MO Calculations Research Articles

ChemInform Abstract: Molecular Determinants for Drug-Receptor Interactions. Part 13. X-Ray Molecular Structure of Naltrexone Malonate and Quantum Chemical Studies of the Conformations of the Pure Narcotic Antagonists Naloxone and Naltrexone.

The X-ray molecular structure of naltrexone malonate [(C20H24NO4)+(C3H3O4)–] was determined. In the T-shape molecule, the D and E rings adopt a chair conformation. Ring B is enveloped (Cs). The configuration of the N-cyclopropylmethyl group is equatorial. The preferred conformations and the energetics of the conformational processes of naloxone and naltrexone, as non-protonated (NLX and NTRX) and protonated (NLXH+ and NTRXH+) species, were studied using semiempirical MO calculations of the AM-1 type. The solid-state conformations about the N-substituent groups were not retained in calculations for isolated molecules. The NTRXH+ species was more conformationally restricted than was NLXH+ about the N–C(17) and, especially, C(17)–C(18) bonds. The conformational isoenergy maps calculated for NLX and NTRX showed that the energy barriers were lower, and that the minima zones were larger and shallower with respect to the cationic species. The present findings provided some insights into the possible conformation–activity requirements for µ-pure opioid antagonists.

ChemInform Abstract: Unimolecular Thermal Decomposition of Ethyl Vinyl Ether and the Consecutive Thermal Reaction of the Intermediary Product Acetaldehyde: Shock Wave Experiment and ab initio Calculation

The thermal decomposition of ethyl vinyl ether has been investigated behind reflected shocks over the temperature range from 890 to 1300 K and at a total gas density of approximately 1.0×10 −5 mol cm −3 by spectroscopic and gas chromatographic analyses. It was confirmed that the main process of the decomposition was the molecular elimination path (1) to form acetaldehyde and ethylene, CH 2 =CH−O−C 2 H 5 →CH 3 CHO+C 2 H 4 . The simple bond fission path (2) with the higher critical energy height, CH 2 =CH−O−C 2 H 5 →CH 2 CHO+C 2 H 5 , did not compete with path 1, at least in the temperature region below 1140 K. Ab initio MO calculations were performed on the 3-21 G level for four conformations of reactant and several transition states and products. The calculated Arrhenius parameters showed that the initial unimolecular step proceeded via the rigid six-center transition state; these parameters were in good agreement with the observed

ChemInform Abstract: Intensities in Local Mode Overtone Spectra: Propane

The gas phase vibrational overtone spectrum of propane is measured using conventional near infrared (NIR) spectroscopy for the ΔvCH=2–5 regions and intracavity dye laser photoacoustic spectroscopy (IDL‐PAS) for the ΔvCH=5 and 6 regions. The peaks are assigned in terms of the local mode model. Experimental oscillator strengths are compared to values calculated for the CH‐stretching components of the spectrum. The calculations use a harmonically coupled, anharmonic oscillator local mode model to obtain the vibrational wave functions, and ab initio MO calculations at the SCF level with a 6‐31G* basis set to obtain the dipole moment function. The importance of intermanifold coupling is explored. The calculations can account for the fall‐off in intensity with increasing v, and can give a reasonably quantitative account of the relative intensities of the individual peaks within a given vibrational manifold. The questions of the relative intensities of primary and secondary CH bonds, and of the relative intensities of different methyl CH bonds are also explored.

ChemInform Abstract: Ab initio MO Calculations of the Internal Rotational Potential of Biphenyl Using Polarized Basis Sets with Electron Correlation Correction.

ChemInform Abstract: Ab initio MO Calculations of the Internal Rotational Potential of Biphenyl Using Polarized Basis Sets with Electron Correlation Correction.

