5024
J. Phys. Chem. 1991, 95, 5024-5031
Gas-Phase Photoacoustic Overtone Spectroscopy of Ethylamine, Ethylamine-N,N-d,, EthyCP,2,t-d,-amhe, and Trlethylamlne Lewis C. Baylor and Eric Weitz* Department of Chemistry, Northwestern University, Evanston, Illinois 60208 (Received: May 18, 1990; In Final Form: January 22, 1991)
Intracavity dye laser photoacoustic spectra have been obtained for the v = 5 and 6 C-H stretching regions of ethylamine, ethylamine-N,N-d,, and ethyl-2,2,2-d3-amine. One methyl absorption and two methylene C-H absorptions were observed. The methylene peaks represent C-H bonds gauche to the nitrogen lone pair (higher frequency) and trans to the nitrogen lone pair (lower frequency). Two N-H peaks were observed and are assigned as the N-H trans to the methyl group (higher frequency) and the N-H gauche to the methyl group (lower frequency). Intracavity dye laser photoacoustic spectra have been obtained for the u = 5-7 C-H stretching region of triethylamine. Standard infrared techniques were used to obtain the fundamental and lower overtones (v = 1-4). Two peaks were observed in the overtone spectra for u 2 3 and were assigned as the methyl C-H stretch (high frequency) and the methylene C-H stretch (low frequency). The positions of both absorptions were fit to the local-mode model equation, and harmonic frequencies (w,) and anharmonicity constants (&) are reported.
Introduction The conventional normal-made model’ views a molecular vibration as a superposition of motion of every atom in the molecule. The normal-mode model works very well in describing the fundamental vibrational spectra of molecules but predicts crowded complex spectra for the overtone levels, complexity that is not typically present. As an alternative to this picture, the local-mode model was first formulated over 50 years ago.2 In this model, molecular vibrations are viewed as a localized motion of two bonded atoms. As a two-parameter model, it predicts simple spectra dominated by the overtones of X-H (X = C, N, and 0) stretching modes. These modes are typically the most anharmonic in a molecule, and hence they have the largest oscillator strength and thus carry most of the intensity in the overtone regions. The normal-mode and local-mode models of vibration represent the endpoints on a continuum of vibrational behavior ranging from involvement of all atoms in the molecule in each vibrational mode to the involvement of only the two atoms attached to a particular bond. The advent of laser excitation sources in the early 1970s coupled with sensitive photoaco~stic~ or thermal lens detection4 methods made investigation of the high overtone absorptions practical and spurred renewed interest in the local-mode description of vibrational modes. Throughout the past two decades there has been extensive work on identifying and characterizing local modes. Henry and coworkers first applied the local-mode model to benzene5s6and later to ammonia, methane, methylene chloride and some methylsubstituted benzenes.’+ A variety of alkanes have been studied to determine steric and substituent effects on the local C-H The dependence of the local-mode frequency on en(1) Herzberg, G. Infrared and Raman Spectra of Polyatomic Molecules; Van Nostrand: New York, 1945. (2) Mccke, R. 2.Phys. 1936, 99, 217. (3) See,for example: West, G. A.; garret, J. J.; Sietcrt, D.R.; Reddy, K. V. Rev. Sd. Instrum. 1983, 54, 797. (4) See. for example: Long, M. E.; Swofford, R. L.; Albrecht, A. L. Science 1976. 191, 183. (5) Henry, B. R.; Siebrand, W. J . Chem. Phys. 1968, 49, 5369. (6) Hayward, R. J.; Henry, B. R.; Siebrand, W. J . Mol. Specrrosc. 1973,
46, 207. (7) Hayward, R. J.; Henry, 9. R. J . Mol. Specrrosc. 1974, 50, 58. (8) Hayward, R. J.; Henry, 9. R. J. Mol. Spectrosc. 1975, 57, 221. (9) Hayward, R. J.; Henry, 9. R. Chem. Phys. 1976, 12. 387. (IO) Wong, J. S.;Moore, C. 9. J. Chem. Phys. 1982, 77, 603. (11) Henry, 9. R.; Hung, 1. F.; MacPhail, R. A.; Strauss, H. L. J . Am. Chem. Soc. 1980, 102, 515. (12) Greenlay, W. R. A.; Henry, B. R. J . Chem. Phys. 1978, 69, 82. (13) Henry, 9. R.; Miller, R. J. D. Chem. Phys. Lett. 1978, 60, 81. (14) Fang, H. L.; Swofford, R. L. J . Chem. Phys. 1980, 73, 2607.
