Kinetics of the iodine atom catalyzed gas phase geometrical isomerization of diiodoethylene. Rotation about a single bond

The spectrophotometric determination of the rate of iodine atom catalyzed geometrical isomerization of diiodoethylene in the gas phase from 502.8 to 609.1°K leads to a rate constant for the bimolecular reaction between I and trans-diiodoethylene of log k_t?c(M^?1 sec^?1) = 8.85 ± 0.12 ? (11.01 ± 0.30)/θ. Estimates of the entropy and enthalpy change for the addition of I atoms to trans-diiodoethylene (process a.b) lead to log K_a.b(M^?1) = ?2.99 ? 4.0/θ, and thus to log k_c (sec^?1) = log k_t?c – log K_ab = 11.8 ?7.0/θ for the rate constant for rotation about the single bond in the adduct radical. The theory for calculation of the rotation rate constant is presented and it is shown that while the exact value depends on the barrier height, a value of 6.8 kcal/mole for this quantity leads to log k (sec^?1) = 11.8 ?6.7/θ. The activation energy points to a better value of the group contribution to heat of formation of the group C -(I)₂(H)(C) than one based on bond additivity.     

Study of Unusually High Rotational Barriers about S?N Bonds in Nonafluorobutane‐1‐sulfonamides: The Electronic Nature of the Torsional Effect

Slow rotation about the S? N bond in N,N‐disubstituted nonafluorobutane‐1‐sulfonamides 1 can easily be detected by NMR measurements at room temperature. This effect causes magnetic nonequivalence of otherwise identical geminal substituents in symmetrical staggered ground‐state conformation A . The torsional barriers determined (62–71 kJ?mol^?1) proved to be the highest ever observed for sulfonamide moieties. They are comparable to the values routinely measured for carboxylic acid amides or carbamates. The restricted rotation is interpreted as result (n_N? d_S)‐π and of n_N? σ interactions, which develop substantial S,N double‐bond character in sulfonamides 1 . The S,N binding interaction is increased by the highly electron‐withdrawing effect of the perfluorobutyl group. The anticipated symmetry of the ground‐state conformation A and the considerable shortening of the S? N bond (1.59 Å) compared to the mean value in sulfonamides (1.63 Å) are confirmed by single‐crystal X‐ray study of N,N‐dibenzylnonafluorobutane‐1‐sulfonamide ( 1c ).     

Mechanism of the Thermal Z⇌E Isomerization of a Stable Silene; Experiment and Theory

The E and Z geometric isomers of a stable silene (tBu₂MeSi)(tBuMe₂Si)Si=CH(1‐Ad) ( 1 ) were synthesized and characterized spectroscopically. The thermal Z to E isomerization of 1 was studied both experimentally and computationally using DFT methods. The measured activation parameters for the 1Z ? 1E isomerization are: E_a=24.4 kcal mol^?1, ΔH^≠=23.7 kcal mol^?1, ΔS^≠=?13.2 e.u. Based on comparison of the experimental and DFT calculated (at BP86‐D3BJ/def2‐TZVP(‐f)//BP86‐D3BJ/def2‐TZVP(‐f)) activation parameters, the Z?E isomerization of 1 proceeds through an unusual (unprecedented for alkenes) migration–rotation–migration mechanism (via a silylene intermediate), rather than through the classic rotation mechanism common for alkenes.     

