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The far-infrared vapour phase spectra of aniline-ND2 and aniline-NHD
Volume 49, number 3
CHEMICAL PHYSICS LETTERS
THE FAR-INFRARED
1 August 1977
VAPOUR PHASE SPECTRA OF ANILINE-ND2 AND ANILINE-NHD
R.A. KYDD and P.J. KRUEGER Department of Chemistry.
UtCwrsity of Gzlgaty.
Gd..ar_.v.Alberta. Camia T.&VIN4 Received 10 March 1977 Revised manuscript received 4 May 1977
Room temperature vapour phase infrared spectra of aniline-NDz, aniline-NHD, and aniline-NH2 from 12 cm-t to 650 CI’II-’are presented. Transitions in the inversion vibration are assigned. Barriers to inversion are cal+ated and found be slightly lower for aniline-NH*.
1. Introduction The far-infrared
vapour
phase spectrum
of aniline
twice in 1976 [ 1.21 with very different results. We have been engaged in more comprehensive related studies, and our experimental results and analysis of the aniline spectrum completely confirm those presented by Larsen et al. [2 j . We have also obtained the spectrum of aniline-NDa, and spectra of samples which contain a sufficiently high proportion of the partially deuterated aniline-NHD molecules to permit the transitions due to this species to be identified We also wish to present the results of calculations of the energy levels of the inversion vibration which were done with a rather different potential function than that used for aniline-NH2 by Larsen et al. was reported
the weaker bands. The resolution was 1 cm-1 _ The accuracy with which the line positions could be measured is estimated to be *OS cm-* _ Aniline-ND, was obtained from Merck, Sharp, and Dohme (Canada) Ltd. with isotopic purity better than 99%. Small amounts of aniline-NHD usually arose from partial exchange in the sample chamber when anilineND, was used. However, it could be prepared in higher concentration by exchange with a mixture of D20 and H,O.
3. Results of &line-ND, between 100 and 650 cm -t and of a sample containing a relatively high ccncentration of aniline-NHD are presented in fig. 1. Our The spectra
spectrum parison,
2. Experimental Spectra were obtained with a Digilab FTS 16 farinfrared interferometer fitted with a Wilks variable path gas cell. Normally the spectra were run at saturated vapour pressure at room temperature. Path lengths used for the measurement of positions varied from 0.75 m (for the strongest bands) to the maximum path length available (23.25 m). AU spectra presented in this paper were obtained at 20.25 m path to show
to
of the normal though
portions
molecule
is included
of this spectrum
for com-
have been
previously [2], with a somewhat poorer signal-to-noise ratio. The spectra of these samples between 12 cm-l and 100 cm-l have also been obtained. No absorption at all was seen between 50 cm-l and 100 cm-l ; the spectra of the deuterated species between 12 cm-l and 50 cm-l are given as fig. 2. The spectrum of z aniline-NH2 in this region is not shown, as the important feature - the absorption at 40.8 cm-t corresponding to the Vi = 0 + 1 transition (Ui being the quantum published
539
Votume 49, number 3
CljEMICAL
PHYSICS LETTERS-
1 August 1977
ANILINE - ND2 I
I
1
I
1
ANILINE - NH0
ANILINE - NH2
1
3
2clo
300
400
500
600
cm‘
Fig. 1. Vapour phase infrared spectra of adines from 100 cm-l to 650 cm-’ at 20 m path. The spectrum marked ani&x+NHD cxx&iins a retatively high fraction of that species_ Asterisks m&Iines due to ani&ze_NHz present as an impurity in the other .%nples. Circles mark linesdue to aniline-N& impurities, and squares mark lines which arise from aniline-NHD impurities preseg in other samples.
