Radiative lifetimes have been measured for the stable vibrational levels of the A 1 sigma - electronic state of C2N2. They range from 1.3 microsec for the 4(sub 0)(sup 1) level to 0.66 microsec for the 1(sub 0)(sup 1) 4(sub 0)(sup 1) level and in general decrease with increasing vibrational excitation. Self-quenching rate constants range from gas kinetic to ten times that. Foreign gas quenching rates are slower. Observation of emission from the 1(sub 0)(sup 1) 4(sub 0)(sup 1) level sets a lower limit for the heat of formation of CN of 439.11 kJ/mol

Barts, Samuel A.

Halpern, Joshua B.

Pinnex, Karen V.

English

NASA Technical Reports Server

N90=26765
THE HEAT OF FORMATION OF CN RADICALS AND
RADIATIVE LIFETIMES OF THE A 1_" STATE OF C2N 2
SAMUEL A. BARTS, KAREN V. PINNEX AND JOSHUA B. HALPERN
Howard University, Dept. of Chemistry, Washington, DC 20059, USA
ABSTRACT
Radiative lifetimes have been measured for the stable vibrational levels of the A1_,- electronic
state of C2N 2. They range from 1.3 p,sfor the 401 level to 0.66 I_Sfor the 101401 level and in general
decrease with increasing vibrational excitation. Self-quenching rate constants range from gas kinetic
to ten times that. Foreign gas quenching rates are slower. Observation of emission from the 101 401
level sets a lower limit for the heat of formation of CN of 439.11 kJ mol1.
INTRODUCTION
Simple nitriles play important roles in the photochemistry of nitrogen-methane planetary
atmospheres. Cyanogen is a photochemically important molecule found in the atmosphere of Titan,
and in other astronomical systems 1. Cyanogen is also an extremely simple tetra-atomic system in
which predissociation can be studied. Potential energy surfaces have been calculated for all of the
excited states which means that models of dissociation dynamics can be tested in detail 2.
We have recently discovered that this molecule has a strong and distinctive emission spectrum.
The lowest lying vibrational bands of A 1_- state of cyanogen (C2N2) fluoresce. The absorption,
emission and photodissociation yield spectra of C2N 2 have been measured between 220 and 210 nm,
spanning the region between the 401 and 101401 bands of the A1_..-*- X1_.+ system3. A recent paper
deals with the emission and fluorescence spectra of the 401 state and its radiative lifetime 4. In this
paper we report emission lifetimes of all the stable vibrational bands.
EXPERIMENTAL
The apparatus was similar to that described in Reference 5. C2N 2 from Matheson was purified
by freeze-thaw cycling. The cyanogen gas flow from a 3 liter reservoir was controlled by a flowmeter.
The pressure was monitored by a capacitance manometer.
The excitation source was a Spectra Physics PDL-2 dye laser pumped by a DCR-11 Nd-YAG
system. Dye laser light was frequency doubled in a KDP crystal and Raman shifted in about 10
atmospheres of hydrogen. For this experiment the third anti-Stokes shifted beam was used.
Emission from C2N 2 was detected directly through an unfiltered 11 stage EMR VUV
photomultiplier. Emission decay lifetimes were measured by fixing the exciting laser on the absorption
line and scanning the boxcar gate delay or by using a DSP 2100 100 MHz transient digitizer. The
221
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experiments were controlled by an IBM PC\XT microcomputer based data acquisition system
constructed in our laboratory. The firing of the lasers and the dye laser scanning was controlled by the
microcomputer. Absorption spectra were measured directly on a Cary 2390 UV-VIS-NIR
spectrophotometer.
RESULTS
Figure 1 shows an absorption spectra measured on the Cary 2390 spectrophotometer between
200 and 230 nm. The lower half of the diagram shows the difference between spectra measured at 20
and 70 °C when the cell was heated by water from a thermostated bath. This identifies lines that
originate in excited vibrational states and makes the assignment of the vibrational bands quite easy.
Extra bands at wavelengths below 210 nm belong to the B 1 u *- X1 _ ÷ transition. We are in the process
of setting up a VUV monochromator with better (0.02 nm) resolution to be able to analyze this second
electronic transition. Figure 2 shows the emission spectrum from the 101401 band and the yield of CN
fragments as a function of wavelength. Figure 3 shows the yield of CN fragments as a function of
wavelength in the region of the 101401 band.
Table I lists the measured radiative lifetimes and quenching rate constants.
TABLE I
LIFETIMES AND QUENCHING RATES OF CYANOGEN A 1_..-
BAND WAVELENGTH VIBRATIONAL LIFETIME QUENCHING
ENERGY GAS
nm cm -1 I_S
QUENCHING RATE
cm3-s/molecule
40_ 218.9 276 1.34 C2N 2 4.3
40' He 1.7
40_ Ar 1.9
40 ' N2 2.5
401 CH 4 3.4
412 220.1 519 0.72 C2N 2 8.4
403 216.6 770 0.67 C2N 2 10.4
