Tomasz Pawlak

Structural correlations for 1H, 13C and 15N NMR coordination shifts in Au(III), Pd(II) and Pt(II) chloride complexes with lutidines and collidine

1H, 13C and 15N NMR studies of gold(III), palladium(II) and platinum(II) chloride complexes with dimethylpyridines (lutidines: 2,3‐lutidine, 2,3lut; 2,4‐lutidine, 2,4lut; 3,5‐lutidine, 3,5lut; 2,6‐lutidine, 2,6lut) and 2,4,6‐trimethylpyridine (2,4,6‐collidine, 2,4,6col) having general formulae [AuLCl3], trans‐[PdL2Cl2] and trans‐/cis‐[PtL2Cl2] were performed and the respective chemical shifts (δ1H, δ13C, δ15N) reported. The deshielding of protons and carbons, as well as the shielding of nitrogens was observed. The 1H, 13C and 15N NMR coordination shifts (Δ1Hcoord, Δ13Ccoord, Δ15Ncoord; Δcoord = δcomplex − δligand) were discussed in relation to some structural features of the title complexes, such as the type of the central atom [Au(III), Pd(II), Pt(II)], geometry (trans‐ or cis‐), metal‐nitrogen bond lengths and the position of both methyl groups in the pyridine ring system. Copyright

Simple and accurate determination of X-H distances under ultra-fast MAS NMR.

We demonstrate that a very simple experiment, Cross-Polarization with Variable Contact-time (CP-VC), is very efficient at ultra-fast MAS (νR ≥ 60 kHz) to measure accurately the C-H and N-H distances, and to analyze the dynamics of bio-molecules. This experiment can be performed with samples that are either (13)C or (15)N labeled or without any labeling. The method is very robust experimentally with respect to imperfect Hartman-Hahn setting, and presents a large scaling factor allowing a better dipolar determination, especially for long C-H or N-H distances, or for CH3 or NH3 moieties with three-site hopping. At ultra-fast MAS, it can be used quantitatively in a 2D way, because its scaling factor is then little dependent on the offsets. This robustness with respect to offset is related to the ultra-fast spinning speed, and hence to the related small rotor diameter. Indeed, these two specifications lead to efficient n = ±1 zero-quantum Hartman-Hahn CP-transfers with large RF-fields on proton and carbon or nitrogen channels, and large dipolar scaling factor.

Understanding the electronic factors responsible for ligand spin-orbit NMR shielding in transition-metal complexes

The significant role of relativistic effects in altering the NMR chemical shifts of light nuclei in heavy-element compounds has been recognized for a long time; however, full understanding of this phenomenon in relation to the electronic structure has not been achieved. In this study, the recently observed qualitative differences between the platinum and gold compounds in the magnitude and the sign of spin-orbit-induced (SO) nuclear magnetic shielding at the vicinal light atom ((13)C, (15)N), σ(SO)(LA), are explained by the contractions of 6s and 6p atomic orbitals in Au complexes, originating in the larger Au nuclear charge and stronger scalar relativistic effects in gold complexes. This leads to the chemical activation of metal 6s and 6p atomic orbitals in Au complexes and their larger participation in bonding with the ligand, which modulates the propagation of metal-induced SO effects on the NMR signal of the LA via the Spin-Orbit/Fermi Contact (SO/FC) mechanism. The magnitude of the σ(SO)(LA) in these square-planar complexes can be understood on the basis of a balance between various metal-based 5d → 5d* and 6p → 6p* orbital magnetic couplings. The large and positive σ(SO)(LA) in platinum complexes is dominated by the shielding platinum-based 5d → 5d* magnetic couplings, whereas small or negative σ(SO)(LA) in gold complexes is related to the deshielding contribution of the gold-based 6p → 6p* magnetic couplings. Further, it is demonstrated that σ(SO)(LA) correlates quantitatively with the extent of M-LA electron sharing that is the covalence of the M-LA bond (characterized by the QTAIM delocalization index, DI). The present findings will contribute to further understanding of the origin and propagation of the relativistic effects influencing the experimental NMR parameters in heavy-element systems.

