J. Phys. Chem. 1985,89, 1741-1748 has previously been invoked to explain Occurrence of an unusual Stokes shift in HT phase TC crystals that vanished in course of the phase t r a n s i t i ~ n . ~While ~ , ~ ~subsequent fluorescence studies of Kalinowski et a1.8 argue against the original suggestion that the LT structure is locally established in the vicinity of an excited TC molecule existing in a HT structure, they are compatible with site relaxation of an excited TC molecule. Further support for this concept comes from the energetics of singlet exciton fission into a pair of triplet states in the HT phase of TC.9J0 Since the energy of two triplet excitons is 20 200 cm-' and the average value of the activation energy required to reach the 2T1 state from the SIstate is about 1500 cm-l?O the latter state must be located near (23) N. J. Kruse and G. J. Small, J . Chem. Phys., 56, 2985 (1972). (24) R. W. Olson, H. W. H. Lee, F. G.Patterson, M. D. Fayer, R. M. Shelby, D. P. Burum, and R. M. Macfrlane, J . Chem. Phys., 77,2283 (1982). (25) B. E. Kohler and J. K. B. Snow, J . Chem. Phys., 79, 2134 (1983). (26) H. B. Levinsky and D. A. Wienma, J. Chem. Phys., 79,2677 (1983). (27) R. Jankowiak and H. Bhssler, Chem. Phys., 89, 81 (1984). (28) H. MLlller and H. Bhssler, Chem. Phys., 36, 312 (1975). (29) H. Muller and H. BBssler, J. Lumin., 12/13, 259 (1976).
1741
18 700 crn-', approximately 500 cm-I less than the position of the low-energy Davydov component in absorption. On the other hand, the diffusion coefficient reported for singlet exciton motion within the (001) lattice plane of the HT phase of T C is larger than expected if the exciton were localized by virtue of a lattice dist ~ r t i o n . Clearly, ~~ more spectroscopic work is required to ascertain whether spontaneous site reorientation occurs in the HT phase of TC.
Acknowledgment. This work was stimulated by discussions with Professor J. M. Thomas and Drs. R. Eiermann, W. Hofberger, R. Jankowiak, and G. M. Parkinson. Sample preparation by Dipl. Phys. J. Lange is gratefully acknowledged as is financial support by the Fonds der Chemischen Industrie. Registry No. Tetracene, 92-24-0. (30) Y. Tomkiewicz, R. P. Groff, and P. Avakian, J . Chem. Phys., 54, 4504 (1971). (31) A. J. Campillo, R. Hyer, S. L. Shapino, and C. E. Swenberg, Chem. Phys. Lett., 48, 495 (1977).
Infrared Matrix Isolation Study of the 1:l Molecular Complexes of the Hydrogen Halides with Methyl-Substituted Cyclopropanes Candace E. Truscott and Bruce S. Ault* Department of Chemistry, University of Cincinnati, Cincinnati, Ohio 45221 (Received: November 26, 1984)
The matrix isolation technique has been employed to isolate and characterize the hydrogen-bonded complexes formed between the hydrogen halides and a variety of methyl-substituted cyclopropanes. In each case, the complex was characterized by a shifting to lower energy of the hydrogen stretching frequency of the hydrogen halides; shifts slightly larger than those observed for the cyclopropanehydrogen halide complexes were noted. In addition, a number of perturbed vibrational modes of the substituted cyclopropane were detected, and, as anticipated, primarily vibrations of the ring skeleton were affected. These perturbed modes, along with symmetry considerations, were used to deduce a likely structure for several of the complexes; for methylcyclopropane and 1,l-dimethylcyclopropane,hydrogen bonding to the carbon-carbon bond adjacent to the site of substitution was indicated. For several systems, a 1:2 complex was detected as well, but no definite structural information could be obtained.
Introduction There has been considerable interest in the past two decades in the role that the hydrogen halides play in the catalytic isomerization of cyclopropane to Recently, both matrix i ~ o l a t i o n and ~ , ~ gas-phase supersonic nozzle techniques6,' were applied to investigate the initial 1:l complex between the hydrogen halides and cyclopropane, which might be the initial species formed in the catalytic isomerization. These studies all obtained evidence for a hydrogen-bonded complex, in which the hydrogen halide lies in the plane of the three-membered cyclopropane ring and is hydrogen bonded through the acid hydrogen to the midpoint of one of the carbon-carbon bonds. Similar and additional questions might be raised concerning the interaction of the hydrogen halides with methyl-substituted cyclopropanes. A number of these are catalytically isomerized by the hydrogen halides, while addition is known to occur in a few c a ~ e s . 8 9 ~In addition, if a hydrogen-bonded complex is formed, (1) Ross, R. A.; Stimson, V. R. J . Chem. SOC.1962, 1602. (2) Maccoll, A.; Ross, R. A. J. Am. Chem. SOC.1965, 87, 4997. (3) Lewis, D. K.; Bosch, H. N.; Hossenlopp, J. M. J . Phys. Chem. 1982, 86, 803. (4) Truscott, C. E.; Ault, B. S. J . Phys. Chem. 1984, 88, 2323. ( 5 ) Barnes, A. J.; Paulson, S.L. Chem. Phys. Lett. 1983, 99, 326. (6) Legon,A. C.; Aldrich, P. D.; Flygare, W. H. J . Am. Chem. Soc. 1982, 104, 1486. (7) Legon, A. C. J . Phys. Chem. 1983,87, 2064. (8) Skell, P. S.; Day, J. C.; Shea, J. S. J . Am. Chem. SOC.1976, 98, 1195. (9) Bullivant, J.; Shapiro, J. S.; Swinbourne, E. S. J. Am. Chem. Soc. 1969, 91. 7703.
