Complete Line Shape Analysis Research Articles

The torsional barrier around the phenyl–C+bond in phenylcarbenium ions

The barrier to rotation around the phenyl–carbenium carbon bond in a series of carbenium ions R1R2C+Ph (with R1 and/or R2= H, CH3, cyclopropyl, and C6H4X) has been measured by complete line-shape analysis of the n.m.r. spectra or by the spin saturation transfer method. ΔG‡ Values range from 36 to 78 kJ mol–1. The results are discussed in terms of steric and electronic contributions and the relationship of the 13C chemical shift of the carbenium centre with the σc+ values of the para-substituent is analysed.

Bond shift kinetics in cyclo-octatetraene by dynamic NMR in liquid crystalline solvents

A complete line shape analysis of the dynamic NMR spectra of cyclo-octatetraene dissolved in nematic liquid crystalline solvents is presented. The analysis involves simulation of the dynamic NMR spectra in the presence of the bond shift reaction. This is performed using a newly developed group theoretical method. Comparison of the simulated and experimental spectra yield the following kinetic parameters for the bond shift process in cyclo-octatetraene: 1/τ (0 °C) = 2.3×102 s−1; ΔE = 10.62 kcal/mol; ΔH≠ = 10.0 kcal/mol; ΔS≠ = −9.7 e.u.

D‐NMR‐Untersuchungen der behinderten Rotation am NC(X)‐Bindungsfragment. XV. Beeinflussung der Rotationsbarriere im NR2C(X)R1‐Bindungsfragment durch elektronische und sterische Substituenteneffekte

d‐N.M.R. Investigations of the Restricted Rotation at the NC(X)‐Bonding Fragment. XV. Influence of the Rotational Barrier in the NR2C(X)–R1‐Bonding Fragment by Electronic and Steric Substituent Effects.The rotational barriers ΔG≠ of the restricted rotation about partial C,N‐double bonds are determined for 4 series of urea‐derivatives, and analogous compounds by complete lineshape analysis and approximation equations. Some general conclusions are drawn about the validity range of the approximations independence on the low temperature splitting Δν, the possible corrections within the margin of error and the internal temperature measurement. The dependence of the determined free enthalpies of activation about the effect of heteroatoms X, the NR2‐substituents and the electronic and steric influence of the substituents R1 are discussed.

Ring reversal of cis-cyclohexane-1,2,3,4,5,6-hexacarboxylic acid and its hexamethyl ester

cis-Cyclohexane-1,2,3,4,5,6-hexacarboxylic acid and its hexamethyl ester have been synthesized and shown, by n.m.r., to exist in solution at room temperature as an equilibrium of slowly exchanging chair conformations. From complete line shape analysis of the 13C n.m.r. spectra measured at different temperatures the activation parameters have been determined. A relatively high value (ca. 17 kcal mol–1) of free energy of activation has been found for both molecules, by analogy with the hexamethylcyclohexane case. The energy barrier of the acid has been calculated with the method of molecular mechanics and the computer program MOLBD3: the value obtained (16 kcal mol–1) is a slight overestimate by comparison with the observed value of 13–14.5 kcal mol–1.

Dynamic behaviour of bis(DIOP)palladium and its reaction with alkenes

The dynamic behaviour of bis(DIOP)palladium is studied by complete line-shape analysis of its 31P NMR spectrum. Its reaction with trans-disubstituted alkenes gives diastereomeric complexes, the selectivity depending on the nature of the substituents.

Molecular vibrations and conformation of primary dithiocarbamate ester. Infrared and NMR studies on S-methyl dithiocarbamate

The i.r. spectra of a primary dithiocarbamate ester namely, S-methyl dithiocarbamate (SMDTC) and its N-dideuterated compound have been measured between 4000 and 30 cm −1. Spectra in solution and at liquid nitrogen temperature have also been obtained. Assignment of all the fundamentals has been proposed and supported from a full normal coordinate analysis. The band assignments for SMDTC have been compared with those of related molecules and the characteristic bands of primary thioamides are derived. Conformational flexibility of SMDTC has been examined by i.r. and proton NMR spectroscopy. The hindered rotation around the CN bond has been studied by a complete line shape analysis. The magnitude of NH 2 and CH 3 torsional barriers is also estimated from vibrational frequencies and force constants.

