50 Years of Chemistry in Opole
Structural and electron aspects
of nitrogen–nitrogen bond
Katarzyna GAJDA, Bartosz ZARYCHTA*, Krzysztof EJSMONT – Faculty of Chemistry, Opole
University, Opole
Please cite as: CHEMIK 2014, 68, 4, 363–368
Introduction
Nitrogen, like many other chemical elements, has catenation
ability. The variety and multiplicity of molecular structures with
nitrogen-nitrogen bond is significantly smaller than for carbon. Such
compounds are of particular interest as potential high-energy density
materials (HEDM) [1], because they can undergo exothermic reaction
Nx → (x/2)N2 releasing 50 kcal/mol or more per one nitrogen atom
[2]. However, their practical application is limited by their stability.
Known are chain, cyclic as well as cage systems containing nitrogennitrogen bond. The experimental studies have proven that it is possible
to obtain ions N3− to N5+ in laboratory conditions [3÷5] as well as
polymer networks [6] but only under high pressure (110 GPa). Based
on the crystallographic resources of Cambridge Structural Database
(CSD) [7], one can conclude that the largest family of chemical
compounds involves systems with two nitrogen atoms connected
to each other (approx. 85% of all structures of type Nx, where x ≥ 2).
Among these system, the largest group are hydrazo, azo, azoxy and
azodioxy compounds and compounds containing nitramine group.
The first four groups form oxidation reduction series in relation
to each other. Hydrazo functional group contains two nitrogen atoms
sp3 connected by σ bond and each of them forms additional chemical
bond with hydrogen atom. While two nitrogen atoms of hybridization
sp2 each of which has nonbonding electron pair and which are doublebounded to each other form bivalent nitrogen group. The molecules
of chemical compounds containing this functional group may exist as
two geometric isomers E and Z. Azoxy functional group is formed as
a result of formation of chemical bond between oxygen atom and free
electron pair of one of azo group nitrogen atoms. Azoxy compounds,
called also diazene N-oxides also show geometric isomerism Z and
E with the respect to nitrogen-nitrogen bond. If both nitrogen atoms
of azo group form chemical bond with oxygen atom (>N2O2), such
group is called azodioxy. It is the most oxidized form of nitrogennitrogen bond [8–14]. The different order of connecting oxygen and
nitrogen atoms within the group of >N2O2 (N–NO2) can be found
in family of compounds called nitramines. Nitramine group forms
in these compound four-core π-electron system that may interact
with substituents of amide nitrogen atom. The studies of properties
of N-N bond and effect of nitramine group on ring aromaticity in
family of nitramine phenyl derivatives are also motivated by the desire
to explain the mechanism of so called nitramine rearrangement.
Description
The functional groups containing nitrogen-nitrogen bond presented in Figure 1 were selected for structural and electron analysis.
In order to determine changes in nitrogen-nitrogen bond length
in systems shown in Figure 1, the average bond length values were
calculated based on data from CSD and determined using quantum
mechanical calculations. These calculations were carried out by means
of MP2 [15] method with basis set 6–311++G(d,p) using software
Gaussian09 [16] with full optimization without using constraints for
molecule geometry, i.e. bond lengths, valence or dihedral angles .
Corresponding author:
Bartosz ZARYCHTA, Ph.D. e-mail: [email protected]
366 •
Fig. 1. Molecular diagram of studied compounds: (a) hydrazo;
(b) nitramine; (c) azoxy; (d) azodioxy; (e) azo compounds
The tool allowing to “look” deeper into the electron structure of
the molecule is the topological analysis of electron density distribution
based on theory of Atoms in Molecules (AIM) [17–19]. This theory
based on the three-dimensional electron density function (ρ) defines
basic chemical concepts such as: atom, chemical bond and nonbonding
interaction. The advantage of the AIM theory is its versatility. The
electron density function in molecules might be analysed both on
the basis of diffraction experiment as well as quantum mechanical
methods. In the analysis, two of four stationary points of electron
density function have greatest relevance, i.e. Ring Critical Point
(RCB) and Bond Critical Point (BCP) that is located between two
adjacent atom nuclei. The critical points correspond to electron
density function maximum, saddle points and local minimum and
can be determined based on the topology of the electron density
distribution. In order to characterize them appropriately, it is necessary
to calculate Hessian, i.e. matrix of second-order derivatives of density
functions with respect to all possible combinations of coordinates.
