NMR spin-spin coupling constants in polymethine dyes as polarity indicators

Herein, we explore the use of spin-spin coupling constants (SSCCs) in merocyanine (MCYNE) dyes as indicators of polarity. For this purpose, we use Car-Parrinello hybrid quantum mechanics/molecular mechanics (QM/MM) to determine the structures of MCYNE in solvents of different polarity, followed by computations of the SSCCs by using QM/MM linear-response theory. The molecular geometry of MCYNE switches between neutral, cyanine-like, and zwitterionic depending on the polarity of the solvent. This structural variation is clearly reflected in the proton SSCCs in the polymethine backbone, which are highly sensitive to the dielectric nature of the environment; this mechanism can be used as a "polarity indicator" for different microenvironments. This result is highlighted by computing the SSCCs of the MCYNE probe in the cavity of the beta-lactoglobulin protein. The computed SSCCs clearly indicate a non-polar hydrophobic dielectric nature of this cavity.     

Basis-set dependence of nuclear spin-spin coupling constants

The convergence of NMR indirect spin-spin coupling constants with the extension of the basis set is analyzed, based on calculations carried out at the multiconfigurational self-consistent-field level for the HF and H₂O systems. For the dominant and difficult Fermi-contact contribution, the standard correlation-consistent basis sets of electronic-structure theory are not suitable, lacking flexibility in the core region. Improved but not satisfactory convergence of the couplings is observed when decontracting the s functions of the correlation-consistent cc-pVXZ basis sets for 2≤X≤6. Next, by systematically extending these basis sets with tight s functions, new sets are obtained that are sufficiently flexible for accurate calculations of indirect nuclear spin-spin couplings, without sacrificing the smooth convergence behavior of the correlation-consistent basis sets. Received: 22 September 1997 / Accepted: 30 December 1997     

MBOHO calculations of phosphorus-carbon nuclear spin-spin coupling constants

Summary The novel generalized correlation of the nuclear spin-spin coupling constants with the atomic hybrids and net charges is employed to give a new relationship for calculating the directly bonded phosphorus-carbon coupling constants by use of the maximum bond order hybrid orbital procedure together with the extended Hückel molecular orbital calculation. The calculated coupling constants of phosphorus-carbon are all in good agreement with the experimental data, which shows that the new relationship obtained in the present paper is quite satisfactory for calculation of the phosphorus-carbon coupling constants.The project was supported by the National Natural Science Foundation of China and the Excellent University Teacher's Foundation of State Education Commission of China     

Stereochemical dependence of NMR geminal spin-spin coupling constants

In this work it was sought to explore the versatility of geminal spin-spin coupling constants, (2)J(XY) SSCCs, as probes for stereochemical studies. A set of compounds, where their experimental (2)J(XY) SSCCs through the X-C-Y molecular fragment are predicted to be sensitive to hyperconjugative interactions involving either bonding or antibonding orbitals containing the C carbon atom ('coupling pathway'), were analyzed. SSCC calculations were performed for some selected examples using the second order polarization propagator approximation (SOPPA) method or within the DFT-B3LYP framework. Hyperconjugative interactions were calculated within the Natural Bond Orbital (NBO) approach. Results are condensed in two qualitative rules: Rule I(M)-hyperconjugative interactions transferring charge into the coupling pathway yield a positive increase to the Fermi contact (FC), contribution to (2)K(XY) reduced spin-spin coupling constants (RSSCC), and Rule II(M)-hyperconjugative interactions transferring charge from the coupling pathway yield a negative increase to the FC contribution to (2)K(XY) RSSCC.     

Ab initio study of the NMR shielding constants and spin-spin coupling constants in cyclopropene

Summary Ab initio calculations of parameters which characterize the NMR spectrum are presented for the cyclopropene molecule. The London orbitals CHF (or GIAO-CHF, Gauge-Independent Atomic Orbital Coupled Hartree-Fock) results for the shielding constants are in good agreement with the experimental data, accurately determined, and with otherab initio values. The calculations of the NMR spin-spin coupling constants have been performed using the Multiconfiguration Time-Dependent Hartree-Fock (MC TDHF) approach. Different basis sets and MC SCF wavefunctions were used to estimate the accuracy of the results. Good agreement is obtained with the coupling constants estimated using the available experimental data.Dedicated to Professor Werner Kutzelnigg on the occasion of his 60th birthday     

Convergence of the perturbation theory expansion for spin-spin coupling constants

The convergence of the full perturbation theory expansion for the Fermi contact term in the spin-spin coupling constant of hydrogen fluoride has been studied. By examining the contribution of higher virtual states as the basis set is expanded to 18 atomic orbitals, it is shown that at this level of approximation the expansion does not converge. The need to also establish convergence before accepting values calculated from configuration interaction wavefunctions is discussed.The Puerto Rico Nuclear Center is operated by the University of Puerto Rico for the United States Atomic Energy Commission under Contract No. AT(40-1)-1833.     

