Structures cristallines et moléculaires des muscs macrocycliques. I. La cis-civettone et les variétés polymorphes α et β de sa 2,4-dinitrophénylhydrazone

Crystal and Molecular Structure of Macrocyclic Musks. I. cis-Civetone and polymorphous α- and β-forms of his 2,4-dinitrophenylhydrazone cis-Civetone (C₁₇H₃₀O) forms tetragonal plastic crystals, space groupe 14¹; a = 9.95(4), c = 32.79(1) Å; Z = 8. The plastic phase exists in a wide temperature range and 731 reflexions could be collected at 153 K. The highly disordered structure model was obtained by the use of direct methods. The molecules appear as ring-shaped diffuse electron-density distributions located in special position. Two polymorphous crystalline forms were isolated for the 2,4-dinitrophenyl-hydrazone of cis-civetone (DNPHCC). Both forms are triclinic, space group P1 . Z = 2 (α-Form: a = 6.279(5), b = 12.605(8), c = 15.253(10) Å, α = 105.49(7). β = 100.31 (6), γ = 91.23(7)°; β-Form: a = 7.950(2). b = 8.405 (2). c = 18.233(4) Å, α = 100.28(2), β = 92.29(3), γ = 94.18(2)°). The structures were solved by direct methods and refined to R = 0.11. Each polymorph is associated with a different quinquangular conformation of the macrocycle. In the crystals the intermolecular interactions between macrocycles and aromatic substituents are minimized, the DNPH group being oriented in a face-to-face arrangement across a centre of symmetry. Empirical force field calculations show that the overall intluence of the DNPH moiety on the attached cycle does not significantly modify its conformation with regard to that of the ketone itself.     

Tordanone,un stéroïde replié avec une jonction de cycle A/B β-cis (5β) et B/C α-cis(8α)

Tordanone, a Twice Bent Steroid Structure with Ring A/B β-cis(5β)- and Ring B/C α-cis(8α)-Fused The 3β, 14α, 25-trihydroxy-5β, 8α-cholestan-6-one ( = tordanone; 4 ) has been prepared by stereospecific hydrogenation of 3β, 14α, 25-trihydroxy-5β-cholesta-7,22ξ-dien-6-one ( 5 ). This is the first stereospecific synthesis of a B/C cis-fused steroid belonging to the 5β, 8α -cholestane group with a H-atom at positions 5β (A/B cis-fused) and 8α. The resulting twice bent structure shows a particularly strong steric hindrance of the β-face where CH₃(18) at the C/D ring junction and H_β? C(7) of the B ring are very close to each other. Structural features and mechanistic aspects of the hydrogenation are discussed.     

Un nouvel exemple de transposition: l'épimérisation en C(3) d'hydroxy-3-δ1-pyrazolines dérivées de sucres

A Novel Example of Reversible Ring Opening: The Epimerization at C(3) of Sugar 3-Hydroxy-Δ¹-pyrazolines Reaction of 1 (either geometrical isomer) with hydrazine followed by in situ Ag₂O oxidation led to two pairs of interconverting isomers 4 ? 5 and 6 ? 7 . By the same treatment, (Z)- 10 and (or) (E)- 10 gave the pair 11 ? 12 . Acetylation of 4 ? 5 led to a non interconverting mixture of 8 and 9 . This fact, and the lack of incorporation of ¹⁸O when the epimerization took place in the presence of H₂¹⁸O indicated that the most probable mechanism consisted in a reversible ring opening ( D ? E ? F ). The kinetic parameters of these reactions are given and structural assignments proposed for the new compounds.     

The Crystal and Molecular Structure of 3-Oxo-17β-acetoxy-Δ4-14α-methyl-8α, 9β, 10α, 13α-estrene

The crystal and molecular structure of 3-oxo-17β-acetoxy-Δ⁴-14α-methyl-8α, 9β, 10α, 13α-estrene, C₂₁H₃₀O₃, has been determined by X-ray diffraction analysis. The crystals belong to the orthorhombic space group P2₁2₁2₁, with the cell dimensions a = 12.093 Å, b = 19.667 Å, c = 7.746 Å; Z = 4. Intensity data were collected at room temperature with an automatic four-circle diffractometer. The structure was solved by direct methods and the parameters were refined by least-squares analysis. All the hydrogen atoms were included in the refinement. The final R value was 0.038 for 1413 observed reflections. The conformation of ring A is intermediate between a half-chair and a 1, 2-diplanar form. The hydrogens at C(9) and C(10) are anti, the B/C ring junction is trans, and rings B and C adopt chair conformations. Ring D is cis fused and is halfway between C₂ and C_s forms.     

