Trimethoxybenzene complexes of pentafluorophenylchlorocarbene

Pentafluorophenylchlorocarbene, generated by laser flash photolysis (LFP) of pentafluorophenylchlorodiazirine, formed π-type complexes with 1,3,5-trimethoxybenzene in pentane. The carbene and carbene complexes were in equilibrium with K = 3.21 × 10(5) M(-1) at 294 K. From the temperature dependence of K, ΔH° = -10.2 kcal/mol, ΔS° = -9.5 eu, and ΔG° = -7.4 kcal/mol at 298 K. The carbene complexes were characterized by UV-vis spectroscopy and computational analysis.     

Charge transfer complex formation between p-chloranil and 1,n-di(9-anthryl)alkanes

Dimer model compounds of polyvinylanthracenes (1,n-di(9-anthryl)alkanes, when n=1-5) were synthesized to model the effects of distance and orientation between anthracene groups in polymeric systems. Charge transfer (CT) complexes of anthracene, 9-methylanthracene and 1,n-di(9-anthryl)alkanes with p-chloranil (p-CHL) have been investigated spectrophotometrically in dichloromethane. The colored products are measured spectrophotometrically at different wavelength depending on the electronic transition between donors and acceptor. The formation constants of the CT complexes were determined by the Benesi-Hildebrand equation. The thermodynamic parameters were calculated by Van't Hoff equation. Stochiometries of the complexes formed between donors and acceptor were defined by the Job's method of the continuous variation and found in 1:1 complexation with donor and acceptor at the maximum absorption bands.     

On the lack of conjugation stabilization in polyynes (polyacetylenes)

By analogy to conjugated polyenes, conjugative stabilization of polyynes with the -CC-CC- group might be expected to be substantial. On the contrary, consistent with our recent report of a surprising lack of conjugative stabilization in butadiyne, we find by G3(MP2) calculations and by comparisons with available experimental data from these and other laboratories that the ground-state stabilization of conjugated polyynes is in fact quite small, amounting to <1 kcal mol(-)(1). By similar calculations, the 2,4-pentadiyn-1-yl radical shows no enhanced stabilization relative to 2-propyn-1-yl radical, despite the potential stabilization of the odd electron by two conjugated triple bonds and unlike the behavior of 2,4-pentadien-1-yl radical. The thermochemistry of straight-chain alkynes and polyynes is very self-consistent. Enthalpies of hydrogenation, leading to enthalpies of formation, are predictable with a high degree of accuracy (absolute mean deviation = +/-0.39 kcal mol(-)(1) vs theoretical values and +/-0.52 vs experimental) from three molecular structure enthalpies and one conjugation stabilization parameter.     

Mechanism of the hydration of carbon dioxide: direct participation of H2O versus microsolvation

Thermochemical parameters of carbonic acid and the stationary points on the neutral hydration pathways of carbon dioxide, CO 2 + nH 2O --> H 2CO 3 + ( n - 1)H 2O, with n = 1, 2, 3, and 4, were calculated using geometries optimized at the MP2/aug-cc-pVTZ level. Coupled-cluster theory (CCSD(T)) energies were extrapolated to the complete basis set limit in most cases and then used to evaluate heats of formation. A high energy barrier of approximately 50 kcal/mol was predicted for the addition of one water molecule to CO 2 ( n = 1). This barrier is lowered in cyclic H-bonded systems of CO 2 with water dimer and water trimer in which preassociation complexes are formed with binding energies of approximately 7 and 15 kcal/mol, respectively. For n = 2, a trimeric six-member cyclic transition state has an energy barrier of approximately 33 (gas phase) and a free energy barrier of approximately 31 (in a continuum solvent model of water at 298 K) kcal/mol, relative to the precomplex. For n = 3, two reactive pathways are possible with the first having all three water molecules involved in hydrogen transfer via an eight-member cycle, and in the second, the third water molecule is not directly involved in the hydrogen transfer but solvates the n = 2 transition state. In the gas phase, the two transition states have comparable energies of approximately 15 kcal/mol relative to separated reactants. The first path is favored over in aqueous solution by approximately 5 kcal/mol in free energy due to the formation of a structure resembling a (HCO 3 (-)/H 3OH 2O (+)) ion pair. Bulk solvation reduces the free energy barrier of the first path by approximately 10 kcal/mol for a free energy barrier of approximately 22 kcal/mol for the (CO 2 + 3H 2O) aq reaction. For n = 4, the transition state, in which a three-water chain takes part in the hydrogen transfer while the fourth water microsolvates the cluster, is energetically more favored than transition states incorporating two or four active water molecules. An energy barrier of approximately 20 (gas phase) and a free energy barrier of approximately 19 (in water) kcal/mol were derived for the CO 2 + 4H 2O reaction, and again formation of an ion pair is important. The calculated results confirm the crucial role of direct participation of three water molecules ( n = 3) in the eight-member cyclic TS for the CO 2 hydration reaction. Carbonic acid and its water complexes are consistently higher in energy (by approximately 6-7 kcal/mol) than the corresponding CO 2 complexes and can undergo more facile water-assisted dehydration processes.     

