- Email: [email protected]
Association and Fermi resonance of the simplest liquid primary aliphatic amines and their N-deutero analogues
pp.
Spectrochimica Acta, Vol. 36A, 899 to 901 @ Pergamon Press Ltd. 1980. Printed in Great Britain
0584-8539/80/1001-0899$02.00/O
Association
primary
and Fermi resonance of the simplest liquid amines and their N-deutero analogues
aliphatic
H. WOLFF, UDO SCHMIDT and E. WOLFF Physikalisch-chemisches
Institut,
Universitat
Heidelberg,
(Receioed
D-69
Heidelberg, Federal Republic of Germany
11 March 1980)
Abstract-The ix. spectra of liquid CH,ND,, C,H,ND, and n-C,H,ND, in the region of ND, stretching vibrations are compared to the spectra of the non-deuterated analogues in the region of NH, stretching vibrations. The observations in the spectra and the results of a Fermi resonance analysis are compatible with the interpretation that the symmetric ND, stretching vibration and the overtone of the ND, deformation vibration of the simple primary amines cross over with selfassociation. CF,CH,NH, and CF,CH,ND, having been investigated as models for the amine association show such a behavior only for the mixed association with strong acceptors.
The symmetric NH, stretching vibration and the overtone of the NH2 deformation vibration of associated primary aliphatic amines form a Fermi resonance doublet [l]. As far as in view of the mixing of the eigenfunctions the doublet bands can still be assigned to the symmetric vibration and the overtone, the latter lies in the i.r. spectra of liquid CH3NH2, C&H,NH* and n-C,H,NH, on the lowfrequency side of the symmetric vibration [2]. If the Fermi resonance analysis is based on the spectra of the undiluted compounds at -20°C [2], the ratio p of the i.r. intensities of the overtone and the symmetric vibration is found between 0.43 and 0.65. In calculating [l, 31 with these values and the measured frequencies [2], unperturbed symmetric vibrations are obtained which are shifted by 70-75 cm-’ to lower wave numbers in comparison to the monomeric frequencies (Table 1). These shifts are 30-35 cm-’ larger than the measured ones and, therefore, reveal the association of the amines more clearly than the observed values. Table 1. Results of the Fermi resonance analysis for undiluted liquid primary aliphatic amines and their Ndeutero-derivatives; frequencies are given in cm-’ for -20°C; vS and 213 = observed frequencies of the symmetric vibration and of the overtone of the deformation vibration; p = intensity ratio of 2S and v,; ‘vS and “26 = calculated unperturbed frequencies; Av, = association shift of the unperturbed symmetric vibration relative to the frequency of the monomers in Ccl,* Amine
v,
3288 CH,NH, 3278 CaHsNH, nC,H,NH,3283 2381 CH,ND, 2370 C,HsND, nC,H,ND,2372
2s
p
ov*
“2s
AvS
3197 3188 3193 2442 2432 2429
0.43 0.46 0.65 0.24 0.52 0.51
3261 3250 3248 2393 2391 2391
3224 3216 3228 2430 2411 2410
76 71 75 56 48 46
*The values of vS and 2S and of the monomeric frequencies used for the calculation of Av$ are obtained from Ref. [2] for the non-deuterated amines; the values for the deuterated compounds are taken from Table 2. The p-values have been obtained from the spectra of reference [2] and the spectra of this work by band separation. %A) 36,IOpr