Methyl 2‐O‐β‐d‐glucopyranosyl‐α‐l‐rhamnopyranoside

The overall conformation of the title compound, C₁₃H₂₄O₁₀, is described by the glycosidic torsion angles ?_H (H1_g—C1_g—O2_r—C2_r) and ψ_H (C1_g—O2_r—C2_r—H2_r), which have values of 13.6 and 16.1°, respectively. The former is significantly different from the value predicted by consideration of the exo‐anomeric effect (?_H～ 60°) and from that in solution (?_H～ 50°), as determined previously by NMR spectroscopy. An intramolecular O3_r—H?O2_g hydrogen bond may help to stabilize the conformation in the solid state. The orientation of the hydroxy­methyl group of the glucose residue is gauche–gauche, with a torsion angle ω (O5_g—C5_g—C6_g—O6_g) of ?70.4 (4)°. Both pyranose rings are in their expected chair conformations, i.e.⁴C₁ for d ‐glucose and ¹C₄ for l ‐rhamnose.     

σ‐Insertive Mechanism versus Concerted Non‐insertive Mechanism in the Intramolecular Hydroamination of Aminoalkenes Catalyzed by Phenoxyamine Magnesium Complexes: A Synthetic and Computational Study

The phenoxyamine magnesium complexes [{ONN}MgCH₂Ph] ( 4 a : {ONN}=2,4‐tBu₂‐6‐(CH₂NMeCH₂CH₂NMe₂)C₆H₂O^?; 4 b : {ONN}=4‐tBu‐2‐(CH₂NMeCH₂CH₂NMe₂)‐6‐(SiPh₃)C₆H₂O^?) have been prepared and investigated with respect to their catalytic activity in the intramolecular hydroamination of aminoalkenes. The sterically more shielded triphenylsilyl‐substituted complex 4 b exhibits better thermal stability and higher catalytic activity. Kinetic investigations using complex 4 b in the cyclisation of 1‐allylcyclohexyl)methylamine ( 5 b ), respectively, 2,2‐dimethylpent‐4‐en‐1‐amine ( 5 c ), reveal a first‐order rate dependence on substrate and catalyst concentration. A significant primary kinetic isotope effect of 3.9±0.2 in the cyclisation of 5 b suggests significant N?H bond disruption in the rate‐determining transition state. The stoichiometric reaction of 4 b with 5 c revealed that at least two substrate molecules are required per magnesium centre to facilitate cyclisation. The reaction mechanism was further scrutinized computationally by examination of two rivalling mechanistic pathways. One scenario involves a coordinated amine molecule assisting in a concerted non‐insertive N?C ring closure with concurrent amino proton transfer from the amine onto the olefin, effectively combining the insertion and protonolysis step to a single step. The alternative mechanistic scenario involves a reversible olefin insertion step followed by rate‐determining protonolysis. DFT reveals that a proton‐assisted concerted N?C/C?H bond‐forming pathway is energetically prohibitive in comparison to the kinetically less demanding σ‐insertive pathway (ΔΔG^≠=5.6 kcal mol^?1). Thus, the σ‐insertive pathway is likely traversed exclusively. The DFT predicted total barrier of 23.1 kcal mol^?1 (relative to the {ONN}Mg pyrrolide catalyst resting state) for magnesium?alkyl bond aminolysis matches the experimentally determined Eyring parameter (ΔG^≠=24.1(±0.6) kcal mol^?1 (298 K)) gratifyingly well.     

