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Complementary Use of Modern Spectroscopy and Theory in the Study of Rovibrational Levels of BF 3 Robynne Kirkpatrick a, Tony Masiello b, Alfons Weber c,

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Presentation on theme: "Complementary Use of Modern Spectroscopy and Theory in the Study of Rovibrational Levels of BF 3 Robynne Kirkpatrick a, Tony Masiello b, Alfons Weber c,"— Presentation transcript:

1 Complementary Use of Modern Spectroscopy and Theory in the Study of Rovibrational Levels of BF 3 Robynne Kirkpatrick a, Tony Masiello b, Alfons Weber c, and Joseph W. Nibler a a Department of Chemistry, Oregon State University b Pacific Northwest National Laboratory c National Institute of Standards and Technology, MD

2 Goals and Methods Push the limits of experiment to see how closely ab initio methods model experiment Use isotopic substitution to gain additional information about molecular potentials  How? Use modern, high resolution ( cm -1 ) spectroscopy to study “simple” molecules of high symmetry, such as BF 3

3 Raman and IR active modes of group D 3h AX 3 molecules E (R, IR) A 2  (IR) E (R, IR) 1 Exclusively Raman Active A 1

4 Consider SO 3 -- an intriguing molecule! 32 S 16 O 3 S O Raman Shift / cm S 16 O 3 32 S 16 O 3 1 CARS Q-Branch

5 32 S 18 O 3 34 S 18 O Raman Shift / cm Raman Shift / cm 32 S 18 O 3 34 S 18 O 3 What causes this complex structure? 1 CARS Q-Branch Q 1 ≠    J(J+1)+(  C 1 -  B 1 )K 2 + higher terms

6 Perturbations to 1 (SO 3 ) deduced using the CARS Q-Branch 1 A 1 ' Fermi resonance Coriolis l-resonance 2 4 (l=0) A 1 ' 2 2 A 1 ' 2 4 (l=2) E ' E '  Let’s examine the CARS Q-Branches of 10 BF 3 and 11 BF 3

7 CARS Experiment Vibrational energy, i Anti-Stokes (AS) energy, S    0 S A Sample Induced dipole in sample ↔ Non-Linear optical interaction ↔  E +  2 +  E ( 0 ) E ( 0 ) E (S ) CARS Intensity ·Monitor CARS beam ·Scan Stokes beam · Keep green beam at a constant frequency

8 ◦ Long pulse → Very high spectral resolution (~0.001 cm -1 ) Tunable Ring dye laser Integrator Nd: YAG PMT Photodiode I 2 cell Sample Filter Ar + laser Dye cell Amplification of Stokes beam Computer Experimental Setup ◦ Nd:YAG output locked to single frequency

9 Predict structure according to: Q 1 = 1 +  B 1 J(J+1)+(  C 1 -  B 1 )K 2 + higher terms With intensities I ~ C g(J,K) (2J+1) exp[-hF 0 (J,K)/kT]) Significant perturbations not evident for 10 BF 3 

10 IR studies on BF 3 (Masiello, Maki, Blake) give 1 parameters indirectly from various transitions: Ground State Energy 1  E'      2 ''     E'

11 1 Q-Branch of 10 BF 3 What do we predict for 11 BF 3 ?

12 ≈0.2 cm -1 Interesting Frequency Shift Observed with Isotopic Substitution at the Center of Mass!

13 Due to an unrecognized Fermi resonance? Due to changes in anharmonicity constants? xxxx  1 Shift: ► IR data ► ab initio calculations Answer these questions by making use of

14 Ask: How well do Measured x ij ’s and isotopic shifts correspond to results of ab initio (Gaussian 03) calculations? ► Instruct Gaussian 03 to compute anharmonicities (and other ro-vibrational parameters) using the anharm option and B3LYP/cc-pVTZ Problem : anharm only works for asymmetric tops Solution: Small distortion ( Å ) of one BF 3 bond

15 Vibrational constants in cm for 10 BF 3 and 11 BF 3 constantexp.theoryexp.theory  x x x x  1 -  ( 10 BF 3 ) - 1 ( 11 BF 3 )-0.198exp theory 10 BF 3 11 BF xxxx  (Hard to get) (Easy to get) What about other anharmonic shifts?

16 Anharmonic shifts (cm -1 ) 10 BF 3 constantExp.B3LYP/Exp.-calc % diff cc-pVTZ.     Conclusion: theory gives excellent values for anharmonic shifts!

17 Vibration-rotation constants in cm -1 for 10 BF 3 Constant Exp.Theory %Diff BeBe  1   2   3   4  CeCe  1   2   3   4  Coriolis constants  33 z  44 z B v = B e –  i  i (v i + d i )+ higher terms

18 Rotational distortion constants (cm -1 ) for ground state of 10 BF 3 Exp.Theory % diff D J x D JK x D K x H J x H JK x H KJ x H K x Since parameters are well-determined by theory, can we ab initio calcs. to accurately assess the potential surface?

19 We can be confident such higher order terms in the potential are well-defined by ab initio calculations. 10 BF 3 11 BF 3 modek ii k iii k iiii K ii k iii k iiii QkQkQkV 4 iiiii 3 iiii 2 iiii  k ii  ↔  i  k iii, k iiii  ↔  x ii

20 Symmetric BF stretch Out-of-plane bend

21 In-plane bend Anti-symmetric BF stretch

22 Conclusions ● CARS spectra of BF 3 confirm validity of 1 parameters deduced indirectly from IR studies ●  shift reproduced by ab initio calculations ● BF 3 parameters (D’s, H’s,  ’s, x’s,  ’s, …) in excellent agreement with ab initio anharmonic values ● Results indicate theory can give very useful estimates of higher-order parameters needed for the analysis of complex ro-vibrational spectra.


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