Experimental findings by others suggest that OH radicals are formed in unexpected abundance on or near surfaces of 1–50 µm microdroplets comprised of pure water, but the mechanism by which these radicals are generated is not yet fully resolved. In this work, we examine two possibilities using ab initio electronic structure methods: (1) electron transfer (ET) from a microdroplet surface-bound OH anion to a nearby H3O+ cation and (2) proton transfer (PT) from such a H3O+ cation to a nearby OH anion. Our findings suggest that both processes are possible but only if the droplet’s underlying water molecules comprising the microdroplet provide little screening of the Coulomb interaction between the anion and cation once they reach ∼10 Å of one another. In the ET event, an OH radical is formed directly; for PT, the OH formation occurs because the new O–H bond formed by the transferred proton is created at a bond length sufficiently elongated to permit homolytic cleavage. Both the ET and PT pathways predict that H atoms will also be formed. Finally, we discuss the roles played by strong local electric fields in mechanisms that have previously been proposed and that occur in our two mechanisms.

1.
J. K.
Lee
,
K. L.
Walker
,
H. S.
Han
,
J.
Kang
,
F. B.
Prinz
,
R. M.
Waymouth
,
H. G.
Nam
, and
R. N.
Zare
, “
Spontaneous generation of hydrogen peroxide from aqueous microdroplets
,”
Proc. Natl. Acad. Sci. U. S. A.
116
,
19294
19298
(
2019
).
2.
M. A.
Mehrgardi
,
M.
Mofidfar
, and
R. N.
Zare
, “
Sprayed water microdroplets are able to generate hydrogen peroxide spontaneously
,”
J. Am. Chem. Soc.
144
,
7606
7609
(
2022
).
3.
H.
Hao
,
I.
Leven
, and
T.
Head-Gordon
, “
Can electric fields drive chemistry for an aqueous microdroplet?
,”
Nat. Commun.
13
,
280
(
2022
).
4.
S. M.
Kathmann
,
I. F. W.
Kuo
, and
C. J.
Mundy
, “
Electronic effects on the surface potential at the Vapor–Liquid interface of water
,”
J. Am. Chem. Soc.
130
,
16556
16561
(
2008
).
5.
P. L.
Geissler
,
C.
Dellago
,
D.
Chandler
,
J.
Hutter
, and
M.
Parrinello
, “
Autoionization in liquid water
,”
Science
291
,
2121
2124
(
2001
).
6.
A.
Hassanali
,
M. K.
Prakash
,
H.
Eshet
, and
M.
Parrinello
, “
On the recombination of hydronium and hydroxide ions in water
,”
Proc. Natl. Acad. Sci. U. S. A.
108
,
20410
20415
(
2011
).
7.
C.
Møller
and
M. S.
Plesset
, “
Note on an approximation treatment for many-electron systems
,”
Phys. Rev.
46
,
618
622
(
1934
).
8.
M.
Head-Gordon
,
J. A.
Pople
, and
M. J.
Frisch
, “
MP2 energy evaluation by direct methods
,”
Chem. Phys. Lett.
153
,
503
506
(
1988
).
9.
M. J.
Frisch
,
M.
Head-Gordon
, and
J. A.
Pople
, “
A direct MP2 gradient method
,”
Chem. Phys. Lett.
166
,
275
280
(
1990
).
10.
R. A.
Kendall
,
T. H.
Dunning
, Jr.
, and
R. J.
Harrison
, “
Electron affinities of the first‐row atoms revisited. Systematic basis sets and wave functions
,”
J. Chem. Phys.
96
,
6796
6806
(
1992
).
11.
M. W.
Schmidt
and
K.
Ruedenberg
, “
Effective convergence to complete orbital bases and to the atomic Hartree–Fock limit through systematic sequences of Gaussian primitives
,”
J. Chem. Phys.
71
,
3951
3962
(
1979
).
12.
M.
Gutowski
and
J.
Simons
, “
Double-Rydberg anions: Ground-state electronic and geometric stabilities
,”
J. Chem. Phys.
93
,
3874
3880
(
1990
).
13.
D.
Hegarty
and
M. A.
Robb
, “
Application of unitary group methods to configuration interaction calculations
,”
Mol. Phys.
38
,
1795
1812
(
1979
).
14.
R. H. A.
Eade
and
M. A.
Robb
, “
Direct minimization in MC SCF theory. The quasi-Newton method
,”
Chem. Phys. Lett.
83
,
362
368
(
1981
).
15.
H. B.
Schlegel
and
M. A.
Robb
, “
MC SCF gradient optimization of the H2CO → H2 + CO transition structure
,”
Chem. Phys. Lett.
93
,
43
46
(
1982
).
16.
F.
Bernardi
,
A.