Cavitand‐Based Pd‐Pyridyl Coordination Capsules: Guest‐Induced Homo‐ or Heterocapsule Selection and Applications of Homocapsules to the Protection of a Photosensitive Guest and Chiral Capsule Formation

A 2 : 4 mixture of tetrakis[4‐(4‐pyridyl)phenyl]cavitand ( 1 ) or tetrakis[4‐(4‐pyridyl)phenylethynyl]cavitand ( 2 ) and Pd(dppp)(OTf)₂ self‐assembles into a homocapsule { 1 ₂ ? [Pd(dppp)]₄}⁸⁺ ? (TfO^?)₈ ( C1 ) or { 2 ₂ ? [Pd(dppp)]₄}⁸⁺ ? (TfO^?)₈ ( C2 ), respectively, through Pd?Npy coordination bonds. A 1 : 1 : 4 mixture of 1 , 2 , and Pd(dppp)(OTf)₂ produced a mixture of homocapsules C1 , C2 , and a heterocapsule { 1 ? 2 ? [Pd(dppp)]₄}⁸⁺ ? (TfO^?)₈ ( C3 ) in a 1 : 1 : 0.98 mole ratio. Selective formation (self‐sorting) of homocapsules C1 and C2 or heterocapsule C3 was controlled by guest‐induced encapsulation under thermodynamic control. Applications of Pd?Npy coordination capsules with the use of 1 were demonstrated. Capsule C1 serves as a guard nanocontainer for trans‐4,4′‐diacetoxyazobenzene to protect against the trans‐to‐cis photoisomerization by encapsulation. A chiral capsule { 1 ₂ ? [Pd((R)‐BINAP)]₄}⁸⁺ ? (TfO^?)₈ ( C5 ) was also constructed. Capsule C5 induces supramolecular chirality with respect to prochiral 2,2′‐bis(alkoxycarbonyl)‐4,4′‐bis(1‐propynyl)biphenyls by diastereomeric encapsulation through the asymmetric suppression of rotation around the axis of the prochiral biphenyl moiety.     

DR. Maurits Dekker: A Pioneer in Scientific Publishing

Abstract

Organic salts as initiators [A? ⁺B^?: Ph?₃C⁺ClO^? ₄, Ph?₃C⁺SbCl^? ₆, (?)Sp? ⁺ClO^? ₄, and (?)(Sp)?₂ ³⁺(ClO₄)^? ₃] and catalysts [A ⁺B?^?: (+)CSA?; A? ⁺B?^?: ph?₃C⁺(+)CSA?^? and (?)Sp?⁺(+)CSA?^?] are prepared and characterized by the dissociation constant (K _d), fraction of free ions (α), and specific rotation. The asymmetrically stereoselective induction of the initiators and catalysts in the polymerization of N-vinylcarbazole is in the order A? ⁺B?^? > A ⁺B?^? > A? ⁺B^?. The specific rotations of the poly(N-vinylcarbazole) (PVCZ)s obtained are generally in this order.     

Chloromethane–Water Adduct: Rotational Spectrum,Weak Hydrogen Bonds,and Internal Dynamics

The rotational spectra of four isotopologues of the 1:1 complex between chloromethane and water revealed the presence of only one rotamer in a pulsed jet expansion. The two subunits are linked through two weak hydrogen bonds, O? H???Cl (R_H???Cl=2.638(2) Å) and C? H???O (R_H???O=2.501(2) Å), forming a five‐membered ring. All transitions display the hyperfine structure due to the ³⁵Cl (or ³⁷Cl) nuclear quadrupole effects. Dynamical features in the spectrum are caused by two large‐amplitude motions. Each component line appears as an asymmetric doublet with a relative intensity ratio of 1:3. The splittings led to the determination of barrier to internal rotation of water around its symmetry axis, V₂=320(10) cm^?1. Finally, an unexpected small value of the inertial defect (?0.96 uŲ rather than ?3.22 uŲ) allowed the estimation of the barrier to the internal rotation of the CH₃ group, V₃≈8 cm^?1.     

