Magnitude Of ΔG Research Articles

Negative Hyperconjugation in Acidity of Polyfluorinated Alkanes. A Natural Bond Orbital Analysis

Abstract Natural bond orbital (NBO) analysis has been applied to various polyfluorinated carbanions. The E(2)[LP(Cα)→σ*(Cβ-F) or σ*(Cβ-Cγ)] values that are interaction energies between a lone pair NBO at the anionic center carbon and the σ*(Cβ-F) or σ*(Cβ-Cγ) NBO increased with the corresponding Cβ-F or Cβ-Cγ bond distances, respectively, being consistent with the molecular orbital theory on negative hyperconjugation. The total E(2) values for the interactions of a lone pair orbital with all σ*(Cβ-F) and σ*(Cβ-Cγ) orbitals were linearly correlated with gas-phase acidities of the corresponding alkanes, giving two lines for the carbanions having no β-fluorine atom and for the primary and secondary carbanions having β-fluorine atoms (slopes of 0.57 and 1.56, respectively). The major E(2)[LP(Cα)→σ*(Cβ-F)] values in the respective anions were found to be linearly correlated with the ΔG°β-F values as an empirical measure of β-fluorine negative hyperconjugation obtained from an increased acidity of the molecule owing to the presence of β-fluorine. However, the magnitude of ΔG°β-F was much smaller than the E(2)[LP(Cα)→σ*(Cβ-F)] value, indicating that the absolute values of the β-fluorine negative hyperconjugation are smaller than the E(2) interaction energies.

Isomerism in platinum(II) complexes of asymmetrically 1,3-disubstituted thiourea dianion ligands

A series of thiourea dianion complexes [Pt{SC(NPh)NR}(PPh3)2] (R=Me, Et, Prn, Pri, Bun, But and p-tol) was synthesised from cis-[PtCl2(PPh3)2] and the asymmetrically disubstituted thioureas PhNHC(S)NHR in the presence of triethylamine base. The complexes were studied by 1H and 31P{1H} NMR, IR and Raman spectroscopies, and ESI mass spectrometry. In each case, the NMR spectrum of a freshly-prepared CDCl3 solution shows the presence of an initially formed, kinetically favoured isomer, which then converts to the thermodynamically favoured isomer. For all cases except when R=But, the initial isomer has the R substituent in a distal position, viz. [Pt{SC(=NR)NPh}(PPh3)2], which then isomerises in solution to the thermodynamically favoured proximal isomer [Pt{SC(=NPh)NR}(PPh3)2]. However, when R=But, the thermodynamically-favoured isomer has the distal configuration, [Pt{SC(=NBut)NPh}(PPh3)2]. DFT-optimised geometries of each isomer allowed the calculation of the difference in Gibbs free energy between the distal and proximal isomers, ΔG(=Gdist−Gprox), which is positive for all compounds except R=But. The rate of isomerisation can be qualitatively correlated with the magnitude of ΔG. Trends in the values of ΔG are discussed in terms of substituent steric effects.

Removal of methyl green dye molecule from aqueous system using reduced graphene oxide as an efficient adsorbent: Kinetics, isotherm and thermodynamic parameters

Dyes released from different industries are one of the major pollutants in the water system. So removal of dye before disposal to the main water stream is necessary. In this study, adsorption technique was adopted for removal of the cationic dye from aqueous system using reduced graphene oxide (rGO) as adsorbent material. The kinetic study of adsorption shows that the present system follows pseudo second order kinetic (linear) model. Adsorption isotherm investigation indicates that the adsorption of methyl green onto rGO nanosheets follows the isotherm models in the order Toth>Sips>Dubinin–Radushkevich (D–R)>Scatchard>Langmuir>Temkin>Freundlich model in the pH range 4–6. However, the order changes to D–R>Scatchard>Toth>Sips>Langmuir>Temkin>Freundlich model at pH 7 and 9. The adsorption capacity of methyl green onto the rGO nanosheets increases with increasing pH of the medium due to the change in the surface properties. Presence of other ions like Na+, Ca2+, Mg2+ and SO42− also influenced on adsorption due to the change in the surface properties. The magnitude of ΔG° values are found to be in the characteristic range of physisorption nature of adsorption. Further, the mechanism of the adsorption process was investigated by FTIR spectroscopy and it indicates that the adsorption is due to electrostatic interaction between methyl green dye molecule and rGO nanosheets. X-ray radial distribution function (RDF) analysis revealed the formation regular molecular packets of graphene. X-ray diffraction (XRD) analysis of the rGO nanosheets before and after adsorption of the dye molecule indicates that some textural changes occur in the rGO nanosheets upon adsorption of the methyl green dye molecule.

