Robert B. Jordan

Kinetics and mechanism of the reaction of aqueous iron(III) with ascorbic acid

The kinetics of the reaction of iron(III) perchlorate with ascorbic acid in dilute aqueous acid have been studied by stopped-flow spectrophotometry at 16.1°C, under conditions of [iron(III)] ((2−7)×10 −3 M) greater than [ascorbic acid] (2×10 −4 M) and [H + ] between 0.01 and 0.15 M. The reaction is biphasic with a rapid increase in absorbance at 560 nm followed by a decrease. The faster reaction is assigned to formation of an iron(III)-ascorbate complex consistent with previous observations

Rotating-frame relaxation rates of solvent molecules in solutions of paramagnetic ions undergoing solvent exchange

Abstract The theory is developed for the dependence of the rotating-frame relaxation rate (R1ϱ) of bulk solvent nuclei on the magnitude of the spin-locking field H 1 (= − ω 1 γ for a dilute solution of a paramagnetic ion dissolved in a solvent which undergoes exchange between bulk and coordinated sites. When certain conditions are satisfied R1ϱ will vary with ω1, according to R1ϱ = R1ϱ∞ + Sϱ2(R2 − R1ϱ)ω1−2, 2, where R1ϱ∞ = R2s + PmrmR2m(R2m + rm)−1, Sϱs2 = (R1m + rm)(R2m + rm)−1 {(R2m + rm)2 + Δωm2}, R1m, R2m are the relaxation rates in the coordinated site, Δωm is the chemical shift between the bulk and coordinated sites, rm is the solvent exchange rate from a coordinated site, R2s is the sum of the pure solvent and outer-sphere relaxation rates, and Pm = (no. coordinated solvent molecules) × (metal-ion molality/solvent molality). Data for cobalt(II) in CH3OD and nickel(II) in CH3CN are used to show that the R1ϱ measurements are especially useful in the determination of Δωm. The latter can be combined with bulk solvent shifts to determine the number of coordinated solvent molecules.

EPR Linewidth Study of Vanadyl Complexes in Various Solvents

The temperature dependences of the EPR linewidths for vanadyl acetylacetonate, VO(acac)2, have been measured in ethanol, n‐propanol, isopropanol, n‐butanol, t‐butanol, trifluoroethanol, and trichloroethanol. A similar study has been carried out on vandayl trifluoroacetylacetonate, VO(tfac)2, in methanol and the solvated vanadyl ion, VO2+, in water. The EPR linewidths have been measured at 25°C for VO2+ in dimethyl sulfoxide, trimethyl phosphate, acetonitrile, N ,N‐dimethylformamide, and methanol. The results are interpreted in terms of the theory proposed by Kivelson et al. to obtain rotational correlation times (τR) for the various systems. The results generally conform to the predictions of the modified Debye theory, in that τR and solvent visocity show the expected temperature dependence. Only VO(tfac)2 in methanol appears to be anomalous in this respect. A detailed analysis has shown that one of the parameters (γ) in Kivelsons expression for the linewidths is less sensitive to experimental error and ...

NMR and EPR Line‐Broadening Effects of Vanadyl Ion in N,N‐Dimethylformamide

The temperature dependence of the NMR linewidths of the formyl proton in solutions of VO2+ in N,N‐dimethylformamide (DMF) indicates that proton relaxation is controlled by outer‐sphere dipole–dipole interactions, chemical exchange, and inner‐sphere dipole–dipole interactions, in the temperature ranges −55°–+6°, +7°–107°, and 108°–150°, respectively. At 25°C the rate constant for chemical exchange is (2.3 / n) × 103sec−1, ΔH† = 7.25 kcal / mole, and ΔS† = (−18.2 − 4.58 logn) eu, where n is the number of exchanging DMF molecules. The linewidths of the EPR spectrum of VO2+ in DMF solutions have been interpreted in terms of the theory developed by Kivelson. The rotational correlation time, τc = 1.2 × 10−10sec, calculated from this theory is shown to be consistent with the observed EPR linewidths and adequately explains the dipole–dipole line broadening observed in the NMR spectra.

