Structure and conformation of 1-Bromopropane: A gas-phase electron-diffraction investigation using microwave-spectroscopy data and results from ab initio calculations as constraints

1-Bromopropane has been studied by gas-phase electron diffraction (ED) at 24C. Earlier published values for rotational constants from microwave spectroscopy (MW), together with results from ab initio molecular-orbital calculations, have been included in the ED analysis. Two conformers with C-C-C-Br torsion angles of 180 (anti) or 66.0(17) (gauche) have been observed. The results obtained for the bond distances (r _g) and valence angles () from this combined ED/MW analysis, with the ab initio results used as constraints are r (C-H)=1.114(9) å,r(C₁-C₂)=1.521(5) å,r(C₂-C₃)=1.535(5) å,r (C₁-Br)=1.962(6) å, <(C-C-C)anti,=110.0(11), <(C-C-C)gauche=113.3(11), < (C-C-Br)anti=111.1(6), < (C-C-Br)gauche=112.1(6), <C₂-C₁-H=112.1 (ab initio value), <C₂-C₃-H=111.4 (ab initio value), <H-C₂-H=107.0 (ab initio value). Error limits are given as 2, where (standard deviation) includes estimates of uncertainties in voltage/height measurements and correlation in the experimental data. The observed amountof gauche conformer was 64(14)%. Using the entropy difference between conformers obtained in the ab initio calculations, this composition corresponds to an energy difference of E=E _anti —E _gauche=0.03(36) kcal/mol. The results are compared with those earlier obtained for other 1-halopropanes.     

Molecular structure of aniline in the gaseous phase: A concerted study by electron diffraction and ab initio molecular orbital calculations

The molecular structure of free aniline has been investigated by gas-phase electron diffraction and ab initio MO calculations at the HF and MP2 levels of theory, using the 6-31G^*(6D) basis set. Least-squares refinement of a model withC _s symmetry, with constraints from MP2 calculations, has led to an accurate determination of the C-C-C angle at theipso position of the benzene ring, =119.0±0.2 (where the uncertainty represents total error). This parameter provides information on the extent of the interaction between the nitrogen lone pair and the system of the benzene ring, and could not be determined accurately by microwave spectroscopy. The angles at theortho, meta, andpara positions of the ring are 120.3±0.1, 120.7±0.1, and 119.0±0.3, respectively. Important bond distances are r _g(C-C)=1.398±0.003 å andr _g(C-N) =1.407±0.003 å. The effective dihedral angle between the H-N-H plane and the ring plane, averaged over the large-amplitude inversion motion of the amino group, is ¦¦=44±4. The equilibrium dihedral angle is calculated to be 41.8 at the HF level and 43.6 at the MP2 level, in agreement with far-infrared spectroscopic information. The MO calculations predict that the differencer(C_ortho-C_meta) -r(C_ipso-C_ortho) is 0.008–0.009 å. They also indicate that the nitrogen atom is displaced from the ring plane, on the side opposite to the amino hydrogens. The displacement is 0.049 å at the HF level and 0.072 å at the MP2 level. The two calculations, however, yield very different patterns for the minute deviations from planarity of the ring carbons.     

Modeling chemoadaptive behavior: X-ray structure of the trimethylamine oxide-urea (1∶4) complex

The structure of a trimethylamine oxide (TMAO)-urea 14 complex was investigated by x-ray crystallographic methods. C₃H₉NO, 4CH₄N₂O, M_r=315.33, monoclinic P2₁/n, a=11.004(2), b=9.8259, c=15.419(2),=106.88(4), V=1595.4(2) å₃, Z=4, D_x=1.313(1) g cm^–3, monochromatized Cu (=1.5418 å),=8.94 cm^–1, F(OOO)=680, temperature = 110K, final R=0.067, for 1786 unique observed reflections. Based on these studies and comparison with related compounds a model of the interaction of urea and TMAO with proteins, especially enzymes, is proposed. This is important in the physiology of many organisms, especially elasmobranchs. The counteracting effect of TMAO on urea perturbation of protein structure and function is interpreted in terms of water structure as a balance between the structure breaking effect of urea and the structure-stabilizing effect of TMAO.     

