Investigation of rotational isomerism along the C-O bond in α-aryloxycarbonyl compounds

Electrical and electrooptical characteristics of -phenoxycyclohexanone, -phenoxyacetone,p-Me-,p-Cl-,p-Br-,p-NO ₂ -phenoxycyclohexanones, andp-NO ₂ -phenoxyacetone in the solution in CCl ₄ were determined; these were utilized for the conformational analysis of the general structural fragment Ar-O-C-C=O. The presence of conformers with theac- andsp-structure at the C _sp 3-C _sp 2 bond was established in the equilibrium. The content of thesp-form increases with the increase in the polar character of the substituent at thep-position of the benzene ring. Thegauche isomer (sp-G orsp-G) is realized along the C-O bond in it. In theac-conformers, both thegauche (ac-G) and the trans (ac-T) structures can be realized along the C-O bond.A. E. Arbuzov Institute of Organic and Physical Chemistry, Kazan Scientific Center, Russian Academy of Sciences, Kazan' 420083. Translated from Izvestiya Akademii Nauk, Seriya Khimicheskaya, No. 3, pp. 612–620, March, 1992.     

Gas Electron Diffraction and Quantum-Mechanical Study of Dimethyloxalate

The geometrical structure and conformation of dimethyloxalate, CH₃OC(O)–C(O)OCH₃, have been studied by gas electron diffraction (GED) and quantum-chemical calculations (MP2 and B3LYP methods with 6-31G* and cc-pVTZ basis sets). The GED analysis with a dynamic model (T = 323 K) results in a mixture of two planar conformers, anti (C_2h symmetry) and syn (C_2v symmetry) orientation of the two C=O bonds. The energy difference between these conformers is 0.02(0.18) kcal/mol and barrier to internal rotation around the C–C bond is 0.44(0.41) kcal/mol. The CH₃ groups occupy synperiplanar positions with respect to the C=O bonds. The following main geometrical parameters for the anti conformer (Å and degrees) have been derived: r_g(C–C) = 1.532(3), r_g(C=O) = 1.203(2), r_g(C_sp3–O) = 1.436(3), r_g(C_sp2–O) = 1.333(3), (C_sp2–C_sp2–O) = 111.9(1.9), (C_sp2–O–C_sp3) = 116.3(1.6), (O–C= O) = 127.0(1.8).This paper is devoted to the 75th anniversary of gas electron diffraction method.     

Zinc(II) complexes derived from imidazo[1,2-a]pyridin-2-ylacetic acid (HIP-2-ac): [Zn(IP-2-ac)2(H2O)] and unexpectedly, [Zn3(IP-2-ac)6(H2O)]·11H2O

Two zinc(II) complexes based on imidazo[1,2-a]pyridin-2-ylacetate (IP-2-ac), [Zn(IP-2-ac)₂(H₂O)] (1) and [Zn₃(IP-2-ac)₆(H₂O)]·11H₂O (2), were synthesized and characterized by single-crystal X-ray diffraction. In both 1 and 2, zinc(II) ions are five-coordinate with N₂O₃ donor set, best described as a distorted trigonal-bipyramidal geometry. In 1, two IP-2-ac ligands chelate zinc(II) through a N,O donor set, whereas in 2, both bidentate and μ-bridging binding modes of IP-2-ac are observed. The crystal of 1 comprises discrete Zn(IP-2-ac)₂(H₂O) coordination entities combined into layers by hydrogen bonds. Inter-layer stabilization of the 3-D crystal lattice is provided by weak C–H?O contacts and π?π interactions. The structure of 2 consists of discrete trinuclear Zn₃(IP-2-ac)₆(H₂O) coordination entities joined into crystal lattice by multiple water molecules.

Compound 1 was characterized by FTIR and FT-Raman spectroscopy, and in terms of thermal stability. Furthermore, its antibacterial activity was tested against selected gram-positive, gram-negative bacteria, and Candida albicans yeast and compared with activity of previously reported [M(IP-2-ac)₂(H₂O)₂]·2H₂O (M = Co, Ni, Mn, Cd) complexes.     

