Theoretical study of the excited‐state double proton transfer in the (3‐methyl‐7‐azaindole)‐(7‐azaindole) heterodimer

Excited‐state double proton transfer (ESDPT) in the (3‐methyl‐7‐azaindole)‐(7‐azaindole) heterodimer is theoretically investigated by the long‐range corrected time‐dependent density functional theory method and the complete‐active‐space second‐order perturbation theory method. The calculated potential energy profiles exhibit a lower barrier for the concerted mechanism in the locally excited state than for the stepwise mechanism through the charge‐transfer state. This result suggests that the ESDPT in the isolated heterodimer is likely to follow the former mechanism, as has been exhibited for the ESDPT in the homodimer of 7‐azaindole. © 2012 Wiley Periodicals, Inc.     

Cyano Analogues of 7‐Azaindole: Probing Excited‐State Charge‐Coupled Proton Transfer Reactions in Protic Solvents

The interplay between excited‐state charge and proton transfer reactions in protic solvents is investigated in a series of 7‐azaindole (7AI) derivatives: 3‐cyano‐7‐azaindole (3CNAI), 5‐cyano‐7‐azaindole (5CNAI), 3,5‐dicyano‐7‐azaindole (3,5CNAI) and dicyanoethenyl‐7‐azaindole (DiCNAI). Similar to 7AI, 3CNAI and 3,5CNAI undergo methanol catalyzed excited‐state double proton transfer (ESDPT), resulting in dual (normal and proton transfer) emission. Conversely, ESDPT is prohibited for 5CNAI and DiCNAI in methanol, as supported by a unique normal emission with high quantum efficiency. Instead, the normal emission undergoes prominent solvatochromism. Detailed relaxation dynamics and temperature dependent studies are carried out. The results conclude that significant excited‐state charge transfer (ESCT) takes place for both 5CNAI and DiCNAI. The charge‐transfer specie possesses a different dipole moment from that of the proton‐transfer tautomer species. Upon reaching the equilibrium polarization, there exists a solvent‐polarity induced barrier during the proton‐transfer tautomerization, and ESDPT is prohibited for 5CNAI and DiCNAI during the excited‐state lifespan. The result is remarkably different from 7AI, which is also unique among most excited‐state charge/proton transfer coupled systems studied to date.     

Excited-state double-proton transfer in the 7-azaindole dimer in the gas phase. 2. Cooperative nature of double-proton transfer revealed by H/D kinetic isotopic effects

The dispersed fluorescence (DF) spectra of the 7-azaindole dimer (7AI2) and deuterated dimers 7AI2-hd and 7AI2-dd, where hd and dd indicate the deuteration of an imino proton and two imino protons, have been measured in a supersonic free jet expansion. The undeuterated 7AI2-hh dimer exhibits only the tautomer fluorescence, but both the normal and tautomer fluorescence have been detected by exciting the origins of 7AI2-h*d, 7AI2-hd* and 7AI2-dd in the S1-S0 region, where h* and d* indicate the localization of the excitation on 7AI-h or 7AI-d moiety. The DF spectra indicate that 7AI2-h*d and 7AI2-hd* undergo excited-state proton/deuteron transfer (ESPDT), while excited-state double-deuteron transfer (ESDDT) occurs in 7AI2-dd. The H/D kinetic isotopic effects on ESDPT have been investigated by measuring the intensity ratios of the normal fluorescence to the tautomer fluorescence. The ESPDT rate is about 1/60th of the ESDPT rate, and the ESDDT rate is about 1/12th of the ESPDT rate, where ESPDT rate is an average of the rates for 7AI2-h*d and 7AI2-hd*. The observed H/D kinetic isotope effects imply that the ESDPT reaction of 7AI2 has a "cooperative" nature; i.e., the motion of the two moving protons strongly couples each other through the electron motions. The difference in the estimated ESPDT reaction rates, 9.8 x 10(9) and 6.9 x 109 s(-1) for 7AI2-h*d and 7AI2-hd*, respectively, is consistent with the concerted mechanism rather than the stepwise mechanism.     

