A Chirped-Pulse millimeter-Wave (CPmmW) spectrometer is applied to the study of chemical reaction products that result from pyrolysis in a Chen nozzle heated to 1000–1800 K. Millimeter-wave rotational spectroscopy unambiguously determines, for each polar reaction product, the species, the conformers, relative concentrations, conversion percentage from precursor to each product, and, in some cases, vibrational state population distributions. A chirped-pulse spectrometer can, within the frequency range of a single chirp, sample spectral regions of up to ∼10 GHz and simultaneously detect many reaction products. Here we introduce a modification to the CPmmW technique in which multiple chirps of different spectral content are applied to a molecular beam pulse that contains the pyrolysis reaction products. This technique allows for controlled allocation of its sensitivity to specific molecular transitions and effectively doubles the bandwidth of the spectrometer. As an example, the pyrolysis reaction of ethyl nitrite, CH3CH2ONO, is studied, and CH3CHO, H2CO, and HNO products are simultaneously observed and quantified, exploiting the multi-chirp CPmmW technique. Rotational and vibrational temperatures of some product molecules are determined. Subsequent to supersonic expansion from the heated nozzle, acetaldehyde molecules display a rotational temperature of 4 ± 1 K. Vibrational temperatures are found to be controlled by the collisional cooling in the expansion, and to be both species- and vibrational mode-dependent. Rotational transitions of vibrationally excited formaldehyde in levels ν4, 2ν4, 3ν4, ν2, ν3, and ν6 are observed and effective vibrational temperatures for modes 2, 3, 4, and 6 are determined and discussed.

Highly accurate calculations are reported for properties of vinylidene (H2C═C:), specifically the position of its zero-point vibrational level relative to that of acetylene and its equilibrium structure and ground state rotational constants. The isomerization energy of vinylidene calculated at the HEAT-456QP level of theory is 43.53 ± 0.15 kcal mol–1, in agreement with the previous best estimate, but associated with a much smaller uncertainty. In addition, the thermochemical calculations presented here also allow a determination of the H2CC–H bond energy of the vinyl radical at the HEAT-345(Q) level of theory, which is 77.7 ± 0.3 kcal mol–1. The equilibrium structure of vinylidene, estimated with an additivity scheme that includes treatment of correlation effects beyond CCSD(T) as well as relativistic and adiabatic (diagonal Born–Oppenheimer correction) contributions, is rCC = 1.2982 ± 0.0003 Å, rCH = 1.0844 ± 0.0003 Å, and θCCH = 120.05 ± 0.05°, with zero-point rotational constants (including vibrational contributions and electronic contributions to the moment of inertia) estimated to be A0 = 9.4925 ± 0.0150 cm–1, B0 = 1.3217 ± 0.0017 cm–1, and C0 = 1.1602 ± 0.0016 cm–1.

We review recent research on the acetylene S1 state that illustrates how mechanistic rather than phenomenological information about intersystem crossing (ISC) may be obtained directly from frequency-domain spectra. The focus is on the dynamically rich “doorway-mediated” ISC domain that lies between isolated spectroscopic spin–orbit perturbations and statistical-limit interactions between one singlet “bright state” and a quasi-continuum of triplet “dark states”. New and improved experimental and data processing techniques permit the statistical-model curtain to be drawn back to reveal mechanistically explicit pathways via one or more identifiable, hence, manipulatable, doorway states, between a user-selected bright state and the undifferentiated bath of dark states.

Rotational analyses are reported for a number of newly-discovered vibrational levels of the S1-trans (Ã1Au) state of C2H2. These levels are combinations where the Franck–Condon active ν2′ and ν3′ vibrational modes are excited together with the low-lying bending vibrations, ν4′ and ν6′. The structures of the bands are complicated by strong a- and b  -axis Coriolis coupling, as well as Darling–Dennison resonance for those bands that involve overtones of the bending vibrations. The most interesting result is the strong anharmonicity in the combinations of ν3′ (trans bend, ag  ) and ν6′ (in-plane cis bend, bu). This anharmonicity presumably represents the approach of the molecule to the trans–cis isomerization barrier, where ab initio   results have predicted the transition state to be half-linear, corresponding to simultaneous excitation of ν3′ and ν6′. The anharmonicity also causes difficulty in the least squares fitting of some of the polyads, because the simple model of Coriolis coupling and Darling–Dennison resonance starts to break down. The effective Darling–Dennison parameter, K4466, is found to increase rapidly with excitation of ν3′, while many small centrifugal distortion terms have had to be included in the least squares fits in order to reproduce the rotational structure correctly. Fermi resonances become important where the K-structures of different polyads overlap, as happens with the 2131B1 and 31B3 polyads (B = 4 or 6). The aim of this work is to establish the detailed vibrational level structure of the S1-trans state in order to search for possible S1-cis (1A2) levels. This work, along with results from other workers, identifies at least one K sub-level of every single vibrational level expected up to a vibrational energy of 3500 cm−1.

A Chirped-Pulse millimeter-Wave (CPmmW) spectrometer is applied to the study of chemical reaction products that result from pyrolysis in a Chen nozzle heated to 1000–1800 K. Millimeter-wave rotational spectroscopy unambiguously determines, for each polar reaction product, the species, the conformers, relative concentrations, conversion percentage from precursor to each product, and, in some cases, vibrational state population distributions. A chirped-pulse spectrometer can, within the frequency range of a single chirp, sample spectral regions of up to ∼10 GHz and simultaneously detect many reaction products. Here we introduce a modification to the CPmmW technique in which multiple chirps of different spectral content are applied to a molecular beam pulse that contains the pyrolysis reaction products. This technique allows for controlled allocation of its sensitivity to specific molecular transitions and effectively doubles the bandwidth of the spectrometer. As an example, the pyrolysis reaction of ethyl nitrite, CH3CH2ONO, is studied, and CH3CHO, H2CO, and HNO products are simultaneously observed and quantified, exploiting the multi-chirp CPmmW technique. Rotational and vibrational temperatures of some product molecules are determined. Subsequent to supersonic expansion from the heated nozzle, acetaldehyde molecules display a rotational temperature of 4 ± 1 K. Vibrational temperatures are found to be controlled by the collisional cooling in the expansion, and to be both species- and vibrational mode-dependent. Rotational transitions of vibrationally excited formaldehyde in levels ν4, 2ν4, 3ν4, ν2, ν3, and ν6 are observed and effective vibrational temperatures for modes 2, 3, 4, and 6 are determined and discussed.