Conformational Flexibility of the Acetoxyphenyl Group Studied by Statistical Analysis of Crystal Structure Data

The preferred conformation of the acetoxyphenyl fragment shows a planar acctoxy (AcO) group perpendicular to the Ph ring. Steric hindrance strongly limits and affects rotation about the C? OCOCH₃ bond: if the A_cO group twists away from its perpendicular conformation, the carbonyl oxygen moves to keep the nonbonded distance to the atoms in ortho-position maximal. This is achieved by some twisting about the O? COCH₃ bond correlated with angle bending at the ipso-C-atom and ester O-atom. The maximum observed deviation away from the perpendicular conformation is ～ 47°. In the case of one or two ortho-hydrogens, deviations of the AcO group from the perpendicular conformation tend to be larger than in the case of two substituents.     

4‐Fluoro‐2‐(phospho­no­methyl)­benzene­sulfonic acid monohydrate

The title compound, C₇H₈FO₆PS·H₂O, contains both phospho­nic and sulfonic acid functionalities. An extensive network of O—H?O hydrogen bonds is present in the crystal structure. The three acidic protons are associated with the phospho­nate group. Two protons experience typical hydrogen‐bond contacts with the sulfonate‐O atoms, while the third has a longer covalent bond of 1.05 (3) Å to the phospho­nate‐O atom and a short hydrogen‐bond contact of 1.38 (3) Å to the water O atom (all O—H?O angles are in the range 162–175°). The sulfonate group is positioned so that one S—O bond is nearly coplanar with the phenyl ring [torsion angle O—S—C—C ?8.6 (2)°]. The phospho­nate group is oriented approximately perpendicular to the ring [torsion angle P—C—C—C 99.2 (2)°] with one P—O bond anti to the benzyl C—C bond. The mol­ecules pack in layers in the b–c plane with the water mol­ecules in between adjacent pairs of inverted layers.     

Ab initio studies of structural features not easily amenable to experiment: Part 6. Quantitative estimate of the effect of bond delocalization on structure and hyperconjugative interaction of the amide group

The structural changes, which occur in the amide unit when the NH₂-group is twisted out of plane by rotation about the NC bond, have been determined by comparing the completely relaxed ab initio geometries of planar and perpendicular formamide and acetamide. In the perpendicular conformation, in which the π-electron amide resonance is uncoupled, the NC bond distance is 0.080.09 Å longer than in the planar form; the CO bond distance is about 0.01 Å shorter; NH distances are about 0.01 Å longer; and HNC angles are 510° smaller, whereas the CNO angle is relatively constant. Because of the apparent invariance of CH₃-hyperconjugation effects in planar and perpendicular acetamide, it is tentatively postulated that anomeric orbital interactive effects (involving the lone pair on NH, the CO π-electron pair and antibonding π*-group-orbitals on C(α) in NHC(HR)C(O)), which should be an important factor in determining peptide chain conformation, do not vary significantly with small deviations from amide group planarity.     

Crystal structure of poly-p-xylylene. I. The α form

The molecular conformation and the crystal structure of α-form poly-p-xylylene has been determined by x-ray diffraction. The polymer has a monoclinic unit cell with a = 5.92, b = 10.64, c (fiber axis) = 6.55 Å, and β = 134.7°. Two chains pass through the unit cell, and the space groups is C2/m. The packing fraction is 0.705. One monomer unit makes up the fiber identity period and the internal rotation angles are 0° and 90° for the ? CH₂? CH₂? and ? CH₂? ?? bonds, respectively. All benzene rings are in parallel orientation, perpendicular to the ac plane.     

Ab initio studies on polymers. VII. Polyoxymethylene

The structure of an isolated, infinite polyoxymethylene chain has been investigated with the aid of the ab initio crystal orbital method applying a basis set of double-zeta quality. Restricting the primitive unit cell to a single CH₂O group, conformational potential curves as a function of the torsional angle have been evaluated. Only a single minimum closely corresponding to an all-gauche structure was detected. The all-trans conformation is a maximum on the energy curve for simultaneous rotation around C? O single bonds. Detailed geometry optimization in the vicinity of the all-gauche conformation led to the following structure: r_CO = r_OC = 1.425 Å, r_CH = 1.072 Å, ∠HCH = 111.7°, ∠OCO = 110.9°, ∠COC = 115.1°, and τ_OCOC = 70.75°. The computed torsional angle τ_OCOC lies midway between the hexagonal (78.2°) and the orthorhombic (63.5°) modification of solid polyoxymethylene.     

