A new approach toward transition state spectroscopy

Abstract

10Chirped-Pulse millimetre-Wave (CPmmW) rotational spectroscopy provides a new class of information about photolysis transition state(s). Measured intensities in rotational spectra determine species-isomervibrational populations, provided that rotational populations can be thermalized. The formation and detection of S 0 vinylidene is discussed in the limits of low and high initial rotational excitation. CPmmW 15 spectra of 193 nm photolysis of Vinyl Cyanide (Acrylonitrile) contain J=0-1 transitions in more than 20 vibrational levels of HCN, HNC, but no transitions in vinylidene or highly excited local-bender vibrational levels of acetylene. Reasons for the non-observation of the vinylidene co-product of HCN are discussed. A. Introduction 20Chirped-Pulse millimetre-Wave (CPmmW) spectroscopy 1-3 is capable of determining the relative species-conformervibrational level populations of all polar products of a photolysis reaction. These experimentally determined populations encode the structures of the transition states 25 that are most important at each photolysis wavelength or combination of wavelengths. 4 The isomer-conformervibrational level population information obtained from a CPmmW spectrum is more complete than what is obtainable by mass spectrometry 5 and free of the need for 30 the transition strength and quantum yield determinations that are required for most laser-based population measurements. However, difficulties exist in the use of populations determined by CPmmW spectroscopy to characterize transition states. B. Chirped Pulse SpectroscopyChirped Pulse Fourier Transform Microwave Spectroscopy 55 is a revolutionary technique developed in the research group of Brooks Pate at the University of Virginia. 2,3 In its initial implementation, the frequency of a microwave pulse is chirped linearly in time over several GHz. This microwave pulse is broadcast into a gas phase molecular 60 sample, polarizing all two-level systems with frequencies within the spectral interval of the chirped pulse. These polarizations relax by Free Induction Decay (FID), which is a voltage vs. time signal that is collected, downconverted by mixing with a local oscillator (heterodyne where  is the electric dipole moment,  0 is the peak microwave electric field, N 1,2 is the population density 75 difference between levels 1 and 2,  is the chirp rate, A is

The neutral photochemistry of nitriles, amines and imines in the atmosphere of Titan

“…There is likely to be another efficient neutral HNC production mechanism: C 2 H 3 CN photodissociation. Indeed, Wilhelm et al (2009) detected unambiguously HNC IR emission in their C 2 H 3 CN photodissociation experiment at 193 nm with a HCN/ HNC ratio equal to 3.34 in good agreement with theoretical calculations from Homayoon et al (2011) as well as direct HNC absorption by the Chirped-Pulse millimetre-Wave study of C 2 H 3 CN photodissociation (Prozument et al, 2013). The dissociative recombination reaction of HCNH + is an efficient source of HNC as HCNH + is the main ion in Titan's upper atmosphere and the branching ratio for HNC production is estimated to be close to 30% (Semaniak et al, 2001;Mendes et al, 2012;Barger et al, 2003).…”

“…Nevertheless, the study of Blank et al (1998) has clearly shown the presence of cyanoacetylene as a photofragment so that it is more likely that a four-center transition state dominates the cyanoacetylene elimination channel, in contrast to the observation that a three-center transition state dominates the acetylene elimination channel. A recent study of C 2 H 3 CN photodissociation at 193 nm using Chirped-Pulse millimetre-Wave (Prozument et al, 2013) absorption spectroscopy clearly shows the production of HCN and HNC but without quantifying the relative yields of these two species. The branching ratios used in the present work are estimated, taking into account the various experimental studies.…”

Rotational spectral studies of O(1D) insertion reactions with methane and ethylene: Methanol and vinyl alcohol in a supersonic expansion

“…Surprisingly, there are only a few examples of millimeter/submillimeter wave spectrometers that have been coupled to lasers [61][62][63][64][65]. A schematic diagram of the experimental setup is shown in Figure 2.…”

Direct detection of Rydberg–Rydberg millimeter-wave transitions in a buffer gas cooled molecular beam

“…The advantages of chirped-pulse spectroscopy, namely the combination of broadband excitation, multiplexed heterodyne detection, high resolution, and meaningful relative intensities, have been demonstrated in pure rotation spectra of large and small molecules [24,25]. Distinct from conventional millimeterwave spectroscopy that employs a thermal detector or other indirect detection schemes (typically ion detection), chirped-pulse techniques make it possible to 6 capture an entire spectrum (400,000 50 kHz resolution elements) in a single chirp (20 GHz bandwidth) rather than sequentially, one resolution element at a time.…”