8002
J. Phys. Chem. 1995, 99, 8002-8012
Spectroscopy and Photochemistry of the
C3*H20 Complex
in Argon Matrices
Jan Szczepanski, Scott Ekern, and Martin Vala* Department of Chemistry and The Center f o r Chemical Physics, University of Florida, Gainesville, Florida 3261 I Received: January 4, 1995; In Final Form: February 28, 1995@
The C3aH20 complex has been formed in argon matrices and studied via FTIR spectroscopy. Five vibrational bands due to the complex have been identified and compared with the results from ab initio calculations (HF/6-3 lG* level) and normal coordinate force constant adjustment calculations. Eight isotopomeric peaks resulting from I2C/l3C substitution are observed for the asymmetric CC stretching mode at 2052 cm-' and lead to the conclusion that the geometry of the complex is planar and asymmetric. Photolysis of the complex at 405.4 nm results in the appearance of two intermediates, transoid and cisoid rotamers of 3-hydroxypropadienylidene (HPD), and two stable molecules, propynal and C30. The latter two have been observed previously in interstellar space. The existence of the intermediates was established by comparison with ab initio theoretical calculations of the vibrational frequencies and intensities, 12C/'3C isotopic studies, and photolytic behavior. Photolysis of the cisoid rotamer produces the transoid rotamer, with spontaneous reversion in the dark. The temperature dependence of the kinetic behavior of the dark interconversion process yields a very low activation energy. A b initio theory predicts a much different value, leading to the suggestion that the solid-state rotamerization process may occur via hydrogen tunneling. The implications of this finding for the production of molecules of astrophysical importance on dust particles in the interstellar medium are discussed.
I. Introduction Interest in small carbon clusters has grown rapidly since the early pioneering work of Weltner and co-workers on the vibrational, electronic, and electron spin resonance spectroscopy of these species isolated in rare gas matrices.' Much of the current interest revolves around the structure, size, and widespread involvement of these clusters in various processes. They have been implicated in the formation of the fullerenes, in flame chemistry, in soot production, and in interstellar species.2-6 Carbon cluster work has proceeded rapidly in the past several years. The linearity (or near-linearity) of gaseous small carbon clusters (n 5 9) is now generally accepted. The structure of C3 has been investigated i n t e n ~ e l y . ~The - ~ very low value of the bending frequence (v2 M 62 cm-I) in the gas phase suggests that the molecule is floppy. Recent c a l c ~ l a t i o n sshow ~~'~ that the bending potential of free linear C3 is flat up to 160". We have recently measured'' the V I v3 combination mode of all six I2C/l3Cisotopomers of C3 and have concluded that in Ar and Kr matrices the molecule is bent with a bond angle of 160". Although the main focus in the recent past has been on the structures of the carbon clusters, little attention has been given to their reactivities. The reaction of small carbon clusters (Cn, n 5 5) with H2O in Ar matrices has, however, been investigated experimentally by Ortman, Hauge, Margrave, and Kafafi (OHMK).12 These workers concluded that certain carbon species, such as atoms in their 3P ground states or diatomic molecules in their IZg+ground or 3Eg- excited states, do not react with H20. However, atomic carbon in its ID excited state is well-known to react with H20 to form CO, H2, and f01maldehyde.I~In their study of C3 with H20, OHMK assigned the 2052-cm-I infrared band to the CC asymmetric stretch of a C3*H20complex. In work performed at about the same time, we deduced that a different species was re~ponsible,'~ but here
+
@
Abstract published in Advance ACS Absrrucrs, April 15, 1995.
we present evidence which confirms OHMK's conclusion. During photolysis with radiation of wavelengths > 400 nm, OHMK observed that the 2052-cm-' band decreased in intensity at the same time that new bands grew in at 1999.8, 1992.8, 1459.6, 1252.5, and 1061.1 cm-I. It was suggested that these bands originated from an intermediate, hydroxyethynylcarbene (HEC) (1, cf. Figure l), a photoproduct of the C3-H2O complex. Later, Liu, Zhou, and Pulay (hereafter LZP) reportedf5 on a thorough and insightful ab initio investigation of 1 and the cisoid (2) and transoid (3) rotamersI6 of 3-hydroxypropadienylidene (HPD). From a comparison of OHMK's experimental spectra with their calculated IR frequencies and intensities, LZP suggested that 2 and 3, not 1, were responsible for the above IR bands. In the present paper, we report on our investigation of the photochemistry of the C3*H20complex. Besides the primary issue of the identification of the intermediate(s), there remain a number of other unsettled problems. (1) Besides the asymmetric CC stretching modes, what are the other experimental IR frequencies of the C3eH2O complex? (2) What are the true identities of the 1999.8- and 1992.8-cm-' bands: is one a site band, as OHMK suggest, or is one the CC asymmetric stretch of transoid HPD and the other a similar mode in cisoid HPD, as LZP suggest? (3) Can the cisoid and transoid rotamers be differentiated experimentally? (4) Why do only wavelengths in the 400-nm region induce the C3*H20 photoreaction? (5) What are the structures of the C3eH2O complex and the cisoid and transoid HPD rotamers in an Ar matrix? (6) Why are three IR bands predicted for the two rotamers in the 1200-1300cm-' region, yet only two were observed by OHMK? In this paper, we address these questions. Our experimental and theoretical procedures are outlined in sections I1 and 111. We present the evidence for the C3eH2O complex in section IVA and for the cisoid and transoid rotamers of HPD in sections IVB and IVC and discuss evidence for the rotamerization process in section IVD. We discuss the possible significance
0022-3654/95/2099-8002$09.00/0 0 1995 American Chemical Society
J. Phys. Chem., Vol. 99, No. 20, 1995 8003
C3-H20Complex in Argon Matrices
1 lJ
5 Figure 1. Structures of the intermediates hydroxyethynylcarbene(HEC,
l), cisoid 3-hydroxypropadienylidene (c-HPD, 2), transoid 3-hydroxypropadienylidene (t-HPD, 3), the CyH2O complex (4), and the transition state (5).
of our observations in section V. Finally, conclusions are drawn in section VI.
11. Experimental Procedures
-
n / n , = (25
+ 1) exp(-AE/kT)
3600
3'100
3800 WAVENUMBERS [ c m - I ]
2000
IS00
+
Figure 2. Portion of the 10 K matrix infrared spectra of 12C,/Ar H20 (0.1%) in the asymmetric CC stretch region (ca. 2000 cm-I) and in the OH asymmetric (v3)and symmetric stretch ( V I )region (ca. 3700
cm-I). Monomer (M) and dimer (D) bands of water are marked; CyH20 complex bands are starred. Inset shows the v2 + v3 combination band of H20.
