New perspectives in the PAW/GIPAW approach: J(P-O-Si) coupling constants, antisymmetric parts of shift tensors and NQR predictions.

In 2001, Pickard and Mauri implemented the gauge including projected augmented wave (GIPAW) protocol for first-principles calculations of NMR parameters using periodic boundary conditions (chemical shift anisotropy and electric field gradient tensors). In this paper, three potentially interesting perspectives in connection with PAW/GIPAW in solid-state NMR and pure nuclear quadrupole resonance (NQR) are presented: (i) the calculation of J coupling tensors in inorganic solids; (ii) the calculation of the antisymmetric part of chemical shift tensors and (iii) the prediction of (14)N and (35)Cl pure NQR resonances including dynamics. We believe that these topics should open new insights in the combination of GIPAW, NMR/NQR crystallography, temperature effects and dynamics. Points (i), (ii) and (iii) will be illustrated by selected examples: (i) chemical shift tensors and heteronuclear (2)J(P-O-Si) coupling constants in the case of silicophosphates and calcium phosphates [Si(5)O(PO(4))(6), SiP(2)O(7) polymorphs and α-Ca(PO(3))(2)]; (ii) antisymmetric chemical shift tensors in cyclopropene derivatives, C(3)X(4) (X = H, Cl, F) and (iii) (14)N and (35)Cl NQR predictions in the case of RDX (C(3)H(6)N(6)O(6)), ß-HMX (C(4)H(8)N(8)O(8)), α-NTO (C(2)H(2)N(4)O(3)) and AlOPCl(6). RDX, ß-HMX and α-NTO are explosive compounds.

Probing heteronuclear (15)N-(17)O and (13)C-(17)O connectivities and proximities by solid-state NMR spectroscopy.

Heteronuclear solid-state magic-angle spinning (MAS) NMR experiments for probing (15)N-(17)O dipolar and J couplings are presented for [(2)H(NH(3)),1-(13)C,(15)N,(17)O(2)]glycine.(2)HCl and [(15)N(2),(17)O(2)]uracil. Two-dimensional (15)N-(17)O correlation spectra are obtained using the R(3)-HMQC experiment; for glycine.(2)HCl, the intensity of the resolved peaks for the CO and C-O(2)H (17)O resonances corresponds to the relative magnitude of the respective (15)N-(17)O dipolar couplings. (17)O-(15)N REDOR curves are presented for glycine.(2)HCl; fits of the initial buildup (DeltaS/S < 0.2) yield effective dipolar couplings in agreement with (+/-20%) the root-sum-squared dipolar couplings determined from the crystal structure. Experimental (15)N-(17)O REAPDOR curves for the (15)N resonances in glycine.(2)HCl and uracil fit well to the universal curve presented by Goldbourt et al. (J. Am. Chem. Soc. 2003, 125, 11194). Heteronuclear (13)C-(17)O and (15)N-(17)O J couplings were experimentally determined from fits of the quotient of the integrated intensity obtained in a heteronuclear and a homonuclear spin-echo experiment, S(Q)(tau) = S(HET)(tau)/S(HOM)(tau). For glycine.(2)HCl, (1)J(CO) was determined as 24.7 +/- 0.2 and 25.3 +/- 0.3 Hz for the CO and C-O(2)H resonances, respectively, while for uracil, the average of the two NH...O hydrogen-bond-mediated J couplings was determined as 5.1 +/- 0.6 Hz. In addition, two-bond intramolecular J couplings, (2)J(OO) = 8.8 +/- 0.9 Hz and (2)J(N1,N3) = 2.7 +/- 0.1 Hz, were determined for glycine.(2)HCl and uracil, respectively. Excellent agreement was found with J couplings calculated using the CASTEP code using geometrically optimized crystal structures for glycine.HCl [(1)J(CO)(CO) = 24.9 Hz, (1)J(CO)(COH) = 27.5 Hz, (2)J(OO) = 7.9 Hz] and uracil [(2h)J(N1,O4) = 6.1 Hz, (2h)J(N3,O4) = 4.6 Hz, (2)J(N1,N3) = 2.7 Hz].

Density functional theory calculations of hydrogen-bond-mediated NMR J coupling in the solid state.

A recently developed method for calculating NMR J coupling in solid-state systems is applied to calculate hydrogen-bond-mediated (2h) J NN couplings across intra- or intermolecular N-H...N hydrogen bonds in two 6-aminofulvene-1-aldimine derivatives and the ribbon structure formed by a deoxyguanosine derivative. Excellent quantitative agreement is observed between the calculated solid-state J couplings and those previously determined experimentally in two recent spin-echo magic-angle-spinning NMR studies ( Brown, S. P. ; et al. Chem. Commun. 2002, 1852-1853 and Pham, T. N. ; et al. Phys. Chem. Chem. Phys. 2007, 9, 3416-3423 ). For the 6-aminofulvene-1-aldimines, the differences in (2h) J NN couplings in pyrrole and triazole derivatives are reproduced, while for the guanosine ribbons, an increase in (2h) J NN is correlated with a decrease in the N-H...N hydrogen-bond distance. J couplings are additionally calculated for isolated molecules of the 6-aminofulevene-1-aldimines extracted from the crystal with and without further geometry optimization. Importantly, it is shown that experimentally observed differences between J couplings determined by solution- and solid-state NMR are not solely due to differences in geometry; long-range electrostatic effects of the crystal lattice are shown to be significant also. J couplings that are yet to be experimentally measured are calculated. Notably, (2h) J NO couplings across N-H...O hydrogen bonds are found to be of a similar magnitude to (2h) J NN couplings, suggesting that their utilization and quantitative determination should be experimentally feasible.

A first principles theory of nuclear magnetic resonance J-coupling in solid-state systems.

A method to calculate NMR J-coupling constants from first principles in extended systems is presented. It is based on density functional theory and is formulated within a planewave-pseudopotential framework. The all-electron properties are recovered using the projector augmented wave approach. The method is validated by comparison with existing quantum chemical calculations of solution-state systems and with experimental data. The approach has also been applied to the silicophosphate, Si(5)O(PO(4))(6), giving (31)P-(29)Si-couplings which are in excellent agreement with experiment.

Assigning carbon-13 NMR spectra to crystal structures by the INADEQUATE pulse sequence and first principles computation: a case study of two forms of testosterone.

A (13)C CPMAS NMR experiment at high field (11.7 T) has produced significantly improved dispersion for the alpha form of testosterone, allowing revisions and extensions to be made to the assignments. Correlations shown by an INADEQUATE two-dimensional spectrum, recorded at 16.5 T, have allowed the components of most of the doublet signals to be grouped into two sets (for the two crystallographically independent molecules). First-principles computations, employing a fully solid-state approach, have been used to obtain values for the crystallographic splittings, which are discussed in relation to the experimental values. This procedure enables assignments to the two groups to be suggested for all but one of the remaining doublet signals. It also allows the two sets of signals to be identified specifically to the two independent molecules in the crystal structure. Computations were also carried out for the beta form of testosterone (a dihydrate). The shift differences between the alpha and beta forms were compared with the experimental data, with encouraging results. Comparisons were also made between computed and experimental shielding anisotropies and asymmetries for three of the carbons of the alpha form. The methodology has a high potential for future applications, though more examples need to be evaluated before general conclusions can be drawn.