2825
J. Phys. Chem. 1986,90, 2825-2829
"[ 2
(1
+ $)"'-1 1 1 (A.l)
where s = 4?re2/wakT, z is the dielectric constant of water, a is the head group area, R is the radius of the micelle, e is the unit charge, k is Boltzmann's constant, T is the temperature, K = (8~n&/zkT)'/~,and 4 = (cmc salt concentration). If we take the SDS-NaC1-oil interface and assume no or little oil uptake in the micelles, the calculated interfacial tensions are y = 5.4, 4.6, 3.4, 2.3, and 1 dyn/cm at salt concentrations of 0, 0.01, 0.03, 0.1, and 0.3 M, respectively. The value 4.6 is in good agreement with a measured value y 5 for the SDS-O.1 M NaC1-heptane interface."*3s There is an extreme paucity of data on this and similar well-defined systems (in fact, only one data point and no aggregation numbers). Even though the numbers will change a little due to attractive or repulsive interactions and entropic contributions, theory seems to be on the right track for this
+
-
problem. The predicted dependence of y on salt concentration in the presence of excess oil is extremely weak (logarithmic) in accord with observation. Thus, if we take a presumed oil-in-water 500 8, and head group area 50 A2, we "droplet" size of R have y = 0.3, 0.2, and 0.1 dyn/cm at concentrations of 0.1, 1, and 3 M salt, respectively. For SDS-alcohol-water-oil microemulsions a typical "drop" size is R = 100 8, at 0.6 M NaC1.I2 The predicted value of y is 1 dyn/cm, much larger than the experimental value of y 0.1, This disagreement is worse at higher salt concentrations which give ultralow surface tension. The discrepancy between theory and experiment seems to be real. It is resolved if one admits that the interpretation of scattering data (from which structure is deduced) depends on the model assumed. There can be very real difficulties in interpreting QELS experiments for micellar and microemulsion system^.^^,^' If these microemulsions are bicontinuous rather than droplets, with then the possibility of very low overall curvature, the problem no longer exists.
-
-
--
SPECTROSCOPY AND STRUCTURE Matrix Isolation Investigation of the Complexes of the Molecular Hatogens with Cyclopropane and Its Derivatives Bruce S. Ault Department of Chemistry, University of Cincinnati, Cincinnati, Ohio 45221 (Received: September 30, 1985; In Final Form: February 28, 1986)
Single- and twin-jet deposition techniques have been employed in conjunction with matrix isolation to investigate the reaction products arising from the codeposition of cyclopropane and its derivatives with CI2,CIF, and Br,. In each case, the infrared spectra indicated formation of an isolated 1:l complex with perturbed modes of both the acid and base subunits detected. For example, in the C1F.c-C3H6complex the stretching mode of the complexed ClF subunit shifted 29 cm-I to lower energy, while vl0 and v l l of cyclopropane each shifted approximately 16 cm-'. The spectra suggested a mode of interaction and coordination that was comparable to that deduced for the analogous hydrogen halide complexes, namely interaction with the midpoint of one of the carbon-carbon bonds, with the halogen lying in the plane of the three-membered ring. The halogens were noted to perturb the cyclopropane subunit to a degree equal to or greater than that caused by the hydrogen halides, while ClF did not perturb the base subunit substantially more than did the nonpolar C12 and Br2 molecules.
