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15N isotope effects on the vibrational frequencies of aniline and assignments of the frequencies of its nh2 group
?N isotope effects on the vibrational frequencies of aniline and assignments of the frequencies of its NH, group M. Tsunor Depamen*of Chemistry,FaGultyof Science, Tokyo University,
Hongo, T&p,
Japan
(Received 4 Jtwauu y 1960)
A~~~~~-X~fr~6d a~~~t~o~~ of aniline in which 31.9 per cent of r&rogen is 15N and 3&l per ccmt is.1% were axmined in dilute Ccl, and CS, solutions by means of a grating instrument. For fivo bands at 3481.4, 3395-2, 161&Q, 1276.1 and 1114.6 cm-l appreciableisotope shifts were observed. It was shown that these bands are to be assigned,respectively,to NH, antisymmetric stretohing, NW, symmetric stretching, NH, bending, C-N stretching and NH, rocking (or twisting} vibrations. Introduction
ONE of the interesting problems in the infrared study of aniline is that of the structure of the aromatic NH, group. It has been shown that in the NH, group of methylamine (a representative aliphatic amine) the nitrogen valence bonds are pyramidal, its atomic orbital being approximately sp3-hybridized, whilst in the NH, group of form~mide (a representative amides the nitrogen valence bonds are planar, its atomic orbital being near to ~~z-hyb~~~ation [l--3]. Of the NH, group of aniline (a representative aromatic amine), however, the geometrical and electronic structures are not yet clear. One approach to this problem is to inquire, where the frequencies of the aromatic NH, group of aniline appear, and whether these frequencies are close to those of the pyramidal-type NH, group or to those of the planar-type NH, group. Previous work on the infrared spectra of aniline, N-deuterated aniline and related compounds in the gaseous [4] and liquid [G]states and in solutions [5, 61 has not yet given conclusive answers to the questions. In order to obtain new information on the nature of the normal vibrations in aniline, examinations were made by the writer of the effects of 15N substitution for i*N on its vibrational frequencies. The purpose of this paper is to present the results of the examinations and to show a reasonable set of assignments of the NE, frequencies in aniline. Tn the last part a comparison is to be made of the NH, frequencies thus assigned in aniline with those of methylamine and formamide. The assignments shown here enable us to estimate the internal potential constants of the NH, group of aniline, which will be given in a later paper with a discussion of the structure of the aromatic NH, group. [l] x1. SKIMODA,T. NISHIXAWA and T. ITOH,J. Phys. Sot. Japan 9, 974 (1954); T. Prop, Ibid. 11, 264 (1956); T. ifE3I=AWA, Ibid. 12, 668 (1957); D. R. Lxn~,J. C&i% P&/s. 27, 343 (1957). [23 R. J. KVR~ZTD and E. B. WILSON, J. Chem. Phys. 27, 685 (1957); J. LADELL and 3% POST, A&T C7y.e. 7, 559 (1954).
--1
1290
J 1
1280
I
I
12m
1260
Wovenumber,
cm-’
-
1”N isotope effects on the vibrational frequencies of aniline
path length for every 31*9°/o-15N-aniline solution was adjusted to be 99.6/6&l times that for the corresponding solution of ordinary aniline, so that the absorbance due to the 14N-aniline is equal in these two systems, and the absorbance due to the 16N-aniline is easily read as the difference in the observed absorbance between the two (see Fig. 1). A precise infrared absorption measurement was carried out in the range of 3600-600 cm-l using a Perkin-Elmer 112G grating spectrometer, with a 75 lines/mm grating and a KBr fore-prism. The wavelength calibration for the spectrometer was made with known frequencies of 296 absorption lines reported in a previous paper [8]. The measurement in the 600-450 cm-l region was made by a Hilger H800 spectrometer with a KBr prism. Wave
Cd
number,
Fig. 2. Infrared absorption spectrum of aniline in its dilute carbon tetrachloride (3600-1400 cm-l) and carbon disulphide (1400-460 cm-l) solutions! mostly determined by the use of a grating instrument. Dotted lines indicate the portlons of the spectrum where the measurement was less accurate than in other portions because of the strong absorptions of the solvent (CS,) or because any grating instrument was not used there. The arrows indicate the bands which show appreciable 16N isotope shifts. ReSUltS
The infrared spectrum of aniline in dilute solutions (solvents used being carbon tetrachloride in the 3600-1400 cm-l region and carbon disulphide in the 1400450 cm-l region) is shown in Fig. 2. As is seen in the figure, there are thirty-three bands observed in the spectral range of 3600-450 cm-l. One of these at about 493 cm-l is out of the range of the grating instrument. The frequencies of five others at about 1190, 1173, 926, 901 and 870 cm-l cannot be measured accurately because of interference by the solvents. The remaining twenty-seven absorption bands were subjected to the precise measurement. Five of these show appreciable isotope shifts (AY > 1 cm-l). Their frequencies and shifts are given in Table 1. The amounts of the isotope shifts are given not only by the frequency, Av (cm-l), but also by Ail I( 15N) --I( 14N) F= I( 14N) where c being the light velocity absorption.
