Subject: Re: CCL:1,5-Diaminotetrazole aminogroup structure

From: Per-Ola Norrby <[email-masked]>
Date: Mon, 13 Aug 2001 08:33:03 +0200

Hi,

Alexander Kulak wrote:
>                "N6" ------>  NH2
>                              |
>                            // \
>                            N   N - NH2  <------ "N5"
>                            |   |
>                            N = N
>
>X-ray investigation of this compound shows that N6 aminogroup
>is planar (angles = 118,118,119) and lies in the tetrazole plane.
>
>But MP2/6-31G(d,p) calculation gives the non-planar structure of that
>aminogroup (112,113,114 degrees), although H atoms lie close to the 
>ring plane,
>in contrast with N5, where they lie at the different sides of the plane.
>
>The HOMO is combined from p(z)-AO of ring's nitrogens and p(z) of N6,
>confirming common assumpion about Pi-conjugation of ring and N6.
>At the same time localization (Boys or Pipek-Mezey) gives a clear
>picture of lone-pair at N6.
>
>Could anyone please answer the QUESTIONS:
>
>1) Why calculated N6 is not planar while nature says it is planar?
>2) Why it CAN be planar?

	First of all, MP2 is not the final computational answer, and 
also very important, X-ray is not the final answer about nature.  It 
is quite common to find that "planar" X-ray structures are in fact 
pyramidal when investigated in gas phase (amides are good examples of 
this).

	In your case, planarity will be favored by crystal packing, 
and by hydrogen bonding which increases the conjugation.  Ways to 
address this computationally would include investigating the planar 
form (lock it in Cs symmetry, compare the energy and vibrations to 
the planar form), or to add hydrogen bonds (preferably you should 
calculate the crystal with periodic boundaries; no, I don't have a 
program for that, but I believe there are solid-state DFT programs 
that can do it).

	One more point: The X-ray does not show you the position of 
the hydrogen nuclei, but if it is a very good quality X-ray, you may 
see the maximum density of the electrons forming the N-H bond.  It is 
well known that this point does NOT coincide with the position of the 
hydrogen nucleus.  Hydrogen positions are always systematically 
"wrong" by 0.1-0.2 A in any high-quality X-ray structure.

	Best regards,

	Per-Ola Norrby
-- 

^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Per-Ola Norrby, Assoc. Professor, http://compchem.dfh.dk/PeO/
Technical University of Denmark, Dept. of Chem., Org. Chem.
Building 201, Kemitorvet, DK-2800 Kgs. Lyngby, Denmark
Email: [email-masked]  tel +45-45252123,  fax +45-45933968