Computing the IR and RAMAN Spectra of Ethylene



in this video I will compute the infrared and Raman spectra of the ethylene molecule using gas view and Gaussian double-click gauss field and then click this R this is organic functional groups the default is a formula functional group but we're going to use a different functional group look for ch2 was a double bond here it's gonna make a three molecule for you you can make one three butadiene easily by clicking one of the hydrogens but I'm gonna do a ctrl Z to undo now click calculate set up to do a vibrational frequency calculation you have to optimize the structure first so we're gonna do opt + F req and then it's gonna compute the vibrational frequencies and also infrared spectrum of the molecule to also compute the Raman spectrum you need additional key word here so fre Q equals Raman and then it's gonna compute Rama as well how about the method we're gonna use a more accurate DFT method then watch you we're gonna choose b3o IP the basis that we're going to use our larger basis and 6:31 Gd this means D type of polarization functions on carbon and we click Submit and save and then we're gonna save this I ran this calculation before so I'm gonna overwrite the previous input file f-fine GJ f GJ f stands for gaussian job file and i'm gonna click Save overwrite it yes submit the calculation yes the log file this is one of the output files already exists overwrite it yes I'm running optimization right now after the optimization this is running frequency Gaussian job has completed do you want to close the Gaussian window yes now do you want to open one of this two files check file is a binary file log file is a text file both can be opened by gaussview the log file can also be visualized in notepad or WordPad so I'm gonna just click OK here to get the check file now you have the optimized structure here this is the input this is output and let's look at the bond distance here one point three to five one point three is three zero so you need the bond distance is optimized all right check file and then let's look at the results we can look at the summary charts diffusion this one will give you the intensity and this one give you the vibrations let's look at the summary first it's gonna tell you the method the basis at the energy the energy gradient the type of moment dipole moment is zero due to the symmetry of the molecule with a d2h point group and then let's click results in vibrations click OK here and you will see all those numbers in this video there are six atoms therefore there are eight emotions 18 minus three translation minus three rotation you have 12 hours you know modes and this are not really frequencies those are wave numbers in the unit of reciprocal centimeters and you have this relative infrared activity here relative Rama activity here and if you see a non zero infrared activity the Romans their activity must be zero and vice versa if you see this non zero rom activities here the infrared activity must be zero there's a reason for molecule with a version center such as ethylene the infrared activity and Raman activity are mutually exclusive now let's look at a few vibrational modes let's look at the first one number 12 with the highest vibrational frequency start animation and this mode changes the type of moment therefore it's infrared active but it does not change the polarizability of the molecule therefore it's Raman inactive let's look at number 10 this mode changes the polarizability and therefore it's Rama active but the type of moment of this molecule during the evaporation remains zero unchanged therefore it's IRR inactive let's look at some others number six again if you look at this vibrational mode the type of moment remains unchanged therefore it's I are inactive however its polarizability changes therefore it's Raman active can both IR and Raman activities be zero at the same time yes it's possible so over here this is a vibrational mode number four and this one happens to be IR inactive and Raman inactive now I'm going to stop the animation and I'm gonna show you the IR spectrum and Raman spectrum here so again you can see the IR peaks and the Raman Peaks are at different positions again this is because of this IR activity and Raman activity being mutually exclusive or any molecule with a inversion Center now I'm gonna show you a another YouTube video I made before this youtube video titled molecular symmetry IR in Ramah activity of the vibrational modes of c2h4 explains why the Raman and I our activities are mutually exclusive if you have a inversion Center in the molecule and then you can tell that all those Raman active modes have G symmetry all those IR active modes vibrational modes have a u symmetry Y so for this molecule to be infrared active during vibration you must have this marginal modes with a u symmetry either this one or this one or this one you look for x y&z which correspond to you the dipole moment for the vibrational mode to be Rama active you look for a kind of nonzero elements in a 3 by 3 matrix of polarizability in that matrix you have diagonal terms x squared Y squared Z squared the off diagonal terms are x y XC y z so if you see nonzero x squared Y squared Z squared or X Y X C Y Z they all have G symmetries and then you have ROM active so again this info active vibration modes or have a you symmetry the Raman active modes or have G symmetry therefore they are mutually exclusive what about a vibrational mode was a you symmetry so if you look at this a you symmetry it's neither IR no or Raman active

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