ChemInform Abstract: Conformation and Conformational Energies of Dimethoxymethane and 1,1- Dimethoxyethane

Abstract The 13 C− 1 H NMR vicinal coupling constant 3 J C H 3 OC H 2 was measured for dimethoxymethane CH 3 OCH 2 OCH 3 in the gas phase as well as in various solvents. The observed values tend to decrease slightly with temperature. The energy difference E σ between the gg and tg forms was estimated to be −2.5 ± 0.2 kcal mol −1 in the gas phase. The coupling constant parameters determined concomitantly are as follows: ( J T + J G− )/2 = 6.7 ± 0.1 and J G = 2.0 Hz. Values of E σ observed in nonpolar solvents were found to fall in the same range. A somewhat smaller value (ca. −2.2 kcal mol −1 ) was derived from the measurements in (CD 3 ) 2 SO. These values are reasonably consistent with the results of recent MO calculations by Wiberg and Murcko. A similar 13 C NMR analysis of 1,1-dimethoxyethane CH 3 OCH(CH 3 )OCH 3 indicates that the g− g and tg conformers exists in nearly equal amounts, i.e. E τ = E g− g − E tg ≅ 0 kcal mol −1 . In the latter compound, the anomeric conformation ( g − ) involves unfavorable steric interactions around the g− bond.

ChemInform Abstract: Nucleophilic Displacement in Polyhalogenoaromatic Compounds. Part 14. Chloroazulenes.

Methoxydechlorination of 5,6-dichloroazulene in NaOMe–MeOH shows second-order kinetics, apparently proceeding by the SNAr mechanism in which the attack of the nucleophile, and not the loss of chloride ion, is rate-determining. No evidence of a Meisenheimer-type intermediate is found. The rate of the reaction is not affected by light, nor by the presence of nitrobenzene; radical contributions to the process are therefore unlikely. The corresponding reactions of 4,5,6,7- and 1,3,5,6-tetrachloroazulene, and of 1- and 3-nitro-5,6-dichloroazulene all show second-order kinetics and are all accelerated compared to the parent compound; 1,3,4,5,6,7-hexachloroazulene reacted too rapidly for measurement. As expected, the extent of such acceleration is conditioned not only by the nature of the substituent but also by its position. Unexpectedly, chlorine substituent effects are clearly greater from the five-membered ring than from the ring undergoing attack.Semi-empirical all-valence-electron SCF–MO calculations are presented which rationalise the orientation of attack and describe the observed substituent effects.

ChemInform Abstract: Torsional Angle and Conformation in 2-Vinylanthracene Studied by NMR Spectroscopy and MO Calculations.

ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

ChemInform Abstract: Asymmetric Synthesis of α-Methyl Carboxylic Acid Derivatives. Stereochemistry in Acidic Ring Opening of Epoxides.

Abstract Optically active α-methyl carboxylic acid derivatives were prepared via reaction of microbially produced epoxides with trimethylaluminum. Stereochemistry of the ring opening reaction was discussed through an MO calculation.

ChemInform Abstract: Triplet Intermediates in cis-trans Photoisomerization of 3- Chrysenylethylenes.

The effect of substitution of 3-chrysenyl group (Ar) on unsaturated double bonds in their triplet sensitized isomerization was investigated for Ar–CH=CH–R, R=′Bu (BC) and Ph (SC) by means of stationary irradiations, laser flash photolyses, and MO calculations. Both olefins undergo mutual isomerization between their cis- and trans-isomers. However, BC has a unique triplet energy surface with two decay funnels at the planar trans (3t*) and twisted (3p*) conformations, which are in equilibrium with each other; 3t* undergoes either unimolecular decay to the ground-state trans isomer or triplet energy transfer to the cis isomer (quantum chain process) and 3p* decays to the ground state to give the cis and trans isomers. Tn←T1 absorptions observed for BC and SC are mainly attributed to 3t* and 3p*, respectively, on the basis of semiempirical MO calculations. The populations at 3t* and 3p* are estimated.

ChemInform Abstract: Trifluoropropene Oxide as a Trifluoromethyl Source Preparation of Optically Active Alcohols.

Abstract Optically active 3,3,3-trifluoropropene oxide(TFPO) was converted to 1,1,1-trifluoro-2-ols via Grignard type or Friedel-Crafts type reaction. The regio-selectivity of the latter reaction was discussed through an MO calculation.

ChemInform Abstract: Gaseous Diphosphorus and Triphosphorus Sulfide: Generation and Identification by Neutralization-Reionization Mass Spectrometry.

Abstract The dissociative ionization by electron impact (70 eV) of tetraphosphorus trisulfide, P4S3, yields [P3S]+ and [PZS]+0 ions whose structures have been investigated by means of Coliisionai Activation (CA) mass spectra. Using the technique of Neutralization-Reionization mass spectrometry (NRMS) it is shown that both ions can be reduced to the corresponding neutral molecules. Thus, triphosphorus sulfide and diphosphorus sulfide are viable molecules in the rarefied gas phase. The results of ab initio MO calculations were used to interpret the experimental findings. Tetrahedral (C3v) and triangular (C2v) structures are proposed for the [P,S]+ and [PZS]+0 ions respectively.

