Recent experiments have shown that in the oxygen isotopic exchange reaction for O(^1D) + CO_2 the elastic channel is approximately 50% that of the inelastic channel [Perri et al., 2003]. We propose an analogous oxygen atom exchange reaction for the isoelectronic O(^1D) + N_2O system to explain the mass-independent isotopic fractionation (MIF) in atmospheric N_2O. We apply quantum chemical methods to compute the energetics of the potential energy surfaces on which the O(^1D) + N_2O reaction occurs. Preliminary modeling results indicate that oxygen isotopic exchange via O(^1D) + N_2O can account for the MIF oxygen anomaly if the oxygen atom isotopic exchange rate is 30–50% that of the total rate for the reactive channels

Blake, Geoffrey A.

Liang, Mao-Chang

Miller, Charles E.

Muller, Richard P.

Yung, Yuk L.

English

Caltech Authors - Main

Evidence for O-atom exchange in the O(1D) + N2O reaction as the
source of mass-independent isotopic fractionation in atmospheric N2O
Yuk L. Yung, Mao-Chang Liang, and Geoffrey A. Blake1
Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California, USA
Richard P. Muller
Computational Materials and Molecular Biology, Sandia National Laboratories, Albuquerque, New Mexico, USA
Charles E. Miller
Atmospheric Chemistry Element, Jet Propulsion Laboratory, Pasadena, California, USA
Received 7 July 2004; revised 21 August 2004; accepted 31 August 2004; published 8 October 2004.
[1] Recent experiments have shown that in the oxygen
isotopic exchange reaction for O(1D) + CO2 the elastic
channel is approximately 50% that of the inelastic channel
[Perri et al., 2003]. We propose an analogous oxygen atom
exchange reaction for the isoelectronic O(1D) + N2O system
to explain the mass-independent isotopic fractionation
(MIF) in atmospheric N2O. We apply quantum chemical
methods to compute the energetics of the potential energy
surfaces on which the O(1D) + N2O reaction occurs.
Preliminary modeling results indicate that oxygen isotopic
exchange via O(1D) + N2O can account for the MIF oxygen
anomaly if the oxygen atom isotopic exchange rate is 30–
50% that of the total rate for the reactive channels. INDEX
TERMS: 0317 Atmospheric Composition and Structure: Chemical
kinetic and photochemical properties; 0322 Atmospheric
Composition and Structure: Constituent sources and sinks; 0341
Atmospheric Composition and Structure: Middle atmosphere—
constituent transport and chemistry (3334).Citation: Yung, Y. L.,
M.-C. Liang, G. A. Blake, R. P. Muller, and C. E. Miller (2004),
Evidence for O-atom exchange in the O(1D) + N2O reaction as the
source of mass-independent isotopic fractionation in atmospheric
N2O, Geophys. Res. Lett., 31, L19106, doi:10.1029/
2004GL020950.
1. Introduction
[2] Understanding the isotopic fractionation of atmo-
spheric nitrous oxide (N2O) is important for constraining
the budget for its sources and sinks [see, e.g., Kim and
Craig, 1993; Stein and Yung, 2003]. At present the mass-
dependent fractionations have been explained satisfactorily
[Blake et al., 2003; McLinden et al., 2003; Morgan et al.,
2004; Liang et al., 2004], but there has not been a definitive
explanation of the mass-independent fractionation (MIF) of
the oxygen isotopic anomaly of atmospheric N2O.
[3] The N2O MIF anomaly was discovered by Cliff and
Thiemens [1997], whose notation we follow. (See Thiemens
et al. [2001] for an overview of the physical basis and
applications of MIF to terrestrial and extraterrestrial
environments.) The mass-dependent fractionation for
oxygen is given by
d17O ¼ 0:515 d18O:
The oxygen anomaly is defined as the residual from the
above equation, or
