J . Phys. Chem. 1991.95, 2687-2692 to 9 1 1.3 cm-' in oxygen- 18 and has a d 6 / d 8ratio of 1.044 00. Two bands due to P408are observed a t higher energies than P406 at 1002.8 and 994.0 cm-I. These bands shift to 953.2 and 952.0 cm-' with oxygen-18 and have d 6 / v 1 * ratios of 1.041 11 and 1.044 12, respectively. An additional P40s band, due to the terminal oxygen stretching motion, was observed at 1383 cm-I. Perhaps the most interesting aspect of these results is that clusters containing two P2 molecules and two to five O2molecules react to produce the P4oxides containing even numbers of oxygen atoms unlike the reaction of P2 with (02), or (02), that can eliminate an oxygen atom to produce the stable P203 or P 2 0 5molecules. In the case of the (P2)(02)3reaction, the lack of a +6 oxidation state for phosphorus requires the expulsion of an oxygen atom and the stable P205molecule results. Conclusions Ultraviolet photolysis of P, and 1% or 2% O2in argon matrices produced oxo-bridged P203and two isomers of oxo-bridged P204. Diphosphorus trioxide is a new species identified from this work for the first time. Product identification is based on spectroscopic and mechanistic evidence. The reaction of P2 in 5% oxygen matrices produced P205and P40, ( n = 4, 6 , 8, 10) in addition to P203 and the two P204 isomers. The concentration dependence of the reaction provides evidence that the reaction proceeds through clusters of the form (P2),(02), where x = 1, 2 and y = 2-5. It is interesting to note that, even at 12 K, P2 and dioxygen clusters react to produce the thermodynamically favored closed-shell products regardless of the size of the cluster. Radicals like PO, PO,, and PO, are absent from the spectra, as are the energetically less favored P-P bonded isomers of P2O3 and P2O4. Clearly, there is sufficient conversion of chemical potential energy of the (P2)(02)2cluster to intramolecular vibrational and rotational energy to account for the ejection of an oxygen atom to form P203and isomerization of P2O4, but the exothermicity is insufficient to dissociate the P204 molecule into PO2 fragments that exit the matrix cage. This is in direct contrast to the spontaneous matrix reaction of P2 and ozone, where the primary products are small free-radical fragments PO, PO,, PO3, and P 2 0 , but P203 is not observed, even though it is the stoichiometric product.I6 The reaction of P2 and oxygen is initiated by dipole-forbidden states of the phosphorus dimer, br3Z[, W'A,, and A'II,, which become partially allowed by interacting with oxygen in a reactive cluster. The reaction is 4 times as prolific with 220-1000-nm photolysis than it is with 290-1000-nm radiation, reflecting the stronger transition moment of the A'II, state of P2.
2687
The vibrational fundamentals of the transient PZO3, P204(X,Yr), and P205molecules are listed in Table 111. The frequencies and the relative intensities of the observed bands are compared to ab initio calculated spectra? Three modes, the antisymmetric P-0-P stretch and the symmetric and antisymmetric -PO2 stretching motions, are critical in the identification of the P20, ( n = 3,4, 5 ) oxides. The antisymmetric -PO2 stretch is a stalwart band that has little interaction with the rest of the molecule. The frequencies of this band for HOPO,, CIPO,, P204(X,Y,Y'), and P205occur in the 1450-cm-I region and show little effect from the adjacent groups. However, the antisymmetric P U P stretch is sensitive to the oxidation state of phosphorus and occurs a t higher wavenumbers when phosphorus is in the pentavalent oxidation state. The band appears at 859.4 cm-' for P203 and a t 916 and 1008 cm-' for P204isomers, and a t 970 cm-' for P205. This band is heavily mixed with the symmetric -PO2 stretch in P20, and the out-of-phase symmetric -PO2 stretch in P205.The intensity of the antisymmetric P-O-P stretch, which is characteristically high, is decreased in molecules where such mixing occurs. In P203the P-0-P stretch is a relatively pure motion and the band shows an extremely high intensity; this intensity is lowered in P204(X)and P205while the intensity of the symmetric -PO2 stretch is increased. Perhaps the symmetric -PO2 motion, which would otherwise be weak, borrows intensity from the P U P stretch. These observations are in agreement with calculated spectra as wek9 The matrix isolation technique, conceived over 30 years ago for the study of reactive species,29has proven its usefulness for the isolation and characterization of highly reactive phosphorus oxide species generated, but not characterized, by Robert Boyle over 300 years ago.' In addition to being a medium for the physical and chemical isolation of reactive molecules, the matrix effectively quenches reaction exothermicity and allows new molecules to be trapped that would ecompose in the analogous gas-phase process.
Acknowledgment. We gratefully acknowledge financial support from N.S.F. Grant CHE 88-20764 and helpful discussions with L. L. Lohr, Jr. L.A. is grateful to George Pimentel for getting him started in matrix isolation spectroscopy. Registry No. P2,12185-09-0; O,,7782-44-7; P203, 1314-24-5; P204, 12137-38-1; P404, 131794-01-9; P406,12440-00-5; P408,70983-17-4; P4OI0,16752-60-6; P205, 1314-56-3. (29) Whittle, E.; Dows, D. A.; Pimentel, G. C. J . Chem. Phys. 1954, 22, 1943.
