# Interstellar Detection of the Highly Polar Five-Membered Ring
Cyanocyclopentadiene

Michael C. McCarthy, Kin Long Kelvin Lee, Ryan A. Loomis,
Andrew M. Burkhardt, Christopher N. Shingledecker,
Steven B. Charnley, Martin A. Cordiner, Eric Herbst,
Sergei Kalenskii, Eric R. Willis, Ci Xue,
Anthony J. Remijan, and Brett A. McGuire

> Much like six-membered rings, five-membered rings are ubiquitous in organic
> chemistry, frequently serving as the building blocks for larger molecules,
> including many of biochemical importance. From a combination of laboratory
> rotational spectroscopy and a sensitive spectral line survey in the radio
> band toward the starless cloud core TMC-1, we report the astronomical
> detection of 1-cyano-1,3-cyclopentadiene, $c$-C5H5CN, a highly polar, cyano
> derivative of cyclopentadiene, $c$-C5H6. The derived abundance of $c$-C5H5CN
> is far greater than predicted from astrochemical models which well reproduce
> the abundance of many carbon chains. This finding implies either an
> important production mechanism or a large reservoir of aromatic material may
> need to be considered. The apparent absence of its closely-related isomer,
> 2-cyano-1,3-cyclopentadiene, may arise from its lower stability or be
> indicative of a more selective pathway for formation of the 1-cyano isomer,
> perhaps one starting from acyclic precursors. The absence of N-heterocycles
> such as pyrrole and pyridine is discussed in light of the astronomical
> finding.

### Main

The recent astronomical detection of benzonitrile, $c$-C6H6CN (?), has at
least partially resolved a long-standing conundrum in astrochemistry — the
apparent absence of five and six-membered rings, which are the building blocks
of organic chemistry on earth. For example, of the more than 135 M compounds
registered in Chemical Abstract Service, it is estimated (?, ?) that nearly
80% contain at least one of these two rings. In contrast, nearly 20 acyclic
compounds with a comparable number of carbon atoms to benzene, $c$-C6H6, were
known astronomically (?) prior to the discovery of benzonitrile. Although
benzene is nonpolar, and hence cannot be detected via its rotational
transitions, its five-membered ring analog cyclopentadiene ($c$-C5H6) is
weakly polar, with a small permanent dipole moment ($\mu_{b}=0.420$ D; Ref. ?,
?), and a well-known rotational spectrum (?). Nevertheless, like with benzene,
replacing a single H atom in cyclopentadiene with a CN group imparts the three
possible cyanocyclopentadiene variants with substantial dipole moments,
regardless of the substitution site, meaning that their rotational lines are
roughly 100 times more conspicuous relative to those of the hydrocarbon parent
in space at the same abundance.

Unlike benzene, cyclopentadiene is a highly reactive diene which readily
dimerizes at room temperature via a Diels-Alder reaction (?). It is widely
used in synthetic organic chemistry, often for stereoselective purposes, while
its anion $c$-C5H5^- is central to organometallic chemistry owing to the
stability of metallocenes, such as derivatives of ferrocene Fe(C5H5)2 (?),
which are regularly used for the catalytic synthesis of asymmetric and chiral
molecules. Functionalized derivatives of cyclopentadiene have been the subject
of many studies [Ref. (?) and references therein]; substitution at the
1-carbon position is thermodynamically favored (?) because it is fully
conjugated with the $\pi$-electron system, as opposed to the other two
positions which are either cross-conjugated (2-carbon) or not conjugated
(5-carbon).

During the course of a laboratory study to identify products formed in an
electrical discharge starting from benzene and molecular nitrogen (?),
evidence was found for the two cyanocyclopentadienes shown in Fig. 1. Using
cavity Fourier transform microwave spectroscopy, the rotational spectrum of
each was subsequently measured at very high spectral resolution ($\sim$0.1
ppm) between 5 and 40 GHz, work which yielded highly accurate values for the
three rotational constants, several of the leading quartic centrifugal
distortion terms, and the two nitrogen quadrupole tensor elements (Tables 4
and 5). Confirmation of the elemental composition and structure for both
molecules is provided by the extremely close agreement of the rotational
constants given in Table 1 with those calculated theoretically and derived
from previous measurements, although the earlier spectral analyses (?, ?) were
performed at considerably lower resolution (40-100 kHz or 2-5 ppm) compared to
the present work (2 kHz, or 0.1 ppm). With the new measurements, the
rotational spectrum of both molecules can now be calculated to better than
0.05 km s-1 at 20 GHz in terms of equivalent radial velocity, more than
sufficient for an astronomical search in the coldest, most quiescent molecular
clouds where lines may be as narrow as 0.3–0.4 km s-1 FWHM.