540
Volume 49, number 3
CHEMlCAL PHYSICS LETTERS
t
c
ae (b)
(a)
-NHD
- ND,
WAVENUMBER
-
Fig. 2. Vapour phase infrared spectra of (a) aniline-ND2 and (b) a sample containing some anline-NHD, from 12 cm-r to 50 cm-‘. number for the inversion) lished [2] _
- has already been pub-
4. Discussion The assignments of the various transitions in the inversion vibration of aniline-NH2 are obvious from the spectra, and apart from very minor differences in the measured positions, our assignments agree completely with those presented by Larsen et al [2]. The sharp line at 13.4 cm-l
in the spectrum
of
aniline-ND2 must represent the ui = 0 + 1 transition_ The line at 337.5 cm-1 must correspond to the ui = 0 --L2 transition, in excellent agreement with the results from the electronic spectra [3,4] _ Another sharp line is seen at 324.5 cm-l. This must arise from the Ui = 1 + 2 transition, the separation of these two lines being 13.0 cm-l, in acceptable agreement with the observed 0-l separation. The sharp line at 443.8 cm-l represents the Ui = 1 + 3 transition (the vapour phase electronic spectra values are 435 cm-l [3] and 439 cm-l 143) and the Ui = 0 + 3 transition is responsible for the line at 457.0 cm-l. As confirmation of these assignments, a relatively weak but sharp line is located at 120.2 cm-l,
1 August 1977
corresponding almost perfectly to the 2-3 separation deduced from the other assigned transitions. It should be noted that Quack and Stockberger [4] tentatively suggested a O-l separation of 13 cm-1 from their vapour-phase fluorescence work, but were uncertain about its correctness because they were unable to find a potential energy function which would fit this result. Quack and Stockbeger also suggest that the level u = 3 in the inversion vibration is doubled by Fermi resonance, with the higher component occurring near 545 cm-l _We do not agree with this interpretation, for the following reason: If the two levels involved in this postulated resonance were to interact strongly enough to produce a separation of nearly 100 cm-r, as Quack and Stockberger suggest, then the corresponding wavefunctions would be sufficiently mixed that transitions to the upper level should have intensities at least comparable to transitions to the lower level. The transitions from Ui = 0 and ui = 1 to the “lower” level produce 457.0 cm-l and 443.8 cm-l lines, which are two of the strongest absorptions below 650 cm-l. The corresponding transitions to the “upper” level would be expected near 540 cm-l, but it is clear that all the lines in this vicinity are due to the aniline-NHD molecule (compare figs. 1 a and 1b). The nearest lines attributable to aniline-ND2 occur at 510.3 cm-l and 491.5 cm-‘, but since these are not separated by 13 cm-’ they cannot represent transitions from ur = 0 and Ui = 1, respectively. We believe that the ui = 3 level is not perturbed by Fermi resonance, and will show in the next section that 457.0 cm-’ is not an unreasonable position for this level. The lines at 363.6 cm-l and 387.3 cm-r in the spectra of various samples of deuterated aniline vary considerably in intensity relative to the lines already discussed, depending apparently on the extent of deuteration of the amino group. They must be due to aniline-NHD. If, as seems likely, they represent the Vi = 1 + 2 and Ui = 0 + 2 transitions in the inversion of this molecule, then the O-l separation would be 23.7 cm-l. There is indeed some evidence of an absorption in this region (see fig. 2) but because of the weakness of the line (due to the relatively small fraction of aniline-NHD molecules present in the sample used) and the somewhat noisy background in this region, it is hard to measure accurately; our best estimate of the position is 24.4 cm-r. Another line which is clearly due to the aniline-NHD molecule is seen at 199.0 cm-l. We believe this rep 541
CHEMKAL PHYSICS LET’fERS
Voiume 49, number 3
resents the Ui = 2 -+ 3 transition in this molecule, correspondirtg to the 276.3 cm-’ Iine in the spectrum of aniline-NH2 and the 120.2 cm-l line seen in anihneND2. The only possible alternative explanation for this line - that it arises from the out-of-plane motion of the enrire amino goup - can be ruled out because that mode must be responsible for the 216.5 cm-’ line in aniline-NHz and the 203.8 cm-l line in anilineNDz, and ‘should occur between those wavenumbers in _the partially deuterated species.. The calculated position of the ui = 3 level in anihneMD is then 586.3 cm-l. indeed there is a rather weak line seen at 587.4 cm-l which may represent the q = 0 -+ 3 absorption in this molecule, and another very weak band at 564.2 cm -I (seen as a shoulder on the side of another band) which could represent the Ui = I --f 3 transition. The energy level diagrams for the inversion vibration of each molecule are collected in fig_ 3. All the transitions indicated have been observed. ANILINE -ND2
-NHD
-387.3 2-
i -u
rt
$
4 .24.4 .ClO
s
Fig. 3. Energyleveldiagram for the anilines. The quantum num-
ber for the inversion vibration, uj, is given at the left of each fine; the wavenumber (cm-‘) above the ground state is given at the right. All transitions marked by arrows were observed, and the transition wavenumbers are indicated.
