Tentative identification of C5N2 in rare gas matrices

Chemical Physics

ELSEVIER Chemical Physics 184 ( 1994) 233-245

Tentative identification of C5N2 in rare gas matrices

Alice M. Smith, Comelia Engel, Anton Thoma, Giinter Schallmoser, Brent E. Wurfel, Vladimir E. Bondybey

Institutfiir Physikalische und Theoretische Chemie, Technische UniversitUt Miinchen, D-85747 Garching, Germany

Received 13 December 1993

Abstract

A new cyanocarbon is formed in a discharge of dicyanoacetylene ( NC4N) and studied in rare gas matrices through its visible absorption, laser-induced fluorescence, and laser excitation spectra. The spectrum, with origin at 22737 cm-’ in an argon matrix, exhibits strong progressions in a vibrational mode around 2000 cm- ’ , typical for a linear molecule with multiple bonds. Isotopic studies suggest that the carrier contains two equivalent nitrogen atoms and probably five carbons. They also point to a centrosymmetric molecule with two equivalent carbon atom pairs. Given this information, we assign the carrier of this spectrum tentatively to a linear cumulenic NC5N or CN&NC. Chemical evidence suggests it may be formed by the reaction of C atoms with N&N or its isomer N&NC.

1. Iutroduction

More than one hundred distinct molecules have been detected in the interstellar medium. The chemistry of interstellar clouds, with its low particle densities and presence of high energy radiation differs substantially from a typical “laboratory chemistry” [ 11. Many of the abundant species are highly reactive intermediates, metastable isomers, radicals or molecular ions, and their laboratory studies are therefore often difficult. Molecules of the type C& and C&H are dispropor- tionately represented among the species detected in interstellar space. Their abundance seems to decrease only very slowly with chain length [ 2,3]. For example, the most complex molecule unambiguously identified to date is the 13-atom linearHC,,N [4].

Recently, we have reported the spectra of the metastable isomers of dicyanoacetylene, NC,N [ 51. We have also attempted to generate the radical species C3N and C,NC by discharging NC4N. The major path of decomposition of dicyanoacetylene upon photolysis or

electron impact is believed to be breakage of one of the single CC bonds, NC,N+C,N+CN [6,7]. Matrix spectra of discharged samples of dicyanoacetylene all exhibited very strong visible spectra of the CN radical. In view of this, we have briefly believed that a new intense visible absorption and fluorescence spectrum also present in these samples might be due to one of the C,N isomers.

Studies using isotopically enriched samples have, however, shown clearly, that the carrier of the spectrum contains two nitrogens, and probably five carbon atoms. In the following article we report our spectral observations, and present evidence suggesting that the carrier is a linear species of the formula C,NP with equivalent nitrogen atoms.

2. Experimental

Our setup for matrix-isolation spectroscopy using an electric discharge-pulsed jet source of transients is

0301-0104/94/$07.00 8 1994 Elsevier Science B.V. All rights reserved SSDIO301-0104 (94)00096-S

234 A.M. Smith et al. /Chemical Physics 184 (1994) 233-245

described thoroughly by Thoma et al. [ 81. Briefly, the matrix gases are admitted through a pulsed electromag- netic valve (Bosch). The gas pulse then enters a 1 mm diameter, 10 mm long teflon discharge channel, which houses two aluminum disk-shaped electrodes separated by 1 mm. In the presence of the gas pulse, a discharge ignites between the electrodes, that are held at a constant potential difference of 50&1000 V. The discharge products are trapped in a solid matrix. Typically 3 mmol were deposited with a pulse repetition rate of 2.5-5 Hz and an opening time of the valve of 5-15 ms. Most of the matrix experiments described here were performed in argon, a few in neon, and in one case in nitrogen. The matrices are deposited on a KC1 substrate held at 12-16 K using an APD Cryogenics HC-2D closed cycle refrigerator for the argon and nitrogen matrix studies. For the neon matrix studies, the matrix was held at 5.5 K using a modified RMC LTS-lC-4.5 closed cycle refrigerator.

laser. For the emission measurements, the laser output was intensity stabilized (LS- 100, Cambridge Research and Instrumentation) and focus& onto the matrix with a 20 cm lens.

The NC$l and HC3N precursor materials were prepared by standard methods using the methylacetyle- necarboxylates as starting material [ 9,101. HCN, HL3CN, and HC”N were prepared from the appropriate KCN salt (99% for isotopic compounds, purchased from Promochem). 13CzHz (99%) and i5N2 (99%) were purchased from Promochem and H&N (99 -t %), acrylonitrile, from Aldrich. All the precursors were purified by repeated freeze-pump-thaw cycles prior to use. The matrix gas, either argon (99.999%), neon (99.998%) or nitrogen (99.999%)) was used as obtained from Messer Griesheim. Typical dilutions (precursor/matrix gas) of 1/400-l /600 were used.