0022-365419 1 /2095-5024$02.50/0
vironment has been observed in alkenes”-21and alkynes?2-M The acetylenic C-H oscillator of propyne and 3,3,3-trifluoropropyne has proven to be a very good local mode, fitting the simple twoparameter model even at the fundamental level.zcz6 The effect of molecular conformation has been examined by In these McKean and cc-workers through deuteration studies, all but one C-H bond in a moleule is isotopically substituted with deuterium, thus ‘isolating” the remaining C-H oscillator. This is a useful technique for observing C-H oscillators in different environments in the same molecule. The isolated frequency of these oscillators, Y ~ is,often a good approximation to the frequency of a ‘local mode” at u = 1; that is, it will often fall close to the best-fit line of a Birge-Sponer plot of the C-H overtones.29 The effect of heteroatoms on C-H oscillators has been investigated in studies of C-H overtones of alcohols,3w32ethers,’, M(CH,), compounds (M = N, P, AS)^'^^' and various other amines.35 One important feature of these heteroatoms is the presence of a lone pair of electrons. These electrons influence bond lengths and vibrational frequencies in the compound through the “trans” This is the weakening of a bond trans to (15) Perry, J. W.; Moll, D. J.; Kupprmann, A.; Zewail, A. H. J . Chem. Phys. 1985,82, 1195. 116) Manzanares I. C.: Yamasaki. N. L. S.: Weitz. E. Chem. Phvs. Lett. 19b. 144.43. (17) Fang, H. L.; Swofford, R. L.; McDevitt, M.; Anderson, A. B. J . Phys. Chem. 1985, 89, 225. (18) Fang, H. L.; Swofford, R. L. Appl. Opt. 1982, 21, 55. (19) Duncan, J. L.; Mills, I. M. Chem. Phys. Lert. 1988, 145, 347. 120) Fang. H. L.: Comuton. D. A. C. J . Phvs. Chem. 1988.. 92.. 7185. i21j Bayior, L. C.; Wciiz, E. J . Phys. Chem: 1990, 94, 6209. (22) Diibal, H.-R.; Quack, M. Chem. Phys. Lert. 1982, 90,370. (23) von Puttkamer, K.; Diibal,H.-R.; Quack, M. Faraday Discuss.Chem. Soc. 1983, 75, 191. (24) Manzanares I, C.; Yamasaki, N. L. S.; Weitz, E. J . Phys. Chem. 1986, 90,3953. (25) Hofmann. P.; Gcrber, R. B.; Ratner, M. A.; Baylor, L. C.; Weitz, E. J . Chem. Phys. 1988,88,7434. (26) Baylor, L. C.; Weitz, E.; Hofmann, P. J. Chem. Phys. 1989, 90,615. (27) McKean, D.C. Chem. Soc. Reo. 1978, 7 , 399. (28) McKean, D.C. Int. 1.Chem. Kinet. 1989, 21, 445. (29) Birge, R. T.; Sponer, H. Phys. Reo. 1926, 28, 259. (30) Fang, H. L.; Swofford, R. L. Chem. Phys. Lett. 1984, 105, 5. (31) Fang, H. L.; Meister, D.M.; Swofford, R. L. J . Phys. Chem. 1984, 88, 405. (32) Fang, H. L.; Compton, D. A. C. J . Phys. Chem. 1988, 92, 6518. (33) Fang, H. L.; Meister, D. M.; Swofford, R. L. J . Phys. Chem. 1984, 88, 410. (34) Manzanares I, C.; Yamasaki, N . L. S.; Weitz, E. J . Phys. Chem. 19117. 91. . - , 1959. -- -- . (35) Fang, H. L.; Swofford, R. L.; Compton, D. A. C. Chem. Phys. Leu. 1984, 108, 539. (36) McKean, D. C. Spectrochim. Acta, Part A 1975, 31, 861.
