Nonempirical study on the potential energy surface of 5-methyl-1,3-dioxane

Quantum-chemical study on the potential energy surface of 5-methyl-1,3-dioxane at the nonempirical RHF//STO-3G, RHF//3-21G, RHF//6-31G(d), RHF//6-31G(d,p), and MP2//6-31G(d,p) levels of theory revealed two energy-equivalent paths of conformational transformation of the equatorial and axial chair conformers. Potential barriers to these processes were estimated. The δG° value for the methyl substituent on C⁵ in 1,3-dioxane ring, determined on the basis of the experimental (NMR) and calculated vicinal 1H-1H coupling constants, was very consistent with published data     

AB Initio and MNDO calculations of atomic interactions in the CH3OCOCl molecule

The CH₃OCOCl molecule is calculated by ab initio methods using the split-valence basis sets at RHF/3-21G//RHF/3-21G, RHF/6-31G^*//RHF/6-31G^*, and RHF/6-311G^*//RHF/6-31G^* levels of theory and in the MNDO approximation. The optimized geometry of the molecule is consistent with the experimental data. The populations of the p-AOs of this molecule and the MO compositions show that the electron distribution in this molecule should be interpreted without considering the conjugation between the lone electron pairs of the Cl or O atoms and the π-electron system of the carbonyl group. The asymmetry parameters of the electric field gradient on the³⁵Cl nucleus were calculated using the Cl p-AO populations and compared with the corresponding experimental value. Instite of Technical Chemistry, Ural Branch, Russian Academy of Sciences. Translated fromZhurnal Struktumoi Khimii, Vol. 37, No. 4, pp. 646–651, July–August, 1996. Translated by I. Izvekova     

<Emphasis Type="Italic">N′,N′</Emphasis>-dialkylhydrazides and their protonated forms: <Emphasis Type="Italic">ab initio</Emphasis> calculations and studies by physical methods

RHF/6-31G(d) and MP2/6-31G(d) quantum-chemical calculations of N′,N′-dimethylethanohydrazide molecule and also MP2/6-31G(d) calculations of its protonated forms were performed. With the use of IR and ¹H NMR spectroscopy the series of N′,N′-dialkylhydrazides and their salts with HCl and H₂SO₄ were studied. The position of proton addition was determined. The experimental results were compared with the calculated findings.     

Ab initio molecular orbital calculations for 3,6-dihydro-1,2-dithiin and 3,6-dihydro-1,2-dioxin

Optimized geometries and total energies for the conformers of 3,6-dihydro-1,2-dithiin ( 2 ) and 3,6-dihydro-1,2-dioxin ( 3 ) were calculated at several ab initio MO levels: RHF/3-21G(*), RHF/6-31G*, MP2/6-31G*, and MP2/6-31G*/ /RHF/3-21G(*). For the dioxin, in addition to the above levels the corresponding nonextended basis sets ab initio methods were also carried out. The dithiin results are compared with those of simple disulfanes, HSSH and (CH₃)₂S₂, whose optimized geometries agree closely with the observed structures, which is the gauche (C₂ symmetry). For the disulfanes, the gauche geometries from RHF/3-21G(*) are in good agreement with the observed structure while the RHF/3-21G results best fit the dioxin. Pertinent structural data at the RHF/3-21G(*) for the half-chair (C₂) dithiin are: bond lengths, ? SS? , ? CS? , ? CC?, and ? C?C? , 2.050, 1.817, 1.515, and 1.317 Å, respectively; bond angles, CSS, ?CCS, and C?CS, 98.0, 114.2, and 127.8°, respectively; CSSC dihedral angle of 63.2°; and twist angle of 36.5°. The total energy for half-chair dithiin at MP2/6-31G*//RHF/3-21G(*) is less than the planar (C_2v) and the half-boat (C_s) structures by 69.67 and 29.05 kJ/mol, respectively. The calculated structural data (vs. observed) at RHF/3-21G for the half-chair dioxin are: bond lengths, ? OO? , ? CO? , ? CC?, and C?C, 1.464 (1.463), 1.454, 1.509, and 1.313 Å (1.338 Å), respectively; bond angles, COO, ?CCO, and C?CO, 105.0, 109.8 (110.3), and 120.7° (119.9°), respectively; COOC dihedral angle of 79.7° (80 ± 2°); and twist angle of 39.0 (38.3°). The total energy for half-chair dioxin at MP2/6-31G//RHF/3-21G is less than the planar and the half-boat structures by 70.35 and 42.85 kJ/mol, respectively. The total energies calculated at the extended basis sets (*) ab initio levels for the C₂ symmetry dioxin are much lower than those of the nonextended basis sets. © John Wiley & Sons, Inc.     