The experimental apparatus used has been described previo u ~ l yso , ~only ~ the essentials are given here. The I2C/l3Cmixed isotope carbon species were produced by pulsed laser (NdlYAG, DCR- 11, Spectra Physics) vaporization (with combined 1064and 532-nm beams) of pressed pellets formed from powders of I2C (Alfa, 99.9995% pure) and/or I3C (Isotec, 98.4% atom %). Matrices were prepared by trapping the vaporized species on a cryogenically-cooled (10 K) BaF2 window (transparent range: 200 nm to 14 pm) together with the Ar isolant gas containing 0.1-0.5% H20. Small carbon clusters (Cn, n 5 9) are easily generated by pulsed laser ablation (Nd/YAG, 10 mJ/cm2) of graphite and then readily trapped in cryogenic rare gas matrices. The electronic (200- 1000-nm) and vibrational (700-7000cm-') absorption spectra of the trapped species were measured using a Cary 17 near IR/visible/UV spectrophotometer (0.51.O-nm resolution) and a Nicolet 7199 Fourier transform infrared (FTIR)spectrometer (0.25- or 1.O-cm-' resolution), respectively, with a crossed beam c~nfiguration.'~ Photolysis of the matrices was carried out with a mediumpressure mercury lamp with a 5-cm-long H20 heat filter and color filters (Veb Jena) or with a 1-kW Xe lamp plus a 0.25-m monochromator (2-mm slit) tuned over the 380-550nm region. For a number of the measurements reported here, accurate temperatures are important. In matrices, temperature measurement has always been problematic. Temperatures are most often measured by thermocouples connected to the cold finger or cold window of the cryostat, with the assumption that the temperature of the species in the matrix is the same. A preferable procedure would be to use a species already embedded in the matrix as an in situ probe of the temperature. We report such a procedure here. The approach is based on the relative intensities of two IR bands which involve the hindered rotational levels of the asymmetric v3 OH stretching mode of water. The two are the 1-1 20 (3776-cm-l) and 00 1-1 (3756-cm-I) transitions of H20/Ar (cf.Figure 2 ) . Their relative intensities are governed by the Boltzmann population
-
0
(1)
where nJ denotes the number of molecules of the Jth (hindered) rotational level and AE is the energy difference between the two initial states. The initial state of the 3756-cm-I transition is J = 0, and for the 3776-cm-' transition, it is J = 1. The
relative intensities are thus given by n,/n, = 3 exp(-AEIkr)
where hE = BJ(J given by
(2)
+ 1) = 20 cm-I, and temperature (in K) is
T = -28.777 ln-'(Aint(3776 cm-')/3Aj,,(3756 cm-I))
(3)
where 4,,,(3776 cm-')/Ain,(3756 cm-') =
is the ratio of the integral intensities of the 3776- and 3756cm-' water bands. With this approach, the lowest temperature of our matrix was 10.0 K, which was 1.2 K higher than the reading of the thermocouple attached to the copper metal window holder. All temperatures quoted here have thus been corrected using this internal temperature monitor. This procedure should only be used on a completely relaxed matrix, Le., after the intensities of the 3776- and 3756-cm-' bands are stable. In our apparatus, this occurred only 6-8 h after deposition.
111. Theoretical Procedures Ab initio total energy calculations and geometry optimizations were carried out on the C3*H;?Ocomplex (4), the transoid (3) and cisoid (2) HPD intermediates, and the transition state (5) between them (cf.Figure 1) using the GAUSSIAN 92 program package.I8 The calculations were carried out at the HF/6-31G* level; the optimized geometrical parameters are collected in Table 1. Our optimized HPD structures are very similar to those calculated by LZP. The C36H20 complex has C, symmetry and is predicted to be bound by 1.4 kcdmol (zero-point-corrected). Other C3*H2Ostructures involving the interaction of the oxygen with the terminal carbon or the hydrogen or oxygen with the central carbon of C3 were tested and no other stationary points located. Vibrational frequencies were calculated at the HF/6-3 lG* level to determine the nature of the stationary points and to help confirm experimental isotopomer assignments. Scaling factors for the calculated frequencies of the various isotopomers
8004 J. Phys. Chem., Vol. 99, No. 20, I995
Szczepanski et al.
TABLE 1: Structural Parameters" Obtained after Geometry Optimization for the CyH20 Complex, Transition State, and 3-Hydroxypropadienylidene(HPD) Rotamers in Their Electronic Ground States HPD
Paramb Rc,c2 RC2C3 RC3H~
ROH~ RH,O Rcgo
LCjCzC3 LC~C~HI LC~HIO LHiOHz LC3OH2 LHIC~O LC3HOH
TS
C3.H20 1.280 1.273 2.492 0.947 0.949 180.0 186.0 174.6 105.4
0.0
1.272 1.295 1.257 0.956 1.176 1.667 175.0 200.1 86.4 115.8 108.9
transoid
cisoid
1.258 1.330 1.075 0.953
1.260 1.324 1.079 0.949
1.307 180.7 122.8
1.314 184.1 120.7
110.4 111.6
111.7 115.8
93.1
total energies -189.369 60 -189.285 27 -189.465 66 -189.456 91 All bond lengths in angstroms, angles in degrees, and energies (HF/ 6-31G*) in hartrees. Atom numbering: C1,terminal carbon; C2, middle carbon; C3, terminal carbon, reactive end.
were determined by minimizing the differences between the calculated and experimental frequencies.