0 199 1 American Chemical Society
Photoacoustic Overtone Spectroscopy of Amines
The Journal of Physical Chemistry, Vol. 95, No. 13, 1991 5025
“.D Me
H
trans
gauche
F i i 1. The two gas-phase conformers of ethylamine, trans and gauche.
the electron lone pair of the heteroatom, thus lowering the frequency and lengthening the bond. The trans effect has been observed in a number of nitrogencontaining compounds including methylamine, dimethylamine, and t r i m e t h ~ l a m i n e ~and ~ ’ ~more recently in P(CH3)3and A s ( C H ~ ) ~It. has ~ ~ also been observed in alcohol^^^.^^ and ethers.” Interestingly, in all these cases in which the C-H absorptions were split, the C-H oscillators were in a methyl group directly bonded to the heteroatom. In propene and cis-propene-1,2,-d2the u electrons of the double bond have the same effect as a lone p?ir, weakening the out-of-plane methyl C-H bond^.'^^^^ This study is an extension of prior work on the M(CH3)3(M = N, P, As) compounds,34in which the lone pair weakened the in-plane C-H bond and split the methyl C-H absorptions. It is interesting to observe the trans effect in compounds with ethyl groups and to examine how an anisotropic environment affects the methyl and methylene C-H oscillators. Are the methyl C-H absorptions still split, and how are the methylene C-H absorptions affected? In a number of prior studies of compounds containing ethyl groups bonded to oxygen, no evidence of splitting has been seen in either the methyl C-H or the methylene C-H overtone s p e ~ t r a . ~This * ~ ~has been the case even when some of the methylene C-H bonds were trans to lone pairs, as in diethyl ether.33 On the other hand, a study of diethylamine did find splitting of the methyl C-H absorpti0n.3~Ethylamine was chosen as a simple case with one ethyl group and a heteroatom lone pair. Companion studies on ethylamine-N,N-d2and ethyl-2,2,2-d3-amine were undertaken to aid in identification of the ethylamine absorptions. The results of the ethylamine study were then applied in an attempt to assign the spectra of a more complex compound, triethylamine. Although the homologous compound n-propylamine has been thoroughly analyzed in the fundamental region,38there is little prior work on the vibrational spectroscopy of ethylamine and its deuterated forms. Hamada et al. have analyzed the vibrational spectrum of both trans and gauche ethylamine below 2000 cm-I 39 and have also examined its structure by gas-phase electron diffra~tion.~’The trans and gauche forms are illustrated in Figure 1. Krueger and Jan analyzed the C-H and C-D stretching regions of ethylamine and eth~l-2,2,2-d~-amine,’~ and Konarski studied the influence of the nitrogen lone pair on ethylamine r ~ t a m e r s . ~Internal ~ * ~ rotation in eight isotopically substituted ethylamines, including the three in this study, has been examined by Tsuboi et a1.@ The microwave spectra of both trans and gauche ethylamine and eth~1amine-NJV-d~ have been ~ t u d i e d . ’ ~ . Al~ though no isolated C-H frequencies have been determined for ethylamine, they have been determined for several methylamine^!^ (37) McKean, D. C.; Biedermann, S.;Bargcr, H. Specrrochim. Acra, Parr A 1974, 30. 845. (38) Sato, N.; Hamada, Y.; Tsuboi, M. Specrrochim. Acfa, Parr A 1987, 43. 943. (39) Hamada, Y.; Hashiguchi, K.; Hirakawa, A. Y.; Tsuboi, M.; Nakata, M.; Tasumi, M.; Kato, S.;Morokuma, K. J. Mol. Specfrosc. 1983, 102, 123. (40) Tsuboi, M.; Tamagake, K.; Hirakawa, A. Y.; Yamaguchi, J.; Nakagawa. H.; Manocha, A. S.;Tuazon, E. C.; Fateley, W. G. J. Chem. Phys. 1975.63. 5177. .. (41) Hamada, Y.; Tsuboi, M.; Yamanouchi, K.;Kuchitsu, K. J. Mol. Srrucr. 1986, 146, 253. (42) Krueger, P. J.; Jan, J. Cun. J. Chem. 1970, 48, 3229. (43) Konarski, J. J. Mol. Sfrucr. 1971, 7 , 337. (44) Konanki. J. Chem. Phys. Left. 1971, 12, 249. (45) Fischer, E.; Botskor, I. J. Mol. Specrrosc. 1982. 91, 116. (46) Fischer, E.;Botskor, I. J. Mol. Spectrosc. 1984, 104, 226. (47) McKean, D. C.; Ellis, 1. A. J . Mol. Sfrucr. 1975. 29. 81.
-. .
