A Low Temperature Infrared Study of Hydrogen Bonding in N-Alkylacetamides

Transcription

1 A Low Temperature Infrared Study of Hydrogen Bonding in N-Alkylacetamides MARIE-CLAUDE BERNARD-HOUPLAIN AND C. SANDORFY Dkporfemenf cle Cl~imie, Utziuer:c.ifk rle Mot~fr.kol, Monfrkal, Quibec. Received April 19, 1973 Can. J. Chem. Downloaded from by on 11/23/17 The infrared spectra of N-niethylacetamide and two other secondary amides were measured in solution at temperatures ranging from 22 to 'C in both the fundamental and the overtone regions. At least two hydrogen bonded species are found as association increases with decreasing temperature. The effect of hydrogen bond formation on the anharnionicity of the NH stretching vibration and on the NH stretching - NH bending coupling constant is examined. Les spectres infrarouges du N-methylacetamide et de deux autres arnides secondaires ont kt6 mesurks en solution, a des temperatures allant de 22 a "C, a la fois dans la region de la fondamentale et dans celle des harmoniques. On a Ctabli I'existence d'au moins deux especes associees par liaison hydrogene en abaissant la temperature. On a examine I'effet de la liaison hydrogene sur I'anharmonicitk de la vibration de valence NH, et sur la constante de couplage entre les vibrations NH de valence et de deforniation. Can. J. Chem (1973) Introduction In three previous publications (1-3) we reported the results of a low temperature infrared solution studv of the self-association of secondary amines. In the present paper we are dealing mainly with N-methylacetamide (NMA), although results obtained on N-ethylacetamide and N-methylpivalamide will also be mentioned. In a sense, hydrogen bonding in NMA is still a case of self-association, since the same molecule is proton donor and acceptor at the same time. The donor and acceptor sites are different, however, so that the case of amides is intermediate between self-association as it exists in alcohols or amines and association between different molecules. There is general agreement that hydrogen bonding in the case of amides is of the N-H---0 type. It is generally admitted too that the CONH group is coplanar because of an appreciable degree of conjugation between the n electrons of the carbonyl group and the nitrogen lone pairs. In N-alkyl-amides cis-trans isomerism is possible around the C-N bond. The bands due to the fundamental vibrations of dissolved secondary amides were thoroughly studied by several authors (see ref. 4). Attention has also been paid to the overtones of these vibrations (5-8). We are now presenting the results of a combined low temperature and overtone study, similar to those which were carried out in this laboratory on amines, alcohols (9, lo), and thiols (11). As in our previous experiments the solvent was a 1 : 1 mixture of CCI~F and CF,Br-CF,Br (FR) which sets to a glass at low temperatures making it possible to measure solution spectra down to "C. As before we were seeking information on changes in association with decreasing temperature and on the anharrnonicity of the vibrations. The following synibols will be used in connection with the vibrations of an X-H---Y system: v, the X-H stretching vibration; v, the X-H in-plane bending vibration; v, the X---Y bridge stretching vibration. The fundamentals and overtones will be called v?', v?,... where the upper indices stand for the vibrational quantum numbers of the lower and upper levels. Combinations will be referred to as v, + v,, v, + 2v,,... The anharmonicity and coupling constants will be called XI,, XI,,... There is certain confusion in the literature concerning the signs of anharmonicity constants and our own previous publications are no exception. We shall henceforth take a constant to be negative if its existence diminishes a vibrational frequency and positive if it increases it. Thus anharmonicity constants will be generally negative. Experimental The aniides were supplied by Eastman Organic Chemicals; N-methyl- and N-ethylacetamides were distilled and dried under vacuum. N-methylpivalamide was used without further purification. The solvents were 1 : 1 mixtures of CCI,F (Freon 11)

