The ultraviolet photochemistry of phenylacetylene and the enthalpy of formation of 1,3,5-hexatriyne

Article abstract of DOI:10.1021/ja0017312

The ultraviolet photochemistry of phenylacetylene was studied in a molecular beam at 193 nm. The only primary photofragments observed were HCCH (acetylene) and C6H4. Some of the C₆H₄ molecules were found to decompose to 1,3,5-hexatriyne and molecular hydrogen. An enthalpy of formation of ΔH_f ≤ 160 ± 4 kcal mol^-1 was determined for 1,3,5=hexatriyne from the energetic threshold for this process. This experimentally determined value agrees well with our ab initio calculations performed at the G2 level of theory. Angular distribution measurements for the HCCH + C₆H₄ channel yielded an i sotropic distribution and were attributed to a long-lived intermediate and ground-state dissociation. An exhaustive search yielded no evidence for the phenyl + ethynyl or, the atomic hydrogen elimination channels even though these were observed in the pyrolytic studies of phenylacetylene.

Full text of DOI:10.1021/ja0017312

J. Am. Chem. Soc. 2001, 123, 671-676
671
The Ultraviolet Photochemistry of Phenylacetylene and the Enthalpy
of Formation of 1,3,5-Hexatriyne
Osman Sorkhabi, Fei Qi, Abbas H. Rizvi, and Arthur G. Suits*,†
Contribution from the Chemical Sciences DiVision, Ernest Orlando Lawrence Berkeley National
Laboratory, Berkeley, California 94720
ReceiVed May 18, 2000
Abstract: The ultraviolet photochemistry of phenylacetylene was studied in a molecular beam at 193 nm. The
only primary photofragments observed were HCCH (acetylene) and C6H4. Some of the C6H4 molecules were
found to decompose to 1,3,5-hexatriyne and molecular hydrogen. An enthalpy of formation of ∆Hf e 160 (
4 kcal mol-1 was determined for 1,3,5-hexatriyne from the energetic threshold for this process. This
experimentally determined value agrees well with our ab initio calculations performed at the G2 level of
theory. Angular distribution measurements for the HCCH + C6H4 channel yielded an isotropic distribution
and were attributed to a long-lived intermediate and ground-state dissociation. An exhaustive search yielded
no evidence for the phenyl + ethynyl or the atomic hydrogen elimination channels even though these were
observed in the pyrolytic studies of phenylacetylene [Hofmann, J.; Zimmermann, G.; Guthier, K.; Hebgen, P.;
Homann, K. H. Liebigs Ann. 1995, 631, 1995. Guthier, K.; Hebgen, P.; Hofmann, K. H.; Zimmermann, G.
Liebigs Ann. 1995, 637, 1995].
Introduction
phenylacetylene, among these were the free radicals phenyl,
phenylvinyl, and o-, m-, and p-ethynylphenyl.11 Assuming that
these transient species are primary products, these observations
indicate the presence of several atomic hydrogen elimination
channels. Pyrolytic studies, however, provide limited and at
times ambiguous results for the primary processes due to the
presence of multiple collisions and subsequent competition with
secondary and tertiary chemical reactions. It is therefore
desirable to unambiguously determine the identity of the primary
products from the ground-state unimolecular dissociation of
phenylacetylene.
Phenylacetylene is an aromatic molecule with an unsaturated
side group and is commonly used as a precursor for the synthesis
of the polymer polyphenylacetylene.1-6 Like other cyclic
aromatic hydrocarbons, phenylacetylene is believed to be an
important contributor to the formation of polycyclic aromatic
hydrocarbons (PAH) in combustion and in interstellar space.7-13
In the initial pyrolytic studies of phenylacetylene, acetylene and
benzene were identified as end products and it was postulated
that further reactions involving them led to cyclization reactions
and PAH formation.7-9 Subsequently, Hofmann et al. carried
out a detailed analysis of the reaction products from the pyrolysis
of phenylacetylene.10 PAHs with up to four rings were observed
and their results confirmed that pyrolysis of phenylacetylene
could indeed lead to PAH formation. In a collaborative study
with Hofmann et al., Zimmermann and co-workers identified
several transient products for the unimolecular dissociation of
We have carried out an ultraviolet (UV) photochemical
investigation of phenylacetylene using photofragment transla-
tional spectroscopy (PTS) on the Chemical Dynamics Beamline
at the Advanced Light Source in Berkeley. This investigation
is part of an ongoing effort to understand the UV photochemistry
of large polyatomic aromatic molecules.14,15 In such systems,
dissociation occurs mostly via the ground state potential energy
surface (PES) following internal conversion from the initially
prepared excited electronic state.14-16 This allows for the study
of the unimolecular dissociation on the ground-state PES under
collision free conditions. Therefore, the photochemistry of large
polyatomic molecules using PTS is a powerful technique for
the determination of the primary processes in the unimolecular
dissociation of these molecules in an unambiguous manner.