Infrared Matrix Isolation Studles of Molecular Interactions: Alkynes with Halide Anlons Mei-Lee H. Jeng and Bruce S. Auk* Department of Chemistry, University of Cincinnati, Cincinnati, Ohio 45221 (Received: August 7. 1990) The matrix isolation technique has been combined with the salt/molecule technique to isolate and characterize the products formed between a series of I-alkynes and the cesium halides. When acetylene was employed, hydrogen-bonded complexes of the form C2H2-.X- were formed, with frequency shifts of the hydrogen-bonded C-H group which were much greater than for analogous complexes with neutral bases. When alkynes containing electron-withdrawing groups were employed, complexes involving nucleophilic attack rather than hydrogen bonding were observed, although for the CsF/CF,CCH system evidence was obtained for both types of complexes. When an alkyne containing an electron-donating group was used, only the hydrogen-bonded complex was observed. Introduction Hydrogen bonding is a very important intermolecular interaction and as such has generated substantial interest over the years.' Recent studies from this laboratory have investigated the hydrogen bonding of alkynic C-H groups to a number of lone To whom correspondence should be addressed.
0022-3654/91/2095-2687$02.50/0
electron pair donors containing N, 0, P, S,As, and Se While the observed complexes were weakly bound, they dc"n(1) Pimentel, G. C.; McClellan, A. L. The Hydrogen Bond; W.H. Freeman
co.: San Francisco, 1960.
(2) Delaat, A. M.; Auk, B. S.J. Am. Chem. Soc. 1987, 109, 4232. (3) Jeng, M.-L.; Delaat, A. M.; Ault, E. S.J. Phys. Chem. 1989,93,3997.
0 1991 American Chemical Society
2688 The Journal of Physical Chemistry, Vol. 95, No. 7, 1991 strated all of the characteristics of hydrogen bonding. Halide anions are among the strongest bases known, based on gas-phase proton affinities, and are known to participate in hydrogen bonding.6 The interaction of a halide anion, particularly F,with an alkyne might well lead to the strongest hydrogen bond involving a C-H group. The salt/molecule reaction technique,"" in conjunction with matrix isolation, was developed to study the product of halide anion transfer from an alkali-metal halide salt to either a Bronsted or Lewis acid anionic species formed in this manner include the (HF2)-, C O F r , and SiFf anions, each ion paired with the alkali-metal cation. These earlier studies demonstrated that the greatest yields arose with the cesium cation and that the spectrum of the anion was largely unaffected by the presence of Cs+. For the unsymmetrical hydrogen bihalide anions, (XHX')-,two different product anions were postulated, corresponding to a double-minimum potential. In these cases, the cation may have served to stabilize one of the product anions, in that recent gas-phase and theoretical studies of these anions have only reported a single species. The acidity of an alkynic hydrogen is much less than that of a hydrogen halide, so whether the interaction of X-with an alkyne will parallel that of X-with a hydrogen halide is not apparent. Also, the fact that alkynes may undergo nucleophilic attack at the carbon-carbon triple bond has been known and widely used in organic chemistry for many years.I2 Halide anions are strong nucleophiles so that nucleophilic addition may also occur. Recently, researched3J4have calculated the structure and energetics of isomers of C 2 H 2 Fby ab initio techniques. Schleyer and K0d3 reported that the hydrogen-bonded complex was bound by approximately 20 kcal/mol while the 8-fluorovinyl carbanion was found to be unstable with respect to C2H2+ F.More recently, Roy and McMahonI4 showed that while the hydrogen-bonded species C2H2.-F is the most stable structure, the a- and 0fluorovinyl carbanions are also bound species. The matrix isolation techniqueI5J6was developed to isolate and characterize reactive intermediates and should be able to isolate the products of the reaction of X- with a series of alkynes. Consequently, a study was undertaken to determine whether hydrogen-bond formation or nucleophilic attack would occur, as a function of the substituent on the alkyne.
Experimental Section All of the experiments in the current study were performed using a standard matrix isolation apparatus which has been described." The alkali-metal halide salts employed in this study were CsF, RbF (both Alfa), CsCl (Fisher), and CsI (Aldrich). C2H2 (Matheson), C2D2(MSD), and CF3CCH (PCR) were purified by vacuum distillation and several freeze-thaw cycles at 77 K. Propargyl chloride (CH,ClCCH) and tert-butylacetylene (both Aldrich) were purified by one or more freeze-thaw cycles. CF3CCD was prepared by leaving CF3CCH vapor in contact overnight with a 50% NaOD/D,O (MSD) solution.'* Argon was used as the matrix gas in all experiments and was used without further purification. (4) Jeng, M.-L.; Auk, B. S.J . Phys. Chem. 1989, 93, 5426. (5) Jeng, M.-L. H.; Ault, B. S.J. Phys. Chem. 1990, 94, 1323.