Introduction Numerous studies]-" in the past two decades have shown that the isomerization of cyclopropane to propene is catalyzed by both Lewis and Bronsted acids, notably HCI, HBr, BC13, and BBr3. In order to more thoroughly understand this catalytic action, and to characterize the interaction between cyclopropane and the hydrogen halides, the 1:l complexes of cyclopropane with the hydrogen halides have been studied extensively in recent Ross, R. A.; Stimson, V. R.J . Chem. SOC.1962, 1602. (2) Maccoll, A.; Ross, R.A. J . Am. Chem. Soe. 1965, 84, 4997. (3) Stimson, V. R.; Taylor, E. C. Aust. J . Chem. 1976, 29, 2557. (4) Lewis, D. K.; Bosch, H. N.; Hossenlopp, J. M. J . Phys. Chem. 1982, 86, 803. (5) Truscott, C. E.; Ault, B. S. J. Phys. Chem. 1984, 88, 2323. (6) Barnes, A. J.; Paulson, S. L. Chem. Phys. Lert. 1983, 99, 326. (7) Buxton, L. W.; Aldrich, P. D.; Shea, J. A.; Legon, A. C.; Flygare, W. H. J . Chem. Phys. 1981, 75, 2681. (1)
0022-3654/86/2090-2825$01.50/0
In these complexes, the cyclopropane unit serves as an electron donor or base, forming a hydrogen bond to the hydrogen halide so that the hydrogen halide lies in the plane of the C3ring, bonded to the midpoint of one of the carbon-carbon bonds. The molecular halogens are well-known Lewis acids and form charge-transfer complexes with a large range of bases, particularly for I, and Br2.9-I' One might, then, anticipate a substantial interaction between cyclopropane and the molecular halogens, yet the dark reaction of c-C3H6with Br2 at -78 "coccurs very ~ l o w l y , ' and ~~'~ no reaction has been reported between c-C3H6and Clz. However, Aleksanyan and Gevorkyan have presented preliminary evidence (8) Legon, A. C. J . Phys. Chem. 1983, 87, 2064. (9) Benesi, H. A.; Hildebrand, K. 0. Acra Chem. SOC.1949, 71, 2703. (IO) Mulliken, R. s. J . Phys. Chem.1952, 56, 801. (11) Mulliken, R.S.; Person, W. B. J. Am. Chem. SOC. 1969, 91, 3409. (12) Deno, N. C.; Lincoln, D. N. J . Am. Chem. SOC.1966, 88, 5357. (13) Skell, P. S.; Day, J. C.;Shea, K. J. J . Am. Chem. SOC.1976,98, 1195.
0 1986 American Chemical Society
2826 The Journal of Physical Chemistry, Vol. 90, No. 13, 1986
Ault
(a) Ar/CI2 c
e
530
450
490
,
410
,
.
530
.
490
.
.
450
.
410
ENERGY (crn-1) Figure 2. Infrared spectra, over the region 390-550 cm", of the product of codeposition of CI, with cyclopropane and its methyl derivatives. L.
-i.--
1500
1450
140-
1050 ENERGY
,
1000
950
900
850
1
800
1
(cm-1)
Figure 1. Infrared spectra, over selected spectral regions, of the products arising from the codeposition of cyclopropane with C12(middle trace) and Br2 (bottom trace) compared to a blank experiment of c-C,H,, all in
I/-
argon matrices.
of a complex between cyclopropane and C12, after the cocondensation of the pure materials onto a low-temperature surface.14 The matrix isolation t e c h n i q ~ e l ~ -has ~ ' proven very effective in the characterization of weakly bound molecular complexes and may be the only direct means by which to observe and study complexes between the molecular halogens and cyclopropane. In addition, recent matrix studies'* have identified for the first time complexes of the highly reactive interhalogen CIF. A comparison of the complexes of Br2 and C12 with those of C1F will provide some insight into the role of electrostatic interactions (greater for ClF due to its nonzero dipole moment) as compared to charget r a n ~ f e r . ' ~Consequently, a study was undertaken to characterize the complexes formed between cyclopropane and its derivatives and the molecular halogens C12, Br2, and CIF, employing the matrix isolation technique. Experimental Section All of the experiments in the current study were performed on a conventional matrix isolation apparatus, which has been described previously.zo The gaseous reactants, c-C3H6 (Matheson), C4Hs (methylcyclopropane, Columbia), Clz (Matheson), and CIF (Pennwalt) were condensed a t 77 K and subjected to several freeze-thaw cycles prior to sample preparation. For the liquid reagents 1,l -dimethylcyclopropane (Pfaltz and Bauer), bromocyclopropane (Aldrich), and Br2 (Fisher), the vapor pressure above the distilled liquid was used for sample preparation. Argon was employed as the matrix gas in all experiments and was used without further purification. Experiments were conducted in both the single-jet and twin-jet modes, a t a flow rate of roughly 2 mmol/h for 20-24 h. Both survey and high-resolution scans were recorded on a Beckman IR12 or Perkin-Elmer 983, at approximately 1-cm-' resolution. In several experiments, samples were then warmed to 35 K to allow limited diffusion, followed by recooling to 14 K and the recording of additional spectra. Finally, several samples were irradiated with the H20filtered light of a 200-W medium-pressure Hg arc after deposition, for 1-3 h. (14) 371. (15) (16) (17)
Aleksanyan, V. T.; Gevorkyan, B. 2. Zh. Struck. Khim. 1976, 17,
Andrews, L. A. J . Mol. Struct. 1983, 100, 281. Auk, B. S. Inorg. Chem. 1981, 20, 2817. Barnes, A. J. In Molecular Interactions, Vol. 1, Ratajczak, H., Orville-Thomas, W. J., Eds.; Wiley: New York, 1980; Chapter 9. (18) (a) Machara, N. P.; Ault, B. S. Inorg. Chem. 1985, 24, 4251. (b) Machara, N. P.; Ault, B. S., to be submitted for publication. (19) Umeyama, H.; Morokuma, K.; Yamabe, S. J . Am. Chem. SOC.1977, 99, 330. (20) Auk, B. S. J . Am. Chem. SOC.1978, 100, 2426.