there is more than one possible site of coordination, as in general the carbon-carbon bonds are no longer equivalent. It would be of interest to isolate complexes of the hydrogen halides with a number of substituted cyclopropanes, not only to obtain the infrared spectrum of the complex, but also to determine the site of coordination. Several solution phase studies have been conducted employing the substituted cyclopropanes and a mixture of HBr and Br2, but no consistent pattern has emerged for the ring opening reaction.1°'12 Matrix isolation has proven an invaluable technique for the characterization of many acid/base systems, for both Bronsted and Lewis bases."18 In addition, many of the complexes which might be formed here are not accessible through gas-phase supersonic nozzle techniques, in that rapid reaction occurs in the gas phase at room temperature. By comparison, twin-jet deposition in conjunction with matrix isolation eliminates this obstacle and makes the investigation of these complexes feasible. With this in mind, a study was undertaken to characterize the reaction products of the hydrogen halides with methylcyclopropane (MCP), (10) Lambert, J. B.; Iwanetz, B. A. J . Org. Chem. 1972, 37, 4083. (11) Rossi, R. A. J . Phys. Chem. 1979, 83, 2554. (12) Renk, E.; Shafer, P. R.; Graham, W. H.; Mazur, R. H.; Roberts, J. D. J. Am. Chem. SOC.1961,83, 1987. (1 3) Andrews, L. J. Mol. Struct. 1983, 100, 28 1. (14) Auk, B. S.; Pimentel, G. C. J . Phys. Chem. 1973, 77, 1649. (15) Barnes, A. J. J . Mol. Struct. 1983, 100, 259. (16) Ault, B. S. Inorg. Chem. 1981, 20, 2817. (17) Auk, B. S. J. Am. Chem. SOC.1983, 105, 5742. (18) Sass, C. S.; Auk, B. S. J . Phys. Chem. 1984,88, 432.
0022-3654/85/2089,1741$01.50/0 0 1985 American Chemical Society
1742 The Journal of Physical Chemistry, Vol. 89, No. 9, 1985
Truscott and Auk
1,l -dimethylcyclopropane (1,l-DMCP), trans- 1,2-dimethylcyclopropane (trans-1,2-DMCP), cis- 1,2-dimethylcyclopropane (cis-l,2-DMCP), and 1,1,2,2-tetramethylcyclopropane(TMCP).
Experimental Section The experiments in this study were carried out with a conventional matrix isolation apparatus which has been described previo~sly.'~All of the reagents used in this study, HCI, HBr, and HF (all Matheson), methylcyclopropane (Columbia), 1,ldimethylcyclopropane (Pfaltz and Bauer), trans- 1,2-dimethylcyclopropane, cis-1,2-dimethylcyclopropane,and 1,1,2,2-tetramethylcyclopropane (API), were subjected to one or more freeze-thaw cycles at 77 K prior to sample preparation to remove any impurities. Argon and nitrogen were used without further purification as the matrix gases in this study. Samples were deposited by single- or twin-jet deposition for 20 to 24 h at a flow rate of roughly 2 mmol h-I. In single-jet deposition experiments, the reactants were premixed in the vacuum line prior to deposition, while in twin-jet deposition experiments the reactants were d e p o s i t e d from separate vacuum lines so that mixing occurred in front of the cold window which was maintained at 15 K. Both high-resolution and survey scans were recorded on either a Beckman IR-12 or a PE 983 infrared spectrophotometer, each with a resolution of 1 cm-'. In most experiments, the matrix was then annealed or warmed to between 35 and 40 K to allow limited diffusion of the trapped species and then recooled to 15 K, and additional spectra were recorded. Normal coordinate calculations were carried out at the University of Cincinnati computing center with a program from the National Research Council of Canada. Results Before investigating the reaction products of the hydrogen halides with methyl-substituted cyclopropanes, blank spectra were run of each reactant alone in either argon or nitrogen matrices. In each case, good agreement was found with literature spectra.20-26 HCl + Methyicyclopropane (MCP). These two reactants were studied in five twin-jet deposition experiments into argon matrices, over a wide range of concentrations (from 1000/1 to 100/1 for each reactant). In a typical, dilute experiment, a product triplet was observed somewhat to lower energies of the HCI monomer absorption, at 2789,2766, and 2750 an-'. In addition, two distinct product absorptions were noted in lOOO-cm-' region, near absorptions of parent MPC. These bands were located at 1031 and 844 cm-', along with shoulders at 1091, 826, and 766 cm-'. When this sample was annealed, all of these product absorptions were noted to grow, and the triplet near 2770 an-'showed some shifting of intensity. In subsequent experiments, the concentration of each reactant was varied in a systematic fashion; the above product bands were observed to grow when the concentration of either or both reactants were increased. However, the three components of the 2770-cm-' triplet varied somewhat from experiment to experiment, with no clear trend. Finally, in one experiment a matrix containing HCI and MCP was irradiated with the full light of a 200-W Hg arc, but no changes in the product bands or increase in yield of product could be detected. This system was also studied in a single-jet experiment, in which the two reactants were mixed in a single manifold prior to sample deposition. In this experiment, a number of intense new absorptions were detected which did not correspond to either the absorptions of parent MCP and HCI, or to absorptions of the reaction product detected in ~
II
NJHCI / C . H ~ = 1 0 0 0 / 5 / 1
I
2d60
2f80
11
N2/HCI /C4H8=1000/5/1 ANNEALED
27'00
si0
aao
&40
'
-
860
ENERGY (CM-')
Figure 1. Infrared spectra of the reaction products arising from the codeposition of samples of NJHCI with N,/methylcyclopropane, compared to blank spectra in selected spectral regions. The bottom trace shows the effect of annealing on the spectrum presented in the middle trace.
