Photochemical overview

The atmospheric chemistry of nitrogen is quite complex and involves literally hundreds or thousands of chemical reactions. Although the fluxes are much smaller than the biological fluxes, these processes are important for a variety of reasons, including impacts on climate, stratospheric ozone, and photochemical smog. In this section we present an overview of the most important processes. 

A. Mills and S. La Hunte, An Overview of Semiconductor Photocatalysis ,Photochem Photobiol A Chemistry, 1997, 108, 1. 

In the USA, the Clean Air Act of 1970 established air-quality standards for six major pollutants particulate matter, sulfur oxides, carbon monoxide, nitrogen oxides, hydrocarbons, and photochemical oxidants. It also set standards for automobile emissions - the major source of carbon monoxide, hydrocarbons, and nitrogen oxides. An overview of the major standards is given in Tab. 10.2. The levels of, for example, the European Union (1996) are easily achieved with the present catalysts. The more challenging standards, up to those for the ultralow emission vehicle, are within reach, but zero-emission will probably only be attainable for a hydrogen-powered vehicle. 

Mills A, Le Hunte S (1997) An overview of semiconductor photocatalysis. J Photochem Photobiol A 108 1-35... 

Most of the AIMD simulations described in the literature have assumed that Newtonian dynamics was sufficient for the nuclei. While this is often justified, there are important cases where the quantum mechanical nature of the nuclei is crucial for even a qualitative understanding. For example, tunneling is intrinsically quantum mechanical and can be important in chemistry involving proton transfer. A second area where nuclei must be described quantum mechanically is when the BOA breaks down, as is always the case when multiple coupled electronic states participate in chemistry. In particular, photochemical processes are often dominated by conical intersections [14,15], where two electronic states are exactly degenerate and the BOA fails. In this chapter, we discuss our recent development of the ab initio multiple spawning (AIMS) method which solves the elecronic and nuclear Schrodinger equations simultaneously this makes AIMD approaches applicable for problems where quantum mechanical effects of both electrons and nuclei are important. We present an overview of what has been achieved, and make a special effort to point out areas where further improvements can be made. Theoretical aspects of the AIMS method are... 

The reaction pathways by which the net transfer of a hydrogen atom from an amine to a photoexcited ketone has been extensively examined in the nanosecond [23, 25-30], picosecond [20, 22, 31-33], and femtosecond [24] time domains. The following mechanism, as it pertains to the photochemical reduction of benzophenone (Bp) by N, A-dimethylaniline (DMA), is derived from these numerous studies. Only an overview of the mechanism will be presented. The details of the studies leading to the mechanism will not be given for specifics, the reader is referred to the original literature. 

Carmichael, G. R L. K. Peters, and R. D. Saylor, The STEM-II Regional Scale Acid Deposition and Photochemical Oxidant Model. I. An Overview of Model Development and Applications, Atmos. Enciron., 25, 2077-2090 (1991). 

We shall begin with a closer look at electronic excitation, some aspects of which were discussed in Section 9-9. Because transfer of electronic energy from one molecule to another is a basic process in photochemistry, we will discuss energy transfer also before giving an overview of representative photochemical reactions. The closely related phenomena of chemiluminescence and bioluminescence then will be described. Finally, there will be a discussion of several important applications of photochemistry. 

The Photochemical Activity and solar Ultraviolet Radiation (PAUR I) and Photochemical Activity and solar Ultraviolet Radiation Modulation Factors (PAUR II) projects had the aim of studying various aspects of ultraviolet radiation and photochemistry interrelationships. PAUR I aimed at studying the interrelationships between total ozone, UV-B radiation, aerosol load, air pollutants, photodissociation rates of N02 and 03 and tropospheric ozone. PAUR II has the aim of studying the interactions between UV-B, total ozone, tropospheric ozone and photochemical activity in the presence of alternating maritime and Saharan aerosols. The present paper presents the main concepts underlying the two projects, the approach followed and a brief overview of some of the results obtained so far. Further, the main results of PAUR I that are relevant to tropospheric ozone chemistry over the Eastern Mediterranean are presented. 

The high surface potentials and differential polarities of molecular assemblies such as micelles, vesicles and microemulsions suggest that they may be of use in effecting charge separation after a photochemical redox event either by preferential electrostatic repulsion of one of the products or by differential solubilities of the two products in the different phases. This area of research has been extensively reviewed325 330 and we give a brief overview of the use of these systems. 

Electron transfer, photochemically induced, 1, 246 Electron-transfer equilibirum method, overview, 1, 817 Electron-transfer reactions Ga, In, T1 complexes, 3, 301 with Ge-Ge bonds, 3, 795 zinc species, 2, 315 Electron-transfer salts... 

In this review, the photophysical and photochemical properties of selected gold systems will be described and discussed, so as to provide readers with a broad overview of the great diversity of these systems in these aspects. 

The two most important methods of both probing and stimulating supramolecular devices are photochemical and electrochemical techniques. The most prevalent events to occur in such devices are electron, energy and proton transfer, as well as molecular rearrangement. Any of these events can provide the basis for forms of transducable output on which molecular electronic devices may be based. Therefore, the theories most commonly applied to such electrochemically and photochemically triggered events are outlined in this chapter. In addition, an overview of the mechanisms by which such events occur is provided, identifying the molecular or physical parameters required to make such events feasible in a supramolecular structure. 

Most of the mass spectrometry applications for combinatorial chemistry will be described in the following sections of this chapter. Here we will give a short overview of MS techniques utilized for the characterization of resin-bound molecules. The majority of publications in this field describe applications of matrix-assisted laser desorption ionization (MALDI), combined with time-of-flight (TOF) detection. The major difference of MS application for analysis of resin-bound molecules from the above-described NMR and IR applications is that analyte should not be covalently bound to solid support prior to mass measurement. Detachment of compound molecules from resin can be done chemically (for example, by bead exposure to TFA vapors) [30,31] or photochemically, such that cleavage, desorption, and ionization of molecules occur simultaneously upon stimulation by laser radiation [32], Since the... 

Calzavara-Pinton PG, Venturini M, Sala R. A comprehensive overview of photodynamic therapy in the treatment of superficial fungal infections of the skin. J Photochem Photo-biol B 2005 78 1-6. 

In this overview and review of tropospheric photochemistry, we will examine a limited set of important homogeneous and heterogeneous photochemical reactions of relevance in the troposphere (Table 1). An expanded array of photochemical reactions is considered viable in the upper atmosphere (e.g., stratosphere) due to exposure to actinic radiation at wavelengths below 290 nm. A brief summary of a limited subset of this array of possible photochemical reactions will be provided in this review. 

Some of the topics addressed here have been reviewed by other authors. Studies of photoinduced processes (direct or indirect) of chlorophenols carried out before 1998 have been covered comprehensively [8] this work will only cursorily be treated here. A detailed overview of the photochemical behavior of phenylurea herbicides was in press at the time of writing this article [9]. The related subject of the photodegradation of pharmaceuticals in the aquatic environment has been reviewed very recently [10]. 

---