J . Phys. Chem. 1990, 94, 7973-7977
7973
Reactions of Small Carbon Clusters with Water in Cryogenic Matrices. The FTIR Spectrum of Hydroxyethynylcarbene Bryan J. Ortman, Westvaco Research Laboratory, Johns Hopkins Road, Laurel, Maryland 20707
Robert H. Hauge, John L. Margrave,* Department of Chemistry and Rice Quantum Institute. Rice University, P.O. Box 1892, Houston, Texas 77251
and Zakya H. Kafafi* Naval Research Laboratory, Code 6551, Washington, D.C. 20375 (Received: February 9, 1990)
Reactions of monatomic, diatomic, and triatomic carbon with H20, H2I80,and D 2 0 in argon matrices at 12 K have been investigated by Fourier transform infrared spectroscopy. The absence of infrared absorption peaks due to Cl,2/H20product(s) suggest that carbon atoms and diatomic carbon molecules do not react with H 2 0 . However, triatomic carbon was found to form a C,(H,O) adduct with H 2 0 . This C3(H20)complex rearranged upon photolysis (A > 400 nm) and formed the hydroxyethynylcarbene, HCEC-C-OH. Vis/UV photolysis caused the rearrangement of this reaction intermediate via the elimination of the hydroxyl hydrogen into propynal. A parallel photolytic reaction, possibly through a different C3(H20) complex, and leading to the total dehydrogenation of C3(H20)and/or HCS-C-OH via the formation of C30, was also observed. C4 and C5 were present in small amounts in the argon matrix but no evidence for complexation or reaction with water was observed.
1. Introduction
In the past few years, there has been a revived interest in the studies of the formation, stability, and spectra of very small carbon cluster^.^-^^ Spectroscopic data on the simple triatomic carbon cluster are still limited. Recently, even the structure of groundstate C3has raised some interesting questions. Is it linear? Or is it quasilinear? Or is it bent? For a long time C3was thought to be a linear and floppy molecule due to its very low ground-state bending frequency, w 2 62 cm-I. Recent ab initio calculations
-
( I ) Jacox, M. E. J . Pfiys. Chem. ReJ Data 1984, 13(4), 945. (2) Kraemer. W. P.; Bunker, P. R.; Yoshimine, M. J . Mol. Spectrosc. 1984, 107, 191. (3) Kraetschmer, W.; Sorg, N.; Huffman, D. R. Surf. Sci. 1985, 156(2). (4) Beardsworth, R.; Bunker, P. R.; Jensen, P.; Kraemer, W. P. J . Mol. Spectrosc. 1986, 118, SO. (5) Magers, D. H.; Harrison, R. J.; Bartlett, R. J. J . Cfiem. Pfiys. 1986, 84, 3284. (6) Kraetschmer, W.; Nachtigall, K. NATO ASI Ser. C (Polycyclic Aromat. Hydrocarbons Astrophys.) 1987, 75, No. 191. (7) Michalska, D.; Chojnacki, H.; Hess Jr., B. A.; Schaad, L. J. Chem. Pfiys. Lett. 1987, 141, 356.
