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Hyperfine excitation of C15N by para-H2 (Flower et al, 2015)



Display rate coefficients as : Display rate coefficients graphically : Data information :
  • C15N initial level labelled from 2 to 34
  • C15N final level labelled from 1 to 33
  • H2 initial level labelled from 1 to 1
  • H2 final level labelled from 1 to 1
  • 28 temperatures between 5 K and 150 K
  • Units : cm3 s-1

C15N

H2


C15N


N/A

N/A

References :

None

The 2D PES used was adapted from the 12CN-H2 4D PES described in Kalugina et al, 2012, 2013.

The calculation of the 4D PES assumes that both 12CN and H2 are rigid rotors. For H2, the bond distance is fixed to the value averaged over the fundamental vibration mode, i.e. rHH = 1.4487 a0. For CN, the equilibrium distance is taken, i.e. rCN = 2.2144 a0 (this value was also assumed when building the C15N-H2 PES). The reactive channel towards HCN + H is ignored, given that this reaction have an activation energy of at least 1100 cm-1 (Ter Horst et al, 1996).

The 12CN-H2 4D PES was calculated at the RCCSD(T) / aug-cc-pVTZ level of theory. The basis was supplemented with mid bond functions. The basis set superposition error (BSSE) was corrected at all geometries with the Boys and Bernardi counterpoise procedure (Boys and Bernardi 1970). The AVTZ + bond functions was found to reproduce higher basis sets calculations (i.e. AV5Z) within ~1-2 cm-1 in the potential well.

The calculations were performed at regularly spaced distances in the range 4-16 a0. At each distance, the angular dependence was sampled on a predefined grid for the 3 angles that describe the 4D PES. A 2D PES is then built from the 4D PES by averaging the ab initio points over the two angles that describe the H2 rotation. At this point, the coordinates of the ab initio points were modified to account for the shift of mass center between the 12CN-H2 and C15N-H2 systems. The new 2D PES specific to C15N-H2 was then fitted with a 9-term angular expansion over Legendre polynomials, thanks to the fitting procedure described by Werner et al, 1989. The fit was performed accounting for all the ab initio points calculated up to 16 a0. Above this intermolecular distance, the anisotropic terms with \lambda > 2 were damped exponentially to zero while the first three terms were extrapolated with R-6 and R-7 dependences.

References :


Close-coupling (CC) calculations were performed by solving the nuclear spin-free coupled set of differential equations. The CC calculations consider explicitely the fine structure.

The 15N atom possesses a I=1/2 spin which induces a hyperfine structure whose corresponding cross sections were obtained through a recoupling technique, in a two step procedure. First, CC calculations were performed just taking into account the fine structure energy levels of C15N, which leads to obtain the scattering matrix elements S^J(N j l,N' j' l'). The hyperfine structure is subsequently introduced and the scattering matrix elements are recoupled according to the methodology described by Corey & McCourt 1983.

References :


not specified

not specified

The methodology used typically allows the determination of rate coefficients with a few percent accuracy, with respect to the PES used.

In the current case, an additional source of error comes from the PES, which is not specifically tailored for C15N (the bond length used corresponds to the 12CN bond length averaged over the fundamental mode of vibration). This typically induces uncertainties of a few 10% (see e.g. scribano et al, 2010).

Additionally, the dynamical calculations neglect the j=2 state of H2 and it was shown by Kalugina et al. 2013 that in the particular case of CN-H2 collisions, this induces 30% uncertainties in rate coefficients.

Presentation

Hyperfine de-excitation rate coefficients are provided for the first 34 hyperfine levels of C15N (up to N=8) in collision with para-H2, for temperatures in the range 5-150K.


References

NB : the main reference is displayed in red
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Status Version Date
Documentation modified 1 2016-03-17