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Society: Washington, DC, 1982; pp 489-515. (53) Katz, J. J.; Shipman, L. L.; Cotton, T. M.; Janson, T. R. In The Porphyrinr; Dolphin, D., Ed.; Academic: New York, 1978; Vol. V, pp 401-458. (54) Barkigia, K. M.; Cbantranupong, L.; Smith, K. M.; Fajer, J. J . Am.
Chem. Soc. 1988. I IO, 7566. (55) Gudowska-Nowak, E.; Newton, M. D.; Fajer, J. J. Phys. Ch” 1990, 94, 5795.
(56) Plato, M.; Mobius,K.; Michel-Beyerle, M. E.; Bixon. M.; Jortner, J.
J . Am. Chem. Soc. 1988, 110, 7279.
Absorption of Supersonically Expanded Cyclopentadiene in the Vacuum-Ultraviolet Region
E.Sominska, V. Kelner, and A. Gedanken* Department of Chemistry, Bar- Ilan University, Ramat Gan 52900, Israel (Received: August 25, 1992)
The absorption spectrum of cyclopentadiene (CPD) was measured over the wavelength region of 1680-1460 A. The spectrum was investigated at room temperature and at low temperatures by supersonically expanding pure cyclopentadiene vapors. In the cold beam spectrum, a few of the absorption bands disappear. The assignment of the observed Rydberg transitions is aided by this simplification of the spectrum. The disappearance of these absorption bands is explained as being caused by the effect identified by Vaida (J. Phys. Chem. 1989,93, 510) as the cluster-induced potential shifts (CIPS). The effect operates in cyclopentadiene contrary to what was observed in methyl iodide and acetone.
introduction The current interest in the spectroscopy of dienes and simple polyenes has concentrated mostly on the low-lying valence transiti0ns.2~This interest was extended recently to the lower Rydberg states by applying multiphoton ionization (MPI) measurements to their study. This technique was employed, for example, in the study of the 3p Rydberg manifold of cyclopentadiene4q5for which the combination of one-photon absorption measurements4 and two-photon MP15 experiments have enabled the successful assignment of all three origins of the manifold. Cyclopentadiene, the simplest stable S-cis diene conformer, reveals the most complicated spectrum of all dienes. This is revealed in the Rydberg 3p region as well as for higher Rydbergs. Price,6 who was the first to measure the higher Rydbergs of cyclic dienes, determined the ionization potential of cyclopentadiene from the Rydberg series of 69 550 cm-’ (8.623eV). The ionization potential measured by Derrick et al.’ using photoelectron spectroscopy differs from Price’s: and a new value of 69 102 cm-’ (8.566 0.01 eV) was given. Following these results, the absorption spectrum of cyclopentadiene was reinterpreted? The proposed Rydberg series converging to the first IP is assigned as a promotion of a la2 (T electron) to an npbz molecular orbital where the observed n’s are 3-8.* A careful check of the proposed series shows that the quantum defect varies from 0.57for n = 3 to 0.38 for n = 8. Two short series converging to the second IP were also assigned in the 1680-1460-A region? Another unique feature which is detected in the Rydberg series of cyclopentadiene is a rich vibrational structure. This structure is not limited to the lower principal quantum numbers but can be observed also at n = 7 and n = 8 members of the Rydberg seriese6S8 The possibility of clarifying somewhat the assignment of the higher Rydberg by cooling the internal degrees of freedom has led us to study the direct absorption of a supersonically expanded beam of cyclopentadiene. According to our observation, the cold spectrum does not show additional features when compared with the static room temperature spectrum. On the contrary, we could observe absorption bands that were washed out in the cold spectrum. Whether the disappearance of these bands is due to their nature as hot bands or due to the interaction with excited states of the dimers will be discussed in this paper.
