3997
J . Phys. Chem. 1989, 93, 3997-4000
Infrared Matrix Isolation Study of Hydrogen Bonds Involving C-H Bonds: Alkynes with Nitrogen Bases Mei-Lee H. Jeng, Andrea M. DeLaat, and Bruce S. Ault* Department of Chemistry, University of Cincinnati, Cincinnati, Ohio 45221 (Received: November 8, 1988)
The matrix isolation technique has been successfully employed to isolate and characterize the hydrogen-bonded complexes formed between acetylenes and a variety of nitrogen-containing bases. Coordination of the acetylenic hydrogen to the nitrogen atom of the base was evidenced by red-shifts of the C-H stretching mode. These shifts, in the range of 50-200 cm-I, were distinct and variable with different alkynes and bases employed, indicating the accessibility of a range of complexes. The hydrogen-bonding interaction of alkynes with NH3 and (CH3)3Nwas considerably greater than that in previous studies of alkynes with oxygen counterparts. For the weak base, acetonitrile, the red-shift with substituted alkynes varied in the order phenylacetylene > acetylene > tert-butylacetylene > methylacetylene, which is consistent with gas-phase acidity data for the alkynes. For the stronger bases, ammonia and trimethylamine, the reverse order, methylacetylene > tert-butylacetylene, was observed.
Twin jet codeposition, in which the reagents were codeposited from separate vacuum lines, was used for all experiments. The Hydrogen bonds are most frequently formed with the highly deposition was carried out at the rate roughly of 2 mmol/h, and electronegative elements N, 0,and F. Since the electronegativities the time ranged from 20 to 24 h. Infrared spectra were recorded of carbon and hydrogen are comparable, the possibility that a C-H on a Perkin-Elmer 983 infrared spectrometer with a resolution group may serve as a proton donor has generated substantial of 2 cm-I. In some experiments, the sample was annealed to 32 interest.l-" Many studies have been directed toward a more K and then recooled to 17 K, and an additional spectrum was complete understanding of the hydrogen-bonding interaction recorded. because of its importance to chemistry, biology, and physics.I2 The degree of hydrogen bonding shown by any C-H bond will be dependent on the reaction partner or base. To date, only a Results relatively small number of bases have been explored in this regard. Blank experiments of all reagents were conducted prior to any In a previous reportI3 from this laboratory, complexes of a set of codeposition experiments in this study. The resulting blank spectra alkynes with a series of oxygen-containing bases were examined. were in good agreement with available literature ~ p e c t r a , as ~~-~~ Distinct shifts to lower frequency for the alkyne C-H stretching mode were observed. These shifts, in the range of 50-100 cm-I, indicated that the hydrogen-bonding strength was less than those observed previously for hydrogen-bonded complexes. The com(1) Sapse, A. M.; Jain, D. C. Chem. Phys. Letr. 1966, 124, 517. plexes of C2H2and CF3H with NH3 in a supersonic expansion (2) Frisch, M.J.; Pople, J. A,; Del Bene, J. E. J . Chem. Phys. 1983, 78, 4063. system have been studied by Klemperer and c o - w ~ r k e r swhile ,~ (3) Truscott, C. E.; Ault, B. S. J. Phys. Chem. 1984, 88, 2323. Legon and Regolo have reported the microwave spectrum of the (4) Manceron, L.; Andrews, L. J . Phys. Chem. 1985, 89, 4094. complex of C2H2with (CH3),N, and Barnes" has found evidence (5) Peterson, K. I.; Klemperer, W. J . Chem. Phys. 1986, 85, 725. for hydrogen bond formation between CF3H and N H 3 in an argon (6) Peterson, K. I.; Klemperer, W. J . Chem. Phys. 1984, 81, 3842. matrix. To more fully understand this phenomenon, the interaction (7) Bach, S. B.; Auk, B. S. J . Phys. Chem. 1984,88, 3600. of the alkynic C-H group with a wider range of bases should be ( 8 ) Fraser, G. T.; Leopold, K. R.; Nelson, D. D., Jr.; Tung, A,; Klemperer, considered. Also, nitrogen bases are known to be substantially W. J . Chem. Phys. 1984, 80, 3073. (9) (a) Fraser, G. T.; Leopold, K. R.; Klemperer, W. J. Chem. Phys. 1984, stronger than their oxygen counterparts and should lead to the 80, 1423. (b) Fraser, G. T.; Lovas, F. J.; Suenram, R. D.; Nekon, D. D., Jr.; formation of stronger hydrogen bonds. Klemperer, W. J . Chem. Phys. 1986, 84, 5983. Infrared spectroscopy has emerged as one of the most effective (IO) Legon, A. C.; Rego, C. A. J . Mol. Strucr. 1988, 189, 137. experimental tools for the study of hydrogen bonding, in that ( I I ) Paulson, S. L.; Barnes, A. J . J . Mol. Srruct. 1982, 80, 151. hydrogen bond formation gives rise to distinct, readily identifiable (12) Pimentel, G. C.; McClellan, A. L. The Hydrogen Bond; W. H. spectral features.I2 The matrix isolation technique14-16has been Sreeman Co.: San Francisco, 1960. used repeatedly for the study of weakly bound c o m p l e ~ e s , ~ ~ - ~ ~ (13) DeLaat. A. M.; Ault, B. S. J . A m . Chem. S o t . 1987, 109. 4232. (14) Craddock, S.; Hinchliffe, A. Matrix Isolation; Cambridge University including hydrogen-bonded c o m p l e ~ e s . ' ~This . ~ ~present ~ ~ study Press: New York, 1975. was undertaken to examine the interaction between alkynes and ( 1 5 ) Vibrational Spectroscopy of Trapped Species; Hallam, H., Ed.; nitrogen-containing bases after isolation in inert matrices. In Wiley: New York, 1973. addition, this study will allow a detailed comparison of the hy(16) Andrews, L. Annu. Rev. Phys. Chem. 1971, 22, 109. drogen-bonded complexes of alkynes with oxygen- and nitro(17) Auk, B. S. J . Am. Chem. S o t . 1983, 105, 5742. gen-containing bases. (18) Auk, B. S. J . Phys. Chem. 1986, 90, 2825. Introduction
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Experimental Section
All of the experiments were carried out on a conventional matrix isolation system which has been described p r e v i o ~ s l y . ~The ~ reagents employed in this study were NH3, (CH3),N, C2H2,C3H, (all Matheson), C H 3 C N (Fisher), tert-butylacetylene C 6 H I o (Wiley Organics), and phenylacetylene C8H6(Aldrich). All were subjected to one or more freeze-thaw cycles prior to sample preparation. Argon and nitrogen were used as matrix gases without further purification. *Author to whom correspondence should be addressed.