IV. Results
A. The C3.HzO Complex. Figure 2 (right) shows the asymmetric CC stretch region (around 2000 cm-I) for a number of carbon clusters formed in an Ar matrix at 10 K. The initial distribution of cluster sizes in the matrix depends on the power density of the laser beam, the matrix deposition temperature, and the Ar gas pressure in the deposition chamber. Aggregation of the clusters is readily achieved by annealing the matrix. This results from diffusion of the C, species in solid Ar which increases rapidly with increasing temperature in the 20-46 K rangeI9 and from the exothermic heats of formation of the carbon clusters.20 In earlier work, many of the IR bands observed in this region were assigned using matrix and/or gaseous samples. These assignments are given in Figure 2 and compiled in Table 2. Water is ubiquitous in matrix isolation experiments. Its removal is exceedingly difficult due to its (seemingly) high affinity for all surfaces. Its presence is, however, easily noted by the appearance of IR bands in the OH stretch region (around 3700 cm-I). Figure 2 (left) shows the IR bands of matrixisolated noninteracting H20 monomers (M) and dimers (D) for a sample formed from a mixture of Ar f 0.1% H20 f C, clusters. The inset shows the region for the v2 f v3 combination bands of noninteracting H20. In addition to known bands attributable to water and carbon clusters, the figure also shows two bands at 2052.3 and 3598.0 cm-' (starred). The 2052cm-' band has been assigned previously by OHMK to the asymmetric CC stretch of the C3*H2O complex. Here we confirm this assignment and, in addition, determine a (almost) complete set of experimental vibrational frequencies of the complex, compare these to a set of theoretical ab initio frequencies, and establish the geometry of the complex. 1. The 2052.3-cm-' Band. The vibrational frequencies and intensities for the C3*H2O complex, computed using the optimized complex geometry (cf. Table I), are given in Table 3. Twelve modes are predicted, but only five intra-species modes were observed; the calculated inter-moiety modes and the C3 bending mode are included in the table but were not observed since their frequencies are too low for our spectrometer. The predictions include three perturbed OH modes (at
1626.8,3604, and 3705.2 cm-') and two perturbed CC stretches (at 1216 and 2052.3 cm-I). The 2052-cm-I mode is predicted to be the most intense, with three others about one-tenth as intense and one practically zero. These bands are compared in the table to the experimental bands assigned to the C3*H2O complex. A discussion of these assignments follows below. A mixed 12C/'3Cisotope run aided in the assignments of the bands and helped in establishing the geometry of the complex. For a symmetric complex (e.g., with the H20 bonded symmetrically to the middle carbon), six isotopomeric peaks are expected for the asymmetric CC stretch. However, for an asymmetrically-bonded complex (e.g., with the H20 attached to one end of the C3), eight isotopomeric peaks are predicted. The experimental spectrum of the 12,13C*H20 complex in the asymmetric CC stretch region is shown in Figure 3. The band positions, collected in Table 4, are compared to a normal coordinate force constant adjustment c a l ~ u l a t i o nand ~ ~ the ab initio frequencies. Both the force constant fit and the ab initio frequencies for a planar asymmetrical complex are close to the experimental values, leading to the conclusion that the 2052.3cm-' band is due to the asymmetric CC stretch of a planar asymmetrical complex. 2. The 3598-cm-' Band. The "before photolysis" spectra in Figures 4-6 give different regions of the IR spectrum of a C,/H2O/Ar matrix for a higher H20 concentration (0.2%) than in Figure 2 (0.1%). The intensities of the 2052.4- and 3597.6cm-' bands both increase with increasing water concentration, while the 2038.9-cm-' band (due to uncomplexed C3) decreases. Forty different spectral runs, including photolysis and annealing experiments, established a positive correlation (correlation coefficient = 0.97) between the 3598-cm-' band and the 2052.4cm-' band intensities. The 3598-cm-' band thus arises from the C3aH20 complex and, because of its proximity to the calculated frequency at 3604 cm-' and the close match of experimental and calculated relative intensities, is attributed to the OH symmetric stretch of the complex. 3. The 3712-cm-' Band. The OH asymmetric stretch in the complex is predicted to appear at 3705.2 cm-' (cf. Table 3). The experimental spectrum is very crowded in this region, with monomer, dimer, and trimer H20 bands appearing (Figure 4). Upon photolysis (during which the complex is transformed into other species, vide infra), a change can be detected in this region (cf. Figure 4, difference spectrum). The change is complicated by overlap with the 37 11.5-cm-' band (1-1 00 transition of the v3 mode in the H2O monomer) which is expected to remain unchanged upon photolysis. There are several reasons for believing that the negative peak in the difference spectrum contains a contribution from the temperature-dependent 371 1.5-cm-' band and the 3712.2-cm-' C3eH20 complex band. Namely, (1) the intensity ratio of the 3755.7cm-' (00 1-1) band to the 3777.9-cm-I (1-1 2-2) band of the H20 monomer is larger in spectrum a than in spectrum b, indicating that the temperature is lower in a. This is reasonable since a was recorded an hour later than b, after the matrix had had more time to cool. (2) The intensity ratio of the 3777.9and 3711.5-cm-' bands is known3' to be 1.4 at 15 K in an Ar matrix. While it is not possible to determine this ratio directly here, it can be deduced from the v2 v3 combination bands (cf. inset, Figure 2). The 5345.2-cm-' (v2 3776.4) band is slightly more intense than the 5280.1-cm-' (v2 371 1.5) band (here shifted to 3710.8 cm-I); it is expected that the 3776.4and 3710.8-cm-l bands should be in approximately the same ratio. But, as the difference spectrum in Figure 4 shows, the intensity ratio is inverted. Thus, it is reasonable to conclude that another species is contributing to this band, e.g., the C3.H20
-
-
-
+
+
+
J. Phys. Chem., Vol. 99, No. 20, 1995 8005
C36H2O Complex in Argon Matrices
TABLE 2: Experimental Frequencies and Vibrational Assignments for Small Carbon Clusters in Their Ground Electronic State C cluster c3 (I$+)
structure
exptl freq (Ar matrix), cm-I
linear, bent in Arl'
2038.9Ia 1214.0"
~801l,21
c5 (lZg+)
linear linear
c6 (3xg-)
linear
c 7 (I&+)
linear or near-linear
Ce ( 3 2 g - )
linear or near-linear linear or near-linear
c 4 (3Xg-l
c9 "+)
mode description
remarks
asym. st., v3 (au) sym. st., V I (ag) bending, v2 (nu)
1543.425 2 16426 1446.629 1952.528 1197.329 2 128.119,30 189427,28
asym. st., v3 (au) asym. st., v3 (au) asym. stretch, v4 (a") asym. st., v4 (a,,) asym. st., v5 (au) asym. st., v4 (a,,) asym. st., v5 (a,)
1998.433,34
asym. st., v6 (a,)
C3 is linear in gas phase9 v3 = 2040.02 cm-' 22 V I = 1224.5 cm-I 23 v:, = 63.4165 cm-' 24 all in gas phase 1548.9368 cm-I, gas phase3" 2169.44 cm-I, gas p h a ~ e ~ . ~ ~ 1959.858 52 cm-I, gas phase3" 2138.1951 cm-I, gas p h a ~ e ~ ' . ~ * 1898.3758 cm-I, gas phase3* expected structure 2014.277 964 cm-l in gas phase35
TABLE 3: Comparison of Experimental (Ar Matrix) and Calculated (HF/6-31G*) IR Frequencies and Intensities for the C3-H20 Complex cm-'
intensity (exptl rel)
v,,lca cm-'
intensity calcdb k d m o l
mode descripn
3712.2 3598.0 2052.3 1593.4 1214.2'
(-0.1) (0.09) (1.0) (0.11) -0.0
3705.2 3604.0 2052.3 1626.8 1216.0 270.0 203.2 144.0 138.7 83.9 37.2 33.8
154.7(0.11) 82.0(0.06) 1450.5(1.0) 117.4(0.08) 2.9(0.002) 168.7(0.12) 74.3(0.05) 7.9(0.005) 29.9(0.02) 1.29(0.001) 8.3(0.006) 0.2(0.000)
OH asym. st. OH sym. st. CC asym. st. HOH bend CC sym. st.
vexp,
a Calculated frequencies scaled by 0.8856. Relative intensities in parentheses. 'Expected value based on the 3258.9-cm-' ( V I v,) combination band of C3 perturbed by H2O in the CyH20 complex (see text).
