J. Phys. Chem. 1992,96,9565-9568
9565
Formation of Coordlnatlvely Unsaturated H,Fe-(CO),, In Solld Matrlxes at 77 K by Irradlatlon of H 2 F e ~ ( C 0 ) 1 1 )A: Model for the Photoreaction Postulated To Form H,Fe*(CO),, on the Surface of Slllca Sa-
Yamamoto,**tPyotaka Asakura,* Atsuhiko Nitta,t and Ham K u r d *
Central Research Institute, Mitsui Toatsu Chemicals, Inc.. 1190 Kasama-cho, Sakae-ku, Yokohama 247, Japan, and Department of Chemistry, Faculty of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyou- ku, Tokyo 11 3, Japan (Received: June 1, 1992; In Final Form August 17, 1992)
The photochemical reaction of dihydroirontriosmiumtridecacarbonyl, H~FCOS~(CO)~~, was investigated by Uv-visible and FT-IR absorption spectroscopiesat 77 K both in a polystyrene film and a 3-methylpentane matrix. Upon Uv-visible light irradiation (irrespective of the excitation wavelength from 313 to 435 nm), H2FeOs3(CO)13undergoes clean CO dissociation yielding selectively a single product. One CO is released from each parent carbonyl cluster that reacts. Upon warming to 298 K the eficient recombination of CO trapped in nearby sites with the photoproduct completely regenerates the parent cluster. From these results, the photoproduct is identified as the coordinatively unsaturated cluster H ~ F C O S ~ ( Cwith O)~~ CO ligand. The W-visibleand IR spectra of matrix-isolated a arordinativelyunsaturated site at t h e 4 atom having no H2FCOS3(CO)12 agree with that of the photoproduct formed when H2F&’(m)13 adsorbed on the surfaceof silica is irradiated. This spectral agreement allows the photoproduct on silica to be assigned as H~F&’(CO)I~,which supports our previous postulate about the photochemical process occumng when the H2FeOs3(CO)13/silicasystem is irradiated.
IntrodUCtiOD
Thermal treatments have traditionally been used in the prep aration of catalytically active metal carbonyl species on oxide supp0rts.l As an alternative, a photochemical approach is expected to provide several advantages over traditional thermal treatment for the preparation of such surface species because of selective bond excitation. Motivated by this expectation, we have been investigating the photochemical transformation of polynuclear metal carbonyls to catalytically active sjxciea on oxide supports.Recently,.we investigated the photochemical reaction of H2F&’(Co)l’ (depicted in 1 ) adsorbed on silica and found that
1
2
UV-visible tight irradiation causes H ~ F C O S ~ ( Cto O )undergo ~~ clean CO diapociation to yield a single subcarbony1species on the surface.’ On the basiis of IR, UV-visible, and EXAFS spectroscopic analyses, as well as chemical analysis of the gas evolved, we concluded that the coordinatively unsaturated cluster H2F+ Os’(CO)12 was p r o d u d on the surface of silica. We ascribed the selective formation and stabilization of the unstable coordinatively unsaturated H2FeOs3(CO)12to the efficient release of dissociated CO and interaction of the photoproduct with the surface hydroxy group of silica. However, the structure of the photoproduct has not been determined beyond doubt to be H2FeOs’(CO)12 because it has not yet been isolated in a stable crystalline form nor have its IR and Uv-visible spectra yet been reported. To confii our hypothesis that the unsaturated cluster is fonncd on the surface of silica,the spectmecop‘ccharactclizatim and elucidation of the chemical reactivity of the coordinatively unsaturated H2FeOs3(C0)12are necessary. The matrix isolation technique is a wetl-catablishcd method for determining the structure of unstable intermediates. With this technique, an unstable intermediate is formed by photolysis of a pnrent m o l d e isdated in a solid matrix. several coordinatively Mitsui T a b u Chemicals. ‘The University of Tokyo.
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unsaturated metal carbonyl species have been trapped and characteked by using this technique with f r o m rare gasea‘ and hydrocarbons5 as well as polymer films6 as the solid matrix. Therefore, it was expected that this technique will allow the coordinatively unsaturated H2FeOs3(CO)12 to be isolated and characterized by UV-visible and IR spectroscopies. We conducted this study on the photochemical reaction of
H2F&S3(CO)l’byusingapolystynnefilmandaZmethylpeQtane matrix at 77 K for furthcr confirmation of our earlier identification of the photoproduct from H2F&S3(CO),3 on silica. In this study, we observed for the first time the Uv-visible and FT-IRabsorptioa spectra Of Coordinetively unsaturated H~FCOS’(CO)~~ formed by the excitation wavelength independent dissociation of CO from H2FeOs3(CO)lp.On the basis of the results obtained in this and previous work, the special features of the photochemistry of H2FeOs3(CO)13are described.
