SUPPLEMENTARY MATERIAL
Here we present details of the synthesis of H
3
C
4
N. We show the photoelectron spectrum of
H
2
C
4
N

(FIG. S1), the polarization dependence of the photoelectron spectrum of HC
4
N

(FIG.
S2), the full simulated photoelectron spectrum of HC
4
N

(FIG. S3), one-dimensional potential cuts along the HCC bend coordinate of HCCN

and HCCN (FIG. S4), and the full simulated photoelectron spectra of DCCN

and HCCN

(FIG. S5). We also present tables containing the calculated equilibrium geometries of HCCN

(DCCN

) and HCCN (DCCN) (TABLE S1) and the frequencies of HCCN (TABLE S2) and DCCN (TABLE S3).
Synthesis of Tetrolonitrile (Methyl Cyanoacetylene; CH
3
-C≡C-C≡N).
Tetrolamide.
1 Into a sealable round bottom flask is added 30 mL of concentrated ammonium hydroxide. To this is added ethyl 2-butynoate (10 g, 107 mmol). The flask is sealed and the mixture is stirred vigorously at room temperature for 24 h. The stir bar is then removed and the ammonium hydroxide/H
2
O solution is removed under high vacuum to yield an off-white solid. The crude product is used in the following dehydration without further purification.
Yield: 6.2 g (75 mmol, 85% yield). 1 H NMR (300 MHz, Me
2
SOd
6
) δ 7.83 (bs, 1H), 7.38 (bs,
1H) 1.92 (s, 3H); 13 C NMR (75 MHz) δ 154.3, 82.3, 75.9, 3.0.
1

Tetrolonitrile.
2 Into a 100 mL flame-dried round bottom flask under nitrogen atmosphere is weighed tetrolamide (6.2 g, 74.6 mmol), sea sand (20 g), and P
2
O
5
(10.6 g, 74.6 mmol). The three solid components are thoroughly mixed by shaking the flask with a septum to seal the top. The flask is then attached to a U-shaped tube leading to a cold finger for condensation of the nitrile. A vacuum (0.2 mmHg) is applied to the apparatus to pull the dehydration product from the reaction. The cold finger is chilled to 77 K while the reaction flask is warmed to 150 °C. Much like acetonitrile, tetrolonitrile is prone to both radical and cationic polymerization. A small amount of water contaminant is acceptable to inhibit the cationic polymerization, whereas radical polymerization can be inhibited by adding 1% weight 4,4’thiobis(2tert -butyl-5-methylphenol). Samples were sealed in glass ampoules, packed in dry ice, shipped to Boulder, and refrigerated upon arrival. Yield: 3.6 g (55.3 mmol, 74% yield). 1 H
NMR (300 MHz, CDCl
3
) δ 2.05 (s, 3H); 13 C NMR (75 MHz) δ 105.5, 83.5, 76.8, 4.3.
1.
Strübing, D.; Neumann, H.; Klaus, S.; Hübner, S.; Beller, M., A facile and efficient synthesis of enyne-reaction precursors by multicomponent reactions. Tetrahedron 2005,
61 (48), 11333-11344.
2.
Wentrup, C.; Crow, W. D., Hetarylnitrenes--I : Ring contraction and fragmentation in nitrenodiazines. Tetrahedron 1970, 26 (20), 4915-4924.
2

FIG. S1. Photoelectron spectrum (364 nm) of H
2
C
4
N

, collected at the magic angle at 150 K.
3

FIG. S2. Polarization Dependence of the Photoelectron Spectrum of HC
4
N

. Photoelectron spectra were collected at 300 K at

= 0° (cyan trace) and

= 90° (gray trace) and scaled for the measured

anisotropy at eBE = 2.8 eV.
4

FIG. S3. Franck-Condon Simulation of the 150 K Photoelectron Spectrum of HC
4
N

.
Geometries, frequencies, and K″ displacements used in the simulation are listed in Tables 2 – 3.
~
The simulation uses the experimental electron affinity and term energy of the X
3 A″ and ã 1 A′ states of HC
4
N.
5

FIG. S4. One-Dimensional Potential Cut along the H-C-CN Bend Coordinate. Energies are relative to the minimum on the anion surface. The