For the laser excitation scans, in which the emission is analyzed by the FIX, the dye or intracavity-doubled titanium-sapphire laser wavelength is stepped by a three-plate birefrigent filter and an emission spectrum recorded at each step. For the excitation scans using the ring dye laser, the laser stepsize was = 0.3 cm- ‘. For the excitation spectra using the intracavity-doubled titanium-sapphire laser, achieving such a fine stepsize was more difficult. For the initial experiments, stepping a three-plate bitefrigent filter and peaking the doubling crystal using a homebuilt computer-controlled setup led to excitation spectra with resolution limited to about 3 cm-‘. Later experiments were performed using more tuning elements and the Coherent-Autoscan option. This yielded a laser linewidth unnecessarily narrow for the present experiments ( = 1 MHz) and made continuous scanning over wider regions more difficult. Nev- ertheless we were able to overcome these difficulties and generate excitation spectra with sufficient resolution of = 1 cm- ‘. Furthermore, the sufficiency of this resolution was tested by continuous scans with 1 MHz resolution in which the total emission was detected by a lock-in amplifier and sent to a computer. If peaks were detected, then finer excitation scans using our FIX over a limited wavenumber range were performed.

3. Results and discussion

Absorption spectra in the infrared to near UV (550- The starting point of the present study were experi- 31000 cm-‘) were recorded with a Bruker IFS 120 ments in which NC,N, dicyanoacetylene, was dis- HR Fourier transform spectrometer (FTS) . The laser- charged in the hope of producing the elusive C,N induced fluorescence was collected by a parabolic mir- radical. According to previous gas phase studies, the ror and analyzed by the PTS using a photomultiplier parent compound breaks up readily to form CN + C3N tube. Optical cut-off, bandpass filters, and a holo- [ 6,7]. When we observed a new strong absorption in graphic notch filter (to suppress the internal HeNe the visible, in addition to the strong absorptions of CN, interference) were used to select the spectral region of we initially believed that we succeeded in detecting interest. The visible absorption and emission spectra C3N. The emission spectrum with origin at 22737 cm- ’ were recorded at 1 cm - ’ resolution. The samples were was suggestive of a linear compound containing triple excited using two different laser systems; for the region bonds. Subsequent studies, however, rather than con- 2150&24400 cm-‘, an Ar+ laser (multiline-UV) firming this assignment, quickly cast considerable pumped ring dye (stilbene-3) laser and for the region doubt on it. Chemical evidence, comparison with the- 24400-28200 cm-‘, an Ar+ laser (multiline-Vis) oretical predictions, and finally and most conclusively, pumped intracavity-doubled titanium-sapphire ring isotopic studies have forced us to reject C3N as the

A.M. Smith et al. /Chemical Physics 184 (1994) 233-245 235

carrier of the spectrum and look for alternative assignments.

3.1. Chemical evidence

A variety of precursors and procedures were tried to produce the spectrum; the results are summarized in Table 1. Among the precursor molecules tried and procedures used, a discharge of dicyanoacetylene turned out to be by far the most efficient for producing the 22737 cm-’ absorber. Annealing of the discharged samples resulted in a considerable increase in the intensity of the unknown spectrum, in some cases by up to a factor of three, as demonstrated in Fig. 1. Intuitively, if C,N, primary product of the parent molecule decomposition, was the carrier of the spectrum, one might expect its concentration to decrease upon annealing, as for instance that of the CN and C3 radicals. In situ W photolysis (both with a pulsed laser and with a high pressure mercury lamp) of the NC,N parent yielded virtually no absorption at 22737 cm- ‘, even though C3N is presumably of the primary products. This led instead to the production of NC;NC and CNC,NC through the efficient cage effect of the argon matrix

[51.

Table 1 Production of 22737 cm-’ absorber

A number of experiments were carried out to estab- lish whether the spectrum carrier contained other elements besides nitrogen and carbon. The presence of hydrogen atoms in the precursor always had a negative effect on the yield of the absorption, as seen in Table 1. Also, there was no positive correlation with oxygen- containing impurities, and the spectrum did not depend on the material of the electrodes in the discharge experiments. This led to the conclusion that the carrier must have the formula CJ$.

Discharge experiments or photolysis of cyanoace- tylene, HC,N, and acrylonitrile, H&N, both molecules containing three carbons and one nitrogen, proved to be poor methods for producing the spectrum, even though C3N might appear to be one of the logical frag- ment products. The spectrum was observed, albeit also weakly, in experiments discharging acetylene and HCN. While the absorptions were at least two orders of magnitude weaker than in the N&N discharge case, it gave us the opportunity to perform isotopic studies, since HCN and C,H, enriched with r3C or 15N are readily accessible.

A discharge of CzH2 and N2 l/ 1 in argon failed to produce the emitter; a small amount was produced when 10 times more nitrogen was used. Apparently

Precursor * Fragmentation method Relative intensity of

22737 cm-’ absorption

electrical discharge atIer 30 min, 30 K annealiig pulsed laser, 235 nm

in situ-f NC3NC, CN&NC

100

30.5

H-C=C-C=N

H-C=C-C=N

H-C=C-C=N

H&=CH-C=N

HZ&-H/H-&N

H-c&-H/N, l/l H-CX-H/N, l/10

pulsed laser, 242 nm in situ -+ H&NC

Lyman-a, 121 nm in situ + H&NC

electric discharge

electric discharge

electric discharge

after 2 min. 33 K annealing

electric discharge electric discharge after 30 min. 31 K annealing

_b

4

0.6

1.5 2

0.2

’ All diluted in argon for argon matrix studies. b Only detectable in fluorescence.