Remarks on the analysis of dynamic 1H NMR spectra of A3B2 ⇌ C3D2 spin systems. Internal rotation in N,N-diethylbenzamide

A method of analysis of dynamic NMR spectra for A₃B₂ ? C₃D₂ spin systems is proposed. This method is based upon the assumption that the shape of the A–C part of the spectrum is the same as for the AB₂ ? CD₂ spin system and the shape of the B–D part is the same as for the A₃B ? C₃D system. By means of this simplification the iterative total line shape analysis of dynamic NMR spectra by the use of the least-squares method becomes feasible. The method proposed is applied to the determination of the thermodynamic parameters of activation for internal rotation in N,N-diethylbenzamide in CD₃CN (ΔG = 62.6 kJ mol^?1 ΔH^≠ = 62 kJ mol^?1, ΔS^≠ = ?1 J/mol^?1 K^?1). The determination of natural line widths is discussed in detail and a method of estimation of the effect of systematic errors on the results is proposed.     

[Pd(Fmes)2(tmeda)]: A Case of Intermittent C?H⋅⋅⋅F?C Hydrogen‐Bond Interaction in Solution

The X‐ray structure of the title compound [Pd(Fmes)₂(tmeda)] (Fmes=2,4,6‐tris(trifluoromethyl)phenyl; tmeda=N,N,N′,N′‐tetramethylethylenediamine) shows the existence of uncommon C? H???F? C hydrogen‐bond interactions between methyl groups of the TMEDA ligand and ortho‐CF₃ groups of the Fmes ligand. The ¹⁹F NMR spectra in CD₂Cl₂ at very low temperature (157 K) detect restricted rotation for the two ortho‐CF₃ groups involved in hydrogen bonding, which might suggest that the hydrogen bond is responsible for this hindrance to rotation. However, a theoretical study of the hydrogen‐bond energy shows that it is too weak (about 7 kJ mol^?1) to account for the rotational barrier observed (ΔH^≠=26.8 kJ mol^?1), and it is the steric hindrance associated with the puckering of the TMEDA ligand that should be held responsible for most of the rotational barrier. At higher temperatures the rotation becomes fast, which requires that the hydrogen bond is continuously being split up and restored and exists only intermittently, following the pulse of the conformational changes of TMEDA.     

Sels de Pyrylium. XVIII. Etude par RMN du Carbone-13 de Sels d'Aminopyrylium et des Barrières de Rotation dans quelques Cations N,N-Diméthylaminopyrylium

The C-2—N bond of 2-N,N-dimethylaminopyrylium cations has a partial π character due to the conjugation of the nitrogen lone-pair with the ring. The values of ΔG^≠, ΔH^≠, ΔS^≠ parameters related to the corresponding hindered rotation have been determined by ¹³C NMR total bandshape analysis. This conjugation decreases the electrophilic character of carbon C-4 so that the displacement of the alkoxy group is no longer possible. Such a hindered rotation also exists in 4-N,N-dimethylaminopyrylium cations and the corresponding ΔG^≠ parameters have been evaluated. Comparison of these two cationic species shows that hindered rotation around the C—N bond is larger in position 4 than in position 2. Furthermore, the barrier to internal rotation around the C-2? N bond decreases with increasing electron donating power of the substituent at position 4. ΔG^≠ values decreases from 19.1 kcal mol^?1 (79.9 kJ mol^?1) to 12.6 kcal mol^?1 (52.7 kJ mol^?1) according to the following sequence for the R-4 substituents: -C₆H₅, -CH₃, -OCH₃, -N(CH₃)₂.     

Résonance Magnétique Nucléaire en phase nématique. Exploration de la barrière d'empêchement à la libre rotation pour le fluorure d'acroyle et l'acroléine

The NMR spectrum of acrolein and acroyl fluoride (CH₂?CH? COX with X?H and F) oriented in a nematic phase has been measured and information about conformational equilibrium s-cis ? s-trans has been obtained. The barrier to internal rotation of the COX group has been studied with various hypotheses. Good agreement between experimental and calculated spectra has been obtained using the potential equation V(?) = Σ_nV_n(1 – cos n?)/2, with V₁ = ?200 cal mol^?1, V₂ = 1500 cal mol^?1 and V₃ = 400 cal mol^?1 for the fluorine compound, and V₁ = 1200 cal mol^?1, V₂ = 3000 cal mol^?1 and V₃ = 2000 cal mol^?1 for acrolein; this last compound is found to be mostly in the s-trans conformation.     

