Crystal structure of an optically active non-symmetric liquid crystal dimer: cholesteryl 5-[4-(4-n-heptylphenylethynyl)phenoxy]pentanoate

The crystal structure of cholesteryl 5-[4-(4-n-heptylphenylethynyl)phenoxy]- pentanoate (C₅₃H₇₆O₃) was determined by direct methods using single crystal X-ray diffraction data. It crystallises in the triclinic system with space group P1 and Z?=?2. The unit cell parameters are: a?=?10.6791(5), b?=?13.0903(7), c?=?18.6430(9)Å, α?=?94.413(3), β?=?98.222(3), γ?=?112.987(3)°. The final reliability factor was R?=?0.0510 for 7284 observed reflections and the goodness of fit was equal to 1.062. The asymmetric unit cell of the compound was found to contain two symmetry-independent molecules, A and B. In both molecules, the six-membered rings of the cholesterol moiety are conformationally very similar. However, pronounced differences were observed in the conformation of the five-membered ring, which is a half-chair in molecule A and assumes an envelope conformation in molecule B. In both molecules, the phenyl rings are planar. The dihedral angle between the two phenyl rings is 43.4(2) and 42.7(2)° for molecules A and B, respectively. The packing of molecules in the crystalline state was found to be a precursor to the smectic A phase structure. The crystal structure is stabilised by inter-molecular C–H…O and C?H…π interactions.     

Synthesis, 13C NMR spectrum and crystal structure of 10-Phenylpyrido[3,2-b][1,4]benzothiazine 5-Oxide

The compound 10-phenylpyrido[3,2-b][1,4]benzothiazine 5-oxide, 1 , has been obtained in nearly quantitative yield oxidation of 10-phenylpyrido[3,2-b][1,4]benzothiazine with oxygen in dioxane solution. The ¹³C nmr chemical shift assignments of 1 are reported. Its structure has been determined by X-ray single crystal methods. The crystals of 1 are monoclinic, space group P2₁/n. There are four molecules in a unit-cell of dimensions a = 12.347(3), b = 12.947(3), c = 8.987(1)Å, β = 106.73(1)° and V = 1375.8(5) ų. The central ring is in a boat conformation and the sulfoxide oxygen atom occupies the axial position. The folding angle between the planes of the pyrido and the benzo planes is 161.55(9)°.     

Transition Metal Halide Salts of 2-Amino-5-Substituted-Pyridines: Synthesis,Crystal Structure and Magnetic Properties of Two Polymorphs of (5-IAP)2CuCl4[5-IAP = 2-Amino-5-Iodopyridinium]

The reaction of anhydrous CuCl₂ with HCl and 2-amino-5-iodopyridine in 1-propanol resulted in two polymorphs of (5-IAP)₂[CuCl₄] [5-IAP = 2-amino-5-iodopyridinium], one as red rods (1) and one as orange prisms (2). The two polymorphs have been characterized by single crystal x-ray diffraction and variable temperature magnetic susceptibility. Crystal data: triclinic, P-1, F(000) = 606, MoK _α (λ = 0.71073 Å): for (1): a = 9.248(6), b = 10.202(7), c = 10.368(7) Å, α = 102.451(9), β = 96.662(8), γ = 96.725(8)°, V = 938.4(11) &Aringsup3;, Z = 2, D_calc = 2.291 Mg/m³, μ = 5.020 mm^?1, R = 0.0392 for [|I|≥2σ(I)]. For (2): a = 8.966(2), b = 10.755(2), c = 11.359(2) Å, α = 89.63(3), β = 68.04(3), γ = 69.07(3)°, V = 938.6(3) &Aringsup3;, Z = 2, D _calc = 2.291 Mg/m³ , μ = 5.019 mm^?1 , R = 0.0217 for [|I|≥2σ(I)]. The crystal packing for both polymorphs suggests that their magnetic behavior should be that of weakly interacting alternating linear chains. Temperature dependent susceptibility measurements yield: for 1 , C = 0.43, g = 2.14, θ = m 1.98; for 2, C = 0.45, g = 2.19, θ = ?4.15.     