Structural and magnetic anomalies among the spin-chain compounds, Ca3Co1+xIr1?xO6

The results of X-ray diffraction, andac anddc magnetisation as a function of temperature are reported for a new class of spin-chain oxides, Ca₃Co_1+x Ir_1−x O₆. While thex = 0.0, 0.3, 0.5 and 1.0 are found to form in the K₄CdCl₆-derived rhombhohedral (space group ) structure, thex = 0.7 composition is found to undergo a monoclinic distortion in contrast to a literature report. Apparently, the change in the crystal symmetry withx manifests itself as a change in the sign of paramagnetic Curie temperature for this composition as though magnetic coupling sensitively depends on such crystallographic distortions. All the compositions exhibit spin-glass anomalies with an unusuallylarge frequency dependence of the peak temperature inac susceptibility in a temperature range below 50 K, interestingly obeying Vogel-Fulcher relationship even for the stoichiometric compounds. Dedicated to Professor C N R Rao on his 70th birthday     

Tautomeric and conformational properties of dimethyl malonate, CH3OC(O)-CH2-C(O)OCH3: electron diffraction and quantum chemical study

The geometric structure of dimethyl malonate, CH₃OC(O)-CH₂-C(O)OCH₃, was studied by gas electron diffraction (GED) and quantum chemical methods. We conclude that only diketo conformers exist in the gas phase. According to the GED refinement, a mixture of two diketo conformers is present: 69(10)% (ac,ac) conformer with C₂ symmetry (both CO bonds anticlinal relative to the opposite C-C bond) and 31(10)% (sp,ac) conformer with C₁ symmetry (one CO bond with synperiplanar, the other CO bond with anticlinal orientation). Ab initio calculations, however, predict a preference of the (sp,ac) conformer rather than the (ac,ac) form.     

Ab initio Calculations of the Geometry,Electronic Structure,and Vibrational Spectrum of (Acetoxymethyl)triflouorosilane

The geometries and electronic structures of various conformers of (acetoxymetyl)trifluorosilane, CH₃C(O)OCH₂SiF₃, and also the frequencies and intensities of its vibration bands were studied by ab initio [RHF(6-31G*)] calculations. According to the experimental spectra and calculation results, the most stable is the conformation in which the donor-acceptor interaction = OSi is realized. Analysis of the geometry, atomic charges, and molecular orbital energies shows that this donor-acceptor bond is mainly due to interaction of the lone electron pairs of the carbonyl oxygen atom with the vacant * orbitals of the C-Si-F _a moiety.     

Conformational Analysis,Infrared, and Fluorescence Spectra of 1-Phenyl-1,2-propandione 1-oxime and Related Tautomers: Experimental and Theoretical Study