Structure and Energetics of β-Diketonates. XII. Structure of Lanthanide tris-Dipivaloylmethanates for Er(thd)3 Used as an Example

A simultaneous electron diffraction and mass spectroscopic study of saturated vapors of erbium tris-dipivalylmethanate has revealed that at 136(5)°C, the vapor consists solely of Er(thd)₃ molecules. Electron diffraction data may be described by two alternative models (of C_3 and D_3 symmetry), for which r_a, r_g, and r structural parameters have been determined. D₃ symmetry is recognized to be preferable for free Er(thd)₃ molecules. The main structural parameters of the model are r (Er – O) 2.218(5), r (O – C) 1.279(5), r (C – C_r) 1.404(6), r (C – C_t) 1.512(3), r(C_t – C_m) 1.542(5), r (rm C_m– H) 1.804(4) , The ErO 75.0(0.4)°. The ErO₆ coordination polyhedron has a structure close to an antiprism. A rotational angle of the O–O–O trigonal face relative to the position in a regular prism is 20.7(0.8)°. Possible reasons for the differences in the structure of Er(thd)₃ molecules in the gas phase and crystal are discussed.     

Triterpene glycosides of Thalictrum squarrosum. IV. Structures of squarrosides A1, A2, B1, and B2

Four new cycloartane glycosides have been isolated from a methanolic extract ofThalictrum squarrosum Stephan ex Willd.: squarroside A1 (I) — (21R, 22S, 23R)-3-(-D-glucopyranosyloxy)-21-methoxy-21,23-epoxycycloart-24-ene-22,30-diol, C₃₀H₆₀O₁₀; squarroside A2 (II) — the (21S)-epimer of compound (I); squarroside B1 (III) (21R, 22S, 23R)-3gb-[O--L-rhamnopyranosyl-(1 6)--D-glucopyranosyloxy]-21-methoxy-21,23-epoxycycloart-24-ene-22,30-diol, C₄₃H₇₀O₁₄; and squarroside B2 (IV) — the (21S)-epimer of compound (III). The proposed structures were determined on the basis of¹H and¹³C NMR spectroscopy, FAB mass spectrometry, and chemical transformations.Irkutsk Institute of Organic Chemistry, Siberian Branch, USSR Academy of Sciences. Translated from Khimiya Prirodnykh Soedinenii, No. 4, pp. 516–523, July–August, 1989.     

The synthesis and structure of the [2.2]paracyclophane cluster Ru6C(CO)13(μ 3-η 2:η 2:η 2-C16H16)(PPh3) and a study of the 1H-n.m.r. spectra of [2.2]paracyclophane and benzene clusters

Summary The [2.2]paracyclophane cluster, Ru₆C(CO)₁₄( ₃- ^{2 2 2}-C₁₆H₁₆) (1), undergoes reaction with Me₃NO and triphenylphosphine to yield Ru₆C(CO)₁₃( ₃- ^{2 2 2}-C₁₆H₁₆)(PPh₃) (2), which may also be produced from (1) by thermolysis with PPh₃ in THF. Compound (2) has been fully characterized in solution by spectroscopy and in the solid state by a single crystal X-ray diffraction analysis at 277 K, and its structure is compared with that of the parent cluster, (1). Using the same synthetic procedures, the tricyclohexylphosphine analogue, Ru₆C(CO)₁₃( ₃- ^{2 2 2}-C₁₆H₁₆)(PCy₃) (3), has also been prepared and characterized spectroscopically. A comparison of the chemical shifts of the 577-01 protons in the ¹H-n.m.r. spectra of compounds (1)–(3) together with a variety of other [2.2]paracyclophane and benzene clusters has been made.     

