Abstract:AM^ calculations for various intermolecular and internally hydrogen-bonding species are compared with experiment. The calculations generally provide a useful, but not perfect, means for modeling H-bonds. The average stabilization energy for 33 gas-phase hydrations of ions was 1.4, whereas that for amination of 10 ions was 4.2 kcal/mol, too weak when compared to experimental results. H-bonding distances have a tendency to be long. The tendency of AM^ to predict multicentered H-bonds is discussed in the context … Show more
“…Initial calculations on hydrated merocyanines which contain hydrogen bonds between the oxygen of the merocyanine and one or two water molecules at the AM1/COSMO level in water (ε = 78.4) gave only weakly bonded complexes with O16···H 2 O distances of around 2.10 Å versus distances of approximately 1.57 and 1.75 Å found in the hydrated crystal structure. (This overestimation of the intermolecular hydrogen-bonding distances is a weakness of the AM1 method which extends also to the intramolecular hydrogen-bonding distances found between the phenolic hydrogen and either the carbonyl oxygen in salicylaldehyde or the imino nitrogen in salicylaldoxime, with calculated values of 2.02 and 2.11 Å, respectively, versus experimental values of 1.70 and 1.83 Å, respectively 54,55 .) 4 Calculated ground and first excited state energies (kcal mol -1 ) for merocyanine 1 in acetone (2), N , N -dimethylformamide (3), dimethyl sulfoxide (4), and ethylene carbonate (5) relative to the values calculated in pyridine (1).…”
The spectroscopic properties of solvent soluble derivatives of Brooker's simple merocyanine 4-[(1-methyl-4(1H)-pyridinylidene)ethylidene]-2,5-cyclohexadien-1-one, which can in principle exist in two distinct canonical forms, have been assessed both experimentally and theoretically using molecular orbital methods. 1 H and 13 C NMR evidence in a range of solvents suggests that the merocyanine exists as a resonance hybrid which is weighted toward the zwitterion even in solvents of low dielectric constants. In protic solvents, the large hypsochromic shift observed for the merocyanine in the visible region arises from both a dielectric effect and a hydrogen bonding effect. Theoretically, the PM3/COSMO method gives a reasonable account of the structure and spectroscopic shifts of the merocyanine in aprotic solvents. The large shifts observed arise because solvents with large dielectric constants have a much greater stabilizing effect on the more polar ground state of the merocyanine than they do on the first excited state. While the same method predicts stable hydrogen-bonded structures for a dihydrate and hexahydrate, it is unable to reproduce the known hypsochromic shift for these solvated species. In contrast, a version of the CNDO/S method does predict the correct trends on hydration though the magnitude of the effect is less than that found experimentally.Recent studies have strongly suggested that one of the main factors which contribute to the large solvatochromic shift is a
“…Initial calculations on hydrated merocyanines which contain hydrogen bonds between the oxygen of the merocyanine and one or two water molecules at the AM1/COSMO level in water (ε = 78.4) gave only weakly bonded complexes with O16···H 2 O distances of around 2.10 Å versus distances of approximately 1.57 and 1.75 Å found in the hydrated crystal structure. (This overestimation of the intermolecular hydrogen-bonding distances is a weakness of the AM1 method which extends also to the intramolecular hydrogen-bonding distances found between the phenolic hydrogen and either the carbonyl oxygen in salicylaldehyde or the imino nitrogen in salicylaldoxime, with calculated values of 2.02 and 2.11 Å, respectively, versus experimental values of 1.70 and 1.83 Å, respectively 54,55 .) 4 Calculated ground and first excited state energies (kcal mol -1 ) for merocyanine 1 in acetone (2), N , N -dimethylformamide (3), dimethyl sulfoxide (4), and ethylene carbonate (5) relative to the values calculated in pyridine (1).…”
The spectroscopic properties of solvent soluble derivatives of Brooker's simple merocyanine 4-[(1-methyl-4(1H)-pyridinylidene)ethylidene]-2,5-cyclohexadien-1-one, which can in principle exist in two distinct canonical forms, have been assessed both experimentally and theoretically using molecular orbital methods. 1 H and 13 C NMR evidence in a range of solvents suggests that the merocyanine exists as a resonance hybrid which is weighted toward the zwitterion even in solvents of low dielectric constants. In protic solvents, the large hypsochromic shift observed for the merocyanine in the visible region arises from both a dielectric effect and a hydrogen bonding effect. Theoretically, the PM3/COSMO method gives a reasonable account of the structure and spectroscopic shifts of the merocyanine in aprotic solvents. The large shifts observed arise because solvents with large dielectric constants have a much greater stabilizing effect on the more polar ground state of the merocyanine than they do on the first excited state. While the same method predicts stable hydrogen-bonded structures for a dihydrate and hexahydrate, it is unable to reproduce the known hypsochromic shift for these solvated species. In contrast, a version of the CNDO/S method does predict the correct trends on hydration though the magnitude of the effect is less than that found experimentally.Recent studies have strongly suggested that one of the main factors which contribute to the large solvatochromic shift is a
“…The applicability of the AM 1 method to H-bonding studies has been reviewed elsewhere. 18 The AMI method has previously been used with success in several hydrogen-bonding studies19 including modeling of the H-bonding between molecules of various nitroanilines in the crystalline state. 2 We explicitly considered all internal coordinates for the semiempirical optimizations.…”
“…ß • was equalized to ß -• Geometry optimization was obtained by means of AMI, using MOP AC (A General Molecular Orbital Package, QCPE 455, version 4.10). The method provides a reliable evaluation of hydrogen-bonded systems (Dewar et al, 1985; ; Ventura et al, 1989;Dannenberg & Evlet, 1992).…”
The electronic properties of 2,4,5-trihydroxyphenylalanine quinone (TPQ), the cofactor of bovine serum amine oxidase [Janes, S. M., Mu, D., Wemmer, D., Smith, A. J., Kaur, S., Maltby, D., Burlingame, A. L., & Klinman, J. P. (1990) Science 248, 981-987], and some adducts with hydrazines were investigated by means of low-molecular-weight models and semiempirical molecular orbital calculation methods. The enzyme visible band was assigned to the first pi-->pi* transition of the cofactor in p-quinonic form, with the C-4 hydroxyl ionized and hydrogen bonded either to a water molecule or to a basic protein residue. The spectra of the protein adducts with some substituted hydrazines were well accounted for by assuming the inhibitor bound to the C-5 carbonyl, usually in azo form. The adduct with the unsubstituted hydrazine was instead assigned an o-quinone hydrazone form, stabilized by an internal hydrogen bond between the amino group and the ortho carbonyl oxygen, a larger electron delocalization, and formation of a hydrogen bond at the C-6 ionized hydroxyl. On the basis of these assignments, the reaction of the protein with benzylhydrazine [Morpurgo, L., Agostinelli, E., Muccigrosso, J., Martini, F., Mondovi, B., & Avigliano, L. (1989) Biochem. J. 260, 19-25] was rewritten. All examined electronic transitions, though highly sensitive to cofactor ionization and hydrogen bonding, could be accounted for without introducing perturbations due to copper. This confirms that copper is not within bonding distance of the oxidized cofactor.
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