I = 4&wv2 and v the observed
[8] S. M~zmm~aan,Rep& on the Perkin-Elmer Corporation, Norwalk (1959).
Grating
507
frequency
(in cm-l)
Spectrophotometer
Model
at the maximum
112-G.
Perkin-Elmer
M. TSUBOI Table 1. 16N isotope shifts in the vibrational frequencies of aniline in dilute Ccl, and CS, solutions Frequency, v (cm-l)
Shift
14N species
15N species
Av (cm-l)
3481.4 3395.2 1618.9 1276.1 1114.6
3471.7 3389.8 1612.9 1270.9 1112.2
-9.7 -5.4 -6.0 -5.2 -2.4
Assignment AA/A0
-0.00556 -0.00318 -0.0074 -0.0081 -0.0043
NH, antisym. stretch NH, sym. stretch NH, bend (*C-N str.) C-N str. ( fNH, bend) NH, rock (or twist) ( *CH in-plane bend)
An estimation of the isotope effects on some ideal vibrations along symmetry co-ordinates According to a perturbation treatment for the isotope shift due to small mass ,change [9], the values of A&/&O for the kth normal vibration may be taken to be *equal to the diagonal element AkX of the matrix A, where A = Lo-' AGL;l ,or
AM = WO-%(LO-~)W II’
AGw
1
in which L is the L-matrix for the i4N species that connects ,co-ordinates S, and the set of normal co-ordinates Qk, and AG = G -
(I) a set of symmetry
Go
,G and Go being the G-matrices for the 16N and 14N species, respectively. If a certain normal co-ordinate QI, is nearly equal to a certain symmetry co-ordinate S,, then all are nearly zero except (Lo-l)kt, which is nearly equal to l/d(G”,,) the (LO-%, .since (Lo),,(ro),,
= GOlck. Therefore,
in this case,
(2) The molecular symmetry of the aniline molecule is CZUor C, depending upon whether the NH, group is planar or pyramidal. For each of these the symmetry ,co-ordinates, according to the usual expression, may be given as in Table 2. In Table 3 are given the values of A&/&O calculated from equation (2) on the basis of the following three geometrically different models of the molecule: (1) Planar (C,,) model with the HNH angle (6) 120’. (2) Planar (C,,) model with the HNH angle (0) 107”. (3) Pyramidal (C,) model in which the HNH and two CNH angles are all tetrahedral (109@). X9] E. B. WILSON, J. C. DECIUSand P. C. CROSS,Molecular (1955).
508
Vibrations p. 188. McGraw-Hill, New York
W
isotope effects on the vibrational frequencies of aniline
Assignments of the NE, group frequencies 1. NH, stretching frequencies Two bands observed at 3481.4 and 3395.2 cm-l are assigned to the NH, stretching vibrations since they are in the expected frequency region and show 1hN isotope shifts. Each of the NH, symmetric and antisymmetric stretching modes is considered to be almost separated from other vibrational modes in the normal vibrations in the aniline molecule. Hence the Ai1/3L”values calculated from equation (2) for these two vibrational modes have much more significance than the values for other Table 2.
Some of the symmetry
For C,,model
NH, NH, C-N NH, NH, NH, NH,
%(
4
Bl i 4
co-ordinates in the aniline molecule
NH, NH, C-N NH, NH, NH, NH,
symmetric stretching bending stretching wagging antisymmetric stretching rocking twisting
Table 3.
