J. Phys. Chem. 1984, 88, 2323-2329 will surely have more positive Eo values than +240 mV, the k, values for these should also lie in the diffusion-controlled limit. Consequently, it is expected that log kf for the reaction of diazasemiquinones with O2will be proportional to the Eo value of the corresponding diazaquinone. Applying the Marcus equation18
AG* =
(:
1
+ T) AGO
to k, and kf in eq I, we obtain a Xo value of 24 kcal/mol. This compares very favorably to Xo = 18 f 2 kcal/mol as determined for a series of quinone^.^ The present Xo value is compatible with that of an electron transfer without any significant change of bonds if the mean radii of the solvated species are assumed to be about 4 A. All in all, the reaction examined in this work appears to be (18) N. Sutin, Acc. Chem. Res., 1, 225 (1968).
2323
fully consistent with an electron-transfer process. Previously, the rate constant for reaction 2 was determined3 to be (5 f 2) X lo7 M-I s-l . Yield measurements indicated that this reaction could involve an electron t r a n ~ f e r .Applying ~ the Marcus equation to reaction 2 and utilizing +0.200 V as the Eo value of the O p / H 0 2 - couple,19we predict the rate constant k2 to be lo7 M-’ s-l. Thus, we conclude that the rate-determining step of reaction 2 may very well be an electron transfer. Registry No. 1, 521-31-3; 3, 89596-65-6; 4, 60851-83-4; 5, 2784611062-77-4; 02,7782-44-7; C03-., 16518-46-0. 29-3; 02--, (19) D. T. Sawyer and E. J. Nanni in “Oxygen and Oxy-Radicals in Chemistry and Biology”, Academic Press, New York, 1981, p 15. (20) N. Anbar, F. Ross, and A. B. Ross, Eds., Natl. Stand. ReJ Data Ser. (U. S., Natl. Bur. Stand.) NSRDS-NBS 51 (1975). (21) N. Anbar, M. Bambenek, and A. B. Ross, Eds., Natl. Stand. Ref. Data Ser. (U.S. Natl. Bur. Stand.) NSRDS-NBS 43 (1973). (22) F. Ross and A. B. Ross, Eds., Natl. Stand. ReJ Data Ser. (U.S. Natl. Bur. Stand.) NSRDS-NBS 59 (1977).
Infrared Matrix Isolation Study of the 1:l Molecular Complexes of the Hydrogen Halides and Hydrogen Cyanide with Cyclopropane Candace E. Truscott and Bruce S. Auk* Department of Chemistry, University of Cincinnati, Cincinnati, Ohio 45221 (Received: September 12, 1983)
The matrix isolation technique has been used to isolate and characterize the 1: 1 hydrogen-bonded complexes between the hydrogen halides (from HF to HI) and hydrogen cyanide and cyclopropane, c-C3H6. In each case, a perturbed vibrational mode of the complexed acid was observed, shifted 10-200 cm-’ to lower energies from the position of the corresponding “free” acid. The magnitude of the shift correlated directly with the strength of interaction and dipole moment of the acid but not with the proton affinity of the halide anion. Several perturbed modes of the cyclopropanesubunit in the complex were observed, and again the magnitude of shift correlated with strength of interaction. For the two most strongly bound complexes, HF.c-C,H6 and HC1.c-C3H6,the low-frequency bending mode was identified as well, near 400 cm-]. The spectra are consistent with the gas-phase structure from rotational spectroscopy, in which the hydrogen of the hydrogen halide is bonded to the midpoint of one carbon-carbon bond, and the hydrogen halide lies in the plane of the three-membered carbon ring. In addition, evidence was obtained for the existence of a 2:l complex 2HX.c-C3H6, but no structural determination could be made.
Introduction Cyclopropane, c-C3H6, has been known for years to exhibit unusual chemical properties due to the high degree of strain in its three-membered ring.’ These properties are olefinic in nature, and various models proposed for the bonding in c-C3H6all suggest that substantial electron density resides outside of the line connecting the carbon nuclei (Le., a bent b ~ n d ) . ~Theoretical4 .~ and experimental5 measurements confirm, in a general sense, these predictions and suggest that protonation might well occur in these regions of high electron density. Stimson6s7and others have shown (1) (a) Bernett, W. J. Chem. Educ. 1967,44, 17. (b) Lukina, M. Y . Russ. Chem. Rev. (Engl. Transl.) 1962, 31, 419. (2) (a) Stevens, R.M.; Switkes, E.; Laws, E. A.; Lipscomb, W. N. J. Am. Chem. SOC.1971.93, 2603. (b) Coulson, C. A,; Moffitt, W. E. Philos. Mag. 1949, 40, 1. (3) Walsh, A. D.Nature (London) 1947, 159, 167, 712. (4) (a) Marsmann, H.; Robert, J.-B.; van Wazer, J. R. Tetrahedron 1971, 27, 4377. (b) Dewar, M. J. S. J. Am. Chem. SOC.1971, 93, 6685. (5) Hartman, A.; Hirshfeld, F. L. Acta Crystallogr. 1966, 20, 80. (6) (a) Maccoll, A,; Ross, R. A. J . Am. Chem. SOC.1965,87, 4997. (b) Ross, R. A.; Stimson, V . R. J. Chem. SOC.1962, 1602. (7) (a) Johnson, R.L.; Stimson, V. R. Aust. J . Chem. 1975, 28,447 and references therein. (b) Lewis, D. K.; Bosch, H. N.; Hossenlopp, J. M. J . Phys. Chem. 1982, 86, 803. (c) Stimson, V. R.;Taylor, E. C. Aust. J . Chem. 1976, 29, 2557.
