Life in the Gas Phase

Particulate organic carbon comprises a large number of compounds having significant variations in volatility as a result, a number of these compounds can be present in both the gas and particulate phases. The ability of such semivolatile compounds to coexist in both phases complicates the distinction between primary and secondary OC. Strictly speaking, secondary OC starts its atmospheric life in the gas phase as a VOC, undergoes one or more chemical transformations in the gas phase to a less volatile compound, and finally transfers to the particulate phase by condensation or nucleation. Therefore the term... 

In the recoil system chemically stable products containing a single silylene unit predominate, since the low concentration of intermediates precludes dimerization. Thermally generated SiF2 is quite unreactive, with a 150-s half-life in the gas-phase (12), so nudeogenic SiF2 may also be reacting before it is thermally equilibrated. 

The unusually stable a-lactone bis(trifluoromethyl)acetolactone (48) has been synthesized by photolysis of the malonyl peroxide 47." It has a half-life in the gas phase of 8 h at 24°C, decarbonylating to give hexafluoroacetone (20). Its relatively... 

Frequently, when the enol content is high, both forms can be isolated. The pure keto form of acetoacetic ester melts at — 39°C, while the enol is a liquid even at — 78°C. Each can be kept at room temperature for days if catalysts such as acids or bases are rigorously excluded.Even the simplest enol, vinyl alcohol (CH2= CHOH), has been prepared in the gas phase at room temperature, where it has a half-life of 30min. " The enol Me2C=CCHOH is indefinitely stable in the solid state at —78°C and has a half-life of 24h in the liquid state at 25°C. When both forms cannot be isolated, the extent of enolization is often measured by NMR. 

Life as we know it depends on this existence of water as a liquid. Biochemical processes require free movement of chemicals, which cannot occur in the solid phase. Biochemical stmctures contain many interlocking parts that would not be stable in the gas phase. Thus, the liquid phase is best suited for life. Moreover, water is an excellent solvent, particularly for molecules that can form hydrogen bonds. As we describe in Chapter JA, the molecular building blocks of living matter are rich in groups that form hydrogen bonds. This allows biological molecules to be synthesized, move about, and assemble into complex structures, all in aqueous solution. 

Nitric acid is the only oxyacid of nitrogen which is at all stable in the free condition. The kinetics of its decomposition are discussed below. Nitrous acid exists in the gas phase, although it cannot be isolated. Information on the kinetics of its decomposition is restricted to an estimate103 of the half-life of the forward reaction in the rapid gas-phase equilibrium... 

Noncovalent interactions play a key role in biodisciplines. A celebrated example is the secondary structure of proteins. The 20 natural amino acids are each characterized by different structures with more or less acidic or basic, hydrophilic or hydrophobic functionalities and thus capable of different intermolecular interactions. Due to the formation of hydrogen bonds between nearby C=0 and N-H groups, protein polypeptide backbones can be twisted into a-helixes, even in the gas phase in the absence of any solvent." A protein function is determined more directly by its three-dimensional structure and dynamics than by its sequence of amino acids. Three-dimensional structures are strongly influenced by weak non-covalent interactions between side functionalities, but the central importance of these weak interactions is by no means limited to structural effects. Life relies on biological specificity, which arises from the fact that individual biomolecules communicate through non-covalent interactions." " Molecular and chiral recognition rely on... 

Chemical/Physical. Begins to polymerize at 80.2 °C (Weast, 1986). Slowly hydrolyzes in water forming methyl alcohol and acrylic acid (Morrison and Boyd, 1971). Based on a hydrolysis rate constant of 0.0779/M-h at pH 9 at 25 °C, an estimated half-life of 2.8 yr at pH 7 was reported (Roy, 1972). The reported rate constant for the reaction of methacrylonitrile with ozone in the gas phase is 2.91 x lO cm moFsec (Munshi et al, 1989a). 

Photolytic. Irradiation of vinyl chloride in the presence of nitrogen dioxide for 160 min produced formic acid, HCl, carbon monoxide, formaldehyde, ozone, and trace amounts of formyl chloride and nitric acid. In the presence of ozone, however, vinyl chloride photooxidized to carbon monoxide, formaldehyde, formic acid, and small amounts of HCl (Gay et al, 1976). Reported photooxidation products in the troposphere include hydrogen chloride and/or formyl chloride (U.S. EPA, 1985). In the presence of moisture, formyl chloride will decompose to carbon monoxide and HCl (Morrison and Boyd, 1971). Vinyl chloride reacts rapidly with OH radicals in the atmosphere. Based on a reaction rate of 6.6 x lO" cmVmolecule-sec, the estimated half-life for this reaction at 299 K is 1.5 d (Perry et al., 1977). Vinyl chloride reacts also with ozone and NO3 in the gas-phase. Sanhueza et al. (1976) reported a rate constant of 6.5 x 10 cmVmolecule-sec for the reaction with OH radicals in air at 295 K. Atkinson et al. (1988) reported a rate constant of 4.45 X 10cmVmolecule-sec for the reaction with NO3 radicals in air at 298 K. 