* Address correspondence to this author. E-mail: arthur.suits@sunysb.edu.
† Permanent address: Department of Chemistry, State University of New
York, Stony Brook, NY 11794, and Chemistry Department, Brookhaven
National Laboratory, Upton, NY 11973.
(1) Simionescu, C. I.; Percec, V. Prog. Polym. Sci. 1982, 8, 133.
(2) Masuda, T.; Higashimura, T. AdV. Polym. Sci. 1987, 81, 121.
(3) Furlani, A.; Licoccia, S.; Russo, M. V.; Camus, A.; Marisch, N. J.
Polym. Sci. Polym. Chem. Ed. 1986, 24, 991.
(4) Shivasubramaniam, V.; Sundararajan, G. J. Mol. Catal. 1991, 65,
205.
(5) Lachmann, G.; Duplessis, J. A. K.; Dutoit, C. J. J. Mol. Catal. 1990,
58, 143.
Several vibronic bands of phenylacetylene have been observed
in the UV and are attributed to excitation of the ring electrons
with vibrational coupling to the C-C stretch of the acetylenic
moiety.17,18 These features were also revealed in the theoretical
(6) Zhang, J.; Fu, K. Polym. Bull. 1994, 32, 63.
(7) Hopf, H.; Musso, H. Angew. Chem. 1969, 81, 704.
(8) Hopf, H.; Musso, H. Angew Chem. Int. Ed. 1969, 8, 680.
(9) Herzler, J.; Frank, P. Ber. Bunsen-Ges. Phys. Chem. 1992, 96, 1333.
(10) Hofmann, J.; Zimmermann, G.; Guthier, K.; Hebgen, P.; Homann,
K. H. Liebigs. Ann. 1995, 1995, 631.
(14) Sorkhabi, O.; Qei, F.; Rizvi, A. H.; Suits, A. G. J. Chem. Phys.
1999, 111, 100.
(11) Guthier, K.; Hebgen, P.; Homann, K. H.; Hofmann, J.; Zimmermann,
G. Liebigs. Ann. 1995, 1995, 637.
(12) Frenklach, M.; Feigelson, E. D. Astrophys. J. 1989, 341, 372.
(13) Goeres, A.; Keller, R.; Sedlmary, E.; Gail, H. P. Polycyclic Aromat.
Compd. 1996, 8, 129.
(15) Qei, F.; Sorkhabi, O.; Rizvi, A. H.; Suits, A. G. J. Phys. Chem.
1999, 103, 8351.
(16) Blank, D. A.; North, S. W.; Lee, Y. T. Chem. Phys. 1994, 187, 35.
(17) Swiderek, P.; Gootz, B.; Winterling, H. Chem. Phys. Lett. 1998,
285, 246.
10.1021/ja0017312 CCC: $20.00 © 2001 American Chemical Society
Published on Web 01/09/2001

672 J. Am. Chem. Soc., Vol. 123, No. 4, 2001
Sorkhabi et al.
sociation of phenylacetylene. In recent years, much effort has
gone into the determination of the thermochemistry and heats
of formations of diacetylene, triacetylene, and other unsaturated
hydrocarbons.24-28 Our result for the heat of formation of
triacetylene is the first experimentally determined value for this
quantity.
Experimental Section
All experiments were conducted at beamline 9.0.2.1 of the Advanced
Light Source with a rotatable source molecular beam machine that was
described in detail in an earlier publication.29 This machine can be used
to study reactive scattering30,31 and photodissociation dynamics.14,32 In
the present photodissociation experiment, helium was bubbled through
a phenylacetylene sample at room temperature. At this temperature and
a total pressure of 800 Torr, a 1% molecular beam of phenylacetylene/
He was generated. This mixture was fed through a pulsed valve (General
Valve) and expanded from a nozzle heated to ∼100 °C in a differentially
pumped source region and into the main chamber. The pressure in the
source chamber was maintained at 2 × 10-4 Torr with the beam on.
The molecular beam was collimated with two skimmers (0.03” and
0.02”) and its velocity and speed ratio were measured by using either
a chopper wheel or a laser hole-burning method. Both methods gave
consistent results, and typical values for the velocity and speed ratio
were found to be 1870 ( 40 m s-1 and 11 ( 2, respectively.
