Complete Line Shape Research Articles

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.

Tunable dye laser spectroscopy of atomic calcium: collisional redistribution of radiation

By tuning a pulsed dye laser through the resonance line of calcium at 422.7 nm the authors have investigated the spectral redistribution of laser radiation by calcium vapour. Two main components were resolved in the side emission from an oven containing calcium vapour and argon buffer gas. These are produced by Rayleigh scattering and by collision-induced fluorescence. The intensity of the Rayleigh component was found to vary as Delta -2 where Delta = omega 21- omega L is the laser detuning from resonance. In contrast the intensity of the collision-induced fluorescence showed a markedly asymmetric dependence on detuning. Detailed studies of the collision-induced fluorescence have enabled the complete line shape of the calcium resonance line perturbed by argon to be determined. The interpretation of this data in terms of a frequency-dependent collision rate, gamma E( Delta ), is discussed. This paper also reports calculations of theoretical line profiles using the unified Franck-Condon theory and trial interatomic difference potentials. The comparison of the experimental and theoretical line profiles enables the range of validity of the unified Franck-Condon theory to be assessed and the form of the optimum Ca-Ar difference potential to be selected.

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.

Theory for broadband detection of ion cyclotron resonance signals

A complete line shape theory is developed for the transient response of a new type of ion cyclotron resonance (ICR) detector circuit. The detector is basically a balanced capacitance bridge which is sensitive to the abundance of gaseous ions stored in a static magnetic ion trap. For the first time, the equations of motion of ions in the ICR analyzer cell are shown to be coupled to the circuit equations of the detector. Also, the effect of nonreactive ion–molecule collisions on line shapes and on the transient response of the detector are analyzed and shown to allow measurement of ion–molecule collisions frequencies as a function of ion translational energy. One of the most important features of the capacitance bridge detector is its broadband sensitivity to a wide range of ion cyclotron resonance frequencies. This allows a mass spectrum of ions stored in the ICR analyzer cell to be obtained by scanning the frequency ω1 of the irradiating rf electric field at a fixed magnetic field strength. The capacitance bridge circuit can serve not only as a direct replacement for marginal oscillator circuits traditionally used in ICR experiments, but also as the detector for recently developed Fourier Transform and Rapid Scan ICR experiments.

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.

Magnetic field dependence of phosphorescence intensity and optical nuclear polarization (ONP) as a result of level-anti-crossing (LAC) and relaxation i

The complete lineshape of level-anti-crossing (LAC)-signals in phosphorescence and optical nuclear polarization (ONP) from excited triplet states of aromatic molecules is analyzed in detail. LAC is established to be induced by either off-diagonal Zeeman interaction or hyperfine coupling. It is shown that the LAC-lineshape exhibits characteristic features, mainly the kind of asymmetry around the centre field which allows an alternative way to distinguish the two types of interaction. For the case of axially symmetric hyperfine coupling an analytical result could be derived which treats the effects of all off-diagonal hyperfine elements including that which can usually be neglected near the avoided level-crossing region but may become important on the low field side. The concept of constructive relaxation introduced earlier is extended. It is shown to enhance the lineshape effects associated with the different interactions responsible for ONP. First experimental examples for the predicted lineshape phenomena are presented.

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.

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.

Conformational study of a series of 6-substituted 5,6,7,12-tetrahydrodibenzo[a,d]-cyclooctenes by nuclear magnetic resonance spectroscopy

The conformational properties of seven 5,6,7,12-tetrahydrodibenzo[a,d]cyclooctenes bearing substituents at C-6 have been studied using variable temperature proton nmr spectroscopy. The position of the equilibrium between boat-chair (BC) and twist-boat (TB) conformers has been measured in several solvents. In contrast to the unsubstituted dibenzocyclooctene, appreciable amounts of TB conformer are present when C-6 is substituted by alkyl, hydroxy, or cyano substituents. Measurements at different temperatures showed the TB form to possess the greater entropy. Barriers to the BC → TB interconversion were determined by coalescence studies, using both approximate and complete line shape methods. Barriers ranged from 10.1 kcal/mol for the 6-keto derivative to 16.7 kcal/mol for the 6-hydroxy-6-methyl derivative.

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.

Barriers to nitrogen inversion in six-membered rings. Ring and nitrogen inversion in some methylene bridged bisheterocycles

A complete temperature dependent line shape analysis of 2,2′-bistetrahydro-1,2-oxazinylmethane and coalescence temperature measurements on 2,2′-bis-3,6-dihydro-1,2-oxazinylmethane, 2,2′-bis-1,2-oxazolidinyimethane and 3,3′-bistetrahydro-1,3-oxazinylmethane are reported. Joining the nitrogen atoms in the two heterocyclic rings via a methylene bridge has a negligible effect upon the ring inversion barrier, but substantially lowers the nitrogen inversion barrier. The relevance of these results to the current controversy over the barrier to nitrogen inversion in six-membered rings is discussed.

Electron spin resonance studies on the interactions between radical ion pairs and macrocyclic polyethers. Part 2. Intramolecular cation exchange in metal ketyl ion pairs

The effect of complexation by the polyether dibenzo-18-crown-6 of radical ion pairs from dicarbonyl derivatives has been studied by electron resonance spectroscopy. In the case of the potassium and lithium salts only small variations have been observed both in the hyperfine splitting constants and in the rate of intramolecular migration of the counterion from one carbonyl group to the other. Drastic changes, mainly in the migration rate, were however observed for the sodium salts. From a complete lineshape analysis of the e.s.r. spectra at several temperatures, the effect of complexation on the activation parameters for the intramolecular exchange was determined. In the light of these results we postulate, for the crown complexes of carbonyl derivatives, a structure more realistic than that proposed in our previous paper.

13C nuclear magnetic resonance studies. 61. The effect of solute–solvent association on internal rotation in substituted benzaldehydes

Complete lineshape analyses of the 13C nmr signals of the ortho and meta carbons have been carried out for two p-substituted benzaldehydes in methylene chloride-d2 and toluene-d8 solutions over a wide temperature range. The activation parameters have been determined for the formyl group rotation and large negative values are found for ΔS≠ when the latter solvent is used. These results suggest that solute–solvent association has a steric effect on the transition state.