J . Phys. Chem. 1990, 94, 199-203 The primary purpose of our calculations of rotational strengths of allene-1,3-d2 was to illuminate the conditions under which the simple FPC, LMO/C, and APT models might yield predictions of usable accuracy. As discussed above, the results for this molecule do not support the general use of these models. Nevertheless, in some of the molecules studied using the FPC and LMO/C models some success has been achieved in replicating some regions of experimental VCD Much of that agreement has been found for higher energy, “characteristic” modes.5 At the level of sign pattern, considering only the C-H stretch and asymmetric C==C=C stretch modes, the same would be true of allene-l,3-d2. In a similar comparison of theoretical VCD calculationsI2it was found that FPC and a priori calculations of VCD for trans-cyclopropane-f ,2-d2had a consistent sign pattern for modes lying above 1000 cm-I. In some studies, the molecular
199
geometry and force field were not accurately known and the significance of the conclusions arrived at is unclear. However, in a few cases-such as, for example, the study of trans-cyclobutane-f ,2-d2-accurate geometries and force fields were availThe reasons for these specific successes in predicting VCD spectra from either FPC or LMO/C models remain unclear and deserve further study. Acknowledgment. This work was supported by grants to T.A.K. from the National Science Foundation (CHE 84-12087) and to P.J.S. from the National Science Foundation, National Institutes of Health, NATO, and the San Diego Supercomputer Center. We thank Professor James Chickos for the gift of a sample of allene- f ,3-d2. Registry No. Allene-I,3-d2,19487-21-9.
Infrared Matrix Isolation Investigation of the Molecular Complexes of CIF with Benzene and Its Derivatives Hebi Bai and Bruce S. Ault* Department of Chemistry, University of Cincinnati, Cincinnati, Ohio 45221 (Received: May 5, 1989)
The matrix isolation technique has been employed to investigate the complexes formed between C1F and benzene, along with a series of substituted benzenes. 1:l adducts were formed in each case, and characterized by the shift of the C1F stretching mode upon complexation. For example, for the CIF.C6H6complex this mode was observed at 710 cm-I, shifted from 770 cm-l for parent CIF. A number of perturbed vibrational modes of the base subunit were detected as well, and this indicated that the CIF subunit sits axially above the benzene ring, interacting with the a-electron density. For the complex of CIF with C6HSBr,two different 1:l complexes were tentatively identified, one in which the CIF is coordinated to the ring, and one in which the CIF is coordinated to the bromine. The magnitude of the shift of the CIF stretching mode varied with the substituent on the ring in a manner which agreed with the electronic character of the substituent.
Introduction Molecular interactions and the formation of molecular complexes or adducts have been of ongoing interest to chemists for many y e a r ~ . I - ~ These complexes are often described in the electron donor-acceptor framework developed by Mulliken and others. Electron donors may be either lone pair u-donors or 7-electron donors from localized or delocalized *-electron systems. Stable complexes of benzene and its derivatives have been known for a number of years,6-8 often with the heavy halogens I2 and Br,. Several of these complexes have been characterized in cryogenic matrices as well, where isolated 1:l complexes can be trapped and s t ~ d i e d . ” ~ Fredin and Nelander have suggested9J0 that the complexes of c1, and Br, with C6H6 are oblique, while the l2 complex is axial. Further, they suggest that IC1 forms two complexes with C6H6, one oblique with the c1 nearer the ring, and one axial with the I nearer the ring. However, a study by Brown and Person showed15that it is necessary to use very high dilutions to isolate monomeric C6H6,suggesting that the complexes observed by Fredin and Nelander might not be 1:l complexes. Chlorine monofluoride, CIF, is a more reactive interhalogen and has proven to be an excellent probe of Lewis acid-base interactions.’620 ClF serves as a Lewis acid as it accepts electron density into its u* antibonding orbital, lowering the CI-F force constant and stretching frequency. Consequently, the experimentally determined position of this stretching mode is a sensitive indicator of the strength of the molecular interaction. Andrews and co-workers21,22 have used H F extensively as a probe of hydrogen bonding to a wide variety of electron donors, including substituted benzenes; the Lewis acid complexes of CIF provide Author to whom correspondence should be addressed.
