Chemical Shielding Constants Research Articles

Theoretical and experimental NMR chemical shifts of norsanguinarine and norchelerythrine

Norchelerythrine and norsanguinarine, tertiary benzo[ c]phenanthridine alkaloids, were examined by gradient-selected 2D NMR spectroscopy and the later also by extensive theoretical calculations. 1H, 13C and 15N chemical shifts assignments of the title isoquinoline alkaloids based on NOE and multiple-bond chemical-shift correlation experiments (GSQMBC) are reported. Various methods were used for the NMR chemical shifts calculations. Molecular mechanics (MM3 forcefield), AM1 method and Ab initio methods were used for optimizing the geometry. Chemical shielding constants were computed by density functional theory, GIAO and IGLO approaches were used. Chemical shifts calculated by all methods display good qualitative agreement with experimentally determined values. The best overall agreement was achieved when geometry was optimized by RHF/6-31G** method and chemical shielding constants were calculated by B3LYP/6-311G** method, GIAO approach.

Ab initio study of NMR spectra of Li 2S–SiS 2 glass system

We obtained the chemical shielding constants of 29Si in Li 2S–SiS 2 glass system on the basis of molecular orbital calculations. The relative chemical shieldings calculated at the HF/6-31G* level is −16.1 and −23.8 ppm for E(1) and E(0), respectively. These calculations are in good agreement with the previous NMR study: σ( E(2))> σ( E(1))> σ( E(0)). It is found theoretically that incorporation of Li 2S into SiS 2 does not influence the 29Si chemical shift, while incorporation of Li 2O into SiO 2 does to a significant extent. It is also found that the existence of oxygen in Li 2S–SiS 2 glasses increases the chemical shielding of 29Si by about 25 ppm, which is in good agreement with the experimental result.

Calculations of [formula omitted] NMR shieldings in polyfluoro-1-lithioethenes : Role of geometry, solvation and comparison with experimental shifts

19 F NMR chemical shielding constants in ( E)-1,2-difluoroethenyllithium ( 1) and trifluoroethenyllithium ( 2) have been calculated by the SOS-DFPT-IGLO 1 For explanation of acronym SOS-DFPT-IGLO see Section 4. 1 method and employed for an assignment of individual signals in experimental 19 F NMR spectra of polyfluoro-1-lithioethenes. The role of solvation on equilibrium geometries and consequently on the shielding constants is discussed. In comparison to HF 2 Acronym HF stands here for computations at the Hartree–Fock level, acronym MP2 for computations including correlation using second-order Møller–Plesset correction, acronym DFT for computations using Density Functional Theory. 2 or MP2 methods, DFT calculated absolute shielding values for fluoroalkenes differ from the experimental values by an opposite offset error. All methods gave acceptable relative shielding values.

ONIOM as an efficient tool for calculating NMR chemical shielding constants in large molecules

The ONIOM approach is used to derive an expression for the NMR chemical shielding tensor in a molecule subdivided into n-layers, each of which can be described at a different level of theory. The two-layer ONIOM2(MP2-GIAO:HF-GIAO) variant, in which a small part of the molecule containing the nuclei of interest is described at the MP2-GIAO level of theory, and the rest – using the HF-GIAO approach – is tested through calculations of absolute isotropic 13 C , 17 O , 19 F , and proton NMR chemical shieldings in the water dimer, ethanol, acetone, acrolein, fluorobenzene, and naphthalene. The results show that with an appropriate partitioning this scheme furnishes shieldings which represent close approximations to the corresponding MP2-GIAO values for the entire molecule and offers a highly efficient tool for accurate shielding calculations on large molecules.

