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