Tricarbonylrhenium(I) halide complexes of chiral non-racemic 2-(dioxolanyl)-6-(dioxanyl)pyridine ligands: synthesis, NMR and DFT calculations

The chiral non-racemic O/N/O donor ligands 2-[(4 R,5 R)-4,5-dimethyl-1,3-dioxolan-2-yl]-6-[(4 R,6 R)-4,6-dimethyl-1,3-dioxan-2-yl]pyridine and 2-[(4 R,5 R)-4,5-dimethyl-1,3-dioxolan-2-deuteryl]-6-[(4 R,6 R)-4,6-dimethyl-1,3-dioxan-2-yl]pyridine were prepared in a stepwise fashion form 2,6-dibromopyridine. Reaction with the pentacarbonylhalogenorhenium(I) compounds yields the complexes [ReX(CO) 3L], in which the ligands act in a N/O bidentate chelate fashion. There are eight possible diastereoisomers, three of which are observable in solution. DFT calculations indicate that the relative stability of the diastereoisomers is SR 5>RR 5>SS 5≈RS 5>RS 6>SS 6>RR 6>SR 6. Above ambient temperature, a dynamic process leads to the exchange of 2 of the 3 diastereoisomers: the free energy of activation is ca. 79 kJ mol −1. The results of the DFT calculations and the magnitude of ΔG ‡ suggest the dynamic process to be the flip of the co-ordinated acetal ring.

Gas-Phase Dynamic NMR Study of the Internal Rotation in N-Trifluoroacetylpyrrolidine

The barrier to the internal rotation around the C−N bond in N-trifluoroacetylpyrrolidine (TFAPYR) was characterized using dynamic NMR spectroscopy. Kinetic parameters, ΔG⧧298 = 71.4(1.2) kJ mol-1, ΔH⧧ = 67.9(1.4) kJ mol-1, and ΔS⧧ = −11.8(4.0) J mol-1), were found to be lower in the gas phase than in solution. The pyrrolidine ring substituent in TFAPYR reduces the bulkiness around the C−N amide bond, stabilizing the ground state and increasing the value of the internal rotation barrier, ΔG⧧298, compared to N,N-dimethyl- and N,N-diethyltrifluoroacetamide. The magnitude of ΔG⧧298 in TFAPYR is also comparable to that found in primary amide bonds in peptides, emphasizing the importance of having a relatively small energy difference between cis and trans conformations in proline's role as a switch in protein signaling.

Nitrosoanisoles. Sensitive indicators of dimerisation criteria for C-nitrosoarenes

A series of mono- and di-methyl substituted p-nitrosoanisoles was synthesised either by direct nitrosation of the anisoles with NO+HSO4− or by mild oxidation (H2O2–Mo catalyst) of the appropriate amines. The nature of these compounds in solution was established by 1H and 13C NMR spectroscopy. Variable temperature 1H dynamic NMR studies provided thermodynamic data for the cases of dimer–monomer solution equilibria, and ΔG‡ data for the restricted rotation of the NO group in the monomers. Magnitudes of ΔG‡ were in the range 28–41 kJ mol−1 and were shown to be sensitive predictors of self-dimerisation tendencies of these compounds. This activation parameter, together with the 14,15N and 17O chemical shifts of the NO group, was shown to be a reliable predictor of self-dimerisation of nitroso compounds in general. The NO bond length and the wavenumber of the IR-active NO stretching mode were found to be somewhat less reliable predictors of this behaviour.

Dynamic NMR investigations of fluxionality of 2-(dimethoxymethyl)pyridine and 2,6-bis(dimethoxymethyl)pyridine in rhenium(I) and platinum(IV) complexes †

The ligands 2-(dimethoxymethyl)pyridine (L1) and 2,6-bis(dimethoxymethyl)pyridine (L2) formed bidentate chelate complexes with the isoelectronic transition metal moieties ReIX(CO)3 and PtIVXMe3 (X = halide). The complexes [ReX(CO)3L1] (X = Cl or Br), [PtXMe3L1] (X = Br or I) and [ReBr(CO)3L2] in organic solvents were shown by NMR to undergo fluxional processes which interconvert co-ordinated and pendant OMe groups. Rates and activation energies of these fluxions were measured by NMR methods (1-dimensional bandshape analysis or 2-dimensional exchange spectroscopy). Magnitudes of ΔG‡ (298.15 K) for the fluxions were in the range 59–85 kJ mol−1 with the order being ReI–L1 > ReI–L2 > PtIV–L1. A carbon–carbon bond rotation mechanism is proposed for the fluxions in the L1 complexes and concerted C–C bond rotation/metallotropic shift processes for the L2 complexes. Crystal structures of [ReCl(CO)3L1] and [PtIMe3L1] revealed distorted octahedral metal centres with N–M–O ‘bite’ angles of 73–75° and a trans relationship of the axial halide and pendant OMe group.