Electron distribution in 3,4-dimethylanisole molecule according to quantum-chemical calculations

Calculations of two forms of 3,4-dimethylanisole molecule with full geometry optimization are performed by AM1, RHF/6-31G(d), and MP2/6-31G(d) methods. The form with cis location of H₃C-O and C¹–C⁶ bonds is somewhat more preferable energetically than that with the trans location. By the RHF/6-31G(d) method, this molecule is also calculated with fixed rotation angles of methoxy group around the C-O bond. It is shown that, in the first form, the negative charge on the C⁶ atom is considerably smaller than on C², while in the second form the charges are practically equal, which does not confirm the concerted effects of steric and electronic factors in reactions of 3,4-dimethylanisole with methyl isocyanate and isobutyraldehyde. The charge difference is due to the effect of methyl group in the 3-position of the aromatic ring and to the orientation of the methoxy group toward geminal C-C bonds of the ring.     

Nonempirical approach to the conformational analysis of 2,4-dimethyl-1,3,2-dioxaborinane and its oxonium ions

Using nonempirical quantum-chemical approximations RHF//STO-3G, 3-21G, 6-31G(d) and MP2//6-31G(d,p) a conformation isomerism of 2,4-dimethyl-1,3,2-dioxaborinane and its oxonium ions was studied. It was shown that the potential energy surface of the studied molecules has minima corresponding to equatorial (main minimum) and axial sofa forms and maxima corresponding to equatorial and axial conformations of 2,5-twist forms. Calculated values of the barriers of internal rotation of methyl group at the ring C⁴ atom were found. It was also established that the heat of protonation of the cyclic boric ester was smaller than of the non-boric analog, cis-2,4-dimethyl-1,3-dioxane, owing to decrease in basicity of the oxygen atoms in the cyclic boric ester due to partial double bond character of B-O bond.     

Electronic effects of conjugated enones on their reactivity in transformations of ADDN type

Using RHF/3-21G, RHF/6-31G(d, p), MP2/6-31G(d, p), B3LYP/6-31G(fd, p) approximations the structure and ¹³C NMR spectra of 2-alkylsubstituted cyclohexene-2-ones and 2-alkylacroleins are studied and calculated. In the series of 2-alkylcyclohexene-2-ones the effect of the substituent on a deviation from coplanarity of the C=C-C=O fragment is more expressed in comparison with 2-alkylacroleins. This deviation (5°) is not enough to explain the observed properties of 2-alkylcyclohexene-2-ones due to disturbed conjugation. The particular behavior of (R)-4-mentenone in reactions of 1,4-addition and ozonolysis is explained by a more expressed +I-effect of the alkyl substituent in α-position.     

