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The infrared spectrum of ammonia isolated in argon and nitrogen matrices
8pectrochimlcs A&, Vol. 2OA.pp. 1633to 1672.PergsmonPrees1073.Printedin NorthernIreland
spectrum of ammonia isolated in argon and nitrogen matrices
The infmed
J. A. CU~LEY and A. D. E. PULLIN Chemistry Department, Monaah University, Clayton, Victoria Australia 3168 (Received10 July
1972)
Ab&&---The infrared spectrum of ammonia, isolated in argon and nitrogen matrices, has been investigated in the v, vibrational region. The temperature dependence of the lines in an sxgon matrix is explained by molecularrotation of NH,. This is in contrast to the nitrogen matrix where no rotation occurs. The effect of adding nitrogen or oxygen impurities to an argon/ammonia matrix has also been examined. INTRODUCTION
ammonia rotates in nitrogen and argon matrices has been disputed for many years [l-5]. On the basis of careful temperature recycling experiments we believe that it does rotate in an argon matrix but not in a nitrogen matrix. We can also correlate the early work of MULLIGAN et al.[l]with the more recent work of ABOUAF-MAU~IXet. al [6]. WHETHER
or not
The matrices were formed by spraying a gas mixture at room temperature onto a cold (~14’K) CsI or KBr window of an Air Products and Chemicals model AC-3L110 cryotip through a Hoke needle valve at a rate which varied from 17 to 1 mmole/ hr. The spectra were independent of both the spray on rate and the two window materials. Temperature variation was achieved by controlling the flow rate and the backpressure of the helium gas, and was measured by a standardized gold-O.07 atomic ‘A iron vs. copper thermocouple imbedded in the cold centre window. The spectra, in the range 1200-800 cm-l were recorded on a P.E. 521 spectrophotometer which was cslibrsted with a sample of gaseous ammonia. The resolution for all the expanded scale spectra, was slightly better than 1-Ocm-l, and the fiequencies are accurate to at least f0.5 cm-l. NUCLEARSPIN CONSIDERATIONS Considering only the nuclear spin of the hydrogen atoms of ammonia., two spin isomers can be obtained-that with 1 = Q or I = 4. Anslysing the molecule under the D, rotational sub-group the nuclear spin isomer 1 = Q belongs to the Al spin species and the isomer I = + belongs to the E spin species (Fig. 1). If nuclear spin interconversion can take place then, as the temperature approaches O’K, the population of the J = 0, K = 0 level will increase at the expense of the other levels. [l] [2] [3] [4] [5]
D. G. R. H. L.
E. MILLIUAN,R. M. HEXTERand K. DRESSLER, J. Chem. Phya. 34, 1009 (1961). C. PIMENTAL,M. 0. BULANINand M. VAN THIEL,J. Chem. Phys. 86,600 (1962). E. MEREDITH,Ph.D. thesis (1963), University of Michigan. P. HOPKINS,R. F. CURLand K. S. PITZER,J. Chem. Phys. 48, 2959 (1968). ABOU~-M~RQUIN, H. DUBOSTand F. LEGAY, Chem. Phys. Letter8 7, 61 (1970). 1665
J. A. CUGLEYand A. D. E.
1666
J=2---
T
J-2----
J=
+
I---.
PULLIN
J=2 -
-+
J=2 -
r.- +
J=l
5
J=l
--
v, =I J=O J- I
+
-- .__+
J*O
‘
‘
J.2-
-
_..
;
Rq
F?(I;)
f?cog v2
‘PCI;)
*o J=l
?
J=2 =: ocl;,,.
--
R(I;) o(I;)
- _
;
J*l
i--t
=
f K=O Al spin species
K-i E spin species
Fig. 1. The rotational energy level scheme for ammonia in the ground and first excited state of Q. Only the vibration-rotation transitionsof concernin this commnniccttionare included.
However if no nuclear spin interconversion can occur under the experimental conditions, then molecules with spin I = + will be trapped in the J = 1, K = 0 level. To compare the experimental results with the predictions of theory it is useful to consider in some detail what happens to the population of the rotational levels of a cold gas when the temperature is raised from 0°K. If no interconversion takes place then the number of molecules in the J = 1, K = 0 level is increased at the expense of molecules in the J = 0, K = 0 level. In consequence the transition R( O,-) should decrease in intensity whilst P( lo+) and R( lo+) should increase. Molecules in the J = 1, K = 1 level can only go to the J = 2, K = 1 or J = 2, K = 2 levels, and hence transitions R(l,*) and &(ll*) should slightly decrease in intensity. For complete spin interconversion the J = 0, K = 0 level loses molecules to both the J = 1, K = 0 and the J = 1 K = 1 levels. Thus the R(O,-) transition should decrease in intensity whilst the &lo+), P(&,+), &(lI*) and R(ll*) transitions increase in intensity.