Table 1: Comparison of experimental and theoretical rotational constants of 1- and 2-cyano-CPD. Constants are given in MHz, with values in parentheses corresponding to 1$\sigma$ uncertainty. The constants in this study were derived used a $A$-reduced Hamiltonian in the $I^{r}$ representation, A complete set of experimental spectroscopic constants and associated uncertainties is given in Table 4 and 5. Theoretical values are calculated at the $\omega$B97X-D/cc-pVQZ level of theory, with anharmonic vibrational corrections computed at the same level using second-order vibrational perturbation theory (VPT2). | 1-cyano-CPD
---|---
Constant | This Work | Ref. ? | Ref. ? | Theoretical
$A_{0}$ | 8352.98(2) | 8356(5) | 8353.97(9) | 8424.05
$B_{0}$ | 1904.2514(3) | 1904.24(2) | 1904.24(1) | 1915.81
$C_{0}$ | 1565.3659(3) | 1565.36(2) | 1565.38(1) | 1575.55
| 2-cyano-CPD |
| This Work | Ref. ? | Theoretical |
$A_{0}$ | 8235.66(4) | 8235.0(13) | 8279.26 |
$B_{0}$ | 1902.0718(2) | 1902.07(2) | 1916.03 |
$C_{0}$ | 1559.6502(2) | 1559.67(2) | 1570.48 |

Concurrent with the laboratory work, a large-scale, high sensitivity spectral
line survey, Green Bank Telescope Observations Hunting for Aromatic Molecules
(GOTHAM), predominately in the K (18-27 GHz) and Ka bands (26-40 GHz), has
been underway since Fall 2018 toward the molecule-rich starless cloud core
TMC-1 using the 100 m Robert Byrd Green Bank Telescope (GBT) — the same source
and radio telescope used to detect benzonitrile (?). Although only ${\sim}$30%
complete to date, where there is frequency coverage, it is roughly an order of
magnitude more sensitive than a survey towards the same source using the
Nobeyama 45 m telescope (?). The higher sensitivity arises primarily from
three factors: the larger collection area of the 100 m dish, a beam size
better matched to the source, and uniformly adopting a velocity resolution
(0.05 km s-1) that is appropriate in this narrow line source. The resolution
in the Nobeyama survey of 0.22–1.26 km s-1 was frequently a factor of 2-4
times too low. An overview of the GOTHAM survey, full observational details,
and a discussion of the data reduction are described in (?).

### Results

A detailed description of the data analysis procedures and statistics are
presented in (?) and summarized in the Methods. Briefly, a Markov Chain Monte
Carlo (MCMC) fit is performed to the data using transitions of the target
species which have predicted flux $\geq 5$% of the strongest line. In
agreement with prior work (?, ?), we detect and fit four distinct velocity
components in the source (after accounting for hyperfine structure) with four
column densities and four source sizes. A uniform linewidth and excitation
temperature is used to reduce the number of parameters being fit. The MCMC fit
to 1-cyano-CPD showed a significant detection in in all four velocity
components. This is shown visually in Fig. 2, where we see compelling evidence
for 1-cyano-CPD, where as the isomer 2-cyano-1,3-cyclopentadiene (2-cyano-CPD)
is not strongly detected.

At our current detection sensitivity, we estimate an upper limit for 2-cyano-
CPD roughly 1/3 that of 1-cyano-CPD, taking into account relatively small
differences in the dipole moments and partition functions between the two. As
such, it is not yet possible to establish with certainty if 1-cyano-CPD is
formed selectively or if the apparent absence of 2-cyano-CPD is simply a
question of sensitivity. Although it may have no bearing on the astronomical
observations, 2-cyano-CPD was found to be approximately four times less
abundant than the 1-cyano-CPD in our laboratory study (?) under identical
experimental conditions; in the earlier study by Sakaizumi et al. (?), the
ratio was 2:1, with 1-cyano-CPD also being more abundant.

To quantify the relative stabilities of the three cyanocyclopentadiene
isomers, energetics have been calculated with the B3LYP variant of the G3
thermochemical method (G3//B3LYP), which has been shown to yield reliable and
highly accurate (${\sim}$4 kJ/mol) energetics (?, ?). As indicated in Fig. 1,
1-cyano-CPD is the most stable of the three, with 2-cyano-CPD computed to lie
higher in energy by only 5 kJ mol-1 (600 K), which is consistent with the
energy ordering from the NMR findings of Wentrup and Crow (?), and conjugation
arguments. 5-cyano-1,3-cyclopentadiene (5-cyano-CPD) is considerably less
stable (26 kJ/mol, or ${\sim}$3130 K), and presumably explains why this isomer
was not detected in our laboratory study or prior work; for this reason, it is
not considered further. Both 1- and 2-cyano-CPD have comparably large dipole
moments along their $a$-inertial axis [4.15(15) D and 4.36(25) D,
respectively; Ref. ?].