The photolysis laser was an ArF excimer (193 nm, Lambda Physik
LPX 220i), focused to a spot 2 × 4 mm and aligned perpendicular to
the plane containing the molecular beam and detector axis, on the axis
of rotation of the molecular beam source. Photofragments entering the
triply differentially pumped detector region (9 × 10-11 Torr) were
photoionized 15.2 cm from the interaction region by using tunable
synchrotron radiation. The characteristics of the light source are
discussed in detail elsewhere and include an intensity of 1016 photons/s
(quasi-continuous), an energy bandwidth of 2.2%, and a cross section
in the probe region of 0.2 × 0.1 mm.33 The tunability of the light source
allowed for selective ionization of products and very low background
counts. The photoionized products were mass selected by using a
quadrupole mass filter and the ions were counted with a Daly ion
counter.34 Time-of-flight of the products was measured with a multi-
channel scaler (EG&G Ortec Turbo MCS). The bin width for the MCS
was fixed at 1 µs for the measurements reported here. Timing sequences
for the laser, pulsed valve, and the MCS were maintained by a digital
delay generator (Stanford Research Systems, Inc. Model 535). Eight
quartz plates fixed at Brewster’s angle were used for the polarization
measurements to give 87 ( 5% polarized light. To rotate the angle of
polarization with respect to the detector axis, a half-wave plate was
used (Karl Lambrecht). Phenylacetylene (98%) was obtained from
Aldrich and used without further purification.
Figure 1. Energy diagram for the photodissociation of phenylacetylene
at a wavelength of 193 nm. Two possible primary pathways leading to
m/e 76 + m/e 26 momentum matched fragments are shown. The path
leading to HCCH + A or B is higher in energy than the one leading to
HCCH + C. In this figure, A, B, and C refer to (Z)-3-hexene-1,5-
diyne, (E)-3-hexene-1,5-diyne, and benzyne, respectively. The energet-
ics were calculated using literature values for ∆Hf of A, B, C, and
acetylene. The energetic threshold for the secondary process (i.e. the
one leading to C6H2 + HCCH + H2) was determined from the
experimentally measured center-of-mass translational energy distribution
of Figure 4. The energetic cutoff for the secondary decomposition is
also shown along with the P(E) of Figure 4.
investigation of Narayanan et al.19 The UV spectroscopy and
vibrational structure of the ground and excited states of
phenylacetylene have been well-established yet little is known
about its photochemistry.19-23 We have carried out an investiga-
tion of the photodissociation of phenylacetylene at 193 nm to
gain more insight into the primary processes involved and to
extend our study of polyatomic aromatic molecules to phenyl-
acetylene. At 193 nm, the vibronic transitions involve those of
1
1
1
1
the D A1 r X A1 or E A1 r X A1 systems.17 At this
wavelength, we identified one primary photochemical pathway:
C6H5-CCH + hV193 f C6H4 + HCCH
∆Hrxn ) -37.9 kcal mol-1 (1)
The UV photochemistry of phenylacetylene is surprisingly
simple. Reaction 1 produces two closed-shell molecules: acety-
lene (HCCH) and (E)-3-hexene-1,5-diyne, (Z)-3-hexene-1,5-
diyne (C6H4), and/or benzyne. For a fraction of the C6H4
products, the amount of energy available in this process is
sufficient to induce a secondary reaction:
Ab initio calculations were performed with either the Q-Chem
software package35 run on a LINUX based 500 MHz personal computer
C6H4* f C6H2 + H2
(2)
(24) Ball, D. W. J. Mol. Struct. Theochem. 1997, 417, 107.
(25) Nicholaides, A.; Radom, L. Mol. Phys. 1996, 88, 759.
(26) Schulman, J. M.; Disch, R. L. J. Mol. Struct. Theochem. 1992, 91,
173.
(27) Curtiss, L. A.; Raghavachari, K.; Redfern, P. S.; Rassolov, V.; Pople,
J. A. J. Chem. Phys. 1998, 109, 7764.
(28) Kiefer, J. H.; Sidhu, S. S.; Kern, R. D.; Xie, K.; Chen, H.; Harding,
L. B. Combust. Sci. Technol. 1992, 82, 101.
(29) Yang, X.; Lin, J.; Lee, Y. T.; Blank, D. A.; Suits, A. G.; Wodke,
A. M. ReV. Sci. Instrum. 1997, 68, 3317.
(30) Hemmi, N.; Suits, A. G. J. Chem. Phys. 1998, 109, 5338.
(31) Blank, D. A.; Hemmi, N.; Suits, A. G.; Lee, Y. T. Chem. Phys.
1998, 231, 261.
where C6H4* is internally excited C6H4 and C6H2 has been
characterized as 1,3,5-hexatriyne which is also referred to as
triacetylene in the literature. From the energetic threshold for
reaction 2, an upper limit of 160 ( 4 kcal mol-1 for the enthalpy
of formation of 1,3,5-hexatriyne was obtained. Quantum ab initio
calculations at the G2 level of theory agree well with this value.
Figure 1 shows the energy diagram for the 193 nm photodis-
(18) Leopold, D. G.; Hemley, R. J.; Vaida, V.; Roebber, J. L. J. Chem.
Phys. 1981, 75, 4758.
(19) Narayanan, K.; Chang, G. C.; Shieh, K. C.; Tung, C. C.; Tzeng,
W. B. Spectrochem. Acta A 1996, 52, 1703.
(20) King W.; So, S. P. J. Mol. Spectrosc. 1971, 38, 543.
