Subject: SUMMARY: 1,5-Diaminotetrazole aminogroup structure

From: Alexander Kulak <[email-masked]>
Date: Fri, 14 Sep 2001 13:09:49 +0300 (EEST)

Hello, All.

Please excuse me for a delay.
In brief, my question was:

AK>                "N6" ------>  NH2
AK>                              |
AK>                            // \
AK>                            N   N - NH2  <------ "N5"
AK>                            |   |
AK>                            N = N
AK>
AK> X-ray investigation of this compound shows that N6 aminogroup
AK> is planar (angles = 118,118,119) and lies in the tetrazole plane.
AK>
AK> But MP2/6-31G(d,p) calculation gives the non-planar structure of that
AK> aminogroup (112,113,114 degrees), although H atoms lie close to the ring plane,
AK> 1) Why calculated N6 is not planar while nature says it is planar?
AK> 2) Why it CAN be planar?

I'm very grateful for all who has replied (listed in random order):

Jeremy Greenwood, John McKelvey, Stanislaw Oldziej, Per-Ola Norrby,
Dave Shobe, Thomas HЭbner, Stefan Fau, Laurence Cuffe, Alan Shusterman.

In general, there are three points in answers:

1. Solid state structure differs from those of single molecule,
because of crystal packing and intermolecular interaction.
Unfortunately, currently we have no computational resources to calculate
gradients of oligomers or crystal of this compound.

2. Diffraction experiment does not tell much about NH2 conformation even
in solid state, because
	a. Positions of H atoms are not directly and exactly predictable.
However, our experimentators (ref. 1 below) tell me that inspite of
these positions are calculated, one can trust the results. Besides,
the diffraction experiment at 5-aminotetrazole hydrate (ref. 2) shows
the same NH2 structure.
	b. Those positions are averaged during experiment, and NH2 transition
barriers are usually very small. Really, I've estimated the enthalpy of
thansition between experimental and lowest energy conformation to be as low
as 5.0 kJ/mol at MP2/6-31G** theory level.

3. MP2/6-31G** is not the final computational answer. I need to carry
out calculations in larger basis and at better correlation level.

Thank you all for help!

REFERENCES

1.
A.S. Lyakhov, P.N. Gaponik, S.V. Voitekhovich. 1,5-Diamino-1-{H}-1,2,3,4-
tetrazole. Acta Cryst. C57, 2001, P. 185--186.

2.
D.D. Bray, J.G. White. Refinement of the Structure of 5-Aminotetrazole
Monohydrate. Acta Cryst. B35, 1979, P. 3089--3091.

b.w., Alexander Kulak   [ http://i.am/kulak ]