ChemInform Abstract: Trimethylamine N-Oxide, Structure and Bonding Investigated by Gas Electron Diffraction and ab initio MO Calculations.

ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

ChemInform Abstract: The Difference in Strength of BF3-Complexes of Aromatic and Aliphatic Aldehydes.

Abstract ab initio MO computations have shown that the difference in energy between syn and anti complexation is much larger for benzaldehyde/BF3 than for acetaldehyde/BF3. The BO bond strength is much greater for the anti PhCHO·BF3 than for its syn isomer and for all the CH3CHO·BF3 complexes.

ChemInform Abstract: Remarkable Effect of Fluorine and Chlorine Atoms on the Stability of 1H-Phosphirene

Ab initio MO calculations at the MP4/6–31G** level show that both fluorine and chlorine atoms exert a strong stabilizing effect on the three-membered ring 1H-phosphirene relative to its isomers. While unsubstituted 1H-phosphirene (12H) is the least stable C2H3P isomer, 1-fluoro-1H-phosphirene (12F) is calculated to be the global minimum of the C2H2FP potential energy surface; 1-chloro-1H-phosphirene (12Cl) is the third most stable of the six C2H2ClP isomers calculated. The remarkable stability of 12F is attributable to the particular strength of the P—F bond.

ChemInform Abstract: Structure of the Cyclohexa-1,4-diene Radical Cation: An ab initio Molecular Orbital and Electron Paramagnetic Resonance Study.

The structure of the radical cation of cyclohexa-1,4-diene, (1+), was explored using ab initio MO calculations and EPR spectroscopy. Geometry optimizations were carried out on neutral 1 and 1+ at the (U)HF/6-31G(d, p) and MP2/6-31G(d, p) levels of theory. The ground states of both 1 and 1+ were found to possess D2h structures, in agreement with experimental data. The ground electronic state of 1+ was found to be 2B1u and not 2B3g, and this is the result of through-bond orbital interactions. The computed MP2/6-31G(d, p) proton isotropic hfs values for 1+ were found to be in good agreement with the experimental values. The EPR spectra and proton hfs values for 1+ are similar to those obtained earlier by Shida et al.

ChemInform Abstract: Molecular and Electronic Structure of Less Common Phosphorus Halides

Abstract Molecular and electronic structures of several known and unknown phosphorus halides are described on the basis of ab initio MO calculations and available experimental data. Trends in bonding and molecular structures have been investigated for the title molecules. Dipole moments and normal mode frequencies are also reported for several molecules in the series.

ChemInform Abstract: Homo- and Heteroatomic Chalcogen Rings

Abstract This review describes recent progress in the chemistry of homo- and heterocyclic chalcogen species summarizing their preparation, structural characterization, bonding, and spectroscopic properties. Although it is possible to prepare stoichiometrically pure chalcogen rings, many reactions result in the formation of complicated mixtures. The crystal and molecular structures of many homocyclic rings are known. All heterocyclic rings, however, are disordered and therefore it has not been possible to determine accurate bond parameters for them. The identification of individual molecular species and the composition of mixtures of chalcogen ring molecules have been studied successfully with HPLC and Raman spectroscopy, as well as with 77 Se and 125 Te NMR spectroscopy. With development of computing techniques, it has been possible to perform MO calculations with increasing sophistication. They have yielded reliable predictions on ground state geometries and electronic structures of different chalcogen rings. The emphasis of research in recent years has shifted from purely structural studies towards understanding the facile interconversion reactions that take place between the different chalcogen rings. The experimental and theoretical evidence on the mechanism of these ring interconversion pathways are discussed.