Electron deficient bridges involving silylenes: a theoretical study

Ab initio and density functional studies show that silylenes can form complexes with BH(3) and the resultant complexes possess 3c-2e bridges. The complexation energy for the formation of the these H-bridged structures is in the range of 18-46 kcal/mol. The characteristics of the electron deficient bridges depend on the substituents attached to the silylenes. With an increase in the pi-donating capacity of the substituents, the exothermicity of complex formation gets reduced but the kinetic stability of the H-bridged structures increase. The natural bond orbital analysis shows that all the H-bridged structures are associated with sigma(B-H)-->ppi(Si) second-order delocalization, which is responsible for the origin of the 3c-2e bonds. The complexation energies of the silylene-BH(3) complexes have been shown to have a correlation to the singlet-triplet energy gaps of silylenes.     

Enthalpies of formation of gas-phase N3, N3-, N5+, and N5- from Ab initio molecular orbital theory, stability predictions for N5(+)N3(-) and N5(+)N5(-), and experimental evidence for the instability of N5(+)N3(-)

Ab initio molecular orbital theory has been used to calculate accurate enthalpies of formation and adiabatic electron affinities or ionization potentials for N3, N3-, N5+, and N5- from total atomization energies. The calculated heats of formation of the gas-phase molecules/ions at 0 K are DeltaHf(N3(2Pi)) = 109.2, DeltaHf(N3-(1sigma+)) = 47.4, DeltaHf(N5-(1A1')) = 62.3, and DeltaHf(N5+(1A1)) = 353.3 kcal/mol with an estimated error bar of +/-1 kcal/mol. For comparison purposes, the error in the calculated bond energy for N2 is 0.72 kcal/mol. Born-Haber cycle calculations, using estimated lattice energies and the adiabatic ionization potentials of the anions and electron affinities of the cations, enable reliable stability predictions for the hypothetical N5(+)N3(-) and N5(+)N5(-) salts. The calculations show that neither salt can be stabilized and that both should decompose spontaneously into N3 radicals and N2. This conclusion was experimentally confirmed for the N5(+)N3(-) salt by low-temperature metathetical reactions between N5SbF6 and alkali metal azides in different solvents, resulting in violent reactions with spontaneous nitrogen evolution. It is emphasized that one needs to use adiabatic ionization potentials and electron affinities instead of vertical potentials and affinities for salt stability predictions when the formed radicals are not vibrationally stable. This is the case for the N5 radicals where the energy difference between vertical and adiabatic potentials amounts to about 100 kcal/mol per N5.     

