Why do bond angle values change?

[irisc23]

I was just doing the worksheet and it appeared that every time I minimize the energy, the bond angles change slightly (± around 0.007º each time). Why does this occur? Aren't the minimum energy values unique (probably solved for from optimization of some energy functions)? Or can different bond angles still produce the same minimum energy value? Thanks.

[Robert]

I googled energy minimization and several websites state:

"Perform an energy minimization of the system, by iteratively adjusting atom coordinates. Iterations are terminated when one of the stopping criteria is satisfied. At that point the configuration will hopefully be in local potential energy minimum"

Since it's a local energy minimum, its not going to be a precise number every time.

Maybe that helps?

[irisc23]

Well it seems like even the local energy minimum should be a single point-- imagine plotting the energy of all of the different conformations of cyclohexane, for example. Now connect them with the reaction profile diagrams, there will be some peaks (transition states, representing activation energy barriers), and some troughs (the local minima, representing the different conformations). So I get that this is a local minimum, and that's because the molecule must acquire a significant amount of energy to pass the transition state, even if it would be going to a more thermodynamically favorable product. However, it still seems like the local minimum corresponds to 1 particular point for a given range. So basically my question is-- why does the fact that it is a local energy minimum have to do with the bond angles changing/not being precise? Thanks.

[y3chem]

I've noticed the change in bond angles too, however, when I ask Lavelle which angle to I put dow, he says to average the two numbers. My guess is that since molecules are not stationary (unlike our structures in chem3D), there will be variations in bond angles when the molecules rotate, move, and vibrate. However, this is just a really broad extrapolation. I want to know the reason behind the bond angle value change too.

[Chem_Mod]

When we draw a molecule on Chem3D, we see only a static depiction of that molecule in whatever conformation we've drawn or specified. When we tell Chem3D to minimize the energy, the program runs through quite a few iterations of calculations to settle on the final minimized structure which we see as a new static conformation. While it's true that there is one true "minimum energy," the value we get from our calculations need not be exactly that value. (Think about too: when we set the angle to 5 degrees and minimize, does the structure go to the absolute most stable conformation or settle in another energy minimum? Why?) We can tell the program to be more specific with the calculations (which also means more iterations and more time waiting for the result) and get closer to the exact minimum energy. This would give more consistent values for bond angles and lengths but take much longer. For our purposes, why wait half an hour or even longer for angles with 0.001 degree variation when we can get 0.007 degree variation with only about thirty seconds of wait time?