A 1H and 13C NMR study of intramolecular rotations in syn- and anti-o-tolyl-di-tert-butyl-carbinols

The syn and anti rotamers of o-tolyl-di-tert-butylcarbinol, 2a and 2b, respectively, have been studied by ¹H NMR at 200 MHz and by natural abundance ¹³C NMR at 50 MHz. ¹H-{¹H} NOE enhancement factors are consistent with the known structures and the calculated geometries of these compounds. The relaxation time, T₁, of the 2-Me protons in 2b is unexpectedly higher than that for 2a. The ¹³C relaxation times of the 2-Me and the quaternary carbon of the tert-butyl group are also both higher in 2b than in 2a, suggesting that the rotation of these groups is faster in the less stable isomer. The activation energies for t-Bu rotation, measured by ¹H DNMR, agree with this conclusion. Further confirmation is provided by theoretical calculation of the 2-Me and t-Bu rotation barriers based on Allinger's MM2 force field. Comparison of measured ΔG? values from this work and from the literature with MM2-calculated ΔH? values indicates that this force field systematically underestimates rotation barriers in open-chain systems by a factor of approximately 0.64.     

Conformational studies by nuclear magnetic resonance—IV: Restricted rotation in vinylogous thioamides

Enamino -thial and -thiones R¹C(S)CH?CHNR (R¹ = H or alkyl; R² = Me or Et) have been shown by NMR spectra to exist in two rotational forms, s-cis and s-trans, the populations of the latter being approximately the same as in the case of the parent oxa analogues. An increase of the order of 2 to 4 Kcal/mole in the heights of C? C and C? N rotation barriers (ΔG*) was found on comparing the title compounds with their oxa analogues. IR spectra failed as a tool to establish the rotational equilibrium. IR absorption bands of the ν_{C? C}, ν_{C? H} (in the NMe₂ group) and γ_HC?CH vibrations have been found, but the ν_C?S band could not be assigned unambiguously. Anomalies concerning the frequency and intensity of the ν_C?C band are discussed.     

Synthesis,characterization and dynamic stereochemistry of thermochromic tris(dithiocarbamato)vanadium(III) complexes stereochemistry of thermochromic tris(dithiocarbamato)vanadium(III) complexes

The vanadium(III) complexes, V(S₂CNMe₂)₃ (1) and V(S₂CN ⁱ Pr₂)₃ (2) were prepared and characterized by analysis, IR, electronic and ¹H NMR spectra. The complexes show reversible thermochromic behaviour. MM2 calculations were used to simulate the molecular structure of 1. For 2, variable temperature ¹H NMR revealed hindered rotation about C–N bonds. The rotational energy barrier (38?kJ?mol^?1) was obtained by molecular mechanics force-field calculations.     

Huge dielectric response and molecular motions in paddle-wheel [Cu(II)2(adamantylcarboxylate)4(DMF)2]⋅(DMF)2