0022-3654/90/2094-0199$02.50/0
an excellent parallel to these hydrogen-bonded complexes. Additionally, theoretical calculations have been carried out by several g r o ~ p son ~ ~complexes ,~~ of CIF, including the complex of C1F with benzene. These calculations suggest a structure which is axial 6 symmetry of the benzene ring. and which preserves the c Consequently, a study was undertaken to isolate and characterize
(1) Jensen, W. B. The Lewis Acid-Base Concepts, an Oueruiew; WileyInterscience: New York, 1980. (2) Mulliken, R. S . J . Am. Chem. SOC.1950, 72, 600. (3) Collin, J.; DOr, L. J . Chem. Phys. 1955, 23, 397. (4) Ferguson, E. E. J . Chem. Phys. 1956, 25, 577. (5) Ferguson, E. E. Spectrochim. Acta 1957, 10, 123. (6) Ferguson, E. E. J . Chem. Phys. 1957, 26, 1357. (7) Hassel, 0.;Stromme, K. 0. Acta Chem. Scand. 1958, 12, 1146. (8) Hassel, 0.;Stromme, K. 0. Acta Chem. Scand. 1959, 13, 1781. (9) Fredin, L.; Nelander, B. Mol. Phys. 1974, 77, 885. (10) Fredin, L.; Nelander, B. J . Am. Chem. SOC.1974, 96, 1672. (11) Brown, K. G.; Person, W. B. J . Chem. Phys. 1977, 66, 8876. (12) Engdahl, A.; Nelander, B. J . Chem. Phys. 1982, 77, 1649. (13) Engdahl, A.; Nelander, B. J . Phys. Chem. 1982, 86, 670. (14) Engdahl, A,; Nelander, B. J . Chem. Phys. 1983, 78, 6563. ( 1 5 ) Brown, K. G.; Person, W. B. Spectrochim. Acta, Parf A 1978, 3 4 4 117. (16) (17) (18) (19) (20) (21) (22) (23) Pitman (24)
Machara, N. P.; Ault, B. S . Inorg. Chem. 1985, 24, 4251. Machara, N. P.; Ault, B. S . J . Phys. Chem. 1987, 91, 2046. Machara, N. P.; Auk, B. S . J . Phys. Chem. 1988, 92, 73. Machara, N. P.; Auk, B. S . J . Phys. Chem. 1988, 92, 2439. Ault, B. S. J . Phys. Chem. 1987, 91, 4723. Andrews, L. J . Mol. Struct. 1983, 100, 281. Andrews, L.; Johnson, G. L. J . Phys. Chem. 1982, 86, 3380. Molecular Interactions; Ratajczak, H., Orville-Thomas, W. J., Eds.; Press: London, 1981; Vol. 1. Lucchese, R. L.; Schaefer, H. F. J . Am. Chem. SOC.1975, 97,7205.
0 1990 American Chemical Society
200
The Journal of Physical Chemistry, Vol. 94, No. I , 1990
Bai and Ault TABLE I: Band Positions and Assignments for the 1:l Complex of CIF with Benzene product band parent band uosition. cm-’ position. cm-I shift assignment” 676 +8 684 763, 770 -59 705, 71 1 859 847 +I2 998 992 +6 I010 t 2 1012 1390 +5 1395 +IO 1521 1531 1812 +I6 1828 +IO 1956 I966
C,H6 band assignments from ref 26.
u A-ICIF :230
‘i’l
-
LAAWeH;lOOO
300
72C
640
.&.___i 54,: 600
i~
759
WAVENUMBERS
720
580
( CW l )
Figure 1. Infrared spectra, in the CI-F stretching region, of matrices into which CIF has been codeposited with C6H6 (left hand side trace c) and with C6(CH3)6 (right hand side traces b and c). Traces a, left and right-hand sides, and b, left-hand side, are spectra of blank experiments of the reagents. Bands due to the 1:l complex are labeled c. The concentration of C6(CH3)6is not indicated, since it could not be precisely determined (see text).
the 1 : 1 molecular complex of CIF with benzene, as well as with a series of substituted benzenes, to determine the effect of substituents on the molecular interaction.
