Evidence for Photolytic Production of Cyclic-N 3

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

2. 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

3. Some background on all Nitrogen Chemistry …especially rings

4. The Nitrogen atom as a chemical building block • N is iso-electronic with CH If benzene, Then, why not Hexa-azabenzene

5. Basic Problem of Stability with all-Nitrogen Ring Allotropes

6. 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. Zvesti47, 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. Berichte129, 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. A101, 4460-4464 (1997). • M. Bittererova et al., J. Phys. Chem. A104, 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. A105, 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).

7. Hexa-aza diazide 189 kcal/mol Hexa-aza Dewar-benzene 244 kcal/mol Hexa-azabenzene 212 kcal/mole Hexa-aza Prismane 323 kcal/mol Hexa-aza bicyclopropenyl 245 kcal/mol Many interesting allotropes have been predicted by theory Stable Stable ? ? ? Motoi Tobita and Rodney J. Bartlett J. Phys. Chem. A 2001, 105, 4107-4113

8. N10 N8

9. Poly-Nitrogen Chemistry • Limited number of allotropes belonging to this family have been synthesized and identified. N≡N N=N=N N=N=N- -0.11 +1 0.22 0.33

10. N5+ Synthesis proved by IR and crystal structures.

11. N5- Identified in fragmentation of electrospray ionization mass spectra.

12. Tetra-azahedrane (tetrazete): The search continues • 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

13. Matrix Isolation • Nitrogen discharges quenched on cold surface • IR spectra recorded • Compared to theoretical predictions Very recent work from Radziszewski appears promising

14. Theoretical simulation of isotopic IR spectrum of Td - N4

15. 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)

16. 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.

17. ׀ ׀ + ׀ + + Geometric Phase Effect GBO BO 2A2 2B1 ׀ + + ׀ ׀ + ׀ + + + 2A2 2B1 + + + + + ׀ + 2B1 2A2 Babikov et al. , J. Chem. Phys., 121, (24), 22 December 2004

18. #3: GPE, A2, 1502 cm-1 #2: GPE, A1 1401 cm-1 #1: GPE , E, 1325cm-1 Vibrational Wave-functions With and Without the Geometric Phase Effect #1: BO A1 1310 cm-1 #3: E 1561 cm-1 #2: E 1364 cm-1 Babikov et al. , J. Chem. Phys., 121, (24), 22 December 2004

19. Up to now, no conclusive experimental evidence Surprisingly, no effort has been made to exploit UV photolysis to make this metastable compound.

20. Theoretical predictions about cyclic N3 Zhang, Morokuma and Wodtke (in press)

21. 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

22. Velocity Map imaging of Cl from ClN3→Cl+N3 …thermochemistry of Cl/N/N/N

23. Velocity Map Ion Imaging

24. Inverse-Abel Transformation • 3D-Distribution • 2D-Projection: • Cut through 3D-Distribution: Inverse Abel-Transformation Using BASEX alla Reisler M. C. Escher

25. N2O Photodissociation N2O + h N2 (X1g+) + O (1D2) • Velocity Map b ~ -1 w/o centroiding w/ centroiding “Improved two-dimensional product imaging: The real-time ion-counting method”, Chang BY, Hoetzlein RC, Mueller JA, Geiser JD, Houston PL, RSI69 (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

26. N2O (0,1,0) N2O (0,0,0) Comparison to Cornell Experiments Determines the N2-O bond energy within several cm-1 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, RSI69 (4): 1665-1670 APR 1998

27. ClN3 absorption spectrum 2A”1A’ 5.6 eV 2A’1A’ 5.1 eV 1A”1A’ 3.1 eV Theoretical calculations of Zhang and Morokuma Cl-atom N-atom N2 Experimental Absorption Spectrum

28.  Experiments with 6 eV photons: Formation of N2( J=68 ) + NCl(X3S and a1D) • Parallel transition: b=1.96 • P(a)/P(X) = 0.78/0.22

29. Thermochemistry of ClN3  N2 + NCl • Maximum release of translational energy provides accurate thermochemistry • ClN3  N2(X) +NCl: DE = -0.93eV • ClN3N2(a) +NCl: DE = 0.22eV

30. e b=1.1 Imaging of ClN3 + 2 hn ClN3+ + e-NCl+ + N2 confirms this thermochemistry NCl+

31. Symmetrized image  Reconstructed v-map Velocity Map Image of Cl from ClN3 N3 + Cl(2P1/2) Two components Internally cold linear N3

32. D0(Cl-N3) from Velocity Map Imaging • Ef is known from laser wavelength. • EMAX is derived

33. Thermochemistry of the Cl/N/N/N • Zero Kelvin Heats of Formation • All heats of formation now known within  0.1 eV Predicted by Bittererova et al.

34. Velocity Map Image of Cl(2P3/2) • Bimodal energy distribution • Angular Distributions parallel but not identical 45% of Eava in translation 80% of Eava in translation

35. Photofragmentation translation spectroscopy Establishing the decomposition pathways important in ClN3 photolysis.

36. 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, Q • TOF reflects laboratory speeds, from which we extract the c.m. frame translational energy release, P(ET)

37. NCl+ observed, but weak! ClN3 + hn → N2+NCl(1D) minor b = - 0.3 600 Eava 75 kcal/mol in products of this reaction!

38. Cl+-TOF, 50o: Cl + N3 is dominant channel • Consistent with VMI, bimodal TOF observed • ClN3 + hn → • Lin-N3 + Cl • HEF-N3+ Cl • ClN3 + hn → • NCl + N2 • NCl+ hn → N+Cl b = 1.7 500 b= 0.4

39. N3+, bimodal N3 distribution • ClN3 + hn → • lin-N3 + Cl • HEF-N3+ Cl • Long-lived HEF N3 b = 1.7 500 b= 0.4

40. 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.

41. Wavelength Dependence • VMI at 235 nm summed over Cl (2PJ) • PTS at 248 nm. • Both Features shifted by change in photon energy.

42. N2+, unimolecular decomposition and photolysis of N3 • N3 → N2 + N(4S) • N3 → N2 + N(2D) • N3 + hn → N2+N(2D) 300

43. N+, unimolecular decomposition and photolysis of N3 • N3 → N2 + N(4S) • N3 → N2 + N(2D) • N3 + hn → N2+N(2D) 500

44. N3 Secondary photodissociation • Data fit by two models • lin-N3 + hn→N(2D)+N2 • HEF-N3 + hn→N(2D)+N2 Evidence suggests the selective photo-dissociation of HEF-N3 at 248 nm

45. Primary and Secondary dissociation channels of 248 nm photolysis of ClN3 • ClN3 + hn → NCl+ N2 • NCl + hn → N + Cl • ClN3 → Cl+ N3 (low energy form) • ClN3 → Cl+ N3 (high energy form) • N3 → N2 + N(4S) • N3 → N2 + N(2D) • N3 + hn → N2+N(2D)

46. VUV synchrotron photoionization based photofragmentation translational spectroscopy Two thresholds in photo-ionization for N3

47. 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

48. Cl+ and N3+TOF N3+ Cl+ • Bimodal features seen again • N3 observed with much better S/N • Two forms of N3 well resolved in the TOF distribution

49. 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

50. Two photoionization thresholds for N3 produced in ClN3 photolysis CYCLIC N3/N3+ theory Tosi, 2004 Krylov & Babikov, 2005 N3 neutral TOF N3+ photoionization yield ● fast channel  slow channel John Dyke, 1982 LINEAR N3 Experiment With Jim Jr-Min Lin at Hsinchu, NSRRC in Taiwan