(8) Smith, R. S.; Anselment, M.; DiMauro, L. F.; Frye, J. M.; Sears, T. J. J . Chem. Phys. 1987,87. 4435; 1988, 89. 2591. (9) Pacchioni, G.;Koutecky, J. J . Chem. Pfiys. 1988, 88, 1066. (IO) Matsumura, K.; Kanamori, H.; Kawaguchi, K.; Hirota, E. J . Cfiem. Phys. 1988,89, 3491. ( I I ) Vala, M.; Chandrasekhar, T. M.; Szczepanski, J.; Van Zee, R.; Weltner Jr., W . J . Cfiem. Pfiys. 1989, 90, 595 and references therein. (12) Heath, J. R.; Cooksy, A. L.; Gruebele, M. H. W.; Schmuttenmaer, C. A.; Saykally, R. J. Science 1989, 244, 564. (13) Bernath, P. F.; Hinkle, K. H.; Keady, J. J. Science 1989, 244, 562. (14) Moazzen-Ahmadi, N.; McKellar, A. R. W.; Armano, T. J. Cfiem. Phys. 1989, 91, 2140. (15) Rohlfing, E. A.; Goldsmith, J. E. M. J. Chem. Phys. 1989, 90,6804. (16) Kawaguchi, K.; Matsumura, K.; Kanamori, H.; Hirota, E. J . Chem. Phys. 1989, 91. 1953. (17) Lemire, G.W.; Fu, Z.; Hamrick, Y. M.; Taylor, S.; Morse, M. D. J . Pfiys. Chem., in press. (18) Jensen, P. Cfiem. Pfiys. Lett., in press. (19) Saykally, R. J. Private communication. (20) Shen, L. N.; Graham, W. R. M. J. Cfiem. Phys. 1989, 91, 5 1 1 5 . (21) Cheung, H. M.; Graham, W. R. M. J . Chem. Pfiys. 1989,91,6664. (22) Kurtz, J.; Huffman, D. R. J. Cfiem. Pfiys. 1990, 92, 30. (23) Weltner, Jr., W.; Van Zee, R. J. Cfiem. Rev. 1989, 89, 1713.
0022-3654/90/2094-7973$02.50/0
have shown that C3 is bent with an equilibrium bond angle of 162' and that there is a small barrier to linearity of the order of 21 cm-1.294It was suggested that C3 becomes more rigid upon symmetric stretching and undergoes bending when stretched asymmetrically. These theoretical results agreed with the recent experimental studies by Rohlfing and G01dsmith.l~Using stimulated emission pumping spectroscopy (SEP), they showed that there is a strong coupling between bending and stretching in jet-cooled C3. Using a Morse oscillator-rigid bender internal dynamics (MORBID) Hamiltonian to refine the potential energy of the ground state of C3, Jensen used the experimental data of Smith et aL8 and Matsumura et a1.I0 and calculated a barrier to linearity of 16.5 cm-I and an equilibrium bond angle of 162.5O.'* Using diode-laser absorption spectroscopy on a supersonic carbon cluster beam, recent studies at Berkeley have confirmed the bent structure of C3.I9 Similarly, C4 has been the subject of some controversial studies, particularly with respect to its ground-state s t r u c t ~ r e . ' * ~ * ~ ~ ~ ~ ~ Ab initio calculations have supported two geometries that lie very close in energy: one is a linear cumulene structure and the other is a rhombus.' Recent FTIRZoand EPRZ1matrix studies have supported a slightly bent geometry. The structure of C5has been unequivocally determined to be linear based on the elegant matrix isolation studies of Vala et a1.I' In a combined infrared and ultraviolet-visible matrix isolation study, Kurtz and Huffmad' have correlated the IR bands measured at 1952 and 1998 cm-I and assigned to nearly linear C6 and C8with two new UV bands at -2465 and -3075 A, respectively. The present paper focuses on the reactions and photochemistry of small carbon clusters with water in inert gas matrices. Little is known about the reactivity of these carbon species. There have been few theoretical studies on the proposed products of the reactions of c1,24s25 and C328-31 with H 2 0 . However, only in the (24) Ahmed, S. N.; McKee, M. L.; Shevlin, P. B. J. Am. Cfiem.Soc. 1983, 105, 3942. (25) Frisch, M. J.; Krishnan, R.; Pople, J. A. J . Pfiys. Cfiem. 1981, 85, 1467. . .- . (26) Bouma, W. J.; Nobes, R. H.; Radom, L.; Woodward, C. E. J. Org. Cfiem. 1982, 47, 1869. (27) Tanaka. K.; Yoshimine, M. J. A m . Cfiem. SOC.1980, 102, 7655. (28) Radom, L. Aust. J. Cfiem. 1978, 31, I . (29) Komornick, A.; Dykstra, E. E.; Vincent, M. A,; Radom, L. J . A m . Cfiem. SOC.1981, 103, 1652.