(6) Allerhand, A.; Schleyer, P. v. R.J . Am. Chem. Soc. 1963.85, 1233. (7) Ault, B. S.Matrix Isolation Studies of Alkali Halide Molecules With Lewis Acids and Bases. ACS Symp. Ser. 1982, No. 179, 327. (8) Ault. B. S.Inorg. Chem. 1982. 21, 756. (9) (a) Auk 9. S.J . Phys. Chem. 1978,82,844. (b) Ault, 9. S.J . Phys. Chem. 1980,84, 3448. (10) Auk, B. S.J . Phys. Chem. 1979,83, 837. ( 1 I ) Ellison, C. M.; Ault, 9. S.J . Phys. Chem. 1979, 83, 832. ( 1 2) Dickstein, J. I.; Miller, S.I. In The Chemistry of the Carbon-Carbon Triple Bond; Patei, S.,Ed.; John Wiley and Sons: New York, 1978; Part 2, Chapter 19. (13) Schleyer, P. v. R.; Kos, A. Tetrahedron 1983, 39, 1141. (14) Roy, M.; McMahon, T. B. Can J . Chem. 1985.63.708. (15) Craddock, S.;Hinchliffe, A. 3. Matrix Isolation; Cambridge University P r a : New York, 1975. (16) Andrews, L. Annu. Reu. Phys. Chem. 1971, 22, 109. (17) Ault, B. S.J . Am. Chem. SOC.1978, 100, 2426. (18) Berney, C. V.; Cousins, L. R.;Miller, F. A. Spectrochim. Acto 1%3, 19. 2019.
Jeng and Ault
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F w 1. Infrared spectra, over selected spectral regions, of the products arising from the codeposition of CsF with Ar/C2H2 (bottom trace) compared to blank spectra of each reagent alone in solid argon (top traces). The alkali-metal halide salts were loaded in a stainless steel Knudsen cell which was placed in a resistively heated oven within the vacuum jacket of the closed cycle refrigerator. The vacuum jacket was evacuated and the oven heated to a temperature slightly lower than that to be used for deposition, to outgas the material. The vaporization temperature was about 500 OC for CsF, CsCl, and CsI and 520 OC for RbF, which corresponded to about a 1-pm vapor pressure for the cesium salts and somewhat less for RbF. The samples were deposited onto a CsI cold window held around 15 K at a rate of 2 mmol/h for 20-24 h. Infrared spectra were recorded on a Perkin-Elmer 983 infrared spectrophotometer at 2-cm-' resolution.
Results Before the reactions of the alkali-metal halide salt molecules with the alkynes were investigated, blank experiments were conducted for each of the reagents alone in argon. The resulting spectra were in good agreement with literature spectra, as well as with spectra recorded previously in this laboratory. CsF was characterizedi9by an intense monomer band near 3 13 cm-' and dimer bands at 205 and 245 cm-I. CsCl and CsI have no absorptions above 200 cm-' but could be monitored by observing the known bands" of CsCl.H20 and CsI-H20from the reaction with residual, impurity H20.RbF monomer was observedZoat 347 cm-I, with the dimer absorbing at 233 and 270 cm-I. C2H2Reactions. The deposition of CsF with C2H2over a range of concentrations gave rise to braod bands of medium intensity at 2800,2856,2873, and 3325 cm-l with a shoulder at 2848 cm-l band. A medium-intensity feature was observed at 1920 cm-l, along with a weak doublet at 1931, 1937 cm-'. In addition, sharp new bands were noted at 632, 636, 920, and 1778 cm-I. Increasing the concentration of either reagent intensified most of the product bands, but not the 2800-cm-l band. Also, the 632, 636-cm-' doublet was dominated by the 636-cm-' component. CsF was d e p o s i t e d with samples of Ar/C2D2 in three experiments and led to new absorptions at 694,2226,2246,2280, and 2296 cm-I. Also, the intensity of the parent C2D2band at 514 cm-I was apparently increased. Figure 1 shows representative spectra for this pair of reactants. CsCl was codeposited with a sample of Ar/C2H2 = 500 and gave rise to two intense absorptions at 3050 and 3338 cm-'. An intense absorption was also observed at 1951 cm-' along with a weak feature at 1944 cm-' and a shoulder at 1940 cm-l. Bands due to the C2H2.H20 complex at 784 and 794 cm-' were broadened by apparent new, weak features near 816 and 832 cm-I. Also, a weak C2H2parent absorption at 634 cm-' was greatly enhanced, with a maximum at 633 cm-I. When the Ar/C2H2 concentration was increased to 200/1 and the CsCl level was kept constant, all of the product bands were intensified. When the C2H2 (19) Auk, 9. S.; Andrews, L. J . Am. Chem. Soc. 1976, 98, 1591. (20) Schaber, H.; Martin, T. P. J . Chem. Phys. 1979, 70, 2029.
