Synthesis and photochemical reaction of cyclic oligomers: Synthesis and photopolymerization of novel C‐methylcalix[4]resorcinarene and p‐alkylcalix[n]arene derivatives containing spiro ortho ether groups

New photoreactive calixarene derivatives containing spiro ortho ester groups (calixarenes 3a–3c ) were synthesized by the reaction of 2‐bromomethyl‐1,4,6‐trioxaspiro[4.4]nonane with 2,8,14,20‐tetramethyl‐4,6,10,12,16,18,22,24‐octakis(carboxymethoxy)calix[4]resorcinarene, 5,11,17,23,29,35‐hexamethyl‐37,38,39,40,41,42‐hexakis(carboxymethoxy)calix[6]arene, and 5,11,17,23,29,35,41,47‐octa‐tert‐butyl‐49,50,51,52, 53,54,55,56‐octakis‐(carboxymethoxy)calix[8]arene, which were prepared by the reaction of C‐methylcalix[4]resorcinarene, p‐methylcalix[6]arene, and p‐tert‐butylcalix[8]arene, respectively. The thermal stability of the obtained calixarene derivatives containing spiro ortho ester groups was examined with thermogravimetric analysis, and it was found that these calixarene derivatives had good thermal stability. The photoinitiated cationic polymerization of spiro ortho ester groups in calixarene derivatives 3a–3c was examined with certain photoacid generators in the film state. Interestingly enough, the reaction of calixarene derivatives did not proceed with only photoirradiation; however, the reaction proceeded smoothly when the photoirradiation was followed by heating. Furthermore, calixarene 3a , composed of a C‐methylcalix[4]resorcinarene structure, showed the highest photochemical reactivity in this reaction system. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 1293–1302, 2002     

Assembling an isomer grid: the isomorphous 4‐, 3‐ and 2‐fluoro‐N′‐(4‐pyridyl)benzamides

The three title isomers, 4‐, (I), 3‐, (II), and 2‐fluoro‐N′‐(4‐pyridyl)benzamide, (III), all C₁₂H₉FN₂O, crystallize in the P2₁/c space group (No. 14) with similar unit‐cell parameters and are isomorphous and isostructural at the primary hydrogen‐bonding level. An intramolecular C—H...O=C interaction is present in all three isomers [C...O = 2.8681 (17)–2.884 (2) Å and C—H...O117–118°], with an additional N—H...F [N...F = 2.7544 (15) Å] interaction in (III). Intermolecular amide–pyridine N—H...N hydrogen bonds link molecules into one‐dimensional zigzag chains [graph set C(6)] along the [010] direction as the primary hydrogen bond [N...N = 3.022 (2), 3.049 (2) and 3.0213 (17) Å]. These are augmented in (I) by C—H...π(arene) and cyclic C—F...π(arene) contacts about inversion centres, in (II) by C—F...F—C interactions [C...F = 3.037 (2) Å] and weaker C—H...π(arene)/C—H...F contacts, and in (III) by C—H...π(arene) and C=O...O=C interactions, linking the alternating chains into two‐dimensional sheets. Typical amide N—H...O=C hydrogen bonds [as C(4) chains] are not present [N...O = 3.438 (2) Å in (I), 3.562 (2) Å in (II) and 3.7854 (16) Å in (III)]; the C=O group is effectively shielded and only participates in weaker interactions/contacts. This series is unusual as the three isomers are isomorphous (having similar unit‐cell parameters, packing and alignment), but they differ in their interactions and contacts at the secondary level.     

Structure of a Discrete 8:6 La(Iii): P-Sulfonatocalix[4]Arene Complex

Abstract

The crystal structure of a complex formed between La(III) and p-sulfonatocalix[4]arene is reported. The complex cristallizes in the monoclinic space group P2₁/n, a = 15.091(1), b = 28.550(3), c = 30.167(3) Å, β = 90.008(2)°, V = 12997(2) ų, ρ_calc. (g cm^?3) = 1.792, Z = 2. Refinement led to R₁ value of 0.0961 for 13930 unique reflections. It forms discrete units with 8 La(III) cations coordinated by 6 calixarene ligands. The calixarenes are organized in an up-down configuration, bridged by La(III), forming layers of 3 repeating calixarene units and channels along the a axis. The units are linked by hydrogen-bonds.     