D17O ¼ d17O 0:515 d18O:
Cliff and Thiemens [1997] discovered that tropospheric N2O
samples contained D17O  1%. Subsequent measurements
confirmed and extended the result into the stratosphere
[Cliff et al., 1999; Ro¨ckmann et al., 2001]. The latter
reference gives D17O = 1.0 ± 0.2% at d18O = 20.7 ± 0.3%.
[4] Several mechanisms involving stratospheric N2O
sources have been proposed to explain the observed MIF:
NO*2 + N2 [e.g., Zellner et al., 1992], O(
1D) + N2 [e.g., Zipf
and Prasad, 1998], NH2 + NO2 [Ro¨ckmann et al., 2001],
and others. These processes have been summarized and
thoroughly discussed by McLinden et al. [2003].
[5] Recently, Perri et al. [2003] studied the isotopic
exchange reactions for oxygen (O = 16O; Q = 18O) and CO2:
Q 1Dð Þ þ CO2 ! O 3Pð Þ þ COQ
! O 1Dð Þ þ COQ:
They determined that the inelastic and the elastic channels
comprise 68 and 32% of the total exchange cross section,
respectively. We argue by analogy that isotopic exchange
reactions also occur for the isoelectronic reaction:
Q 1D
 þ N2O! Oþ N2Q; ð1Þ
where the product O is the sum of O(3P) and O(1D). The
chemical reaction between O(1D) and N2O is well known to
have two product channels [Sander et al., 2000]
N2Oþ O 1D
 ! 2NO ð2aÞ
! N2 þ O2: ð2bÞ
The rate coefficients are k2a = 6.7  1011 and k2b = 4.9 
1011 cm3 s1, summing to a total of k2 =1.21010 cm3 s1.
There have been no reported experimental studies of (1).
GEOPHYSICAL RESEARCH LETTERS, VOL. 31, L19106, doi:10.1029/2004GL020950, 2004
1Also at Division of Chemistry and Chemical Engineering, California
Institute of Technology, Pasadena, California, USA.
Copyright 2004 by the American Geophysical Union.
0094-8276/04/2004GL020950
L19106 1 of 4
[6] In this paper we use quantum chemical methods to
investigate the structure and energetics of the potential
energy surfaces for the O(1D) + N2O reaction, to assess
the possibility of the isotopic exchange reaction. Using the
Caltech/JPL two-dimensional (2-D) model, we also carry
out a modeling study to estimate the rate coefficient of the
isotopic exchange reaction that would be required to explain
the observed D17O.
2. Computational Chemistry Calculations
[7] To better understand the potential energy surfaces on
which the N2O + O(
1D) reaction takes place, we have used
quantum chemistry methods to investigate the structures
and energetics of various intermediates. We have used
B3LYP density functional theory (DFT) [Hohenberg and
Kohn, 1964; Kohn and Sham, 1965; Becke, 1993] with
Dunning’s cc-pVTZ(-f) basis set [Dunning, 1989], as
implemented in the Jaguar program suite (Jaguar 5.5,
Schrodinger, LLC, Portland, Oregon, 2003), to obtain
optimized structures for all stable species and transition
states, including zero-point energy corrections. We then
performed single-point coupled-cluster calculations at the
SD(T) level (CC-SD(T)) with the Molpro program (version
2002.1) using the optimized DFT structures to obtain more
accurate energies for all stable species and transition states.
[8] The results are summarized in Figure 1. Overall, the
calculated energetics are within the expected uncertainty
limits. For N2O + O(
1D) ! N2 + O2, DEexpt = 124.5,
DEcalc = 128.2, Dexpt
calc – = 3.7 kcal/mole, while for
(NO + NO)–(N2 + O2), DEexpt = 43.2, DEcalc = 43.0,
Dexpt
calc – = 0.2 kcal/mole. Performance is slightly worse for
singlet-triplet differences, but still within a factor of 2 of the
estimated uncertainty. All intermediates and products are
predicted to be exothermic with respect to the reactants and
the reaction proceeds with no significant barrier, in
agreement with experimental reaction kinetics [Sander et