Synthesis and crystal structure of Na2[RuNOCl5] · 6H2O

A procedure for the synthesis of Na₂[RuNOCl₅]·6H₂O is described. The complex was studied by IR spectroscopy and by thermal, X-ray phase, and X-ray diffraction analyses. Crystal data: a = 26.740(7), b = 31.581(7), c= 6.918(3) Å, V = 5842(3) ų space group Fdd2. Z = 16, d_calc= 2.103, d_exp = 2.112 g/cm³. The structure consists of [RuNOCl₅]^2? complex cations, Na⁺ cations, and crystallization water molecules. The compound is stable in conditions of high humidity of atmosphere but irreversibly loses up to four water molecules when stored in dry atmosphere. Removal of two water molecules is accompanied by crystal decay and changes in the crystal structure. Heating the substance above 100°C leads to a hygroscopic anhydrous complex Na₂[RuNOCl₅], which is stable below t = 375°C.     

Powder X‐ray study of racemic (2RS,3RS)‐5‐amino‐3‐[4‐(3‐methoxyphenyl)piperazin‐1‐yl]‐1,2,3,4‐tetrahydronaphthalen‐2‐ol

The structure of the title benzovesamicol analogue, C₂₁H₂₇N₃O₂, an important compound for the diagnosis of Alzheimer's disease, has been determined by X‐ray powder diffraction. The title compound was firstly synthesized and characterized by spectroscopic methods (FT–IR, and ¹³C and ¹H NMR). The compound is a racemic mixture of enantiomers which crystallizes in the monoclinic system in a centrosymmetric space group (P2₁/c). Crystallography, in particular powder X‐ray diffraction, was pivotal in revealing that the enantio‐resolution did not succeed. The piperazine ring is in a chair conformation, while the cyclohexene ring assumes a half‐chair conformation. The crystal packing is dominated by intermolecular O—H...N hydrogen bonding which links molecules along the c direction.     

5‐Ethynyl‐2′‐deoxycytidine: a DNA building block with a `clickable' side chain

The title compound [systematic name: 4‐amino‐1‐(2‐deoxy‐β‐d ‐erythro‐pentofuranosyl)‐5‐ethynylpyrimidin‐2(1H)‐one], C₁₁H₁₃N₃O₄, shows two conformations in the crystalline state. The N‐glycosylic bonds of both conformers adopt similar conformations, with χ = −149.2 (1)° for conformer (I‐1) and −151.4 (1)° for conformer (I‐2), both in the anti range. The sugar residue of (I‐1) shows a C2′‐endo envelope conformation (²E, S‐type), with P = 164.7 (1)° and τ_m = 36.9 (1)°, while (I‐2) shows a major C3′‐exo sugar pucker (C3′‐exo‐C2′‐endo, ₃T², S‐type), with P = 189.2 (1)° and τ_m = 33.3 (1)°. Both conformers participate in the formation of a layered three‐dimensional crystal structure with a chain‐like arrangement of the conformers. The ethynyl groups do not participate in hydrogen bonding, but are arranged in proximal positions.     

Synthesis and Crystal Structures of O‐2′,3′‐Cyclic Cyclopentanone and Cyclohexanone Ketals of the Cytostatic 5‐Fluorouridine

The synthesis of two O‐2′,3′‐cyclic ketals, i.e., 5 and 6 , of the cytostatic 5‐fluorouridine ( 2 ), carrying a cyclopentane and/or a cyclohexane ring, respectively, is described. The novel compounds were characterized by ¹H‐, ¹⁹F‐, and ¹³C‐NMR, and UV spectroscopy, as well as by elemental analyses. Their crystal structures were determined by X‐ray analysis. Both compounds 5 and 6 show an anti‐conformation at the N‐glycosidic bond which is biased from +ac to +ap compared to the parent nucleoside 2 . The sugar puckering is changed from ^2′E to _3′E going along with a reduction of the puckering amplitude τ_m by ca. 10–13° due to the ketalization. The conformation about the sugar exocyclic bond C(4′)? C(5′) of 5 and 6 remains unchanged, i.e., g⁺, compared with compound 2 .     

