Broadband Nitrile Reaction Screening. Kiera Matthews Hanifah Hendricks

Transcription

1 Broadband Nitrile Reaction Screening Kiera Matthews Hanifah Hendricks

2 Acknowledgements Brooks Pate - University of Virginia Justin Neill University of Virginia Sarah Payne - University of Virginia Michael McCarthy Harvard-Smithsonian CfA Kristin Morgan University of Virginia Shirley Cauley University of Virginia Anthony Remijan National Radio Astronomy Observatory This work was supported in part by the National Science Foundation Center for Chemical Innovation through award CHE , University of Virginia, and the Virginia-North Carolina Alliance LSAMP Program.

3 Rotational spectroscopy a Crash Course Quantum mechanics tells us that atoms and molecules have quantized energy. Molecules and atoms only absorb/emit energy in discrete amounts In contrast to, for example, a macroscopic object like the heating coil on a stove, which emits a continuum of energies If molecules were like a heating coil, they would absorb all light but they only absorb in what appear to be lines in the prism Quantum regime Emission spectrum of a heating coil for various temperatures Absorption spectrum for the sun and emission spectra for hydrogen, helium, mercury and uranium atoms, respectively

4 Rotational spectroscopy a Crash Course

5 Rotational spectroscopy a Crash Course In the microwave region, we measure the different energies of a molecule s rotation. This is extremely sensitive to the geometry of the molecule For instance, the energy required for a molecule to go from not rotating at all to the lowest energy (think: slowest) rotating state is heavily dependent on the size of the molecule. CONSIDER: If a figure skater s moment of inertia is large, e.g. when her arms are spread out, her angular momentum is low. However, when her arms are folded in, her angular momentum Increases. We measure the arms of the molecule using three rotational constants, A, B & C (each proportional to the moment of inertia for the molecule for each of the three Cartesian axes X, Y and Z). The larger the constants, the smaller the molecule. The smaller the constants, the larger the molecule s size and the lower energies its rotations are. TAKE HOME POINT: Rotational spectroscopy offers rich structural information about the studied molecule.

6 (ABOVE) Theory gives us a prediction of our spectrum, the only input being the predicted structure of the molecule And we get an experimental result in great agreement to theory.

7 Rotational spectroscopy in action! Lactide experimental spectrum (right, black) Plotted against each isotopically substituted atom position (in color) Each set of colored lines represents the experimental spectrum of lactide with the atom colored below isotopically substituted (center of symmetry allows assignment of two equivalent atoms with single substituted spectrum) Gives you the direct molecular structure of lactide

8 Introduction Why broadband? Complex mixture analysis Overlap with GBT and evla data Nitrile Chemistry Easier to detect in the interstellar medium High dipole moments. Make up a large percentage of the known interstellar molecules Size and low rotational temperatures under lab conditions favor our lab frequency range GHz and GHz

9 Introduction con t. Electrical discharge chemistry of H 2 S and CH 3 CN suggests: Correlation between molecules synthesized in the discharge and interstellar survey species. 20 out of 26 synthesized molecules are interstellar Identifying known molecules and understanding their formation pathways can lead to new interstellar detections. The molecular carriers can then be identified using chemical principles.

10 CP-FTMW Spectrometer Schematic of the GHz CP-FTMW Spectrometer Schematic of the GHz CP-FTMW Spectrometer Gordon G. Brown, Brian C. Dian, Kevin O. Douglass, Scott M. Geyer, Steven T. Shipman, and Brooks H. Pate. Rev. Sci. Instrum. 79, , (2008).

11 CP-FTMW Spectrometer We detect molecular emissions in a way similar to that of FT-NMR. 300W microwave chirp pulse Molecular excitation Molecule relaxes and emits a time-domain emission signal as it relaxes (time vs intensity) Amplification, then convert to frequency vs intensity spectrum Femtowatt emission (10-15 W!)

12 Discharge 2 kv DC discharge Before expansion, molecules are forced through this discharge M.C. McCarthy, W. Chen, M.J. Travers, and P.Thaddeus, Ap. J. Supp. Series, 129, (2000).

13 Reaction Chemistry H 2 S / CH 3 CN discharge SH v=0 SH v=1 CH 3 CH 2 CN HCCCN CH 2 CHCN CH 2 CHNC In the CH 3 CN / H 2 S discharge, radicals are formed by homolytic bond cleavage. Those radicals recombine to form new molecules, some of which are interstellar (see figure to the left).