Expekentd section The metal carbonyl cluster H2FeOs3(CO)13was prepared according to the method reported in the literature? The 3methylpentane (Cica-reagent, Kanto Chemical) was dried over CaC12and then distilled from Na metal and LiAIH, powder prior to use. Benzene (Uvasol, Merck) was used without further purification. The carbonyl cluster was dissolved in 3-methylpcntane with the concentration limited to about 0.1 mM to avoid aggregation of cluster molecules when the sample was cooled to 77 K. The solution was deoxygenated by bubbling helium through it for about 30 min. The solution was then loaded into an IR cell containingsapphircwindows(2.0 mm p a t h le@) or into a quartz cell (10 mm X 10 mm) under an argon atmosphere in a drybox (DRI-LAB equipped with HE493 DRI-TRAIN, Vacuum Atmospheres Co.). The use of 3-methylpentane as the solvent for matrix isolation of coordinatively unsaturated carbonyl clusters is common. However, 3-mcthylpcntanc has an IR abeurption band at 2132 cm-’that overlap with the weak stretching absorption arising from trapped CO. Since the intensity of the 3-methylpentane absorption increases drastically with decreasing temperature, it is difficult to observe the weak abmrption resulting from the dissociated CO trapped in the matrix. As an alternative matrix system, we used a polystyrene f h to isolate the carbonyl cluster. Polystyrene films arc t I ” t in the uv-visiblc region above 300 nm, and thcy haveno IR absorption bands at 2132~11-I. Moreover,it is possible to prepare homogmaw films that contain high ~ t r of theacarbonyl ~ cluster and which do not d e r from molecule aggregation when cooled. This makes it possible Ca 1992 American Chemical Society
Yamamoto et al.
9566 The Journal of Physical Chemistry, Vol. 96, No. 23, 1992 1.5
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400 600 Wavelength(nm1
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F w 1. UV-visible spectral changes accompanying 435-nm irradiation
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2000 1900 Wavenum ber (cm-'1
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2000 Wave number (c m-' )
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of H2FeOs3(C0)13in a polystyrene film at 77 K. Irradiation time (min): (-) 0, (---) 10, (---) 30, and (---) 60.
to more precisely observe the weak IR absorption band arising from dissociated CO trapped in the matrix. Polystyrene films of the metal carbonyl were prepared by mixing a benzene solution of the metal carbonyl (4 mL) with a benzene solution of polystyrene (4 mL, 20 wt 5% TOPOREX 555-57, MW = 29 300, Mitsui Toatsu Chemicals, Inc.). The solution was then transferred to a glass plate, and the benzene was allowed to evaporate at room temperature and pressure for about one day. The partially dried film was then transferred to an evacuable vessel, and the remaining benzene was removed by evacuation (1 X Torr) at room temperature. A typical film thickness was about 200 pm. Irradiation was done by mounting a sample (solution or film) in a cryostat (DN1704, Oxford) and irradiating the sample (in a helium atmosphere) with an ultra-high-pressure Hg lamp (BLM-SOOD, 500 W, WACOM) through one of three different filter combinations that transmitted light at 313, 366, and 435 nm. The filter combination providing the 313-nm light was made with aqueous solutions of K2Cr04(0.27 g/L) and Na2C03(1 g/L)* and a glass filter (UV-D33S, Toshiba). The 366-nm light was obtained by using a water filter in combination with a two glass filters (UV-D33S and W-35, Toshiba). The 435-nm light was obtained by using an aqueous solution of CuS04 (transmittance at 366 nm = 0.82)8 and a cutoff filter (L42, Toshiba). The optical path length of the quartz cell for the aqueous filter solutions was 5 cm. The UV-visible spectra were recorded on a S h i m a h W-2200 spectrophotometer,and the IR spectra were recorded on a JASCO FT/IR-8300 FT-IR spectrometer. 