Veronica Vaida

Perspective: Water cluster mediated atmospheric chemistry

The importance of water in atmospheric and environmental chemistry initiated recent studies with results documenting catalysis, suppression and anti-catalysis of thermal and photochemical reactions due to hydrogen bonding of reagents with water. Water, even one water molecule in binary complexes, has been shown by quantum chemistry to stabilize the transition state and lower its energy. However, new results underscore the need to evaluate the relative competing rates between reaction and dissipation to elucidate the role of water in chemistry. Water clusters have been used successfully as models for reactions in gas-phase, in aqueous condensed phases and at aqueous surfaces. Opportunities for experimental and theoretical chemical physics to make fundamental new discoveries abound. Work in this field is timely given the importance of water in atmospheric and environmental chemistry.

Direct absorption spectroscopy of jet‐cooled polyenes. II. The 1 1B+u←1 1A−g transitions of butadienes and hexatrienes

In the present paper, we report the direct absorption spectra of the 1 1B+u←1 1A−g transitions of gas phase butadiene, deuterated and methylated butadienes, and the cis and trans isomers of hexatriene cooled to low rotational and vibrational temperatures in supersonic molecular jets. These jet absorption spectra allow the more accurate determinations of Franck–Condon factors, upper state vibrational intervals and vibronic band homogeneous widths. We discuss the experimental constraints that the measurements reported here and in the previous paper of this series impose on theoretical models of the equilibrium structures and relaxation dynamics of the 1 1B+u excited states of the small linear polyenes.

Dissociation of NH3 to NH2+H

Potential energy, dipole moment, and electronic transition moment surfaces for the lowest dissociative pathways of the singlet X and A states of NH3 yielding NH2 (X 2B1,A 2A1) +H(2S) products have been calculated using complete active space MCSCF ab initio wave functions. The A state dissociation proceeds via a minimum barrier at the following planar geometry: αHNH =113°, rNH =1.042 A (in the NH2 fragment), and RNH =1.323 A (in the dissociation coordinate). The barrier height is calculated to be 3226 cm−1 with an expected accuracy of about 300 cm−1. The barrier height increases with increasing out‐of‐plane angle. Close to the barrier there are strong variations of the shapes of the dipole moment and transition moment surfaces. The minimum energy path through the X–A conical intersection follows planar geometries. Along this pathway the angle αHNH decreases, but the distance rNH in the NH2 fragment hardly changes. The crossing distance RcNH of the X and A states in planar structures depends strong...

Theoretical A 1A‘2–X 1A1 absorption and emission spectrum of ammonia

Potential energy, electric dipole moment, and electronic transition moment surfaces have been calculated for the A and X states of NH3 from CASSCF and CEPA electronic wave functions. Anharmonic vibrational term values, Franck–Condon factors, and A–X radiative transition probabilities for the symmetric stretching and bending modes of NH3 and ND3 have been evaluated. The theoretical absorption spectra at room and low temperatures agree well with experimental data. The symmetric stretching mode in the A state has only small intensities in the A–X absorption spectrum. Emission rates from various initial vibronic levels of the A state are given. The ab initio electric dipole moment surfaces for the ground state of NH3 have been used to compute transition moments, which are in good agreement with experimental data.

The photoreactivity of chlorine dioxide

Determining the detailed photoreactivity of radicals that are of importance in atmospheric processes requires information from both laboratory and field measurements and theoretical calculations. Laboratory experiments and quantum calculations have been used to develop a comprehensive understanding of the photoreactivity of chlorine dioxide (OCIO). The photoreactivity is strongly dependent on the medium (gas phase, liquid solution, or cryogenic matrix). These data reveal details of the complex chemistry of OCIO. The potential role of this radical in stratospheric ozone depletion is discussed in accord with these laboratory measurements.