The formation of the triplet state of benzene in the gas phase was proven only recently, but this has now been accomplished by three independent methods.24-27 These methods will be discussed shortly in more detail, but it should be mentioned in passing that the mean life of the triplet state in the gas phase of triplet benzene is far shorter than it is in a glassy matrix and is indeed between 10-5 and 10-4 sec.26d,2S... 

The photolysis of diazoalkanes both in the gas phase and in solution is a carbenoid reaction. Moreover, the results of EPR-spectroscopic investigations (Section IIB) demonstrate that triplet carbenes can be generated by irradiation of diazoalkanes. That the reactive intermediates in carbenoid reactions are free carbenes is usually taken as self-evident. While such an assumption is probably wholly justified in most cases, it is worth remembering that both in the gas phase and in solvents such as n-hexane, the electronic absorption spectra of simple diazoalkanes show definite fine structure (Bradley etal., 1964a). This implies that the photo-excited state is bonding (Hoffmann, 1966) and consequently may have a life-time long enough to enable it to react directly with another molecule... 

Must a standard liquid of some kind serve as the matrix for life Can a supercritical fluid serve as well Can life exist in the gas phase In solid bodies, including ice ... 

Conceptually, it is the atomic number and the electronic configuration of an element that define its position in the Periodic Table. Since they cannot be measured for the very heavy elements, information on its chemical behavior is often used to place an element in a chemical group. Unfortunately, with increasing nuclear charge the cross sections and the production rates drop so rapidly that such chemical information can be accessed only for elements with a half-life of the order of at least few seconds and longer. In this case, some fast chemistry techniques are used. They are based on the principle of chromatographic separations either in the gas phase exploiting the differences in volatility of heavy element compounds, or in the aqueous... 

For simple carbonyl compounds, the equilibrium between an aldehyde or a ketone and its corresponding enol is usually so shifted towards the keto form that the amount of enol at equilibrium can neither be measured nor detected by spectroscopy. Nevertheless, as recently emphasised by Hart (1979), this does not mean that the enol cannot exist free, not in equilibrium with ketones and aldehydes. Several examples of kinetically stable enols in the gas phase or in aprotic solvents have been reported. Broadly speaking, it appears that enols have relatively large life-times when they are prepared in proton-free media [e.g. the half-life of acetone enol was reported to be 14 s in acetonitrile (Laroff and Fischer, 1973 Blank et al., 1975) and 200 s in the gas phase (MacMillan et al., 1964)]. These life-times are related to an enhanced intramolecular rearrangement, indicated by the very high energies of activation (85 kcal mol-1 for acetaldehyde-vinyl alcohol tautomerization) which have been calculated (Bouma et al., 1977 Klopman and Andreozzi, 1979) It has therefore been possible to determine most of the spectroscopic properties of simple enols [ H nmr,l3C nmr (CIDNP technique), IR and microwave spectra of vinyl alcohol... 

We present experimental results on photophysical deactivation pathways of uracil and thymine bases in the gas phase and in solvent/solute complexes. After photoexcitation to the S2 state, a bare molecule is tunneled into and trapped in a dark state with a lifetime of tens to hundreds of nanoseconds. The nature of this dark state is most likely a low lying nn state. Solvent molecules affect the decay pathways by increasing IC from the S2 to the dark state and then further to the ground state, or directly from S2 to S0. The lifetimes of the S2 state and the dark state are both decreased with the addition of only one or two water molecules. When more than four water molecules are attached, the photophysics of these hydrated clusters rapidly approaches that in the condensed phase. This model is now confirmed from other gas phase and liquid phase experiments, as well as from theoretical calculations. This result offers a new interpretation on the origin of the photostability of nucleic acid bases. Although we believe photochemical stability is a major natural selective force, the reason that the nucleic acid bases have been chosen is not because of their intrinsic stability. Rather, it is the stability of the overall system, with a significant contribution from the environment, that has allowed the carriers of the genetic code to survive, accumulate, and eventually evolve into life s complicated form. 

Life, as we know it, is based on replication, which on a molecular level is realized by DNA base pairing. However, the scheme with four nucleobases of DNA does not necessarily represent the only way to achieve molecular replication [1], Furthermore, there are alternate pairing schemes and structures and interactions between the bases that can lead to mutations, for example by proton transfers that lead to different tautomers. For all these reasons the study of interactions between individual nucleobases and of the properties of isolated base pairs at the most fundamental level is important and such studies are possible in the gas phase. 

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