4P038
C4Cl revisited in a SAC-CI general-R light
Sundaram Arulmozhiraja,* Masahiro Ehara, and Hiroshi Nakatsuji
Department of Synthetic Chemistry and Biological Chemistry, Faculty of Engineering,
Kyoto University, Kyoto
C2nH carbon-chain series radicals have attracted much attention experimentally andtheoretically, partially due to the interesting energy separation feature between the ground andthe first excited states, 2Σ and 2Π: the energy separation and the order of these states vary with
the number of carbon atoms. Attention has also been paid on chlorinated carbon-chain series,C2nCl, to investigate the substitution effects on the energy separation and the nature of theground state structures. After a few contradictory reports, a combined experimental andtheoretical study1 has concluded that the C2Cl, the first member of the C2nCl carbon-chainseries, has a bent structure due to a strong vibronic coupling between the two nearby states.
However, such a consistent conclusion has not been arrived in a similar combined study on
the next member of the C2nCl series, C4Cl.2 A recent study2 concluded that while experimental
results, the rotational transitions and hyperfine coupling constant observed by
Fourier-transform microwave (FTMW) spectroscope, support that the C4Cl radical has theground state corresponding to 2Π and the molecular structure is bent due to the vibronic
coupling, none of the high-level ab initio calculations support the bent structure. We carriedout the present study to solve the existing contradiction between the theory and theexperiment by obtaining the accurate ground state structure.
At first, the C4Cl radical structure was optimized by using BLYP, BPW91, and B3LYP
density functionals to clear a few earlier doubts. Many different basis sets, 6-311G(d),Dunning’s correlation-consistent polarized valence triple-zeta (cc-pVTZ), augmented valence
triple-zeta (aug-cc-pVTZ), quadruple-zeta (aug-cc-pVQZ), and quintuple-zeta (aug-cc-pV5Z)
basis sets, were utilized for this purpose. Finally, we used high-level ab initio
symmetry-adapted cluster (SAC) General-R method3 which includes up to quadruple
excitations for the optimization of the radical structure.
The results obtained using density functionals, especially using pure functionals, showthat the radical has a bent structure. The results derived using BLYP functionals werepresented in the Table 1. The bent structure is well pronounced: the θ(C1C2C3) bond angle iscalculated to be 166.2o at BLYP/Aug-cc-pV5Z level. The linear structure has an imaginary
frequency. The reliability of the results obtained using density functionals were verified bystudying related radicals whose experimental/reliable values are available in the literature.
Finally, the C4Cl ground state structure was optimized using SAC general-R theory withcc-pVTZ basis set and the results were tabulated in Table 2. The table clearly shows thenon-linear nature of the radical structure. It has been confirmed that the results derived forC2Cl radical using the same level of theory agree very well with the established reliableresults. The foregoing results clearly support the conclusions arrived in the earlier
experimental study1 that the C4Cl has the ground state correlating 2Π with a bent molecularstructure. The next nearby lowest state, with 2Σ symmetry, has also been optimized using SAC(SD-R) theory. It is shown to be linear in nature. The vertical transition energies of a few
other excited states have also been calculated.
However, an earlier study2 using MRCI and CCSD(T) predicted a linear C4Cl radicalstructure with 2Σ and 2Π symmetry, respectively. We thoroughly investigated this problem. We
also studied the structure using CCSD/cc-pVTZ level. The CCSD results show that the C4Cl
has a linear structure and it is shown to be global minima. The <S2> value (the degree of spincontamination) was calculated to be 1.14 in contrast to 0.75 for a pure doublet. Inclusion oftriples through CCSD(T) was unable to rectify the spin contamination problem. So it ispossible that the calculated state may not be a pure doublet state and in this context webelieve that earlier CCSD(T) results may not be trusted. It is worth to mention here that thequartet state obtained using SAC(SD-R) theory is shown to have a linear structure.
Table 1. C4Cl structures using BLYP functional with different basis sets. R in Å; θ in degrees.
Parameter

6-311G(d)cc-pVTZ

Aug-cc-pVTZ

Aug-cc-pVQZ

Aug-cc-pV5Z

R(C1C2)
R(C2C3)
R(C3C4)R(C4Cl)θ(C1C2C3)
θ(C2C3C4)
θ(C3C4Cl)

1.285
1.336
1.238
1.636
164.7
178.1
179.6

1.282
1.333
1.237
1.630
166.0
178.8
179.6

1.283
1.332
1.237
1.629
166.1
178.8
179.7

1.282
1.332
1.237
1.625
166.2
178.8
179.8

1.282
1.332
1.237
1.622
166.2
178.8
179.8

Table 2. C4Cl structure at SAC-CI(General-R)/cc-pVTZ level. R in Å; θ in degrees.
Parameter

General-R/cc-pVTZ

R(C1C2)
R(C2C3)
R(C3C4)R(C4Cl)θ(C1C2C3)
θ(C2C3C4)
θ(C3C4Cl)

1.293
1.341
1.223
1.639
172.6
179.6
179.3

The present study using DFT with large basis sets up to
Aug-cc-pV5Z basis set and the high-level ab initio throughthe SAC General-R theory which includes up to quadruple
excitations provide a bent ground state structure for theC4Cl radical. The results overcome the earlier contradiction
between the theory and experiment and clearly support theexperimental conclusion that the C4Cl radical has theground state corresponding to 2Π symmetry but the molecular structure is bent.
References:
1. Sumiyoshi, Y.; Ueno, T.; and Endo, Y. J. Chem. Phys. 2003, 119, 1426.
2. Sumiyoshi, Y.; Katoh, K.; Endo, Y. Chem. Phys. Lett. 2005, 414, 82.
3. Nakatsuji, H. Chem. Phys. Lett. 1991, 177, 331; and also the references therein.