Raman Spectroscopy of Liquids

Transcription

1 Chemistry 357 Spring 2013 Raman Spectroscopy of Liquids Lab TA: Paul Dent

2 PURPOSE: You will investigate Raman light scattering of several different molecular liquids. You will also determine the vibrational frequencies and depolarization ratios of the Raman active vibrational modes of these materials. And finally, you will identify all the Raman active vibrational modes for each liquid and compare the experimental results with the spectra obtained with a Gaussian calculation. BACKGROUND: Please read the Theory for "Experiment 35: Raman Spectroscopy: Vibrational Spectrum of CCl4" found in your Shoemaker's Experiments in Physical Chemistry. It is important that you understand the information given to you in this section, as it will aid in understanding of Raman spectroscopy, as well as in writing of the lab report. Also review Sections 19.8 of your Engel & Reid, Physical Chemistry book. DURING THE LAB: You will acquire Raman spectra for 7 liquids using a Raman spectrometer, which are: CH3CN & CD3CN C6H6 & C6D6 CCl3H & CCl3D CCl4 You will also do Gaussian calculations for all of the cases measured (use Opt & Freq method with Raman turned on; the Method DFT b3lyp 6-31G(2d,2p) and 4 processors). Put all of the experimental spectra into an Excel file while the Gaussian calculations are running. It might be best for one student to run three spectra and then begin calculations for these. The next student does the same and then the third. The sequence can be as follow: 3 students: (a) CH3CN & CD3CN plus CCl4; (b) C6H6 & C6D6 plus CCl4; (c) CCl3H & CCl3D plus CCl4 2 students: (i) CH3CN, CD3CN, CCl4 & CCl3H (ii) C6H6, C6D6, CCl3H & CCl3D The first student can then measure the other samples and so on as time permits. You will each perform the Gaussian calculations and collect the data on all 7 samples. The use of the spectrometer will be demonstrated to you before you start the experiment. During your experiment, always make sure that the laser is on. The red beam should be visible from the reflection of the meniscus. The simplified instructions for using the spectrometer are as follow: 1/ Turn the spectrometer on (back right of the box). 2/ Open up the DeltaNu software. It will open to display a stock spectrum. The red box on the top right hand corner will turn blue once the laser is ready. Choose high resolution. 3/ Under Acquisition (or using one of the tabs on the left side of the window), click on Continuous. Run it for at least five minutes. This will stabilize the temperature of the laser. In

3 the top left hand corner, a green box should appear. This indicates that a spectrum is being acquired. 4/ Once the laser is stabilized, insert an empty cuvette, and press Reference (either using Acquisition drop down menu, or one of the tabs on the left side of the window). You have to do this only once; this will subtract the spectrum of an empty cuvette from the samples you will measure next. 5/ Insert your first sample. Press Acquire. This will collect the normal Raman spectrum. 6/ Once the spectrum is acquired, under Save, pick save as ASCII. This format will open in Excel (although you might have to Open As, and then pick Excel as your software of choice). 7/ Next thing is to acquire a polarized spectrum for the sample. To do this, click on Polarization (either from Acquisition drop down menu, or from one of the tabs on the left side of the window). 8/ Save this spectrum as well. 9/ To acquire spectra for other samples, repeat steps 5 through 8. Precautions: As tempting as it might be, do not look directly at the laser; it can cause serious damage to the eyes. Also, handle the cuvettes containing samples with caution; all of the liquids are toxic and some are carcinogenic. SUPPLEMENTARY CALCULATIONS FOR THE LAB: You should run Gaussian calculations for all the liquids you have analyzed. Once you build the necessary molecule in GaussView, run calculations using Opt+Freq job type. Make sure you select YES under Compute Raman (besides vibrational frequencies, this option will give you Raman intensities and depolarization ratios for each mode of vibration). DFT b3lyp 63-1G(2d,2p) is fine with Raman turned on as noted above. The molecules should each be symmetrized. The spectra can be saved and plotted on top of the experimental spectra. You may have to adjust the spectral linewidth to make this comparison more convincing. In order to account for the mass of D in the deuterated compounds, you should open the.out (or.log) file for the run of the H compound. This should now be set for job type frequency. Change the title to indicate that you are now dealing with the deuterated form. Select edit, save the file with the new name and in the atom list put (Iso=2) after each H atom, i.e., H(Iso=2). HANDLING OF THE RESULTS: Plot the data acquired in the lab as a Scatter plot (scatter with smooth lines is the preferred type). In the experiment you measure a Raman spectrum and a depolarization ratio data set. You plot only the measured Raman spectrum. On top of that, you plot the computed Raman activity spectrum that has been scaled to the maximum peak from the experiment. To get the computed spectrum, you need to open the.out (or.log) file in GaussView, click on results and then vibrations. The listed vibration data is first (frequency, IR intensity, Raman intensity, etc.) Click on Spectrum at the bottom and scan down to the Raman spectrum, right click on the spectrum and save as ASCII or CSV. Open in Excel and plot.

