Dynamics of Hydrogen Bonding in Three-Component Nanorotors

The dynamics of hydrogen bonding do not only play an important role in many biochemical processes but also in Nature's multicomponent machines. Here, a three-component nanorotor is presented where both the self-assembly and rotational dynamics are guided by hydrogen bonding. In the rate-limiting step of the rotational exchange, two phenolic O-H–N,N_{(phenanthroline)} hydrogen bonds are cleaved, a process that was followed by variable-temperature ¹H NMR spectroscopy. Activation data (ΔG^≠₂₉₈=46.7 kJ mol⁻¹ at 298 K, ΔH^≠=55.3 kJ mol⁻¹, and ΔS^≠=28.8 J mol⁻¹ K⁻¹) were determined, furnishing a rotational exchange frequency of k₂₉₈=40.0 kHz. Fully reversible disassembly/assembly of the nanorotor was achieved by addition of 5.0 equivalents of trifluoroacetic acid (TFA)/1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) over three cycles.     

Kinetic study of the ligand substitution on the trimethylplatinum(iv) complexes with unidentate ligands

Ligand substitution kinetics for the reaction [Pt^IVMe₃(X)(NN)]+NaY=[Pt^IVMe₃(Y)(NN)]+NaX, where NN=bipy or phen, X=MeO⁻, CH₃COO⁻, or HCOO⁻, and Y=SCN⁻ or N₃⁻, has been studied in methanol at various temperatures. The kinetic parameters for the reaction are as follows. The reaction of [PtMe₃(OMe)(phen)] with NaSCN: k₁=36.1±10.0 s⁻¹; ΔH₁^≠=65.9±14.2 kJ mol⁻¹; ΔS₁^≠=6±47 J mol⁻¹ K⁻¹; k₋₂=0.0355±0.0034 s⁻¹; ΔH₋₂^≠=63.8±1.1 kJ mol⁻¹; ΔS₋₂^≠=−58.8±3.6 J mol⁻¹ K⁻¹; and k₋₁/k₂=148±19. The reaction of [PtMe₃(OAc)(bipy)] with NaN₃: k₁=26.2±0.1 s⁻¹; ΔH₁^≠=60.5±6.6 kJ mol⁻¹; ΔS₁^≠=−14±22 J mol⁻¹K⁻¹; k₋₂=0.134±0.081 s⁻¹; ΔH₋₂^≠=74.1±24.3 kJ mol⁻¹; ΔS₋₂^≠=−10±82 J mol⁻¹K⁻¹; and k₋₁/k₂=0.479±0.012. The reaction of [PtMe₃(OAc)(bipy)] with NaSCN: k₁=26.4±0.3 s⁻¹; ΔH₁^≠=59.6±6.7 kJ mol⁻¹; ΔS₁^≠=−17±23 J mol⁻¹K⁻¹; k₋₂=0.174±0.200 s⁻¹; ΔH₋₂^≠=62.7±10.3 kJ mol⁻¹; ΔS₋₂^≠=−48±35 J mol⁻¹K⁻¹; and k₋₁/k₂=1.01±0.08. The reaction of [PtMe₃(OOCH)(bipy)] with NaN₃: k₁=36.8±0.3 s⁻¹; ΔH₁^≠=66.4±4.7 kJ mol⁻¹; ΔS₁^≠=7±16 J mol⁻¹K⁻¹; k₋₂=0.164±0.076 s⁻¹; ΔH₋₂^≠=47.0±18.1 kJ mol⁻¹; ΔS₋₂^≠=−101±61 J mol⁻¹ K⁻¹; and k₋₁/k₂=5.90±0.18. The reaction of [PtMe₃(OOCH)(bipy)] with NaSCN: k₁ =33.5±0.2 s⁻¹; ΔH₁^≠=58.0±0.4 kJ mol⁻¹; ΔS₁^≠=−20.5±1.6 J mol⁻¹ K⁻¹; k₋₂=0.222±0.083 s⁻¹; ΔH₋₂^≠=54.9±6.3 kJ mol⁻¹; ΔS₋₂^≠=−73.0±21.3 J mol⁻¹ K⁻¹; and k₋₁/k₂=12.0±0.3. Conditional pseudo-first-order rate constant k₀ increased linearly with the concentration of NaY, while it decreased drastically with the concentration of NaX. Some plausible mechanisms were examined, and the following mechanism was proposed. [Note to reader: Please see article pdf to view this scheme.] © 1998 John Wiley & Sons, Inc. Int J Chem Kinet 30: 523–532, 1998     