403 Ar 6.7
2114 10 218.3 632 1.25 C2N 2 9.9
2114nl 218.6 908 0.94 C2N 2 12.5
201401 214.7 0.68 Ar 3.1
101401 208.3 2606 0.66 C2N 2 13.0
222
1.0
0.0
+0.10
0.00
-0.05
t
!
I 1
I I I
I1
I I
1 on2o24o 1
I lon2o14o 1
I I 1°n4°3
lon2o1410I IloO+o,
I I ;°n+12
I
I
[ 1on423
I
I I
1 on410
1 on421
I
i mosanoo,r secru m +anot
, , ,
46,000
FREQUENCY cm-1
FIGURE 1. Absorption spectrum of C2N 2
223
IO P Branc_
Branch
'_" IX 10
\
WAVELENGTH
Figure 2. CYANOGEN EMISSION SPECTRUM
_J
Z
m
O3
5
|
Q Branch
Brancn
WAVELENGTH
Figure 3. CN FRAGMENT QUANTUM YIELD
224
DISCUSSION
Using photodissociative excitation Davis and Okabe measured a dissociation energy of 5.58 __+
0.05 eV and a threshold wavelength of 222.2 nm 6. Eres, Gurnick and McDonald observed the
rotational distribution of CN (X2_ +) fragments in the 193 nm photolysis of C2N27. Based on the
highest observed fragment rotational state they calculate a bond strength of 5.83 __+0.04 eV and a
photodissociation limit of 212.7 __+1.5 nm. Most of the uncertainty is related to the 1 nm bandwidth of
the excimer laser. Recently Lin, Johnston and Jackson have used a narrower bandwidth doubled dye
laser to photodissociate cyanogen at 206 nm 8. Their fragment LIF spectra implies an upper limit to the
bond energy of 5.70 eV and a dissociation limit of 217.5 nm, assuming that there were no hot band
contributions to the photolysis. The observation of rotational structure in bands above 218 nm9 show
that the actual dissociation limit must lie below this point.
- The observation of strong fluorescence from the 101401 band at 209.5 nm is strong evidence
that at least some rotational states of this level are bound. As can be seen in Figures.2and 3 the Q
branch of the emission spectrum is narrower than the Q branch in the fragment yield spectrum. The P
and R branches in the emission spectrum are much weaker than those in the fragment yield spectrum.
This also shows that low rotational states of the 101401 band are bound and high rotational states are
dissociative. Thus, we assign a lower limit of the NC-CN bond strength of 5.92 eV. Using the standard
value of the heat of formation for C2N 2 of 307.223 __+1.80 kJ mo1-1,10 this yield a lower limit of 439.11
__+2.00 kJ mol"1 for the heat of formation of CN radicals, as compared to Davis and Okabe's value of
423 __+4 kJ mo1-1.6
The radiative lifetimes range from 1.4 I_S in the 401 band to 0.6 I_Sfor the 101 401 band. Self
quenching rates range from gas kinetic (4.3 x 10-10 cm3/molecule_s) for the 401 band to 13.0 x 10"10
cm3/molecule-s for the 101401 band. Foreign gas quenching rates for the 401 band have been
measured against He, Ar, N 2 and CH 4. As expected the quenching rate constants increase as the
molecular weight and the complexity of the quenching gas, with the rate for quenching against He
being the slowest and that against CH4 being the fastest.
CONCLUSIONS
Earlier work done in our laboratory has demonstrated that dicyanoacetylene (C4N2) will also
have several bound vibrational states in the A 1_..+ electronic state. We are currently synthesizing a
sample of this compound in order to look for the fluorescence. Cyanoacetylene is known to have no
bound levels in its first excited singlet state, and cannot be monitored by fluorescence. The UV
emission of simple nitriles offers a method of remote detection, and will also be useful in the laboratory
for study of such reactions as CN + HCN -- > C2N 2 + H.
225
ACKNOWLEDGEMENTS
Parts of this work were supported by NASA under Grant NAG-785, and grants from the
Department of Energy and the Howard University Research Development Program. S.A. Barts was
supported by a Danforth Foundation Fellowship. K. V. Pinnix was supported by a grant from the
Department of Education.
REFERENCES
1. Y.L. Yung, M. Allen and J. P. Pinto, Astophys. J. Supp., 55, 465 (1984).
2. C.E. Dateo, Thesis, University of California, Berkeley, 1987.
3. S.A. Barts and J. B. Halpern, to appear in Proceedings of the 1989
NASA/HB CU Colloquium.
4. S.A. Barts and J. B. Halpern, Chem. Phys. Letts., 161,207 (1989).
5. J.A. Russell, I. A. McLaren, W. M. Jackson and J. B. Halpern, J. Phys. Chem., 91,
3248 (1987).
6. D.D. Davis and H. Okabe, J. Chem. Phys., 49, 5526 (1968).
7. D. Eres, M. Gurnick and J. D. McDonald, J. Chem. Phys., 81, 5552 (1984).
8. H. Lin, E. A. Johnson and W. M. Jackson, Chem. Phys. Letts., 152, 477 (1988)
9. G.B. Fish, G. J. Cartwright, A. D. Walsh and P. A. Warsop, J. Mol. Spec., 41,20 (1972).
10. D.R. Stull, E. F. Westrum, Jr. and G. C. Sinke, "JANAF Thermochemical Tables",
2nd Edition, Natl. Stand. Ref. Data Ser., Natl. Bur. Stand. (US) 37 (1971).
226


The heat of formation of CN radicals and radiative lifetimes of the A 1 Sigma - state of C2N2

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