Nitrosodifluoroamine, F2N2O

Article abstract of DOI:10.1021/jp994240g

Nitrous oxide, N₂O, was observed to react with F₂ under UV irradiation at -196 °C to form initially F₂N₂O which in accord with earlier reports from the literature decomposes above -140 °C to form N₂F₄ and NO. The apparently different reaction behavior of N₂O toward H radicals (→H₂O+N₂) and F radicals (→F₂N₂O→N₂F₄+NO) can be accounted for by thermodynamic considerations [average bond energies (kcal mol^-1): O-H (114.2)>N-H (93.3); N-F (66.4)>O-F (44.7)]. A reaction mechanism for the formation and decomposition of F₂N₂O from N₂O and F₂ (present study) and from N₂F₄ and NO has been suggested using valence bond considerations. The structure of F₂N₂O was fully optimized at the electron correlated MP2(FULL)/cc-pVTZ level of theory and was shown to possess C₁ symmetry. A frequency analysis clearly shows the C₁ structure to represent a true minimum (NIMAG = 0), whereas the earlier reported planar C_s structure was shown to represent a first-order transition state (NIMAG = 1).

Full text of DOI:10.1021/jp994240g

3406
J. Phys. Chem. A 2000, 104, 3406-3409
Nitrosodifluoroamine, F2N2O†,‡
§
⊥
,§
Margaret-Jane Crawford, Richard D. Harcourt, and Thomas M. Klap o1 tke*
Institute of Inorganic Chemistry, UniVersity of Munich, Butenandtstrasse 5-13 (Haus D),
D-81377, Munich, Germany, and School of Chemistry, UniVersity of Melbourne,
ParkVille, Victoria 3052, Australia
ReceiVed: December 1, 1999; In Final Form: February 7, 2000
Nitrous oxide, N
which in accord with earlier reports from the literature decomposes above -140 °C to form N
The apparently different reaction behavior of N O toward H radicals (fH O + N ) and F radicals (fF
f N + NO) can be accounted for by thermodynamic considerations [average bond energies (kcal mol ):
O-H (114.2) > N-H (93.3); N-F (66.4) > O-F (44.7)]. A reaction mechanism for the formation and
decomposition of F O from N O and F (present study) and from N and NO has been suggested using
valence bond considerations. The structure of F O was fully optimized at the electron correlated MP2(FULL)/
cc-pVTZ level of theory and was shown to possess C symmetry. A frequency analysis clearly shows the C
2
O, was observed to react with F
2
under UV irradiation at -196 °C to form initially F
2 2
N O
and NO.
2 4
F
2
2
2
2 2
N O
-
1
2 4
F
N
2 2
2
2
2 4
F
2 2
N
1
1
structure to represent a true minimum (NIMAG ) 0), whereas the earlier reported planar C
s
structure was
shown to represent a first-order transition state (NIMAG ) 1).
Introduction
SCHEME 1: Valence bond structures 1-4
It is well-known that compared to other oxides of nitrogen,
nitrous oxide, N2O, is relatively unreactive and inert toward
5
-7
halogens and alkali metals at room temperature.
With
hydrogen, however, the main reaction that occurs at high
temperature is the formation of N2 and H2O (eq 1). The
N O + H f N + H O
(1)
2
2
2
2
mechanism of this reaction is not simply dependent on the
dissociation of nitrous oxide, and for the propagation, the
following mechanism which is based on the presence of H
radicals has been suggested (eqs 2 and 3).7 More recently, ab
-9
ization indicated in 2 leads to 3, which participates in resonance
with 4, thereby generating N2 and OH as products.11
H + N O f OH + N2
(2)
(3)
2
In the present contribution we want to report on an experi-
mental study concerning the reactivity of N2O with fluorine
radicals at low temperature. In the theoretical study we analyze
the observed reaction behavior of N2O with F radicals and
compare it with the reactivity of N2O toward H radicals.
OH + H f H O + H
2
2
initio studies have been performed for the reaction of H radicals
with N2O (eq 2) for which a direct mechanism was calculated
10
to proceed via an NNOH transition state. The calculated
structure for this transition state (NN, 1.13 Å; NO, 1.23 Å; OH,
Results and Discussion
1
.52 Å; NNO, 1.48°; NOH, 113°)10 is in accord with that which
After an equimolar mixture of N2O and F2 is irradiated at
is implied by resonance between the increased-valence structures
and 2 (Scheme 1). At the transition state, the potential energy
curves for structures 1 and 2 cross. Therefore the valence bond
VB) description of the transition state involves equal contribu-
tions from these two structures. The electronic reorgan-
-
196 °C with UV radiation, a dark blue liquid was observed
to form in small amounts. This liquid was observed to
1
decompose at temperatures above -140 °C. The high thermal
instability and dark blue color identified this compound as being
the known compound nitrosodifluoroamine, F2NNO.
(
1a,b
The
main reported route to forming F2NNO in the literature is the
*
To whom correspondence should be addressed. Fax: intl. + 49 (0)89
reaction of N2F4 with a 10-fold excess of NO at 310 °C followed
2
180 7492. E-mail: tmk@cup.uni-muenchen.de.
1,2
by quenching at -196 °C (eq 4). To investigate the decom-
†
This work was in part presented as a talk at the 12th European
Symposium on Fluorine Chemistry in Berlin (Aug 29 to Sep 2, 1998).
1
‡
We dedicate this paper to Professor Dr. Paul v. R. Schleyer on the
/ N F + NO f F NNO
(4)
2
2
4
2
occasion of his 70th birthday.
§
University of Munich.
University of Melbourne. (Fax: intl. + 61 (0)3 9347 5180. E-mail:
⊥
position products of the thermally highly unstable dark blue
liquid, a room temperature gas-phase IR spectrum was recorded
harcus@rubens.its.unimelb.edu.au).
1
0.1021/jp994240g CCC: $19.00 © 2000 American Chemical Society
Published on Web 03/28/2000

