Solvent Contribution to Vibrational Dynamics in Terminal Acetylenes |
| There are two contributing processes to the total vibrational energy relaxation of solvated molecules. IVR (intramolecular vibrational-energy redistribution) is a process that occurs within the solute molecule even in the absence of solvent. The rate of this process is molecule-dependent. Interactions of the solute with the solvent lead to VER (vibrational energy relaxation), whereby energy is removed from the solute and the remaining energy can be found in lower energy vibrational states. The rate of this process is solvent-dependent. These two relaxation mechanisms BOTH occur in solution, giving a total relaxation rate that depends on both the intramolecular redistribution (IVR) rate and the intermolecular relaxation (VER) rate. For more detailed explanation of these processes, click here. |
kIVR |
kVER |
kTOT |
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Intramolecular Dynamics |
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Intermolecular Dynamics
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Total Vibrational Dynamics |
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Measurements on isolated molecules in the gas phase give the IVR rate in terms of the lifetime (t) of the first excited state of the acetylenic CH stretch (see left). In dilute CCl4 solution, the lifetime we measure is related to the total vibrational energy relaxation rate, which is the combination of the IVR rate and the VER rate (see right- rollover to overlay gas and solution data). For experimental details, click here. |
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| The lifetime measured for gas-phase propargyl fluoride is 89ps and when this molecule is solvated, the lifetime measured is 35ps. From this data, we can see that solvent plays a fairly significant role in the total vibrational energy relaxation measured for propargyl fluoride in solution. Now let's compare this to another molecule- methylbutynol... |
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On the left is a plot of the gas (red) and solution (black) data taken for propargyl fluoride. The IVR rate is fairly slow and the solution contribution is obvious. Methylbutynol (shown at right: gas in red, solution in black) is structurally quite similar, but its IVR rate is much faster. It is clear from the fact that the gas and solution data exactly overlap that there is no change in the relaxation rate measured when this molecule is solvated. The IVR rate for methylbutynol is so fast that solvent effects can not contribute to the vibrational energy relaxation and even in solution, the intramolecular dynamics are in full control. |
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| Without the gas phase measurements, the relaxation rate measured in solution would be attributed to solvent effects. However, the gas phase data of methylbutynol clearly tells us that the only relaxation mechanism contributing to the rate we measure is the intramolecular (IVR) process!!! In terms of our simple model of vibrational dynamics in solution, kTOT = kIVR + kVER, it is the IVR rate difference between these molecules that changes the extent to which the VER rate is observed. |
We have seen that a change in the IVR rate of a molecule will affect the extent to which solvent can contribute to vibrational energy relaxation. By using different solvents, we can examine the effects of changing the VER rate. Shown to the right is a plot of propargyl chloride data taken in five solvents. The intramolecular rate is constant, since that is dependent on the molecule alone, so the difference in measured total vibrational energy relaxation is due to the different VER rates of the solvents. |
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To gain a better understanding of vibrational dynamics in a more general sense, we need to look at the interplay between IVR and VER in a series of molecules (wide range of IVR values) and for many solvents (wide range of VER values). We study a series of 10 terminal acetylenes in 10 different solvents and compare the initial relaxation rates in gas and dilute solution. These rate comparisons can be seen in the 10 small plots below. |
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