Summary. Conformational analysis and frequency calculation were achieved for 1-phenyl-1,2-propandione 1-oxime and its four tautomers: 1-nitroso-1-phenyl-1-propen-2-ol, 1-nitroso-1-phenyl-2-propanone, 2-hydroxy-1-phenyl-propenone oxime, and 3-nitroso-3-phenyl-propen-2-ol. Calculations were carried out at the Hartree–Fock (HF), Density Functional Theory (B3LYP), and the second-order M?ller–Plesset perturbation (MP2) levels of theory using 6-31G^* and 6-311G^** basis sets. Five conformers with no imaginary vibrational frequency were obtained by free rotations around three single bonds of 1-phenyl-1,2-propandione-1-oxime: Ph–C(NOH)C(O)CH₃, PhC(NOH)–C(O)CH₃, and PhC(N–OH)C(O)CH₃. Similarly, eight structures with no imaginary vibrational frequency were encountered upon rotations around three single bonds of 1-nitroso-1-phenyl-1-propen-2-ol: Ph–C(NO)C(OH)CH₃, PhC(N–O)C(OH)CH₃, and PhC(NO)C(–OH)CH₃. In the same manner, six minima were found through rotations around three single bonds of 1-nitroso-1-phenyl-2-propanone: Ph–CH(NO)C(O)CH₃, PhCH(–NO)C(O)CH₃, and PhCH(NO)–C(O)CH₃. Also, two minima were found through rotations around four single bonds of 2-hydroxy-1-phenyl-propenone oxime: Ph–C(NOH)C(OH)CH₂, PhC(N–OH)C(OH)CH₂, PhC(NOH)–C(OH)CH₂, and Ph-C(NOH)C(–OH)CH₂. Finally, two minima were found through rotations around four single bonds of 3-nitroso-3-phenyl-propen-2-ol: Ph–CH(NO)C(OH)CH₂, PhCH(–NO)C(OH)CH₂, PhCH(NO)–C(OH)CH₂, and PhCH(NO)C(–OH)CH₂. Interconversions within the above sets of conformers were probed through scanning (one and/or two dimensional), and/or QST3 techniques. The order of the stability of global minima encountered was: 1,2-propandione-1-oxime > 1-nitroso-1-phenyl-2-propanone > 1-nitroso-1-phenyl-1-propen-2-ol > 2-hydroxy-1-phenyl-propenone oxime > 3-nitroso-3-phenyl-propen-2-ol. Hydrogen bonding appears significant in tautomers of 1-nitroso-1-phenyl-1-propen-2-ol and 2-hydroxy-1-phenyl-propenone oxime. The CIS simulated λ_max for the first excited singlet state (S₁) of 1-phenyl-1,2-propandione 1-oxime is 300.4 nm, which was comparable to its experimental λ_max of 312.0 nm. The calculated IR spectra of 1-phenyl-1,2-propandione 1-oxime and its tautomers were compared to the experimental spectra.     

Conformational investigation of phenoxycyclohexan-2-one andp-Cl- andp-NO2-phenoxycyclohexan-2-one by molecular mechanics; x-ray crystallographic analysis ofp-NO2-phenoxycyclohexan-2-one

The preferred conformers of phenoxycyclohexan-2-one and p-Cl- andp-NO₂-phenoxycyclohexan-2-ones were calculated by the MM2(87) method. The results are compared with the data from experimental conformational investigations in solution. According to the calculation, the compounds are described by a multicomponent equilibrium, including thesp-G conformer with a planar orientation of the anisole fragment and a mixture of theac-G andT conformers both with planar and with orthogonal arrangement of the aryl group. The structure ofsp-G(0°)p-NO₂-phenoxycyclohexan-2-one was determined by x-ray crystallographic analysis.A. E. Arbuzov Institute of Organic and Physical Chemistry, Kazan' Scientific Center, Russian Academy of Sciences, 420083 Kazan'. Translated fromIzvestiya Akademii Nauk, Seriya Khimicheskaya, No. 6, pp. 1359–1364, June, 1992.     

AM1 study of photoelectronic spectra

Ionization energies of four model compounds with known conformations,i.e., benzo-1,3-dioxole, 2-methoxyphenol, benzo-1,3-dioxole-5-carbaldehyde (piperonal), and 4-hydroxy-3-methoxybenzaldehyde, have been calculated by theab initio AM1 method, using the formalized scheme of configuration interactions. It has been demonstrated that this method is adequate for the study of photoelectronic spectra of methoxy(hydroxy)-substituted benzenes. The ionization energies of 1,2-dimethoxybenzene, 3,4-dimethoxybenzaldehyde, and 4-hydroxy-3,5-dimethoxybenzaldehyde have been calculated for various orientations of theo-methoxy(hydroxy) groups. It has been revealed that three first ionization potentials corresponding to the states with vacancies on the -MO depend on the torsion angle. It has been established by comparison of calculated and experimental ionization potentials that in gas the compounds with adjacent methoxy groups have one O-Me bond parallel with the plane of the benzene ring, while another group is nearly perpendicular to this plane. Conformations of the heavy-atom framework are planar for gaseous molecules with adjacent methoxy and hydroxy groups.For Part 7, see Ref. 1.Translated fromIzyestiya Akademii Nauk. Seriya Khimicheskaya, No. 12, pp. 2365–2368, December, 1995.     