Ab initio calculations of electron-spin magnetic moments for Li, Be and B hydrides inX 2?+ states

Electron-spin magnetic moments, in the form ofg-shifts, have been computed at the ROHF level for theX2∑+ states of LiH+, BeH2+, LiH−, BeH and BH+. A perturbative approach, complete to second-order in appropriate Breit-Pauli operators, has been used. Retention of two-centre integrals has proven vital. First-order terms are important, especially in describing the negativeg∥ shifts observed experimentally in2∑+ molecules. The relativistic mass correction dominates in first-order, except for LiH− where the two-electron spin-Zeeman gauge correction supersedes. Second-order terms contribute negatively, and only to the Δg⊥ component. Along the isoelectronic series LiH− → BeH → BH+, the magnitude of Δg⊥ increases due to the dependence of spin-orbit coupling on nuclear charge. The relation ofg-shifts to electronic structure and bonding is explored.

Fluxional rhenium(I) tricarbonyl halide complexes of pyrazolylpyridine ligands. A detailed nuclear magnetic resonance investigation

2,6-Bis(pyrazol-1-yl)pyridine (bppy) reacted with pentacarbonylhalogenorhenium(I) complexes under mild conditions to form stable octahedral complexes of type fac-[ReX(CO)3(bppy)](X = Cl, Br or I) in which bppy acts as a bidentate chelate ligand. In solution these tricarbonyl complexes are fluxional with bppy oscillating between equivalent bidentate bonding modes by a twist mechanism involving the breaking/making of two Re–N bonds. 2,6-Bis(3,5-dimethylpyrazol-1-yl)pyridine (tmbppy) and 2-(3, 5-dimethylpyrazol-1-yl)-6-(pyrazol-1-yl)pyridine (dmbppy) formed analogous fluxional complexes. The 1,4-Re–N metallotropic shifts of [ReX(CO)3(dmbppy)] interconvert chemically distinct complexes with different solution populations. Under more severe reaction conditions the complex cis-[ReBr(CO)2(tmbppy)] was formed in which the ligand adopts its more usual terdentate behaviour. Rates and activation energies of these ‘tick-lock’ twist fluxions have been investigated by one- and two-dimensional NMR methods. Activation energies are dependent on the relative donor strengths of the N atoms, with magnitudes of ΔG‡(298.15 K) being 55–56 (bppy), ≈69 (dmbppy) and 70–77 kJ mol–1(tmbppy).

The heteronuclear cluster chemistry of the Group 1 B metals. Part 12. Variable temperature nuclear magnetic resonance spectroscopic studies of the dynamic behaviour of the mixed-metal cluster compounds [M2Ru4(µ3-H)2(µ-L)(CO)12][M = Cu or Ag, L = Ph2P(CH2)nPPh2(n= 1–6) or cis-Ph2PCHCHPPh2; M = Cu, L = Ph2As (CH2)nPPh2(n= 1 or 2)] in solution