(21) Singh, H.; Laposa, J. D. J. Luminesc. 1972, 5, 32.
(22) Powers, D. E.; Hopkins, J. B.; Smalley, R. E. J. Chem. Phys. 1981,
74, 5971.
(32) Sun, W. Z.; Yokoyama, K.; Robinson, J. C.; Suits, A. G.; Neumark,
D. M. J. Chem. Phys. 1999, 110, 4363.
(33) Heimann, P. A.; Koike, M.; Hsu, C. W.; Evans, M.; Ng, C. Y.;
Blank, D. A.; Yang, X. M.; Flaim, C.; Suits, A. G.; Padmore, H. A.; Lee,
Y. T. ReV. Sci. Instrum. 1997, 68, 1945.
(34) Lee, Y. T.; McDonald, J. D.; LeBreton, P. R.; Herschbach, D. R.
ReV. Sci. Instrum. 1969, 40, 1402.
(23) Chia, L.; Goodman, L. J. Chem. Phys. 1982, 76, 4745.

The UltraViolet Photochemistry of Phenylacetylene
J. Am. Chem. Soc., Vol. 123, No. 4, 2001 673
or the Gaussian 98 software package36 run on the National Energy
Research Scientific Computing Center’s Cray supercomputer parallel
vector processor cluster (http://www.nersc.gov). Calculations were
carried out at several levels of theory for the ∆Hf of acetylene,
diacetylene, and triacetylene. Of all the calculations performed, the G2
results were found to be the most accurate and the results will be
provided in the next section.
Results and Discussion
Signal was observed for m/e 26, 74, and 76 corresponding
to HCCH, C6H2, and C6H4, respectively. The signal for each of
these fragments was measured as a function of laser pulse energy
and data were recorded in the single-photon regime. In the
presentation of the data, the open circles represent the experi-
mental data and the solid lines represent the best fit to the data.
An energy diagram for the photodissociation of phenylacetylene
at 193 nm is provided in Figure 1.
Photochemistry. The measured TOF spectra for the primary
photoproducts are shown in Figures 2 and 3. To convert from
the laboratory-frame TOF distribution (such as those in Figure
2) to the center-of-mass translational energy distribution, P(E),
the forward convolution method was applied to the data.37,38
The best fits to the C2H2 data (solid lines in Figures 2 and 3)
resulted in the P(E) shown in Figure 4a. This distribution has
an average translational energy of 12 kcal mol-1 and extends
out to a maximum value of about 40 kcal mol-1. If C2H2 and
C6H4 are momentum matched photofragments, then the same
P(E) should fit both data sets. Attempts were made to fit the
C6H4 data with the distribution of Figure 4a; however, the slow
C6H4 fragments could not be accounted for (refer to Figure 3).
To get the best fit for the C6H4 fragment, the slower portion of
the P(E) of Figure 4a had to be removed. This depletion of the
distribution suggests the presence of a secondary dissociation
process since the slower fragments will have enough internal
energy to induce this process [see reaction 2]. Indeed, as seen
in the TOF spectrum of Figure 5, the slow portion of the
distribution appears as C6H2; that is the only available secondary
decomposition product. The translational energy distribution
obtained for C6H2 is shown in Figure 4c. The P(E)’s for the
C6H4 and C6H2 necessarily sum to give the C2H2 distribution
of Figure 4a. This is true in this case because even though the
C6H2 is a secondary fragment, it is only necessary to fit the
primary step in that process since there is negligible recoil
imparted to the heavy fragment in the second step. The P(E)
shown in Figure 4c for C6H2 is, in fact, a primary P(E) for
those fragments that go on to undergo secondary decomposition.
These results indicate that the primary C6H4 fragments formed
Figure 2. Time-of-flight spectra of m/e 26 measured at laboratory
angles of 15° and 30°. Both of spectra were measured with a
photoionization probe energy of 12.5 eV. Open circles represent the
experimental data and the solid line represents the forward convolution
fit to the data.
with sufficient internal energy decompose to C6H2 and H2. The
energetics of this process will be discussed in more detail in
the next section.
The probable isomers for C6H4 are (E)-3-hexene-1,5-yne, (Z)-
3-hexene-1,5-yne, and benzyne. Therefore, reaction 1 may be
rewritten as:
C6H5-CCH + hV193 f (E)-3-hexene-1,5-yne + HCCH
∆Hrxn ) -38 kcal mol-1 (1a)
f (Z)-3-hexene-1,5-yne + HCCH
∆Hrxn )-38 kcal mol-1 (1b)
(35) White, C. A.; Kong, J.; Maurice, D. R.; Adams, T. R.; Baker, J.;
Challacombe, M.; Schwegler, E.; Dombroski, J. P.; Ochsenfeld, C.; Oumi,
M.; Furlani, T. R.; Florian, J.; Adamson, R. D.; Nair, N.; Lee, A. M.;
Ishikawa, N.; Graham, R. L.; Warshel, A.; Johnson, B. G.; Gill, P. M. W.;
Head-Gordon, M. Q-Chem, Version 1.2; Q.-Chem, Inc.: Pittsburgh, PA,
1998.