In order to compare the resonance of the normal amines to that of the N-deuterated analogues, the spectra of liquid CH,ND,, C,H,ND, and n-C3H,ND2 and of carbon tetrachloride solutions of these compounds were determined by the technique used for the non-deuterated amines [4]. The spectra obtained as a function of the temperature, which for -20°C are reproduced in Figs. 1 and 2 with the extinction coefficients and in Table 2 with the measured wave numbers, considerably deviate from the spectra of the NH2 compounds in the region of NH2 stretching vibrations. However, in the spectrum of CH,ND, obtained for the highest dilution (Fig. 1, curve 1) the bands at 2537 and 2449 cm-’ doubtless belong to the antisymmetric and the symmetric ND1 stretching vibrations of the monomers; the ND stretching vibration of CH,NHD in a dilute solution with Ccl, is situated almost exactly in the middle of the two coupled vibrations (Table 3). The weak and comparatively broad band at 2388 cm-’ strongly increases in intensity with enhancement of the amine concentration (curves 2-5) and, therefore, possibly represents a band of associated CH3ND2. At any rate, for the monomeric amine in the solution the overtone can be assumed to appear at a lower wave number than the symmetric vibration; a value of about 2430 cm-’ results if 15-20 cm-’ for the solvent shift and approximately as large an amount for the anharmonicity are subtracted from 2468 cm-‘, which is double the value of the deformation fundamental of the gaseous amine [5]. With the association, i.e. the transition from curve 1 to curve 5, apart from the absorption at 2380-2390 cm-’ also the absorption at 24402450 cm-’ increases in intensity. However, even for the undiluted amine for which the maximum is measured at 2442 cm-‘, the absorption attains only about a quarter of the intensity of the lowfrequency absorption measured for the undiluted compound at 2381 cm-‘. Since with Fermi resonance merely an intensity equalization results and since the overtone borrows its intensity from the 899
900
H. WOLFF, U. SCHMIDT and
E. WOLFF
Table 2. Infrared bands (cm-‘) of dilute solutions of C,H,ND, and n-C,H,ND, in CCI, and of the CH,ND,, undiluted compounds at -20°C; x is the mole fraction of amine* Amine
x
.
0.0226 1 GH&D, 0.0318 1 n-CsH,NDz0.0371 1
CH&Dz
up””
v:”
v.““”
2~“’
t
v:”
2537 2528 2530 -
2513 2508 2514
2449 2439 2437 -
2442 2432 2429
2388 2384 2382 2376 -
2381 2370 2372
* vFon and yF”n = monomeric frequencies of the antisymmetric and the symmetric ND, stretching vibrations; Y:6”, Y?, and 2P = associated frequencies of the antisymmetric ND, stretching vibration, the symmetric ND, stretching vibration, and the overtone of the ND, deformation vibration; t See text.
6 -
2366
2600
2700
cm-l Fig. 1. ND, stretching vibration region of CH,ND, in solutions with CCI, and for the undiluted state, at -20°C +20”(z): 1. 0.0244 mot l-t, 2. (concentrations at 0.243 mall-‘, 3. 1.96moll-‘, 4. 8.53moll-‘, 5. 22.9 mol I-’ (undiluted). The spectra are compensated for CC& absorption.
