Astron. Astrophys. 333, 1007-1015 (1998)

2. The formation of H₂ NC (¹ A₁) and HCNH ()

For the stationary points on the C (² P) + NH₃ H₂ NC (¹ A₁) + H HCNH () + H potential energy surface, the absolute and relative energies, corrected for zero-point vibrational energies, are given in Table 2, while the molecular structures and the energy profile of the reaction path are depicted Fig. 3 and Fig. 4, respectively (see appendix for computational details). From Fig. 4, one can see that there is no long-range barrier on the portion of the potential energy surface leading from reactants to the 1² component of H₃ NC (X² E). The reaction path to H₂ NC (¹ A₁) is obtained through the in-plane stretch of one of the NH bonds. Once formed, H₂ NC (¹ A₁) can isomerize to a more stable linear geometry (Fig. 4) by going over a transition state . The transition state nature of with respect to dissociation to H₂ NC (¹ A₁) + H and of with respect to isomerisation into HCNH is confirmed by the vibrational analysis in Table 3.

Fig. 3. MP3/6-311 + + G(d,p) stable and transition structures along the 1 2 H3 NC surface. For H3 NC (2 ) are also reported the MP2/6-31G(d,p) optimized geometries (between brackets) for comparison. Italic numbers correspond to the rotational constants (in Ghz) calculated at the MP3/6-311 + + G(d,p) energy level. Bond lengths are in Angstroms and angles in degrees.

Fig. 4. Energy profile (in Kcal/mol) for the C (2 P) + NH3 H2 NC (1 A1) + H HCNH (1 S ) + H reaction, calculated at the MP4SDTQ/6-311 + + G(3df,3pd) using MP3/6-311 + + G(d,p) optimized geometries. Scaled zero point energies are taken in account.

Table 2. Absolute and relative energies calculated at the MP4SDTQ/6-311 + + G(3df,3pd) level for the lowest 2 surface

Table 3. MP2/6-31G(d,p) harmonic vibrational frequencies

From the energy profile of Fig. 4, it is obvious that the transition structures and (H₂ lie much lower in energy than the initial reactants, (68.9 and 72.5 kcal/mol below reactants, respectively). It is therefore clear that the vibrationally excited [H₃ NC ] complex (energy minimum) formed by the collision will easily overcome the corresponding barrier opposing the formation of H₂ NC (¹ A₁) + H. The subsequent isomerization to linear HCNH () is a more complex problem since it depends on the amount of relative translational energy between H₂ NC (¹ A₁) + H, as well as on other matters. This isomerization will be discussed in Sect. 4.

© European Southern Observatory (ESO) 1998

Online publication: April 28, 1998

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