Carbenes (R=C:) are a class of chemical compounds that possess a neutral carbon atom with two unshared valence electrons. Due to the reactivity of the electron lone pair, these molecules are highly reactive, and play important roles in many organic reactions[174]. This often involves an intramolecular 1,2-hydrogen shift, where a migrating hydrogen moves across the double bond towards the lone pair electrons - a mechanism that plays a critical role in more complex organic reactions such as dehydration of alcohols (or hydration of alkenes), and the gas-phase combustion of hydrocarbons[175,176]. The simplest unsaturated carbene is the vinylidene molecule, making the vinylidene-acetylene isomerisation reaction an ideal prototype for studying this 1,2-H migration[177].
The study of vinylidene provides a novel example of transition-state spectroscopy. Transitions state theory (TST) states that the dynamics of a reaction are controlled by the formation of reactive intermediates, at the rate determining step. Therefore, studying these intermediate states, and their associated chemistry, is vital to the understanding and possibly controlling of how large multi-step organic reactions proceed. Vinylidene is widely invoked as a reaction intermediate in a host of different chemical processes, making it an attractive target for spectroscopic studies[178–182]. But, by definition these intermediate species are transient in nature, making them unsuitable for measurement via most common spectroscopic techniques. However, if a transient species possesses a stable parent anion (as is the case for vinylidene), it is possible to use photoelectron spectroscopy of the negative ion to obtain information about the neutral intermediate state.
This makes the vinylidene-acetylene isomerisation an appealing benchmark unimolec- ular process in chemical physics, and as such, has been the focus of many theoretical studies[183–189]. However, while the existence of acetylene has been known and studied for well over a century[177,189], the existence of the vinylidene isomer as a stable bound molecule was much less certain[172,190]. Numerous attempts failed to isolate the vinylidene molecule[171], and hence it was long believed to be extremely difficult, if not impossible, to observe vinylidene either chemically or spectroscopically[172] .
The difficulty in observing vinylidene directly, arises from the remarkably low barrier to isomerisation (∼0.1−0.2 eV[53,191]). Isomerisation reactions of carbenes involving 1,2- hydrogen shifts are known to occur very rapidly[174], and it has long been established that if vinylidene indeed exists, its lifetime would be very short[172,178,192]. However, despite these difficulties, the first spectroscopic measurement of isolated vinylidene was achieved in 1983 by the Lineberger group at JILA[193], by applying photoelectron spectroscopy to the vinylidene anion. This measurement, shown in Fig. 8.1, reveals some of the vibrational structure of the vinylidene isomer, and confirmed the existence of vinylidene as a bound molecule[190,193]. The deuterated species D
2CC was also measured, and found to show similar spectroscopic features. Importantly this result confirmed that the structure of neutral vinylidene could be accessed via spectroscopy of the negative ion.
1.4 1.6 1.8 2.0 2.2 Electron Kinetic Energy (eV)
Photoelectron Counts (arb.)
H2CC
D2CC
Figure 8.1: The first spectroscopic measurement of the vinylidene molecule, recorded by the Lineberger group in 1983 via photodetachment of the vinylidene anion at 488 nm, reproduced from Ref[193].
The measurement in Fig. 8.1 finally gave a definitive answer about the existence of vinylidene, however this was really only the start of the vinylidene story, with attention now turned to studying the lifetime and dynamics of this deceptively complex molecule.
§8.1 The story of vinylidene 125
8.1.1 The lifetime conundrum
In 1989 a seminal second photoelectron spectrum of the vinylidene ion was published by the Lineberger group, where implementation of a new ion source created a colder, higher density packets of ions, and greatly improved spectral resolution[11]. The high resolution photoelectron spectrum, presented in Fig. 8.2, is able to resolve more of the vibrational structure of the molecule, with a much lower background. However, the electron kinetic energy resolution remained insufficient to fully resolve the rotational band shape of these vibrational resonances. This result was greeted with much fanfare, as the resolved peak widths provided a prediction for the lifetime of the vinylidene molecule. The experimental peak widths observed in Fig. 8.2 have three key contributions, with broadening due to the unresolved rotational structure, the instrumental resolution, and the lifetime of the state. The lifetime broadening arises from the uncertainty principle, ∆E∆τ ≥~/2, and results in a Lorentzian line shape with width Γ1/2 = 1/2πτ, where τ is the lifetime of the state.