236 A.M. Smith et al. /Chemical Physics 184 (1994) 233-245

Fig. 1. FIIR visible absorption spectrum from a sample of discharged dicyaaoacetyleae (NC,N) in aa argon matrix. The effect of annealing to 30 K is shown (a) before annealing, (b) after annealing: the well-known absorptions of C3 and CN decrease by 40% whereas the absorptions tentatively assigned here to a symmetric C5NZ species increase by about 300%. The CSN2 visible absorption frequencies are listed in Table 3.

more nitrogen was needed since the Nz molecule frag- ments inefficiently in the discharge due to its strong bond. All of the above results support the conclusion that the nitrogen is incorporated into the emitter in the form of CN groups and no NN bonding as in CNN occurs.

This product was also efficiently formed in a discharge of N&N in neon, permitting Ne matrix studies. Rather unexpectedly, a very small amount of this unknown species was also produced in a discharge of C6;6 in nitrogen, permitting N2 matrix studies. As shown in Table 5, the minimal shifts of both the origin and ground state frequencies with respect to the argon matrix values and the fact that the transitions remain sharp in argon and nitrogen matrices imply a nonionic carrier, eliminating, for example, the ion of NC,N or its metastable isomers.

3.2. Vibrational structure in excitation and emission spectra

The sharp vibrational structure in the emission and laser excitation spectra of the unknown species is rich

in information. (See Figs. 1 and 2 and Tables 2 and 3). The absorption-laser excitation spectrum is dominated by a strong progression with a frequency of 1960 cm- ‘. Regardless of which vibronic mode is excited, the emission occurs from the vibrationless origin, as typical for polyatomic molecules in rare gas matrices [ 11 I.

The structure in the emission spectrum is similar to that in the absorption spectrum, but the dominant vibrational mode appears to be doubled, probably by Fermi resonance, with components at 2071.4 and 2030.9 cm- ‘. In the overtone region of the emission spectrum one sees the characteristic “Fermi polyads” [ 12 I. Also a vibration of 1639.5 cm- ’ appears fairly prominently in the spectrum, its overtone and combination with the Fermi resonance pair being also among the prominent bands. At least six additional bands are detected between 500 and 900 cm- ‘, but these are at least two orders of magnitude weaker than the bands of the main progression. Several conclusions can be made based on the observed spectra:

1. The spectrum suggest a linear molecule. In bent species, the bond angle is usually quite sensitive to the

400

300

3 5 .e 200 I .- E al

100


6000 4000 2000 0 I I

6 >-

0

16000

III .I. ,I 111 Il. 1, II.

18000 20000 22ooa frequency (cm-‘)

Fig. 2. Laser-induced fluorescence spectrum of C,N, excited at the @ at 22737 cm-‘. The prominent frequencies (the Fermi-resonance pair, VI and 4. and ~3)~ the much weaker low frequency modes ( v~- up), and a few of the prominent ovetione and combination bands are Welled. The remaining assignments and complete emission frequencies and intensities are listed in Table 2.

electronic excitation, and progressions of low frequency bending vibrations appear prominently in their spectra.

2. The dominant mode near 2000 cm- ’ is appropriate for a CC or CN triple bond stretching vibration or a multiple bond stretching vibration of a cumulenic molecule such as C3 or CS.

3. Given the number of vibrational frequencies (9) detected in fluorescence, the unknown species could easily be larger than the four-atomic C3N molecule.

4. Even though a large number of infrared bands (see below) were detected in the spectrum of the discharged NC$I samples, no coincidences with the intervals observed in the visible emission spectrum could be found. This suggests a molecule with an inversion center, since in this case, the totally symmetric vibrations which are detected in fluorescence are forbidden in the infrared.

3.3. Infrared spectrum of discharged N&N

Approximately 150 infrared peaks from 600 to 4000 cm-’ were observed in the argon matrix-IR spectrum

of discharged NC& many of them unidentified. Based on comparison of discharged and non-discharged spectra of NC,N, a fairly large fraction (50-8096) of the NC,N was decomposed in the discharge. Some of the transient species identified include (we list the vibrational frequency in cm- ‘, and when known, an ab initio intensity, in km/mol, is listed in parentheses) :

c4 1543.4

C6 1952.5

c*o 1952.4

cs 2039.1 (990 [15])

CN 2044.4 (21 [17])

NC3NC 2044.8 (186 [19])

CNCN 2054.2 (165 [ 131)

CNCzNC 2114.9 (553 [ 141)

C, 2128.0

238

C5 2164.1 (3008 [16])

A.M. Smith et al. /Chemical Physics 184 (1994) 233-245

Table 3 Visible absorption spectrum of C5N2 in an argon matrix *

C30 2242.9 (1621 [ 181)

C3H 3237.0

HCN, H&N, COz, CO, N,O and C,H, comprise the stable products and impurities identified.