Symmetry co-ordinate
NH, antisym. str. NH, sym. str. NH, bend C-N str. NH, rock (B,) or NH, twist (A”)
For C, model
I
symmetric stretching bending stretching wagging antisymmetric stretching twisting rooking
A’ I A” I
Calculated and observed isotope shifts, AL/I0 of aniline
I
AI/IO, calculated for the model
I
Pyramidal with 6 = 109.5”
I--~-0.00645 ~~ -0.00230 -0.0065 -0.0307 -0.0111
/ I
-0.00566 -0.00322 -0.0057 -0.0307
-0~00582 -0~00304 -0.0052 -0.0307
- 0.0126
-0~0080
!
Observed AL/IO
Frequency (cm+)
-0.00556 -0~00318 -0.0074 -0~0081
3481.4 3395.2 1618.9 1276.1
-0.0043
1114.6
vibrational modes. Both of these Al/A0 values depend almost solely upon the value of the HNH angle (0) of the NH, group in question. Since the angle 0 here is no doubt obtuse, the Al/Lo value for the antisymmetric stretching frequency must be Therefore the frequency greater than that for the symmetric stretching frequency. 3481.4 cm-l, with the greater Al/I0 value, is assigned to the antisymmetric stretching vibration, and the frequency 3395.2 cm-r with smaller Al/il” value to the symmetric one. As is seen in Table 3, the observed values of AI./i1Oare in better agreement with those calculated on the assumption that 8 = 107” than with those calculated on the assumption that 0 = 120” or 8 = 109.5”. A more detailed discussion on this point will be given. in a later paper. 509
M. TSUBOI
The NH, bending f~quen~y is expected to be at about 1600 cm-r and the C-N stretching frequency in the 1200-1300 cm-l region. Two bands observed at 16184 and 1276.1 om-l are in the expected regions and show definite l&N isotope shifts. Therefore these may be assigned respectively to the NH, bending and C-N stretching vibrations. However the observed Alz/P value is greater for the former and smaller for the latter than the corresponding value calculated from equation (2) (see Table 3). This may be interpreted as indicating that the NH, bending and C-N stretching modes are coupled with each other in the normal vibrations of the aniline molecule. This view is supported also by the examination of the Ndeuteration effects (see Section 5) below. 3. NH, $oc~~~g(B,) or ~~~~~~g(A”) ~r~q~e~~ A weak band observed at 11146 om-l in the CS, solution is considered to be related to the NH, rocking (if the NH, group is in a planar form} or NH, twisting (if the NW, group is in a p~amidal form) mode for the fo~o~ng reasons: (a) It shows definite IaN isotope shift. (b) Its frequency is in the expected region [l&14]. (0) Its counterpart in the liquid spectrum is observed at a little higher frequency, 11I9 cm-r, with much broader width and a little weaker intensity.* This is just as is expected for the NH, rocking for testing) band. (d) The band at 1119 cm-l just mentioned disappears on N-deuteration [5], As may be seen in TabIe 3, however, the observed value of A~~~0for this band is smaller than the calculated one. Hence, it is evident that the normal vibration for -this band is not the pure NH, rocking (or twisting) vibration but is a vibration in which the NH, rocking (or t~stlng) mode is coupled with some other B, (or A”) vibrational modes. A weak band at 1053am--l in the liquid spectrum, whose counterpart is found at 105544cm-l in the solution spectrum, also disappears on N-deuteration [5]. For this band, however, the frequency shift in going from liquid to solution takes place in the opposite direction to that expected for the NH, rocking {or testing) band [5].* In addition, this band does not show any appreciable l&N isotope shift (Av < 0.5 cm-l). Therefore this band is not considered as the one to which the NH, rocking (or testing) vibration gives the largest contribution. However, it is probable that the NH, rocking (or testing) vibration would give the second largest contribution to this normal vibration at 1055.8 cm-l. The view that the NH8 rocking (or twisting) mode participates in both the * The writer made infrared measurements on liquid and gaseous aniline, too. -be detailed elsewhere.