1. W-Visible Absorption Spectral Analysis. Figure 1 shows the W-visible absorption spectral change of H2FeOs3(CO)13in a polystyrene upon 435-nm irradiation at 77 K. The same spectral change was also observed when H2FeOs3(C0)13in a 3-methylpentane matrix was irradiated with 435-nm light. Irradiations at 366 and 313 nm induced the same absorption spectral changes as those shown in Figure 1. The UV-visible spectra of H2Feos3(co)13 at 77 K in either a polystyrene film or a 3-methylpentane matrix contain a series of broad absorption bands at 354, 390, and 450 nm. Upon 435-nm irradiation, the absorption spectra change and show clear isosbestic points at 330 and 370 nm. A new band that is weak and broad appears at 770 nm. The main absorption bands become broader and their intensity decreases. In addition, warming the samples to 298 K leads to disappearance of the weak broad band at 770 nm and complete regeneration of the UV-visible absorption spectrum of H2FeOs3(CO)13. 2. IFT-IR Absorptioa Spectral Analysis. Figure 2a shows the FT-IR absorption spectral changes of H2FeOs3(CO)13 at 77 K in a polystyrene film upon 435-nm irradiation. The same spectral changes were observed upon 366- and 3 13-nm IR radiation. The FT-IR absorption spectrum of the cluster in a 3-methylpentane
v.v
2200
1900
Figure 2. (a, top) FT-IR absorption spectral changes accompanying 435-nm irradiation of H2FeOs3(C0)13in a polystyrene film at 77 K. Irradiation time (min): (1) 0, (2) 10, (3) 20, (4) 40, and ( 5 ) 100. (b, bottom) FT-IR difference spectrum accompanying 435-nm irradiation of a polystyrene film at 77 K. Irradiation time (min): (1) 10, (2) 20, (3) 40, and (4) 100.
matrix and its corresponding spectral changes upon irradiation are essentially the same as those observed for the cluster in a polystyrene film, although the spectral resolution is higher than that obtained for samples in polystyrene films. This difference in the spectral resolution can be attributed to solvent effects because the spectrum of H2FeOs3(CO)13 in benzene coincides well with that of the metal carbonyl in a polystyrene film at 298 K. The FT-IR absorption spectrum of the polystyrene film containing H2FeOs3(CO)13exhibits bands associated with terminal CO groups at 21 17.1 (vw),2088.2 (s), 2073.7 (s), 2039.0 (s), 2023.6 (sh), 2013.9 (sh), and 1987.9 (w, br) cm-', as well as bands at 1864.4 (w) and 1828.7 (m) cm-' that are associated with the symmetric and antisymmetric vibration modes of CO bridging Fe and Os atoms, respectively.12 Upon 435-nm irradiation, the FT-IR absorption spectrum changes showing clear isosbestic points. The FT-IR absorption spectrum of the photoproduct in the polystyrene film exhibits bands associated with terminal CO groups at 2109.4 (vw),2087.2 (s, sh), 2077.6 (s), 2055.4 (s), 2039.0 (s, sh), 2023.6 (s), and 1990.8 (m, br). The bands associated with the CO bridging Fe and Os atoms can be observed at 1860.6 (w) and 1828.7 (m) cm-',respectively. The intensity of the lower frequency antisymmetric vibration mode of bridging COS decreased drastically, while the intensity of the symmetric mode decreased only slightly and was red-shifted. An additional absorption peak that is weak can be observed at 2132 cm-' in the FT-IR absorption spectrum of the photoirradiated 3-methylpentane matrix and polystyrene film samples. This can be recognized more clearly in the difference spectrum obtained by subtracting the spectrum of the initial carbonyl cluster from that of the photoirradiated samples, as shown in Figure 2b. The interference pattern in the base line arising from the polystyrene film makes it difficult to observe weak absorption peaks. Howewer, it is obvious that the intensity at 2132 cm-' increases with in.creasing irradiation time. This peak is assigned to CO frozen in the matrix. The total amount of released CO was estimated to be the same as the initial amount of H2FeOs3(CO)13 by using the absorbance at 2132 cm-'for the 3-methylpentane matrix sample
Photochemical Reaction of H2FeOs3(CO)13
The Journal of Physical Chemistry, Vol. 96, No. 23, I992 9567
TABLE I: lT-IR Spcetd h t r Of H I F ~ ( C O ) I ,H z F a ( C O ) l h rad tk PboO. Sili~r sample temp (K) peak position (an-') HZFb3(CO) I3 in polystyrene film 3-methylpentane benzene cyclohexane" hexad on S i O i HZFeOs3(C0)12 in polystyrene film 3-methylpcntane photoproduct on Si02 a Reference
12.