Experimental Section All of the experiments described here were carried out on matrix isolation equipment which has been described p r e v i o ~ s l y . CIF ~~ (Ozark-Mahoning) was introduced as a gas into the vacuum line and purified by freezethaw cycles at 77 K. Benzene, C6H6 (Baker and Adams); benzene-d6, C6D6 (Merck); toluene, C6H,CH, (MCB); toluene-& C6DSCD3(Aldrich); and hexafluorobenzene, C6F6(Aldrich), were degassed by freezethaw cycles at 77 K prior to sample preparation. The full room temperature vapor pressures of bromobenzene, C6H# (Eastman), and cyclopropylbenzene, C3HSC6H,(Aldrich), were used to make samples of these reagents, which led to a small uncertainty in the concentration of these samples. Hexamethylbenzene, C6(CH3)6 (Eastman Kodak), did not have sufficient room temperature vapor pressure for sample preparation. Consequently, a sample of solid hexamethylbenzene was placed in a stainless steel finger and joined to the argon deposition line near the entrance to the vacuum vessel. In other experiments, this compound was placed in a Knudsen cell within the vacuum vessel and heated slightly. Argon was used as the matrix gas in all experiments and was used without further purification. Twin jet deposition was used for all experiments, with deposition typically for 20-24 h onto a 15 K cold window. Final infrared spectra were recorded on an IBM 98 FTIR at 1 cm-l resolution. Several samples were then annealed to 35 K and recooled, followed by the recording of additional spectra.
I
,
780
500
.IC
740
703
460
%0
W A V E NUMBER(
c M-’ )
Figure 2. Infrared spectrum of the products of deposition of CIF with toluene into argon matrices, over selected spectral regions (lower trace) compared to a blank of toluene over the same regions (upper trace).
Annealing these blank experiments had very little affect on the resultant spectra. C1F Benzene. Samples of Ar/C6H6 and Ar/CIF were codeposited in a number of experiments, over a wide range of concentrations. In each experiment, the most prominent new spectral feature was a sharp doublet at 71 1 and 705 cm-I, with a full width at half-maximum of 2 cm-I, and approximately a 3:l intensity ratio. In addition, a strong new absorption was observed at 684 cm-I, near a mode of parent benzene at 676 cm-I. A representative spectrum for this pair of reagents is shown in Figure 1 . When the concentrations of both reagents were increased from a IOW Of IOOO/I + IOOO/l Up as high as AT/C& = 333 + Ar/CIF = 250,additional weak bands were noted at 859, 998, 1012, 1395, 1531, 1830, and 1966 cm-]. All of these new absorptions (listed also in Table I) grew or were reduced in intensity in a manner proportional to the change in concentration from one experiment to the next. A sample of Ar/ClF was codeposited with a sample of Ar/C6D6 in one experiment; the sharp doublet observed above with C6H6 Results was observed unshifted, at 7 1 1 and 705 cm-I. On the other hand, the product absorption described above at 684 cm-I disappeared, Prior to any codeposition experiments, blank experiments were and a comparable, strong band was noted at 504 cm-I, near the conducted of each reagent alone in argon. The infrared spectra 496-cm-’ absorption of parent C6D6. which were obtained were in good agreement with matrix spectra, CIF + Toluene. Samples of CIF in argon were codeposited with where available, and with gas-phase spectra for the r e ~ t . * ~ - ~ O samples of toluene in argon in a number of experiments over a (25) Ault, B. S. J . Am. Chem. SOC.1978, 100, 2426. (26) Andrews, L.; Johnson, G. L.; Davis, S . R. J. Phys. Chem. 1985,89, 1706. (27) Davis, S. R.; Andrews, L. J . Phys. Chem. 1986, 90, 2600. (28) Sass, C. E.; Ault, B. S . J . Phys. Chem. 1987, 91, 3207.
