1758
H. Lutz, . Bréhéret, and L. Lindqvist
been described does not exist. Although no scattering center was found, the conclusion must be reached that alkali metals do not perturb the solvent structure in a detectable manner in the concentration region studied.
(1930). (11) M. G. Costeanu, C. R. Acad. Sci., 207, 285 (1938). (12) S. Kinumaki and K. Aida, Sci. Rep. Res. Inst. Tokohu Univ., 6, 186
(1954).
(13) C. A. Flint, R. . (14)
Acknowledgment. We gratefully acknowledge the financial support of The Robert A. Welch Foundation. We are indebted to the University of Dallas for the loan of the Spex 1401.
References and Notes (1) Metal-Ammonia Solutions, Physicochem. Prop., Colloq. Weyl II, 1970, 1 (1971). (2) D. A. Copeland, N. R. Kestner. and J. Jortner, J. Chem. Phys., 53, 1189 (1970). (3) P. F. Rusch, Université Cathoiique de Lille, France, private commu-
J. Phys. Chem. 1973.77:1758-1762. Downloaded from pubs.acs.org by UNIV OF KANSAS on 01/19/19. For personal use only.
nication.
(4) G. C. Klick and J. H. Schulman, “Solid State Physics," Vol. 5, Academic Press, New York, N. Y., 1957, p 97. (5) T. A. Beckman and K. S. Pitzer, J. Phys. Chem., 65,1527 (1961). (6) D. F. Burow and J. J. Lagowski, J. Phys. Chem., 72,. 169 (1968). (7) P. F. Rusch, Ph.D. Dissertation, The University of Texas at Austin, 1971. (8) P. Daure, Ann. Phys., 12, 375 (1929). (9) S. Bhagavantam, Indian J. Phys., 5, 54 (1930). (10) A* Dadieu and K. W. Kohlrausch. Naturwissenschaften, 18, 154
(15) (16) (17) (18) (19) (20) .
(21) (22) (23) (24) (25) (26) (27) (28) (29) (30)
B. Small, and H. L. Welsh, Can. J. Phys., 32, 653 (1954). G. Seillier, M. Ceccaldi, and J. P. Lelcknam, Method. Phys. Anal. (GAM), 4, 388 (1968). T. Birchall and I. Drummond, J. Chem. Soc. A, 1859 (1970). B. Bettigm'es and F. Wallart, C. R. Acad. Sci., 271, 640 (1970). R. Quinn and J. J. Lagowski, J. Phys. Chem., 73, 2326 (1969). D. F. Burow and J. J. Lagowski, Advan. Chem. Ser., No. 50, 1 (1965). G. Herzberg, “Infrared and Raman Spectra,” Van Nostrand, New York, N. Y., 1966. . H. Claasen, H. Selig, and J. Shanier, Appl. Spectrosc., 23, 8 (1969). C. D. Allemand, Appl. Spectrosc., 24,348 (1970). W. F. Murphy, . V. Evans, and P. Bender, J. Chem. Phys., 47, 1836 (1967). A. F. Slomba, C. D. Hinman, and E. H. Siegler, Proc. Conf. Anal. Chem. Appl. Spectrosc., (1965). A. E. Douglas and D. H. Hank, J. Opt. Soc. Amer., 38, 281 (1948). M. Gold and W. L. Jolly, Inorg. Chem., 1,818 (1962). D. A. Kleinman, Phys. Rev. A, 134, 423 (1964). J. Brandmüller and H. Moser, “EinfUhrung in die Raman-spektroskopie," Dr. Dietrich Steinkopff Verlag, Darmstadt, 1962. J. M. Worlock and S. P. S. Porto, Phys. Rev. Lett., 15, 697 (1965). S. Radhakrishna and . K. Sehgal, Phys. Lett. A, 29, 286 (1969). R. Catterall, Metal-Ammonia Solutions; Physicochem. Prop., Colloq. Weyl II, 1970, 1 (1971).