The temperature‐dependent dynamic properties of [Cu^II₂(ADCOO)₄(DMF)₂]?(DMF)₂ ( 1 ) and [Cu^II₂(ADCOO)₄(AcOEt)₂] ( 2 ) crystals were examined by X‐ray crystallography, ¹H NMR spectroscopy, and measurements of the dielectric constants and magnetic susceptibilities (ADCOO=adamantane carboxylate, DMF=N,N‐dimethylformamide, and AcOEt=ethyl acetate). In both crystals, four ADCOO groups bridged a binuclear Cu^II? Cu^II bond, forming a paddle‐wheel [Cu^II₂(ADCOO)₄] structure. The oxygen atoms of two DMF molecules in crystal 1 and two AcOEt molecules in crystal 2 were coordinated at axial positions of the [Cu^II₂(ADCOO)₄] moiety, forming [Cu^II₂(ADCOO)₄(DMF)₂] and [Cu^II₂(ADCOO)₄(AcOEt)₂], respectively. Two additional DMF molecules were included in the unit cell of crystal 1 , whereas AcOEt was not included in the unit cell of crystal 2 . The structural analyses of crystal 1 at 300 K showed three‐fold rotation of the adamantyl groups, whereas rotation of the adamantyl groups of crystal 2 at 300 K was not observed. Thermogravimetric measurements of crystal 1 indicated a gradual elimination of DMF upon increasing the temperature above 300 K. The dynamic behavior of the crystallized DMF yielded significant temperature‐dependent dielectric responses in crystal 1 , which showed a huge dielectric peak at 358 K in the heating process. In contrast, only small frequency‐dependent dielectric responses were observed in crystal 2 because of the freezing of the molecular rotation of the adamantyl groups. The magnetic behavior was dominated by the strong antiferromagnetic coupling between the two S=1/2 spins of the Cu^II? Cu^II site, with magnetic exchange energies (J) of ?265 K (crystal 1 ) and ?277 K (crystal 2 ).     

Pd(II)‐catalyzed polymerization of optically active norbornene carboxylic acid esters

(?)‐(1S,2R)‐Norbornene‐2‐carboxylic acid alkyl esters (alkyl = Me, Bz, L ‐menthyl, or D ‐menthyl) were successfully prepared by the Diels–Alder reaction of cyclopentadiene with (R)‐(?)‐pantolactone‐O‐yl acrylate followed by epimerization and column chromatography. The enantiomeric excess was 99.9%. These monomers were polymerized by Pd(II)‐based catalysts, and high yields of the polymers were obtained. The methyl ester gave an optically active polymer of high optical rotation (monomer [α]_D = ?24.7, polymer [α]_D = ?98.5). This high rotation value of the polymer was attributed to the isotactic chain regulation of the polymer. This high rotation was not observed with methyl esters prepared by the transesterification of menthyl esters. The stereoregular polymer exhibited notable resonance peaks at 39 ppm in ¹³C NMR spectra. No crystallinity was observed. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 1263–1270, 2006     

Conformational Equilibria and Large‐Amplitude Motions in Dimers of Carboxylic Acids: Rotational Spectrum of Acetic Acid–Difluoroacetic Acid

We report the rotational spectra of two conformers of the acetic acid–difluoroacetic acid adduct (CH₃COOH–CHF₂COOH) and supply information on its internal dynamics. The two conformers differ from each other, depending on the trans or gauche orientation of the terminal ?CHF₂ group. Both conformers display splittings of the rotational transitions, due to the internal rotation of the methyl group of acetic acid. The corresponding barriers are determined to be V₃(trans)=99.8(3) and V₃(gauche)=90.5(9) cm^?1 (where V₃ is the methyl rotation barrier height). The gauche form displays a further doubling of the rotational transitions, due to the tunneling motion of the ?CHF₂ group between its two equivalent conformations. The corresponding B₂ barrier is estimated to be 108(2) cm^?1. The increase in the distance between the two monomers upon OH→OD deuteration (the Ubbelohde effect) is determined.     

Calculation of the magneto–optical activity of the hydrogen molecule

We calculated the Verdet constant of the hydrogen molecule. We found that the angle of rotation may be represented by ?₀ = A₀Hv²/(v ? v²)² where A₀ is 3.23 × 10^?33 microminutes sec² per oersted-cm-atmosphere. This is in agreement with the experimental observations where ?₀ is given by the same frequency dependence with the constant A₀ × 2.72 = 10^?33 in terms of the same units.     