+
I
complex. Because of its proximity to the predicted 3705.2cm-' band, we ascribe the 3712.2-cm-' band to the OH asymmetric stretch of the complex. 4. The 1214.3- and 1593.4-cm-' Bands. A perturbed HOH bend is predicted for the complex at 1626.8 cm-' (cj. Table 3) with an intensity intermediate between the two OH stretch modes. A band at 1593.4 cm-' (cf.Figure 7), having the correct behavior, can be assigned to this mode. With photolysis, this band decreases in intensity, in parallel with the 2052.3-cm-' band, as expected. A symmetric CC stretch is predicted at 1216 cm-I and is expected to have very low intensity. Its presence can, however, be determined from the V I v3 combination region. Two weak bands are observed in this region: one at 3245.2 cm-I, which has been previously assigned",'* to the V I v3 combination band in "noninteracting" C3; the other at 3258.9 cm-I, which we assign here to the v1 v3 combination band of the complex. To determine the frequency of the v1 mode, we use the anharmonicity factor found" for the combination mode of C3 in Ar (Le., - 7.7 cm-I). If this value is correct for the complex also, we expect that the CC symmetric mode frequency in C3sH2O should be found at 3258.9 - 2052.4 7.7 cm-I = 1214.2 cm-'. This is very close to the V I value of 1214.0 cm-I for C3 in Ar and to the 1216-cm-' value calculated here for the complex. Furthermore, the 3258.9-cm-' band decreases upon photolysis of the matrix, as expected. Thus, we assign the CC symmetric stretch in the complex to 1214.2 cm-I deduced from the V I vg combination band. 5. Electronic Spectrum. OHMK showed previously'* that photolysis with A > 420 nm produces no reaction, but with A
+
+
+
+
+
I
2050
I
I
,
,
#
I
I
I
I
2025
,
2000
,
,
,
,
,
,
1975
iJ/ c rn-1 Figure 3. Portion of the annealed (32 K) matrix infrared spectra of I2C/l3C/Ar ([12C]:[13C]= 1:l) (top) and of I2C/Ar (12 K) (bottom) in the asymmetric CC stretching region. In the top spectrum, the C3 isotopomers are marked by inverted full triangles, while the C3*H2O complex isotopomers are marked by solid circles. In the lower spectrum, the marked bands are due to C3sH20 (2052 cm-I), C3 (2039 cm-I), and C9 (1998 cm-I). Resolution = 0.25 cm-I. > 400 nm, photochemical activity is observed. Since photolysis in the 400-500-nm region destroys the C3.H20 complex, the electronic band of the complex responsible for activation was sought by scanning this region before and after photolysis. Figure 8 shows that a band at 405.4 nm disappears upon photolysis. Furthermore, this band correlates well with the 2052.3-cm-' band (correlation coefficient = 0.95). The integral intensity ratio, Z(405.4 nm)/Z(2052.3 cm-I), is found to be ca. 70. The 405.4-nm band is here assigned to the electronic band origin of the complex, a transition analogous to the Ill IZ transition of uncomplexed C3 at 408.4 nm.' The above observation demonstrates that the rearrangement of the C30H20 complex to the rotamers of HPD and/or C3O (vide infra) requires the excess energy acquired from excitation to an excited electronic state of the complex. 6. Transition State between C3 and H20. Using ab initio calculations (HF/6-31G* level), the singlet potential surface for the C3*H2O complex has been investigated and the transition state (TS) for its formation from C3 and H20 located. The optimized TS geometry found is sketched in Figure 1, and its geometrical parameters are given in Table 1. The C3*H20
-
Szczepanski et al.
8006 J. Phys. Chem., Vol. 99, No. 20, 1995 c,o
TABLE 4: Experimental (Ar Matrix, 11 K) and Calculated (HF/6-31G*) IR Frequencies for the CC Asymmetric Complexes Stretching Mode of the 12/13C3.H20
wcrbcm-' (HF/6-31 G*, cm-' const fit. cm-' cm-1) 2052.3 2052.82 (+0.52) 2052.3 (0.0) 2040.0 2040.19 (+0.19) 2040.1 (f0.1) 2038.5 2038.87 (+0.37) 2038.2 (-0.3) 2026.0 2025.33 (-0.67) 2025.5 (-0.5) 2000.5 2000.69 (+0.19) 1998.9 (-1.6) 1988.2 1987.86(-0.54) 1986.5 (-1.7) 1986.5 1986.26 (-0.24) 1984.6(-1.9) 1973.3 1972.40(-0.90) 1971.5(-1.8)
HC,CHO
C,.H,O
I-HPD
1
cm-'
vCalc/
veXp,
(FG
+ force
isotooomer 12-12-12-1-16-1 13-12-12-1-16-1 12-12-13-1-16-1 13-12-13-1-16-1 12-13-12-1-16-1 13-13-12-1-16-1 12-13-13-1-16-1 13-13-13-1-16-1
Program from ref 36; deviations from experimental frequencies given in parentheses; RMS deviations of all eight bands is 0.44 cm-I. Force constants found in force constant adjustment calculation: ACICz) = 11.213;f(C2-C3) = 10.932;f(C3.H1)= 2.079;AHl-0) = 9.483; f(0-H2) = 5.33l;f(CI-C2, C2-C3) = 1.239, andf(H1-0, 0-Hz) = -1.208 in units of lo2N m-I. Scaled by 0.8856 factor. Deviations from experimental frequencies given in parentheses.
.2
u Lrl
3 .l3 0 CT
I
1 After 1 Photolysis
.1
I
I
zioo
2200
2000
WAVENUMBERS [ o m -11
+
Figure 5. Portion of the 12 K matrix infrared spectra of '2C,/Ar H20 (0.2%)before photolysis and after photolysis (Hg lamp + filter: ?, > 390 nm). Bands which have grown upon photolysis are due to "c30 (2243.2 cm-I), propynal (2108.4 cm-I), and transoid HPD (1992.5 cm-I). The C3aH2O complex band (2052.4 cm-I) decreases upon photolysis. The C3*(HzO),(n = 2 and 3) bands are marked by stars.
* i l
.2
1
3800
3700
3800
3000
1
'
Before Photolysis(b)
3400
WAVENUMBERS [ c m -11
Figure 4. Portion of the 12 K matrix infrared spectra of I2C,/Ar + H20 (0.2%) in the OH stretch region before photolysis (b) and after photolysis (a). The monomer (M), dimer (D), and trimer (T) bands of water are so marked. The starred bands are the OH stretchs in the CyH20 complex. The positions of several bands are shifted compared to the positions in Figure 2 due to the higher concentration of H20 used.
