Pure Rotational Spectroscopy of the CH2CN Radical Extended to the Sub-Millimeter Wave Spectral Region

Rotational spectroscopy of methyl tert-butyl ether with a new Ka band chirped-pulse Fourier transform microwave spectrometer

Chirped-pulse Fourier transform microwave (CP-FTMW) spectroscopy is a powerful tool for performing broadband gas-phase rotational spectroscopy, and its applications include discovery of new molecules, complex mixture analysis, and exploration of fundamental molecular physics. Here we report the development of a new Ka band (26.5-40 GHz) CP-FTMW spectrometer that is equipped with a pulsed supersonic expansion source and a heated reservoir for low-volatility samples. The spectrometer is built around a 150 W traveling wave tube amplifier and has an instantaneous bandwidth that covers the entire Ka band spectral range. To test the performance of the spectrometer, the rotational spectrum of methyl tert-butyl ether (MTBE), a former gasoline additive and environmental pollutant, has been measured for the first time in this spectral range. Over 1000 spectroscopic transitions have been measured and assigned to the vibrational ground state and a newly-identified torsionally excited state; all transitions were fit using the XIAM program to a root-mean-square deviation of 22 kHz. The spectrum displays internal rotation splitting, nominally forbidden transitions, and an intriguing axis-switching effect between the ground and torsionally excited state that is a consequence of MTBE's extreme near-prolate nature. Finally, the sensitivity of the spectrometer enabled detection of all singly-substituted 13C and 18O isotopologues in natural abundance. This set of isotopic spectra allowed for a partial r0 structure involving the heavy atoms to be derived, resolving a structural discrepancy in the literature between previous microwave and electron diffraction measurements.

High-Resolution Spectroscopic Investigation of the CH2CHO Radical in the Sub-Millimeter Region

In this work, the pure rotational spectrum of the vinoxy radical (CH2CHO) has been studied at millimeter and sub-millimeter wavelengths (110-860 GHz). CH2CHO was produced by H-abstraction from acetaldehyde (CH3CHO) using atomic fluorine in a double-pass absorption cell at room temperature. A Zeeman-modulation spectrometer, in which an external magnetic field generated inside the absorption cell is amplitude-modulated, was used to record the pure rotational transitions of the radical. The recorded spectra are devoid of signals from closed-shell species, allowing for relatively fast acquisitions over wide spectral windows. Transitions involving values of the rotational quantum numbers N″ and Ka″ up to 41 and 18, respectively, were measured and combined with all available high-resolution literature data (both pure rotation and ground-state combination differences from ro-vibration) to greatly improve the modeling of the CH2CHO spectrum. The combined experimental line list is fit using a semirigid rotor Hamiltonian, and the results are compared to quantum chemical calculations. This laboratory study provides the spectroscopic information needed to search for CH2CHO in various interstellar environments, from cold (e.g., typically 10 K for dense molecular clouds) to warm (e.g., ∼200 K for hot corinos) objects.

Direct Probes of π-Delocalization in Prototypical Resonance-Stabilized Radicals: Hyperfine-Resolved Microwave Spectroscopy of Isotopic Propargyl and Cyanomethyl

Delocalization of the unpaired electron in π-conjugated radicals has profound implications for their chemistry, but direct and quantitative characterization of this electronic structure in isolated molecules remains challenging. We apply hyperfine-resolved microwave rotational spectroscopy to rigorously probe π-delocalization in propargyl, CH2CCH, a prototypical resonance-stabilized radical and key reactive intermediate. Using the spectroscopic constants derived from the high-resolution cavity Fourier transform microwave measurements of an exhaustive set of 13C- and 2H-substituted isotopologues, together with high-level ab initio calculations of zero-point vibrational effects, we derive its precise semiexperimental equilibrium geometry and quantitatively characterize the spatial distribution of its unpaired electron. Our results highlight the importance of considering both spin-polarization and orbital-following contributions when interpreting the isotropic hyperfine coupling constants of π radicals. These physical insights are strengthened by a parallel analysis of the isoelectronic species cyanomethyl, CH2CN, using new 13C measurements also reported in this work. A detailed comparison of the structure and electronic properties of propargyl, cyanomethyl, and other closely related species allows us to correlate trends in their chemical bonding and electronic structure with critical changes in their reactivity and thermochemistry.

3-Fluoro-2H-azirine: Generation, Characterization, and Photochemistry

The elusive 3-fluoro-2H-azirine, cyclic NCH2CF, has been generated through the stepwise decomposition of the acryloyl azide CH2CFC(O)N3 in an N2-matrix at 10 K. The characterization of cyclic NCH2CF with matrix-isolation IR spectroscopy is supported by 15N isotope labeling and the calculations with density functional theory (DFT) at the B3LYP/6-311++G(3df,3pd) level of theory. Upon irradiation at 193 nm, cyclic NCH2CF undergoes ring opening by forming the more stable nitrile isomer CH2FCN. In contrast to the photodecomposition reactions, the high-vacuum flash pyrolysis of CH2CFC(O)N3 in the gas phase at 500 °C yields the Curtius rearrangement product CH2CFNCO along with secondary fragmentation to the atmospherically relevant fluorocarbonyl radical (FCO) and cyanomethyl radical (CH2CN). Calculations on the potential energy profile for the decomposition reactions of CH2CFC(O)N3 demonstrate that the excessive energy, arising from the highly exothermic Curtius rearrangement of the azide, plays a key role in driving further dissociation reactions of CH2CFNCO by overcoming the formidable barriers (>50 kcal mol-1) under the pyrolysis conditions.