Gas Phase Structures of Peroxides: Experiments and Computational Problems

Gas‐phase structures of several organic and inorganic peroxides X‐O‐O‐X and X‐O‐O‐X′, which have been determined experimentally by gas electron diffraction and/or microwave spectroscopy, are discussed. The O?O bond length in these peroxides varies from 1.481(8) Å in Me₃SiOOSiMe₃ to 1.214(2) Å in FOOF and the dihedral angle ?(XO‐OX) between 0° in HC(O)O‐OH and near 180° in Bu^tO‐OBu^t. Some of the peroxides cause problems for quantum chemistry, since several computational methods fail to reproduce the experimental structures. Extreme examples are MeO‐OMe and FO‐OF. In the case of MeO‐OMe only about half of the more than 100 computational methods reported in the literature reproduce the experimentally determined double‐minimum shape of the torsional potential around the O?O bond correctly. For FO‐OF only a small number of close to 200 computational methods reproduce the O?O and O?F bond lengths better than ±0.02 Å.     

Ab initioSTO-3G optimization of planar,perpendicular, and twisted molecular structures of biphenyl

The complete molecular structure of biphenyl, characterized by 12 independent parameters, has been derived by ab initio gradient techniques using a STO -3G basis set for coplanar, perpendicular, and minimum energy conformations with the constraint of planar phenyl ring units and a C₂ symmetry axis along the CC interring bond. The minimum torsional angle obtained was ? = 38.63° with torsional energy barriers of 8.59 and 10.04 kJ/mol for ? = 0° and ? = 90°, respectively.     

2‐(4‐Acetyl‐3,5‐di­hydroxy‐2‐methyl­phenoxy)‐4,6‐di­methoxy‐3‐methyl­benzoic acid

In the title compound, C₁₉H₂₀O₈, the benzene rings are nearly perpendicular to each other [dihedral angle 80.2 (2)°]. The carboxy group is twisted out while both the methoxy and acetyl groups are almost coplanar with their attached benzene rings. The hydroxy group is involved in an intramolecular O—H?O hydrogen bond with the acetyl O atom and the compound is connected through an intermolecular O—H?O contact to form a dimer. The crystal structure is stabilized by intermolecular O—H?O hydrogen bonds.     

Synthese von Nitrenkomplexen mit N-Trimethylsilylanilin. II. Charakterisierung und Kristallstruktur der Rhenium(V)-phenylnitren-Komplexe mer-[Re(NPh)Cl3(NH2Ph)(Ph3P)] und trans-[Re(NPh)(OMe)Cl2(Ph3P)2]

Synthesis of Phenylnitrene Complexes with N-Trimethylsilylaniline. II. Characterization and Crystal Structure of the Rhenium(V) Complexes mer-[Re(NPh)Cl₃(NH₂Ph)(Ph₃P)] and trans-[Re(NPh)(OMe)Cl₂(Ph₃P)₂] Reaction of [ReOCl₃(Ph₃P)₂] with N-trimethylsilylaniline yields mer-[Re(NPh)Cl₃(Ph₃P)₂], which reacts under air with excess of N-trimethylsilylaniline to form [Re(NPh)Cl₃ · (NH₂Ph)(Ph₃P)]. Crystallization from CH₂Cl₂/MeOH affords [Re(NPh)(OMe)Cl₂(Ph₃P)₂] as an additional product. [Re(NPh)Cl₃(NH₂Ph)(Ph₃P)] crystallizes in the monoclinic space group P2₁/n with a = 1 192.3(3); b = 1 918.9(3); c = 1 266.3(3) pm; β = 101.71(1)°; Z = 4. The rhenium atom has a distorted octahedral environment with the Cl atoms in meridional positions. The phenyl nitrene ligand is coordinated with an almost linear arrangement Re? N1? C40 = 166.8(6)° and with a bond distance Re?N = 170.5(6) pm. [Re(NPh)(OMe)Cl₂(Ph₃P)₂] · 1/2CH₂Cl₂ crystallizes in the triclinic space group P1 : a = 1 103.1(4); b = 1 227.9(4); c = 1 711.3(5) pm; α = 70.48(3)°; β = 72.71(3)°; γ = 80.03(3)°; Z = 2. The rhenium atom exhibits a distorted octahedral coordination with the Cl atoms and the phosphine ligands in trans positions. As a consequence of the competition of the nitrene ligand and the trans-coordinated methoxy group the Re?;N bond length is slightly lengthened to 173.2(7) pm, while the Re? O bond length of 193.4(6) pm is short. The bond angles Re? N? C70 and Re? O? C80 are 173.3(7)° and 139.1(7)°, respectively.     

