Mechanism of Framework Oxygen Exchange in Fe‐Zeolites: A Combined DFT and Mass Spectrometry Study

The role of framework oxygen atoms in N₂O decomposition [N₂O(g)→N₂(g) and 1/2O₂(g)] over Fe‐ferrierite is investigated employing a combined experimental (N₂¹⁸O decomposition in batch experiments followed by mass spectroscopy measurements) and theoretical (density functional theory calculations) approach. The occurrence of the isotope exchange indicates that framework oxygen atoms are involved in the N₂O decomposition catalyzed by Fe‐ferrierite. Our study, using an Fe‐ferrierite sample with iron exclusively present as Fe^II cations accommodated in the cationic sites, shows that the mobility of framework oxygen atoms in the temperature range: 553 to 593 K is limited to the four framework oxygen atoms of the two AlO₄^? tetrahedra forming cationic sites that accomodate Fe^II. They exchange with the Fe extra‐framework ¹⁸O atom originating from the decomposed N₂¹⁸O. We found, using DFT calculations, that O₂ molecules facilitate the oxygen exchange. However, the corresponding calculated energy barrier of 87 kcal mol^?1 is still very high and it is higher than the assumed experimental value based on the occurrence of the sluggish oxygen exchange at 553 K.     

Protonation Switching to the Least‐Basic Heteroatom of Carbamate through Cationic Hydrogen Bonding Promotes the Formation of Isocyanate Cations

We found that phenethylcarbamates that bear ortho‐salicylate as an ether group (carbamoyl salicylates) dramatically accelerate O?C bond dissociation in strong acid to facilitate generation of isocyanate cation (N‐protonated isocyanates), which undergo subsequent intramolecular aromatic electrophilic cyclization to give dihydroisoquinolones. To generate isocyanate cations from carbamates in acidic media as electrophiles for aromatic substitution, protonation at the ether oxygen, the least basic heteroatom, is essential to promote C?O bond cleavage. However, the carbonyl oxygen of carbamates, the most basic site, is protonated exclusively in strong acids. We found that the protonation site can be shifted to an alternative basic atom by linking methyl salicylate to the ether oxygen of carbamate. The methyl ester oxygen ortho to the phenolic (ether) oxygen of salicylate is as basic as the carbamate carbonyl oxygen, and we found that monoprotonation at the methyl ester oxygen in strong acid resulted in the formation of an intramolecular cationic hydrogen bond (>C?O⁺?H???O<) with the phenolic ether oxygen. This facilitates O?C bond dissociation of phenethylcarbamates, thereby promoting isocyanate cation formation. In contrast, superacid‐mediated diprotonation at the methyl ester oxygen of the salicylate and the carbonyl oxygen of the carbamate afforded a rather stable dication, which did not readily undergo C?O bond dissociation. This is an unprecedented and unknown case in which the monocation has greater reactivity than the dication.     

Specific intramolecular C-H...O interactions in 1-vinyl-2-formylimidazole and 1-vinyl-2-formylbenzimidazole studied by1H and13C NMR spectral data

According to the¹H and¹³C NMR spectral data, the vinyl group in 1-vinyl-2-formylimidazole and 1-vinyl-2-formylbenzimidazole istrans-oriented with respect to the formyl fragment, while the carbonyl group occupies theanti-position with respect to the N atom of pyridine cycle. A specific intramolecular C—H...O interaction of a weak hydrogen bond type is realized between the -H atom of the vinyl group and O atom of the carbonyl group.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 5, pp. 1197–1201, May, 1996.     

Multiple isotope effect study of the acid-catalyzed hydrolysis of methyl formate

Multiple isotope effects have been measured for the acid-catalyzed hydrolysis of methyl formate in 0.5 M HCl at 20 degrees C. The isotope effects in the present investigation include the carbonyl carbon (13k = 1.028 +/- 0.001), the carbonyl oxygen (18k = 0.9945 +/- 0.0009), the nucleophile oxygen (18k = 0.995 +/- 0.001), and the formyl hydrogen ((D)k = 0.81 +/- 0.02). Determination of the carbonyl carbon, carbonyl oxygen, and formyl hydrogen isotope effects was performed via isotopic analysis of residual substrate. However, determination of the oxygen nucleophile isotope effect required analysis of the oxygen atoms of the product (formic acid), which exchange with the solvent (water) under acid conditions. This necessitated measurement of the rate of exchange of these oxygen atoms under the conditions for hydrolysis (k(ex) = 0.0723 min(-1)) and correction of the raw isotope ratios measured during the nucleophile-O isotope effect experiment. These results, along with the previously reported isotope effect for the leaving oxygen (18k = 1.0009) and the ratio of the rate of hydrolysis to that of exchange of the carbonyl oxygen with water (k(h)/k(ex) = 11.3), give a detailed picture of the transition-state structure for the reaction.     

