Matrix isolation study of complexation and exchange reactions of

The sec-butylamine CH3 absorption is broad compared with that of isopropylamine. Both have the same number of CH3 groups, and it seems that the additi...
0 downloads 0 Views 690KB Size
Download PDF
3080

J . Phys. Chem. 1991, 95, 3080-3084

from it. The latter has the absorption peak at 13 298 cm-' and the former at 13 055 cm-'. The close vicinity of the nitrogen lone pair has the effect of weakening the C-H bond and causing a decrease in frequency. This effect was discussed by Fang et aL2 n-Butylamine has three CH2 groups, and the one closest to the N absorbs at 13 055 cm-l exactly as the n-propylamine. The other CH2groups have the maximum of the absorption curve at 13 141 cm-I. The relative ratio of the areas confirm the assignment of the peaks. The sec-butylamine CH3absorption is broad compared with that of isopropylamine. Both have the same number of CH3 groups, and it seems that the additional CH, group in sec-butylamine overlaps the major CH3 peak. The single C-H is well separated and appears at 13 080 cm-l. There is a small shoulder at 13 076 cm-I in the isopropylamine spectrum which might indicate the lone C-H on the central carbon atom. In summary, the CH(5) methyl peak appears at the highest frequency followed by the methylenic CH(5) and then the isolated C-H. Methylenic groups close to the N appear at lower energy than those further removed. However, the opposite trend is true for the methyl groups; ;.e., CH3 groups close to N have slightly higher transition energies than those removed from it (a and a' in our notation of Figure 9 and Table I). Photoacoustic spectra of gaseous tert-butylamine and its D2 isotopomer reveal splitting of the CH(5) peak into three peaks at 13415, 13513,and 13560cm-I. Thereare threeconfigurational nonequivalent C-H bonds in this molecule. Six H atoms are farther from the nitrogen, two are close to the lone electron pair, and one lies between the two H atoms on the nitrogen. The expected intensity ratio of 6:2:1 is observed experimentally. The

peak at 13 424 cm-' belongs to the C-H furthest from the NH2 group. n-Butylamine has the CH3 group furthest removed from the NH2 group; the 13 424 cm-l is intense as expected and there is a peak at 13 490 cm-I smaller in intensity which might belong to the positional isomers whereby the C-H group is closer to the NH2. There is a succession of overlapping peaks in the range 13400-13 200 cm-l which belong to the various CH2 groups. sec-Butylamine has the three peaks as in tert-butylamine. They are, however, unresolved since the molecule contains two kinds of CH3 groups with overlapping absorptions. In conclusion, the spectra of five aliphatic primary amines in the liquid and in the gas phase are reported. It is found that the N-H stretch overtone shifts to higher wavenumbers when less bulky groups surround it. A combination of NH(4) stretch and the bending mode could account for a small absorption peak in the region of 12 830 cm-I. The CH(5) energies are higher in CH3 followed by CH,, and the lowest is the single C-H. Photoacoustic spectra reveal splitting of the CH(5) absorption which can be interpreted by allowing positional isomers. Acknowledgment. We thank Ms. Alison Geyh for preparing the deuterated tert-butylamine, Ms. Kathy Hansen for running the liquid spectra, and Mr. Jeff Dage for help in preparing the figures. This work was supported in part by the US-Israel Binational Science Foundation, the Fund for Promotion of Research at Technion, the German Israeli Foundation for Scientific Research and Development, and by the Office of Naval Research. Registry No. i-PrNH,, 75-31-0; PrNH2, 107-10-8; BuNH2, 109-73-9; s - B u N H ~ ,13952-84-6; f-BuNH,, 75-64-9.