Über die Temperaturabhängigkeit der 13C‐NMR.‐Spektren von Tetracarbonyl (η‐(Z)‐cycloocten)eisen und (Z)‐cycloocten

On the Temperature Dependence of the 13C‐NMR.‐Spectra of Tetracarbonyl (η‐(Z)‐cyclooctene)iron and of (Z)‐CycloocteneReaction of (Z)‐cyclooctene (1) with Fe2(CO)9 in pentane at 0° yielded tetracarbonyl(η‐(Z)‐cyclooctene)iron (2) as a yellow oil which can be stored over a longer period only at −78°.It is shown that the title compounds (1 and 2, respectively) are fluxional.The activation parameters for the conformational C‐atom site exchange of (Z)‐cyclooctene (1) and tetracarbonyl (η‐(Z)‐cyclooctene)iron (2) (in CCl2F2) have been determined between 113 K and 151 K for 1 and between 151 K and 205 K for 2, respectively, by a complete line shape analysis of the temperature dependent proton noise‐decoupled 13C‐NMR. signals of the olefinic C‐atom. The kinetic data and activation parameters are given in Tables 1 and 2.

Intramolekularer Carbonylgruppenaustausch in Tetracarbonyleisen‐Komplexen von Äthylen und einfachen (E)‐konfigurierten Olefinen

Intramolecular Exchange of Carbonyl Group Sites in Tetracarbonyliron Complexes of Ethylene and Simple (E)‐Configurated OlefinsThe activation parameters of the intramolecular exchange of carbonyl group sites in the tetracarbonyliron complexes 2–5 of ethylene, (E)‐2‐butene, (E)‐4‐octene and (E)‐cyclooctene, respectively have been determined between 180 and 115 K by a complete line shape analysis of the 13C‐NMR, signals of the CO‐groups. The temperature dependence of the rates (k (2), k (3), k (4) and k (5), respectively) of this intramolecular exchange process can be described by the following equations (Ea in J/mol): Further data are given in Tables 3 and 4.Furthermore, a good linear correlation between the energies of activation of the carbonyl exchange process in 1,2‐disubstituted olefins and the corresponding averaged wavenumbers ( (CO)) of the carbonyl stretching vibrations or of the corresponding chemical 13C‐shifts (δ (CO)) of the carbonyl group at fast exchange in the complexes seems to exist.

Hindered rotation in the N(2,2,2‐trichlorocarboethoxy)nortropanes. An NMR study

AbstractA rarely occurring type of hindered rotation was observed by 1H NMR in some N‒(2,2,2‒trichlorocarboethoxy)nortropanes. A complete lineshape analysis gave the activation energy Ea = 56.9 kJ mol‒1 for the rotation around the CH2OCO bond.

Barriers to rotation about the N—CO bond in N-vinyl amides; a new two-site approximation method

The barriers to rotation about the N—CO bonds in a series of N-vinyl amides have been determined by a study of their proton spectra at variable temperatures. A complete LACN 3 analysis of compound 1 gave the parameters used in a complete line shape (CLS) analysis to obtain ΔG≠. A new approximate method was shown to give a value for ΔG that agreed within 0.1 kcal/mol of that obtained by CLS. The approximate method was employed to determine ΔG≠ for other members of the series. The barrier in the N-vinyl amide 1 was found to be 2.5 kcal/mol higher than that in its saturated analog. Variations within the series of N-vinyl amides amounted to only 1.2 kcal/mol. A steric effect is proposed for the increase in barrier height. The new approximation formula was tested for the general case of unequal singlet coalescence and found to give k's accurate to within 4% for Δν > 10 and to within 10% at lower values of Δν.