0022-3654/84/2088-2323$01.50/0
that the isomerization of cyclopropane to propylene in the gas phase is catalyzed by both Lewis acids and Brernsted acids, such as the hydrogen halides. In particular, he found that the activation barrier was lowered by roughly 25 kcal/mol by the addition of HBr in catalytic amounts and that addition of BBr3 increased the rate by an additional factor of 10. He was led to invoke a weakly bound gas-phase complex, in contrast to the protonated complex which has been invoked to explain the solution-phase chemistry of c-C3H,. The gas-phase proton affinity of cyclopropane8 has been measured to be 179 kcal/mol, which while very high for a hydrocarbon is still not sufficient to remove a proton from any of the hydrogen halides. Rather, a hydrogen-bonded complex seems reasonable for the interaction leading to the gas-phase catalytically induced isomerization; hydrogen bonding has been postulated as being responsible for the anesthetic action of cycl~propane.~ Rotational spectroscopy of weakly bound complexes formed through free-jet expansions and supersonic nozzles has provided structural information on a number of hydrogen-bonded com(8) (a) Chong, S. L.; Franklin, J. L. J. Am. Chem. SOC.1972, 94, 6347. (b) Dymerski, P. D.; Prinstein, R. M.; Bente, P. F.; McLafferty, F. W. J. Am. Chem. SOC.1976, 98, 6834. (9) Hobza, P.; Mulder, F.; Sandorfy, C. J. Am. Chem. SOC.1981, 103, 1360.
0 1984 American Chemical Society
2324
The Journal of Physical Chemistry, Vol. 88, No. 11, 1984
plexes. Flygare's group,lO-l'and others,12has recently published the rotational spectra and structures of the HCl and H F complexes of c-C3H6and was able to confirm that the complex is weakly hydrogen bonded, with the acid hydrogen interacting with the midpoint of one of the carbon-carbon bonds, for a C2, structure. The infrared spectrum of such a complex would be of considerable interest, in that the structure and bonding may be verified and the potential function for the molecule deduced. Also, the degree of perturbation of the cyclopropane subunit in the complex may be determined through the splitting of doubly degenerate modes of the isolated molecule and activation of modes inactive under D3hsymmetry. In addition, while no infrared studies in solution have been carried out on hydrogen-bonded cyclopropane complexes, several solution studies have investigated the hydrogen bonding between cyclopropyl derivatives (such as cyclopropyl carbinol) and phenols; shifts of the 0-H stretching mode on the order of 15-60 cm-I were 0b~erved.l~ Matrix isolation infrared spectr~scopyl~ provides the most direct means for the isolation and spectroscopic characterization of weakly bound complexes involving either Lewis's-17or Bran~ted'*,'~ acids and a variety of bases. Barnes and co-workers20reported briefly the infrared spectrum of the HIc-C3H6complex several years ago as part of their investigation of HI complexes in argon matrices and quite recently reported cyclopropane complexes with HC1 and HBr,21although no deuterium studies were attempted. With this background in mind, a study was undertaken to thoroughly characterize the hydrogen-bonded complexes of cyclopropane with the hydrogen halides and hydrogen-bonded complexes of cyclopropane with the hydrogen halides and hydrogen cyanide, and their deuterium analogues, in argon and nitrogen matrices at 14 K.
Experimental Section All ofthe experiments conducted in the present study were carried out on a conventional matrix isolation apparatus which has been described previously.22 The gases employed in this study, HF, HCl, HBr, HI, HCN, and c-C3H6(all from Matheson except HCN), were all subjected to one or more freeze-thaw cycles at 77 K prior to sample preparation. For deuterium studies, the stainless-steel vacuum line was thoroughly deuterated with D 2 0 and flamed, followed by exposure to the deuterated gas of interest, before a sample was prepared. D / H ratios between 2:l and 4:l were obtained in this fashion; DCl, DBr, and DI were all obtained commercially (Merck) and purified through freeze-thaw cycles. DCN was prepared by exposing H C N to a well-deuterated vacuum line; exchange led to a deuteration ratio of roughly 1:l. Argon and nitrogen were used as the matrix gases in all experiments and were used without further purification. Experiments were conducted in both the single-jet and twin-jet modes; in the former mode the two reactant gases were premixed in a single vessel prior to deposition, while in the latter mode the two reactants were codeposited from separate vacuum lines. Samples were deposited at roughly 2 mmol/h of total mixture for 20-24 h, prior to obtaining final spectra. Both survey scans ~~
(IO) Legon, A. C.; Aldrich, P. D.; Flygare, W. H. J . Am. Chem. SOC.1982, 104, 1486. (1 1) Buxton, L. W.; Aldrich, P. D.; Shea, J. A.; Legon, A. C.; Flygare, W. H. J. Chem. Phys. 1981, 75, 2681. (12) Legon, A. C. J . Phys. Chem. 1983, 87, 2064. (13) (a) Joris, L.; von Schleyer, R.; Gleiter, R. J. Am. Chem. SOC.1968, 90, 327. (b) von Schleyer, R.; Trifan, P. S . ; Bacskai, R. Ibid. 1958,80,6691. (14) (a) Craddock, S.; Hinchliffe, A. J. *Matrix Isolation"; Cambridge University Press: New York, 1975. (b) Andrews, L. Annu. Rev. Phys. Chem. 1971, 22, 109. (15) Ault, B. S. Inorg. Chem. 1981, 20, 2817. (16) Lorenz, T.; Auk, B. S . Inorg. Chem. 1982, 21, 1758. (17) McNair, A. M.; Au!t, B. S . Inorg. Chem. 1982, 21, 1762. (18) Andrews, L. J. Mol. Strucr. 1983, 100, 281. (19) Barnes, A. J. J. Mol. Struct. 1983, 100, 259. (20) Barnes, A. J.; Davies, J. B.; Hallam, H. E.; Howells, J. D. R. J. Chem. Soc., Faraday Trans 2 1974, 70, 1682. (21) Barnes, A. J.; Paulson, S. L Chem. Phys. Lett. 1983, 99, 326. (22) Ault, B. S J . Am. Chem. SOC.1978, 100, 2426.