The infrared spectrum of ammonia isolated in argon and nitrogen matrices
1667
RESULTS
Argon matrix The effect of temperature cycling for an argon/ammonia ratio of 1150/l is shown in Fig. 2. The spectrum is fist recorded at the lowest temperature possible, the temperature is raised to a required value, the spectrum recorded and the temperature
Wavenum ber,
cm-l
Fig. 2. Temperature recycling for the argon-ammonia system with M/A The spectra are of one sample, recorded in the order 1-7.
= 1000.
is lowered. This raising and lowering operation is then repeated for a new temperature. Two lines, at 1000 and 1019 cm-l, show an irreversible temperature dependence. Their intensities grow as the temperature is raised but do not diminish when the temperature is lowered. These lines are more intense when the matrix/absorber (M/A) ratio is lower and hence are reasonably assigned to dimers or multimers. It is these lines that both Milligan et al. and Hopkins et al. observe and assign to rotating NH,. At higher temperatures additional dimer/multimer lines at 1049 and 1067 cm-l appear. The lines at 962 and 1014 cm-l increase quite remarkably when the temperature is raised and decrease when it is lowered. The line at 975 cm-l shows the reverse effect. These observations are consistent with transitions P(lO+) and R(l$) arising
J. A. Cuarx~u and A. D. E, PULLIN
1663
from the J = 1, K = 0 level and the transition R(O,-)arising from the J = 0, K = 0 level respectively. The other three lines in the spectra are approximately temperature independent and are assigned to transitions arising from the J = 1, g = 1 level of the ground state.* From the assignment of the observed lines one can calculate approximate parameters involved in the calculation of the frequencies in the mibtrix (Tables 1 md 2). Table 1. Comparison of rotational constants in gas phase and effective constants for the matrix B”
B’
C”_C
matrix
9.4
87
0.5
23.3
969.3
gas IQ1
9.94
9.8
0.08
368
960.0
E”inv i-E’inv
Y@(em-l)
@ refers to the ground vibrational state. ’ refers to the upper vibrational state. v0 is the frequency correspondingto the vibrational change alone. All values in wavenumbers (cm-l). Table 2 veal.3
%bs Matrix
(cm-l)
Matrix (cm-‘)
956.3
956.3
961.9 97670 9796 992.9
962.0 975.0 979.6 991.1
1014.2
1014.2
+ X(Jr*) vibration rotation line (X = refer to the lower level of the transition.
Assignment
Q(lf) P(lo+) RO,)
Q&-l
R(lf) R(lo+) i R(l,-)
P, Q or R) where thenos. in bracketa
It is reasonable to expect only a small change in B”, from 9.9 cm-l in the gas phase to 9.4 cm-l in the matrix, since if the lattice grossly distorts the tmpped molecule then rotation would not be expected. For the pg vibration of ammonk the relative intensities of the ~bration~lrotational lines can be csJou.lsted from
* At 13’K the line at 966 cm-l appears to be split in these spectra. When the temperature is raised the splitting disappears but occurs again when the temperature is lowered. Some of the other spectra also appeared to show a sphtting of this bmnd but the phenomenon couId not be confirmed. [S] ALLEN and CROSS,MoZec&xr V&rotors, p. 102, Wiley, New York (1963). [7] B. &IEYER,Low Xemperatwe Sped-oacopy, p. 110, Klsevier, New York (1971). [8] ARZA RON and 0. SCHNEPP,J. Chem. P&y-s.46,3991 (1967). [S]W. S BENEDIOTa;ndEARLE X. PLYLER, Cm. J. P&p. 35,1236 (1967).