The results will
IlO] C. L. ANUELL, N. SESWPASLD, A. YAWAQOCHI,!I!. SHIMB~NOUCHI, T. MIYAZAWA and S. MI~~~ECI~~A, Trans. B’araday 800. S8, 589 (1967); A. YAMACWCHI,Nippon Kagak~ Zaashi 78, 672, 694 (1957). Ill] A. YAXAQUCEI,T. MIYAZAWA,T. SHIMANOUOHI &nd S. M~zwsarx~, Spect~~chim. Acta 10,170 (1967); A. YAMAOVQHI,NGppon Kqpkz~ Zasshi 78, 1319, 1467 (1957). :flZ] T. MIYAZAWA, i.Vi~on &g&i.6 ZassG 76, 821 (1955). 1131 J. C. EVANS, J. Chem. P&y& 22, 1228 (1954). $141 A. P. aRAY and It. C. LORD,J. Chem. PhyS. 26,690 (1967); A. ~AMXOUCEI, N$?~O% KCX@U &‘I#& 80, 1105 (1959).
nN isotope effects on the vibrations1 frequencies of aniline
normal vibrations 11146 and 10554 cm-l is supported also by the results of the examination of the ~-deuteration effects (see Section 5 below). 4. N& wagging freqaelzcy The NH, wagging frequency is expected in the region of 700-900 cm-i. No band is found in the solution spectrum nor in the vapour spectrum* that is to be assigned to the NH, wagging mode in this region. However, a broad band with its centre at about 700 cm-l is observed for the liquid state of aniline [5]*, but does not appear for the liquid state of ND,-aniline [5]. A similar band is observed for Table 4.
A snnunary of the N-deutexation effects on the liquid spectrum of aniline TSl. and their interpretation
Bands which appear in the spectrum of undeuterated aniline but not in that of N-deutorated aniline
Bands which appear in the spectrum of N-deuterated aniline but not in that of undeuterated aniline
Product rule test
Frequency (cm-r)
Assignment
Frequency (cm-r)
Assignment
Observed ratio
Calculated r&i0
_ 1621 1277 1
1119 1063 I
700
NH, bend and c--N str. NH, rook (or twist) and CH in-plane bend
NH, wag
1301
-_
1621 x 122 1301 x 1150
1150
-_ 1 1078 797
CH in-phme bend ND, rock (or twist)
B ;
1
_ 1.38
1119 x 1054 ~= 1 *3, 1078 x 797
1.36
1.36
<600
formamide in its liquid and solid states [lZ], but not in the vapour [133. This band is reasonably assigned to the NH2 wagging mode of formamide [12J. Therefore it is very probable that the broad band of liquid aniline around 700 cm-1 is due to the NH, wagging mode.
Infrared absorption of N-deuterated aniline was observed by CALIEANO and in its liquid state. The results show that all the bands of liquid aniline shift only very slightly on N-deuteration except those given in Table 4. Instead of the two bands at 1621 and 1277 cm-l of undeuterated liqnid aniline, which are oonsidered to correspond, respectively, to the 1618.9 and 1276.1 cm-1 bands already mentioned (see Section 2), two bands are found at 1301 and 1150 om-l in N-deuterated liquid aniline. These may be assigned, respectively, to almost pure C-N stretching and ND, bending vibrations. Instead of the two bands of undeuterated aniline at 1119 and 1053 cm-l already mentioned (see
MOCCIA [S]
* See footnote p. 510. 511
M. TSUBOI
Section 3) two bands are found at 1078 and 797 em-l in N-deuterated aniline. These may be assigned, respectively, to a B, (or A”) vibration (probably a C-H in-plane bending vibration) and the ND, rocking (or twisting) vibration. The ~signments presented here are in satisfactory agreement with the product. rule, as may be seen from the last column of Table 4. Table 5. Comparison of the NH, frequencies(cm-l) of formamide, aniline and methylamine
Assignment
NH, antisym. str. NH, sym. str. NH, bend C-N str. NH, rock (planes) or twist (pyramidal) NH, w&g
XXCONH, (gas)
3545 3450 1572 1253 1059 (700 in liq.)
ffP4
(in dil. soln.)
3498.0 3415.3 1619 1273
3481.4 3395-2 1618.9 1276.1
3427 3361 1623 1044
1114.6 -
l&Z5 780
-
_-
(‘700 in liquid)
~ -
As has been stated, five frequencies of aniline in dilute solutions are assigned to its NH, vibrations. Four of these are ako identified in the spectrum of the gas.* These frequencies are given in Table 5, together with the corresponding frequencies of formamide [I37 and methylamine [14] in their gaseous states. A comparison of the NH, stretching frequencies shows that the NH, group of aniline has an intermediate structure between those of the NH, groups of formamide and methylamine. On the basis of the NH, deformation frequencies and C-N stretching frequencies, the NH, group of aniline seems to be close to that of formamide rather than to that of methylamine. Acl&owEedgement~-- The writer wishes to express his sincere thanks to Professor SAN-ICHIRO MIZUSEXMA and Professor TAKEEIIEO SH~ANOUC~~for their v&able suggestions. * See footnotep. 510.