Matrix Isolation Study of Complexation and Exchange Reactions of Molecular Halogens with tert-Butyl Halides Hebi Bai and Bruce S. Ault* Department of Chemistry, University of Cincinnati, Cincinnati, Ohio 45221 (Received: August 22, 1990)

The 1 / 1 molecular complexes of tert-butyl chloride and bromide with CIF, CI2,and Br2 have been isolated and characterized in argon matrices following twin-jet deposition. Perturbed vibrational modes of the tert-butyl halide, including the carbon-halogen stretching and the CH3 rocking modes, were observed upon complex formation. For the CIF complexes, the red-shifted CI-F stretching mode was observed as well. The twin-jet codeposition of (CH3)3Brwith C12 and CIF led to the exchange reaction product (CH3)3CCI,despite the very short mixing time in these experiments. The single-jet codeposition of these pairs of reactants led to further reaction, forming terr-butyl fluoride in the CIF experiments and 1,2-dichlor0-2-methylpropanein the reactions of (CH3),CC1 and (CH3)3CBrwith CI2. While the halogen-exchange reactions are very facile, these results support the intermediacy of molecular complexes during the reaction process.

Introduction The tert-butyl halides form an interesting class of compounds whose chemistry is in part determined by the ability of the tert-butyl group to stabilize a positive charge.l The tert-butyl halides are known to form complexes with certain strong Lewis acids,13 including BF3 and GaBr3. With other Lewis acids, rapid halogen-exchange reactions have been observed.& Complexation to Bronsted acids occurs as well,' and the hydrogen-bonded complexes of the tert-butyl halides with the hydrogen halides have been isolated in inert matrices.s In these studies, halogen-exchange reactions were also observed when certain pairs of hydrogen halideltert-butyl halide were premixed in the gas phase at room temperature. Molecular halogens are well-known as Lewis acids by virtue of the ability of the u* antibonding orbital to accept electron density. A wide range of charge-transfer complexes of the heavy *To whom correspondence should be addressed.

halogens Br2 and I2 have been examined over the On the other hand, much less is known about the ability of the lighter halogens, including CIF and CI2, to form molecular complexes. Recent studiesl2-I8 in this laboratory have shown that CIF is an (1) Butler, G. B.; Berlin, K. D. Fundamenruls o/ Organic Chemistry; Ronald Press Co.: New York, 1972; p 175. (2) Perkampus, Von H. H.; Baumgarten, E. Be?. Bumen-Ges. Phys. Chem. 1964, 68, 496. ( 3 ) Nakane, R.; Kurihari, 0.;Natsubori. A. J . Phys. Chem. 1964, 68, 2876. (4) Goldstein, M.; Haines, L. I. B.; Hemmings, J. A. G. J . Chem. SOC., Dalton Trans. 1972, 2260. ( 5 ) Choi, S.U.; Pae, Y. Bull Korean Chem. SOC.1982, 3, 144. (6) Kwun, 0. C.; Choi, S.U. J . Phys. Chem. 1968, 72, 3148. (7) Krueger, P. J.; Mettee, H. D. Can. J . Chem. 1964, 42, 288. (8) Ault, B. S.;Sass, C. E. J . Phys. Chem. 1987, 91, 1063. (9) Benesi, H. A.; Hildebrand, J. H. J . Am. Chem. Soc. 1949, 71, 2703. (10) Mulliken, R. S.J . Phys. Chem. 1952, 56, 801. ( 1 1) Tames, M. Mol. Assoc. 1979, 2, 331.