Bond length alternation and aromaticity in large annulenes

Properties of [4n] and [4n+2]annulenes were studied as a function of n for up to [66]annulene using Hartree–Fock and density functional theory in the generalized gradient approximation (DFT-GGA). In the 4n+2 series a “transition” from delocalized to localized structures occurs at 4n+2=30. Various indices of aromaticity, including NMR chemical shifts, bond localization, and aromatic stabilization energy (ASE) were monitored. π-bond localization occurs not due to a dramatic decrease of ASE as n increases, but rather as a result of a pseudo-Jahn–Teller (PJT) effect that sets in as the HOMO-LUMO gap decreases with increasing size. The NMR measures of aromaticity (difference between inner and outer 1H chemical shielding constants and the nucleus-independent chemical shifts, NICS) are reduced in the localized structures in comparison to the delocalized ones. The gradual nature of this “transition” is also implied by the relatively large values of the NMR measures of aromaticity that approach zero only gradually for larger size annulenes. Therefore intermediate size annulenes, such as [30]annulene are predicted to have a localized structure and aromatic properties at the same time showing the delocalized structure is not a necessary condition to be aromatic.

Calculation of the chemical shielding constants of 95Mo in the clusters of [MoO mS 4 − m(CuX) n] 2− ( X = CN, Cl; m = 0–2, n = 1–3) by the finite perturbation theory-CNDO/2 method

By using a semi-empirical molecular orbital method within the framework of the finite perturbation theory (FPT), the FPT-CNDO/2 method, which has been successfully adapted to the study of the 95Mo chemical shielding in molybdate and thiomolybdate [MoO m S 4 − m ] 2− ( m = 0–4) anions, theoretical calculation of the chemical shielding constants of 95Mo in the MoCuS clusters [MoO m S 4 − m (CuX) n ] 2− ( X = CN, Cl; m = 0–2, n = 1–3) has been carried out to extend the study of MoCuS clusters. Factors determining the regular change of 95Mo chemical shielding constants in the MoCuS clusters are discussed and the Mo d-orbital exponent is deduced to be the most sensitive one. The deduction is verified in the calculation by varying the Mo d-orbital exponent. The scaling coefficient of the linear regression between the calculated and the experimental results changes from 2.956 to 0.834 as the Mo d-orbital exponent reduces from 3.22 to 3.05, and the value of 3.10 produces the best linear regression. The deviation in the linear regression is attributed to the basic approximation of the CNDO/2 semi-empirical method.

FPT-CNDO/2 studies of 59Co chemical shielding constants in hexacoordinated diamagnetic Co(III) complexes

The 59Co nuclear shielding constants of five hexacoordinated diamagnetic cobalt(III) complexes have been calculated using a semiempirical molecular orbital method within the framework of the finite perturbation theory, the FPT-CNDO/ 2 method. Experimental and calculated chemical shifts referenced to the cobalticyanide anion gave good agreement. The difference between the calculated and the experimental chemical shifts is attributed to the underestimation of the radial factor. The origin of the shielding on the cobalt nucleus is discussed in terms of the 3d orbital population. The shielding anisotropy in the compounds with low symmetry is also surveyed.

Deuterium isotope effects on the 119Sn shielding constants and spin–spin coupling constants in stannane and the stannonium cation

Deuterium induced isotope effects on the 119Sn chemical shielding constants have been measured for stannane and the stannonium cation; they are found to be approximately −0.403 ppm/D and −0.05 ppm/D respectively. The 119Sn shifts in the series SnDnH4−n (n ≤ 4) deviate from additivity as predicted by Jameson and Osten. The primary and secondary isotope effects on Sn–H spin–spin coupling for stannane were obtained and are −2.8 Hz and −1.7 Hz respectively. The primary isotope effect for Sn–H spin–spin coupling for the stannonium cation was found to be −11.6 ± 7 Hz; an accurate value for the secondary isotope effect on the spin–spin coupling could not be obtained. The derivative of the 119Sn shielding constant and J (Sn,H) with respect to extensions in the equilibrium bond length have been calculated for stannane.

Analysis of Complex NMR Spectra. An Iterative Method

A method for analyzing complex NMR spectra, which utilizes an iterative technique on the spin energy levels, is described. We show how the experimental energy levels and their errors are derived from the observed spectrum and the assignment of the various transitions. We discuss several techniques and methods that are helpful in obtaining this assignment. Sum rules for the energy levels and the intensities of the transitions, useful in the analysis, are also pointed out. With the iterative procedure, which uses the eigenvectors of an approximate Hamiltonian, we then calculate the changes in the chemical shielding constants and the spin—spin coupling constants necessary to adjust the approximate energy levels toward the experimental ones. The probable errors in the derived magnetic parameters are also readily determined.