After discarding the assigned peaks, some 120 peaks, mostly weaker, remain. These have to be examined for their correlation with the 22737 cm-’ band and other visible absorptions of the unknown species. This was done using a program written for this purpose [ 201. A number of infrared absorption bands could be elimi-

fib. Intensity ha - fi&) Assignment

22737.3 3.64 0.0 @ 23245.8 0.11 508.5 9: 23338.5 0.17 601.2 8; 24697.8 1.56 1960.5 1;

0.38 3911.7 12 26649.0 0

’ All frequencies, S, in cm -I; accuracy f 0.2 cm-‘. Intensities in

arbitrary units.

nated since they appeared in experiments using other precursor species (see Table 1) for which the visible

Table 2 Emission spectrum of CSNZ in an argon matrix ’

%bS Intensity fi* - Lr Assignment Intensity i&$-L Assignment

22215.4 0.92 521.9 22212.1 0.76 525.2 22206.8 1.65 530.5 I 22135.4 0.36 601.9 22127.7 0.49 609.6 1

21988.8 1.11 748.5 21935.8 0.77 801.5 21878.0 0.58 859.3 21866.4 2.06 870.9 21097.9 94.32 1639.5 20706.4 355.47 2030.9 20665.9 452.54 2071.4 20577.2 1.11 2160.1 20572.4 0.76 2164.9 20569.4 0.25 2167.9 20353. I 0.32 2388.0 20317.3 0.29 2420.0 20229.1 0.57 2498.8 20185.7 2.75 2551.6 20178.0 0.68 2558.3 20146.2 5.31 2591.1

20140.4 0.76 2596.9 20099.4 0.82 2637.9 20089.3 5.87 2648.0 20060.9 0.75 2676.4 20052.7 0.73 2684.6 19960.1 1.25 2777.2 19920.6 1.53 2816.7 19910.6 0.40 2826.4 19901.3 1.04 2836.0 19863.2 1.05 2874.1 19842.5 0.74 2894.8 19830.4 2.79 2906.9 1

v8

3

%

V5

V4

9

Y

Vl

y+530

P, + 748 v, + 801 q+871 y+525 vz + 530 v, + 525 v, + 530 v*+602 v2+610 v, +602 v, +610 q+748 v, + 748

y+801 v, + 801 y+871

19800.0 0.56 2937.3 19788.5 1.43 2948.8 1

19481.8 7.96 3225.5 19454.4 27.35 3288.9 1

19077.8 26.75 3659.5 19035.9 44.27 3701.4 18704.2 61.63 4033.1 18640.1 147.58 4097.2 18588.0 33.43 4149.3 18557.1 0.42 4180.2 18515.7 1.36 4221.6 18173.7 0.26 4563.3 18086.1 1.60 4651.2 18016.1 0.84 4721.2 17872.1 0.63 4865.2 17820.0 1.12 4917.3 17767.9 0.40 4969.4 17474.8 6.08 5262.5 17450.7 5.69 5286.6 17419.3 0.37 5317.9 17399.0 3.03 5338.2 17085.2 2.37 5652.1 17073.2 0.24 5664.1 17017.7 6.92 5719.6 16968.6 2.01 5768.8 16776.2 0.31 5961.1 16716.9 6.22 6020.4 16703.4 0.87 6033.9 16629.6 13.30 6107.7 16851.9 9.33 6155.4 16506.0 0.96 6231.3

v, + 871

2 v3

v2+ v3

VI+ ti 2% v*+ VI

2 VI v*+ y+530 v,+ q+530 2vz+530 2v,+530

3 v3

v,+2Y

2Y+ v3

Y+Y+Y 29+ 4

39 v,+2v,

2 v, + vz

3 VI

’ All frequencies, F, in cm-‘; accuracy f 0.2 cm- ‘, Intensities in arbitrary units. For the emission measurements, F,_ = 22737.3 cm --I.

A.M. Smith et al. / Chemkal Physics 184 (I 994) 233-245 239

absorption at 22737 cm- ’ was very weak or absent. More than ten infrared/visible spectra involving dis-

charged samples of NC,N with various concentrations and various annealing and photolysis histories, were compared. Two infrared peaks at 2152.0 and 2041.1 cm-’ showed the best correlation with the 22737 cm- ’ absorption. They exhibited correlation coefficients > 0.9 with the visible absorptions and with each other.

3.4. Isotopic studies

The number of atoms in a molecule can, in principle, be decided unambiguously with the help of isotopic studies. As noted above, the fact that the 22737 cm-’ absorption can be observed, albeit with a relatively low yield, by discharging samples containing HCN and C,H,, allowed us to study spectra of isotopically enriched samples.

3 4 I ‘jN enrichment studies . . . Fig. 3 shows an excitation spectrum of a sample

generated by discharging a mixture of HC “N/HC15N with C,H, and by scanning a ring dye (stilbene-3) laser over the 0: region. The figure represents an intensity

contour map, with excitation wavenumber on the vertical axis and emission wavenumber on the horizontal axis. The long slanted appearance of the emission is due to the inhomogenous site distribution and the small wavenumber range of excitation compared to emission which is displayed in this particular plot. If the displayed x and y range happened to be identical, then the site distribution of the emission peaks would lie along the 45” diagonal.