Hydrated Complexes: Relevance to Atmospheric Chemistry and Climate

Intermolecular interactions in molecular clusters can significantly alter the optical properties and reaction dynamics of the constituent monomers. As a result, both atmospheric chemistry and climate may be affected. We review here the spectroscopy and photochemistry of molecular hydrates and point to their possible relevance to the Earths atmosphere. Data needed to model the atmospheric effects of hydrates includes calculation of both their abundance and their absorption cross-section. We discuss these issues and examine the potential spectroscopic effects of hydrates in the atmosphere, as well as the fundamental work needed to quantify better the magnitude of these effects.

Photoisomerization of OCIO: a possible mechanism for polar ozone depletion

CONNECTIONS between polar ozone depletion and halocarbon chemistry have been established by a number of studies1–10. Recent attempts to account quantitatively for the observed rate of ozone decline in Antarctica in terms of known photochemical processes have not been entirely successful, and it seems that further chemical ozone-depleting mechanisms may be needed, particularly if the transport of ozone into the polar regions competes with chemical losses. Spectroscopic and photochemical data indicate that photolysis of OCIO may provide a further ozone loss mechanism that has not previously been considered. Here we report laboratory studies of OCIO spectroscopy and photoproducts which suggest that atomic Cl and O2 are formed to some extent in the photodis-sociation process. This evidence points towards possible photo-isomerization to the unstable species ClOO, (or at least to a similar metastable intermediate) probably by way of the 2B2 excited state of OClO, thus reinforcing the idea that photolysis of OC1O may contribute to polar ozone depletion.

Ultraviolet absorption spectroscopy of dissociating molecules: Effects of cluster formation on the photodissociation of CH3I

The ultraviolet absorption spectra of jet‐cooled CH3I, (CH3I)2, and (CH3I)n, n>2 are measured in the valence A state and the Rydberg B state. A significant blue shift of the valence state is observed upon cluster formation. The magnitude of this shift yields a dimer bond strength of at least 500 cm−1 which implies extensive dimer formation at room temperature and moderate pressures. The methyl iodide B‐state spectrum consists of a single progression in the ν2 mode. Dimerization retards the predissociation and therefore increases the excited state lifetime; consequently many new modes are observed which in the monomer are coupled to the dissociation. The implications of this result to the photodissociation dynamics are discussed. The Rydberg spectra of (CH3I)n, n>2 show asymmetric line shapes which may be due to interferences between the B state and an underlying continuum. The nature of these interferences and the possible indentity of this continuum are discussed.

The ultraviolet absorption spectrum of the à 1A‘2←X̃ 1A1 transition of jet-cooled ammonia

The A←X absorption spectra of NH3 and ND3, recorded in a cold molecular jet, are presented. Vibrational band progressions resolvable up to v’2=14 appear. No other vibrations are present, either alone or in combinations. Relative band intensities for v2 progressions are recorded, and the homogeneous lifetime broadenings of vibrational levels of the A state are reported. The FWHM linewidths span 34–293 cm−1 over all bands of NH3 and 30–135 cm−1 over the v’2=2 through 14 bands of ND3. In general, the rate of dissociation increases nonlinearly with vibrational energy. The band intensity alternation, previously observed only in matrix spectra below 15 K, has been observed in these very cold gas phase samples.

Twilight observations suggest unknown sources of HOx

Measurements of the concentrations of OH and HO_(2) (HO_(x)) in the high-latitude lower stratosphere imply the existence of unknown photolytic sources of HO_(x). The strength of the additional HO_(x) source required to match the observations depends only weakly on solar zenith angle (SZA) for 80° < SZA < 93°. The wavelengths responsible for producing this HO_(x) must be longer than 650 nm because the flux at shorter wavelengths is significantly attenuated at high SZA by scattering and absorption. Provided that the sources involve only a single photon, the strength of the bonds being broken must be < 45 kcal mole^(−1). We speculate that peroxynitric acid (HNO_4) dissociates after excitation to an unknown excited state with an integrated band cross section of 2-3 × 10^(−20) cm^(2) molecule^(−1) nm (650 < λ < 1250 nm).