1. C. T. Dewberry, G. S. Grubbs II and S. A. Cooke
FORBIDDEN TRANSITIONS IN THE VERY RICH PURE ROTATIONAL SPECTRUM OF TRANS-1-IODOPERFLUOROPROPANE
C. T. Dewberry, G. S. Grubbs II and S. A. Cooke
Department of Chemistry, University of North Texas, Denton, TX

2. Background
In 1957 Javan proposes the use of D J = +3 or +2 transitions in relation to stimulated emission (PR 107 (1957) 1579)
In 1966 Oka observes a D J = +3 transition in ethyl iodide. (JCP 45 (1966) 752)
A handful of molecules have been studied in which forbidden transitions are observable including 1-iodopropane. (M. Fujitake and M. Hayashi 127 (1988) 112)

3. Forbidden transitions
Require:
Near degeneracy.
Energy levels involved must be connected via a component of the nuclear quadrupole coupling tensor.

4. Trans-1-iodoperfluoropropane
Drawn with PMIFST, free from the PROSPE website

5. Experimental methods
Search Accelerated Correct Intensity Fourier Transform Microwave (SACI-FTMW) Spectroscopy.
Smitty Grubbs
Talk WF13
RSI 78 (2007)

6. Experimental methods
Fourier Transform Radiofrequency FTRF Spectroscopy.
Chris Dewberry
Talk MH13
JMS 242 (2007) 39

7. Experimental methods Traditional Balle-Flygare-type FTMW Spectroscopy
Circuit design follows that of Jens Uwe Grabow
CP 342 (2007) 71

8. Results

9. Results

10. Results
776 transitions for the parent between 1 and 17 GHz. Energy levels covering J = 1 to 50. Both a- and b- types, Q and R branches.
32, 58 and 51 transitions for the carbon-13 C(1), C(2) and C(3) species, respectively.
Watson S Hamiltonian (κ = -0.97), Ir representation. Use Herb Picketts SPFIT/SPCAT.

11. Spectroscopic Parameters of trans-1-iodoperfluoropropane

12. Forbidden transitions
JK-1,K+1F : 16 9,7 31/2 ← 13 10, 4 29/2

13. Forbidden transitions

14. Forbidden transitions
My .int file:

15. The .egy file

16. The 16 9,7 31/2 ← 13 10, 4 29/2 Transition
Pmix is for 16 9,7 31/2 and for 13 10, 4 29/2
Find that the 15 9,6 29/2 level is just MHz above the 13 10, 4 29/2 level.
H. P. Benz et al.
JMS 21 (1966) 156
A Fortran 77 code shows a non-zero matrix element
< 13 10, 4 29/2 | HQ | 15 9,6 29/2 > = MHz.
The magnitude of this matrix element scales only with the manitude of χab .
16 9,7 31/2 ← 15 9, 6 29/2 is an allowed a-type transition.

17. Similar arguments prevail for all of the forbidden transitions observed.

18. The 14 4,10 23/2 ← 12 5, 7 21/2 Transition

19. Conclusions
First rotational spectroscopy on trans-1-iodoperfluoropropane.
Numerous dipole forbidden transitions observed. (Some were very strong).
Rationalization appeals to near degeneracies between energy levels that are linked via χab .
Possible candidate for use in stimulated emission experiments.
SPFIT/SPCAT calculations show that the strength and number of dipole forbidden transitions can be enhanced by increasing the magnitude of χab , and/or increasing the number of near degeneracies.
Studies initiated on the butane and pentane analogs.

20. Acknowledgements
Profs. Mike Gerry, Jon Hougen and Stew Novick for insightful comments.
$$ NSF CHE