Hongo, T&p,
Japan
(Received 4 Jtwauu y 1960)
A~~~~~-X~fr~6d a~~~t~o~~ of aniline in which 31.9 per cent of r&rogen is 15N and 3&l per ccmt is.1% were axmined in dilute Ccl, and CS, solutions by means of a grating instrument. For fivo bands at 3481.4, 3395-2, 161&Q, 1276.1 and 1114.6 cm-l appreciableisotope shifts were observed. It was shown that these bands are to be assigned,respectively,to NH, antisymmetric stretohing, NW, symmetric stretching, NH, bending, C-N stretching and NH, rocking (or twisting} vibrations. Introduction
ONE of the interesting problems in the infrared study of aniline is that of the structure of the aromatic NH, group. It has been shown that in the NH, group of methylamine (a representative aliphatic amine) the nitrogen valence bonds are pyramidal, its atomic orbital being approximately sp3-hybridized, whilst in the NH, group of form~mide (a representative amides the nitrogen valence bonds are planar, its atomic orbital being near to ~~z-hyb~~~ation [l--3]. Of the NH, group of aniline (a representative aromatic amine), however, the geometrical and electronic structures are not yet clear. One approach to this problem is to inquire, where the frequencies of the aromatic NH, group of aniline appear, and whether these frequencies are close to those of the pyramidal-type NH, group or to those of the planar-type NH, group. Previous work on the infrared spectra of aniline, N-deuterated aniline and related compounds in the gaseous [4] and liquid [G]states and in solutions [5, 61 has not yet given conclusive answers to the questions. In order to obtain new information on the nature of the normal vibrations in aniline, examinations were made by the writer of the effects of 15N substitution for i*N on its vibrational frequencies. The purpose of this paper is to present the results of the examinations and to show a reasonable set of assignments of the NE, frequencies in aniline. Tn the last part a comparison is to be made of the NH, frequencies thus assigned in aniline with those of methylamine and formamide. The assignments shown here enable us to estimate the internal potential constants of the NH, group of aniline, which will be given in a later paper with a discussion of the structure of the aromatic NH, group. [l] x1. SKIMODA,T. NISHIXAWA and T. ITOH,J. Phys. Sot. Japan 9, 974 (1954); T. Prop, Ibid. 11, 264 (1956); T. ifE3I=AWA, Ibid. 12, 668 (1957); D. R. Lxn~,J. C&i% P&/s. 27, 343 (1957). [23 R. J. KVR~ZTD and E. B. WILSON, J. Chem. Phys. 27, 685 (1957); J. LADELL and 3% POST, A&T C7y.e. 7, 559 (1954).
--1
1290
J 1
1280
I
I
12m
1260
Wovenumber,
cm-’
-
1”N isotope effects on the vibrational frequencies of aniline
path length for every 31*9°/o-15N-aniline solution was adjusted to be 99.6/6&l times that for the corresponding solution of ordinary aniline, so that the absorbance due to the 14N-aniline is equal in these two systems, and the absorbance due to the 16N-aniline is easily read as the difference in the observed absorbance between the two (see Fig. 1). A precise infrared absorption measurement was carried out in the range of 3600-600 cm-l using a Perkin-Elmer 112G grating spectrometer, with a 75 lines/mm grating and a KBr fore-prism. The wavelength calibration for the spectrometer was made with known frequencies of 296 absorption lines reported in a previous paper [8]. The measurement in the 600-450 cm-l region was made by a Hilger H800 spectrometer with a KBr prism. Wave
Cd
number,
Fig. 2. Infrared absorption spectrum of aniline in its dilute carbon tetrachloride (3600-1400 cm-l) and carbon disulphide (1400-460 cm-l) solutions! mostly determined by the use of a grating instrument. Dotted lines indicate the portlons of the spectrum where the measurement was less accurate than in other portions because of the strong absorptions of the solvent (CS,) or because any grating instrument was not used there. The arrows indicate the bands which show appreciable 16N isotope shifts. ReSUltS
The infrared spectrum of aniline in dilute solutions (solvents used being carbon tetrachloride in the 3600-1400 cm-l region and carbon disulphide in the 1400450 cm-l region) is shown in Fig. 2. As is seen in the figure, there are thirty-three bands observed in the spectral range of 3600-450 cm-l. One of these at about 493 cm-l is out of the range of the grating instrument. The frequencies of five others at about 1190, 1173, 926, 901 and 870 cm-l cannot be measured accurately because of interference by the solvents. The remaining twenty-seven absorption bands were subjected to the precise measurement. Five of these show appreciable isotope shifts (AY > 1 cm-l). Their frequencies and shifts are given in Table 1. The amounts of the isotope shifts are given not only by the frequency, Av (cm-l), but also by Ail I( 15N) --I( 14N) F= I( 14N) where c being the light velocity absorption.