Synthesis,Structure, and Molecular Recognition of S6‐ and (SO2)6‐Corona[6](het)arenes: Control of Macrocyclic Conformation and Properties by the Oxidation State of the Bridging Heteroatoms

We report herein the synthesis, structure, and molecular recognition of S₆‐ and (SO₂)₆‐corona[6](het)arenes, and demonstrate a unique and efficient strategy of regulating macrocyclic conformation and properties by adjusting the oxidation state of the heteroatom linkages. The one‐pot nucleophilic aromatic substitution reaction of 1,4‐benzenedithiol derivatives, biphenyl‐4,4′‐dithiol and 9,9‐dipropyl‐9H‐fluorene‐2,7‐dithiol with 3,6‐dichlorotetrazine afforded S₆‐corona[3]arene[3]tetrazines. These compounds underwent inverse‐electron‐demand Diels–Alder reaction with enamines and norbornadiene to produce S₆‐corona[3]arene[3]pyridazines. Facile oxidation of sulfide linkages yielded (SO₂)₆‐corona[3]arene[3]pyridazines. All corona[6](het)arenes adopted generally hexagonal macrocyclic ring structures; however, their electronic properties and conformation could be fine‐tuned by altering the oxidation state of the sulfur linkages. Whereas (SO₂)₆‐corona[3]arene[3]pyridazines were electron‐deficient, S₆‐corona[3]arene[3]pyridazines acted as electron‐rich macrocyclic hosts that recognized various organic cations in both aqueous and organic solutions.     

Simultaneous endo and exo Complex Formation of Pyridine[4]arene Dimers with Neutral and Anionic Guests

The formation of complexes between hexafluorophosphate (PF₆⁻) and tetraisobutyloctahydroxypyridine[4]arene has been thoroughly studied in the gas phase (ESI‐QTOF‐MS, IM‐MS, DFT calculations), in the solid state (X‐ray crystallography), and in chloroform solution (¹H, ¹⁹F, and DOSY NMR spectroscopy). In all states of matter, simultaneous endo complexation of solvent molecules and exo complexation of a PF₆⁻ anion within a pyridine[4]arene dimer was observed. While similar ternary complexes are often observed in the solid state, this is a unique example of such behavior in the gas phase.     

Two related lithium calixarene complexes, [p‐tert‐butylcalix[4]arene(OMe)(OH)2(OLi)]2·4MeCN and {p‐tert‐butylcalix[4]arene(OH)2(OLi)[OLi(NCMe)2]}2·8MeCN,determined using synchrotron radiation

The crystal structures of acetonitrile solvates of two related lithium calixarene complexes have been determined by low‐temperature single‐crystal X‐ray diffraction using synchrotron radiation. Bis(μ‐5,11,17,23‐tetra‐tert‐butyl‐26,28‐dihydroxy‐25‐methoxy‐27‐oxidocalix[4]arene)dilithium(I) acetonitrile tetrasolvate, [Li₂(C₄₅H₅₇O₄)₂]·4C₂H₃N or [p‐tert‐butylcalix[4]arene(OMe)(OH)₂(OLi)]₂·4MeCN, (I), crystallizes with the complex across a centre of symmetry and with four molecules of unbound acetonitrile of crystallization per complex. Tetraacetonitrilebis(μ‐5,11,17,23‐tetra‐tert‐butyl‐26,28‐dihydroxy‐25,27‐dioxidocalix[4]arene)tetralithium(I) acetonitrile octasolvate, [Li₄(C₄₄H₅₄O₄)₂(C₂H₃N)₄]·8C₂H₃N or {p‐tert‐butylcalix[4]arene(OH)₂(OLi)[OLi(NCMe)₂]}₂·8MeCN, (II), also crystallizes with the complex lying across a centre of symmetry and contains eight molecules of unbound acetonitrile per complex plus four more directly bound to two of the lithium ions, two on each ion. The cores of both complexes are partially supported by O—H...O hydrogen bonds. The methoxy methyl groups in (I) prevent the binding of any more than two Li⁺ ions, while the corresponding two O‐atom sites in (II) bind an extra Li⁺ ion each, making four in total. The calixarene cone adopts an undistorted cone conformation in (I), but an elliptical one in (II).     

