Mitchio and Stan were incredible mentors: they taught me how to do exact science with high impact. They taught me how to constantly question my work and always strive for the best possible results. Bill is the perfect person to work at Caltech: he pushes students to high levels of performance, is a skilled trumpet player and teacher, and knows how to make everything fun.
In the years since then, you have taught him everything he knows: how to learn, how to work hard, how to enjoy life, how to succeed in your endeavors, how to endure life's trials, how to value friends and how to love family. CRDS was used to make the first measurements of the OH stretch and A-X electronic spectra and the kinetics of hydroxymethylperoxy chemistry. CRDS was used to make the first measurements of the electronic A-X spectrum of -HOC4H8OO• and pure OH stretching spectra of all four radicals.
Part 1—Introduction and Description of Cavity Ringdown Spectrometer
Chapter 1—Introduction
HCHO: Spectroscopy, Kinetics, and Electronic Structure of HOCH 2 OO•
The activation barrier for reaction 1.10 is lower than dissociation back to HO2 + HCHO. The combination of these two factors indicates that the isomerization reaction will occur in the atmosphere and that reaction 1.9 should be faster at reduced temperatures. Previous experiments have measured the kinetics of HO2 + HCHO through the FTIR spectra of the end products24 or through the BX band of the direct reaction product, hydroxymethylperoxy (HOCH2OO• or HMP).25, 26 Despite these measurements, there is still considerable uncertainty in the activation energy. One source of this uncertainty is the interference of other species (HO2, CH3O2) within the structureless B X spectrum of HMP. We used CRDS for the first detection of the OH stretching vibrational spectrum and the AX electronic spectrum.
Third, anomalous behavior in the relative kinetic data is observed at low [O2] (less than 1 torr); the yield of the isomerization product increases as [O2] increases. We used CRDS for the first detection of the AX -HOC4H8OO• band, the isomerization product of n-butoxy. The results of our electronic structure calculations show clear differences in the OH stretching absorption cross sections for HOR• and HOROO• and clear patterns of how the relative locations of the hydroxyl and peroxy groups affect the intensity of the OH stretching.
Chapter 2—Description of Cavity Ringdown Spectrometer
The position of this peak is sensitive to the local chemical environment of the peroxy group. Finishing mirrors (Los Gatos Research, Newport, ATFilms, Layertec) were placed in commercially available mounts (Los Gatos Research) and attached to the ends of the kinetics cell. The cavity ringdown spectrometer's performance is intimately related to the reflectivity of the mirrors used.
In the center of the mirror beam, the reflection is the greatest and the lifetime of the revival will be the longest. To determine mirror reflectivity, vacuum depletion data is recorded across the entire range of mirrors. Typically, the bandwidth of the detector should be greater than 5/ to avoid this effect.
Part 2—Quantum Chemistry Studies of Peroxynitrous Acid (HOONO)
Chapter 3 – A three-dimensional potential energy surface and dipole moment surface for modeling the torsion-torsional dipole moment surface for modeling the torsion-torsion.
Chapter 3—A 3-Dimensional Potential Energy Surface and Dipole Moment Surface for Modeling the Torsion-Torsion Dipole Moment Surface for Modeling the Torsion-Torsion
The shape of the potential energy surface and the minimum energy path are analyzed to determine the magnitude of the coupling. To obtain this we need to map approximately 6000 cm−1 of the potential energy surface. Our three-dimensional potential energy surface was a function of the HOON dihedral angle (HOON), the OONO dihedral angle (OONO), and the OH bond length (rOH).
First, we present the potential energy surface as a function of the two dihedral angles, and comment on the coupling of the two torsional normal modes. Third, we show the logic behind the choice of the level of theory used for the calculation of the dipole moment surface. The plot of the potential energy surface of HOONO as a function of the HOON and OONO dihedral angles is illustrated in Figure 3.3.
This criterion was met when the endpoints of the potential energy surface were calculated (gray stars in Figure 3.3). The first feature is the shape of the potential energy well near the cis-cis HOONO minimum. Third, our potential energy surface sheds light on the nature of the cis-perp conformer of HOONO.
Our potential energy surface is a superset of the McCoy surface, allowing us to determine the nature of the cis-perp minimum they calculated. The HF-aug-cc-pVTZ dipole derivative is larger than any other method. Further from the potential energy minimum, the wavefunctions turn to the axis, indicating the decoupling of the two rotational modes.
These surfaces are functions of the two torsional modes (HOON, OONO) and the OH stretching mode (OH).
Part 3—Spectroscopy, Kinetics, and Quantum Chemistry of the Hydroxymethylperoxy Radical (HOCH 2 OO•, HMP)
Chapter 4 — Reduction Spectra of the Stretching OH Electron Cavity and A-X Reduction Spectra of the Hydroxymethylperoxy Radical of the Hydroxymethylperoxy Radical.
Chapter 4—The OH Stretch and A-X Electronic Cavity Ringdown Spectra of the Hydroxymethylperoxy Radical Ringdown Spectra of the Hydroxymethylperoxy Radical
The experimental and theoretical studies on HO2 + HCHO paint a consistent picture of the reaction mechanism (Reaction 4.1). This chapter (Chapter 4) describes the first detection of the 1 and A-X bands of HMP via cavity ring-off spectroscopy. 1 (OH stretching) spectrum of HOCH2OO• (HMP), the product of HO2 + HCHO, taken under our most ideal conditions.