0 1990 American Chemical Society
7974
The Journal of Physical Chemistry, Vol. 94, No. 20, 1990
Ortman et al. ALUMINA THERMOCOUPLE INSULATOR
4
TO VACUUM FTlR SPECTROMETER 1
MATRIX BLOCK
END VIEW OF THE SAMPLE PELLET
WIRES
SET SCREW +
T
E
FIVE-SIDED SHIELDED P E R MIRROR
SAMPLE HOLDING RING
Graphite pellet wires inside sample holder
NaCl WINDOW
LIQUID N, SHIELD
NaCl LENS
I co,
I
L ~ E BEAM R
PUMPING CHAMBER
i Figure 1 . Top view of the multisurface matrix isolation apparatus using a CO, laser for graphite vaporization.
case of C , / H 2 0 has there been an experimental study on C , reactivity. Using ab initio calculations and gas-phase experimental results, Ahmed, McKee, and Shevlin showed that C('D), and not C(,P), reacted with water to produce C O + H2 and formaldehyde.24 In the present study, a convenient method of studying the reactivity and resultant products of triplet C , , C, and C, with a reactant gas (in this case HzO) is described. By the use of laser-vaporization matrix-isolation techniques, carbon atoms and small clusters were prepared in argon matrices doped with H,O a t 12 K. The matrix-isolation technique, coupled with Fourier transform infrared spectrometry, affords a method whereby adducts, reaction intermediates, and final products can be analyzed. From the FTlR spectra, it was concluded that ground-state triplet C , and ground-state singlet and first excited-state triplet C2 do not react with H,O, while C3 forms a C 3 ( H 2 0 ) adduct. The photochemistry of the C,( HzO) adduct was then studied via broad-band irradiation of the matrices with a medium-pressure Hg-discharge lamp. It was found that Cj(H2O) undergoes two parallel reactions. In the first, C, inserts into the H - 0 bond of H 2 0 to give the previously uncharacterized carbene, HC=CCOH, which then rearranges upon vis/UV photolysis to form propynal. In the second reaction, C 3 0 was formed as the final dehydrogenation product. 11. Experimental Section A schematic diagram of the CO, laser-vaporization matrix-
isolation apparatus used in these experiments is presented in Figure 1. A complete description of the multisurface matrix-isolation apparatus has been given by Hauge et al.,, The present system differs only in the method of generating the carbon vapors: a C 0 2 laser, rather than a resistively heated furnace, is used as a heating source. In this design, the laser beam passes through a 250-mm NaCl focusing lens and then a NaCl window before entering the laser-vaporization vacuum chamber, where it is reflected from a copper mirror onto the 2.5 X 3.0 mm graphite sample. One of five samples mounted on a movable rod can be accessed by (30) Farnell, L.; Radom, L. Chem. Phys. Lett. 1982, 91. 373; 1983, 99, 516. (31) Greenberg, A.; Tomkins. R. P. T.; Dobrovolny, M.; Liebman, J . F. J . Am. Chem. Soc. 1983, 105, 6 8 5 5 . (32) Hauge, R. H.; Fredin, L.: Kafafi, 2. H.: Margrave, J. L. Appl. Spectrosc. 1986. 40, 5 8 8 .
Figure 2. A close-up view of the sample holder where a graphite pellet is held by alumina-insulated thin tungsten wires.