Complexation of methylparathion and bis(2‐hydroxyethyl)sulfide by the tridentate lewis acid [(o‐C6F4Hg)3]

The interaction of trimeric perflu‐ oro‐ortho‐phenylene mercury ( 1 ) with bis(2‐hydroxy‐ ethyl)sulfide (S((CH₂)₂OH)₂) in dichloromethane and methylparathion (SP(OMe)₂(p‐C₆H₄NO₂)) in 1,2‐dichloroethane leads to the crystallization of [ 1 ⋅ (S((CH₂)₂OH)₂)] and [ 1 ⋅ (μ₃‐SP(OMe)₂(p‐C₆H₄‐ NO₂))₂], respectively. These two adducts have been characterized by elemental analysis and single crystal X‐ray diffraction. The structure of [ 1 ⋅ S((CH₂)₂OH)₂] shows that the bis(2‐hydroxyethyl)sulfide molecule interacts with the mercury centers of 1 by formation of a Hg–S interaction of 3.138(4) Å. Association of the two components is further strengthened by the coordination of one of the oxygen atoms of the bis(2‐hydroxyethyl)sulfide molecule. This oxygen atom interacts simultaneously with three mercury centers of 1 with Hg–O distances ranging from 2.889(8) to 3.142(9) Å. In the lattice, molecules of [ 1 ⋅ (S((CH₂)₂OH)₂)] associate with compact cofacial dimers with Hg–Hg metallophilic contacts of 3.794 Å and 4.076 Å. The structure of [ 1 ⋅ (μ₃‐SP(OMe)₂(p‐C₆H₄NO₂))₂] is that of a 2:1 complex in which two molecules of methylparathion are triply coordinated via their sulfur atom to the mercury centers of 1 on either side of the molecular plane. The Hg–S contacts fall within the range of 3.278 and 3.651 Å. © 2005 Wiley Periodicals, Inc. 16:292–297, 2005; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/hc.20125     

In‐ and Out‐of‐Cavity Interactions by Modulating the Size of Ruthenium Metallarectangles

Cationic (arene)ruthenium‐based tetranuclear complexes of the general formula [Ru₄(η⁶‐p‐cymene)₄(μ‐N∩N)₂(μ‐OO∩OO)₂]⁴⁺ were obtained from the dinuclear (arene)ruthenium complexes [Ru₂(η⁶‐p‐cymene)₂(μ‐OO∩OO)₂Cl₂] (p‐cymene=1‐methyl‐4‐(1‐methylethyl)benzene, OO∩OO=5,8‐dihydroxy‐1,4‐naphthoquinonato(2?), 9,10‐dihydroxy‐1,4‐anthraquinonato(2?), or 6,11‐dihydroxynaphthacene‐5,12‐dionato(2?)) by reaction with pyrazine or bipyridine linkers (N∩N=pyrazine, 4,4′‐bipyridine, 4,4′‐[(1E)‐ethene‐1,2‐diyl]bis[pyridine]) in the presence of silver trifluoromethanesulfonate (CF₃SO₃Ag) (Scheme). All complexes 4 – 12 were isolated in good yield as CF₃SO salts, and characterized by NMR and IR spectroscopy. The host–guest properties of the metallarectangles incorporating 4,4′‐bipyridine and (4,4′‐[(1E)‐ethene‐1,2‐diyl]bis[pyridine] linkers were studied in solution by means of multiple NMR experiments (1D, ROESY, and DOSY). The largest metallarectangles 10 – 12 incorporating (4,4′‐[(1E)‐ethene‐1,2‐diyl]bis[pyridine] linkers are able to host an anthracene, pyrene, perylene, or coronene molecule in their cavity, while the medium‐size metallarectangles 7 – 9 incorporating 4,4′‐bipyridine linkers are only able to encapsulate anthracene. However, out‐of‐cavity interactions are observed between these 4,4′‐bipyridine‐containing rectangles and pyrene, perylene, or coronene. In contrast, the small pyrazine‐containing metallarectangles 4 – 6 show no interaction in solution with this series of planar aromatic molecules.     