vibration, this observation is evidence of the fact that the overtone of associated CH,ND, occurs on the high-frequency side of the symmetric vibration. The value of 2393 cm-’ is calculated for the unperturbed symmetric vibration from the pvalue of 0.24 and the measured frequencies (Table 1). An association shift of 56 cm-’ corrected for Fermi resonance is found instead of the observed shift of 68 cm-‘; the lower value is the consequence of the reversed positions of the symmetric vibration and the overtone. Based on the shift of 56 cm-’ and that of 76 cm-’ for CH3NH2 (Table l), a value of approximately l/a results for Av(CH,NDJAv(CH3NH2), while the ratio of the observed shifts is considerably higher. Further, it agrees with reasonable expectations that the overtone calculated as 2430 cm-’ (Table 1) coincides with the value estimated for the monomeric overtone; there is little influence of association on the deformation vibration of the simple primary amines [l, 61. The fact that the lags behind that of 69 cm-’ value of 56cm-’ which is calculated from the data of Table 3 for the association shift of the ND stretching vibration of
symmetric
CH,NHD, is a consequence of the coupling splitting of the symmetric and the antisymmetric stretching vibrations; the coupling splitting reduces the symmetric frequency of monomers more strongly than that of 1: 1 complexes; these associated species can be assumed to predominate in the undiluted amine, as only the symmetric stretching vibration is significantly lowered in comparison to the monomeric frequency [l]. The small Fermi resonance correction of only 12 cm-’ for p = 0.24 can be explained from the low value of the perturbation term; this term, double the value of which yields the splitting with exact resonance, is approximately half as large for the N-deuterated amines as for the non-deuterated amines; it amounts to only about 25 cm-’ [l]. In the spectra of C,H,ND, [Fig. 2(a)] a relatively sharp band at 2382-2384 cm-‘, which for the higher concentrations is observed as a shoulder of the low-frequency band of the triplet in the region of ND2 stretching vibrations, probably belongs to a combination tone. However, the assignment and the Fermi resonance analysis of the bands of CzH5ND, and of n-C3H,ND2 [Fig. 2(b)] are analogous to the assignment and to the analysis of CH,ND,. As shown by Table 1, the association shifts of the corrected symmetric vibrations of the two homologues are likewise reduced and amount to approximately 50 cm-‘; they, therefore, come nearer to expectations. Above all a cross-over of Table 3. NH and ND-stretching vibration bands (cm-‘) of CH,NHD, C,H,NHD, and n-C,H,NHD in dilute solutions with Ccl,, and ND stretching vibration band of undiluted (associated) CH,NHD* Amine
v(NH)~“”
v(ND)~O”
v(ND)BS
CH,NHD C,H,NHD n-C,H,NHD
3368 3358 3361
2491 2482 2484
2422 _
* v(NH)~O” and v(ND)~“” = monomeric NH and ND frequency. frequencies, u(ND)Bse = associated
ND
Association and Fermi resonance of the simplest liquid primary aliphatic amines
a
901
n-C3bND2
C2YO2
i
w
cm-1 Fig. 2. (a) ND, stretching vibration region of C2HSND, in solutions with Ccl, and for the undiluted state, at -2OT: 1. 0.349 mol I-‘, 2. 1.12 mol I-‘, 3. 11.8 mol I-‘, 4. 16.2 mol I-’ (undiluted); (b) ND, stretching vibration region of n-C,H,ND, in solutions with CCI, and for the undiluted state, ai -20°C: (1) 0.393 mol I-‘, (2) 1.03 mol I-’ (3) 12.8 mol I-’ (undiluted). The spectra are compensated for Ccl, absorption. the symmetric vibration and the overtone of the REFERENCES deformation vibration is again detected with the [l] H. WOLFF and D. HORN, Ser. Bunsenges. Physik. self-association of the two compounds. The differChem. 71, 467 (1967); 72, 419 (1968). ences in the spectra of CH,ND*, C2H5ND2, and [2] H. WOLFF and U. SCHMIDT,Ber. Bunsenges. Physik. n-C,H,ND, compared to those of CHsNH2, Chem. 68, 579 (1964). [3] J. 0 VEXFND, in: Infrared Spectroscopy and Molecular C2H5NH,, and n-C,H,NH, can easily be explained Structure (Edited by M. DAVIES). Elsevier, Amsterfrom this behavior which only with the mixed asdam (2 963) sociafion witi strong acoqtors is obszved for 14j The ND, compounds were prepared in the same way CF,CZ4&Hz as+!. ‘&C&Y%!&, SW ASS&& SW &TV as I’n vapor pressure investigations of the amine association [H. WOLFF and A. H~PFNER, Ber. Bunamine association [ 11. Acknowledgements-We gratefully acknowledge support of this work by Deutsche Forschungsgemeinschaft, Bad Go&eCDeg. *PYFL&S &T mm% _~&v.&&K?&xJ 4~ BAS"r, LubH;@xiren.