542
1 August 1977
5. Potential function calculations Larsen et al. [Z] used a potential function for aniline-NH2 which describes the inversion motion as a rotation of the amino hydrogen about an axis in the Cg HsN plane through the nitrogen atom and perpendicular to the NC bond. We have chosen the more traditional method of using a simple doubfe-turn harmonic well potential with a gaussian barrier. Quack and Stockberger [4] used such a potential function in their discussion of the aniline inversion, but were handicapped because the tables from which they obtained the energy levels (those tables compiled by Coor et al. [S] ) were not complete. We have written a program, based on the method suggested by Coon et al., to carry out this calculation for any values of the parameters B, P, and go. (The barrier height in cm-l is equal to &,-,; p determines the relative steepness of the walls of the barrier and of the potential weli itself. See the original paper [5] for a complete description of these parameters.) To determine the best values of these three parameters the following procedure was used: For a given value of B, p was chosen arbitrarity; v. (which has the effect of a scaling parameter) was then selected to make the caIcuIation tit the observed Ui = 0 + 2 transition. This transition was chosen because it was always easy to measure accurately. Different values of p (and thus vo) were then tried, until the best fit possible for az1levels for that value of S was obtained. Then a new value of B was chosen, and the procedure repeated until the optimum values of all three parameters were found. The results for aniline-NH2 and aniline-ND2 are collected in table 1. For aniline-NH2 the agreement with the observed energy levels is exceptionally good. It is also remark;ible for the barrier heights calculated with the two rather dissimilar potential functions are virtually identical. This fact, combined with the excellent agreement of the observed and calculated energy levels, gives confidence in the calculated barrier height; we believe that it is correct to within +5 cm-l. For aniline-NHD and aniline-ND2 the agreement of the calculated and observed wavenumbers is also excellent. However, since we feel the precise location of the Ui =50 + 1 transition in these molecules is not quite as wea dete~ed as in aniline-NH2 (because of experimental difficulties - low energy, detector sensitivity,
Volume 49. number 3
CHEMICAL PHYSICS LETTERS
Table 1 Observed and calculated -
energy levelsa) Aniline-NH2 this work
Ui= 0 1 2 3 barrier height (cm-’ ) B P v&cm-‘)
1 August 1977
Aniline-NHD
Aniline-ND2
ref. 121 b)
obs.
CdC_
obs.
Cilk.
oh.
talc.
obs.
talc.
0.0 40.8 423.8 700.1
0.0 40.8 423.84) 700.1
0.0 40.8 423.0 699.2
0.0 40.8C) 422.6 c, 693.4
0.0 24.4 387.3 587.4
0.0 13.4 387.3d) 586.8
0.0 13.4 337s 457.0
0.0 13.0 337.54) 457.0
525.9 0.5435 0.23 967.6
524.4
550.3 1.089 0.65 505.3
543.4 1.805 1.20 301.0
a) The observed values are, where possible, those for the transition direct from the ui =-0 level. h) Larsen et al. presented their results in terms of transitions. The energy levels shoan here are calculated from their results. C) These values were used in making the fit (see ref. [2] ). d) Parameter ~0 scaled to make this calculated value agree with observed value.
etc.) we feel the barrier heights are not quite as reliable as for aniline-NH2. changes
(The barrier height
quite quickly
with changes
Acknowledgement
calculated
in this transition
frequency.)