The Journal of Physical Chemistry, Vol. 95, No. 7, 1991 2689
Interactions of Alkynes with Halide Ions
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concentration was reduced to 1000, or the CsCl vaporization temperature decreased, the same product bands were observed, with lower intensity. C2H2was also codeposited with CsI and led to a broad band of medium intensity at 1955 cm-l with a weak band at 1950 cm-I. The absorptions of C2H2.H20at 784 and 794 cm-I were broadened with a new maximum at 782 cm-l. A medium-intensity feature was observed at 720 cm-I, along with a strong absorption at 628 cm-I and a weak band at 643 cm-I. In the C-H stretching region, a broad band with maxima at 3182 and 3 188 cm-I was observed, along with a weak band at 31 36 cm-I. Increasing the C2H2concentration intensified all of the product bands except the 3 136-cm-' band. CF3CCH Reactions. The codeposition of a sample of Ar/ CF3CCH = 2000 with CsF resulted in a weak doublet at 3225, 3232 cm-', near the absorption of parent dimer at 3218 cm-l. In addition, a distinct, broad absorption was noted at 2637 cm-I along with nearby doublet a t 2657, 2666 cm-I. Near the parent triple-bond stretch, new bands were observed at 2050 (weak), 2068 (sharp, intense), 2076 (sharp, medium), and 2098 cm-I (broad), along with a shoulder a t 21 10 cm-'. A very intense, sharp absorption was noted at 1848 cm-l while in the C-F stretching region, the parent 1245-cm-l band was broadened with two shoulders at 1250 and 1254 cm-I. New bands were noted at 1139, 1142, and 1146 cm-l near the parent 1147-cm-' band. The parent shoulder at 1161 cm-' was seen as a distinct, sharp band. When the concentration of CF3CCH was increased to 500/ 1 or 200/ 1 , the same set of product bands was observed, with an increase in intensity. Additional shoulders were observed at 1840, 1842, and 3220 cm-l. Spectra are shown in Figure 2 for this pair of reactants. CsF was also codeposited with samples of Ar/CF,CCD at several different concentrations between 200/ 1 and 2000/ 1 . In each case, some CF,CCH was also present due to exchange so that the bands reported above for the CsF/CF,CCH system were observed. In addition, a distinct band was observed at 1 150 cm-l which broadened the parent C-F stretches at 1147 and 1166 cm-I, along with a very weak absorption a t 1877 cm-I. The intensity ratio of parent bands at 141 8 and 1335 cm-I increased from 1 / 3 to 1 / 1 and even to 4/1 in the most dilute experiment. Increasing the concentration of CF3CCD led to a more distinct 1877-cm-' band, along with bands at 1810 and 1820 cm-l. For comparison, CF3CCH was codeposited with RbF in two experiments. The concentration of RbF in these experiments was distinctly less than CsF in the above experiments. A new, intense absorption was observed at 1852 cm-l, along with weak bands at 2050,2068,2100,21 IO, 3225, and 3232 cm-I. Broadening of the parent bands at 1146, 1166, and 1245 cm-' was noted as well. A sample of Ar/CF3CCH = 2000 was codeposited with CsCl and led to an intense absorption a t 2132 cm-l which overlapped an absorption of CF3CCH dimer. Weak features were seen a t
21 15,3170 (broad), and 3198 cm-I. Broadening of the 1166-cm-I parent band was also observed, along with enhancement of the 1127- and 1132-cm-l shoulders. When the concentration of CF3CCH was increased, the same set of product bands was observed with greater intensity. Annealing the sample also led to growth of the new absorptions. CsI was codeposited with CF3CCH in argon and gave rise to new weak absorptions at 3123 and 3180 cm-'. A new peak was noted at 2143 cm-' that overlapped and intensified the parent bands at 2138 and 2146 cm-'. Broadening of the parent 1166-cm-' band was noted, with a sharp, weak band at 1161 cm-I. A new sharp absorption was detected at 1 1 10 cm-l along with a weak band at 1125 cm-I. CsI was also codeposited with samples of Ar/CF3CCD = 2000 and 500. In addition to the product bands reported above (due to residual CF3CCH), the intensity ratio of the parent 1975/1968-cm-I bands was altered, with the 1968 cm-' now being more intense. Two shoulders a t 1954 and 1960 cm-I and a weak band at 1935 cm-I were also observed. CH2C1CCH Reactions. The codeposition of a sample of Ar/CH,CICCH = 500 with CsF gave rise to a broad band at 2923 cm-l along with a medium absorption at 2084 cm-I and a shoulder at 2096 cm-l. The parent 718-cm-l band was broadened, and a weak band at 700 cm-' was noted. An increase of the CH2CICCH concentration to 200/1 led to more intense product bands, while a reduction in CsF concentration led to a reduction in intensity. The codeposition of propargyl chloride with CsCl resulted in a weak feature at 3170 cm-I and a medium, broad absorption at 2926 cm-l. A medium-intensity doublet was observed at 21 18 and 2 122 cm-I, while the 7 18-cm-I parent band was broadened by a broad band with a maximum at 698 cm-I. The codeposition of CHzCICCH with CsI gave rise to a very broad absorption of medium intensity at 2930 cm-l and an intense band at 2128 cm-' with a shoulder at 2122 cm-'. A broadening of the 718-cm-I band was again noted. Figure 3 shows spectra resulting from the codeposition of propargyl chloride with these cesium halides. (CH3),CCCH CsF. When CsF was codeposited with samples of Ar/(CH,),CCH + 500 and 200, no new distinct product bands were observed in the C-H stretching or bending regions. A very weak feature was observed at 2075 cm:l which grew when the CsF concentration was increased. Tables I and I1 summarize the band positions for the reaction of the cesium halides with C2H2 and CF3CCH, respectively.
+
Discussion The codepositJon of the cesium halides and RbF with the alkynes in this study produced a number of new infrared absorptions that cannot be attributed to either parent species and must be assigned to one or more reaction products. Since there are multiple possible product species, and since the spectra were
2690 The Journal of Physical Chemistry, Vol. 95, No. 7, 1991
1 CH$lCCH.
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CsCl 3338 3050
assignment C-H sym str 3182 C-H antisym str 3136 C-H str, aggregate 1944, 1951 1950, 1955 C C triple bond str C-H-X- bend, overtone 816,832 782 C-H-X- bend 633 628 C-H sym bend 333
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Infrared-inactive modes for parent C2H2. 'Most intense component of site split triplet. C l nkcal/mol.
