[1] In the mid-1970s, it was recognized that chlorofluorocarbons (CFCs) were strong greenhouse gases that could have substantial impacts on radiative forcing of climate change, as well as being substances that deplete stratospheric ozone. Around a decade later, this group of radiatively active compounds was expanded to include a large number of replacements for ozone-depleting substances such as chlorocarbons, hydrochlorocarbons, hydrochlorofluorocarbons (HCFCs), hydrofluorocarbons (HFCs), perfluorocarbons (PFCs), bromofluorocarbons, and bromochlorofluorocarbons. This paper systematically reviews the published literature concerning the radiative efficiencies (REs) of CFCs, bromofluorocarbons and bromochlorofluorocarbons (halons), HCFCs, HFCs, PFCs, sulfur hexafluoride, nitrogen trifluoride, and related halogen containing compounds. In addition, we provide a comprehensive and self-consistent set of new calculations of REs and global warming potentials (GWPs) for these compounds, mostly employing atmospheric lifetimes taken from the available literature. We also present global temperature change potentials for selected gases. Infrared absorption spectra used in the RE calculations were taken from databases and individual studies and from experimental and ab initio computational studies. Evaluations of REs and GWPs are presented for more than 200 compounds. Our calculations yield REs significantly (> 5%) different from those in the Intergovernmental Panel on Climate Change Fourth Assessment Report (AR4) for 49 compounds. We present new RE values for more than 100 gases which were not included in AR4. A widely used simple method to calculate REs and GWPs from absorption spectra and atmospheric lifetimes is assessed and updated. This is the most comprehensive review of the radiative efficiencies and global warming potentials of halogenated compounds performed to date.

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Global warming potentials and radiative efficiencies of halocarbons and related compounds: a comprehensive review

Modelling of molecular emission from interstellar clouds requires the calculation of rates for excitation by collisions with the most abundant species. The present paper focuses on the calculation of rate coefficients for rotational excitation of the hydrogen cyanide (HCN) and hydrogen isocyanide (HNC) molecules in their ground vibrational state in collision with He. The calculations are based on new two-dimensional potential energy surfaces obtained from highly correlated ab initio calculations. Calculations of pure rotational (de)excitation cross-sections of HCN and HNC by He were performed using the essentially exact close-coupling method. Cross-sections for transitions among the eight first rotational levels of HCN and HNC were calculated for kinetic energies up to 1000 cm−1. These cross-sections were used to determine collisional rate constants for temperatures ranging from 5 to 100 K. A propensity for even Δj transitions is observed in the case of HCN–He collisions whereas a propensity for odd Δj transitions is observed in the case of HNC–He collisions. The consequences for astrophysical models are evaluated and it is shown that the use of HCN rate coefficients to interpret HNC observations can lead to significant inaccuracies in the determination of the HNC abundance, in particular, in cold dark clouds for which the new HNC rates show that the j= 1 − 0 line of this species will be more easily excited by collisions than HCN. An important result of the new HNC–He rates is that the HNC/HCN abundance ratio derived from observations in cold clouds has to be revised from >1 to ≃1, in good agreement with detailed chemical models available in the literature.

The rotational excitation of HCN and HNC by He: new insights on the HCN/HNC abundance ratio in molecular clouds

Helium is the only substance that has been observed on macroscopic scale to form the fourth state of matter, the superfluid state. However, until recently superfluid helium had not found any practical applications, mainly because it expels all other atoms or molecules. Only in the 1990s was it discovered that it is possible to mix in other substances with superfluid helium if helium is prepared as small droplets, called nanodroplets, containing only a few thousand atoms. This discovery led to the development of a new and very powerful experimental technique, called helium-nanodroplet spectroscopy. Superfluid helium creates a gentle matrix around the impurities and – due to superfluidity and to very weak interactions of helium atoms with other atoms or molecules – allows measurements of the spectra with precision not much lower than in the gas phase. Consequently, helium-nanodroplet spectroscopy enables very accurate probing of molecules or clusters which cannot be investigated in the gas phase due to thei...

Interplay between theory and experiment in investigations of molecules embedded in superfluid helium nanodroplets