3,4,6‐Tri‐O‐acetyl‐1,2‐O‐[1‐(exo‐ethoxy)ethylidene]‐β‐d‐mannopyranose 0.11‐hydrate

The title compound, C₁₆H₂₄O₁₀·0.11H₂O, is a key intermediate in the synthesis of 2‐deoxy‐2‐[¹⁸F]fluoro‐d ‐glucose (¹⁸F‐FDG), which is the most widely used molecular‐imaging probe for positron emission tomography (PET). The crystal structure has two independent molecules (A and B) in the asymmetric unit, with closely comparable geometries. The pyranose ring adopts a ⁴C₁ conformation [Cremer–Pople puckering parameters: Q = 0.553 (2) Å, θ = 16.2 (2)° and ϕ = 290.4 (8)° for molecule A, and Q = 0.529 (2) Å, θ =15.3 (3)° and ϕ = 268.2 (9)° for molecule B], and the dioxolane ring adopts an envelope conformation. The chiral centre in the dioxolane ring, introduced during the synthesis of the compound, has an R configuration, with the ethoxy group exo to the mannopyranose ring. The asymmetric unit also contains one water molecule with a refined site‐occupancy factor of 0.222 (8), which bridges between molecules A and B via O—H...O hydrogen bonds.     

Studies on metal—acetylene complexes : VI. Crystal and molecular structure of [diethylbis(1-pyrazolyl)borato]methyl(1-phenylpropyne)-platinum(II), [(C2H5)2B(N2C3H3)2](CH3)Pt(C6H5CCCH3)

The crystal and molecular structure of [diethylbis(1-pyrazolyl)borato]methyl(1-phenylpropyne)platinum(II), ([(C₂H₅)₂B(N₂C₃H₃)₂](CH₃)Pt(C₆H₅CCCH₃)), has been determined by a single crystal X-ray diffraction study using diffractometer techniques. The compound crystallizes in the monoclinic space group P2₁/c with a = 13.239(6), b = 11.077(5), c = 15.619)7) Å and β = 114.53(2)°. The observed density of 1.71(2) g cm^?3 agrees well with the calculated value, 1.687 g cm^?3, assuming four molecules in the cell. A conventional agreement factor of 0.036 was obtained by least-squares refinement on F using 3289 observations and 194 variables. The coordination about the platinum atom is square planar, if the acetylene is assumed to occupy one coordination site. The substituents of the acetylene are cis-bent away from the Pt atom, the methyl substituent by 17.7(1.0)°, and the phenyl substituent by 21.2(9)°. The coordinated triple bond length is 1.227(10) Å. These results indicate that the acetylene is moderately perturbed on coordination, consistent with the observation that Δν(CC) is 211 cm^?1. The conformation of the ring formed by the bidentate polypyrazolylborate ligand is that of a “shallow” boat. One of the methylene H atoms on the ethyl substituents on the polypyrazolylborate ligand when placed in an idealized position is 2.65 Å distant from the Pt atom.     

The synthesis and structure of 5-hydroxy-2-methyl-2-(2-pyridinyl)hexahydro-1,3-pyrimidine

The synthesis and the x-ray structure of 5-hydroxy-2-methyl-(2-pyridinyl)hexahydro-1,3-pyrimidine are reported. The compound was prepared by reaction of 2-acetylpyridine and 1,3-diamino-2-hydroxypropane in a 2:1 molar ratio. The colourless crystals are monoclinic, space group P2₁/c with cell parameters, a = 10.385(5), b = 11.171(5), c = 17.415(6)Å, β = 93.05(37)° for Z = 8. The asymmetric unit of the structure is composed of two independent molecules of the compound which are in the chair form and adopt the same conformation with equatorial -OH and -CH₃ groups and axial pyridine substituents. The packing of the molecule seems to be controlled by two independent hydrogen-bonding sequences.     

Crystal structure of a liquid crystal non‐symmetric dimer: cholesteryl 4‐[4‐(4‐n‐butylphenylethynyl)phenoxy]butanoate

The crystal structure of cholesteryl 4‐[4‐(4‐n‐butylphenylethynyl)phenoxy]butanoate [phase sequence: Cr 155°C (46.1?J?g^?1) SmA 186.8°C (1.5?J?g^?1) TGB‐N* 204.7 (6?J?g^?1) I] has been solved from single crystal X‐ray diffraction data. The compound crystallizes in the monoclinic space group P2₁ with unit cell parameters: a?=?13.129(2), b?=?9.3904(10), c?=?17.4121(8)?Å, β?=?92.790(7)°, Z?=?2. The structure has been solved by direct methods and refined to R?=?0.0606 for 3?250 observed reflections. The bond distances and angles are in good agreement with the corresponding values for compounds containing phenyl and cholesterol moieties. The phenyl rings A and B are planar. The dihedral angle between the least‐squares planes of the two phenyl rings is 28°. The cholesterol moiety has the usual structure: the C and E rings have chair conformations, and the D and F rings adopt half‐chair conformations. The molecules in the unit cell are arranged in an antiparallel manner. The crystal structure is stabilized by an intermolecular C–H…O contact of 2.989(10)?Å.     