!
Y
I
A
770
730
690
"
I
ai0
ENERGY
I
I
770
CIF + 1.1 DMCP ,
,
730
,
,
690
(crn-1)
Figure 3. Infrared spectra of the CI-F stretching region (670-810 cm-I) for four different experiments. The upper left trace shows a blank experiment of CIF in argon, while the lower left shows the spectrum arising from the codeposition of CIF and c-C3H,. The upper right spectrum is from the codeposition of CIF with methylcyclopropane, while the lower right is taken from a similar experiment employing CIF and 1,l-di-
methylcyclopropane. Results Prior to the codeposition of the cyclopropane derivatives with the molecular halogens, blank spectra were recorded at representative dilutions in argon. The spectra of all of these reagents have been recorded in argon matrices in this laboratory, as well as in the gas phase in other l a b o r a t o r i e ~ , ~ ~and ' ~ ~the ~ 'current -~~ spectra were in good agreement. Cyclopropane Halogens. c-C3H6was codeposited with C12 in several single-jet experiments into argon matrices, over a wide range of concentrations. When a total dilution of Ar/ClZ/c-C3H6 = 500/ 1/ 1 was employed, a number of new infrared absorptions were noted. The most prominent were moderately intense bands at 845 and 1037 cm-I, while a medium-intensity band was noted at 1456 cm-l, as can be seen in Figure 1. In addition, weak absorptions were observed at 428, 787, and 1179 cm-I. It should be pointed out that, in these experiments, no indication of impurity HCI was detected, nor was the intense 2771-cm-' absorption of the c-C3H6.HCl c o m p l e ~ When .~ the concentration of the reagents was increased to Ar/C12/c-C3H6 = 1000/5/2, the same set of
+
(21) Prochaska, E. S.; Andrews, L.; Smyrl, N. R.; Mamantov, G. InorR. Chem. 1978, 17, 970. (22) Duncan, J. L.; Burns, G. R. J . Mol. Spectrosc. 1969, 30, 253. (23) Truscott. C. E.;Auk. B. S. J . Phvs. Chem. 1985.89, 1741. (24) Wurrey, C. J.; Berry, R. J.; Yeh, Y . Y.; Little, T. S.; Kalasinsky, V . F. J . Raman Spectrosc. 1983, 14,81.
