Comment on the possible origin of an anomalous isotope effect in hydrogen bonds

It is shown that the electrical anharmonicity in H-bonds which was assumed as an explanation of the observed anomalous isotope effect cannot be due only to the dynamical coupling between the high and low energetic modes in an H-bond. This anharmonicity must therefore be intrinsically connected with the large enhancement of the total intensity of the infrared spectrum as was suggested elsewhere.     

Quantum-chemical analysis of vibrational intensity distributions in the S0-S1 absorption spectra and raman excitation profiles of azulene

The vibronic, Franck—Condon and Dushinsky activity of the totally modes of azulene in S₀-S₁ transition is investigated by quantum-chemical methods. It is found that the 900 and 1400 cm⁻¹ mode mixing proposed earlier to explain the relative intensities of Raman fundamentals in the region of the S₁ state is not supported by the calculations. The alternative mechanism arising from vibronic activity of the a₁ modes produces signs of the non-Condon parameters in agreement with those found from fitting the Raman excitation profiles; however, the absolute value of this parameter for the 900 cm⁻¹ mode is almost one order of magnitude too small. Thus, although quantum-chemical calculations favour non-Condon effects over Dushinsky effects in shaping the relative intensities of Raman fundamentals, they do not give a conclusive answer. It is argued that such an answer can be obtained from measurements of the 900 + 1400 cm⁻¹ combination band Raman excitation profiles.     

Conformational pathways of saturated six-membered rings. A static and dynamical density functional study

The conformation of the six-membered ring of pyranosyl sugars has pronounced effects on the physical and chemical properties of carbohydrates. We present a method to determine key features of the potential energy surfaces, such as transition states associated with the inversion pathways of the model compounds cyclohexane, tetrahydropyran, p-dioxane, m-dioxane, s-trioxane, and 2-oxanol. Finally, we make the first determination of the pathways for inversion of penta-O-methyl-alpha-D-glucopyranose and penta-O-methyl-beta-D-glucopyranose. For both anomers, a transition state with five coplanar atoms with appreciable (O)E character was found. The method is based on constrained Car-Parrinello ab initio molecular dynamics, as implemented in the projector augmented-wave (PAW) method. The constraints are derived from the normal modes of six-membered rings and are described in terms of the canonical conformations (1)C(4) chair, (1,4)B boat, and (O)S(2) skew-boat. The PAW derived trajectories are in agreement with previous suggestions in the literature that pseudorotation is an important feature of such conformational interconversions. The dynamic nature as well as the internal coordinate-based constraints provide a method which can reliably accommodate pseudorotation. To determine semiquantitative energies, we recalculate key conformations using standard quantum mechanical calculations while keeping the ring dihedral angles frozen at their values found in the dynamics. In all cases where experimental barriers are known, our results are in excellent agreement.     

Mechanism and dynamics of azobenzene photoisomerization

The excited-state dynamics of trans-azobenzene were investigated by femtosecond time-resolved photoelectron spectroscopy and ab initio molecular dynamics. Two near-degenerate pipi* excited states, S2 and S3,4, were identified in a region hitherto associated with only one excited state. These results help to explain contradictory reports about the photoisomerization mechanism and the wavelength dependence of the quantum yield. A new model for the isomerization mechanism is proposed.     

Synthesis and characterization of N2S3X-Fe models of iron-containing nitrile hydratase