0022-3654/89/2093-3997$01.50/0
(19) Sass, C. S.; Ault, B. S. J . Phys. Chem. 1986, 90, 4533. (20) Andrews, L. J. Mol. Srruct. 1983, 100, 281. (21) Johnson, G.; Andrews, L. J . Am. Chem. SOC.1983, 105, 163. (22) Johnson, G.;Andrews, L. J . Am. Chem. SOC.1982, 104, 3043. (23) Barnes, A. J . J . Mol. Strucr. 1983, 100, 259. (24) Barnes, A. J.; Paulson, S. L. Chem. Phys. Lett. 1983, 99, 326. (25) Ault, B. S. J . Am. Cheni. S o t . 1978, 100, 2426. (26) Herzburg, G . Infrared and Raman Spectra of Polyatomics; Van Nostrand: New York, 1975; Vol. 2, p 290. (27) Sanssey, J.; Lamotte, J.; Lavalley, J . C. Spectrochim. Acta, Parr A 1976. 32A. 763. (28) Sheppard, N. J. Chem. Phys. 1949, 17, 455. (29) Allen, A. D.; Cook, C. D. Can. J . Chem. 1963, 41, 1084
0 1989 American Chemical Society
3998 The Journal of Physical Chemistry, Vol. 93, No. 10, 1989
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Figure 1. Infrared spectra of the products arising from the codeposition of C2H2and N(CH,), into argon matrices, compared to blank spectra
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well as with spectra previously recorded in this laboratory. In general, the dominant species in each blank spectrum was the monomer; for the alkynes some dimerization was noted as well, particularly for C2H2. Spectra of samples of Ar/NH3 showed some indication of dimerization, as well as the hindered rotational structure which researchers have reported and characterized previously. Dimerization and rotational structure led to some additional spectral congestion, but product absorptions could still be readily identified. C2H2+ (CH,),N. The twin jet codeposition of C2H2 with (CH3),N, each at a concentration of 500/1, into an argon matrix gave rise to a new doublet at 3133 and 31 16 cm-I, which was shifted to lower energy relative to the parent =C-H stretching mode ( u s ) , 3300 and 3285 cm-l. Near the C=C stretching mode of (C2H2)2,1966 cm-I, a weak broad new band at 1955 cm-' was observed. No distinct new features were noted near the parent C-N stretching and alkynic C-H bending vibrations. Dilution of the C2H2to 1000/ 1 in argon led to similar product bands and about the same relative intensity ratio of the 3 1 3 3 / 3 1 1 6 - ~ m - ~ doublet; changing the concentration of (CH,),N to 200/1 or I OOO/ 1 and keeping that of C2H2constant resulted in the increase of the intensity of the 3133-cm-I band relative to the 31 16-cm-I band. Figure 1 shows representative infrared spectra of a matrix containing this pair of reactants. C2H2+ NH,. C2H2was codeposited with NH3, at concentrations of 1OOO/ 1 and 500/ I , respectively, and several new peaks were observed. In the lower frequency region to us, an intense broad band at 3 170 cm-' was noted. A new feature was observed at 1960 cm-I with an intensity greater than that of the parent (C2H2),absorption. In addition, two weak peaks were found at 823 and 1003 cm-l, shifted to higher energy from the C-H bending mode of C 2 H 2and the symmetric N-H bending mode of NH,, respectively. NH3 was also codeposited with samples of partially deuteriated acetylene, with a D / H ratio of approximately 2, so that the ratio of species was roughly C2D2:C2HD:C2H2 = 4:4: 1. In the C-H and C-D stretching regions, new product bands were noted at 3192, 2503 (medium), and 2380 (intense) cm-l. No distinct additional features could be found in the low-energy region. The spectra are compared in Figure 2 for the reaction products of acetylene with NH, and CH,CN. C2H2 + CH,CN. C2H2 and CH3CN were codeposited at concentrations of 2000/1 and 200/1, respectively; this led to the disappearance of a doublet that was noted at 3264, 3212 cm-' in (30) Colthup, N. 8 . Appl. Spectrosc. 1976, 30, 589. ( 3 I ) Goldfarb, T. D.: Khare, B. N . J . Chem. Phys. 1967, 46, 3379. (32) Ribbegard. G . Chem. Phys. 1975, R , 185. (33) Schriver, L.: Schriver. A.; Perchard, J . P. J . Chem. Soc.. Faraday Trans 2 1985, R I . 1407.
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