the twin-jet experiments. Comparison to literature spectra demonstrated that these absorptions were due to the product species 2-chlorob~tane.~~ The HCl/MCP reaction system was also investigated in nitrogen matrices, at three different concentrations, by twin-jet deposition. When relatively dilute samples were employed (500/1), an intense product band was observed at 2749 ern-', along with weaker bands at 1091 and 1028 cm-I. When this sample was annealed to 37 K and recooled to 15 K, additional product bands were observed a t 2730, 937, 827, and 795 cm-I, and an intensification of the original three product bands was noted. When the N,/HCI ratio was increased to 100/1, additional new product absorptions were noted at 1057, 846, and 767 cm-I, in addition to all of the product bands observed above, with the exception of the 2730-cm-' band, which was only detected after matrices containing HCI and MCP were annealed. Figure 1 shows representative spectra obtained after the twin-jet deposition of HCI and MCP. HCl 1 ,I -Dimethylcyclopropane ( 1 ,I-DMCP). The reaction of HCI with 1,l-DMCP was investigated in two twin-jet argon matrix experiments. In the more dilute experiment, moderately intense product bands were observed at 2762 and 2742 cm-' in the HCI stretching region, along with quite intense product absorptions at 1012 and 931 cm-' and weak absorptions 2698, 1319, 1241, 1151, 1033, 919, and 828 cm-'. After this sample was annealed and recooled, a sharp new band was observed at 2775 cm-I, while the absorption at 2762 cm-' was no longer present. The product absorption at 2742 cm-' grew in intensity, as did all of the product absorptions in the lower energy, DMCP region. When the concentration of HC1 was increased to Ar/HC1 = 200, the same set of product bands was observed in the DMCP
+
:
~~~~~~~
(19) Ault, B. S. J . Am. Chem. SOC.1978, 100, 2426. (20) Conden, F. E.; Smith, E. J . Am. Chem. Soc. 1947, 69, 965. (21) Bush, S.F.; Dixon, P. W. Spectrochim. Acta, Parr A 1978,34A, 515. (22) Durig, J. R.; Neuse, A. B.; Milani-Nejad, F. J. J. Mol. Strucr. 1981, 72, 57. (23) Aleksanyan, V. T.; Aliev, M. P.;Lukena, M. Yu.; Nesmeyonova, 0. A.; Khotinakaya, G. A. Akad. Nauk. SSSR Chem. Ser. 1968, 176. (24) Giradet, C.; Maillard, D.; Shriver, A.; Perchard, J. P. J . Chem. Phys. 1979. 70. -, -IS - I- 1(25) Barnes, A. J.; Hallam, H. E.; Scrimshaw, G. F. Trans. Faraday S O ~ . 1969, 65, 3172. (26) Andrews, L.; Johnson, G. L. Chem. Phys. Leu. 1983, 96, 133.
(27) Shipman, J. J.; Folt, V. L.; Krimm, S. Spectrochim. Acta, Part A 1968, 24A, 437.
1:l Complexes of H X with Methyl-Substituted Cyclopropanes
The Journal of Physical Chemistry, Vol. 89, No. 9, 1985 1743
Arl1,lDMCP = 9 0
A r l IJDMC P =500
Ar/HCI=200+Ar/l,lDMCP=500
ANNEALED
990 930 870 810 ENERGY (CM-1) Figure 2. Infrared spectra of the reaction products arising from the codeposition of HC1 and 1,l-DMCP into argon matrices. The top two traces show blank spectra of each reagent, while the third trace shows a typical reaction run, which is contrasted in the bottom trace to the spectrum obtained after 2720
2600"
annealing. region, while in the upper HCl region, some changes were noted. A single broad band was detected a t 2762 cm-I, with a weak counterpart at 2698 cm-I. After the sample was annealed, however, product bands were noted at 2750 and 2774 cm-', in place of the band at 2762 cm-I. In addition, the band at 2698 cm-' was no longer present, but a new, more intense band was observed at 2714 cm-I. Finally, it should be noted that in the lower energy, DMCP region, the product absorption a t 919 cm-I did not maintain a constant intensity ratio to the remaining product bands upon annealing. In contrast to the above HCl/MCP results, when HCl and 1,I-DMCP were premixed and deposited in a single-jet experiment, the spectra obtained were nearly identical with those resulting from twin-jet deposition, with the same product and parent absorptions. HCI trans-l,2-Dimethylcyclopropane (trans-l,2-DMCP). When the HCl and trans- 1,2-DMCP were codeposited from separate vacuum lines into an argon matrix, new absorptions were noted which could not be attributed to either of the parent species. When the concentration of each reactant was 500/1 in argon, a moderately intense band was noted at 2773 cm-', as well as weak absorptions a t 1089, 1026, 877, and 859 cm-I, as well as a broadening or shoulder on the low-energy side of the 763-cm-l parent absorption. When this sample was annealed, the 2773-cm-' product band disappeared, and a considerable intensification of the parent trans-12-DMCP parent band at 2750 cm-l was noted. However, when a blank of trans-1,2-DMCP was annealed in argon, a comparable intensification was not observed, suggesting the presence of a second absorption near 2750 cm-' in the HCl/trans-1,2-DMCP experiment. It was also noted that the product absorptions in the low-energy region increased in intensity upon annealing. This system was also studied in an experiment in which the concentration of trans- 1 ,2-DMCP was held constant, and the concentration of HC1 was increased. In this experiment, a considerable growth of the band at 2750 cm-l was noted, again
+
suggestive of a second, underlying absorption. In the lower-energy region, the same set of product bands was observed, with increased intensity but a constant intensity ratio. Similar results were obtained when these two reactants were codeposited from separate vacuum lines into a nitrogen matrix. An intense doublet was observed at 2737 and 2727 cm-I, which coalesced into a single intense band at 2735 cm-' upon annealing. A weaker counterpart was observed at 2680 cm-', which did increase slightly upon annealing. In addition, product bands were noted at 1090, 1031,976,876, and 758 cm-' as well as a shoulder at 859 cm-I, in the trans-1,2-DMCP spectral region. In general, these product bands were sharper and more distinct in the nitrogen matrix experiments than in argon and are depicted in Figure 2. HCI cis-l,2-Dimethylcyclopropane(cis-l,2 DMCP). The twin-jet codeposition of these two reactants into a nitrogen matrix at dilutions of 500/1 each gave rise to a moderately intense, broad feature at 2753 cm-', as well as a sharp, moderately intense band at 1066 cm-', and a shoulder at 1021 cm-I. When the sample was annealed, the 2753-cm-' band sharpened and shifted, slightly, to 2748 cm-', which was within the bandwidth of the 2753-cm-' band contour. HCI + 1,I ,2,2-Tetramethylcyclopropane (TMCP). These two reactants were studied in a single experiment using nitrogen as the matrix gas. A very intense product band was observed at 2748 cm-I, with an intensity nearly equal to that of the HCl monomer parent band, while a weaker counterpart was noted at 2678 cm-'. Two intense product bands were observed at 935 and 849 cm-', shifted to lower energy from T M C P fundamentals, as well as a shoulder at 1022 cm-'. HBr + Methylcyclopropane. The codeposition of samples of N2/HBr = 500 with N2/MCP = 500 gave rise to an intense new absorption at 2459 cm-I, as well as weak absorptions in the MCP region at 1033 and 833 cm-I. After the sample was annealed, the intensity of these bands increased significantly, as can be seen in Figure 3, while new distinct absorptions were detected at 2421, 979,935,845, and 793 cm-'. In subsequent experiments at higher
+
1744 The Journal of Physical Chemistry, Vol. 89,No.9, 1985
I
Truscott and Ault
I l l
I
1, 2840
y,
N$HCI= 200
2720
+
I*
N2/TMCP = 500 I
I
26'60 1020
960
I
I
900
84C
ENERCY (CM -1) Figure 3. Infrared spectrum of the reactant products in the twin jet deposition of HCI and TMCP into N, matrices; in the bottom trace, compared to blank spectra of the two reagents, in the upper two traces.