The free energies of activation (ΔG‡) for the intramolecular metal core rearrangements which the clusters [M2Ru4(µ3-H)2(µ-L)(CO)12][M = Cu or Ag, L = Ph2P(CH2)nPPh2(n= 1–6); M = Cu, L = Ph2 As (CH2)nPPh2(n= 1 or 2)] undergo in solution have been calculated by bandshape analysis of variable temperature 31P-{1H} or 1H n.m.r. spectra or estimated from the coalescence temperature in 31P-{1H} n.m.r. spectra. Changes in the nature of the bidentate ligands attached to the coinage metals cause relatively large alterations of up to ca. 13 and ca. 1O kJ mol–1 in the values of ΔG‡ for the copper- and silver-containing species, respectively. The formal replacement of two PPh3 groups in [Ag2Ru4(µ3-H)2(CO)12(PPh3)2] by a Ph2PCH2PPh2 or Ph2P(CH2)2PPh2 ligand reduces the value of ΔG‡ by ca. 8 and ca. 4 kJ mol–1, respectively. These decreases, together with the previously reported distortions in metal framework geometry from capped trigonal bipyramidal to or towards capped square-based pyramidal, which occur for the same formal change of ligands in the analogous gold-containing species, provide some indirect evidence to support a restricted Berry pseudo-rotation mechanism for the Group 1 B metal site-exchange process in the clusters studied. The new heteronuclear cluster compounds [M2Ru4H2(µ-cis- Ph2PCHCHPPh2)(CO)12][M = Cu (2O), Ag (21), or Au (22)] have been synthesized and the effect of the cis-Ph2PCHCHPPh2 ligand on the value of ΔG‡ for the metal core rearrangement of (21) and on the skeletal geometry adopted by (22) is also consistent with the restricted Berry pseudorotation mechanism. The magnitude of ΔG‡ for the silver-containing species (21) is ca. 7 kJ mol–1 lower than that for the analogous PPh3-ligated cluster and the metal framework geometry of the gold-containing compound (22) is altered from capped trigonal bipyramidal to capped squarebased pyramidal when two PPh3 groups are formally replaced by the cis-Ph2PCHCHPPh2 ligand. In addition, variable temperature 1H and 31P-{1H} n.m.r. spectroscopic studies show that the hydrido ligands in the copper and silver clusters containing Ph2E(CH2)nPPh2(E = P, n= 1–6; E = As, n= 1 or 2) are rendered equivalent by dynamic behaviour which involves conformational changes in the methylene backbones of the bidentate ligands and that the cis-Ph2PCHCHPPh2 ligand in (21) undergoes a novel fluxional process involving intramolecular site exchange of the two phosphorus atoms between the two silver atoms.

A dynamic nuclear magnetic resonance study of 1,3-intramolecular shifts in pentacarbonyl-chromium(O) and -tungsten(O) complexes of β-2,4,6-trimethyl-1,3,5-trithian, 1,3,5-trithian, 1,3-dithian and 2-methyl-1,3-dithian: the x-ray crystal structure of [W(CO) 5{SCH(Me)SCH(Me)SCH(Me)}

Variable temperature 1H NMR spectroscopy has been used in the study of 1,3-intramolecular shifts of the M(CO) 5 moiety in complexes of the general formula [M(CO) 5L], (M = Cr or w), L = SCH 2SCH 2SC H 2, SCH 2SCH 2CH 2C H 2 and SCH(Me)SCH 2CH 2C H 2. For the 1,3,5-trithian complexes precise energy barriers for the process have been obtained by detailed computer simulation of the static and dynamic spectra. Our results suggest that the magnitude of ΔG ≠ (298.15 K) for the 1,3-shift is largely dependent upon the skeletal flexibility of the ligand system. In this context we have investigated the X-ray crystal structure of the highly substituted trithian complex [W(CO) 5{β- SCH(Me)SCH(Me)SC H(Me)}].

A variable temperature 1H NMR study of stereochemical non-rigidity in group VI metal pentacarbonyl complexes of 1,3,5,7-Tetrathian; [M(CO)5(H2)] (M=Cr OR W)

Abstract Stereochemical non-rigidity in the complexes [M(CO)5( SCH 2 SCH 2 SCH 2 SC H2)] (M = Cr or W) has been studied by dynamic nuclear magnetic resonance spectroscopy. In the temperature range ca. −100 to 120°C two intramolecular processes were observed, namely pyramidal atomic inversion about sulphur atoms, and a commutation of the M(CO)5 moiety between coordination sites on different sulphur atoms. Accurate energy barriers for both processes have been obtained by detailed computer simulations of the static and the dynamic spectra. The magnitudes of ΔG≠ (298.15 K) are compared with those reported for related complexes. The exact nature of the M(CO)5 shift cannot be unambiguously assigned from the observed spectral line-shape changes. Two possible mechanisms are proposed and discussed.

Nuclear magnetic resonance investigation of the conformational dynamic properties of benzocycloheptenones and related compounds

Free energy barriers for ring inversion were determined from dnmr studies of 3,4-benzo-cycloheptenone (4) and for deuterated derivatives 6 and 7 of 4,5-benzocycloheptenone (5). The investigation of 2,3-benzocycloheptenone (3) revealed negative results. The magnitude of ΔG≠ for 4 and the trend observed for 5, 6, and 7 strongly suggest that the chair conformation of these molecules interconverts through a wagging motion of the C(5) end of the benzocycloheptene seven-membered ring.