,
Complexes of Molecular Halogens with Cyclopropane product bands was noted, and all appeared to grow a t approximately the same rate. When the concentration was further increased to 200/ 1/ 1, additional intensity was noted for each of these absorptions and the 428-cm-l product band appeared to have unresolved structure on the low-energy side, as shown in Figure 2. The d e p o s i t i o n of samples of Ar/ClF with Ar/c-C,H6 was carried out in the twin-jet mode, due to the likelihood of direction reaction in the gas phase. In a typical experiment, with M / R = 500 for each reagent, a quite intense doublet was noted at 733 and 739 cm-I, with an intensity ratio of 3:l indicative of chlorine isotopic splitting, as shown in Figure 3. In addition, strong product absorptions were noted at 845 and 1038 cm-', while a weak absorption was noted at 1180 cm-I. As the concentration of each reagent was increased up to 200/1, the intensity of each of the product absorptions described above was observed to grow, and at a comparable rate. However, since the 845- and 1038-cm-I absorptions lie near strong parent modes of c-C3H6, some broadening and smearing of the spectrum was noted in these regions at high concentration. Finally, one matrix containing this pair of reactants was irradiated with the H20-filteredoutput of a Hg arc lamp for 1.5 h after deposition. No change in the spectrum was noted; the bands due to the reaction product were not diminished, and no new absorptions were observed. Cyclopropane was codeposited with samples of Ar/Br2 in both single-jet and twin-jet experiments, over a wide range of concentrations. Adsorption of Br, to the stainless steel vacuum lines made exact determination of the concentrations difficult, but the values cited are probably correct to within 10%. In a single-jet experiment, with total concentration of 500/ 1/ 1, several new, quite intense product absorptions were noted, a t 849, 1036, and 1456 cm-I. In addition, weak features were detected at 773 and 1180 cm-'. When the concentration of either or both reagents was increased, all of these product bands were observed to grow, and a t the same relative rate. When the same pair of reactants were deposited from separate vacuum lines in twin-jet experiments, at the same relative concentrations, no changes were noted in the spectrum. Finally, a similar sample was subjected to irradiation during the entire length of deposition, 20 h, with the same light source; this procedure did not alter the final spectrum in any detectable way. Methylcyclopropane ( M C P ) Halogens. The single-jet codeposition of MCP with C12at a total dilution of 500/ 1/ 1 in argon gave rise to four distinct product absorptions, at 434, 842, 932, and 1022 cm-'. The latter three were close to parent modes of M C P and not as cleanly resolved as the 434-cm-I band, but nonetheless they were distinct. As the concentration of C12was increased, so that the total dilution was Ar/C12/MCP = 1000/5/2, the same set of product absorptions were noted, with increased intensity. The upper three product bands were now more clearly resolved than in the preceding experiment. The codeposition of MCP with CIF in a twin-jet experiment at concentrations of 500/1 each in argon gave rise to an intense, sharp doublet with 3:l relative intensity, at 735 and 729 cm-l. In addition, product absorptions were noted near several modes of parent MCP, at 841, 1029, and 1051 cm-l. MCP was also codeposited with Br2 in two single-jet experiments; product absorptions were noted a t 841, 931, 972, and 1011 cm-' in each. In addition, at higher concentrations, a possible weak product band was noted near 285 cm-', with a band width of approximately 6 cm-I. 1,l-Dimethylcyclopropane(1,l-DMCP) Halogens. The codeposition of a sample of Ar/Br2 = 500 with a sample of Ar/ 1, I-DMCP = 500 in a twin-jet experiment gave rise to several distinct product absorptions of moderate intensity, at 826, 91 8, 927, 1003, and 1009 cm-I, as well as a weak absorption at approximately 260 cm-I. As the concentration of reactants was increased, these product bands grew in intensity, although the band near 260 cm-' remained quite weak. The codepositon of 1,lDMCP with ClF into argon matrices in a twin-jet experiment likewise gave rise to distinct product absorptions, the most intense of which was a strong 3:l doublet at 693 and 699 cm-l, as can
+
+
The Journal of Physical Chemistry, Vol. 90, No. 13, 1986 2827 TABLE I: Band Positions for the 1:l Complexes of Cyclopropanes with C12, CIF, and Br2
species C-CqHACl, - - -
428'
c-C~H&IF
733, 739 762, 768
Vacid
c-C3H,-Br2
vacid"
545d
313d
MCP.Cl2
434
545d
MCP-CIF
729, 735 762, 768
MCP-Br2
285c
313d
l,l-DMCP.C12
394
545d
1,l-DMCP.ClF 693, 699 762, 768 1,1-DMCP.Br2 26OC 313d BCPCIF
684, 690 762, 168
vbaoc
787, 845, 1037, 1179, 1456 845, 1038, 1180 773, 849, 1036, 1180, 1456 842, 932, 1012 841, 1029, 1051 841, 931, 971, 1011 826, 928, 1009 833, 1006 826, 927, 1009 544
:sa%
739.d 863, 1023, 1188,d 1432 863, 1023, 1188d 739,d 863, 1023, 1180: 1432 852, 927, 1016 852, 1016, 1041 852, 927, 981, 1016 838, 935, 1016 838, 1016 838, 935, 1016 550
"Position of the free or uncomplexed parent acid or base. bBand positions in cm-'. CTentativeassignment, see text. dInfrared inactive (in parent). be seen in Figure 2. In addition, weaker but distinct absorptions were noted at, 833 and 1006 cm-', near parent modes of 1,lDMCP at 838 and 1016 cm-I. In addition, C12 was codeposited with 1,l-DMCP in a twin-jet experiment; product absorptions were noted a t 394, 826, 928, and 1009 cm-I, the latter three being analogous to product bands described above. Finally, 1,l -DMCP was codeposited with C12 in a single-jet experiment, at a total dilution of 500/1/1 in argon. The spectrum of the resulting mixture bore little resemblance to either reagent; rather, a gasphase reaction apparently occurred and the product has been tentatively identifiedI5 as l-chloro-3-methyl-2-butene(prenyl chloride); in addition, moderately intense bands near 2880 cm-' were observed which could be assigned to HC1. These indicate that ring opening occurred followed by HC1 elimination, yielding prenyl chloride. Bromocyclopropane (BCP) CIF. The codeposition of C1F with BCP in a twin-jet experiment was conducted at dilutions of 500/ 1 each in argon; the resulting spectrum showed an intense 3:l doublet at 684 and 690 cm-I, as well as a strong band at 544 cm-I, just to the low-energy side of the intense parent mode of BCP at 500 cm-I. No other product bands were detected anywhere in the spectrum. All of the band positions cited above are tabulated in Table I.