On the asymptotic behavior of Hartree-Fock orbitals

Summary Handy et al. have shown that the asymptotic behavior of Hartree-Fock orbitals is controlled by the energy of highest occupied molecular orbital, except for the atomic case in which onlys-orbitals are occupied. However, their proof is not complete at one point. This point is clarified, and a more unified derivation is given. Further, we discuss the preexponential factorr of the leading asymptotic termr exp[–r], where =(–2_HOMO)^1/2 and _HOMO is the orbital energy of HOMO. New results are obtained for linear molecules, and the results of several authors for atoms and non-linear molecules are reproduced.     

Synthesis,crystal structure and reaction of differentially ring-substituted titanocene dichlorides

Summary A new class of differentially ring-substituted titanocene dichlorides of the type CpCpTiCl₂ has been prepared by reacting CpM (M=Li, K; Cp=RC₅H₄) with CpTiCl₃ (Cp=MeOCH₂CH₂C₅H₄). The crystal structure of Cp(PhCEt₂C₅H₄)TiCl₂, the first example, has been determined by x-ray diffraction and refined to a final R factor of 0.0532 for 2304 reflections. The crystal belongs to the monoclinic space group C _2h ⁵ –P2₁/c with unit cell parameters:a 10.205(6),b 13.003(5),c 17.591(9) Å, 101.91(4)°, V=2284 ų, Z=4 and Dc=1.32 g/cm^–3. Replacement of one neomenthyl in bis(neomenthyl-cyclopentadienyl)titanium dichloride by one MeOCH₂CH₂ — raises the e.e. value from 5.8% to 11.2% in the catalytic asymmetric hydrogenation of 2-phenyl-1-butene.     

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.     

The molecular complex of dibenz[a,c]anthracene and trinitrobenzene

The crystal structure of the complex between the polycyclic aromatic hydrocarbon di-benz[a,c]anthracene and 1,3,5-trinitrobenzene is reported. The crystals are triclinic, space group P¯1 with unit cell dimensionsa=7.277(2) å,b=11.237(6) å, andc=13.902(5) å,= 104.13(4),=96.04(3), and =95.15(2). Diffraction data were measured at 121(2) K. The structure was determined and refined toR ₁=0.046. The structure consists of layers containing both dibenz[a,c]anthracene and 1,3,5-trinitrobenzene molecules, interconnected within a layer by C-H O interactions. Layers stack on one another so that dibenz[a,c]anthracene molecules are sandwiched between 1,3,5-trinitrobenzene molecules and vice versa. The average distance between molecules in these sandwiches is 3.23 å.     

19F,31P,1H NMR and X-ray crystallographic studies of (pentafluorophenyl)3−n (4-pyrazol-1-yl-2,3,5,6-tetrafluorophenyl) n=1,2,3 Phosphines

New phosphines1–3 have been synthetized by reaction of pyrazolate anion with tris(pentafluorophenyl)phosphine and characterized by¹H,³¹P, and¹⁹F NMR studies.¹⁹F NMR spectral data contribute to the evidence for apara-substitution of tetrafluorophenyl rings. The crystal structure of tris(4-pyrazol-1-yl-2,3,5,6-tetrafluorophenyl)phosphine 1 has been determined, proving that the assignment based on spectroscopic data was correct: C₂₇H₉F₁₂N₆P,M _r = 676.37, monoclinic, space group P2_l/c,a=10.754(2) å,b=10.316(2) å,c = 23.598(5) å,=95.36(3),V=2607(1), å³,Z=4,R ₁=0.042, andwR ₂=0.122.     

Reactivity and flexibility in platinum metal clusters

The synthesis, structural properties, and fluxional behaviour of platinum-triosmium and platinum-triruthenium clusters derived from Os₃Pt(-H)₂ (CO)₁₀(PR₃) and Ru₃Pt(-H)(-CC ^t Bu)(CO)₉ (dppe) and related species are described.     

