Transcript Slide 1

Evidence for Photolytic
Production of Cyclic-N3
Dr. Petros Samartzis, Dr. Nils Hansen,
Yuanyuan Ji, Alec M. Wodtke
Dept. of Chemistry and Biochemistry
UCSB, Santa Barbara CA 93106
Air Force Office of Scientific Research
Outline

Background
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Poly-nitrogen allotropes are rare… …ring structures even more so.
Three experiments provide evidence for photochemical
production of cyclic N3
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Velocity Map Imaging
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Photofragmentation translational spectroscopy
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Thermochemistry of all molecules made from one Cl atom and three N
atoms.
Primary and Secondary decomposition pathways resulting from ClN3
photolysis
VUV synchrotron photoionization based photofragmentation
translational spectroscopy

Two photo-ionization thresholds for N3
Some background on all
Nitrogen Chemistry
…especially rings
The Nitrogen atom as a chemical building
block

N is iso-electronic with CH
If benzene,
HC
HC
H
C
C
H
Then, why not
Hexa-azabenzene
CH
N
CH
N
N
N
N
N
Basic Problem of Stability with allNitrogen Ring Allotropes
N
 << 0
N
H
C
HC
N
N
0
CH
HC
N
CH
+
+
N
CH
Theory on Cyclic Nitrogen Allotropes
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T. J. Lee et al., J. Chem. Phys. 94, 1215-1221 (1991).
W. J. Lauderdale et al., J. Phys. Chem. 96, 1173-1178 (1992).
D. R. Yarkony, J. Am. Chem. Soc. 114, 5406-5411 (1992).
R. Klein et al., Chem. Pap.-Chem. Zvesti 47, 143-148 (1993).
K. M. Dunn et al., J. Chem. Phys. 102, 4904-4908 (1995).
M. N. Glukhovtsev et al., Inorg. Chem. 35, 7124-7133 (1996).
A. A. Korkin et al., J. Phys. Chem. 100, 5702-5714 (1996).
M. T. Nguyen et al., Chem. Berichte 129, 1157-1159 (1996).
J. Wasilewski, J. Chem. Phys. 105, 10969-10982 (1996).
A. Larson et al., J. Chem. Soc.-Faraday Trans. 93, 2963-2966 (1997).
M. L. Leininger et al., J. Phys. Chem. A 101, 4460-4464 (1997).
M. Bittererova et al., J. Phys. Chem. A 104, 11999-12005 (2000).
M. Bittererova et al., Chem. Phys. Lett. 340, 597-603 (2001).
M. Bittererova et al., Chem. Phys. Lett. 347, 220-228 (2001).
T. J. Lee et al., Chem. Phys. Lett. 345, 295-302 (2001).
H. Ostmark et al., J. Raman Spectrosc. 32, 195-199 (2001).
M. Tobita et al., J. Phys. Chem. A 105, 4107-4113 (2001).
M. Bittererova et al., J. Chem. Phys. 116, 9740-9748 (2002).
T. J. Lee et al., Chem. Phys. Lett. 357, 319-325 (2002).
Many interesting allotropes have been predicted
by theory
Stable
Stable
?
Hexa-aza diazide
189 kcal/mol
Hexa-azabenzene
212 kcal/mole
?
Hexa-aza Dewar-benzene
244 kcal/mol
?
Motoi Tobita and Rodney J.
Bartlett J. Phys. Chem. A 2001,
105, 4107-4113
Hexa-aza Prismane
323 kcal/mol
Hexa-aza bicyclopropenyl
245 kcal/mol
N8
N10
Poly-Nitrogen Chemistry
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Limited number of allotropes
belonging to this family have
been synthesized and
identified.
N≡N
-0.11
N
N=N=N
N=N=N-
N
N
+1
0.22
N
0.33
N
N5+ Synthesis proved by IR and crystal
structures.
N5- Identified in fragmentation of
electrospray ionization mass spectra.
Tetra-azahedrane (tetrazete): The search
continues
Obeys the octet rule.
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Dissociation to 2N2 releases
760 kJ/mol. (Interesting
HEDM candidate)
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Must proceed over 250
kJ/mole barrier to be spinallowed
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Spin-forbidden channels
have lower barriers…
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Produce excited electronic
state products
N
N
N
N
Matrix Isolation
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Nitrogen discharges
quenched on cold
surface
IR spectra recorded
Compared to
theoretical predictions
Very recent work from Radziszewski appears promising
Theoretical simulation of isotopic IR
spectrum of Td - N4
Cyclic-N3: the “simplest” all-Nitrogen ring
allotrope and precursor to Td-N4
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C2v Symmetry
Bound by 1 eV if “spin conserved”
@1 eV barrier to linearization
precursor to tetra-azahedrane
Bittererova, Östmark and Brinck, J. Chem. Phys. 116 9740 (2002)
Pseudo-rotation in cyclic N3
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Energy minimum exhibits C2v symmetry
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Shallow barrier through to other isomers.
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Barrier lower than zero-point energy
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Molecule exhibits pseudo-rotation
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Photochemical angular distribution will be
broadened
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All N-atoms are equally likely to leave
Babikov, Morokuma, Zhang… several recent papers have appeared.
Geometric Phase Effect
GBO
BO