+
Discussion The growth of moderate intensity, distinct infrared absorptions upon the twin-jet codeposition of Br,, CI2, and ClF with cyclopropane and several of its derivatives indicates the formation of one or more products. A typical system might be the c-C3H6/C12 system, where product bands were noted at 428, 787, 845, 1037, 1179, and 1456 cm-I. In this system, as in all of the reaction pairs reported here, all of the product bands grew in at the same rate as the concentration of one or both of the reagents was increased (except, possibly, for the Br2-1 ,1-DMCP system). This indicates that only a single reaction product was formed, as might have been anticipated, given the high dilutions at which these experiments were conducted. Certainly, many previous studies have shown that only a single reactive collision is likely to occur during the deposition process when high dilutions are employed. The nature of the reaction product can be inferred from the observation that each of the product bands observed fell near a fundamental vibration of either the halogen or the cyclopropane. For example, the product doublet at 733 and 739 cm-l in the C-C3&/ClF experiments lies slightly to the red of the parent ClF fundamental at 762 and 768 cm-I, while the 845-cm-I product absorption lies 18 cm-' below the cyclopropane fundamental v l Some of the
2828
The Journal of Physical Chemistry, Vol. 90, No. 13, 1986
weaker product absorptions were observed near infrared inactive fundamentals of either reagent, indicating activation of these vibrational modes. All of these observations point toward a reaction product in which both the halogen and the cyclopropane remain structurally intact but are perturbed by the presence of the reaction partner. This indicates the formation of a molecular complex, similar in nature to the previously reported complexes. The high dilutions employed also strongly point to a complex with 1:l stoichiometry, i.e. a 1:l acid/base complex with the halogen serving as a Lewis acid, and the cyclopropane as a Lewis base. The identification of complexes of cyclopropane and its derivatives with Lewis acids, and in particular the molecular halogens, marks the first time that these compounds have been observed, other than the tentative observation of the CI2-c-C3H6complex by Aleksanyan and Gevorkyan.I4 Additional product bands in the Br2/l ,I-DMCP experiments, at 918 and 1003 cm-', appeared to be favored at higher Br2 concentrations, and since Br, is known to dimerize readilyz6 may be assigned to a 2:l complex involving two Br, molecules and one cyclopropane. The difficulty in preparing Ar/Br2 samples of precisely known concentration due to Br2 adsorption may also have contributed to formation of these larger complexes.26 Nonetheless, the major product in each of the systems studied can be, as noted above, assigned to the 1:1 complex of the molecular halogens with cyclopropane and its derivatives. Band Assignments The product absorptions for each system can readily be divided into those vibrations lying near fundamentals of the cyclopropane and the single absorption lying near the one fundamental of the halogens. The latter band is readily assigned to the perturbed halogen in the complex, and in each case a shift to lower energy is observed, consistent with the transfer of electron density from the cyclopropane to the halogen. This absorption was most distinctive in the C1F complexes, where the parent mode itself is infrared active, and moderately intense. The perturbed CI-F stretches in the reaction products were all quite intense and appeared as distinct 3:l doublets, due to the 35C1-37CIsplitting from a single chlorine atom in natural abundance. This splitting, roughly 6 cm-I for each system, is identical with the splitting for parent CIF and further supports the above conclusion that the halogen remains intact during product formation. The C1, complexes with cyclopropane, MCP, and 1,l-DMCP each had a weak infrared absorption near 430 cm-l, with a slight indication of structure on the low-energy side. This is best assigned to the CI-C1 stretching mode, activated by complex formation. The unresolved structure on the low-energy side is readily attributed to the presence of chlorine isotopes in the complex. Whether these isotopes are equivalent or inequivalent cannot be determined from the spectra, but structural considerations (see below) suggest that they are likely to be inequivalent. The low intensity of this product band relative to other product bands observed for the complex is consistent with the fact that this mode is forbidden for parent C12 and is likely to be only weakly activated