0022-365419112095-3080$02.50/0 0 1991 American Chemical Society

Reactions of Molecular Halogens with tert-Butyl Halides excellent probe of Lewis acid-base interactions; transfer of electron density of CIF reduces the CI-F stretching force constant and vibrational frequency. Consequently, the pasition of this stretching mode is a sensitive indicator of the degree of electron transfer and the strength of the intermolecular interaction. The matrix isolation technique'*22 is ideally suited for the study of molecular interactions, especially for reactive species, and was used for the CIF studies mentioned above. The rapidly condensing matrix typically stabilizes initially formed complexes and prevents further reaction. In fact, even complexes of F2 have been isolated in this manner.,, Consequently, a study was undertaken to characterize the interaction between the molecular halogens CIF, Cl,, and Br2 with terr-butyl chloride and tert-butyl bromide. Experimental Section The experiments in this study were all carried out on conventional matrix isolation apparatus, which has been de~cribed.2~ClF (Pennwalt) and CI2(Matheson) were introduced into the stainless steel vacuum line from lecture bottles and purified by freeze-thaw cycles at 77 K. Samples of bromine (Fisher), tert-butyl chloride, and tert-butyl bromide (both Aldrich) were prepared from the vapor above the purified liquid. Argon was used as the matrix gas in all experiments and was used without further purification. Experiments were conducted in both the single-jet and the twin-jet modes. In the former, the two reagent gases were premixed in a single vacuum line, argon was added, and the mixture was deposited onto the 14 K cold surface. In the latter, the two reagents were diluted in argon in separate vacuum lines and then deposited simultaneously through separate nozzles onto the cold surface. The distance from the tip of each deposition line to the cold surface was approximately 4 cm. Matrix samples were deposited for 20-22 h onto a CsI cold window before final spectra were recorded on an IBM 98 Fourier transform infrared spectrometer at I-cm-' resolution. Most samples were then annealed to 32 K and recooled, followed by the recording of additional spectra. Results Prior to the codeposition of the tert-butyl halides with the molecular halogens, blank spectra were obtained for each reagent alone in an argon matrix. The matrix infrared spectra obtained for CIF, tert-butyl chloride, and tert-butyl bromide were in good agreement with previous studies in this laboratory and in othetxZJ6 Several absorptions of impurity isobutene, including those at 440 and 888 cm-I, were noted in some experiments involving the tert-butyl halides. C12and Br2 are infrared inactive, and no distinct absorptions were observed in blank spectra of these reagents. tert-Butyl Chloride Reactions. The twin-jet codeposition of a sample of Ar/(CH,)$CI with a sample of Ar/CIF produced a number of new absorptions not present in the blank spectra of either reagent. A sharp doublet was noted a t 706, 713 cm-I, to the red of the parent CI-F stretching mode near 770 cm-I, with a full width at half-maximum of 2 cm-' for each component of the doublet and an intensity ratio of approximately 1/3 after annealing. A very strong multiplet centered at 1154 cm-' was noted near the parent multiplet centered at 1163 cm-I. Additional Machara, N. P.; Auk, B. S . Inorg. Chem. 1985, 24, 4251. Machara, N. P.; Ault, B. S.J. Phys. Chem. 1987, 91, 2046. Machara, N. P.; Ault, B. S.J. Phys. Chem. 1988, 92, 73. Machara, N. P.;Ault, B. S. J. Phys. Chem. 1988, 92, 2439. Ault, B. S . J. fhys. Chem. 1987, 91, 4723. Bai, H.; Auk, B. S . J. Mol. Strucr. 1989, 196, 47. Bai, H.; Auk, B. S . J. Phys. Chem. 1990, 94, 199. Hallam, H., Ed. Vibrarional Spectroscopy of Trapped Species; Wiley: New York. 1971. . - -.__, .- - . (20) Craddock, S.;Hinchliffe, A. Matrix Isolation; Cambridge University Press: New York, 1973. (21) Moskowits, M., Ozin, G.,Eds. Cryochemistry; Wiley: New York, (12) (13) (14) (15) (16) (17) (18) (19)

1 Q76 _-.-.

(22) Andrews, L. Annu. Reu. Phys. Chem. 1971, 22, 109. (23) (a) Andrews, L.; Lascola, R.J. Am. Chem. Soc. 1987,109,6243. (b) Hunt, R. D.; Andrews, L. J. Phys. Chem. 1988, 92, 3769. (24) A u k 9. S. J. Am. Chem. Soc. 1978, 100, 2426. (25) (a) Evans, J. C.; Lo,G.Y.-S. J. Am. Chem. Soc. 1966,88,2118. (b) Bertie, J. E.; Sunder, S. Can. J. Chem. 1973, 51, 3344. (26) Sheppard, N. Tram. Faraday Soc. 1950, 46, 521.

+

The Journal of Physical Chemistry, Vol. 95, No. 8, 1991 3081

rV

-I

I a4 v)

C

I . 1200 1150

1400

%

800

'

700

II

800

ENERGY ( CM-l)

Figure 1. Infrared spectra arising from twin-jet codeposition of (CH,),CCI with molecular halogens into argon matrices: (a) Ar/CIF = 500/1; (b) Ar/(CH,),CCI = 500/1; (c) Ar/(CH,),CCI/CIF = 1000/ 2/1; (d) Ar/(CH3),CCI/CI2 = 500/1/1; (e) Ar/(CH,),CCI/Br2 = 500/1/ I .