The determination of chemical exchange rates by N.M.R. over a large temperature range by a combination of complete line shape analysis and relaxation time (T1and T2) measurements

The determination of chemical exchange rates by N.M.R. over a large temperature range by a combination of complete line shape analysis and relaxation time (T₁and T₂) measurements

Dynamic 19F NMR studies The torsional barrier in pentafluorobenzaldehyde, its conjugate acid and protonated pentafluoroacetophenone

An upper limit of the barrier to internal rotation around the phenyl-carbonyl bond in pentalfluorobenzaldehyde dissolved in a freon mixture has been estimated from low temperature 19F NMR study. Protonation of this compound increases drastically the free energy of activation ΔG≠. Complete lineshape analysis leads to ΔG≠ (273 K) = 60.4 kJ/mol, comparable to the value obtained for protonated benzaldehyde. This result, as well as those obtained by CNDO/2 calculations support the conclusions that protonated pentafluorobenzaldehyde is planar in the ground state. This is not the case for protonated pentafluoroacetophenone in which the lower barrier height when compared to protonated acetophenon has been related to the steric strain and dipole repulsion.

The torsional barrier in aromatic carbonyl compounds. VI. Carbon‐13 and 1H DNMR study of protonated alkyl and haloalkyl p‐tolyl ketones

AbstractThe free energy of activation of the torsional barrier around the phenyl–carbonyl bond in a series of stable protonated α‐substituted p‐methylacetophenones (p‐tolyl‐ where R = CH3, C2H5, i‐C3H7, t‐C4H9, CH2Cl, CHCl2, CCl3, CH2F, CHF2, CF3 and CH2Br) has been evaluated by complete lineshape analysis of 13C or proton low temperature NMR spectra. α‐Substitution by an alkyl group leads to a decrease of the barrier height due to both steric and electronic effects. For the halo compounds, steric and electronic effects of the substituents are in competition, leading to a very high ΔG≠ value in protonated α,α,α‐trifluoro‐p‐methylacetophenone (63.2 kJ mol−1). In addition, the twist angle of the protonated carbonyl group has been tentatively evaluated by the use of the (modified) Stothers's method.

Über die Temperaturabhängigkeit der 13C‐NMR.‐Spektren von [5–6‐η‐(1Z, 5E)‐Cyclooctadien]tetracarbonyleisen und (1Z, 5E)‐Cyclooctadien

On the Temperature Dependence of the 13C‐NMR. Spectra of [5‐6‐η‐(1Z,5E)‐Cyclooctadien] and of (lZ, 5E)‐CyclooctadieneThe activation parameters of the conformational ring inversion process (simultaneous rotation around the C(3), C(4) and C(7), C(8) bonds; cf. Scheme 1) of the title compounds (1 and 2, respectively) have been determined between 275 L and 155 K by a complete line shape analysis of the temperature dependent proton noise‐modulated decoupled 13C‐NMR. spectra of 1 and 2. The temperature dependence of the rates (k(1) and k(2), respectively) of the inversion process can be described by the following equations (no influence of the solvents was observed; Ea in J/mol): . Further data are given in Tables 1 and 2.The carbonyl groups of the complex 1 show at 180 K, where the ring inversion process is frozen out, a single line at 211 ppm, i. e. the coalescence temperature of the carbonyl groups must be < 180 K.

The solvation of the dioxouranium(VI) ion by dimethyl sulfoxide

The species UO2(DMSO) 5 2+ is shown from1H NMR studies to be the predominant dioxouranium(VI) species existing in dilute anhydrous acetonedimethyl sulfoxide (DMSO) solutions, and this result is compared with data reported for the analogous water-acetone-dimethyl sulfoxide system. Complete line-shape analyses of exchange-modified1H NMR line shapes indicate that the mechanism for DMSO exchange on UO2(DMSO) 5 2+ is probably of theD orI D type. A typical set of rate parameters arek ex (260°K) =273±14 sec−1, ΔH #=38.9±0.5 kJ-mole−1, and ΔS #=−47.5±1.8 J-oK−1-mole−1 for a solution in which [UO2(DMSO)5 2+], [DMSO], and [d 6 acetone] are, respectively, 0.01155, 0.0875, and 13.00 moles-dm−3.