senges. Physik. Chem. 69, 710 (1965)]. T5l A. P. GRAY and R. C. LORD, J. C&em. Phvs. 26. 690
Spectrochimica Acta, Vol. 36A, 899 to 901 @ Pergamon Press Ltd. 1980. Printed in Great Britain
0584-8539/80/1001-0899$02.00/O
Association
primary
and Fermi resonance of the simplest liquid amines and their N-deutero analogues
aliphatic
H. WOLFF, UDO SCHMIDT and E. WOLFF Physikalisch-chemisches
Institut,
Universitat
Heidelberg,
(Receioed
D-69
Heidelberg, Federal Republic of Germany
11 March 1980)
Abstract-The ix. spectra of liquid CH,ND,, C,H,ND, and n-C,H,ND, in the region of ND, stretching vibrations are compared to the spectra of the non-deuterated analogues in the region of NH, stretching vibrations. The observations in the spectra and the results of a Fermi resonance analysis are compatible with the interpretation that the symmetric ND, stretching vibration and the overtone of the ND, deformation vibration of the simple primary amines cross over with selfassociation. CF,CH,NH, and CF,CH,ND, having been investigated as models for the amine association show such a behavior only for the mixed association with strong acceptors.
The symmetric NH, stretching vibration and the overtone of the NH2 deformation vibration of associated primary aliphatic amines form a Fermi resonance doublet [l]. As far as in view of the mixing of the eigenfunctions the doublet bands can still be assigned to the symmetric vibration and the overtone, the latter lies in the i.r. spectra of liquid CH3NH2, C&H,NH* and n-C,H,NH, on the lowfrequency side of the symmetric vibration [2]. If the Fermi resonance analysis is based on the spectra of the undiluted compounds at -20°C [2], the ratio p of the i.r. intensities of the overtone and the symmetric vibration is found between 0.43 and 0.65. In calculating [l, 31 with these values and the measured frequencies [2], unperturbed symmetric vibrations are obtained which are shifted by 70-75 cm-’ to lower wave numbers in comparison to the monomeric frequencies (Table 1). These shifts are 30-35 cm-’ larger than the measured ones and, therefore, reveal the association of the amines more clearly than the observed values. Table 1. Results of the Fermi resonance analysis for undiluted liquid primary aliphatic amines and their Ndeutero-derivatives; frequencies are given in cm-’ for -20°C; vS and 213 = observed frequencies of the symmetric vibration and of the overtone of the deformation vibration; p = intensity ratio of 2S and v,; ‘vS and “26 = calculated unperturbed frequencies; Av, = association shift of the unperturbed symmetric vibration relative to the frequency of the monomers in Ccl,* Amine
v,
3288 CH,NH, 3278 CaHsNH, nC,H,NH,3283 2381 CH,ND, 2370 C,HsND, nC,H,ND,2372
2s
p
ov*
“2s
AvS
3197 3188 3193 2442 2432 2429
0.43 0.46 0.65 0.24 0.52 0.51
3261 3250 3248 2393 2391 2391
3224 3216 3228 2430 2411 2410
76 71 75 56 48 46
*The values of vS and 2S and of the monomeric frequencies used for the calculation of Av$ are obtained from Ref. [2] for the non-deuterated amines; the values for the deuterated compounds are taken from Table 2. The p-values have been obtained from the spectra of reference [2] and the spectra of this work by band separation. %A) 36,IOpr