Functionally Rigid and Degenerate Molecular Shuttles

The preparation and dynamic behavior of two functionally rigid and degenerate [2]rotaxanes ( 1⋅ 4 PF₆ and 2⋅ 4 PF₆) in which a π-electron deficient tetracationic cyclophane, cyclobis(paraquat-p-phenylene) (CBPQT⁴⁺) ring, shuttles back and forth between two π-electron-rich naphthalene (NP) stations by making the passage along an ethynyl-phenylene-(PH)-ethynyl or butadiyne rod, are described. The [2]rotaxanes were synthesized by using the clipping approach to template-directed synthesis, and were characterized by NMR spectroscopic and mass spectrometric analyses. ¹H NMR spectra of both [2]rotaxanes show evidence for the formation of mechanically interlocked structures, resulting in the upfield shifts of the resonances for key protons on the dumbbell-shaped components. In particular, the signals for the peri protons on the NP units in the dumbbell-shaped components experienced significant upfield shifts at low temperatures, just as has been observed in the flexible [2]rotaxanes. Interestingly, the resonances for the same protons did not exhibit a significant upfield shift at 298 K, but rather only a modest shift. This phenomenon arises from the much reduced binding of the ethynyl-NP unit compared to the 1,5-dioxy-NP unit. This effect, in turn, increases the shuttling rate when compared to the 1,5-dioxy-NP-based rotaxane systems investigated previously. The kinetic and thermodynamic data of the shuttling behavior of the CBPQT⁴⁺ ring between the NP units were obtained by variable-temperature NMR spectroscopy and using the coalescence method to calculate the free energies of activation (ΔG_c^≠) of 9.6 and 10.3 kcal mol⁻¹ for 1⋅ 4 PF₆ and 2⋅ 4 PF₆, respectively, probed by using the rotaxane's α-bipyridinium protons. The solid-state structure of the free dumbbell-shaped compound ( 3 ) shows the fully rigid ethynyl-PH-ethynyl linker with a length (8.1 Å) twice as long as that (3.8 Å) of the butadiyne linker. Full-atomistic simulations were carried out with the DREIDING force field (FF) to probe the degenerate molecular shuttling processes, and afforded shuttling energy barriers (ΔG^≠=10.4 kcal mol⁻¹ for 1⋅ 4 PF₆ and 2⋅ 4 PF₆) that are in good agreement with the experimental values (ΔG_c^≠=9.6 and 10.3 kcal mol⁻¹ for 1⋅ 4 PF₆ and 2⋅ 4 PF₆, respectively, probed by using their α-bipyridinium protons).     

Low temperature 13C NMR study of 1-(3-pentyloxyphenylcarbamoyloxy)-2-dialkyl- aminocyclohexanes

Cyclohexane and piperidine ring reversal in 1-(3-pentyloxyphenylcarbamoyloxy)-2-dialkylaminocyclohexanes was investigated by ¹³C NMR. An unusually low conformational energy ΔG = 0.59 kJ mol^?1 and activation parameters ΔG^≠₂₁₈ = 43.8 ± 0.4 kJ mol^?1, ΔH^≠ = 48.9 ± 2.5 kJ mol^?1 and ΔS^≠ = 23 ± 9 J mol^?1 K^?1 were found for the diequatorial to diaxial transition of the cyclohexane ring in the trans-pyrrolidinyl derivative. In the trans-piperidinyl derivative, ΔG₂₂₂ = 44.7 ± 0.5 KJ mol^?1, ΔH^≠ = 55.7 ± 6.3 kJ mol^?1 and ΔS^≠ = 51 ± 21 J mol^?1 K^?1 was found for the piperidine ring reversal from the non-equivalence of the α-carbons.     