Résonance Magnétique Nucléaire en phase nématique. Exploration de la barrière d'empêchement à la libre rotation de la monofluoroacétone

The NMR spectra of monochloro-, monobromo- and monofluoroacetone (CH₃? CO? CH₂X with X = Cl, Br, F) oriented in a nematic phase have been measured and the direct dipolar coupling constants determined. The barrier to internal rotation for the CH₂F group has been studied for fluorine compound using various hypotheses. The best agreement with IR data has been obtained using the potential equation V(θ) = Σ_n V_n × (1 – cos nθ)/2 and a Boltzmann distribution of the CH₂F group (V₁ = 250 ± 50 cal.mol^?1, V₂ = 1650 ± 100 cal.mol^?1, V₃ = ?1000 ± 100 cal.mol^?1).     

Facile P−C/C−H Bond‐Cleavage Reactivity of Nickel Bis(diphosphine) Complexes

Unusual cleavage of P?C and C?H bonds of the P₂N₂ ligand, in heteroleptic [Ni(P₂N₂)(diphosphine)]²⁺ complexes under mild conditions, results in the formation of an iminium formyl nickelate featuring a C,P,P‐tridentate coordination mode. The structures of both the heteroleptic [Ni(P₂N₂)(diphosphine)]²⁺ complexes and the resulting iminium formyl nickelate have been characterized by NMR spectroscopy and single‐crystal X‐ray diffraction analysis. Density functional theory (DFT) calculations were employed to investigate the mechanism of the P?C/C?H bond cleavage, which involves C?H bond cleavage, hydride rotation, Ni?C/P?H bond formation, and P?C bond cleavage.     

X-ray and conformation analysis of the new trinuclear cluster of osmium Os3(μ,η2-OCC6H5)(η3-C3H5)(CO)9

The crystal structure of the Os₃(μ,η²-O=CC₆H₅)(η³-C₃H₅)(CO)₉ cluster synthesized by the reaction of the (μ-H)Os₃(μ-O=CC₆H₅)(CO)₁₀ complex with allylamine in chloroform was determined by X-ray analysis. Prolonged storage of the reaction mixture led to N-C bond cleavage in allylamine and η³-addition of the allyl fragment at one of the Os atoms (Os-C 2.246 ?, 2.248 ?, and 2.273 ?). The unit cell parameters of the complex are a = 9.494(1) ?, b = 10.479(1) ?, c = 12.474(2) ?, α = 84.55(1)°, β = 70.08(1)°, γ = 70.72(1)°, V = 1255.8(4), ?³, space group P , Z = 2; C₁₉H₁₀O₁₀Os₃; d _calc = 2.922 g/cm³, 3085 I _hkl > 2σ _I of 3611 collected reflections; R = 0.0252. The structure of Os₃(μ,η²-O=CC₆H₅)(η³-C₃H₅)(CO)₉ is molecular. The plane of the Os₃ triangle and the OsCOOs plane are connected according to the “butterfly” principle with an angle of 103.4° between them. The Os-Os distances in the cluster core vary from 2.836(1) ? to 2.844(1) ?; the Os-C_carb distances are 1.88(1)–1.97(1) ?; the distances to the atoms of the bridging ligands are Os-C 2.11(1) ?, Os-O 2.14(1) ?; the O-C bridging bond is 1.24(1) ?. of the Os₃(μ,η²-O=CC₆H₅)(η³-C₃H₅)(CO)₉ triosmium cluster were studied theoretically. The potential curve of the internal rotation of the allyl ligand relative to the Os(1)-C(9) bond was determined. The rotation barrier of the allyl ligand in crystal relative to the Os(1)-C(9) bond is 8.38 kJ/mol, and the rotation of the ligand is not hindered. The effects of the intra-and intermolecular interactions on the conformation state of the cluster complex are considered. Original Russian Text Copyright ? 2008 by V. A. Maksakov, N. V. Pervukhina, N. V. Podberezskaya, M. Yu. Afonin, V. A. Potemkin, and V. P. Kirin __________ Translated from Zhurnal Strukturnoi Khimii, Vol. 49, No. 5, pp. 926–932, September–October, 2008.