I = 4&wv2 and v the observed
[8] S. M~zmm~aan,Rep& on the Perkin-Elmer Corporation, Norwalk (1959).
Grating
507
frequency
(in cm-l)
Spectrophotometer
Model
at the maximum
112-G.
Perkin-Elmer
M. TSUBOI Table 1. 16N isotope shifts in the vibrational frequencies of aniline in dilute Ccl, and CS, solutions Frequency, v (cm-l)
Shift
14N species
15N species
Av (cm-l)
3481.4 3395.2 1618.9 1276.1 1114.6
3471.7 3389.8 1612.9 1270.9 1112.2
-9.7 -5.4 -6.0 -5.2 -2.4
Assignment AA/A0
-0.00556 -0.00318 -0.0074 -0.0081 -0.0043
NH, antisym. stretch NH, sym. stretch NH, bend (*C-N str.) C-N str. ( fNH, bend) NH, rock (or twist) ( *CH in-plane bend)
An estimation of the isotope effects on some ideal vibrations along symmetry co-ordinates According to a perturbation treatment for the isotope shift due to small mass ,change [9], the values of A&/&O for the kth normal vibration may be taken to be *equal to the diagonal element AkX of the matrix A, where A = Lo-' AGL;l ,or
AM = WO-%(LO-~)W II’
AGw
1
in which L is the L-matrix for the i4N species that connects ,co-ordinates S, and the set of normal co-ordinates Qk, and AG = G -
(I) a set of symmetry
Go
,G and Go being the G-matrices for the 16N and 14N species, respectively. If a certain normal co-ordinate QI, is nearly equal to a certain symmetry co-ordinate S,, then all are nearly zero except (Lo-l)kt, which is nearly equal to l/d(G”,,) the (LO-%, .since (Lo),,(ro),,
= GOlck. Therefore,
in this case,
(2) The molecular symmetry of the aniline molecule is CZUor C, depending upon whether the NH, group is planar or pyramidal. For each of these the symmetry ,co-ordinates, according to the usual expression, may be given as in Table 2. In Table 3 are given the values of A&/&O calculated from equation (2) on the basis of the following three geometrically different models of the molecule: (1) Planar (C,,) model with the HNH angle (6) 120’. (2) Planar (C,,) model with the HNH angle (0) 107”. (3) Pyramidal (C,) model in which the HNH and two CNH angles are all tetrahedral (109@). X9] E. B. WILSON, J. C. DECIUSand P. C. CROSS,Molecular (1955).
508
Vibrations p. 188. McGraw-Hill, New York
W
isotope effects on the vibrational frequencies of aniline
Assignments of the NE, group frequencies 1. NH, stretching frequencies Two bands observed at 3481.4 and 3395.2 cm-l are assigned to the NH, stretching vibrations since they are in the expected frequency region and show 1hN isotope shifts. Each of the NH, symmetric and antisymmetric stretching modes is considered to be almost separated from other vibrational modes in the normal vibrations in the aniline molecule. Hence the Ai1/3L”values calculated from equation (2) for these two vibrational modes have much more significance than the values for other Table 2.
Some of the symmetry
For C,,model
NH, NH, C-N NH, NH, NH, NH,
%(
4
Bl i 4
co-ordinates in the aniline molecule
NH, NH, C-N NH, NH, NH, NH,
symmetric stretching bending stretching wagging antisymmetric stretching rocking twisting
Table 3.
Symmetry co-ordinate
NH, antisym. str. NH, sym. str. NH, bend C-N str. NH, rock (B,) or NH, twist (A”)
For C, model
I
symmetric stretching bending stretching wagging antisymmetric stretching twisting rooking
A’ I A” I
Calculated and observed isotope shifts, AL/I0 of aniline
I
AI/IO, calculated for the model
I
Pyramidal with 6 = 109.5”
I--~-0.00645 ~~ -0.00230 -0.0065 -0.0307 -0.0111
/ I
-0.00566 -0.00322 -0.0057 -0.0307
-0~00582 -0~00304 -0.0052 -0.0307
- 0.0126
-0~0080
!