Our estimate of the uncertainty in the barrier heights for aniline-NHD and aniline-ND2 is i-10 cm-l. Thus it appears there is a real difference in the barrier heights of these molecules, at least between anilineNH2 and the other two deuterated analogues. It is not surprising to find a lower barrier for aniline-NH2 than for the deuterated species, since it is generally accepted that deuterium attracts electrons slightly less than does ordinary hydrogen. One would then expect to find a slightly lower electron density on the nitrogen atom in aniline-NH2, thus lowering the barrier to inversion. In fact, one would expect aniline-ND2 to have a higher barrier than aniline-NHD, based on the same reasoning , which is not the result we obtained. However, since the difference between the calculated barrier heights for these two molecules in only 6.9 cm-l, well within our estimate of the uncertainty, it is not possible to con-
The financial support of the National Research Council of Canada is gratefully acknowledged.
References [l]
[2] [3] [4] [S]
C. Belorgeot, P. Quintard, P. Delmorme and V. Lorenzelli, Cm. J. Spectry. 21 (1976) 119; C. Belorgeot, P. Quintardand J. Gerbier, Compt. Rend. Acad. Sci. (Paris) 269B (1970) 110.5. N.W_ Larsen, I7.L. Hansen and F.M. Nicolaisen, Chem. Phys. Letters 43 (1976) 584. J.C.D. Brand, D.R. Williams and T.J. Cook, J. Mol. Spectry. 20 (1966) 359. M. Quack and M Stockberger, J. Mol. Spectry. 43 (1972) 87. J.B. Coon, N.W. Naugle and R.D. McKenzie, J. Mol. Spectry. 20 (1966) 107.
clude which moiecule has, in reality, the higher barrier.
543
CHEMICAL PHYSICS LETTERS
THE FAR-INFRARED
1 August 1977
VAPOUR PHASE SPECTRA OF ANILINE-ND2 AND ANILINE-NHD
R.A. KYDD and P.J. KRUEGER Department of Chemistry.
UtCwrsity of Gzlgaty.
Gd..ar_.v.Alberta. Camia T.&VIN4 Received 10 March 1977 Revised manuscript received 4 May 1977
Room temperature vapour phase infrared spectra of aniline-NDz, aniline-NHD, and aniline-NH2 from 12 cm-t to 650 CI’II-’are presented. Transitions in the inversion vibration are assigned. Barriers to inversion are cal+ated and found be slightly lower for aniline-NH*.
1. Introduction The far-infrared
vapour
phase spectrum
of aniline
twice in 1976 [ 1.21 with very different results. We have been engaged in more comprehensive related studies, and our experimental results and analysis of the aniline spectrum completely confirm those presented by Larsen et al. [2 j . We have also obtained the spectrum of aniline-NDa, and spectra of samples which contain a sufficiently high proportion of the partially deuterated aniline-NHD molecules to permit the transitions due to this species to be identified We also wish to present the results of calculations of the energy levels of the inversion vibration which were done with a rather different potential function than that used for aniline-NH2 by Larsen et al. was reported
the weaker bands. The resolution was 1 cm-1 _ The accuracy with which the line positions could be measured is estimated to be *OS cm-* _ Aniline-ND, was obtained from Merck, Sharp, and Dohme (Canada) Ltd. with isotopic purity better than 99%. Small amounts of aniline-NHD usually arose from partial exchange in the sample chamber when anilineND, was used. However, it could be prepared in higher concentration by exchange with a mixture of D20 and H,O.
