The Spectra of Dow704 Diffusion Pump Oil

For a wide range of research and production applications, Dow704 Diffusion Pump Oil (Tetramethyl tetraphenyl trisiloxane) is used in large and medium diffusion pumps since a lower attainable pressure rationalizes the added expense of this excellent fluid.

The highlights of the Dow704 Diffusion Pump Oil are:

• Increased resistance to hydrolysis and oxidation at operating temperature
• A suitable fluid for aggressive applications that require ultimate pressures (without trapping) in the 10^-7 Torr range
• Chamber pressures can approach 10^-11 Torr range with first class LN₂ traps
• Applications include diffusion bonding, vacuum metal casting and instruments

Anasazi Instrument engineers use this sample frequently for demonstrating the use of multinuclear experiments on the EFT spectrometer. This sample offers the ability to run single and two-dimensional experiments on ¹H, ¹³C, and ²⁹Si.

Figure 1 shows the 60MHz proton spectrum for Dow704 diffusion pump oil below. All of the resonances are well resolved. Each of the two methyl resonances have six identical protons. The assignments are H_8,9:0.60_ppm, and H_6,7:0.15_ppm.

Figure 1. Proton spectrum Dow704 pump oil

It is observed that there is a significant amount of symmetry in the tetramethyl-tetraphenyl-trisiloxane molecule. In Figure 2, each group’s numbering is duplicated to make assignments less complex.

The ¹³C spectrum as shown in Figure 3 was obtained in 1min. All the peaks are resolved clearly. While evidence is not completely clear, these assignments can be made: C₁:137.4ppm, C₂:133.8ppm, C₄:129.4ppm, C₃:127.6ppm, C₆:1.29ppm, C₅:0.72ppm.

Figure 2. Tetramethyl tetraphenyl trisiloxane molecule

Figure 3. C13 spectra of Dow704 Diffusion pump oil

This can be confirmed while observing the 2D 29Si X-HETCOR spectrum. Unlike in most organic molecules where a substituted ring carbon has a lower amplitude than the other peaks on the ring, the carbon at position C₁ has a similar amplitude when compared to the other carbons on the ring.

Heteronuclear Correlation (HETCOR)

The COSY experiment and the HETCOR experiment are similar, the only difference is that it concerns coupling between two different nuclei types. This technique is normally used for correlation of proton resonances with those of directly bonded carbon-13 (or other) nuclei.

The contour plot axes represent the proton and other X-nucleus chemical shift ranges and signals occur at coordinates corresponding to the shifts of the bonded pairs of nuclei. In this experiment, X-nucleus signals are detected instead of those of the protons. Modifying the delays in HETCOR can provide long range coupling information (couplings between proton and X-nucleus through 2-3 bonds).

The HETCOR spectrum for Dow704 is shown below (Figure 4). The results of the HETCOR strengthen the assignments made in the C13 spectrum. The peak at 137.4ppm indicates that there are no directly attached protons associated with that resonance.

Figure 4. C13-H1 Heteronuclear Correlation (HETCOR) of Dow704 diffusion pump oil in CDCI3

X-Nucleus Polarization Transfer (XPT)

Multinuclear spectra are obtained by modern NMR spectrometers in a routine manner. However it is tougher to obtain ¹³C-NMR spectrum than a ¹H-NMR spectrum for two main reasons.

Firstly the natural abundance of a number of NMR active isotopes is low, so there are fewer NMR-active nuclei per mole of compound to absorb energy. Secondly, the inherent signal intensity per nucleus is generally very low.

Thus NMR spectroscopists are seeking methods to increase the signal intensity of carbon and other X-nuclei. All of the methods they have developed involve a phenomenon known as polarization transfer whereby magnetization is transferred via spin-spin couplings.

In Figure 5, the ²⁹Si 1D acquisition (bottom) and the ²⁹Si XPT acquisition (top) show the advantages of the X-Nucleus Polarization Transfer experiment. The two silicon assignments in the sample are done based on peak height with the interior ²⁹Si at -18ppm and the two exterior ²⁹Si at -12ppm. Both the experiments were done uninterrupted on the same sample with the same parameters. The S/N ratio for both the spectra are 29.6 and 74.0, respectively.

Figure 5. Si29 1D and XPT spectra of Dow704 diffusion pump oil in CDCI3

²⁹Si X-Nucleus Heteronuclear Correlation (X-HETCOR)

The Heteronuclear Correlation (HETCOR) experiment is similar to the COSY experiment with the difference that it concerns coupling between two different types of nuclei. This method is generally used to correlate proton resonances with those of directly bonded carbon-13 (or other) nuclei.

The axes of the contour plot represent the proton and other X-nucleus chemical shift ranges and signals occur at coordinates corresponding to the shifts of the bonded pairs of nuclei. In this experiment, X-nucleus signals are detected instead of those of the protons.

The ¹H-²⁹Si X-HETCOR spectrum for Dow704 is shown in Figure 6. The results of the X-HETCOR strengthen the assignments made in the proton and C13 spectra. The ²⁹Si peak at -12ppm shows coupling to the phenyl resonance as well as the proton peak at 0.60ppm. The ²⁹Si peak at -18ppm is shows coupling to the methyl resonance at 0.15ppm.

Figure 6. Si29-H1 X-Nucleus Heteronuclear Correlation (X-HETCOR) of Dow704 pump oil

About Anasazi Instruments

Anasazi Instruments has been providing high quality, rugged, easy-to-use 60 and 90 MHz NMR spectrometers and upgrades to the educational and industrial markets. These instruments have been successfully implemented at hundreds or institutions ranging from large companies and top-tier universities to community colleges throughout North and South America.

In research environments, the Eft is a cost-effective workhorse for synthetic and analytical laboratories. These permanent magnet based FT-NMR spectrometers have applications in industrial labs for quality testing or as a "walk-up" NMR resource. Crucial to the success of the Eft is that, over the lifetime of the instrument, the total annual cost is fixed, whereas for a supercon-based NMR, annual costs increase.

In education, the Eft gives thousands of undergraduates the hands-on opportunity to learn to acquire and analyze FT-NMR data. Additionally, the wide appeal of the Eft spectrometer is due to the ease of obtaining high quality NMR spectra on an instrument that does not required cryogens and has minimal maintenance requirements.

This information has been sourced, reviewed and adapted from materials provided by Anasazi Instruments.

For more information on this source, please visit Anasazi Instruments.