Monday, June 21, 2010

J. Phys. Chem. A

Solid-State NMR Spectra and Long, Intra-Dimer Bonding in the π-[TTF]22+ (TTF = Tetrathiafulvalene) Dication

Merrill D. Halling, Joshua D. Bell, Ronald J. Pugmire, David M. Grant* and Joel S. Miller

J. Phys. Chem. A, 2010, 114 (24), pp 6622–6629
DOI: 10.1021/jp910509f
Publication Date (Web): June 2, 2010

The 13C chemical-shift tensor principal values for TTF and π-[TTF]22+ (TTF = tetrathiafulvalene) dimer dications have been measured in order to better understand the electronic structure and long intradimer bonding of these TTF-based dimer structures. The structure of π-[TTF]22+ is abnormal due to its two C−C and four S−S ca. 3.4 Å intradimer separations, which is less than the sum of the sulfur van der Waals radii, and has a singlet 1A1g electronic ground state. This study of TTF and [TTF]22+ was conducted to determine how the NMR chemical-shift tensor principal values change as a function of electronic structure. This study also establishes a better understanding of the interactions that lead to spin-pairing of the monomeric radical units. The density functional theory (DFT) calculated nuclear shielding tensors are correlated with the experimentally determined principal chemical-shift values. The embedded ion method (EIM) was used to investigate the electrostatic lattice potential in [TTF]22+. These theoretical methods provide information on the tensor magnitudes and orientations of their tensor principal values with respect to the molecular frame. The experimental chemical-shift principal values agree with the calculated quantum mechanical chemical-shielding principal values, within typical errors commonly seen for this class of molecular system. Relatively weak Wiberg bond orders between the two [TTF]+ components of the dimer dication correlate with the long bonds linking the two [TTF]+ monomers and substantiate the claim that there is weak multicenter bonding present.

Interpretation of Indirect Nuclear Spin−Spin Couplings in Isomers of Adenine: Novel Approach to Analyze Coupling Electron Deformation Density Using Localized Molecular Orbitals

Radek Marek*, Aneka Kstkov, Kateina Malikov, Jaromr Touek, Jaromr Marek, Michal Hocek, Olga L. Malkina* and Vladimir G. Malkin

J. Phys. Chem. A, 2010, 114 (24), pp 6689–6700
DOI: 10.1021/jp102186r
Publication Date (Web): June 2, 2010

Adenine, an essential building block of nucleic acids present in all living systems, can occur in several tautomeric forms. The phenomenon of tautomerism can be investigated by several experimental methods, including nuclear magnetic resonance. In this study, long-range 1H−13C and 1H−15N coupling constants for N-alkyl derivatives related to four tautomers of adenine are investigated in DMSO and DMF solutions. To investigate the structural dependence of the coupling constants and to understand how polarization propagates in the system, Fermi contact (FC) terms were calculated for the individual isomers and analyzed by using density functional theory (DFT), and the coupling pathways were visualized using real-space functions. The coupling electron deformation densities (CDD) of several 1H−X (X = 13C, 15N) pairs are evaluated and compared. In order to analyze the CDD in more detail, a new approach to break down the CDD into contributions from Boys or Pipek−Mezey localized molecular orbitals (LMOs) has been developed. A similar approach has been applied to split the value of the FC contribution to the J coupling into the LMO contributions. On the basis of chemical concepts, the contributions of σ-bonds, π-electrons, and lone pairs of electrons are discussed. The lone pair of electrons at the nitrogen atom contributes significantly to the 1H−C═15N coupling, whereas the 1H−C═N−13C coupling is affected in a somewhat different way. Surprisingly, the contribution of the intervening C═N bond to the FC term for 1H−C═15N coupling originates exclusively in σ-electrons, with a vanishingly small contribution calculated for the π-electrons of this fragment. This behavior is rationalized by introducing the concept of “hard and soft J elements” derived from the polarizability of the individual components.

NMR Spectroscopic Parameters of Molecular Systems with Strong Hydrogen Bonds

Natalia Zarycz and Gustavo A. Aucar*

J. Phys. Chem. A, Article ASAP
DOI: 10.1021/jp1019334
Publication Date (Web): June 16, 2010

A series of closed H-bonded molecules that have (or not) delocalized bonds were studied. The dependence of both NMR spectroscopic parameters σ and J-couplings, and also the energy stability of such molecules with H-bond strength, were analyzed. The selected basic geometrical structure was that of malonaldehyde. From its full optimized geometry, the corresponding geometry of 3-OH propanal was obtained, fixing either the d(O−O) distance or a more extended local geometry and then optimizing the other part of the whole structure. Nitromalonaldehyde and nitromalonamide were also studied because they should have stronger H-bonds and their basic structure is also malonaldehyde. The last one also has electronic effects that may be varied by rotating the amino groups. By doing this it is possible to show that the effects on acidity of donors are more important than the equivalent effects on the basicity of acceptors. It is also shown that J-couplings that involve atoms close to the H-bond have important noncontact contributions that must be included in order to reproduce total J values. Noncontact contributions are more important than the Fermi contact (FC) one for J(O−O) in malonaldehyde. In nitromalonamide all three terms, FC, paramagnetic spin-orbital, and spin-dipolar are of the same order of magnitude when both amino groups are rotated. This does not happen for its planar configuration. Nuclear magnetic shielding of the hydrogen belonging to the H-bond is quite sensitive to it. The magnetic behavior of such hydrogen atom is modified when it is part of a closed H-bonded molecule. Then a relationship between the H-bond strength with the paramagnetic contributions of the shieldings of both atoms, C and O of the donor substructure, was obtained. We have found a cubic correlation between σp (C) of the C−O donor bond with σ (H) of the H-bonded hydrogen. It is observed that both the noncontact J-coupling contributions and shieldings on atoms belonging to the donor substructure, give a clear evidence about the presence of the resonance phenomenon in the model compounds that have been studied, malonaldehyde, nitromalonaldehyde, and nitromalonamide.

Spin-State-Corrected Gaussian-Type Orbital Basis Sets

Marcel Swart*, Mireia Gell, Josep M. Luis and Miquel Sola

J. Phys. Chem. A, Article ASAP
DOI: 10.1021/jp102712z
Publication Date (Web): June 16, 2010

Recently, we reported that the basis set has a profound influence on the computed values for spin-state splittings [ J. Phys. Chem. A 2008, 112, 6384]. In particular, small Gaussian-type orbital (GTO) basis sets were shown to be unreliable for the prediction of them. Here, we report simple modifications of the small Pople-type Gaussian-type orbital basis sets (3-21G, 3-21G*, 6-31G, 6-31G*), which correct their faulty behavior for the spin-state energies. We have investigated the basis sets for a set of 13 first-row transition-metal complexes for which reliable reference data have been obtained at the OPBE/TZ2P(STO) level. For several systems, we have used single and double spin-contamination corrections to avoid ambiguity of the results because of spin contamination, that is, the energies and geometries were obtained for the pure spin states. The spin ground states as predicted by the spin-state-corrected GTO basis sets (s6-31G, s6-31G*) are in complete agreement with the reference Slater-type orbital (STO) data, while those of the original basis sets and a recent modification by Baker and Pulay (m6-31G*) are not for all cases. The spin-state-corrected GTO basis sets also improve upon the original and modified basis sets for the accuracy of geometry optimization, while the accuracy of the vibrational frequencies is as good or better. At a limited additional cost, one therefore obtains very reliable results for these important spin-state energies.

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