Melanie Schnell

Ab initio MRD-CI study of excited states of chloromethanol ClCH2OH and photofragmentation along C–O and C–Cl cleavage

Abstract Large-scale ab initio multi-reference configuration interaction calculations are carried out for ground and excited states of chloromethanol ClCH 2 OH to investigate photofragmentation processes relevant to atmospheric chemistry. Five low-lying excited states ( 1 3 A ″ , 1 3 A ′ , 1 1 A ″ , 2 1 A ′ and 2 3 A ′ ) in the energy range between 6.8 and 8.5 eV are found to be highly repulsive for C–Cl elongation leading to CH 2 OH ( X 2 A ′ ) and Cl ( 2 P ) . Photodissociation along the C–O bond leading to CH 2 Cl ( X 2 B 2 ) and OH ( X 2 Π) has to overcome a small barrier of about 0.3 eV because the low-lying excited states 1 1 A ″ , 1 3 A ′ and 1 3 A ″ become repulsive only after the C–O bond is elongated by about 0.2 A.

Ab initio CCSD(T) and MRD-CI study of excited states and the electronic spectrum of linear C5+

Abstract Large-scale ab initio coupled cluster and multi-reference configuration interaction calculations (MRD-CI) are carried out to determine the equilibrium geometry and the vertical electronic spectrum of linear C5+. Contrary to prior theoretical estimates we find three low-lying states within an energy range of 0.3 eV: 2 Σ u + , 2 Σ g + and 2 Π g and a symmetric arrangement of nuclei. Transitions from 2 Σ u + to these low-lying states are dipole-allowed; sizeable oscillator strengths are computed for the 2 Π + g ← X 2 Σ u + transition at 2.62 eV and the 2 Σ g ← X 2 Σ u + transition at 3.36 eV and should give a guide to spectroscopic identification of linear C5+.

The electronic spectrum of chloromethanol ClCH2OH in comparison with isomeric methylhypochlorite CH3OCl

Large-scale multi-reference configuration interaction calculations are employed to differentiate in the electronic spectrum between chloromethanol ClCH2OH and its isomer methylhypochlorite CH3OCl by certain fingerprints. The first dipole-allowed transitions of the CH3OCl spectrum are obtained at 4.13 eV(11A″ ← X1A1) and 5.50eV (21 A′ ← X1A′). in reasonable agreement with prior calculations (4.24eV and 5.58eV) and in accordance with experimental measurements of 4.02 eV and 5.27 eV. For the yet to be observed spectrum of ClCH2OH the corresponding transitions are predicted at higher energies of 7.32eV for 11A″ ← X1A′ and 7.95eV for 21A′ ← X1A′ and 7.98eV for 21A″ ← X1A′.

Ab initio MRD-CI investigation of the electronic spectrum of 1-chloromethyl hypochlorite ClCH2OCl in comparison with isomeric dichloromethanol Cl2CHOH

The electronic absorption spectrum of 1-chloromethyl hypochlorite ClCH2OCl is computed with multi-reference configuration interaction (MRD-CI) methods and compared to isomeric dichloromethanol. While the first dipole-allowed transitions of dichloromethanol Cl2CHOH are obtained above 7 eV, the corresponding transitions of ClCH2OCl are obtained at much lower energy, namely 3.66eV (11A″ ←[Xtilde]1A′), 4.96eV (21A′←[Xtilde]1A′), 6.04eV (21A″←[Xtilde]1A′) and 6.67eV (31A′ ← [Xtilde]1 A′). A further strong transition is computed at 7.82eV (41A′ ← [Xtilde]1A′) with a relatively large oscillator strength of ƒ = 0.4.

Ab initio and RRKM studies of decomposition and interconversion pathways of ClCH2OH and CH3OCl isomeric species

The photofragmentation and photointerconversion pathways of the CH3OCl and ClCH2OH species are studied using quantum mechanical and RRKM theories. Ten possible channels are investigated in total and the relative importance of the various pathways in stratospheric photochemistry is discussed.

The electronic spectrum of trichloromethanol Cl3COH: an ab initio study

Abstract The electronic spectrum of trichloromethanol Cl3COH is predicted by multi-reference configuration interaction calculations up to 8.5 eV. The first dipole-allowed transitions are calculated at 6.8 eV (1 1 A ″← X 1 A ′ ) and 7.1 eV (2 1 A ″← X 1 A ′ ) followed by two transitions at 7.2 eV (2 1 A ′ ← X 1 A ′ ) and 7.7 eV (3 1 A ′ ← X 1 A ′ ) . The largest oscillator strength (f=0.145) is obtained for the σ→σ*-type excitation 3 1 A ″← X 1 A ′ computed around 8 eV. The corresponding triplet states are also given.

Ab initio investigation of the photofragmentation of bromomethanol

Abstract Ab initio multi-reference configuration interaction calculations are carried out for ground and excited states of bromomethanol BrCH 2 OH to investigate photofragmentation processes relevant to atmospheric chemistry. Five low-lying excited states with vertical excitation energies between 5.8 and 7.4 eV ( 1 1 A ″, 1 3 A ″, 2 1 A ′ , 1 3 A ′ , 2 3 A ′ ) are found to be highly repulsive for C–Br elongation leading to CH 2 OH(X 2 A ′ ) and Br (X 2 P). Photodissociation along the C–O bond leading to BrCH 2 (X 2 B 2 ) and OH (X 2 Π) has to overcome a barrier of about 0.6–0.7 eV because the low-lying excited states 1 1 A ″, 1 3 A ′ and 1 3 A ″ become repulsive only after the C–O bond is elongated by about 0.2 A.

Photofragmentation of Dichloromethanol Cl2CHOH Along C–O and C–Cl Cleavages: A Theoretical Study¶

Multireference configuration interaction calculations are carried out for ground and excited states of dichloromethanol, Cl2CHOH, to investigate two important photofragmentation processes relevant to atmospheric chemistry. Five low‐lying excited states (11A″, 21A′, 13A″, 23A″ and 13A′) in the energy range between 6.4 and 7.5 eV are found to be highly repulsive for C–Cl elongation, leading to ClCHOH (X2A) and Cl (X2P). Photodissociation along the C–O bond resulting in CHCl2 (X2A′) and OH (X2Π) has to overcome a barrier of about 0.5 eV because the low‐lying excited states 11A″, 13A′ and 13A″ become repulsive only after the C–O bond is elongated by about 0.3 Å.