Reports: ND650650-ND6: Chirped-Pulse Millimeter-Wave Spectroscopy: A Universal Probe of Photofragmentation and Pyrolysis Mechanisms via Species and Vibrational Population Distributions

Robert W. Field, Massachusetts Institute of Technology

PRF Grant Report

We have demonstrated two major new applications of chirped-pulse millimeter-wave (CPmmW) spectroscopy of exceptional relevance to basic petroleum research.

A wealth of information about the identities and relative abundances of reaction products, in particular the relative populations of species, isomers, and vibrational levels, which we obtain by CPmmW spectroscopy, reveals the reaction mechanism. Paradigm-changing results are observed for a classical unimolecular photolysis reaction, which illustrate a systematically accessible experimental path to the study of bimolecular pyrolysis reactions relevant to biomass decomposition.

We demonstrate isomer- and vibrational level-specific spectroscopy of the products of 193 nm photolysis of vinyl cyanide. This fundamental chemical reaction has attracted the attention of researchers for decades. HCN was believed to be a dominant product of 193 nm photolysis via a three-center transition state mechanism, where the CN group captures the hydrogen from the adjacent carbon of the substituted ethylene. In contrast, a four-center mechanism, where the CN group abstracts the hydrogen atom of the other carbon atom of the ethylene, was believed to be the dominant mechanism responsible for HNC products. In our investigation of this photolysis reaction, we recorded CPmmW spectra of the photolysis products of normal (CH2=CH-CN) and singly-deuterated (CH2=CD-CN) vinyl cyanide. Surprisingly, the spectra in both cases showed almost equal yields of HCN and approximately the same distribution of population among vibrational levels. This observation is clearly incompatible with the commonly accepted three- vs. four-center competition between the mechanisms of vinyl cyanide photolysis. We find that when the hydrogen atom that would be expected to give HCN through the three-center mechanism is replaced by deuterium there is negligible effect on the HCN product yield. The DCN spectrum was observed as well, although it was of significantly lower intensity than the HCN spectrum and had a different vibrational population distribution.

The key advantages of CPmmW spectroscopy, which include rapid acquisition of broad bandwidth spectra, are demonstrated to be indispensible for fundamental chemistry research. The J=0-1 rotational transitions of vibrationally excited HCN molecules, which are products of the photolysis reaction, are spread in a vibrational pattern that covers several GHz in the millimeter-wave spectral range. The ability to simultaneously record these transitions enabled us to observee and distinguish the unique signatures of the four-center and the three-center transition states along the vinyl cyanide dissociation path. The slit jet enables systematic variation of the amount of rotational cooling, by carefully positioning the photolysis laser with respect to the collisional region of the supersonic expansion, were crucial in these experiments.

Our application of CPmmW spectroscopy to products of pyrolysis reactions is the second major new direction initiated during the grant period. We have demonstrated the unsuspected importance of bimolecular reactions with H-atoms in a Peter Chen type pyrolysis nozzle. Previously unobserved reactions with H-atoms were observed and the reaction yields were quantified.

The broad bandwidth of CPmmW spectroscopy facilitates parallel detection of many pyrolysis reaction products and their vibrational population distributions (VPD). We found, however, that, in contrast with photolysis reaction experiments, the pyrolysis VPD contains almost no information about the pyrolysis reaction transition state. The reason for this is the collisional relaxation of both the rotational and vibrational populations during the supersonic expansion from the Chen nozzle. For example, we found that the VPD of the formaldehyde product of methyl nitrite pyrolysis

CH3ONO ^ [CH3O] + NO ^ CH2O + H + NO

is very similar to the VPD of formaldehyde that was pre-mixed with the carrier gas and ejected into vacuum through a heated pyrolysis nozzle.

The VPD of formaldehyde in both cases, however, was found to be remarkable and counterintuitive. The common expectation is that vibrational levels that lie higher than about 500 cm-1 above the ground state are not significantly thermalized in the supersonic expansion. However, in our CPmmW studies we found that the degree of relaxation is highly molecule- and vibrational mode-specific. In formaldehyde, the out-of-plane bending mode is highly populated and its corresponding vibrational temperature does not fall below 50% of the temperature of the nozzle (1500 K). Other vibrational modes of formaldehyde, however, relax to 10–20% of the nozzle temperature. The non-Boltzmann VPD in formaldehyde can be attributed to the high symmetry of the molecule, which imposes restrictions on transitions among quantum states during the collisional relaxation, and the unique role played by Coriolis interactions between some of the vibrational states.

Broadband detection of the pyrolysis reaction products in a CPmmW experiment proved to be an information-rich path toward the discovery of new chemical mechanisms at elevated temperatures. Pyrolysis of biomass is commonly used to reduce the molecular weight of cellulose as an essential step in its conversion into biofuels.

Our pyrolysis jet CPmmW results have revealed the importance of chemistry mediated by hydrogen atoms. In collaboration with the research group of Prof. G. B. Ellison we have investigated the thermal decomposition of acetaldehyde. It is known to isomerize to vinyl alcohol, which unimolecularly decays into ketene by eliminating H2. We have discovered that acetaldehyde thermally decomposes to formaldehyde at very high (1800 K) temperature; however, thermochemistry prohibits such a unimolecular reaction. In fact, it is the free hydrogen atoms from dissociation of acetaldehyde that react with other acetaldehyde molecules and produce the formaldehyde detected in our CPmmW experiments. To investigate the H-atom addition reactions, we have added methyl nitrite, which is a source of free H-atoms at moderate temperatures in the Chen nozzle, to our acetaldehyde sample. This resulted in orders of magnitude larger formaldehyde yields at significantly lower (1200 K) temperature. We conclude that free H-atoms catalyze the cracking of acetaldehyde:

CH3CHO + H ^ CH2O + CH3

Isotopically substituted acetaldehyde was used to distinguish its CH2O product from that produced in the pyrolysis of methyl nitrite. The relative intensities of the precursor and product molecules measured in the CPmmW spectrum make it possible, after some calibration, to deduce the yields of different pyrolysis reactions. These initial observations open a new direction of research in which pyrolytic reactions that involve H-atoms can be discovered and quantified by CPmmW spectroscopy.