in the complex. Recent ~ t u d i e s have ~ ~ - reported ~~ the observation of this mode for other C1, complexes, although with smaller shifts from parent CI,. This discrepancy in magnitude of shift is a point of some concern and, indeed, could cast some doubt on the assignment of these absorptions. However, the C1-CI stretching mode should be activated in the 1:l complex, and no other absorptions were noted in this region (see Figure 2). While weak, these bands were certainly reproducible. Moreover, the indication of unresolved chlorine isotopic structure is further indication of assignment to this mode. Recent studiesiabin this laboratory of the complexs of C12 and CIF with alkyl sulfides have shown that (25) Melendez, E . ; Pardo, C. An. Quim. 1972, 68, 1283. (26) Ault, B. S.:Howard, W. F.; Andrews, L. J . Mol. Spectrosc. 1975, 55, 217. (27) Agarwal, U. P.: Barnes, A. J.: Orville-Thomas, W. J. Can. J . Chem. 1985, 63, 1705. ( 2 8 ) Holroyd, S . ; Barnes, A. J.; Suzuki, S.; Orville-Thomas, W. J. J . Roman Spectrosc. 1982, 12, 162. (29) Nelander, B., private communication.
Ault TABLE II: Comparison of Band Shifts (cm-I) for the Halogen and Hydrogen Halide Complexes of Cyclopropane
sDecies Auoa C-C3H6*HCI 29 c-C,H6.HBr c-C~H~SHF c - C ~ H ~ C I ~ 24 c-CjH&lF 24 c-C3H6.Br2 MCPeHCI MCPeHBr MCP-HF MCP-CI, MCP-CIF MCP.Br2
Au,~ 15 14 21 14 15 13
A u 1~
ref
-10
5 5 5 b b b
-10
6 -18 -18 -14
-9" -10 -1
-10 -1 1 -1 I
23 23 23 b b b
OAv = uFOmplCX- uprent. bThis work. cFor the MCP complex, Auk, represents the shift of the antisymmetric ring deformation mode at 852 cm-'.
such large shifts, and larger, can occur. For example, codeposition of Clzand (CH3)2Sinto argon matrices gave rise to a moderately intense absorption near 360 cm-I, which has been assigned to the CI-C1 stretching mode, while the complex of CIF with (CH3)2S has vClF at 470 cm-l. Although such large shifts are unusual, one must recognize that the nature of the interaction is significantly dependent on the similarities in orbital energies of the HOMO of the base and the LUMO of the acid. If the match is quite close, such large shifts might well occur. Finally, the Br, experiments with MCP and 1,l-DMCP gave rise to weak absorptions below 300 cm-', at 285 and 260 cm-'. respectively. The fundamental vibration of parent Br, falls at 313 cm-I and is infrared inactive. The location and low intensity of these product absorption suggest assignment to the perturbed Br-Br stretching mode of the coordinated Br2 subunit in the complex, but the overall low intensity of these bands requires that these assignments be somewhat tentative. The remaining product absorptions in each system lie near vibrational modes (allowed or forbidden) of the parent cyclopropane. For example, in the c-C3H6experiments, a product band was observed at 845, 845, and 849 cm-' for the complexes with CI,, ClF, and Br2, respectively. These lie just to the low-energy side of the antisymmetric ring deformation mode of parent c-C3H6 at 863 cm-' and are readily assigned to the counterpart mode in the complex. The remaining product bands in the c-C3H6experiments, near 787, 1037, 1180, and 1450 cm-], are assigned as the perturbed counterparts of the parent modes (the 1180-cm-' mode being the infrared inactive ring breathing mode). Similar assignments can be made for the complexes of the halogens with the substituted cyclopropanes, in that each product band fell within 10-20 cm-' of one of the parent modes of the cyclopropane. For example, all three halogens gave rise to a product band near 840 cm-' in a complex with MCP; the corresponding parent mode occurs at 852 cm-' and has been shown to be particularly sensitive to complex formation.23 The product band positions for the complexes of cyclopropane and the methyl-substituted cyclopropanes with CI2,CIF, and Br2 are presented in Table I. Finally, for the complex of bromocyclopropane with CIF, only one vibrational mode of BCP was perturbed. This was the intense C-Br stretching mode, which occurs at 550 cm-I for parent BCP and shifted to 544 cm-' in the 1:l complex. None of the ring deformation modes of BCP were affected by complex formation, an observation which agrees with the previous studies of hydrogen halide complexes of BCP.30 Structure of the Complexes Several possible geometric arrangements of the acid and base subunits in these complexes might be envisioned; the infrared spectra here given strong indication of the structure but cannot provide a precise geometry. In all of these systems, the best model is that of the hydrogen halide-cyclopropane complexes, which are (30) Truscott, C. E.: Ault, B. S. J . Phys. Chem. 1986, 90, 2566.