concentration, all of these product bands were observed to grow, and at the same rate, except for the product band at 2421 cm-'. This band was quite weak at low HBr levels and grew more rapidly than the remaining product bands as the level of HBr was increased. This system was also characterized in argon matrices, although a generally lower yield of product was noted in argon relative to nitrogen. In the upper, HBr region, no clear-cut new product band was observed. However, a broadening of the HBr dimer band at 2467 cm-I was noted, an effect which was also seen upon annealing the matrix. The behavior of this band was compared closely to that observed in blank experiments of Ar/HBr, before and after annealing, and close inspection suggests that an additional product absorption must lie within the band contour of the HBr dimer band. In the M C P region near 1000 cm-I, weak absorptions were noted at 1103, 1050, 1028, 1022, and 834 cm-'. HBr + 1 ,I-Dimethylcyclopropane.The reaction between HBr and 1,l-DMCP was studied by twin-jet deposition using nitrogen as the matrix gas, and dilutions of 500/1 for each reactant. In the H-Br stretching region, an intense product band was observed at 2464 cm-I, with a weaker counterpart at 2453 cm-'. However, when this sample was annealed, this 2464-cm-' band split into a doublet, with components at 2470 and 2463 cm-'. In the lower-energy region, a number of product bands were detected, shifted slightly from fundamental vibrations of parent 1,l-DMCP. These were located at 1320, 1022, 1013, 969, 926, 892, 883, and 831 cm-I and, while of relatively low intensity, were quite sharp. HBr + trans-l,2-Dimethylcyclopropane. These two reactants were investigated in two twin-jet experiments, both employing nitrogen matrices. In each experiment, the dominant new spectral feature was a band at 2445 cm-I, with a shoulder at 2438 cm-I, while additional, weaker product bands were observed in the lower-energy region, at 1021, 969, 880, and 758 cm-'. When this
sample was annealed, the band at 2445 cm-' decreased in intensity, while the low-energy counterpart at 2438 cm-* grew. In addition, a new, relatively weak band was observed at 2406 cm-', while the spectral region associated with trans-1,2-DMCP vibrations broadened sufficiently that it was difficult to determine the effect of annealing on the product bands. HBr I ,I,2,2-Tetramethylcyclopropane.HBr was codeposited with TMCP in two twin-jet experiments into nitrogen matrices. In both experiments, the most intense new spectral feature was an absorption at 2450 cm-', which was much more intense than the absorption of unreacted monomeric HBr, as can be seen in Figure 4. In the region associated with vibrations of parent TMCP, new absorptions were noted near parent bands of TMCP, at 1112, 969, 934, and 848 cm-', and were both sharp and of moderate intensity. After the sample was annealed, all of these features were seen to grow and maintain a constant intensity ratio. In addition, one new absorption was noted at 2406 cm-I. In the second experiment, at higher overall concentrations, the same set of product bands was detected, with uniformly increased intensity. HF + Methylcyclopropane. This set of reactants was studied in a total of seven twin-jet experiments, all into argon matrices, as well as several single-jet experiments. The concentration of MCP was held constant at 500/1, while the concentration of the Ar/HF samples was very difficult to determine, due to the strong tendency of H F to absorb on the wall of the vacuum line. This sequence of experiments was interspersed with several blank experiments of HF in argon, to provide reliable comparison. The most notable feature in all of these experiments was a sharp, very intense band at 3716 cm-I, which varied in intensity as the level of H F in the sample varied. In addition, product bands were noted near certain of the fundamentals of MCP, at 1095, 1025, 848, and 818 cm-', along with two absorptions of moderate intensity at 355 and 485 cm-'. Finally, as the level of HF in the sample
+
1:1 Complexes of HX with Methyl-Substituted Cyclopropanes
The Journal of Physical Chemistry, Vol. 89, No. 9, 1985 1745 TABLE I1 Band Positions" of the Ring Vibrations of the Methyl-Substituted Cyclopropanes in 1:l Complexes with the Hydrogen Halides
acid species methylcyclopropane
1180' 931' 855' 1,l-dimethylcyclopropane' 1323 1061 837 trans- 1,2-dimethylcy~lopropane~ 1218 913 922 cis- 1,2-dimethylcy~lspropane~ 1220 915 915 1,1,2,2-tetramethylcyclopropane 961' 857' 1188 cyclopropane' 866 866
HCI
HBr
HF
931 846 1319
935 845 1320
848
828
831
916
969
849 1185 853 866
848 1185 853 866
869 866
"Band positions in cm-I. 'Tentative assignments, see text. CBand assignments from ref 21. dBand assignments from ref 22. eBand assignments and discussion from ref 4.