Cation-π interactions of curved polycyclic systems: M (M=Li and Na) ion complexation with buckybowls

B3LYP/6-311+G** calculations on alkali metal ion (Li⁺ and Na⁺) complexation with corannulene and sumanene indicate stronger binding compared to [5]-radialene or benzene. The dependence of binding to the convex and concave site is marginal, albeit the preference was consistent for convex binding in the range of 1-4 kcal/mol. The bowl-to-bowl inversion barriers are only marginally affected, below 2 kcal/mol, by metal ion complexation.     

Synthesis, spectroscopic and thermal studies of the reactions of the donors piperazine and N,N′-dimethylpiperazine with σ- and π-acceptors

The interactions of the electron donors piperazine (PIP) and N,N′-dimethylpiperazine (DMPIP) with the σ-acceptor iodine and the π-acceptors tetracyanoethylene (TCNE) and 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) were studied spectrophotometrically in chloroform at 25 °C. The electronic and infrared spectra of the resulting charge-transfer complexes were recorded, in addition to thermal analysis. The results obtained showed that the stoichiometries of the reactions are not fixed and depend on the nature of both the donor and the acceptor. The formed CT-complexes have the formulas of , [(PIP)(TCNE)₂], [(PIP)(DDQ)₂], , [(DMPIP)(TCNE)₂] and [(DMPIP)(DDQ)₂]. A general mechanism explaining the formation of triiodide complexes was suggested.     

Interaction of <Emphasis Type="Italic">L</Emphasis>-cysteine with selenious and selenic acids: A study by the density functional theory method

Using the density functional theory method at the B3LYP/6-31G(d,p) theory level, the formation of hydrogen bonded complexes of L-cysteine with selenious and selenic acids is studied. In both cases, complexes formed through the carboxyl group of cysteine mostly arise, their enthalpy of formation being of -19kcal/mol to -21 kcal/mol and the free energy of -6kcal/mol to -9kcal/mol. The primary act of interaction in the system of hydroxyl-containing selenium compound — α-aminoacid, including the mutual orientation of reactant molecules and the formation of intermolecular hydrogen bonds is likely to a serve as prerequisite for the thiol group to be able to participate in the next stages (including deeper chemical transformations) of biologically significant reactions.     

Experimental determination of the heat of hydrogenation of phenylcyclobutadiene

The heat of hydrogenation of phenylcyclobutadiene (DeltaH degrees (hyd) = 57.4 +/- 4.9 kcal mol(-1)) was determined via a thermodynamic cycle by carrying out gas-phase measurements on 1-phenylcyclobuten-3-yl cation. This leads to an antiaromatic destabilization energy of 27 +/- 5 kcal mol(-1), a difference of 9.6 +/- 4.9 kcal mol(-1) for the first and second C-H bond dissociation energies of 1-phenylcyclobutene, and an estimate of 96 +/- 5 kcal mol(-1) for the heat of formation of cyclobutadiene. These results are compared to G3, G3(MP2), and B3LYP computations and represent the first experimental measurements of the energy of a monocyclic cyclobutadiene.     

Computational investigation of hydrogen abstraction from 2-aminoethanol by the 1,5-dideoxyribose-5-yl radical: a model study of a reaction occurring in the active site of ethanolamine ammonia lyase

