NMR-spectra solid state

Figure 4 shows representative solid state NMR spectra of crystalline vanadates. 

Figure 4 Local microstruetures and experimental solid state NMR spectra in crystalline vanadium oxide compounds.

Figure 5 Solid state NMR spectra of Vanadium oxide on y-alumina as a function of vanadium loading (wt.%) and surface coverage 0. Note the gradual emergence of the six-coordinated vanadium site with increased loading.

In general, the specific information that can be obtained from a simple solid state NMR experiment depends on the personality of the nuclear isotope under study. In many cases, solid state NMR spectra are not as straightforwardly interpretable as in the preceding example. Furthermore, disordered materials, such as thin films. 

Figure 3.66 Enriched 1SN solid-state NMR spectra of (a) [Pt(en)Cl4] (b) [Pt(en)Cl2] (c) [Pt(en)Cl2][Pt(en)Cl4]. (Reprinted with permission from Inorg. Chem., 1992,31,4281. Copyright (1992) American Chemical Society.)...

The CP/MAS NMR spectra are an important source of information regarding the structure of cellulose and its polymorphos. A number of groups have investigated these spectra 11 15) and also reviews on the subject have been published 16 17>. For an orientation in the field Table 1 shows the most important features of the solid-state NMR spectra of cellulose I, II and IV and in Fig. 3 the numeration of the carbon atoms of the cellulose basic unit is given. It is evident that the polymorphs... 

Not only relaxation T1 data, but chemical shifts also give indications regarding the effect of water on cellulose. In Fig. 7 (and Table 3) 22) some data are collected for the solid-state NMR spectra of moist cellulose and for comparison, also of cellulose acetate under similar conditions. 

Fig. 9. Solid-state NMR spectra of stiff chain aromatic polyesters containing sulfur bonds and tentative assignements of their signals, 401. A contact time of 2 ms and a pulse repetition time of 10 s were used...

The authors are grateful to the Robert A. Welch Foundation, the National Science Foundation, the Phillips Petroleum Foundation, and Haldor Topsoe A/S (Denmark) for financial support. We express our thanks to the Exxon Research and Engineering Co. for measuring the solid state NMR spectra and the Shell Development Company for the ESCA measurements. Finally, we acknowledge the participation of Dr. Zinfer R. Ismagilov in part of the cyclohexane studies. 

F ure 10. solid-state NMR spectra of calcium pectate in the solid (A) and gel (B) forms. Inset resolution-enhanced spectra. The gel concentration was 290 g/1. 

We thank Prof. Janet Blumel, Texas A M University, and the Gesellschaft Deutscher Chemiker for allowing us to reproduce solid state NMR spectra. In addition we thank Klaus Jurkschat and Bernhard Lippert and their groups for making available samples of organometallic molecules. Thanks also go to Andrea Bokelmann and Bernhard Griewel for their valuable technical help. 

Here, we have taken advantage of the low silanol content in pure silica LTA and CHA type zeolites synthesized in fluoride medium to get better resolved 29Si and H solid state NMR spectra in the silanol region. This has allowed us to investigate the nature of the silanol groups in zeolites and their interaction with water molecules. 

Solid-state NMR spectra of 2H, where the main concern is to refocus the reasonably small first-order broadening, are commonly obtained by measuring the quadrupolar echo on a static sample. In such a case, the o pq frequencies can be approximated... 

When different Si(nM) sites (n = 0—4) can be resolved in the 29Si solid state NMR spectra of tetrahedral framework silicates, then the Si/M ratio can be determined by the equation [21]... 

When the crystallography of compounds related by polymorphism is such that nuclei in the two structures are magnetically nonequivalent, it will follow that the resonances of these nuclei will not be equivalent. Since it is normally not difficult to assign organic functional groups to observed resonances, solid state NMR spectra can be used to deduce the nature of polymorphic variations, especially when the polymorphism is conformational in nature. Such information is extremely valuable at the early states of drug development when solved single crystal structures for each polymorph or solvate species may not yet be available. 

Manufacturers of today s modem NMR spectrometers normally offer a number of different models of instruments that are capable of measuring solid state NMR spectra. Usually, a dedicated solid state NMR instrument is available along with solution-phase models that are capable of solids work with the purchase of an additional solids accessory package. For any pharmaceutical company that is contemplating the purchase of an NMR for solids work, it is this author s opinion... 

The use of solid state NMR for the investigation of polymorphism is easily understood based on the following model. If a compound exists in two, true polymorphic forms, labeled as A and B, each crystalline form is conformationally different. This means for instance, that a carbon nucleus in form A may be situated in a slightly different molecular geometry compared with the same carbon nucleus in form B. Although the connectivity of the carbon nucleus is the same in each form, the local environment may be different. Since the local environment may be different, this leads to a different chemical shift interaction for each carbon, and ultimately, a different isotropic chemical shift for the same carbon atom in the two different polymorphic forms. If one is able to obtain pure material for the two forms, analysis and spectral assignment of the solid state NMR spectra of the two forms can lead to the origin of the conformational differences in the two polymorphs. Solid state NMR is thus an important tool in conjunction with thermal analysis, optical microscopy, infrared (IR) spectroscopy, and powder... 

Fig. 9 Examples of simplifying solid state NMR spectra by the TOSS and delayed decoupling pulse sequences. Shown is a comparison of the 31P CP/MAS NMR spectrum of fosinopril sodium utilizing the standard pulse sequence (A) and the TOSS routine (B). Also shown is the full 13C CP/MAS NMR spectrum of fosinopril sodium (C) and the nonprotonated carbon spectrum (D) obtained from the delayed decoupling pulse sequence utilizing a 80 /us delay time. Signals due to the methyl carbon resonances (0-30 ppm) are not completely eliminated due to the rapid methyl group rotation, which reduces the carbon-proton dipolar couplings.

Once the proton Tx and Tcli values are determined for a polymorphic system, physical mixtures of the two polymorphs can be generated (calibration samples). Subsequent acquisition of the solid state NMR spectra under quantitative conditions yields signal intensities representative of the amount of each solid state phase... 

Finally, a series of papers have been published on the solid state NMR spectra of a number of analgesic drugs. Jagannathan recorded the solid state 13C NMR spectrum of acetaminophen in bulk and dosage forms [60], From the solution-phase NMR spectrum, assignments of the solid state NMR resonances could be made in addition to explanations for the doublet structure of some resonances (dipolar coupling). Spectra of the dosage product from two sources indicated... 

Figure 3. Solid-state NMR spectra of the polymer (a) one hour after reaction, (b) one week after reaction (N2 storage), and (c) one week following reaction and...

The exact nature of the alkylidenes formed on various oxide surfaces is still uncertain, as is the nature of the alkylidenes responsible for the often observed metathesis activity. Mo(N)(CH2CMe3)3 also has been employed as a precursor to a surface-bound species believed to be of the type Mo(NH)(CHCMe3)(CH2CMe3) (Osurf) [115]. Although the alkylidene carbon atom could not be observed in solid state NMR spectra, which is typical of surface supported alkylidenes, reaction with acetone to give 2,4,4-trimethylpent-2-ene quantitatively confirmed the presence of the reactive neopentylidene complex. Such species would initiate various metathesis reactions when prepared on partially dehydroxylated silica. 

---