I

1500

h

1400

"

1200

ilm *

900

c Bw'

700

@m

J

E N E RGY IC m - l )

Figure 2. Infrared spectra arising from single-jet codeposition of (CH,),CCI with molecular halogens: (a) Ar/(CH,),CCI = 500/1; (b) Ar/(CH,),CCI/CIF = 500/1/1; (c) Ar/(CH,),CCI/CI2 = 500/1/1; (d) Ar/(CH3),CCI/Br2 = 500/1/1.

weak bands were observed a t 571, 573, 802, 1242, 1372, 1374, and 1464 cm-I near absorptions of the parent (CH,),CCI at 578, 808, 814, 1239, 1371, and 1458 cm-I. This experiment was repeated at a variety of sample concentrations and led to these same product absorptions with the same relative intensities. The twin-jet codeposition of Ar/(CH,),CCI with Ar/CI2 led to two new product doublets, at 569, 572 and at 1 159, 1 155 cm-I, near parent modes at 578 and 1163 cm-I. The intensity of the higher energy doublet was comparable to that of the nearby parent mode, even when a dilution ratio of 500/1/1 was employed. The twin-jet codeposition of tert-butyl chloride with Br, led to similar product absorptions at 573, 1156, and 1 157 cm-l but with considerably lower intensities. In all of the above experiments, all product bands grew in intensity at approximately the same rate when parent concentrations were increased. Moreover, the product absorptions grew and the parent absorptions decreased in intensity upon sample annealing. Figure 1 shows representative spectra arising from the twin-jet codeposition of tert-butyl chloride with these molecular halogens. When tert-butyl chloride was codeposited with ClF in the single-jet mode at a concentration ratio of 500/1/ 1, no absorption

3082 The Journal of Physical Chemistry, Vol. 95, No. 8, 1991

Bai and Ault

in a final spectrum which matched very well that obtained from the (CH3),CC1 + Cl, system, namely new bands near 614,630, 731, 739, 985, 1117, 1385, 1420, 1436, and 1460 cm-l. The single-jet codeposition of (CH3)3CBr with Br2 produced a broadening of the parent absorptions but led to no new product bands. A trace of (CH,),CCl was observed as a residual left in the vacuum line in early experiments, but it disappeared as the vacuum system was passivated and conditioned.

I . . I . . . . l 1180

1140

580

540

500

E N E R G Y (cm-1)

Figure 3. Infrared spectra arising from the twin-jet codeposition of (CH,),CBr with molecular halogens into argon matrices: (a) Ar/ (CH,),CBr = 500/1; (b) Ar/(CH,),CBr/ClF = 500/1/1; (c) Ar/ (CH3),CBr/Cl2 = 500/1/1: (d) Ar/(CH,)3CBr/Br2 = 5 0 0 / 1 / 1 .

of parent CIF was observed. Two new bands were noted at 1198 and 1262 cm-I, along with a broadening of the absorptions of parent (CH3),CC1. On the other hand, single-jet codeposition of tert-butyl chloride with C1, in argon gave rise to an entirely different spectrum; the absorptions of parent (CH3),CCI were greatly reduced, and a strong new doublet was observed at 11 14, 1 1 17 cm-l. In addition, new absorptions of weak-to-medium intensity were noted near 614 (multiplet), 630,731,739,814,985, 1385, 1420, 1436, and 1460 cm-l. Single-jet codeposition experiments with (CH3),CCI and Br2 led to new features at 614, 1 1 12, and 1388 cm-I, along with a broadening of the parent absorptions. A shoulder was also noted on the low-energy side of the 614-cm-l absorption. Figure 2 shows spectra arising from the single-jet deposition of (CH,),CCl with CIF, C12, and Br,. tert-Butyl Bromide Reactions. The twin-jet codeposition of a sample of Ar/(CH3),CBr with a sample of Ar/CIF led to numerous new infrared absorptions, the majority of which matched bands of (CH3)3CClprecisely. In addition, the 3/1 doublet noted above at 706,7 13 cm-' was again observed, but with significantly reduced intensity, as were the other product bands reported above for the (CH3),CC1 ClF system. Also, new product absorptions were recorded at 5 13, 1 142, 1245, and 1374 cm-', near bands of parent (CH,),CBr. The twin-jet deposition of (CH3),CBr with C1, into argon matrices also led to absorptions due to (CH3),CCI as well as the absorptions noted above for the (CH3)3CCI+ C12 system. Weak bands were also noted near 513, 1142, 1245, and 1374 cm-' as in the (CH,),CBr + ClF twin-jet experiments. All of these product bands grew in proportion to the concentration of ClF or Cl,; the rate of product formation was somewhat greater with Cl, than with CIF. The twin-jet codeposition of (CH3)$Br with Br2 led to a single new weak absorption at 1144 cm-l. Figure 3 shows representative spectra for these twin-jet experiments. The single-jet codeposition of a sample of 500/ 1 / 1 Ar/CIF/ (CH3),CBr led to new product peaks at 870, 882, 1192, 1198, and 1262 cm-', while the absorptions of parent (CH,),CBr were greatly reduced and no absorption due to parent CIF was observed. The single-jet deposition of tert-butyl bromide with C12resulted

