Electronic States of CH and NH

Abstract

Ab initio calculations (LCAO–MO–SCF) are performed on a series of valence levels of the molecules CH and NH+. Correlation energies are estimated semiempirically from corresponding atomic data. Close agreement with experiment is found for known states for a series of molecular properties such as equilibrium internuclear distances, vibration frequencies, term values, and dissociation energies. A low-lying Σ4− state in CH is calculated to lie 7500 cm−1 above the X 2Π state. No observable quartet transition could be found for CH, while for NH+ a Π4−Σ4− transition should occur in the region of 1000 Å. Other qualitative differences in the observed spectra of the two molecules are discussed. Finally a value of D00(NH+) = 3.4 eV is calculated.

Ab initioMRD-CI study of the electronic spectrum of SiH

“…Because of the XH bonding character of the lo MO, one expects a larger r e value compared with the lowest a 4y,-state. Accordingly the present MRD-CI study predicts an elongation of c. 0.8 a o with respect to the lowest quartet state, while similar increases can be estimated from figure 2 of [39] containing results for the CH radical. This second 4y-state correlates with the 5S u (nsnp a) atomic state of C and Si respectively, for which the excitation energy relative to the X apg state [22] is nearly the same in both systems (that is 4-18 eV in C and 4.13 eV in Si).…”

“…In the case of Sill the MRD-CI calculations place the 2 4E-state 5.55 eV above the ground state, or roughly 1.40 eV below the Si (sSu) limit. In the case of CH only the theoretical study of Liu and Verhaegen [39] is available for this state, according to which its energy is computed to be 0.90 eV greater than that of its dissociation limit; in this connection it should be noted, however, that these authors have expressed the opinion that this state has been somewhat less satisfactorily described than the other low-lying species. All in all it at least seems likely that the energy relationships between the bound 2 4E-molecular state and its dissociation products is quite different in Sill than in CH.…”

Electron-impact dissociation and ionization of NH+: formation of N+and N2+

“…The comparison of total ionization cross sections along an isoelectronic sequence relies on the Thomson's classical scaling law (Thomson 1912), which predicts that the total cross sections scale according to the inverse of the square of the corresponding ionization threshold energy I i (eV). The energy threshold is 10.6eV (Liu and Verhaegen 1970) for CH that is about two times lower than the present 20eV for NH + . Consequently, the total ionization cross section for CH should be about four times larger than the one of NH + and not only two times.…”

“…It can be concluded from the present study that the comparison of the two isoelectronic molecular species is not successful and that the classical formula is inappropriate for molecular ions. Differences among the potential energy diagrams for CH and for NH + which have been pointed out by Liu and Verhaegen (1970) may explain such a discrepancy.…”

Dynamic Electron Correlation: A Fragments-in-Molecules Approach

“…When the nonorthogonality is treated correctly, E is obtained by diagonalizing HFJ defined as 1 2 (17) where S p p is the overlap matrix element between reference configurations P and P' . While eq 17 performs less well than eq 16 in practice, the effective Hamiltonian in eq 17 does yield a better eigenvector since it includes the correlation correction.…”