Observed AL/IO
Frequency (cm+)
-0.00556 -0~00318 -0.0074 -0~0081
3481.4 3395.2 1618.9 1276.1
-0.0043
1114.6
vibrational modes. Both of these Al/A0 values depend almost solely upon the value of the HNH angle (0) of the NH, group in question. Since the angle 0 here is no doubt obtuse, the Al/Lo value for the antisymmetric stretching frequency must be Therefore the frequency greater than that for the symmetric stretching frequency. 3481.4 cm-l, with the greater Al/I0 value, is assigned to the antisymmetric stretching vibration, and the frequency 3395.2 cm-r with smaller Al/il” value to the symmetric one. As is seen in Table 3, the observed values of AI./i1Oare in better agreement with those calculated on the assumption that 8 = 107” than with those calculated on the assumption that 0 = 120” or 8 = 109.5”. A more detailed discussion on this point will be given. in a later paper. 509
M. TSUBOI
The NH, bending f~quen~y is expected to be at about 1600 cm-r and the C-N stretching frequency in the 1200-1300 cm-l region. Two bands observed at 16184 and 1276.1 om-l are in the expected regions and show definite l&N isotope shifts. Therefore these may be assigned respectively to the NH, bending and C-N stretching vibrations. However the observed Alz/P value is greater for the former and smaller for the latter than the corresponding value calculated from equation (2) (see Table 3). This may be interpreted as indicating that the NH, bending and C-N stretching modes are coupled with each other in the normal vibrations of the aniline molecule. This view is supported also by the examination of the Ndeuteration effects (see Section 5) below. 3. NH, $oc~~~g(B,) or ~~~~~~g(A”) ~r~q~e~~ A weak band observed at 11146 om-l in the CS, solution is considered to be related to the NH, rocking (if the NH, group is in a planar form} or NH, twisting (if the NW, group is in a p~amidal form) mode for the fo~o~ng reasons: (a) It shows definite IaN isotope shift. (b) Its frequency is in the expected region [l&14]. (0) Its counterpart in the liquid spectrum is observed at a little higher frequency, 11I9 cm-r, with much broader width and a little weaker intensity.* This is just as is expected for the NH, rocking for testing) band. (d) The band at 1119 cm-l just mentioned disappears on N-deuteration [5], As may be seen in TabIe 3, however, the observed value of A~~~0for this band is smaller than the calculated one. Hence, it is evident that the normal vibration for -this band is not the pure NH, rocking (or twisting) vibration but is a vibration in which the NH, rocking (or t~stlng) mode is coupled with some other B, (or A”) vibrational modes. A weak band at 1053am--l in the liquid spectrum, whose counterpart is found at 105544cm-l in the solution spectrum, also disappears on N-deuteration [5]. For this band, however, the frequency shift in going from liquid to solution takes place in the opposite direction to that expected for the NH, rocking {or testing) band [5].* In addition, this band does not show any appreciable l&N isotope shift (Av < 0.5 cm-l). Therefore this band is not considered as the one to which the NH, rocking (or testing) vibration gives the largest contribution. However, it is probable that the NH, rocking (or testing) vibration would give the second largest contribution to this normal vibration at 1055.8 cm-l. The view that the NH8 rocking (or twisting) mode participates in both the * The writer made infrared measurements on liquid and gaseous aniline, too. -be detailed elsewhere.
The results will
IlO] C. L. ANUELL, N. SESWPASLD, A. YAWAQOCHI,!I!. SHIMB~NOUCHI, T. MIYAZAWA and S. MI~~~ECI~~A, Trans. B’araday 800. S8, 589 (1967); A. YAMACWCHI,Nippon Kagak~ Zaashi 78, 672, 694 (1957). Ill] A. YAXAQUCEI,T. MIYAZAWA,T. SHIMANOUOHI &nd S. M~zwsarx~, Spect~~chim. Acta 10,170 (1967); A. YAMAOVQHI,NGppon Kqpkz~ Zasshi 78, 1319, 1467 (1957). :flZ] T. MIYAZAWA, i.Vi~on &g&i.6 ZassG 76, 821 (1955). 1131 J. C. EVANS, J. Chem. P&y& 22, 1228 (1954). $141 A. P. aRAY and It. C. LORD,J. Chem. PhyS. 26,690 (1967); A. ~AMXOUCEI, N$?~O% KCX@U &‘I#& 80, 1105 (1959).