Effects of Solvent and Substituents on the Absorption Spectra of Triplet Acetophenone and the Acetophenone Ketyl Radical Studied by Nanosecond Laser Photolysis Hanspeter Lutz,1 Emilienne Bréhéret, and Lars Lindqvist* Laboratoire de Photophysique Moléculaire du C.N.R.S,, Université de Paris-Sud, 91405 Orsay, France (Received December 8, 1972} Publication costs assisted by the Centre National de la Recherche Scientifique
The triplet absorption spectra of p-trifluoromethylacetophenone, acetophenone, and p-methyl- and pmethoxyacetophenone in nonpolar solvents were measured by nanosecond laser photolysis using the third and fourth harmonics of a Nd glass laser. Acetophenone was also studied in acetonitrile and water. In the cases where the energy of the lowest 3( , *) state is lower than that of the lowest 3( , *) state, the triplet absorption spectra are characterized by two weak absorption bands at about 410 and 450 nm and by a strong absorption appearing below 330 nm. When the 3( , *) state is lowest, the triplet has a structureless, weak absorption in the visible and a strong maximum at about 350 nm. The spectra of the ketyl radicals of p-trifluoromethylacetophenone, acetophenone, and p-methylacetophenone were determined in cyclohexane. The spectra are all very similar and resemble the triplet absorption spectra of the ketones which have lowest triplets of 3( , *) nature. The rate constants of hydrogen abstraction from 2-propanol by the triplet state of these latter ketones in benzene solutions containing 2 M 2-propanol are 6.2 X 106, 1.2 X 106, and 1.3 x 10® -1 sec-1, respectively. The results indicate that measurements of triplet absorption spectra may be useful in determining the 3( , *) vs. 3( , *) nature of the lowest triplet state of alkyl phenyl ketones.
Introduction It is known that many alkyl phenyl ketones have almost isoenergetic lowest 3( , *) and 3( , *) states, a property leading to interesting spectroscopic and chemical consequences. Information about the nature of the lowest triplet state of these compounds has been obtained mainly The Journal of Physical Chemistry, Vol. 77, No. 14, 1973
from studies of phosphoresence,2-6 singlet-triplet absorption,7-8 and zero-field splitting.9-10 Triplet absorption spectra are expected also to be of value in characterizing the triplet state; however, only few measurements of triplet absorption spectra of alkyl phenyl ketones have been reported previously.11·12
1759
Absorption Spectra of Triplet Acetophenone and the Acetophenone Ketyl Radical
In the present study the triplet absorption spectra of acetophenone and a number of acetophenone derivatives were determined using the nanosecond laser photolysis method. The lowest triplet state of acetophenone in nonpolar solvents is considered to be a 3( , *) state;2·38 the second triplet, situated only a few hundred wave numbers above the lowest triplet state, is considered to be a 3( , *) state.® Vibronic coupling between these two states is efficient and produces a mixed character of the lowest triplet state.13 A consequence of the energetic proximity of the triplet levels is that even small perturbations, e.g., substituent5·14 or solvent9 effects, may produce level inversion or changes in the extent of vibronic coupling between the two lowest lying triplet states. The effects of such perturbations on the triplet absorption spectrum were determined in the present study by comparing the absorption spectrum of triplet acetophenone in nonpolar solvents to those of para-substituted acetophenones containing electron-withdrawing (CF3") or electron-donating groups (CH3™, CH3O™). Solvent effects were determined from measurements of the acetophenone triplet absorption spectrum in solvents of varying polarity (cyclohexane, acetonitrile, and water). By the use of these solvents and substituents, it was possible to vary the nature of the triplet state progressively from a predominantly 3(n,ir*) to a
predominantly 3(
, *) configuration.