2,2′‐Anhydro‐1‐(3′,5′‐di‐O‐acetyl‐β‐D‐arabinofuranosyl)uracil,a cyclouridine nucleoside with a C4′‐endo furanosyl conformation

2,2′‐Anhydro‐1‐(3′,5′‐di‐O‐acetyl‐β‐D‐arabinofuranosyl)uracil, C₁₃H₁₄N₂O₇, was obtained by refluxing 2′,3′‐O‐(methoxymethylene)uridine in acetic anhydride. The structure exhibits a nearly perfect C4′‐endo (⁴E) conformation. The best four‐atom plane of the five‐membered furanose ring is O—C—C—C, involving the C atoms of the fused five‐membered oxazolidine ring, and the torsion angle is only −0.4 (2)°. The oxazolidine ring is essentially coplanar with the six‐membered uracil ring [r.m.s. deviation = 0.012 (5) Å and dihedral angle = −3.2 (3)°]. The conformation at the exocyclic C—C bond is gauche–trans which is stabilized by various C—H...π and C—O...π interactions.     

<!?tlsb=‐0.06pt>Bromido(η5‐carboxycyclopentadienyl)dinitrosylchromium(0) and (η5‐benzoylcyclopentadienyl)bromidodinitrosylchromium(0)

In the structures of each of the title compounds, [CrBr(C₆H₅O₂)(NO)₂], (I), and [CrBr(C₁₂H₉O)(NO)₂], (II), one of the nitrosyl groups is located at a site away from the exocyclic carbonyl C atom of the cyclopentadienyl (Cp) ring, with twist angles of 174.5 (3) and 172.5 (1)°. The observed orientation is surprising, since the NO group is expected to be situated trans to an electron‐rich C atom in the ring. The organic carbonyl plane is turned away from the Cp ring plane by 5.6 (8) and 15.2 (3)°in (I) and (II), respectively. The exocyclic C—C bond in (I) is bent out of the Cp ring plane towards the Cr atom by 2.8 (3)°, but is coplanar with the Cp ring in (II); the angle is 0.1 (1)°.     

Calculations on the diphenylmethyl anion

Semiempirical and ab initio calculations for the diphenylmethyl anion and related species are reported. MINDO/3 calculations indicate a coplanar anion with an enlarged bond angle of ～139° to counteract the steric repulsions. MNDO calculations reveal an expanded bond angle with somewhat greater twist (21°). The ab initio calculations (STO -3G level) reveal an expanded central bond angle with an intermediate degree of phenyl twist. The results are compared with experimental data for this and related anions.     

p‐Cresyl 2,3‐an­hydro‐5‐deoxy‐5‐phthal­imido‐1‐thio‐α‐d‐lyxo­furan­oside

The crystal structure of the title compound, C₂₀H₁₇NO₄S, (I), was determined in order to compare the solution and solid‐state conformations. The mol­ecule was synthesized as a building block for incorporation into oligosaccharides comprised of conformationally restricted furan­ose residues. The furan­ose ring adopts an envelope conformation with the ring O atom displaced above the plane (an ^OE conformation). The pseudorotational phase angle (P) is 88.6° and the puckering amplitude (τ_m) is 31.5°. The C₂—C₁—S—C(Ph) torsion angle is ?163.2 (2)°, which places the aglycone in the exo‐anomeric effect preferred position. The C1—S—C14 bond angle is 99.02 (13)° and the plane of the cresyl moiety is oriented nearly parallel to the four in‐plane atoms of the furan­ose ring envelope. The orientation about the C4—C5 bond is gauche–gauche [Bock & Duus (1994). J. Carbohydr. Chem. 13 , 513–543].     