The emission is shown in the region of the strong Fermi resonance pair near 20700 cm-‘. In addition to emission bands due to species containing only 14N (A) or only “N (C), well-known from experiments with the pure isotopic species, a third, intermediate band (B) was observed (compare Table 4). This indicates that two equivalent N atoms are present in the carrier of the 22737 cm-’ absorber and conclusively eliminates C3N. Also the relative intensities of the three excitation maxima point to two equivalent nitrogens. Given the starting ratio of HCi4N/HCL5N of l/1.9, determined by integrating HCN’s infrared bending fundamental, an intensity ratio of 217.617.2 would be expected for a molecule with two equivalent nitrogens. This agrees very well with the observed intensity ratio of 2/g/7,

22732

22745

20660 20680 20700 20720 20740

Emission lcni’l

Fig. 3. A contour plot of a laser excitation scan over the 0: transition of C5N2 from a sample enriched with “N. &HJHCN/HC”N (31 I / 1.9 mixture) diluted in argon was discharged. The emission is shown in the region of the strong Fermi resonance pair ( Y, and v*). Instead of the two pair of bands expected for the previously studied pure ‘*N (A) and pure “N (C) specie-s, a third pair (B) is observed, clearly indicating that the carrier of the emission is not C,N, but rather has the formula CJV, with equivalent nitrogens.

240 A.M. Smith et al. /Chemical Physics I84 (1994) 233-245

obtained by rationing the sum of the integrals of the two emission bands in Fermi resonance with excitation at their appropriate 0:‘s.

3 4 2 “C enrichment studies . . . Fig. 4 shows the result of an experiment with a sam-

ple enriched to 50% with 13C using the mixture C,H,/ ‘3CzHJHCN/H13CN. In this experiment, an intracavity doubled titanium-sapphire ring laser was stepped in 2-3 cm-’ steps in the region of the 1; absorption band at 24698 cm- ‘, and for each excitation wavelength the fluorescence spectrum was recorded with the FTS. The figure represents an intensity contour map, with excitation wavenumber on the vertical axis and emission wavenumber on the horizontal axis. The emission is shown in the region of the strongest peak, namely at the $ .

To get oriented in this figure one can note that taking a horizontal section from right to left yields an emission spectrum observed by exciting with a laser tuned to the frequency indicated by the ordinate. A vertical section, on the other hand, represents an excitation spectrum one would obtain by monitoring the frequency indicated by the abscissa, and scanning the laser. Finally,

-i E ”

24670 E .-

? u Y

24650 w

24630

I 1

22720 22740 22760

Emission (cm-‘)

Fig. 4. A contour plot of a laser excitation scan over the 1: transition of C5N, using a sample enriched to 50% with ‘?. The strongest emission band, the @, is shown here. As labelled. thirteen unique

carbon isotopomers (compare Table 4) could be unambiguously identified through analysis of the emission spectra. These results demonstrate that the emitter must contain at least four, but probably

five, carbon atoms.

a section taken at =r 45” (exactly 45” for the same x and y axes wavenumber range) from bottom left to top right implies constant difference between excitation and emission wavelength. This represents the inhomogeneous site distribution, and as mentioned above, two distinct sites would give two distinct maxima on such a diagonal section.

Comparison of these emission spectra, excited in the 1: band, with an earlier excitation scan using less i3C ( C2H,/H’3CN l/ 1 mixture) and excited in the 0: band proved to be very productive in identifying and assign- ing frequencies of the individual isotopomers. as given in Table 4. For example, at the 1; excitation energy of 24667.4 cm- I, three species emit, albeit with differing 0: frequencies and differing ground state frequencies, specifically v,- y (compare peaks 6-g in Fig. 4 and Table 4). Although it is not clear in the 1: excitation scan alone which 0: belongs to which v,-v~, in the earlier excitation scan over the $, these species appeared separately and comparison of the two sets of emission spectra allows for a clearcut identification of the individual isotopomers through their characteristic emission frequencies. In this manner, at least thirteen isotopomers could be identified in Fig. 4; their frequencies are listed in Table 4. It is clear from previous measurements with pure isotopic compounds that species 1 is the pure “C emitter. Similarly species 13 is identified as the pure 13C emitter, and as expected, it is furthest shifted from the pure “C species.

A molecule with three carbon atoms, all of them inequivalent, could give at most eight distinct isotopomers. These results indicate that the emitter must have at least four carbon atoms. Whereas a C,N2 species with one equivalent C atom pair could yield at most twelve distinct absorptions, one with four nonequivalent C atoms would give sixteen different isotopomers. It is, however, difficult to conceive a realistic molecule consistent with the observed spectrum with two equivalent nitrogens and four carbons, all of them nonequivalent. Furthermore, the three likeliest linear NC4N isomers are now known, and are observed in the matrix. Their infrared absorptions show no correlation with the visible spectrum; when they are produced efficiently through in situ photolysis of NC&, this visible spectrum is not observed [5]. We therefore arrive at the conclusion, that the carrier of the spectrum probably contains five carbon atoms.


Table 4 r3C and “N isotopic spectral frequencies and shifts for CrN2 ’

Label Species b

1 GN,

2 ‘%X,N,

3 (2)

4 ‘%X,N,

5 (2)

6 (3)

7 (2)

8 (2)

9 (3)

10 (4)

11 (3-4)

12 (4)

13 “CsN2

A=1 C,N,

B Ci4N’sN

C C:’ N 2

D 13C:‘NI

Upper state

laser’

