Transcript VNCO.ppt
Dynamic Rotational Spectroscopy of Vinyl Isocyanate: IR-Chirped-Pulse Fourier Transform Microwave Double Resonance Gordon G. Brown, Justin L. Neill, Steven T. Shipman, and Brooks H. Pate University of Virginia Department of Chemistry Vinyl Isocyanate Torsional Potential E of laser = 3000-3200 cm-1 Delocalized torsional wavefunctions Localized torsional wavefunctions Ground State Parameters Species trans cis Preda Expb Preda Expb A (MHz) 65195.93 62584.051 22484.09 20146.8 B (MHz) 2390.11 2437.730 2909.83 3017.267 C (MHz) 2305.58 2346.507 2576.40 2689.513 a (D) 1.99 2.35 b (D) 0.87 0.27 aa (MHz) 2.92 2.63c 2.73 bb (MHz) -1.59 -1.49c -1.35 Barrier to isomerization (cm-1) 577.2 aCalculated 401.6 with Gaussian 03 using b3lyp/6-31++g(d,p). constants from C. Kirby and H.W. Kroto. J. Mol. Spec., 70, 216-228 (1978). cFit from CP-FTMW data using SPFIT. bRotational Experimental Procedure • Find laser absorption using a Balle-Flygare cavity -Use Gaussian 03W predicted and experimental dipole derivative directions to aid in interpretation • Observe upper state rotational spectra on a chirped-pulse Fourier transform microwave spectrometer -Intensity distribution and upper state hyperfine patterns (if resolved) provide information about the geometry of the upper state • Why vinyl isocyanate? -Barrier much lower than previous DRS studies (cyclopropanecarboxaldehyde, allyl cyanide, methyl vinyl ether) -The possibility exists of creating an asymmetric gyroscope (where internal angular momentum is generated), rather than simply observing coalescence 0.02 cm-1 IR-FTMW Double Resonance Spectrum anharmonic starred—fundamentals unstarred—combination bands Band Contour—a-type R(2) Monitor 202101 trans P(2) P(1) R(1) Contour assumes no change in rotational constants upon vibrational excitation; fits well because B and C rotational constants do not change significantly compared to the laser bandwidth (~600 MHz) Band Contour—a-b hybrid A B Misses on the band contour of the b component because the A rotational constant can change drastically with a small geometry shift due to vibrational excitation; Band Contour—a-b hybrid A Misses on the band contour of the b component because the A rotational constant can change drastically with a small geometry shift due to vibrational excitation; B Fits with a vibrationally averaged A constant in the upper state of 60.7 GHz A B : 1 : 1.9 q q 2 2 GSD Band Summary Experimental Intensities Calculated Intensities BO (cm-1) A (mV) B (mV) 3164.42 41 0 3153.25 10 0 3132.10 14 26 3114.29 38 0 3100.52 9 0 3099.25 6 0 3092.88 10 0 3088-3091 ? 3056.95 3047.10 Fundamental (cm-1) A (km/mol) B (km/mol) 3130.23 (C-H str) 1.16 2.99 3040.30 (C-H str) 0.85 1.00 2999.16 (C-H str) 4.54 8.12 2305.21 (N=C=O str) 1525.68 10.36 1671.91 (C=C bend) 158.26 0.00 Combination band (cm-1) Strong Normal Mode Contributor ? 3173.15 C=C bend 3 6 3169.93 N=C=O str 1 1 3093.56 C=C bend 3007.83 N=C=O str b3lyp/6-31g++(d,p) anharmonic (rediagonalized with cubic force constants) GSD Band Summary Experimental Intensities Calculated Intensities BO (cm-1) A (mV) B (mV) 3164.42 41 0 3153.25 10 0 3132.10 14 26 3114.29 38 0 3100.52 9 0 3099.25 6 0 3092.88 10 0 3088-3091 ? 3056.95 3047.10 Fundamental (cm-1) A (km/mol) B (km/mol) 3130.23 (C-H str) 1.16 2.99 3040.30 (C-H str) 0.85 1.00 2999.16 (C-H str) 4.54 8.12 2305.21 (N=C=O str) 1525.68 10.36 1671.91 (C=C bend) 158.26 0.00 Combination band (cm-1) Strong Normal Mode Contributor ? 