‫׀‬
+
+
2A
2
expi 23  
2A
2
2B
1
2B
1

+
‫׀‬

2A
2
Babikov et al. , J. Chem. Phys., 121, (24), 22 December 2004
+
+
2B
1
Vibrational Wave-functions With and
Without the Geometric Phase Effect
#1: BO A1 1310 cm-1
#1: GPE , E, 1325cm-1
#2: E 1364 cm-1
#2: GPE, A1 1401 cm-1
#3: E 1561 cm-1
#3: GPE, A2, 1502 cm-1
Babikov et al. , J. Chem. Phys., 121, (24), 22 December 2004
Up to now, no conclusive
experimental evidence
Surprisingly, no effort has been made to
exploit UV photolysis to make this
metastable compound.
Theoretical predictions about cyclic N3
Eneryg (kcal/mol)
63.38 (65.72)
D0_C2v_TS
70.00
62.18 (64.85)
D0_Cs_TS
59.10
MSX_C2v_2A2/4B1_1
58.90
58.56
60.00
N2
+N(2D)
MSX_C2v_2B1/4A2
59.64 (61.76)
D0_Cv_TS
MSX_Cs_2A"/4A"_1
52.47
56.49 (58.98)
N2+N(2D)
MSX_Cs_2A"/4A"_2
50.00
CI(2B1/2A2)
45.35
45.86 (48.69)
Q1_Cs_TS
47.39
MSX_C2v_2A2/4B1_2
40.00
43.82 (46.18)
Q1_4B1
30.53 (33.09)
30.00
D0_2A2_1
D0_2B1
D0_2B1
30.28 (32.20)
30.28 (32.20)
20.00
10.00
-0.23 (2.26)
0.00
N2+N(4S)
linear N3 0.00 (0.00)
Figure 3, JCP, Zhang
Zhang, Morokuma and Wodtke (in press)
N2+N(4S)
Three experimental approaches
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Velocity Map Imaging
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Photofragmentation translational spectroscopy
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Thermochemistry of all molecules made from one Cl atom
and three N atoms.
Primary and Secondary decomposition pathways resulting
from ClN3 photolysis
VUV synchrotron photoionization based
photofragmentation translational spectroscopy
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Two photo-ionization thresholds for N3
Velocity Map imaging of Cl
from ClN3→Cl+N3
…thermochemistry of Cl/N/N/N
Velocity Map Ion Imaging
Photolysis-Detection
Ew
Laser
Molecular Beam
3D-Product
Distribution
2D-Projection
Inverse-Abel Transformation
 3D-Distribution
 2D-Projection:
 Cut through 3D-Distribution:
M. C. Escher
Inverse Abel-Transformation
Using BASEX alla Reisler
N2O Photodissociation
N2O + h  N2 (X 1g+) + O (1D2)
 Velocity Map
50
55
b ~ -1
60
65
75
70
52
202.6
w/o centroiding
w/ centroiding
202.8
203.0
80
56
203.2
60
203.4
v' = 0
N'
v' = 1
85
65
71
N'
203.6
203.8
Wavelength / nm
“Improved two-dimensional product imaging: The real-time ion-counting method”, Chang BY, Hoetzlein RC,
Mueller JA, Geiser JD, Houston PL, RSI 69 (4): 1665-1670 APR 1998
“Photodissociation of N2O: J-dependent anisotropy revealed in N2 photofragment images”, Neyer DW,
Heck AJR, Chandler DW, JCP, 110 (7): 3411-3417 FEB 15 1999
Comparison to Cornell Experiments
Determines the N2-O bond energy within several cm-1
N2O (0,1,0)
N2O (0,0,0)
Santa Barbara machine
Cornell machine*
“Improved two-dimensional product imaging: The real-time ion-counting method”, Chang BY, Hoetzlein RC,
Mueller JA, Geiser JD, Houston PL, RSI 69 (4): 1665-1670 APR 1998
*
ClN3 absorption spectrum
1A”1A’
3.1 eV
S2
Theoretical calculations of
Zhang and Morokuma
S0
S1
N2
N-atom
S3
2A’1A’
5.1 eV
Cl-atom
2A”1A’
5.6 eV
6
Experimental
Absorption Spectrum
5
4
3
E / eV
2
1
0
Experiments with 6 eV photons: Formation of
N2( J=68 ) + NCl(X3 and a1)
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Parallel transition: b=1.96
P(a)/P(X) = 0.78/0.22
N2 + NCl(X)
N2 + NCl(a)
Ereac.(1) = 0.21 eV

