57-12-5Relevant articles and documents
Jenkins,Harris
, p. 4439 (1955)
Halpern, J.,Pribranic, M.
, p. 96 - 98 (1971)
Entropy-Driven Proton-Transfer Reactions
Meot-Ner, Michael
, p. 6580 - 6585 (1991)
The reation between kinetics and thermochemistry in fat reactions is examined, including reactions with substantial entropy changes.Rate constants for such reactions, in the range of (0.02-3.0) * 10-9 cm2 s-1, were measured by pulsed high-pressure mass spectrometry.The following relations were observed: (1) The reaction efficiency in either direction is controlled uniquely and completely by the overall reaction free energy change.Specifically, the efficieny r is determined by the equilibrium constant according to r= K/(1+K). (2) The sum of reaction efficiencies in the forward (exergonic) and reverse (endergonic) directions is near unity (rf + rr = ca. 1).These relations are obseved in anionic and cationic systems, in reactions with ΔH0 up to 12 kcal/mol and with ΔS0 up to 15 cal/(mol K).Consistent with (1), reactions that are endothermic up to 7 kcal/mol can nevertheless proceed near the collision rate, when positive entropy changes make the reactions exergonic.The entropy canges are effective regardless of their stuctural origin.Relations analogous to (1) and (2) are also derived for reactions with multiple channels that proceed without significant barriers through a common intermediate.
-
Burgess, W. M.,Holden, F. R.
, p. 459 - 462 (1937)
-
Mechanistic diversity covering 15 orders of magnitude in rates: Cyanide exchange on [M(CN)4]2- (M = Ni, Pd, and Pt)
Monlien, Florence J.,Helm, Lothar,Abou-Hamdan, Amira,Merbach, Andre E.
, p. 1717 - 1727 (2002)
Kinetic studies of cyanide exchange on [M(CN)4]2- square-planar complexes (M = Pt, Pd, and Ni) were performed as a function of pH by 13C NMR. The [Pt(CN)4]2- complex has a purely second-order rate law, with CN- as acting as the nucleophile, with the following kinetic parameters: (k2Pt,CN)298 = 11 ± 1 s-1 mol-1 kg, ΔH2? Pt,CN = 25.1 ± 1 kJ mol-1, ΔS2? Pt,CN = -142 ± 4 J mol-1 K-1, and ΔV2? Pt,CN = -27 ± 2 cm3 mol-1. The Pd(II) metal center has the same behavior down to pH 6. The kinetic parameters are as follows: (k2Pd,CN)298 = 82 ± 2 s-1 mol-1 kg, ΔH2? Pd,CN = 23.5 ± 1 kJ mol-1, ΔS2? Pd,CN = -129 ± 5 J mol-1 K-1, and ΔV2? Pd,CN = -22 ± 2 cm3 mol-1. At low pH, the tetracyanopalladate is protonated (pKa Pd(4,H) = 3.0 ± 0.3) to form [Pd(CN)3HCN]-. The rate law of the cyanide exchange on the protonated complex is also purely second order, with (k2PdH,CN)298 = (4.5 ± 1.3) × 103 s-1 mol-1 kg. [Ni(CN)4]2- is involved in various equilibrium reactions, such as the formation of [Ni(CN)5]3-, [Ni(CN)3HCN]-, and [Ni(CN)2(HCN)2] complexes. Our 13C NMR measurements have allowed us to determine that the rate constant leading to the formation of [Ni(CN)5]3- is k2Ni(4),CN = (2.3 ± 0.1) × 106 s-1 mol-1 kg when the following activation parameters are used: ΔH2? Ni,CN = 21.6 ± 1 kJ mol-1, ΔS2? Ni,CN = -51 ± 7 J mol-1 K-1, and ΔV2? Ni,CN = -19 ± 2 cm3 mol-1. The rate constant of the back reaction is k-2Ni(4),CN = 14 × 106 s-1. The rate law pertaining to [Ni(CN)2(HCN)2] was found to be second order at pH 3.8, and the value of the rate constant is (k2 Ni(4,2H),CN,)298 = (63 ± 15) × 106 s-1 mol-1 kg when ΔH2? Ni(4,2H),CN = 47.3 ± 1 kJ mol-1, ΔS2? Ni(4,2H),CN = 63 ± 3 J mol-1 K-1, and ΔV2? Ni(4,2H),CN = -6 ± 1 cm3 mol-1. The cyanide-exchange rate constant on [M(CN)4]2- for Pt, Pd, and Ni increases in a 1:7:200 000 ratio. This trend