+
~~
~
~~
(29) Cesaro, S. N.; Martini, B.; Bencivenni, L.; Spoliti, M.; Maltese, M. Spectrochim. Acta, Part A 1980, 36A, 165. (30) Maltese, M.; Cesaro, S . N.; Sbaraglia, M.; Spoliti, M. Spectrochim. Acta, Parr A 1979, 34A, 1
Molecular Complexes of ClF with Benzene
The Journal of Physical Chemistry, Vol. 94, No. 1, 1990 201 absorption at 700 cm-l which partially overlapped a parent doublet of cyclopropylbenzene at 691,696cm-I. While overlapped, this new product absorption was nonetheless distinct. In addition, another parent band at 892 cm-' was clearly enhanced in the twin jet experiments relative to the blank of cyclopropylbenzene. As the concentrations of the reagents were increased, these features became proportionally more intense. ClF + Hexafluorobenzene. No product bands were detected in the spectrum taken after codeposition of a sample of Ar/ClF with a sample of Ar/C6F6.
ArlCIF= 2 0 0
1080
1000
-
760
680
600
440
1
400
W A V E NUMBERS ( C M - ' )
Figure 3. Infrared spectrum of the matrix formed by the codeposition of CIF with C6HSBr,each diluted in argon (bottom trace) compared to spectra of the blank experiments of the two reagents (upper traces), over
selected spectral regions. range of concentrations. The most distinctive new feature was a sharp (2cm-l fwhm) absorption at 705 cm-I, as shown in Figure 2, along with a medium-intensity counterpart at 697 cm-'. In addition, somewhat weaker but distinct product absorptions were noted at 468 and 738 cm-l. When the concentration of either reagent was altered, the intensities of these features changed in approximately the same ratio. Four codeposition experiments were carried out with C6D5CD3 and C1F at different dilutions in argon. The dominant absorption in these experiments was again the doublet near 695,703cm-', although this doublet was somewhat broader than in the above experiments with C6H5CH3. Additional new, weaker product bands were noted at 659,721,and 1593 cm-l. Annealing this sample to 35 K led to only slight changes in intensity of these product absorptions. CIF Bromobenzene. The codeposition of samples of Ar/CIF with those of Ar/C6H5Br led to the formation of two distinct doublets, at 714,720 and 706,710cm-'. Each doublet appeared with a 1:3 intensity ratio, the higher energy component being the more intense in each case. At higher sample concentrations, additional absorptions were noted at 462,472,747, 1021, 1070, and 1468 cm-', quite near absorptions of parent C6H5Br,as shown in Figure 3c. There was no significant change in the intensity ratio of the two product doublets as the concentrations of the two reagents were varied. CIF + Hexamethylbenzene. When C6(CH3)6 was codeposited with a sample of Ar/CIF = 100, two product doublets were observed, at 647,652and 636,639cm-l. No other product absorptions were noted in the spectrum of this pair of reagents. When the concentration of ClF was reduced to 500/1,the intensities of both doublets were reduced. However, the intensity of the 636,639cm-l doublet was reduced more than the doublet at 647,652cm-I. When the concentration of hexamethylbenzene was increased at a constant level of CIF, the doublet at 647,652 cm-l grew in intensity, while the lower energy doublet was reduced to a shoulder on the more intense doublet, as shown in Figure 1, b and c. CIF + Cyclopropylbenzene. The codeposition of a sample of Ar/CIF with a sample of Ar/C3H5C6H5led to a distinct new
+
Discussion All of the product bands described above were observed upon codeposition of C1F with benzene and the different substituted benzenes employed in this study but were not observed when the individual reagents were deposited alone in argon. These new absorptions, then, can be assigned to a reaction product for each system. For each pair of reagents, the dominant absorption was a moderately intense doublet near 700 cm-l, somewhat to the red of the absorption of parent C1F near 770 cm-'. Other, weaker absorptions were noted as well, in each case within 10-20 cm-l of one of the fundamentals of the benzene reagent. These spectral features are characteristic of the formation of a molecular complex, where the two subunits in the complex are perturbed but maintain their basic structural i n t e g r i t ~ . ~ Similar ' results have been observed for the reactions of CIF with both lone pair donors as well as localized ?r-electron donors.16-20 In addition, the dominant