K.E.R / eV
3.0
3.5
4.0
4.5
5.0
5.5
6.0 6.5
E(a-X)
7.0
Thermochemistry of ClN3  N2 + NCl
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Maximum release of
translational energy
provides accurate
thermochemistry
ClN3  N2(X) +NCl:
E = -0.93eV
ClN3N2(a) +NCl:
E = 0.22eV
Imaging of ClN3 + 2 h  ClN3+ + e-  NCl+ +
N2 confirms this thermochemistry
NCl+
b=1.1

Two components
MAX
ET
0.0
0.5
1.0
1.5
Cl* Translational Energy / eV
2.0
Reconstructed v-map
Internally cold
linear N3
Symmetrized image
Velocity Map Image of Cl from ClN3 N3 +
Cl(2P1/2)
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D0(Cl-N3) from Velocity Map Imaging
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E is known from laser
wavelength.
EMAX is derived
mCl vMAX
= mN3 vMAX
Cl
N3
2
1
1
MAX 2



mCl v MAX
=
m
v
Cl
N3
N3 
2
2
mClEMAX
= mN3 EMAX
Cl
N3
2 mN3 + mCl
1

D0 (Cl - N3 ) = h - mCl vMAX
Cl
2
mN3
Thermochemistry of the Cl/N/N/N
Predicted by Bittererova et al.
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Zero Kelvin
Heats of
Formation
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All heats of
formation now
known within
 0.1 eV
Velocity Map Image of
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2
Cl( P3/2)
Bimodal energy distribution
Angular Distributions parallel but not identical
45% of Eava in translation
80% of Eava in translation
Photofragmentation
translation spectroscopy
Establishing the decomposition
pathways important in ClN3 photolysis.
Photofragmentation Translational
Spectroscopy
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Electron bombardment
ionization of
photofragments provides
universal detection
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With Ion fragmentation
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Detector is rotate-able to
accept products recoiling at
different angles, Q
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TOF reflects laboratory
speeds, from which we
extract the c.m. frame
translational energy
release, P(ET)
NCl+ observed, but weak!
ClN3 + h→ N2+NCl(1) minor
0.0005
0.0003
Probability / a.u.
Signal (a.u.)
b = - 0.3
Data
NCl
0.0004
600
0.0002
Eava
0.0001
0.0000
0
50
100
150
TOF s
200
250
300
0
25
50
75
Translational Energy / kcal*mol
100
-1
75 kcal/mol in products of this reaction!
Cl+-TOF, 50o:
Cl + N3 is dominant channel
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Consistent with
VMI, bimodal
TOF observed
ClN3 + h →
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Lin-N3 + Cl
HEF-N3+ Cl
b = 1.7
0.012
Ion counts/laser shot
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DATA
lin. N3
HEF N3
0.010
NCl sec. photodiss.
Total
0.008
500
0.006
0.004
b = 0.4
0.002
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ClN3 + h →


NCl + N2
NCl+ h → N+Cl
0.000
0
50
100
150
TOF s
200
250
300
+
N3 ,
ClN3 + h →
0.008
lin-N3 + Cl
HEF-N3+ Cl
0.007
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Long-lived
HEF N3
Ion Counts / laser shot
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bimodal N3 distribution
b = 1.7
Data
Total
lin N3
0.006
HEF N3
0.005
500
0.004
0.003
0.002
b = 0.4
0.001
0.000
0
50
100
150
TOF s
200
250
300
Translational Energy Distributions
of ClN3→Cl+ N3
M1v1 = M2v2

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Experiments at m/z=42
(N3+) and m/z=35 (Cl+)
are fundamentally
redundant.
Yet differences arise
Mass 35
Mass 42
0.04
0.03
Probability

0.02
0.01

Likely due to N3
dissociation.
0.00
0
10
20
30
40
50
Center of mass energy [kcal]
60
70

PTS at 248 nm.

