evidence for photolytic production of cyclic-n 3
DESCRIPTION
Evidence for Photolytic Production of Cyclic-N 3. 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 - PowerPoint PPT PresentationTRANSCRIPT
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
Poly-nitrogen allotropes are rare… …ring structures even more so.
Three experiments provide evidence for photochemical production of cyclic N3
Velocity Map Imaging Thermochemistry of all molecules made from one Cl atom and three N
atoms.
Photofragmentation translational spectroscopy 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
N
NN
N
NN
CH
CHCH
CH
CHCH
If benzene, Then, why notHexa-azabenzene
Basic Problem of Stability with all-Nitrogen Ring Allotropes
CHCH
CH
CH
CHCH+
0
NN
N NN N+
<< 0
Theory on Cyclic Nitrogen Allotropes
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
Hexa-azabenzene212 kcal/mole
Hexa-aza Dewar-benzene244 kcal/mol
Hexa-aza Prismane323 kcal/mol
Hexa-aza bicyclopropenyl245 kcal/mol
Hexa-aza diazide189 kcal/mol
Motoi Tobita and Rodney J. Bartlett J. Phys. Chem. A 2001, 105, 4107-4113
Stable
Stable
??
?
N8
N10
Poly-Nitrogen Chemistry
Limited number of allotropes belonging to this family have been synthesized and identified.
N≡N
N=N=N
N=N=NN
NN
NN0.33
0.22
0.11+1
N5+ Synthesis proved by IR and
crystal structures.
N5 Identified in fragmentation
of electrospray ionization mass spectra.
Tetra-azahedrane (tetrazete): The search continues
N
N
NN
Obeys the octet rule.
Dissociation to 2N2 releases 760 kJ/mol. (Interesting HEDM candidate)
Must proceed over 250 kJ/mole barrier to be spin-allowed
Spin-forbidden channels have lower barriers…
Produce excited electronic state products
Matrix Isolation
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
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
Energy minimum exhibits C2v symmetry
Shallow barrier through to other isomers.
Barrier lower than zero-point energy
Molecule exhibits pseudo-rotation
Photochemical angular distribution will be broadened
All N-atoms are equally likely to leave
Babikov, Morokuma, Zhang… several recent papers have appeared.
׀׀
++
+ 2B12A2
2B1
2B1 2A2
2A2
׀
+׀
+׀
+
++
++
++
׀׀
++
+׀
23exp i
BO GBO
Geometric Phase Effect
Babikov et al. , J. Chem. Phys., 121, (24), 22 December 2004
#1: BO A1 1310 cm-1 #2: E 1364 cm-1 #3: E 1561 cm-1
Vibrational Wave-functions With and Without the Geometric Phase Effect
#1: GPE , E, 1325cm-1 #2: GPE, A1 1401 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
0.00
10.00
20.00
30.00
40.00
50.00
60.00
70.00
Eneryg (kcal/mol)
56.49 (58.98)
-0.23 (2.26)
N2+N(4S)
N2+N(2D)
linear N3
59.64 (61.76)
D0_Cv_TS
D0_2B1
D0_C2v_TS
N2+N(2D)
N2+N(4S)
30.28 (32.20)
63.38 (65.72)62.18 (64.85)
45.35
CI(2B1/2A2)
30.28 (32.20)
30.53 (33.09)
D0_2A2_1
0.00 (0.00)
58.56
MSX_C2v_2B1/4A2
Q1_4B1
43.82 (46.18)
Q1_Cs_TS45.86 (48.69)
58.9059.10
MSX_C2v_2A2/4B1_1
MSX_Cs_2A"/4A"_1
MSX_C2v_2A2/4B1_2
47.39
52.47MSX_Cs_2A"/4A"_2
D0_2B1
D0_Cs_TS
Figure 3, JCP, Zhang
Zhang, Morokuma and Wodtke (in press)
Three experimental approaches Velocity Map Imaging
Thermochemistry of all molecules made from one Cl atom and three N atoms.