Mechanistic Aspects of Ligand Substitution on the Hydroxopentaaquarhodium(III) Ion in Aqueous Solution by Sulfur‐Containing Bioactive Ligands

The kinetics of the interactions between three sulfur‐containing ligands, thioglycolic acid, 2‐thiouracil, glutathione, and the title complex, have been studied spectrophotometrically in aqueous medium as a function of the concentrations of the ligands, temperature, and pH at constant ionic strength. The reactions follow a two‐step process in which the first step is ligand‐dependent and the second step is ligand‐independent chelation. Rate constants (k₁ ～10^?3 s^?1 and k₂ ～10^?5 s^?1) and activation parameters (for thioglycolic acid: ΔH₁^≠ = 22.4 ± 3.0 kJ mol^?1, ΔS₁^≠ = ?220 ± 11 J K^?1 mol^?1, ΔH₂^≠ = 38.5 ± 1.3 kJ mol^?1, ΔS₂^≠ = ?204 ± 4 J K^?1 mol^?1; for 2‐thiouracil: ΔH₁^≠ = 42.2 ± 2.0 kJ mol^?1, ΔS₁^≠ = ?169 ± 6 J K^?1 mol^?1, ΔH₂^≠ = 66.1 ± 0.5 kJ mol^?1, ΔS₂^≠ = ?124 ± 2 J K^?1 mol^?1; for glutathione: ΔH₁^≠ = 47.2 ± 1.7 kJ mol^?1, ΔS₁^≠ = ?155 ± 5 J K^?1mol^?1, ΔH₂^≠ = 73.5 ± 1.1 kJ mol^?1, ΔS₂^≠ = ?105 ± 3 J K^?1 mol^?1) were calculated. Based on the kinetic and activation parameters, an associative interchange mechanism is proposed for the interaction processes. The products of the reactions have been characterized from IR and ESI mass spectroscopic analysis. A rate law involving the outer sphere association complex formation has been established as      

The kinetics of pentoxyl oxidation by hypochlorite ions

The kinetics of pentoxyl (I) oxidation in aqueous media under the action of hypochlorite ions was studied at pH 8.8 and 273–298 K. The order of the reaction with respect to both participants was found to be one. The temperature dependence of the reaction rate obeyed the Arrhenius law. The reaction activation parameters were found to be E _a=11.08 kJ/mol, ΔH ^≠=8.73 kJ/mol, ΔS ^≠=?200.70 J/(mol K), and ΔG ^≠=66.88 kJ/mol. Reaction stoichiometry was studied, the chemical characteristics of the process considered, and a mechanism of the oxidative transformation of I under the action of OCl^? suggested.     

Kinetics and Mechanism Study of the Substitution Reactions of the Chloride Ligand in Dichloro(5,10,15,20)‐tetraphenyl‐21H,23H‐porphinato)tin(IV) by Organic Bases in Dimethylformamide Solvent

Substitution reactions of a Cl⁻ ligand in [SnCl₂(tpp)] (tpp=5,10,15,20‐tetraphenyl‐21H,23H‐porphinato(2−)) by five organic bases i.e., butylamine (BuNH₂), sec‐butylamine (^sBuNH₂), tert‐butylamine (^tBuNH₂), dibutylamine (Bu₂NH), and tributylamine (Bu₃N), as entering nucleophile in dimethylformamide at I=0.1M (NaNO₃) and 30–55° were studied. The second‐order rate constants for the substitution of a Cl⁻ ligand were found to be (36.86±1.14)⋅10⁻³, (32.91±0.79)⋅10⁻³, (22.21±0.58)⋅10⁻³, (19.09±0.66)⋅10⁻³, and (1.36±0.08)⋅10⁻³ M ⁻¹s⁻¹ at 40° for BuNH₂, ^tBuNH₂, ^sBuNH₂, Bu₂NH, and Bu₃N, respectively. In a temperature‐dependence study, the activation parameters ΔH^≠ and ΔS^≠ for the reaction of [SnCl₂(tpp)] with the organic bases were determined as 38.61±4.79 kJ mol⁻¹ and −150.40±15.46 J K⁻¹mol⁻¹ for BuNH₂, 40.95±4.79 kJ mol⁻¹ and −143.75±15.46 J K⁻¹mol⁻¹ for ^tBuNH₂, 30.88±2.43 kJ mol⁻¹ and −179.00±7.82 J K⁻¹mol⁻¹ for ^sBuNH₂, 26.56±2.97 kJ mol⁻¹ and −194.05±9.39 J K⁻¹mol⁻¹ for Bu₂NH, and 39.37±2.25 kJ mol⁻¹ and −174.68±7.07 J K⁻¹ mol⁻¹ for Bu₃N. From the linear rate dependence on the concentration of the bases, the span of k₂ values, and the large negative values of the activation entropy, an associative (A) mechanism is deduced for the ligand substitution.     