The steric effect on the ring opening process of the decarboxylation of cis-carbonato-bis(diamine)cobalt(III) ions

Abstract The aquation of carbonatotetraaminecobalt(III) ions; [Co(N 2 ) 2 CO 3 ] + where (N 2 ) is 1,1,2,2-tetramethylethylenediamine (tme) and N,N′ -dimethylethylenediamine (bmen) has been studied in aqueous 1.0 M HClO 4 /NaClO 4 . For tme, the HClO 4 is 0.01-0.20 M and the temperature is 20, 25, 30 and 35 °C; for bmen, the HClO 4 is 0.15-0.55 M and the temperature is 55 and 63 °C. Both complexes hydrolyse to form the cis -diaqua product, and the rate law is d(ln[complex])/d t = k o + k 1 [H 3 O + ]. The values of the rate constant (25 °C), Δ H ≠ (kcal mol −1 ) and ΔS ≠ (cal mol −1 K −1 ) are: for [Co(tme) 2 CO 3 ] + , k o =2.59×10 −4 s −1 , Δ H o ≠ =18.6±1.8, ΔS o ≠ =−12.6±8.5; k 1 =2.86×10 −2 M −1 s −1 , ΔH 1 ≠ =ll.4±1.0, ΔS 1 ≠ =−27.5±3.4; for [Co(bmen) 2 CO 3 ] + , k o =1.33×10 −6 s −1 , ΔH o ≠ =4.6, ΔS o ≠ = −66; k 1 =5.54× 10 −6 M −1 s −1 , ΔH 1 ≠ =28.3, ΔS 1 ≠ =12.2. The tme system shows a deuterium isotope effect with k 1 D / k 1 H =2.2, consistent with a rapid pre-equilibrium protonation followed by rate controlling ring opening. The variations of k 1 with the amine ligand are in the order (en) 2 ⋍(pn) 2 >(tme) 2 ⪢(bmen) 2 . Since the latter two systems have almost the same electron donor ability, based on their pK a values, their large reactivity difference must be ascribed to steric effects of the −N(CH 3 ) groups in bmen. The data from a large number of previous studies of such carbonate chelate ring openings have been reanalysed, and the reactivity patterns are discussed.

Phosphorus‐31 Nuclear Magnetic Relaxation Times in the PBr3–PBr2Cl–PBrCl2–PCl3 System

The NMR transverse and longitudinal relaxation times of 31P in a liquid mixture of PCl3 and PBr3, and in a liquid mixture of PBr3, PBr2Cl, PBrCl2, and PCl3 were measured at 40.5 MHz as a function of temperature from 80 to −100°C. Transverse relaxation times of 35Cl in a liquid mixture of PCl3 and PBr3 and in neat liquid PCl3 were measured over the temperature region 70–29°C. The 35Cl relaxation time in PCl3 is entirely due to a quadrupolar interaction mechanism, thus the reorientational correlation time of the PCl3 molecule is determined from the 1/T2 values and the known quadrupole coupling constant. The 31P transverse relaxation time is controlled by a scalar coupling interaction mechanism while the longitudinal relaxation time is controlled by a spin—rotation interaction mechanism at high temperatures and a combination of dipolar, scalar coupling, and anisotropic chemical shift interaction mechanisms at lower temperatures. The scalar coupling constants calculated for P–35Cl in PCl3 and P–79Br in PBr3 a...

Nuclear relaxation rates in the hexaamminecobalt(III)-dimethyl sulfoxide system

Abstract In the Co(NH 3 ) 6 (ClO 4 ) 3 - d 6 -DMSO system the longitudinal relaxation rates have been measured for cobalt-59, ammine protons, and solvent deuterons. The proton transverse relaxation rates have been measured as a function of the 180° pulse separation in the Carr-Purcell pulse train to yield values for the nitrogen-14 relaxation rates and 14 NH coupling constant. The relaxation rates have been determined as a function of concentration (0.1 to 0.5 molal) and temperature (25–60°C). The results are interpreted in detail to determine the relaxation mechanism and correlation time controlling the relaxation rates.