Electron diffraction study of the molecular structure ofo-chloroanisole using a dynamic nonparametrized model

The geometrical parameters of the o-chloroanisole molecule were determined by gas phase electron diffraction in terms of the dynamic model using vibrational spectroscopy data and quantum chemical calculations. A new approach based on Tikhonov's regularization method is used to explicitly define the internal rotation potential of the methoxy group. It was found that the nonparametric internal rotation potential has two minima, one of which corresponds to the planar (?=0°) and another to orthogonal (?=90°) orientation of the O?CH₃ bond relative to the plane of the benzene ring. The difference between the energies of the orthogonal and planar conformers is 0.9–1.0 kcal/mole, and the height of rotation barriers at ??65° is 1.4–1.6 kcal/mole, which confirms the results of quantum chemical calculations, indicating that the orthogonal conformer is present in substantial amounts (～30%). The following basic geometrical parameters were found (r_a in Å, ∠_α in deg, the error equals 3σ): r(C?C)_ave=1.398(4); r(O?C_Ph)=1.358(36); r(O?C_Me)=1.426(21);r(C?Cl)=1.733(4);r(C?H)_Ph=1.086(6);r(C?H)_Me=1.095(6); ∠CC_OC_Cl=118.7(2.2); ∠C_OCC=119.9(2.5); ∠C_OC_ClC=121.5(1.1); ∠COC=117.6(2.6); ∠C_OCCl=119.1(2.1); ∠CCO=124.7(1.2). The results are compared with the data for related compounds. Stereochemical features of o-anisoles that are responsible for the orthogonal conformer are discussed.     

Ab initio studies of structural features not easily amenable to experiment. 25. Conformational analysis of methyl propanoate and comparison with the methyl ester of glycine

The molecular geometries of three conformations of methyl propanoate (MEP) (C? C? C?O torsions of 0°, 120°, and 180°) and the potential-energy surfaces of MEP (C? C? C?O torsions) and of the methyl ester of glycine (MEG) (N? C? C?O torsions) have been determined by ab initio gradient calculations at the 4-21G level. MEP has conformational energy minima at 0° and 120° of the C? C? C?O torsion, while the 60–90° range and 180° are energy maxima. For MEG there are two minima (at 0° and 180°) and one barrier to N? C? C?O rotation in the 60–90° range. The N? C? C?O barrier height is about twice as high (4 kcal/mol) as the C? C? C?O barrier. The 180° N? C? C?O minimum is characteristically wide and flat allowing for considerable flexibility of the N? C? C?O torsion in the 150–210° range. This flexibility could be of potential importance for polypeptide systems, since the N? C? C?O angles of helical forms are usually found in this region. The molecular structures of the methyl ester group CH₃OC(?O)CHRR′ in several systems are compared and found to be rather constant when R ? H and R′ ? H, CH₃, CH₃CH₂; or when R ? NH₂ and R′ ? H, CH₃, or CH(CH₃)₂.     

2,4‐Bis(α,α‐di­methyl­benzyl)­phenol

The structure of the title compound, 2,4‐bis(1‐methyl‐1‐phenylethyl)phenol, C₂₄H₂₆O, was found to have a torsion angle of 129.95 (13)° for the C—C bond that connects the benzyl carbon to the phenol ring ortho to the OH group. A value of ～50° was expected from molecular mechanics calculations. Intermolecular interactions, in particular O—H?O and edge–face π bonding, may contribute to this discrepancy. Intramolecular O—H?π bonding is also observed.     

The crystal and molecular structure of dioxo bis (2,2,6,6 - tetramethylheptane -3,5 dionato)methanol uranium (VI)

UO₂(thd)₂ CH₃OH (thd = tetramethylheptane-3,5-dione) is monoclinic, with a = 10.602(11), b = 22.883(20), c = 12.054(11) Å and β = 105.90(3)°, Z = 4 and space group P2₁/c. The structure, which is molecular, was solved by conventional Patterson and Fourier techniques with 3173 independent (hkl) reflexions collected with MoKα radiation (λ = 0.7107 Å), and refined to R = σ(|Fo|-|Fc|)/σ|Fo| = 0.093. The uranium coordination polyhedron is a pentagonal bipyramid, with UO (carbonyl) distances between 2.25 and 2.37 Å and a longer UO (methanol) distance of 2.50 Å. The uranyl group is linear (uranyl angle 179.3(8)°). The pentagon oxygen atoms and uranium do not form a planar system, as there are deviations of up to 0.17 Å from the mean plane. If the methanol oxygen atom O(7) is excluded from the plane calculation, the remaining atoms are more nearly planar. The four carbonyl oxygens are coplanar, with uranium 0.08 Å from their plane. The methanol oxygen is 0.28(4) Å from this second plane.The two (thd) molecules, excluding methyl carbons, are planar and are inclined at 43.6° to each other in a boat form and at 29.1 and 14.5° to the pentagonal plane. The methanol CO bond is inclined at 133° to the UO bond, confirming the ligand is the neutral CH₃OH molecule, and not CHO^?₃.     

