Properties of Diazocarbene [CNN] and the Diazomethyl Radical [HCNN] via Ion Chemistry
and Spectroscopy
Eileen P. Clifford,
§
Paul G. Wenthold,
¶
W. Carl Lineberger,*
,¶
George A. Petersson,*
,†
Katherine M. Broadus,
‡
Steven R. Kass,*
,‡
Shuji Kato,
§
Charles H. DePuy,*
,§
Veronica M. Bierbaum,*
,§
and G. Barney Ellison*
,§
Department of Chemistry & Biochemistry, UniVersity of Colorado, Boulder, Colorado 80309-0215, JILA and
the Department of Chemistry & Biochemistry, UniVersity of Colorado, Boulder, Colorado 80309-0440,
Hall Atwater Laboratories of Chemistry, Wesleyan UniVersity, Middletown, Connecticut 06459-0180, and
Department of Chemistry, UniVersity of Minnesota, Minneapolis, Minnesota 55455
ReceiVed: NoVember 20, 1997; In Final Form: February 20, 1998
We have used negative ion photoelectron spectroscopy to measure the electron affinities of diazocarbene and
the diazomethyl radical: EA( ˜
3
Σ
-
CNN) ) 1.771 ( 0.010 eV, EA( ˜
2
A′′ HCNN) ) 1.685 ( 0.006 eV,
and EA( ˜
2
A′′ DCNN) ) 1.678 ( 0.006 eV. Our experimental findings are accurately reproduced by complete
basis set (CBS) ab initio electronic structure calculations: EA( ˜
3
Σ
-
CNN) ) 1.83 ( 0.03 eV, EA( ˜
2
A′′
HCNN) ) 1.69 ( 0.03 eV. We make use of the electron affinities of CNN and HCNN, together with the gas
phase acidity of diazomethane, ∆
acid
H
298
(HCHN
2
) ) 372.2 ( 2.1 kcal mol
-1
(CBS calculated value ) 373.4
( 0.7), to find the bond enthalpies of H
2
CNN. We find DH
298
(H-CHN
2
) equal to 97 ( 2 kcal mol
-1
, which
closely agrees with the CBS-QCI/APNO-calculated value [DH
298
(H-CHN
2
) ) 98.5 ( 0.7 kcal mol
-1
]. From
proton transfer experiments in a Fourier transform mass spectrometer and a tandem flowing afterglow-selected
ion flow tube (FA-SIFT), we find ∆
acid
H
298
(HCNN) ) 352 ( 4 kcal mol
-1
in agreement with the CBS-QCI/
APNO-calculated value of 351.8 ( 0.7 kcal mol
-1
. Use of the experimental electron affinity, EA(CNN),
leads to the CH bond enthalpy of the cyanoamino radical, DH
298
(H-CNN) ) 79 ( 4 kcal mol
-1
which is in
excellent agreement with the CBS-QCI/APNO-calculated value: DH
298
(H-CNN) ) 78.7 ( 0.7 kcal mol
-1
.
If we adopt the CBS-QCI/APNO value for ∆
f
H
298
(CH
2
N
2
) [64.1 ( 0.7 kcal mol
-1
] as our reference, we
obtain ∆
f
H
298
(HCN
2
) ) 110 ( 2 kcal mol
-1
and ∆
f
H
298
(CN
2
) ) 136 ( 5 kcal mol
-1
, which are again in
agreement with the CBS-QCI/APNO values: ∆
f
H
298
(HCN
2
) ) 110.5 ( 0.7 kcal mol
-1
and ∆
f
H
298
(CN
2
) )
138.4 ( 0.7 kcal mol
-1
. We recommend revised experimental values for ∆
f
H
0
(HCN) ) 30.9 ( 0.7 kcal
mol
-1
and ∆
f
H
298
(HCN) ) 30.8 ( 0.7 kcal mol
-1
and find that the reaction CH (
2
Π) + N
2
f HCN + N(
4
S)
to be slightly endothermic, ∆
rxn
H
0
) 1.6 ( 0.7 kcal mol
-1
.
I. Introduction
In an internal combustion engine, air is the source of most
of the N atoms which produce nitric oxide. Three mechanisms
are proposed
1
for the production of NO and these are designated
as “fuel NO”, “thermal NO”, and “prompt NO.” Nitrogen-
containing fuels (“fuel NO”) will have obvious routes to produce
NO
x
but most fuels are hydrocarbons. It has been known
1
for
some time that “thermal” NO is generated in the post-
combustion region by the Zeldovich mechanism involving O
atoms and N
2
.
In most combustion processes, hydrocarbons are degraded to
produce radicals such as C, CH, and CH
2
. The “prompt”
formation of NO at the flame front is now recognized to involve
chemistry of the CH radical via the Fenimore mechanism:
2,3
The reaction of the CH radical with N
2
in (2a) provides an
interesting problem because spin is not conserved. This reaction
is written explicitly in (3)
The organic radical CH is known to cleave N
2
to generate
HCN and N atoms, and it is conjectured that the adduct HCNN
is an important intermediate in this process. The diazomethyl
(HCNN) and diazocarbene (CNN) radicals are important
combustion species because they provide low energy paths to
cleave N
2
to produce N atoms which are then rapidly oxidized
to nitric oxide.
4
Thus the reaction of CH with N
2
to produce N
§
Department of Chemistry & Biochemistry, University of Colorado.
¶
JILA and the Department of Chemistry & Biochemistry, University of
Colorado.
†
Wesleyan University.
‡
University of Minnesota.
O + N
2
f NO + N (1a)
N + O
2
f NO + O (1b)
CH + N
2
f HCN + N (2a)
HCN + O f NCO + H (2b)
NCO + H h NH + CO (2c)
NH + H h N + H
2
(2d)
N + OH f NO + H (2e)
CH(
2
Π
1/2
) + N
2
(
1
Σ
g
+
) f HCN( ˜
1
Σ
g
) + N(
4
S
3/2
) (3)
7100 J. Phys. Chem. A 1998, 102, 7100-7112
S1089-5639(98)00273-4 CCC: $15.00 © 1998 American Chemical Society
Published on Web 08/18/1998