Abstract:Ultra-wideline 27Al NMR experiments are conducted on coordination compounds with 27Al nuclei possessing immense quadrupolar interactions that result from exceptionally nonspherical coordination environments. NMR spectra are acquired using a methodology involving frequency-stepped, piecewise acquisition of NMR spectra with Hahn-echo or quadrupolar Carr-Purcell Meiboom-Gill (QCPMG) pulse sequences, which is applicable to any half-integer quadrupolar nucleus with extremely broad NMR powder patterns. Despite the l… Show more
“…In Fig. 6, a graphical representation of theV-tensor is shown for The sign pattern and orientation of the EFG tensor of the four-coordinate 27 Al compound shown in Fig. 4 matches exactly the predictions from this model; due to the electronegativity of the nitrogen ligands one expects less electron density in the aluminum sp 3 hybrids pointing towards nitrogen than in those pointing towards the carbons.…”
Section: Basic Models Sign Patternssupporting
confidence: 68%
“…To visually compare EFG tensor magnitudes between different systems, such as the three-, four-, and five-coordinate aluminum centers that will be discussed in Section "Al EFG Tensors", it is best to use a common scaling factor. Figure 4 shows simulated SSNMR spectra for a set of hypothetical 27 Al compounds all having the same V 33 but varying η. Like these simulated spectra, the shapes of real spectra reveal the "shape" (asymmetry) of the EFG tensor.…”
Section: Tensor Plotsmentioning
confidence: 99%
“…This result is also easily obtained by considering (i) that the EFG contribution of an sp 3 orbital along its directional vector is − 3 /2, (ii) that with respect to this direction the other three orbitals are symmetry equivalent, and (iii) that for equal occupations of all four orbitals the diagonal tensor components must vanish. For the 4-coordinate 27 Al system shown in Fig. 4, the nearest neighbor atoms are two carbons on one side and two nitrogens on the other side.…”
Section: Basic Models Sign Patternsmentioning
confidence: 99%
“…More recent discussions have noted the connections between the quadrupolar interaction and the local symmetry, coordination numbers and bonding arrangements (27)(28)(29); however, only comparisons of series of structurally and/or chemically similar systems are valid in this context. Therefore, there is a need for the further development for systematic investigations of quadrupolar interactions for a variety of atom types in an assortment of coordination environments, as well as robust computational methods for exploring these systems.…”
ABSTRACT:This article is concerned with the analysis of electric field gradients (EFGs) using first-principles theory along with model calculations. Simple atomic orbital (AO) models for the EFG are developed in the spirit of the Townes-Dailey (TD) analysis and applied to various sets of sp n hybrid orbitals and to atomic d orbital shells. These AO models are then combined with modern analysis methods rooted in first principles theory which provide accurate localized molecular orbital contributions to the EFG. It is shown by density functional computations how such analyses of the EFG for a variety of typical structural motifs can provide an intuitive way of understanding the chemical origin of the magnitude and the sign of EFG tensors at atomic nuclei, as well as of their orientation with respect to the molecular coordinate frame. The utility of graphical visualizations of EFG tensors is also emphasized. The systems that are investigated span the range from very small molecules (carbon and sulfur EFGs in CO, CS, OCS) to small-and medium-sized molecules (nitrogen and aluminum EFGs in ammonia, methyl-cyanide and -isocyanide, aluminum AlX 3 model systems and various alumino-organic systems), to the metal atom field gradient in transition metal complexes with Ru and Nb and a variety of ligands.
“…In Fig. 6, a graphical representation of theV-tensor is shown for The sign pattern and orientation of the EFG tensor of the four-coordinate 27 Al compound shown in Fig. 4 matches exactly the predictions from this model; due to the electronegativity of the nitrogen ligands one expects less electron density in the aluminum sp 3 hybrids pointing towards nitrogen than in those pointing towards the carbons.…”
Section: Basic Models Sign Patternssupporting
confidence: 68%
“…To visually compare EFG tensor magnitudes between different systems, such as the three-, four-, and five-coordinate aluminum centers that will be discussed in Section "Al EFG Tensors", it is best to use a common scaling factor. Figure 4 shows simulated SSNMR spectra for a set of hypothetical 27 Al compounds all having the same V 33 but varying η. Like these simulated spectra, the shapes of real spectra reveal the "shape" (asymmetry) of the EFG tensor.…”
Section: Tensor Plotsmentioning
confidence: 99%
“…This result is also easily obtained by considering (i) that the EFG contribution of an sp 3 orbital along its directional vector is − 3 /2, (ii) that with respect to this direction the other three orbitals are symmetry equivalent, and (iii) that for equal occupations of all four orbitals the diagonal tensor components must vanish. For the 4-coordinate 27 Al system shown in Fig. 4, the nearest neighbor atoms are two carbons on one side and two nitrogens on the other side.…”
Section: Basic Models Sign Patternsmentioning
confidence: 99%
“…More recent discussions have noted the connections between the quadrupolar interaction and the local symmetry, coordination numbers and bonding arrangements (27)(28)(29); however, only comparisons of series of structurally and/or chemically similar systems are valid in this context. Therefore, there is a need for the further development for systematic investigations of quadrupolar interactions for a variety of atom types in an assortment of coordination environments, as well as robust computational methods for exploring these systems.…”
ABSTRACT:This article is concerned with the analysis of electric field gradients (EFGs) using first-principles theory along with model calculations. Simple atomic orbital (AO) models for the EFG are developed in the spirit of the Townes-Dailey (TD) analysis and applied to various sets of sp n hybrid orbitals and to atomic d orbital shells. These AO models are then combined with modern analysis methods rooted in first principles theory which provide accurate localized molecular orbital contributions to the EFG. It is shown by density functional computations how such analyses of the EFG for a variety of typical structural motifs can provide an intuitive way of understanding the chemical origin of the magnitude and the sign of EFG tensors at atomic nuclei, as well as of their orientation with respect to the molecular coordinate frame. The utility of graphical visualizations of EFG tensors is also emphasized. The systems that are investigated span the range from very small molecules (carbon and sulfur EFGs in CO, CS, OCS) to small-and medium-sized molecules (nitrogen and aluminum EFGs in ammonia, methyl-cyanide and -isocyanide, aluminum AlX 3 model systems and various alumino-organic systems), to the metal atom field gradient in transition metal complexes with Ru and Nb and a variety of ligands.
“…Due to the limited excitation bandwidths associated with the WURST pulses, all spectra were acquired using the previously mentioned VOCS method, 98,99 with frequency increments equal to an integer multiple of the spikelet spacings arising from the CPMG portion of the pulse sequence. 141 The experimental times ranged from as long as 40 hours to as short as 8.4 hours, with a mean of approximately 16 hours. Further experimental details are given in the Supporting Information (Tables S1 -S3).…”
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