1. Structural studies of CH 3 SiF 2 -X (X = NCO, Cl) by microwave spectroscopy Gamil A. Guirgis, Jason S. Overby College of Charleston Nathan A. Seifert, Daniel P. Zaleski, Brooks H. Pate University of Virginia Michael H. Palmer University of Edinburgh Rebecca A. Peebles, Sean A. Peebles, Lena F. Elmuti, Daniel A. Obenchain Eastern Illinois University

2. Introduction: CH 3 SiF 2 NCO Simple molecules of the isocyanate and related classes, such as CH 3 NCO and CH 3 NCS show extremely non-rigid behavior due to both low-lying torsional and C-NCO bending modes First excited state of CNC bend for CH 3 NCO is only 200 cm -1 + anharmonic potential CH 3 barrier to rotation at equilibrium is small, only ~21 cm -1 Traditional semi-rigid rotor fit can only fit K = 0 for low J 1 Most recent model Hamiltonian fit only gets RMS to 3.0 MHz with data up to 40 GHz! (J max = 3) 2 Similar silanes, such as H 3 SiNCO 3 and HF 2 SiNCO 4 (RC10, 2010), can be fit using a typical Watson semi-rigid Hamiltonian, but exhibit abnormally large distortion constants. H 3 SiNCO: D JK = 642.0(3) kHz; HF 2 SiNCO: Δ JK = 455.44(25) kHz For comparison, acetyl isocyanate: D JK = 8.6492 kHz 5 Where does CH 3 F 2 SiNCO fall in terms of rigidity? 1.R. G. Lett, W. H. Flygare, J. Chem. Phys. 47 (1967) 4730. 2.J. Koput. J. Mol. Spectrosc. 115 (1986) 131. 3.J.A. Duckett, A. G. Robiette, M. C. L. Gerry, J. Mol. Spectrosc. 90 (1981) 374. 4.G. A. Guirgis, et al. J. Mol. Struct. 983 (2010) 5. 5.B. M. Landsberg, K. Iqbal. J. C. S. Faraday II. 76 (1980) 1208.

3. Introduction: CH 3 SiF 2 Cl Detected as impurity from isocyanate synthesis (see next slide for details) Provides a good comparison point for various other structural studies of halogenated silanes and hydrocarbons: Naturally, CH 3 SiF 2 Cl should be an intermediate case in both rotor barrier and structural parameters. Isocyanate is often called a “pseudohalogen”. How does it compare to the chloride analogue?

4. Experimental 6.5-18 GHz arrangement 6 (right), 400 000 avg spectrum. CH 3 SiF 2 Cl was detected in the microwave spectrum, likely as a fluorination byproduct (NCO is halogen-like in its reactivity) Spectrum was taken using approximately a 0.2% concentration of product vapor pressurized to 7 atm in Ne. 6. Brown, G. G.; Dian, B.C.; Douglass, K. O.; Geyer, S. M.; Shipman, S. T.; Pate, B. H. Rev. Sci. Instru. 2008, 79, 053103. Initial isocyanate spectroscopy performed at Eastern Illinois University Spectra taken on 480 MHz CP-FTMW and Balle-Flygare cavity Isotopologue measurements performed at UVa: CH 3 SiF 2 NCO was prepared by the reaction of MeSiCl 3 with AgOCN to form MeSiCl 2 NCO, which was then fluorinated using SbF 3 to form the desired product.

5. Results – CH 3 SiF 2 NCO High barrier methyl rotor No excited vibrational states observed SiNC bending mode calculated to be 380 cm -1 Some B-type K a = 2 E state lines missing Remaining satellites fit as E-state forbidden c-type transitions Very near prolate (κ = –0.98) so when asymmetry splitting ≃ A-E splitting, K state mixing occurs between asymmetry and torsional interactions Analyzed in detail in ethanol by Pearson et al. a ) a) J.C. Pearson, K. V. L. N. Sastry, M. Winnewisser, E. Herbst, F. C. De Lucia. J. Chem. Phys. Ref. Data. 1995, 24, 1.