Open metallocenes XI. A pentadienyl compound of heavier rare earth element [2,4-(CH3)2-η5-C5H5]3Tb·1/2THF

The reaction of TbCl₃ with K[2,4-(CH₃)₂ C₅H₅] at 0°C in THF followed by crystallization at ?90°C led to a pale yellow hexagonal prismatic crystalline product [2,4-[CH₃)₂-η⁵-C₅H₅]₃-Tb·1/ 2THF , which is highly sensitive to air and water and rapidly efflorescent at ambient temperature. The single crystal X-ray diffraction data of the compound have been collected at low temperature (-60°C) and the crystal structure has been solved by heavy atom method. It belongs to triclinic system, space group P1 with lattice parameters α=8.477 Å , b=12.583 Å , c=12.858 Å , α=118.08°, β=91.38°, γ=108.75°, V=1120.36 ų and Z=2. Least-squares refinement converged to a final value R=0.043. The compound possesses an idealized C_3h symmetry with three 2,4dimethylpentadienyl ligands bound to the central terbium atom in a pentahapto mode (η⁵). Each unit cell contains two molecules of [2,4-(CH₃)₂-η⁵-C₅H₅]₃Tb and one molecule of solvent THF, of which the role in the lattice has been discussed in detail.     

N‐tert‐Butyl‐N′‐[5‐cyano‐2‐(4‐methylphenoxy)phenylsulfonyl]urea,a new TXA2 receptor antagonist

The title compound, C₁₉H₂₁N₃O₄S, crystallizes in the space group P2/c with two molecules in the asymmetric unit. The conformation of both molecules is very similar and is mainly determined by an intramolecular N—H...O hydrogen bond between a urea N atom and a sulfonyl O atom. The O and second N atom of the urea groups are involved in dimer formation via N—H...O hydrogen bonds. The intramolecular hydrogen‐bonding motif and conformation of the C—SO₂—NH(C=O)—NH—C fragment are explored and compared using the Cambridge Structural Database and theoretical calculations. The crystal packing is characterized by π–π stacking between the 5‐cyanobenzene rings.     

Structural study of 5-methoxyfuro[3,2-e]imidazo[1,2-c]pyrimidine

The nmr and X-ray diffraction data of the novel ring system furo[3,2-e]imidazo[1,2-c]pyrimidine are reported in this paper. The crystal and molecular structure of this compound (C₉H₇N₃O₂·1/2H₂O) has been solved at room temperature. Crystals are monoclinic in a space group of P2/n with cell constants a = 9.982(8), b = 13.526(9) and c = 13.981(9) Å, β = 107.44(5)°, V = 1800.9 ų, Z = 8, D_x = 1.462 Mg·m^?3. The structure was solved by full matrix least square refinement giving a final R = 0.054 (Rw = 0.069) for 1263 reflections (I>3.0σ(I)). The compound is essentially planar, existing in two slightly structural different forms, A and B. These are held in pairs with symmetry related molecules by hydrogen bonds formed with two water molecules. Then the dimeric units are stacked parallel to the c axis to form the tridimensional packing.     

[Be3(μ3‐O)3(MeCN)6{Be(MeCN)3}3](I)6 – ein Berylliumkomplex mit Cyclo‐(Be3O3)‐Kern und sein Hydrolyseprodukt [Be(H2O)4](I)2·2MeCN

