Geometries of the excited electronic states of HCN

Abstract

Ab initio quantum mechanical electronic structure calculations have been carried out for the ground state and 12 low-lying (< 10 eV) excited states of HCN. A contract,ed gaussian basis set of essentially double zeta quality was employed. A new theoretical approach, which sho1.,1.ld be widely applicable, was applied to the excited electronic states. First one selects a physically meaningful set of orbitals, which, hopefully, will be about equally suitable for all the electronic states of interest. After selecting a single configuration to described each electronic state, configuration interaction, is performed including all configurations differing by one orbital from any of the selected reference configurations. The method appears to be one of the simplest capable of treating several states of the same symmetry. (1) where the triplet coupling of the unfilled 5cr and 2TI orbitals has been indicated.If one has obtained the correct SCF wave function for the 3 rr state then the following single excitations will have vanishing matrix elements with ~SCF:(lcr ncr 1 r+) 2cr 2 3cr 2 2 4 4cr ln ( 5cr 2TI 3rr)1cr 2 (2cr ncr lE+) 3cr 2 4cr 2 1TI 4 (5cr 2n 3rr)1cr 2 2cr 2 ( 3cr ncr lE+) 4cr 2 1TI 4 (5cr 2TI 3rr) (4) 1cr 2 2cr 2 3<i2 (4cr ncr lE+) ln 4 (5cr 2n 3rr)1cr 2 2cr 2 3<12 4cr 2 3 1 + (ln m7T E ) ( scr 2n 3 rr ) (6) I 1cr 2 2cr 2 3cr 2 4cr 2 1n 4 (ncr 2n 3 rr)-3- LBL-1983 where ncr represent,s. 6cr, 7cr, 8cr, 9cr, .•. , and mn represents 37f, 4n, 5n, 6n, Note that the 3 n coupling of the last two orbitals is .retained in all the (8) above configuratiC?ns. By analogy with the closed-shell case, then, the iterative natural orbital procedure applied to a wave function consisting of all configurations ( 1) Therefore, scree double excitations will be included with respect to any one particular SCF configuration.The procedure described would appear to be a relatively simple but presumably reliable method for the prediction of the geometries of the excited states of molecuJes. In the present research we have applied this method to the valence excited states of HCN.

Abinitio calculation of the vibrational structure in the electronic spectra of HCN and DCN between 1700 and 2000 Å

“…It is also seen from Fig. 1 that occupation of the 7ar species (or the corresponding MO in acetylene), as found in many instances before (5,7,8,16,17), always results in a bent upper state, in complete agreement with qualitative MO theory. On the other hand, Fig.…”

“…by roughly 0.15 eV (1300-1400 cm-I). The earlier calculated To result of Schwenzer et al (5) is 0.7 eV below the present CI value (Table 4).…”

“…Theoretical studies (5,6) dealing with four HCN states of A' and three of A" symmetry, on the other hand, predict that the second 'A" state of this system should prefer a linear or nearly line& geometry. Furthermore this finding is in complete agreement with the qualitative predictions of molecular orbital theory (7,8), but of course stands in strong contrast with the value of 114.5" derived by Herzberg and Innes (4) for the equilibrium internuclear angle of the P state.…”

Electron collision with the HCN and HNC molecules using theR-matrix method

“…There have been a number of previous studies on the electronically excited states of HCN (Schwenzer et al 1974, Nayak et al 2005. The most comprehensive appears to be the recent study by Nayak et al (2005) who aimed at characterizing the excited triplet states of HCN.…”

THEORETICAL STUDIES OF $\tilde{A}{}^1 A^{\prime\prime}\to\tilde{X}{}^1 A^\prime$ RESONANCE EMISSION SPECTRA OF HCN/DCN USING SINGLE LANCZOS PROPAGATION METHOD

“…This electronic state is known from absorption spectroscopic studies to have a 1 A character, 11 an assignment that has also been confirmed by several ab initio studies. [12][13][14][15][16] Although the perpendicular transition between theà andX-states is forbidden at linearity, the bent excited state is ideal for probing highly excited bending levels in the ground electronic state. This situation is similar to the much-studied and isoelectronic acetylene.…”