1. Rotationally Resolved Diode Laser Jet Spectroscopy of Propadienone (CH 2 CCO) in the 3 Band Region P. J. O’Sullivan, R. J. Livingstone, Z. Lui, P. B. Davies Department of Chemistry, University of Cambridge, Lensfield Road, Cambridge CB2 1EW, UK

2. Introduction Propadienone is a member of the cumolenone series (H 2 C n O), is short lived and has a non rigid kinked backbone. Microwave spectra measured in the low frequency ν 12 bending mode : Brown, Godfrey and Champion, J. Mol. Spec., 123, 93 (1987) Infrared spectra in N 2 and Ar matrices; no rotationally resolved gas phase spectra Pulse Pyrolysis Infrared Laser Jet Spectroscopy used to produce a rotationally resolved spectrum

3. The Kinked Structure of Propadienone (PPD)

4. Double-Well Potential for PPD 1 Vibrational levels split by quantum tunnelling through inversion barrier. Lowest pair of vibrational levels separated by 3.7GHz (0.123cm -1 ). Splitting of subsequent levels dependent on energy in relation to the inversion barrier and component of vibration along inversion axis. 1 R.D. Brown, P.D. Godfrey, R. Champion J.Mol.Spec 123, 93 (1987)

5. 4000 3500 3000 2500 2000 1500 Wavenumber (cm -1 ) Absorbance 0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 Infrared Spectrum of Matrix-isolated PPD 1 1 O.L. Chapman, M.D. Miller, S.M. Pitzenberger J. Am. Chem. Soc. 109, 6867 (1987) argon matrix Freq. (cm -1 ) Intensity 3049 3024 2978 2974 2603 0.58 217714.21 212572.06 2079 0.83 1959 0.68 1690 1.16 1444 0.65 1046 3.44 980 2.51 903 0.40 668 - 473 0.77 nitrogen matrix Freq. (cm -1 ) Intensity 3037 3026 2978 2968 2604 0.51 2172 6.17 212482.93 2075 0.95 1973 0.33 1685 0.87 1457 0.17 1047 2.25 988 2.40 910 0.15 674 0.27 479 0.65 1.37 1.62 1.18 1.16

6. Experimental

7. Experimental – Flash Pyrolysis Nozzle 1 Ceramic tube: 1mm i.d., 2mm o.d., 42mm long Heating coil: thin NiCr wire, insulated by MgO, inside a stainless steel sheath. 10mm long, 3.5 Ω Applied power of 10W → 1500K inside tube. 1. P. Chen, S.D. Colson, W.A.Chupka, J.A. Berson J.Phys.Chem. 90, 2319 (1986)

8. Experimental – Signal Processing A: Averaged absorption signal at a peak position. B: Trace A with a 1.4ms time delay. C: The subtraction of Trace B from Trace A. D: Resultant smoothed Trace. Gas Pulse A B C (=A-B) D 35 Hz msec

9. Synthesis of Propadienone based on pyrolysis of either (ii) diazotetronic acid: Synthesis of Propadienone based on pyrolysis of either (i) 2,2-dimethyl-5-methylene-1,3-dioxan-4,6-dione: Chemistry Synthesis of Propadienone based on pyrolysis of either (iii) acrylic anhydride:

10. Chemistry All three starting compounds have to be synthesised in the laboratory but only acrylic anhydride (iii) is stable for more than a few days and was therefore the preferred reagent. Conditions:  Acrylic anhydride heated to 80 o C  Nozzle temperature of 600 o C (heating length = 20mm)  Backing pressure of argon = 2.5 bar  Nozzle-laser distance = 5mm

11. Spectral Details Continuous spectral coverage from 2123.2cm -1 to 2132.9cm -1, using several modes from a single diode laser. Over 180 intense lines observed and measured with an estimated accuracy of 0.0003cm -1. Several series of regularly spaced lines separated by the order of (B + C) = 0.288 cm -1 for the ground state. Absorption signals disappeared below 450 o C.

12. Sample Spectra 2127.0 2127.5 2128.0 2128.5 2129.0 2129.5 Frequency (cm -1 )

13. Loomis-Wood Plot of Hypothetical PPD R-branch Q-branch P-branch Upper state constants identical to ground state constants: Very regular series of lines with no distortion, as expected. Indication of approximate magnitude of asymmetry doubling. Q-branch expected, although not intense and may be dispersed in real spectrum.