3. Results of &line-ND, between 100 and 650 cm -t and of a sample containing a relatively high ccncentration of aniline-NHD are presented in fig. 1. Our The spectra
spectrum parison,
2. Experimental Spectra were obtained with a Digilab FTS 16 farinfrared interferometer fitted with a Wilks variable path gas cell. Normally the spectra were run at saturated vapour pressure at room temperature. Path lengths used for the measurement of positions varied from 0.75 m (for the strongest bands) to the maximum path length available (23.25 m). AU spectra presented in this paper were obtained at 20.25 m path to show
to
of the normal though
portions
molecule
is included
of this spectrum
for com-
have been
previously [2], with a somewhat poorer signal-to-noise ratio. The spectra of these samples between 12 cm-l and 100 cm-l have also been obtained. No absorption at all was seen between 50 cm-l and 100 cm-l ; the spectra of the deuterated species between 12 cm-l and 50 cm-l are given as fig. 2. The spectrum of z aniline-NH2 in this region is not shown, as the important feature - the absorption at 40.8 cm-t corresponding to the Vi = 0 + 1 transition (Ui being the quantum published
539
Votume 49, number 3
CljEMICAL
PHYSICS LETTERS-
1 August 1977
ANILINE - ND2 I
I
1
I
1
ANILINE - NH0
ANILINE - NH2
1
3
2clo
300
400
500
600
cm‘
Fig. 1. Vapour phase infrared spectra of adines from 100 cm-l to 650 cm-’ at 20 m path. The spectrum marked ani&x+NHD cxx&iins a retatively high fraction of that species_ Asterisks m&Iines due to ani&ze_NHz present as an impurity in the other .%nples. Circles mark linesdue to aniline-N& impurities, and squares mark lines which arise from aniline-NHD impurities preseg in other samples.
540
Volume 49, number 3
CHEMlCAL PHYSICS LETTERS
t
c
ae (b)
(a)
-NHD
- ND,
WAVENUMBER
-
Fig. 2. Vapour phase infrared spectra of (a) aniline-ND2 and (b) a sample containing some anline-NHD, from 12 cm-r to 50 cm-‘. number for the inversion) lished [2] _
- has already been pub-
4. Discussion The assignments of the various transitions in the inversion vibration of aniline-NH2 are obvious from the spectra, and apart from very minor differences in the measured positions, our assignments agree completely with those presented by Larsen et al [2]. The sharp line at 13.4 cm-l
in the spectrum
of
aniline-ND2 must represent the ui = 0 + 1 transition_ The line at 337.5 cm-1 must correspond to the ui = 0 --L2 transition, in excellent agreement with the results from the electronic spectra [3,4] _ Another sharp line is seen at 324.5 cm-l. This must arise from the Ui = 1 + 2 transition, the separation of these two lines being 13.0 cm-l, in acceptable agreement with the observed 0-l separation. The sharp line at 443.8 cm-l represents the Ui = 1 + 3 transition (the vapour phase electronic spectra values are 435 cm-l [3] and 439 cm-l 143) and the Ui = 0 + 3 transition is responsible for the line at 457.0 cm-l. As confirmation of these assignments, a relatively weak but sharp line is located at 120.2 cm-l,
1 August 1977