A structural systematic study of three isomers of difluoro‐N‐(4‐pyridyl)benzamide

The isomers 2,3‐, (I), 2,4‐, (II), and 2,5‐difluoro‐N‐(4‐pyridyl)benzamide, (III), all with formula C₁₂H₈F₂N₂O, all exhibit intramolecular C—H...O=C and N—H...F contacts [both with S(6) motifs]. In (I), intermolecular N—H...O=C interactions form one‐dimensional chains along [010] [N...O = 3.0181 (16) Å], with weaker C—H...N interactions linking the chains into sheets parallel to the [001] plane, further linked into pairs via C—H...F contacts about inversion centres; a three‐dimensional herring‐bone network forms via C—H...π(py) (py is pyridyl) interactions. In (II), weak aromatic C—H...N(py) interactions form one‐dimensional zigzag chains along [001]; no other interactions with H...N/O/F < 2.50 Å are present, apart from long N/C—H...O=C and C—H...F contacts. In (III), N—H...N(py) interactions form one‐dimensional zigzag chains [as C(6) chains] along [010] augmented by a myriad of weak C—H...π(arene) and O=C...O=C interactions and C—H...O/N/F contacts. Compound (III) is isomorphous with the parent N‐(4‐pyridyl)benzamide [Noveron, Lah, Del Sesto, Arif, Miller & Stang (2002). J. Am. Chem. Soc. 124 , 6613–6625] and the three 2/3/4‐fluoro‐N‐(4‐pyridyl)benzamides [Donnelly, Gallagher & Lough (2008). Acta Cryst. C 64 , o335–o340]. The study expands our series of fluoro(pyridyl)benzamides and augments our understanding of the competition between strong hydrogen‐bond formation and weaker influences on crystal packing.     

Bis‐tert‐Alcohol‐Functionalized Crown‐6‐Calix[4]arene: An Organic Promoter for Nucleophilic Fluorination

A bis‐tert‐alcohol‐functionalized crown‐6‐calix[4]arene (BACCA) was designed and prepared as a multifunctional organic promoter for nucleophilic fluorinations with CsF. By formation of a CsF/BACCA complex, BACCA could release a significantly active and selective fluoride source for S_N2 fluorination reactions. The origin of the promoting effects of BACCA was studied by quantum chemical methods. The role of BACCA was revealed to be separation of the metal fluoride to a large distance (>8 Å), thereby producing an essentially “free” F^?. The synergistic actions of the crown‐6‐calix[4]arene subunit (whose O atoms coordinate the counter‐cation Cs⁺) and the terminal tert‐alcohol OH groups (forming controlled hydrogen bonds with F^?) of BACCA led to tremendous efficiency in S_N2 fluorination of base‐sensitive substrates.     

杯芳烃接枝超细SiO2的制备及其对Cu2+、Ag+萃取性能的研究

通过对杯[4]芳烃以及杯[6]芳烃上缘进行烯丙基化和硅氢加成2步衍生化反应得到硅氢化杯[4]芳烃以及硅氢化杯[6]芳烃,再将这2种硅氢化杯芳烃衍生物分别接枝到超细SiO₂上。在不同的pH值条件下,就2种杯芳烃接枝超细SiO₂衍生物对水合Cu²⁺及Ag⁺的萃取性能进行了研究。结果表明,与对叔丁基杯芳烃相比较,杯芳烃接枝超细SiO₂衍生物对Cu²⁺和Ag⁺的萃取率都有所提高,其中杯[4]芳烃接枝超细SiO₂对Ag⁺的最高萃取率达到98.78%,杯[6]芳烃接枝超细SiO₂对Cu²⁺的最高萃取率达到67.74%。     

Complexation Between (O‐Methyl)6‐2,6‐Helic[6]arene and Tertiary Ammonium Salts: Acid/Base‐ or Chloride‐Ion‐Responsive Host–Guest Systems and Synthesis of [2]Rotaxane

Complexation between (O ‐methyl)₆‐2,6‐helic[6]arene and a series of tertiary ammonium salts was described. It was found that the macrocycle could form stable complexes with the tested aromatic and aliphatic tertiary ammonium salts, which were evidenced by ¹H NMR spectra, ESI mass spectra, and DFT calculations. In particular, the binding and release process of the guests in the complexes could be efficiently controlled by acid/base or chloride ions, which represents the first acid/base‐ and chloride‐ion‐responsive host–guest systems based on macrocyclic arenes and protonated tertiary ammonium salts. Moreover, the first 2,6‐helic[6]arene‐based [2]rotaxane was also synthesized from the condensation between the host–guest complex and isocyanate.     