(36) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb,
M. A.; Cheeseman, J. R.; Zakrzewski, V. G.; Montgomery, J. A., Jr.;
Stratmann, R. E.; Burant, J. C.; Dapprich, S.; Millam, J. M.; Daniels, A.
D.; Kudin, K. N.; Strain, M. C.; Farkas, O.; Tomasi, J.; Barone, V.; Cossi,
M.; Cammi, R.; Mennucci, B.; Pomelli, C.; Adamo, C.; Clifford, S.;
Ochterski, J.; Petersson, G. A.; Ayala, P. Y.; Cui, Q.; Morokuma, K.; Malick,
D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Cioslowski, J.;
Ortiz, J. V.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi,
I.; Gomperts, R.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.;
Peng, C. Y.; Nanayakkara, A.; Gonzalez, C.; Challacombe, M.; Gill, P. M.
W.; Johnson, B.; Chen, W.; Wong, M. W.; Andres, J. L.; Gonzalez, C.;
Head-Gordon, M.; Replogle, E. S.; Pople, J. A. Gaussian 98, Revision
A.5,Gaussian 98, ReVision A.5; Gaussian, Inc.: Pittsburgh, PA, 1998.
(37) Wodtke, A. M. Ph.D. Thesis, University of California at Berkeley,
1986.
f benzyne + HCCH ∆Hrxn )-62 kcal mol-1 (1c)
Often the center-of-mass translational energy distribution of
photofragments extends out to the maximum available energy,
so these limits are sometimes used both to obtain thermochemi-
cal information and to identify likely product isomers.16,32 The
distribution which best fits the C6H4 data extends out to about
40 kcal mol-1 and matches well with the available energy for
reactions 1a and 1b. Given this fact alone, one may be tempted
to identify the likely isomers of C6H4 as (E)-3-hexene-1,5-diyne
and (Z)-3-hexene-1,5-diyne. For certain molecules, measurement
of their photoionization efficiency (PIE) spectra can further help
in assigning the identity of a given molecule.14,32 PIE spectra
may be conveniently measured by tuning the VUV undulator
radiation while monitoring the integrated ion signal. The PIE
spectrum of C6H4 was measured and the ionization threshold
(38) Zhao, X. Ph.D. Thesis, University of California at Berkeley, 1988.

674 J. Am. Chem. Soc., Vol. 123, No. 4, 2001
Sorkhabi et al.
Figure 4. The fits shown in Figures 2 and 3 resulted in the center-
of-mass translational energy distributions, P(E), shown here: (a) the
P(E) used to fit m/e 26 (HCCH) fragment; (b) the P(E) used to fit the
m/e 76 (C6H4) product in Figure 3; and (c) the P(E) used to fit the m/e
74 (C6H4) product in Figure 5.
Figure 3. Time-of-flight spectra of m/e 76 measured at laboratory
angles of 10°, 15°, and 20°. These spectra were measured with a
photoionization probe energy of 10 eV. A MgF2 optical filter was used
to filter all photons with energies of 11.2 eV or higher. Open circles
represent the experimental data and the solid line represents the forward
convolution fit to the data.
and the anisotropy parameter, â, was determined as described
in a previous publication.14 A â parameter of nearly zero was
obtained and most likely implies a lifetime for the intermediate
at least as long as several rotational periods. The observation
of a â parameter of zero, closed shell products, and an
exponentially falling translational energy distribution all suggest
that the dissociation may occur via the ground-state surface after
internal conversion for the 193-nm photodissociation of phen-
ylacetylene. The UV excitation of phenylacetylene involves a
π f π* transition and the delocalized electronic system extends
to the acetylenic moiety.17-19 Since this transition involves the
excitation of an electron from a bonding to an antibonding
orbital, a decrease in the C-C bond order of the conjugated
system will lead to a geometry change in the excited state. This
geometry change could enhance the probability of internal
conversion to high-lying vibrational levels of the ground
electronic state of phenylacetylene (due to a larger Franck-
Condon overlap integral). Our data indicate the presence of
internally excited C6H4 which could arise from the dissociation
of vibrationally excited, “hot”, phenylacetylene. Although a
detailed mechanism of the dissociation is beyond the scope of
the present study, the data suggest the following picture. After
excitation of phenylacetylene by the 193-nm photon to the D
1A1 or E 1A1 states, internal conversion to the ground electronic
was found to occur at about 9 eV. Ionization potentials (IP) for
(E)-3-hexene-1,5-yne, (Z)-3-hexene-1,5-yne, and benzyne are
9.07 ( 0.02,39 9.10 ( 0.02,39 and 9.03 ( 0.05 eV,40 respectively.