Truscott and Ault
jl
'
'
.
' - - '
3760 3 6 8 0
'
1060
'
'.
'^.'
1020 E N E R G Y (cm-1)
1
Ar/HF ( m o r e dilute) *
'
'";e;
420
'
'
I
Figure 1. Infrared spectra taken after the codeposition of HF and c-C3H6 into argon matrices, compared to blank spectra of each reagent alone in argon. The bottom trace shows a second codeposition experiment, with lower HF level.
and high-resolution scans over the regions of interest were obtained on a Beckman IR 12 infrared spectrophotometer at approximately 1-cm-' resolution. Normal-coordinate calculations were carried on at the University of Cincinnati computing center using a program from the National Research Council of Canada, employing a general valance force field.
Results Prior to the codeposition of any of the hydrogen halides and c-C3H6into argon matrices, blank experiments were conducted on each of the reagents employed in this study. While such experiments had been conducted in this laboratory in the past,23,24 additional blanks were run at varying concentrations as needed and were in good agreement with the many literature spectra which have been r e p ~ r t e d . ~ ~Four ~ ~ blank " ~ ~ experiments were conducted with c-C3H6in either argon or nitrogen, at concentrations ranging from 200:l to 2000:1, and all were in good agreement with literature ~ p e c t r a . ~ " HF + c-C3H6. When a sample of A r / H F = 1000 was codeposited with a sample of Ar/c-C3H6 = 1000 in a twin-jet experiment, a number of new infrared absorptions were observed which could not be ascribed to either parent species. An intense band was noted at 3753 cm-' with a weaker counterpart at 3715 cm-', in addition to moderately intense bands at 1044 and 869 cm-l, near intense parent bands of c-C3H6. In addition, quite intense absorptions were noted at 374 and 389 cm-I and were somewhat broader than the previously mentioned bands. Finally, two quite weak absorptions were observed in this experiment, at (23) Ault, B. S. J. Phys. Chem. 1979, 83, 837. (24) Ault, B. S. J. Phys. Chem. 1979, 83, 2634. (25) Andrews, L.; Johnson, G. L. Chem. Phys. Lett. 1983, 96, 133. (26) Maillard, D.; Schriver, A,; Perchard, J. P.; Girardet, C. J . Chem. Phys. 1979, 71, 505. (27) Girardet, C.; Maillard, D.; Schriver, A.; Perchard, J. P. J . Chem. Phys. 1979, 70, 1511. (28) Mann, D. E.; Acquista, N.; White, D. J . Chem. Phys. 1966, 44, 3453. (29) Barnes, A. J.; Hallam, H. E.; Scrimshaw, G. F. Trans. Faraday. SOC. 1969, 65, 3172. (30) (a) Duncan, J. L.; Burns, G. R. J . Mol. Spectrosc. 1969, 30, 253. (b) Duncan, J. L.; McKean, D. C. Ibid. 1968, 27, 117. (c) Brumant, J. C.R . Hebd. Seances Acad. Sci., Ser. B 1972, 274, 637. (d) Baker, A. W.; Lord, R. C. J . Chem. Phys. 1955, 23, 1636.
The Journal of Physical Chemistry, Vol. 88. No. 11, 1984 2325
z l
c
I
i -
,
2900
2820
, 2740
, A _ _ ,
,
,
,
A
,
,
1040 1000w 860 ENERGY (cm 1)
1
, A
1
820"440
1
I
400
Figure 2. Infrared spectra of the products of codeposition of HC1 and cyclopropane into argon matrices, in either single- or twin-jet mode, compared to blank spectra of each reactant alone in argon, over spectral regions of interest.
3495 and 1055 cm-I. This experiment was repeated three times in the single-jet mode but with different concentrations (both greater than and less than the 1OOO:l values originally used). The results of these experiments were quite similar to those described above; however, it was noted that the bands at 3753, 3715, 1044, 869, 389, and 374 cm-' maintained a constant intensity ratio to one another, but the two weak bands at 3495 and 1055 cm-' showed a different concentration dependence. Specifically, these two bands were favored at relatively high ratios of H F to c-CJH6. H F and c-C3H6were also premixed with argon in a single-jet experiment on four occasions, and the spectra obtained were nearly identical with those obtained through twin-jet experiments. The single-jet mode may have increased the product yield slightly, but the difficulty in determining the exact H F concentration makes a definitive statement impossible. Certainly, in these single-jet experiments no new product bands were noted, and the two sets of bands showed the same concentration dependence as in the twin-jet experiments. Figure 1 shows spectra of reaction products of the codeposition of H F and c-C3H6in argon. Finally, HF and cyclopropane were investigated in a single-jet experiment, employing a nitrogen matrix. A weak set of product bands was noted near the argon matrix band positions but shifted slightly to lower energies. Overall, the yield was less than at comparable concentrations in argon, and the product bands were somewhat broader. HCI c-C3H6. These two reactants were codeposited in a number of experiments into argon matrices, employing both singleand twin-jet deposition. In a typical twin-jet experiment, with both values of M/R = 500, a number of new product absorptions were noted at 420, 850,853, 1038, 1042, 2718, 2773, 3015, and 3084 cm-I, the latter two being shoulders on the sides of the two intense c-C3H6parent bands in the 300O-cm-' region. The concentrations were then varied over roughly a factor of 10, and just as in the H F experiments, two distinct sets of bands were noted. The first set, dominant at high dilutions, consisted of the bands at 420, 853, 1038, 2773, 3015, and 3084 cm-I, the most intense of which was the band at 2773 cm-', which was fully absorbing in the most productive experiments as can be seen in Figure 2. The second set of bands, favored at high HC1 concentrations, consisted of bands at 850, 1042, and 2718 cm-'. At a total dilution
+
i
Y
, 2160
,
. 2080
,
, 2000
,
.
