1 The r 0 Structural Parameters of Equatorial Bromocyclobutane, Conformational Stability from Temperature Dependent Infrared Spectra of Xenon Solutions, and Vibrational Assignments Arindam Ganguly, Ph.D. Candidate Molecular Spectroscopy Laboratory Department of Chemistry, University of Missouri-Kansas City, Missouri 64110, USA

2 Outline Background and Motivation Objectives Conformational Stability Structural Parameters Conclusions & Future Directions

3 Background and Motivation Rothschild’s Microwave and Vibrational 1 Investigations of Monosubstituted Cyclobutanes in early 1960s. Microwave investigation identified the presence of only the equatorial conformer while vibrational studies provided limited evidence for the axial conformer. Durig’s 2 group utilizing deuteration and carried out comprehensive vibrational assignment, again only for the equatorial conformer. Klaeboe 3 utilizing ab initio scaled force fields from cyclobutane identified some bands for the axial conformer. Durig’s 4 group carried out temperature dependent Raman gas study, to obtain  H (~350 cm -1 ) between the equatorial and axial conformer, and proposed a double well potential for the ring puckering vibration. 1.W.G. Rothschild, B.P. Dailey, J. Chem. Phys. 36 (1962) 2931,44 (1966) 2213, 45 (1966) J.R. Durig, W.H. Green, J. Chem. Phys. 47 (1967) P. Klaeboe, et al. J. Raman Spectrosc. 20 (1989) J.R. Durig, et al. J. Raman Spectrosc. 20 (1989) 757.

4 Objectives Obtain  H with relatively low uncertainty utilizing the Noble Gas Spectroscopy 5,6. Obtain complete structural parameters for the equatorial conformer and predict for the axial utilizing the A&M (Ab initio & Microwave) program. Utilizing the  H obtained along with the observed frequencies for the excited state transitions for the ring puckering vibrations in the equatorial well a double well potential should be obtained. 5. M.O. Bulanin, J. Mol. Struct. 73 (1995) J.R. Durig, et al. J. Phys. Chem. 99 (1995) 578.

5 Methods and Experiments Theoretical calculations for predicting the conformational stability utilizing ab initio MP2(full) and Density Functional Theory by the B3LYP method. MP2(full)/6-31G(d) for predicting vibrational frequencies, infrared intensities and depolarization values. Liquefied noble gas studies are carried out using Bruker IFS-66 spectrometer equipped with a DTGS detector. A specially designed cryostat cell.

6 Conformational Stability Advantages :- Solvent has no absorption bands. Usually several conformer pairs can be measured. Very low uncertainty of determined values. Very accurate measurement of the temperature. Little interaction of solvent with solute molecules. Infrared bands are very narrow. Small enthalpy changes can be measured. Suppress hot-bands. Suppress overtones and combination bands. Limitations:- Limited solubility of many polar molecules. Difficult to have very dry xenon so water can interfere. At low temperatures sample may deposit on the window.

7 Conformational Stability

8 Fig. 1.

9 Conformational Stability Fig. 2.

10 Conformational Stability Fig. 3.

11 Conformational Stability

12 Structural Parameters A & M 7 (Ab initio and Microwave) ab initio MP2(full)/6-311+G(d,p) 8 calculations predict the r 0 structural parameters for more than fifty C-H distances better than 0.002Å compared to the experimentally determined values. We combine the ground state rotational constants obtained from Microwave Spectroscopy with ab initio predicted structure, which leaves only the heavy atom parameters to be determined. 7. J.R. Durig, et al. J. Phys. Chem. A 103 (1999) J.R. Durig, et al. Struct. Chem. 15 (2004) 149.

13 Structural Parameters Fig. 4.

14 Structural Parameters

15

16 9. W.Caminati, et al. Chem. Phys. Lett. 141 (1987) J.R. Durig, et al. Spectrochim. Acta 71A (2008) J. R. Durig, et al. Struct. Chem. 19 (2008) J. R. Durig, et al. J. Mol. Struct. 922 (2008) J.R. Durig, et al. J. Mol. Struct. 923 (2009) This Study (Accepted Manuscript) J. Mol. Struct. (2009).

17 Potential Function for the Ring Puckering mode

18 Fig. 4.

19 Conclusions and Future directions In the present study we obtained a much lower  H = 291 cm -1 utilizing Noble Gas Spectroscopy. We report the structural parameters for the equatorial conformer and provide a platform for reinvestigation of the microwave spectra in order to identify microwave spectra for the axial conformer. An improved and more reliable ring puckering potential function has been obtained utilizing the current value of the  H.

20 Acknowledgements Thank you all for your attention. Dr. J.R. Durig, Curators’ Professor of Chemistry and Geosciences, UMKC Dr. Peter Groner, Associate Professor, UMKC Molecular Spectroscopy Research Group at UMKC

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22 Equatorial C γ C α = 2.111Å  C γ C α Br = 150.1° Axial C γ C α = 2.149Å  C γ C α Br = 106.4°

23 For a binary conformational equilibrium : A B then the equilibrium constant K ΔG 0 = -RT ln K ΔG 0 = ΔH 0 – TΔS 0 The integrated intensity I A of an infrared band due to species A is given by: I A = n A α A l ( l = 4cm (Xe), 7cm (Kr)) ln(I A /I B ) = - (ΔH 0 /RT) + ln(α A /α B ) + ln(g A /g B ) + ΔS 0 /R ln(I A /I B ) = - (ΔH 0 /RT) + c A plot of ln(I A /I B ) with (1/T), the slope of the line equals to - ΔH 0 /R.