Platinum-195 nuclear magnetic resonance spectroscopy (platinum NMR or
195Pt NMR) is a
spectroscopic technique which is used for the detection and characterisation of
platinum compounds
Platinum is a chemical element with the symbol Pt and atomic number 78. It is a dense, malleable, ductile, highly unreactive, precious, silverish-white transition metal. Its name originates from Spanish , a diminutive of "silver".
Platinu ...
. The sensitivity of the technique and therefore its diagnostic utility have increased significantly starting from the 1970s, with
195Pt NMR nowadays considered the method of choice for structural elucidation of Pt species in solution.
Examples of compounds routinely characterised with the method include platinum
clusters and
organoplatinum Organoplatinum chemistry is the chemistry of organometallic compounds containing a carbon to platinum chemical bond, and the study of platinum as a catalyst in organic reactions. Organoplatinum compounds exist in oxidation state 0 to IV, with oxida ...
species such as Pt
II-based
antitumour agents.
Additional applications of
195Pt NMR include
kinetic
Kinetic (Ancient Greek: κίνησις “kinesis”, movement or to move) may refer to:
* Kinetic theory of gases, Kinetic theory, describing a gas as particles in random motion
* Kinetic energy, the energy of an object that it possesses due to i ...
and mechanistic studies or investigations on drug
binding.
195Pt magnetic properties
Among the naturally occurring
isotopes of platinum
Naturally occurring platinum (78Pt) consists of five stable isotopes (192Pt, 194Pt, 195Pt, 196Pt, 198Pt) and one very long-lived (half-life 6.50×1011 years) radioisotope (190Pt). There are also 34 known synthetic radioisotopes, the longest-lived ...
,
195Pt is the most abundant (33.8%) and the only one with non-zero
spin
Spin or spinning most often refers to:
* Spinning (textiles), the creation of yarn or thread by twisting fibers together, traditionally by hand spinning
* Spin, the rotation of an object around a central axis
* Spin (propaganda), an intentionally b ...
''I''=1/2.
The
magnetic properties
Magnetism is the class of physical attributes that are mediated by a magnetic field, which refers to the capacity to induce attractive and repulsive phenomena in other entities. Electric currents and the magnetic moments of elementary particles ...
of the nucleus are considered favourable; the high natural abundance coupled with a medium
gyromagnetic ratio
In physics, the gyromagnetic ratio (also sometimes known as the magnetogyric ratio in other disciplines) of a particle or system is the ratio of its magnetic moment to its angular momentum, and it is often denoted by the symbol , gamma. Its SI u ...
(5.768×10
7 rad T
−1 s
−1) result in good
195Pt NMR signal receptivity, 19 times that of
13C (but still only 0.0034 times that of
1H).
The resonance frequency (relative to a 100 MHz
1H NMR instrument) is approximately 21.4 MHz, close to the
13C resonance at 25.1 MHz.
Chemical shifts
The
chemical shifts of
195Pt nuclei span a very large range of over 13000
ppm (''cf''. with ~300 ppm range for
13C).
The NMR signals are also very sharp and highly sensitive to the platinum chemical environment (
oxidation state
In chemistry, the oxidation state, or oxidation number, is the hypothetical charge of an atom if all of its bonds to different atoms were fully ionic. It describes the degree of oxidation (loss of electrons) of an atom in a chemical compound. C ...
,
ligand
In coordination chemistry, a ligand is an ion or molecule (functional group) that binds to a central metal atom to form a coordination complex. The bonding with the metal generally involves formal donation of one or more of the ligand's electr ...
identity and
field strength In physics, field strength means the ''magnitude'' of a vector-valued field (e.g., in volts per meter, V/m, for an electric field ''E'').
For example, an electromagnetic field results in both electric field strength and magnetic field strength.
As ...
,
coordination number, etc.).
Therefore, substituting even very similar ligands can result in shift changes in the order of hundreds of ppm which stand out on the spectrum and are easily monitored.
The
reference compound typically chosen for
195Pt NMR experiments is 1.2 M
sodium hexachloroplatinate(IV) (Na
2PtCl
6) in
D2O; this platinum(IV) complex is preferred due to its commercial availability, chemical stability, lower price relative to other platinum compounds, and high
solubility
In chemistry, solubility is the ability of a substance, the solute, to form a solution with another substance, the solvent. Insolubility is the opposite property, the inability of the solute to form such a solution.
The extent of the solubil ...
which enables spectrum recording within minutes.
Less soluble ionic platinum complexes have spectrum recording times of about an hour, whereas the borderline insoluble neutral complexes may require overnight measurements.
The high sensitivity of the experiment means that contributions from different
chlorine isotopes in the reference compound or other species can be resolved at high magnetic field strengths, giving a ±5 ppm uncertainty in reported shift values (which is, however, negligible in view of the 13000 ppm overall range).
Couplings
Coupling of
195Pt to
1H,
13C,
31P,
19F or
15N has been reported through one up to four bonds (
1''J'' to
4''J'') and is commonly studied to provide additional structural information for platinum complexes.
The ~34% abundance of
195Pt (with the remaining 66% of natural Pt being NMR-inactive) means that this coupling appears in the respective
1H/
31P/
15N/
13C NMR spectra as satellite peaks (''cf.''
13C satellites) which, for example, result in 17:66:17 patterns for singlets.
The
trans influence In inorganic chemistry, the trans effect is the increased lability of ligands that are trans to certain other ligands, which can thus be regarded as trans-directing ligands. It is attributed to electronic effects and it is most notable in square pl ...
in 16 e
− square planar Pt
II complexes has been studied by comparing the magnitude of coupling constants in the ''cis''- and ''trans''- isomers.
Complicated homonuclear couplings ranging from 60 to 9000 Hz for
1''J''(
195Pt–
195Pt) are of interest in the context of platinum cluster compounds.
References
{{NMR by isotope
Nuclear magnetic resonance
Platinum