High resolution infrared spectrum and molecular structure of dichloroacetylene

The infrared spectrum of dichloroacetylene, prepared by the pyrolysis of dichloromaleic anhydride at 1000°C, has been recorded at both low and high resolution. In the low resolution spectrum a number of combination bands not previously observed have been assigned, the infrared active fundamentals have been reassigned, and a center wavenumber value has been determined for the Raman activev ₁ fundamental. The high resolution spectra of a number of fundamental bands, summation bands, and one difference band for the isotopomers,³⁵CICC³⁵CI and³⁵CICC³⁷CI, have been assigned, while a more limited number of bands has been assigned for the species³⁷CICC³⁷CI and³⁵CI¹³CC³⁵CI. The resultant rotational and vibration-rotation constants have been used to obtainr _o,r _e,r _s, and partial r_s structural parameters. The most reliable bond lengths are obtained from the partialr _s treatment and are 164.105(53) pm for the C-CI bond and 119.203(79) pm for the CC bond.     

Triterpene glycosides ofScheffleropsis angkae. II. Structure of glycosides L-E1, L-E2, L-K1, and L-K2

The structures of four triterpene glycosides from leaves ofScheffleropsis angkae (Araliaceae) are established using chemical and NMR methods. The structures 3-O--D-glucopyranosyl-(1-3)-O--L-arabinopyranosides of oleanic and ursolic acids and their 28-O--L-rhamnopyranosyl-(1-4)-O--gentiobiosyl ethers are proposed for L-E₁, L-E₂, L-K₁, and L-K₂, respectively. L-K₁ and L-K₂ are new triterpene glycosides.Translated from Khimiya Prirodnykh Soedinenii, No. 3, pp. 239–241, May–June, 2000.     

Synthesis of dicationic triple-decker complexes with a central B-cyclohexyl-substituted borole ligand

Stacking reactions of the dicationic fragments [LM]²⁺ (LM = (-C₆H₆)Ru, (-C₆H₃Me₃)Ru, or (-C₅Me₅)Rh) with the complex (-C₅H₅)Co(-C₄H₄BCy) (Cy = cyclo-C₆H₁₁) afforded new dicationic 30-electron triple-decker complexes [(-C₅H₅)Co(-:-C₄H₄BCy)ML](BF₄)₂ containing a cyclohexyl-substituted borole ligand in the central position.     

Molecular structure of 1,1-dimethylsilacyclopentene-3,4-oxide from gas-phase electron diffraction

The molecular structure of 1,1-dimethylsilacyclopentene-3,4-oxide has been determined by electron diffraction in the gas phase. The experimental data are consistent withC _s molecular symmetry and boat conformation with a flattened end at the silicon atom. The flap angles characterizing the orientation of C-Si-C and C-O-C planes with respect to the four coplanar carbon atoms of the ring are 16.6 ± 0.6 and 73.3 ± 0.6, respectively. Bond lengths (r_g) are Si-C₆, 1.866 ±0.008; Si-C₂, 1.899 ± 0.008; C₂-C₃, 1.513 ± 0.005; C₃-C₄ (bridge), 1.477 ± 0.013; C-O, 1.443 ± 0.007; (C-H)_mean 1.116 ± 0.003 å. Bond angles are <C₅-Si-C₂, 96.2 ± 0.4; <Si-C₂-C₃, 103.9 ± 0.3; <C₂-C₃-C₄, 116.5 ± 0.3; <C₃-C₄-O, 59.2 ± 0.5; zC₄-C₃-H₉, 109.0 ± 3.5; <C₂-C₃-H₉, 132.9 ± 3.1; <C₆-Si-C₁₂, 114.6 ± 0.8; <Si-C₆-H₁₅, 109.7 ± 0.9.     