+

Discussion Both single- and twin-jet codeposition of the molecular halogens CIF, C12, and Br2 with tert-butyl chloride and tert-butyl bromide gave rise to distinct new infrared absorptions that could not be attributed to either parent species and must be assigned to reaction products. Moreover, for a given pair of reagents, these product absorptions grew with increasing reactant concentration and maintained a constant intensity ratio to one another, indicating that a single product was formed for each system. However, results for single-jet deposition were in general quite different from those for twin-jet deposition. Consequently, these two types of experiments will be considered separately. Twin-Jet Reactions. When (CH3)$C1 was codeposited with C1F in the twin-jet mode, a sharp, distinct 3/1 doublet was observed at 706, 713 cm-I, as were weak absorptions near modes of the parent tert-butyl chloride. These spectral features are all indicative of the formation of a molecular complex where the two subunits are perturbed yet retain their structural i n t e g r i t ~ . ~ ~ . ~ ~ Similar results have been observed for the twin-jet codeposition of CIF with a large number of bases, and in each case formation of an isolated 1/1 complex was indicated.I2-l8 The doublet near 710 cm-' is assigned to the CI-F stretching mode, red-shifted from the parent position near 770 cm-I. The doublet structure is due to the two isotopes of chlorine in natural abundance; the 3/1 intensity ratio matches the ratio in natural abundance of 35C1/37C1. The splitting of 7 f 1 cm-I agrees well with the 6 f 1 cm-l isotopic splitting of the ClF parent absorption in solid argon. The most intense absorption of the perturbed (CH,),CCl subunit in the complex fell near 1154 cm-l, to the red of a parent mode at 1163 cm-I. This has variably been assigned in the literature to a CH3 rocking mode25and to the CC, stretching mode;% normal-coordinate calculations suggest that the former assignment is more likely.2sa The high intensity of this product absorption (more intense than the parent absorption in several experiments) suggests that there is substantial intensification of this mode upon complexation, either from a change in geometry of the rert-butyl chloride subunit in the complex or from charge redistribution upon complex formation. At the same time, CIF has been shown to interact end-on through the chlorine end of the molecule,12-18such that the nominal C,, symmetry of the subunit is likely to be retained. A reduction in symmetry would be apparent in splittings of the degenerate modes of the tert-butyl halide. While small splittings were observed, these were present in the parent spectrum as well and are most likely due to trapping on multiple matrix sites (site splitting). To a first approximation, then, the symmetry of the molecular complex is likely to be the same as for the parent, nominally C3". The twin-jet codeposition of (CH,),CCl with C12 led to the formation of a 1/ 1 complex as well with perturbations to the C-Cl stretching and CH3 rocking modes of the parent. No activation of the CI-CI stretching mode of C12 was noted, which is not unexpected for a weak molecular complex. The twin-jet codeposition of tert-butyl chloride with Br2 led to similar results, namely perturbations of the C-CI stretching and CH, rocking modes of the (CH3),CCI subunit, and again indicates formation of a 1/1 molecular complex. Quite different results were obtained from the twin-jet codeposition of tert-butyl bromide with either CIF or CI2. The major products in each of these cases were tert-butyl chloride and the ~~~

~~

(27) Jensen, W. B. The Lewis Acid-Base Concepts: an chrerview; WileyInterscience: New York, 1980. ( 2 8 ) A u k B. S. Rev. Chem. Intermed. 1988, 9, 233.