absorption persisted at very high dilutions in argon and the product bands maintained a constant intensity ratio to one another (except for the C6(CH3)3system). These observations point to the formation of an isolated 1:l complex, particularly since earlier studies have shown that most molecules undergo at most one reactive collision during the deposition process at these concentrations. Consequently, the major product absorptions reported here are assigned to the isolated 1:l complex between CIF and the benzene derivative employed. In the case of hexamethylbenzene, the 647,652cm-' doublet persisted when the concentration of CIF was reduced and/or the concentration of C6(CH3)6 was increased, whereas the intensity of the 636,639cm-l doublet was diminished greatly under these conditions. Consequently, the 647,652-cm-I doublet is assigned to the 1:l complex, while the 636,639-cm-1 doublet is assigned to a complex involving more than one C1F molecule, most likely the 1:2complex. As noted above, the dominant absorption for each system studied was observed near 700 cm-', to the red of the parent C1F stretching mode. Moreover, for almost all of the systems studied, this absorption appeared as a cleanly resolved 3:l doublet. Similar observations were made in the previous studies of C1F complexes with lone electron pair ( u ) donors and were assigned to the perturbed C1-F stretching mode in the complex. C1F acts as a Lewis acid, accepting electron density into its lowest unoccupied molecular orbital, the u* orbital. This weakens the ClF bond and reduces the stretching force constant, leading to a red shift of the vibration. The 3:l doublet structure is due to the two isotopes of chlorine in natural abundance, j5CI and 37Cl, in a 3:l ratio. The magnitude of the splitting, approximately 6 cm-' at 700 cm-l, is just that observed for parent C1F near 770 cm-l, as anticipated. Consequently, the corresponding assignment is made here, namely the strong absorption near 700 cm-' for each system is assigned to the C1-F stretching mode of the CIF subunit in the 1:l molecular complex. A number of additional, weaker product absorptions were noted for each system, such as the bands at 684,859,998,1012, 1395, 1531, 1830,and 1966 cm-' for the C1F.C6H6 complex. These bands all lie near vibrational modes of the C6H6 subunit, some of which are infrared active in the parent, and some of which are infrared forbidden in the ~ a r e n t . ' ~A, ~similar ~ statement can be made for the additional weaker product bands observed for B. S . Reu. Chem. Inrermed. 1988, 9, 2 3 3 . ( 3 2 ) Herzberg, G . Infrared and Raman Spectra of Polyatomic Molecules; Van Nostrand: New York, 1945. (31) Ault,
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The Journal of Physical Chemistry, Vol. 94, No. 1 , 1990
the other systems studied. All of these can be assigned to vibrational modes of the perturbed base subunit in its complex with CIF. Previous studies have shown that the base subunit is generally perturbed less than the acid subunit in a molecular complex, so that only small shifts are anticipated. In addition, for the highly symmetric base benzene, the symmetry must be lowered by complexation to CIF. This can lead to activation of otherwise inactive modes. For two systems, bromobenzene and hexamethylbenzene, two doublets were observed in the CIF stretching region. In the C6H5Br + CIF system, the two doublets maintained a constant intensity ratio as the concentration of each reagent was varied. This argues that the absorbers responsible for the two doublets have the same stoichiometry, namely 1 :1. Bromobenzene is the one base in the current study which has two potential basic sites, the bromine atom and the *-electron density of the ring. Consequently, the pair of doublets may well be assigned to the CIF stretching mode of CIF molecules coordinated to either the ring or the Br atom. Determination of which is the more basic site cannot be readily made, and consequently assignment of one doublet to the Br-bound CIF subunit and one to the *-electronbound CIF cannot be made. It is also noteworthy that a number of vibrations of the C6H5Br subunit were perturbed and shifted as well. These include the C-Br stretching mode, as would be expected for a Br-bound complex, and several ring modes, as would be expected for the interaction of CIF with the a-electron density of the ring. These