### Astrochemical implications

Astrochemical modeling has been carried out to estimate the abundance of
1-cyano-CPD in TMC-1. However, it is apparent from Fig. 3 that the abundance
inferred from observations vastly exceeds what our simulations predict,
implying a far richer aromatic inventory for TMC-1 than previously expected.
This result is surprising since the same models are in very good agreement
with observations for many carbon-chain molecules at reasonable timescales for
the source (104-105 years) (?, ?), with the long carbon chain HC11N being a
possible exception (?) where the models actually overpredict the abundance
relative to observation. The best agreement for 1-cyano-CPD is achieved at
unrealistically early times in a simulation ($<10^{4}$ years) when a
substantial initial abundance of CPD is assumed ($\sim$0.5% of the carbon
budget). Not surprisingly, the abundances of 1- and 2-cyano-CPD are nearly
identical since, in the absence of a laboratory measurement, it was necessary
to assume equal branching for cyanation of CPD. Even so, our models produce
slightly more 1-cyano-CPD than 2-cyano-CPD due to the marginally larger dipole
moment, and therefore faster rate of destruction with ions, of the latter
isomer (?).

$c$-C5H5CN is now the fourth organic ring, along with benzonitrile and 1- and
2-cyanonaphthalene (?), that has been observed in TMC-1 with an abundance that
exceeds, sometimes greatly so, predictions from our most up-to-date models.
Although not directly comparable, the same situation appears to be true for
benzonitrile in other pre-stellar cores (?). In light of these findings, other
production pathways for cyclic molecules may need to be explored or a large
unseen reservoir of aromatic material, such as benzene, may need to be
invoked. The surprisingly high abundances of cyclic species, deviations from
previous astrochemical models, and promising potential pathways are further
discussed in Ref. ?.

In this context, it is intriguing that no evidence has been found in TMC–1 to
date for well-known N-heterocycles (defined here as a cyclic compound in which
one or more of the heavy atoms of the ring contains an atom other than carbon)
of comparable size to 1-cyano-CPD and benzonitrile, such as pyrrole (C4H4NH,
$\mu=1.80$) and pyridine (C5H5N, $\mu=2.19$; see Fig. 3), differences in their
polarity aside (?). Indeed, upper limits of their column densities from the
GOTHAM survey are of the same order as the measured column densities for
1-cyano-CPD and the cyanonaphthalene isomers (?). One possibility is that the
lack of nitrogen incorporation into the heavy atom ring may indicate that much
of the chemically active nitrogen is not available in the form of NH or NH2,
since the reaction of these radicals with butadiene (?) is thought to be an
important pathway to pyrrole; an analogous argument may hold for pyridine. A
second possibility is that cynation of pre-existing aromatic material to
produce $c$-C5H5CN and other CN-functionalized rings is efficient.
Nevertheless, in either scenario, the preponderance of nitrile terminated
chains and now rings highlights the key role CN radical apparently plays as a
chemically active sink for nitrogen in TMC–1.

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### Acknowledgements

The authors thank anonymous reviewers for helpful comments. M.C.M. and K.L.K.
Lee acknowledge support from NSF grant AST-1908576 and NASA grant
80NSSC18K0396. A.M.B. acknowledges support from the Smithsonian Institution as
a Submillimeter Array (SMA) Fellow. C.N.S. thanks the Alexander von Humboldt
Stiftung/Foundation for their generous support, as well as V. Wakelam for use
of the NAUTILUS v1.1 code. S.B.C. and M.A.C. were supported by the NASA
Astrobiology Institute through the Goddard Center for Astrobiology. E.H.
thanks the National Science Foundation for support through grant AST-1906489.
C.X. is a Grote Reber Fellow, and support for this work was provided by the
NSF through the Grote Reber Fellowship Program administered by Associated
Universities, Inc./National Radio Astronomy Observatory and the Virginia Space
Grant Consortium. Support for B.A.M. was provided by NASA through Hubble
Fellowship grant #HST-HF2-51396 awarded by the Space Telescope Science
Institute, which is operated by the Association of Universities for Research
in Astronomy, Inc., for NASA, under contract NAS5-26555. The National Radio
Astronomy Observatory is a facility of the National Science Foundation
operated under cooperative agreement by Associated Universities, Inc. The
Green Bank Observatory is a facility of the National Science Foundation
operated under cooperative agreement by Associated Universities, Inc.