298 77 298 77 298 298 77 77 298
2114.2 (4, 2085.3 (s), 2071.8 (s), 2037.0 (vs), 2023.6 (sh), 2013.9 (sh), 1987.9 (w),1867.3 (vw), 1835.5 (m) 2117.9 (vw), 2088.2 (8). 2073.7 (s), 2039.0 (a), 2023.6 (sh), 2013.9 (sh), 1987.9 (w), 1864.4 (vw), 1828.7 (m) 2086.2 (s), 2071.8 (s), 2039.9 (vs), 2032.3 (m), 2025.5 (m), 2015.9 (m), 1994.6 (w),1876.9 (vw), 1846.1 (m) 2117.13 (vw),2087.2 (s), 2072.8 (8). 2039.0 (vs), 2031.3 (m), 2024.5 (m), 2013.9 (m), 1992.7 (w),1869.3 (vw), 1836.5 (m) 2115.2 (vw), 2086.3 (8). 2071.8 (s), 2037.0 (vs), 2023.6 (sh), 2013.9 (sh), 1987.9 (w),1867.3 (vw),1836.5 (m) 2086 (s), 2072 (s), 2040 (vs), 2032 (m), 2025 (m), 2015 (w), 1994 (w),1875 (w) 2114 (vw), 2087 (81,2073 (s), 2041 (vs), 2034 (m). 2027 (m), 2017 (m), 1994 (w), 1875 (w), 1846 (m) 2118.1 (vw), 2090.1 (s), 2077.6 (a), 2044.8 (vs), 1865.4 (m), 1809.5 (m) 2109.4 (vw), 2087.2 (s, sh), 2077.6 (s), 2055.4 (s), 2039.0 (8, sh), 2023.6 (8) 1990.8 (m. br), 1860.6 (w), 1828.7 (m) 2086.3 (s), 2073.7 (s), 2054.4 (m), 2038.0 (s), 2022.6 (vs), 2011.0 (w), 1983.0 (w). 1869.3 (w), 1830.7 (m) 2086 (m, sh), 2076 (s), 2064 (s), 2042 (m,sh), 2027 (vs), 2010 (sh)
Reference 7. Reference 3.
and the value of the extinction coefficient t (350 M-'cm-1)5cfor CO trapped in methylpentane. Warming either the irradiated 3-methylpentane matrix or the polystyrene film samples to about 180 K followed by refreezing to 77 K led to complete regeneration of the spectrum of HzFeOs3(C0)13.In addition, the absorption due to released CO was no longer observed.
Discussion The spectral analyses provide great insight into the nature of the photochemical processes that occur when H2FeOs3(C0)13is irradiated. The W-visible and FT-IR spectral changes showed clear isosbestic points when the samples were irradiated at 435 nm. Such spectral changes imply that the reaction is clean and that there are no secondary products. Furthermore, when the photoproduct in the polystyrene film or 3-methylpentane matrix was warmed, IR spectroscopy showed that the parent cluster H2FeOs3(C0)13 was completely regenerated as the released CO disappeared. The recombination reaction was also demonstrated by the UV-visible absorption spectral changea which showed that was regenerated upon warming the parent cluster H2FeOs3(CO)13 the samples, while the weak absorption band around at 770 nm due to the photoproduct disappeared. The complete regeneration of the parent H Z F ~ O ~ ~ ( indicates C O ) ~ ~that the FeOs3 metal framework is still intact in the photoproduct. The fact that the total amount of CO formed is equal to the initial amount of H2FeOs3(CO)13implies that one CO is released from each H2FeOs3(C0)13molecule. From these findings, it can be concluded that the photoproduct from H2FeOS3(C0)13is the coordinatively unsaturated cluster H2FeOs3(CO)12.The weak and broad band centered around 770 nm for the photoproduct supports this assignment since a similar broad band has also been reported for other coordinatively unsaturated metal carbonyl From thest results, one can conclude that the primary photochemical event is the selective dissociation of a single CO ligand from the parent carbonyl cluster. This result is the formation of the coordinatively unsaturated cluster HzFeOs3(C0)12as shown ineq 1. H2FeOs3(CO)13
+
HzFeOs3(C0)1z CO
(1)
Both the coordinatively unsaturated H2FeOs3(C0)1zand CO are trapped near each other in the polystyrene film or 3methylpmtane matrix at 77 K. Upon warming, the recombination of these two species occurs easily to completely regenerate the parent cluster HzFeOs3(CO)13.The fact that the recombination reaction occurs wen at 180 K reveals that the coordinatively unsaturated cluster H2FeOs3(CO)12is highly reactive. The coordinatively unsaturated H2FeOs3(C0)12 shows characteristic peaks at 1860.6 (w) and 1828.7 (m) cm-'associated with the CO bridging Fe and Os atoms (see Figure 2a). This implies that the CO ligand that is lost upon irradiationis a terminal CO. The coordinativcly Unsaturated site is most likely on the Os( 1) atom which does not have a bridging CO as depicted in