2 BERNARD-HOUPLAIN AND SANDORF :Y: LOW TEMPERATURE I.R. STUDY 3641 and C,Br,F, (Freon 11482) (FR) obtained from Dupont de Nemours Co. of Canada. They were distilled and kept over phosphorus pentoxide. N-methylacetamide-(I6 (CD,CONHCD3) was purchased from Merck, Sharp and Dohme of Canada Ltd. and was of a minimum isotopic purity of 99%. N-rnethylacetamide-N-rl was prepared by exchange with heavy water (12, 13) and was of a rninin~um isotopic purity of 95x. All products were kept and handled in an inert atmosphere to avoid moisture adsorption.the solutions were prepared gravinletrically in volumetric flasks and the concentration was corrected for solvent contraction, admitting a linear dependence of volume with temperature. The NH fundamentals were measured on a Perkin Elmer 621 spectrometer and the overtones on a Cary 17 instrument. Other experimental details were given in previous publications from this laboratory (14, 15). The Fundamentals NMA exists practically only in the truns form (16). The free NH fi~ndamental is at 3476 cm-' in CCI, solution (17). At room temperature, with increasing concentration an associated band appears at 3370 cin-' and moves gradually to lower frequencies. At about 1 M the maximuni reaches 3300 cm-' and remains there even for the pure liquid. This large shift and the shape of the band indicates that association does not stop at the dimer stage but that higher polymers are successively formed. In sufficiently concentrated solutions a shoulder is observed at 3225 cm-' which has been assigned by Cannon (18) and by Fillaux and De LozC (19) to a combination of the amide I (mainly C=O stretching) and arnide I1 (mainly NH bending) bands. Another band, at 3100 cm-', was the target for much discussion (20-23). It has been assigned to the first overtone of amide 11 in Fermi resonance with the associated NH stretching vibration or to a combination of the NH stretching vibration with a low frequency bridge vibration. Figure 1 shows the spectra of a M solution of NMA in FR at different temperatures. The free band is at 3482 cm-' at 22 "C and has a half-width of 18 cm-i. When temperature is decreased, the associated band becomes gradually stronger and moves from 3330 cm-' at 22 "C to 3256 crn- ' at "C. The half-width of this band goes from I20 cm-' at 22 "C to 80 cm-' at C. This is likely to be due to the elimination of certain solute-solvent configurations with unfavorable Boltzmann factors and to the slowdown of thermal motion. CH, CONHCH, CONC M SOLV CCL, F - C, Br, F, FIG. 1. The infrared spectrum ( cm- ') of N-methylacetamide at different temperatures in a 1 :I mixture of CC13F and CF,Br-CF,Br. Concentration at 22 "C: M; cell length: mm. The intensity of the 3100cni-' band also increases with decreasing temperature. However, this band moves slightly to higher frequencies with decreasing temperature. This all but rules out its assignment to an NH stretching vibration or to a combination containing that vibration. It disappears upon deuteration, however, showing that it is an NH band. At "C the ratio of the intensities of the NH stretching band and of the 3100cm-' band is practically independent of concentration between 0.02 and 0.4 M. Its intensity increases with decreasing temperature showing that it cannot belong to a difference tone. These facts are all in favor of the assignment of the 3100 cm-' band to the first overtone of the amide I1 vibration. As temperature decreases the NH stretching band shifts lower, the overtone shifts higher, so that their mutual distance diminishes. Fermi resonance becomes more important and the relative intensity of the overtone with respect to v, increases (Fig. I). At the lowest temperature the 3100cm-' band has two peaks, at 3100 and 3120 cm-i. This could be due to two different associated species as indicated by the existence of two carbonyl bands (see below). We carried out the same measurements on NMA-d6 (Fig. 2). The NH stretching region remained the same while the 3100 cm-' band underwent a shift of about 20 cm-' to lower wavenumbers. Since, at the same time, the amide I1 band shifts by about 10 cm-', this amounts to an additional argument in favor of the assignment of the 3100 cm-' band to the first overtone of the amide I1 vibration.