Dimeric Calixarenes: A New Family of Major‐Groove Binders

A new class of potent DNA binding agents is presented. Dimeric calix[4]arenes with cationic groups at their upper rims and flexible alkyl bridges can be synthesized from triply acyl‐protected calix[4]arene tetramines in relatively short synthetic sequences (3–5 steps). The compounds attach themselves to double‐stranded nucleic acids in a noncovalent fashion, with micro‐ to nanomolar affinities. Guanidinium headgroups with their extended hydrogen‐bonding “fingers” are more powerful than ammonium groups, and the benzylamine series is superior to the anilinium series (see below). The new ligands easily distinguish between RNA and various DNA types, and produce characteristic changes in UV/Vis, fluorescence, CD, as well as NMR spectra. Especially extended oligonucleotides of more than 100 base pairs are bound with affinities increasing from RNA (10 μM K_d)<AT‐rich (1 μM )<GC‐rich DNA double strands (100–10 nM ). Ethidium bromide displacement studies confirm this order. CE₅₀ values are remarkably low (1–4 μM ), and are more than 300 times lower than that of spermine, which is a typical backbone binder. Stoichiometries are rather high (one calixarene dimer per two BP), suggesting a potential aggregation of bound ligands inside the major groove. Most UV/Vis melting curves display an inverted shape, and start from drastically enhanced absorption intensities for the DNA complexes. DAPI displacement studies prove that up to one equivalent of calixarene dimer can be accommodated in the dye‐loaded DNA. RNA complexation by calixarene dimers is accompanied by a drastic CD spectral transition from the typical A‐form to a perfect B‐signature, providing further experimental evidence for major‐groove binding. The orientation of the ligands can be deduced from NMR titrations and is reproduced in Monte‐Carlo simulations on 1:1 complexes in water.     

Structure and solution dynamics of a “P–N–P” ligand attached to calix[4]arene scaffold and its palladium dichloride complex

A new diphosphazane ligand, Ph₂PN(CHMe₂)P(Ph){1,3-dimethoxycalix[4]arene} (2), in which the P–N–P unit is appended to calix[4]arene backbone, and its PdCl₂ complex (3) have been synthesized. The structures of the conformers of both the compounds in solution have been elucidated with the help of two-dimensional NMR experiments. Single crystal X-ray diffraction studies reveal the partial cone and cone conformations of the calixarene unit in the ligand (2) and the Pd-complex (3), respectively.     

Encapsulation of Pyrene‐Functionalized Poly(benzyl ether) Dendrons into a Water‐Soluble Organometallic Cage

Two generations of lipophilic pyrenyl functionalized poly(benzyl ether) dendrimers (P₁ and P₂) have been synthesized. The thermal properties of the two functionalized dendrimers have been investigated, and the pyrenyl group of the dendritic molecules encapsulated in the arene–ruthenium metalla‐cage, [Ru₆(p‐cymene)₆(tpt)₂(donq)₃]⁶⁺ ([ 1 ]⁶⁺) (tpt=2,4,6‐tri(pyridin‐4‐yl)‐1,3,5‐triazine; donq=5,8‐dioxydo‐1,4‐naphthoquinonato). The host–guest properties of [P₁⊂ 1 ]⁶⁺ and [P₂⊂ 1 ]⁶⁺ were studied in solution by NMR and UV/Vis spectroscopic methods, thus allowing the determination of the affinity constants. Moreover, the cytotoxicity of these water‐soluble host–guest systems was evaluated on human ovarian cancer cells.     

Synthesis,Crystal Structure and Complexing Properties of Calix[4]arene Schiff Bases

Four p‐tert‐butylcalix[4]arene derivatives with different Schiff base groups at the lower rim were efficiently prepared in three steps. p‐tert‐Butylcalix[4]arene was firstly O‐peralkylated with ω‐haloalkylphthalimide in the system of NaH/DMF to give calixarene tetraalkylphthalimides, which were in turn hydrazinolyzed to give tetraaminoalkylcalixarenes. Then by condensation of the latter with salicylaldehyde and 2‐hydroxy‐1‐naphthaldehyde, a series of calixarene Schiff bases were obtained in satisfying yields. The complexing properties of these Schiff bases for transition metal ions were investigated with UV spectroscopy.     