Difference (a-b)
&P 1400
1200 1000 WAVENUMBERS [ c m -11
800
+
Figure 6. Portion of the 12 K matrix infrared spectra of 12C,/Ar H20 (0.2%) before photolysis (b) and after photolysis (Hg lamp + filter: A > 390 nm) (a). All bands in the difference spectrum are due to transoid HPD.
complex is planar with a long C. *Hbond (2.492 A), while the TS geometry is nonplanar with the C. .H bond decreased clearly also be photolysis products. We show below that they significantly to 1.257 A and the oxygen swung around to interact are due to transoid 3-hydroxypropadienylidene (t-HPD). with the terminal carbon with a 1.666-A bond length. The LZP have shownI5 on the basis of extensive ab initio calculated energy barrier of 52.01 k c d m o l ( l 8 190 cm-I) from calculations that one of the major products of photolysis of the complex to TS represents a substantial barrier and explains why C3-H2O complex is probably t-HPD. In Table 5, we present photochemical excitation is required to drive the reaction experimental frequencies and intensities and results from LZP's forward. From the TS, there is a large exothermicity to the calculation and from our ab initio HF/6-31G* calculation for two intermediates, the cisoid and transoid HPDs. The latter is t-HPD. In Figure 7, a graphical comparison is given. It can predicted to be somewhat more stable (-55.94 kcal/mol) than be seen that the correspondence for the bands at 1992.5, 1460.8, the former (-50.64 kcal/mol). 1280.5, 1223.1, and 1016.3 cm-I is quite good. Frequencies match reasonably well, with the worst off by e 3 0 cm-I. This B. The Transoid Rotamer of 3-Hydroxypropadienylidene (t-HPD). Photolysis of the matrix-isolated mixture described is quite acceptable considering that anharmonicity factors were above with 1 > 390 nm leads to a decrease in the bands at not included in either calculation. The relative intensities are also close to the calculated distribution for both the HF/6-3 1G* 2066.8, 2056.2, 2052.4, 3597.6, and 3712.2 cm-' (cf. Figures and DZPfdiffuse approaches. 4 and 5). The latter three are due to the C3*H20complex, while the former two are tentatively assigned here to the Cy(H2O)3 Further support for this assignment comes from 12C/'3C and C3.(H20)2 complexes, respectively. As a result of this isotopic studies. Eight bands are expected if I3C is substituted for I2C at every position in HPD. In Figure 9 are shown the photolysis, new bands appear at 2243.2,2108.4, 1992.5, 1688.4, I2C/l3C mixed isotope runs before and after photolysis. The 1460.8, 1280.5, 1223.1, 1016.3, and 940.1 cm-' (cf. Figures spectrum shows a series of eight negative-going 5-7). The 2243.2- and 2108.4-cm-' bands have p r e v i o ~ s l y ~ ~ ~difference '~ been attributed to the asymmetric CC stretch of C30 and CC peaks, attributed above to the isotopomers of the C3eH2O complex. Also shown in the difference spectrum is a series of stretch of propynal, respectively. The remaining peaks must
J. Phys. Chem., Vol. 99, No. 20, 1995 8007
CyH20 Complex in Argon Matrices 0
I
-
Expt. Difference Spectrum( Photolyzed Uaphotolyzed)
I
I 1
1500
2000
1000
WAVENUMBERS [ c m -11
Figure 7. Comparison of the experimental (upper) and theoretical (lower) spectra of transoid HPD. The singly starred bands are due to the C y H 2 0 complex, the doubly starred bands to propynal, and the bands marked by solid circles to transoid HPD (cf. Table 5 ) .
o J 370
380
390
400
410
420
WAVELENGTH [ n m ]
Figure 8. Electronic spectra (AImatrix, 10 K) for l2C3and the '2C3.H20 complex before photolysis (upper) and after photolysis (lower). Band at 405.4 nm is assigned to the '2C3*H20complex.
eight positive-going peaks, listed in Table 6. The table also gives the results of the ab initio calculation of the IR frequencies for the eight isotopomers of t-HPD; the results are reasonably good, although the two outliers at -9.1 and +4.5 cm-' are troubling. We believe that this may be due to a perturbation of the matrix cage on the geometry of the molecule which is not accounted for in the calculation. As confirmation of this assignment, the isotopomeric band structure was sought for the next-most intense t-HPD band at 1223.1 cm-'. A set of five weak bands was found in this region and is displayed in Figure 10. The energy spread of these bands is much smaller than the spread in the 1992.5-cm-' range (5.5 cm-' vs 84.6 cm-I). This is because the 1223.1-cm-' mode is a combination HCO/CCH bending vibration, while the 1992.5-cm-' one is an asymmetric CC stretching vibration. Despite the presence of several large discrepancies between the calculated and experimental isotopomeric bands in the 1992.5-cm-' region, the matchup of the five experimental frequencies and intensities and the theoretical values given in Table 5 and the match up between the observed and calculated peaks given in Table 6 are sufficient grounds to conclude that the set of peaks at 1992.5, 1460.8, 1280.5, 1223.1, and 1016.3 cm-' originate from t-HPD. C. The Cisoid Rotamer of 3-Hydroxypropadienylidene (c-HPD). Normally, photolysis of the sample/matrix is performed just prior to a spectral scan. Ocassionally, however, photoproducts are produced whose lifetimes are so short that
they are not observable under such an experimental protocol. Therefore, scans were taken during photolysis. The matrix sample window was aligned at 45" to the photolysis beam and the IR probe beam. Figures 11 and 12 show the results of this approach. Two new bands at 1999.6 and 1254.3 cm-' grow in, while the five bands, attributable to t-HPD, all decrease in parallel. With the photolysis light off, this process is reversed. This strongly suggests a light-induced process, with spontaneous reversion in the dark. The species producing the 1999.6- and 1254.3-cm-' bands will be shown below to be cisoid HPD. LZP have p r e d i ~ t e dthat ' ~ HPD may also exist in the cisoid form. Their calculations of the IR frequencies and intensities for cisoid HPD, together with ours, are given in Table 5. The most intense bands are predicted to lie at 1987.2 and 1279.8 cm-' (HF/6-31G*), the former being about twice as intense as the latter. This is just the pattern observed for the two new bands at 1999.6 and 1254.3 cm-' (cf.next section). Further support for the attribution of these bands to c-HPD comes from '*C/I3Cisotopic studies. Figure 13 gives the spectra of the 12C/'3Cisotopically-substituted species recorded (after 30 min of photolysis) with and without the photolysis lamp. The difference spectrum shows the negative-going peaks of t-HPD, beginning at 1992.5 cm-' and stretching to 1907.9 cm-I. Also shown is a set of eight positive-going peaks starting at 1999.6 cm-' and ending at 1923.5 cm-', which are created as a result of the photolysis (cf.also Table 6). The negative-going peak due to t-HPD at 1923.4 (seen clearly in Figure 9) is not seen in this spectrum due to overlap with the 1923.5-cm-' isotopomeric peak of c-HPD. Table 6 lists the results from the ab initio calculation of the eight c-HPD isotopomeric bands. The correspondence is quite satisfactory. Although the isotopomeric structure in the 1254.3-cm-' region was searched for, none was detected, presumably because of its expected weak intensity. In summary, we have shown that photolysis of the C3*H;?O complex leads to c30 (as previously reported'* by OHMK)and also to cisoid and transoid isomers of HPD. Cisoid HPD spontaneously isomerizes to the transoid form in the dark. Once formed, t-HPD can be further photoexcited and converted to c-HPD. These findings are consistent with the picture determined via ab initio calculations, where it was found that the transoid rotamer is more stable than the cisoid by ca. 5 kcaV mol. The transition state between the two is nonplanar, with the rotatable hydroxy hydrogen being almost perpendicular to the rest of the molecule. One intriguing point is the substantial predicted barrier of 8.29 kcal/mol between the cisoid and the transoid rotamers, which is somehow surmounted spontaneously in the dark in the low-temperature matrix. This point is explored further in the next section. D. The Rotamerization Process. 1. Kinetic Behavior. In this section, the process by which the two HPD rotamers interconvert is investigated. By focusing on similar vibrational modes in the two rotamers, we first discuss the kinetic behavior of the CC asymmetric stretching modes. At long times after extinguishing the photolysis source (Le., 2 h), the absorbance of the 1999.6-cm-l c-HPD band is a factor of 1.26 x lo-* smaller than the absorbance of the 1992.6-cm-' t-HPD band, which is at a maximum. The difference spectra (A, - A,) for both rotamers as a function of time ( t = 0 defined as the photolysis cutoff) are plotted in Figure 14. The positive peak (1999.6 cm-l, c-HPD) is at a maximum at I = 0 and decreases with time. The negative peak (1992.6 cm-I, t-HPD) is at a minimum at t = 0 and increases to a maximum at t = 00 (Le., 2 h). (Recall that since AA (EAr - A,) is plotted, an increasing absorbance will appear as a negative peak decreasing in
8008 J. Phys. Chem., Vol. 99, No. 20, I995
Szczepanski et al.