AB initio study of thermodynamic stability and structure of cage molecules B4N4H8 and Be4O4H8

Stable molecular structures of heterocubane systems B₄N₄H₈ 2 and Be₄O₄H₈, isoelectronic to the cubane molecule, are investigated by ab initio (RHF/6-31G**, MP2(full)/6-31C**, and MP2(full)/6-311+ + G**) methods and are shown to be highly thetmodynamically stable. Decomposition of structure 2 into two 1,3,2,4-diazadiboroethidine molecules 6 or four iminobomne NBNH molecules 11 is an endothermal process taking 10.1 (RHF/6-31G**), 39.6 (MP2(full)/6-31G**) kcaUmole and 140.6 (RHF/6-31G**), 161.4 (MP2(full)/6-31G**) kcal/mole, respectively. Decomposition of structure 3 into two 1,3,2,4-dioxydi-beryllothidine molecules 12 or four molecules 13 is also an endothermal reaction taking 22.1 (RHF/6-31G**), 39.8 (MP2(full)/6-31G**) kcal/mole and 127.1 (RHF/6-31G**), 155.2 (MP2(full)/631G**) kcal/mole, respectively. The geometrical characteristics of simple molecules BeH₂ 15, Be₂ 16 and 17, Be₂H₂ 18, Be₂H₄ 19, BeO 20, and Be₂O₂ 21 are calculated. Translated from Zhumal Struktumoi Khim ii, Vol. 41, No. 1, pp. 3-13, January–February, 2000     

Conformational Isomerism and Chlorotropy Mechanisms of Chloromethyl- and Trichloromethyldichlorophosphines

³⁵Cl NQR spectroscopy and MP2//RHF/6-31⁺⁺G(d,p) and MNDO-PM3 calculations were used to study the conformational and chlorotropic isomerism of chlorodimethyldichlorophosphine (I) and trichloromethyldichlorophosphine (II). The experimental ³⁵Cl NQR spectrum is in complete accord with the staggered conformation of phosphine II obtained using an RHF/6-31⁺⁺G(d,p) calculation. The rotational barrier of the CCl₃ group is 38.1 kJ/mol. On the other hand, the spectrum of phosphine I is in accord with a gauche conformation, which agrees with experiment only upon taking account of electron correlation (MP2). The ylide and phosphinic chlorotropic isomers for I and II are thermodynamically stable with greater stability found for II. The chlorotropic phosphine–ylide conversion in system I proceeds exclusively through a sigmatropic transition state in qualitative accord with nonempirical and semiempirical calculations. Such a conversion is theoretically possible in system II by means of dissociation of the P⁺—C^– ylide bond.     

Structure of N-chloromethyllactams and features of the interaction of atoms in them as a result of ab initio calculations

Quantum-chemical calculations have been carried out by the RHF/6-31G(d) and MP2/6-31+G(d) methods of molecules of N-chloromethylpyrrolidone, N-chloromethylcaprolactam, N-chloromethyl-succinimide, and N-chloromethylphthalimide with full optimization of their geometry, and also N-chloromethylpyrrolidone molecule by the RHF/6-31G(d) method at various angles of rotation of the CH₂Cl group around the C―N bond. It was shown that the lower frequencies of the ³⁵Cl NQR of the first two molecules in comparison with the later are mainly determined by the high populations of the p _σ -orbitals of their Cl atoms. The population of the orbitals of the unshared electron pair of the N atom is practically unchanged on rotating the CH₂Cl group, but the N atom polarizes the C―Cl bond in the indicated molecule. This does not confirm the supposed p,σ*-conjugation in the Cl―C―N grouping. Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 10, pp. 1537–1544, October, 2008.     

Thermochemical and theoretical study of some methyldiazines

The standard (p ⁰ = 0.1 MPa) molar enthalpies of formation for the liquid 2,3-dimethylpyrazine and trimethylpyrazine and the crystalline 2,3-dimethylquinoxaline and tetramethylpyrazine were derived from the standard molar enthalpies of combustion, in oxygen, atT=298.15 K, measured by static-bomb combustion calorimetry. The standard molar enthalpies of vaporization or of sublimation for the same compounds were determined by Calvet microcalorimetry. Ab initio full geometry optimization at the 3-21G and 6-31G^* levels were also performed for all the methylpyrazine isomers. MP2/RHF/3-21G//3-21G and DFT energies were also calculated for all the methylpyrazine isomers, thus allowing us to estimate their isodesmic resonance energies.     