Since the IPs of these isomers are so close in value, however,
we are unable to verify our assignment of the identity of the
C6H4 fragment solely based on the PIE data. In fact, even if
the initial step results in formation of the linear C6H4 species,
at the energies at which the bulk of the C6H4 is formed (see
Figure 1) rearrangement to benzyne is certainly a possibility.
Identification of the product thus may not reveal the nature of
the primary product in any case.
Figure 6 shows the PIE spectrum for the C6H2 fragment. The
most likely structure for C6H2 is 1,3,5-hexatriyne since the
threshold of ionization from its PIE curve matches well with
the reported IP of 9.50 ( 0.02 eV for 1,3,5-hexatriyne.41
Spectroscopic characterization of the products may be necessary
to establish their identities with greater certainty. The spectros-
copy of open chain and cyclic unsaturated hydrocarbons is well-
known, thus, IR, visible, and/or UV spectroscopic techniques
may be employed to measure and characterize their identity.42-49
Angular distribution measurements for reaction 1 were made
1
state (X A1) produces hot phenylacetylene. Subsequently, the
(39) Roth, W. R.; Adamczak, O.; Breuckmann, R.; Lennartz, H. W.;
Boese, R. Chem. Ber. 1991, 124, 2499.
(40) Zhang, X.; Chen, P. J. Am. Chem. Soc. 1992, 114, 3147.
(41) Bieri, G.; Burger, F.; Heilbronner, E.; Maier, J. P. HelV. Chim. Acta
1977, 60, 2213.
(45) Nam, H. H.; Leroi, G. E. J. Mol. Struct. 1987, 157, 301 and
references therein.
(46) Wenthold, P. G.; Squires, R. R.; Lineberger, W. C. J. Am. Chem.
Soc. 1998, 120, 5279.
(42) Hass, S.; Winnewisser, G.; Yamada, K M. T.; Matsumura, K. J.
Mol. Spectrosc. 1994, 167, 176.
(43) Delpech, C.; Guillemin, J. C.; Paillous, P.; Khlifi, M. Spectroc. Acta
A 1994, 50, 1095.
(47) Marquardt, R.; Sander, W.; Kraka, E. Angew. Chem. 1996, 108,
825.
(48) Arrington, C. A.; Ramos, C.; Robinson, A. D.; Zwier, T. S. J. Phys.
Chem. A 1999, 103, 1294.
(44) Radziszewski, J. G.; Hess, B. A.; Zahradnik, R. J. J. Am. Chem.
Soc. 1992, 114, 52 and references therein.
(49) Arrington, C. A.; Ramos, C.; Robinson, A. D.; Zwier, T. S J. Phys.
Chem. A 1998, 102, 3315.

The UltraViolet Photochemistry of Phenylacetylene
J. Am. Chem. Soc., Vol. 123, No. 4, 2001 675
Figure 5. Time-of-flight spectrum of m/e 74 (C6H2) measured at a
laboratory angle of 5°. The residual distribution of Figure 4c is
momentum matched with the data shown here. Refer to the text for
more details.
Figure 6. To get the photoionization efficiency spectrum for m/e 74,
the integrated ion signal at m/e 74 was measured as a function of the
photon energy. Note that the ionization onset of this curve occurs at
about 9 eV and matches well with the ionization potential for 1,3,5-
hexatriene (i.e. 9.5 eV).
hot phenylacetylene decomposes to vibrationally excited C6H4
and HCCH [see reaction 1]. This process would require the
opening of the ring and is consistent with characterization of
C6H4 as either (E)- or (Z)-3-hexene-1,5-diyne as discussed
above. Some of the C6H4 products have sufficient internal
energy to undergo secondary dissociation [see reaction 2] to
produce 1,3,5-hexatriyne and molecular hydrogen.
HCCH + H2 + Ecutoff, where Ephoton and Ecutoff are the photon
energy and maximum total product translational energies,
respectively. We can determine the enthalpy of formation of
C6H2 as:
Ground-state unimolecular dissociation of phenylacetylene
has been studied recently by pyrolysis and the results may be
compared with the results obtained here.7-11 Frank and co-
workers and Hopf et al. identified acetylene and benzene as
the end products in their pyrolytic studies.7-9 More recently,
PAHs were also identified as end products.10 In a subsequent
study, Guthier et al. reported the observation of several free
radicals including phenyl, phenylvinyl, and o-, m-, and p-
ethynylphenyl with phenyl being the major radical product.11
The observation of phenyl indicates the presence of the phenyl
+ ethynyl channel as well as several H atom elimination
channels if these radicals are generated from primary processes.