I,
Y
N2/HBr = 200 N2/TMCP = 500 ANNEALED
I n I I 1 2420" 970 910 ac ENERGY (CM-1) Figure 4. Infrared spectra of the reaction product arising from the
2500
codeposition of HBr and TMCP into Nz matrices. The top two traces show the blank spectra of the two reagents, while the third shows the spectrum of the reaction mixture. The bottom trace shows the spectrum recorded after the samples were annealed and recooled to 15 K. TABLE I: Position (cm-') of the Hydrogen Halide Stretching Freouencv in 1:l Comdexes with Methvl-Substituted Cvclopropanes base
methylcyclopropane
HC1
2149 2115" 1,l-dimethylcyclopropane trans- 1,2-dirnethylcyclopropane 2750 2748 cis-1,2-dimethylcyclopropane 1,1,2,2-tetramethylcyclopropane 2148 2166 cyclopropane 2852
acid HBr 2459 2412, 2464 2438 2450 2474" 2556"
HF 3116"
3753" 3954"
"Argon matrix spectra; all other in N2 matrices. was increased, additional product bands were observed at 1050, 1039, and 796 cm-'. Selected band positions for the products of the reaction of the hydrogen halides with methyl-substituted cyclopropanes are given in Tables I and 11. Discussion The codeposition of the hydrogen halides with a variety of methyl-substituted cyclopropanes into inert matrices gave rise to a number of new infrared absorptions which could not be attributed to either parent species, suggesting the formation of one or more product species. Since the hydrogen halides are known to catalyze the isomerization of many of the substituted cyclopropanes used in this study, the spectra were compared to appropriate literature spectra. In only one case, the single-jet reaction of HCl with methylcyclopropane, was there evidence for an isomerization or addition reaction. Rather, the spectra obtained
in this study were indicative of perturbed acid and base (methyl-substituted cyclopropane) subunits, where the basic structural integrity of each species has been retained. Such results point to the formation of a molecular complex, such as has been observed for the reaction of the hydrogen halides with cyclopropane, where a hydrogen-bonding interaction was i n d i ~ a t e d . ~ For all of the systems studied here, the product absorptions can be divided into two sets based upon concentration dependence and diffusion behavior, which suggests the formation of more than one product species. The first set was typified by the bands at 2775, 1319, 1241, 1151, 1033, 1012, 931, and 828 cm-' in the HC1+ 1,l-DMCP system, and the second set by bands a t 2742 and 919 cm-' in this system. The first set of bands was favored when relatively dilute samples were employed, and when the ratio of HC1 to 1,l-DMCP was equal to one or less. By comparison, the second set of bands was clearly favored at high HCl concentrations and showed a marked increase in intensity after annealing. This behavior is indicative of aggregate formation and is best assigned to a 2:l complex of HCl with 1,l-DMCP. The first set of bands, which persisted in experiments with total dilutions up to 1000/ 1/ 1 or more can be readily assigned to a 1:1 molecular complex between the hydrogen halide and substituted cyclopropane. Similar results were obtained in the previous study with cyclopropane: where 1:l complex formation was dominant, but a 2:l complex could be observed a t high hydrogen halide concentration or after annealing. While discussed in more detail below, it should be noted that the degree of perturbation of the hydrogen halide subunit and cyclopropane subunit in the previous study was very similar to that observed here, which also suggests molecular complex formation. The observation here of these 1:l hydrogen bonded complexes marks the first time these species have been observed, and for several matrix isolation provides probably the only means for their synthesis and characterization. The structure of the HX-c-C3H6complex was determined to involve hydrogen bond formation from the hydrogen halide to the electron density at the midpoint of one of the carbon-carbon bonds, with the hydrogen halide lying in the plane of the three-membered ring. In the present study, while a qualitatively similar structure might be suggested, the carbon-carbon bonds are no longer equivalent, and more than one site of coordination is possible. After the vibrational assignments for each species have been made (below), the structure of each of the complexes will be discussed. Band Assignments ~ e ~ ~ ~ ~ c ~ c Z oHCI, ~ ~HBr, o ~and a HF n e were , ~ all, ~ .in turn, codeposited with MCP into inert matrices; for all three systems
1746 The Journal of Physical Chemistry, Vol. 89, No. 9, 1985
the most intense product band was located 80-250 cm-I below the H-X stretching mode of the parent hydrogen halide. In a hydrogen-bonding interaction, the most characteristic spectral change is a shift of lower energy of the hydrogen stretching motion, often accompanied by substantial intensification.28 Hence the spectra are strongly indicative of hydrogen bond formation, and the intense spectra feature is readily assigned to the perturbed H-X stretching mode in the complex. A number of product bands were also noted below 1200 cm-' when any of the three acids were employed. In all cases, these product absorptions fell within 10-20 cm-' of a parent vibrational mode of MCP and can readily be assigned to the analogous vibration of the complexed MCP. Unfortunately, no vibrational analysis has been carried out to date for MCP, so that assignment to specific vibrations is more difficult. However, the antisymmetric ring deformation mode of cyclopropane a t 866 cm-I, which is doubly degenerate in D3hsymmetry, must split into an A' A" pair in C, symmetry (A, + B2 in C2u).29While this splitting in methylcyclopropane need not be centrosymmetric about the 866-cm-' cyclopropane position, these two modes are expected in 800-1000-~m-~ spectra region. Using this reference point and making comparison to a number of mono- and disubstituted cyclopropanes strongly suggests that the parent modes at 931 and 855 cm-' are the best candidates for these two ring vibrations. In addition, both were substantially perturbed by complexation to the hydrogen halide, which is anticipated for the ring modes. For the HCLMCP complex, these modes were observed a t 846 and 937 cm-I, while for the HBr complex, bands were detected at 845 and 935 cm-'. For the H F complex, only a perturbed counterpart of the lower band was observed, a t 848 cm-I, likely as a consequence of lower yield in the HF experiments. In the HF/MCP experiments, two low-energy bands were observed as well, at 355 and 485 cm-I. Previous s t ~ d i e s 'have ~ * ~shown ~ that the librational or bending modes of the hydrogen bond in HF complexes lie between 200 and 700 cm-I; for the HF.c-C3H6 complex, these modes4 were located at 374 and 389 cm-I. While the splitting between these two modes (in-plane and out-of-plane) is greater for the HF-MCP complex studies here (see discussion below), assignment of these two bands to the two anticipated librational modes of the hydrogen bond is quite reasonable. 1,l-Dimethylcyclopropane.". In a manner similar to the above MCPsHX system, the most intense product band arising from the codeposition of 1,l-DMCP with either HCl or HBr was an intense absorption to the low-energy side of the parent hydrogen halide. This is again indicative of hydrogen bond formation and is readily assigned to the stretching mode of the coordinated hydrogen halide. Additional product bands were observed in the lower-energy region, generally near fundamentals of the parent 1,l-DMCP. Assignments of these bands is relatively straightforward in that a vibrational analysis of the parent has been carried out.*' The symmetric ring breathing mode of 1,l-DMCP has been identified at 1323 cm-I, and product bands at 1319 and 1320 cm-' for the HC1 and HBr complexes can readily be so assigned. The two lower-energy ring deformation modes, arising from reduction of symmetry from D3hfor cyclopropane to C2, for 1,l -DMCP, have been assigned at 837 and 1061 cm-l for the B2 and A, modes, respectively. Counterparts of the B2 mode were observed at 828 and 831 cm-' for the HC1 and HBr complexes of l,l-DMCP, respectively, while no counterparts of the upper, A, mode were detected. This may be a consequence of the number of parent and product bands in this region or due to misassignment of the vibrational spectrum of 1,l-DMCP. In particular, the 934-cm-' parent band showed counterparts at 931 and 926 cm-' for the HCl and HBr complexes, respectively, and might well have been assigned to the ring deformation mode (the literature assignment is to the totally symmetry CH3 rocking mode). In addition, perturbed counterparts of the CH2 rock, the CC2 antisymmetric
+
(28) Pimentel, G. C.; McClellan, A. "The Hydrogen Bond";W. H. Freeman: San Francisco, 1960. (29). Nakamoto, K. "Infrared and Raman Spectra of Inorganic and Coordination Compounds"; Wiley-Interscience: New York, 1979, 3rd ed. (30) Andrews, L.; Johnson, G. L. J . Chem. Phys. 1982, 76, 5767.