Photofragmentation translational spectroscopy Primary and Secondary decomposition pathways resulting
from ClN3 photolysis
VUV synchrotron photoionization based photofragmentation translational spectroscopy 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
Molecular Beam
3D-Product Distribution
Photolysis-Detection
LaserEw
2D-Projection
Inverse-Abel Transformation
Inverse Abel-Transformation Using BASEX alla ReislerM. C. Escher
3D-Distribution
2D-Projection:
Cut through 3D-Distribution:
N2O + h N2 (X 1g+) + O (1D2)
Velocity Map
w/ centroidingw/o centroiding
“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
~ 1
N2O Photodissociation
“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
N2O (0,0,0)
N2O (0,1,0)
Comparison to Cornell Experiments
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
Determines the N2-O bond energy within several cm-1
ClN3 absorption spectrum
0123456E / eV
S 0S 1S 2S 3
1A”1A’3.1 eV
2A’1A’5.1 eV
2A”1A’5.6 eV
ExperimentalAbsorption Spectrum
Theoretical calculations of Zhang and Morokuma
Cl-a
tom
N-a
tom
N2
Experiments with 6 eV photons: Formation of N2( J=68 ) + NCl(X3 and a1) Parallel transition: P(a)/P(X) = 0.78/0.22
Thermochemistry of ClN3 N2 + NCl
Maximum release of translational energy provides accurate thermochemistry
ClN3 N2(X) +NCl:
E = 0.93eV
ClN3N2(a) +NCl:
E = 0.22eV
Imaging of ClN3 + 2 h ClN3 + e
NCl + N2 confirms this thermochemistry
=1.1
NCl
Velocity Map Image of Cl from ClN3 N3 + Cl(2P1/2)
Sym
met
rized
imag
eR
econ
stru
cted
v-m
ap
0.0 0.5 1.0 1.5 2.0
EMAX
T
Cl* Translational Energy / eV
Two components
Inte
rnal
ly c
old
linea
r N
3
D0(Cl-N3) from Velocity Map Imaging
E is known from laser wavelength.
EMAX is derived
N3
ClN32MAXClCl30 m
mmvm
21
h)N(ClD
v mvm MAXN3N3
MAXClCl
22 MAXN3N3
MAXClCl v m
21
vm21
MAXN3N3
MAXClCl E mEm
Thermochemistry of the Cl/N/N/N
Zero Kelvin Heats of Formation
All heats of formation now known within 0.1 eV
Predicted by B
ittererova et al.
Velocity Map Image of Cl(2P3/2) Bimodal energy distribution Angular Distributions parallel but not identical
80% of Eava in translation
45% of Eava in translation
Photofragmentation translation spectroscopy
Establishing the decomposition pathways important in ClN3 photolysis.
Photofragmentation Translational Spectroscopy Electron bombardment
ionization of photofragments provides universal detection With Ion fragmentation
Detector is rotate-able to accept products recoiling at different angles,
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 50 100 150 200 250 300
0.0000
0.0001
0.0002
0.0003
0.0004
0.0005
Sig
na
l (a
.u.)
TOF s
Data NCl
0 25 50 75 100
Pro
babi
lity
/ a.u
.
Translational Energy / kcal*mol-1
600Eava
75 kcal/mol in products of this reaction!
= 0.3
Cl+-TOF, 50o: Cl + N3 is dominant
channel Consistent with VMI, bimodal TOF observed
ClN3 + h → Lin-N3 + Cl HEF-N3+ Cl
ClN3 + h → NCl + N2
NCl+ h → N+Cl
0 50 100 150 200 250 300
0.000
0.002
0.004
0.006
0.008
0.010
0.012
Ion
co
un
ts/la
se
r s
ho
t
TOF s
DATA lin. N
3
HEF N3
NCl sec. photodiss. Total
500
= 1.7
= 0.4
N3+, bimodal N3 distribution
ClN3 + h → lin-N3 + Cl
HEF-N3+ Cl
Long-lived HEF N3
0 50 100 150 200 250 300
0.000
0.001
0.002
0.003
0.004
0.005
0.006
0.007
0.008
Ion
Co
un
ts /
las
er
sh
ot
TOF s
Data Total lin N
3
HEF N3
500
= 1.7
= 0.4
Translational Energy Distributionsof ClN3→Cl+ N3 M1v1 = M2v2
Experiments at m/z=42 (N3
+) and m/z=35 (Cl+)
are fundamentally redundant.