### Contributions

M.C.M. and K.L.K.L. performed the laboratory experiments, theoretical
calculations, and wrote the paper with the help of A.M.B. and C.N.S. A.M.B and
B.A.M. performed the astronomical observations and subsequent analysis. E. H.
determined and/or estimated rate coefficients and is the originator for many
of the chemical simulations. A.M.B. and C.N.S. contributed or undertook the
astronomical modeling and simulations. E.R.W., M.A.C., S.B.C., S.K., C.X. and
B.A.M. contributed to the design of the GOTHAM survey, and helped revise the
manuscript.

### Corresponding author and requests for materials

Correspondence to M.C. McCarthy<EMAIL_ADDRESS>and B.A. McGuire
(brettmc@mit.edu).

### Author Information

Center for Astrophysics $\mid$ Harvard & Smithsonian, Cambridge, MA 02138, USA
Michael C. McCarthy, Kin Long Kelvin Lee, Andrew M. Burkhardt, Brett A.
McGuire

National Radio Astronomy Observatory, Charlottesville, VA 22903, USA
Ryan A. Loomis, Anthony J. Remijan, Brett A. McGuire

Department of Physics and Astronomy, Benedictine College, Atchison, KS 66002,
USA
Christopher N. Shingledecker

Center for Astrochemical Studies, Max Planck Institute for Extraterrestrial
Physics, Garching, Germany
Christopher N. Shingledecker

Institute for Theoretical Chemistry, University of Stuttgart, Stuttgart,
Germany
Christopher N. Shingledecker

Astrochemistry Laboratory and the Goddard Center for Astrobiology, NASA
Goddard Space Flight Center, Greenbelt, MD 20771, USA
Steven B. Charnley and Martin A. Cordiner

Institute for Astrophysics and Computational Sciences, The Catholic University
of America, Washington, DC 20064, USA
Martin A. Cordiner

Department of Chemistry, University of Virginia, Charlottesville, VA 22904,
USA
Eric Herbst, Eric R. Willis, Ci Xue

Department of Astronomy, University of Virginia, Charlottesville, VA 22904,
USA
Eric Herbst

Astro Space Center, Lebedev Physical Institute, Russian Academy of Sciences,
Moscow, Russia
Sergei Kalenskii

Department of Chemistry, Massachusetts Institute of Technology, Cambridge, MA
02139, USA
Brett A. McGuire

Figure 1: Geometric structures of the two low-lying cyanocyclopentadiene
isomers, along with their relative stabilities calculated theoretically at the
G3//B3LYP level of theory. Owing to its substantially lower stability, the
third isomer, 5-cyano-CPD, is not shown. Dipole moments are from Ref. ?.
Figure 2: Velocity-stacked spectra of 1-cyano-CPD and 2-cyano-CPD and the
impulse response function of the stacked spectra. Left panel: Velocity-stacked
spectra are in black, with the corresponding stack of the simulation using the
best-fit parameters to the individual lines in red. The data have been
uniformly sampled to a resolution of 0.02 km s-1. The intensity scale is the
signal-to-noise ratio of the spectrum at any given velocity. Right panel: The
impulse response function of the stacked spectrum generated using the
simulated line profile as a matched filter. The intensity scale is the signal-
to-noise ratio of the response function when centered at a given velocity. The
peak of the impulse response function provides a minimum significance for the
detection of 5.8$\sigma$. Bottom row shows the same, but for the 2-cyano-CPD
upper limit (2.9$\sigma$). See Loomis et al. (?) for details. Figure 3:
Abundance predictions for 1-cyano-CPD and 2-cyano-CPD from two chemical models
in comparison to those derived from observations of TMC-1. Results for
1-cyano-CPD are indicated in red, while those for 2-cyano-CPD are in black.
Left panel: model results from a full chemical treatment. Abundances derived
from the MCMC analysis of the observational data are represented by dotted
lines. Right panel: Results from the same model but one including an initial
abundance of CPD equal to $X(\ce{c-C5H6})_{t=0}=1.5\times 10^{-7}$, or about
0.5% of the total carbon budget, chosen to best reproduce the abundances
obtained from our MCMC analysis.

### Methods

#### Spectral Stacking Routine

Full details of the methodology used to detect and quantify molecules in our
spectra are provided in Loomis et al. (?), including assumptions made in
simulating the spectra and in determining physical parameters of the source,
priors used in the fitting analysis, and an in-depth examination of the
robustness of the methodology to interlopers and false-positives. Briefly, we
first perform a Markov-Chain Monte Carlo (MCMC) fit to the lines of strongly-
detected and optically thin HC9N and benzonitrile in our data. Model spectra
for these fiducial molecules, and for all other species, are generated using
the formalisms outlined in Turner (?), which includes corrections for optical
depth, and adjusted for the effects of beam dilution. The specific transition
parameters for each species are obtained from spectral line catalogs primarily
pulled from publicly accessible databases (spec.jpl.nasa.gov and
https://cdms.astro.uni-koeln.de/classic/). In some cases, we have generated
these catalogs directly from the parameters provided from laboratory work
outlined in individual publications. In almost all cases, the substantial
number of transitions used in the analysis (100s - 1000s) makes in impractical
to provide a table of parameters in-text. Instead, the interested reader is
referred to the catalog files in the online supplementary data which contain
all of the required information in a machine-readable format.