14 Discharge Reaction Chemistry

15 Dehydrogenation + CH 3 CH 2 CN CH 3 CH 2 CN + Ethyl cyanide ΔE (kj/mol) Energy Released 421 Energy Pool 421 Vinyl cyanide ΔE (kj/mol) Energy Consumed 164 Energy Pool 257 CH 3 CH 2 CN CH 2 CHCN H 2 + Cyanoacetylene ΔE (kj/mol) Energy Consumed 196 Energy Pool 61 CH 2 CHCN HCCCN H 2

16 Isomerization When CH 3 CN (methyl cyanide) homolytically cleaves into CH 3 and CN radicals, there is a probability that they can recombine (back into CH 3 CN) and then isomerize. Recombination releases 578 kj/mol (calculated) Ketenimine and methyl isocyanide are detected experimentally Possible Direct Pathway: CH 2 CN + H CH 2 CNH Figure taken from: Doughty et al. J. Phys. Chem. 98, (1994).

17 Radical-Neutral Reactions There is evidence for addition of hydrogen atom to CH 3 CN at the nitrile position Early ESR study shows evidence for CH 3 CHN radical formation from CH 3 CN and a hydrogen source Experiment done on 77K ices with VUV radiation Addition of one more hydrogen to CH 3 CHN to form CH 3 CHNH which is energetically favorable This molecule is detected in experiment and in the interstellar medium; there is also evidence for the precursor radical CH 3 CHN in the PRIMOS survey 1. Svejda et al. J. Phys. Chem. 74, (1970). 2. GBT PRIMOS Survey,

18 Ethanimine

19 Radical-Radical Recombination Radical Recombination from H 2 S / CH 3 CN discharge CH 2 CN (cyanomethyl) and SH (mercapto) radicals Mercaptoacetonitrile (HSCH 2 CN) Experimentally detected Dehydrogenation across the CS bond Cyanothioformaldehyde (HCSCN) Experimentally detected

20 Cyanothioformaldehyde + SH CH 2 CN HSCH 2 CN Mercaptoacetonitrile ΔE (kj/mol) Energy Released 322 Energy Pool Cyanothioformaldehyde ΔE (kj/mol) Energy Consumed 145 Energy Pool 177 HSCH 2 CN HCSCN H 2

21 Comparison with Literature MP2/ G(d,p) Experiment Bogey et al (1989) A (MHz) B (MHz) C (MHz) ΔJ (khz) ΔJK (khz) δk(khz) Χaa (MHz) (Xbb-Xcc) (MHz) M. Bogey et al. J. Am. Chem. Soc., 111, , (1989).

22 Comparison with Literature MP2/ G(d,p) Experiment Bogey et al (1989) A (MHz) B (MHz) C (MHz) ΔJ (khz) ΔJK (khz) δk(khz) Χaa (MHz) (Xbb-Xcc) (MHz) M. Bogey et al. J. Am. Chem. Soc., 111, , (1989).

23 Sulfur-34 X 20,000 magnification

24 Nitrogen-15 H 2 S / CH 3 C 15 N 260 k avg discharge H 2 S / CH 3 C 15 N 260 k avg discharge Full Spectrum b type Transitions

25 Kraitchman Substitution Structure The atomic positions of the nitrogen and sulfur atoms in HCSCN were obtained, as shown in the figure on the left. The colored circles represent atom position based on MP2/ G(d,p) calculation. The small white circles represent the experimentally determined atom positions, which confirms the assignment of the vibrational ground state of HCSCN.

26 Comparison with Literature MP2/ G(d,p) Experiment Bogey et al (1989) A (MHz) B (MHz) C (MHz) ΔJ (khz) ΔJK (khz) δk(khz) Χaa (MHz) (Xbb-Xcc) (MHz) M. Bogey et al. J. Am. Chem. Soc., 111, , (1989).

27 Conclusions Similar formation chemistry in regions such as Sgr B2(N) Chemical processes identified: Dehydrogenation Isomerization Radical-neutral reactions Radical-radical recombination Based on chemistry, two new molecules have been identified: Mercaptoacetonitrile and Cyanothioformaldehyde These are potential candidates for interstellar detection even though they don t appear in the PRIMOS survey. HCSCN is confirmed by obtaining the atom positions of sulfur and nitrogen.