Measurement of long-range 1H-19F scalar coupling constants and their glycosidic torsion dependence in 5-fluoropyrimidine-substituted RNA

Long-range scalar 5J(H1',F) couplings were observed in 5-fluoropyrimidine-substituted RNA. We developed a novel S3E-19F-alpha,beta-edited NOESY experiment for quantitation of these long-range scalar 5J(H1',F) couplings, where the J-couplings can be extracted from inspection of intraresidual (H1',H6) NOE cross-peaks. Quantum chemical calculations were exploited to investigate the relation between scalar couplings and conformations around the glycosidic bond in oligonucleotides. The theoretical dependence of the observed 5J(H1',F) couplings on the torsion angle chi can be described by a generalized Karplus relationship. The corresponding density functional theory (DFT) analysis is outlined. Additional NMR experiments facilitating the resonance assignments of 5-fluoropyrimidine-substituted RNAs are described, and chemical shift changes due to altered shielding in the presence of fluorine-19 (19F) are presented.     

Nuclear spin-spin coupling constants in a coupled-cluster polarization propagator approach

A recently derived coupled-cluster doubles polarization propagator approximation (CCDPPA) for excitation energies and transition moments is extended to the calculation of nuclear spin-spin coupling constants. As expected, in cases where the perturbation is small (e.g. C₂H₂) the difference between results obtained using the present scheme and results obtained using the perturbative second-order polarization propagator approach (SOPPA) is small (1-5%). However, for CH⁺ the difference is found to be more than 40%.     

13C-13C spin-spin coupling constants in 3-carene

Novosibirsk Institute of Organic Chemistry, Siberian Branch, USSR Academy of Sciences. Translated from Khimiya Prirodnykh Soedinenii, Vol. 6, pp. 853–854, November–December, 1989.     

The relative importance of delocalization terms in proton spin-spin coupling constants

An expression for the atom-atom poiarizability between the1s orbitals of geminal or vicinal protons has been derived by perturbation theory using thePople-Santry Theory of electron systems. The coupling constants calculated on this basis agree well with these obtained from full molecular orbital calculations. Expressions have been obtained for the variation of vicinal coupling constants with dihedral angle in ethane and the HCC bond angle and C-C bond length in ethylene, and these agree well with corresponding expressions obtained using the VB method.

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Zusammenfassung Es wurde ein Ausdruck für die Atom-Atom-Polarisierbarkeit zwischen den1s Orbitalen geminaler bzw. vicinaler Protonen abgeleitet mittels Störungsrechnung unter Benutzung derPople-Santry-Theorie für-Elektronensysteme. Die auf dieser Basis berechneten Koppelungskonstanten stimmen gut mit denen aus einer vollständigen MO Rechnung überein. Es wurden Ausdrücke erhalten für die Änderung der vicinalen Koppelungskonstanten mit dem Diederwinkel in Äthan sowie dem HOC Bindungswinkel und der C-C Bindungsiänge in Äthylen, die gut mit den entsprechenden Ausdrücken aus der VB Methode übereinstimmen.

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Résumé A l'aide de la théorie dePople etSantry sur les systèmes d'électrons, nous dérivons de la théorie des perturbations une expression pour la polarisabiiité atome-atome entre les orbitales1s des protons géminaux ou vicinaux. Les constantes de couplage ici calculées s'accordent bien à celles obtenues des calculs OM complets. Les expressions obtenues pour la variation du couplage vicinal avec l'angle dihédrique dans l'éthane, et l'angle HCC et la distance C-C dans l'éthylène, s'accordent bien aux formules correspondantes de la méthode de la mésomérie.

     

The calculation of indirect nuclear spin-spin coupling constants in large molecules

We present calculations of indirect nuclear spin-spin coupling constants in large molecular systems, performed using density functional theory. Such calculations, which have become possible because of the use of linear-scaling techniques in the evaluation of the Coulomb and exchange-correlation contributions to the electronic energy, allow us to study indirect spin-spin couplings in molecules of biological interest, without having to construct artificial model systems. In addition to presenting a statistical analysis of the large number of short-range coupling constants in large molecular systems, we analyse the asymptotic dependence of the indirect nuclear spin-spin coupling constants on the internuclear separation. In particular, we demonstrate that, in a sufficiently large one-electron basis set, the indirect spin-spin coupling constants become proportional to the inverse cube of the internuclear separation, even though the diamagnetic and paramagnetic spin-orbit contributions to the spin-spin coupling constants separately decay as the inverse square of this separation. By contrast, the triplet Fermi contact and spin-dipole contributions to the indirect spin-spin coupling constants decay exponentially and as the inverse cube of the internuclear separation, respectively. Thus, whereas short-range indirect spin-spin coupling constants are usually dominated by the Fermi contact contribution, long-range coupling constants are always dominated by the negative diamagnetic spin-orbit contribution and by the positive paramagnetic spin-orbit contribution, with small spin-dipole and negligible Fermi contact contributions.