A
1960 1920 ENERGY (cm-1)
,
,
1040
,
,
1000 880
840
Figure 3. Infrared spectra of the reaction products of DCI with cyclopropane in argon matrices, compared to blank spectra of each reagent, over the spectral regions of interest.
of Ar/HCl/c-C3H6 = 1000:1:1, the ratio of optical densities of the 2718-cm-' band to the 2773-cm-' band was 0.07, but at a total concentration of 500:5:1, this ratio had increased to 0.23. One matrix sample was subjected to annealing and recooling; the first set of product bands diminished in intensity, while the second set of bands grew distinctly. In a few experiments, with relatively high concentrations and hence high yields of product, three additional weak absorptions were noted in the infrared spectrum at 740, 1185, and 1461 cm-I. However, these absorptions were too weak to allow determination as to whether they correlated with the first or second set of product absorptions. Experiments were also conducted in the single-jet mode for this system; similar results were obtained to the twin-jet mode, and only a slight enhancement of the product bands was noted. The same concentration dependence was observed for the two sets of bands, and no new product bands were detected in the spectrum. Several nitrogen matrix experiments were carried out as well employing HCI and c-C3H6. In this case, product yields were considerably less, but new absorptions were noted at 2766, 2726, 1044, 1039, and 859 cm-', which match quite closely to the argon matrix positions. However, the intensities of the 2726- and 1044-cm-' bands were sufficiently low that a thorough concentration study was not possible. D e l + c-C3H6. Several experiments were conducted using both single- and twin-jet modes, in which DCI and c-C3H6were codeposited into argon matrices. Since the level of deuteration was not loo%, the product bands described above were noted in all experiments. In addition, two new features were observed at 1960 and 2007 cm-'; the relative intensities and band shapes of these two bands were very similar to those of the pair at 271 8 and 2773 cm-' in the above HCl experiments as shown in Figure 3. No new product bands were noted in any experiment, although in the highest yield experiments, weak absorptions were noted near 740, 1185, and 1460 cm-'. HBr + c-C3H6. The reaction between HBr and c-C3H6was studied by a series of single-jet experiments employing argon matrices over the concentration range of Ar/HBr/c-C3H6 = 500:l:l to 1000:20:1 to 1000:1:5. Four product bands were noted in each of these nine experiments at 853, 1037, 2435, and 2474 cm-I, the latter two resembling the 2718- and 2773-cm-' pair of bands observed in the HCl experiments. The concentration dependence was similar as well; the 2435-cm-' band was distinctly
Truscott and Ault
2326 The Journal of Physical Chemistry, Vol. 88, No. 1 1 , 1984
TABLE I: Product Band Positions and Assignments (cm-I) for the 1:l Complexes of Cyclopropane with the Hydrogen Halides vl0a’b acid us (free acid) v t v i ,a*c VBU HF 1044 869 374, 389 3954 3753 HC1 2870 2773 1038 853 420d HCle 1039 859 2856 2166 2077 2007 1038 853 DC1 HBr 2556 2474 1037 853 1037 853 DBr 1839 1780 2204 2191 1036 855 HI 1036 855 DI 1570 1560 1040 856 HCN 3306 3238 1040 856 DCN 2740 2691
AriHBr
I1
Band designations:
Ar/DBr
us, stretching mode of
HX subunit in complex;
v l 0 , antisymmetric ring stretching mode; v I 1 ,CH, rocking mode of cyclopropane in complex; vB, bending mode of hydrogen bond. ul0
for free cyclopropane in argon absorbs at 1025 cm-’. c u l l for free cyclopropane in argon absorbs at 864 cm-’. dTentative assignment. eNitrogen matrix results.
I 2500
2420
1800
1760 ‘ 7 2 0 1060 E NE R G Y ( c m-1)
1020
880
840
Figure 4. Infrared spectra of the reaction products of codeposition of HBr and DBr with cyclopropane into argon matrices, compared to blank spectra of the reagent alone in argon. favored at high ratios of HBr/c-C3H6, while the latter three bands were favored at high dilutions and at low HBr/c-C3H6 ratios. Figure 4 shows typical spectra of Ar/HBr/c-C3H6 samples. In addition, one sample was annealed to approximately 38 K and recooled to 14 K. The band at 2435 cm-’ roughly doubled in intensity, while a slight decrease was noted in the remaining product bands. Also, a weak absorption was noted after annealing a t approximately 1055 cm-’. Finally, in the highest yield experiments, weak absorptions were noted, just as in the HC1 and DCl experiments, near 740 and 1185 cm-I. To investigate the photolytic behavior of the product species in this system, two samples were irradiated with the H20-filtered output of a 200-W medium-pressure mercury arc lamp. One sample was irradiated after deposition; the second sample was irradiated during the full 20 h of deposition. No new product bands were observed in either case, and the yield of product was not measurably sensitive to irradiation. DBr c-C3H6. The codeposition of DBr and C-C3H6in a single-jet experiment in argon gave rise to a similar set of absorptions to those with HBr. In addition, two new absorptions were detected at 1780 and 1764 cm-I, with a general shape and intensity ratio quite close to the 2435- and 2474-cm-’ pair of bands observed in the HBr experiment (these bands were also noted in the present experiment due to incomplete deuteration of the vacuum system). HCI HBr C-c3H6. In four single-jet experiments, c-C3H6 was codeposited with a mixture of HCl and HBr over a considerable range of concentrations of each reactant. All of the product bands described in the different sections above were noted in these mixed experiments, but no new product bands were detected anywhere in the spectrum. HI + c-C3H6. These two reactants were studied in a single-jet experiment, to confirm the results reported earlier by Barnes and co-workers.20 A weak absorption was noted near 2191 cm-’, as well as absorptions at 855 and 1036 cm-I. The former two bands agree with the work of Barnes; the latter does not, as Barnes reported a band on the low-energy side of the c-C3Hpparent band. DZ c-C,H,. This system was studied in a single-jet experiment to determine the deuterium mass dependence of each of the three product bands noted above. The bands at 855 and 1036 cm-I did not show any deuterium shift, while a possible weak deuterium
+
+
+
+
counterpart of the 2191-cm-I band was detected near 1560 cm-I. However, this spectral region is partially obscured by atmospheric and matrix isolated HzO, and the observation of this band must be considered to be tentative. HCN C-C,H,. H C N was codeposited in a single-jet experiment with c-C3H6at a total dilution of 500:l:l in argon. Quite intense product bands were noted at 856, 1040, and 3238 cm-I; no other bands were observed in this spectrum. DCN c-C3H6. These two reactants were also codeposited in single-jet experiments; due to the method of formation of DCN, considerable H C N was present as well. Consequently, the above set of bands observed with H C N was noted in this experiment; only one new product band was observed. This was detected at 2691 cm-’ and had an intensity ratio to the 3238-cm-I H C N product band that was identical with the ratio of intensities of the H C N and DCN parent bands. All of the band positions noted in the Results section are tabulated in Table I.