Hydrogen abstraction from 2-aminoethanol by the 5'-deoxyadenosyl radical, which is formed upon Co--C bond homolysis in coenzyme B(12), was investigated by theoretical means with employment of the DFT (B3LYP) and ab initio (MP2) approaches. As a model system for the 5'-deoxyadenosyl moiety the computationally less demanding 1,5-dideoxyribose was employed; two conformers, which differ in ring conformation (C2- and C3-endo), were considered. If hydrogen is abstracted from "free" substrate by the C2-endo conformer of the 1,5-dideoxyribose-5-yl radical, the activation enthalpy is 16.7 kcal mol(-1); with the C3-endo counterpart, the value is 17.3 kcal mol(-1). These energetic requirements are slightly above the activation enthalpy limit (15 kcal mol(-1)) determined experimentally for the rate-determining step of the sequence, that is, hydrogen delivery from 5'-deoxyadenosine to the product radical. The activation enthalpy is lower when the substrate interacts with at least one amino acid from the active site. According to the computations, when a His model system partially protonates the substrate the activation enthalpy is 4.5 kcal mol(-1) for the C3-endo conformer and 5.8 kcal mol(-1) for the C2-endo counterpart. As hydrogen abstraction from the fully as well as the partially protonated substrate is preceded by the formation of quite stable encounter complexes, the actual activation barriers are around 13-15 kcal mol(-1). A synergistic interaction of 2-aminoethanol with two amino acids where His partially protonates the NH(2) group and Asp partially deprotonates the OH group of the substrate results in an activation enthalpy of 12.4 kcal mol(-1) for the C3-endo conformer and 13.2 kcal mol(-1) for the C2-endo counterpart. However, if encounter complexes exist in the active site, the actual activation barriers are much higher (>25 kcal mol(-1)) than that reported for the rate-determining step. These findings together with previous computations suggest that the energetics of the initial hydrogen abstraction decrease with an interaction of the substrate with only a protonating auxiliary, but for the rearrangement of the radical the synergistic effects of two auxiliaries are essential to pull the barrier below the limit of 15 kcal mol(-1).     

Charge-transfer complex formation between p-chloranil and 1,n-dicarbazolylalkanes

Dimer model compounds of polyvinylcarbazoles (1,n-di(N-carbazolyl)alkanes, when n=1-5) were synthesized to model the effects of distance and orientation between carbazole groups in polymeric systems. Charge-transfer (CT) complexes of carbazole, N-ethylcarbazole and 1,n-di(N-carbazolyl)alkanes with p-chloranil (p-CHL) have been investigated spectrophotometrically in dichloromethane. The colored products are measured spectrophotometrically at different wavelength depending on the electronic transition between donors and acceptor. The formation constants of the CT complexes were determined by the Benesi-Hildebrand equation. The thermodynamic parameters were calculated by Van't Hoff equation. Stochiometries of the complexes formed between donors and acceptor were defined by the Job's method of the continuous variation and found in 1:1 complexation with donor and acceptor at the maximum absorption bands.     

Synthesis and Diels-Alder Reactivity of 2,3,5,6-Tetramethylidenenorbornane

Pd-catalyzed double carbomethoxylation of the Diels-Alder adduct of cyclo-pentadiene and maleic anhydride yielded the methyl norbornane-2,3-endo-5, 6-exo-tetracarboxylate ( 4 ) which was transformed in three steps into 2,3,5,6-tetramethyl-idenenorbornane ( 1 ). The cycloaddition of tetracyanoethylene (TCNE) to 1 giving the corresponding monoadduct 7 was 364 times faster (toluene, 25°) than the addition of TCNE to 7 yielding the bis-adduct 9 . Similar reactivity trends were observed for the additions of TCNE to the less reactive 2,3,5,6-tetramethylidene-7-oxanorbornane ( 2 ). The following second order rate constants (toluene, 25°) and activation parameters were obtained for: 1 + TCNE → 7 : k₁ = (255 + 5) 10^?4 mol^?1 · s^?1, ΔH≠ = (12.2 ± 0.5) kcal/mol, ΔS≠ = (?24.8 ± 1.6) eu.; 7 + TCNE → 9 , k₂ = (0.7 ± 0.02) 10^?4 mol^?1 · s^?1, ΔH≠ = (14.1 ± 1.0) kcal/mol, ΔS≠ = ( ?30 ± 3.5) eu.; 2 + TCNE → 8 : k₁ = (1.5 ± 0.03) 10^?4 mol^?1 · s^?1, ΔH≠ = (14.8 ± 0.7) kcal/mol, ΔS≠ = (?26.4 ± 2.3) eu.; 8 + TCNE → 10 ; k₂ = (0.004 ± 0.0002) 10^?4 mol^?1 · s^?1, ΔH≠ = (17 ± 1.5) kcal/mol, ΔS≠ = (?30 ± 4) eu. The possible origins of the relatively large rate ratios k₁/k₂ are discussed briefly.     

