Structure elucidation with lanthanide-induced shifts. 4—Bound shifts vs relative shifts

The use of lanthanide shift reagents (LSR's) to obtain additional structural information from nuclear magnetic resonance studies has gained widespread acceptance. However, there has not been general agreement with regard to the most appropriate methodology for analysis of the shifted NMR spectra. We present arguments that only the bound shifts (Δ₁) corresponding to the LS complex should be used for correlation of lanthanide-induced shifts with molecular structure by means of the pseudocontact equation. Several examples are discussed of compounds for which the relative induced shifts are dependent on the concentration of LSR. For such cases it is not possible for both Δ₁ and Δ₂ (the bound shift corresponding to the LS₂ complex) to correlate successfully with the correct structure. Alternative methods of obtaining bound shifts are critically evaluated.     

Chemical shifts in methylcyclohexane

In order to determine the effect of a single methyl group on the chemical shifts of protons in a cyclohexane ring, methylcyclohexane-1,2,3,3,5,5,-d₆ has been synthesized. The protons in the 4-position and the 2,6-equatorial protons are not significantly different from those in ring-frozen cyclohexane. The 2,6-axial protons, however, experience an up-field shift of about 15 Hz from the position of the other axial protons. These observations are discussed in terms of the structure of methylcyclohexane.     

稠环芳烃系列化合物化学位移的计算 IV.^1H化学位移加合性规律的量子力学基础

Based on a starting point of the McWeeny Huckel-London perturbation theory approximation, a formula similar to the fourmula of additivity rule for the calculations of the chemical shifts of condensed aromatic hydrocarbons has been derived. The quantum mechanical basis of additivity rule is thus indicated by this formula. It is shown that by using the concept of aromatic shift and sigma ratio, this method for calculation of proton chemical shifts has the virtues of definite physical meaning and simple calculation. It can also be extended to the calculation of the proton chemical shifts of substituents fr the molecules of this series.     

Field shifts in hafnium II

By means of classical interference spectroscopy, using enriched isotope samples, the isotope shift between¹⁷⁸Hf and¹⁸⁰Hf has been measured for 33 transitions in the Hf II spectrum. For the pure Russell-Saunders terms 5d ²6s ⁴ F and² F the parametric analysis yields a field-shift difference of 17(2) mK produced by the second-order interaction of the electrostatic operator and the field-shift operator. Semi-empirical calculations based on the non-relativistic Hartree-Fock method reproduce this value as well as the experimental field shifts if a factor of 1.68(6) is used to scale theab initio electron densities at the nucleus. The corresponding factor for the Hf atom is much smaller. This leads to a re-evaluation of screening ratios for Hf and to a more accurate value of the nuclear parameter λ^178,180 (Hf)=0.072(4) fm².     

Lanthanide-induced shifts of diastereotopic groups

The chemical shift differences of diastereotopic protons in aliphatic acyclic alcohols, mostly of the type R? CH₂? CR′R″? (CH₂)_nOH, have been investigated. Fairly small amounts of Eu(dpm)₃ cause the spectra of these materials to simplify dramatically; indeed, even diastereotopic protons rather far removed form the hydroxyl group give discrete signals in the presence of the shift reagent. Large shift differences were realized in the γ-protons (n = 1) and the δ-protons (n = 2), particularly if R is bulky and R' and R″ have different steric requirements. Semi-quantitative conformational preferences can be determined from the data obtained.     

Sigmatropic hydrogen shifts in aryltetraphenylcyclopentadienes

Dynamic NMR has revealed intramolecular migrations of hydrogen atom over the periphery of the five-membered ring in 5-(p-tolyl)-1,2,3,4-tetraphenylcyclopentadiene in a deuteronitrobenzene solution with energy barrier ΔG ₁₈₀ ^≠ = 24.8 kcal/mol. Quantum-chemical DFT calculations B3LYP/6-311++G** have shown that such migrations in 1,2,3,4,5-pentaphenylcyclopentadiene in the gas phase occur in a chiral conformation of propeller type by the mechanism of 1,5-sigmatropic hydrogen shifts with retention of configuration through asymmetric transition state with energy barrier ΔE _ZPE ^≠ = 25.9 kcal/mol. Enantiomers P and M can readily interconvert into each other (ΔE _ZPE ^≠ = 3.9 kcal/mol) owing to synchronous flip rotations of the phenyl groups.     