corresponding almost perfectly to the 2-3 separation deduced from the other assigned transitions. It should be noted that Quack and Stockberger [4] tentatively suggested a O-l separation of 13 cm-1 from their vapour-phase fluorescence work, but were uncertain about its correctness because they were unable to find a potential energy function which would fit this result. Quack and Stockbeger also suggest that the level u = 3 in the inversion vibration is doubled by Fermi resonance, with the higher component occurring near 545 cm-l _We do not agree with this interpretation, for the following reason: If the two levels involved in this postulated resonance were to interact strongly enough to produce a separation of nearly 100 cm-r, as Quack and Stockberger suggest, then the corresponding wavefunctions would be sufficiently mixed that transitions to the upper level should have intensities at least comparable to transitions to the lower level. The transitions from Ui = 0 and ui = 1 to the “lower” level produce 457.0 cm-l and 443.8 cm-l lines, which are two of the strongest absorptions below 650 cm-l. The corresponding transitions to the “upper” level would be expected near 540 cm-l, but it is clear that all the lines in this vicinity are due to the aniline-NHD molecule (compare figs. 1 a and 1b). The nearest lines attributable to aniline-ND2 occur at 510.3 cm-l and 491.5 cm-‘, but since these are not separated by 13 cm-’ they cannot represent transitions from ur = 0 and Ui = 1, respectively. We believe that the ui = 3 level is not perturbed by Fermi resonance, and will show in the next section that 457.0 cm-’ is not an unreasonable position for this level. The lines at 363.6 cm-l and 387.3 cm-r in the spectra of various samples of deuterated aniline vary considerably in intensity relative to the lines already discussed, depending apparently on the extent of deuteration of the amino group. They must be due to aniline-NHD. If, as seems likely, they represent the Vi = 1 + 2 and Ui = 0 + 2 transitions in the inversion of this molecule, then the O-l separation would be 23.7 cm-l. There is indeed some evidence of an absorption in this region (see fig. 2) but because of the weakness of the line (due to the relatively small fraction of aniline-NHD molecules present in the sample used) and the somewhat noisy background in this region, it is hard to measure accurately; our best estimate of the position is 24.4 cm-r. Another line which is clearly due to the aniline-NHD molecule is seen at 199.0 cm-l. We believe this rep 541
CHEMKAL PHYSICS LET’fERS
Voiume 49, number 3
resents the Ui = 2 -+ 3 transition in this molecule, correspondirtg to the 276.3 cm-’ Iine in the spectrum of aniline-NH2 and the 120.2 cm-l line seen in anihneND2. The only possible alternative explanation for this line - that it arises from the out-of-plane motion of the enrire amino goup - can be ruled out because that mode must be responsible for the 216.5 cm-’ line in aniline-NHz and the 203.8 cm-l line in anilineNDz, and ‘should occur between those wavenumbers in _the partially deuterated species.. The calculated position of the ui = 3 level in anihneMD is then 586.3 cm-l. indeed there is a rather weak line seen at 587.4 cm-l which may represent the q = 0 -+ 3 absorption in this molecule, and another very weak band at 564.2 cm -I (seen as a shoulder on the side of another band) which could represent the Ui = I --f 3 transition. The energy level diagrams for the inversion vibration of each molecule are collected in fig_ 3. All the transitions indicated have been observed. ANILINE -ND2
-NHD
-387.3 2-
i -u
rt
$
4 .24.4 .ClO
s
Fig. 3. Energyleveldiagram for the anilines. The quantum num-
ber for the inversion vibration, uj, is given at the left of each fine; the wavenumber (cm-‘) above the ground state is given at the right. All transitions marked by arrows were observed, and the transition wavenumbers are indicated.
