Comment pubs.acs.org/JPCA
Reply to “Comment on ‘Photolysis of Polycyclic Aromatic Hydrocarbons on Water and Ice Surfaces’ and on ‘Nonchromophoric Organic Matter Suppresses Polycyclic Aromatic Hydrocarbon Photolysis in Ice and at Ice Surfaces’” D. J. Donaldson*,† and Tara F. Kahan*,‡ †
Department of Chemistry, University of Toronto, Toronto, Ontario M5S 3H6, Canada Department of Chemistry, Syracuse University, Syracuse, New York 13244, United States
‡
J. Phys. Chem. A 2014, 118 (9), 1638−1643. DOI: 10.1021/jp500263h J. Phys. Chem. A 2007, 111 (7), 1277−1285. DOI: 10.1021/jp066660t J. Phys. Chem. A 2015, 119. DOI: 10.1021/jp5b08276 argue that the anthracene fluorescence observed via energy transfer to a trace anthracene impurity from naphthalene is from “monomeric” (that is, similar to dilute aqueous solution) anthracene (because it is present in such low concentrations). However, as we reported in KD,1 when anthracene is deposited onto the ice surface its spectrum does not vary with deposition time, even at very short times with little deposited, where the S/N is poor; the emission spectrum is at all times that of excimeric anthracene. This suggests that any anthracene which is present at the ice surface that displays the “monomeric” spectrum has emission intensity too weak for us to have observed. Finally, we note that even if emission at longer wavelengths is due to anthracene impurities in the naphthalene, it does not change the larger stories reported by us, in the commentedupon papers and subsequent work, concerning the different solvation and reactive environment presented by frozen versus liquid water surfaces. Given our observations of self-associated benzene5 and anthracene,1 neither disputed by Krausko et al.,3 it is expected that naphthalene should also display such behavior. Indeed those authors have recently reported this,6 in basic agreement with the discussion presented in Ardura et al.4
In their Comment on the papers of Kahan and Donaldson1 (KD) and Malley and Kahan,2 Krausko et al.3 suggest that the assignment of long-wavelength emission observed1,2 from naphthalene at the surface of frozen water to self-associated naphthalene is incorrect, and that the observed emission features are better assigned to an anthracene impurity, which is excited via energy transfer from photoexcited naphthalene. They support this reassignment by showing spectra of mixed naphthalene/anthracene solutions (in a 4000:1 concentration ratio), which indeed display anthracene emission features that bear some similarity to those reported by us.1,2 Although we cannot eliminate the possibility that some impurity was responsible for the long-wavelength emission we reported from naphthalene deposited from the gas phase onto ice surfaces1 or excluded to such surfaces during freezing,2 we offer here a few arguments which make the assignment of this emission to anthracene impurities not entirely unambiguous. First, given that the vapor pressure of naphthalene is 104 times higher than that of anthracene, if we assume, for the point of argument, that there is an ∼1% anthracene impurity in the naphthalene sample used by KD, the anthracene concentration in the gas phase is predicted to be 10−6 that of naphthalene, assuming ideal behavior. One then expects a similar ratio of naphthalene to anthracene to be deposited on the ice surface from the gas phase. By contrast, exclusion of these aromatics from solution is expected to be similar for both, so one would expect ∼1% anthracene impurity at the ice surface when these are excluded from the frozen solution. In spite of this 104-fold difference in expected anthracene concentrations at the ice surface, we observed roughly similar intensities at long wavelengths (relative to monomeric emission at shorter wavelengths) when naphthalene was excluded from a frozen solution2 or deposited from the gas phase.1 Second, the spectra of the frozen naphthalene-anthracene mixture and solid naphthalene crystals displayed in Figure 1b of the Comment3 show essentially the same features as those reported in the commented papers1,2 and assigned there to selfassociated naphthalene. However, inspection of Figures 3 and 4 in KD,1 and Figure 1 in Ardura et al.4 shows that the longwavelength features in the emission spectrum of naphthalene deposited to the ice surface do not correspond to the spectra of anthracene at the ice surface displayed in KD.1 One might © XXXX American Chemical Society
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AUTHOR INFORMATION
Notes
The authors declare no competing financial interest.
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REFERENCES
(1) Kahan, T. F.; Donaldson, D. J. Photolysis of polycyclic aromatic hydrocarbons on water and ice surfaces. J. Phys. Chem. A 2007, 111, 1277−1285. (2) Malley, P. P. A.; Kahan, T. F. Nonchromophoric Organic Matter Suppresses Polycyclic Aromatic Hydrocarbon Photolysis in Ice and at Ice Surfaces. J. Phys. Chem. A 2014, 118, 1638−1643. (3) Krausko, J.; Bičanová, G.; Heger, D. Comment on “Photolysis of Polycyclic Aromatic Hydrocarbons on Water and Ice Surfaces” and on “Nonchromophoric Organic Matter Suppresses Polycyclic Aromatic Hydrocarbon Photolysis in Ice and at Ice Surfaces. J. Phys. Chem. A, 2015, 119, DOI: 10.1021/jp5b08276.
Received: September 16, 2015
A
DOI: 10.1021/acs.jpca.5b09045 J. Phys. Chem. A XXXX, XXX, XXX−XXX
Comment
The Journal of Physical Chemistry A (4) Ardura, D.; Kahan, T. F.; Donaldson, D. J. Self-Association of Naphthalene at the Air-Ice Interface. J. Phys. Chem. A 2009, 113, 7353−7359. (5) Kahan, T. F.; Donaldson, D. J. Benzene Photolysis on Ice: Implications for the Fate of Organic Contaminants in the Winter. Environ. Sci. Technol. 2010, 44, 3819−3824. (6) Krausko, J.; Malongwe, J. K.; Bicanova, G.; Klan, P.; Nachtigallova, D.; Heger, D. Spectroscopic Properties of Naphthalene on the Surface of Ice Grains Revisited: A Combined Experimental Computational Approach. J. Phys. Chem. A 2015, 119, 8565−8578.
B
DOI: 10.1021/acs.jpca.5b09045 J. Phys. Chem. A XXXX, XXX, XXX−XXX