C-C bond formation at polynuclear metal centers

Studies on C-C bond formation between simple hydrocarbon species such as CH₂, C=CH₂, CH=CH₂, CH₂=CH₂, CH₂=C=CH₂ and CHCH at a diruthenium center suggest that the process is promoted when the dimetal center can readily compensate for the two electrons lost in the formation of the new C-C bond. Thus, whereas -CH₂ and ethene combine only under forcing conditions, the combination of -CH₂ with allene or ethyne, which have additional -electrons available for coordination, occurs readily at room temperature. Likewise, the availability of uncoordinated -electrons in -C=CH₂ allows vinylidene to link rapidly with ethene at room temperature. Alkyne complexes [Ru₂(CO)(-RCCR)(-C₅H₅)₂] (R=CF₃ or Ph) react only under vigorous conditions with additional alkyne to give [Ru₂(CO)(-C₄R₄) (-C₅H₅)₂], but give these same species at room temperature in the presence of acid, shown to be due to the intermediacy of highly reactive 30-electron -vinyl cations. Thermally, alkyne linking proceedsvia three-alkyne species [Ru₂(-C₆R₆)(-C₅H₅)₂] to a four-alkyne complex [Ru₂(-C₈R₈)(-C₅H₅)₂], containing an unprecedented C₈ ligand composed of a C₆ ring with a C₂ tail. Treatment of [Ru₂(CO)(-RCCR)(-C₅H₅)₂] with unsaturated metal fragments gives trimetal complexes such as [Ru₃(CO)₅(₃-CF₃CCCF₃) (-C₅H₅)₂]. The MeCN derivative of this species undergoes unusual linking processes on reaction with additional alkyne to giveinter alia [Ru₃(CO)₃(₃-CCF₃){₃-C₃(CF₃)₃}(-C₅H₅)₂], arising from alkyne cleavage, and [Ru₃(CO)₃{₃-C₄(CF₃)₂(CO₂Me)₂}(-C₅H₅)₂], a closo-pentagonal bipyramidal Ru₃C₄ cluster.     

Transformations of formaldehyde and glycolaldehyde during the hydroformylation of formaldehyde in the presence of rhodium catalysts

Hydroformylation of formaldehyde to give glycolaldehyde (GA) in the presence of RhCl(PPh₃)₃, RhCl(CO)(PPh₃)₂, or the RhCl₃ + PPh₃ system inN,N-dimethylacetamide was studied. The hydroformylation is accompanied by the Cannizzaro-Tishchenko reaction, condensation of CH₂O with GA to give C₃-C₁₆ polyoxyaldehydes (POA), and dimerization of GA. The formation of POA, which probably occurs through coordination of GA with a Rh atom, predominates among the side reactions. The optimum conditions for hydroformylation of CH₂O were found to be: RhCl₃ + PPh₃ as the catalyst,T 383 K, 12MPa, [H₂O] 1.8 mol L^–1, [Rh] 2.5 · 10^–3 g-at. L^–1, and [CH₂O] 0.03 g L^–1. At a substrate conversion of 62–67 %, the selectivity of GA formation reaches 96 %, and the yield is 60–65 %.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 1, pp. 75–78, January, 1995.     

Synthesis of Co3(CO)7(μ-η3-Allyl)(μ3-X) Complexes (X=S, Se) and Structure of the Selenium Derivative

The complexes Co₃(CO)₉( ₃-X) (X=S, Se) can be reduced to the corresponding anionic species [Co₃(CO)₉( ₃-X)]^–, which react with allyl bromide to give Co₃(CO)₇(- ³-C₃H₅)( ₃-X) (X=S, Se). These are the first two cobalt complexes containing the bridging - ³-allyl ligand. The structure of the selenium complex was determined by X-ray crystallography. Crystal data for Co₃(CO)₇(- ³-C₃H₅)( ₃-Se) are as follows: space group P2₁/c, a=9.051(2) Å, b=8.102(2) Å, c=21.27(4) Å, =93.82(3)°, Z=4, and R=0.0565 for 2491 observed reflections.     

Structure and Energetics of β–Diketonates. XI. Molecular Structure of Sc(thd)3 from Gas–Phase Electron Diffraction Data

Electron–diffraction and mass–spectrometric studies of saturated vapor of scandium tris–dipivaloyl–methanate showed that at 135(5)°, the vapor contains only monomeric Sc(thd)₃, whose structural parameters r _a , r _g , and r were determined. The internuclear distances in the chelate ring were found to be rSc=O) = 2.066(5), r(O=C) = 1.272(3), and r(C=C_r) = 1.385(3). The ScO₆ coordination polyhedron has a D ₃ symmetry configuration close to a regular antiprism. The angle of rotation of the O=O=O trigonal faces relative to their position in a regular prism is 25.7(1.5)°.