In order to compare the resonance of the normal amines to that of the N-deuterated analogues, the spectra of liquid CH,ND,, C,H,ND, and n-C3H,ND2 and of carbon tetrachloride solutions of these compounds were determined by the technique used for the non-deuterated amines [4]. The spectra obtained as a function of the temperature, which for -20°C are reproduced in Figs. 1 and 2 with the extinction coefficients and in Table 2 with the measured wave numbers, considerably deviate from the spectra of the NH2 compounds in the region of NH2 stretching vibrations. However, in the spectrum of CH,ND, obtained for the highest dilution (Fig. 1, curve 1) the bands at 2537 and 2449 cm-’ doubtless belong to the antisymmetric and the symmetric ND1 stretching vibrations of the monomers; the ND stretching vibration of CH,NHD in a dilute solution with Ccl, is situated almost exactly in the middle of the two coupled vibrations (Table 3). The weak and comparatively broad band at 2388 cm-’ strongly increases in intensity with enhancement of the amine concentration (curves 2-5) and, therefore, possibly represents a band of associated CH3ND2. At any rate, for the monomeric amine in the solution the overtone can be assumed to appear at a lower wave number than the symmetric vibration; a value of about 2430 cm-’ results if 15-20 cm-’ for the solvent shift and approximately as large an amount for the anharmonicity are subtracted from 2468 cm-‘, which is double the value of the deformation fundamental of the gaseous amine [5]. With the association, i.e. the transition from curve 1 to curve 5, apart from the absorption at 2380-2390 cm-’ also the absorption at 24402450 cm-’ increases in intensity. However, even for the undiluted amine for which the maximum is measured at 2442 cm-‘, the absorption attains only about a quarter of the intensity of the lowfrequency absorption measured for the undiluted compound at 2381 cm-‘. Since with Fermi resonance merely an intensity equalization results and since the overtone borrows its intensity from the 899
900
H. WOLFF, U. SCHMIDT and
E. WOLFF
Table 2. Infrared bands (cm-‘) of dilute solutions of C,H,ND, and n-C,H,ND, in CCI, and of the CH,ND,, undiluted compounds at -20°C; x is the mole fraction of amine* Amine
x
.
0.0226 1 GH&D, 0.0318 1 n-CsH,NDz0.0371 1
CH&Dz
up””
v:”
v.““”
2~“’
t
v:”
2537 2528 2530 -
2513 2508 2514
2449 2439 2437 -
2442 2432 2429
2388 2384 2382 2376 -
2381 2370 2372
* vFon and yF”n = monomeric frequencies of the antisymmetric and the symmetric ND, stretching vibrations; Y:6”, Y?, and 2P = associated frequencies of the antisymmetric ND, stretching vibration, the symmetric ND, stretching vibration, and the overtone of the ND, deformation vibration; t See text.
6 -
2366
2600
2700
cm-l Fig. 1. ND, stretching vibration region of CH,ND, in solutions with CCI, and for the undiluted state, at -20°C +20”(z): 1. 0.0244 mot l-t, 2. (concentrations at 0.243 mall-‘, 3. 1.96moll-‘, 4. 8.53moll-‘, 5. 22.9 mol I-’ (undiluted). The spectra are compensated for CC& absorption.
vibration, this observation is evidence of the fact that the overtone of associated CH,ND, occurs on the high-frequency side of the symmetric vibration. The value of 2393 cm-’ is calculated for the unperturbed symmetric vibration from the pvalue of 0.24 and the measured frequencies (Table 1). An association shift of 56 cm-’ corrected for Fermi resonance is found instead of the observed shift of 68 cm-‘; the lower value is the consequence of the reversed positions of the symmetric vibration and the overtone. Based on the shift of 56 cm-’ and that of 76 cm-’ for CH3NH2 (Table l), a value of approximately l/a results for Av(CH,NDJAv(CH3NH2), while the ratio of the observed shifts is considerably higher. Further, it agrees with reasonable expectations that the overtone calculated as 2430 cm-’ (Table 1) coincides with the value estimated for the monomeric overtone; there is little influence of association on the deformation vibration of the simple primary amines [l, 61. The fact that the lags behind that of 69 cm-’ value of 56cm-’ which is calculated from the data of Table 3 for the association shift of the ND stretching vibration of
