Question

Chemists tried for a long time to make molecular compounds containing silicon- silicon double bonds, they finally succeeded in 1981 . The trick is having large, bulky R groups on the silicon atoms to make \(\mathrm{R}_{2} \mathrm{Si}=\mathrm{SiR}_{2}\) compounds. What experiments could you do to prove that a new compound has a silicon-silicon double bond rather than a silicon-silicon single bond?

Short answer

To prove that a new compound has a silicon-silicon double bond rather than a single bond, a combination of X-Ray crystallography, infrared (IR) spectroscopy, and nuclear magnetic resonance (NMR) spectroscopy can be used. X-Ray crystallography can show differences in bond lengths, with double bonds being shorter than single bonds. IR spectroscopy can reveal unique absorption frequencies associated with double bonds compared to single bonds. NMR spectroscopy can provide information about the bonding environment of the silicon atoms, and signals consistent with a double bond support the presence of a silicon-silicon double bond in the compound.

Step-by-step solution

1. X-Ray Crystallography

: X-Ray crystallography is a technique used to determine the atomic and molecular structure of a crystal. This experiment can help prove that the new compound has a silicon-silicon double bond by showing the differences in bond lengths between single and double bonds. The bond length for a silicon-silicon double bond is shorter than that of a single bond. If the new compound has a shorter bond length than the one typically observed for a silicon-silicon single bond, it provides evidence that the compound has a silicon-silicon double bond.

2. Infrared Spectroscopy

: Infrared (IR) spectroscopy is an experimental technique that measures the vibrational frequencies of molecules, which corresponds to the stretching and bending of their bonds. Different types of bonds have characteristic IR absorption frequencies. The presence of a silicon-silicon double bond would exhibit a unique IR absorption frequency compared to a silicon-silicon single bond. Analyzing the IR spectrum of the new compound, we can verify the presence of a silicon-silicon double bond by comparing the observed absorption frequencies with known values for single and double silicon-silicon bonds.

3. Nuclear Magnetic Resonance (NMR) Spectroscopy

: NMR spectroscopy is a technique that provides information about the chemical environment and connectivity of atoms in a molecule. The NMR spectrum of a compound with a silicon-silicon double bond would differ from that of a compound with a silicon-silicon single bond. By analyzing the chemical shifts and coupling patterns of silicon NMR signals, we can infer the bonding environment around the silicon atoms. If the NMR signals in the new compound are consistent with the presence of a silicon-silicon double bond, this supports the idea that the new compound has a double bond rather than a single bond. In conclusion, a combination of X-Ray crystallography, IR spectroscopy, and NMR spectroscopy experiments can provide evidence for the presence of a silicon-silicon double bond in the new compound. Comparing results from these experiments to known data for single and double silicon-silicon bonds can confirm the bond type in the synthesized compound.

Key concepts

X-Ray Crystallography

X-Ray Crystallography is a powerful technique utilized to unveil the atomic and molecular makeup of crystals. Chemists highly value it, especially when verifying the presence of silicon-silicon bonds. To detect a silicon-silicon double bond, scientists often rely on comparing bond lengths. Generally, a double bond is shorter than a single bond.
To conduct this experiment, a crystal of the compound is exposed to X-rays. The X-rays are diffracted by the crystals, creating a pattern that can be captured on film or an electronic detector. This pattern is then analyzed to reveal the structure of the crystal, including the lengths of the bonds between atoms.
The crucial clue here is the bond length. If the measured silicon-silicon bond length is shorter than the typical length of a silicon-silicon single bond, this indicates the presence of a double bond. This method offers a visual map of the atomic structure, providing clear evidence to support or refute the existence of a silicon-silicon double bond.

Infrared Spectroscopy

Infrared Spectroscopy (IR) is a key analytical technique used to investigate molecular vibrations, identifying unique absorption bands corresponding to different types of chemical bonds. When looking for a silicon-silicon double bond, this technique proves invaluable because each bond type in a molecule has a characteristic absorption frequency in the IR spectrum.
During an IR experiment, an IR beam passes through the compound, allowing the molecule's bonds to absorb energy at specific frequencies, causing them to vibrate. By recording this absorption, spectra are produced, reflecting distinct peaks based on the bond types present.

• The stretching frequency of a silicon-silicon double bond differs from that of a single bond.
• Analyzing these frequencies helps identify the specific type of bond present.

Comparing the experimental data to standard values for absorption of silicon-silicon bonds enables chemists to confirm the presence of a double bond. A molecule containing such a bond will exhibit unique absorption peaks that are distinguishable from those of single bonds.

Nuclear Magnetic Resonance Spectroscopy

Nuclear Magnetic Resonance (NMR) Spectroscopy is another valuable tool for exploring the structural details of molecules. This technique is particularly useful for understanding the chemical environment around atoms, such as the silicon atoms in compounds suspected of containing silicon-silicon double bonds.
NMR works by detecting magnetic properties of certain atomic nuclei. When placed in a magnetic field, nuclei such as those in silicon atoms can absorb radiofrequency radiation, resulting in an NMR spectrum. This spectrum contains peaks that correspond to the chemical shifts of different atoms, revealing their environment and connectivity.
The presence of a silicon-silicon double bond affects these shifts and the coupling patterns in the spectrum.

• Evaluation of silicon NMR signals helps uncover the bonding scenario.
• Consistent chemical shifts and coupling patterns associated with double bonds offer strong evidence to suggest their existence.

In essence, by studying the NMR spectrum, chemists can visualize the silicon connectivity and confirm whether a silicon-silicon bond is indeed a double bond.