TABLE 5: Experimental (Ar Matrix) and Calculated IR Frequencies and Intensities for Transoid and Cisoid 3-Hydroxypropadienylidine(HPD)in Their Electronic Ground States transoid HPD
cisoid HPD
HF/6-31G* Vexp,
vcalc:
cm-I
cm-'
1992.5(1.0) 1460.8(0.14) 1280.5(0.04) 1223.1(0.16) 1016.3(0.12)
DZP+diffuse*
HF/6-31G*
v , , ] ~ , ~intensity, km/mol cm-'
intensity, km/mol
3619.7 120.4(0.11) 3051.2 10.3(0.01) 1974.0 1104.7(1.0) 1487.9 292.0(0.26) 1311.5 63.8(0.06) 1222.6 265.5(0.24) 1023.4 109.5(0.10) 997.0 0.4(0.0004) 655.5 200.3(0.18) 589.7 40.8(0.04) 196.0 2.6(0.002) 177.2 3.8(0.003)
vcaIc,~ cm-I
V,,p,
cm-'
3669 127(0.10) 3080 6(0.005) 1940 1274(1.0) 1999.6(1.0) 1482 355(0.28) 1309 l05(0.08) 1254.3(0.35) 1216 284(0.22) 1018 91(0.07) 995 1(O.OO08) 657.5 193(0.15) 596.0 46(0.04) 267.2 l(O.0008) 21 1.4 3(0.002)
v , , ] ~ , ~intensity, cm-I km/mol
mode descripn
3714 221(0.17) 3020 17(0.013) 1951 1284(1.0) 1495 116(0.09) 1281 628(0.49) 1240 104(0.08) 1001 l(O.001) 956.3 2(0.002) 15(0.01) 594.5 494.0 106(0.08) 32(0.02) 267.8 2(0.002) 213.5
OH st. CH st. CC asym st. COH bend, CH st. COH bend, CO st. HCO, CCH bend cc,co St. CH wagging HOCC, HOCH, tors. CCO bend CCCO bend CCC bend
+ diffuse sp function of Pople basis set functions was used.
Table 5) that this mode in c-HPD should be approximately twice as intense as in t-HPD. The theoretical calculations predict both intensities and frequencies for the two HPD isomers reasonably accurately. The dark reaction relaxation time and (crudely) its temperature dependence may be determined by using the data in Figure 14. Both data sets were fit to the first-order expression
1-HPD Ln
intensity, km/mol
3676.9 208.4(0.19) 3000.0 21.9(0.02) 1987.2 1120.0(1.O) 1507.3 82.3(0.07) 1279.8 494.9(0.44) 1249.1 178.8(0.16) 1011.5 2.2(0.002) 968.3 2.2(0.002) 592.3 13.1(0.01) 489.5 116.2(0.10) 210.5 15.2(0.01) 193.0 1.1(0.001)
a Frequencies scaled by 0.897 factor. From ref 15. The Dunning's double-t(DZ) Frequencies have been scaled by 0.9. Relative intensities in parentheses
..e
e.
DZP+diffuseb
IA, - A,I = IA, - A,] exp(-t/t)
A
I (
I
Before Photolysis
--
L I
Photolysis -c---h-s& 2000 1950 WAVENUMBERS [ c m -11
2050
1900
Figure 9. Portion of the 12 K matrix infrared spectra of I2C/"C/Ar
+
HzO (0.15%) (['2C]:['3C] = 1:2) before photolysis and after 30 min of photolysis (Hg lamp filter: , I> 390 nm). Upper spectrum is the difference spectrum (after - before). The negative bands (filled triangles) are the asymmetric CC stretchs of the 12,'3C3*H20 complex isotopomers. The positive peaks (solid circles) in the 1992.5- 1907.9cm-' region are the asymmetric CC stretchs of the 12.'3C3-substituted t-HPD isotopomers.
+
amplitude with time.) The fact that the integral intensity of the 1999.6-cm-I (c-HPD) positive peak is approximately equal to the integral intensity of the 1992.6-cm-' (t-HPD) negative peak indicates that the product of the concentration and the transition strength for this mode is about the same for both cisoid and transoid isomers. Table 5 shows that calculations predict the same transition strength of the CC asymmetric stretch for both isomers. Next, the behavior of the combined COH bendKO stretch modes in the two rotamers is investigated. The difference spectrum (cf. Figure 12) shows that the integral intensity of the 1254.3-cm-' band (c-HPD) is approximately twice as intense as the 1223.1-cm-' band (t-HPD), if one assumes that the concentration changes (IAc(c-HPD)I = IAc(t-HPD)I) in the light-induced process are the same for both rotamers. The latter is a reasonable assumption if the rotamers interconvert and are not formed by any other process. Both calculations predict (cf.
where t is the relaxation (appearance/disappearance) time for either rotamer. If one rotamer converts directly into the other, the relaxation times should be the same for both. Figure 15 shows the plot of lnlA, - A,I vs time for both isomers at two different matrix temperatures. The slopes (= -UT) at a given temperature are the same for each rotamer. Hence, it can be concluded that the c-HPD conformer relaxes completely to t-HPD with a relaxation time of z = 10.8 min ( k = 1.54 x s-I) at 11 K and t = 8.2 min ( k = 2.03 x s-I) at 20 K. 2. Cisoid-to-TransoidActivation Barrier. Although higher temperature measurements were attempted, aggregation of the carbon clusters interferes at temperatures higher than 20 K." In addition, other reaction channels, probably different for the two isomers, open up. We observed different values for the relaxation times for the two conformers at higher temperatures. In addition, we found that the 1999.6-cm-' (c-HPD) peak of A, - A , was more intense than the 1992.6-cm-' (t-HPD) peak at elevated temperatures. Despite this difficulty, it is possible to obtain a rough estimate of the activation barrier for the cisoid-to-transoid HPD isomerization from the 11 and 20 K runs. Using
l/t(T) = A exp(-AE/RT) we find AE = 0.06 0.02 kJ/mol; this very small value leads to the question of whether there is a barrier at all. This value is equivalent to cu. 5 cm-'. There are two possible explanations for such a low value. First, it could simply represent a lower bound on the activation energy since the energies used in the IR probe beam in this experiment (7000-700 cm-I) are much greater than this value. Or, alternatively, the process involved in the conversion could involve hydrogen tunneling for which there should be no barrier. Both LZP and Herbst (vide infra) have suggested the possibility of such a mechanism for this system or systems like this one.