Dimethylformamide-Exchange Mechanism on Some First-Row Divalent Transition-Metal Ions

The effect of temperature on the dimethylformamide exchange on Mn(DMF) and Fe(DMF) has been studied by ¹³C- and ¹⁷O-NMR, respectively, yielding the following kinetic parameters: k²⁹⁸ equals; (2.2±0.2). 10⁶ S^?1, ΔH^≠ = 34.6 ± 1.3 kJ mol^?1, ΔS^≠ = ?7.4 ± 4.8 J K^?1mol^?1 for Mn²⁺ and K²⁹⁸ = (9.7 ± 0.2).10⁵ S^?1, Delta;H^≠ = 43.0 ± 0.9 kJ mol^?1, ΔS^≠ = + 13.8 ± 2.8 J K^?1mol^?1 for Fe²⁺. The volumes of activation, ΔV^≠ in cm³mol^?1, derived from high-pressure NMR on these metal ions, together with the previously published activation volumes for Co²⁺ and Ni²⁺ (+2.4 ± 0.2 (Mn²⁺), +8.5 ± 0.4 (Fe²⁺) +9.2 ± 0.3 (Co²⁺), + 9.1 ± 0.3 (Ni²⁺)) give evidence for a dissociative activation mode for DMF exchange on these high-spin first-row transition-metal divalent ions. The small positive ΔV^≠ value observed for DMF exchange on Mn²⁺ seems to indicate that a mechanistic changeover also occurs along the series, (probably from I_d to D), as for the other solvents previously studied (I_a to I_d, for H₂O, MeOH, MeCN). This changeover is shifted to the earlier elements of the series, due to more pronounced steric crowding for dimethylformamide hexasolvates.     

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

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

Steric and electronic contributions to barriers of internal rotation in 1,8-dibenzoylnaphthalenes

Restricted rotation about the naphthalenylcarbonyl bonds in the title compounds resulted in mixtures of cis and trans rotamers, the equilibrium and the rotational barriers depending on the substituents. For 2,7-dimethyl-1,8-di-(p-toluoyl)-naphthalene (1) ΔH° = 3.66 ± 0.14 kJ mol^?1, ΔS° = 1.67 ± 0.63 J mol^?1 K^?1, ΔH^≠_ct = 55.5 ± 1.3 kJ mol^?1, ΔH^≠_ct = 51.9 ± 1.3 kJ mol^?1, ΔS^≠_ct = ?41.3±4.1 J mol^?1 K^?1 and ΔS^≠_ct = ?42.9±4.1 J mol^?1 K^?1. The rotation about the phenylcarbonyl bond requires ΔH^≠ = ?56.9±4.4 kJ mol^?1 and ΔS^≠ = ?20.5±15.3 J mol^?1 K^?1 for the cis rotamer, and ΔH^≠ = 43.5Δ0.4 kJ mol^?1 and ΔS^≠ =± ?22.4Δ1.3 J mol^?1 K^?1 for the trans rotamer. The role of electronic factors is likely to be virtually the same for both these rotamers but steric interaction between the two phenyl rings occurs in the cis rotamer only. Hence, the difference of the activation enthalpies obtained for the cis and trans rotamers, ΔΔH^?1 = 13.4 kJ mol^?1, provides a basis for the estimation of the role of steric factors in this rotation. For the tetracarboxylic acid 2 and its tetramethyl ester 3 the equilibrium is even more shifted towards the trans form because of enhanced steric and electrostatic interactions between the substituents in the cis form. The barriers for the rotation around the phenylcarbonyl bond and the cis-trans isomerization are lowered; an explanation for this result is presented.     