Theoretical Study on the Substituent Effect on the Excited‐State Proton Transfer in the 7‐Azaindole‐Water Derivatives

The first excited‐state proton transfer (ESPT ) in 7AI ‐H₂O complex and its derivatives, in which the hydrogen atom at the C₂ position in pyrrole ring was replaced by halogen atom X (X = F, Cl, Br), were studied at the TD ‐M06‐2X/6‐31 + G(d, p) level. The double proton transfer took place in a concerted but asynchronous protolysis pathway. The vibrational‐mode selectivity of excited‐state double proton transfer in the model system was confirmed. The specific vibrational‐mode could shorten the reaction path and accelerate the reaction rate. The substituent effects on the excited‐state proton transfer process were discussed. When the H atom at C₂ position in 7AI ‐H₂O complex was replaced by halogen atom, some geometrical parameters changed obviously, the barrier height of ESDPT reduced, and the asynchronicity of proton transfer enlarged. The above changes correlated with the Pauling electronegativity of halogen atom.     

The Host/Guest Type of Excited‐State Proton Transfer; a General Review

Contemporary progress regarding guest/host types of excited‐state double proton transfer has been reviewed, among which are the biprotonic transfer within doubly H‐bonded host/guest complexes, the transfer through a solvent bridge relay, the intramolecular double proton transfer and solvation dynamics coupled proton transfer. Of particular emphases are the photophysical and photochemical properties of excited‐state double proton transfer (ESDPT) in 7‐azaindole and its corresponding analogues. From the chemical aspect, two types of ESDPT reaction, namely the catalytic and non‐catalytic types of ESDPT, have been classified and reviewed separately. For the case of static host/guest hydrogen‐bonded complexes both hydrogen‐bonding strength and configuration (i.e. geometry) play key roles in accounting for the reaction dynamics. In addition to the dynamical concern, excited‐state thermodynamics are of importance to fine‐tune the proton transfer reaction in the non‐catalytic host/guest type of ESDPT. The mechanisms of protic solvent assisted ESDPT, depending on host molecules and proton‐transfer models, have been reviewed where the plausible resolution is deduced. Particular attention has been given to the excited‐state proton transfer dynamics in pure water, aiming at its future perspective in biological applications. Finally, the differentiation in mechanism between solvent diffusive reorganization and solvent relaxation to affect the host/guest ESPT dynamics is made and discussed in de tail.     

Excited-state double-proton transfer in the 7-azaindole dimer in the gas phase. 3. Reaction mechanism studied by picosecond time-resolved REMPI spectroscopy

The excited-state double-proton-transfer (ESDPT) reaction in the jet-cooled 7-azaindole dimer (7AI2) has been investigated with picosecond time-resolved resonance-enhanced multiphoton ionization spectroscopy. The observed decay profiles of 7AI2 by exciting the origin and the intermolecular stretch fundamental in the S1 state are well reproduced by single-exponential functions with time constants of 1.9 +/- 0.9 ps and 860 +/- 300 fs, respectively. This result provides clear evidence of the concerted mechanism of ESDPT in 7AI2.     

η5‐(3)‐1‐Methyl‐1,2‐dicarbollyl‐η5‐2′,5′‐dimethylpyrrolylcobalt(III)

In the title compound, (η⁵‐2,5‐di­methyl­pyrrolyl)[(7,8,9,10,11‐η)‐7‐methyl‐7,8‐dicarba‐nido‐undecaborato]­cobalt(III), [3‐Co{η⁵‐[2,5‐(CH₃)₂‐NC₄H₂]}‐1‐CH₃‐1,2‐C₂B₉H₁₀] or [Co(C₃H₁₃B₉)(C₆H₈N)], the Co^III atom is sandwiched between the pentagonal faces of the pyrrolyl and dicarbollide ligands, resulting in a neutral mol­ecule. The C—C distance in the dicarbollide cage is 1.649 (3) Å.     