is modified at low pH, and the palladium becomes 400 times more reactive than the platinum because of the formation of [Pd(CN)3HCN]-. For all cyanide exchanges on tetracyano complexes (A mechanism) and on their protonated forms (IIIa mechanisms), we have always observed a pure second-order rate law: first order for the complex and first order for CN-. The nucleophilic attack by HCN or solvation by H2O is at least nine or six orders of magnitude slower, respectively than is nucleophilic attack by CN- for Pt(II), Pd(II), and Ni(II), respectively.
Carbon Monoxide Dehydrogenase Reduces Cyanate to Cyanide
Ciaccafava, Alexandre,Tombolelli, Daria,Domnik, Lilith,Jeoung, Jae-Hun,Dobbek, Holger,Mroginski, Maria-Andrea,Zebger, Ingo,Hildebrandt, Peter
, p. 7398 - 7401 (2017)
The biocatalytic function of carbon monoxide dehydrogenase (CODH) has a high environmental relevance owing to its ability to reduce CO2. Despite numerous studies on CODH over the past decades, its catalytic mechanism is not yet fully understood. In the present combined spectroscopic and theoretical study, we report first evidences for a cyanate (NCO?) to cyanide (CN?) reduction at the C-cluster. The adduct remains bound to the catalytic center to form the so-called CN?-inhibited state. Notably, this conversion does not occur in crystals of the Carboxydothermus hydrogenoformans CODH enzyme (CODHIICh), as indicated by the lack of the corresponding CN? stretching mode. The transformation of NCO?, which also acts as an inhibitor of the two-electron-reduced Cred2 state of CODH, could thus mimic CO2 turnover and open new perspectives for elucidation of the detailed catalytic mechanism of CODH.
Stranks, D. R.,Harris, G. M.
, p. 2015 - 2016 (1953)
Difluorocarbene-based cyanodifluoromethylation of alkenes induced by a dual-functional Cu-catalyst
Zhang, Min,Lin, Jin-Hong,Jin, Chuan-Ming,Xiao, Ji-Chang
, p. 2649 - 2652 (2021)
Although cyanofluoroalkylation has received increasing attention, a toxic cyanation reagent is usually required. Herein, a Cu-catalyzed difluorocarbene-based cyanodifluoromethylation of alkenes with BrCF2CO2Et/NH4HCO3under photocatalytic conditions is described. BrCF2CO2Et and NH4HCO3serve as a carbon source and a nitrogen source of the nitrile group, respectively, avoiding the use of a stoichiometric toxic cyanation reagent. The Cu-complex plays a dual role. It is not only a photocatalyst, but also a coupling catalyst for the formation of a C-CN bond.
Catalytic Polarographic Wave of Fe(II) in Neutral Thiocyanate Solutions at Dropping Mercury Electrode
Himeno, Sadayuki,Saito, Atsuyoshi
, p. 1715 - 1719 (1981)
The electrochemical behavior of Fe(II) in neutral thiocyanate solutions has been investigated at a dropping mercury electrode (DME).It was found that Fe(II) in neutral thiocyanate solutions gave a catalytic polarographic wave at potentials prior to the main Fe(II) reduction wave.The mechanism of the catalytic process involves the chemical reduction of thiocyanate ions with Fe(OH)2,aq at the electrode surface.Controlled potential electrolysis suggests that the reduction of thiocyanate ions proceeds with the formation of sulfide and cyanide ions.Sulfide ions produced at the electrode surface can react with Fe(II) diffusing to the electrode to form FeS.The discharge of this is responsible for the catalytic current, while cyanide ions have no essential role in the catalytic process.The effects of surface active substances and iodate ions on the catalytic wave are also discussed.