Reactions of Molecular Halogens with terr-Butyl Halides complex of tert-butyl chloride with CIF or C12, respectively. This observation indicates that the halogen-exchange reaction must be very rapid, in that the mixing region for the two reactants was very short, 2-4 cm, before matrix condensation. Since the two reagents condense onto a cryogenic surface, the effective temperature of the two reactants upon collision is likely very low. Consequently, the halogen-exchange reaction must have a very low activation barrier. At the same time, some evidence of complex formation with (CH,),CBr was observed for both CIF and CI,, such as a perturbed C-Br stretching mode at 513 cm-I. This demonstrates that molecular complexes of fert-butyl bromide with ClF and CI2can be isolated, presumably if sufficient energy has been removed from the reagents prior to reactive collision. These complexes, then, are likely intermediates in the halogenexchange reaction. Trends in the magnitude of shift of vibrational modes upon complex formation have been used to indicate relative strengths of interaction in the complex.28 In the current study, differences in shifts from one complex to the next were not large; the difference in shifts of the C-CI stretching mode near 570 cm-' for the (CH3),CC1 complexes with CIF, C12, and Br, was only 1 cm-l, suggesting similar strengths of interaction. The red shift of the CI-F stretching mode in the (CH3),CCI-C1F complex, to 713 cm-I, was quite similar to that observed earlier for the complex of ClF with benzene.'* Overall, the spectra indicate that while complex formation did occur,the tert-butyl halides are only weakly basic. N o experimental proton affinities are available for the tert-butyl halides; one theoretical c a l c ~ l a t i o nplaced ~ ~ the proton affinity of terr-butyl chloride at 194 kcal/mol. Since the proton affinity of C6H6 is 181 kcal/mol,'O this calculated value for tert-butyl halide may be somewhat too large, on the basis of shifts of the CI-F stretching mode in the respective complexes. Single-Jet Reactions. The single-jet codeposition of pairs of reactants in this study gave rise to distinctly different results than for twin-jet deposition. The single-jet codeposition of either (CH3),CC1 or (CH,),CBr with CIF gave rise to new absorptions at 882, 1198, and 1262 cm-', along with a considerable reduction in the parent alkyl halide absorptions, and no ClF absorption. The new bands match precisely the strongest absorptions of tert-butyl fluoride,8 indicating that reaction has occurred, consuming the ClF and most of the tert-butyl chloride or bromide. (CH,),CF is itself quite reactive and may have partially decomposed on the walls of the vacuum line, leading to relatively low product yield. In any event, a distinct room-temperature reaction occurred rather than the complexation reaction that was observed upon twin-jet codeposition. This, too, argues that the molecular complex may well be an intermediate in the halogen-exchange reaction. The single-jet codeposition of Clz with (CH3),CCI led to a completely new set of product absorptions, many of which were not near parent bands of tert-butyl chloride. This indicates that a distinct chemical reaction occurred rather than the complex formation that occ~rredin the twin-jet mode. (CH3),CCI is known to react photochemically with CI, to yield 1,2-dichloro-2methylpropane; the spectrum obtained here upon single-jet deposition matches very well the 80 K spectrum of this compo~nd.~' Since the vacuum manifold/deposition line is stainless steel, the reaction is undoubtedly not photochemical. Rather, it may occur slowly and homogeneously in the gas phase, yielding the dichloroalkane product, or it may occur heterogeneously on the metal surfaces (the metal surfaces were passivated and conditioned with the reagents used prior to a given experiment). Interestingly, the single-jet codeposition of (CH3)3CBrwith CI, yielded a spectrum identical with that of (CH3)3CCI + CI,, namely that of 1,2-dichloro-2-methylpropane. The earlier twin-jet experiments demonstrated that halogen exchange takes place very rapidly, leading to (CH3)3CCland (presumably) BrCI. In a single-jet experiment, this latter very reactive species may disproportionate to Br2 and C12 or decompose on the walls of the vacuum manifold; CI2 (as ~~

(29) Jorgensen, W. L. Chem. Phys. Lett. 1978, 53, 525. (30) Lias, S.G.; Liebman, J. F.;Levin, R. D. J . Phys. Chem. ReJ Data 1984, 13,695. (31) Crowder, G . A.; Richardson, M.T. J . Mol. Srrucr. 1982, 78, 229.