Molecular conformations of jet-cooled 2-methylindan and 2-phenylindan

The molecular conformations of jet-cooled 2-methylindan (2MI) and 2-phenylindan (2PI) have been studied using resonant-enhanced two-photon ionization spectroscopy in combination with ab initio calculations. Both axial (2MI_ax) and equatorial (2MI_eq) conformers of 2MI have been observed. A 2MI_eq/2MI_ax conformer ratio of 2.3 was estimated at 298 K, leading to the energy difference, \Updelta E = E _{2 \textMI\textax} - E _{2 \textMI\texteq} \Updelta E = E_{{ 2 {\text{MI}}_{\text{ax}} }} - E_{{ 2 {\text{MI}}_{\text{eq}} }} , of 0.49 kcal/mol. Ab initio calculations predicted three stable conformers of 2PI: two equatorial conformers (2PI_eq0 and 2PI_eq90), and one axial conformer (2PI_ax). Only the axial conformer of 2PI (2PI_ax) was experimentally observed. The indan ring of 2PI_ax is slightly more planar than the indan rings of the two equatorial conformers of 2PI because of the intramolecular C_sp2–H/π interactions in 2PI_ax. The equatorial conformers of 2PI relax to the more stable axial conformer because of the high pre-expansion temperature (383 K), and relatively low barrier (1.68 kcal/mol) to axial–equatorial interconversion. The barrier (2.33 kcal/mol) to axial–equatorial interconversion in 2MI is high enough to prevent conformational relaxation at the pre-expansion temperature of 298 K. Intramolecular C–H/π interactions are found to be more important in determining the conformational preference of 2PI than 2MI; this can be attributed to the higher acidity of the C_sp2–H bond than that of C_sp3–H bond.     

19F NMR study ofcis- andtrans- effects of substituted triphenylphosphine ligands intrans-(4-fluorophenyl)triarylphosphine-tri(4-fluorophenyl)phosphineplatinum 4-fluorothiophenoxides

A number of compounds of the type oftrans-4-FC₆H₄Pt(PAr₃)₂SC₆H₄F-4, where Ar is a substituted phenyl group, have been prepared starting from the corresponding chlorides. By exchange reactions oftrans-4-FC₆H₄Pt[P(C₆H₄F-4)₃]₂SC₆H₄F-4 with the above-mentioned compounds or Ar₃P,trans-4-FC₆H₄Pt[P(C₆H₄F-4)₃][PAr₃]SC₆H₄F-4 have been generated in solution. For the latter compounds, the effect of Ar₃P oncis- andtrans-ligands has been studied by the¹⁹F NMR technique. It has been shown that thecis- andtrans-effects of Ar₃P run parallel and are well described by pK _a values and ionization potentials of the unshared electron pair in Ar₃P, as well as by ⁰ constants of the aryl groups.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 7, pp. 1359–1363, July. 1995.     

Theoretical study of the structure of (O→Si)-(acetoxymethyl)trifluorosilane dimers

According to the data of quantum chemical calculations (HF, DFT), the isolated sp,sp conformer of (acetoxymethyl)trifluorosilane — pentacoordinated silicon compound — forms dimers with two intermolecular bonds Si-F→Si. The =O→Si intramolecular coordination bonds become stronger, which leads to the general stabilizing effect. In polar media, dimer formation is energetically unfavorable relative to monomer formation.     

1,N6‐Etheno‐2′‐deoxytubercidin hemihydrate

The title compound [systematic name: 7‐(2‐deoxy‐β‐d ‐erythro‐pentofuranosyl)‐7H‐imidazo[1,2‐c]pyrrolo[2,3‐d]pyrimidine hemihydrate], 2C₁₃H₁₄N₄O₃·H₂O or (I)·0.5H₂O, shows two similar conformations in the asymmetric unit. These two conformers are connected through one water molecule by hydrogen bonds. The N‐glycosylic bonds of both conformers show an almost identical anti conformation with χ = −107.7 (2)° for conformer (I‐1) and −107.0 (2)° for conformer (I‐2). The sugar moiety adopts an unusual N‐type (C3′‐endo) sugar pucker for 2′‐deoxyribonucleosides, with P = 36.8 (2)° and τ_m = 40.6 (1)° for conformer (I‐1), and P = 34.5 (2)° and τ_m = 41.4 (1)° for conformer (I‐2). Both conformers and the solvent molecule participate in the formation of a three‐dimensional pattern with a `chain'‐like arrangement of the conformers. The structure is stabilized by intermolecular O—H...O and O—H...N hydrogen bonds, together with weak C—H...O contacts.     