Kinetic shifts and metastable transitions

Differences in the appearance potentials of normal fragment ions and of metastable transitions leading to the formation of the same fragment ions have been measured in the mass spectra of a number of compounds. The differences varied from 0.1 to 1.0 eV and were attributed to the measurable part of the kinetic shift. An approximate correlation was found between the intensity of a ‘metastable peak’ and the observed difference in appearance potentials, and it is suggested that the full kinetic shift for processes giving rise to intense ‘metastable peaks’ is substantially larger than the measurable part. The implications of this in thermochemical calculations are discussed.     

31P chemical shifts in N-aryliminotriphenylphosphoranes

³¹P chemical shifts in N-aryliminotriphenylphosphoranes and the corresponding N-methylated phosphonium salts are presented. They can be correlated with those theories of the bonding in phosphorus-nitrogen ylides which involve partial double bonding as a result of p^π–d^π overlap.     

15N chemical shifts in aziridines

For a number of 1-substituted aziridines and also some 1,2-disubstituted aziridines it has been shown that electron-donating substituents on the nitrogen atom produce a downfield shift of the ¹⁵N resonance. The ¹⁵N chemical shifts of aziridines correlate with the ¹⁵N shifts in N,N-dimethylamines and primary amines as well as with the ¹⁷O shifts in oxiranes. A correlation is also observed between the ¹⁵N chemical shifts and the electronegativity of the substituents on the nitrogen atom.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 10, 1336–1339, October, 1988.     

NMR chemical shifts. Substituted acetylenes

The MPW1PW91/6-311+G(2d,p) and MP2/6-311+G(2d,p) GIAO nuclear shieldings for a series of monosubstituted acetylenes have been calculated using the MP2/6-311G(2d,p) geometries. Axially symmetric substituents such as fluorine may lead to large changes in the isotropic shielding but have little effect on the tensor component (zz) about the C[triple bond]C bond axis. On the other hand, substituents such as vinyl and aldehyde groups lead to essentially no difference in the isotropic shielding but are calculated to give a large zz paramagnetic shift to the terminal carbon of the acetylene group, without having much effect on the inner carbon. The tensor components of the chemical shifts for trimethylsilylacetylene, methoxyacetylene, and propiolaldehyde have been measured and are in reasonable agreement with the calculations. The downfield shift at the terminal carbon of propiolaldehyde along with a small upfield shift at the adjacent carbon has been found to result from the coupling of the in-plane pi MO of the acetylene with the pi* orbital that has a node near the central carbon. The tensor components for acetonitrile also have been measured, and the shielding of cyano and acetylenic carbons are compared.     

Chemical shifts in uranium Auger transitions

Uranium metal and uranium dioxide surfaces have been examined using Auger electron spectroscopy. An essentially oxide free uranium surface can be produced by heating the metal to 800°C in a vacuum of ≈ 1 × 10⁻⁸ NM⁻² while subsequent oxidation is shown to produce chemical shifts in the uranium Auger peak positions.     

Excited state polarizabilities from solvatochromic shifts

A new method is proposed to estimate the polarizability (α_e) of a molecule in an excited state using solvatochromic shift measurements and McRae's equation. In the earlier methods the contribution due to polarizability was not considered. In view of this, the proposed method is also expected to give a better estimation of excited state electric dipole moment (μ_e) and the (θ) angle between excited and ground state electric dipole moments, μ_e and μ_g apart from giving values of polarizability of the molecules in the excited state. This method has been applied in the case of the La band of p-nitro aniline and the results for all the parameters are found to be satisfactory and of right order in comparison with that reported in literature.     

Lanthanide-induced shifts in proton NMR spectra—IV: Praseodymium-induced shifts to lower applied fields and obscured chemical shifts from lanthanide-induced shift (LIS) data

Praseodymium(III) tris(dipivalomethanato) [Pr(DPM)₃] reduces the complex overlapping aromatic absorptions in the proton NMR spectrum of a monosubstituted naphthalene, cis-3-(1-naphthyl)-1,3,5,5-tetramethlcyclohexan-1-ol( 1 ) to a virtually first-order pattern, whereas Eu(DPM)₃ at the same and higher molar concentrations does not completely resolve the aromatic proton signals. Plots of the Pr(DPM)₃-induced shifts measured in carbon tetrachloride solution versus molar equivalents of Pr(DPM)₃ added may be extrapolated to provide accurate chemical shifts in the absence of added lanthanide shift reagent (LSR). The qualitative conformational conclusions from the Pr(DPM)₃ shifts agree with those from the Eu(DPM)₃-induced shifts, but there are detailed differences in the relative lanthanide-induced shifts (LIS) for different proton types in the molecule.