Experimental Section Materials. Acetophenone (Eastman) and p-methylacedistilled; p-triflutophenone (Fluka) were doubly vacuum oromethyl- and p-methoxyacetophenone (Fluka) were purified by sublimation. Benzene, cyclohexane, 2-propanol, acetonitrile (Merck Uvasol), and perfluoromethylcyclohexane (Peninsular Chemresearch) were used without further purification. Water was doubly distilled. Solutions were degassed by three freeze-thaw cycles with intermittent saturation by argon. Laser Photolysis Equipment. The excitation light source was a Q-switehed Nd glass laser (Compagnie Générale d’Electricité, Model VD 231, maximum energy 60 J) emitting at 1058 nm; the pulse half-width was 35 nsec. The third (353 nm) and fourth (265 nm) harmonics, generated by means of KDP crystals, were used in the present study, at energies in the order of 1-20 mJ. Relative values of the laser energy were obtained by measuring the integrated photocurrent from a photomultiplier tube (EMI 9781 B) exposed to a small fraction of the ultraviolet laser beam. An optical system projected the laser beam, reduced to a height of 4 mm and a width of 8 mm, on one side of a 10-mm square silica cell with polished sides, containing the sample. The monitoring light was obtained by discharging a 320-µ capacitor (1-1.8 kV) across a xenon flash tube (Verre et Quartz, VQX 65 N) in series with an inductance of approximately 100 µ ; the flash half-width was 650 µß . Triggering was made such that the laser pulse occurred during the period of maximum intensity of the analysis flash. The analyzing light passed in a crossed-beam arrangement through a 2 mm wide section of the sample cell, close to the laser entrance window. A narrow wavelength band (2 nm) of the transmitted light was selected by means of a monochromator (Jarrell-Ash, Model 82-410, //3.5, / = 25 cm); the intensity in this band was measured using a photomultiplier tube (Radiotechnique, 150 UVP or 150 CVP). The output of the seventh dynode, terminated by 75 , was displayed on an oscillo-
scope (Fairchild, Model 777, 100 MHz). The overall time resolution of the detector system was ca. 10 nsec. were obtained from oscilrange of wavelengths (every 3-10
Transient absorption spectra loscope recordings over
a
nm). A linear correction of the transient optical densities was applied for variations in the laser energy.
Results and Discussion (A) Effect of Substituents on the Acetophenone Triplet Absorption. Laser excitation of acetophenone and some of its para-substituted derivatives in nonpolar solvents produced the end-of-pulse transient absorption spectra shown in Figure 1. The ultraviolet part of the spectra was obtained after excitation by the 265-nm harmonic. The solute concentration was chosen such that an OD of 1 to 2 a 1-cm path (0.002-0.006 M). The was obtained across transient absorption in the visible is very weak and it was for this reason found necessary to use the higher output of the 353-nm harmonic and higher solute concentrations (0.1-0.2 M) to obtain measurable absorptions in this spectral range. Aerated solutions gave the same end-of-pulse spectra as the degassed solutions. Measurements between 600 and 1000 nm did not reveal any perceptible absorption in this spectral range. Our previous study12 established that the transient spectrum observed immediately after laser irradiation of acetophenone is due to the triplet state. The spectra in Figure 1 are by analogy attributed to the lowest triplet state of the corresponding compounds. In the case of p-trifluoromethylacetophenone, measurements of the triplet spectrum in hydrogen-donating solvents was impracticable due to the high triplet reactivity of this compound. Perfluoromethylcyclohexane was chosen as solvent in the determination of the ultraviolet part (Figure la, dotted line) and benzene, because of the higher solubility of the ketone in this solvent, in the determination of the visible part (Figure la, full line) of the triplet spectrum of this ketone. Estimates of the extinction coefficients given in Figures 1 and 2 were obtained by comparison of the triplet absorption intensities of the acetophenone derivatives with the intensity of the benzophenone triplet absorption,12 measured at equal laser excitation energies. It was assumed that the extinction coefficient of triplet benzophenone in cyclohexane has the same value at 533 nm as that determined in benzene solution15 (7630 M_1 cm-1). It was also assumed that the triplet yield was unity for all the compounds studied. It is seen from Figure 1 that substitution in the benzene nucleus leads to pronounced changes in the triplet absorption of acetophenone. Substitution by the electron-withdrawing trifluoromethyl group produces a shift of the strong ultraviolet absorption to shorter wavelengths. The electron-donating methyl and methoxy groups instead lead to complete disappearance of the structure of the absorption in the visible while a high-intensity absorption band shows up in the near ultraviolet around 350 nm. These spectral changes may be discussed by considering the effect of substituents on the electronic nature of the lowest triplet state.