observations support assignment of the two doublets to two different structural isomers of the complex. This isomerism has also been observed by Andrews for H F complexes of substituted b e n z e r ~ e s . ~ It ~ .should ~ ~ , ~ be ~ noted, though, that the separation of the two doublets is 10 cm-l and could be explained by site splitting in the argon matrix. However, site splittings were not observed for any other 1:1 complex and this interpretation cannot readily account for the number of perturbed fundamentals of the bromobenzene subunit. Consequently, assignment to two different structural isomers of the 1:l complex of CIF with C6H5Br is preferred. Some inferences as to the structures of the 1:l complexes may be made from the spectra as well, particularly for the CIF.C6H6 complex. Fredin and Nelander s ~ g g e s t e d ~both , ’ ~ axial and oblique structures for the complexes of the heavier halogens with benzene (and did not study any substituted benzenes). Their evidence for an oblique structure for the complexes of Br2, CI2, and IC1 with benzene is that most of the infrared-inactive fundamentals of benzene were activated in the complex, suggesting a symmetry lower than c6”. On the other hand, only the A,, and El, modes were activated in the I,.C6H6 complex, which was taken as an indication of axial symmetry. Andrews studied26the HFsC6H6 complex in solid argon and concluded that the H F is axially bound to the benzene ring, while Klemperer and c o - ~ o r k e r studied s~~ this complex in a molecular beam and reached the same conclusion. CIF as a Lewis acid has been shown to mirror closely the Bransted acid HF in terms of its ability to form complexes, and the spectra here support an axial structure for the CIF.C6H6 complex. First, the only modes of C6H6 which were perturbed and shifted in the complex were out-of-plane vibrations, particularly of the ring hydrogens. This is anticipated for an axial structure, while for an oblique structure both in- and out-of-plane modes would be perturbed (as was the case for several of the complexes studied by Nelander). Second, the mode of C6H6 which was most strongly activated in the complex (but forbidden in the parent) was u,,, of E,, symmetry. This is precisely the result expected for a reduction in symmetry from &),to c6,, Le., axial complexation. Finally, the magnitude and direction of the shifts of perturbed modes of benzene matched very closely those observed for the HF.C6H, complex, where the structure is well-known. All of these arguments point to a structure which is axial, and an overall C6, symmetry for the complex. Certainly there was no (33) Davis, S . R.;Andrews, L. J . Mol. Srruci. 1987, 157, 103. (34) Baiocchi, F. A.; Williams, J. H.; Klemperer, W. J . Phys. Chem. 1983, 87. 2079.
Bai and Ault TABLE 11: Comparison of the Position of the Cl-F Stretching Mode of ClF Complexed to Substituted Benzenes with the Analogous HF Complexes Aulu x
CiH;CH3 C6H5Br C6(CH3)6 C3H,C,H5 C2HdC a
705 720 710 652 700 662
65 50 60 118 70 108
8.4 6.5 7.8 15.3 9.1 13.3
3781 3800 3775
138 119 164
3.5 4.2 4.2
3730
224
5.7
From ref 20, 26, and 27
evidence obtained for two structural isomers as Fredin and Nelander propose for the complex. Since the chlorine atom in CIF carries a net positive charge, it represents the “acidic” end of the CIF molecule. As such, it is likely that the CIF subunit is oriented with the chlorine end directed toward the .rr-electron density of the ring. For the complexes of CIF with toluene, where a c 6 axis is not present, a conclusion as to the structure of the complex cannot be as definitive. Nonetheless, it is likely that the CIF sits above the *-electron density of the ring and is relatively close to axial but distorted somewhat by the methyl group. Inasmuch as 1:l complexes of CIF with cy~lopropane~~ and with benzene have been isolated, the complex of CIF with cyclopropylbenzenecould involve interaction with either the cyclopropyl group or with the *-electron density of the ring. However, the cyclopropyl group in cyclopropylbenzene is known to stabilize positive charge, increasing the basicity of the phenyl ring.36 Metal ions are known37 to complex with the phenyl ring, and HCI has been shown to hydrogen bond to the a-electrons of the phenyl ring.38 Unfortunately, the most diagnostic mode for such complexation is the out-of-plane C-H deformation mode of the benzene moiety. This