Single crystals of [Be₃(μ₃‐O)₃(MeCN)₆{Be(MeCN)₃}₃](I)₆·4CH₃CN ( 1 ·4CH₃CN) were obtained in low yield by the reaction of beryllium powder with iodine in acetonitrile suspension, which probably result from traces of beryllium oxide containing the applied beryllium metal. The compound 1 ·4CH₃CN forms moisture sensitive, colourless crystal needles, which were characterized by IR spectroscopy and X‐ray diffraction (Space group Pnma, Z = 4, lattice dimensions at 100(2) K: a = 2317.4(1), b = 2491.4(1), c = 1190.6(1) pm, R₁ = 0.0315). The hexaiodide complex cation 1 ⁶⁺consists of a cyclo‐Be₃O₃ core with slightly distorted chair conformation, stabilized by coordination of two acetonitrile ligands at each of the beryllium atoms and by a {Be(CH₃CN)₃}²⁺ cation at each of the oxygen atoms. This unique coordination behaviour results in coplanar OBe₃ units with short Be–O distances of 155.0 pm and 153.6 pm on average of bond lengths within the cyclo‐Be₃O₃ unit and of the peripheric BeO bonds, respectively. Exposure of compound 1 ·4CH₃CN to moist air leads to small orange crystal plates of [Be(H₂O)₄]I₂·2CH₃CN ( 3 ·2CH₃CN). According to the crystal structure determination (Space group C2/c, Z = 4, lattice dimensions at 100(2) K: a = 1220.7(1), b = 735.0(1), c = 1608.5(1) pm, β = 97.97(1)°, R₁ = 0.0394), all hydrogen atoms of the dication [Be(H₂O)₄]²⁺ are involved to form O–H ··· N and O–H ··· I hydrogen bonds with the acetonitrile molecules and the iodide ions, respectively. Quantum chemical calculations (B3LYP/6‐311+G**) at the model [Be₃(μ₃‐O)₃(HCN)₆{Be(HCN)₃}₃]⁶⁺ show that chair and boat conformation are stable and that the distorted chair conformation is stabilized by packing effects.     

Darstellung,Kristallstruktur und spektroskopische Charakterisierung von [(H3C)3Si]NH(BCl2)

Preparation, Crystal Structure, and Spectroscopic Characterization of [(H₃C)₃Si]NH(BCl₂) [(H₃C)₃Si]NH(BCl₂) is formed during the reaction of boron trichloride with hexamethyldisilazane at low temperatures. The compound crystallizes monoclinic in space group P2₁/c with a = 953.8(2) pm, b = 1059.9(1) pm, c = 867.4(1) pm, β = 99.40(2)°; Z = 4. According to the single crystal X-ray diffraction analysis (1275 symmetry independent reflections, R = 0.040), [(H₃C)₃Si]NH(BCl₂) exists as a dimer, in the crystal. The dimers have site symmetry C_i and, within the margins of error of the structure determination, point symmetry C_2h. A characteristic feature is a planar, almost square B–N–B–N four-membered ring with the trimethylsilyl groups in trans position. The compound has been characterised by MS-, ¹H-, ¹³C-, ¹¹B-, and ²⁹Si-NMR-spectroscopy.     

Ferrocene Compounds. XXVII. Synthesis and Structure of 2-(Ferrocenylmethyl)propane-1,3-diol

The title compound was obtained by reduction of diethyl (ferrocenylmethyl)malonate with lithium aluminium hydride in diethyl ether. The structure of this novel ferrocene derivative was assigned by means of elemental analysis, IR, [¹H]NMR, and [¹³C]NMR spectroscopy. The structure was also confirmed by a single crystal X-ray study. The compound crystallizes in monoclinic P2₁/a space group with unit cell dimensions: a = 9.7360(6), b = 27.040(5), c = 14.767(3) Å, = 103.835(6)°, V = 3774.8(11) ų, Z = 12. The asymmetric unit contains three crystallographically independent molecules. In the ferrocenyl moieties, the Fe–C bond distance values are in the range 2.006(5)—2.051(3) Å and C–C distances in the range 1.366(7)–1.425(4) Å. The cyclopentadienyl rings in each of the molecules are mutually twisted by about 13° from the eclipsed conformation. The hydroxyl groups are involved in the intermolecular O–H...O hydrogen bond formation with O-O distances in the range 2.686(3)–2.801(4) Å forming infinite two-dimensional network in a [0 0 1] plane. The crystal structure is additionally stabilized by C–H-O weak intermolecular hydrogen bonds.     

