Marco Nonella

Photodissociation dynamics of methylnitrite (CH3O–NO) in the 300–400 nm range: An abinitio quantum mechanical study

We report the results of a two‐dimensional, quantal study of the photodissociation of CH3O–NO within the first continuum (S0→S1, 300–400 nm) taking into account only the O–N and the N=O separations. The S1 potential energy surface is taken from recent ab initio calculations. The calculated absorption spectrum consists of two band progressions of narrow resonance lines with widths of ∼0.3 and ∼5 meV, respectively. These resonances can be associated with excitation of the O–N bond (m=0,1) and excitation of the N=O chromophore (n*=0,1,2,...). The intensities of the m=1 band are negligibly small compared to those of the m=0 band. The decay mechanism in the two cases is different: The m=0 resonances decay primarily via vibrational predissociation, i.e., a nonadiabatic transition from n* to n*−1, and yield NO products with a preferential population of the (n*−1) level. The m=1 resonances decay mainly via tunneling through a potential barrier yielding preferentially NO products in state n*. Several of the theore...

Photodissociation of CH3ONO in the first absorption band: A three‐dimensional classical trajectory study

The photodissociation of c i s‐CH3 ONO following excitation into the first absorption band near 350 nm is investigated by means of classical trajectories and an a b i n i t i opotential energy surface. The calculations include the O–N coordinate, the N=O coordinate, and the ONO bending angle as variables whilst the internal degrees of freedom of the CH3 O moiety are kept fixed. The calculated lifetimes range from 120 to 410 femtoseconds for excitation of the n*=4 to n*=0 vibrational states of the terminal NO group in the intermediate complex. They agree well with the lifetimes estimated from the anisotropy parameter β. The ONO bending degree of freedom has only a small effect on the lifetime of the complex. The final vibrational state (n) distribution of the NO fragment exhibits a systematic energy dependence which manifests itself in a propensity for the excitation of level n=n*−1 that is in excellent agreement with the measurement. Two‐dimensional calculations for a fixed ONO bending angle cannot satisfactorily reproduce these experimental findings. The rotational state distributions are highly inverted with maxima around j∼30–35 depending slightly on the initial state (n*) and the final state (n) of NO. The overall agreement with the measured distributions is satisfactory. The results of this study emphasize the importance of the bending degree of freedom in the dissociation of CH3 ONO and by revealing the interplay of the three active vibrational modes they provide a detailed picture of the predissociation mechanism in a polyatomic molecule.

Photodissociation of methylnitrite: an mc scf calculation of the S1 potential surface

Abstract Using the MC SCF method, the S 1 potential-energy surface of cis-methylnitrite has been calculated to be dissociative, giving rise to the process CH 3 ONO→ CH 3 O + NO. Features of this surface are discussed in the light of recent experimental findings.

Diffuse vibrational structures in photoabsorption spectra: A comparison of CH3ONO and CH3SNO using two‐dimensional ab initio potential energy surfaces

We investigated the photodissociation of methyl nitrite (CH3 ONO) and methyl thionitrite (CH3 SNO) within the first absorption band (S1 ←S0 ). The calculations were based on a two‐dimensional model including the O–NO/S–NO and N=O bond distances as active coordinates. The S1 ‐potential energy surfaces were calculated with quantum chemical methods and the dynamical calculations were performed exactly within the time‐independent approach. The main emphasis is on the origin of diffuse vibrational structure in the photoabsorption spectrum of both molecules. A low potential barrier of 0.086 eV along the O–NO dissociation coordinate in CH3 ONO prevents immediate dissociation and leads to an initial state dependent lifetime for the excited complex of 100–250 fs corresponding to 3–8 NO vibrational periods. CH3 ONO decays nonadiabatically via vibrational predissociation. The absorption spectrum of CH3 ONO is dominated by narrow Feshbach‐like scattering resonances which can be characterized by two quantum numbers, m...

Photodissociation of ClNO in the S1 state: A quantum‐mechanical ab initio study

We investigated the photodissociation of ClNO via the S1 electronic state using a three‐dimensional (3D) ab initio potential‐energy surface (PES). The dissociation is found to be fast and direct. In the Franck–Condon (FC) region the slope of the potential along the dissociation path is relatively small giving rise to narrow partial absorption peaks. The total absorption spectrum therefore exhibits a broad vibrational structure which is in perfect agreement with recent measurements. The vibrational excitation of the NO fragment is small and can be qualitatively described within the adiabatic approximation. It is found to be very sensitive to the vibrational FC factor in the transition region. The rotational state distribution of NO is highly inverted with a peak around j=30. It is readily explained by the rotational reflection principle. The experimental results are satisfactorily reproduced by our calculations which underlines the overall quality of the calculated PES. Minor adjustments are necessary, how...