*
Experimental Section The apparatus which was employed in our experiments has been described el~ewhere.~ Its light source is a modified Hinterreger
lamp through which hydrogen gas flows. To overcome the difficulty of many sharp hydrogen emission lines at X < 1675 A,an automatic gain control feedback unit is employed. This unit adjusts the input voltage to the photomultiplier and keep the dc signal constant at a desired level. The vacuum-UV radiation intersects a pulsed molecular beam at 1-2 cm from the valve. The signal is fed into a lock-in amplifier (Stanford Research SR510), whose reference signal is taken from the pulsed valve system. The static absorption measurements were camed out in a singlebeam mode. The accuracy of the peak positions at A < 1675 A is 1 2 A due to difficulties impaped by the sharp hydrogen emission lines. Cyclopentadiene was cracked at 170 OC from the dimer and was vacuum-distilled before use. The expansion of CPD was carried out from its equilibrium vapor pressure at room temperature or a lower temperature. S i n c eit is well-known that CPD undergoes a slow dimerization at room temperature, the expansion was carried out only from freshly cracked samples. The indication that the concentration of the monomer is close to 10096 is its vapor pressure, which was 280 Torr at -20 OC. A second indication of the high concentration of the monomer is the structured absorption spectrum. Pickett et al.lz failed to observe the absorption spectrum of the vapors of the CPD dimers. They attributed this to its low vapor pressure. They did,12 however, observe the dimer absorption spectrum in solution. The spectrum of CPD dimers in the T T* region was diffuse and structureltss, while the monomer showed a vibrational structure. The CPD molecule is unique in that it undergoes a Diels-Alder condensation and forms a dimer. The CIPS effect, which was discussed above, is caused by the formation of van der Waals molecules.
-
Results .ad Discussion In Figure 1 we present the direct absorption of supersonically expanded cyclopentadiene,as well as the static absorption of the molecule. Since Price’s6 spectrum showed only photographic plates, only the wavelengths of the peaks observed can be compared with the previous results. In general, there is good agreement between the toom temperature spectra, although in the two spectra our peak wavelengths are 1-2 A lower than Price’s. The comparison between the room temperature and the cold spectra shows that few absorption bands are missing from the cold spectrum. On the other hand, the narrowing of the bands in the cold spectra reveals the splitting of the most intense band (1575 A) into two absorption bands. The first absorption peak at 1664 A (60096
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The Journal of Physical Chemistry, Vol. 96, No. 25, 1992 10241
Absorption Spectrum of Cyclopentadiene
TABLE I: Energies d Assignments of the Higher Rydberg T d t i o m of Cycbpent8diene
x (A)
1700
1650
1600
Wavelength
6)
1550
I
1500
Figure 1. Absorption spectra of cyclopentadiene. Static room temperature spectrum (--.), spectra resolution 2.6 A. Absorption spectra of
1664 1644 1634.5 1627 1612 1604" 1597.5" 1591" 1575 1572b 1549.5 1542 1534.5 1527 1519 1507 1502 1497 1492
(cm-I) 60096 60827 61 181 61463 62035 62344 62598 62852 63492 63613 64537 64851 65168 65488 65833 66357 66578 66800 67024
Y
assignment origin 4p 10'0(4p) 7'0(4p) 4'0(4p) 72~(4p)and 10'04'0(4p) 7'04'0(4p) 420(4p) 7'?(4p) origin 5p origin 5p' 7'0(5p) 4'0(Sp) 1020(5p) 720(5p)and origin 6p 6L04'0(5p) 10'0(6p) origin 7p 4',(6p) 1020(6p)
Av (an-') 731 1085 1367 1940 2248 2502 2756 1045 1359 1676 1996 2341 869 1312 1536
"This band is observed only in the room temperature spectrum. bThis band is observed only in the cold spectrum.
supersonically expanded beam: (-) stagnation pressure 280 Torr, spectral resolution 2.6 A; (---) stagnation pressure, 170 Torr, spectral resolution 2.6 A.
cm-') is assigned as an excitation to an allowed 4p Rydberg. This assignment is based on the following arguments. The quantum defect is 6 = 0.51, which is typical for an np Rydberg. The peaks cannot be assigned as a 4s Rydberg because the ns series in cyclopentadiene has an Az symmetry and is therefore forbidden from the ground state, and indeed the 3p manifold is very intense, while the 3s is hardly observed. The last argument is that the vibrational envelope which appears in the 3p4s5is very similar to what is observed in the 4p series. It is worth noting at this stage that a hot band does not appear to the red of the 4p origin. Since the transition to the 4p Rydbergs is electric dipole allowed, we expect the vibrational envelope to present the excitation of any totally symmetric vibrations. Indeed, the spacing of 731, 1085, and 1367 cm-'from the origin would be attributed to the activity of the vlo, v7, and v4 vibrations," respectively. Their ground-state frequencies are 802,1106, and 1500 c m - I , and they are described as the ring deformation, CH wagging, and the C = C stretching modes, respectively. The absorption bands at higher energies are all assigned as combinations or overtones of the active vibrations. The absorption bands at 1604, 1597.5, and 1591 A disappear in the cold spectrum. The immediate explanation would be that these bands are hot bands belonging to the origin of the next member of the Rydberg series, the 5p, whose origin is located at 1575 A. An attempt to assign these bands as hot bands to the 1575 A origin would have yielded red shifts of 1148, 894, and 640 cm-I, respectively. For the 1148- and 894-cm-' vibrations, a reasonable fit to ground-state totallysymmetric vibrations can be found (v," = 1106 cm-I, Y{ = 915 cm-I). However, we reject the idea of assigning the bands as hot bands for the following reasons. Their intensity at room temperature, which is determined by the Boltzmann factor, is much larger than the calculated values for vibrations of 640, 894, and 1148 cm-'. A second reason is that hot bands were not observed, either for the 4p Rydberg or for the ion, and since there is a striking similarity of the vibrational structure of the 4p, Sp Rydbergs and the ion, one would expect it to be manifested in the hot region as well. We therefore conclude that these bands are built up on the 4p origin and represent spacings of 2248,2502, and 2758 cm-' above the origin. Their assignment is shown in Table I. It is worth noting that the vibrational envelope which is stretched over 3300 cm-' for the 3p Rydberg is somewhat smaller at the 4p level. The reason for the disappearance of these bands upon cooling is related to the effect identified by Vaida et al. as Wuster-induced potential shifts". However, this effect will be used here in the opposite way to that previously used. In Vaida's case the excited