The diagonalization of such matrix leads to obtaining three Hessian
eigenvalues, which rank (number of non-zero eigenvalues) and
signature (sum of non-zero signs of Hessian eigenvalues) characterizes
these points [19]. For RCP these parameters have form of (3,+1),
while for BCP – (3,-1). The value of electron density in these points
is a measure of electron charge. The chemical bonds exhibiting more
covalent character have usually higher value of critical point electron
density in comparison to ionic bonds.
The systems where nitrogen-nitrogen bond plays a role of
bridge between aromatic rings are perfect model for description and
analysis of electron effects resulting from their mutual interaction.
In order to determine the impact on aromaticity of phenyl rings
connected to analysed functional groups using the geometric
aromaticity criterion, the HOMA index (Harmonic Oscillator Model
of Aromaticity) defined by the equation 1 was used.
(1)
where n is a number of summed up chemical bonds; α – normalizing constant (for bonds C–C α = 257.7) set in such way that
HOMA = 0 for nonaromatic system and HOMA = 1 for system
where all bond lengths are equal to optimum value Ropt assuming
completely aromatic system (for bonds C–C, Roptequals to 1.388 Å); Ri –
length of i-th bond [20,21].
nr 4/2014 • tom 68
Table 1
Average values of N–N bond length from database CSD and for
structures optimized using quantum mechanical methods and
corresponding values of critical point electron density. The values are
listed in decreasing order of average N–N bond length
System
hydrazo
nitramine
azodioxy
azoxy
azo
CSD, Å
1.399
1.384
1.313
1.262
1.249
MP2, Å
1.426
1.406
1.375
1.296
1.259
ρ, au
0.308
0.331
0.345
0.392
0.410
The Table 2 summarizes values of HOMA index for phenyl rings
connected to analysed functional groups containing nitrogen-nitrogen
bond. The aromaticity index HOMA is the same for hydrazo and
azo compounds. This indicates that these groups have identical
impact on aromaticity of phenyl rings, they are substituted with
and no participation of π bond in delocalization of N-N bridge for
azo compounds. HOMA index for phenyl ring of azoxycompounds
equals to 0.970 for ring with oxidised nitrogen atom and 0.911 for
ring connected to another nitrogen atom. This difference may be
a result of resonance occurring within the molecule. It means that the
ring distant from N-N group oxygen atom might lose aromaticity due
to the formation of quinoid structure.
Fig. 2. Selected resonance structures for azoxybenzene
The value of aromaticity index for phenyl ring connected to oxidised
nitrogen atom is the same as for azodioxy compounds (0.970).
Asymmetry of interaction between azoxy bridge with phenyl rings
should be noted. Among analyzed systems, the greatest changes in
aromaticity are observed for the ring not connected to unoxidised
nitrogen atom, while the smallest for the ring connected to oxidised
nitrogen atom of azoxy bridge.
nr 4/2014 • tom 68
Table 2
Average values of HOMA indices for phenyl rings in analyzed compounds
System
HOMA
hydrazo nitramine azodioxy
0.951
0.962
0.970
azoxy
azo
0.970 (NO) 0.911 (N)
0.951
Analysis of aromaticity index HOMA for nitramines confirms
formation of conjugated system C6H5NNO2. This index for all studied
nitramine molecules varies in the range of 0.962–0.966, while for
flat molecule of N–phenylnitramine its value decreases to 0.952 (for
perpendicular arrangement HOMA = 0.964).
Summary
For structural and electron analysis of functional groups containing
nitrogen-nitrogen bond compounds containing following groups were
selected: hydrazo, azo, azoxy, azodioxy and nitramine group. In order
to establish changes of nitrogen-nitrogen bond length, its average
values were calculated based on quantum mechanical calculations and
database CSD [7]. Moreover, the impact of analysed functional groups
on aromaticity of connected with them phenyl rings was determined
by calculations of aromaticity index HOMA.
The greatest nitrogen-nitrogen bond length is observed for hydrazo
compounds, while the shortest bonds are in azo compounds. The
other groups have intermediate length values. Large correlation was
observed between values of critical point electron density of nitrogennitrogen bond and its length. Aromaticity index HOMA increases in
compound series from azoxy, through azo/hydrazo, nitramine up
to azodioxy, indicating decreasing impact of these groups on π-electron
delocalization of connected with them phenyl rings. Furthermore,
for azoxy group variation of interaction with aromatic systems was
observed, while for nitramines influence of mutual arrangement of ring
and connected functional group was proven.