Excited-state hydrogen relay along a blended-alcohol chain as a model system of a proton wire: deuterium effect on the reaction dynamics

The excited-state deuteron transfer (ESDT) of deuterated 7-hydroxyquinoline (7DQ) along a heterogeneous hydrogen (H)-bonded chain composed of two deuterated alcohol (ROD) molecules having different acidities, as a model system of a proton wire consisting of diverse amino acids, has been investigated. To understand dynamic differences between deuteron transfer and proton transfer, solvent-inventory experiments have been performed with variation of the combination as well as the composition of alcohols in a H-bonded mixed-alcohol chain. Deuteron transfer from the adjacent ROD molecule to the basic imino group of 7DQ via tunneling, which is the rate-determining step, initiates ESDT, and subsequent barrierless deuteron relay from the acidic enolic group of 7DQ to the alkoxide moiety along the H-bonded chain completes ESDT. Whereas the acceleration of the reaction has been observed in excited-state proton transfer because of the accumulated proton-donating abilities of two alcohol molecules in a H-bonded chain by a push-ahead effect, such acceleration is not observed in ESDT. Because the energy barrier of deuteron relay is much higher than that of proton relay due to the low zero-point energy of 7DQ·(ROD)(2) and a deuteron is twice as heavy as a proton, it is hard for a deuteron to pass through the barrier via tunneling. Moreover, both the H-bonding ability and the acidity of ROD molecules are so weak that their deuteron-donating abilities cannot be accumulated at the rate-determining step of ESDT. Consequently, the rate constant of ESDT is determined mostly by the acidity of the ROD molecule H-bonded directly to the imino group of 7DQ.     

Synthesis,in vitro urease inhibitory activity and molecular docking of 3,5‐disubstituted thiadiazine‐2‐thiones

A series of 3,5‐disubstituted‐tetrahydro‐thiadiazine‐2‐thione ( 1 ‐ 16 ) have been synthesized, characterized by elemental analysis, infrared (IR), UV‐visible, ¹H NMR, ¹³C NMR, and MS spectroscopic techniques, and screened against jack bean urease. Among 16 compounds, compounds ( 1 ), ( 2 ), ( 3 ), ( 4 ), ( 6 ), ( 7 ), and ( 9 ) demonstrated excellent urease inhibitory activity with IC₅₀ values (9.8 ± 0.5, 11.0 ± 0.6, 16.0 ± 1.5, 17.2 ± 0.5, 15.4 ± 0.5, 19.7 ± 0.4, and 15.8 ± 0.2μM), respectively, even better than the standard thiourea (IC₅₀ = 21 ± 0.01μM). However, compound ( 8 ) shows an almost same level of inhibition (IC₅₀ = 22.9 ± 0.3μM), as like standard. In this work, we reported for the first time urease inhibitory activity of thiadiazine thiones and its molecular docking studies.     

Three‐dimensional networks in 5‐methylimidazolium 3‐carboxy‐4‐hydroxybenzenesulfonate and bis(5‐methylimidazolium) 3‐carboxylato‐4‐hydroxybenzenesulfonate

The two title proton‐transfer compounds, 5‐methylimidazolium 3‐carboxy‐4‐hydroxybenzenesulfonate, C₄H₇N₂⁺·C₇H₅O₆S⁻, (I), and bis(5‐methylimidazolium) 3‐carboxylato‐4‐hydroxybenzenesulfonate, 2C₄H₇N₂⁺·C₇H₅O₆S²⁻, (II), are each organized into a three‐dimensional network by a combination of X—H...O (X = O, N or C) hydrogen bonds, and π–π and C—H...π interactions.     

Proton‐transfer compounds of 8‐hydroxy‐7‐iodoquinoline‐5‐sulfonic acid (ferron) with 4‐chloroaniline and 4‐bromoaniline

The crystal structures of the proton‐transfer compounds of ferron (8‐hydroxy‐7‐iodoquinoline‐5‐sulfonic acid) with 4‐chloroaniline and 4‐bromoaniline, namely 4‐chloroanilinium 8‐hydroxy‐7‐iodoquinoline‐5‐sulfonate monohydrate, C₆H₇ClN⁺·C₉H₅INO₄S⁻·H₂O, and 4‐bromoanilinium 8‐hydroxy‐7‐iodoquinoline‐5‐sulfonate monohydrate, C₆H₇BrN⁺·C₉H₅INO₄S⁻·H₂O, have been determined. The compounds are isomorphous and comprise sheets of hydrogen‐bonded cations, anions and water molecules which are extended into a three‐dimensional framework structure through centrosymmetric R₂²(10) O—H...N hydrogen‐bonded ferron dimer interactions.     