5‐Amino‐4‐(4‐diethyl­amino­phenyl)‐2‐(4‐hydroxy­phenyl)‐7‐(pyrrolidin‐1‐yl)‐1,6‐naphthyridine‐8‐carbo­nitrile

In the title compound, C₂₉H₃₀N₆O, the naphthyridine moiety is planar with a dihedral angle between the fused rings of 1.9 (1)°. The phenol ring is nearly coplanar, while the diethyl­amino­phenyl substituent is orthogonal to the central naphthyridine ring and the pyrrolidine ring makes an angle of 11.2 (1)° with it. The O atom of the hydroxy substituent is coplanar with the phenyl ring to which it is attached. The molecular structure is stabilized by a C—H?N‐type intramolecular hydrogen bond and the packing is stabilized by intermolecular C—H?π, O—H?N and N—H?O hydrogen bonds.     

Polysulfonylamine. LXXII [1]. Triphenylcarbenium- und Triphenylphosphonium-di(fluorsulfonyl)amid: Zwei Kristallstrukturen mit geordneten (FSO2)2N⊖-Lagen

Polysulfonyl Amines. LXXII. Triphenylcarbenium and Triphenylphosphonium Di(fluorosulfonyl)amides: Two Crystal Structures with Ordered (FSO₂)₂N^? Sites Treatment of HN(SO₂F)₂ in CH₂Cl₂ with Ph₃P, Ph₃PO or collidine (=B) affords the compounds Ph₃PH^⊕[(FSO₂)₂N]^? ( 3 ), Ph₃PO · HN(SO₂F)₂, and BH^⊕[(FSO₂)₂N]^? ( 7 ). The carbenium salt Ph₃C^⊕[(FSO₂)₂N]^? ( 5 ), obtained by metathesis of Ph₃CBr with [(C₆H₆)AgN(SO₂F)₂] in CH₂Cl₂, crystallizes from chloroform/petroleum ether as a monosolvate Ph₃C^⊕[(FSO₂)₂N]^? · CHCl₃ ( 6 ). In presence of a sterically hindered base, viz. collidine, 5 is a suitable reagent for the tritylation of molecules containing weakly activated H atoms (e. g.: MeCN → Ph₃CCH₂CN, acetone → tritylacetone; co-product: 7 ). The crystal structures of the ionic solids 3 (monoclinic, space group P2₁/n) and 6 (monoclinic, P2₁/c) were determined by X-ray diffraction at ?130°C; the structure refinements were not impaired by the notorious tendency of the (FSO₂)₂N moiety towards crystallographic disorder. As in the known structure of the tetraphenylarsonium salt, the anion of 3 and 6 adopts a staggered conformation of approximately C₂ symmetry (averages of all values: S? N? S 121.4°, N? S 156.2, S? O 141.6, S? F 156.6 pm). The crystal packing of 6 displays a three-centre C? H(…?O)₂ hydrogen bond between the CHCl₃ molecule and two oxygen atoms of a single anion, resulting in a six-membered ring [R₁²(6) pattern; H …? O 234 and 262 pm]. The crystal of 3 contains one-dimensional arrays of alternating cations and anions connected by a three-centre P? H(…?O)₂ bond [C(6) pattern; H …? O 237 and 254 pm]. The Ph₃C^⊕ cation of 6 is propeller-shaped, with three coplanar central bonds (mean C? C 144.5 pm) and interplanar angles of 52.7, 56.4 and 60.1° between the phenyl groups.     

X-ray scattering from the long spacing of completely crystalline oligoethylene glycol di-n-alkyl ethers

Completely crystalline samples of oligoethylene glycol di-n-alkyl ethers, H(CH₂)_nO-(CH₂CH₂O)_m(CH₂)_nH, where m is 9 or 15 and n is in the range 12-18, were orientated in capillaries by slow crystallization in a temperature gradient. X-ray scattering from the long spacings of the oriented samples, rotated continuously through 360°, was concentrated on the equator of a Debye–Scherrer photograph and many orders of reflection could be seen (e.g., up to order 30). The intensities of these reflections were analyzed by use of a model electron density distribution through the layer structure. Thus it was shown that the central oxyethylene block, in helical conformation, is oriented normal to the layer end plane, while the end alkyl bolcks, in planar zigzag conformation, are tilted relative to the layer end plane at an angle in the range 70°–64°.