J. Phys. Chem., Vol. 99, No. 20, 1995 8009
Cs'H20 Complex in Argon Matrices
TABLE 6: Experimental (Ar Matrix) and Calculated (HF/6-31G*) IR Frequencies for All Isotopomers of Cisoid and Transoid 3-Hydroxypropadienylidene (HPD) for the Most Intense CC Asymmetric Stretch, CCH HCO Bend (Transoid HPD), and HCO Bend CO Stretch (Cisoid HPD) Modes
+
+
transoid HPD
vexp,cm-l 1992.5 1984.8 1974.7 1965.8 1937.6 1923.4 1918.5 1907.9 1223.1 1220.2 1221.7 1218.6 1221.7 1218.9 1221.7 1217.6
isotopomer 12-12-12-1 - 16- 1 12- 12- 13- 1- 16- 1 13- 12- 12- 1-16- 1 13-12-13-1-16-1 12-13-12-1-16-1 12-13-13-1-16-1 13-13-12-1-16-1 13-13-13-1-16-1 12-12-12-1-16-1 12-12-13-1-16-1 13-12-12-1-16-1 13-12-13-1-16-1 12-13-12-1-16-1 12-13-13-1-16-1 13-13-12-1-16-1 13-13-13-1-16-1
cisoid HPD - Vcak +2.8 +1.5 $4.5 +2.5 -1.3 -9.1 -0.4 -4.0 0.0 +o. 1 -0.2 -0.1 -0.7 +0.2 +0.5 -0.4
vcalc. cm-' 1989.7(0.99)",' 1983.3(0.95) 1970.2(1.00) 1963.3(0.96) 1938.9(0.95) 1932.5(0.91) 1918.9(0.96) 1911.9(0.92) 1223.1(0.85)'$ 1220.1(0.97) 1221.9(0.87) 1218.7(0.99) 1222.4(0.86) 1218.7(0.97) 1221.2(0.88) 1218.0(1.OO)
cm-l 1999.6 1991.3 1982.0 1973.1 1950.2 1942.2 1932.8 1923.5 1254.3
Vexp
vCdc.cm-l 2000.5(1.O)d 1992.5(0.96) 1982.7(1.00) 1973.9(0.96) 1950.4(0.95) 1942.0(0.91) 1932.3(0.96) 1922.9(0.91) 1254.3(0.97)'$ 1247.1(0.73) 1254.2(0.98) 1246.9(0.75) 1253.7(0.99) 1246.4(0.74) 1253.7(1.OO) 1246.2(0.77)
vexp.
-Vdc -0.9 -1.2 -0.7 -0.9 -0.2 +0.2 +0.5 +0.6 0.0
vexp
All frequencies for this mode scaled by 0.904. Relative intensities in parentheses. Scaled by 0.897 factor. Scaled by 0.903 factor. e Scaled by 0.879 factor. 'Due to the overlapped isotopomer bands, the assignment of the experimental frequencies for the CCH+HCO bend mode is tentative.
8
f *$
0
V
.2
* c
W V
.002
m
4f
3
3
.1
1230
1220
I
1
I
1
Differenee(5D)
c,. H ,o
(
1240
Photolysis
&Before
C,.
1210
H ,O
WAVENUMBERS [ c m -13 10
Figure 10. Portion of the 12 K matrix infrared spectra of 12C/13C/Ar H20 (0.15%) ([1zC]:[13C] = 1:2) after 30 min of photolysis (Hg lamp
+ + filter:
1 > 390 nm). Isotopomeric bands are due to transoid HPD
and are assigned in Table 6.
3. Stabilization Energy Difference. The difference in stabilization energies for the two rotamers may be determined from the intensity distribution of their CC asymmetric stretching vibrations. At long times (Le., 2 h) after the photolysis is terminated, the 1992.6-cm-' band is about 79 times more intense than the 1999.6-cm-' band. Taking the integral intensities of these two bands to be equal, the equilibrium constant for the two isomers at 11 K is
K , , , = [c-HPD]/[t-HPD] = 1.26 x lo-*
(7)
From this value, the standard free-energy difference for the two rotamers can be obtained from
AGIIKo= -RTln KllK= 0.4 f 0.15 kJ/mol
(8)
where AGI'KO is the standard free-energy difference between c-HPD and t-HPD conformers in equilibrium in an Ar matrix. Figure 16 shows a schematic diagram of the potential energy curve involved in the rotamerization process. The cisoid rotamer is less stable than the transoid. The experimental value of 0.4 kJ/mol is comparable to the standard free-energy difference of 0.12 kJ/mol for the two hydroxy rotamers of aminohydroxy-
9250
2100
HA"UYBERS
' '' I 1;
k - . - / L I- .
1 2050
,
1 2000
[ c m -11
Figure 11. Portion of the 12 K matrix infrared spectra of IZC,/Ar
+ +
H20 (0.2%) before and after 30 min of photolysis (Hg lamp filter: 1 > 390 nm) but recorded during photolysis. The starred bands are due to the I2C3*H20complex (2052.4 and 3258.9 cm-I) and probably to the 12C3*(H20)2 complex (2056.2 cm-I). The 33245.2- and 3258.9-cm-I bands are the V I v 3 combination bands in C3 and the C3sH20 complex, respectively. Bands due to c-HPD and t-HPD are so marked.
+
9-methylguanine in Ar at 12 K.37 It may be compared also with the theoretical value of the internal energy difference between the two rotamers. LZP in their ab initio studiesI5 of the HPD products predicted that the cisoid form should lie about 23.4 kJ/mol above the transoid form. This calculated difference in stabilization energies is practically independent of method; RHF, MP2, MP3, or MP4(SD) give the same results to f1.3 kJ/mol and are very different from the experimental value. While the matrix phase is expected to influence this value somewhat, this large discrepancy points to the possibility of a different mechanism involved in the conversion process. LZP also located the transition state for the isomerization of cisoid-to-transoid HPD. They calculated that the energy of the transition state lies 46 kJ/mol above the cisoid singlet ground state (MP2DZP level). The almost negligible value of the activation barrier we find here (0.06 kJ/mol) is also very different from the calculated one. As LZP point out, the
8010 J. Phys. Chem., Vol. 99, No. 20, 1995
Szczepanski et al.
I
c-HPD
p1
2P 3
0
After 30 min.Photolysis (hv,) .2 1
I
After 30 min. Photolysis (hv..)
0 1400
1200
,
2050
+
Figure 12. Portion of the 12 K matrix infrared of '2C/13C/Ar H20 (0.15%) ([12C]:['3C]= 1:2) before photolysis and after 30 min of filter: , I > 390 nm) but recorded during photolysis (Hg lamp photolysis. The 1254.3-cm-I band is due to the COH bend CO
+
stretch of c-HPD.