Theoretical study of pyridine and 4,4'-bipyridine adsorption on the lewis acid sites of alumina surfaces based on ab initio and density functional cluster calculations

The interactions of pyridine and 4,4'-bipyridine with the Lewis acid sites of alumina surfaces are investigated using ab initio and density functional calculations. Four cluster models of different sizes and shapes are chosen to represent the Lewis acid sites: three hydrogenated clusters Al(OH)(3), Al(4)O(9)H(6), and Al(10)O(21)H(12) and one non-hydrogenated cluster Al(4)O(6). The Hartree-Fock (HF) and B3LYP approaches with two basis sets 6-31G and 6-31+G are used to calculate the geometries, the electronic structures, the vibrational frequencies, and the adsorption energies of the complexes formed upon interaction of pyridine or 4,4'-bipyridine ligands on the cluster surfaces. Electronic structures are determined by the electrostatic potential (ESP) analysis of charges. Adsorption energies are calculated with corrections made for zero-point energies (ZPE) and basis set superposition error (BSSE). The ESP analysis of atomic charges reveals that the charge-transfer effects are more important in Lewis complexes formed with Al(4)O(6) cluster than in those formed with hydrogenated clusters Al(OH)(3), Al(4)O(9)H(6), and Al(10)O(21)H(12). The significantly larger charge transferred from pyridine or 4,4'-bipyridine ligand to Al(4)O(6) cluster should increase the adsorption energy of these complexes. Consequently, at all levels of calculation, the adsorption energies of pyridine and 4,4'-bipyridine complexed to Al(4)O(6) cluster ( approximately 46 kcal/mol), which compare very well to experiment, are strongly larger than those obtained for both pyridine and 4,4'-bipyridine ligands complexed to Al(OH)(3) (32 kcal/mol), Al(4)O(9)H(6) (24 kcal/mol) and Al(10)O(21)H(12) (25 kcal/mol) clusters. The corrected adsorption energy is found to be insensitive to basis set and electron correlation effects. It essentially depends on the ionic character of the cluster model rather than on its size. For 4,4'-bipyridine complexes, similar results to those obtained for pyridine are found, and the geometry and the amount of charge of the unbound pyridyl ring are unchanged upon complexation. The calculated vibrational frequencies and frequency shifts are little sensitive to the size and shape of the cluster model. The two ring stretching modes 8a and 19b of pyridine and 4,4'-bipyridine observed in the 1400-1600 cm(-1) region are the most affected modes upon adsorption, in good agreement with the available infrared and Raman data.     

On the mechanism of (PCP)Ir-catalyzed acceptorless dehydrogenation of alkanes: a combined computational and experimental study