Constitution moléculaire d'α,ω-difluoropolydiméthyl-N-méthylsilazanes. Equilibres obtenus par une réaction de redistribution

Unlike the α,ω-dihalogenopolydimethylsiloxanes, the α,ω-dichloropolydimethyl-N-methylsilazanes show a net preference for cyclic species with respect to linear structures at equilibrium. The aim of this study is to evaluate the perturbations in the molecular constitution of these α,ω-dihalogenopolydimethyl-N-methylsilazanes resulting from the substitution of the terminal chlorine atoms by fluorine atoms. This polymeric family was prepared by reacting (CH₃)₂SiF₂ with nonamethylsilazane [(CH₃)₂SiNCH₃]₃. The redistribution of the fluorine atoms with the bridging methylimino groups reached an equilibrium after about 5 months' heating at 150°C for all the samples prepared. The relative abundance of the various molecular species and fragments at equilibrium was deduced from the quantitative analysis of the proton nuclear magnetic resonance (NMR) spectra. The molecular constitution at equilibrium is described by two constants. The first, K = [neso] [middles in chains]/[terminal moieties]² = (2.8 ± 0.8) 10^?2, shows that the presence of terminal fluorine atoms is unfavorable to the formation of short chains. On the other hand, the trimeric cyclic species [(CH₃)₂SiNCH₃]₃ are found to be highly favored (K°₃ = 550 ± 100 mole/liter). These observations further confirm that the equilibrium constants which control the noncyclic part of polymeric families depend little on the functionality of the substituents exchanged [for example, on changing from ? N(CH₃)₂ to ? NCH₃? ] when the reorganization heat order is one.     

Synthese von diastereo- und enantio-selektiv deuterierten β,ε-, β,β-, β,γ- und γ,γ-Carotinen

Synthesis of Diastereo- and Enantioselectively Deuterated β,ε-, β,β-, β,γ- and γ,γ-Carotenes We describe the synthesis of (1′R, 6′S)-[16′, 16′, 16′-²H₃]-β, εcarotene, (1R, 1′R)-[16, 16, 16, 16′, 16′, 16′-²H₆]-β, β-carotene, (1′R, 6′S)-[16′, 16′, 16′-²H₃]-γ, γ-carotene and (1R, 1′R, 6S, 6′S)-[16, 16, 16, 16′, 16′, 16′-²H₆]-γ, γ-carotene by a multistep degradation of (4R, 5S, 10S)-[18, 18, 18-²H₃]-didehydroabietane to optically active deuterated β-, ε- and γ-C₁₁-endgroups and subsequent building up according to schemes \documentclass{article}\pagestyle{empty}\begin{document}${\rm C}_{11} \to {\rm C}_{14}^{C_{\mathop {26}\limits_ \to }} \to {\rm C}_{40} $\end{document} and C₁₁ → C₁₄; C₁₄+C₁₂+C₁₄→C₄₀. NMR.- and chiroptical data allow the identification of the geminal methyl groups in all these compounds. The optical activity of all-(E)-[²H₆]-β,β-carotene, which is solely due to the isotopically different substituent not directly attached to the chiral centres, is demonstrated by a significant CD.-effect at low temperature. Therefore, if an enzymatic cyclization of [17, 17, 17, 17′, 17′, 17′-²H₆]lycopine can be achieved, the steric course of the cyclization step would be derivable from NMR.- and CD.-spectra with very small samples of the isolated cyclic carotenes. A general scheme for the possible course of the cyclization steps is presented.     

Differentiation of Boc‐protected α,δ‐/δ,α‐ and β,δ‐/δ,β‐hybrid peptide positional isomers by electrospray ionization tandem mass spectrometry

Two new series of Boc‐N‐α,δ‐/δ,α‐ and β,δ‐/δ,β‐hybrid peptides containing repeats of L ‐Ala‐δ⁵‐Caa/δ⁵‐Caa‐L ‐Ala and β³‐Caa‐δ⁵‐Caa/δ⁵‐Caa‐β³‐Caa (L ‐Ala = L ‐alanine, Caa = C‐linked carbo amino acid derived from D ‐xylose) have been differentiated by both positive and negative ion electrospray ionization (ESI) ion trap tandem mass spectrometry (MS/MS). MSⁿ spectra of protonated isomeric peptides produce characteristic fragmentation involving the peptide backbone, the Boc‐group, and the side chain. The dipeptide positional isomers are differentiated by the collision‐induced dissociation (CID) of the protonated peptides. The loss of 2‐methylprop‐1‐ene is more pronounced for Boc‐NH‐L ‐Ala‐δ‐Caa‐OCH₃ (1), whereas it is totally absent for its positional isomer Boc‐NH‐δ‐Caa‐L ‐Ala‐OCH₃ (7), instead it shows significant loss of t‐butanol. On the other hand, second isomeric pair shows significant loss of t‐butanol and loss of acetone for Boc‐NH‐δ‐Caa‐β‐Caa‐OCH₃ (18), whereas these are insignificant for its positional isomer Boc‐NH‐β‐Caa‐δ‐Caa‐OCH₃ (13). The tetra‐ and hexapeptide positional isomers also show significant differences in MS² and MS³ CID spectra. It is observed that ‘b’ ions are abundant when oxazolone structures are formed through five‐membered cyclic transition state and cyclization process for larger ‘b’ ions led to its insignificant abundance. However, b₁⁺ ion is formed in case of δ,α‐dipeptide that may have a six‐membered substituted piperidone ion structure. Furthermore, ESI negative ion MS/MS has also been found to be useful for differentiating these isomeric peptide acids. Thus, the results of MS/MS of pairs of di‐, tetra‐, and hexapeptide positional isomers provide peptide sequencing information and distinguish the positional isomers. Copyright © 2010 John Wiley & Sons, Ltd.     