The kinetics of the OH radical and Cl atom reactions with nine fluorinated ethers have been studied by the relative rate method at 298 K and 1013 hPa using gas chromatography-mass spectroscopy (GC-MS) detection: k(OH + CH3CH2OCF3) = (1.55 +/- 0.25) x 10(-13), k(OH + CF3CH2OCH3) = (5.7 +/- 0.8) x 10(-13),k(OH + CF3CH2OCHF2) = (9.1 +/- 1.1) x 10(-15), k(OH + CF3CHFOCHF2) = (6.5 +/- 0.8) x 10(-15), k(OH + CHF2CHFOCF3) = (6.8 +/- 1.1) x 10(-15), k(OH + CF3CHFOCF3) < 1 x 10(-15), k(OH + CF3CHFCF2OCHF2) = (1.69 +/- 0.26) x 10(-14), k(OH + CF3CHFCF2OCH2CH3) = (1.47 +/- 0.13) x 10(-13), k(OH + CF3CF2CF2OCHFCF3) < 1 x 10(-15), k(Cl + CH3CH2OCF3) = (2.2 +/- 0.8) x 10(-12), k(Cl + CF3CH2OCH3) = (1.8 +/- 0.9) x 10(-11), k(Cl + CF3CH2OCHF2) = (1.5 +/- 0.4) x 10(-14), k(Cl + CF3CHFOCHF2) = (1.1 +/- 1.9) x 10(-15), k(Cl + CHF2CHFOCF3) = (1.2 +/- 2.0) x 10(-15), k(Cl + CF3CHFOCF3) < 3 x 10(-15), k(Cl + CF3CHFCF2OCHF2) < 6 x 10(-16), k(Cl + CF3CHFCF2OCH2CH3) = (3.1 +/- 1.1) x 10(-12), and k(Cl + CF3CF2CF2OCHFCF3) < 3 x 10(-15) cm3 molecule(-1) s(-1). The error limits include three standard deviations (3 sigma) from the statistical data analyses, as well as the errors in the rate coefficients of the reference compounds that are used. Infrared absorption cross sections and estimates of the trophospheric lifetimes and the global warming potentials of the fluorinated ethers are presented. The atmospheric degradation of the compounds is discussed.

Atmospheric Chemistry of Hydrofluoroethers: Reaction of a Series of Hydrofluoroethers with OH Radicals and Cl Atoms, Atmospheric Lifetimes, and Global Warming Potentials

The study of weakly bound complexes and clusters promises to provide a bridge between the properties of isolated molecules and those of dense phases. We describe briefly the method of pulsed nozzle Fourier transform microwave spectroscopy, which we use in our laboratory to study rotational and low-lying van der Waals vibrational spectra of weakly bound complexes and clusters. Systematic studies of three different classes of clusters are described: (i) mixed rare gas dimers, trimers, and tetramers, where the transition dipole moment is entirely provided by the weak van der Waals interactions; (ii) (rare gas)1,2,3–ammonia clusters where the internal motions of the ammonia subunit are affected by the rare gas atom solvation; (iii) (helium)1,2,3–molecule clusters, which provide a basis for the study of larger helium atoms containing clusters. We also discuss the effect of three-body non-additive contributions on spectroscopic parameters, such as dipole moments, rotational constants, and nuclear quadrupole cou...

Microwave spectroscopy of ternary and quaternary van der Waals clusters

Millimeter-wave absorption spectroscopy combined with a pulsed-jet expansion technique was applied to measure the internal-rotation band of He–HCN in the frequency region of 95–125 GHz. In total 13 rovibrational lines, split into nitrogen nuclear hyperfine structure, were observed for the fundamental internal-rotation band, j=1−0. The observed transition frequencies were analyzed including their hyperfine splitting to yield an intermolecular potential energy surface, as improved from the one given by a coupled-cluster single double (triple) ab initio calculation. The surface obtained has a global minimum in the linear configuration (He⋅⋅⋅H–C–N) with a well depth of 30.2 cm−1, and a saddle point located in the antilinear configuration (H–C–N⋅⋅⋅He) which is higher by 8.91 cm−1 in energy than the global minimum. The distance Rm between the He atom and the center of mass of HCN along the minimum energy path shows a strong angular dependence; Rm is 4.169 and 4.040 A in the linear and antilinear forms, respecti...

Millimeter-wave spectroscopy of the internal-rotation band of the He–HCN complex and the intermolecular potential energy surface

Ab initio G-2 calculations have been carried out for a number of elementary reactions relevant to low-temperature silane oxidation. The calculations have been made to identify the chain-branching process which enables the spontaneous ignition of silane at room temperature. Extensive calculations have also been made for the reactions of major intermediates with hydrogen atoms and water vapor.

Ab Initio Energetic Calculations of Elementary Reactions Relevant to Low-Temperature Silane Oxidation by Gaussian-2 Theory

An excellent linear correlation was found between calculated and experimental total infrared intensities in the region of 500–2000 cm−1 for five partially fluorinated ethyl methyl ethers, HFC-134a, and CFC-11. The calculated results using ab initio molecular orbital method with D95V**(Huzinaga-Dunning’s valence double zeta) show the best linear correlation among basis sets examined in this study. The correlation coefficient with D95V** is 0.9993 although the calculated values are on the average 32% higher than experimental ones.

Mutsumi Aoyagi

Papers

Millimeter-wave spectroscopy of the internal-rotation band of the He–HCN complex and the intermolecular potential energy surface

Ab Initio Energetic Calculations of Elementary Reactions Relevant to Low-Temperature Silane Oxidation by Gaussian-2 Theory

Estimation of Total Infrared Intensities of Fluorinated Ethyl Methyl Ethers.