How to properly compute the resonance energy within the ab initio valence bond theory: a response to the ZHJVL paper

Valence bond (VB) theory describes a conjugated system by a set of electron-localized Lewis resonance structures. VB assumes that the magnitude of the intramolecular electron delocalization can be measured in terms of a resonance energy (RE), taken to be the energy difference between the real conjugated system (delocalized) and the corresponding most stable virtual resonance structure (localized). Proper RE estimates within VB theory require both delocalized and localized states to be defined at the same theoretical level, and the definition of the localized state to closely correspond to the intuitive picture of the corresponding VB structure. In contrast, the VB-delocal and VB-local computational approaches adopted by Zielinski, et al. [preceding paper in this issue] used definitions for either the delocalized or the localized states which, in our view, depart from the intuitive chemical picture. Consequently, their RE estimates are much lower than seemingly appropriate experimental evaluations with which they strongly disagree. Very large basis sets approaching completeness blur the boundaries among resonance structures and result in “basis set artifact” problems within any variant of VB theory. However, block-localized wavefunction (BLW) computations with mid-size basis sets not only exhibit insignificant variations with theoretical levels, but the resulting RE estimates also are justified by comparisons with those employing experimental data and MO computations. We stress that RE differs from the aromatic stabilization energy (ASE). The RE measures the total stabilization of an aromatic system, whereas ASE measures only the part of the RE that exceeds that of appropriate conjugated (but non-aromatic) reference molecules.

Silyloxyamines as sources of silyl radicals: ESR spin-trapping, laser flash photolysis investigation, and photopolymerization ability

Two silyloxyamines derived from 8-(pentamethyldisilyloxy)-julolidine and diethyl 3-(pentamethyldisilyloxy)-aniline are proposed as new sources of silyl radicals. The decomposition mechanism, excited state processes and the radical generation are explored by steady state photolysis, laser flash photolysis (LFP), electron spin resonance (ESR), and MO calculations. The SiSi bond cleavage is clearly demonstrated. The formation of a radical cation on the amine moiety is also observed. Moreover, these compounds work as efficient Type I and Type II photoinitiators (PI) of free radical photopolymerization (FRP). Copyright © 2010 John Wiley & Sons, Ltd.

Radiation and Quantum Chemical Studies of Chalcone Derivatives

The reactions of oxidizing radicals ((*)OH, Br(2)(*-), and SO(4)(*-)) with -OH-, -CH(3)-, or -NH(2)-substituted indole chalcones and hydroxy benzenoid chalcones were studied by radiation and quantum chemical methods. The (*)OH radical was found to react by addition at diffusion-controlled rates (k = 1.1-1.7 x 10(10) dm(3) mol(-1) s(-1)), but Br(2)(*-) radical reacted by 2 orders of magnitude lower. Quantum chemical calculations at the B3LYP/6-31+G(d,p) level of theory have shown that the (C2-OH)(*), (C11-OH)(*), and (C10-OH)(*) adducts of the indole chalcones and the (C7-OH)(*) and (C8-OH)(*) adducts of the hydroxy benzenoid chalcones are more stable with DeltaH = -39 to -28 kcal mol(-1) and DeltaG = -32 to -19 kcal mol(-1). This suggests that (*)OH addition to the alpha,beta-unsaturated bond is a major reaction channel in both types of chalcones and is barrierless. The stability and lack of dehydration of the (*)OH adducts arise from two factors: strong frontier orbital interaction due to the low energy gap between interacting orbitals and the negligible Coulombic repulsion due to small absolute values of Mulliken charges. The transient absorption spectrum measured in the (*)OH radical reaction with all the indole chalcone derivatives exhibited a maximum at 390 nm, which is in excellent agreement with the computed value (394 nm). The formation of three phenolic products under steady-state radiolysis is in line with the three stable (*)OH adducts predicted by theory. Independent of the substituent, identical spectra (lambda(max) = 330-360 and approximately 580 nm) were obtained on one-electron oxidation of the three indole chalcones. MO calculations predict the deprotonation from the -NH group is more efficient than from the substituent due to the larger electron density on the N1 atom forming the chalcone indolyl radical. Its reduction potential was determined to be 0.56 V from the ABTS(*-)/ABTS(2-) couple. In benzenoid chalcones, the (*)OH adduct spectrum is characterized by a peak at 270 nm and a broad maximum centered in the range 430-450 nm with an intense bleaching at 340 nm. The spectrum formed by electron transfer in these derivatives with lambda(max) = 280 and 380 nm (epsilon(280) = 5000 dm(3) mol(-1) cm(-1) and epsilon(380) = 700 dm(3) mol(-1) cm(-1)) was assigned to its phenoxyl radical. Our pulse radiolysis experiments in combination with quantum chemical calculations demonstrate that chalcones are efficient scavengers of damaging oxyl radicals.