Thermodynamic Stability and Physicochemical Characterization of Ligand (4S)‐4‐Benzyl‐3,6,10‐tris(carboxymethyl)‐3,6,10‐triazadodecanedioic Acid (H5[(S)‐4‐Bz‐ttda]) and Its Complexes Formed with Lanthanides,Calcium(II), Zinc(II), and Copper(II) Ions

A derivative of H₅ttda (=3,6,10‐tris(carboxymethyl)‐3,6,10‐triazadodecanedioic acid=N‐{2‐[bis(carboxymethyl)amino]ethyl}‐N‐{3‐[bis(carboxymethyl)amino]propyl}glycine), H₅[(S)‐4‐Bz‐ttda] (=(4S)‐4‐benzyl‐3,6,10‐tris(carboxymethyl)‐3,6,10‐triazadodecanedioic acid=N‐{(2S)‐2‐[bis(carboxymethyl)amino]‐3‐phenylpropyl}‐N‐{3‐[bis(carboxymethyl)amino]propyl}glycine; 1 ) carrying a benzyl group was synthesized and characterized. The stability constants of the complexes formed with Ca²⁺, Zn²⁺, Cu²⁺, and Gd³⁺ were determined by potentiometric methods at 25.0±0.1° and 0.1M ionic strength in Me₄NNO₃. The observed water proton relaxivity value of [Gd{(S)‐4‐Bz‐ttda}]²⁻ was constant with respect to pH changes over the range pH 4.5–12.0. From the ¹⁷O‐NMR chemical shift of H₂O induced by [Dy{(S)‐4‐Bz‐ttda}]²⁻ at pH 6.80, the presence of 0.9 inner‐sphere water molecules was deduced. The water proton spin‐lattice relaxation rate for [Gd{(S)‐4‐Bz‐ttda}]²⁻ at 37.0±0.1° and 20 MHz was 4.90±0.05 mM ⁻¹ s⁻¹. The EPR transverse electronic relaxation rate and ¹⁷O‐NMR transverse‐relaxation time for the exchange lifetime of the coordinated H₂O molecule (τ_M), and ²H‐NMR longitudinal‐relaxation rate of the deuterated diamagnetic lanthanum complex for the rotational correlation time (τ_R) were thoroughly investigated, and the results were compared with those previously reported for the other lanthanide(III) complexes. The exchange lifetime (τ_M) for [Gd{(S)‐4‐Bz‐ttda}]²⁻ (2.3±1.3 ns) was significantly shorter than that of the [Gd(dtpa)(H₂O)]²⁻ complex (dtpa=diethylenetriaminepentaacetic acid). The rotational correlation time τ_R for [Gd{(S)‐4‐Bz‐ttda}]²⁻ (70±6 ps) was slightly longer than that of the [Gd(dtpa)(H₂O)]²⁻ complex. The marked increase of relaxivity of [Gd{(S)‐4‐Bz‐ttda}]²⁻ mainly resulted from its longer rotational time rather than from its fast water‐exchange rate. The noncovalent interaction between human serum albumin (HSA) and the [Gd{(S)‐4‐Bz‐ttda}]²⁻ complex containing the hydrophobic substituent was investigated by measuring the solvent proton relaxation rate of the aqueous solutions. The association constant (K_A) was less than 100 M ⁻¹, indicating a weaker interaction of [Gd{(S)‐4‐Bz‐ttda}]²⁻ with HSA.     