Low-temperature studies of the lifetime, structure, and polarization of the phosphorescence of acetophenone in hydrocarbons indicate that the lowest triplet state of the ketone is of 3( , *) nature under these conditions.13·2·3 This state is expected to be strongly perturbed by a nearby 3( , *) state. The closeness of the two states is demonThe Journal of Physical Chemistry, Vol. 77, No. 14, 1973
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H. Lutz, . Bréhéret, and L. Llndqvist
WAVELENGTH (nm) Figure 1. Triplet absorption spectra of para-substituted acetophenones in aerated solutions determined by nanosecond laser photolysis (end-of-pulse spectra): (a) p-trifluoromethylacetophenone in perfluoromethylcyclohexane (----) and in benzene (-); (b) acetophenone in cyclohexane; (c) p-methylacetophenone in cyclohexane; and (d) p-methoxyacetophenone in cyclohexane.
Figure 2. Triplet absorption spectra of acetophenone in aerated polar solvents determined by nanosecond laser photolysis (endof-pulse spectra): (a) acetophenone In acetonitrile and (b) acetophenone in water.
strated by the ease of inversion of the states with change of solvent; in the more polar EPA glass the lowest triplet is a 3( ,7 *) state according to a study of the polarization of the phosphorescence.13 The appearance of vibrations assigned to nonplanar modes in the 3{ , *) phosphorescence of acetophenone in EPA13 and some substituted acetophenones16 reveals a strong vibronic coupling between the 3( , *) and 3( , *) configurations. p-Trifluoromethylacetophenone has been studied using phosphorescence excitation techniques.14 It was found that the energy of the 3( , *) state of acetophenone is decreased by the substituent; no evidence was obtained as to the location of the upper 3( , *) state. From the fact that trifluoromethyl substitution produces a blue shift of the absorption maximum of the 1La «— 4A transition,17 one may expect a similar increase in energy of the corresponding 3( , *) state 8La. It follows that the energy spacing between the low-lying 3( , *) and 3( , *) levels should increase when a trifluoromethyl group is introduced in the acetophenone molecule, thus bringing about a reduction in the vibronic coupling between the two states. The enhancement of the 3( , *) character of the lowest triplet state is further supported by the very short phosphorescence lifetime of p-trifluoromethylacetophenone.14 rse Journal of Physical Chemistry, Vol.
