Interesting stability and structural trends have been found with MP 2 / 6-31 G^* calculations on

Interesting stability and structural trends have been found...

Key Concepts

Natural Population Analysis (NPA)

Natural Population Analysis is a computational method used to examine the distribution of electron density within a molecule. This technique provides insight into the partial charges and bonding characteristics of atoms by partitioning the molecule’s electron density into contributions from individual atoms. NPA is important for understanding the electronic structure and interactions between atoms, particularly in systems where polarization and charge transfer play significant roles in chemical behavior.

Substituent Effects and Electronegativity

Substituent effects refer to the influence that different groups attached to a molecule exert on its overall stability, reactivity, and structure. Group electronegativity is a key parameter in these effects, as it quantifies the tendency of a substituent to attract electrons. Variations in electronegativity can alter bond angles and electronic distributions, thereby affecting the energy and geometry of the molecule. Understanding these effects is essential for predicting and rationalizing trends in molecular properties.

Ab Initio Methods (MP2/6-31G*)

This concept involves using ab initio computational techniques to predict molecular properties and behaviors. The MP2 (second-order Møller-Plesset perturbation theory) method, combined with the 6-31G* basis set, provides a balance between computational efficiency and accuracy in estimating electron correlation effects within molecules. This approach is commonly used to explore the electronic structure, stability, and geometric parameters of molecules, providing insights that complement experimental observations.

Isodesmic Reaction

An isodesmic reaction is a type of hypothetical chemical reaction used in computational chemistry where the types and numbers of bonds remain constant between reactants and products. This reaction scheme is particularly useful in isolating and comparing the effects of substituents on molecular stability by minimizing systematic errors. By maintaining similar bonding environments, researchers can focus on the energetic contributions of different substituents without the confounding influences of changing bond types.

Transcript

00:03 All right, so let's calculate the change in energy with this equation right here.

00:11 Okay? so to do this, we need to look at first the different bonds that are found in these molecules.

00:17 One thing that i like to do is sometimes it's nice to draw these out so you can kind of see what's going on.

00:23 So make these kind of draw out the loose structures or whatever with these.

00:31 No need to really draw the electrons on there, so i guess you're not really doing that when it comes to the...

00:45 Here, let's erase it.

00:47 When it comes to the lewis structures, let me erase down right there.

00:53 Okay, good.

00:55 Just so it kind of looks nice there.

00:56 So we have our hydrogen, hydrogen, hydrogen.

00:59 So i'm just drawing this one out right there.

01:01 I know it's going to form this of ch3.

01:03 So c bonded to three hydrogen, bonded to three hydrogen, bonded nitrogen, then nitrogen's going to be double bonded with that carbon there, is going to then form this molecule right here, where we have our carbon, three hydrogen.

01:27 Okay, did i miss anything there? i want to verify here.

01:44 Okay, so i think, yeah, i just swapped.

01:46 Okay, very good.

01:47 So what we're going to do is we're going to then look at these.

01:49 I like to write down all the different bonds that are found in this molecule.

01:55 Let's do that in blue.

01:56 So we have our hydrogen bonded to carbon.

01:59 We have three of those.

02:01 We have our carbon that's bonded to nitrogen.

02:03 And we have a nitrogen that's double bonded to carbon.

02:07 Right over here, we have the same kind of thing going here.

02:13 We have carbon bonded to hydrogen.

02:16 We have a carbon bonded to carbon, and then we have a carbon double bonded to nitrogen.

02:22 Okay.

02:23 And so then you need to look up these values.

02:25 Here and so that's what we're done to do.

02:28 So let's write those in red.

02:30 So carbon and hydrogen we have that as a 413 and this is in kilojoules per mole and this is the energy that's required to either break or form these bonds.

02:44 Carbon and nitrogen is 305.

02:50 Carbon nitrogen double bond 615.

02:59 Carbon hydrogen we see that's 413.

03:02 Carbon carbon, 347.

03:10 And then the same thing right there, 615.

03:13 Okay.

03:14 So we've got our values here.

03:15 Now what we've got to do is look and say, okay, well, here's the bonds on this side of the equation.

03:20 These are bonds that will need energy to break those...