References
1. Strout D. L.: Why Isn’t the N20 Dodecahedron Ideal for Three-Coordinate
Nitrogen?. J. Phys. Chem. A 2005, 109, 1478–1480.
2. Fau S., Bartlett R. J.: Possible products of the end-on-addition of N3− to N5+.
J. Phys. Chem. A 2001, 105, 4096–4106.
3. Christe K. O., Wilson W. W., Sheehy J. A.; Boatz J. A.: N5+: A Novel Homoleptic
Polynitrogen Ion as a High Energy Density Material. Angew. Chem., Int. Ed.
1999, 38, 2004–2009.
4. Vij A., Pavlovich J. G., Wilson W. W., Vij V., Christe K. O.: Experimental
Detection of the Pentaazacyclopentadienide (Pentazolate) Anion, cyclo-N5−
Angew. Chem., Int. Ed. 2002, 41, 3051–3054.
5. Butler R. N., Stephens J. C., Burke L. A.: First generation of pentazole (HN5,
pentazolic acid), the final azole, and a zinc pentazolate salt in solution: A new
N-dearylation of 1-(p-methoxyphenyl) pyrazoles, a 2-(p-methoxyphenyl)
tetrazole and application of the methodology to 1-(p-methoxyphenyl)
pentazole. Chem. Commun. 2003, 8, 1016–1017.
6. Eremets M. I., Gavriliuk A. G., Trojan I. A., Dzivenko D. A. Boehler R.:
Single-bonded cubic form of nitrogen. Nature Mater. 2004, 3, 558–563.
7. Allen F. H.: The Cambridge Structural Database: a quarter of a milion crystal
structures and rising. Acta Cryst. 2002, B58, 380–388.
8. Shorter J.: The chemistry of hydrazo-, azo- and azoxy- groups S. Patai, Ed.,
J. Wiley, N. York, 1997, 2, 367.
9. Kyzioł J.B., Ejsmont. K.: 9-Methyl-3-phenyldiazenyl-9H-carbazole; X-ray and
DFT calculated structures. Acta. Cryst. 2007, C63, 77–79.
10. Ejsmont K., Domański A.A., Kyzioł J.B., Zaleski J.: Disorder in the crystals of
trans-4-fluoroazoxybenzene. Synthesis, Spectral Properties, Crystal Structures
and DFT Calculations J. Mol. Struct., 2005, 753, 92–98.
11. Ejsmont K., Domański A., Kyzioł J.B., Zaleski J.: 4-Hydroxy-ONNazoxybenzene Acta Cryst., 2000, C56, 697–699.
12. Domański A., Ejsmont K., Kyzioł J.B., Zaleski J.: Two trans-4aminoazoxybenzenes. Acta Cryst., 2001, C57, 467–470.
13. Ejsmont K., Broda M., Domański A.A., Kyzioł J.B., Zaleski J.: Orthorhombic
polymorphs of two trans-4-aminoazoxybenzenes. Acta Cryst., 2002, C58, 545–548.
• 367
50 Years of Chemistry in Opole
The Table 1 summarizes average N-N bond lengths and
corresponding values of electron density in studied compound
groups. The greatest length is observed for hydrazo compounds,
while the shortest one for azo compounds. This is due to the fact
that in hydrazo systems there is single bond between nitrogen atoms,
while in azo compounds – double bond. For azoxy and azodioxy
compounds, the bond lengths have the intermediate values between
single and double bond. In nitramine structures, the average distance
between nitrogen atoms is shorter by 0.020 Å than for hydrazo
compounds, however it is clearly longer (by 0.147 Å) than average
value for azo compounds. As the bond gets shorter, the value
of electron density in bond critical point increases. The lengths
determined by quantum mechanical calculations have similar values
as ones taken from database CSD [7]. The longest bond length
(1.460 Å) was observed for an unsubstituted nitroamine molecule.