symmetric
CH,NHD, is a consequence of the coupling splitting of the symmetric and the antisymmetric stretching vibrations; the coupling splitting reduces the symmetric frequency of monomers more strongly than that of 1: 1 complexes; these associated species can be assumed to predominate in the undiluted amine, as only the symmetric stretching vibration is significantly lowered in comparison to the monomeric frequency [l]. The small Fermi resonance correction of only 12 cm-’ for p = 0.24 can be explained from the low value of the perturbation term; this term, double the value of which yields the splitting with exact resonance, is approximately half as large for the N-deuterated amines as for the non-deuterated amines; it amounts to only about 25 cm-’ [l]. In the spectra of C,H,ND, [Fig. 2(a)] a relatively sharp band at 2382-2384 cm-‘, which for the higher concentrations is observed as a shoulder of the low-frequency band of the triplet in the region of ND2 stretching vibrations, probably belongs to a combination tone. However, the assignment and the Fermi resonance analysis of the bands of CzH5ND, and of n-C3H,ND2 [Fig. 2(b)] are analogous to the assignment and to the analysis of CH,ND,. As shown by Table 1, the association shifts of the corrected symmetric vibrations of the two homologues are likewise reduced and amount to approximately 50 cm-‘; they, therefore, come nearer to expectations. Above all a cross-over of Table 3. NH and ND-stretching vibration bands (cm-‘) of CH,NHD, C,H,NHD, and n-C,H,NHD in dilute solutions with Ccl,, and ND stretching vibration band of undiluted (associated) CH,NHD* Amine
v(NH)~“”
v(ND)~O”
v(ND)BS
CH,NHD C,H,NHD n-C,H,NHD
3368 3358 3361
2491 2482 2484
2422 _
* v(NH)~O” and v(ND)~“” = monomeric NH and ND frequency. frequencies, u(ND)Bse = associated
ND
Association and Fermi resonance of the simplest liquid primary aliphatic amines
a
901
n-C3bND2
C2YO2
i
w
cm-1 Fig. 2. (a) ND, stretching vibration region of C2HSND, in solutions with Ccl, and for the undiluted state, at -2OT: 1. 0.349 mol I-‘, 2. 1.12 mol I-‘, 3. 11.8 mol I-‘, 4. 16.2 mol I-’ (undiluted); (b) ND, stretching vibration region of n-C,H,ND, in solutions with CCI, and for the undiluted state, ai -20°C: (1) 0.393 mol I-‘, (2) 1.03 mol I-’ (3) 12.8 mol I-’ (undiluted). The spectra are compensated for Ccl, absorption. the symmetric vibration and the overtone of the REFERENCES deformation vibration is again detected with the [l] H. WOLFF and D. HORN, Ser. Bunsenges. Physik. self-association of the two compounds. The differChem. 71, 467 (1967); 72, 419 (1968). ences in the spectra of CH,ND*, C2H5ND2, and [2] H. WOLFF and U. SCHMIDT,Ber. Bunsenges. Physik. n-C,H,ND, compared to those of CHsNH2, Chem. 68, 579 (1964). [3] J. 0 VEXFND, in: Infrared Spectroscopy and Molecular C2H5NH,, and n-C,H,NH, can easily be explained Structure (Edited by M. DAVIES). Elsevier, Amsterfrom this behavior which only with the mixed asdam (2 963) sociafion witi strong acoqtors is obszved for 14j The ND, compounds were prepared in the same way CF,CZ4&Hz as+!. ‘&C&Y%!&, SW ASS&& SW &TV as I’n vapor pressure investigations of the amine association [H. WOLFF and A. H~PFNER, Ber. Bunamine association [ 11. Acknowledgements-We gratefully acknowledge support of this work by Deutsche Forschungsgemeinschaft, Bad Go&eCDeg. *PYFL&S &T mm% _~&v.&&K?&xJ 4~ BAS"r, LubH;@xiren.
senges. Physik. Chem. 69, 710 (1965)]. T5l A. P. GRAY and R. C. LORD, J. C&em. Phvs. 26. 690