Electronic and Spatial Structure of Piperidine and Its Substituted Derivatives from the Results of ab initio Calculations

The results of non empirical quantum-chemical calculations using the RHF/6-31G(d) and MP2/6-31G(d) methods do not agree with proposals for the axial position of the H atom on the N atom in the piperidine molecule. According to RHF/6-31G(d) calculations for the N-methylpiperidine molecule and its chloro-substituted derivatives an equatorially placed methyl group is energetically more favored than an axial. The axial C-Cl and C-H bonds in these molecules are longer than the equatorial. The ³⁵ Cl NQR frequencies for the axial Cl atoms are lower than the equatorial. The ³⁵ Cl NQR frequency of the axial chlorine atom in 2-chloro-1-methylpiperidine is anomalously low. This is chiefly due to the high population density of its p σ-orbital and this is a result of the polarization of the C-Cl bond via the N atom unshared electron pair directly through the field. The effect of a similar unshared electron pair on the parameters of the C-Cl bond in the ClCH₂NH₂ molecule has been studied by the RHF/6-31(g) method for different angles of rotation of the ClCH₂ group around the C-N bond. __________ Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 7, pp. 1044–1052, July, 2005.     

Conformational analysis of <Emphasis Type="Italic">cis-</Emphasis> and <Emphasis Type="Italic">trans</Emphasis>-isomers of 2,5-dimethyl-1,3-dioxane

The conformational isomerization of cis- and trans-isomers of 2,5-dimethyl-1,3-dioxane was investigated by means of ab initio RHF//6-31G(d) and PBE//ccpVDZ quantum-chemical methods. It has been shown that in comparison with cis-form the potential energy surface of trans-isomer has a less number of stationary points and includes the main minimum belonging to the diequatorial chair form. In the case of cisisomer the main minimum corresponds to the C 2e5a chair form. The values of potentional barriers of conformational isomerization were also determined. On the basis of experimental (NMR ¹H) and theoretical vicinal coupling constants we determine the value ΔG ⁰ of chair-chair inversion for the trans-isomer molecule.     

Structure and Conformational Stability of 1,3-Dihalo-2,2,4,4-tetrachloro-1λ3,3λ3-diphosphetidines

The optimal geometry of isomeric molecules of (XP-CCl₂)₂ with X = F, Cl, Br was determined by RHF/6-31G(d) calculations. With X = F and Cl, the electronic correlation was considered on the MP2/6-31G(d) level. The P₂C₂ ring is nonplanar. With X = Cl and Br, the trans conformation is energetically preferable compared to the two possible cis conformations: by 7.8 and 14.2 kJ mol^{- 1} for X = Cl and by 7.5 and 14.1 kJ mol^{- 1} with X = Br. respectively. With X = F, the calculated energies of the cis and trans forms are very close.     

The Hartree-Fock dissociation of F2

We examine the stability of the restricted Hartree-Fock (RHF) wave function for F₂ in the vicinity of the equilibrium internuclear distance (R=R _e ) and the shape of the unrestricted Hartree-Fock (UHF) potential energy curve for the same system. The results depend on the basis set: With a split valence plus polarization basis, 6-31G(d), the RHF wave function is unstable at R _e , and the UHF potential curve is purely dissociative. When the basis is extended to 6-311+G(3d) or 6-311+G(3df), the RHF wave function becomes stable, and the UHF potential curve acquires a local maximum for R slightly (0.02 å) greater than R _e . The local maximum, however, is only 0.1 kcal/mol higher than the local minimum at R=R _e .     