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9568
J. Phys. Chem. 1992,96,9568-9571
remains an intemting point that needs to be further clarified. We
H2FeOS3(C0)13/Si02 HzFeOs3(C0)1z/Si02+ CO (2) occurring on the surface of ~ i l i c a .It~ is worth pointing out that this is essentially the same as the reaction shown in eq 1. The initial photochemical process occurring in matrixes at 77 K,and on the surface of silica, involves the dissociation of one CO ligand. The matrix systems allow the photoproduct to be isolated and studied. In a similar manner, on the surface of silica the initial photoproduct HzFeOs3(C0)12appears to be stable and its spectra can be obtained. This stabilization in the silica system can be rationalized as involving the surface hydroxyl group which are the only functionality available to act as a stabilizing ligand" as mentioned in our previous paper.3 This stabilization prevents disproportionation of the tetranuclear photoproduct to smaller clusters. In addition, the dissociated CO is easily lost from the surface because there is no possibility for a cage effect on the open silica surface. The loss of CO from the surface prewents a geminate recombination of the coordinatively unsaturated cluster HzFe O S ~ ( C Owith ) ~ ~the released CO. The formation of H2FeOs3(CO)12on the surface of silica and its observation are similar in many ways to the solid matrix systems at 77 K.
arenowstudyingfurtberthephotochemistryof~carbonyl clusters on the surface of silica and in solid matrixes to address this interesting point. The results will be published elsewhere.
Acknowledgment. We thank Mr. H. Yao of the Central R e starch Institute of Mitsui Toatsu Chemicals, Inc., for preparation
of the polystyrene film sample.
Refemma urd Notes (1) Gam, B. C.; Guczi, L.;Knozinger, H. Metal Clwters in Catalysis; Elrevia: Amsterdam. 1986. (2) (a) Yamamoto, S.;Lewis,R. M.;Hotta, H.; Kuroda, H. Inorg. Chem. 1989.28, 3091. (b) Yamamoto, S.; Lewis, R. M.; Hotta, H.; Kuroda, H. Vacuum lm,41,65. (c) Yamamoto, S.;Lewis, R. M.;Nabata, Y.; Hotta, H.; Kuroda, H. Inorg. Chem. 1998,29,4342. (3) Yamamoto, S.;Miyamoto, Y.; Koizumi, M.;Lewis, R. M.;Morioka, Y .; Asakura, K.; Kuroda, H. to be published in J. P h y . Chem. (4) Burdett, J. K.; Pmtz, R. N.; Poliakoff, N.; Turner, J. J. Pure Appl. Chem. 1977,49,271. (5) (a) Hcpp, A. F.; Wnghton,M.S. J. Am. Chem.Soc. 1983,105,5934. (b) Bentsen, J. G.; Wnghton, M.S.J . Am. Chem.Soc.1S4,106,4041. (c) Bentsen, J. G.; Wrighton, M. S.J. Am. Chetn. Sor. 1987,109,4518. (6) Hooker, R. H.; Mahmoud, K. A.; Rest, A. J. J. Chem. Soc., Chem. Commun. 1983, 1022. (7) Burkhardt, E. W.; Geoffroy, G. L. J. Organornet. Chem. 1980,198, 179. ( 8 ) Mum, S. L. H d M of Photorhedtrv; Marcel Dckkcr, Inc.: New York, 1973. (9) (a) Abrahamson, H. B.; Frada, C.; Ginley, D. S.;Gray, H. B.; Lilienthal, J.; Tyler, D. R.; Wrighton, M.S. Inorg. Chem. 1977, 16, 1554. (b) Tyler, D. R.; Levenson, R. A,; Gray, H. B. J. Am. Chem. Soc. 1978,100, 7888. (c) Delley, B.; Manning, M.C.; Ellis,D. E.; Berkowitz, J.; Trogler, W. C. Inorg. Chem. 1982,21,2247. (10) Foley, H. C.; Geoffroy, G. L. 1.Am. Chem. Soc. 1981,103,7176. (1 1) M m y , G. L.; Gladfclter, W. L. J. Am. Chem. Sor. 19n. 99,7565. (12) The photochemical reaction of H2FcCh3(C0),, a h b e d on the surface of silica ie independent of the loading of the metal carbonyl. Thus, we present here the spectrum of the photoproduct from 2 wt 96 H2FcCh3(C0),3 on silica becaw the spectral nsdutim of this -le with lowgcarbonyl load* b better than that for samples with higher carbonyl loadings. (13) ForwmpIe,thewcakpeakat2109~n-~ sceainthespaanunofthe polyltyrsacfhcannot hobraved in the spectra ofthe-on dica or in that of the 3-methylpentane matrix sample. The caw for the lack of the weak ahrption peak at 2109 cm-I is not clear. (14) JacLson, R. L.;Truaheim, M.R. J. Am. Chem. Soc. 1982,104,6590.