As mentioned above, photodissociation of phenylacetylene most
likely occurs on the ground-state surface. If so, then phenyl as
well as phenylvinyl and o-, m-, and p-ethynylphenyl should also
be observed in our PTS measurements. However, an exhaustive
search yielded no evidence for the phenyl + ethynyl or the
atomic hydrogen elimination channels. Guthier et al. used a
radical scavenger method to characterize the radicals generated
in their pyrolytic studies of phenylacetylene. Although this is a
powerful method for the characterization of the free radicals
produced, it does not distinguish whether these free radicals
are primary products. This disagreement between our PTS
results and the pyrolysis studies suggests that the radical
products may not be the result of a primary process.
∆Hf (C6H2) ) ∆Hf(C6H5CCH) + 148 kcal/mol -
∆Hf(C2H2) - ∆Hf(H2) - Ecutoff
all values at 0 K. Using ∆Hf(C6H5CCH) ) 79.4 kcal/mol,50
∆Hf(C2H2) ) 54.5 kcal/mol,51 and Etrans ) 13 kcal/mol, we find
∆Hf(0 K)(C6H2) ) 160 ( 4 kcal/mol. The error in this value
was determined by checking the sensitivity of the fit in Figure
5 to Ecutoff
.
Theoretical calculations were employed to give us a better
handle on the thermochemistry described above. Single-point
calculations were performed at the MP2/6-311G(d,p), CISD/6-
311G(d,p), B3LYP/6-311G(d,p), and G2 levels of theory for
acetylene, 1,3-butadiyne (diacetylene), and 1,3,5-hexatriyne
(triacetylene). Enthalpies of formation were calculated using
these values according to the atomization52-57 and formation
reaction procedures.56-59 These procedures gave satisfactory
results only for acetylene. Therefore, an alternative procedure
was employed. Using the following reaction
(50) Heat of formation of phenylacetylene is from the following,
extrapolated to (0 K): Afeefy, H. Y.; Liebman, J. F.; Stein, S. E., Neutral
Thermochemical Data. In NIST Chemistry WebBook, NIST Standard
Reference Database Number 69; Mallard, W. G., Linstrom, P. J., Eds.;
February 2000, National Institute of Standards and Technology, Gaithersburg
MD, 20899 (http://webbook.nist.gov).
(51) Thermodynamic Properties of IndiVidual Substances, 4th ed.;
Gurvich, L. V., Veyts, I. V., Alcock, C. B., Iorish, V. S., Eds.;
Hemisphere: New York, 1991; Vol. 2.
(52) Pople J. A.; Curtiss, L. A. J. Chem. Phys. 1991, 95, 4385.
(53) Curtiss, L. A.; Nobes, R. H.; Pople, J. A.; Radom, L. J. Chem. Phys.
1992, 97, 6766.
(54) Wong M. W.; Radom, L. J. Am. Chem. Soc. 1993, 115, 1507.
(55) Glukhovtsev, M. N.; Pross, A.; Radom, L. J. Phys. Chem. 1996,
100, 3498.
Thermochemistry. It was mentioned above that the second-
ary dissociation of C6H4 [ reaction ] occurred as a result of the
large amount of internal energy released in this fragment. The
distributions of Figures 4b and 4c clearly demonstrate this. A
striking feature of these distributions is the clear energetic cutoff
for the secondary dissociation. At this cutoff energy we assume
that the C6H2 + H2 products have no internal energy and we
can determine the enthalpy of formation for the C6H2 fragment
from the following reaction: C6H5C2H + Ephoton f C6H2 +
(56) Schlege H. B.; Skancke, A. J. Am. Chem. Soc. 1993, 115, 7465.
(57) Nicolaides, A.; Rauk, A.; Glukhovtsev, M. N.; Radom, L. J. Phys.
Chem. 1996, 100, 17460.

676 J. Am. Chem. Soc., Vol. 123, No. 4, 2001
nC2H2 f C2nH2 + (2n - 1)H2
Sorkhabi et al.
Table 1. A Comparison of the Experimental and Calculated ∆Hf
(0 K) for Acetylene, 1,3-Butadiyne (Diacetylene), and
1,3,5-Hexatriyne (Triacetylene)
where n ) 2 for 1,3-butadiyne and n ) 3 for 1,3,5-hexatriyne,
we can write
enthalpy of formation, ∆Hf/kcal mol-1
B3LYP/6-311G(d,p)
G2
experiment
∆Hrxn ) (2n - 1)∆Hf(H2) + ∆Hf(C2nH2) - n∆Hf(C2H2)
acetylene
1,3-butadiyne
1,3,5-hexatriyne
61.7
115
171
58.3
109
161
54.4a
111b
160c
Solving for ∆Hf(C2nH2) and substituting {G2(C2nH2) +
(2n - 1)G2(H2) - nG2(C2H2)} for ∆Hrxn, we get
a Reference 62. b Reference 63. c Determined from the experimental
data presented in this paper (refer to the text and Figure 4).
∆Hf(C2nH2) ) G2(C2nH2) + (2n - 1)G2(H2) -
nG2(C2H2) + n∆Hf(C2H2)
for the process, we can envision no plausible alternative
explanations to account for the observation of m/e 62 closely
matching the absence of m/e 64 in the spectra.