nN isotope effects on the vibrations1 frequencies of aniline
normal vibrations 11146 and 10554 cm-l is supported also by the results of the examination of the ~-deuteration effects (see Section 5 below). 4. N& wagging freqaelzcy The NH, wagging frequency is expected in the region of 700-900 cm-i. No band is found in the solution spectrum nor in the vapour spectrum* that is to be assigned to the NH, wagging mode in this region. However, a broad band with its centre at about 700 cm-l is observed for the liquid state of aniline [5]*, but does not appear for the liquid state of ND,-aniline [5]. A similar band is observed for Table 4.
A snnunary of the N-deutexation effects on the liquid spectrum of aniline TSl. and their interpretation
Bands which appear in the spectrum of undeuterated aniline but not in that of N-deutorated aniline
Bands which appear in the spectrum of N-deuterated aniline but not in that of undeuterated aniline
Product rule test
Frequency (cm-r)
Assignment
Frequency (cm-r)
Assignment
Observed ratio
Calculated r&i0
_ 1621 1277 1
1119 1063 I
700
NH, bend and c--N str. NH, rook (or twist) and CH in-plane bend
NH, wag
1301
-_
1621 x 122 1301 x 1150
1150
-_ 1 1078 797
CH in-phme bend ND, rock (or twist)
B ;
1
_ 1.38
1119 x 1054 ~= 1 *3, 1078 x 797
1.36
1.36
<600
formamide in its liquid and solid states [lZ], but not in the vapour [133. This band is reasonably assigned to the NH2 wagging mode of formamide [12J. Therefore it is very probable that the broad band of liquid aniline around 700 cm-1 is due to the NH, wagging mode.
Infrared absorption of N-deuterated aniline was observed by CALIEANO and in its liquid state. The results show that all the bands of liquid aniline shift only very slightly on N-deuteration except those given in Table 4. Instead of the two bands at 1621 and 1277 cm-l of undeuterated liqnid aniline, which are oonsidered to correspond, respectively, to the 1618.9 and 1276.1 cm-1 bands already mentioned (see Section 2), two bands are found at 1301 and 1150 om-l in N-deuterated liquid aniline. These may be assigned, respectively, to almost pure C-N stretching and ND, bending vibrations. Instead of the two bands of undeuterated aniline at 1119 and 1053 cm-l already mentioned (see
MOCCIA [S]
* See footnote p. 510. 511
M. TSUBOI
Section 3) two bands are found at 1078 and 797 em-l in N-deuterated aniline. These may be assigned, respectively, to a B, (or A”) vibration (probably a C-H in-plane bending vibration) and the ND, rocking (or twisting) vibration. The ~signments presented here are in satisfactory agreement with the product. rule, as may be seen from the last column of Table 4. Table 5. Comparison of the NH, frequencies(cm-l) of formamide, aniline and methylamine
Assignment
NH, antisym. str. NH, sym. str. NH, bend C-N str. NH, rock (planes) or twist (pyramidal) NH, w&g
XXCONH, (gas)
3545 3450 1572 1253 1059 (700 in liq.)
ffP4
(in dil. soln.)
3498.0 3415.3 1619 1273
3481.4 3395-2 1618.9 1276.1
3427 3361 1623 1044
1114.6 -
l&Z5 780
-
_-
(‘700 in liquid)
~ -
As has been stated, five frequencies of aniline in dilute solutions are assigned to its NH, vibrations. Four of these are ako identified in the spectrum of the gas.* These frequencies are given in Table 5, together with the corresponding frequencies of formamide [I37 and methylamine [14] in their gaseous states. A comparison of the NH, stretching frequencies shows that the NH, group of aniline has an intermediate structure between those of the NH, groups of formamide and methylamine. On the basis of the NH, deformation frequencies and C-N stretching frequencies, the NH, group of aniline seems to be close to that of formamide rather than to that of methylamine. Acl&owEedgement~-- The writer wishes to express his sincere thanks to Professor SAN-ICHIRO MIZUSEXMA and Professor TAKEEIIEO SH~ANOUC~~for their v&able suggestions. * See footnotep. 510.