Pincer complexes of the type ((R)PCP)IrH(2), where ((R)PCP)Ir is [eta(3)-2,6-(R(2)PCH(2))(2)C(6)H(3)]Ir, are the most effective catalysts reported to date for the "acceptorless" dehydrogenation of alkanes to yield alkenes and free H(2). We calculate (DFT/B3LYP) that associative (A) reactions of ((Me)PCP)IrH(2) with model linear (propane, n-PrH) and cyclic (cyclohexane, CyH) alkanes may proceed via classical Ir(V) and nonclassical Ir(III)(eta(2)-H(2)) intermediates. A dissociative (D) pathway proceeds via initial loss of H(2), followed by C-H addition to ((Me)PCP)Ir. Although a slightly higher energy barrier (DeltaE(+ +)) is computed for the D pathway, the calculated free-energy barrier (DeltaG(+ +)) for the D pathway is significantly lower than that of the A pathway. Under standard thermodynamic conditions (STP), C-H addition via the D pathway has DeltaG(o)(+ +) = 36.3 kcal/mol for CyH (35.1 kcal/mol for n-PrH). However, acceptorless dehydrogenation of alkanes is thermodynamically impossible at STP. At conditions under which acceptorless dehydrogenation is thermodynamically possible (for example, T = 150 degrees C and P(H)2 = 1.0 x 10(-7) atm), DeltaG(+ +) for C-H addition to ((Me)PCP)Ir (plus a molecule of free H(2)) is very low (17.5 kcal/mol for CyH, 16.7 kcal/mol for n-PrH). Under these conditions, the rate-determining step for the D pathway is the loss of H(2) from ((Me)PCP)IrH(2) with DeltaG(D)(+ +) approximately DeltaH(D)(+ +) = 27.2 kcal/mol. For CyH, the calculated DeltaG(o)(+ +) for C-H addition to ((Me)PCP)IrH(2) on the A pathway is 35.2 kcal/mol (32.7 kcal/mol for n-PrH). At catalytic conditions, the calculated free energies of C-H addition are 31.3 and 33.7 kcal/mol for CyH and n-PrH addition, respectively. Elimination of H(2) from the resulting "seven-coordinate" Ir-species must proceed with an activation enthalpy at least as large as the enthalpy change of the elimination step itself (DeltaH approximately 11-13 kcal/mol), and with a small entropy of activation. The free energy of activation for H(2) elimination (DeltaG(A)(+ +)) is hence found to be greater than ca. 36 kcal/mol for both CyH and n-PrH under catalytic conditions. The overall free-energy barrier of the A pathway is calculated to be higher than that of the D pathway by ca. 9 kcal/mol. Reversible C-H(D) addition to ((R)PCP)IrH(2) is predicted to lead to H/D exchange, because the barriers for hydride scrambling are extremely low in the "seven-coordinate" polyhydrides. In agreement with calculation, H/D exchange is observed experimentally for several deuteriohydrocarbons with the following order of rates: C(6)D(6) > mesitylene-d(12) > n-decane-d(22) > cyclohexane-d(12). Because H/D exchange in cyclohexane-d(12) solution is not observed even after 1 week at 180 degrees C, we estimate that the experimental barrier to cyclohexane C-D addition is greater than 36.4 kcal/mol. This value is considerably greater than the experimental barrier for the full catalytic dehydrogenation cycle for cycloalkanes (ca. 31 kcal/mol). Thus, the experimental evidence, in agreement with calculation, strongly indicates that the A pathway is not kinetically viable as a segment of the "acceptorless" dehydrogenation cycle.     

Thermochemistry of the initial steps of methylaluminoxane formation. Aluminoxanes and cycloaluminoxanes by methane elimination from dimethylaluminum hydroxide and its dimeric aggregates

Results are presented of ab initio studies at levels MP2(full)/6-31G* and MP2(full)/6-311G** of the hydrolysis of trimethylaluminum (TMA, 1) to dimethylaluminumhydroxide (DMAH, 2) and of the intramolecular 1,2-elimination of CH(4) from 2 itself to form methylaluminumoxide 3, from its dimeric aggregate 4 to form hydroxytrimethyldialuminoxane 5 and dimethylcyclodialuminoxane 6, and from its TMA aggregate 7 to form 8 and/or 9, the cyclic and open isomers of tetramethyldialuminoxane, respectively. Each methane elimination creates one new Lewis acid site, and dimethylether is used as a model oxygen-donor molecule to assess the most important effects of product stabilization by Lewis donor coordination. It is found that the irreversible formation of aggregate 4 (ΔG(298) = -29.2 kcal/mol) is about three times more exergonic than the reversible formation of aggregate 7 (ΔG(298) = -9.9 kcal/mol), that the reaction free enthalpies for the formations of 5 (ΔG(298) = -9.0 kcal/mol) and 6 (ΔG(298) = -18.8 kcal/mol) both are predicted to be quite clearly exergonic, and that there is a significant thermodynamic preference (ΔG(298) = -7.2 kcal/mol) for the formation of 6 over ring-opening of 5 to hydroxytrimethyldialuminoxane 10. The mechanism for oligomerization is discussed based on the bonding properties of dimeric aggregates and involves the homologation of HO-free aluminoxane with DMAH (i.e., 9 to 13), and any initially formed hydroxydialuminoxane 10 is easily capped to trialuminoxane 13. Our studies are consistent with and provide support for Sinn's proposal for the formation of oligoaluminoxanes, and in addition, the results point to the crucial role played by the kinetic stability of 5 and the possibility to form cyclodialuminoxane 6. Dialuminoxanes 9 and 10 are reversed-polarity heterocumulenes, and intramolecular O→Al dative bonding competes successfully with Al complexation by Lewis donors. Intramolecular O→Al dative bonding is impeded in cyclodialuminoxane 6, and the dicoordinate oxygen in 6 is a strong Lewis donor. Ethylene polymerization catalysts contain highly oxophilic transition metals, and our studies suggest that these transition metal catalysts should discriminate strongly in favor of cycloaluminoxane-O donors even if these are present only in small concentrations in the methylaluminoxane (MAO) cocatalyst.     