Nitrosodifluoroamine
J. Phys. Chem. A, Vol. 104, No. 15, 2000 3407
which showed unreacted N2O (2260, 2230, 1300, 1150, 600
TABLE 1: F
2
N
2
O Molecule in C
s
vs C
1
Symmetry
-
1
cm ; cf. refs 12a,b) starting material and small amounts of
a
C
s
-
1
N2F4 (1025, 1015, 998, 738, 540 cm ; cf. refs 12c-e) and
imag
freq/
-
1
NO (1960 cm , cf. ref 12f), which correspond to the decom-
position products as were observed in the literature. In addition
traces of SiF4, NO2, and HF (IR) as well as traces of N2 but no
F2 were identified by gas discharge (Note: A very small leak,
i.e. traces of air could also have caused the appearance of O2/
N2). Therefore, from our observations, in contrast to the
literature, N2O does indeed react with fluorine at low temper-
ature (eq 5), however, only if the reaction mixture is irradiated
method/
basis set
∆E(C
s
- C
1
-1
)/
)
-1
b
-E/au
cm
C
1
-E/au (kcal mol
PM3/VSTO-3G*
HF/4-31G
HF/6-31G*
MP2/6-31G*
MP2(FULL)/
cc-pVTZ
-464i
+8.4
381.783 563 -464i 381.812 534
382.330 612 -552i 382.366 103
383.185 934 -483i 383.223 029
383.685 778 -451i 383.716 675
+18.2
+22.3
+23.3
+19.4
a
type structures NIMAG ) 1. b For all C
type structures
1
For all C
s
F + N O f F NNO
(5)
2
2
2
NIMAG ) 0.
2 1
TABLE 2: Structural Parameters Calculated for F NNO (C
with UV; without UV irradiation no reaction was observed to
occur. Whereas N2O does not react with fluorine under ambient
conditions (atmospheric pressure at room temperature), we found
that at -196 °C F2 does indeed react with N2O to form initially
F2N2O (eq 5), which above -140° decomposes to yield N2F4
and NO (see above) (eq 6; note, eq 6 is the reverse of eq 4).
Symmetry) at MP2(FULL)/cc-pVTZ Level of Theory
1
bond
type
ν/cm-
a
d/Å
angle
∠ /deg mode
(rel int)
95 (1)
N3-F1 1.387 F1-N3-F2
N3-F2 1.369 F1-N3-N4
N3-N4 1.769 F2-N3-N4
N4-O5 1.149 N3-N4-O5
103.1
94.9
ν
1
ν
2
ν
3
ν
4
ν
5
ν
6
ν
7
ν
8
ν
9
260 (11)
306 (14)
398 (20)
539 (1)
99.1
1
109.6
F N O f / N F + NO
(6)
2
2
2
2 4
F1-N3-N4-O5 -115.7
F2-N3-N4-O5 -11.5
709 (21)
Since reaction 5 was only observed under UV irradiation, the
presence of F radicals can be assumed to be essential for the
reaction to occur. This can be seen in analogy to the reaction
mechanism proposed for the reaction of N2O with H2 which
requires the presence of H radicals. However, the products
formed (H2O and N2 vs N2F4 and NO) indicate different reaction
mechanisms for the hydrogen and fluorine reaction, where in
one case the H radicals attack the oxygen atom of N2O whereas
in the other reaction the F radicals apparently attack the terminal
N atom of the N2O. This different reaction behavior is surprising
at a first glance; however, if the average bond energies of the
binary molecules H2O, NH3, OF2, and NF3 are compared, it
950 (132)
1033 (72)
1817 (233)
a
Relative IR intensities in kilometers per mole; zpe ) 8.7 kcal mol-1
.
-1
becomes apparent the O-H bond (114.2 kcal mol ) is stronger
Figure 1. Structure of F
2 1
NNO (C symmetry) calculated at the
-
1
than the N-H bond (93.3 kcal mol ), whereas the N-F bond
MP2(FULL)/cc-pVTZ level of theory.
-
1
(
66.4 kcal mol ) is much stronger than the O-F bond (44.7