Complexes of Molecular Halogens with Cyclopropane
The Journal of Physical Chemistry, Vol. 90, No. 13, 1986 2829
structurally well characterized in inert mat rice^,^*^*^^ nnd in molecular Unfortunately, one of the key points supporting the C,, structure for these H X complexes, the removal of the removal of the degeneracy of the H X librational mode (a point not brought out in ref 5, but discussed in ref 23) is not applicable here. Nonetheless, it is significant that the vibrational modes of the cyclopropane which were perturbed by coordination to the hydrogen halides were the same modes perturbed in the halogen complexes. Moreover, the direction of perturbation was the same in each case, and the magnitudes, while not identical, were comparable. For example, the complex of HCl with c-C3H6gave rise5 to perturbed base modes at 740, 853, 1038, 1185, and 1461 cm-I, which compare quite well with the product bands associated with the C12.c-C3H6complex, at 787, 845, 1037, 1179, and 1456 cm-I. Similar agreement can be noted for the CIF and Br2 complex of c-C3H6. This suggests that the interaction of the molecular halogens with cyclopropane is of the same fundamental type as for the hydrogen halides, namely coordination to the midpoint of one of the C-C bonds of the ring, and lying in the plane of the three-membered ring. Since the halogens are u* acceptors,31and have been shown to coordinate in an end-on fashion, the most likely geometry is one in which the molecular axis of the halogen is perpendicular to one of the C-C bonds. Similar considerations hold for the complexes of the methylsubstituted cyclopropanes with C12, ClF, and Brz. Again, the analogy to the hydrogen halide complexes of these bases holds quite well, with the same base modes perturbed, and by approximately the same amount. In the case of the H X complexes, it was argued23that the coordination must be at the C-C bond adjacent to the site of substitution, in that symmetry considerations require that the B, ring deformation mode be unaffected by coordination opposite to the site of substitution, while both the A I and B2 modes may be shifted if the coordination is adjacent. Since the spectra showed clearly in the case of the H X complexes, and show clearly in the present systems, that both modes are perturbed, coordination adjacent to the site of methyl substitution is the preferred geometry. Finally, the fact that in the CIF-BCP complex only the C-Br mode was shifted and the ring modes were not affected argues strongly for coordination at the bromine substituent and not on the ring itself. This conclusion is in agreement with studies of the hydrogen halide complexes of BCP and related compounds.30 In view of the infrared spectral evidence, andd comparison to previous studies, the structural conclusions reached above are on sound footing. However, a molecular beam microwave spectrum can provide a more thorough structural determination, and it is hoped that the present work will generate interest in this type of experiment.