We detect four distinct velocity components ($v_{lsr}$) for nearly all
molecules, in agreement with prior observations of the source (?). We also
simultaneously fit for column densities ($N_{T}$), source sizes
($\theta_{s}$), excitation temperatures ($T_{ex}$), and linewidth ($\Delta
V$). The derived parameters for HC9N and $c$-C6H5CN are then used as Gaussian
priors for MCMC fits to other linear and cyclic species of interest,
respectively. In general, we find that the linewidths ($\sim$0.1–0.3 km s-1)
and excitation temperatures ($T_{ex}=$ 5–7 K) are consistent across velocity
components and molecular species. Both this trend, and the derived values, are
in excellent agreement with prior observations of the source (?, ?, ?).

For nearly all species, there are many more transitions covered by our spectra
than are visible above the local RMS noise level of the observations. We
therefore extract a small portion of the observations centered around each
spectral line, disregarding any windows that have a spectral feature
$>$5$\sigma$, to avoid interloping signals from other species. An SNR weighted
average of these spectra is then performed based on the expected intensity of
the line (derived from the MCMC parameters) and the local RMS noise of the
observations. For the purposes of this analysis, largely due to hyperfine
splitting, we treat the signals on a per-line basis rather than a per-
transition basis. This result is that the stacked feature is somewhat
broadened, as the hyperfine components and velocity components are not
collapsed, but there is no over-counting of flux. This results in a
substantial increase to overall SNR, with the spectrum now encapsulating the
total information content of all observed lines, rather than only that from
the brightest lines. Finally, the model spectra are stacked using identical
weights, and that stacked model is used as a matched filter which is cross-
correlated with the stacked observations. The resulting impulse response
spectrum provides a lower limit statistical significance to the detection.
Because the filter contains the same broadened hyperfine and velocity
structure as the stack, there is no loss in significance. Below, we provide a
number of details pertinent to the MCMC fitting and stacking analyses
performed for the specific molecules discussed in this work.

Table 2 shows the total number of transitions (including hyperfine components)
of the molecules analyzed in this paper that were covered by GOTHAM
observations at the time of analysis and were above our predicted flux
threshold of 5%, as discussed in (?). Also included are the number of
transitions, if any, that were coincident with interfering transitions of
other species, and the total number of lines used after excluding interlopers.
Observational data windowed around these transitions, spectroscopic properties
of each transition, and the partition function used in the MCMC analysis are
provided in the Harvard Dataverse repository (?).

#### Reaction pathways and energetics

Possible formation pathways for 1-cyano- and 2-cyano-CPD under interstellar
conditions have been investigated by calculating the reaction between
cyclopentadiene and the CN radical, by analogy to benzonitrile formation (?,
?, ?). For this and other potential surface calculations, the growing string
method (?, ?, ?) has been used: it is computationally efficient, allows
various reactants to be rapidly surveyed and reaction barriers identified, and
yields approximate reaction enthalpies between stationary points. Stationary
points are then subsequently refined with the G3//B3LYP method. As shown in
Fig. 4, the $c$-C5H6 + CN reaction to yield either 1-cyano or 2-cyano-CPD is
found to be both exothermic and barrierless, irrespective of the stability of
the the two isomers.

On the operative assumption that 1-cyano-CPD is formed from $c$-C5H6, the
reaction pathways that might produce this five-membered hydrocarbon ring in
space have also been investigated, since they apparently have not been
considered in current chemical models for dark clouds. Two possible reactions
are shown in Fig. 5. The red trace follows the reaction between ethylene
(H2C=CH2) and propargyl radical (HC#CCH2), which combines barrierlessly to
form an acyclic intermediate. Prior to CPD formation, however, this
intermediate must undergo ring-closure which requires surmounting a barrier in
excess of its initial energy (${\sim}$33 kJ/mol, or 4000 K), implying this
pathway is not viable in TMC-1. The blue trace follows the reaction between
gauche-butadiene and CH radical, which is calculated to produce CPD
exothermically and barrierlessly via subsequent hydrogen atom loss. Due to
large energy barriers, the formation of CPD via reaction of allyl radical and
acetylene (?) can also be neglected. It should be emphasized that reactions
with large activation barrier may take place on grains by the bombardment of
cosmic rays and internal UV photons (?, ?). Though we have not considered the
effects of such reactions involving excited, suprathermal species here, it is
possible that the contribution of these processes to the abundances of gas-
phase aromatic molecules could be non-trivial, even in cold cores such as
TMC-1.

