# How to obtain molecule length?

1. Mar 8, 2016

### diekatnoe

Hello everyone!

The goal is to calculate the length of octanthiols in full stretched all-trans configuration. The structural formula is HS-C_7H_17-CH_3 witch is an alkyl chain terminated with sulfur.

I used Avogadro software to build and model the structure. After i let it measure the distance from the head sulfur atom to the last carbon but i wonder how correct the results are. It says 10.xx Angström.

How would you perform such a calculation?

Greetings

2. Mar 8, 2016

### Staff: Mentor

There some discussion here along with a table describing measured length for single and double bonds based on atomic number:

http://chemwiki.ucdavis.edu/Core/Th...es_of_Chemical_Bonding/Bond_Order_and_Lengths

and the table:

There are probably other things beside bond length such as its geometry and how its folded that you must consider as well in your calculation. However, someone more knowledgeable than me will have to answer that.

3. Mar 9, 2016

### Staff: Mentor

If it is just a full stretched all-trans configuration it sounds like a simple geometry problem - angles are known and bond lengths are quite constant in such a setup.

Are you sure about the -C7H17- part? Not -C7H14- ?

Last edited: Mar 10, 2016
4. Mar 10, 2016

### diekatnoe

Yeah its $H_{14}$ thank you!

So what i extraxt after reading the articles is the following method: add covalent radii of S-C and C-C bonds and divide it by 2 to get the bond length. Then apply geometric calculation witch respects the angles to get the total length.

Thank you!