24698.3

24684.4

- 13.9

24680.0

- 18.3

24680.0

- 18.3

24617.1

-21.2

24667.4

- 30.9

24667.4

- 30.9

24667.4

- 30.9

24661.2

-37.1

24654.5

-46.6

24651.7

-46.6 24645.3

- 53.0

24635.0

-63.3

G

22731.3

22740.6

22737.6

22134.6

22132.5

22733. I

22738.4

22743.1

22735.9

22736.9

22734.8

22737.7

22736.3

22737.3

22139.9

22742.6

22740.2

Lower state

v3

1639.5

1639.3

-0.2

1639.2

-0.3

1628.9

- 10.6

1621.4

- 18.1

1585.8

-53.7

1628.4

-11.1

1639.0

-0.5

1620.4

-19.1

1585.8

-53.7

1585.6

-53.9 1620.3

- 19.2

1585.4

-54.1

1639.5

1635.2

-4.3

1631.0

-8.5

1577.7

-61.8

v2 VI

2030.9 2071.4

1991.6 2027.2

2061.5

- 24.4

1984.5 2050.6

- 33.6

2023.3 2064.4

-7.3

2006.4 2057.7

- 19.1

2015.9 2057.9

- 14.3

1987.7 2055.5

- 29.6

1995.8 2046.9

- 29.8

1983.4 2044.8

-37.1

1986.5 2047.9

-34.0

2041.6

1983.1 2021.3

49.0

1976.1 2014.5

-55.9

2030.9 2071.4

2025.4 2062.8

-7.1

2011.1 2051.4

- 19.9

1955.9 1994.2

-76.1

* All values in cm-‘. The isotopic shift for the Fermi resonance pair is given as the shift of the center of gravity from that of the “CSL4N2

species (species 1) .

b The number in parentheses denotes the likely number of carbon- 13 atoms. See text.

’ Exact 1; resonances cannot be given, since the laser was scanned here in steps of 2-3 cm-‘. The frequencies given, however, can be taken as

the resonances to this accuracy. For the species with no 1; frequency given, the fluorescence was excited at the 0:.

A CSNz molecule with two pairs of equivalent car- First we must address the question as to why in an bons would give twenty distinct isotopomers. There are experiment with enrichment to 50% with r3C, thirteen several possible molecules fulfilling this requirement. distinct isotopomers were observed rather than the Remaining with the assumed linear structure, the cen- expected twenty. Several factors, both inherent and trosymmetric CCNCNCC, CNCCCNC, and synthetic, could play a role here. The inherent factors NCCCCCN are the alternatives. Such species also ful- are symmetry-related. For an allowed electronic tran- fill the requirement of two equivalent nitrogens. We sition in a centrosymmetric C5N2 species, only the will now examine the available evidence in light of this totally symmetric vibrational modes will be active. possibility. Such modes will not involve motion of the central atom,

242 A.M. Smith etal. /Chemical Physics 184 (1994) 233-245

i.e. isotopomers differing only in the mass of the central atom will have almost identical totally symmetric vibrations. As an example, ab initio calculations predict for the symmetric stretches of Cs, vibrational shifts of only0.1 cm-’ upon 13C substitution at the central atom [ 161. An isotopomer with shifts of this magnitude could not be resolved in our argon matrix spectra, even for the 1: transition, given the inhomogeneous absorption linewidths of = 5 cm-’ (fwhm), and emission linewidths of = 2.3 cm- ’ (fwhm) . This implies that the twenty possible distinct species may thus be divided into ten pairs of the type MNlZM’N and MN1 3M’N’ (where M, M’ and N, N’ are the two pairs of equivalent carbons), with very similar frequencies. At first glance, this would imply that in a laser excitation scan over the V, (symmetric stretch) in the excited electronic state, only ten of the twenty possible isotopomers would be resolved. One should not forget however, that the zero point energies of species MN12M’N’ and MN13M’N’ do differ, since the asymmetric stretching and bending modes involve motion of the central atom. This zero point energy difference leads to a minor shift in the 0: band (see Table 4), which carries over to the 1;. The above pairs, if resolved, would then appear in the contour map of the type shown in Fig. 4 as signals displaced along the 45” diagonal, similar to different “sites”. The observation of thirteen isotopomers, instead of ten or twenty, thus implies that some closely lying signals are resolved and others not.

Perhaps more likely, however, is the interplay with a set of external factors. The experiment employed a mixture of i3CzHz and ‘*C2H2, rather than an equili- brated mixture. The production of all of the twenty isotopomers may therefore not be statistical, and the abundance of some may be very low or completely missing. Even with a statistical distribution, the expected intensities of all the isotopomers would not be equal: twelve have a statistical weight of 2, and the remaining eight weight of 1. It is clear, however, that we observe at least two of the species with a weight of 1, that is the all carbon-12 and all carbon-13 species. Regarding experimental factors, it must also be mentioned that at the time when the 50% i3C experiment was performed, we could only step the laser in incre- ments of 1 S-2 cm- ’ giving, after frequency doubling, a resolution of 3-4 cm-‘. In such a coarse scan, some of the weak isotopomer resonances may have been missed.

Several interesting observations can be made upon examining Table 4. The Fermi resonance pair shifts by - 55.9 cm-’ in i3CsN2. This is close to 5/2 times the shift for C515N 29 - 19.9 cm-‘. If one assumes that these modes involve motion of all the atoms in the linear chain, these shifts are consistent with the C5N2 formula. It may also be noted that if one sums the isotopic shifts of the (q-y) modes for each isotopomer, it is possible to divide the species listed in Table 4 into groups of molecules containing O-5 13C carbon atoms, assuming a redshift of 24-34 cm-’ for each “C atom. Using this reasoning, the likely number of carbon-13 atoms contained in each species is given in parentheses in Table 4.