Conclrmion Using the matrix isolation technique, we were able for the first time to observe the W-visible and IR absorption spectra of the coordinatively unsaturated cluster H2FeOs3(CO)L2. FT-IR spectroscopy revealed that H2FeOs3(C0)12has the same two bridging CO ligands as the parent HzFeOs3(C0)13cluster. H2FeOs3(CO)lzhas a structure in which the unsaturated site is at the Os atom which does not have a bridging CO ligand. It was found that HzFeOs3(CO)lzis so reactive that it recombines with CO even at the low temperature of 180 K. From the similarity of the UV-visible and IR spectra of the matrix isolate photoproduct with the spectra of the photoproduct formed on silica, we can assign the photoproduct on silica as H2FeOs3(CO)11.This c o n f i i our previous postulate about the photochemical reaction of H2FeOs3(CO)13on the surface of silica. Finally, the nature of the stabilization of the coordinatively unsaturated cluster H2FeOs3(C0)1zby the silica surface, even at room temperature,
Estimation of Heats of Formation for Vinyl Derivatives Yu-Ran Luo and John L. Holmes* Department of Chemistry, University of Ottawa, Ottawa, Ontario, Canada KIN 6N5 (Received: May 7, 1992; In Final Form August 18, 1992) The heats of formation for vinyl compounds can be estimated by the following equation: AfHO(C2HsX)= AfHO(C6H5X)
- 7.1 (f1.5)kcal/mol. This new relationship provides a useful supplement to group additivity rules.
For X = H, CH3, and CzHSthe difference between the heats of formation were 6.9, 7.1, and 7.3 kcal mol-', respectively. Using the average value, 7.1 kcal mol-', and the AfW values for phenyl halides, AfW for CzH3F,CzH3C1,and CzH3Brwere estimated. The work was followed by a discussionof heterocycle aromaticity.' In another ampilation,8 George reported a similar mult, namely 1.0060&F[C6H&] -7.3 1.5 kcalmol-', A&P[CHzCHX] b a d upon 12 data points. However these earlier studies did not use all the available experimental data and did not consider possible discrepanciesand uncertainties. The present work discusses the data from 37 molecular pairs.
Introduction There are relatively few experimental values for the standard heats of formation, A$P, for molecules containing the vinyl group.' The recent difficulty in estimating 4W for vinyl iodid& has prompted us critically to evaluate all available data on vinylic molecules with the intent of finding an appropriate reliable additivity term or similar empirical relationship which can be widely used. Two approaches have been employed in this work. The first involves the use of a recently described electmmgativity (EN) scale and the correspondingcovalent potential V,.4J This has bcen used to correlate heats of formation and bond strengths for a wide range of compounds. The second a p c h involved the empirical comparison of the available experimental enthalpy data for molecules containing a formal vinyl group with their phenyl analogues. Some progress has been reported in two books. Six years ago Liebmand compared AfHO(CHzCH-X) with AfW(C6Hs-x). 0022-3654/92/2096-9568$03.00/0
*
R d Q urd Dbcuesion Table I gives the available experimental data for vinyl and phenyl derivatives, together with the estimated errors. In an earlier study by Luo and Benson4it was found that a linear correlation (8
1992 American Chemical Society