Excited-State Double Proton Transfer in 1-Azacarbazole Hydrogen Bonded Complexes

Excited-state double proton transfer (ESDPT) has been studied in a variety of 1-azacarbazole (1AC) hosted hydrogen-bonded complexes. In 1 AC/carboxylic acids hydrogen bonded complexes, large association constants of > 1.0 × 10⁴ M^?1 are measured in the ground state and the rate of ESDPT is » 5.0 × 10⁹ s^?1, resulting in a dominant proton-transfer tautomer emission. In several 1 AC/lactam hydrogen bonded complexes, however, spectral and dynamic results show the existence of a fast excited-state equilibrium between normal and proton-transfer tautomer states. The result can be tentatively rationalized by a non-catalytic ESDPT mechanism incorporating tautomerization energy of the guest molecule.     

Covalent‐Bond Formation in the Course of the Inhibition of δ‐Chymotrypsin with trans‐Decalin‐Type Organophosphates: 31P‐NMR Evidence

Earlier investigations have shown that the irreversible inhibition of δ‐chymotrypsin with the axially substituted trans‐3‐(2,4‐dinitrophenoxy)‐2,4‐dioxa‐3λ⁵‐phosphabicyclo[4.4.0]decan‐3‐one (=2‐(2,4‐dinitrophenoxy)hexahydro‐4H‐1,3,2‐benzodioxaphosphorin 2‐oxide) proceeds under inversion of the configuration at the P‐atom. Since this assignment is based on the comparison of the respective chemical shifts with model compounds, the covalent nature of the binding interaction between enzyme and inhibitor was formulated in analogy. To prove this assumption, inhibition experiments were performed with the deuterated inhibitor (±)‐trans‐3‐(2,4‐dinitrophenoxy)‐2,4‐dioxa‐3λ⁵‐phospha(1,5,5‐²H₃)bicyclo[4.4.0]decan‐3‐one ((±)‐ 6a ). ³¹P{²H}‐NMR‐Spectroscopic monitoring of the reaction of stoichiometric amounts of the enzyme with (±)‐ 6a at pH 7.8 yielded the diastereoisomeric adducts 9 (−3.88 ppm) and 9′ (−3.96 ppm). Comparing the ³¹P chemical shifts of the corresponding deuterated covalent phosphoserine model compounds 8a/8a′ (−6.70 ppm, axial) and 8b/8b′ (−4.11/−4.13 ppm, equatorial) confirmed the inversion of the configuration at the P‐atom. ¹H‐Correlated ³¹P{²H}‐NMR spectra revealed a cross peak of the Ser¹⁹⁵‐H₂ (4.45 ppm) with the P‐atom of the inhibitor at −3.88/−3.96 ppm, thus establishing the covalent nature of the Ser¹⁹⁵−O−P bond.     

Excited‐State Proton‐Relay Dynamics of 7‐Hydroxyquinoline Embedded in a Solid Matrix of Poly(2‐hydroxyethyl methacrylate)

Excited 7‐hydroxyquinoline embedded in a solid matrix of poly(2‐hydroxyethyl methacrylate) undergoes a proton‐relay reaction efficiently to form its keto tautomer. However, the reaction mechanism depends on the torsional conformation and the microscopic environment of the molecule at the moment of excitation. Whereas the bridged cis‐enol form undergoes proton relay immediately on absorption of a photon to produce its tautomeric keto species, the unbridged cis form requires 120 ps for bridge formation via solvent reorganization prior to proton relay. Furthermore, the trans form needs 1000 ps for tautomerization because it requires an activated (11 kJ mol^?1) torsional motion to change into its cis form prior to bridge formation and proton relay. Torsional motion rather than solvent reorganization determines the proton relay rate of the trans‐form of the molecule.     

