Figure 1. Percent integrated infrared intensities in the atmospheric IR window of (1) CH4, (2) CH3F, (3) CH2F2, (4) CHF3, and (5) CF4.
The Origin Of Global Warming By Means Of Molecular Selection
Sustainability stretches through greener chemistry. Imagine having a choice in designing environmentally friendly materials. This opportunity is presented in "Identifying the Molecular Origin of Global Warming" scheduled for the November 12's ACS Journal of Physical Chemistry. The approach taken by Partha Bera et al. seeks to explain how fundamental properties influence molecular absorption in the atmospheric window. What are the major factors that make some molecules more effective greenhouse gases (GHGs)?
Is it possible, for example, to design materials with minimal absorption and shorter lifetimes in the atmosphere? Some studies have looked at how to minimize the atmospheric lifetime of materials but none, the authors note, had "addressed how to minimize the absorption capabilities of molecular species in the atmospheric window." For purposes of this study, they defined the atmospheric infrared (IR) window to be 800−1400 cm−1.
Chlorofluorocarbons (CFCs), hydrofluorocarbons (HFCs), perfluorocarbons (PFCs), nitrogen fluorides, and other known atmospheric trace molecules are included in this paper. The researchers observed first that "certain bond stretch vibrational modes are ideally suited for occurring in the atmospheric IR window region. These modes include C−F, C−Cl, C−Br, S−F, and N−F vibrational stretches among other vibrational modes. Some bond angle bending and torsions fall within the atmospheric window and contribute to global warming albeit in a significantly lesser way."
Another observation was from investigating the IR vibrational frequencies and intensities of some of the most common HFCs and CFCs: (1) the IR absorption intensities within the IR atmospheric window are much larger than that of CO2, and therefore they are much more potent absorption agents than CO2; (2) the percentage of the integrated vibrational absorption intensity that falls within the atmospheric window increases, to almost 99% of the total IR intensity, as more and more halogens are involved. These factors, promoted by very long atmospheric lifetimes, contribute to making the HFCs, CFCs, PFCs, and other GHGs extremely potent compared to CO2 on a per molecule basis.
The data reported in Table 1 highlight that these molecules possess long atmospheric lifetimes but more importantly they strongly absorb radiation in the atmospheric window. The HFCs, CFCs, PFCs, and sulfur and nitrogen fluorides absorb in the atmospheric window, where no other atmospheric molecules absorb, and do so very effectively. The fact that "for many of these molecules more than 85−90% of their IR absorption occurs in the atmospheric window" deserves emphasis.
A study of the HFCs and PFCs listed in Table 2 reveals that with introduction of each F atom more and more vibrational modes occur in the atmospheric window by virtue of the C−F bond stretch falling within the 800−1400 cm−1 region, while simultaneously each C−F stretch is becoming more intense. Figure 1 shows here that as one introduces F, going from CH4 to CF4, the percentage of the total amount of IR intensity within the atmospheric window increases from 34.6% [my correction to the paper's "20%"] up to 98.8% [my correction to the paper's "almost 100%"]. [Note: This correction, which I mentioned last week on telephone to a coauthor, is important in that the effect is 2.9 times and not 5 times as the paper's sentence would imply.]
In summary, the highly polarized nature of the X−F bond leads to a large dipole derivative; increasing the number of fluorine atoms bonded to a given central atom increases the total or integrated dipole derivative linearly as a function of the number of F atoms, leading to a nonlinear increase in IR intensity. Therefore, a combination of factors contribute toward making fluorine-containing compounds the most effective global warming agents. These factors are:
(1) the X−F stretching frequencies falling within the atmospheric IR window.
(2) the strong electronegativity of F.
(3) the typically long atmospheric lifetimes of compounds containing many fluorine atoms.
Nitrogen trifluoride has a global warming potential of 17,200 times that of CO2 over a 100 year period. Its atmospheric lifetime is 740 years. (Image Credit: Wikipedia)