0.3 0.4 0.5 0.6 0.7 0.8 0.9
Electron Binding Energy (eV) 0.0 0.1 0.2 0.3 0.4 0.5 0.6 Ph ot oe lec tro n Co un ts (1 0 3) H2CC X1A1 S 301 R 611 000 R 620 640 S310 210 320 220310 220 CH 110
(a) 1989 photoelectron spectrum of the vinylidene molecule, with vibrational structure of the ground
˜ X1A1 state resolved[11] 0.46 0.48 0.50 0.52 0.54
electron binding energy (eV) 0.0 0.2 0.4 0.6 0.8 1.0 relative intensity
X
1A
1(b) Rotational band contour (− − −) fitted to experimental data (−−−). 25−120 cm−1
of excess broadening is attributed to lifetime broadening.
Figure 8.2: Improved high resolution photoelectron spectrum of vinylidene at 351.1 nm measured by the Lineberger group in 1989, reproduced from Ref[11]. Observed peak widths suggest a vinylidene lifetime ofτ = 0.04−0.2 ps.
Rotational contours were constructed to estimate the amount of spectral broadening due to rotational transitions (shown in blue in Fig. 8.2). Convolving the rotational con- tour with the known instrument resolution function (Lorentzian broadening with 9meV FWHM) leaves Γ1/2∼25−120 cm−1 of additional broadening present on top of the rota- tional/instrument width, with this extra broadening attributed to the short lifetime of the vinylidene isomer. This was supported by comparison between the photoelectron spectra of H2CC and D2CC which showed the extra broadening was only present in the H2CC spectrum (and only in the ground electronic state), suggesting that it must therefore be linked to the isomerisation process. The excess linewidth corresponds to an estimated lifetime of τ = 0.04−0.2ps[11]. It was carefully noted by the authors, however that it is possible that all of the observed broadening may be accounted for by the rotational con- tour plus instrument resolution, due to large uncertainties in the simulations. Despite this careful qualification, subsequent research interpreted all of the observed additional width as definite measured lifetime broadening[172,175]. As such, vinylidene lifetimes ranging
from a few picoseconds to as short as ∼100 femtoseconds have been cited as being exper- imentally measured in the PES spectrum[55,194,195]. This consensus for a sub-picosecond vinylidene lifetime was generally agreed upon for 9 years, until a subsequent coulomb explosion imaging study appeared to alarmingly contradict this interpretation.
In 1998 the Vager research group at the Weizmann Institute[196,197]reinvestigated the stability of the vinylidene molecule using a coulomb explosion imaging (CEI) apparatus, which also accessed the transient neutral vinylidene state by exploiting the stable negative ion. Vinylidene anions were produced in a microwave discharge source, mass separated, photodetached, and then accelerated towards a thin strip of gold foil. The subsequent impact strips the remaining electrons from the molecule, with the position and arrival times of the resulting cationic fragments (2×H+and 2×C6+) measured on a multi-particle detector[12]. From the coincidental temporal and spatial information, the geometry of the molecule at the point of contact with the gold foil is calculated.
The CEI experiment provided an extraordinary result - even when photodetachment (forming the transient neutral vinylidene molecule) occurred 3.5 µs before the collision, ∼ 50% of the molecules retained the vinylidene geometry at the point of contact[12]. This surprising observation appeared to contrast with the commonly accepted notion of vinylidene being a short-lived isomer with a sub-picosecond lifetime.
Many future studies have attempted to reconcile the stark discrepancy between these two experimental results, with little success[53,189,198]. One reason for this is the con- tinued difficulty in observing the vinylidene molecule experimentally[188], with the stable vinylidene anion hard to produce[199]. Difficulties also arise from the complexity of the physics involved in the isomerisation. As the barrier to isomerisation lies∼2 eV above the acetylene minimum, vinylidene is believed to tunnel through a small barrier, into a quasi- continuum of acetylene vibrational states. However, calculations and experimental studies that focused on the acetylene side of the barrier by Field and co-workers suggest that vinylidene may only be able to interact with a select few vibrational door-way states on the acetylene side[200,201]. Consequently, this process is not a simple textbook tunnelling process, where vinylidene tunnels through a barrier into a continuum of states. Instead, periodic reversible dynamics are expected, with predicted long-lived eigenstates consist- ing of both vinylidene and acetylene character[53]. Finding evidence of these long-lived eigenstates holds the key to understanding the dynamics of the isomerisation process, and is the main aim of the work in this section of the thesis. This requires a concerted effort using multiple experimental techniques on both sides of the vinylidene-acetylene barrier, coupled with state of the art full quantum dynamic calculations.