#### Astrochemical Modeling

We have used the NAUTILUS-v1.1 program (?), together with a modified version
the KIDA 2014 network (?) to predict the abundance of 1-cyano-CPD and other
species in our GOTHAM observations (?, ?, ?). This network includes the
barrierless, exothermic reaction of CPD with CN radical to yield 1-cyano-CPD
and 2-cyano-CPD , which we assume has a rate coefficient of $\sim 3\times
10^{-10}$ cm-3 s-1, which is reasonable assuming the reaction occurs with
every collision (?, ?). Pathways that yield CPD from acyclic precursors are
also explicitly included in our model, as are destruction reactions with ions
(?) and depletion onto grains.

Standard physical conditions relevant to TMC-1 were used in our simulations,
e.g.,* a gas temperature of $T_{\mathrm{gas}}=T_{\mathrm{dust}}=10$ K, gas
density of $2\times 10^{4}$ cm-3, and standard cosmic ray ionization rate of
$1.3\times 10^{-17}$ s-1 (?, ?). For initial elemental abundances, we use the
values from Hincelin et al. (?), except for the initial atomic oxygen, where
we have chosen an initial value of $X(\mathrm{O})_{t=0}\approx 1.5\times
10^{-4}$, resulting in a slightly carbon rich C/O $\approx 1.1$ by optimizing
to best reproduce the observed cyanopolyyne abundances (?). To test the
possible influence of CPD inherited from earlier stages of cloud evolution, we
have also run a second simulation including an initial abundance of CPD,
$X(\ce{c-C5H6})_{t=0}=1.5\times 10^{-7}$, which was chosen to best reproduce
the abundances obtained from our MCMC analysis. This is equivalent to
approximately 0.5% of the entire carbon budget in these simulations.

### Data statement

The datasets analyzed during the current study are available in the Green Bank
Telescope archive (https://archive.nrao.edu/archive/advquery.jsp). A user
manual for their reduction and analysis is available as well
(https://greenbankobservatory.org/science/gbt-observers/visitor-facilities-
policies/data-reduction-gbt-using-idl/). The complete, reduced survey data at
X-band is available as Supplementary Information in (?). The individual
portions of reduced spectra used in the analysis of the individual species
presented here is available in the Harvard Dataverse Archive (?).

### Code statement

All the codes used in the MCMC fitting and stacking analysis presented in this
paper are open source and publicly available at
https://github.com/ryanaloomis/TMC1_mcmc_fitting.

### Extended Data

Table 2: Total number of transitions of a given species within the range of the GOTHAM data, number of interfering lines, and total number included in MCMC fit. | Transitions Covered | Interfering Lines | Total Transitions
---|---|---|---
Molecule | By GOTHAM | In Data | Used in MCMC
1-Cyano-CPD | 111 | 0 | 111
2-Cyano-CPD | 108 | 0 | 108
cyclopentadiene | 21 | 0 | 21
pyridine | 76 | 0 | 76
pyrrole | 34 | 0 | 34

Figure 4: Thermochemistry (0 K) for the reaction between CN radical and CPD.
The calculated has been performed at the G3//B3LYP level of theory, and
energies in kJ/mol are given relative to the reactant asymptote. The reaction
bifuricates as CN attacks CPD barrierlessly, forming a radical intermediate.
Subsequent hydrogen atom loss yields 1-cyano and 2-cyano-CPD. Figure 5:
Potential energy surface for the formation of CPD at 0 K. Reaction energies
are given relative to the product (CPD + H atom) asymptote. The red trace
corresponds to reaction between propargyl radical (HCCCH2) and ethylene
(H2C=CH2), and the blue trace represents gauche-butadiene (H2C(CH)2CH2)
reacting with CH radical.

## Supplementary Information

### Analysis Results

The resulting best-fit parameters of the MCMC analysis carried out for
1-cyano-CPD are given in Supplementary Table 1. The stacked spectrum and
matched filter results are shown in Supplementary Figure 2. A corner plot of
the parameter covariances for the 1-cyano-CPD MCMC fit is shown in
Supplementary Figure 1.