Metabolism of Deuterated Isomeric 6,7‐Dihydroxydodecanoic Acids in Saccharomyces cerevisiae – Diastereo‐ and Enantioselective Formation and Characterization of 5‐Hydroxydecano‐4‐lactone (=4,5‐Dihydro‐5‐(1‐hydroxyhexyl)furan‐2(3H)‐one) Isomers

The chemical synthesis of deuterated isomeric 6,7‐dihydroxydodecanoic acid methyl esters 1 and the subsequent metabolism of esters 1 and the corresponding acids 1a in liquid cultures of the yeast Saccharomyces cerevisiae was investigated. Incubation experiments with (6R,7R)‐ or (6S,7S)‐6,7‐dihydroxy(6,7‐²H₂)dodecanoic acid methyl ester ((6R,7R)‐ or (6S,7S)‐(6,7‐²H₂)‐ 1 , resp.) and (±)‐threo‐ or (±)‐erythro‐6,7‐dihydroxy(6,7‐²H₂)dodecanoic acid ((±)‐threo‐ or (±)‐erythro‐(6,7‐²H₂)‐ 1a , resp.) elucidated their metabolic pathway in yeast (Tables 1–3). The main products were isomeric ²H‐labeled 5‐hydroxydecano‐4‐lactones 2 . The absolute configuration of the four isomeric lactones 2 was assigned by chemical synthesis via Sharpless asymmetric dihydroxylation and chiral gas chromatography (Lipodex ^® E). The enantiomers of threo‐ 2 were separated without derivatization on Lipodex ^® E; in contrast, the enantiomers of erythro‐ 2 could be separated only after transformation to their 5‐O‐(trifluoroacetyl) derivatives. Biotransformation of the methyl ester (6R,7R)‐(6,7‐²H₂)‐ 1 led to (4R,5R)‐ and (4S,5R)‐(2,5‐²H₂)‐ 2 (ratio ca. 4 : 1; Table 2). Estimation of the label content and position of (4S,5R)‐(2,5‐²H₂)‐ 2 showed 95% label at C(5), 68% label at C(2), and no ²H at C(4) (Table 2). Therefore, oxidation and subsequent reduction with inversion at C(4) of 4,5‐dihydroxydecanoic acid and transfer of ²H from C(4) to C(2) is postulated. The 5‐hydroxydecano‐4‐lactones 2 are of biochemical importance: during the fermentation of Streptomyces griseus, (4S,5R)‐ 2 , known as L‐factor, occurs temporarily before the antibiotic production, and (?)‐muricatacin (=(4R,5R)‐5‐hydroxy‐heptadecano‐4‐lactone), a homologue of (4R,5R)‐ 2 , is an anticancer agent.     

Three Modes of Proton Transfer in One Chromophore: Photoinduced Tautomerization in 2‐(1H‐Pyrazol‐5‐yl)Pyridines,Their Dimers and Alcohol Complexes

Studies of 2‐(1H‐pyrazol‐5‐yl)pyridine (PPP) and its derivatives 2‐(4‐methyl‐1H‐pyrazol‐5‐yl)pyridine (MPP) and 2‐(3‐bromo‐1H‐pyrazol‐5‐yl)pyridine (BPP) by stationary and time‐resolved UV/Vis spectroscopic methods, and quantum chemical computations show that this class of compounds provides a rare example of molecules that exhibit three types of photoreactions: 1) excited‐state intramolecular proton transfer (ESIPT) in the syn form of MPP, 2) excited‐state intermolecular double‐proton transfer (ESDPT) in the dimers of PPP in nonpolar media, as well as 3) solvent‐assisted double‐proton transfer in hydrogen‐bonded 1:1 complexes of PPP and MPP with alcoholic partners. The excited‐state processes are manifested by the appearance of a dual luminescence and a bimodal irreversible kinetic coupling of the two fluorescence bands. Ground‐state syn–anti equilibria are detected and discussed. The fraction of the higher‐energy anti form varies for different derivatives and is strongly dependent on the solvent polarity and hydrogen‐bond donor or acceptor abilities.     