77, No. 14, 1973
Triplet state assignments have been made for p-methylacetophenone in 3-methylpentane glass at 77°K.13 It is concluded from the results that the lowest triplet state in this case is a 3( , *) state, coupled vibronically to a closelying 3( , *) state. The 3( , *) assignment of the lowest triplet state of p-methylacetophenone is also supported by its phosphorescence excitation spectrum.14 The lowest triplet of p-methoxyacetophenone in a moderately polar solvent is considered to be a 3( , *) state.5·16 The 3( , *) state was located 2100 cm™1 above this state.5 Due to the large energy gap between the two states, vibronic interactions are expected to be small, and variations in the solvent polarity thus should not have a great influence on the orbital character of the triplet state. Indeed, it has been shown that the phosphorescence lifetime of this ketone is almost the same in 3-methylpentane glass as in EPA, demonstrating the lack of influence of the solvent on the triplet state character in this case.13 Making use of the assignments cited above in discussing the absorption spectra given in Figure 1, we note that the absorption spectra of the ketones with lowest triplet states of 3( , *) nature (p-trifluoromethylacetophenone and acetophenone) show two absorption bands in the visible. The presence of these bands seems to be directly connected with the 3( , *) nature of the triplet state since no such bands are observed in the cases where the 3( , *) state is lowest. It may also be noted that the visible band system is most pronounced in the triplet absorption spectrum of
p-trifluoromethylacetophenone in which vibronic perturbation of the lowest 3( , *) state is expected to be weak. The ketones with lowest triplet state of 3( , *) nature (p-methyl- and p-methoxyacetophenone) have triplet absorption spectra showing a strong maximum at about 350 nm and weak structureless absorption in the visible. The triplet absorption spectra in the present study were obtained at room temperature whereas the triplet state assignments discussed above were obtained from low-temperature studies. It has been shown,3b·18 that the overall orbital character may vary with the temperature when the triplet states are closely spaced. In the absence of quantitative estimates of such effects, their possible influence on the triplet nature was neglected in the interpretation of the results. A verification of the above assignments and of their validity at room temperature conditions may be obtained by determining the hydrogen-abstracting power of the triplet state, since it is known that hydrogen abstrae-
Absorption Spectra of Triplet Acetophenone and the Acetophenone Ketyl Radical
1761
Figure 3. Ketyl radical absorption spectra of para-substituted acetophenones in degassed cyclohexane; (a) p-trifluoromethylacetophenone (measured 1 psec after laser excitation); (b) acetophenone (measured 1.4 psec after laser excitation); and (c) p-methylacetophenone (measured 10 psec after laser excitation).
tion from the solvent by an aromatic ketone in the triplet state will be efficient only if the state has 3(n,ir*) orbital character. The hydrogen-abstracting efficiency is obtained conveniently by the nanosecond laser technique from measurements of the triplet decay rate. We determined in this manner the rate constants of photoreduction, kT, of p-trifluoromethylacetophenone, acetophenone, and pmethylacetophenone. Details of the procedure have been described elsewhere.19 The measurements were made in degassed solutions of these compounds (0.1 M) in benzene solutions that were 2 Min 2-propanol. The following values were obtained Compound p-Trifluoromethylacetophenone Acetophenone p-Methylacetophenone
& r,
1
6.2 X
sec ~1 10®
1.2 x 10® 1.3 X 10®
information regarding the effect of ring substituents on the triplet state reactivity of acetophenone was obtained by Yang and Dusenbery14 from quantum yield studies. These authors reported somewhat lower values of kr than those obtained in the present work; the relative importance of kT is about the same, however. The strong decrease of kr in going from p-trifluoromethylacetophenone to p-methylacetophenone is consistent with the proposed change of the orbital character of the lowest triplet state from 3( , *) to 3( , *). The high reactivity of triplet p-trifluoromethylacetophenone compared to that of acetophenone may be attributed to the reduced perturbation of the lowest 3( , *) by the upper 3( , *) state. The lowest triplet state of p-methylacetophenone is expected to be a 3( , *) state, yet p-methylacetophenone shows significant photoreactivity. This behavior has been attributed to the vibronic coupling of the lowest 3( , *) with the closely spaced 3( , *) state.14 (B) Effect of Solvent Polarity on the Acetophenone Triplet Absorption. Figure 2 shows the triplet absorption spectra of acetophenone in the polar solvents acetonitrile (a) and water (b). The triplet absorption spectrum of acetophenone in ethanol has been given elsewhere.12 The spectra were measured under the same experimental conditions as the spectra shown in Figure 1. The wavelength of the triplet absorption peak in water is in agreement with recent flash photolysis results.20 It is seen that the absorption spectra of triplet acetophenone in polar sol-