vertically positioning the sample holder. The samples are positioned at a 30' angle with respect to the matrix block. Six alumina-insulated tungsten wires support the graphite samples in the sample holder (Figure 2), thus minimizing heat loss to the rod. Through a viewing window in the laser-vaporization chamber, one can make optical pyrometric measurements to determine the temperature of the sample. The spot at the center of the sample (0.5 mm in diameter) is approximately 200 times hotter than the rest of the sample. Carbon vapor species passing from an aperture of the liquid N, trap are cocondensed with the reactant gas and an excess of an inert gas onto one of the surfaces of the multisurface matrix block. During this deposition, matrix gas and the reactant gas ( H 2 0 ) are passed through 1 /4-in. stainless steel tubes directed at the matrix surface. In a typical experiment, the water concentration was held constant by controlling its flow through a needle valve, while the carbon concentration was varied by adjusting the laser power. Usually three matrices with three different carbon concentrations were prepared by vaporizing the graphite sample at three different temperatures. The sample rod was then vertically moved to a new sample position for the next series of experiments. During the last deposition, the highest concentration of carbon vapors was achieved by raising the C 0 2 laser power to the point where a hole was burnt through the graphite sample. The operating temperature varied between 2250 and 2650 OC for the lowest and highest deposition rates of carbon vapors. A quartz crystal microbalance, directly mounted on the cold block, was used to determine the molar ratios of carbon vapors to water to argon. The concentration of HzO was varied from 0 to I O parts per thousand argon. Separate graphite samples were dedicated to quartz crystal measurements. From the thermodythe namic equilibrium values given in the JANAF concentrations of C,, C2, and C, species in the argon matrix were estimated. During the laser burn-through of the graphite sample, Le., the highest achievable deposition rate, this concentration was measured to be 0.38,O. 13, 1.55 parts per thousand argon for C,, C,, and C3, respectively. In a typical experiment, carbon, water, and argon were cocondensed onto a polished rhodium-plated copper surface maintained at 12 K by the use of a closed-cycle helium refrigerator. After a 15-min deposition, the matrix block was rotated 180' and an infrared spectrum was measured over the range 4000-450 cm-l with a vacuum IBM IR-98 Fourier transform infrared spectrometer. Visible or ultraviolet photolysis was applied after deposition by exposing the matrices to a medium-pressure 100-W Hg lamp. Cutoff filters (400 and 500 nm), as well as a band-pass filter for the range 380-280 nm, were used in the photolysis experiments. (33) J A N A F Thermochemical Tables, 2nd ed.; NSRDS-NBS 31; National Bureau of Standards: Washington, DC, 1977. (34) Engdahl, A,; Nelander, B. Chem. Phys. Left. 1983, 100, 129. (35) Thompson, K. R.; Dekock. R. L.; Weltner, Jr., W. J. Am. G e m . Soc. 1971. 93. 4688.
The Journal of Physical Chemistry, Vol. 94, No. 20, I990 7975
Reactions of Small Carbon Clusters with Water
AJ H-CEC-C-H
4
-OH
m
I
a
am a
B-A
3250
I
I
2100
2000
i;(cm-') Figure 3. FTIR absorption spectra of various gaseous species produced by laser vaporization from a graphite pellet and cocondensed with excess argon (a) in the absence and (b) in the presence of water. The carbon source was an isotropic form of graphite, called Graphnol, obtained from Raytheon. H2I80(98% purity) and D 2 0 (99.8% purity), both obtained from Cambridge Isotope Laboratories, were also deposited with carbon in excess argon (99.9998% purity) obtained from Matheson Gas Products. 111. Results A portion of the FTIR spectrum of laser-vaporized graphite in solid argon at 12 K is shown in part a of Figure 3. The band located at 3289 cm-l is assigned to the C-H stretching mode of acetylene. C2H2is formed from reaction of C2 with residual H2, on the matrix surface. It is speculated that residual H2 is present in the system from either cracking of the pump oil on hot surfaces or outgassing of the graphite sample. Further experiments with excess H 2 and D2 verified the above conclusion.36 C O was also present as a residual gas as shown from its characteristic IR absorption band. In addition to these impurities' bands observed in the region above 2000 cm-I, absorption bands appeared at 3245, 2164,2039, and 2034 cm-I. All these bands varied with the carbon concentration. With the exception of the 2164-cm-I peak, the other three bands retained the same relative intensity upon variation of the carbon concentration. The doublet at 2034 and 2039 cm-' has been previously observed and assigned to the asymmetric stretch of C3. Hence the 3245-cm-I band must also be associated with C3. A possible assignment is the combination mode of the symmetric and asymmetric stretch of triatomic carbon. The 21 64-cm-l band has been recently and unequivocally reassigned to CS." Figure 3b shows a portion of the FTIR spectrum of laser-vaporized graphite with H 2 0 added in solid argon. The water concentration was varied between 2 and 10 parts per thousand argon. In all these experiments, only three new bands were observed upon adding either H 2 0 , H2I80,or D 2 0 and were located at 3240, 2149, and 2052 cm-l, respectively. The 3240- and 2 149-cm-I bands have been previously reported and assigned to the C2H2(H20)30and CO(H20)31 adducts, respectively. The presence of C O in the spectra is due to the continual outgassing of the graphite sample and not to the reaction between carbon and water. This conclusion is further supported by H2I80 experiments where no CI8O was formed. On the other hand, acetylene was also always present, but directly dependent on the carbon concentration. This is further discussed in the C 2 / H 2 0 (36) (a) Ortman, B. J . Ph.D. Thesis, Rice University, May, 1987. (b) Ortman, B. J.; Hauge, R. H.; Margrave, J. L. J . Quant. Spectrosr. Radiat. Tramfer 1988, 40, 439. (c) Ortman, 9. J.; Kafafi, Z. H.; Hauge, R. H.; Margrave, J. L. Unpublished results.