As discussed in the Chemistry section, one of the byproducts in our experiment is H2O2, and these peaks belong to the 1/5 bands (OH-symmetric and antisymmetric stretches) of H2O2. 1 (OH stretch) spectra of HOCH2OO• (HMP), the product of HO2 + HCHO, taken under poor conditions. Despite interference from HO2, we see four clean absorption bands in the rest of the spectrum.
The left panel is measured to the red of the tape head and is subject to more noise near the peak. The right panel is measured at the peak absorption and is the best measure of the peak kinetics. We observe excellent agreement between experiment and simulation, supporting our assignment of the band to HMP.
Similar to the 1 band, we can estimate the absorption cross sections of the A-X bands in Figure 4.9. Despite the difference in band positions, the similarities in band shapes give us more confidence in our assignment of the observed absorption to the 1 band of HMP. Therefore, we observe good agreement in the line strength of the pure A-X electronic transition between HMP and CH3OO•.
In this chapter, we reported the first detection of the 1 (OH stretch) vibrational and A-X electronic spectra of the hydroxymethylperoxy radical (HOCH2OO•, HMP), a product of HO2 + HCHO.
Chapter 5—Predicted A-X Transition Frequencies and 2-Dimensional Torsion-Torsion Potential Energy Surfaces of 2-Dimensional Torsion-Torsion Potential Energy Surfaces of
In this thesis chapter, we present two-dimensional potential energy surfaces for the ground (X) and first excited (A) states of HMP, as a function of the OCOH and OOCO dihedral angles. We generated potential energy surfaces of HMP as a function of the two dihedral angles OCOH and OOCO by performing relaxed energy scans (i.e. for each set of dihedral angles (OCOH, OOCO), all other molecular coordinates were allowed to relaxed). Based on the results of these potential energy surfaces, we investigated the global minimum of the X state and the associated potential energy well of the A state in greater detail to obtain information about the A-X transition frequency.
Figures 5.1-5.3 show a series of 2-dimensional potential energy surfaces of the HMP as a function of the two dihedral angles OCOH and OOCO. B3LYP/6-31+G(d,p) HMP potential energy surfaces for state X (left) and state A (right), as a function of dihedral angles OCOH and OOCO. CCSD/6-31+G(d,p) (top) and B3LYP/cc-pVDZ (bottom) HMP potential energy surfaces for state X (left) and state A (right), as a function of dihedral angles OCOH and OOCO.
There are three non-equivalent minima on the potential energy surface representing rotation of the peroxy group, similar to the three equivalent minima observed for CH3OO•.127 The global minimum conformer (Figure 5.1, labeled Conformer A) has an internal hydrogen bond. Finally, we note that the positions of the minima on the X mode (the positions of the letters) do not match the minima on the A mode. This is likely due to the relative inflexibility of the cc-pVDZ basis set (no polarization or diffuse functions) compared to the 6-31+G(d,p) basis set.
Finally, we extend the methods presented in this chapter to generate potential energy surfaces and predict the spectroscopic properties of 2-HIPP. Similar to HMP, the global minimum of state A corresponds to the equivalent of conformer B. 123 We currently recommend CCSD or W1 for calculating A-X transition frequencies and calculating potential energy surfaces.
Chapter 6—Kinetics of HO2 + HCHO and Further Reaction of the Hydroxymethylperoxy Radical (HOCH2OO•) the Hydroxymethylperoxy Radical (HOCH2OO•).
We now turn our attention to the kinetics of HMP: both formation (Reaction 6.1) and destruction (Reactions 6.2-6.6). We can use the spectroscopic bands of HMP from Chapter 4 to measure the kinetics of Reactions 6.1-6.6. This thesis chapter describes the first kinetic measurements of the formation and destruction of HMP, as measured via the 1 vibrational and A-X electronic bands of HMP.
Tunable MIR light used to measure the kinetics via the 1 absorption of HMP was generated using an optical parametric amplifier. We also show that the absorption in the 1 region remains constant for a long time despite the formation of HCOOH, indicating that the 1 band of HMP cannot be used to measure its destruction rate. These results clearly demonstrate the destruction of HMP, and we infer a lifetime of HMP under our experimental conditions.
At short times we have shown (Chapter 4) that the measured spectrum should be representative of HMP. The uncertainty on this rate constant includes the spread of the k1 data, 20% uncertainty on the IR absorption cross section of HMP, and 10% uncertainty on k1 due to the approximations made to obtain equation 6.22. As explained in the introduction, our kinetics measurements using the A-X bands are well suited to measure HMP destruction and therefore the lifetime of HMP in our experiment.
Although we were able to derive an accurate (±20%) expression for the rate constant of HMP formation, we cannot do the same for HMP destruction for two reasons. We cannot determine individual destruction rate constants; rather, we can only examine the overall destruction of HMP. Therefore, the lifetime of HMP will be greater than a simple analysis of the rate constants of HMP destruction.
Due to the multiple destruction pathways of HMP and regeneration of HO2 (reactions 6.2-6.6), it is difficult to estimate whether this lifetime is reasonable or not.