High-level computational study on the thermochemistry of saturated and unsaturated three- and four-membered nitrogen and phosphorus rings

The heats of formation and strain energies for saturated and unsaturated three- and four-membered nitrogen and phosphorus rings have been calculated using G2 theory. G2 heats of formation (ΔH_f298) of triaziridine [(NH)₃], triazirine (N₃H), tetrazetidine [(NH)₄], and tetrazetine (N₄H₂) are 405.0, 453.7, 522.5, and 514.1 kJ mol⁻¹, respectively. Tetrazetidine is unstable (121.5 kJ mol⁻¹ at 298 K) with respect to its dissociation into two trans-diazene (N₂H₂) molecules. The dissociation of tetrazetine into molecular nitrogen and trans-diazene is highly exothermic (ΔH₂₉₈ = −308.3 kJ mol⁻¹ calculated using G2 theory). G2 heats of formation (ΔH_f298) of cyclotriphosphane [(PH)₃], cyclotriphosphene (P₃H), cyclotetraphosphane [(PH)₄], and cyclotetraphosphene (P₄H₂) are 80.7, 167.2, 102.7, and 170.7 kJ mol⁻¹, respectively. Cyclotetraphosphane and cyclotetraphosphene are stabilized by 145.8 and 101.2 kJ mol⁻¹ relative to their dissociations into two diphosphene molecules or into diphosphene (HP(DOUBLE BOND)PH) and diphosphorus (P₂), respectively. The strain energies of triaziridine [(NH)₃], triazirine (N₃H), tetrazetidine [(NH)₄], and tetrazetine (N₄H₂) were calculated to be 115.0, 198.3, 135.8, and 162.0 kJ mol⁻¹, respectively (at 298 K). While the strain energies of the nitrogen three-membered rings in triaziridine and triazirine are smaller than the strain energies of cyclopropane (117.4 kJ mol⁻¹) and cyclopropene (232.2 kJ mol⁻¹), the strain energies of the nitrogen four-membered rings in tetrazetidine and tetrazetine are larger than those of cyclobutane (110.2 kJ mol⁻¹) and cyclobutene (132.0 kJ mol⁻¹). In contrast to higher strain in cyclopropane as compared with cyclobutane, triaziridine is less strained than tetrazetidine. The strain energies of cyclotriphosphane [(PH)₃, 21.8 kJ mol⁻¹], cyclotriphosphene (P₃H, 34.6 kJ mol⁻¹), cyclotetraphosphane [(PH)₄, 24.1 kJ mol⁻¹], and cyclotetraphosphene (P₄H₂, 18.5 kJ mol⁻¹), calculated at the G2 level are considerably smaller than those of their carbon and nitrogen analog. Cyclotetraphosphene containing the P(DOUBLE BOND)P double bond is less strained than cyclotetraphosphane, in sharp contrast to the ratio between the strain energies for the analogous unsaturated and saturated carbon and nitrogen rings. © 1997 John Wiley & Sons, Inc. Int J Quant Chem 62 : 373–384, 1997     

Enantiomerization of an inherently chiral resorcarene derivative: determination of the interconversion barrier by computer simulation of the dynamic HPLC experiment

The inherently chiral tetrabenzoxazine resorcarene derivative 1 shows characteristic plateau-formation during enantioselective HPLC separation on the chiral stationary phase Chiralpak AD. By computer assisted peak form analysis of the elution profiles, obtained from temperature dependent dynamic HPLC (DHPLC) experiments, with ChromWin, the enantiomerization barrier ΔG^#(298 K)=92±2 kJ mol⁻¹ and the activation parameters ΔH^#=53.0±1.8 kJ mol⁻¹ and ΔS^#=−131±14 J (K mol)⁻¹ were determined.     