2W
a
W
z W
J
5 Iz
W
IO
n
BOND LENGTH
Figure 2. Schematic representation of the perturbed and unperturbed potential energy curves. The solid lincs represent the unperturbed potential curves of the bound and repulsive excited states (the bound state is a 4p Rydberg state). The dashed lines are the corresponding statea effected by the CIPS mechanism.
bound state of the monomer is crossed by a dissociative state. The presence of a cluster influences both the bound and the dissociative states. In her picture the bound state is more stabilized than the dissociative state. In CPD the opposite occurs, viz., the presence of CPD dimers has a bigger effect on the unbound state and causes the crossing of the attractive potential at 2000 cm-'above the J = 0 level instead of 3000 cm-'in the bare monomer. In Figure 2 a schematic representation of the bound and repulsive states is depicted. The nature of the dissociative state, which is a p parently more sensitive to the presence of another molecule, is a higher Rydberg state. The excited electron orbiting at a very long distance from the molecular plane will be perturbed more strongly by environmental molecules. The CPD molecule is unique because on its expansion it can either form van der Waals molecules or undergo a Diels-Alder condensation and form a dimer. The aforementioned CIPS effect is caused by the van der Waals molecules. The Diels-Alder reaction, whose rate is slow at room temperature, does not occur at the low temperatures reached in the supersonic expansion. The indication that the condensation is minimal is the structured spectrum which is almost identical to the static absorption spectrum of CPD. The next member of the Rydberg series, the SP, appears in the room temperature spectrum at 1575 A and upon cooling reveals a splitting of about 150 cm-I. This splitting is attributed to the appearance of two origins, the Bl (5py) and the B2 (5pJ. This splitting is 460 cm-' for the n = 3 Rydberg and is not observed for the n = 4. Whether it is unobserved in the n = 4 Rydberg because the bands are more strongly predissociated or it is indeed
J. Phys. Chem. 1992,96, 10242-10246
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smaller than the n = 5 remains an open question. Galasso," who has correctly given the size and ordering of the 3p origins, has not provided any information regarding the dependence of this splitting on the principal quantum number, and it is difficult to substantiate our interpretation by theoretical results. Since the splitting is observed only under the cooling condition and was not observed men at higher resolution, the possibility that this feature is due to a transition associated with a dimer cannot be ruled out. Thus, the narrowing of the rotational envelope and the formation of a dimer am responsible for this absorption band The remaining question is, however, why such a splitting is not observed for the n = 4 and higher Rydberg member. The possibility that the +15OCm-l band is a sequence band or is due to a drastic change in the vibrational frequency is ruled out for obvious reasons. However, it is worth noting that if this band is indeed associated with a dimer its origin is blueshifted compared to the monomer. The higher members of the Rydberg series belong to the same np manifold as the lower ones. The span of the vibrational structure decreases with the size of the Rydberg orbital and for higher n's it diminishes to about lo00 cm-'. We now turn to the nature of the active vibrations. Unlike the 3p manifold where non-totally symmetric vibrations were assigned3 as active in the spectrum (with Av = l), here all the active vibrations in the higher Rydbergs are totally symmetric. This indicates that the configuration of the molecules in these higher states is more similar to the ground state. The configuration of the molecule in the higher Rydberg state can also be deduced from the active vibrational modes and the similarity to the vibration structure of the ion. From the distribution of the absorption intensities it seems that the C==C bonds in the higher Rydbergs and ionic states are slightly elongated in respect to the ground state. Our results concur with Derrick's fndings that the Rydberg formula is not obeyed very well for the np series, according to the reported ionization potential.*
Conclusion The higher energy excitations of cyclopentadiene show a well-developed Rydberg series. The ionization potential that is obtained from the best fit to the Rydberg series differs from the ionization potential measured photoelectronically. A rich vibrational structure is observed through the Rydberg series. Evidence is provided for the influence of van der Waals molecules on the spectrum of the monomer. The disappearance of a few vibrational lines in the cold spectrum is accounted for by the application of the CIPS effect. Acknowledgment. We would like to thank the Ministry of Absorption's Center for Science Absorption for their support of Dr. E. Sominska during the course of this research. Registry No. Cyclopentadiene, 542-92-7.