al., 2000]. For O(1D) attack on the terminal N atom, there is
a van der Waals entrance channel that leads to a trans-ONNO
intermediate (IV) that lies only 	20 kcal/mole above the
2NO product channel. Lying close in energy to the trans-
ONNO structure (IV) is a cyclic intermediate (V) in which
one of the oxygen atoms is bound to both N atoms. None of
these structures can lead directly to oxygen atom exchange,
and no barriers are predicted along this reaction pathway.
[9] For O(1D) attack on the central N atom, however,
there is a C2v intermediate (II) that lies 	27 kcal/mole
below the entrance channel. The structure of this inter-
mediate is similar to that the C2v symmetry of CO3
intermediate thought to promote oxygen atom exchange in
the O(1D) + CO2 reaction. There is a 	15 kcal/mole barrier
that leads directly to the N2 + O2 products at the level of
theory employed. Although this barrier lies below the
entrance channel energy, this barrier can still cause elastic
(i.e., non-reactive) scattering. If the C2v intermediate (II) is
sufficiently long-lived (of order the vibrational period(s), or
a few picoseconds), its symmetry should make oxygen atom
exchange possible on the singlet surface. Quenching to the
triplet surface may also lead to oxygen atom exchange, but
is not calculated here.
[10] The degree to which these processes occur will be
sensitive not only to the height and shape of the potential
barrier that leads to N2 + O2, but also to the coupling of this
path on the potential energy surface to other barriers that
connect to the cyclic intermediate (V) and thereby to the NO
product channel. In principle, the degree of exchange could
be calculated by multi-dimensional reactive scattering
calculations, but a much finer sampling of the potential
energy surface would be required. Such calculations are
beyond the scope of this work; however, the calculations
presented here illustrate the plausibility of isotopic
exchange via (1) and motivate future experimental study
of isotopic dependencies in the O(1D) + N2O system.
3. Photochemical Modeling Results and
Discussion
[11] The Caltech/JPL 2-D model for simulating the dis-
tribution of N2O and its isotopologues and isotopomers has
been described elsewhere [Morgan et al., 2004]. The model
has 18 latitudes from pole to pole, and 40 layers from 1000
to 0.01 mbar. The wind fields are derived from the
reanalysis product from the National Center for Environ-
mental Prediction and Department of Energy [Jiang et al.,
2004]. Four new reactions are added to the model. They are
reaction (1) and
O 1D
 þ N2Q! Qþ N2O ð3Þ
P 1D
 þ N2O! Oþ N2P ð4Þ
O 1D
 þ N2P! Pþ N2O; ð5Þ
Figure 1. A summary of the calculations for the O(1D) +
N2O singlet reaction surface. All energies are in kcal/mol,
and are the results of CC-SD(T) calculations with zero-point
energy corrections from B3LYP density functional theory.
Transition states for the 2NO product channel are connect
by the dotted curve as a guide to the eye, the N2 + O2
product channel is highlighted by the solid curve. Experi-
mental values for the reactant and product energetics are
summarized in the table at center.
L19106 LIANG ET AL.: N2O ISOTOPIC ANOMALY L19106
2 of 4
where P = 17O. These four reactions describe the forward
and reverse exchange for 18O and 17O with N2
16O. For lack
of information, we assume k1 = k3 = k4 = k5 = gk2, where
g is an unknown constant.
[12] One may ask why the addition of these four reactions
could produce an MIF in N2O. The reason is simple.
Suppose N2O were in photochemical equilibrium with
O(1D), then the reactions (1), (3), (4) and (5) imply
N2Q=N2O ¼ Q 1D
 