-1 7
kcal mol ). So the initial attack of the H radical at the O atom
this study it was observed that F2N2O decomposes at temper-
atures above -140 °C to form N2F4 and NO (eq 6).1
,2
of N2O can well be explained by the fact that the average O-H
-1
bond is ca. 20 kcal mol stronger than the average N-H bond,
whereas in the case of an F radical attacking the N2O molecule
it is exactly the other way around with the average N-F bond
The structure of F2NNO calculated using quantum chemi-
cal calculations (HF/4-31G) is wrongly reported in the liter-
3
,4
ature as being planar with Cs symmetry. As a frequency
analysis of this planar structure (HF/4-31G, MP2/6-31+G*,
MP2(FULL)/cc-pVTZ) shows NIMAG ) 1, this structure
merely represents a transition state (Table 1). Recalculation of
this compound as the more likely nonplanar structure (C1
-
1
being ca. 20 kcal mol stronger than the average O-F bond.
(
(
For a comparative ab initio study at configuration interaction
CI) level including polarization functions of the systems ONH3
vs ONF3, see ref 13.)
1
4
Note: The reliability of this crude average bond energy
approach is nicely demonstrated by a comparison of the
symmetry) at MP2(FULL)/cc-pVTZ generates a structure of
F2NNO which does represent a true minimum for this compound
(Figure 1, Tables 1 and 2), therefore, we here report on the
structure of F2NNO as predicted by quantum chemical calcula-
tions.
exp
experimental reaction enthalpy ∆H and the reaction enthalpy
which has been calculated using a simple Born-Haber cycle
abe
∆
H
with the above average bond energies for the following
exp
6
reaction: 3H2O + 2NF3 f 2NH3 + 3OF2; ∆H ) +195.5,
The bonding situation in the nonplanar F2N2O was also
abe
-1
15a-c
∆H
) +255.6 kcal mol . This (average bond energy) method
investigated with the help of an NBO analysis.
In view of
only represents a crude estimate since the bond energies in
radicals often differ significantly from those in closed shell
molecules. For example, the N-H bond energy in N2H is much
weaker than in N2H2.5
the high thermal instability of this molecule and its ready
dissociation into NF2 and NO radicals, it was interesting to
investigate whether the natural bond orbital (NBO) analysis
would give insight into the bonding situation. The RESONANCE
keyword was activated to permit strongly delocalized structures.
The NBO analysis nicely accounts for the rather long and weak
N-N bond in F2N2O. The standard Lewis structure to which
the NBO analysis corresponds is in agreement with that shown
in Scheme 2 (7) (see below). This resonance structure has
reasonably well-localized σ NBOs (1.982-1.999 electrons), but
one severely depleted π lone pair centered at the oxygen atom
,6
Since the F2N2O molecule which is formed in reaction 4 is
stable at low temperatures only (T < -140 °C; it decomposes
according to eq 6 to form N2F4 and NO), this finding is nicely
in agreement with an earlier study which reports on the
formation of the thermally highly unstable F2N2O from a
thermolysis reaction of N2F4 and NO followed by subsequent
quenching of the F2N2O formed at -196 °C (eq 6).1 Also in
-3