3 3642 CAN. J. CHEM. VOL. 51, 1973 E L MOL-I CM' CH3CONHCH3 CONC M E SOLV. CC13F-C,Br,F, g 0.3 a CC,CONHCO, CONC M CM' FIG. 2. The infrared spectrum ( cm-') of N-methylacetamide-rlc, at different temperatures in a 1 : 1 mixture of CC13F and CF2Br-CF2Br. Concentration at 22 "C: M; cell length: mm , CM FIG. 3. The infrared spectrum ( cm- ') of N-methylacetamide at different temperatures in a 1:1 mixture of CC13F and CF2Br-CF2Br. Concentration at 22 "C: M; cell length: mm. While the frequency difference between the free NH stretching band measured at 22 "C and the associated band measured at "C is about 230 cm-', it is only 60 cm-' for the amide I1 band (1582 cm-' at "C) and, as expected, it is in the opposite sense. The associated amide I1 band is not very sensitive to temperature changes, it shifts only by about 25 cni-' from 22 to "C toward higher frequencies (Fig. 3). vlo' and, v: are in Fermi resonance and we shall need their unperturbed values. For computing these we supposed that without Fermi resonance the intensity of, v: would be negligible and then used the related equations as given, for example, by Overend (p , in ref. 24). The band areas were planimetered out and are, of course, approximate. Table 1 gives the observed and unperturbed valires for v," and ' v: as well as the observed values of vzo'. p stands for the ratio of the observed intensities of v,o1 and v:,. From the unperturbed frequencies of v202 given in Table 1 we computed the anharmonicity constant for v,. It has the low value of about - 10 cm- at "C. The free amide I (carbonyl) band is found at 1690 cm- ' at room temperature. As temperature is lowered two associated bands appear successively (Fig. 3). The one of higher frequencies varies little with temperature while the one of lower frequency shifts rapidly to lower values. At "C the two bands are at 1650 and 1620 cm- ' respectively. Overtones and Combination Tones In very dilute CCl, solutions the free vlo2 band is at 6810 cm-' (half-width 30 cm-'). With increasing concentration three more bands appear; they are at about 6550, 6380, and 6280 cm-' for the pure liquid. In order to interpret these spectra we measured those of NMA-N-d and NMA-4. In alcohols NH + CH type combination bands complicate the respective part of the spectrum (25). This is not the case for amines but the stronger hydrogen bonds present in the amides make them an intermediate case and we wanted to ascertain if such combinations are present in NMA. The result was that the 6280 cm-' band disappeared for both of the deuterated molecules so it must be NH + CH. The two other bands (6550 and 6380 cm-') remained in NMA-d,, however. Figures 4 and 5 show the spectra of NMA (0.095 M) and NMA-d6 (0.221 M) in the NH stretching overtone region, at different temperatures in FR. The free band is at cm - ' at room temperature, yielding an anharmonicity constant (vlo2/2) - vlo' = -72 cm-l, a value similar to those found for amines. The free band is still well distinguished at "C while at similar concentrations its fundamental had disappeared at - 60 "C. This again underscores the fact that the relative intensity of the free bands is greater at the overtone level than at the level of the fundamental (1, 25). The comparison of the spectra of NMA and NMA-cl, shows again that the band at about 6250cm-' of the former is due to NH + CH combinations. The relative intensity of the two remaining associated bands varies with temperature. Their frequencies shift from 6550 to

4 BERNARD-HOUPLAIN AND SANDORFY: LOW TEMPERATURE I.R. STUDY 3643 TABLE I. Observed frequencies, unperturbed frequencies, and experimental intensity ratios (p) for N-methylacetamide T vlol v202 vlol v202 v201 ("c) observed observed P unperturbed unperturbed observed Can. J. Chem. Downloaded from by on 11/23/17 W I,T MAX 6819 I :,, i j.22"c 20-, m I I I :,., CH3CONHCH3 I j CONC 0095 M,, 8, 15- m SOLV CCI3F -C2Br2F4., <,..,,-- FIG. 4. The infrared spectrum ( crn-i) of N-methylacetamide at different temperatures in a 1 :1 mixture of CC13F and CF,Br-CF2Br. Concentration at 22 "C: M; cell length: 8.5 cm. CD3CONHCD3 CONC M SOLV CCI3F- C2Br2F4 FIG. 5. The infrared spectrum ( ~m-~) of N-methylacetamide-d, at different temperatures in a 1 :I mixture of CCI3F and CF,Br-CF2Br. Concentration at 22 "C: M: cell length: 8.5 cm and from 6375 to 6282 cm-' respectively from - 20 to - I90 "C. The first associated band (6500 cm-' at "C) might receive contributions from the NH stretching overtone of the less associated species, from N-H---O=C end groups and from the v, + 2v2 combination of the more highly associated species. We assign the second associated band (6282 cm - ' at "C) to the first NH stretching overtone of the more highly associated species whose existence has been inferred from observations made in the fundamental region. We shall call these species oligomer and polymer respectively but we shall make no attempt to estimate the number of monomers contained in the adducts. The higher frequency band (6500 cm-' at "C) cannot be the overtone of the polymer fundamental. This would yield an anharmonicity constant of -6 cm-' with the observed fre- quencies; if we use the unperturbed value of v, we obtain a positive anharmonicity. Both would contradict all previous observations. Furthermore the very moderate temperature shift of this band (much less than that of the fundamental) also contradicts its assignment to the polymer. Still another contribution to the apparent intensity of the 6500 cm-' band might be the polymer v, + 2vC,, combination. The v, + vc=, combination, however, was too weak to be observed and the contribution of v, + 2vC,, is therefore not expected to be significant. For all these reasons we assign the 6282 cm-' band to the first overtone of the polymer NH stretching vibration. The rapid shift to lower frequencies of the "polymer" band with decreasing temperature is characteristic of stretching overtones (93 cm-' from - 20 to "C). The shift of v, + v,, for example, is much slower: only about 30 cm-'. At the same time v, shifts by 50 cm-' to lower frequencies and v, by 24 cm-' to higher frequencies. Asselin and Sandorfy (26) made similar observations for alcohols. Lucazeau and Sandorfy (27) gave examples to the effect that the shifting of combination bands is usually

5 3644 CAN. J. CHEM. VOL. 51, 1973 CONC M SOLV. CCI,F-C2Br2F, CM' FIG. 6. The infrared spectrum ( cm-i) of N-methylpivalamide at different temperatures in a 1 :I mixture of CC1,F and CF2Br-CF2Br. Concentration at 22 "C: M; cell length: mm. intermediate between that of their constituting fundamentals. In order to ascertain if there are bands due to on anharmonic coupling constants in the case of secondary amides. The calculation of the anharmonic coupling constants is hampered by the fact that both the fundamental and the overtone of v, might be influenced by Fermi resonance-with v:2 and v, + 2v2 respectively-and all we can do is to compute upper and lower limits for XI,, X,,, and XI, using first the observed values of the frequencies and then their unperturbed values. For the three-atomic model the frequencies are given, to the second order, by the following formulae if o, and w2 are the harmonic frequencies of v, and v2 and the Xij are the anharmonic coupling constants. [3] v, + v2 = 0, + 0, + 2x1, + 2x2, + 2X XI, + 9 X23 [4] vio2 = 20, + 6X,, + XI, + X13 simultaneous excitation in the overtone region similar to those which were found for alcohols [5] vzo2 = x22 + X12 + X23 (15) we carried out an isotopic isolation experiment measuring the spectrum of a 1 :9 mixture of NMA and NMA-N-d from 22 to "C. No significant changes were observed. This eliminates this possibility which was unlikely anyway, since no two N-H bonds are directly linked together in the amide polymers or oligomers. Measurements similar to those reported for NMA were also carried out on N-ethylacetamide. Since the results were practically the same as for NMA we shall not describe them. N-methylpivalamide yielded one interesting observation. In this sterically hindered molecule v, is at higher frequencies since the hydrogen bonds are weaker, 3340 cin-' at "C (Fig. 6). The v2 deformation band is at lower frequencies, 1545 cm- ' at "C. Therefore v202 is more widely separated from vl and it is much weaker as for NMA. This shows that there is practically no Fermi resonance in this case and, on the other hand it corroborates the assumption that Fermi resonance does take place for NMA between vlo' and v202. Discussion The main purpose of our work has been to examine the effect of hydrogen bond formation The observed frequencies are, all for "C and for the polymeric species of NMA, From these we obtain Using these constants we can estimate the value of vl + 2v2. The sum of the observed vlol and twice the observed v201 is: , + 2x11 + 4x Xl2 + +XI, + X2, = x 1582 = 6420cm-'