References and Notes (1) Vaida, V.; Donaldson, D. J.; Sapers, S. P.; Naaman, R.;Child, M.S. J . Phys. Chem. 1989,93, 510. (2) Hudson, B. S.;Kohler, B. E.; Schulten, K.In Excited Stares; Lim, E. C.,'Ed.; Academic: New York, 1982; Vol. 6, p 1. (3) Docring, J. P.; Sabljic, A.; McDiarmid, R.J. Phys. Chem. 1985,83,
2147.
(4) McDiarmid, R.;Sabljic, A. J. Phys. Chem. 1991, 95, 6455. (5) Sabljic, A.; McDiannid, R.;Gcdanken, A. J. Phys. Chem. 1992, 96, 2442. (6) Price, W. C.; Walsh, A. D. Proc. R. Soc. London Ser. A 1941,179, 101. (7) Edquist, 0.;Lindholm, E.; Selin, L. E.; kbrink, L. Phys. Scr. 1970, I, 25. (8) Derrick, P.J.; &brink, L.; Edquist, 0.;Jonsson, B. 6.; Lmdholm, E. Inr. J. Mass.Specrrom. Ion. Phys. 1971, 6, 203. (9) Maulem, R.;Gedanken, A. Chem. Phys. Lerr. 1992, 188, 383. (10) Castcllucci,E.; Mamli, P.; Fortunato, B.;Gallinela, E.;Miront, P. Spectrochim. Acra 1975, ZIA, 451. (11) Galasso, V . Chem. Phys. 1991,153, 13. (12) Pickett, L. W.; Paddock, E.; Sacklcr,E. J. Am. Chem. Soc. 1941,63, 1073.
A Density.FunctionaiInvestigation of the Structure of Sulfur Chlorides SCI, and SCI,' (/I = 1-6) Cennady L.Gutsev Institute of Chemical Physics at Chernogolovka, Chernogolwka, Moscow Region 142432 (Received: February 3, 1992)
Calculations of the electronic and geometrical structure of the ground and first excited states of the title compounds are camed out within a local spin density functional approach (LSDA). Fragmentation energies, vertical electron affinities (EA), and adiabatic EAs of the neutral sulfur chloridca as well as fmt ionization potentials (FIP) of the anions are evaluated. In the calculations of the and energies of fragmentation through diftmnt decay channels, the nonlocal gradient c " to the LSDA exchange are included. According to the results of the calculations the SCl< anion is stable toward both the loss of an extra electron and dissociation, although its neutral parent appears to be unstable. All the sulfur chlorides, SCl,, are less stable than their congener SF, ( n = 1-6). The same is true for the anions of both series. The adiabatic EA of the sulfur chlorides are high enough, and beginning with SC13it prevails over the EA of the most electronegative atoms, i.e. halogens. The results of calculations are in good accordance with scare experimental data available for the neutral sulfur chlorides.
Inh.odpetion Contrary to the series SF, (n = l d ) , which is quite well studied by both experimental and theoretical methods, the valent isoelectronic sulfur chlorida series has received much leas attention. Experimentally, it seems to be related with the less stability of the sulfur chlorides, which d d a t e at relatively low temperatures.' For theoretical calculations by traditional ab initio and post-HF methods, these compounds present a challenge due to the necessity of using large basis set expansions.
The experimental geometry is known for the ground states of SClz and SC12s-5 only. The existence of SC13has been suggested in the SC& (SC1331tCl) salt.' Neither SC1, nor SCll have been The highest sulfur chlorida SCls invatigated in the gas phaa~.'.~ and SCb have not been observed, though their congeners SFs and SF6 are known to be quite stable chemical compounds.' The electronic structure of the sulfur hexachloride has been investigated within the multiple-scattereds and discrete variat i ~ n a l ~X,-methods. .'~ These calculations have been carried out
0022-3654/92/2096- 10242$03.00/0 Q 1992 American Chemical Society