3408 J. Phys. Chem. A, Vol. 104, No. 15, 2000
Crawford et al.
SCHEME 2: Valence bond structures 5-7
Figure 2. Negative hyperconjugation in F
N4). (For an alternative formulation of negative hyperconjugation, see
ref 26).
2
N
2
O: π-LP(O5) w σ*(N3-
SCHEME 3: Valence bond structures for NF2 + NO f
F2NNO
no explosion was observed in our studies, all covalent N-F
compounds and high oxidizers should be handled with respect,
and fully protective clothing should be warn at all times.
F2 (Solvay) and N2O (Aldrich) were used as obtained. The
experimental techniques and apparatus used in this study have
1
6
been described elsewhere.
Using a Monel vacuum line, 2.0 mmol of N2O were
condensed at -196 °C into a prepassivated (ClF3, 100 °C, 2
atm, 2 h) 15 mL glass one-bulb vessel fitted with a Young valve.
At -196 °C 2.0 mmol of F2 was condensed into the reaction
vessel. The reaction vessel was then irradiated at -196 °C for
2
h using a Hg high-pressure 1000 W UV lamp (254 nm) which
(
π-LP(O5), 1.778 electrons) and corresponding high occupancy
was submerged together with the reaction vessel in a large steel
Dewar vessel containing liquid nitrogen. All glass parts were
made of quartz glass, and the distance between the UV lamp
and the reaction vessel was no more than 1 cm. A gas cell IR
spectrum was recorded at room temperature after all volatile
materials had been removed under vacuum at -196 °C.
in one σ* antibond (σ*(N3-N4), 0.260 electrons). Therefore,
the most significant type of donor-acceptor interaction (negative
hyperconjugation, anomeric effect)1
5d-f
in F2N2O that is re-
sponsible for the non-Lewis contributions is π-LP(O5) w
σ*(N3-N4) (Figure 2). This intramolecular donor-acceptor
interaction causes the NO bond to be stronger than a double
bond and the N-N bond to be weaker than a single bond. This
finding is also nicely in accord with VB structure 10 (Scheme
Computational Methods
The calculations were performed with the program package
3
, see below) which contains an N-O bond with a bond order
greater than 2 and a fractional N-N bond (b < 1). Structure
0 is obtained by spin-pairing the odd electron of F2N with the
17
Gaussian 98. The structure of F2N2O, its energy and zero point
energy (zpe), and the vibrational data were calculated at the
1
18
electron correlated MP2(FULL) level of theory using a
odd electron of NO. The latter electron occupies an antibonding
MO. Moreover, it is also in agreement with the decomposition
of the F2N2O molecule occurring at the long and weak N-N
bond forming NF2 and NO.
19
polarized standard triple-ú cc-pVTZ basis set.
The usually very poor agreement between experiment and
uncorrelated HF calculation for fluorine containing nonmetal
molecules clearly shows the great importance of electron
correlation for accurate predictions for fluorine containing
The formation of F2N2O from both (i) N2F4 and NO and from
(
ii) N2O and F2 can also be rationalized from the valence bond
20
nonmetal compounds. (Note: The F2 molecule is unbound if
11,23b,24
point of view.
5
The best VB structure for N2O is structure
21
electron correlation is not taken into account. ) Whereas the
in Scheme 2. Electronic excitation of N2O and dissociation
CCSD(T) method has generally been shown to be reliable for
of F2 leads to the formation of F2NNO with VB structure 8, via
VB structures 6 and 7.
22,23
covalently bound nonmetal compounds,
often the less
expensive MP2 method in combination with a double-ú basis
The formation and decomposition of F2NNO from N2F4
set gives very good structural results and vibrational frequencies.
(which is known to dissociate easily into radicals) and ground-
22,23
Since the contribution of the core orbitals to the binding
24b
state NO (VB structure 9) can on the other hand easily be
described by the spin-pairing mechanism shown in Scheme 3
(
25
energy is structure-dependent, the frozen core approximation
was assumed not to allow sufficient variational freedom in the
calculations in order to correctly determinate the energy and
structure for F2N2O, for which we expected to have a relatively
flat potential energy surface. Therefore we applied the more
expensive MP2(FULL) method, where all electrons are included
in a correlation calculation.
cf. F + NO f FNO).24b
Experimental Methods
Caution: Many N-F compounds are highly explosive and
often are very toxic. Fluorine, F2, is a strong oxidizer. Although

Nitrosodifluoroamine
J. Phys. Chem. A, Vol. 104, No. 15, 2000 3409
Acknowledgment. Financial support from the Deutsche
Forschungsgemeinschaft (KL 636/6-1), the Fonds der Chemis-
chen Industrie, Ludwig-Maximilians UniVersity, and the Uni-
Versity of Melbourne for a Wilsmore Fellowship to T.M.K. is
gratefully acknowledged. We also thank Professor Joel Liebman
for many inspired discussions and for his help in improving
the manuscript.
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