J.A. Ladd

The 1H and 7Li magnetic resonance spectra of some aryllithiums

Abstract The 1H magnetic resonance spectra of phenyllithium, meta- and para-tolyllithium and the corresponding chloro derivatives have been analysed and, where possible, the results have been compared with previous work. Serious errors have been found in the work of Fraenkel et al. The 7Li magnetic resonance spectra of these compounds have also been obtained and an observed correlation of the 1H and 7Li chemical shifts has been interpreted in terms of the ionic character of the carbon—lithium bond.

Reversible proton transfer phenomenon in the 2,4-dichlorophenol-triethylamine hydrogen-bonded complex studied by low-temperature 1H NMR spectroscopy

Abstract Low-temperature 1 H NMR studies of the bridging OHN signal in the hydrogen-bonded complex formed between 2,4-dichlorophenol and triethylamine in C 2 H 5 Cl solution have demonstrated for the first time that separate signals for the molecular and ion-pair forms of this type of system can be observed. The temperature dependence of these signals leads to thermodynamic and kinetic quantities which suggest that the potential energy profile of the hydrogen bond is roughly symmetric with a double minimum.

Molecular parameters and bond structure: Part 7

Abstract Using accurately known force constants and bond lengths a relation of the form f = 37.3/ r 5.71 mdyneA˚ −1 has been obtained for nitrogen-nitrogen bonds. A molecular orbital study has been carried out on nitrous oxide providing a satisfactory description of both the bonding and the low dipole moment whose sense is predicted to be N XXX N → 0.

Reversible proton transfer in the 2,3,5,6-tetrachlorophenol-N,N-dimethylaniline hydrogen-bonded complex studied by low-temperature 1H NMR spectroscopy

Abstract Low temperature 1 H NMR studies of the bridging OHN signal in the hydrogen-bonded complex formed between 2,3,5,6-tetrachlorophenol and N,N -dimethylaniline in C 2 H 5 Cl solution have shown that separate signals for the molecular and ion-pair forms of the complex can be observed below −135°C (138 K). Analyses of the observed lineshapes have yielded values for the thermodynamic quantities Δ H °, Δ S ° as well as for the activation quantities Δ H ‡ , Δ S ‡ .

Structure and dynamics of hindered organosilicon compounds. The conformations of symmetrical (Me3Si)3C and (PhMe2Si)3C derivatives

Abstract At low temperatures the methyl region of the 1 H and 13 C NMR spectra of (Me 3 Si) 3 CSiCl 3 and the 1 H NMR spectrum of (Me 3 Si) 3 CSiBr 3 each show three signals of equal intensity and the 29 SiNMR spectrum of the trichloride shows only one Me 3 Si signal. These data are consistent with the methyls within each Me 3 Si group becoming inequivalent. Compounds containing the (PhMe 2 Si) 3 C group are able to adopt different conformations at low temperature; in (PhMe 2 Si) 3 CSiCl 3 the phenyl groups mesh together while in (PhMe 2 Si) 3 CBr they are separated by methyl groups. These different arrangements of the ligand can readily be distinguished by 1 H NMR spectroscopy, and the conformation in the case of (PhMe 2 Si) 3 CSiCl 3 has been confirmed in the solid state by X-ray crystallography.

Dynamic NMR: comparison of the experimental barriers to internal rotation in N,N-dimethylformamide with those calculated by the ab initio SCF MO method

Abstract The GAUSSIAN 70 package of ab initio computer programs has been used to determine the total energies, force constants and dipole moments of the ground and transition states of DMF following complete geometry optimisation. The 4–31G split-valence basis set was employed. There is good agreement between the resulting geometry and that determined by electron diffraction. The barriers to internal rotation about the CN amide and NMe bonds so derived are compared with those determined experimentally by NMR methods.

Infrared band intensities and bond polarities: Part 2. Bond moment constants in linear triatomic molecules using zero-order bond moment theory

Abstract Bond moment constants have been calculated for XCN (X = H, D, Cl, Br) molecules using zero-order bond moment theory.

A double-fitting procedure for two-site exchange analysis

Abstract The total lineshape (TLS) analysis of a two-site system undergoing chemical exchange is subject to difficulties associated with the temperature dependence of the lineshape parameters. A logarithmic approach is developed in order to deal with this problem and its use is demonstrated by application to the case of N,N -dimethyltrichloroacetamide. The extraction of the temperature dependence of T 2 at each site from the TLS results provides information on the molecular reorientation which could otherwise have been overlooked.

Nuclear magnetic resonance spectra of tropic acid and some derivatives

Abstract The NMR spectra of tropic acid, its methyl ester and acetyltropic acid methyl ester were measured at 100 and 220 MHz in various solvents. The spectra were analysed by means of an iterative computer procedure. The results indicate, for all the compounds studied, a predominance of the conformation where the phenyl and hydroxyl (or acetoxyl) groups are in anti.positions to each other. The solvent and concentration effects upon the vicinal coupling constants and hence upon the position of the conformational equilibria are rather weak.

NMR studies of 7Li chemical exchange between phenyllithium and p-tolyllithium: The degree of association of phenyllithium in diethyl ether solution

Abstract An investigation of the temperature dependence of the 7Li NMR spectra of equimolar mixtures of phenyl- and p-tolyllithium in ether solution reveals that only two species exist in rapid equilibrium, the kinetics of which are consistent with a bimolecular mechanism having an Arrhenius activation energy of 29.5 ± kJ/mol. The results are consistent with monomeric aryllithium molecules rather than dimeric as has been suggested on the basis of ebullioscopic measurements.