1. Development of a QCL-Based Spectrometer for Analysis of Biogenic Volatile Organic Compounds
Michael Iranpour, Minh Tran, Marcus Pereira, Jacob Stewart
Department Of Chemistry, Connecticut College
ISMS 2017

2. What are BVOCs and why are they important?
Naturally occurring compounds emitted by plants
Impact atmospheric processes
React to form various radicals and ozone
Help to defend against biotic stress

3. Isoprene is the first target molecule
The simplest BVOC
Composes half of all BVOC emission
Responsible for a myriad of atmospheric processes

4. Isoprene is the precursor to processed BVOCs and ozone
OH, NO3, O3
Oxidizing isoprene produces more stable BVOCs. This process produces atomic oxygen which will react with molecular oxygen to form ozone
NO2 + hv → NO + O(3P)

5. Oxidized BVOCs form aerosols
Processed BVOCs condense to form secondary aerosols.
Aerosols impact local climates through the absorption and reflection of radiation
Of particular importance within the context of global climate change is the scientists' belief that "isoprene and monoterpenes, which constitute a major fraction of BVOCs, might confer protection against high temperatures" by acting "as scavengers of reactive oxygen species produced [within plants] under high temperatures."  If this is inde
As a result," they continue, "there should be a net cooling of the Earth's surface during the day because of radiation interception," noting that Shallcross and Monks (2000) "have suggested that one of the reasons plants emit the aerosol isoprene might be to cool the surroundings in addition to any physiological or evaporative effects that might cool the plant directly."

6. Objectives Use IR spectroscopy to analyze BVOCs
Use absorbance information to determine BVOC concentrations in the atmosphere
Determine structural information about isoprene from high-resolution spectra

7. Instrumental setup

8. Methods of scanning the laser
Using laser controller
Using piezo
Wavelength Modulation

9. High resolution IR spectrum of isoprene
Acquired using direct absorption spectroscopy
Very strong Q-branch
Asymmetric top
First high-resolution IR spectrum of isoprene
Matches previously reported low resolution spectrum
Limited frequency accuracy (imprecise motor for sweeping grating in laser)

10. Calibrated isoprene spectrum

11. High resolution spectrum demonstrates agreement with Low resolution spectrum
First high-resolution IR spectrum of isoprene
Matches previously reported low resolution spectrum
Limited frequency accuracy (imprecise motor for sweeping grating in laser)
PNNL data from Brauer et al., Atmos. Meas. Tech., 7, , 2014.
PNNL data from Brauer et al., Atmos. Meas. Tech., 7, , 2014.

12. High resolution spectrum of the Q branch area demonstrates more complex structure
Q-branch tails off towards the R-branch
Rotational structure is actually separate vibrational bands
Additional vibrational bands could be caused by hot bands, combination bands, or presence of a gauche conformer.
Hot bands
What may initially be seen as simply just rotational structure, is notably additional vibrational bands in the Q branch. We also believe that combination bands may be responsible for these as well
Use comp to predict geometry and vibrational freqs. Step further to diff rotational constant for each different vibrational state. Know ground state because of microwave. Predict excited staea s well gives idea of how much constants will change

13. Vibrational Frequency
High resolution spectrum of the Q branch area demonstrates more complex structure
Vibrational Frequency
Boltzmann Factor
152.7 cm-1
0.48
200 cm-1
0.38
280 cm-1
0.27
Hot bands
What may initially be seen as simply just rotational structure, is notably additional vibrational bands in the Q branch. We also believe that combination bands may be responsible for these as well
Use comp to predict geometry and vibrational freqs. Step further to diff rotational constant for each different vibrational state. Know ground state because of microwave. Predict excited staea s well gives idea of how much constants will change
Allodi, Marco A., Karl N. Kirschner, and George C. Shields. “Thermodynamics of the Hydroxyl Radical Addition to Isoprene.” The journal of physical chemistry. A (2008): 7064–7071. PMC. Web. 15 June 2017.

14. Additional vibrational bands may suggest presence of the s-gauche conformer
Exists at a higher energy state than the trans- conformer. ∆G = 2.45 kJ/mol
Previous theoretical calculations predict an abundance of approximately 5% for this conformer at room temperature
No IR spectroscopy performed on this conformer in the gas phase
Gauche conformer
The quantum number J refers to the total angular momentum, as before. Since there are three independent moments of inertia, there are two other independent quantum numbers to consider, but the term values for an asymmetric rotor cannot be derived in closed form. They are obtained by individual matrix diagonalization for each J value. Formulae are available for molecules whose shape approximates to that of a symmetric top
Fitting the spectra to the theoretical expressions gives numerical values of the angular moments of inertia from which very precise values of molecular bond lengths and angles can be derived in favorable cases. In the presence of an electrostatic field there is Stark splitting which allows molecular electric dipole moments to be determined.
Trans conformer
Brauer,et al.: Quantitative infrared absorption cross sections of isoprene for atmospheric measurements, Atmos. Meas. Tech., 7, , 2014.

15. Future Work Assign the calibrated isoprene spectrum
Pursue computational methods to help determine the cause(s) of the additional vibrational bands
Increase the sensitivity of the spectrometer
Acquire isoprene spectra in hypothermic conditions with supersonic expansion

16. Conclusions
BVOC’s are highly relevant to atmospheric processes, influencing reactions that may produce aerosols, ozone, and other atmospheric species
Acquired and calibrated the first high-resolution IR spectrum of isoprene
Developed a QCL-based IR spectrometer that can obtain high-resolution spectra with high sensitivity

17. Acknowledgements Stewart Research Group Dave Lewis
Connecticut College Chemistry Department