14. Loomis-Wood Plot PPD cm -1 (B+C = 0.29cm -1 )

15. Q-branch structures in CCO Antisym. Stretch of Ketene 1 1. J.L. Duncan, A.M.Ferguson, J.Harper, K.H.Tonge, F.Hegelund J. Mol. Spec. 122, 72 (1987) Q-branch structures of isotopically-substituted ketenes suggest an unusual arrangement of the K a sub-bands. If propadienone is similarly erratic, this could account for:  A dispersed and unidentified Q-branch  The apparent lack of a relationship between the observed series in the Loomis-Wood plot.

16. Ground State Combination Differences (cm -1 ) for PPD J Kc’ – J Kc” Microwave Infrared Obs-Calc (10 4 ) K a = 0 2 2 – 0 0 0.8651 0.8675 23 3 3 – 1 1 1.4419 1.4422 3 4 4 – 2 2 2.0186 2.0194 8 5 5 – 3 3 2.5951 2.5958 7 6 6 – 4 4 3.1716 3.1723 7 7 7 – 5 5 3.7479 3.7480 1 8 8 – 6 6 4.3240 4.3212 -28 9 9 – 7 7 4.9000 4.9025 26 10 10 – 8 8 5.4757 5.4789 32 11 11 – 9 9 6.0511 6.0501 10 12 12 – 10 10 6.6262 6.6075 -187 13 13 – 11 11 14 14 – 12 12 15 15 – 13 13 8.3469 8.3467 -2 J Kc’ – J Kc” Microwave Infrared Obs-Calc (10 4 ) K a = 1 3 3 – 1 1 1.4313 1.4335 22 3 2 – 1 0 1.4530 1.4521 -9 4 4 – 2 2 2.0038 2.0035 -3 4 3 – 2 1 2.0341 2.0340 -1 5 5 – 3 3 2.5762 2.5747 -15 5 4 – 3 2 2.6153 2.6176 23 6 6 – 4 4 3.1487 3.1481 -6 6 5 – 4 3 3.1964 3.1954 -10 7 7 – 5 5 3.7212 3.7213 1 7 6 – 5 4 3.7775 3.7769 -6 8 8 – 6 6 4.2936 4.2932 -4 8 7 – 6 5 4.3586 4.3581 -5 9 9 – 7 7 4.8660 4.8683 23 9 8 – 7 6 4.9396 4.9415 19 10 10 – 8 8 10 9 – 8 7 5.5207 5.5204 -3 11 11 – 9 9 6.0107 5.9963 -144 11 10 – 9 8 6.1016 6.1011 -5 12 12 – 10 10 12 11 – 10 9 6.6826 6.6823 -2

17. Molecular Constants (cm -1 ) of PPD υ 12 = 0 υ 12 = 1 Ground State υ 3 =1 Ground State υ 3 =1 v 0 2127.690641(32)2127.690624(32) A5.122646(2)5.006123(113)5.112639(3)4.99517(117) B0.146336(1)0.145570(6)0.1463560(1)0.145595(6) C0.142036(1)0.142583(5)0.1420173(1)0.142561(5) Δ (amuǺ 2 )0.197-0.9190.221-0.888 D J (x10-8)5.82(10)-60.9(13)5.79(10)-60.5(13) D JK (x10-5)-2.621(13)-101.51(17)-2.596(10)-101.57(17) D K (x10-1)-1.1284(33)4.8595(16)-1.1292(33)4.8486(16) D 1 (x10-9)7.61(17)-4642(21)-7.17(17)-4669(21) H kj (x10-7)1.03(67)-981.5(24)-9.21(67)-977.5(25) κ -0.998-0.999-0.998-0.999 Lines135135 Rms0.00280.0028

18. Rotationally resolved spectrum of the ν 3 C-O stretching fundamental of propadienone recorded Combination Differences and Loomis Wood plots lead to assignment of many K a = 0 and 1 P-and R-branch lines Structure of the IR spectrum and of the upper state constants indicate a significant change in the ν 12 inter-conversion potential for the upper level and/or perturbations of the upper level Next step? Analysis of congested region about 4.5 cm -1 below band origin – possible intersystem v 12 = 0-1 lines ( Δ K a = -1) Conclusions

19. Ray Livingstone Zhuan Liu Paul O’Sullivan Iain Bell £££££ EPSRC Acknowledgements