Yet differences arise
Likely due to N3
dissociation.
0 10 20 30 40 50 60 700.00
0.01
0.02
0.03
0.04
Mass 35 Mass 42
Pro
babi
lity
Center of mass energy [kcal]
Wavelength Dependence
VMI at 235 nm summed over Cl (2PJ)
PTS at 248 nm.
Both Features shifted by change in photon energy.
0 10 20 30 40 50 60 70 80
Etrans /kcal*mol-1
VMI-Experiment (235 nm) PTS-Experiment (248 nm)
EM
AX
T =
69
kcal
/mol
EM
AX
T =
38
kcal
/mol
N2+, unimolecular
decomposition and photolysis of N3
N3 → N2 + N(4S)
N3 → N2 + N(2D)
N3 + h→ N2+N(2D)
0 50 100 150 200 250 300
0.000
0.002
0.004
0.006
0.008
0.010
0.012
0.014
0.016
0.018
0.020
Ion
Co
un
ts /
las
er
sh
ot
TOF s
DATA MODELTOF
N3 + h -> N(2D) + N
2
HEF-N3 -> N(4S) + N
2
lin. N3
HEF-N3
NCl+ N2
HEF-N3 -> N(2D) + N
2
300
N+, unimolecular decomposition and photolysis of N3
0 50 100 150 200 250 300
0.000
0.002
0.004
0.006
Ion
co
un
ts /
las
er
sh
ot
TOF s
DATA TOTAL NCl + h -> N + Cl N
3 + h-> N + N
2
N3 -> N(4S) + N
2
N3 -> N(2D) + N
2
lin-N3
HEF-N3
N3 → N2 + N(4S)
N3 → N2 + N(2D)
N3 + h→ N2+N(2D)
500
N3 Secondary photodissociation
Data fit by two models lin-N3 + h→N(2D)+N2
HEF-N3 + h→N(2D)+N2
Evidence suggests the selective photo-dissociation of HEF-N3 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
Bimodal features seen again
N3 observed with much better S/N
Two forms of N3 well resolved in the TOF distribution
0.00
0.01
0.02
0.03
0.04
0.05
0.06
20 30 40 50 60 70 80 90 100
0.00
0.01
0.02
0.03
0.04
0.05
0.06
0.000
0.002
0.004
0.006
0.008
0.010
20 30 40 50 60 70 80 90 100
0.000
0.002
0.004
0.006
0.008
0.010
m/e=35
m/e=35
m/e=42
Cou
nts/
Pas
ses,
pow
er
TOF (s) TOF (s)
m/e=42
Cou
nts/
Pas
ses,
pow
er
N3+ Cl+
TOF spectra of N3 vs. ionization photon energy White light continuum
produces “below threshold ions”
11.07 eV ionization of “fast peak” matches literature value for linear N3
New threshold ~10.6 eV
30 40 50 60 70
0.000
0.005
0.010
9.449.86
10.2710.67
11.0711.49
11.9112.37
12.83
X
Inte
nsi
ty
Time of Flight
Ionization Energy
Two photoionization thresholds for N3 produced in ClN3 photolysis
10.2 10.4 10.6 10.8 11.0 11.2 11.4 11.6
0.0
0.2
0.4
0.6
0.8
1.0
Inte
nsi
ty (
no
rm.)
Θ = 45o
synchrotron photon energy / eV
x4
20 30 40 50 60 70 80 90 100
0.000
0.002
0.004
0.006
0.008
0.010
TOF (s)
m/e=42
Co
un
ts/P
ass
es,
po
we
r
Tosi, 2004Krylov & Babikov, 2005
John Dyke, 1982 LINEAR N3 Experiment
With Jim Jr-Min Lin at Hsinchu, NSRRC in Taiwan
CYCLIC N3/N3+ theory
● fast channel slow channel
N3 neutral TOF
N3+ p
ho
toio
niz
atio
n
yiel
d
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, Dept. of Chemistry and Biochemistry, UCSB, Santa
Barbara CA 93106
Dr. Jason Robinson, Niels Sveum Dan Neumark, 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