+
+
Discussion The codeposition of the hydrogen halides or hydrogen cyanide with cyclopropane into either argon or nitrogen matrices gave rise to infrared absorptions due to a product species. As noted in the Results section, for HF, HCl, and HBr, the product absorptions could be divided into two sets, typified by the sets (1) 420, 853, 1038, 2773, 3015, and 3084 cm-’ and (2) 850, 1042, and 2718 cm-I for the HCl/c-C3H6 system. This observation indicates the formation of two product species, one that is favored at high dilution and high levels of c-C3H6 (set 1) and a second species favored at high concentrations and high hydrogen halide levels (set 2). Moreover, set 2 grew upon annealing and diffusion of a sample, while set 1 decreased. This suggests that set 2 is due to an aggregate species and likely one that contains more than one HX unit per cyclopropane molecule. Set 1, on the other hand, is best assigned to the 1:l reaction product or complex for each system, on the basis of concentration dependence. Only one product band system was observed for H I and HCN, and on the basis of concentrations this set of bands is best assigned to the 1:l complex. This assignment is in agreement with the conclusions20*2iof Barnes and co-workers. One might anticipate the direct reaction of a hydrogen halide with C-C&; possible products include the isomerization product propylene, the addition product propyl halide, and the molecular complex. The product bands in each system can be compared to those of gas-phase and matrix spectra3’ of propylene and the appropriate propyl halide, and this comparison definitely indicates that neither isomerization or addition has occurred. Rather, the spectra are reminiscent of a perturbed acid subunit and a perturbed cyclopropane unit, as would be expected for a weakly bound (31) Ault, B. S., unpublished results.
1:1 HCN/c-C3H6 and HX/c-C3H6 Complexes molecular complex. The 1:1 hydrogen-bonded complexes of HF and HCl with c-C3H6 are both known from supersonic nozzle experiments, lending further support that the species responsible for the first set of absorptions is the 1:l adduct of cyclopropane with each of the hydrogen halides and cyanide. Moreover, as noted below, the spectra are strongly suggestive of hydrogen-bond formation (although weak), in agreement with the gas-phase results.
Band Assignments
The Journal of Physical Chemistry, Vol. 88, No. 11, 1984 2327
H'
'H
CYCLOPROPANE - H X
Figure 5. Structure of the 1:l complex between a hydrogen halide, HX,
and cyclopropane (from ref 10). The product bands observed for the 1:l complex in each of the systems studied here can be divided into two or three categories. comparison of all of the spectra shows clearly that a new absorption Firstly, for each acid studied a very intense absorption was noted is present. This, in principle, could be due to a perturbed v o some 10-200 cm-' below the parent band position, which might v I 1 mode in the complex. However, the assignment to ul: H i be ascribed to a perturbed acid unit in the complex. Secondly, strongly preferred on the basis of intensities, in that the product several medium-to-weak absorptions were noted near intense band was often considerably more intense than v7 and always absorptions of the parent cyclopropane, suggestive of a perturbed considerably less intense than vll, which is certainly the anticipated base subunit in the complex. Thirdly, for the H F and HCl result. Moreover, Barnes observed the same product absorption complexes one or two low-frequency modes in the neighborhood at 855 cm-I for the HI*C-C& complex and also assigned this to of 400 cm-' were observed, not near any parent absorptions. the perturbed vI1 mode in the complex. Finally, it should be noted The vibrational mode u, of the perturbed acid, shifted to lower that this, too, is a doubly degenerate mode that should split if the energies by up to 200 cm-', is diagnostic for the formation of a symmetry of the complex is C2, or lower. This point will be hydrogen bond.32 Such shifts, and shifts as high as 2000 cm-', discussed below. have been noted for many hydrogen-bonded systems and are often correlated with the strength of interaction in the c ~ m p l e x . ~ ~ J ~ For ~ ~ the ~ 1:l complexes HF.c-C3H6 and HCl.C-C3H6, vibrational modes were observed near 400 cm-', far below the lowest parent Moreover, when the deuterated analogue of the acid was studied, absorption. For many hydrogen-bonded complexes, the bending a shift was noted to lower energy; the magnitude of the shift vH/vD mode (or modes) of the hydrogen bond has been observed below for each complex was around 1.38, as expected for a hydrogen 700 cm-1.34 Moreover, this mode is postulated to increase in stretching motion and as was observed for the parent hydrogen frequency as the strength of interaction increases. Consequently, and deuterium halides. It is also noteworthy that the shifts in for the two most strongly bound complexes (from