A new nuclear spin selective reaction of molecular oxygen

During high-temperature (623–673 K) oxidation of polyarylenes (polypyromellitimide and polyphenylquinoxaline), the molecular oxygen is enriched in the¹⁸O nonmagnetic isotope and impoverished in the¹⁷O magnetic isotope. The isotope selection increases with the increase in the degree of conversion of oxygen. The spin-selective reaction responsible for the selection of the¹⁷O isotope is the addition of molecular oxygen to triplet exited aromatic fragments of macromolecules to give endoperoxides. This reaction, which is selective in terms of the electron spin, is also nuclear-spin selective resulting in a magnetic isotope effect. The selection of nonmagnetic isotopes,¹⁶O and¹⁸O, is caused by competition between the reversible and irreversible decomposition of endoperoxide and by the classical isotope effect in these reactions.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 8, pp. 1402–1405, August, 1994.The authors are grateful to E. M. Galimov and I. V. Nikulina for high-quality isotope analyses and to the Russian Foundation for Basic Research for financial support (grant 93-03-5227).     

Breakdown of Chlorophyll: Partial synthesis of a putative intermediary catabolite. Preliminary communication

The partial synthesis of 10,22-dihydro-4,5-dioxo-4,5-secopheophorbide a ( 1 ) from pheophorbide a methyl ester (2) is described. A regioselective, photooxygenolytic reaction of (pheophorbidato a methyl ester)cadmium(II)( 3 ) provides the entry to the crucial 4,5-secoporphinoid structure in form of the (10,22-dihydro-4,5-dioxo-4,5-seco-pheophorbidato a methyl ester)cadmium(II) ( 4 ). The hydride reduction of this 4,5-dioxo-4,5-secophytoporphyrin ester occurs selectively at the ‘eastern’ meso-position to lead (after demetallation) to 10,22-dihydro-4,5-dioxo-4,5-secopheophorbide a methyl ester ( 5 ). This oxobilin-carbaldehyde has the structure assigned earlier to an ester of an isolation form of the red pigment(s) from Chlorella protothecoides. Hydrolysis of the propanoate ester function of 5, selectively catalyzed by pig liver esterase, then yields the title compound 1 . The red tetrapyrrole 1 may represent an intermediary chlorophyll catabolite in degreening plants.     

Unimolecular reactions of ionized methyl allyl ether

The fragmentation of CH₂?CHCH₂OCH₃^+· cation-radicals has been investigated by means of ²H- and ¹³C-labelling experiments and by analysis of collision-induced dissociation spectra. Metastable C₄H₈O^+· species decompose via one of three main channels which involve loss of (a) a hydrogen atom, (b) a methyl radical or (c) a formaldehyde molecule. Extensive, but not complete, exchange of the hydrogen and deuterium atoms in specifically labelled C₄H₈-_nD_nO^+· analogues precedes each of the three fragmentation pathways. The role of distonic ions in the rearrangement steps which bring about hydrogen exchange is discussed. The influence of isotope effects on the relative rates of the major reactions and the associated kinetic energy releases is examined. Only loss of a hydrogen atom is subject to a substantial isotope effect. Elimination of a methyl radical releases a large amount of kinetic energy, as is shown by the broad and dish-topped appearance of the corresponding metastable peak (T_1/2 ≈ 42 kJ mol^?1). The carbon atom of the original methoxy group is specifically expelled in this process. Both the large T_1/2 value and the unusual site selectivity are atypical of methyl and other alkyl radical losses from ionized alkenyl methyl ethers. The carbon atom of the methoxy group also participates specifically in formaldehyde elimination, but the two hydrogen atoms are not always selected from the three contained in the initial methoxy group. The implications of these labelling results for the synchronicity of concert of formaldehyde loss, which can be formu lated as a pericyclic process, is analysed.     