mode lies a t 751 cm-I, very near the absorptions of parent monomeric and dimeric CIF. Consequently, if this mode were perturbed and blue-shifted, as was the case for the benzene complex, it would have been obscured by the intense CIF absorptions. Consequently, coordination of the CIF to the phenyl group is quite likely, but cannot be proven in this case. The 1:l complex of CIF with hexamethylbenzene very likely mirrors that of the CIF-C6H6complex, with the CIF subunit axial 6 axis. A 2:1 complex was also to the *-electron density, on the c observed for this system, which in turn has two possible structures. In the first, the two CIF subunits lie on opposite sides of the ring, each on the C, axis. However, coordination of the first CIF to the ring should reduce the basicity of the ring, and the shift of the CIF stretching mode in the 2:l complex should be less than in the 1:1 complex. Alternatively, the 2:l complex might be viewed as a C 1 F dimer complexed to hexamethylbenzene, analogous to complexes of (HF), with many bases.21,22Here, the second H F or CIF polarizes the first H F or CIF somewhat, leading to a stronger interaction with the base, and a slightly larger shift of the H-F or CI-F stretching mode. Since a larger shift was observed for the 2:l complex than for the 1:1 complex, this latter structural arrangement is more likely. While the shifts of the CIF stretching mode in the complexes observed here were similar, some variation was noted with the different bases employed. These variations followed expectations with respect to substituents employed. Methyl groups are known to be electron donating and should increase the basicity of the ring. Toluene produced a larger shift than benzene, and hexamethylbenzene produced the largest shift observed in this study. On the other hand, a bromine atom is electron withdrawing and should reduce the basicity of the ring. This is in accord with the (35) Ault, B. S. J . Phys. Chem. 1986, 90,2825. ( 3 6 ) Nishida, S.; Moritani, I.; Sato, T. J . Am. Chem. SOC.1967,89,6885. (37) Hanlan, A. S.L.; Vgolick, R. G.; Fulcher, J. G.; Togashi, S.;Bocarsley, A . B.; Gladysz, J. A. Inorg. Chem. 1980, 19, 1543. (38) Sass, C.E.;Ault, B. S . J . Phys. Chem. 1987, 91, 3207.
J . Phys. Chem. 1990, 94, 203-208 shift of the CI-F stretching mode in the CIF.C6H5Br complex, which was less than that in the benzene complex. One would expect C6F6 to be the least basic, and for the C1F/C6F6 system, no complex was observed. Table I1 lists the shifts of the CIF stretching mode observed here and contrasts these with shifts of the H F stretching mode in the analogous hydrogen-bonded complexes. Complexes of CIF with alkenes and alkynes in argon matrices were reported recently,20having a tee-shaped structgure in which the CIF interacts with the *-electron density of the multiple bond. The shifts of the CIF stretching mode in these complexes were consistently larger than those observed here, other than the C1FC2H2 complex, which was very similar to the CIF.C6H6 complex. These shifts indicate that the phenyl group with its delocalized ?r-electrons is less basic than a localized carbon-carbon double bond and is similar to a carbon-carbon triple bond. In addition, substitution of a methyl group on the phenyl ring has less effect on the basicity of the ring than does a methyl group adjacent to a double or triple bond, based on the observed shifts.
lB1
-
203
Conclusions The codeposition of CIF benzene and substituted benzenes into argon matrices has led to the formation of 1 :1 Lewis acid-base complexes. These were characterized by the shift of the CIF subunit in the complex, as well as by the perturbation to certain of the base modes. The complex of CIF with C6H6was determined to have axial, or C, symmetry, while a similar mode of interaction was likely for the remaining complexes. Substituent effects on the magnitude of the shift of the CIF stretching mode followed expectations, and the shifts observed here were overall somewhat less than observed for the complexes of CIF with localized double and triple bonds. Acknowledgment. The authors gratefully acknowledge support of this research by the National Science Foundation under grant C H E 87-21969. Registry NO. CIF, 7790-89-8; ClF.C6H6, 123487-09-2; CIF*C,H&H,, 123487-10-5; CIF.C6H,Br, 123487-1 1-6; ClF-C6(CH&, 123487-12-7; CIFC,H5C6H5, 123487-1 3-8; DS,7782-39-0; Ar, 7440-37-1.