Explanation of Anomalous Gas-Chromatographic (Gc) Behaviour in a Homologous Series of 2-Chloroethyl Phosphonic Acid Dialkyl Esters

Abstract

The study of GC separation of 2-chloroethylphosphonic acid di-n-alkyl (1–5 C atoms) esters (synthesised by us) on silicone stationary phases (OV-1, OV-17, OV-225) revealed a deviation from the expected linear dependence of retention indexes (RI) versus the number of C atoms of the alkyl chain: the first member of the series presents stronger retention than one can expect. This anomalous behaviour was observed especially on polar stationary phase (OV-225, see Figure l), and was emphasised with the increase of the column temperature. In an attempt to rationalise the above mentioned facts, we tried to relate the RI values to a global polarity parameter: the dipole moment, μ. The μ values for ClC₂H₄P(O)(OR)₂ were calculated by a method described in [1] (tested by comparing the calculated μ values with experimental ones for alkyl phosphonic acid dialkylesters), using molecular mechanics (COSMIC package) in the search of the conformational space, AM1 method (MOPAC 6.0) for the μ values of the conformers, and Boltzmann distribution for the global value (see Table I). At low temperature, the μ values are not related to the Kovats indexes. Those calculated at 200°C (column temperature range) demonstrate that, indeed, only in the case of the methyl derivative, the temperature rising led to a higher μ (enhanced population of the more polar conformers: ac position for the C-Ethyl group - ac for the Cl, μ ≈ 27 D, or ±ap for one R, μ ≈ 3.8 ÷ 4.3 D). It can be concluded that dipole-dipole forces contribute to the separation process of the first members of the series.     

Theoretical investigations on structure, electrostatics potentials and vibrational frequencies of Li+ - CH3- O- (CH2- CH2- O)n- CH3 (n=3-7) conformers

Electronic structure, charge distributions and vibrational characteristics of CH₃ O(CH₂ CH₂ O) _n CH₃ (n=3-7) have been derived using the ab initio Hartree Fock and density functional calculations. For tri- to hexaglymes the lowest energy conformers have trans- conformation around the C-C and C-O bonds of the backbone. For heptaglyme (n=7 in the series), however, gauche-conformation around the C-C bonds renders more stability to the conformer and turns out to be 10.1 kJ mol ⁻¹ lower in energy relative to the conformer having trans-orientation around the C-C and C-O bonds. The molecular electrostatic potential topographical investigations reveal deeper minima for the ether oxygen in conformers having the gauche conformation around the C-C bonds over those for the trans- conformers. A change from trans- to gauche-conformation around the C-C bonds of the lowest energy conformer of heptaglyme engenders a triplet of intense bands ∼1,150 cm ⁻¹ in the vibrational spectra. Theoretical calculations predict that Li ⁺ binds strongly to the heptaglyme conformer in the above series. The frequency shifts in the vibrational spectra of CH₃O(CH₂CH₂O) _n CH₃- Li⁺ (n=3-7) conformers have been discussed     

Ground states of molecules. 53.MNDO calculations for molecules containing chlorine

MNDO calculations of heats of formation, dipole moments, ionization potentials, and structures are reported for a wide range of compounds containing chlorine in its characteristic valence state (Cl^I) and one or more of the elements H, B, Be, C, N, O, and F. The calculated errors in the heats of formation and the dipole moments are not significantly greater than those previously reported for compounds containing no chlorine. First vertical ionization potentials were on average 0.95 eV too high. The ordering of higher cationic states was found to be correct, even for species such as Cl₂O, Cl₂, and HOCl, where ab initio–Koopmans' theorem calculations predict the incorrect ordering. The calculated energies and geometries of compounds such as CIF₃ are qualitatively incorrect, probably because of the lack of 3d atomic orbitals in the orbital basis set.     