I
I
1
I
I
2200 2000 1800 1600 1400 1200 1000 i; (cm- ')
Figure 4. FTIR difference spectra reflecting the photochemistry of triatomic carbon cluster with water in solid argon. Molar ratio of C,:H20:Ar 1:10:1000. (A) A > 500 nm; (B) A > 400 nm;and (C) 280 nm < A < 360 nm. Note the formation of propynal with visible photolysis (A > 400 nm) (denoted with asterisks).
-
section. The 2052-cm-I band has also been previously observed, but it was attributed to a CO/carbon-related species.3s This band is located very close to the C3 asymmetric stretch and varies as a function of the water and C3 concentrations. Thus, evidence from the present study suggests that it is related to a carbon/water product, specifically a complex between triatomic carbon and water. No perturbation on the water bending and/or stretching frequencies was detected for the C3(H20) adduct. Difference spectra for a series of photolysis experiments are depicted in Figure 4. For A > 420 nm, no new bands were observed; however, upon photolysis at X > 400 nm, the 2052-cm-' peak disappeared completely with the concomitant growth of a peak at 2243 cm-l assigned to C30,36and two sets of new features located at 1016.1, 1221.7, 1252.5, 1459.6, 1992.8, and 1999.8 cm-l and at 942.5, 1688.6, and 2108.3 cm-l, respectively. Further photolysis of the matrix in the UV (320 nm < X < 380 nm) region produced no new product peaks. However, photolysis in the shorter UV (280 nm < X < 360 nm) region caused the disappearance of the peaks at 1016.1, 1221.7, 1252.5, 1459.6, 1992.8, and 1999.8 cm-] and the further growth of the peaks at 942.5, 1688.6, and 2108.3 cm-I, respectively. Similar experiments were conducted with H21s0 and D 2 0 . Photolysis experiments with D 2 0 as the reactant species were slow and required approximately four times as long. Annealing studies of the unphotolyzed and photolyzed matrices produced a growth in the 2052-cm-I band, while the H,O and C3 absorption bands were reduced.
IV. Discussion 1. Cl Chemistry with H 2 0 . A number of different products might be expected to form in carbon-water reactions, such as H,CO, HCOH, C(H20),or C O and H2. None of these products is detected, indicating that ground-state C I does not react with H 2 0 in solid argon at 12 K. This conclusion is further supported by the experimental and theoretical results of Shevlin et al.24 They showed that C(lD) atoms and not C(3P) atoms were reactive toward H 2 0 in the gaseous state to produce C O + H2 and formaldehyde. They predicted that in the case of the triplet carbon, a C ( H 2 0 ) complex would form and be stabilized by 6.5 kcal/mol. However, the barriers to further reaction are higher than that for dissociation to carbon and water, so triplet carbon atoms should be unreactive toward water. Since Shevlin's results do not preclude the formation of a C ( H 2 0 ) adduct in the gas phase, our matrix experiments could have trapped this C(H,O) adduct if it indeed existed. However, it is possible that the IR
Ortman et al.
