The first infrared spectrum of gas phase, jet-cooled C4 has been measured by high resolution diode laser absorption spectroscopy. Twelve rovibrational transitions are assigned to the nu3(sigmau) antisymmetric stretch of linear 3Sigma - g C4. No evidence is observed for the bent structure of triplet C4 recently observed in a matrix study by Cheung and Graham [J. Chem. Phys. 91, 6664 (1989)]. Indeed, the measured band origin (1548.9368(21) cm^–1) and effective ground state C–C bond length [1.304 31(21)A] are consistent with several ab initio predictions of a rigid, linear, cumulenic structure for this cluster radical

Heath, J. R.

Saykally, R. J.

English

Caltech Authors - Main

The structure of the C4 cluster radical 
J. FL Heath and FL J. Saykally 
Department of Chemistu, University of California, Berkeley, California 94720 
(Received 9 November 1990; accepted 10 December 1990) 
Within the family of small carbon clusters, C, has re- 
ceived a particularly notable amount of attention.* Begin- 
ning with the extended Huckel calculations of Pitzer and 
Clementi in 1958,’ many ab initio calculations have ad- 
dressed the structures and energetics of the two most stable 
isomers.3-13 The most sophisticated calculations now agree 
that the bicyclic rhombus (‘A,) structure and the linear 
triplet (3X; )structures are approximately isoenergetic. 
Prior to 1988, the only experimental information avail- 
able for C, consisted of an EPR and optical study of triplet 
C, isolated in rare gas matrices. l4 However, four important 
experiments have appeared in the past two years. Yang 
et al. l5 have measured the photoelectron spectrum of 
Cc and have found evidence for the linear triplet isomer. 
Algranati et al., I6 have reported the observation of the 
rhombic isomer in a Coulombic explosion experiment of 
photodetached Cc. Shen and Graham” reported a defin- 
itive assignment of the v3(cU ) vibration of the triplet iso- 
mer in a matrix infrared experiment. The most surprising 
new result is that of Cheung and Graham,18 who presented 
evidence from new EPR measurements that the triplet 
state of C4 is actually slightly bent. This is not supported 
by any of the ab initio calculations. 
In this letter, we report the measurement of the v3 (a,) 
fundamental vibration-rotation band of jet-cooled 32; C4 
through diode laser absorption spectroscopy. In this first 
high resolution study of the C4 cluster, we find no evidence 
for nonlinearity. 
The diode laser apparatus used in recent studies of C, 
(Ref. 19), C, (Refs. 20 and 21), and Cg (Ref. 22)has been 
described in several recent publications. In brief, carbon 
clusters are generated by RrF excimer laser ablation of a 
rotating, translating graphite rod placed in the throat of a 
pulsed, planar, argon jet. The expanding carrier gas/ 
carbon cluster mixture intersects 10 passes of a diode laser 
beam over a 15 ,us time interval. The diode laser is focused 
onto a HgCdTe detector and the diode laser power is mon- 
itored by gated integration of the preamplified, filtered de- 
tector output. The diode laser is stepped in ca. 20 MHz 
frequency intervals every 80-100 shots, and the absorption 
signal is averaged over a 300 shot time constant. Immedi- 
ately following the measurement of an absorption line, ab- 
solute and relative frequency calibrations are performed by 
measuring the absorption spectrum of NH, or H,O and the 
fringe spectrum of an air-spaced, temperature-stabilized 
germanium etalon. The measurements have a frequency 
precision of ca. 0.002 cm - *. 
Guided by the IR spectrum of matrix-isolated C, re- 
ported by Shen and Graham,” much of the region between 
1540-1560 cm - i was searched. Twelve rovibrational lines 
assignable to a single vibrational band of a linear triplet 
molecule were observed. The calculated stick spectrum, 
along with typical data, are presented in Fig. 1. The fre- 
quencies and their respective assignments are presented in 
Table I. Derived spectroscopic constants and relevant ab 
initio predictions are presented in Table II. The band was 
assigned to C4 based on the following: 1) the fitted spec- 
troscopic constants are in excellent agreement with theory; 
2) the measured band origin is in excellent agreement with 
both theory and matrix data (corrected for a matrix shift); 
3) the chemistry of this absorber with respect to added 
impurities in the Ar carrier gas is consistent with that of 
other bare carbon clusters. 
The lowest state of linear C4 is 3X; ; consequently, 
except for the lowest J states, each rovibrational transition 
should actually be a sum of three separate transitions cor- 
responding to J=N + 1, iV, and N - 1 states, where N is 
the rotational quantum number, J is the spin plus rotation 
quantum number, and 1 is the electron spin (S) for a 
triplet molecule. The magnitude of the triplet splitting will 
be determined by the spin-rotation constant (y) and the 
spin-spin constant (a). The values of these terms have 
been measured by Graham et ~1.‘~ to be 0.0006(2) cm - ’ 
and 0.128 (3) cm - i, respectively. Assuming similar upper 
and lower state spin splitting constants, only the R (2)) 
P(2) and P(4) splittings are calculated to be greater than 
the resolution of our spectrometer (ca. 100 MHz). Only in 
the case of the P( 4) transition was our sensitivity sufficient 
to detect separated spin components, and this partially re- 
solved transition is shown in Fig. 1. Although no compo- 