the rotational us observed here are small by hydrogen-bonding standards, sugspectra; see below), these bending modes may lie in an accessible gestive of a weak interaction (which will be discussed below). A region. The mode at 420 cm-' for the HC1 complex, which was second diagnostic observation for the formation of a hydrogen bond always quite weak, is tentatively assigned to one of the bending is the intensification of the hydrogen stretching motion v,. This modes, while the intense doublet at 374 and 389 cm-' for the H F mode, typified by the band at 3753 cm-' in the H F system, 2773 complex is assigned either to the two bending modes (in-plane cm-I for HC1, 2474 cm-' for HBr, and 2191 cm-' for HI, was and out-of-plane) of the hydrogen bond or to one bending mode generally very intense and in some experiments was nearly fully of the complex in two slightly different geometries (induced by absorbing. In the HBr system in particular, the product absorption the matrix environment) just as two slightly different geometries at 2474 cm-I was much more intense than the parent absorptions were suggested by the results in the H-F stretching region. For (see Figure 4), but it is likely that only a few percent of the parent the HBr, HI, and HCN complexes, these bending modes should HBr was converted to product. This indicates a much higher be less intense and at lower energies, making their observation absorption coefficient for the complexed hydrogen halide, as exless likely, and no bands were observed in the low-energy region, pected for a hydrogen-bonded species. It should be pointed out down to 200 cm-'. that in the HF experiments, two bands were observed in the H-F stretching region of the 1:l complex, a quite intense band at 3753 Structure of the Complex cm-' and a less intense band at 37 15 crn-'. Since these two bands showed a constant intensity ratio over many experiments, they Rotational spectra of the HF.c-C3H6 and HCl.c-C3H6 commust be assigned to the same species or to two different forms plexes in a supersonic expansion indicate a C,, structure with the of the 1:l complex. This latter interpretation is preferred, and hydrogen halide bonding to the midpoint of one of the carbonsince the difference in band positions is only 38 cm-', the difference carbon bonds, as shown in Figure 5 . These workersl0 also calin the geometry is likely to be very slight. It is likely that such culated a well depth for the complex, in the approximation of a a slightly different geometry is matrix induced and may simply Lennard-Jones potential. A well depth of 1871 cm-' (or 5.34 represent a different site in the argon lattice. Unfortunately, the kcal/mol) was calculated for the HF.C3H6 complex, while this yields in the nitrogen matrix experiments were sufficiently low value dropped to 959 cm-' (2.73 kcal/mol) for the HC1 complex. that such behavior (or lack thereof) could not be observed. The infrared spectra obtained here might serve to confirm this In the spectral region near ulo, the doubly degenerate ring structural finding and also provide a force field for the complex. deformation mode of the parent cyclopropane, a product absorption However, not nearly enough vibrational modes of the complex was noted for each 1:l complex studied here. Invariably, the were detected to provide any meaningful force field. The question product band occurred at higher energy than the isolated parent, of the symmetry of the complex can be addressed in some detail; but the shifts were only of the order of 12-18 cm-'. These are a C3,structure has also been proposed in that the center of the assigned to a ring deformation mode of the c-C3H6unit in the cyclopropane ring contains considerable electron density as well. complex; this mode can no longer be doubly degenerate. SymTwo potentially observable spectroscopic changes3s should occur metry considerations will be presented in a later section. Also, as a C,, complex is formed, the first of which is the splitting into a new product absorption was noted in the region near v l l , the two components of all of the infrared-active doubly degenerate doubly degenerate CH2 wagging mode of the parent at 864 cm-I. modes of the cyclopropane molecule in the complex. The second As can be seen in Figures 1-4, there is a second absorption of the is the activation of at least some of the vibrational modes of the parent in this region, which has been assigned to u7. The product cyclopropane entity which are infrared forbidden for the isolated absorption substantially overlaps this weaker parent band, but close ~
(32) Pimentel, G. C.; McClellan, A. "The Hydrogen Bond"; W. H. Freeman: San Francisco, 1960. (33) Ault, B. S.; Pimentel, G. C. J . Phys. Chern. 1973, 77, 1649.
~
~
~~
~~
(34) Ault, B.S.; Steinback, E.; Pimentel, G. C. J . Phys Chem 1975, 79, 615. (35) Nakamoto, K. "Infrared and Raman Spectra of Inorganic and Coordination Compounds", 3rd ed.; Wiley-Interscience: New York, 1978.