Truscott and Ault stretch, the CHI wag, and the CH2 twisting modes were observed in the higher yield experiments, particularly with HC1. trans-l,2-Dimethylcyclopropane.HX.As in the MCP and 1,l-DMCP systems, the most intense feature for both the HC1 and HBr complexes was observed in the hydrogen halide stretching region and can readily be assigned to the perturbed H-X stretching mode in the hydrogen-bonded complex. In the lower region, assignment is relatively straightforward, as a vibrational analysis" has been performed for trans-1,2-DMCP. The most intense product band in this region was observed at 976 cm-l for the HCl complex, and 969 cm-l for the HBr complex, shifted slightly from the ring deformation mode of the parent at 973 cm-I. The second antisymmetric ring deformation mode has been assigned to a weak fundamental at 922 cm-'; no counterpart corresponding to perturbed trans-1,2-DMCP was observed for this mode. However, this particular mode should have considerable infrared intensity (as did its counterparts for the other substituted cyclopropanes studied here) and one might argue that the 872-cm-I parent fundamental (currently assigned to the CH3 rock) might be best reassigned to the ring mode. Perturbed counterparts of this mode were observed at 877 and 880 cm-' for the HC1 and HBr complexes, respectively. Finally, bands at 1031 and 1021 cm-' for the HCl and HBr complexes have been assigned to the CH2 rocking mode, which has proven to be highly susceptible to perturbation in these hydrogen-bonded complexes. cis-1,Z-DimethyIcyclopropane.HCI. Only HCI was deposited with cis-1,2-DMCP, and the overall yield of product in these experiments was somewhat lower than in the experiments described above. The one intense feature observed at 2748 cm-' is readily assigned to the H-Cl stretching mode of the acid in the hydrogen-bonded complex, and this band lies quite close to the product absorption observed for each of the complexes above. In the lower region, the one distinct product band at 1066 cm-' is assigned to the CH3rocking mode of cis-1,2-DMCP in the molecular complex, shifted 10 cm-l from its unperturbed position. A shoulder on the CH2 rocking mode near 1021 cm-I is likewise assigned to the peturbed base subunit, while no bands were observed which could be assigned to the ring modes in the hydrogen-bonded product. 1,I,2,2-TetramethyEcyclopropane-HX. Both HCl and HBr complexes of TMCP were isolated in this study, and both showed intense hydrogen halide stretching modes of the complexed acid, at 2748 cm-' for HCl and 2450 cm-' for HBr. Unfortunately, a complete vibrational analysis of the parent TMCP has not been carried out, and assignment of the lower region is more difficult and is further complicated by the somewhat larger size and hence more vibrational modes of the parent. Candidates for the Al ring deformation mode include parent features at 1035, 1020,961, and 938 cm-I, while candidates for the B2 ring deformation mode were noted at 857 and 938 cm-I. In an attempt to sort out these possibilities, gas-phase spectra were run on pure TMCP; a vibration of Al symmetry is expected to give a A-typc contour, while a B2 species should produce a C-type c ~ n t o u r . ~ The ' band at 938 cm-' was clearly of A-type contour, while the 857-cm-' band resembled a C-type contour. This strongly suggests that the 857-cm-I band is the B2 ring deformation mode, for which complexed counterparts were observed at 849 and 848 cm-l for the HCl and HBr complexes, respectively. Several of the bands near 1000 cm-' also showed A-type contours, so that picking out the A, ring deformation mode on this basis was not possible. However, analogy to the other substituted methylcyclopropanes studied here indicates that the 961-cm-' band is the best choice, although the 938-cm-I band could not be ruled out. Perturbed counterparts of both parent vibrations were observed, but the exact assignment of this mode does not affect the structural conclusions discussed below. 2 2 Complexes. As noted above, in several experiments a second product species was observed which was assigned to a 2:l complex. Fewer vibrational modes were observed for this species; the most apparent feature was a band in the hydrogen halide stretching ~~
~
~
(31) Herzberg, G. "Infrared and Raman Spectra"; Van Nostrand: New York, 1945.
1:1 Complexes of H X with Methyl-Substituted Cyclopropanes
The Journal of Physical Chemistry, Vol. 89, No. 9, 1985 1747
region. This band was generally 30-50 cm-’ lower in energy than the hydrogen halide stretching frequency in the 1:l complex and is assigned to the stretching mode of one of the HX units hydrogen bonded to the cyclopropane ring. The increased red shift of this mode from the position of the parent vibration has been noted for a number of systems and attributed to increased distortion of the hydrogen-bonded H X unit by the second HX species in the 2:l complex.32 In the lower-energy region, the only system in which sufficient yield of 2:l complex was obtained to observe the perturbed base subunit was the HCl/1,1-DMCP system. Here, a product band at 919 cm-’ showed the characteristics upon annealing of a 2:l complex and is assigned to the CH3 rocking mode of 1,l-DMCP in the 2:l complex.