A series of iron complexes based on the pentadentate ligand 4,7-bis(2'-methyl-2'-mercaptopropyl)-1-thia-4,7-diazacyclononane), (bmmp-TASN)(2)(-), have been synthesized and characterized as models of iron-containing nitrile hydratase (NHase). The chloro derivative [(bmmp-TASN)Fe(III)Cl].0.5EtOH (1) contains a labile chloride which facilitates synthesis of related complexes via substitution reactions. Complex 1 is high-spin, g = 4.28. Addition of NEt(4)CN with 1 in CH(2)Cl(2) results in the cyanide ligated complex [(bmmp-TASN)Fe(III)CN] x 0.5EtOH (2), which shows a single intense nu(CN) band at 2083 cm(-)(1) in the IR region. Complex 2 is low-spin, g(1) = 2.31, g(2) = 2.16, and g(3) = 1.96. Under basic conditions complex 1 affords a mu-oxo bridged dimeric Fe(III) complex [(bmmp-TASN)Fe(III)](2)O (3), which shows an intense band at 799 cm(-)(1). Complex 3 was recrystallized from CH(2)Cl(2)/hexane solution in the triclinic space group P1, with a = 10.5486(15) A, b = 13.0612(19) A, c = 8.1852(12) A, alpha = 96.923(2) degrees, beta = 112.729(2) degrees, gamma = 81.048(2) degrees, and Z = 1. Density functional theory (DFT) calculations of the previously communicated iron-nitrosyl complex [(bmmp-TASN)Fe(III)(NO)][BPh(4)] (4) (Inorg. Chem. 2002, 41, 1039-1041) reveal that the HOMO region is dominated by Fe-S bonding. Complexes 1-4 display irreversible or quasi-reversible reductions in the cyclic voltammograms. All of the iron complexes and the zinc derivative, (bmmp-TASN)Zn (5), display an irreversible oxidation. Complex 5 was crystallized in the monoclinic space group P2(1)/n with a = 9.5759(6) A, b = 20.9790(13) A, c = 10.7113(7) A, beta = 91.283(1) degrees, and Z = 4.     

Interpretation of the vibronic fine structure in the fluorescence spectrum of matrix isolated UF6

The emission spectra of UF₆ in an argon matrix are ascribed to a T_1g state subject to a strong Jahn-Teller effect and a weak crystal field and Herzberg-Teller effects. These effects account quantitatively for the observed triplet structure of the vibronic bands without involving different crystal sites.     

Highly effective quenching of the ultrafast radiationless decay of photoexcited pyrimidine bases by covalent modification: photophysics of 5,6-trimethylenecytosine and 5,6-trimethyleneuracil

5,6-Trimethylenecytosine (TMC) and 5,6-trimethyleneuracil (TMU), in which the twist of the C5-C6 bond (or the pyrimidalization of C5) is strongly hindered, do not exhibit the subpicosecond excited-state lifetime characteristic of the naturally occurring pyrimidine bases. This result demonstrates the important role the out-of-plane deformation of the six-membered ring plays in the ultrafast (subpicosecond) internal conversion of photoexcited nucleobases. The dramatically shorter fluorescence lifetime of TMU ( approximately 30 ps) relative to TMC ( approximately 1.2 ns), in aqueous solution at room temperature, is attributed to the presence in TMU of an efficient, secondary nonradiative decay channel of S(1)(pipi*) involving a low-lying (1)npi* state.     

Structural implications of ring shape, dimension, and metal atom insertion in nanosized cyclic oligothiophenes: joint Raman and density functional theory study

This paper reports on a combined spectroscopic and density functional theory study on a series of cyclo(diacetylenes-oligothiophenes) with different shapes and dimensions. The case of a macrocycle fragmented by the insertion of Pt metallic atoms is illustrated as well. The analysis covers the neutral and oxidized samples. A full assignment of the spectra has been proposed, supported by BP86/cc-pVDZ theoretical calculations. Ring shape and molecular dimension have been related with the C[Double Bond]CC-C structural parameters and Raman wave numbers. Platinum atom insertion within the acetylene spacer seems to soften ring strain owing to the rupture of the strained circular shape. Oxidation causes an overall downshift of the Raman wave numbers likely due to the skeletal relaxation of CC conjugated path, while the ring shape of each macrocycle is conserved upon oxidation.     

The effect of dimerization on the resonance raman excitation profiles of metalloporphyrins

Model calculations are reported showing the effect of dimerization on the absorption spectra and resonance Raman excitation profiles of metalloporhyrins. This effect, which qualitatively resembles that of metal atom substitution or solvent change, is mainly due to mixing of the monomer Q and B states.     

Strong coupling in dimers

Strong coupling in dimers with inclusion of a dependence of the resonance interaction on nuclear coordinates is investigated. Some spectral consequences are discussed.