Kinetics of the reaction of chromium (II) with ethyliodoacetate, iodoacetatopentaamminecobalt (III)

The kinetics and products of the reaction of aqueous chromium (II) with ethyliodoacetate (EIA) and (NH3)5CoO2CCH2I2+ have been studied in 1.0 M LiClO4/HClO4, with rate constants reported at 25° C. All these reactions are first order in [Cr2+] and independent of [H+]. The iodine atom abstraction from ethyliodoacetate by chromium(II) gives rate = k [Cr2+] [EIA], k = 0.41 ±0.02 M−1s−1, ΔH∗ = 20.4 ± 5 kJ mol−1, ΔHS∗ = - 184 ± 14J mol−K−1. The reaction of (NH3)5CoO2CCH2I2+ is complicated because of competing iodine atom abstration and cobalt(III) reduction and the reaction is biphasic. The overall rate for the first stage gives k1app = 0.23 ± 0.02 M−1 s−1, ΔHH∗ = 22.0 ± 7.2 kJ mol−1, ΔHS∗ = - 183 ± 20.6 J mol−1K−1. The contributions have been separated by a study of the rate of cobalt(II) production. The first stage yields (NH3)5CoO2CCH2−Cr(OH2)54+ (B1)(k = 0.07 M−1 s−1) and (H2O)5CrO2CCH2I2+ (A1) (k = 0.13 M−1 s−1). For the second stage, k2aap = 0.053 ± 0.004 M−1s−1, ΔH∗ = 195 ± 5.0 kJ mol−1, ΔS∗ = −204 ± 13.4 cal mol−1 K−1 and this stage involves halogen abstraction from A1 (k = 0.045 M−1s−1) and cobalt(III) reduction of B1 (k − 0.012 M−1 s−1). The aquation of (H2O)5CrI2+ catalysed by Cr(II) has been observed as a second stage in the reaction of EIA, and an independent study gives the following kinetic results: rate = k[Cr2+][(H2O)5CrI2+]/[H+], k = 0.020 ± 0.002 s−1, ΔH∗ = 69.9 ± 3.0 kJ mol ΔS∗ = −42.7 ± 13.7 J mol−1 K−1. A rate constant correlation for this pathway for a number of complexes is suggested.

Geschwindigkeitsgesetz und Mechanismus

Nach der Bestimmung des experimentellen Geschwindigkeitsgesetzes erfolgt als nachster Schritt die Formulierung eines hiermit in Einklang stehenden Mechanismus’. Das Geschwindigkeitsgesetz kann den Mechanismus nicht eindeutig definieren, schrankt aber die Moglichkeiten ein. Der vorgeschlagene Mechanismus erlaubt Voraussagen uber Reaktivitatstrends und fuhrt somit zu anderen Experimenten, die zur Uberprufung des Vorschlags durchgefuhrt werden konnen.

Mechanismen metallorganischer Reaktionen

Die in Kapitel 3 diskutierten allgemeinen Prinzipien gelten auch fur die Reaktionen von metallorganischen Komplexen. Da die Komplexe der unterschiedlichen Metallatome innerhalb der Reihen des Periodensystems nur selten vergleichbare Zusammensetzungen aufweisen, beschranken sich die Vergleichsmoglichkeiten gewohnlich auf eine bestimmte Gruppe. Es steht jedoch eine breite Palette an Liganden fur die Untersuchung von Eintritts- und Abgangsgruppeneffekten zur Verfugung. Dieses Gebiet wurde bereits in Lehrbuchern sowie in verschiedenen Ubersichtsartikeln vorgestellt.1–3 Ein Hauptunterschied zu den in Kapitel 3 diskutierten Systemen ist die Loslichkeit vieler dieser Komplexe in organischen Solventien, einschlieslich Kohlenwasserstoffen. Damit kann der komplizierende Faktor der Solvenskoordination minimiert werden; da aber diese Solventien oftmals auserst niedrige Dielektrizitatskonstanten besitzen, ist hier die Wahrscheinlichkeit des Auftretens verschiedener Arten der Praassoziation groser.