Supplementary Table 1: 1-cyano-CPD best-fit parameters from MCMC analysis

Component | $v_{lsr}$ | Size | $N_{T}^{\dagger}$ | $T_{ex}$ | $\Delta V$
---|---|---|---|---|---
(km s-1) | (′′) | (1011 cm-2) | (K) | (km s-1)
C1 | $5.602^{+0.007}_{-0.007}$ | $120^{+69}_{-38}$ | $4.09^{+0.89}_{-0.83}$ | $6.0^{+0.3}_{-0.3}$ | $0.121^{+0.011}_{-0.011}$
C2 | $5.764^{+0.003}_{-0.003}$ | $68^{+16}_{-15}$ | $2.61^{+0.98}_{-0.94}$
C3 | $5.880^{+0.006}_{-0.006}$ | $261^{+67}_{-81}$ | $5.08^{+0.85}_{-0.82}$
C4 | $6.017^{+0.003}_{-0.003}$ | $254^{+65}_{-83}$ | $2.67^{+0.77}_{-0.78}$ |  |
$N_{T}$ (Total)†† |  $1.44^{+0.17}_{-0.17}\times 10^{12}$ cm-2

Note – The quoted uncertainties represent the 16th and 84th percentile
($1\sigma$ for a Gaussian distribution) uncertainties.
†Column density values are highly covariant with the derived source sizes. The
marginalized uncertainties on the column densities are therefore dominated by
the largely unconstrained nature of the source sizes, and not by the signal-
to-noise of the observations. See Fig. 1 for a covariance plot, and Loomis et
al. (?) for a detailed explanation of the methods used to constrain these
quantities and derive the uncertainties.
††Uncertainties derived by adding the uncertainties of the individual
components in quadrature.

Supplementary Figure 1: Parameter covariances and marginalized posterior
distributions for the 1-cyano-CPD MCMC fit. 16th, 50th, and 84th confidence
intervals (corresponding to $\pm$1 sigma for a Gaussian posterior
distribution) are shown as vertical lines.

The resulting best-fit parameters of the MCMC analysis carried out for
2-cyano-CPD are given in Supplementary Table 2. The stacked spectrum and
matched filter results are shown in Supplementary Figure 2. A corner plot of
the parameter covariances for the 2-cyano-CPD MCMC fit is shown in
Supplementary Figure 2.

Supplementary Table 2: 2-cyano-CPD best-fit parameters from MCMC analysis

Component | $v_{lsr}$ | Size | $N_{T}^{\dagger}$ | $T_{ex}$ | $\Delta V$
---|---|---|---|---|---
(km s-1) | (′′) | (1011 cm-2) | (K) | (km s-1)
C1 | $5.593^{+0.008}_{-0.008}$ | $101^{+72}_{-54}$ | $<1.83^{+0.92}_{-0.70}$ | $6.1^{+0.3}_{-0.3}$ | $0.128^{+0.012}_{-0.012}$
C2 | $5.764^{+0.003}_{-0.003}$ | $66^{+16}_{-15}$ | $<1.76^{+0.87}_{-0.75}$
C3 | $5.885^{+0.007}_{-0.007}$ | $263^{+64}_{-85}$ | $<0.77^{+0.60}_{-0.45}$
C4 | $6.017^{+0.003}_{-0.003}$ | $251^{+66}_{-81}$ | $<0.90^{+0.58}_{-0.51}$ |  |
$N_{T}$ (Total)†† |  $<5.26^{+1.51}_{-1.23}\times 10^{11}$ cm-2

Note – The quoted uncertainties represent the 16th and 84th percentile
($1\sigma$ for a Gaussian distribution) uncertainties.
†Column density values are highly covariant with the derived source sizes. The
marginalized uncertainties on the column densities are therefore dominated by
the largely unconstrained nature of the source sizes, and not by the signal-
to-noise of the observations. See Fig. 2 for a covariance plot, and Loomis et
al. (?) for a detailed explanation of the methods used to constrain these
quantities and derive the uncertainties.
††Uncertainties derived by adding the uncertainties of the individual
components in quadrature.

Supplementary Figure 2: Parameter covariances and marginalized posterior
distributions for the 2-cyano-CPD MCMC fit. 16th, 50th, and 84th confidence
intervals (corresponding to $\pm$1 sigma for a Gaussian posterior
distribution) are shown as vertical lines.

### Upper Limits

We have also calculated upper limits to the column densities of
cyclopentadiene, pyridine, and pyrrole. The stacked spectra, corner plots, and
upper limits, including assumed parameters, are presented below in
Supplementary Figure 3, Supplementary Figures 4\- 6, and Supplementary Table
3, respectively. While none of these show detectable signal in our current
survey data, cyclopentadiene does display a small significance to a possible
detectable column in one velocity component, as can be seen in its corner
plot. This may indicate that a detection may be possible with additional
observation.