Preparation of Protected β2‐ and β3‐Homocysteine, β2‐ and β3‐Homohistidine,and β2‐Homoserine for Solid‐Phase Syntheses

The Ser, Cys, and His side chains play decisive roles in the syntheses, structures, and functions of proteins and enzymes. For our structural and biomedical investigations of β‐peptides consisting of amino acids with proteinogenic side chains, we needed to have reliable preparative access to the title compounds. The two β³‐homoamino acid derivatives were obtained by Arndt–Eistert methodology from Boc‐His(Ts)‐OH and Fmoc‐Cys(PMB)‐OH (Schemes 2–4), with the side‐chain functional groups' reactivities requiring special precautions. The β²‐homoamino acids were prepared with the help of the chiral oxazolidinone auxiliary DIOZ by diastereoselective aldol additions of suitable Ti‐enolates to formaldehyde (generated in situ from trioxane) and subsequent functional‐group manipulations. These include OH→O^tBu etherification (for β²hSer; Schemes 5 and 6), OH→STrt replacement (for β²hCys; Scheme 7), and CH₂OH→CH₂N₃→CH₂NH₂ transformations (for β²hHis; Schemes 9–11). Including protection/deprotection/re‐protection reactions, it takes up to ten steps to obtain the enantiomerically pure target compounds from commercial precursors. Unsuccessful approaches, pitfalls, and optimization procedures are also discussed. The final products and the intermediate compounds are fully characterized by retention times (t_R), melting points, optical rotations, HPLC on chiral columns, IR, ¹H‐ and ¹³C‐NMR spectroscopy, mass spectrometry, elemental analyses, and (in some cases) by X‐ray crystal‐structure analysis.     

Etude Structurale de la forme basse Température de NH4Sn2F5 (Type γ)

Structural Study of the Low-temperature Modification of NH₄Sn₂F₅ (Type γ) NH₄Sn₂F₅, monoclinic, space group C2, Cm or C2/m, a = 7.361(4) Å, b = 12.752(6) Å, c = 10.492(5) Å, β = 103.5(7)°, V = 957,2(9) ų, Z = 6. MoKα (= 0.7107 Å) R = 0.036 for 476 observed reflexions. The structure is built from alternative layers [Sn? F] and [NH₄⁺]. Three kinds of fluorine have vacancy positions, that explains the bidimensional conduction in NH₄Sn₂F₅.     

Nuclear magnetic relaxation of α-13C nuclei of helical poly(γ-hexyl-L-glutamate) and poly(γ-benzyl-L-glutamate)

Spin-lattice relaxation times (T₁), spin-spin relaxation times (T₂), and nuclear Overhauser enhancements (NOE), at 75.5 MHz are reported for α-¹³C nuclei of poly (γ-benzyl-L -glutamate) in deuterated dimethylformamide at 60°C and of poly(γ-hexyl-L -glutamate) in cyclohexanone at 48 and 79°C. It is shown that for molecular weights above 10⁵, the polypeptides cannot be considered as essentially rigid helices with internal librational motions; additional backbone flexing motions contribute to the relaxation behavior.     

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.     