Theoretical study of the structure, stability, and infrared spectra of hydrogen-bonded complexes of N-nitrosodiethanolamine (NDELA) with water molecules

A theoretical study of the interaction between the N-nitrosodiethanolamine (NDELA) molecule and one to five water molecules was performed at the B3LYP level using a large polarized basis set. The calculated complexation energies (corrected for BSSE and ZPVE) of NDELA with one, two, three, four, and five water molecules are ?4.62 kcal/mol, ?9.83 kcal/mol, ?15.29 kcal/mol, ?21.60 kcal/mol, and ?25.10 kcal/mol respectively at the B3LYP/6-311++G** level. In all complexes studied, there are red shifts in the vibrational frequencies of the O-Hs of NDELA and water molecules along with increases in the corresponding IR intensities.     

Cyclodextrin and modified cyclodextrin complexes of E-4-tert-butylphenyl-4'-oxyazobenzene: UV-visible, 1H NMR and ab initio studies

alpha-Cyclodextrin, beta-cyclodextrin, N-(6(A)-deoxy-alpha-cyclodextrin-6(A)-yl)-N'6(A)-deoxy-beta-cyclodextrin-6(A)-yl)urea and N,N-bis(6(A)-deoxy-beta-cyclodextrin-6(A)-yl)urea (alphaCD, betaCD, 1 and 2) form inclusion complexes with E-4-tert-butylphenyl-4'-oxyazobenzene, E-3(-). In aqueous solution at pH 10.0, 298.2 K and I = 0.10 mol dm(-3)(NaClO(4)) spectrophotometric UV-visible studies yield the sequential formation constants: K(11) = (2.83 +/- 0.28) x 10(5) dm(3) mol(-1) for alphaCD.E-(-), K(21) = (6.93 +/- 0.06) x 10(3) dm(3) mol(-1) for (alphaCD)(2).E-3(-), K(11) = (1.24 +/- 0.12) x 10(5) dm(3) mol(-1) for betaCD.E-(-), K(21) = (1.22 +/- 0.06) x 10(4) dm(3) mol(-1) for (betaCD)(2).E-(-), K(11) = (3.08 +/- 0.03) x 10(5) dm(3) mol(-1) for .E-3(-), K(11) = (8.05 +/- 0.63) x 10(4) dm(3) mol(-1) for .E-3(-) and K(12) = (2.42 +/- 0.53) x 10(4) dm(3) mol(-1) for .(E-3(-))(2). (1)H ROESY NMR studies show that complexation of E-3(-) in the annuli of alphaCD, betaCD, 1 and 2 occurs. A variable-temperature (1)H NMR study yields k(298 K)= 6.7 +/- 0.5 and 5.7 +/- 0.5 s(-1), DeltaH = 61.7 +/- 2.7 and 88.1 +/- 4.2 kJ mol(-1) and DeltaS = -22.2 +/- 8.7 and 65 +/- 13 J K(-1) mol(-1) for the interconversion of the dominant includomers (complexes with different orientations of alphaCD) of alphaCD.E-3(-) and (alphaCD)(2).E-3(-), respectively. The existence of E-3(-) as the sole isomer was investigated through an ab initio study.     

