3.5 Solid State NMR Experiments
3.5.4 Cross Polarisation and CP HETCOR
While cross polarisation (CP) experiments have not been performed as 1D experiments in this thesis, as is their more common use, where CP is used to enhance more diluted spins [74]. Cross polarisation is however, the basis of the heteronuclear correlation experiment (HETCOR) experiment [75], and therefore requires an explanation of the process involved.
Cross polarisation is a widely used technique in magnetic resonance to enhance the signal of dilute or low natural abundance spins, by transferring the magnetisation from a more abundant spin in the network. This is most commonly done by transferring from protons, which have a large magnetic moment, but can be done with any abundant nuclei [51]. However the experiment can have varying degrees of difficulty for nuclei with spin I > 12 [45]. In the work within CP was form the 31P nuclei to the less
abundant29Si nuclei, while not impossible, it is not really a standard technique due to the weak dipole coupling between the nuclei. In general the signal enhancement that is achievable in a CP experiment can be considered to be a ratio of γI/γS for the two
nuclei involved, whereγI is that of the more abundant spin. As the transfer mechanism
than those further away, therefore it is possible to tell which sites are closer in space than others, it can also be used to exclude sites that don’t take part in the process as those are phase cycled out.
Figure 3.16: Pulse sequence for cross polarisation, an initial π/2(x) pulse rotates the
I spins to thex-y plane, a successivey pulse locks the magnetisation from theI spins, the S spins are then brought into contact for the Hartmann-Hahn match condition, before detection on theS channel.
The theory behind the technique is reasonably complicated [45,50,51,76], especially when MAS is involved, so only a basic explanation is given here. The CP pulse sequence (see Figure 3.16) uses what is called spin locking, spin locking involves an initialx pulse B1(I) to rotate the magnetisation of the more abundant spin I onto the y´axis of the
rotating frame, if a second B1(I) pulse is immediately applied so that it points along
the y´direction and is sufficiently strong enough the magnetisation will remain along
y´as it is not free to precess around z and will only precess around the effective field Bef f which is along y´and is therefore said to be locked, and the magnetisation will
stay locked for how long the pulse is applied for. However, the magnetisation will eventually decay, as theB0 field being much stronger will still cause the magnetisation
to relax back along z this is given the time constant T1ρ. If a pulseB1(S) is applied to
the dilute spins at the same time as the spin lock pulse so that the separate spins are brought into thermal contact, this is done by using what is called the Hartmann-Hahn
(H-H) match condition [76]
γIB1(I) =γSB1(S) (3.19)
It can be seen from Equation 3.19 that the energy transition between the upper and lower levels of the Zeeman splitting for the two spins is now the same, and as the energy of the magnetisation for spinI was created by theB0 spin system then it is too
large to be sustained by the B1(I) field and will decay. However as the spin system
is now coupled to the S spins, energy is conserved, as the I spin system decays it redistributes energy and theS spin system will now also adjust its energy, and as there is no net loss in the system, the S spins gain a large polarisation along the direction B1(S).
There are other advantages to using the CP process, as the T1 of many dilute spins
can often be long, the whole cross polarisation process can use the T1 of the more
abundant spin system. Furthermore, it is not always necessary to use a completely relaxed system for the Ispins, as the magnetisation can still be enhanced as long as there is sufficient build up of longitudinal magnetisation before the transfer takes place. The length of time there needs to be contact between the pulses varies due to the distances and the nuclei involved, and hence the strength of the dipolar coupling. In different systems the contact time can vary from milliseconds to tens of milliseconds, the former are not a problem, however when long contact times are involved, this puts a large amount of stress on the rf circuit, T1ρ can also become a factor and the
polarisation transfer might be insufficient, due to the magnetisation not staying locked long enough for the spins to come into contact. Spinning fast can also complicate matters as dipolar coupling is removed or certainly reduced by rapid spinning, this leads to a loss of efficiency in the CP process. [45, 51]
The CP HETCOR [75] experiment is essentially an extension of the 1D experiment (see Figure 3.17). As with the R125
2pulse sequence a saturation comb was used to make
sure that the31P longitudinal magnetisation was the same before each acquisition. The
difference between the HETCOR and the CP is fairly straight forward, and it can be seen that instead of having the initial B1(I) pulse immediately before the lock pulse,
Figure 3.17: The CP HETCOR experiment is very similar to the CP experiment except that the time t1 between the initial I pulse and the lock pulse is incremented
between each successive data set.