3.4.3. Isotopic structure in natural abundance NC,N samples

In view of the excellent signal/noise ratio in laser- induced fluorescence experiments using a discharge of NC,N, we have searched for the isotopomers containing one 13C atom in samp les of natural isotopic abundance. The number of such isotopic absorptions, and their relative intensities can yield valuable information about the number of distinct carbon atoms, and their statistical weights.

For the experiments described here, we implemented the line scanning capability of the intracavity doubled titanium-sapphire laser using the autoscan feature, so that the chance of missing isotopomer resonances of sufficient intensity was eliminated.

Fig. 5 represents the result of one such search, in which the intracavity doubled titanium-sapphire laser was finely stepped (stepsize = 0.3 cm- ‘) over the 1: region, and the emission in the neighborhood of the strong 0; is monitored. Here we excite the redder of the two sites in a Ne matrix (site 1 in Table 5) and the scan proceeds to the red of the pure ‘%$I, species to search for the singly-substituted carbon-13 species in natural abundance. The strong emission of the pure ‘*CN2 species at 22762.9 cm-’ with excitation max- imum at 24724.2 cm- ’ is partially cut-off at the top of the diagram (compare Table 5). Two isotopic maxima with similar intensities attributable to molecules with one carbon-13 are observed, shifted - 15.9 and - 16.6 cm-’ to lower excitation energies. A comparison of the full emission spectra of these species, excited at their respective maxima, demonstrate that these correspond to isotopic species 2 and 4 in the 13C enriched


experiments as seen in the laser excitation scans in an Ar matrix. (See Table 4 and Fig. 4.) Furthermore these absorptions exhibit about 2% of the intensity of the main, pure ‘% isotopic peak. This result is indicative of a molecule containing two pairs of equivalent carbon atoms, and rules out asymmetric species like CNCNCC. Also the equivalence of the nitrogen atoms, as previously established, makes asymmetric structures unlikely. Similar excitation scans over the 1: band as well as over the 1: and G bands, were performed in an argon matrix and gave identical results, that is, again only isotopic species 2 and 4 were detected and with roughly equal intensities. The isotopic shifts from tire strong pure *‘C signal were, as expected, much larger for the 1: band, and very small for the @ band. The neon matrix studies, as displayed in Fig. 5, present most clearly these results since the individual absorptions are much sharper in neon (fwhm of 3 cm- ’ versus 5 cm-’ in argon), leading to a better resolution of the singly-substituted carbon-13 isotopomers in natural abundance samples. Specifically, in an argon matrix, the isotopomer absorptions are more hidden in the wing of the 50 times stronger absorption of the all carbon- 12 species.

At a first glance, a centrosymmetric CSN2 species should exhibit three singly substituted carbon- 13 spe-

Table 5 Matrix effect on transition fmquencies of C5N2 (all values in cm-‘)

Assignment l Neon matrix Argon matrix Nitrogen matrix

1: site 1 24724.2 24698.3 site 2 24746.3

O0 0 site 1 22762.9 22737.3 22731.8 site 2 22784.0

VI 2070.7 2071.4 2071.6 9 2030.7 2030.9 2032.0 Y 1639.2 1639.5 1639.7 v4 871.7 870.9 VS 860.8 859.3 % 804.7 801.5 3 753.3 748.5 % b 603.2 605.7 V9 b 527.7 527.8

’ The tis are ground state vibrational frequencies, deduced from the emission spectra. b Center of split transition. gee Table 2 for explicit argon values. In neon, the transition to these ground state vibrations am broader in comparison to the other transitions and have a weak shoulder.

ties with statistical weights of 2/2/ 1. Quite analogous to the above discussion however, the species 12-12- 13-12-12 with statistical weight 1 has symmetric modes nearly identical to those of the 12-12-12-12- 12 species. Especially in this particular experiment, it

22 155 22 765 22 775

Emission ( cm-‘)

24 710

Fig. 5. A contour plot of a laser excitation scan over the redder site of the 1; transition of C5N, in a neon matrix using a discharged sample of N&N (natuml abundance). Cut-off at the top is the $ emission of the pure carbon-12 species, which appears at a laser excitation frequency of 24724.2 cm-‘. To lower excitation energies, only detection of species with either one carbon-13 or one nitrogen-15 is expected. What we observe is two unique species with one carbon-13 (corresponding to species 2 and 4 in Table 4 and Fig. 4, as observed in argon) with roughly equal intensities, which provides strong evidence for a linear centrosymmetric C,N2 emitter.

244 A.M. Smith et al. /Chemical Physics I84 (I 994) 233-245

is quite likely that the 12-12-13-12-12 species would not be resolved, since to resolve a signal in the prox- imity of a hundredfold stronger signal is much more difficult that to resolve two signals of comparable intensity in the 50% i3C experiment. Thus on the whole, the isotopic structure studies consistently point to a centrosymmetric C,N1 species.