3173.15 C=C bend 3 6 3169.93 N=C=O str 1 1 3093.56 C=C bend 3007.83 N=C=O str b3lyp/6-31g++(d,p) anharmonic (rediagonalized with cubic force constants) Chirped Pulse Fourier Transform Microwave Spectrometer 10 GHz Bandwidth 9-19 GHz 1-11 GHz 12 GHz Oscilloscope Ground State Rotational Spectrum cis pure rotational signal trans 303202 ~300 times weaker than trans cis 202101 trans 202101 cis 303202 Ground State Hyperfine Structure (trans) 212111 32 F'F'' 202101 21 21 32 10 F'F'' 11 22 11 10 12 22 211110 3164 cm-1 band Monitor 202101 R(1) Laser pumps all three J = 21 a-type transitions at once; population in 101 of GS is much greater than that in 111 or 110, so most of excited population is in 202 3164 cm-1 band: Hyperfine Structure • Resolved hyperfine structure observed in all upper states • Two different patterns observed: 1) K=0 pattern with eQq shifted down from ground state US GS 2) Pattern resembling nothing in the ground state 3164 cm-1 band: Hyperfine Structure (J = 21) Asterisks indicate a pattern unlike the ground state; all other lines have K=0 pattern Upper State Hyperfine There are two clear limits: Rigid Rotor (no K mixing) F'F'' Isotropic K distribution 21 32 ? 11 10 22 12 Hyperfine maintains ground state pattern Experimental Observations Hyperfine collapses; believed to be the expected result for simple coalescence 3132 cm-1 band: b-type Laser pumps R(1) of B band— makes 212 in US A B 3132 cm-1 band: b-type GS: K=0 (largest US) K=1 • Now the K = 1 pattern is observed; also, compression is observed as in 3164 cm-1 band 3114 cm-1 band Most upper state intensity near trans GS; likely not isomerizing Laser pumps P(2) (and P(3))-prepares 202/101 in US 3114 cm-1 band GS: K=0 (largest US) K=1 GS K = 0 pattern without compression observed; regardless, some upper states show hyperfine patterns unlike ground state (similar patterns to upper states in 3164 cm-1 band) Conclusions • A Balle-Flygare FTMW cavity was used to detect laser absorption from 30203180 cm-1 -Of the bands observed, most were pure a-type; a few (and one of the strongest) were a-b hybrids. -C-H stretching modes are predicted to be a-b hybrids, so some of the strongest bands in the region are likely to be combination bands off the a-type isocyanate stretch or C-C bend • Hyperfine-resolved upper state spectra were observed on the chirped pulse-FTMW instrument in the largest IR bands -One band (3114 cm-1) does not appear to induce isomerization, but the frequency spectra on other bands indicate that the upper states are delocalized between trans and cis -Quadrupole hyperfine structure is resolved in all upper states in all bands; most states maintain the qualitative splitting patterns of K=0 or K=1 ground state transitions, but with eQq smaller than for the ground state (or, according to G03W predictions, at any static geometry around the torsional angle) Acknowledgements Pate Lab Funding: -NSF Chemistry and MRI program -Jefferson Scholars Foundation (Justin) Double Pulse Ground State Depletion Bloch Vector Model Initial “/2” “-/2” Double Pulse Ground State Depletion MW pulse sequence FID signal Fourier transform Double Pulse Ground State Depletion Effect of the Laser Laser “/2” “-/2” We can then detect the vector component in the x-y plane as coherent emission against zero background. Room Temperature FTIR C-H stretches; comb. bands C-C stretch; C-H bends N=C=O stretch GSD Band Summary Band Origin (cm-1) A Inten (mV) B Inten (mV) 3164.42 41 0 3153.25 10 0 3132.10 14 26 3114.29 38 0 3100.52 9 0 3099.25 6 0 3092.88 10 0 3088-3091 20-25 0 3056.95 3 6 3047.10 1 1 3132 cm-1 band: a-type Laser pumps R(1) of A band— SHOULD make 202 in US A B 3132 cm-1 band: a-type GS: K=0 (largest US) K=1 • Confident only a-type transitions are being pumped, and J = 1 population is mostly in 101, so seeing a K = 1 hyperfine pattern is a puzzling result; also, this spectrum is not compressed at all • No upper states in this spectrum show the K = 0 hyperfine pattern