Computed and Experimental Chemical Shift Parameters for Rigid and Flexible YAF Peptides in the Solid State

DFT methods were employed to compute the (13)C NMR chemical shift tensor (CST) parameters for crystals of YAF peptides (Tyr-Ala-Phe) with different stereochemistry for the Ala residue. Tyr-D-Ala-Phe 1 crystallizes in the C2 space group while Tyr-L-Ala-Phe crystallizes in either the P2(1)2(1)2 space group (2a) or the P6(5) space group (2b). PISEMA MAS measurements for samples with a natural abundance of (1)H and (13)C nuclei and (2)H QUADECHO experiments for samples with deuterium labeled aromatic rings were used to analyze the geometry and time scale of the molecular motion. At ambient temperature, the tyrosine ring of sample 1 is rigid and the phenylalanine ring undergoes a π-jump, both rings in sample 2a are static, and both rings in sample 2b undergo a fast regime exchange. The theoretical values of the CST were obtained for isolated molecules (IM) and clusters employing the ONIOM approach. The experimental (13)C δ(ii) parameters for all of the samples were measured via a 2D PASS sequence. Significant scatter of the computed versus the experimental (13)C CST parameters was observed for 1 and 2b, while the observed correlation was very good for 2a. In this report, we show that the quality of the (13)C σ(ii)/(13)C δ(ii) correlations, when properly interpreted, can be a source of important information about local molecular motions.

1H, 13C, 15N and 195Pt NMR studies of Au(III) and Pt(II) chloride organometallics with 2-phenylpyridine.

1H, 13C, 15N and 195Pt NMR studies of gold(III) and platinum(II) chloride organometallics with N(1),C(2′)‐chelated, deprotonated 2‐phenylpyridine (2ppy*) of the formulae [Au(2ppy*)Cl2], trans(N,N)‐[Pt(2ppy*)(2ppy)Cl] and trans(S,N)‐[Pt(2ppy*)(DMSO‐d6)Cl] (formed in situ upon dissolving [Pt(2ppy*)(µ‐Cl)]2 in DMSO‐d6) were performed. All signals were unambiguously assigned by HMBC/HSQC methods and the respective 1H, 13C and 15N coordination shifts (i.e. differences between chemical shifts of the same atom in the complex and ligand molecules: Δ1Hcoord = δ1Hcomplex − δ1Hligand, Δ13Ccoord = δ13Ccomplex − δ13Cligand, Δ15Ncoord = δ15Ncomplex − δ15Nligand), as well as 195Pt chemical shifts and 1H‐195Pt coupling constants discussed in relation to the known molecular structures. Characteristic deshielding of nitrogen‐adjacent H(6) protons and metallated C(2′) atoms as well as significant shielding of coordinated N(1) nitrogens is discussed in respect to a large set of literature NMR data available for related cyclometallated compounds. Copyright

1H NMR assignment corrections and 1H, 13C, 15N NMR coordination shifts structural correlations in Fe(II), Ru(II) and Os(II) cationic complexes with 2,2′-bipyridine and 1,10-phenanthroline

1H, 13C and 15N NMR studies of iron(II), ruthenium(II) and osmium(II) tris‐chelated cationic complexes with 2,2′‐bipyridine and 1,10‐phenanthroline of the general formula [M(LL)3]2+ (M = Fe, Ru, Os; LL = bpy, phen) were performed. Inconsistent literature 1H signal assignments were corrected. Significant shielding of nitrogen‐adjacent protons [H(6) in bpy, H(2) in phen] and metal‐bonded nitrogens was observed, being enhanced in the series Ru(II) → Os(II) → Fe(II) for 1H, Fe(II) → Ru(II) → Os(II) for 15N and bpy → phen for both nuclei. The carbons are deshielded, the effect increasing in the order Ru(II) → Os(II) → Fe(II). Copyright