3.5. Vibrational assignments

Since the identity of the carrier of the spectrum remains uncertain, it is not very meaningful to attempt specific assignments of the vibrational modes, labeled arbitrarily z+ v9 in the figures and tables. One can only note that the number of observed modes indicates a relatively large molecule, and is not inconsistent with assignment to CSN2. The absence of infrared bands coincident with intervals deduced from the visible spectrum suggests the presence of a center of symme-

try. Unfortunately, to the best of our knowledge, there

are no ab initio calculations for the CSNz isomers, Com- parison with related species however indicates that vibrational frequencies in the area of 2000 cm- ’ would certainly be expected. As predicted by ab initio calculations, the symmetric stretches of Cii have frequencies of 2008 and 792 cm-’ [ 161. The Fermi doublet at 2030.9 and 2071.4 cm-‘, as well as the 1960.5 cm-’ progression in the visible spectrum surely involve one of these symmetric stretches. The mode at 1639.5 cm - ’ could also be one of the symmetric stretching funda- mentals, or an overtone band. The low frequency modes K-V, may involve low frequency stretches, bending frequencies appearing due to matrix effects, or their even overtones.

The 2152.0 cm-’ (or 2041.1 cm-‘) infrared band which possibly belongs to CSNz would then correspond to the highest frequency asymmetric stretch, analogous to the well-known asymmetric stretch of Cs at 2164 cm-’ in an argon matrix [ 2 11. More specific assignments would, of course, require further studies, and in particular, observation of the low frequency modes in additional isotopically substituted molecules. Up to now, as shown in Table 6, we have succeeded in meas- uring these shifts only for the all carbon-13 and all nitrogens- 15 species, through discharging the appropriate acetylene and HCN isotopically labelled compounds.

Table 6 Ground state vibrational frequencies and isotopic shifts for C,N, (all values in cm-‘)

Mode C5N2 “CSN2

TW 22737.3 22736.3

VI 2071.4 2014.5 - 56.9

Y 2030.9 1976.1 -54.8

* 1639.5 1585.4 - 54.1

V4 870.9 840.8 -30.1

vs 859.3 830.0 -29.3

“6 801.5 772.2 - 29.3

3 748.5 719.9 - 28.6

v8 . 605.7 586.2 - 19.5

%a 527.8 517.7 - 10.1

’ Center of split transition. See Table 2.

C, lSN *

22742.6

205 1.4 - 20.0 2011.1 - 19.8

1631.0 -8.5 867.0 -3.9 856.7 - 2.6

799.1 - 2.4

747.7 -0.8 605.2 -0.5 518.1 -9.7

3.6. Summary: evidence for Cf12

The chemical evidence indicates that the carrier molecule is formed efficiently only in the samples with discharged NC4N, but that it is not generated by its photolysis. Furthermore, it grows in readily upon annealing the discharged samples. This is at least consistent with the CSNz assignment. In the discharged samples, typically only a fraction of the parent is decomposed. Among the products, Cz-C, and CN molecules are identified, and there is little doubt that also copious C atoms are present. We have used more recently other “hotter” discharge units leading to more of the longer chain carbon molecules and we have in each case noted that the appearance of more long chain carbon molecules correlated with the appearance of more CSN2. Again upon annealing, diffusion of carbon atoms is evident through growth of absorptions due to C,-C, and decrease of the C3 absorptions. It is then easy to understand formation of the carrier by the reaction C,N, + C + C,N,. While CCNCNCC would probably not be readily formed in this reaction, the other two symmetric species suggested above, CNCCCNC and NCCCCCN could form easily, in view of the easy isomerization of the CN groups.

One can also understand the much less efficient appearance of this molecule in photolyzed samples, or in discharges from C,NH, molecules or C,H,/HCN mixtures. In this case either recombination with more than one atom, or diffusion of polyatomic molecules (perhaps C3 + 2(CN)) would be required. Further- more, the easily diffusing H atoms might result in


hydrogen containing species and in fact hinder theCSNz production.

As discussed in detail above, the C,N, assignment is also strongly indicated by the isotopic studies. The experiments with 15N enriched samples indicate that the carrier of the spectrum contains two equivalent nitrogen atoms. Experiments with isotopic carbon are indicative of the presence of five carbons. The sum of the evidence suggests strongly a centrosymmetric C,N, species, most likely either NCCCCCN or CNCCCNC.

Additional experimental studies are needed. Future matrix work include a more selective and efficient production of C5N2, through for example the controlled reaction of carbon atoms with NC.,N or its isomers. As a carbon atom source, either laser vaporization of graphite [ 221 or photolysis of cyanogen azide (N&N) [ 23,241 can be used. As demonstrated here in experiments with NC,N natural abundance samples, in neon matrices the inhomogeneous linewidths are reduced by about a factor of 2 over argon matrices, leading to a better resolution of isotopomer absorptions. For this reason, experiments with 13C enrichment in neon matrices would be informative. Very desirable would of course be gas phase observation and studies of the spectrum. Finally, good quality ab initio calculations on the C,N, isomers would be extremely useful.

Acknowledgement

We thank Professor P. Botschwina (University of Gijttingen) for communicating his ab initio results on C3N and &NC and other molecules, and for additional helpful information. We are indebted to K. Fritz for synthesis of most of the nitriles used here and to Dr. J. Silha, Charles University Prague, for initiating the NC$J synthesis. Research support from the Deutsche Forschungsgemeinschaft and the Fonds der Chem- ischen Industrie is also gratefully acknowledged.

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Tentative identification of C5N2 in rare gas matrices

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