Ab initio study of the conformational equilibrium of ethylene glycol

Summary The conformational equilibrium of ethylene glycol (CH₂OHCH₂OH) has been examined by performing geometry optimizations at the 6-31G*, MP2/6-31G* and 6-31G** levels. Final energies have been calculated at the MP3 level with the optimized geometries. The two most stable conformers are atGg andgGg but it is verified that the inclusion of electronic correlations reduces their energy difference of 0.6 kcal/mol at the HF level to less than 0.2 kcal/mol. The possible coexistence of the two most stable conformers is in agreement with some previous studies of Frei et al. For thetXg conformer a detailed analysis of the intramolecular potential as a function of rotation around the C-C bond is also reported.     

An average fock operator technique of approximate open-shell LCAO-MO-SCF calculation

The exchange part of the usual Hartree-Fock potential in the unrestricted Hartree-Fock (UHF) theory is suitablyaveraged to construct an, average one-electron model Hamiltonian which generates a set of spin-restricted one-electron orbitals in a self-consistent manner. These orbitals are then used to calculate the electronic energy of the open-shell system by using the proper functional form for the energy which handles the exchange terms correctly. The eigenvalues ofF ^av can be used for calculating either the spin-polarised or spin-averaged ionisation potentials of different orbitals at theKoopmans’ theorem level of approximation. Comparison ofE ^ac with the UHF-energy shows thatE _UHF<E ^ac in each case revealing some kind of an upper bound nature ofE ^ac. An approximate variational argument is given. Relationship of our model with the hyper-Hartree-Fock theory of slater is explored and the general problem of eliminating ‘self-interaction’ terms in average Fock-operator based theories is discussed.     

Cadmium copper tetrachloride tetrahydrate

The double salt [CuCl₂(H₂O)₂{CdCl₂}]·2H₂O crystallizes in the triclinic rather than the monoclinic system as reported previously. The structure consists of sheets in the ac plane with slightly distorted octahedral CdCl₆ [Cd—Cl 2.5813 (8)–2.6943 (8) Å] connected by Cd—Cl—Cd bridges in the Cd equatorial plane along a , and by Cd—Cl—Cu bridges to layers of square‐planar CuCl₂(H₂O)₂ along c . There are long axial Cu—Cl interactions of 2.8623 (7) Å and additional water of hydration is hydrogen bonded to coordinated water and chloride ligands. The additional water connects the ac sheets into a three‐dimensional network. Both Cd and Cu occupy different sites. The Cu?Cu and Cd?Cd distances are 3.8274 (6) Å.     

Contributions to the chemistry of silicon-sulfur compounds. Part 58

The deviation from tetrahedral configuration at the silicon atom has been studied by modified EHT calculations. The geometry and energy of spiranes M(S₂C₂H₄)₂, M=C, Si were optimized with bond lengths constrained at experimental values starting from experimental or assumed geometry. Additionally Si(SR)₄, R=H, CH₃ as model compounds, have been studied with EHT. Full geometry optimizations with MM2 were also carried out for spiranes M(E₂C₂R₄)₂ and M(E₂C₂H₂)₂, M=C, Si, Ge, E = O, S, Se and R=H, CH₃. The performed MM2 calculations gave no evidence for significant deviation from the ideal spiro configuration. The EHT calculations, on the other hand, show that D₂ conformers of Si(S₂C₂H₄)₂ and Si(SR)₄ are the most stable in somewhat distorted conformation because of the operation of an anomeric effect. The anomeric interactions in S₄ conformers do not cause those spiro angle deviations. The energy differences between D₂ and S₄ conformers are small. The existence of S₄ conformers has been predicted in vapor phase by comparison of the observed and calculated splittings of lone pair ionization potentials.Presented as a poster at 10th IUPAC Conference on Physical Organic Chemistry 5–10 August 1990, Haifa, Israel. For Part 57, see ref. [1].