2328 The Journal of Physical Chemistry, Vol. 88, No. 1I, 1984 species (some of these are doubly degenerate as well, of E” symmetry in the D3hpoint group, and should also split). However, the magnitude of the splitting as well as the degree of activation should be sensitive to the strength of interaction in the complex. It was noted in the Results section that, in the highest yield experiments, weak infrared absorptions were often seen near 740 and 1185 cm-I. These positions lie very near two E” modes of free cycl~propane~~ and represent slight activation by the presence of the complexed hydrogen halide. The intensities of these modes, nonetheless, are still very low, as might be anticipated for a weak interaction. The question of the splitting of the infrared-active degenerate modes, in particular ui0 and u I I , by the perturbing influence of the hydrogen halide must be considered as well. Flygare and co-workersi0~I1 were able to reproduce their rotational spectra of the complex by assuming that the cyclopropane subunit was unperturbed from its free geometry, yet a shift of ul0 of up to 19 cm-’ is substantial (as is a binding energy of 5.4 kcal/mol). Consequently, normal-coordinate calculations were undertaken to model, in a simple fashion, the interaction. First, the normal-coordinate calculations of Duncan and Burns30were reproduced for free cyclopropane, using their reported force field and frequencies. Then, the force constant for the one carbon-carbon bond involved in the hydrogen-bonding interaction was systematically altered (both raised and lowered) while the force constants for the remaining two carbon-carbon bonds were held constant (and equal to one another). The total change in force constant was +0.30 mdyn/A, in steps of 0.05 mdyn/A, and several interesting points were noted. Firstly, as anticipated, the two components of each doubly degenerate mode of the free molecule did split into a doublet, as required for C, symmetry, and this splitting occurred for even small changes in the one force constant (0.05 mdyn/A). Secondly, for only two of the infrared-active doubly degenerate modes (Le,, E’ modes in D3,, symmetry) was the shift from the parent mode greater than 1 cm-l and hence possibly resolvable on the spectrometer employed, these two modes were ul0 and v l i , where product bands were observed. Thirdly, when both ul0 and uI1 split, one component shifted up (or down) for the parent band position and the second component in each case did not shift more than 0.1-0.2 cm-’ from the parent band position. This suggests that although two components are predicted, one lies directly underneath the parent band and cannot be detected. Only the shifted component of the doublet could be and was observed. Finally, these calculations suggest that for a given change in force constant the predicted shift in ul0 should be considerably greater than the shift in v l l , as was observed. All of these observations and calculations, which represent a simple model, are in direct agreement with a C, structure in agreement with the rotational spectra. It should be noted, however, that the spectra obtained here are not inconsistent with a C3, geometry, in that activation of the E” modes is still anticipated, and only one component of each split E’ mode was observed. Nonetheless, on the basis of the gas-phase work and the spectra obtained here, the C, structure is most likely correct. C N D 0 / 2 calculations performed here also support the C , structure as being more stable than the C3, isomer. These observations and calculations are in disagreement with the conclusions of Barnes and Paulson, who reported the shifted ul0 band of cyclopropane in the HCl complex as a doublet at 1039 and 1041 cm-’. They interpret this doublet as both components of the E’ mode, shifted up some 13 cm-l but only slightly split (2 cm-I). In view of this observation, the current spectra were reexamined for all hydrogen halide complexes and for the HCl.C,H6 complex in particular. The reported band at 1038 cm-’ was observed to be sharp, unsplit, and completely symmetrical, with a bandwidth of less than 1 crn-’. Similar results were obtained with HF, HBr, HI, and HCN. Consequently, the interpretation as one component of the split E’ mode suggested above is strongly preferred. It is also noteworthy that Barnes only reports this splitting for the HCI complex, not the HBr or H I complexes, and the splitting is barely perceptible in the reported spectra.*I One explanation for this apparent discrepancy was the fact that the
Truscott and Ault TABLE II: Band Shifts due to Complex Formation in 1:l Cyclopropane Hydrogen Halide Complexes E,b PA,C AuS,’ AuS/ Avlo, kcall kcall p,d acid cm-I v. cm-I mol mol D
HF HC1 HBr HI HCN
201 97 82 13
0.051 0.034 0.032
68
0.021
0.006
19 13 12 11 15
5.34 2.73 2.4Y
371 333 323 313 353
1.91 1.03 0.79 0.38 2.98
“Shift of us from argon matrix Q-branch position of parent HX. Well depth, from ref 11. Proton affinities, from ref 41. “Dipole moment of hydrogen halide. @Fromref 42. present work was, for the most most part, done at a much lower concentration (often 1:l:lOOO) while Barnes and co-workers reported spectra at 1:1:200. The higher concentration might lead to increased perturbations in the matrix and a slight site splitting of the 1038-crn-’ band. At the lower concentrations employed here, such splitting was not observed. In discussing the overall structure of the complex, the possibility of a protonated intermediate complex should be considered. The spectra obtained here are consistent with a quite weak hydrogen bond, without complete proton transfer. The proton affinity of cyclopropanes is 179 kcal/mol, much less, for example, than that of NH3 (205 kcal/m01)~~ where proton transfer has been shown to be absent. Also, the gas-phase rotational spectra were consistent with a H F or HC1 unit whose bond length was unchanged or lengthened just slightly, which certainly would not indicate proton transfer. Finally, if proton transfer did occur in the gas phase, one might anticipate some isotope scrambling in the product when DCl or DBr was codeposited with c-C3H6. No scrambling of this sort was noted (i.e., no c-C3H5D was observed), further ruling against a protonated intermediate. On the other hand, scrambling does occur in solution, and protonated intermediates are commonly i n v ~ k e d . ~ ’ Solvation *~~ by the solvent, particularly in acid solution, must account for the considerable difference between gas-phase and solution-phase behavior.
Spectroscopic Trends This study presents spectroscopic observations for a series of acids with a common base, from which some conclusions may be drawn. For hydrogen-bonded systems, the most common measure19,33is the shift of the hydrogen stretching frequency, Av,, or the relative shift Av,/v,. In addition, in this study one can also employ the magnitude of shift of the perturbed cyclopropane, as measured by the shift of the high-energy component of ul0 near 1040 cm-’. Table I1 makes this comparison, and as can be readily seen, the greatest shifts occur with HF, and they decrease down the halogen family to HI. In the past,33 a correlation with the gas-phase proton affinities of the halides anions X- has been made, but as Table I1 demonstrates, this trend is exactly reversed. Specifically, the argument has been made that the lower the proton affinity of the halide anion, the better proton donor it becomes and the more readily it can form a hydrogen bond. The evidence here is opposite, in that F- has the greatest proton affinity and the strongest interaction. Barnes has notedi9 this phenomenon as well and states that for weak acceptors (such as c-C3H6)proton affinities are not a reliable guide, but for stronger bases this approach works well. Table I1 also lists the dipole moments of the hydrogen halides, and these do correlate directly with the shift in vibrational frequencies. This suggests that, for a weak interaction, the hydrogen bonding is dominated by electrostatics, and for stronger hydrogen bonds a covalent contribution becomes significant. The dipole moment and proton affinity of hydrogen (36) Aue, D. H.; Bowers, M. T. In “Gas Phase Ion Chemistry”; Bowers, M. T., Ed.; Academic Press: New York, 1979; Vol. 2. (37) Collins, C. J. Chem. Rec. 1969, 69, 543. (38) Saunders, M.; Vogel, P.; Hagen, E. L.; Rosenfeld, J. Acc. Chem. Res. 1913, 6, 53.
