Vibrational Dynamics (IVR and VER) of Terminal Acetylenes |
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The terminal acetylene structure is shown above, where R is an organic substituent of varying size. One of the reasons that terminal acetylenes are particularly well-suited to the study of vibrational dynamics is that across the wide range of substituent sizes and structures studied, the vibrational frequencies of the acetylene modes remain constant. This is illustrated in the graph to the right. Here the vibrational frequency of four modes of interest are shown as a function of R group structure. In all modes shown, "CH" corresponds to the acetylenic CH circled in the above diagram. Another advantage of terminal acetylenes is that the chromophore is physically isolated from the rest of the molecule. In solution-phase measurements, this isolation leads to a constant local solvent environment. These two things provide the possibility that the contribution of the solvent to the measured dynamics will be constant across the entire range of molecules studied. A third advantage of studying these terminal acetylenes is that their relatively high vapor pressures allow them to be studied easily in both the gas and solution phase, which makes examination of solvent effect straightforward.
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| The mode of interest in these studies is the acetylenic C-H stretch, which occurs at about 3330cm-1 in gas phase samples, and about 3310cm-1 in solution phase samples. Shown to the left is a level diagram describing a typical 2 color experiment. We use the first pulse at the frequency corresponding to the v=0 – v=1 vibrational transition of interest, which in our case is ~3330cm-1. The second pulse is used as a probe and monitors the v=1 – v=2 vibrational transition, which in this case is ~100cm-1 away at ~3230cm-1. The major benefit of doing this 2 color experiment, as opposed to a 1 color experiment, where you’d both pump and probe at the 0-1 transition, is that in this scheme, we’re able to uniquely probe the population of this excited state. So the population decay that we get is the pure population decay out of the excited state, free from additional elements that would be in the 1 color experiment, such as stimulated emission. To see a diagram of the experimental setup click here.
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Vibrational-Energy Relaxation Measurements in Dilute Solution Population lifetimes of the acetylenic C-H stretch in gas phase and 0.05M carbon tetrachloride solution at room temperature are measured using two-color picosecond transient absorption spectroscopy. The population of the first excited state of the acetylenic C-H stretch normal mode is directly monitored through the absorption of the anharmonically shifted v=1 - v=2 transition. The picosecond transient excited-state absorption spectra can be fit to a single exponential decay expression. |
Intramolecular Vibrational-Energy Redistribution Measurements of Room-Temperature Gas-Phase Molecules IVR rates of gas-phase molecules are also determined by directly monitoring the excited-state population through the transient absorption signal of the anharmonically shifted v=1-2 vibrational transition absorption. However, the interpretation of gas-phase transient-absorption spectra is more complicated. The excited-state population signals for most of the large terminal acetylenes (vibrational state-densities above the IVR threshold of 10 states/cm-1) decay on two time scales. Our analysis indicates that the initial redistribution process involves population transfer from the first excited state of the acetylenic C-H stretch to vibrational states that include 2 quanta in the acetylenic C-H bend modes. The second, slower process corresponds to full population relaxation to essentially the full local vibrational-state density. The initial redistribution rate in room-temperature gases correlate with the single relaxation time scale observed in solution.
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Influence of IVR and VER on Vibrational Relaxation |
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To gain insight into the interplay between the IVR dynamics of isolated molecules and the VER dynamics caused by the solvent, we compare the initial relaxation rates in gas and dilute CCl4 solution in the figure to the right. Note that each data circle on the plot represents a single terminal acetylene molecule's relaxation rates.This analysis shows a linear relationship between the total relaxation rate in solution and the intial redistribution rate of isolated molecules. This correlation is consistent with the simple model for population relaxation:
kTOT = kIVR + kVER , in which the IVR and VER processes act independently to give the total solution relaxation rate. A linear regression analysis of the data (shown in red) points out two key features of the dynamics. The slope determination of 1.02 indicates that the solvent has no effect on the initial IVR rate. The fact that all measurements fall on a single line indicates that single VER rate decribes the solvent contribution to the total relaxation rate in dilute solution for all terminal acetylenes. |
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