similar to those found in acetophenone containelectron-donating substituent in the benzene ring. ing The band structure in the visible observed in cyclohexane (Figure lb) does not appear in the more polar solvents, but instead, a strong absorption maximum is found at
vents
are
an
about 330 nm. This change may be interpreted as being due to the change from 3(n,ir*) to 3( , *) nature of the lowest triplet with increasing solvent polarity. Lamola9 predicted the 3(n,ir*) and 3(t,t*) states of acetophenone to have nearly the same energy in ethanol. This prediction is supported by the observation of dual phosphorescence from acetophenone in ethanol and solvents of similar polarity, such as EPA.21 It appears from recent phosphorescence studies that the 3( , *) state of acetophenone is located even below the 3( , *) state in ethanol-containing glass.13 In a previous study we have presented evidence for a strong solvent dependence of the acetophenone triplet state reactivity and explained this in terms of the extent of mixing of the lowest 3( , *) and 3( , *) states.19 The dependence of the acetophenone triplet absorption on the solvent polarity observed in the present study lends further support to this conclusion. fC) Effects of Substituents on the Acetophenone Ketyl Radical Absorption. In the preceding discussion, the variations in triplet absorption due to substitution were interpreted as being due to changes in the relative positions of the 3( , *) and 8( , *) levels. If other specific substituent effects produce the observed variations one would expect to find similar effects on the acetophenone ketyl radicals. To examine this possibility the ketyl radical spectra of acetophenone and p-trifluoromethyl- and p-methylacetophenone were determined and compared. The ketyl radicals were obtained by laser excitation of the ketones in degassed cyclohexane. Figure 3 shows the transient spectra due to this radical, measured after disappearance of the triplet absorption. The extinction coefficients of the radicals were estimated roughly assuming that the corresponding triplet disappears exclusively to produce the radical. The similarity of the spectra indicates that substitution does not specifically change the electronic configuration of the acetophenone ketyl radical. This observation strengthens the hypothesis that the spectral changes in the triplet absorption arising by substitution are indeed due to 3( , *)-3( , *) level inversion. The
Journal of Physical Chemistry, Vol. 77, No.
14, 1973
1762
Howard D. Mettee
It is seen that the ketyl radicals have spectra very similar to those of the triplet state of 3( , *) nature. Such a relation was already found for benzophenone and was attributed to the similarity in the electron configuration of the 3( , *) state and the ketyl radical.22 Conclusions The present study provides evidence of strong solvent and substituent effects on the absorption spectrum of triplet acetophenone. These effects are shown to be related to variations in the extent of 3( , *) us. 3{ , *) orbital nature of the lowest triplet state. One may conclude from these results that triplet absorption spectra are useful criteria in determining the lowest triplet configurations of alkyl phenyl ketones. Measurements of the decay of the triplet in hydrogendonating solvents show that the changes in the triplet absorption spectra due to solvent and substituent effects are associated with significant changes in the reactivity of the triplet state. These findings are direct evidence that the variations in the acetophenone photoreactivity are due to variations of the electronic configuration of the lowest triplet state.
References and Notes (1) Present address, Laboratory for Research on the Structure of Matter, The University of Pennsylvania, Philadelphia, Pa. 19174. (2) V. G. Krishna, J. Mol. Spectrosc., 13, 296 (1964).
(3) (a) R, Shimada and L, Goodman, J. Chem. Phys., 43, 2027 (1965); (b) M. Koyanagi, R. J. Zwarich, and L. Goodman, ibid., 56, 3044
(1972). (4) T. F. Hunter, Trans. Faraday Soc., 66, 300 (1970). (5) D. R. Kearns and W. A. Case, J. Amer. Chem. Soc., 88, 5087
(1966). (6) W. A, Case and D. R. Kearns, J. Chem. Phys., 52, 2175 (1970). (7) S. Dym and R. M. Hochstrasser, J. Chem. Phys., 51, 2458 (1969). (8) Y. Tanlmoto, H. Kobayashi, S. Nagakura, and T. Azumi, Chem. Phys. Lett., 16, 10 (1972). (9) A. A. Lamola, J. Chem. Phys., 47, 4810 (1967). (10) (a) N. Hirota, Chem. Phys. Lett., 4, 305 (1969); (b) J. B. Gallivan and J. S. Brinen, ibid., 10, 455 (1971); (c) T. H. Cheng and N. Hirota, ibid., 13, 194 (1972); (d) S. W. Mao, T. C. Wong, and N. Hirota, ibid., 13, 199 (1972); (e) T. H, Cheng and N. Hirota, J. Chem. Phys., 56, 5019 (1972). (11) D. S. McClure and P. L. Hanst, J. Chem. Phys., 23, 1772 (1955). (12) H. Lutz and L. Lindqvist, Chem. Commun., 493 (1971). (13) Y. H. Li and E. C. Lim, Chem. Phys. Lett., 7, 15 (1970). (14) N. C. Yang and R. L. Dusenbery, J. Amer. Chem. Soc.. 90, 5899
(1968).