3‐Deoxy‐β‐d‐ribo‐hexopyranose (3‐deoxy‐β‐d‐glucopyranose)

The β‐pyranose form, (III), of 3‐deoxy‐d ‐ribo‐hexose (3‐deoxy‐d ‐glucose), C₆H₁₂O₅, crystallizes from water at 298 K in a slightly distorted ⁴C₁ chair conformation. Structural analyses of (III), β‐d ‐glucopyranose, (IV), and 2‐deoxy‐β‐d ‐arabino‐hexopyranose (2‐deoxy‐β‐d ‐glucopyranose), (V), show significantly different C—O bond torsions involving the anomeric carbon, with the H—C—O—H torsion angle approaching an eclipsed conformation in (III) (−10.9°) compared with 32.8 and 32.5° in (IV) and (V), respectively. Ring carbon deoxygenation significantly affects the endo‐ and exocyclic C—C and C—O bond lengths throughout the pyranose ring, with longer bonds generally observed in the monodeoxygenated species (III) and (V) compared with (IV). These structural changes are attributed to differences in exocyclic C—O bond conformations and/or hydrogen‐bonding patterns superimposed on the direct (intrinsic) effect of monodeoxygenation. The exocyclic hydroxymethyl conformation in (III) (gt) differs from that observed in (IV) and (V) (gg).     

Luminescence de l'europium divalent dans les composés α- et β-RbLu3F10

The effect of the crystal structure upon the luminescence of the divalent europium within the RbLu₃F₁₀ dimorphous matrix has been investigated. The obtained results essentially show that the difference between the rubidium coordination numbers in both α- and β-RbLu₃F₁₀ phases (15 or 16 and 8 or 10 respectively) is responsible for the change over from a 4f⁷ → 4f⁷ emission to a 4f⁶5d¹ → 4f⁷ emission.     

Isomérisations en chimie des sucres. II Fermetures de cycles par attaque nucléophile intramoléculaire sur des carbones sp2 à caractère électrophile: Furannoses hybridés sp2 en C3

When heated with one equivalent of H₂O, the 1,2: 5,6-di-O-isopropylidene-α-D-ribo- and -xylo-hexofuranos-3-uloses loose one molecule of acetone and yield the 3, 6-anhydro-1, 2-O-isopropylidene-α-D-ribo- and -xylo-hexofuranos-3-ulose ketohydrols. The carbonyl group of the starting material seems to provide some kind of intramolecular electrophilic assistance to the hydrolysis of the 5, 6-O-isopropylidene group. When the oxygen of the carbonyl group is replaced by cyanomethylene, an analogous cyclisation takes place under base catalysis, provided that C6 bears a free hydroxyl group.     

Methyl β‐allolactoside [methyl β‐d‐galactopyranosyl‐(1→6)‐β‐d‐glucopyranoside] monohydrate

Methyl β‐allolactoside [methyl β‐d ‐galactopyranosyl‐(1→6)‐β‐d ‐glucopyranoside], (II), was crystallized from water as a monohydrate, C₁₃H₂₄O₁₁·H₂O. The βGalp and βGlcp residues in (II) assume distorted ⁴C₁ chair conformations, with the former more distorted than the latter. Linkage conformation is characterized by ϕ′ (C2_Gal—C1_Gal—O1_Gal—C6_Glc), ψ′ (C1_Gal—O1_Gal—C6_Glc—C5_Glc) and ω (C4_Glc—C5_Glc—C6_Glc—O1_Gal) torsion angles of 172.9 (2), −117.9 (3) and −176.2 (2)°, respectively. The ψ′ and ω values differ significantly from those found in the crystal structure of β‐gentiobiose, (III) [Rohrer et al. (1980). Acta Cryst. B 36 , 650–654]. Structural comparisons of (II) with related disaccharides bound to a mutant β‐galactosidase reveal significant differences in hydroxymethyl conformation and in the degree of ring distortion of the βGlcp residue. Structural comparisons of (II) with a DFT‐optimized structure, (II_C), suggest a link between hydrogen bonding, pyranosyl ring deformation and linkage conformation.     

Electrophilic Alkylations of Vinylsilanes: A Comparison of α‐ and β‐Silyl Effects