{[Cd(NTO)2(CHZ)]·2H2O}n的合成、分子结构和热分解机理

通过3-硝基-1,2,4-三唑-5-酮(NTO)镉与碳酰肼(NH₂NHCONHNH₂,CHZ)反应制备出了新型配合物{[Cd(NTO)₂(CHZ)]·2H₂O}_n,研究了其分子结构和热分解机理。该配合物的晶体属正交晶系,Pbca空间群,晶体学参数:a=0.8623(1)nm,b=1.8259(4)nm,c=1.9997(3)nm     

2-取代氨基-6-甲基-5-氧代-4-正丁基-4,5-二氢-1,2,4-三氮唑并[1,5-a]嘧啶类衍生物的合成

以2-溴丙酸甲酯、α,α-二氯甲基甲醚和胍唑为原料, 经缩合以及环化反应制得2-氨基-6-甲基-5-氧代-4,5-二氢-1,2,4-三氮唑并[1,5-a]嘧啶. 为了提高其在有机溶剂中的溶解性, 该化合物再同1-溴丁烷发生亲核取代反应得到了2-氨基-6-甲基-5-氧代-4-正丁基-4,5-二氢-1,2,4-三氮唑并[1,5-a]嘧啶, 然后与芳基醛和叔丁基异氰发生Ugi多组分反应, 合成了一系列具有潜在催吐活性的2-取代氨基-6-甲基-5-氧代-4-正丁基-4,5-二氢-1,2,4-三氮唑并[1,5-a]嘧啶类衍生物, 产品结构经质谱、核磁共振谱及元素分析确认.     

Mass spectrometry of pyridine compounds–IV: The formation and decomposition of the [M – methyl]+ ion [C8H10N]+ from 4-t-butylpyridine in comparison with that of the [M – methyl]+ ion [C9H11]+ from t-butylbenzene,as studied by D- and 13C-labelling

A strong secondary isotope effect is observed in the preferred loss of methyl vs. trideutero-methyl from the molecular ions of appropriately labelled 4-t-butylpyridine and t-butylbenzene decomposing in the first and second field free regions of a double focusing mass spectrometer. This has been rationalised by invoking the theory of radiationless transitions², which can account for the higher population of activated states responsible for loss of methyl vs. that for trideuteromethyl. ¹³C-Labelling at the central carbon atom of the t-butyl group indicates that the [M – methyl]⁺ ions, decomposing further by elimination of ethylene, cannot be represented exclusively by a pyridylated (or phenylated) cyclopropane ion if present at all. It is concluded that ions with structures generated by 1,2-hydrogen-, 1,2-pyridyl- (or 1,2-phenyl-) and 1,2-methyl shifts must also play a role. D-labelling further shows an extensive randomisation of side-chain hydrogen atoms in the [M-methyl]⁺ ions of 4-t-butylbenzene; in this case, however, the expelled ethylene also contains ring hydrogen atoms (≤2). Presumably this is caused by exchange between the side-chain and ortho-hydrogen atoms in the initially generated phenyldimethylcarbinyl carbenium ion.     

The mass spectrum of isopropyl o-toluate

The formation of an [M + 1]⁺ ion and the fragmentation of isopropyl o-toluate have been investigated by the deuterium labelling technique and kinetic energy release measurements. The hydrogen atom involved in the [M + 1]⁺ ion formation does not originate from a specific part of the molecule, but from all parts. A small amount of hydrogen exchange between the secondary carbon atom in the isopropyl group and the carbon atoms in the tolyl group takes place prior to decomposition of the molecular ion into the m/z 136 ion by a McLafferty rearrangement. Either almost complete scrambling of the hydroxyl hydrogen atom and the methyl hydrogen atoms in tolyl group or an almost equilibrated exchange of the hydroxyl hydrogen atom with one of the remaining hydrogen atoms in tolyl group also takes place prior to the elimination of a water molecule from the intermediate m/z 136 ion.     