(15) R. Bensasson and E. J. Land, Trans. Faraday Soc., 67, 1904
(1971). E. C. Lim, Y. H. Li, and R. Li, J. Chem. Phys., 53, 2443 (1970). N. C. Yang and R. L. Dusenbery, Mol. Photochem., 1, 159 (1969). E. Migirdicyan, Chem. Phys. Lett., 12, 473 (1972). H. Lutz, M. C. Duval, E. Bréhéret, and L. Lindqvist, J. Phys. Chem., 76,821 (1972). (20) . B. Ledger and G. Porter, J. Chem. Soc., Faraday Trans. 1, 68, 539 (1972). (21) (a) N. C. Yang and S. Murov, J. Chem. Phys., 45, 4358 (1966); (b) P. Gacoin and Y. Meyer, C. R. Acad. Sci., 267, 149 (1968); (c) R. N. Griffin, Photochem. Photobiol., 7, 159, 175 (1968); (d) P. J. Wagner, M. J. May, A. Haug, and D. R. Graber, J. Amer. Chem. Soc., 92, 5269 (1970); (e) . E. Long, Y. H. Li, and E. C. Lim, Mol. Photochem., 3, 221 (1971). (22) H. Tsubomura, N. Yamamoto, and S. Tanaka, Chem. Phys. Lett., 1, 309 (1967). (16) (17) (18) (19)
Vapor-Phase Dissociation Energy of (HCN)2 Howard D. Mettee Department of Chemistry, Youngstown State University, Youngstown, Ohio 44503 (Recieved April 2, 1973) Publication costs assisted by The Graduate School, Youngstown State University
The temperature dependence of fundamental infrared absorbance intensities of vi in monomeric and dimeric forms of HCN vapor is used to calculate a ß = -3.80 ± 0.16 kcal/mol. This value is somewhat greater than the earlier experimental value of -2.6 kcal/mol, determined by classical vapor density measurements, and compares more favorably with a recently theoretically computed value of AED" = -3.7 kcal/mol, based on a priori methods.
With the exceptions of the work of Inskeep,1 Bernstein,2
and Dunken and Winde,3 very few measurements of the thermodynamic properties of small, hydrogen-bonded complexes in the vapor phase are currently being reported. Since recent theoretical methods have advanced to the point where a priori calculations may now be performed on such systems,4 there exists a need to have reliable experimental data at hand for comparison. The recent calculations of Kollman, et al.,s on the HCN dimer illustrates the problem rather nicely. For the reac-
tion 2HCN
(HCN)2.
The Journal of Physical Chemistry, Vol. 77, No. 14, 1973
the calculated energy of reaction ( £0° = -3.7 kcal/mol of bonds) could only be compared with an experimental = -2.6 kcal/mol of bonds) determined 34 value ( years ago by Giauque and Ruehrwein6 using vapor density data which were still 10 years older. In view of the acof higher polymers in HCN vapor knowledged occurence such data are somewhat suspect. In fact, the above authors6 caution against taking equilibrium constants based on such measurements too seriously. In the case of HCN the infrared measurements of Hyde and Hornig7 provide an independent means of checking the earlier vapor density value and the more recent calculated one. The v\ mode of monomeric HCN is an example