The infrared spectrum of ammonia isolated in argon and nitrogen matrioea
1669
where n” or n’ stand for all quantum numbers describing the lower or upper state of the transition respectively. The symbol gn is the nuclear spin degeneracy and I(dIMIn')12 is th e rotational line strength [6]. The energy of the state n, E,, can be calculated from the parameters for ammonia in the matrix given in Table 1. For the case of no nuclear spin interconversion it was assumed that + of the total number of molecules were trapped in the E spin species (I = 4) when the sample was deposited. A plot of these calculated intensities in the range lO-31@K is shown in Fig. 3a, b. As the observed line at 1014 cm-l is assumed to be composed of both the R(l,,+) and
bk (a)
I/
(b)
\
’ R(OJ
_Tc _.
R(Q+R(I;) R(C6)
01
Q(ti),R(lJ R(l;LQ(Q
Fig. 3. Computed relative intensities of vibration-rotation traruitions &a 8 fun&ion of temperature for (a) the “no spin conversion” model, and (b) the “oomplete spin convemion” model.
the R(l,-) transition, the calculated relative intensities of these transitions were summed. It appears that the “no spin interconversion” model explains the observed spectra more satisfactorily than the “complete spin interconversion model.” The P(l,,+) transition at 962 cm-l is expected to increase dramatically in intensity in the range from 13 to 23’K, the &(I,*) and R( 1,‘) transitions are expected to decrease slightly as the temperature is raised, and the ratio of the intensity of the R(O,-) transition at 976 cm-1 to the transition at 1014 cm-l more closely resembles the experimental results for the no spin interconversion model. Irnpuritie8 added to an argon matrix
About 25 separate spectra were recorded of NH, trapped in argon and some of the early spectra showed anomalous lines which we thought could have been due to
1670
J. A.
and A. D. E.
CUGLEY
PULLIN
impurities in the argon. To check this we first satisfied ourselves that we could obtain reproducible spectra that did not contain these anomalous lines, then we deliberately added trace amounts of oxygen and nitrogen. Figure 4 shows the spectrum of [Ar]/[NH,] = 1000 with [Ar]/[N,] = 190. The only difference between this and the uncontaminated spectra is a shoulder at 972 cm-l on the low frequency side of the R(O,-) transition. Similarly Fig. 5 with [Ar]/ [0,] = 250 shows a peak at 968 cm-l.
20 -
1010
1000
990
960
Wavenumber,
970
960
Cm-1
Fig. 4. The argon-ammonia system with added nitrogen impurity. [Ar]/[NH,] = 1000, [Ar]/[Nz] = 190. 8
6o-
I
I
I
I
I
I
I
I
1
I
I
I 1010
1
I 1000
I
I 990
I
I 960
970
960
Wavenumber, cm-l Fig. 5. The argon-ammonia,system with added oxygen impurity. [Ar]/[NH,] = 1000, [Ar]/[O,] = 260.
These lines coincided with the anomalous lines that we had originally obtained and probably result from a nitrogen or oxygen molecule being close to an ammonia molecule, and hence able to perturb it enough to stop rotation. Nitrogen matrix The effect of varying the temperature of an 1130/l matrix from 11 to 16 to 21’K is shown in Fig. 6. Even in warming from 11 to 16’K the two lines at 1003 and 986 cm-l, which also show concentration dependence, start to appear, and become more
The infrared spectrum of ammonia isolated in argon and nitrogen matrices
60
-
40
I.
1671
T=II’K
20-
60
I I
I I
3.
T=2l*K
I
I I I
I I I
I
-
60-
20
-
I
I I I I I I I I 1010 IO00 990 980 Wavenumber.
970
960
cm-l
Fig. 6. The effect of increasing temperature for the nitrogen-ammonia system with M/A = 1130.
intense as the matrix is warmed further. They do not disappear when the temperature is lowered and so must be assigned to dimers or multimers. Of the rest, the line at 982 cm-l appears to merge into the line st 986 cm-i, and may well be a crystal effect which disappears when the matrix is annealed [7]. No lines appear below 969 cm-l and the shoulder on the high frequency side of this band does not appesr to show any temperature dependence. Far infrared work from 10 to 60 cm-i which we have just recently performed shows no evidence for rotating NH, in a nitrogen matrix, there being only one line, at 49 cm-l, which is due to the nitrogen lsttice [8]. In contrast, the far infrared spectrum of NH, isolated in an argon matrix [lo] consists of a number of lines, the analysis of which supports the assignments here proposed. [101 J. A. CUQLEY
8
and A. D. E. PULLIN,Chewa.Whys. L&t. 17, 406 (1972).