Synthesis of 1,2,4,5‐Benzenetetrasulfonic Acid (H4B4S) and its Implementation as a Linker for Building Metal‐Organic Frameworks on the Example of [Cu2(B4S)(H2O)8]·0.5H2O

Benzene 1,2,4,5‐tetrasulfonic acid (H₄B4S) was prepared in two steps starting from 1,2,4,5‐Tetrachlorobenzene. Slow evaporation of an aqueous reaction mixture of H₄B4S and Cu₂(OH)₂(CO₃) led to light green single crystals of [Cu₂(B4S)(H₂O)₈] · 0.5H₂O. X‐ray single crystal investigations revealed the compound to be triclinic [P , Z = 1, a = 710.0(1), b = 713.7(1), c = 1077.1(2) pm, α = 98.41(2)°, β = 102.91(2)°, γ = 100.69(2)°]. In the crystal structure the Cu²⁺ ions are coordinated by four water molecules and two monodentate sulfonate anions yielding a tetragonally distorted [CuO₆] octahedron. The anions are connected to further copper ions leading to ladder shaped chains running along the [100] direction. According to DTA/TG investigations the dehydration of the compound is finished at 240 °C and the decomposition of the anhydrous sulfonate starts at 340 °C.     

Experimental and Theoretical Studies of 4‐(1‐benzyl‐5‐methyl‐1H‐1,2,3‐triazol‐4‐yl)‐6‐(2,4‐dichlorophenyl)pyrimidin‐2‐amine: A Potential Antibacterial Agent

The title compound ( 1 ), 4‐(1‐benzyl‐5‐methyl‐1H‐1,2,3‐triazol‐4‐yl)‐6‐(2,4‐dichlorophenyl)pyrimidin‐2‐amine (C₂₀H₁₆Cl₂N₆), was synthesized and structurally characterized by elemental analysis, ¹H NMR and ¹³C NMR and single crystal X‐ray diffraction. The compound crystallizes as a colourless needle shaped in the triclinic system, space group P‐1 with cell constants: a = 10.7557(11) Å, b = 12.7078(17) Å, c = 15.511(2) Å, α = 68.029(4)0, β = 86.637(5)0, γ = 87.869(4)0; V = 1962.4 (4) ų, Z = 4. There are two structurally similar but crystallographically independent molecules (A and B) in the asymmetric unit of the title compound, which is linked via N‐H…Cl hydrogen bond. An intramolecular C‐H…N hydrogen also occurs in each molecule. In the crystal, each of independent molecules forms a centrosymmetric dimer with an R²₂(8) ring motifs through a pair of N‐H…N hydrogen bonds. These dimers are further connected by intermolecular N‐H…Cl and C‐H…Cl hydrogen bonds, forming an infinite two dimensional supramolecular network lying parallel to the [010] plane. The molecular geometry was also optimized using density functional theory (DFT/B3LYP) method with the 6‐311G (d, p) basis set and compared with the experimental data. Mulliken population analyses on atomic charges, HOMO‐LUMO energy levels, Molecular electrostatic potential and chemical reactivity of the title compound were investigated by theoretical calculations. The thermo dynamical properties of the title compound at different temperature have been calculated and corresponding relations between the properties and temperature have also been obtained. The in vitro antibacterial activity has been screened against Gram‐positive (Bacillus cerus and Staphylococcus epidermidis) and Gram‐Negative (Escherichia coli, Acinetobacter baumannii and Proteus vulgaris). The results revealed that the compound exhibited good to moderate antibacterial activity.     

1‐(β‐d‐Erythrofuranosyl)adenosine

The title compound, also known as β‐erythroadenosine, C₉H₁₁N₅O₃, (I), a derivative of β‐adenosine, (II), that lacks the C5′ exocyclic hydroxymethyl (–CH₂OH) substituent, crystallizes from hot ethanol with two independent molecules having different conformations, denoted (IA) and (IB). In (IA), the furanose conformation is ^OT₁–E₁ (C1′‐exo, east), with pseudorotational parameters P and τ_m of 114.4 and 42°, respectively. In contrast, the P and τ_m values are 170.1 and 46°, respectively, in (IB), consistent with a ²E–²T₃ (C2′‐endo, south) conformation. The N‐glycoside conformation is syn (+sc) in (IA) and anti (−ac) in (IB). The crystal structure, determined to a resolution of 2.0 Å, of a cocrystal of (I) bound to the enzyme 5′‐fluorodeoxyadenosine synthase from Streptomyces cattleya shows the furanose ring in a near‐ideal ^OE (east) conformation (P = 90° and τ_m = 42°) and the base in an anti (−ac) conformation.