 Hello, I'm Professor Steven Nashiba, and I'm here to help you out a little bit with this set of ideas associated with factors that influence infrared spectra and to orient you a little bit, I've just kind of drawn a couple of spectra here and the idea is that here's a frequency axis which we're giving in this this frequency called new bar a wave number and the way this is conventionally laid out is that we go from high frequency to low frequency and Also on the vertical axis here It's a it's percent Transmission so in other words if I have a molecule that's not absorbing any light that means it would be a hundred percent transmission and so these two peaks mean that if this wave number and at this wave number the molecule is absorbing and another few Things before we we get into the these criteria is that a strong absorber would mean that this peak goes way down because it absorbs you know more that is to say You know eventually at that reach the bottom that would mean that no light is transmitted So that must be a very strong absorber so again absorption strength has to do with the The size of that that inverted peak and the and the position has to do with with where we are on this this axis So here are the rules The line strength rule which determines how far down that goes is very simple It just says the the larger the difference in electronegativity between two atoms that are bonded But that difference is really large then the line is stronger okay, and and then we have another rule which has to do with the position again and That rule the line position rule says There's two considerations actually a stronger bond Would produce a higher frequency which would be to the left here absorber or lighter masses Tentative produce absorption that will at higher frequencies Okay, so stronger bond or lighter mass produce or correspond to higher new bar higher wave numbers So let's just take a couple of examples here. This is the one we're looking at I see two peaks here This one is a stronger absorber this one's a weaker absorber And let's suppose we know that one of those was carbon double bonded to oxygen and the other one was carbon double bonded to sulfur And let's just look at the line strength rule If you look up the electronegativities of carbon oxygen and sulfur you find that this has a much bigger difference in electronegativity between carbon and oxygen this carbon and sulfur have a very small Difference in electronegativity, so we would expect that the stronger absorber would would be this one Okay, and that is to say this stronger absorber would correspond it to Carbon double bonded to oxygen, so we're kind of thinking that belongs there But let's just check the other rule the line position rule says that stronger bonds Can't really tell because they're both double bonds But how about this criterion a lighter mass produces a higher wave number? Well, the carbons are the same but look we have oxygen here and sulfur here Oxygen is much lighter than sulfur so that would argue for this carbon double bond oxygen to be off to the left So both of these criterias are arguing that this really is the carbon double bond oxygen and that really is the carbon double bond sulfur peak both of those rules Let's have a look at this other spectrum. I put up here So now what we say we know about this is that one of these peaks is carbon single bonded to oxygen And the other one is carbon single bonded to hydrogen Okay, let's let's do our line strength rule again Well, I happen to know that there's a big electronegativity difference between carbon and hydrogen Hardly any between sorry carbon and oxygen Harvey any electronegativity difference between carbon and hydrogen So this one now looks like it belongs to that peak because that's the stronger absorber Okay, let's see how that checks out with the line position rule stronger bond No, we have the same bond strength here, but we do have a lighter mass and And so it's a similar case here I've got a lighter mass here because that's hydrogen that's gonna argue For hydrogen the CH belonging to here, which means that the CO must be here So both of those considerations point to the same Conclusion that that peak must correspond to CO and this peak must correspond to to CH Okay