TABLE 11: Band Positions and Assignments for the Products of the Reaction of the Cesium Haliden witb CF3CCH band position, cm-I parent CsF CsCl Csl 3310 3225, 3232 3170, 3198 3123, 3180 2637O 2158 2098 2076 2050, 2068 2115, 2132 2143 1848 1245 1254 (sh) 1161 1166 1161 1110, 1125 1147 1139, 1146
assignment C-H str, complex A C-H str, complex B C - C str, complex Bb C-C str, complex Cb C - C str, complex Ab C-H str, complex Cb C-F SIP C-F strC C-Fstf
Most intense component of triplet. bTentative assignment; see text. CProximityto parent bands preclude definite assignment to a particular complex.
relatively complex, reactions with each alkyne will be considered separately. C2H2Reactions. The new absorptions observed after codeposition of C2H2with the cesium halides generally fall into three groups: to lower energy from the C-H stretching modes and the carbon-carbon triplebond stretch and to higher energy from the hydrogen bending modes. The magnitude of the shift from the parent mode varied with the alkali-metal halide, as shown in Table I, but this grouping holds for all three halides. These shifts are precisely the infared spectral characteristics of hydrogen bonding,' as is the observed broadening and intensification of the stretching mode of the alkynic hydrogen. Given the very high basicity of the halide anions, and the previous results of the reaction of the cesium halides with hydrogen halides to form the hydrogen bihalide anions,' it is appropriate to assign the present product absorptions to a hydrogen-bonded complex. These product ab-
Jeng and Ault sorptions remained distinct although less intense in the high dilution experiments, suggesting a 1:l stoichiometry for the complex. Assignment of most of the product absorptions is straightforward, based on relative proximity to the parent modes. The bands near 2860, 3050, and 3182 cm-' in the CsF, CsCl, and CsI experiments, respectively, were broad, intense, and to the red of the parent antisymmetric C-H stretching mode at 3285 cm-I. These can readily be assigned to the hydrogen-bonded C-H stretching mode in the C2H2-X- complex. The shift of the band near 2860 cm-' to near 2250 cm-' upon deuteration supports this assignment. An additional band at 2800 cm-' in the CsF/C2H2experiments and 3 136 cm-'in the CsI/C2H2 experiments did not show the same intensity variation with reactant concentration as the dominant absorption and is best assigned to an aggregate species. The bands between 1920 and 1955 cm-' and between 780 and 920 cm-' are readily assigned to the shifted carbon-carbon triple-bond stretch and the bending mode of the hydrogen-bonded hydrogen, based upon the direction and the magnitude of the shifts. The multiplet structure in the 1900-cm-' region could be indicative of the formation of more than one type of complex, but with splitting of only a few wavenumbers, matrix site splitting appears more likely. Moreover, a second distinct complex should have a number of infrared absorptions, yet all that were observed can readily be assigned to the hydrogen-bonded complex. Consequently, site splitting is the most likely origin of the multiplet structure. For each system, two new bands were also observed near 630 and 3330 cm-I, close to infrared-inactive modes of C2H2 For the hydrogen-bonded complex, these correspond to the bend and stretching motions of the hydrogen not involved in the hydrogen bond. Interaction with the halide anion is clearly sufficient to detectably activate these vibrational modes. The shift of the 636and 920-cm-' bending modes to 5 14 and 694 cm-' upon deuteration supports these assignments. Since both of the bending modes are degenerate vibrations, the appearance of a single band for each indicates that the complex has axial or nearly axial symmetry and hence a linear hydrogen bond. One product band for the CsF/CzH2 system remains unassigned, the band at 1778 cm-I. Several explanations may be put forth for this absorption, the most probable being the overtone of the bending mode of the hydrogen-bonded hydrogen. It has been shown2',22that there is a significant intensity enhancement to the overtone of the bend in hydrogen-bonded systems as a consequence of the relatively large amplitude of motion and charge mobility, leading to a high dipole moment derivative and high intensity. In some systems, the overtone of the bend can actually have higher intensity than the fundamental. The fundamental was observed here at 920 cm-I, with an intensity somewhat greater than the 1778-cm-' band. Since this mode, particularly the overtone, should be quite anharmonic. the ratio 1778/920 = 1.93 is certainly reasonable. The lack of observation of a deuterium counterpart for this band is understandable in view of the greater mass of the deuterium atom, lower amplitude of vibration, and better approximation to the harmonic oscillator selection rules. Alternative explanations for the 1778-cm-' band include assignment to the fluorovinyl anion (C,H2F) through F attachment to acetylene or the the acetylide anion through full proton abstraction to form H F Cs+C2H-. However, comparison of the one band here with the known ~ p e c t r u m vof . ~C2H~ rules out that possibility, as does the lack of HF in the matrix spectrum. On the basis of a variety of fluoroalkenes, one can estimate the spectrum of the fluorovinyl anion, and a good match is not found. Finally, one could envision a second hydrogen-bonded form of the complex, analogous to the type I (asymmetric) and type I1 (nearly symmetric or shared) hydrogen bond. The complex assigned above is undoubtedly asymmetric; the type I1 complex should have a very intense hydrogen stretching motion, along with several ad-
+
(21) Thompson, W.E.; Pimentel, G. C. Z. Electrochem. 1960, 61, 748. (22) Nibler, J. W.;Pimentel, G. C. J . Chem. Phys. 1967, 47, 710. (23) (a) Goubeau, V.; Beurer, 0.Z. Anorg. Allg. Chem. 1961,310, 110. (b) Nast, R.;Gremm, J. Z. Anorg. Allg. Chem. 1963, 325, 62. (24) Saykally, R. J. Talk 76, Physics Division ACS National Meeting, New Orleans, 1987.