dipole mechanism, while C1F has a permanent dipole3znear 0.9D, and one would anticipate a substantial dipole-induced dipole contribution. As can be seen in Table 11, this is not the case, particularly for such vibrations as vl0 and u I 1 of cyclopropane, where very similar shifts were noted. Trends in the Lewis basicities of the cyclopropane derivatives can be determined in a similar fashion, by their relative effects on the halogen vibrations. Here, the ClF stretching mode is most notable, due to its high intensity and sensitivity to complexation. The shift in the C1F-MCP complex was quite similar to that in the C1F.c-C3H6complex, a result which is in agreement with the €IC1 and HBr complexes of these bases. However, the shift of the ClF stretching mode in the 1,l-DMCP complex was much larger, roughly double, which suggests a sustantially stronger interaction. This is in agreement with the ability of a tertiary carbon to preferentially stabilize a carbocation, but in disagreement with the HCI and HBr complexes.23 There, shifts of uHCl and uHBr were similar for the 1,l-DMCP complexes as for the c-C3H6and MCP complexes. This discrepancy may lie in a fundamental difference in the nature of the interaction, or with a breakdown in the ability of frequency shifts to measure the strength of interaction. The former option seems more likely, but certainly the latter cannot be ignored. An independent measure of the strength of interaction may shed more light on these possibilities. Finally, the largest shift in the ClF stretching mode was in the complex with BCP, where the interaction was to the bromine atom, rather than to the ring. This agrees well with the hydrogen halide results, where substantially greater shifts were observed for BCP and related compounds, than for the methyl-substituted cyclopropane~.~~ It is of interest to note the high intensity of the Cl-F stretching mode of the CIF subunit in these molecular complexes. In several cases, as can be seen in Figure 2, the intensity of the ClF complexes was equal to or greater than that of the unreacted parent. This suggests that either a high percentage of the CIF reacts during deposition, or that an intensification of the C1-F mode occurs in the complex. Since the intensity of parent C1F in the twin-jet deposition experiments was only slightly less than that of CIF in a blank experiment, the former alternative is not likely. Rather, coordination of ClF to a base appears to increase the absorption coefficient for the C1-F stretch. Similar behavior has been well documented for Bronsted acid complexes (hydrogen bonds)33but has not been noted for complexes of diatomic Lewis acids. Of course, the stretching mode in C1, is activated in these complexes, and hence there is necessarily some increase in absorption coefficient, but the overall intensity of this mode remains quite low compared to the C1F complexes.
Acid/Base Strengths Several comparisons can be made within the current set of observations, as well as by comparing the current data to those obtained in previous studies. All of these use the magnitude of shift of a particular fundamental vibration as an indicator either of the strength of interaction or, if not that, the degree of perturbation and electron rearrangement in the product species. One comparison of note is that of the Bronsted acids (the hydrogen halides) to the Lewis acids (the halogens) and their effect on the vibrational modes of cyclopropane. In nearly every case, the shift obtained in the halogen complex is equal to, or greater than, the shift in the corresponding hydrogen halide complex. For example, the shift of ul0 near 1030 cm-I was just about the same for the Bronsted and Lewis acids, while the shift of u l l near 860 cm-I was somewhat larger for the halogens. To the degree to which these shifts correlate with strength of interaction, one can suggest that these halogen complexes are bound by at least 2-3 kcal/mol and perhaps more.1,8 Within the three halogens employed here, very little differences were noted in their effect on the vibrations of either cyclopropane or its derivatives. C1, and Br,, of course, are nonpolar and must interact through a induced dipole-induced
Conclusions The present study has led to the formation and positive indentification for the first time of 1:l complexes of cyclopropane and its derivatives with the molecular halogens Cl,, CIF, and Br2. Distinct shifts were observed iun the vibrational modes of both the acid and base subunits in the complex, and of a magnitude that was comparable to those observed in the hydrogen halide complexes. The spectra also indicated similar geometries, namely interaction with one of the carbon-carbon bonds of the ring, except for the ClF complex with BCP. Surprisingly, the perturbation of the cyclopropane subunit was not greater for the ClF complexes than for the Cl, and Br2 complexes, despite the substantial dipole moment of C1F. The role of these complexes in the chemistry of cyclopropane and its acid-catalyzed reaction continues to be under investigation in this laboratory.
(31). Jenson, W. G. The Lewis Acid-Base Concepts, an Overview; WileyInterscience: New York, 1980.
Acknowledgment. The author gratefully acknowledges support of this research by the National Science Foundation under Grant C H E 84-00450, and the award of a Dreyfus Teacher-Scholar grant by the Dreyfus Foundation. (32) Gilbert, D. A,; Roberts, A,; Griswold, P. A. Phys. Rev. 1949, 76, 1723. (33) Pimentel, G. C.; McClellan, A. L. The Hydrogen Bond Freeman: San Francisco, 1960.