542
1 August 1977
5. Potential function calculations Larsen et al. [Z] used a potential function for aniline-NH2 which describes the inversion motion as a rotation of the amino hydrogen about an axis in the Cg HsN plane through the nitrogen atom and perpendicular to the NC bond. We have chosen the more traditional method of using a simple doubfe-turn harmonic well potential with a gaussian barrier. Quack and Stockberger [4] used such a potential function in their discussion of the aniline inversion, but were handicapped because the tables from which they obtained the energy levels (those tables compiled by Coor et al. [S] ) were not complete. We have written a program, based on the method suggested by Coon et al., to carry out this calculation for any values of the parameters B, P, and go. (The barrier height in cm-l is equal to &,-,; p determines the relative steepness of the walls of the barrier and of the potential weli itself. See the original paper [5] for a complete description of these parameters.) To determine the best values of these three parameters the following procedure was used: For a given value of B, p was chosen arbitrarity; v. (which has the effect of a scaling parameter) was then selected to make the caIcuIation tit the observed Ui = 0 + 2 transition. This transition was chosen because it was always easy to measure accurately. Different values of p (and thus vo) were then tried, until the best fit possible for az1levels for that value of S was obtained. Then a new value of B was chosen, and the procedure repeated until the optimum values of all three parameters were found. The results for aniline-NH2 and aniline-ND2 are collected in table 1. For aniline-NH2 the agreement with the observed energy levels is exceptionally good. It is also remark;ible for the barrier heights calculated with the two rather dissimilar potential functions are virtually identical. This fact, combined with the excellent agreement of the observed and calculated energy levels, gives confidence in the calculated barrier height; we believe that it is correct to within +5 cm-l. For aniline-NHD and aniline-ND2 the agreement of the calculated and observed wavenumbers is also excellent. However, since we feel the precise location of the Ui =50 + 1 transition in these molecules is not quite as wea dete~ed as in aniline-NH2 (because of experimental difficulties - low energy, detector sensitivity,
Volume 49. number 3
CHEMICAL PHYSICS LETTERS
Table 1 Observed and calculated -
energy levelsa) Aniline-NH2 this work
Ui= 0 1 2 3 barrier height (cm-’ ) B P v&cm-‘)
1 August 1977
Aniline-NHD
Aniline-ND2
ref. 121 b)
obs.
CdC_
obs.
Cilk.
oh.
talc.
obs.
talc.
0.0 40.8 423.8 700.1
0.0 40.8 423.84) 700.1
0.0 40.8 423.0 699.2
0.0 40.8C) 422.6 c, 693.4
0.0 24.4 387.3 587.4
0.0 13.4 387.3d) 586.8
0.0 13.4 337s 457.0
0.0 13.0 337.54) 457.0
525.9 0.5435 0.23 967.6
524.4
550.3 1.089 0.65 505.3
543.4 1.805 1.20 301.0
a) The observed values are, where possible, those for the transition direct from the ui =-0 level. h) Larsen et al. presented their results in terms of transitions. The energy levels shoan here are calculated from their results. C) These values were used in making the fit (see ref. [2] ). d) Parameter ~0 scaled to make this calculated value agree with observed value.
etc.) we feel the barrier heights are not quite as reliable as for aniline-NH2. changes
(The barrier height
quite quickly
with changes
Acknowledgement
calculated
in this transition
frequency.)
Our estimate of the uncertainty in the barrier heights for aniline-NHD and aniline-ND2 is i-10 cm-l. Thus it appears there is a real difference in the barrier heights of these molecules, at least between anilineNH2 and the other two deuterated analogues. It is not surprising to find a lower barrier for aniline-NH2 than for the deuterated species, since it is generally accepted that deuterium attracts electrons slightly less than does ordinary hydrogen. One would then expect to find a slightly lower electron density on the nitrogen atom in aniline-NH2, thus lowering the barrier to inversion. In fact, one would expect aniline-ND2 to have a higher barrier than aniline-NHD, based on the same reasoning , which is not the result we obtained. However, since the difference between the calculated barrier heights for these two molecules in only 6.9 cm-l, well within our estimate of the uncertainty, it is not possible to con-
The financial support of the National Research Council of Canada is gratefully acknowledged.
References [l]
[2] [3] [4] [S]
C. Belorgeot, P. Quintard, P. Delmorme and V. Lorenzelli, Cm. J. Spectry. 21 (1976) 119; C. Belorgeot, P. Quintardand J. Gerbier, Compt. Rend. Acad. Sci. (Paris) 269B (1970) 110.5. N.W_ Larsen, I7.L. Hansen and F.M. Nicolaisen, Chem. Phys. Letters 43 (1976) 584. J.C.D. Brand, D.R. Williams and T.J. Cook, J. Mol. Spectry. 20 (1966) 359. M. Quack and M Stockberger, J. Mol. Spectry. 43 (1972) 87. J.B. Coon, N.W. Naugle and R.D. McKenzie, J. Mol. Spectry. 20 (1966) 107.
clude which moiecule has, in reality, the higher barrier.
543