Interactions of Alkynes with Halide Ions ditional intense absorptions. The 1778-cm-l band would be very tenuous evidence at best for such a complex. Overall, assignment of the 1778-cm-I band to the overtone of the hydrogen bending motion of the hydrogen bond is most appropriate. The frequency shifts of the C-H stretching mode in the hydrogen-bonded complex, hereafter called us, are consistent with the basicity of the halide anions employed here as measured by their gas-phase proton affinities2s (listed in Table I). The magnitude of shift is often correlated with the strength of interaction, and within the three halide anions this trend holds nicely. The trends in the magnitude of shift of the carbon-arbon triple bond stretch and of the hydrogen bending modes are also consistent with the C-H stretching mode. The shifts for the F and Cl- complexes are distinctly larger than for previously studied complexes of acetylene with neutral bases, as would be anticipated from proton affinities. However, the shift Aus in the CzH2.1- complex was on the same order as that in the C2H2.N(CH3), complex even though the proton affinity of I- is approximately 90 kcal/mol higher than that of N(CH3),. An earlier study5 demonstrated that alkynes will hydrogen bond more strongly to hard bases than soft bases; since I- is a very soft base and N(CH3), is a hard base, this may account for the comparable shifts. Nonetheless, these data suggest that the F and C1- complexes with C2H2are significantly more strongly bound than the analogous amine complexes, while the I- complex is similar in strength to the amine complexes. CH2CICCH Reactions. The codeposition of each of the cesium halides with propargyl chloride resulted in spectra that were very similar to one another, with product absorptions near 700, 2100, and 2920 cm-l. The first two lie very close and to the red of the parent C - C l and stretching modes and are readily assigned as such in a complex between the two reagents. The last band, near 2920 cm-I, showed only a small shift from one halide anion to the next, in contrast to the acetylene complexes. This is not the result anticipated for a hydrogen-bonding interaction to the alkynic hydrogen. Also, the shift from the alkynic C-H stretch of the parent was less than for the C2H2--Fcomplex, while a greater shift would have been expected on the basis of previous studies: if the interaction involved hydrogen bonding. In addition, the shift of the C-CI stretching mode was approximately 20 cm-’, which is much larger than would be expected if the interaction was through the alkynic hydrogen. Finally, a blue-shifted hydrogen bending mode would be expected for a hydrogen-bonded complex yet was not observed. The above observations all argue for an interaction which does not involve hydrogen bonding. Moreover, the large shift of the C-Cl stretching mode suggests an interaction at the chloromethyl group. With this in mind, the band observed near 2920 cm-’ for each complex can readily be assigned to the perturbed, red-shifted CHz antisymmetric stretching mode of the CH2CI group down from the parent band position of 2960 cm-I. Nucleophilic substitution of a haloalkane by another halide anion is well-known, so that interaction of the halide nucleophile with propargyl chloride may be occurring at the carbon atom of the chloromethyl group. Such an interaction would lead to a red shift of the C-CI and CH, stretching modes, and also to a small perturbation to the carbon-carbon triple bond, all of which were observed. The magnitudes of the shift of all three perturbed modes were in the order F > Cl- > I-, which is consistent with the nucleophilicity of the halide anions in the gas phase as well as the reactivity of halide anions toward alkyl halide^.^'*^* The precise geometry of the complex, particularly the location and role, if any, of the cation cannot be determined. Nonetheless, the interaction appears to be one involving the halide anion and the carbon of the CH2CI group. (25) Huheey, J . E. Inorgic Chemistry: Principle of Structure and Reactivity, 3rd ed.; Harper and Row: New York. 1983; p 301. (26) Pearson, R. G. Hard andSoft Acids and Bases; Dowden, Hutchinson and Ross: Stroudsburg, PA, 1973. (27) Olmstead, W. N.; Brauman, J. 1. J . Am. Chem. Soc. 1977,99,4219. (28) Tanaka, K.; Mackau, G. 1.; Payzant, J. D.; Bohme, D. K. Can. J . Chem. 1976, 54, 1643.