6. Results – Isotopologues 29 Si 30 Si 13 C 15 N

7. Results – Isocyanate Structure r 0 structure fit with Kisiel’s STRFIT Fluorine positions fit with P cc instead of C r0r0 rsrs Imaginary coordinates in Kraitchman analysis

8. Polynomial fit suggests potential 2 nd minimum; likely to relax to global min. in expansion. Attempts at a full relaxed SiNC PES were not successful here at UVa. Inspection of broadband spectrum does not immediately suggest the existence of a second conformer spectrum Ab Initio Results 157.4°

9. Results – CH 3 SiF 2 Cl K a = 2 b-type E-state lines also disappear, but no new c-type transitions were observed Less prolate than isocyanate (κ = –0.85)  Weaker mixing? High sensitivity (1500:1 S/N on strongest transition) enabled detection of double silicon/chloride isotopologues

10. Results – Isotopologues 13 C 29 Si 30 Si 37 Cl 2 12 -1 01

11. Results – CH 3 SiF 2 Cl Analysis identical to isocyanate Structure in excellent agreement with MP2/6-311G++(d,p) ab initio structure. Small difference between Ab-initio and r s / r 0 values of the rotor angles; suggests rotor tilt due to Cl σ donation into the anti methyl hydrogen Imaginary coordinates for Kraitchman, but errors are less apparent in derived quantities than seen in -NCO

12. Results

13. Conclusions & Future Work Substitution and r 0 structures for CH 3 SiF 2 Cl and CH 3 SiF 2 NCO were determined in natural abundance using CP-FTMW spectroscopy Internal rotation and nuclear hyperfine parameters were fit, for a total global fit with an RMS of 9.0 and 18.3 kHz for the isocyanate and chloride normal species, respectively. Future implications: With proper sensitivity measurements and sufficient sensitivity, substitution structures in natural abundance become a routine process for a given molecule of interest. Reaction product analysis for volatile mixtures. Ablation source  product analysis even for solid product mixtures? Addition of heavy atoms (fluorines) to the C-Si-NCO frame seem to ease the large-amplitude motions and centrifugal distortion seen in other isocyanates. Structural results from this study lend additional evidence to the pseudohalogen nature of the isocyanate group.

14. Thank you for listening! This work was supported by the National Science Foundation MRI-R2 project (0960074).

15. CH3NCO Koput’s fit accounts for transitions up to J = 3, with v bend between 0 and 3 and |m| ≤ 7 (torsional level) “Close coincidences of some CNC bending-torsion-rotation energy levels give rise to strong accidental resonances and some lines have been observed to be shifted as much as several GHz (!) away from the main bunches.”

16. Results – Internal Rotation Used the program XIAM 6 to simultaneously fit internal rotation and nuclear quadrupole effects XIAM uses a “Combined Axis Method” (CAM) to fit the internal rotation; simply: Generates traditional Principal Axis Hamiltonian for system Rotates PA Hamiltonian into the rho-axis system: Rotates system into regime where the Coriolis cross terms disappear Rho-axis roughly collinear with rotor axis in PA system (only exactly true with a symmetric top) Diagonalizes rho-axis Hamiltonian Rotates eigenvalues back to PA Advantages: Very fast for systems with sufficiently high barriers: only need to fit A, B, C, V 3, F and the rotor angle defined in PAM. Also, rotational constants are equivalent to a standard PAM fit with an averaged line center between the A/E states. 6. H. Hartwig, H. Dreizler, Z. Naturforsch 51a (1996) 923. 7. I. Kleiner, J. Mol. Spectrosc. 260 (2010) 1.

17. Notes on XIAM method  Generates cross terms of the form (J i J j + J j J i ) and p α J i in PAM basis We can do a van Vleck transformation to reduce these cross terms, but we get a factored Hamiltonian of the form Rotates into RAM frame using the following angle: Which reduces the cross term -2Fp α ρJ into -2Fρ z p a J z which is diagonal in this symmetric top basis We can then factor the Hamiltonian into four terms H RAM = H T + H r + H cd + H int : We can then do the proper diagonalization of this Hamiltonian to get the appropriate torsion-rotation energy levels, then rotate back to the PA frame to get the typical XIAM output. N.B. XIAM ignores most higher order terms associated with the RAM Hamiltonian. For sufficiently high barriers, you can obtain a good effective fit of the A and E states (as was done here) Adapted from: I. Kleiner, J. Mol. Spectrosc. 260 (2010) 1-18.