1:1 HCN/c-C3H6 and HX/c-C3H6 Complexes cyanide do not fit either trend well; perhaps the net positive charge on the hydrogen atom in each acid would be the ultimate measure of the electrostatic interaction. Comparison might also be made with the HF complexes of unsaturated hydrocarbons; Andrews has o b ~ e r v e d the ~ ~ sH~F~ stretching mode of the C,H2.HF complex at 3746 cm-’ and the HF stretch of the C2H4.HFcomplex at 3731 cm-’. These values are quite close to the present value of 3753 crn-’ for the c-C3H6.HF complex, which is in keeping with the olefinic nature of cyclopropane. The bending modes observed here, at 374 and 389 cm-’, for the HF.c-C3H6 complex also agree well with those of the HF.C2H2 and H F C 2 H 4complexes, where these modes were observed at 382,426 and 396,424 cm-’, respectively. Interestingly, the complexes of H F with C2H2and C2H4are both T-shaped, with the hydrogen of HF hydrogen bonding to the midpoint of the carbon-carbon multiple bond, in a fashion analogous to cyclopropane.
Identification of Second Product Species A second product species was noted in most experiments with HF, HCl, or HBr, and it was observed that this species is favored at high concentrations and high HX/c-C,H6 ratios. This behavior marks the product as an aggregate species, the most likely of which is the 1:2 complex c-C3H6.2HX. If one assumes the C , structure for the 1:1 complex discussed above, two sides remain available for hydrogen bonding without any serious steric hindrance. The addition of a second H X molecule would present approximately twice the perturbation to the cyclopropane ring; it is noteworthy that the ring deformation mode for the 1:2 complex near 1050 cm-’ is shifted approximately twice as far from the parent as is the 1:l complex. A second band assignable to the 1:2 complex (39) (a) Andrews, L.; Johnson, G. L.; Kelsall, B. J. J. Am. Chem. SOC. 1982, 104, 6180. (b) McDonald, S. A.; Johnson, G. L.; Keelan, B. W.; Andrews, L. Ibid. 1980, 102, 2892. (40) (a) Andrews, L.; Johnson, G. L. J . Chem. Phys. 1982,76, 5767. (b) Andrews, L.; Johnson, G . L. J . Phys. Chem. 1982, 86, 3374. (41) Bartmess, J. E.; McIver, R. T. Jr. In “Gas Phase Ion Chemistry”; Bowers, M. T., Ed.; Academic Press: New York, 1979; Vol. 2. (42) Kukolich, S. G.; Shea, J. A,; Aldrich, P. D.; Campbell, G. C.; Read, W. G. “Abstracts of Papers”, 186th National Meeting of the American Chemical Society, Washington, DC, Sept 1, 1983; American Chemical Society: Washington, DC, 1983; PHYS 162.
The Journal of Physical Chemistry, Vol. 88, No. 11, 1984 2329 fell below the H-X stretching mode of the 1:l complex, shifted approximately twice as far from the parent band position. A second H-X stretching mode should be observable in such a 1:2 complex, but only one was noted. A second possible structure for a 1:2 complex would be a complex of cyclopropane with a dimer of the hydrogen halide; in all, insufficient evidence is available to make a structure deteimination for this larger aggregate. Mixed halogen experiments were conducted by codepositing HC1, HBr, and c-C3H6,and no new absorptions were noted which could be ascribed to a mixed 2:l complex. However, the spectral regions under consideration were very complex, and this observation is not definitive.
Conclusions The matrix isolation technique, using either single- or twin-jet deposition, has led to the isolation and characterization of the 1:l complex of the hydrogen halides and cyanide with cyclopropane. These complexes are all weakly bound, and the order of interaction decreases from H F to HI, in agreement with a dipolar or electrostatic interpretation of the hydrogen-bonding interaction. The spectra are consistent with and support the C2, structure determined from gas-phase rotational spectroscopy. However, on the basis of the results presented here, the possibility of a C3,structure cannot definitely be eliminated. In addition, evidence for a 1:2 complex was observed, but a structural determination could not be reached. Further studies into the structure and binding in weak molecular complexes are now in progress to further clarify the role of these molecular complexes in reaction pathways. Note Added in Proof. Several researchers have recently reversed the assignments of vi,, and v i , for cyclopropane. While this discrepancy has not been clearly resolved, the disagreement has only a small qualitative bearing on the results and calculations presented here, and no effect on the conclusions reached. Acknowledgment. We gratefully acknowledge support of this research by the National Science Foundation, through Grant CHE 81001 19. B.S.A. also gratefully acknowledges the Dreyfus Foundation for a Teacher-Scholar Grant. Dr. H. H. Jaffe is acknowledged for his help with the C N D 0 / 2 calculations. Registry No. H F , 7664-39-3; HCI, 7647-01-0; DC1, 7698-05-7; HBr, 10035-10-6; DBr, 13536-59-9; HI, 10034-85-2; DI, 14104-45-1; HCN, 74-90-8; DCN, 3017-23-0; c - C ~ H 75-19-4. ~,