Supplementary Figure 3: From left to right, velocity-stacked spectra of
cyclopentadiene, pyridine, and pyrrole in black, using the best-fit parameters
from the MCMC analysis for the upper limits.
Supplementary Figure 4: Corner plot of column densities in the four velocity
components for cyclopentadiene.
Supplementary Figure 5: Corner plot of column densities in the four velocity
components for pyridine.
Supplementary Figure 6: Corner plot of column densities in the four velocity
components for pyrrole. Supplementary Table 3: Cyclopentadiene, pyridine, and
pyrrole best-fit parameters from MCMC analysis

|  |  |  |  | cyclopentadiene | pyridine | pyrrole
---|---|---|---|---|---|---|---
Component | $v_{lsr}$ | Size | $T_{ex}$ | $\Delta V$ | $N_{T}^{\dagger}$ | $N_{T}^{\dagger}$ | $N_{T}^{\dagger}$
(km s-1) | (′′) | (K) | (km s-1) | (1011 cm-2) | (1011 cm-2) | (1011 cm-2)
C1 | [$5.595$] | [$92$] | [$6.1$] | [$0.121$] |  $<$$214$ |  $<$$0.79$ |  $<$$0.95$
C2 | [$5.764$] | [$68$] | [$6.1$] | [$0.121$] |  $<$$70$ |  $<$$1.92$ |  $<$$0.56$
C3 | [$5.885$] | [$265$] | [$6.1$] | [$0.121$] |  $<$$71$ |  $<$$0.51$ |  $<$$0.24$
C4 | [$6.016$] | [$250$] | [$6.1$] | [$0.121$] |  $<$$276$ |  $<$$2.66$ |  $<$$1.16$
cyclopentadiene $N_{T}$ (Total)††: $630\times 10^{11}$ cm-2
pyridine $N_{T}$ (Total)††: $5.9\times 10^{11}$ cm-2
pyrrole $N_{T}$ (Total)††: $2.9\times 10^{11}$ cm-2

Note – The quoted uncertainties represent the 16th and 84th percentile
($1\sigma$ for a Gaussian distribution) uncertainties. Upper limits are given
as the 97.8th percentile (2$\sigma$) value. Parameters in brackets were held
fixed to the 50th percentile value.
†Column density values are highly covariant with the derived source sizes. The
marginalized uncertainties on the column densities are therefore dominated by
the largely unconstrained nature of the source sizes, and not by the signal-
to-noise of the observations. See Supplementary Figures 4\- 6 for covariance
plots, and (?) for a detailed explanation of the methods used to constrain
these quantities and derive the uncertainties.
††Uncertainties derived by adding the uncertainties of the individual
components in quadrature.

### Laboratory Spectroscopy

Supplementary Tables 4 and 5 provide comparisons of the spectroscopic
constants determined in this work to those from the literature for 1-cyano-CPD
and 2-cyano-CPD, respectively.

Supplementary Table 4: Best-fit spectroscopic constants of 1-cyano-CPD, derived using a $A$-reduced Hamiltonian in the $I^{r}$ representation, in comparison to earlier studies. Constants are given in MHz, with values in parentheses corresponding to 1$\sigma$ uncertainties. Constant | This Work | Ref. ? | Ref. ?
---|---|---|---
$A$ | 8352.98(2) | 8356(5) | 8353.97(9)
$B$ | 1904.2514(3) | 1904.24(2) | 1904.24(1)
$C$ | 1565.3659(3) | 1565.36(2) | 1565.38(1)
$10^{3}\times D_{J}$ | 0.0701(15) | $\cdots$ | 0.08(3)
$10^{3}\times D_{JK}$ | 2.361(15) | $\cdots$ | 2.46(7)
$10^{3}\times d_{J}$ | 0.0120(11) | $\cdots$ | $\cdots$
$10^{3}\times d_{K}$ | 1.21(13) | $\cdots$ | $\cdots$
$\chi_{aa}$(N) | -4.1796(21) | $\cdots$ | $\cdots$
$\chi_{bb}$(N) | 2.3052(26) | $\cdots$ | $\cdots$
Supplementary Table 5: Best-fit spectroscopic constants of 2-cyano-CPD, derived using a $A$-reduced Hamiltonian in the $I^{r}$ representation, in comparison to earlier studies. Constants are given in MHz, with values in parentheses corresponding to 1$\sigma$ uncertainties. Constant | This Work | Ref. ? | Ref. ?
---|---|---|---
$A$ | 8235.66(4) | 8235.0(13) | 8302.7357
$B$ | 1902.0718(2) | 1902.07(2) | 1902.3323
$C$ | 1559.6502(2) | 1559.67(2) | 1562.5153
$10^{3}\times D_{J}$ | 0.0561(37) | $\cdots$ | 0.07(3)
$10^{3}\times D_{JK}$ | 2.286(46) | $\cdots$ | 2.459(8)
$\chi_{aa}$(N) | -4.234(6) | $\cdots$ | $\cdots$
$\chi_{bb}$(N) | 2.236(7) | $\cdots$ | $\cdots$