While the shortest one (1.373 Å) for N-phenylnitramine. This difference
indicates significant impact of substituents of amide nitrogen atom on
the structure of nitramine group (N-N bond). Furthermore, also mutual
arrangement of substituent in relation to the plane of nitro group affects
the length of discussed bond. This is clearly seen for N-phenylnitramine
molecule, in which depending on the orientation (perpendicular/
parallel) the length decreases by 0.041 Å, when nitramine is co-planar
to aromatic ring. This can be due to the fact that two π-electron systems
in such arrangement might interact with each other forming 10 core
system of delocalized electrons.
50 Years of Chemistry in Opole
14. Ejsmont K., Domański A.A., Kyzioł J.B., Zaleski J.: Trans-4-bromo-ONNazoxybenzene at 100 K. Acta Cryst., 2004, C60, 368–370.
15. Møller C., Plesset M.: Note on an approximation treatment for many-electron
systems. Phys. Rev., 1934, 46, 618–622.
16. Gaussian 09, Revision D.01, Frisch M. J., G. W. Trucks, H. B. Schlegel,
G. E. Scuseria, M. A. Robb, J. R. Cheeseman, G. Scalmani, V. Barone, B.
Mennucci, G. A. Petersson, H. Nakatsuji, M. Caricato, X. Li, H. P. Hratchian,
A. F. Izmaylov, J. Bloino, G. Zheng, J. L. Sonnenberg, M. Hada, M. Ehara,
K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O.
Kitao, H. Nakai, T. Vreven, J. A. Montgomery, Jr., J. E. Peralta, F. Ogliaro,
M. Bearpark, J. J. Heyd, E. Brothers, K. N. Kudin, V. N. Staroverov, R.
Kobayashi, J. Normand, K. Raghavachari, A. Rendell, J. C. Burant, S. S.
Iyengar, J. Tomasi, M. Cossi, N. Rega, J. M. Millam, M. Klene, J. E. Knox, J.
B. Cross, V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts, R. E. Stratmann,
O. Yazyev, A. J. Austin, R. Cammi, C. Pomelli, J. W. Ochterski, R. L. Martin,
K. Morokuma, V. G. Zakrzewski, G. A. Voth, P. Salvador, J. J. Dannenberg, S.
Dapprich, A. D. Daniels, Ö. Farkas, J. B. Foresman, J. V. Ortiz, J. Cioslowski,
D. J. Fox, Gaussian, Inc., Wallingford CT, 2009.
17. Bader R. F. W. Atoms in Molecules: A Quantum Theory, Clarendon, Oxford, 1990.
18. Ronald J. Gillespie and Paul L. A. Popelier, Chemical Bonding and Molecular
Geometry: From Lewis to Electron Densities, Oxford University Press: New
York, NY, 2001.
19. Matta C. F. and Boyd R. J. An Introduction to the Quantum Theory of Atoms
in Molecules, in The Quantum Theory of Atoms in Molecules: From Solid State
to DNA and Drug Design, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim,
Germany, 2007.
20. Krygowski T. M.: Crystallographic studies of inter- and intramolecular
interactions reflected In aromatic character of π-electron system J. Chem. Inf.
Comput. Sci., 1993, 33, 70–78.
21. Krygowski T. M.; Cyrański M. K.: Separation of the energetic and geometric
contributions to the aromaticity. Part IV. A general model for the π-electron
systems. Tetrahedron 1996, 52, 10255–10264.
Acknowledgements
Authors would like to thank Wroclaw Centre for Networking and Supercomputing for allowing them to perform quantum mechanical calculations. Katarzyna Gajda
is a recipient of scholarship under programme “Ph.D. scholarships – investment in
scientific staff of Opole voivodeship” for academic year 2013/2014.
12-13 czerwca 2014 rok
Puławski Park Naukowo-Technologiczny
w Puławach
Program Kongresu:
12 czerwca 2014 – I dzień Kongresu PIPC
10:00-10:30 Sesja otwierająca. Wystąpienia gości specjalnych.
10:30-12:00 Debata strategiczna: „Polski Przemysł Chemiczny na tle gospodarki
Unii Europejskiej”
Prezentacja przedstawiciela CEFIC Dr Hubert Mandery,
Dyrektor Generalny
Prezentacja: „ Polski przemysł chemiczny” Dr inż. Tomasz Zieliński,
Prezes Zarządu PIPC
Panel dyskusyjny:
Moderator: Dr inż. Tomasz Zieliński, Prezes Zarządu PIPC
12:00-12:30 Przerwa kawowa
12:30-13:15 Sesja 1: Konkurencja na światowych rynkach chemikaliów
i produktów chemicznych – ochrona europejskiego rynku.