POLYMERIC MOLECULAR PATTERN IN THE CRYSTALS OF A CALCIUM(II) COMPLEX WITH PYRIDINE-3,4-DICARBOXYLATE AND WATER LIGANDS

Abstract

Crystals of triaquamono (μ-pyridine-3,4-dicarboxylato-O,O′,O,O′)(aqua-O)calcium(II) contain molecular ribbons in which two adjacent calcium ions are bridged via oxygen atoms donated by the carboxylate group attached to carbon atom “3” in the pyridine ring. Both oxygen atoms are bidentate, each being coordinated to two caloium ions. In addition, every second pair of calcium(II) ions is bridged by a water oxygen atom. The coordination polyhedron around the calcium(II) ion is pentagonal bipyramidal; its equatorial plane is composed of two bridging oxygen atoms each belonging to the carboxylate group of the adjacent ligands, the bridging water oxygen atom and two coordinated water molecules. Another coordinated water oxygen atom constitutes the apex of the pyramid on one side of the pentagon, while two bridging carboxylate oxygen atoms donated by the same carboxylate group make two apices on the other side of the pentagon. The pyridine hetero-ring nitrogen atom does not participate in coordination to the central ion. Both oxygen atoms of the carboxylate group attached to the carbon atom in position “4” of the pyridine ring are not directly coordinated to the calcium(II) ion and act only as acceptors in the hydrogen bond system.     

Three sterically hindered 6‐amino‐5‐cyano‐2‐methyl‐4‐(1‐naphthyl)‐4H‐pyran‐3‐carboxylate derivatives

In the title compounds, 2‐methoxyethyl 6‐amino‐5‐cyano‐2‐methyl‐4‐(1‐naphthyl)‐4H‐pyran‐3‐carboxylate, C₂₁H₂₀N₂O₄, (II), isopropyl 6‐amino‐5‐cyano‐2‐methyl‐4‐(1‐naphthyl)‐4H‐pyran‐3‐carboxylate, C₂₁H₂₀N₂O₃, (III), and ethyl 6‐amino‐5‐cyano‐2‐methyl‐4‐(1‐naphthyl)‐4H‐pyran‐3‐carboxylate, C₂₀H₁₈N₂O₃, (IV), the heterocyclic pyran ring adopts a flattened boat conformation. In (II) and (III), the carbonyl group and a double bond of the heterocyclic ring are mutually anti, but in (IV) they are mutually syn. The ester O atoms in (II) and (III) and the carbonyl O atom in (IV) participate in intramolecular C—H...O contacts to form six‐membered rings. The dihedral angles between the naphthalene substituent and the closest four atoms of the heterocyclic ring are 73.3 (1), 71.0 (1) and 74.3 (1)° for (II)–(IV), respectively. In all three structures, only one H atom of the NH₂ group takes part in N—H...O [in (II) and (III)] or N—H...N [in (IV)] intermolecular hydrogen bonds, and chains [in (II) and (III)] or dimers [in (IV)] are formed. In (II), weak intermolecular C—H...O and C—H...N hydrogen bonds, and in (III) intermolecular C—H...O hydrogen bonds link the chains into ladders along the a axis.     

Mechanism of the formation of a dehydrated ion by an unusual loss of oxygen at the 4'-carbonyl group of abscisic acid methyl ester in electron ionization mass spectrometry

Methyl ester of abscisic acid (ABA), a plant hormone, gives a dehydrated ion at m/z 260 in electron ionization mass spectrometry (EI-MS). This dehydrated ion had been considered to be derived only from the elimination of the tertiary hydroxyl group at C-1'. We found that 34% of the dehydrated ion was formed by elimination of the oxygen atom at the 4'-carbonyl group, and the remaining 66% by elimination of the 1'-hydroxyl group. This unusual elimination of the carbonyl oxygen was shown with [4'-(18)O]ABA methyl ester. Involvement of the 4'-carbonyl oxygen in dehydration was observed in methyl ester of phaseic acid (PA), a natural metabolite of ABA, but not in 1'-deoxy-ABA methyl ester or isophorone. This suggested that the 1'-hydroxyl group was necessary for the elimination of the 4'-carbonyl oxygen. ABA methyl esters labeled with stable isotopes showed that hydrogen atoms at the 1'-hydroxyl group and at C-4 or -5 or -3' or - 5' or -7' were eliminated with the 4'-carbonyl oxygen. These results allow us to propose a formation mechanism of the dehydrated ion derived from the elimination of 4'-carbonyl oxygen and hydrogen atoms at C-4 and 1'-oxygen in ABA methyl ester as follows: first, ionization at the 1'-hydroxyl group occurs to give an ion radical, and the proton at the 1'-oxygen migrates to the 4'-carbonyl oxygen after the bond fission between C-1'-C-6'; second, migration of the proton at C-4 to the 1'-oxygen is followed by migration of the protons at C-5 and C-7' to C-4 and C-5, respectively; finally, the proton at the 1'-oxygen migrates to the 4'-hydroxyl group, and H(2)O at C-4' is eliminated to give the dehydrated ion. Our findings point out that a dehydrated ion is not always derived from the elimination of a hydroxyl group.     