The Journal of Physical Chemistry, Vol. 95, No. 7, I991 2691
CF3CCHReactions. The reaction of trifluoropropyne with the cesium halides gave rise to a number of product absorptions, as listed in Table 11. As can be seen, reaction with CsF (and RbF) gave rise to more numerous absorptions, suggesting that more than one type of complex was formed with F. This is consistent with the higher proton affinity, nucleophilicity, and reactivity of F. Product bands common to all of the cesium halides occurred to the red of the alkynic C-H stretching mode, in the carbon-carbon triple bond stretching region, and in the CF, stretching region. Several points indicate that these features cannot be assigned to a hydrogen-bonded complex. First, the shift of the C-H stretching mode was quite small, 100-150 cm-I, while a much larger shift would have been anticipated. In hydrogen-bonded complexes with neutral bases, CF3CCH consistently showed the greatest shift as a consequence of the electron-withdrawing ability of the CF3 group. Shifts greater than 500 cm-’ would have been anticipated for a hydrogen-bonded complex between CF3CCH and F. Second, the order of the shifts of the C-H stretch is reversed relative to the proton affinity of the halide anions. As a result, a hydrogen-bonding interaction can be ruled out for these product absorptions. The carbon-carbon triple bond is subject to nucleophilic attack, and this should be enhanced by the presence of the electronwithdrawing CF3 group. Researchersz9 have found that nucleophiles with high polarizabilities have high reactivity toward alkynes, which supports the order of interaction strength, namely, I- > CI- > F. In view of the fact that a hydrogen-bonding interaction cannot account for these three bands, and since nucleophilic attack is quite feasible, these absorptions are assigned to a molecular complex in which the halide anion is interacting with the carbon-carbon triple bond (hereafter complex A). The lack of shift upon substitution of RbF for CsF suggests that the cation is not playing a significant role in the complex. Several additional product absorptions were observed upon codesorption of CsF with CF3CCH, including a broad, distinct absorption near 2640 cm-I, and several absorptions in the triple-bond stretching region. The 2640-cm-’ band shows all of the hallmarks of a hydrogen-bonded complex and falls very close to the position predicted for the CF3CCH-.F complex from earlier complexes of alkynes with neutral bases and the C2HZ.-F complex. Moreover, a deuterium counterpart was located near 1877 cm-I, confirming the hydrogenic nature of the vibration. Consequently, this band and one of the bands in the triple-bond stretching region are assigned to the hydrogen-bonded complex between CF3CCH and F,analogous to the C2H2-F complex (hereafter complex
B)*
One major product absorption for the CsF/CF,CCH reaction pair remains unassigned, the very intense band at 1848 cm-I, with its RbF counterpart at 1852 cm-I. This band appeared to shift to 1418 cm-I; parent CF3CCD absorbs weakly at 1418 cm-I, but this band grew dramatically relative to other parent bands upon codeposition with CsF, indicating that a product band fell at the same location. The ratio 1848/1418 = 1.30 is certainly appropriate for a hydrogenic vibration, either a stretch or bend involving the alkynic hydrogen. Its location, howbver, makes assignment to either complex A or complex B difficult. It could be assigned to the overtone of the bending mode of complex B, yet its high intensity and the absence of a fundamental near 950 cm-’ suggest that this is not likely. One possibility is assignment to a second hydrogen-bonded complex, analogous to the type I and type I1 complexes of the unsymmetrical hydrogen bihalides. The electron-withdrawing ability of the CF, group may increase the acidity of the alkynic hydrogen sufficiently to give rise to both types of hydrogen-bonded complexes. Such a complex would be expected to have the C-H bending mode at quite high energies, probably above 1000 cm-l. The region above 1100 cm-l was largely obscured by the intense parent C-F stretching modes, and the bending mode of this complex might well have been hidden. Some shoulders and weak new features were observed in the region that could possibly be attributable to the bending mode. Nonetheless, (29) Bunnett, J. F. Annu. Rev. Phys. Chem. 1963, 14, 271.
J. Phys. Chem. 1991, 95, 2692-2696
2692
it is very difficult to identify a new product species on the basis of one infrared absorption, and the above discussion of a type I1 complex (complex C) must be regarded as tentative. For the F/CF$CH system, several absorptions were observed in the region 2050-2100 cm-I and are assignable to the triple-bond stretching modes of complexes A, B, and C. However, to uniquely identify which band is associated with which complex is very difficult, and the assignments indicated in Table I1 must be regarded as very tentative. CsHloReactions. tert-Butylacetylene was codeposited with CsF in several experiments, and one weak feature was observed at 2075 cm-l. This is undoubtedly due to a perturbed triplebond stretching vibration. The tert-butyl group is electron-donating in nature, which should lower susceptibility to nucleophilic attack, as well as hydrogen-bonding ability. Earlier studies of tert-butylacetylene with neutral bases have identified hydrogen-bonded complexes with shifts of us less than for the corresponding acetylene complexes. If the product complex involved nucleophilic attack of the triple bond, then a slightly red-shifted alkynic C-H stretch would be anticipated, as for species A, above. This should fall around 100 cm-' to the red of the parent mode and was not observed. If, instead, the complex were hydrogen bonded as in species B, then a shift of some 300-400 cm-I would be anticipated (somewhat less than the 425cm-' shift for the C2H2-F complex). This would shift the alkynic C-H stretching mode to between 2900
and 3000 cm-I, a region obscured by the stretching modes of the three methyl groups of the parent alkyne. Consequently, assignment to a hydrogen-bonded complex fits the available data more effectively and is so tentatively assigned. Summary
The codeposition of CsF, CsCI, and CsI with several alkynes has led to the formation of molecular complexes. For the simplest alkyne, acetylene, a hydrogen-bonded complex was observed for each halide anion, with shifts that were consistent with the basicity of the halide anion. The complexes of CH2CICCH, in contrast, appeared to involve nucleophilic interaction of the halide anion with the carbon of the chloromethyl group. The interaction of CsCl and CsI with CF3CCH also led to a nucleophilic interaction, while the interaction of F with CF3CCH led to hydrogen-bonded and nucleophilic complexes. Acknowledgment. The authors gratefully acknowledge support of this research by the National Science Foundation, under Grant C H E 87-21969. Most importantly, the interest in hydrogen bonding and enthusiasm for chemistry of George Pimentel, which he shared with us, is fondly remembered. Registry No. HCECH, 74-86-2; (CH,),CC=CH, 917-92-0 F3CC e H , 661-54-1; CICH2*CH, 624-65-7; CSF, 13400-13-0; CSCI, 7647-17-8; CSI, 7789-17-5.