=O 1D
  ð6Þ
N2P=N2O ¼ P 1D
 
=O 1D
 
: ð7Þ
In other words, N2O equilibrates isotopically with O(
1D).
Since O(1D) is known to have MIF, its MIF is transferred to
N2O, much as in the case of CO2 [Thiemens et al., 1991;
Yung et al., 1991; Wen and Thiemens, 1993; Yung et al.,
1997]. Mixing in the atmosphere tends to dilute the MIF,
and the net effect is much less than that given by the
isotopic equilibrium. Because dQ  dP 100%, the
induced MIF may still be non-trivial even after extensive
dilution by mixing.
[13] The results of our model for the cases g = 0.3,
0.5 and dQ = dP = 100% are summarized in Figure 2.
The dotted line is the baseline case, i.e., no isotopic
exchange (g = 0). The solid (dashed) line shows the
impact of the new exchange reactions for g = 0.5 (0.3).
These values of g are consistent with the experimentally
determined isotopic exchange reaction for O(1D) + CO2.
Figure 2 illustrates that isotopic exchange reactions in the
range 0.3 < g < 0.5 are of the correct order of magnitude to
explain the bulk of the 2-isotope observations for atmo-
spheric N2O.
[14] In summary, we have demonstrated the feasibility of
transferring oxygen MIF enrichment from O3 to N2O if the
exchange channel were as large as 30–50% of the reactive
channels. Even a smaller isotopic exchange rate coefficient
would still account for part of the observed MIF, leaving the
rest to be explained by other mechanisms. Experimental
verification of our proposal would yield interesting new
insight into the atmospheric chemistry of N2O.
[15] Acknowledgments. We thank R.-L. Shia for helping to run the
2-D model for N2O and M. Gerstell, X. Jiang, and J. Kaiser for helpful
discussions. We also thank the two referees for helpful comments. Special
thanks are due M. H. Thiemens for sending us his data and for many
illuminating conversations. This work was supported in part by NSF grant
ATM-9903790. The extension of the Caltech/JPL 2-D model to the
troposphere was supported by NASA ACMAP grant NAG1-1806. Sandia
is a multiprogram laboratory operated by Sandia Corporation, a Lockheed
Martin Company, for the United States Department of Energy under
Contract No. DE-AC04-94AL85000. RPMwould like to thank Schrodinger,
LLC, for use of the Jaguar program.
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G. A. Blake, M.-C. Liang, and Y. L. Yung, Division of Geological and
Planetary Sciences, California Institute of Technology, MS150-21, 1200 E.
California Boulevard, Pasadena, CA 91125, USA. (mcl@gps.caltech.edu)
C. E. Miller, Atmospheric Chemistry Element, Jet Propulsion Laboratory,
Pasadena, CA 91109, USA.
R. P. Muller, Computational Materials and Molecular Biology, Sandia
National Laboratories, P.O. Box 5800, MS0196, Albuquerque, NM 87185,
USA.
L19106 LIANG ET AL.: N2O ISOTOPIC ANOMALY L19106
4 of 4


Evidence for O-atom exchange in the O(^1D) + N_2O reaction as the source of mass-independent isotopic fractionation in atmospheric N_2O

15N and 18O characteristics of nitrous oxide—A global perspective,

A BornOppenheimer photolysis model of N2O fractionation,

A semianalytic model for photo-induced isotopic fractionation in simple molecules,

Chemical kinetics and photochemical data for use in stratospheric modeling,

Density functional thermochemistry III: The role of exact exchange,

Gaussian basis sets for use in correlated molecular calculations: 1. The atoms boron through neon and hydrogen,

Global modeling of the isotopic analogues of N2O: Stratospheric distributions, budgets, and the 17O-18O mass-independent anomaly,

Inhomogeneous electron gas,

Isotopic fractionation of nitrous oxide in the stratosphere: Comparison between model and observations,

Massindependent isotopic compositions in terrestrial and extraterrestrial solids and their applications,

Oxygen isotope fractionation in stratospheric CO2,

Production, isotopic composition, and atmospheric fate of biologically produced nitrous oxide,

QBO and QBO-annual beat in the tropical total column ozone: A two-dimensional model simulation,

Self-consistent equations including exchange and correlation effects,

The 18O/16O and 17O/16O ratios in atmospheric nitrous oxide: A mass-independent anomaly,

The origin of the anomalous or ‘‘mass-independent’’ oxygen isotope fractionation in tropospheric N2O,

Thiemens

https://authors.library.caltech.edu/34155/1/2004GL020950.pdf

Evidence for O-atom exchange in the O(^1D) + N_2O reaction as the source of mass-independent isotopic fractionation in atmospheric N_2O

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