Design strategies to minimize the radiative efficiency of global warming molecules

Abstract

A strategy is devised to screen molecules based on their radiative efficiency. The methodology should be useful as one additional constraint when determining the best molecule to use for an industrial application. The strategy is based on the results of a recent study where we examined molecular properties of global warming molecules using ab initio electronic structure methods to determine which fundamental molecular properties are important in assessing the radiative efficiency of a molecule. Six classes of perfluorinated compounds are investigated. For similar numbers of fluorine atoms, their absorption of radiation in the IR window decreases according to perfluoroethers > perfluorothioethers≈ sulfur∕carbon compounds > perfluorocarbons > perfluoroolefins > carbon∕nitrogen compounds. Perfluoroethers and hydrofluorethers are shown to possess a large absorption in the IR window due to (i) the C─O bonds are very polar, (ii) the C-O stretches fall within the IR window and have large IR intensity due to their polarity, and (iii) the IR intensity for C-F stretches in which the fluorine atom is bonded to the carbon that is bonded to the oxygen atom is enhanced due to a larger C─F bond polarity. Lengthening the carbon chain leads to a larger overall absorption in the IR window, though the IR intensity per bond is smaller. Finally, for a class of partially fluorinated compounds with a set number of electronegative atoms, the overall absorption in the IR window can vary significantly, as much as a factor of 2, depending on how the fluorine atoms are distributed within the molecule.

A group increment scheme for infrared absorption intensities of greenhouse gases

“…9 It has been shown that by strategic distribution of fluorine atoms along the carbon framework, the infrared radiation absorption capacity can be reduced by a factor of two. 8,9 Based on this knowledge, a group increment scheme for absorption intensities is proposed and investigated in the present study. The idea of a group increment scheme is analogous to…”

“…Finally, based on the analysis of the molecular origin of the strong absorption of radiation within the atmospheric IR window, 8 design strategies were devised to minimize an industrial molecule's radiative efficiency. 9 Due to the relatively shorter atmospheric lifetimes, HFEs have been recommended as alternatives for PFCs and HFCs. 10,11 HFEs generally have an increased reactivity with hydroxyl radicals, which decreases their atmospheric lifetimes.…”

“…12 However, they absorb IR radiation very efficiently in the Earth's atmospheric window wavelength range. 9,13 The carbon-oxygen stretches fall within the atmospheric IRW and have a large IR intensity due to the polarity of the C-O bond. This in combination with the carbon-fluorine vibrational stretch, which also occurs within the atmospheric IRW, makes perfluoroethers (PFEs) and HFEs very effective greenhouse gases.…”

“…This in combination with the carbon-fluorine vibrational stretch, which also occurs within the atmospheric IRW, makes perfluoroethers (PFEs) and HFEs very effective greenhouse gases. 8,9,12,[14][15][16][17][18] For fluorinated hydrocarbons and ethers, molecular structure greatly impacts its ability to absorb IR radiation. 9 It has been shown that by strategic distribution of fluorine atoms along the carbon framework, the infrared radiation absorption capacity can be reduced by a factor of two.…”

“…8,9,12,[14][15][16][17][18] For fluorinated hydrocarbons and ethers, molecular structure greatly impacts its ability to absorb IR radiation. 9 It has been shown that by strategic distribution of fluorine atoms along the carbon framework, the infrared radiation absorption capacity can be reduced by a factor of two. 8,9 Based on this knowledge, a group increment scheme for absorption intensities is proposed and investigated in the present study.…”