Kinetics of substitution of aqua ligands from cis‐diaqua (ethylenediamine)platinum(II) perchlorate by L‐asparagine in aqueous medium

The kinetics of the interaction of L ‐asparagine with [Pt(ethylenediamine)(H₂O)₂]²⁺ have been studied spectrophotometrically as a function of [Pt(ethylenediamine)(H₂O)₂²⁺], [L ‐asparagine], and temperature at pH 4.0, where the substrate complex exists predominantly as the diaqua species and L ‐asparagine as the zwitterion. The substitution reaction shows two consecutive steps: the first step is the ligand‐assisted anation and the second one is the chelation step. Activation parameters for both the steps have been calculated using Eyring equation. The low ΔH₁^≠ (43.59 ± 0.96 kJ mol^?1) and large negative values of ΔS₁^≠ (?116.98 ± 2.9 J K^?1 mol^?1) as well as ΔH₂^≠ (33.78 ± 0.51 kJ mol^?1) and ΔS₂^≠ (?221.43 ± 1.57 J K^?1 mol^?1) indicate an associative mode of activation for both the aqua ligand substitution processes. © 2003 Wiley Periodicals, Inc. Int J Chem Kinet 35: 252–259, 2003     

Bistable or Oscillating State Depending on Station and Temperature in Three‐Station Glycorotaxane Molecular Machines

High‐yield, straightforward synthesis of two‐ and three‐station [2]rotaxane molecular machines based on an anilinium, a triazolium, and a mono‐ or disubstituted pyridinium amide station is reported. In the case of the pH‐sensitive two‐station molecular machines, large‐amplitude movement of the macrocycle occurred. However, the presence of an intermediate third station led, after deprotonation of the anilinium station, and depending on the substitution of the pyridinium amide, either to exclusive localization of the macrocycle around the triazolium station or to oscillatory shuttling of the macrocycle between the triazolium and monosubstituted pyridinium amide station. Variable‐temperature ¹H NMR investigation of the oscillating system was performed in CD₂Cl₂. The exchange between the two stations proved to be fast on the NMR timescale for all considered temperatures (298–193 K). Interestingly, decreasing the temperature displaced the equilibrium between the two translational isomers until a unique location of the macrocycle around the monosubstituted pyridinium amide station was reached. Thermodynamic constants K were evaluated at each temperature: the thermodynamic parameters ΔH and ΔS were extracted from a Van′t Hoff plot, and provided the Gibbs energy ΔG. Arrhenius and Eyring plots afforded kinetic parameters, namely, energies of activation E_a, enthalpies of activation ΔH^≠, and entropies of activation ΔS^≠. The ΔG values deduced from kinetic parameters match very well with the ΔG values determined from thermodynamic parameters. In addition, whereas signal coalescence of pyridinium hydrogen atoms located next to the amide bond was observed at 205 K in the oscillating rotaxane and at 203 K in the two‐station rotaxane with a unique location of the macrocycle around the pyridinium amide, no separation of ¹H NMR signals of the considered hydrogen atoms was seen in the corresponding nonencapsulated thread. It is suggested that the macrocycle acts as a molecular brake for the rotation of the pyridinium–amide bond when it interacts by hydrogen bonding with both the amide NH and the pyridinium hydrogen atoms at the same time.     