7976 The Journal of Physical Chemistry, Vol. 94, No. 20, 1990 TABLE I: Comparison of Frequencies for the Cj.Symmetric Stretch, Y, (em'). in the Gas Phase and in Inert Gas Matrices method of measurement medium p I , cm-I, of C, reference 33 gas phase 1224.5 absorption spectra I235 32 N e matrix emission spectra present study Ar matrix [1206]" infrared spectra ~~~~~
~
a Estimated from the measured frequencies of the combination modes, uI + u3 = 3245 cm-l and the asymmetric stretching mode, u3 = 2039
cm-'.
spectrum of C ( H 2 0 ) like that of C 3 ( H 2 0 )does not exhibit any water bending and stretching absorption bands and thus will not be detected in the IR region explored in this study. Photolysis experiments do not give rise to any new peaks that can be assigned to a photoproduct formed from the rearrangement or fragmentation of a C ( H 2 0 ) complex. Since carbon atoms have no electronically allowed transitions in the UV region, the photochemical study needs to be extended to the vacuum UV before any conclusions can be drawn regarding the photochemistry of the C l / H 2 0 system. 2. C2 Chemistry with H 2 0 . The 3240-cm-I peak, which was assigned to the C 2 H 2 ( H 2 0 adduct, ) was the only IR detectable product involving a reaction between C2 and possibly H 2 0 . As one can deduce from Figure 3a, C2H2,characterized by the H-C stretch at 3289 cm-', is always present, even when no H 2 0 is added. This is due to the fact that residual H2 reacts with C2 on the matrix surface to form C2H2. The residual H2 in the vacuum system turns out to be a convenient tool for deducing the reactivity of C2 with an added reactant. In experiments conducted in which C2 does react with the reactant, as in the case of CO, the C2H2 peak becomes depressed.36 This indicates a competition between H2 and C O for the available C2. Ultimately, at high concentrations of CO, the C2 H2 reaction is completely eliminated. However, in the case of C2 with H 2 0 , the C2H2 peak is not depressed, and a C2H2(H20)peak grows in. Only two reactions seem to be possible and they are C2 + H2 C2H2 + H2O C,H,(H,O) (1)
+
+
C2
+ H2O
TABLE 11: FTIR Measured Frequencies (em-') of Propynal and Its Isotopomers in Solid Argon
~~
+
+
C2(H20)
+ H2
+
C2H,(H,O)
(2)
The first reaction involves the activation of the H-H bond by diatomic carbon and the formation of acetylene. The latter forms an adduct with H 2 0 . In the second reaction, water initially forms an adduct with C2. This C2(H20)complex inserts into the H-H bond of molecular hydrogen and forms the acetylene water complex. One can eliminate the second reaction on the basis of the absence of a C2H2(D20)peak when D 2 0 is used instead of H 2 0 as the reactant. The photolysis study further supports this conclusion due to the absence of any photoproduct that may result from photolysis of C2(H20),a reaction product of C2 and H 2 0 . In conclusion, C2 is found to be unreactive toward H 2 0 in cryogenic argon matrices at 12 K. 3. C3 Chemistry with H 2 0 . With the exception of the 3245 cm-l new absorption band, the infrared spectrum of matrix-isolated C, has been previously This band has been assigned to the combination mode of the symmetric and asymmetric stretch of Cj. Therefore, an estimate of the frequency of the symmetric stretch of C3 may be deduced from v l + v, and v,. Table I shows that the estimated value of v l compares well with those measured in the gas phase by Mererj9 and in neon matrices by Weltner and McLeod.jS This good agreement supports the mode assignment. The doublet absorption bands located at 2039 and 2034 cm-l have been assigned to the asymmetric stretching mode of C3. The splitting in this band is probably due to the interaction of C3 with two different argon matrix sites. In the presence of H 2 0 , a new peak grows in at 2052 cm-I, which is attributed to the asymmetric stretching mode of C3 perturbed by a water molecule, Le., a (37) Dekwk, R. L.; Weltner, Jr., W. J . Am. Chem. SOC.1971, 93, 7106. (38) Weltner. Jr., W.; McCleod, Jr.. D. J. Chem. Phys. 1964, 40, 1305. (39) Merer, A . J. Con.J . Phys. 1962, 45, 4103.