4 If you look in the computed vibrational data table, Raman activity is listed for each vibration and also the depolarization is given. What you will see is that this ratio is 3/4 for some vibrations but smaller values (less depolarized) for the others. You will compare that with the experimental depolarized spectra. What you will find is that all of the calculated vibrations with a depolarization of 3/4 are, in the experiment, plotted in a positive sense. All the others are plotted downward in a negative sense with an amplitude that depends on the extent of depolarization. As in this case, what the red depolarization plot means is that the large feature near 460 cm -1 is polarized (as is possible only for totally symmetric modes) and the pair of features near 780 cm -1 are depolarized (depolarization ratio = 3/4) as is necessarily the case for non-totally symmetric modes. Your write-up should present your results in four groups (presented in any order) CCl4, [C6H6, C6D6], [CH3CN, CD3CN] and [CCl2H2, CCl2D2]. For each group you should have one spectral plot with up to two spectra. You may include the depolarization data. For each molecule you should present a table showing your measured and computed values side by side, include all observed lines and all computed lines that are predicted to have nonzero Raman intensity. Note any observed features that are seen but are not predicted and those that are predicted but not seen. Explain how this might be possible and justify your statement. In this table include an entry that indicates whether the computed transition is expected to be polarized or depolarized (depolarization ratio = 3/4) and for each observed transition give the same information based on the depolarization spectrum. In the two H/D pairs compare the results for the two species. If there are unexplained anomalies (extra bands, missing bands, incorrect depolarization, etc.) is there a parallel discrepancy in both isotopomers? LAB REPORT: The report should be written in ACS format. That is, all equations, figures, and tables should be properly labeled and titled. The structure and format should be similar to the ones you did last semester, which includes Cover Page, Tittle, Abstract, Introduction, Experimental Methods, Results, Calculations, Discussion and References. Abstract should state what you have done (also, show briefly calculated results) and why it is important.

5 The Introduction and Experimental Methods should be written according to the related experiments in the textbook Experiments in Physical Chemistry. In the introduction, please discuss the theory behind, the methodology, and the importance of the experiment. Include any important equations, all of which should be properly labeled. In the experimental section, do not copy word for word from the handout, but rather summarize the procedure in your own words. The results section should be a narrative with your raw data incorporated into it. Include all relevant tables and plots. All of them should be properly labeled and titled. (In this case, the spectra you got from experiment and Gaussian, and the tables of vibrational modes. A sample of how to scale the plots is available on blackboard.) benzene experiment For the discussion section please comment on the technique and the success of the experiment. Compare your experimental results with Gaussian and literature results. (The comparison is more effective with a percent deviation calculated.) Comment on any missing or extra peaks (what is their frequency and what might be the cause of their appearance or their lack, etc.). Comment how the spectra of the deuterated liquids differ from their respective regular forms (shifts, if any, in the frequencies of the peaks; number of peaks, etc.). Extra Credit: Make a comparison between the observed and the computed depolarization ratios. Scale the computed frequency scale to see if a uniform factor accounts for the anharmonicity.