with the other 2D sequences, so that theI spin frequencies are allowed to evolve before the CP transfer takes place, if no increment takes place it is just the 1D one pulse. In addition to this the pulse that is used on the B1(S) channel has a variable amplitude
contact pulse [77] in order to improve the efficiency of the H-H match. The first pulse has a phase cycle of two, whereas theS contact pulse has a phase cycle of eight steps, ensuring that isolated peaks are not visible in the final spectrum. The final Fourier transformed 2D spectrum gives chemical shift information for the observed nucleus in the f2 dimension, and the chemical shift of those spins where the magnetisation was
Chapter 4
31
P MAS NMR Studies of
α-TCP,
with First Principles Calculations
4.1
Introduction
Synthetic calcium phosphate materials generate significant interest in the field of bio- material research [23]. They exhibit both chemical and structural similarities with the calcium apatite like phase that makes up the mineral component of bone, this min- eral phase accounts for approximately 75 % of the bone structure with the other 25 % making up an organic phase [10]. This calcium apatite phase is best described as a form of hydroxyapatite (Ha, Ca10(PO4)6(OH)2) [21], though it is not”pure”and is
found to contain many elements in various quantities [11]. Some elements are only found in trace amounts such as Sr2+,Cu2+, Pb2+ [11], while other elements make up a
more substantial part of the phase Mg2+, Na2+[12]. Bone is not inert and continuously grows just like any other tissue in the body, it does this by undergoing what is called ’remodelling’, this is where bone is continuously broken down by the body through a process known as resorption, whilst new bone grows in its place [13].
Amongst other materials many calcium phosphates are used as biomedical implants, for as well as being similar in chemical composition and structure they are also bioac- tive; this means they interact in a positive way with surrounding tissue when implanted in the body [78]. They are also potentially resorbable, where they are broken down over
time through dissolution or through resorption [2]. These implants can take the form of dense materials, used as coatings for metal implants, where the coating helps fix the metal implant, such as fixing a replacement hip joint in place allowing the existing bone and implant to grow together [79, 80]. Calcium phosphates are also used as bone cements, to help repair extensively damaged bone by filling cavities or defects, these materials are generally highly porous and existing bone grows through the implant, and the implant may eventually be replaced by natural tissue [5].
In recent years tricalcium phosphate (TCP, Ca3(PO4)2) has shown increasing inter-
est as a biomaterial, as it demonstrates both good bioactive and resorbable characteris- tics [17]. There are three poly-morphs of TCP, namelyβ [81],α [1] andα’ [82], though the α’ phase is of little practical interest and only forms at temperatures over 1430◦ C, and reverts back to the α phase when cooled below the transition temperature [82]. While α and β TCP share the same chemical composition they differ considerably in structure, density and solubility [31], leading to quite different biological properties and applications. The α-TCP structure contains a lot of channels and as such is a much more open and less dense material. Both β-TCP and α-TCP find clinical use in dentistry and orthopaedics and come in the form of dense blocks, granules and fine powders for bone cements though α-TCP is more likely to be found as the later since it is the more soluble and reactive of the two polymorphs [17]
As mentioned above β, and α-TCP share the same chemical composition, though there is where the similarities end as they posses very different structures. β-TCP can be considered to be the low temperature phase and α-TCP its high temperature analogue. β-TCP is the more extensively studied of the two phases, however this is more likely due to its ease of synthesis compared to that of the α phase, rather than because of any perceived relative importance. β-TCP is found to be stable at room temperature up to 1125◦ C and crystallizes in the rhombohedral space group R3c with lattice parameters a = 10.439(1), c = 37.375(6) ˚A and Z = 21 [81]. In contrast the α phase is only metastable at room temperature, and if TCP is annealed above 1125◦ C it transforms into the α phase , and special measures have to be taken for cooling, as the structure can revert back to the β phase when returning back through this phase transition temperature [83]. Either the sample has to be cooled carefully, or more
often, quenched quickly to freeze the α phase in place [84]. α-TCP crystallises in the monoclinic space group P21/a with lattice parameters a = 12.887(2), b = 27.280(4), c
= 15.219(2) ˚A, β = 126.20(1)◦ and Z = 24 [1].
A more detailed look at the results from the literature show that the studies are limited to unit cell parameters and refinements with fixed atomic positions, and so conjecture does exist [85, 86], nevertheless Rietveld refinements of these parameters from X-ray diffraction data are in good agreement with the single crystal structure determined by Mathew et al [1] as is the computational study of α-TCP [87]. There exists only one31P MAS NMR study onα-TCP [34] and is not so satisfying, they claim evidence of fifteen31P resonances fromα-TCP, and attribute these to the structure by
Mackay [88] which has sixteen independent phosphorous sites. However the structure proposed by Mackay was found to be incomplete, and was shown by Mathew et al [1] that what Mackay had found was only a third of the actual unit cell with b = b/3, and that the full structure has twelve crystallographic independent phosphorous sites. Therefore it is expected that only a maximum of twelve resonances should be observed and reducing the unit cell to that of the incomplete one found by Mackay would not change this, so undoubtedly some other factor has to explain the extra three peaks, such as impurities or some kind of error.