TABLE III: Comparison of Shiftso of the Hydrogen Halide Stretching Frequencies in the 1:l Complexes of the Hydrogen Halides with Methyl-Substituted Cyclopropanes
Structural Considerations Determination of the structure of the 1:l complexes is one of the more interesting aspects of this study; certainly the evidence presented above leaves no doubt that the hydrogen halide is hydrogen bonded to the base subunit in some fashion. A similar conclusion has been reached for the cyclopropane complex, and both gas-phase rotational spectroscopy6*’ and matrix infrared speetra4J indicate that the hydrogen is bonded to the midpoint of one of the carbon-carbon bonds, in the plane of the threemembered ring. The spectra here for the substituted cyclopropanes were very similar to that obtained for the cyclopropane complexes, particularly for the hydrogen halide stretching mode (see Tables I and 11). Consequently, a similar mode of interaction is strongly suggested. However, since the three carbon-carbon bonds are not equivalent upon methyl substitution, more than one site of interaction is possible. For example, in the MCP and 1,l-DMCP studies, the hydrogen halide may interact with either of the C-C bonds adjacent to the site of substitution, or with the C-C bond opposite the substitution site. The spectra collectively suggest that only one species was formed; i.e. that the hydrogen halide in each case preferentially attacked6 only one site. One might argue that different sites might not be resolvable within the bandwidth of the hydrogen halide stretching mode. However, in many cases these bands were very sharp (1-2 cm-’ half-width) and the electron distribution at the different sites varies sufficiently that different degrees of hydrogen-bonding interaction should occur and be resolved. Some multiplet structure was observed in the hydrogen halide stretching region, which might be assigned to two structural isomers of the product complex. However, this multiplet structure, which was most common in argon matrices, generally disappeared in nitrogen matrices or, if not then, upon annealing. These observations suggest matrix perturbations rather than distinct structural isomers. A key factor in determining the site of interaction is the symmetry of the molecule, and the effect of hydrogen bonding at each of the possible sites of coordination on the ring vibrations. In principle, one would anticipate both the AI and B2 ring vibrations (arising from E‘ mode of cyclopropane, split by methyl substitution) to be perturbed by hydrogen bonding to the hydrogen halide. To determine the magnitudes involved, model normal coordinate calculations were carried out on a three-membered ring, in which two of the bonds were equivalent and one was unique, generating a species to model 1,l-DMCP. The three stretching modes of this ring were then refined to the known ring frequencies of 1,l-DMCP. At this point, one bond in the species was systematically perturbed by either raising or lowering its force constant stepwise. When one of the bonds adjacent to the site of methyl substitution was so perturbed, both ring modes showed measurable shifts, up to 10 cm-I for a perturbation of 0.30 mdyn/A. By comparison, when the bond opposite to the site of coordination was perturbed (Le. the unique bond) over the same range, the A, ring mode was substantially shifted, while the B2 mode showed no shift, to the nearest 0.01 cm-I. Earlier calcul a t i o n ~on~ the cyclopropane-HX complexes were able to reproduce spectra of the complexes with a change in force constant of 0.20
“Band shift in cm-I. bArgon matrix results, all others in N2 matrices. ‘From ref 38.
(32) Maes, G.,private communication.
acid base
HF
cyclopropane methylcyclopropane 1,l-dimethylcyclopropane trans- 1,2-dimethylcyclopropane
201b 238b
cis-1,2-dimethylcyclopropane
1,1,2,2-tetramethylcyclopropane proton affinity of X-, kcal/mol dipole moment of H X / D
371‘ 1.91
HCI 86 103 95b 102 104 104
82b 85 77 115
333‘ 1.03
323‘ 0.79
HBr
91
mdyn/A. Thus, a criterion for distinguishing between coordination opposite vs. adjacent to the site of methyl substitution might be whether or not the B2 ring vibration is measurably perturbed. As can be seen in Table 11, for both C2, species, 1,l-DMCP and TMCP, the B2 ring mode is shifted, and by an amount similar to shifts in the cyclopropane complexes. Consequently, the spectra strongly suggest that attack and hydrogen bonding in the 1,lDMCP-HX complexes must be at one of the carbon-carbon bonds adjacent to the site of methyl substitution, while for the TMCP-HX complexes the hydrogen bonding must be to one of the carbon-carbon bonds joining a substituted and the nonsubstituted carbon. These conclusions are in good agreement with the known chemistry of substituted cyclopropanes, where ring cleavage has been observed between the most highly substituted and least substituted carbon^.^^-^^ This further suggests that the molecular complexes observed here may be the initial species formed in the acid-catalyzed ring-opening reactions of substituted cyclopropanes. Less definite statements can be made about the structure of the remaining complexes, in that the parent methyl-substituted cyclopropane is of lower symmetry, either C, or C2. For the MCP-HX complexes, perturbation of the A’ ring mode at 855 cm-’ is suggestive of coordination adjacent to the methyl group, by analogy to 1,l-DMCP, which is certainly consistent with the formation of 2-chlorobutane when HCl and MCP were premixed in a single vacuum line. Moreover, this structure is consistent with a molecular orbital treatment of cyclopropane which suggests that when an electron-donating s~bstituent,~’ such as a methyl group, is added, the adjacent carbon-carbon bonds are strengthened, and the opposite bond is weakened. The splitting of the two librational modes of the hydrogen bond in the HF-MCP complex sheds some light on this as well. First, the observation of two such modes indicates that the H F subunits lies in a site of symmetry lower than C,, (a point which was not brought out in the previous cyclopropane study, but which substantiates the structural conclusions presented there). The degree of splitting of this mode must reflect the degree of asymmetry in the potential functions in- and out-of-plane. For cyclopropane the splitting was only 15 cm-’, which was comparable to the splitting observed for the H F C 2 Hcomplex.30 ~ The much larger splitting observed here, 130 cm-’, suggests substantially greater asymmetry, which is very likely brought about by interaction with the carbon-carbon bond adjacent to the methyl group. Complexation to the carbonarbon bond opposite to the site of methyl substitution should give rise to a splitting of comparable magnitude to that observed in the (33) Zimmerman, M.P.;Li, H.-T.; Daux, W. L.; Weeks, C. M.; Djerassi, C. J. Am. Chem. SOC.1984, 106, 5602. (34) Saunders, M.; Vogel, P.; Hagen, E.; Rosenfeld, J. Acc. Chem. Res. 1973, 6, 53. (35) DeMayo, P.“Molecular Rearrangements”; Wiley-Interscience: New York, 1963. Richardson, K. S.‘Mechanism and Theory in Organic (36) Lowry, T. H.; Chemistry”; Harper and Row: New York, 1981; Chapter 5. (37) Allen, F.H.Acta Crystallogr., Sect. B 1980, 836, 1. (38) Beauchamp, J. L. Annu. Reu. Phys. Chem. 1971, 22, 527.