Conformation of N‐methyl‐4‐piperidyl 2,4‐dinitrobenzoate

The crystal structure of N‐methyl‐4‐piperidyl 2,4‐di­nitro­benzoate, C₁₃H₁₅N₃O₆, (I), at 130 (2) K reveals that, in the solid state, the mol­ecule exists in the equatorial conformation, (Ieq). Thus, the through‐bond interaction present in the axial conformation, (Iax), is not strong enough to overcome the syn–diaxial interactions between the axial methyl substituent and the axial H atoms on the two piperidyl ring C atoms either side of the ester‐linked ring C atom. The carboxyl­ate group in (I) is orthogonal to the aromatic ring, in contrast with other 2,4‐di­nitro­benzoates, which are coplanar. The piperidyl–ester C—O bond distance is 1.467 (3) Å, which is actually shorter than other equatorial cyclo­hexyl–ester C—O distances. This shorter piperidyl–ester C—O bond distance is due to the reduced electron demand of the orthogonal ester group.     

Die Synthese und Kristallstruktur von Caesium-Praseodymsulfat Über Salze und Doppelsalze der Seltenen Erden, 4. Mitt.

Crystals of the title compound were prepared by heating Cs[Pr(SO₄)₂(H₂O)₃]·H₂O with H₂SO₄ at 300°C. CsPr(SO₄)₂ crystallizes in the orthorhombic space group Pnna witha=9.497(3),b=14.106(5),c=5.457(1) Å,p ₀=4.20,p _c=4.236 gcm^-3,Z=4. The structure, solved by the heavy-atom method from X-ray diffractometer data, was refined by least-squares techniques to R=0.023. The praseodymium atom is eight coordinated by eight oxygen atoms in the form of a distortedArchimedian antiprism. The antiprisms together with sulphur atoms form a layer-like structure parallel to thexz plane. The caesium atoms, which lie between the layers, have a coordination number of fourteen.

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3. Mitt.: Mh. Chem.108, 997 (1977).     

Synthesis and reactivity of some imidazo‐, triazolo‐ and tetrazolo‐isoquinoline derivatives

1‐Acetylirrüno‐3‐methyl‐1H‐isochromene‐4‐carbonitrile, 1 , reacts with glycine ethyl ester under basic conditions to give an imidazo[2,1‐a]isoquinoline derivative, while reaction with hydrazine hydrate in 1,4‐dioxane, with further chemistry, provides access to [1,2,4]triazolo[5,1‐a]isoquinoline, [1,2,4]triazolo[3,4‐a]isoquinoline and tetrazolo[5,1‐a]isoquinoline analogs. Benzene ring nitration and radical bromination of substituent methyl groups were investigated in the four tricycles, with some different positional reactivities being found. Two bromomethyl derivatives so produced were oxidised; ethyl 2‐bromomethyl‐6‐cyano‐5‐methylimidazo[2,1‐a]isoquinoline‐3‐carboxylate gave the anticipated ethyl 6‐cyano‐2‐formyl‐5‐methylimidazo[2,1‐a]isoquinoline‐3‐carboxylate (which reacted further with hydrazine to form a new system, 8,9‐dihydro‐6‐methyl‐8‐oxopyridazino[4′,5′:4,5]imidazo[2,1‐a]isoquinoline‐5‐carbonitrile), while 5‐bromomethyl‐2‐methyl[1,2,4]triazolo[5,1‐a]isoquinoline‐6‐carbonitrile unexpectedly gave directly another new system, 5,6‐dihydro‐5‐hydroxy‐2‐methyl‐7H‐pyrrolo[3,4‐c][1,2,4]triazolo[5,1‐a]isoquinolin‐7‐one.     

