3P076
Electronic spectra and photodissociation of vinyl chloride and its derivatives
Sundaram Arulmozhiraja,* Ryoichi Fukuda, Masahiro Ehara, and Hiroshi Nakatsuji
Department of Synthetic Chemistry and Biological Chemistry, Faculty of Engineering, Kyoto
University, Kyoto
Vinyl chloride (C2H3Cl), VC, is an atmospheric pollutant. Evidence showed that VC
causes cancer in human beings. Because of this, understanding of photochemistry andspectroscopy of VC is important. There were a few studies in this direction in the past
however there are many questions yet to be answered. For example, very recently, π,3s state
has been assigned as the optically bright state.1,2 It is contradictory to the earlier assumptionthat π→π* (transition from C=C bonding to its anti-bonding orbital. For orbital character see
Table 1) excitation is the main electronic transition in VC. Another thing is that though theoptically bright state has generally been thought to be predissociated by the repulsive nσ*(transition from Cl non-bonding to C–Cl anti-bonding orbital), recently it has been proposedthat dissociation in VC happens through πσ* state and not through nσ* state.3 These
foregoing facts reveal that a thorough study to clarify the doubts and assumptions is necessary.
Hence we studied VC to (1) obtain its absorption spectrum by using SAC-CI and to assign theexcitations including π→σ*, π→π*, n→σ*, and n→π* and (2) understand the C–Cl bondbreaking mechanism by studying the potential energy surfaces (PES) of its excited states.
The vertical absorption spectra were obtained by using SAC-CI. Various basis sets up toAug-cc-pVTZ with necessary Rydberg functions on carbon and chlorine atoms were utilizedfor this purpose. PES were obtained mainly using D95(d,p) basis set.
We show the character of n, π, π*, and σ* in Table 1. A few vertical excitation energies
with oscillator strengths along with available experimental and theoretical results aretabulated in Table 2. Our results agree well with the experimental results. Particularly, theenergy (6.96 eV) obtained for the π→π* excitation agrees well with the experimental values(6.7–6.9 eV). These results and the oscillator strengths obtained here clarify that the mainexcitation in VC is the ππ* excitation [with a small contribution of out of plane n(Cl lone pair) inthe ππ* excitation]. The basis sets used in the previous studies were insufficient to describe
the π→π* excitation properly. Other valence excitation (π→σ*, n→σ*, and n→π*) energies
and Rydberg excitation (π→3s, π→3pσ, and π→3pπ) energies were also obtained.
PES (see figure 1 and 2) obtained along the C–Cl bond length clearly shows the formationof avoided crossing between bright ππ* and nσ* states at around 1.85Å. This reveals that theexcited VC transfers from the bound ππ* state to the dissociative nσ* state through levelcrossing. Meanwhile, PES obtained along C=C torsion provides interesting results that thelow lying two states, πσ* and ππ*, mix from the very beginning of torsion. As the twistingangle increases, the mixing becomes stronger and stronger. Hence, C–Cl dissociation can also

takes place through πσ* state after mixing. The study on 2-D PES incorporating both C–Clbond length and C=C twisting is underway.
Table 1. Nature of π, π∗, σ∗, and n orbitals.
C–Cl
NatureC=C
anti-bondinganti-bondinganti-bonding∗anti-bondinganti-bonding

n-bondingσbondingnσ*

10Table 2. Vertical excitation energy (∆E) and oscillator
strength (f) of VC.
NatureSAC-CI
Past studies∆E (eV)
fTheoryExpt.
π,3s/σ*6.81
0.0111
6.682
π,π*
6.96
0.3274
7.46,2 7.403 6.7-6.94
π,σ*/3s
6.99
0.0035
7.27,2 7.263
7.48
0.00067.482π,3pσxπ,3pσy
7.70
0.0102
7.562
n,π*
7.82
0.00057.903
π,3pπ
7.890.0098
7.825n,σ*8.200.0012
8.503

10(a)

11nπ*

Relative energy (eV)

97nσ*

πσ*

6ππ*+πσ*6π*: C=Cσ*: C–Cl4n: Cllone pair

1A´

005431A2´

103040Fig. 1. PES along C=C twisting(b)

´4A

˝3A

´3A8

˝2A

76

´2A

˝

1A

543

´1A

2

120φ(C=CH2) (degrees)

9ππ*

8nπ*+nσ*

8 ππ*
πσ*

210Relative energy (eV)

11πσ*nσ*nπ*

Relative energy (eV)ππ∗1001.6

1.8

2.0

2.2

2.4

2.6

R(C-Cl) (Å)

2.8

3.0

3.2

3.4

1.6

1.8

2.0

2.2

2.4

2.6

2.8

3.0

3.2

3.4

R(C-Cl) (Å)

Fig. 2. PES along C–Cl bond (a) diabatic PES of πσ*, ππ*, nπ*, and nσ* states(b) adiabatic PES of a few low-lying states

References:
1. Chang, J.-L., J. Chem. Phys. 2005, 122, 194321.
2. Chang, J.-L. and Chen, Y.-T., J. Chem. Phys. 2002, 116, 7518.
3. Tonokura, K.; Daniels, L. B.; Suzuki, T.; and Yamashita, K., J. Phys. Chem. A 1997, 101,7754.
4. See the references in Reference 1.
5. Williams, B. A. and Cool, T. A., J. Phys. Chem. 1993, 97, 1270.