The Journal of Physical Chemistry, Vol. 95, No. 8, 1991 3083 TABLE I: Summary of Reaction Products Observed in SingbJet a d Twin-Jet Dewition of Molecular Halonew with tert-Butyl Halide ~~~

reactants (CH,),CCI (CH3),CCI (CH,),CCI (CH,)$Br

+ CIF

+ C1, + Br2 + CIF (CH&CBr + CIz (CH,),CBr + Br, ( C H A C C I + CIF (CH;);CCI + CI, - .

(CH,),CCI (CH3),CBr (CH,),CBr

+ Br2 + CIF + CI2

deposition mode twin-jet twin-jet twin-jet twin-jet twin-jet twin-jet single-jet single-jet single-jet single-jet single-jet

obsd products (CH,),CCI-CIF (CH3),CCI-C12 (CH,),CCI-Br2 (CH3),CCI (CH3),CCICIF (CH,),CBr-CIF" (CH3),Ckl + (CH3)3CCI-C1z + __ (CH,),CBr-C12" (CH3),CBr-Br2' (CHACF

+

+

I ,2-d-i~hloro-2-methylpropane 1,2-dibromo-2-methylpropane" (CH,),CF

1,2-dichloro-2-methylpropane

Minor amounts only.

TABLE II: Bond Dissociation Energies and Standard Heats of Formation for Molecular Halogens and terf-Butyl Halidesa Do(R-X), M I o , Do(R-X), AH?, species

kcal/mol

kcal/mol

species

kcal/mol

kcal/mol

CIF C12 BrCl Br2 (8) HF

60.5 57.9 52.2 46.1 135.0

-13.0 0.0 3.5 7.4 -64.3

HCI HBr (CH,)jCF (CH,),CCI (CH,),CBr

103.0 87.5 108.4 80.6 66.1

-22.1 -8.7 -83.1 -43.7 -31.9

'Data from refs 34-38.

initial reactant or intermediate formed in the exchange reaction) may then react with the tert-butyl chloride to form the final product. The photochemical reaction of tert-butyl bromide with Cl, is r e p ~ r t e d ' ~to. ~yield ~ two bromo chloro isomers of 2methylpropane. These were not apparently observed here, although their spectra are very similar to that of 1,2-dichloro-Z methylpropane, so that detection of these species would be difficult. The single-jet codeposition of (CH3),CI with Br2 led to new absorptions that match well the most intense absorptions of 1,2dibromo-2-methylpropane. Table I summarizes the product species observed for each pair of reactants in single- and twin-jet modes. Halogen-Exchange Reactions. While the twin-jet deposition experiments described represent systems far from chemical equilibrium, thermodynamic data can nonetheless provide some insights into the reactions occurring here. The twin-jet codeposition of C12 or CIF with (CH3),CBr gave rise to the exchange product (CH3)3CCl. While the mixing temperature is not well-known for twin-jet deposition, room-temperature heats of formation and bond dissociation energies both strongly favor (CH,),CCl over (CH,),CBr, as shown in Table 11. The activation energy for the exchange reaction is apparently also low enough to permit rapid reaction. The single-jet reaction of either tert-butyl halide with ClF produced (CH3),CF, in accord with the very large (negative) standard heat of formation of that compound. Thermodynamic arguments indicate that Br2 should not undergo halogen exchange with the tert-butyl halides, a prediction that is in accord with present observations. Earlier studies8 of the hydrogen-bonded complexes of the tert-butyl halides with the hyrogen halides led to results opposite to those obtained here. Namely, when the halogen of the hydrogen halide (Bronsted acid) was heavier than the halogen of the tert-butyl halide, an exchange reaction occurred in the single-jet mode. In the present study, Juneja, P. S.;Hodnett, E. M.J . Am. Chem. Soc. 1967, 89, 5685. Haag, W. 0.;Heiba, E.-A. I. Tetrahedron Lett. 1965, 3683. Eggers, K. W.; Cocks, A. T. Helu. Chim. Acta 1973,56, 1516. Eggers, K. W.;Cocks, A. T. Helu. Chim. Acta 1973, 56, 1537. Stull, D.R.; Westrum, E. F., Jr.; Sinke, G. C. The Chemical Thermodynamics of Organic Compounds; Wiley: New York, 1969. (37) Cox, J. D.; Pilcher, G. Thermochemistry of Organic and Organ& metallic Compounds; Academic: London, 1970. (38) Handbook of Chemistry and Physics, 69th ed.; CRC Press: Boca Raton, FL, 1988. (32) (33) (34) (35) (36)