C-C-N angle is constrained to be linear in this scan, while the remaining internal coordinates are set to the values that minimize the energy of the anion when the

C-C-N angle was 180

. All potential curves were calculated at the
ROMP2/6-311++G(d,p) level of theory/basis.
6

FIG. S5. Franck-Condon Simulations of the Photoelectron Spectra of HCCN

and DCCN

.
Geometries, frequencies, and K″ displacements used in the simulation are listed in Tables S1 –
~
S3. The simulations use the measured electron affinity and term energy of the X
3 A″ and ã 1
A′ states of HCCN and of DCCN.
7

~
TABLE S1. Calculated (ROMP2/6-311++G(d,p)) equilibrium geometry of X
2 A″ HCCN 
(DCCN

~
) and the calculated net geometry change upon photodetachment to the X
3 A″ and ã 1 A′ states of HCCN (DCCN). Bond lengths are given in units of angstrom (Å), and bond angles are given in units of degree (°). Boldfaced entries highlight the internal coordinates that undergo significant change upon photodetachment.
Internal Coordinate
H-C1
HCCN

~
X
2 A″
1.104
Geometry Change
~
X
3 A″ ←
~
X

0.030
2 A″ ã 1 A′ ←
~
X

0.001
2 A″
C1-C2 1.382
1.210

0.050

0.006
+0.026
C2-N

H-C1-C2

C1-C2-N
109.6
172.3
+35.0
+3.0

0.026

1.69

1.14
8

TABLE S2. Calculated (ROMP2/6-311++G(d,p)) unscaled, harmonic frequencies of
2 A″
HCCN

and the
3 A″ and ã 1 A′ states of HCCN. The measured CN stretch frequency is listed in italics below the calculated value. Computed K ″ displacements (PESCAL) are also listed. The
~ computed X
3 A″ ←
~
X
2 A″
K
″ displacements do not accurately represent the geometry change that takes place upon photodetachment and result in a Franck-Condon simulation that does not reproduce the observed triplet progression (Fig. S5).
HCCN
 ~
X
3 A″ ←
~
X
2 A″ ã 1 A′ ←
~
X
2 A″
Mode Description
2 A″, cm -1 
K
″ 3 A″, cm -1 
K
″ 1 A′, cm -1
A′ 
1
CH stretch

2
CN stretch



3
4
5
HCC bend
CC stretch
CCN bend
A″ 
6
CCN bend
3031.6
1764.1
 
0.061446 1631.5
 
0.066178 2227.7
1903(40) a
1032.7 0.16846 260.3

5.1113x10
-6 1065.9
936.5
458.7
457.2
 
 
0.51123
0.18347
0
0.15904 3379.8
1071.0
470.3
416.5
 
0.047076

0.025295
0
0.0035737 3093.3
979.6
404.2
334.8 a
Experiment, this work
9

~
TABLE S3. Calculated (ROMP2/6-311++G(d,p)) unscaled, harmonic frequencies of X
DCCN

~
and the X
3
2 A″
A″ and ã 1 A′ states of DCCN. The measured CN stretch frequency is listed in italics below the calculated value. Computed K
″ displacements (PESCAL) are also listed.
~
The computed X
3 A″ ←
~
X
2 A″ K ″ displacements do not accurately represent the geometry change that takes place upon photodetachment and result in a Franck-Condon simulation that does not reproduce the observed triplet progression (Fig. S5).
DCCN
 ~
X
3 A″ ←
~
X
2 A″ ã 1 A′ ←
~
X
2 A″
Mode Description
2 A″, cm -1 
K
″ 3 A″, cm -1 
K
 1 A′, cm -1
A′ 
1
CD stretch 2226.0
 
0.20592 2498.0
 
0.0062980 2280.4

2
CN stretch 1759.4

0.046471 1620.9
 
0.066722 2216.5
1936(40) a

3
DCC bend 998.4
 
0.089994 204.5

0.020164 984.4

4
CC stretch 776.0

5
CCN bend 418.0
0.61268
0.43067
1011.3
466.1
 
0.046367 825.2

0.012876 378.4
A″ 
6
CCN bend 450.4 0 410.0 0 328.2 a Experiment, this work
10