1672
J. A.
CUULEY
and A. D. E.
PULLIN
CONCLUSION
Ammonia rotates in an argon matrix but not in a nitrogen matrix, and the assignment proposed by Abouaf-Marguin et al. explains the temperature dependence of the observed lines. Acknowledgement-J. A. Cugley wishes to acknowledge the assistance of a Commonwealth postgraduate research award. The authors also thank Professor I. Mills, Dr. A. Hurley and Dr. M. F. O’Dwyer for informative discussions on statistical weights and line intensities.
spectrum of ammonia isolated in argon and nitrogen matrices
The infmed
J. A. CU~LEY and A. D. E. PULLIN Chemistry Department, Monaah University, Clayton, Victoria Australia 3168 (Received10 July
1972)
Ab&&---The infrared spectrum of ammonia, isolated in argon and nitrogen matrices, has been investigated in the v, vibrational region. The temperature dependence of the lines in an sxgon matrix is explained by molecularrotation of NH,. This is in contrast to the nitrogen matrix where no rotation occurs. The effect of adding nitrogen or oxygen impurities to an argon/ammonia matrix has also been examined. INTRODUCTION
ammonia rotates in nitrogen and argon matrices has been disputed for many years [l-5]. On the basis of careful temperature recycling experiments we believe that it does rotate in an argon matrix but not in a nitrogen matrix. We can also correlate the early work of MULLIGAN et al.[l]with the more recent work of ABOUAF-MAU~IXet. al [6]. WHETHER
or not
The matrices were formed by spraying a gas mixture at room temperature onto a cold (~14’K) CsI or KBr window of an Air Products and Chemicals model AC-3L110 cryotip through a Hoke needle valve at a rate which varied from 17 to 1 mmole/ hr. The spectra were independent of both the spray on rate and the two window materials. Temperature variation was achieved by controlling the flow rate and the backpressure of the helium gas, and was measured by a standardized gold-O.07 atomic ‘A iron vs. copper thermocouple imbedded in the cold centre window. The spectra, in the range 1200-800 cm-l were recorded on a P.E. 521 spectrophotometer which was cslibrsted with a sample of gaseous ammonia. The resolution for all the expanded scale spectra, was slightly better than 1-Ocm-l, and the fiequencies are accurate to at least f0.5 cm-l. NUCLEARSPIN CONSIDERATIONS Considering only the nuclear spin of the hydrogen atoms of ammonia., two spin isomers can be obtained-that with 1 = Q or I = 4. Anslysing the molecule under the D, rotational sub-group the nuclear spin isomer 1 = Q belongs to the Al spin species and the isomer I = + belongs to the E spin species (Fig. 1). If nuclear spin interconversion can take place then, as the temperature approaches O’K, the population of the J = 0, K = 0 level will increase at the expense of the other levels. [l] [2] [3] [4] [5]
D. G. R. H. L.
E. MILLIUAN,R. M. HEXTERand K. DRESSLER, J. Chem. Phya. 34, 1009 (1961). C. PIMENTAL,M. 0. BULANINand M. VAN THIEL,J. Chem. Phys. 86,600 (1962). E. MEREDITH,Ph.D. thesis (1963), University of Michigan. P. HOPKINS,R. F. CURLand K. S. PITZER,J. Chem. Phys. 48, 2959 (1968). ABOU~-M~RQUIN, H. DUBOSTand F. LEGAY, Chem. Phys. Letter8 7, 61 (1970). 1665
J. A. CUGLEYand A. D. E.
1666
J=2---
T
J-2----
J=
+
I---.
PULLIN
J=2 -
-+
J=2 -
r.- +
J=l
5
J=l
--
v, =I J=O J- I
+
-- .__+
J*O
‘
‘
J.2-
-
_..
;
Rq
F?(I;)
f?cog v2
‘PCI;)
*o J=l
?
J=2 =: ocl;,,.
--
R(I;) o(I;)
- _
;
J*l
i--t
=
f K=O Al spin species
K-i E spin species
Fig. 1. The rotational energy level scheme for ammonia in the ground and first excited state of Q. Only the vibration-rotation transitionsof concernin this commnniccttionare included.