Herlem, M.,Minet, J. J.,Thiebault, A.,Fave, G.
, p. 203 - 217 (1971)
DeLouise, L. A.,Winograd, N.
, p. 79 - 89 (1985)
Far-ultraviolet Solution Spectroscopy of Thiocyanate
Fox, Malcolm F.,Smith, Clifford B.,Hayon, Elie
, p. 1497 - 1502 (1981)
The far-ultraviolet solution of very dilute thiocyanate ion in a range of solvents shows that there are at least three absorption bands labelled A, D and E.All three bands are shown to have charge-transfer-to-solvent charasteristics, in contrast to some previous reports.The conflicting previous reports of the spectroscoy and photochemistry of thiocyanate ions, together with the current work, are resolved in terms of a spectroscopic transition scheme involving a forbidden at ca. 36 000 cm-1, which is normally extremely weak or not detectable.The transition is only detected at high contrentrations of salt.The first allowed transition is expected to occur at ca. 62 000 cm-1.Within this scheme the c.t.t.s. bands occur (in aqueous solution) at 46 000 and 53 500 cm-1.A further band is observed in red-shifting solvents at ca. 57 000 cm -1.The temperature sensitivities of the three c.t.t.s. bands, relative to the solvated electron, are 0.58, 0.47 and 0.30.
Low-energy dissociative electron attachment to BrCN and CBrCl3: Temperature dependences and reaction dynamics
Parthasarathy,Suess,Hill,Dunning
, p. 7962 - 7968 (2001)
The velocity and angular distributions of negative ions produced due to electron transfer in collisions with Rydberg atoms was measured to investigate low-energy dissociative electron attachment to BrCN and CBrCl3. Monte Carlo collision code wa
Reactions of Azine Anions with Nitrogen and Oxygen Atoms: Implications for Titan's Upper Atmosphere and Interstellar Chemistry
Wang, Zhe-Chen,Cole, Callie A.,Demarais, Nicholas J.,Snow, Theodore P.,Bierbaum, Veronica M.
, p. 10700 - 10709 (2015/09/28)
Azines are important in many extraterrestrial environments, from the atmosphere of Titan to the interstellar medium. They have been implicated as possible carriers of the diffuse interstellar bands in astronomy, indicating their persistence in interstellar space. Most importantly, they constitute the basic building blocks of DNA and RNA, so their chemical reactivity in these environments has significant astrobiological implications. In addition, N and O atoms are widely observed in the ISM and in the ionospheres of planets and moons. However, the chemical reactions of molecular anions with abundant interstellar and atmospheric atomic species are largely unexplored. In this paper, gas-phase reactions of deprotonated anions of benzene, pyridine, pyridazine, pyrimidine, pyrazine, and s-triazine with N and O atoms are studied both experimentally and computationally. In all cases, the major reaction channel is associative electron detachment; these reactions are particularly important since they control the balance between negative ions and free electron densities. The reactions of the azine anions with N atoms exhibit larger rate constants than reactions of corresponding chain anions. The reactions of azine anions with O atoms are even more rapid, with complex product patterns for different reactants. The mechanisms are studied theoretically by employing density functional theory; spin conversion is found to be important in determining some product distributions. The rich gas-phase chemistry observed in this work provides a better understanding of ion-atom reactions and their contributions to ionospheric chemistry as well as the chemical processing that occurs in the boundary layers between diffuse and dense interstellar clouds.