3084

J . Phys. Chem. 1991, 95, 3084-3090

when the molecular halogen (Lewis acid) was lighter than the halogen of the tert-butyl halide, exchange occurred. The bond dissociation energies in Table I1 show that all of the molecular halogens and interhalogens have comparable bond dissociation energies (within 10-15 Kcal/mol), while the C-X bond strength of the tert-butyl halide varies much more dramatically with the halogen. Consequently, the energetics of the exchange reaction between the tert-butyl halides and the molecular halogens is driven by the tert-butyl halide. On the other hand, the bond dissociation energy of the hydrogen halides varies greatly with the halogen and dominates the energetics of the exchange reaction between the hydrogen halides and the tert-butyl halides, favoring the light hydrogen halides. To rationalize their kinetic data, Goldstein, Haines, and Hemmings4 have invoked complex formation in solution as an

Ammonia Activation by Co', Ni+, and Cu'. Adduct Lifetimes

initial step in the exchange reaction between the tert-butyl halides and the boron trihalides. The present study has permitted observation of several of these intermediate complexes as well as the products of the exchange reaction when the reaction conditions were varied. Overall, the experiments here support the concept that the molecular complex plays a role as an initial intermediate in the halogen-exchange reaction, although these reactions appear very rapid at room temperature and below. Acknowledgment. We gratefully acknowledge support of this research by the National Science Foundation through Grant CHE 87-21969. Regisby No. r-BuCI, 507-20-0 f-BuBr, 507-19-7; CIF, 7790-89-8; CI,, 7782-50-5; Br2, 7726-95-6; Z-BuF, 353-61-7; CICH,C(CI)(CH,),, 59437-6.

MS-NH2 Bond Energies and M+-NH3

D. E. Clemmer and P. B. Armentrout*vt Department of Chemistry, University of Utah, Salt Lake City, Utah 84112 (Received: September 10, 1990)

Reactions of Co+, Ni+, and Cu+ with ammonia are studied as a function of translational energy in a guided ion beam tandem mass spectrometer. All three metal ions form MH+ and MNH,' in endothermic reactions, but in contrast to reactions of Sc', Ti', and V+ with ammonia, no MNH+ formation is observed. Thresholds for the cross sections are interpreted to give the 298 K bond energies of Do(Co+-NH,) = 2.66 f 0.09 eV, Do(Ni+-NH2) = 2.41 f 0.08 eV, and Do(Cu+-NH2) = 2.09 f 0.13 eV. Details of the bonding interaction between M+ and NH2 are discussed. At low kinetic energies, formation of MNHp+is also observed and found to be a result of secondary stabilizing collisions. Lifetimes for these adducts are determined and compared to values calculated by RRKM theory. Dual features in the cross section for adduct formation in the reaction of Co+ with NH3 (and ND3) are shown to have drastically different lifetimes and are attributed to different structural isomers, Co+-.NH3 and H-Co+-NH2.

Introduction Recently, several laboratories have reported results concerning single ligand metal amide species for late transition metals. Ions such as FeNH,', CoNH2+,and RuNH2+have been observed in the gas pha& and neutral NiNHz and CuNH, have been studied in cryogenic argon mat rice^.^ Primary amide complexes of these late transition metals are seldom formed in solution! a result that has been attributed to the relatively weak bonds of these spe~ies.4~ Radecki and Allison reached this conclusion based on their observation that gas-phase Co+ does not activate the C-N bond of aminesa5 They speculated that this was a result of a weak Co+-NH2 bond energy,