted. Heats of formation of homologous series [HC_n]⁺, [CH₃C_n]⁺, [CH₂C_n?2CH]⁺, [CH₃C_n?2CH₂]⁺ and [CH₂?CHC_n?3CH₂]⁺ with 1 < n < 6 are calculated to aid prediction of the most stable products of fragmentation of doubly charged cations. The homologous series [CH₂C_n?2CH]⁺ is relatively stable and may account for ready fragmentation of doubly charged ions to [C_nH₃]⁺; alternatively the symmetrical [C₅H₃]⁺ ion [CHCCHCCH]⁺ may be formed. Dicoordinate carbon chains appear to be important stabilizing features for both cations and dications.     

Ground states of molecules: Part XLII. Vibrational frequencies of isotopically-substituted molecules calculated using MINDO/3 force constants

The MINDO/3 method is used to calculate vibrational frequencies for ethylene, water and methylamine and their isotopic derivatives. The calculated ²H, ¹³C, ¹⁵N, and ¹⁸O vibrational shifts are in good agreement with the observed values, which suggests the method can be applied with confidence to the calculation of equilibrium and kinetic isotope effects in chemical reactions.     

Treatment of hydrogen bonding within CNDO/2 and MINDO/3: CNDO/2H and MINDO/3H

A formalism has been developed to treat hydrogen-bonded A—H…?B systems within the CNDO /2 and the MINDO /3 methodologies. In this formalism the interactions are divided into three distinct classes; those between (a) two hydrogen-bonded atoms, (b) one hydrogen-bonded and non-hydrogen-bonded atom, and (c) two non-hydrogen-bonded atoms. The last class of interactions is treated solely by the existing CNDO /2 or MINDO /3 method. For A –H…?B systems, the core resonance integrals are individually parametrized depending upon the class of the interaction. Three types of A—H…?B systems have been thus far parametrized. Nine hydrogen-bonded dimers have been studied using the new formalism and the current CNDO /2 and the MINDO /3 methods. MINDO /3 predicts very large interatomic (A –B) distances for the equilibrium geometry, and relatively small stabilization values for the hydrogen-bond energies. CNDO/2 predicts the reverse. The new formalism for both CNDO /2 and MINDO /3 predicts accurate geometries as well as energies for all nine dimers. The new formalisms are called CNDO /2H and MINDO /3H. A general discussion of the nature of hydrogen bonding as exhibited by CNDO /2H and MINDO /3H is presented.     

Isomer effects in the excess enthalpies of mixtures of alkanes and cycloalkanes

Using the Picker flow calorimeter, excess molar enthalpies H ^E have been obtained at 25°C for mixtures of 1,2-, 1,3- and 1,4-cis- and trans-dimethylcyclohexane and cis- and trans-decalin with n-hexadecane and the highly branched C₁₆ isomer, 2,2,4,4,6,8,8-heptamethylnonane. Values of H ^E are also obtained for cis- and trans-decalin mixed with C₆, C₇, and C₉ isomers. Anomalously low values of H ^E occur for normal alkanes mixed with cycloalkanes in the di-equatorial configuration, suggesting the presence of a negative contribution in H ^E possibly due to a restriction of n-alkane molecular motion by the flat, plate-like cycloalkane.     

Atmospheric pressure chemical ionization of alkanes,alkenes, and cycloalkanes

Normal and cyclic alkanes and alkenes form stable gas-phase ions in air at atmospheric pressure from 40 to 200°C when moisture is below 1 ppm. Ionization of alkanes in a ⁶³Ni source favored charge transfer over proton transfer through pathways involving [M?1]⁺ and [M?3]⁺ ions. Ion mobility spectra for alkanes showed sharp and symmetrical profiles while spectra for alkenes suggested fragmentation. Ion identifications were made by using mass spectrometry, and ionization pathways were supported by using deuterated analogs of alkanes and alkenes. Alkanes were ionized seemingly through a hydrogen abstraction pathway and did not proceed through an alkene intermediate. New methods for interpretation of mobility spectra utilizing ion mobility spectrometry, atmospheric pressure chemical ionization mass spectrometry, chemical ionization mass spectrometry, and ion mobility spectrometry-mass spectrometry data were demonstrated.     

Development and status of MINDO/3 and MNDO

The historical development of MINDO/3 and MNDO is outlined in relation to the parallel developments of the ab initio SCF approach. It is pointed out that both treatments are purely empirical so far as chemistry is concerned and complement one another, MINDO/3 and MNDO allowing calculations to be carried out when ab initio methods of comparable performance are inapplicable because they need one-thousand times more computing time.     

CNDO/2 and MINDO/2 energy band structures of some homopolynucleotides

The all-valence electron band structures of the periodic DNA models polyC, polyT, and polyA have been calculated with the aid of the CNDO /2 and MINDO /2 crystal orbital (CO ) method. According to the obtained results the valence and conduction bands are always of π type. The widths of the bands are usually larger in the CNDO case, than those obtained for the same systems in the PPP CO approximation. On the other hand the MINDO results show similar widths as the PPP ones. The forbidden bandwidths are by 3–4 eV larger in the CNDO /2 case, than in the PPP one, while the MINDO /2 CO method has resulted in similar gaps like the PPP calculation.     

MINDO/3 calculations of ESCA chemical shifts

Calculations of ESCA chemical shifts, using Jolly's equivalent core approximation and the MINDO/3 semi-empirical SCF MO method, have given results in reasonable agreement with experiment.     

MINDO/3 Calculation of carbocation heats of formation

Heats of formation calculated by MINDO /3 are reported for 42 carbocations for which experimental heats of formation have been published. Errors associated with these calculations can be large, with an overall range of ±13 kcal/mol. Correction of systematic errors in the MINDO /3 calculations by means of hydrocarbon models and isodesmic relationships results in a reduction in the range of errors to ±8 kcal/mol. Comparison with experimental heats of reaction of hydride transfer equilibria minimizes experimental errors and gives an average absolute error of 2 kcal/mol with a range of ±3 kcal/mol.     

A MINDO/3 MO study of oxidized flavins

The use of semi-empirical molecular orbital methods for the study of biologically important molecules has increased in recent years. The validity of well-known approximate Hartree-Fock molecular orbital methods such as MINDO/3 and MNDO still remains largely untested for biological molecules. Here we report its first application to the study of the electronic structures of isoalloxazines. Electronic charge distribution, variation of dipole moment, first ionization potential, electron affinity, orbital energies and electronic transitions are examined. Some of these results are compared with experiments. Good correlations with experiments were generally found in net atomic charge distributions, ionization potentials and electronic transitions. As a result of relatively good correlations with experiment application of the MINDO/3 method for extended study of biologically important coenzymes is promising.