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Inelastic neutron scattering spectroscopy (INS) and ab initio methods make great partners
in studying the vibrational properties of molecules. Once a geometry optimization (structure
calculation) and normal mode analysis (calculation of vibrations and their frequencies) has
been completed, the corresponding INS spectrum can be easily simulated. By fitted calculated
peaks to experimental data, ab initio methods can tell us much about the structural and
vibrational properties of the system of study. Furthermore, by analyzing discrepancies
between the calculation and experiment, we can appraise the effectiveness of ab intio
methods and the approximations therein. Through this process, we can develop new techniques
for applying ab initio methods and begin to predict when these new technique will be necessary.
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Dodecahedrane
Dodecahedrane (C20H20) is highly symmetric, providing a problem for optical
spectroscopy.
Due to optical selection rules, only three of thirty unique vibrations are
observable with infrared spectroscopy, and one eight more are seen in Raman
spectroscopy. Since no selection rules exist for INS spectroscopy, all
thirty vibrations are seen, see Figure 2. INS spectroscopy provides us with
an increased number of peaks with which to fit the ab initio calculations to.
When an excellent fit (like that seen in Figure 2) is obtained, our confidence
in vibrational assignments is likewise increased.
[2.2]Paracyclophane
Highly strained structures like that of [2.2]paracyclophane, see Figure 3a, can be
an excellent test of ab initio methods. Paracyclophane undergoes a twisting vibration
between the two energy minima seen in Figure 3b. Due to molecular strain this vibration
is thought to be highly anharmonic, providing a test of ab initio methods. With the combination
of INS spectroscopy, and ab initio methods, confident assignment of the vibration spectrum can be
obtained.
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Figure 1: Dodecahedrane shown with
hydrogens, with transparent hydrogens, and without hydrogens.
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Figure 2: INS spectrum of dodecahedrane, shown in blue.
IR and Raman active vibrations are marked with green lines, and symmetry assignments in gray.
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![[2.2]Paracyclophane, C16H16](images/paracyclophane6.gif)
Figure 3: a) Highly strained [2.2]paracyclophane. b)
Two energy minima.
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