Quantum Chemical Model of a Lysozyme+Substrate Complex and a Cooperative Process in It

This paper presents an ab initio (RHF/6-31G** and MP2(full)6-31G**) and density functional (DFT) study of the structure and energetics of formation of an intermolecular complex which is the simplest model of an active center lysozyme with a substrate. The calculated energy of complex formation is 41.4 (RHF), 53.4 (MP2), and 52.7 kcal/mole (DFT). The proton transfer reaction is a concerted reaction having an energy barrier of 41.1 (RHF), 31.6 (MP2), and 25.3 (DFT) kcal/mole.     

Quantum-chemical study of 1,3-dioxane complexes with two water molecules

Methods PM3, RHF/6-31G(d), and MP2/6-31G(d)//RHF/6-31G(d) were used in calculation of the energy of formation of five 1,3-dioxane complexes with two water molecules formed through hydrogen bonds. The study of the conformational properties of the most stable associate revealed two routes of the chair-chair conformation isomerization. It was shown that the difference between the minima on the potential energy surface in this gase increased, and the barriersto the interconversion decreased as compared to the calculated values for the isolated molecule of 1,3-dioxane.     

Conformational analysis of 2-methyl-5-nitro-1,3,2-dioxaborinane

Quantum-chemical methods HF/6-31G(d), HF/6-31+G(d), MP2/6-31G(d)//HF/6-31G(d), and MP2/6-31+G(d)//HF/6-31+G(d) were used to investigate the conformational isomerization of 2-methyl-5-nitro-1,3,2-dioxaborinane. It has been shown that a potential energy surface of this compound includes two minima: an axial form of semi-chair and equatorial sofa together with a transition state belonging to the conformation of 2,5-twist-form. A comparison between experimental NMR ¹H and theoretical vicinal coupling constants was used to determine the quantitative conformational composition of cyclic boric acid ester and a value of ΔG ⁰ for nitro group at the ring carbon atom C⁵ in CCl₄ and C₆D₅NO₂ solutions.     

Multiple bond migration with participation of a protophilic agent

Ab initio study of the pathways of migration of the double bond in the 3-methylthioprop-1-ene (1) and 3-methoxyprop-1-ene (2) molecules with participation of hydroxide ion was carried out by the RHF/6-31+G^* and MP2/6-31+G//RHF/6-31+G^* methods. Conformational isomerism of the initial molecules and reaction products was considered. The distinctions are discussed in the spatial and electronic structure of intermediate carbanions stabilized (for1) due to the negative hyperconjugation. Stationary points corresponding to complexes between the molecules under study and the hydroxide ion and between the corresponding carbanion and water molecule were localized on the potential energy surfaces of the proton transfer reactions. For2, the single-stage mechanism of prototropic rearrangement involving the H atom of the hydroxide ions was found to be more energetically preferable than the two-stage mechanism, whereas both mechanisms are expected to be equiprobable for1. For Parts 1–3, see Refs. 1–3. Translated fromIzvestiya Akademii Nauk. Seriya Kimicheskaya, No. 3, pp. 407–413, March, 2000.     

Spatial and electronic structure and 35Cl NQR parameters of ethyl(trichlorogermyl) propionate according to Ab initio calculations

Two stable structures of ethyl(trichlorogermyl) propionate have been studied by RHF/6-31G(d) and MP2/6-31G(d) quantum-chemical caclulation with full geometry optimization. The structure with pentacoordinated Ge atom has been more stable than that with tetracoordinated Ge atom. Based on the computation results, the frequencies of ³⁵Cl nuclear quadrupole resonance of the studied compound with pentacoordinated Ge atom has been estimated, it has been in satisfactory agreement with the experimental data. Additionally, the calculations by means of the RHF/6-31G(d) method have been performed at various fixed Ge…O interatomic distances. When the Ge and O coordination centers get closer, both the positive charge on Ge and the negative charge on O increase. The electron density shifts from Ge atom to the axial Cl atom, and the electron density shifst from the carbonyl C atom to the carbonyl oxygen atom. The electron density charge trasfer from O to Ge does not occur.