Investigating alanine-silica interaction by means of first-principles molecular-dynamics simulations.

In our attempts to achieve a detailed understanding of protein-silica interactions at an atomic level we have, as a first step, simulated a small system consisting of one alanine in different protonation states, and a hydroxylated silica surface, using a first-principles molecular-dynamics technique. The simulations are carried out in vacuo as well as in the presence of water molecules. In the case of a negatively charged surface and an alanine cation, an indirect proton transfer from the alanine carboxylic group to the surface takes place. The transfer involves several water molecules revealing an alanine in its zwitterionic state interacting with the neutral surface through indirect hydrogen bonds mediated by water molecules. During the simulation of the zwitterionic state the ammonium group eventually establishes a direct -N-H...O-Si interaction, suggesting that the surface-amino group interaction is stronger than the interaction between the surface and the carboxylic group. In vacuum simulations, the amino group exhibits clearly stronger interactions with the surface than the carboxylic group.

UV-, VIS- and IR-light-induced isomerization of HSNO in a low-temperature matrix

Abstract An infrared spectroscopic investigation has been performed on the trans and cis isomers of thionitrous acid (HSNO) and their D- and 15 NO-isotopic modifications in argon matrices at 12 K. The substances were prepared photolytically from thionylimide (HNSO) isotopes in the matrix. With UV (250 nm), VIS (585 nm), and IR irradiation the cis → trans or the trans → cis isomerization of HSNO was induced, allowing an unequivocal distinction between the closely resembling IR spectra of the trans and cis isomers. Complete sets of fundamental frequencies of both rotamers were obtained and assigned by normal coordinate analysis using the transferable valence force field (TVFF) approach. Parallel to this analysis ab initio calculations on the SCF- and CI-levels were performed to predict energy, geometry, and barrier of internal rotation for the two HSNO rotamers.

An unscaled quantum mechanical harmonic force field for p-benzoquinone

Abstract Structure and harmonic vibrational frequencies of p-benzoquinone have been calculated using quantum chemical ab initio and density functional methods. Our calculations show that a satisfactory description of fundamentals and normal mode compositions is achieved upon consideration of correlation effects by means of Moller-Plesset perturbation expansion (MP2) or by density functional theory (DFT). Furthermore, for correct prediction of CO bondlength and force constant, basis sets augmented by polarization functions are required. Applying such basis sets, MP2 and DFT calculations both give results which are generally in reasonable agreement with experimental data. The quantitatively better agreement, however, is achieved with the computationally less demanding DFT method. This method particularly allows very precise prediction of the experimentally important absorptions in the frequency region between 1500 and 1800 cm−1 and of the isotopic shifts of these vibrations due to 13C or 18O substitution.

The triplet state decay (T1(nπ*) → S0) of benzaldehydes in the dilute gas phase

Abstract The triplet T 1 (nπ * ) decay of benzaldehyde (B) and its isotopomers and were investigated in the dilute vapour phase (≤0.5 Torr) at room temperature. Following excitation the quantum yields of the phosphorescence and photodecomposition, and the rate constants of the phosphorescence and the radiationless T 1 → S o process were determined. Based on these results and in conjunction with theoretical calculations of T 1 → S o rates and previous data obtained on propynal, the decay mechanism of benzaldehyde was analyzed. It is shown that the important accepting modes of the non-radiative T 1 → S a decay are the wagging and the CO stretching modes. In spite of the close vicinity of the T 2 (ππ * ) and the T 1 (nπ * ) states, the non-adiabatic coupling (communication between ring and carbonyl vibrations is not sufficient to influence the relaxed T 1 (nπ * ) decay significantly.

A quantum chemical investigation of structures, vibrational spectra and electron affinities of the radicals of quinone model compounds

In the present study, the first quantum chemical calculations of structures and vibrational spectra of radicals of 1,4-naphthoquinone and 2-methoxy-1,4-benzoquinone that account for electron correlation are presented. In the case of 1,4-naphthoquinone a good agreement between calculated vibrational frequencies and 18O-shifts of the 1,4-naphthoquinone radical (protonated radical anion) with experimental data of a species detected after irradiation of vitamin K1 in solution is found. Our calculations, thus, support the previous assignment. In the case of 2-methoxy-1,4-benzoquinone we have localized the stable conformations with respect to the orientation of the methoxy group and we have determine the harmonic force fields for these structures. Our calculation suggest that, while the frequencies of the two conformers are similar, the 18O-shift of the most intensive absorptions in the spectral region between 1400 and 1700 cm−1 of the two conformers differ significantly and might serve as a tool to distinguish between the two conformers. The applied DFT method is shown to predict electron affinities which are systematically underestimated by 10%.