P tgg’ Figure 2. The three mcst stable conformers of triethylamine. The t,g,g‘ form is the most stable in the gas phase. The dark circles represent heteroatoms; the open circles represent carbon atoms. Hydrogen a t o m are not shown.
Manzanares et al. have studied the gas-phase overtone spectra of trimethylamine.M Fang et al. have studied the overtone spectra of five gaseous amines, including d i e t h ~ l a m i n e . ~ ~ , ’ ~ There is even less literature available on triethylamine. Kumar studied the gas-, and liquid-, and solid-phase conformers of triethylamine by IR and Raman spectroscopy?* The three most stable conformers are shown in Figure 2. IR and Raman spectroscopy have also been used to study the liquid and solid phase conformers of trieth~lamine.4~Bushweller et al. used dynamic NMR spectroscopy to resolve the liquid-phase stereodynamics of trieth~lamine.~~ Experimental Section Both ethylamine and ethylamine-N,N-d2were generated from ethylamine hydrochloride. Ethylamine hydrochloride (Aldrich Chemical Co.) was dissolved in a minimal amount of H 2 0 and reacted with excess solid NaOH to liberate ethylamine gas.39 Ethylamine-N,N-d2 was generated similarly, with D 2 0 (99.8% isotopically pure, Aldrich Chemical Co.) substituted for H 2 0 . Ethylamine-2,2,2-d3 was generated by dissolving ethyl-2,2,2d3-amine hydrochloride (98% isotopically pure, MSD Isotopes, Merck Chemical Division) in a minimal amount of H 2 0 and reacting it with an excess of solid NaOH to liberate the product gas. The identity of all three ethylamines was confirmed by FTIR analysis, which revealed the presence of a small water impurity in each case. Deuteration of the ethylamine-N,N-d2 was not complete, as revealed by small residual N-H stretching peaks and residual N-H bending peaks; however, the fully deuterated amine group was clearly the major product. Triethylamine, specified as 99% pure (Aldrich), was degassed by freeze-pumpthaw cycles and checked for purity by FTIR analysis before use. All ex(48) Kumar, K. Chem. Phys. Lerr. 1971, 9, 504. (49) Crocker, C.; Goggin, P. L. J. Chem. Soc., Dalron Trans. 1977,388. (50) Bushweller. C. H.; Fleischmann, S. H.; Grady, G. L.;McGoff, P.;
Rithner, D.; Whalon, M. R.; Brennan, J. G.; Marcantonio, R. P.;Domingue, R. P. J. Am. Chem. Soc. 1982,104, 6224.
5026 The Journal of Physical Chemistry, Vol. 95, No. 13, 1991
Baylor and Weitz
TABLE I: Observed Frequencies ( c d ) for Gas-Phase Ethylamines ethylamine vibr level
obsd freq
5 5 5
12 996 13245 13 330 I 3 439 13 555
5 5 6 6 6
15256 1 5 338 15415 15 572 15772
0 0
6 6 6
15890 16112 16 196
0
5 5
assg trans
0 0 0
0 0
--
---
vibr level
ethylamine-N,N-d2 obsd freq assg trans
5 5
12815 12982
5
13 325 13442 13 546 13663 13950 15 249 15352
0 0 0
5vNM
5 5 5 5 5 5
6v, 6v,
6 6
15574 15 743
0 0
6v,
6 6
15881 16 074
0
5v, 5v, 5v,
5vNH
'Residual N-H peaks resulting from incomplete deuteration. methylene H.
vNM designates
periments were performed at 21 f 2 OC. The reported spectra were obtained by three techniques. The fundamental spectra of ethylamine, ethylamine-N,N-d2, and ethyl-2,2,2-d3-amine were all recorded on a Mattson Polaris Fourier transform infrared spectrophotometer at pressures of 59, 67, and 52 Torr, respectively. It should be noted that these ethylamines adsorb on the glass walls of the IR and PA cells, so that approximately 30 min is required for the pressure to stabilize after each cell fill. For the IR runs, the cells were filled with an initial pressure (listed above) that yielded a suitable signal intensity, and no special care was exercised to ensure pressure stabilization. For the photoacoustic experiments (described below), the cited gas pressures represent those following stabilization of the cell pressure. The fundamental spectrum of triethylamine was recorded at room temperature in a 10-cm-path-length cell on a Nicolet 7 199 Fourier transform infrared spectrophotometer at a pressure of 8 Torr. Spectra of the lower overtones (