nents of either the R(0) or P(2) lines were observed, the 
measured spin splitting of the P(4) line is sufficient for 
unambiguous assignment of the spectrum. 
A further test of the open shell nature of this cluster 
was done by measuring the effect of a 100-200 G (inho- 
mogeneous) magnetic field on the P(6) absorption line. 
The P(6) line was observed to broaden from a zero-field 
linewidth of 120 MHz to about 200 MHz in the presence of 
the field. 
Since spin splittings were observed for only one rovi- 
brational transition, the rotational constants B”, B’, II’, 
and the band origin v3 were extracted by a least squares fit 
of the center frequency of the observed transitions to the 
energy expression 
E,,=(u+ l/2~v~+B,UVW+ 1)) -D,[NW+ 1)12. 
(1) 
The ground-state centrifugal distortion constant D” was 
not statistically significant, and thus was fixed at 0.0 for the 
final fit. For the P(4) line, splittings were calculated from 
the Schlapp expressionsZ3 for a 38 molecule 
&dJ=N + 1) 
=&- [2NV+ lYWV+3)1 +yW+ 1) (2) 
J. Chem. Phys. 94 (4), 15 February 1991 0021-9606/91/043271-03$03.00 0 1991 American Institute of Physics 3271 
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v~((T,) Fundamental of 3c, C4 -I 
antisymmetric stretch fundamental of C!,. Quoted uncertainties are la. 
““IP 
Experiment Theory 
J-3 45 
9 1548.9368(21) cm.- ’ 1586 (Ref. 13)a, 1741 (Ref. 25)b, 
1349 (Ref. 25)’ 1715 (Ref. 25)d, W lO) 1648 (Ref. 26)‘, 1437 (Ref. 26)F 
1541 (Ref. 26)s ,* B” 0.16542(5) cm- ’ 0.163 (Ref. 25)h, 0.170 (Ref. 25)‘, : 0.163 (Ref. 9)jO.l63 (Ref. 7)k : B’ 0.16486(7) cm-’ , t u 
.d : 
:’ i D’ 87(19)e-08 cm-’ 
I 
I 3 /I re6 1.30431(21) A  (1.308, 1.330, (Ref. 25)h I * (1.283, 1.305,) (Ref. 25)‘, L’ (1.316, 1.297, ), (Ref. 9)j 
(%305, 1.300,) (Ref. 7)’ 
Levels of theorv: (a) M.P2/6-31G. [b) UHF/6-3lG. (‘I ROHF/4s2~1d, Ld) 
-I- - I 
15’ P3.0 00 1551.000 UHF/4s2pld, ?‘) HF/4-21G, trj MP2/4-2lG, (8) ad hoc/hINbO,’ (h) 
bn SCF, “‘D-MBPT(4), ti) SRCI, (‘I HF/3-21G. o-Denotesouter C-C 
Frequency (cm-l) bond length. m- Denotes middle C-C bond length. 
Letters to the Editor 
TABLE II. Measured and predicted molecular constants for the vX (0,) 
FIG. 1. Fitted rovibrational stick spectrum and experimental data of the 
Y~(D” ) antisymmetric stretch fundamental of the ‘2, isomer of C.,. The 
rotational temperature of C, is estimated to be near 30 K. 
&,(J=N) =Evr (3) 
E,,(J=N- l)=E,,- [2&V/(2N--- 1)] -riv (4) 
with spin splitting parameters taken from Ref. 14. The 
deviations of the calculated from the observed transitions 
presented in Table I show a systematically larger error for 
low N. This is most likely due to a broadening of the low N 
lines from unresolved spin multiple&. 
Is the triplet form of C4 linear? 
Cheung and Graham” recently proposed that triplet 
C4 is actually slightly bent, at least when isolated in an 
argon matrix at 12.K. This conclusion was reached on the 
basis of resolving doublet structures in the EPR spectrum 
of 12C4, which were interpreted as evidence of inequivalent 
x and y axes in the molecule. Experiments were conducted 
to test for inequivalent matrix sites and H atom interac- 
tions, and it was concluded that neither phenomena was 
the cause for the splitting. 
TABLE I. Observd frequencies (cm-‘) assigned to the v4 (gu ) anti- 
symmetric stretch fundamental of C!,. The standard deviation of the fit 
was 0.003 06 cm - ‘. Parenthetical values are calculated. 
cm -- ‘, and no lines other than those listed in Table I were 
observed. It thus seems likely that the nonlinearity of trip- 
let C4 reported by Cheung and Graham is due to the ma- 
trix environment. Cheung and Graham do point out that 
other triplet radicals, notably Sic0 and SiNN,24 are bent 
in an argon matrix while the unperturbed, gas phase struc- 
ture is linear. 
ACKNOWLEDGMENTS 
This work was supported by grants from the NASA 
Innovative Science Grants Program (Grant No. 
NAGW. 1022) and by the ONR Cluster Program (Grant 
No. N004-90-J-1368). J.R.H. is supported by the NASA 
SET1 foundation. We thank Dr. Jan Martin for sending us 
his results prior to publication. We would also like to 
obs-calc obs-calc thank Dr. Sherwood Chang and Dr. Jill Tartar for their 
N R(N) (x 10 - 3) P(N) (x 10-3) continued interest in this project. 
0 (1549.2665) 
2 .J=2 1549.9188 3.7 J-2 (1548.2740) 
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3272 
We fitid no evidence for a bent structure in the present 
study, and, indeed, the results presented here strongly sup- 
port ab initio predictions of a linear, cumulenic cluster with 
a 3X; ground state. Although it is not possible to com- 
pletely rule out a bent structure from these experiments, it 
is possible to place an upper lim it on’ how bent the cluster 
can be. If C, were bent by greater than about 6”, the 
K = 1 + 1 transition would originate less than 40 cm - ’ 
above the ground state, resulting in a second set of P- and 
R-branch transitions slightly displaced from those reported 
in Table I. The strongest of these transitions would be 
observable with a signal to noise of at least 3 to 1. Scans 
which were sufficiently thorough to detect such transitions 
were performed through the frequency region 1540-1560 
J. Chem. Phys., Vol. 94, No. 4, 15 February 1991 
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The structure of the C4 cluster radical

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The structure of the C4 cluster radical

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