Activation of the aromatic system by the SO2CF3 group: Kinetics study and structure–reactivity relationships

The rate constants for the reaction of 2,6‐bis(trifluoromethanesulfonyl)‐4‐nitroanisole with some substituted anilines have been measured by spectrophotometric methods in methanol at various temperatures. The data are consistent with the S_NAr mechanism. The effect of substituents on the rate of reaction has been examined. Good linear relationships were obtained from the plots of log k₁ against Hammett σ_para constants values at all temperature with negative ρ values (?1.68 to ?1.11). Activation parameters Δ^≠H varied from 41.6 to 54.3 kJ mol^?1 and Δ^≠S from ?142.7 to ?114.6 J mol^?1 K^?1. The δΔ^≠H and δΔ^≠S reaction constants were determined from the dependence of Δ^≠H and Δ^≠S activation parameters on the σ substituent constants, by analogy with the Hammett equation. A plot of Δ^≠H versus Δ^≠S for the reaction gave good straight line with 177°C isokinetic temperature. © 2010 Wiley Periodicals, Inc. Int J Chem Kinet 42: 203–210, 2010     

Kinetics and mechanism of decomposition of intermediate complex during oxidation of pectate polysaccharide by potassium permanganate in alkaline solutions

The kinetics of decomposition of an [Pect·Mn^VIO₄^2?] intermediate complex have been investigated spectrophotometrically at various temperatures of 15–30°C and a constant ionic strength of 0.1 mol dm^?3. The decomposition reaction was found to be first‐order in the intermediate concentration. The results showed that the rate of reaction was base‐catalyzed. The kinetic parameters have been evaluated and found to be ΔS^≠ = ? 190.06 ± 9.84 J mol^?1 K^?1, ΔH^≠ = 19.75 ± 0.57 kJ mol^?1, and ΔG^≠ = 76.39 ± 3.50 kJ mol^?1, respectively. A reaction mechanism consistent with the results is discussed. © 2002 Wiley Periodicals, Inc. Int J Chem Kinet 35: 67–72, 2003     

Ab initio studies of three-membered ring formation through intramolecular nucleophilic substitution

Three-membered ring (3MR) forming processes of ⁻X(SINGLE BOND)CH₂(SINGLE BOND)CH₂(SINGLE BOND)F and ⁻CH₂(SINGLE BOND)C((SINGLE BOND)Y)(SINGLE BOND)CH₂(SINGLE BOND)F (X(DOUBLE BOND)CH₂, O, or S and Y(DOUBLE BOND)0 or S) through a gas phase neighboring group mechanism (S_Ni) are studied theoretically using the ab initio molecular orbital method with the 6–31+G* basis set. When electron correlation effects are considered, the activation (ΔG^≠) and reaction energies (ΔG⁰) are lowered by ca. 10 kcal mol⁻¹, indicating the importance of the electron correlation effect in these reactions. The contribution of entropy of activation (−TΔS^≠) at 298 K to ΔG^≠ is very small, and the reactions are enthalpy controlled. The ΔG^≠ and ΔG⁰ values for these ring closure processes largely depend on the stabilities of the reactants and the heteroatom acting as a nucleophilic center. The Bell–Evans–Polanyi principle applies well to all these reaction series. © 1997 John Wiley & Sons, Inc. J Comput Chem 18 : 1773–1784, 1997     

Mechanistic insights into photochromic behavior of a ruthenium(II)-pterin complex