Temperature‐induced pseudopolymorphism of molecular salts from a pyridyl bis‐urea macrocycle and naphthalene‐1,5‐disulfonic acid

Molecular salts, often observed as cocrystals, play an important role in the fields of pharmaceutics and materials science, where salt formation is used to tune the properties of active pharmaceutical ingredients (APIs) and improve the stability of solid‐state materials. Salt formation via a proton‐transfer reaction typically alters hydrogen‐bonding motifs and influences supramolecular assembly patterns. We report here the molecular salts formed by the pyridyl bis‐urea macrocycle 3,5,13,15,21,22‐hexaazatricyclo[15.3.1.1^7,11]docosa‐1(21),7(22),8,10,17,19‐hexaene‐4,14‐dione, ( 1 ), and naphthalene‐1,5‐disulfonic acid (H₂NDS) as two salt cocrystal solvates, namely 4,14‐dioxo‐3,5,13,15,21,22‐hexaazatricyclo[15.3.1.1^7,11]docosa‐1(21),7(22),8,10,17,19‐hexaene‐21,22‐diium naphthalene‐1,5‐disulfonate dimethyl sulfoxide disolvate, C₁₆H₂₀N₆O₂²⁺·C₁₀H₆O₆S₂²⁻·2C₂H₆OS, ( 2 ), and the corresponding monosolvate, C₁₆H₂₀N₆O₂²⁺·C₁₀H₆O₆S₂²⁻·C₂H₆OS, ( 3 ). This follows the ΔpK_a rule such that there is a proton transfer from H₂NDS to ( 1 ), forming the reported molecular salts through hydrogen bonding. Prior to salt formation, ( 1 ) is relatively planar and assembles into columnar structures. The salt cocrystal solvates were obtained upon slow cooling of dimethyl sulfoxide–acetonitrile solutions of the molecular components from two temperatures (363 and 393 K). The proton transfer to ( 1 ) significantly alters the conformation of the macrocycle, changing the formerly planar macrocycle into a step‐shaped conformation with trans–cis urea groups in ( 2 ) or into a bowl‐shape conformation with trans–trans urea groups in ( 3 ).     

Hierarchy of hydrogen bonding in bis­(1,4,7‐trioxa‐10‐azoniacyclo­dodecane) bis­(4‐amino­benzoate) trihydrate

In the title compound, 2C₈H₁₈NO₃⁺·2C₇H₆NO₂⁻·3H₂O, proton transfer occurs from the carboxylic acid group of the 4‐amino­benzoic acid (PABA) mol­ecule to the amine group of the macrocycle, resulting in the formation of a salt‐like adduct. The anions are combined into helical chains which are further bound by the water mol­ecules into sheets. The macrocyclic cations are situated between these layers and are bound to the anions both directly and via bridging water mol­ecules. The structure exhibits a diverse system of hydrogen bonding.     

Polar,functionalized diene‐based materials. V. Free‐radical polymerization of 2‐[(N‐benzyl‐n‐methylamino)methyl]‐1,3‐butadiene and copolymerization with styrene

2‐[(N‐Benzyl‐N‐methylamino)methyl]‐1,3‐butadiene (BMAMBD), the first asymmetric tertiary amino‐containing diene‐based monomer, was synthesized by sulfone chemistry and a nickel‐catalyzed Grignard coupling reaction in high purity and good yield. The bulk and solution free‐radical polymerizations of this monomer were studied. Traditional bulk free‐radical polymerization kinetics were observed, giving polymers with 〈M_n〉 values of 21 × 10³ to 48 × 10³ g/mol (where M_n is the number‐average molecular weight) and polydispersity indices near 1.5. In solution polymerization, polymers with higher molecular weights were obtained in cyclohexane than in tetrahydrofuran (THF) because of the higher chain transfer to the solvent. The chain‐transfer constants calculated for cyclohexane and THF were 1.97 × 10^?3 and 5.77 × 10^?3, respectively. To further tailor polymer properties, we also completed copolymerization studies with styrene. Kinetic studies showed that BMAMBD incorporated into the polymer chain at a faster rate than styrene. With the Mayo–Lewis equation, the monomer reactivity ratios of BMAMBD and styrene at 75 °C were determined to be 2.6 ± 0.3 and 0.28 ± 0.02, respectively. Altering the composition of BMAMBD in the copolymer from 17 to 93% caused the glass‐transition temperature of the resulting copolymer to decrease from 64 to ?7 °C. © 2001 John Wiley & Sons, Inc. J Polym Sci Part A: Polym Chem 39: 3227–3238, 2001