However if no nuclear spin interconversion can occur under the experimental conditions, then molecules with spin I = + will be trapped in the J = 1, K = 0 level. To compare the experimental results with the predictions of theory it is useful to consider in some detail what happens to the population of the rotational levels of a cold gas when the temperature is raised from 0°K. If no interconversion takes place then the number of molecules in the J = 1, K = 0 level is increased at the expense of molecules in the J = 0, K = 0 level. In consequence the transition R( O,-) should decrease in intensity whilst P( lo+) and R( lo+) should increase. Molecules in the J = 1, K = 1 level can only go to the J = 2, K = 1 or J = 2, K = 2 levels, and hence transitions R(l,*) and &(ll*) should slightly decrease in intensity. For complete spin interconversion the J = 0, K = 0 level loses molecules to both the J = 1, K = 0 and the J = 1 K = 1 levels. Thus the R(O,-) transition should decrease in intensity whilst the &lo+), P(&,+), &(lI*) and R(ll*) transitions increase in intensity.
The infrared spectrum of ammonia isolated in argon and nitrogen matrices
1667
RESULTS
Argon matrix The effect of temperature cycling for an argon/ammonia ratio of 1150/l is shown in Fig. 2. The spectrum is fist recorded at the lowest temperature possible, the temperature is raised to a required value, the spectrum recorded and the temperature
Wavenum ber,
cm-l
Fig. 2. Temperature recycling for the argon-ammonia system with M/A The spectra are of one sample, recorded in the order 1-7.
= 1000.
is lowered. This raising and lowering operation is then repeated for a new temperature. Two lines, at 1000 and 1019 cm-l, show an irreversible temperature dependence. Their intensities grow as the temperature is raised but do not diminish when the temperature is lowered. These lines are more intense when the matrix/absorber (M/A) ratio is lower and hence are reasonably assigned to dimers or multimers. It is these lines that both Milligan et al. and Hopkins et al. observe and assign to rotating NH,. At higher temperatures additional dimer/multimer lines at 1049 and 1067 cm-l appear. The lines at 962 and 1014 cm-l increase quite remarkably when the temperature is raised and decrease when it is lowered. The line at 975 cm-l shows the reverse effect. These observations are consistent with transitions P(lO+) and R(l$) arising
J. A. Cuarx~u and A. D. E, PULLIN
1663
from the J = 1, K = 0 level and the transition R(O,-)arising from the J = 0, K = 0 level respectively. The other three lines in the spectra are approximately temperature independent and are assigned to transitions arising from the J = 1, g = 1 level of the ground state.* From the assignment of the observed lines one can calculate approximate parameters involved in the calculation of the frequencies in the mibtrix (Tables 1 md 2). Table 1. Comparison of rotational constants in gas phase and effective constants for the matrix B”
B’
C”_C
matrix
9.4
87
0.5
23.3
969.3
gas IQ1
9.94
9.8
0.08
368
960.0
E”inv i-E’inv
Y@(em-l)
@ refers to the ground vibrational state. ’ refers to the upper vibrational state. v0 is the frequency correspondingto the vibrational change alone. All values in wavenumbers (cm-l). Table 2 veal.3
%bs Matrix
(cm-l)
Matrix (cm-‘)
956.3
956.3
961.9 97670 9796 992.9
962.0 975.0 979.6 991.1
1014.2
1014.2
+ X(Jr*) vibration rotation line (X = refer to the lower level of the transition.
Assignment
Q(lf) P(lo+) RO,)
Q&-l
R(lf) R(lo+) i R(l,-)
P, Q or R) where thenos. in bracketa
It is reasonable to expect only a small change in B”, from 9.9 cm-l in the gas phase to 9.4 cm-l in the matrix, since if the lattice grossly distorts the tmpped molecule then rotation would not be expected. For the pg vibration of ammonk the relative intensities of the ~bration~lrotational lines can be csJou.lsted from
* At 13’K the line at 966 cm-l appears to be split in these spectra. When the temperature is raised the splitting disappears but occurs again when the temperature is lowered. Some of the other spectra also appeared to show a sphtting of this bmnd but the phenomenon couId not be confirmed. [S] ALLEN and CROSS,MoZec&xr V&rotors, p. 102, Wiley, New York (1963). [7] B. &IEYER,Low Xemperatwe Sped-oacopy, p. 110, Klsevier, New York (1971). [8] ARZA RON and 0. SCHNEPP,J. Chem. P&y-s.46,3991 (1967). [S]W. S BENEDIOTa;ndEARLE X. PLYLER, Cm. J. P&p. 35,1236 (1967).