CGC str C=O str C-C str
2106.0 1696.9 943.1
2108.3 1688.6 942.5
2108.3 1653.4 938.7
1662.1 868.3
I n the gas phase, ref 34. TABLE 111: ITIR Frequencies (em-') Measured for HCM-C-OH and HC=C-C--'80H in Solid Argon vibrational mode HC=C-C-OH HCEC-C--'*OH 1016.1 1003.8 C-C stretch 1215.4 C-0-H out of plane def 1221.7 1252.5 1230.9 C-0 stretch 1454.3 C-0-H in-plane bend 1459.6 1992.8, 1999.8 1992.1, 1999.3 C=C stretch
C3(H20)complex. N o H 2 0 frequencies were detected for the triatomic carbon-water complex, suggesting little perturbation of the water molecule by C,. Three different photoproducts are observed in the photolysis studies carried out on the C3(H20) adduct in solid argon. Identification of the three different species is straightforward, since the three sets of photoproduct absorption bands exhibit different behavior in the photolysis experiments. One of the photoproducts formed with X > 400 nm photolysis has infrared absorptions in the C-C, C=C, and C - 0 stretching regions, with other peaks possibly in the C-0-H deformation region. A possible candidate for this set of peaks is hydroxyethynylcarbene. This assignment is further confirmed by the UV photorearrangement of this carbene into propynal whose gas phase infrared spectrum has been previously identified.40 Isotopic labeling experiments with Ols support this assignment. The other photolysis product is C 3 0 whose spectrum compares In H 2 0 quite well with the published matrix-isolation spectrum!' experiments, C 3 0 is identified by its absorption at 2243 cm-', while in H 2 l S 0experiments, C3I8Ois formed and identified by its absorption at 2224 cm-'. C 3 0 has been previously produced by the and the codeposition of C O C2,36the photolysis of c302,36 pyrolysis of fumaroyl dichloride!' The present study shows a new photochemical way to synthesize the triatomic carbon monoxide, namely via the C3(H20)photoinduced dehydrogenation reaction. In contrast to C,O being identified by its known argon matrix isolation ~ p e c t r u m , ~propynal ' is recognized by comparing its measured frequencies in an argon matrix with gas-phase frequencies measured by Brand, Callomon, and Watson.40 In addition to the good agreement between the matrix and gas-phase frequencies, the measured oxygen- 18 isotopic shifts support the mode assignment to propynal. These measured frequencies are all listed in Table 11. Absorption bands due to the C-H stretches, along with all the bending modes of propynal, have not been observed. This is not surprising in light of the weak spectrum detected for this molecule in the present study. The absorption due to the C=O stretch is by far the most intense of the three observed bands. Five frequencies were measured for HCEC-C-OH and HC=C-C--l*OH and are listed in Table 111. No absorption bands were observed in the D 2 0 experiments, presumably due to the low absorption coefficients of the deuterated products. The vibrational band at 1252.5 cm-' has been assigned to the C - 0 stretch. This is consistent with the measured large isotopic shift of 21.6 cm-' with H2I80 substitution. The two frequencies at 1221.7 and 1459.6 cm-I have been assigned to the C-0-H outof-plane deformation mode and the C-0-H in-plane bending mode, respectively. These two bands show approximately the same I8Oshift, of the order of 5-6 cm-I. The vibrational bands at 1992.8
+
(40) Brand. J . C. D.; Callomon. J. H.; Watson, J. K . G.Discuss. Faraday So;. 1962, 35, 175. (41) Brown. R. D.: Pullin. D. E.: Rice. E. H. N.; Rodler, M . J. Am. Chem. So:. 1985, 107, 7877
Reactions of Small Carbon Clusters with Water
C, + H20
X>400 nm
c,o + H ~ -
J ..