The pterin‐coordinated ruthenium complex, [Ru^II(dmdmp)(tpa)]⁺ ( 1 ) (Hdmdmp=N,N‐dimethyl‐6,7‐dimethylpterin, tpa=tris(2‐pyridylmethyl)amine), undergoes photochromic isomerization efficiently. The isomeric complex ( 2 ) was fully characterized to reveal an apparent 180° pseudorotation of the pterin ligand. Photoirradiation to the solution of 1 in acetone with incident light at 460 nm resulted in dissociation of one pyridylmethyl arm of the tpa ligand from the Ru^II center to give an intermediate complex, [Ru(dmdmp)(tpa)(acetone)]²⁺ ( I ), accompanied by structural change and the coordination of a solvent molecule to occupy the vacant site. The quantum yield (?) of this photoreaction was determined to be 0.87 %. The subsequent thermal process from intermediate I affords an isomeric complex 2 , as a result of the rotation of the dmdmp^2? ligand and the recoordination of the pyridyl group through structural change. The thermal process obeyed first‐order kinetics, and the rate constant at 298 K was determined to be 5.83×10^?5 s^?1. The activation parameters were determined to be ΔH^≠=81.8 kJ mol^?1 and ΔS^≠=?49.8 J mol^?1 K^?1. The negative ΔS^≠ value indicates that this reaction involves a seven‐coordinate complex in the transition state (i.e., an interchange associative mechanism). The most unique point of this reaction is that the recoordination of the photodissociated pyridylmethyl group occurs only from the direction to give isomer 2 , without going back to starting complex 1 , and thus the reaction proceeds with 100 % conversion efficiency. Upon heating a solution of 2 in acetonitrile, isomer 2 turned back into starting complex 1 . The backward reaction is highly dependent on the solvent: isomer 2 is quite stable and hard to return to 1 in acetone; however, 2 was converted to 1 smoothly by heating in acetonitrile. The activation parameters for the first‐order process in acetonitrile were determined to be ΔH^≠=59.2 kJ mol^?1 and ΔS^≠=?147.4 kJ mol^?1 K^?1. The largely negative ΔS^≠ value suggests the involvement of a seven‐coordinate species with the strongly coordinated acetonitrile molecule in the transition state. Thus, the strength of the coordination of the solvent molecule to the Ru^II center is a determinant factor in the photoisomerization of the Ru^II–pterin complex.     

Dynamic proton magnetic resonance studies on complex spin systems. Non-mutual three-spin exchange in N-(trideuteriomethyl)-2-cyanoaziridine

At room temperature and below, the proton NMR spectrum of N-(trideuteriomethyl)-2-cyanoaziridine consists of two superimposed ABC patterns assignable to two N-invertomers; a single time-averaged ABC pattern is observed at 158.9°C. The static parameters extracted from the spectra in the temperature range from –40.3 to 23.2°C and from the high-temperature spectrum permit the calculation of the thermodynamic quantities ΔH⁰ = ?475±20 cal mol^?1 (?1.987 ± 0.084 kJ mol^?1) and ΔS⁰ = 0.43±0.08 cal mol^?1 K^?1 (1.80±0.33 J mol^?1 K^?1) for the cis ? trans equilibrium. Bandshape analysis of the spectra broadened by non-mutual three-spin exchange in the temperature range from 39.4–137.8°C yields the activation parameters ΔH_tc^≠ = 17.52±0.18 kcal mol^?1 (73.30±0.75 kJ mol^?1), ΔS_tc^≠ = ?2.08±0.50 cal mol^?1 K^?1 (?8.70±2.09 J mol^?1 K^?1) and ΔG_tc^≠ (300 K) = 18.14±0.03 kcal mol^?1 (75.90±0.13 kJ mol^?1) for the trans → cis isomerization. An attempt is made to rationalize the observed entropy data in terms of the principles of statistical thermodynamics.     

Kinetics and mechanism of the interaction of adenosine with cis‐diaqua(cis‐1,2‐diaminocyclohexane)platinum(II) perchlorate in aqueous medium

The kinetics of the interaction of adenosine with cis‐[Pt(cis‐dach)(OH₂)₂]²⁺ (dach = diaminocyclohexane) was studied spectrophotometrically as a function of [cis‐[Pt(cis‐dach)(OH₂)₂]²⁺], [adenosine], and temperature at a particular pH (4.0), where the substrate complex exists predominantly as the diaqua species and the ligand adenosine exists as a neutral molecule. The substitution reaction shows two consecutive steps: the first is the ligand‐assisted anation followed by a chelation step. The activation parameters for both the steps have been evaluated using Eyring equation. The low negative value of ΔH^≠₁ (43.1 ± 1.3 kJ mol^?1) and the large negative value of ΔS^≠₁ (?177 ± 4 J K^?1 mol^?1) along with ΔH^≠₂ (47.9 ± 1.8 kJ mol^?1) and ΔS^≠₂ (?181 ± 6 J K^?1 mol^?1) indicate an associative mode of activation for both the aqua ligand substitution processes. The kinetic study was substantiated by infrared and electrospray ionization mass spectroscopic analysis. © 2011 Wiley Peiodicals, Inc. Int J Chem Kinet 43: 219–229, 2011