J. Phys. Chem. 1985,89, 1748-1752
1748
HF-C3H6complex. These observations support a structure in which the H F is complexed adjacent to the methyl substitution site. For the complexes of trans-1,2-DMCP and cis-l,2-DMCP, very little structural information can be determined, from the combined difficulties of low symmetry and low product yield. By analogy only, one might suggest that the coordination is to the carboncarbon bond connecting one of the substituted carbons with the unsubstituted carbon, but no direct evidence was observed to support this suggestion. Table 111 presents a correlation of the shifts in the hydrogen halide stretching frequency with the different substituted cyclopropanes studied vs. the dipole moment of the hydrogen halide and the proton affinity of the corresponding halide anion. The magnitude of shift has been correlated numerous times28with the strength of interaction and indicates that the H F complexes are most strongly bound. This is in agreement with the gas-phase studies on cyclopropane-HX complexes and correlates with the dipole moment trend within the hydrogen halides. This suggests that, for these rather weakly bound complexes, electrostatic interactions are more important than covalent contributions, which have been shown to be significant for more strongly bound complexes. Trends within the set of methyl-substituted cyclopropanes are less apparent; Table I11 demonstrates that complexes of cyclopropane are slightly less strongly bound than those of the methyl-substituted cyclopropanes, but the number and location of the methyl substituents do not present a clear pattern of increasing basicity.
Conclusions
The matrix isolation technique has provided the first opportunity to isolate and characterize the 1:1 hydrogen-bonded complexes of the hydrogen halides with a number of methyl-substituted cyclopropanes. In all cases, the hydrogen stretching frequency of the HX subunit shifted substantially to lower energy, while certain vibrational modes of the base subunit were perturbed to a measurable degree. These perturbations allowed a determination of the site of interaction for the more highly symmetric (C2J substituted cyclopropanes, namely, between the most and least highly substituted carbons in the three-membered ring. For the remaining complexes, the most likely structure was suggested, but no definite conclusions could be reached. The agreement of the conclusions reached here with previous solution studies of the acid-catalyzed ring opening reactions of substituted cyclopropanes further suggests that the complexes identified here may either represent or closely resemble the initial intermediates in these solution reactions. Acknowledgment. The authors gratefully acknowledge support of this research by the National Science Foundation under Grant C H E 8400450. B.S.A. also acknowledges the Dreyfus Foundation for a Teacher-Scholar Grant, while C.E.T. acknowledges the University of Cincinnati University Research Council for a summer fellowship. Registry No. HC1, 7647-01-0; MCP, 594-1 1-6; 1,l-DMCP, 163094-0; tr~ns-1,2-DMCP,2402-06-4; cis- 1 ,ZDMCP, 930-1 8-7; TMCP, 4127-47-3; HBr, 10035-10-6; HF, 7664-39-3.
pK Values from Acidity Function Type Methods: Inherent Uncertainties John F. Wojcik Department of Chemistry, Villanova University, Villanova, Pennsylvania 19085 (Received: January 24, 1984; In Final Form: December 1 1 , 1984)
The uncertainties inherent in the measurements of thermodynamic pK values by means of excess acidity functions are examined by using a statistical approach. The key to the problem is the mathematical form of the equation relating the activity coefficients of two weak bases. The analysis presented here, using data for 10 weak bases, shows that, although an overlap method for determining pK values is possible from a mathematical viewpoint, experimental exigencies lead to serious problems since the data, derived from limited overlap acidities, give no hint as to the form of the activity coefficient relationship. The magnitude of the uncertainty in the determined pKvalues due to the uncertainty in the form of this relationship is given and is shown to have a high probability of being several pK units for very weak bases.
The relative basicities of organic bases has long been of interest both as a source of information about electronic structures and reactivities and as a means of estimating the amounts of various protonated forms of a base in solutions of different acid concentration. The most common measure of basicity has traditionally been the pK of the conjugate acid of the base. In order to make meaningful comparisons among bases, the px"s must be referred of a common standard state, the usual choice being infinite dilution (hypothetical 1 m). Numerous methods have been devised for the measurement of p 6 s . One requirement for all methods is that measurements are made under conditions where significant amounts of the unprotonated and protonated forms of the base are present simultaneously in solution. A second requirement for obtaining a pK value referred to the infinite dilution standard state is that activity coefficients be taken into account in some fashion since measurements are usually made in terms of concentrations rather than activities. Accounting for activity coefficients can amount to using a form of the Debye-Huckel equation or to extrapolating concentration constants to zero ionic strength, at least in those cases where the solutions are essentially dilute, that is, with 1 < pH 0022-3654/85/2089-1748$01.50/0
< 13. Many interesting bases, however, are very weak, and, in order to achieve the first requirement, they must be studied in concentrated acid. Fulfilling the first requirement complicates achievement of the second one. A promising solution to what might be called the extrapolation problem was presented by Hammett' in terms of acidity functions. In the many years since, these methods have been extensively studied and modified, a recent product of this work being the excess acidity function. The history of these functions along with extensive references has been given in a recent review.2 In a recent a r t i ~ l e however, ,~ the utility of acidity function methods for determining pK values was questioned and it was demonstrated that traditional methods, including excess acidity methods, logically yield not pK values but rather a quantity which is the sum of the desired pK value and an indeterminate constant. In the latter work it was suggested that the value of this indeterminate constant could be substantial, but the method used to (1) Hammett, L. P.; Deyrup, A. J. J . Am. Chem. SOC.1932, 54, 2721. (2) Cox, R. A.; Yates, K. Can. J . Chem. 1983, 61, 2225. (3) Wojcik, J . F.J . Phys. Chem. 1982, 86, 145.
0 1985 American Chemical Society