Generation of Excited NCI by the Reaction of Hydrogen Atoms with NC13 D.B. Exton, J. V. Gilbert, and R. D. Coombe* Department of Chemistry, University of Denver, Denver, Colorado 80208 (Received: August 8, 1990)
Discharge-flow methods were used to study the reaction of gaseous NC13 with e x w s hydrogen atoms. The system produces electronically excited NCl(alA,b'Z+) by a two-step mechanism in which the hydrogen atoms first react with NClp to produce HCI and NCI2 and then with NC12 to produce HC1 and excited NCI. From measurements of the time dependence of NCI b ' F X3Z- chemiluminescence in the system, the two rate constants were found to be (9 f 4) X IO-" cm3 s-I and (4.0 f 0.4) X cm3s-I, both at 300 K. Based on the rates of analogous reactions with NF3 and NF2, the larger rate is tentatively assigned to the H + NC12 reaction. The branching fraction for production of NCl(aIA) by H + NCI2 has a lower limit of 0.15. The propensity for production of NCl(alA) by H + NC12is considered in terms of an addition-dimination mechanism similar to that operative in the analogous H + NF2 reaction.
-
Introduction The production of excited singlet N F by the reaction of hydrogen atoms with NF2 has been studied in many previous experiments." Such processes are of interest because of the very strong dynamic constraints on the distribution of product states and because NF(a'A) has potential utility as an energy carrier in high-energy laser systems. The branching fraction to NF(a) in the H + NF2 reaction has been determined to be in excess of 0.90.a8 The product-state selectivity in this case is thought to arise from an addition-elimination mechanism involving attack of the hydrogen atom on the nitrogen atom of NF2to produce an excited singlet difluoramine intermediate, which eliminates singlet H F to leave excited singlet NF.3*4 This process has been used' to ( I ) Clyne, M. A. A.; White, 1. F. Chem. Phys. Lett. 1970, 6, 465. (2) Malins, R. J.; Setser, D. W. J . Phys. Chem. 1981, 85, 1342. (3) Herbelin, J. M. Chem. Phys. Lett. 1976, 42, 367. (4) Herbelin, J. M.; Cohen, N. Chem. Phys. Lett. 1973, 20, 605. (5) Cheah, C. T.; Clyne, M. A. A.; Whitefield, P. D. J . Chem. SOC., Faraday Trans. 2 1980. 76,711. Cheah, C. T.; Clyne, M. A. A. J . Chem. Soc.. Faraday Trans. 2 1900, 74, 1543. (6) Koffend, J. B.; Gardner, C. E.; Heidner, R. F. J . Chem. Phys. 1985, 83, 2904. (7) Herbelin, J. M.; Spencer, D. J.; Kwok, M. A. J . Appl. Phys. 1977,48, 3050. (8) Heidner, R. F.; Helvajian, H.; Holloway, J. S.; Koffend, J. B. J . Phys. Chem. 1989, 93, 7818.
0022-3654/9l/2095-2692$02.50/0
generate NF(a) densities in excess of 1OIs ~ m - ~In. principle, similar reactions of H atoms with other NX2 or NXY radicals (X,Y = halogen) might also produce high yields of excited singlet N X species. Such reactions have not been explored, however, in large measure because chlorinated or brominated analogues of N2F4 (the normal precursor of NF2) are unknown. Recent work in our laboratory has involved studies of the spectroscopy and reactions of fully halogenated amines NX3 or NX2Y, and techniques for the safe synthesis and manipulation of these energetic compounds have been developed."' Like N2F4, these species can be used as precursors of NX2 or NXY radicals and hence of excited NX nitrenes by reactions with H atoms. This paper presents the results of experiments involving the reaction of NCI3 with excess H atoms. Excited NCI can be produced in this system by a two-step mechanism as follows: H + NCI3 NC12 HCI (1)
-
+ NCl* + HCI
H + NC12 (2) To the extent that reaction 2 proceeds by the addition-elimination (9) Gilbert, J. V.; Wu, X. L.; Stedman, D. H.; Coombe, R. D. J . Phys. Chem. 1987, 91, 4265. (10) Conklin, R. A.; Gilbert, J. V. J . Phys. Chem. 1990, 94, 3027. (11) Gilbert, J. V.; Conklin, R. A.; Wilson, R. D.; Christe, K. 0. J . Fhorine Chem. 1990, 48, 361.
0 1991 American Chemical Society