6 I BERNARD-HOUPLAIN AND SANDORFY: LOW TEMPERATURE I.R. STUDY 3645 This is lower than the v, + 2v2 combination [6] by 2X2, + 2X12. Using the above values for X,, and X,, we have that while the observed, apparent peak is at 6500 cn1-'. If, instead of using the observed frequencies we take the unperturbed values of vlo' and v,02 which are 3221 and 3145 cm-' respectively we obtain X,, = - 80 cm-' X,, = - 10 cm-' XI, = +79 cm-' v, + 2v2 = 6525 cm-' The unperturbed frequencies should not be considered as very accurate. We are dealing with two broad bands whose different parts are affected to different degrees by Fermi resonance. Since the wings facing each other are likely to be affected most the split of the maxima appears to be larger than it is in reality and the unperturbed values lie probably somewhere at halfways between the observed and calculated values. Thus we can give X,, an estimated value which is intermediate between - 10 and -27. We cannot compare this value to the corresponding value of the free species since v202 was not found for the latter. The coupling constant X,, has a value between + 44 and + 79 cm- '. This is a very significant change from -7 cm-' (22 "C) which is the value we obtained for the free species. The case of XI, is more delicate. The mean value of and - 80 cm-' would be - 98 cm-' which we believe is close to its real value. The problem is complicated by the possibility of still another Fermi resonance, between vio2 and v, + 2v2. However, the above calculations show that v, + 2v2 falls into the 6500 cm-' region close to its observed value. Therefore we do not believe that the unperturbed value of vloz is greatly different from the observed one. At any rate, our value of XI, remains somewhat uncertain. In spite of this it seems to be reasonable to conclude to a moderate increase in X,, I upon hydrogen bond formation. What causes 1 the uncertainty is essentially the fact that, unlike in the cases of alcohols and amines, v, + 2v2 I has a frequency higher, not lower, than vlo2. ; Thus while the XI, computed with disregard of Fermi resonance are minimum values for alcohols and amines (because the frequency of the overtones was heightened by thel-esonance) this is not so for amides. The anharmonicity of v, is weak and still negative but hydrogen bonding increases its value. More important than this is the large increase in the value of the coupling constant X,, which becomes positive upon hydrogen bond formation. Since X,, enters the expression of v201 [I] it actually raises its frequency by a nonnegligible amount. The usual Av-AH relationships cover probably a number of such anharmonic contributions. Conclusions Our low temperature measurements give evidence to the existence of at least two hydrogen bonded adducts for non-sterically hindered amides. The pattern of association is closer to that of alcohols than to that of amines. Contrary to amines the overtones of the associated NH stretching vibration are readily identified. Associated amides have well defined in-plane deformation bands and there is strong anharmonic coupling between these and the NH stretching vibration. The sizable increase in the value of these coupling constants upon hydrogen bond formation and the Fermi resonance it causes constitute a significant fact for the understanding of the nature of hydrogen bonding. We acknowledge financial help including a scholarship (M.-C.B.-H.) from the National Research Council of Canada. 1. M-C. BERNARD-HOUPLAIN and C. SANDORFY. J. Chem. Phys. 56, 3412 (1972). 2. M-C. BERNARD-HOUPLAIN, G. B~LANGER, and C. SANDORFY. J. Chem. Phys. 57,530 (1972). 3. M-C. BERNARD-HOUPLAIN and C. SANDORFY. Can. J. Chem. 51, 1075 (1973). 4. L. J. BELLAMY. Advances in infrared frequencies. Methuen and Co., Ltd. London p S. MIZUSHIMA, T. SIMANOUTI, S. NAGAKURA, K. KURATANI, M. TSUBOI, H. BABA, and 0. FUJIOKA. I. Am. Chem. Soc. 72, 3490 (1950). 6. M. DAVIES, I. C. EVANS, and R. L. JONES. Trans. Faraday Soc. 51, 761 ( 1955). 7. I. M. KLOTZ and J. S. FRANZEN. I. Am. Chem. SOC. 84, 3461 (1962). 8. Y. H. SHAW and N. C. LI. Can. I. Chem. 48, 2090 (1970). 9. M. ASSELIN and C. SANDORFY. I. Mol. Struct. 8, 145 (1971).

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