Nuclear Magnetic Resonance Study of Exchanging Systems. XI. 13C NMR Spectra of 9-Ethyltriptycene Derivatives and the Restricted Rotation of the Ethyl Group

Abstract 9-Ethyl-1,4-dimethoxytriptycene, 1,5-dichloro-9-ethyltriptycene, and 9-ethyl-1,4-dimethyltriptycene were prepared. The 13C NMR spectra of these compounds were measured at low temperatures in order to obtain information about the restricted rotation of the ethyl group. It was found that the former two compounds have the meso isomer as well as the dl isomer. Different chemical shifts of the methylene carbons in the ethyl group, C(2) and C(3) of the benzene A ring, and some quaternary carbons in the two isomers of 9-ethyl-1,4-dimethoxytriptycene indicate that the 120°C rotation around Csp3–Csp3 single bond in the ethyl group deforms the benzene or bicyclo[2.2.2]octatriene skeleton. The complete line-shape analyses of the temperature-dependent 13C NMR spectra of this compound were performed, and the thermodynamic static parameters and activation parameters of the ethyl rotation were determined. The correlation between the rotational barriers of the alkyl groups and their bulkiness was briefly discussed.

Internal rotation in conjugated ethylenes

Abstract The kinetic and activation parameters for the rotation about the CC double bond and the C-N single bond in the compounds listed in Table 1 were determined by variable temperature NMR spectroscopy. Complete line shape analyses, using a computer REEP program, were carried out on several systems. The main objective of the present study is to determine the effect of the ring size in II on the two types of kinetic processes.

Nuclear Magnetic Resonance Study of Exchanging Systems. X. Rotational Barriers to trans-cis Isomerization in Esters

Abstract The 13C NMR spectra of t-butyl formate (1), 1,1-diethylpropyl formate (2), and 2,6-dimethylphenyl formate (3) were measured at various temperatures. It was found that the signals of the trans and cis isomers in 1 and 2 are separated at very low temperatures, and that the signal of 3 only shows a slight line broadening. The complete line shape analyses of these temperature-dependent spectra were performed. The activation energies Ea(t-c) of the isomerization from the trans isomer to the cis were determined to be 10.9 kcal/mol for 1 and 7.2 kcal/mol for 2.

Cesium-133 nuclear magnetic resonance study of complexation by cryptand C222 in various solvents: evidence for exclusive and inclusive complexes

The chemical shift of the /sup 133/Cs resonance was studied as a function of the mole ratio of cryptand C222 to Cs/sup +/ at various temperatures in six solvents, pyridine, acetone, propylene carbonate, N,N-dimethylformamide, acetonitrile, and dimethyl sulfoxide. The variation of the chemical shift with the mole ratio was used to determine the apparent complexation constants at temperatures high enough to yield a single, exchange-narrowed line. At lower temperatures, complete line-shape analysis was used to determine apparent exchange rates. The limiting chemical shift at high C222/Cs/sup +/ mole ratios was found to be strongly dependent upon both solvent and temperature but converged to a solvent-independent value of -245 +- 5 ppM (downfield) at low temperatures. The behavior of the chemical shift and the variation of the line shape and apparent activation energy with temperature suggest the presence of two types of 1 : 1 complexes in solution, an exclusive complex in which the ion may interact with the solvent, and an inclusive complex which has a solvent-independent chemical shift. The temperature dependence of both the thermodynamic and the rate parameters is consistent with this model.

Determination of the π barrier to rotation of the Carbonyl group in para-dimethylamino-π-(tricarbonylchronium) Benzaldehyde by 13C NMR spectroscopy

The barrier to rotation around the bond between the formyl group and the complexed ring in para-dimethylamino-π-(tricarbonylchromium) benzaldehyde (I) was determined by 13C NMR spectroscopy at variable temperature and complete line shape analysis. The Δ G ≠ value for the interconversion in I (8.6 kcal mol −1 is smaller than the corresponding value for the free arene(II) (10.2 kcal mol −1. This decrease can be attributed to a loss of conjugation in the complex and/or to an interaction between the formyl group and the Cr(CO) 3 moiety in the transition state.