Modeling the photochemistry of the reference phototoxic drug lomefloxacin by steady-state and time-resolved experiments, and DFT and post-HF calculations

The irradiation in water of 1-ethyl-6,8-difluoro-7(3-methylpiperazino)3-quinolone-2-carboxylic acid (lomefloxacin), a bactericidal agent whose use is limited by its serious phototoxicity (and photomutagenicity in the mouse), leads to formation of the aryl cation in position eight that inserts into the 1-ethyl chain. Trapping of the cation was examined and it was found that chloride and bromide straightforwardly add in position eight, but with iodide and with pyrrole the 1-(2-iodoethyl) and the 1-[2-(2-pyrrolyl)ethyl] derivatives are formed. Flash photolysis reveals the triplet of lomefloxacin, a short-lived species (lambda max=370 nm, tau=40 ns) that generates the triplet cation (lambda max=480 nm, tau approximately 120 ns). The last intermediate is quenched both by halides and by pyrrole. DFT and post-HF methods have shown that the triplet is the lowest state of the cation (Delta G(ST)=13.3 kcal mol(-1)) and intersystem crossing (ISC) to the singlet has no role because a less endothermic process occurs, that is, intramolecular hydrogen abstraction from the N-ethyl chain (9.2 kcal mol(-1)) that finally leads to cyclization. The halides form weak complexes with the triplet cation (kq from 4.9 x 10(8) for Cl(-) to 7.0 x 10(9) m(-1) s(-1) for I-). With Cl(-) and Br(-) ISC occurs in the complex along with C8--X bond formation. However, this latter process is slow with bulky iodide and with neutral pyrrole, and in these cases moderately endothermic electron transfer (ca. 7 kcal mol(-1)) yielding the 8-quinolinyl radical occurs. Hydrogen exchange leads to a new radical on the 1-ethyl chain and to the observed products. These findings suggest that the mutagenic activity of the DNA-intercalated drug involves attack of the photogenerated cation to the heterocyclic bases.     

Acid-base chemistry of a carbenium ion in a zeolite under equilibrium conditions: verification of a theoretical explanation of carbenium ion stability

The 1,3-dimethylcyclopentenyl carbenium ion (C7H11(+)) was reproducibly prepared on zeolite HZSM-5 using a pulse-quench reactor, and then each of a number of bases was coadsorbed into the catalyst channels to either compete with the cation for protonation or to possibly react with it as a nucleophile. For seven bases with proton affinities (PA) between 142 and 212.1 kcal/mol, there was no reaction with C7H11(+). Coadsorption of smaller amounts of dimethylacetamide (PA = 217 kcal/mol) also produced no reaction, but with a higher loading, a proton was transferred from the carbenium ion to the base to leave 1,3-dimethylcyclopenta-1,3-diene in the zeolite as a neutral olefin. Deprotonation was the primary reaction with coadsorption of either pyridine (PA = 222 kcal/mol) or trimethylphosphine (PA = 229.2 kcal/mol). The estimated experimental deprotonation enthalpy for C7H11(+), approximately 217 kcal/mol in the zeolite, is in excellent agreement with MP4/6-311G gas-phase value of 215.6 kcal/mol. Coadsorption of either NH3 (PA = 204.0 kcal/mol) or PH3 (PA = 188 kcal/mol) does not deprotonate the carbenium ion, but these species do react as nucleophiles to form onium ion derivatives of C7H11(+). Analogous onium complexes with pyridine or trimethylphosphine formed in lower yields due to steric constraints in the zeolite channels. The essential experimental observations were all predicted and explained by density functional calculations (B3LYP/6-311G) and extensions of our recently developed theory of carbenium ion stability in zeolites. In addition, we report theoretical geometries for several complexes which contain unusual C-H- - -X hydrogen bonds.