Validation of Relativistic DFT Approaches to the Calculation of NMR Chemical Shifts in Square-Planar Pt2+ and Au3+ Complexes

Recently implemented hybrid density functional methods of calculating nuclear magnetic shielding using the two-component zeroth-order regular approximation approach (J. Phys. Chem. A2009, 113, 11495) have been employed for a series of compounds containing heavy transition-metal atoms. These include Pt(2+), Pd(2+), and Au(3+) organometallics and metal complexes with azines, some of which exhibit interesting biological and catalytic activities. In this study we investigate the effects of geometry, exchange-correlation functional, solvent, and scalar relativistic and spin-orbit corrections on the nuclear magnetic shielding-mainly for (13)C and (15)N atoms connected to a heavy-atom center. Our calculations demonstrate that the B3LYP method using effective core potentials and a cc-pwCVTZ-PP/6-31G** basis set augmented with the polarizable continuum model of the dimethylsulfoxide solvent provides geometries for the complexes in question which are compatible with the experimental NMR results in terms of both the trends and the absolute values of the (13)C shifts. The important role of the exact exchange admixture parameter for hybrid functionals based on B3LYP and PBE0 is investigated systematically for selected Pt(2+) and Au(3+) complexes. The (13)C and (15)N NMR chemical shifts are found to be best reproduced by using a B3LYP or PBE0 approach with 30% and 40-50% exact exchange admixtures for the Pt(2+) and Au(3+) complexes, respectively. The spin-orbit contributions to the (15)N NMR chemical shifts reflect metal-ligand bonding that is much more ionic for the Au(3+) than for the Pt(2+) complex. Finally, an optimized density functional method is applied to a series of transition-metal complexes to estimate the scope and the limitations of the current approach.

Fine refinement of solid-state molecular structures of Leu- and Met-enkephalins by NMR crystallography.

This paper presents a methodology that allows the fine refinement of the crystal and molecular structure for compounds for which the data deposited in the crystallographic bases are of poor quality. Such species belong to the group of samples with molecular disorder. In the Cambridge Crystallographic Data Center (CCDC), there are approximately 22,000 deposited structures with an R-factor over 10. The powerful methodology we present employs crystal data for Leu-enkephalin (two crystallographic forms) with R-factor values of 14.0 and 8.9 and for Met-enkephalin (one form) with an R-factor of 10.5. NMR crystallography was employed in testing the X-ray data and the quality of the structure refinement. The GIPAW (gauge invariant projector augmented wave) method was used to optimize the coordinates of the enkephalins and to compute NMR parameters. As we reveal, this complementary approach makes it possible to generate a reasonable set of new coordinates that better correlate to real samples. This methodology is general and can be employed in the study of each compound possessing magnetically active nuclei.

1H, 13C, 195Pt and 15N NMR structural correlations in Pd(II) and Pt(II) chloride complexes with various alkyl and aryl derivatives of 2,2'-bipyridine and 1,10-phenanthroline.

1H, 13C, 195Pt and 15N NMR studies of platinide(II) (M = Pd, Pt) chloride complexes with such alkyl and aryl derivatives of 2,2′‐bipyridine and 1,10‐phenanthroline as LL = 6,6′‐dimethyl‐bpy, 5,5′‐dimethyl‐bpy, 4,4′‐di‐tert‐butyl‐bpy, 2,9‐dimethyl‐phen, 2,9‐dimethyl‐4,7‐diphenyl‐phen, 3,4,7,8‐tetramethyl‐phen, having the general [M(LL)Cl2] formula were performed and the respective chemical shifts (δ1H, δ13C, δ195Pt, δ15N) reported. 1H high‐frequency coordination shifts (

Study of the thermal processes in molecular crystals of peptides by means of NMR crystallography

\Delta_{\rm {coord}}^{\rm {1H}} = \delta_{\rm {complex}}^{\rm {1H}} - \delta_{\rm {ligand}}^{\rm {1H}}