'A, Two-Photon Spectrum of Gas-Phase Difluorodiazirine H. Sieber, A. E. Bruno: and H. J. Newer* Institut f u r Physikalische und Theoretische Chemie, Technische Universitat Munchen, Lichtenbergstrasse 4, 0-8046 Garching, West Germany (Received: May 15, 1989)
-
The lB, 'A, two-photon spectrum of gas-phase difluorodiazirine, F2CN2,was measured under Doppler-limited resolution between 26 770-and 3 1 420-cm-' two-photon energy. In two-photon absorption unlike one-photon absorption, all symmetry , point group are allowed; thus, a complete set of excited electronicstate vibrational frequencieswas determined. species in the C Assignment was performed on the basis of a hot-band analysis, discussion of the gross features of the rotational contour, and the polarization behavior of the rotational envelope. The CF bond deformation modes v4 and u7 display no pronounced frequency changes in the excited state, whereas the frequencies of the stretching modes involving the CN bond v2, v3, and up strongly decrease in the excited IB, state. This is in line with the dissociation pathway of F2CN2in the 'BI state leading to F2C:-&d N2.
Introduction Difluorodiazirine, F2CN2, is a five-atom molecule with a three-membered ring. It is known to be a difluorocarbene precursor, which can be smoothly added to olefins to form substituted stereospecific difluorocyclopropanes' or perfluorocyclopropanes.2 It can react either with carbon acids to form esters or with alcohols to form ethers under mild condition^.^ The elementary reaction step is the dissociation of difluorodiazirine
yielding molecular nitrogen and the reactive difluorocarbene radical as an intermediate product. The dissociation can be initiated either by pyrolysis (A) or by photolysis (hu) with UV light. There is some evidence that a linear intermediate during the decomposition of difluorodiazirine is involved! In order to understand the dissociation process and the kinetics of the thermal decompositi~n,~ the spectral properties of difluorodiazirine have been the subject of detailed investigations. Although difluorodiazirine dissociates from the excited state, it displays a sharp absorption spectrum between 290 and 370 nm in the range of the allowed SI(IBI) So('A,) n r * transition. One-photon absorptionb1' and emission12spectra have been taken in this part of the spectrum.
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'Present address: Ciba-Geigy AG, FO 3.2 Analytische Forschung, K127.176, CH-4002 Basel, Switzerland.
0022-3654/90/2094-0203$02.50/0
Difluorodiazirine has C, symmetry in the 'Al and in the 'BI statess It is a strongly asymmetric rotor with K" i= -O).08.10 There exist nine fundamental vibrational frequencies of symmetry a , (4), a2 (l), b, (2), and b2 (2). One-photon absorption spectroscopy yielded an incomplete set of excited-state frequencies, since vibrations of b2 symmetry cannot be excited due to the one-photon selection rules. On the other hand, the symmetry selection rules for the two-photon excitation in principle permit transitions to all nine normal modes;13one expects that the twephoton spectrum should resemble the one-photon spectrum with several additional transitions. Contrary to the one-photon absorption, the two-photon absorption displays a strong dependence of the absorption strength ( I ) Mitsch, R. A. J. Am. Chem. SOC.1965, 87, 758. (2) Mitsch, R. A. J. Heterocycl. Chem. 1964, 1 , 271. (3) Mitsch, R. A.; Robertson, J. E.J. Heterocycl. Chem. 1965, 2, 152. (4) Odgen, P. H.; Mitsch, R. A. J. Heterocycl. Chem. 1968, 5, 41. (5) Neuvar, E. W.; Mitsch, R. A. J. Phys. Chem. 1967, 71, 1229. (6) Mitsch, R. A. J. Heterocycl. Chem. 1964, I , 59. (7) Simmons, J. D.; Bartky, I. R.; Bass, A. M. J. Mol. Spectrosc. 1965, 17, 48. (8) Lombardi, J. R.; Klemperer, W.; Robin, M. B.; Basch, H.; Kuebler, N. A. J. Chem. Phys. 1969, 51, 3 3 . (9) Robin, M. B.; Basch, H.; Kuebler, N. A,; Wiberg, K. B.; Ellison, G. B. J. Chem. Phys. 1969, 51, 45. (IO) Hepburn, P. H.; Hollas, J. M. J . Mol. Spectrosc. 1974, 50, 126. ( 1 1 ) Vandersall, M.; Rice, S. A. J. Chem. Phys. 1983, 79, 4845. ( I 2) Hepburn, P. H.; Hollas, J. M.; Thakur, S. N. J. Mol. Spectrosc. 1915, 54, 483. (13) McClain, W. M. J. Chem. Phys. 1971, 55, 2789.
0 1990 American Chemical Society
