Calculation of electronic and vibrational states of chlorine isomers

Гладков, Л. Л.; Kuz'mitskii, V. A.

doi:10.1007/bf02606897

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“…reproduced correctly in the calculations by the CNDO/S method [11][12][13][14][15] as well as in the earlier calculations by the PPP method [16][17][18]. One of the reasons for this disagreement is the absence of x-ray structural data on H 2 Ch.…”

mentioning

confidence: 68%

“…The CNDO/S data and the data of earlier works [11][12][13][14][15] point to the fact that, in all cases, the Soret band of H 2 Ch is mainly due to two ππ * -transitions: G → B y and G → B x . Unlike this, the Soret band of the H 2 P molecule calculated by the CNDO/S and ab initio methods can be due to two or three ππ * -transitions (see [9]).…”

Section: Rhfmentioning

confidence: 87%

“…It should be noted that such restrictions were imposed on the symmetry of the molecule in the RHF ab initio calculations of its ground state [23; 29]; the geometry obtained was then used in the calculations of excited states of H 2 Ch [23,26]. The same restrictions were imposed on the symmetry of the H 2 Ch molecule when its geometry was constructed by the data of x-ray structural analysis [19] and excited states of H 2 Ch were calculated by the CNDO/S semiempirical method [11][12][13][14][15]. It is seen from Fig.…”

Section: Rhfmentioning

confidence: 99%

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Dependence of the Electronic Spectra of the Chlorin Molecule on the Degree of Alternation of the Macrocycle Bond Lengths

Kuz'mitskii

2005

J Appl Spectrosc

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539.19The geometric structure of the chlorin molecule (H 2 Ch) has been calculated by the restricted and unrestricted Hartree-Fock (RHF and UHF) methods with an AM1 Hamiltonian. Transformations of this molecule into excited electronic states have been calculated by the CNDO/S method. The RHF method without symmetry restrictions gives a plane structure for the chlorin macrocycle with an alternation of the lengths of the bonds along the 18-member azacyclopolyene and a C 1h symmetry for the molecule as a whole. The level of the first excited state Q 1 of this structure is substantially shifted (δE Q 1 . +4000 cm -1 ) relative to the Q x level of porphin (H 2 P) toward shorter waves, which is in contradiction with the experimental data, according to which this shift is long-wave and is equal to δE Q x = -400 to -550 cm -1 . The optimization of the geometry of H 2 Ch by the UHF method has shown that it has a structure with an 18-member azacyclopolyene with bonds of equal lengths and a D 2h symmetry. For this geometry of H 2 Ch the calculated shift of the Q x level, equal to δE Q x = -70 cm -1 , is bathochromic and the position of the Q y level is practically exactly coincident with the experimental one. For the geometry calculated by the RHF method with restrictions on the D 2h symmetry of the 18-member azacyclopolyene δE Q x = +180 cm -1 , and for the geometry calculated with restrictions on the highest C 2v symmetry of the H 2 Ch molecule δE Q x = +620 cm -1 . The latter result shows that the "natural" requirement for the C 2v symmetry of the H 2 Ch macrocycle, which is frequently used in various calculations, is inadequate to achieve a quantitative agreement between the calculation and experimental data and, in this case, the lengths of the bonds along the 18-member azacyclopolyene are not equal. Keywords: chlorin, quantum-chemical calculation, restricted and unrestricted Hartree-Fock methods, AM1 CNDO/S methods.Introduction. It has been established in the calculations of monobenzoporphyrin molecules (H 2 MBP) carried out in [1] that their geometric and chemical structures significantly influence the energy of the Q and B states, the transitions to which determine the positions of the absorption bands in the visible and near-UV regions of the spectrum. The geometric structure of the molecules considered was optimized by the restricted and unrestricted HartreeFock (RHF and UHF) methods with a semiempirical AM1 Hamiltonian [2]. The results obtained have an important peculiarity. The RHF method without symmetry restrictions gives a structure of H 2 MBP with an alternation of the lengths of the bonds along the 18-member azacyclopolyene and large (,0.5 eV) shifts of its Q levels relative to the Q levels of porphin (H 2 P) toward shorter waves. In contrast to this, the UHF method gives an H 2 MBP structure with equal lengths of the bonds along the 18-member azacyclopolyene and long-wave shifts of the Q levels that are consistent in value with the experimental shifts [3].It should be noted that the data of calculations o...

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