ECHET96 Search CD [Molecules: None] [Related articles/posters: 009 065 102 036 043 ]


Electronic structure of tetrahydro-v-tetrazines

H.M. Muchall and P. Rademacher

Institut für Organische Chemie, Universität GH Essen, Germany


Photoelectron (PE) spectroscopy [1]

We have measured the He(I) photoelectron spectra of the tetrazines 1 [2],[3] and 2.

PE spectra
PE spectra
Interpretation of PE spectra usually requires correlation of measured ionization potentials and calculated orbital energies using Koopmans' approximation [4]. Molecular orbital energies are taken from semiempirical quantum chemical calculations like PM3 [5]. For tetrazines 1 and 2, this approach does not give satisfactory results.

Tetrazine 1

Semiempirical Calculations

PM3 calculations lead to two conformers for 1, the half-chair is more stable by 0.84 kJ mol-1. In both, lone pairs on N(1) and N(4) occupy axial positions which shows that the 2-tetrazene moiety is only little affected by the conformational change. Therefore, its orbital energies should not be influenced much either. However, this expectation is not met by the PM3 results. Calculated spectra of the two conformers differ considerably (Scheme 1) and correlation coefficients R2 of a linear regression with ionization potentials are rather poor (0.9077 for the half-chair, 0.9333 for the boat). With respect to the enthalpy difference, the experimental spectrum should be that of the mixture of both conformers and according to PM3 should be complex in the low energy region.
scheme 1
As this is not the case, we undertook higher level calculations in order to better explain the experimental data.

Ab initio calculations

For the hypothetical, unsubstituted tetrahydro-v-tetrazine 1h, we performed ab initio HF and - assuming electron correlation could be important - MP2 calculations with the basis set 6-31+G*.

Again, unexpectedly, we obtained similar results regarding orbital energies of the conformers (as for PM3 calculations on 1). For MP2, the differences in orbital energies of the conformers were even more pronounced. When we employed the hybrid method Becke3LYP [6] of the density functional theory (DFT) using the same basis set as above, the situation changed. In contrast to HF and MP2, the two conformers of 1h have almost the same orbital energies (Scheme 2).

scheme 2
This positive result was transferred to 1. According to Becke3LYP/6-31+G*, the two conformers of 1 are very close in total energy, the unsymmetrical boat being more stable by 1.09 kJ mol-1. Between the conformers, there is a maximum difference in orbital energies of about 0.2 eV (Scheme 3), correlation with ionization potentials is good: R2 is 0.9717 (0.9987) with a slope of the straight line of 0.9583 (1.0025) for the half-chair (boat). Differentiation between the conformers by photoelectron spectroscopy is not possible because the Ei values are so similar.
scheme 3
The orbital sequence [2] is the same as in other cyclic 2-tetrazenes: lone pairs on N(1) and N(4) are included in a system which leads to orbitals 3 to 1. Lone pairs on N(2) and N(3) interact to give n-NN and n+NN: 3 > 2 > n-NN > n+NN > 1.

Tetrazine 2

Problems in assigning ionization bands cannot be due to different geometrical structures because all calculations predict 2 to be planar. Rather, there is the possibility of complete conjugation over the heteroatoms which is not taken into account properly by semiempirical or HF ab initio methods. Becke3LYP calculations on the other hand give orbital energies in good agreement with ionization potentials taken from the PE spectrum (Scheme 4).
scheme 4
In comparison to 1, the 2-tetrazene orbitals are stabilized due to the carbonyl substitution. The lone pairs on the oxygen atoms combine to n+O and n-O and are included in the tetrazene orbital sequence: 3 > n+O > 2 > n-NN > n-O > n+NN > 1.


Tetrazines 1 and 2 are small molecules with multiple vicinal electron lone pairs. Semiempirical methods (PM3) reveal difficulties in describing their molecular orbital energies correctly. Ab initio calculations based on HF or perturbation theory (MP2) do not perform better. Good correspondence with experimental ionization potentials is achieved with Becke3LYP (DFT).

The quality of the Becke3LYP calculations can best be estimated when both ionization potentials and orbital energies of 1 and 2 are compared with one another (Scheme 5).

scheme 5


[1] J.H.D. Eland, Photoelectron Spectroscopy, Butterworth, London 1984; C.R. Brundle, A.D. Baker (Eds.), Electron Spectroscopy: Theory, Techniques and Applications, 5 volumes, Academic Press, London 1977-1984.

[2] P. Heymanns, P. Rademacher, Tetrahedron 1986, 42, 2511.

[3] S.F. Nelsen, R. Fibiger, J. Am. Chem. Soc. 1972, 94, 8497.

[4] T. Koopmans, Physica 1934, 1, 104.

[5] J.J.P. Stewart, J. Comp. Chem. 1989, 10, 209.

[6] A.D. Becke, J. Chem. Phys. 1993, 98, 5648; C. Lee, W. Yang, R.G. Parr, Phys. Rev. B 1988, 37, 785.