Tetrameric complexes of germanium(IV) and cobalt(II), Nickel(II), or Zinc(II) with 1,3-Diamino-2-propanol-tetraacertic acid: crystal and molecular structures of [(OH)2Ge2(μ-Hpdta)2Zn2(H2O)4] · 12H2O

Heteronuclear germanium(IV) and Zn(II) (Co(II), Ni(II)) complexes with 1,3-diamino-2-propanol-tetraacetic acid (H₅Hpdta) were synthesized. The compounds were characterized by elemental analysis, thermogravimetry, and IR spectroscopy. X-ray diffraction analysis of the crystals of [(OH)₂Ge₂(??-Hpdta)₂Zn₂(H₂O)₄] · 12H₂O (I) was performed. The crystals are tetragonal, a = 15.2022(9)?, c = 20.932(3) ?, V = 4837.5(7) ?³, Z = 4, space group P4₃, R1 = 0.0449 over 11399 reflections with I > 2??(I). The structural units of the crystal are tetrametric complex molecules [(OH)₂Ge₂(??-Hpdta)₂Zn₂(H₂O)₄] and water molecules of crystallization. The tetramer is composed of two similar neutral dimeric molecules [(OH)Ge(??-Hpdta)Zn(H₂O)₂]. The germanium and zinc atoms in the dimer are linked by the bridging oxygen atom of the deprotonated isopropanol group of the Hpdta^5? ligand (average Ge-O, 1.844(2)?; Zn-O, 2.192(3)?). The coordination sphere of the Ge and Zn atoms contains also one nitrogen atom (average Ge-N, 2.074(4)?; Zn-N, 2.156(3)?), four oxygen atoms belonging to four acetate branches of the octadentate Hpdta^5? ligands including two carboxyl O atoms for each Ge atom (average Ge-O, 1.912(3)?) and two carbonyl O atoms for each Zn (average Zn-O, 2.065(3)?). The coordination polyhedron of each Ge atom is completed to a distorted octahedron by the oxygen atom of the terminal hydroxy group (average Ge-O, 1.772(2)?) and the carboxyl oxygen atom of the bridging acetate branch (average Ge-O, 1.926(3)?) coordinated through carbonyl oxygen to Zn atom (average Zn-O, 2.148(3)?) of the second dimeric molecule. The distorted octahedron around each Zn atom is completed by oxygens of two water molecules at substantially different distances (average Zn-O, 1.984(3) and 2.100(3)?). The structural units are combined by O-H??O hydrogen bonds to form a framework.     

Electron impact induced hydrogen migrations in organic molecules: IV—novel hydrogen transfers in alkyl o-methoxybenzoates

Simultaneous hydrogen transfers—one from the methoxy group and the other from the alkyl group—to both the oxygen atoms of the ester function result in the formation of a common ion at m/z 152 in the alkyl o-methoxybenzoates on electron impact. Expulsion of the formyl radical from this ion leads to a fragment resembling the protonated benzoic acid. Another novel feature in these compounds is the loss of H₂O from the [M? R]⁺ ion which arises through an ortho effect during a secondary fragmentation process.     

Neptunium(V) Perrhenate Complex with 1,10-Phenanthroline [(NpO2)(ReO4)(Phen)(H2O)2]: Crystal Structure and Absorption Spectra

Neptunium(V) perrhenate complex [(NpO₂)(ReO₄)(Phen)(H₂O)₂] was synthesized with 1,10-phenanthroline as a ligand. Its composition and structure were determined by X-ray diffraction analysis. The coordination polyhedron of the Np atom is a pentagonal bipyramid. The nearest surrounding of the neptunoyl ion includes the oxygen atom of the ReO₄ anion, two nitrogen atoms of phenanthroline, and the oxygen atoms of two water molecules. The crystals of the compound are monoclinic. The main crystallographic parameters are the following: space group P2₁/c, unit cell parameters a = 7.288(1) Å, b = 10.513(2) Å, c = 20.936(4) Å, = 96.939(5)°, Z = 4, V = 1592.2(5) ų. Absorption spectra of the compound in visible and IR regions are reported.