Both Features shifted
by change in photon
energy.
VMI-Experiment (235 nm)
PTS-Experiment (248 nm)
= 69 kcal/mol
VMI at 235 nm summed
over Cl (2PJ)
ET
MAX
ET
MAX

= 38 kcal/mol
Wavelength Dependence
0
10
20
30
40
50
Etrans /kcal*mol
-1
60
70
80
N2+, unimolecular decomposition and
photolysis of N3
0.020
DATA
MODELTOF
2
N3 + h -> N( D) + N2
0.018

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N3 → N2 + N(4S)
N3 → N2 + N(2D)
N3 + h→ N2+N(2D)
Ion Counts / laser shot
0.016
4
300
0.014
HEF-N3 -> N( S) + N2
lin. N3
0.012
HEF-N3
0.010
NCl+ N2
2
0.008
HEF-N3 -> N( D) + N2
0.006
0.004
0.002
0.000
0
50
100
150
TOF s
200
250
300
N+, unimolecular decomposition and
photolysis of N3
0.006
DATA
TOTAL
NCl + h -> N + Cl
N3 + h-> N + N2



N3 → N2 + N(4S)
N3 → N2 + N(2D)
N3 + h→ N2+N(2D)
Ion counts / laser shot
500
0.004
4
N3 -> N( S) + N2
2
N3 -> N( D) + N2
lin-N3
HEF-N3
0.002
0.000
0
50
100
150
TOF s
200
250
300
N3 Secondary photodissociation

Data fit by two models
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lin-N3 + h→N(2D)+N2
HEF-N3 + h→N(2D)+N2
Evidence suggests
the selective photodissociation of HEFN3 at 248 nm
Primary and Secondary dissociation
channels of 248 nm photolysis of ClN3

ClN3 + h→ NCl+ N2



NCl + h→ N + Cl
ClN3 → Cl+ N3 (low energy form)
ClN3 → Cl+ N3 (high energy form)



N3 → N2 + N(4S)
N3 → N2 + N(2D)
N3 + h→ N2+N(2D)
VUV synchrotron photoionization
based photofragmentation translational
spectroscopy
Two thresholds in photo-ionization for N3
Experiment nearly unchanged

Instead of electron
impact ionization of
photofragments

We can use tunable
VUV photons for near
threshold ionization


Eliminate ion
fragmentation
Measure ionization
threshold
+
Cl
and
+
N3 TOF
N3+
Cl+
0.010


Bimodal features
seen again
N3 observed with
much better S/N
Two forms of N3
well resolved in the
TOF distribution
m/e=42
0.008
Q=
m/e=35
0.05

Q=

0.04
0.006
0.03
0.004
0.02
0.002
0.01
0.00
0.000
0.010
Counts/Passes, power

Counts/Passes, power
0.06
0.06
m/e=35
m/e=42
0.008
Q=

Q=
0.05

0.04
0.006
0.03
0.004
0.02
0.002
0.01
0.000
0.00
20
30
40
50
60
70
80
TOF (s)
90
100
20
30
40
50
60
70
TOF (s)
80
90
100
TOF spectra of N3 vs. ionization photon
energy
White light continuum
produces “below
threshold ions”
0.010
0.005

11.07 eV ionization of
“fast peak” matches
literature value for
linear N3
0.000
9.44
9.86
10.27
10.67
11.07
rgy
ne
E
n
tio
11.49
11.91
12.37

New threshold ~10.6
eV
12.83
30
40
50
60
Time of Flight
X
70
iza
Ion
Intensity

Two photoionization thresholds for N3
produced in ClN3 photolysis
N3 neutral TOF
1.0
0.010
0.8
x4
Counts/Passes, power
Intensity (norm.)
N3+ photoionization yield
CYCLIC N3/N3+ theory Tosi, 2004
Krylov & Babikov, 2005
0.6
0.4
● fast channel
0.2
 slow
channel
Θ = 45o
0.0
m/e=42
0.008
Q=
0.006
0.004
0.002
0.000
20
30
40
50
60
70
80
TOF (s)
10.2 10.4 10.6 10.8 11.0 11.2 11.4 11.6
synchrotron photon energy / eV
John Dyke, 1982
LINEAR N3 Experiment
With Jim Jr-Min Lin at Hsinchu, NSRRC in Taiwan

90 100
Conclusions





UV photolysis of ClN3 at 248 nm produces Cl and N3
with 0.95 quantum yield.
Primary and Secondary decomposition pathways
have been mapped out
Two energetic forms of N3 seen, whose HF’s are in
agreement with what is known for linear and cyclic
N3
VUV photoionization threshold data also in
agreement with theoretical predictions for linear and
cyclic N3
If indeed we are seeing cyclic-N3, it is long lived.
Acknowledgements

Dr. Petros Samartzis, Dr. Nils Hansen, Yuanyuan Ji,


Dr. Jason Robinson, Niels Sveum Dan Neumark,


Dept. of Chemistry and Biochemistry, UCSB, Santa
Barbara CA 93106
UC Berkeley
Dr. Jim Jr-Min Lin , Tao-Tsung Ching, Chanchal
Chadhuri, Shih-Huang Lee

National Synchrotron Radiation Research Center, Hsinchu
30077, Taiwan, Republic of China
Air Force Office of Scientific Research
National Science Foundation