Ionic Liquids: Designer Solvents

Ionic liquids are room temperature liquids, or low melting point solids, composed solely of ions. They form an exciting new class of materials with interesting properties and a diverse range of applications. Given the high melting points of typical ionic salts (i.e. NaCl has a melting point of ~ 801°C), it is remarkable that we can obtain liquid salts at room temperature. However, this can be readily achieved through careful selection and modulation of the constituent ions. Typically, ionic liquids are composed of bulky and charge delocalised, asymmetric organic cations and inorganic anions.

In theory, the cation and anion combination can be carefully engineered to give an ionic liquid with the desired properties for a specified purpose. This has resulted in ionic liquids being termed "designer solvents". Given the huge range of possible combinations of ions it is impossible to experimentally synthesise and determine the physical and chemical properties of them all. Increasingly, computational chemistry has become an important tool in helping us to understand ionic liquids at the molecular level. Significantly, we may soon begin to be able to use computational chemistry to predict the properties of ionic liquids before they are made.

In this mini-project you will investigate a two related ionic-liquid cations.

• Optimise, and carry out a frequency analysis to confirm you have minima for the cations [N(CH₃)₄]⁺ and [P(CH₃)₄]⁺. Don't forget to apply the correct charge!
• Hint: start with a molecule of methane and substitute the H's and replace the central C with N or P
• Note that you may get formally zero frequencies of ± 20-30 cm^-1. This occurs because the basis set we are using is not quite good enough and there are some very soft vibrational modes. In this case having negative modes less negative than -30 cm^-1 is sufficient for our purposes.
• compute the MOs and carry out an NBO charge analysis (be sure you have NBO charges, not Mulliken!) for both structures. When you selected NBO (under "frequency analysis") you turned on the NBO charge analysis. Under "Results" select charge analysis and then under Type choose NBO. Play around with the options. You can then select colour and/or show numbers. Make sure you use the same colour range for both molecules when you save an image. Use the largest range to visualise both systems (think about why we need to do this)
• Compare the charge distribution for these cations, placing images side by side is not sufficient, list and discuss the charges. The key words here are "compare" and "discuss" just presenting the data is not sufficient, you must interpret your results.
• [NR₄]⁺ (R=alkyl) is often depicted as shown, with the positive charge placed on the nitrogen centre. Based on your results for [N(CH₃)₄]⁺, discuss the validity of this traditional description. You should consider the following:
  • What does the "formal" positive charge on the N represent in the traditional picture?
  • On what atoms is the positive charge actually located for this cation?
• Visualise all the occupied valence MOs and the lowest 5 unoccupied MOs of [N(CH₃)₄]⁺, you do not need to reproduce them all. If you don't know what the valence MOs are ask!
• in your wiki present 3 valence MOs from those you have visualised. Cover a range of MOs showing bonding and antibonding character, pick only occupied MOs.
• Draw a LCAO MO diagram for each of your 3 MOs (using chemdraw). Identify the fragment orbitals of the methyl groups and use this information to simplify your representation of the overall MO as shown below:
  
• optional Comment on or analyse or describe some aspect of this system which you found interesting.