Bottoni
,
J. J. W.
McDouall
,
M. A.
Robb
, and
H. B.
Schlegel
, “
MCSCF gradient calculation of transition structures in organic reactions
,”
Faraday Symp. Chem. Soc.
19
,
137
147
(
1984
).
17.
M. J.
Frisch
,
I. N.
Ragazos
,
M. A.
Robb
, and
H.
Bernhard Schlegel
, “
An evaluation of three direct MC-SCF procedures
,”
Chem. Phys. Lett.
189
,
524
528
(
1992
).
18.
N.
Yamamoto
,
T.
Vreven
,
M. A.
Robb
,
M. J.
Frisch
, and
H.
Bernhard Schlegel
, “
A direct derivative MC-SCF procedure
,”
Chem. Phys. Lett.
250
,
373
378
(
1996
).
19.
J. J. W.
McDouall
,
K.
Peasley
, and
M. A.
Robb
, “
A simple MC SCF perturbation theory: Orthogonal valence bond Møller–Plesset 2 (OVB MP2)
,”
Chem. Phys. Lett.
148
,
183
189
(
1988
).
20.
S.
Miertuš
,
E.
Scrocco
, and
J.
Tomasi
, “
Electrostatic interaction of a solute with a continuum. A direct utilizaion of AB initio molecular potentials for the prevision of solvent effects
,”
Chem. Phys.
55
,
117
129
(
1981
).
21.
S.
Miertuš
and
J.
Tomasi
, “
Approximate evaluations of the electrostatic free energy and internal energy changes in solution processes
,”
Chem. Phys.
65
,
239
245
(
1982
).
22.
M.
Cossi
,
V.
Barone
,
R.
Cammi
, and
J.
Tomasi
, “
Ab initio study of solvated molecules: A new implementation of the polarizable continuum model
,”
Chem. Phys. Lett.
255
,
327
335
(
1996
).
23.
M. J.
Frisch
,
G. W.
Trucks
,
H. B.
Schlegel
,
G. E.
Scuseria
,
M. A.
Robb
,
J. R.
Cheeseman
,
G.
Scalmani
,
V.
Barone
,
G. A.
Petersson
,
H.
Nakatsuji
,
X.
Li
,
M.
Caricato
,
A. V.
Marenich
,
J.
Bloino
,
B. G.
Janesko
,
R.
Gomperts
,
B.
Mennucci
,
H. P.
Hratchian
,
J. V.
Ortiz
,
A. F.
Izmaylov
,
J. L.
Sonnenberg
,
D.
Williams-Young
,
F.
Ding
,
F.
Lipparini
,
F.
Egidi
,
J.
Goings
,
B.
Peng
,
A.
Petrone
,
T.
Henderson
,
D.
Ranasinghe
,
V. G.
Zakrzewski
,
J.
Gao
,
N.
Rega
,
G.
Zheng
,
W.
Liang
,
M.
Hada
,
M.
Ehara
,
K.
Toyota
,
R.
Fukuda
,
J.
Hasegawa
,
M.
Ishida
,
T.
Nakajima
,
Y.
Honda
,
O.
Kitao
,
H.
Nakai
,
T.
Vreven
,
K.
Throssell
,
J. A.
Montgomery
, Jr.
,
J. E.
Peralta
,
F.
Ogliaro
,
M. J.
Bearpark
,
J. J.
Heyd
,
E. N.
Brothers
,
K. N.
Kudin
,
V. N.
Staroverov
,
T. A.
Keith
,
R.
Kobayashi
,
J.
Normand
,
K.
Raghavachari
,
A. P.
Rendell
,
J. C.
Burant
,
S. S.
Iyengar
,
J.
Tomasi
,
M.
Cossi
,
J. M.
Millam
,
M.
Klene
,
C.
Adamo
,
R.
Cammi
,
J. W.
Ochterski
,
R. L.
Martin
,
K.
Morokuma
,
O.
Farkas
,
J. B.
Foresman
, and
D. J.
Fox
,
Gaussian 16, Revision C.01
,
Gaussian, Inc.
,
Wallingford CT
,
2016
.
24.
C.
Gonzalez
and
H. B.
Schlegel
, “
An improved algorithm for reaction path following
,”
J. Chem. Phys.
90
,
2154
2161
(
1989
).
25.
A nice modern treatment can be found in
E. P.
Glasbrenner
and
W. P.
Schleich
, “
The Landau–Zener formula made simple
,”
J. Phys. B: At., Mol. Opt. Phys.
56
(
10
),
104001
(
2023
).
26.
N. I.
Hammer
,
J.-W.
Shin
,
J. M.
Headrick
,
E. G.
Diken
,
J. R.
Roscioli
,
G. H.
Weddle
, and
M. A.
Johnson
, “
How do small water clusters bind an excess electron?
,”
Science
306
,
675
679
(
2004
).
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