Liquid chromatographic behavior of two alanine‐substituted calix[4]arene‐bonded silica gel stationary phases

Two new kinds of alanine‐substituted calix[4]arene stationary phases of 5,11,17,23‐p‐tert‐butyl‐25,27‐bis(l ‐alanine‐methylester‐N‐carbonyl‐methoxy)‐26,28‐dihyroxycalix[4]arene‐bonded silica gel stationary phase (BABS4) and 5, 11, 17, 23‐p‐tert‐butyl‐25,26,27,28‐tetra(l ‐alanine‐methylester‐N‐carbonyl‐methoxy)‐calix[4]arene‐bonded silica gel stationary phase (TABS4) were prepared and characterized in the present study. They were compared with each other and investigated in terms of their chromatographic performance by using polycyclic aromatic hydrocarbons, disubstituted benzene isomers, and mono‐substituted benzenes as solute probes. The results indicated that both BABS4 and TABS4 exhibited multiple interactions with analytes. In addition, the commonly used Tanaka characterization protocol for the evaluation of commercially available stationary phases was applied to evaluate the properties of these two new functionalized calixarene stationary phases. The Tanaka test results were compared with Zorbax Eclipse XDB C₁₈ and Kromasil phenyl columns, respectively. BABS4 has stronger hydrogen‐bonding capacity and ion‐exchange capacity than TABS4, and features weaker hydrophobicity and hydrophobic selectivity. Both of them behave similarly in stereoselectivity. Both BABS4 and TABS4 are weaker than C₁₈ and phenyl stationary phases in hydrophobicity and hydrophobic selectivity.     

Chlor(trifluorphosphan)gold(I): [Au(PF3)Cl]

Chloro(trifluorophosphane)gold(I): [Au(PF₃)Cl] X‐ray quality crystals of [Au(PF₃)Cl] (orthorhombic, Pnma) are obtained from a toluene / pentane solution at 6 °C. According to the result of the X‐ray structural analysis, [Au(PF₃)Cl] contains an almost linear F₃P‐Au‐Cl unit. The shortest Au‐Au contacts between two of these units are 3.3495(9) Å.     

Design and Synthesis of Calixarene‐Based Bis‐alkynyl‐Bridged Dinuclear AuI Isonitrile Complexes as Luminescent Ion Probes by the Modulation of Au⋅⋅⋅Au Interactions

Two calixarene‐based bis‐alkynyl‐bridged Au^I isonitrile complexes with two different crown ether pendants, [{calix[4]arene‐(OCH₂CONH‐C₆H₄C≡C)₂}{Au(CNR)}₂] (R=benzo[15]crown‐5 ( 1 ); R=benzo[18]crown‐6 ( 2 )), together with their related crown‐free analogue 3 (R=C₆H₃(OMe)₂‐3,4) and a mononuclear gold(I) complex 4 with benzo[15]crown‐5 pendant, have been designed and synthesized, and their photophysical properties have been studied. The X‐ray structure of the ligand, calix[4]arene‐(OCH₂CONH‐C₆H₄C?CH)₂ has been determined. The cation‐binding properties of these complexes with various metal ions have been studied using UV/Vis, emission, ¹H NMR, and ESI‐MS techniques, and DFT calculations. A new low‐energy emission band associated with Au???Au interaction could be switched on upon formation of the metal ion‐bound adduct in a sandwich fashion.     

Aminolysis reaction of calix [ 4 ] arene esters and crystal structures and conformational behaviors of calix[4]arene amides

We first make use of aminolysis of calix[4]arene esters to synthesize calix[4]arene amides. When the two ethyl esters of the calix[4]arene esters are aminolysized, the 1, 3-amide derivative is formed selectively. The crystal structures of the calix-[4]arene with two butyl amide (3b) and four butyl amide moieties (4b) were determined. The intermolecular hydrogen bonds make 4b form two-dimensional net work insolid state. The 1H NMR spectra prove that 3b is of a pinched cone conformation, while 4b and tetraheptylamide-calix[4]arene (6b) take fast interconversion between two C2v isomers in solution and appear an apparent cone conformation at room temperature. As decreasing temperature, the interconversion rate decreases gradually and, finally, the interconversion process is frozen at Tc = -10℃, which makes both conformations of 4b and 6b the pinched cone structures. The hydrogen bond improves the interconversion barrier, and the large different values of the potential barrier between 6b and 4b (or 6b) may