H-c~c-C-O-H
i
H-C=C-C
/P ' H
Figure 5. Observed reaction pathways and photochemistry of triatomic carbon cluster with water in solid argon.
and 1999.8 cm-l have been assigned to the C=C stretch. The doublet feature is explained as due to matrix site splitting. No '*Oisotopic shift was measured, as expected for this mode. It should be noted that the C=C stretch is 108 cm-' lower than the corresponding C=C stretch in propynal, which indicates that the C=C bond in the carbene has lost some of its triple bond character. The vibrational band at 1016.1 cm-I, assigned to the C-C stretch, is 1 18.6 cm-' higher than the corresponding C-C single bond in propynal. This is again supporting the idea of an increase in the double bond character of the C-C single bond. The lower C=C and higher C-C stretching frequencies suggest that this compound has a carbene structure ( l ) ,partially resonant with a dicarbene structure (2). H-c=c-~-o-H (1) H-C=C=C-O-H (2)
-
The reaction and photolysis of C3/H20 chemistry is summarized in Figure 5. Photolysis at X > 400 nm causes the rearrangement of C3(HzO) into the intermediate, hydroxyethynylcarbene, which partially photoconverts to the final product, propynal. A competitive photoreaction takes place and leads to the dehydrogenation of C3(H20)and/or HCECCOH with the formation of C 3 0 . It is also possible that the precursor of C 3 0 is another form of C3(H20). Upon further photolysis with UV light, the hydroxycarbene intermediate is totally converted into the final product, propynal. The formation of propynal as the final product is not surprising on the basis of calculated thermodynamic values of Radom2* Using ab initio molecular orbital calculations, he predicted that propynal lies 2.9 kcal/mol lower
The Journal of Physical Chemistry, Vol. 94, No. 20, 1990 7977 and cyclopropenone 3.2 kcal/mol higher than propadienone. Unlike the neutral C 3 H 2 0species, the most stable two isomers of [C,H20]+ were found to be the propadienone cation and the molecule, [CH==CH-CO]+ with no neutral stable c o ~ n t e r p a r t . ~ ~ Ab initio molecular orbital calculations have also predicted that propynal is kinetically stable with respect to dissociation to acetylene and carbon monoxide, but it is thermodynamically less stable than these products. Propynal was recently detected in the cold cloud TMC-I, with an abundance similar to that of the tricarbon monoxide, C30.43 The low kinetic temperature (- I O K) of this nearby dark interstellar cloud suggests that propynal must be formed from an exothermic reaction with very low activation barrier. It is proposed that acetylene cation and carbon monoxide are the reactant precursors that lead to the formation of propynal through a dissociative electron recombination process. V. Concluding Remarks 1. Neither ground-state C I nor C2 forms stable adducts or products with H 2 0 in argon matrices at 12 K. C3 has been shown to have no activation barrier in the formation of an adduct with H 2 0 in solid argon at 12 K. In spite of matrix isolation of larger carbon clusters such as C4 and Cs in small amounts, no IR evidence for complexation or reaction with water was observed. 2. C3(H2O) undergoes a unique sequence of photochemical reactions. In one reaction, the dehydrogenation product C 3 0 is formed. In the second parallel reaction a unique intermediate is formed and isolated for the first time, the hydroxyethynylcarbene HC=C-C-OH. The latter photorearranges with visible or ultraviolet light into propynal. 3. The mechanism of the C3/H20cryogenic reaction leading to the astrophysically important molecule, propynal, has been delineated for the first time. 4. Finally, a new frequency, 3245 cm-', was observed and assigned to the combination mode of the asymmetric and symmetric stretch of the C3 molecule. From this band, the frequency of the symmetric stretch of C3 was estimated to be 1206 cm-I.
Acknowledgment. We acknowledge financial support from the National Science Foundation. Registry No. C, 7440-44-0; C,, 12070-15-4; C,, 12075-35-3; H20, 7732-18-5; HCeCCOH, 128780-74-5. (42) Bouchoux, G.; Hoppilliard, Y.; Flament, J.-P.;Terlouw, J. K.; Valk, F. V. D. J . Phys. Chem. 1986, 90, 1582. (43) Irvine, W. M.; Brown, R. D.; Cragg, D. M.; Friberg, P.; Godfrey, P. D.; Kaifu, N.; Matthews, H . E.: Ohishi, M.: Suzuki, H.; Takeo, H. Astrophys. J . 1989, 335, L89.