The enthalpies of formation for 1,3-butadiyne and 1,3,5-
hexatriyne calculated with this procedure agree well with
experimental values. These results along with experimental
values are summarized in Table 1.
Conclusions
This paper presents the study of the photodissociation
dynamics of phenylacetylene at a wavelength of 193 nm with
photofragment translational spectroscopy (with tunable VUV
as the ionization method). The results presented are part of an
ongoing effort at the Chemical Dynamics Beamline of the
Advanced Light Source to unravel the photodissociation dynam-
ics of cyclic-polyatomic molecules. For such systems, dissocia-
tion via the ground state potential energy surface following
internal conversion is the dominant dissociative pathway. A key
aspect of these studies is the determination of the primary
processes in such events. The 193-nm dissociation of phenyl-
acetylene was found to yield only one primary channel: C6H4
+ C2H2. The primary product fragments were characterized as
(E)- and/or (Z)-3-hexene-1,5-diyne and acetylene, respectively.
The (E)- and/or (Z)-3-hexene-1,5-diyne molecules, with suf-
ficient energy, were found to undergo secondary dissociation
to produce C6H2 + H2. The C6H2 fragment was characterized
as 1,3,5-hexatriyne by measuring its photoion yield spectrum.
A heat of formation (∆Hf)(0 K) of 160 ( 4 kcal mol-1 was
determined for 1,3,5-hexatriyne and this value agrees well with
the results of our ab initio calculations.
The value of 160 ( 4 kcal mol-1 should be taken as an upper
limit for the ∆Hf of 1,3,5-hexatriyne due to the possible presence
of a barrier for the reverse reaction. The reverse reaction could
involve the addition of molecular hydrogen across a triple
bond: a process that typically requires an energy barrier of about
2 eV (46 kcal mol-1).60,61 However, the data suggest that 135
kcal mol-1 may be near the energy minimum of the products
since little energy is released into product translational energy
(see Figure 4c). This presents an apparent dilemma in which
the threshold energy of 135 kcal mol-1 may be near the energy
minimum of the products yet an energy barrier is expected for
the reverse reaction (please refer to Figure 1). Conversely, a
three-center dissociation involving a 3,4-hydride shift in either
(E)- or (Z)-3-hexene-1,5-diyne will have a significantly lower
barrier. The barrier for the reverse reaction will be much lower
than 2 eV if such a hydride shift is involved. Hydride shifts in
ethylene and polyenes are greatly facilitated by the torsional
motion about the CdC bond.62,63 The (E)-/(Z)-3-hexene-1,5-
diyne molecules have sufficient internal energy (as evident from
the distribution of Figure 4b) and it is conceivable that some of
this energy will be in the torsional vibrations of the 4,5 CdC
bond. This motion should greatly facilitate a 4,5-hydride shift
and cause reaction 2 to proceed via a nearly barrier-less path
and, thus, nullify the apparent dilemma mentioned above.
Although it is somewhat surprising that the apparent onset of
dissociation of C6H4 dissociation is near the energy threshold
Acknowledgment. We thank Dr. S. North and Dr. B. Ruscic
for valuable discussions. This work was supported by the
Director, Office of Science, Office of Basic Energy Sciences,
Chemical Sciences Division of the U.S. Department of Energy
under contract No. DE-AC03-76SF00098. The Advanced Light
Source is supported by the Director, Office of Science, Office
of Basic Energy Sciences, Material Sciences Division of the
U.S. Department of Energy under the same contract. This work
also used resources of the National Energy Research Scientific
Computing Center, which is supported by the Director, Office
of Science, Division of Mathematical, Information, and Com-
putational Sciences of the U.S. Department of Energy under
the same contract.
(58) Armstrong, D. A.; Rauk, A.; Yu, D. J. Am. Chem. Soc. 1993, 115,
666.
(59) Yu, D.; Rauk, A.; Armstrong, D. A. Can. J. Chem. 1994, 72, 471.
(60) Chang, A. H. H.; Mebel, A. M.; Yang, X. M.; Lin, S. H.; Lee, Y.
T. J. Chem. Phys. 1998, 109, 2748.
(61) Lin, J. J.; Hwang, D. W.; Lee, Y. T.; Yang, X. J. Chem. Phys.
1998, 109, 2979.
(62) Ohmine, I. J. Chem. Phys. 1985, 83, 2348.
(63) Freund, L.; Klessinger, M. Int. J. Quantum Chem. 1998, 70, 1023.
(64) Chase, M. W., Jr. J. Phys. Chem. Ref. Data 1998, 9, 1-1951.
(65) Kiefer, J. H.; Sidhu, S. S.; Kern, R. D.; Xie, K.; Chen, H.; Harding,
L. B. Combust. Sci. Technol. 1992, 82, 101.
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