L
0-
1000
WAVENUMBERS [ c m -11
+
interconversion could be occurring via hydrogen tunneling, for which there should be no activation barrier.
2000
A
W 1800
1950
WAV!3NUMBERS [ c m -11
Figure 13. Portion of the 12 K matrix infrared spectra of '*C/I3C/Ar H20 (0.15%)([1zC]:[13C] = 1:2) after 30 min of photolysis (Hg lamp filter: 1 > 390 nm) and recorded with the photolysis lamp on (bottom) and off (middle). In the difference spectrum (top) are shown the CC asymmetric stretching modes of the isotopomers of c-HPD (positive peaks) and t-HPD (negative peaks).
+ +
1 ".,.
i
c-HPD
V. Discussion Finally, we discuss the possible significance of our observations with respect to the mechanisms of chemical reaction in the interstellar medium. Both C30 and propynal, the final products of photolysis of the C3*H2O complex, have been observed39 in almost equal abundance in the cold (10 K) molecular cloud, TMC-1. Herbst, Smith, and Adams have postulated40that the following ion-molecule reactions are the principal pathways of formation for C3O and HzC3O:
+ C O - H,C30+ + hv H,C,O+ + e- - C,O + H,
C,H,+
(94 (9b)
I
0
6
10 TfyB
18
20
[ d l
Figure 14. Time dependence of the difference spectrum (A, - A,) in the asymmetric CC stretch region: peaks plotted are 1999.6 cm-I (cHPD) and 1992.5cm-' (t-HPD). The t = 0 and subsequent plots were
+ e- - C,O + H, + H H3C30+ + e- - H,C,O + H
H,C,O+
(lob) (10c)
On the other hand, Brown and co-workers have suggested4' that the most important pathway for C 3 0 production is
+ c, -c30 + c 0 + c4-cc,+ co 0
(1W (1lb)
The estimates by Brown and co-workers4' and by Herbst and Leung4, for the formation of c30 are in good agreement with observations. Irvine et al. point that although the branching ratio in (10) is not known from laboratory studies, the fact that approximately equal abundances of c30 and propynal are observed must mean that (1Oc) is a significant reaction pathway. Our results suggest a possible different mechanism. We propose that the complexation of small carbon clusters (Le., C3) and H20 occurs on the small dust particles prevalent in interstellar clouds. As we have shown, the C3.H20 complex readily forms in low-temperature environments and, when photolyzed, forms the intermediates cisoid and transoid HPD.
scanned after the photolysis lamp was extinguished. Each spectrum was recorded for ca. 10 s (ie., 10 scans), with A , scanned after 2 h. The maximum absorbance occurs for the 1992.5-cm-' band at r = 2 h, at which time the 1999.6-cm-' band displays a minimum absorbance. Further photolysis results in C30 and propynal formation. Desorption from the surface of the dust particles could then take place by any of the mechanisms proposed by others p r e v i ~ u s l y . The ~ ~ matrix environment used in our studies is obviously different from the dust particles, but the conditions of low temperature and exposure to visible and/or ultraviolet radiation are common to both. The low activation energy necessary for cisoid-to-transoid rotamerization of the HPD intermediates is very suggestive that hydrogen tunneling is operative. Herbst has very recently proposeda that hydrogen tunneling could be a viable low-temperature process for the reaction between neutral species of astrophysical importance. If hydrogen tunneling occurs for the low-temperature cisoidto-transoid rotamerization, it may then also be involved in the other reaction steps, e.g., the t-HPD-to-propynal step(s). We are looking into this possibility. Hydrogen tunneling may also be important in the formation of other interstellar species. We are currently searching for other species which may form complexes with water and which may, under photolysis, yield species already known to exist in the interstellar medium.
J. Phys. Chem., Vol. 99, No. 20, 1995 8011
C3*H20Complex in Argon Matrices -5
-
1999.6 cm“:c-HPD
1992.5 em“: I-HPD
-5.5 -6
8
e -6.5 ’+
4: c
-
. .
-7
-7.5
,
. .
,
.
,
I , A E= 0.06kJlmol
-a
which exhibits IR bands at 1999.6 and 1254.3 cm-I. Two sets of eight 12C/13Cisotopomeric bands have been observed for the intense 1992.5-cm-’ t-HPD and 1999.6-cm-’ c-HPD asymmetric CC stretching modes. All observed bands are shown to match ab initio values reasonably well. The relaxation rate (in the dark) of c-HPD to t-HPD has been found to be 1.54 x s-l at 11 K and 2.03 x s-l at 20 K. A lower limit for the activation energy of this process has been established as approximately 0.06 kJ/mol. In addition, the different in standard free energies of the two rotamers has been determined to be 0.4 kJ/mol. Both of these values are much smaller than theoretical estimates. Thus, hydrogen tunneling is proposed as the major mechanism for the lowtemperature rotamerization.
0
5
10
15
Figure 15. Dependence of lnlA, - A,[ YS time after 30 min of photolysis (Hg lamp + filter: A. > 390 nm) of the ‘*C,/Ar + H 2 0
Acknowledgment. We gratefully acknowledge the National Aeronautics and Space Administration and the Petroleum Research Fund, administered by the American Chemical Society, for their support of this research.
(0.15%) matrix sample for T = 11 and 20 K. The A , and A , absorbances are identical to the ones shown in Figure 14.
References and Notes
TIME [min]
‘H
-n
transoid
0
cisoid
rl transoid
a (OH)
Figure 16. Potential energy curve for the OH intemal rotation of the = 0.4 & 0.15 HPD molecule in its electronic ground state. AGI~KO kJ/mol is the standard free-energy difference between c-HPD and t-HPD conformers at equilibrium (Ar matrix, 11 K), and AE = 0.06 & 0.02 kJ/mol is the energy barrier for conversion of the cisoid-to-transoid rotamers.
VI. Conclusions In this paper, the existence of a complex between C3 and H20 in an Ar matrix has been confirmed via 12C/’3Cisotopic labeling studies using FTIR spectroscopy in conjunction with ab initio calculations. LR frequencies attributable to the complex have been observed at 2052.3 cm-’ (asym. CC stretch), 3598 cm-’ (OH sym. stretch), 3712.2 cm-’ (OH asym. stretch), 1593.4 cm-’ (HOH bend), and 1214.3 cm-I (sym. CC stretch). An electronic band at 405.4 nm is assigned to the perturbed Ill - ‘ 2transition of the C3 portion of the complex. The groundstate geometry of the complex is determined to be planar with one hydrogen of H20 weakly bound to a terminal C of C3 with an inter-moiety angle of 175’. Photolysis of the C3*H20 complex results in the formation of c30 and cisoid and transoid 3-hydroxypropadienylidene (HPD). IR peaks assigned to t-HPD have been observed at 1992.5, 1460.8, 1280.5, 1223.1, and 1016.3 cm-l. Further photolysis of t-HPD results in its rotamerization to c-HPD,
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