Case study: „Preferencyjne traktowanie producentów
pozaeuropejskich na rynku europejskim”
Tomasz Kalwat, Prezes Zarządu Synthos S.A.
13:15-14:30 Sesja 2: Energia dla Chemii
Case study: „Efektywność energetyczna vs. koszty polityki
klimatycznej”
- przedstawiciel firmy członkowskiej PIPC lub przedstawiciel
KAPE S.A.
Moderator: Dr inż. Krzysztof Łokaj, Senior Manager PIPC
14.30–15.00 Sesja 3: Polska Chemia w sporcie – Gość Specjalny
15:00–16:00 Lunch
16:00-16:45 Sesja 3: Dystrybucja produktów chemicznych
Case study: Przedstawiciel firmy członkowskiej PIPC
16:45-17:30 Sesja 4: Kształcenie kadr dla przemysłu chemicznego
Katarzyna GAJDA – M.Sc. is a graduate of the Faculty of Chemistry of
the Opole University (2010). She is a Ph.D. student of the Environmental
Doctoral Studies in the Faculty of Chemistry, Opole University. Scientific
interests: structural chemistry, π-electron delocalization and aromaticity
of cyclic systems. She is an author of 1 publication in journals from ISI
Master Journal List.
e-mail: [email protected], phone: +48 774527161
Case study: „Kształcenie kadr dla przemysłu chemicznego”
- Prof. dr hab. Inż. Władysław Wieczorek, Prorektor Politechnika
Warszawska
Moderator: dr inż. Krzysztof Łokaj, Senior Manager PIPC
(„ChemHR – projekt wsparcia dla rozwoju kadr”)
17:30-18:15 Sesja 5: Bezpieczna Chemia – sesja plenerowa
Case study: „Praktyczne aspekty bezpieczeństwa procesowego”
Moderator: Paweł Zawadzki, Kierownik Projektu PIPC
18:15-18:30 Podsumowanie I dnia Kongresu
20:00
* Bartosz ZARYCHTA – Ph.D. is a graduate of the Faculty of Physics,
Mathematics and Chemistry of the Opole University. (2002). He received
Ph.D. degree from the Faculty of Chemistry of the Opole University in
Opole (2008). Currently he works in the Faculty of Chemistry of the Opole
University. Scientific interests: studies of electron density distribution,
structural chemistry. He is an author of 23 publications in journals from
ISI Master Journal List
e-mail: [email protected], phone: +48 774527161
Uroczysta kolacja połączona z wystąpieniem zaproszonego Gościa
Specjalnego
13 czerwca 2014 – II dzień Kongresu PIPC
10:00-11:00 Wizyta w zakładzie produkcyjnym Grupa Azoty Zakłady Azotowe
Puławy S.A.
11:15-11:30 Przerwa kawowa
11.30-12.30 II Debata strategiczna: „Europejski przemysł chemiczny na tle
gospodarki światowej”.
12:30-13:15 Sesja 6: Inwestycje rozwojowe w polskiej chemii
Case study: „Prezentacja przedstawiciela firmy członkowskiej PIPC”
Krzysztof EJSMONT – Ph.D., D.Sc. is a graduate of the Faculty of
Physics, Mathematics and Chemistry of the State Higher Pedagogical
College (1992). He obtained his Ph.D. degree from the Institute of
Low Temperature and Structure Research, PAN in Wroclaw (1999). He
obtained the title of D.Sc. from the Faculty of Chemistry of the University
of Lodz (2013). Currently he works in the Faculty of Chemistry of the
Opole University. Scientific interests: structural chemistry, π-electron
delocalization and aromaticity of cyclic systems. He is an author of 61
publications in journals from ISI Master Journal List.
e-mail: [email protected], phone: +48 774527106
368 •
13.15-13.30 Przerwa kawowa
13:30-14:15 Sesja 7: Innowacje – inwestycje w przyszłość
Case study: „Prezentacja przedstawiciela firmy członkowskiej PIPC”
14:15
Podsumowanie Kongresu / zakończenie Kongresu
14:30
Lunch
Wszelkie informacje znajdą Państwo na stronach:
www.kongrespolskachemia.pl oraz www.pipc.org.pl
nr 4/2014 • tom 68