Kinetics of the reactions of benzhydrylium ions (Aryl₂CH⁺) with the vinylsilanes H₂C?C(CH₃)(SiR₃), H₂C?C(Ph)(SiR₃), and (E)‐PhCH?CHSiMe₃ have been measured photometrically in dichloromethane solution at 20 °C. All reactions follow second‐order kinetics, and the second‐order rate constants correlate linearly with the electrophilicity parameters E of the benzhydrylium ions, thus allowing us to include vinylsilanes in the benzhydrylium‐based nucleophilicity scale. The vinylsilane H₂C?C(CH₃)(SiMe₃), which is attacked by electrophiles at the CH₂ group, reacts one order of magnitude faster than propene, indicating that α‐silyl‐stabilization of the intermediate carbenium ion is significantly weaker than α‐methyl stabilization because H₂C?C(CH₃)₂ is 10³ times more reactive than propene. trans‐β‐(Trimethylsilyl)styrene, which is attacked by electrophiles at the silylated position, is even somewhat less reactive than styrene, showing that the hyperconjugative stabilization of the developing carbocation by the β‐silyl effect is not yet effective in the transition state. As a result, replacement of vinylic hydrogen atoms by SiMe₃ groups affect the nucleophilic reactivities of the corresponding C?C bonds only slightly, and vinylsilanes are significantly less nucleophilic than structurally related allylsilanes.     

μ3‐η2:η2:η2‐Coordination of Primary Silane on a Triruthenium Plane

A μ₃‐η²:η²:η²‐silane complex, [(Cp*Ru)₃(μ₃‐η²:η²:η²‐H₃SitBu)(μ‐H)₃] ( 2 a ; Cp*=η⁵‐C₅Me₅), was synthesized from the reaction of [{Cp*Ru(μ‐H)}₃(μ₃‐H)₂] ( 1 ) with tBuSiH₃. Complex 2 a is the first example of a silane ligand adopting a μ₃‐η²:η²:η² coordination mode. This unprecedented coordination mode was established by NMR and IR spectroscopy as well as X‐ray diffraction analysis and supported by a density functional study. Variable‐temperature NMR analysis implied that 2 a equilibrates with a tautomeric μ₃‐silyl complex ( 3 a ). Although 3 a was not isolated, the corresponding μ₃‐silyl complex, [(Cp*Ru)₃(μ₃‐η²:η²‐H₂SiPh)(H)(μ‐H)₃] ( 3 b ), was obtained from the reaction of 1 with PhSiH₃. Treatment of 2 a with PhSiH₃ resulted in a silane exchange reaction, leading to the formation of 3 b accompanied by the elimination of tBuSiH₃. This result indicates that the μ₃‐silane complex can be regarded as an “arrested” intermediate for the oxidative addition/reductive elimination of a primary silane to a trinuclear site.     

Constitution moléculaire d' α,ω-difluoro-μ-oxo-poly(méthylphosphanoxy diméthylsilanes): Mise en evidence d'une forte tendance à l'alternance régulière des centres siliciés et phosphorés à l'equilibre

The purpose of this study was to gain a closer knowledge of the molecular constitution of the linear fluorine-terminated oxygen-bridged methylphosphanoxy/dimethylsilane polymers, for example, to find evidence for preferential sorting (or, on the contrary, for random scattering) of the substituents and building units. The title polymers were prepared by reaction of MeP(O)F₂ with cyclic dimethylsiloxanes (Me₂SiO)_n (n = 3 or 4). An equilibrium is reached in the redistribution of fluorine vs. bridging oxygen atoms among the phosphorus and silicon-based centers, and among the resulting building units, after about 2 months at 120°C. The excellent resolution of the ¹H-NMR spectra (Fig. 2), even at 60 MHz, allowed identification of seventeen different fragments (Table II). Nineteen equilibrated samples of varied overall compositions (R = F/(Si + P); R′ = P/(Si + P)) have been analyzed (Table IV), and their molecular constitution is described by a set of four basic constants. The fundamental features which govern the structure of these polymers are as follows. (a) The regular (Si-O-P) alternation of the two different centers, which is thermodynamically favored, as shown by the linkage constant K₀ = [Si-O-Si][P-O-P]/[Si-O-P]² ? 10⁴, which describes the sorting of the silicon and phosphorus atoms on the bridging oxygens, and which deviates by four orders of magnitude from its random value of 0.25. (b) A somewhat surprising lack of preferential distribution of fluorine and oxygen between the two centers (K_I = [MeP(O)F₂][Me₂SiO_1/2]₂/[Me₂SiF₂]-[MeP(O)(O_1/2)₂]) differs little from (a), which contrasts with the preferential affinity of fluorine for silicon and oxygen for phosphorus (K_I ? 10⁷) that was found when F atoms and OCH₃ groups were exchanged between the same centers. (c) The sorting of the fluorine atoms and oxygen bridges on each center, to give neso molecules and the terminal and medium building units, resulting in a slight preference for the formation of the terminal units, as expressed by