The infrared spectrum of ammonia isolated in argon and nitrogen matrioea
1669
where n” or n’ stand for all quantum numbers describing the lower or upper state of the transition respectively. The symbol gn is the nuclear spin degeneracy and I(dIMIn')12 is th e rotational line strength [6]. The energy of the state n, E,, can be calculated from the parameters for ammonia in the matrix given in Table 1. For the case of no nuclear spin interconversion it was assumed that + of the total number of molecules were trapped in the E spin species (I = 4) when the sample was deposited. A plot of these calculated intensities in the range lO-31@K is shown in Fig. 3a, b. As the observed line at 1014 cm-l is assumed to be composed of both the R(l,,+) and
bk (a)
I/
(b)
\
’ R(OJ
_Tc _.
R(Q+R(I;) R(C6)
01
Q(ti),R(lJ R(l;LQ(Q
Fig. 3. Computed relative intensities of vibration-rotation traruitions &a 8 fun&ion of temperature for (a) the “no spin conversion” model, and (b) the “oomplete spin convemion” model.
the R(l,-) transition, the calculated relative intensities of these transitions were summed. It appears that the “no spin interconversion” model explains the observed spectra more satisfactorily than the “complete spin interconversion model.” The P(l,,+) transition at 962 cm-l is expected to increase dramatically in intensity in the range from 13 to 23’K, the &(I,*) and R( 1,‘) transitions are expected to decrease slightly as the temperature is raised, and the ratio of the intensity of the R(O,-) transition at 976 cm-1 to the transition at 1014 cm-l more closely resembles the experimental results for the no spin interconversion model. Irnpuritie8 added to an argon matrix
About 25 separate spectra were recorded of NH, trapped in argon and some of the early spectra showed anomalous lines which we thought could have been due to
1670
J. A.
and A. D. E.
CUGLEY
PULLIN
impurities in the argon. To check this we first satisfied ourselves that we could obtain reproducible spectra that did not contain these anomalous lines, then we deliberately added trace amounts of oxygen and nitrogen. Figure 4 shows the spectrum of [Ar]/[NH,] = 1000 with [Ar]/[N,] = 190. The only difference between this and the uncontaminated spectra is a shoulder at 972 cm-l on the low frequency side of the R(O,-) transition. Similarly Fig. 5 with [Ar]/ [0,] = 250 shows a peak at 968 cm-l.
20 -
1010
1000
990
960
Wavenumber,
970
960
Cm-1
Fig. 4. The argon-ammonia system with added nitrogen impurity. [Ar]/[NH,] = 1000, [Ar]/[Nz] = 190. 8
6o-
I
I
I
I
I
I
I
I
1
I
I
I 1010
1
I 1000
I
I 990
I
I 960
970
960
Wavenumber, cm-l Fig. 5. The argon-ammonia,system with added oxygen impurity. [Ar]/[NH,] = 1000, [Ar]/[O,] = 260.
These lines coincided with the anomalous lines that we had originally obtained and probably result from a nitrogen or oxygen molecule being close to an ammonia molecule, and hence able to perturb it enough to stop rotation. Nitrogen matrix The effect of varying the temperature of an 1130/l matrix from 11 to 16 to 21’K is shown in Fig. 6. Even in warming from 11 to 16’K the two lines at 1003 and 986 cm-l, which also show concentration dependence, start to appear, and become more
The infrared spectrum of ammonia isolated in argon and nitrogen matrices
60
-
40
I.
1671
T=II’K
20-
60
I I
I I
3.
T=2l*K
I
I I I
I I I
I
-
60-
20
-
I
I I I I I I I I 1010 IO00 990 980 Wavenumber.
970
960
cm-l
Fig. 6. The effect of increasing temperature for the nitrogen-ammonia system with M/A = 1130.
intense as the matrix is warmed further. They do not disappear when the temperature is lowered and so must be assigned to dimers or multimers. Of the rest, the line at 982 cm-l appears to merge into the line st 986 cm-i, and may well be a crystal effect which disappears when the matrix is annealed [7]. No lines appear below 969 cm-l and the shoulder on the high frequency side of this band does not appesr to show any temperature dependence. Far infrared work from 10 to 60 cm-i which we have just recently performed shows no evidence for rotating NH, in a nitrogen matrix, there being only one line, at 49 cm-l, which is due to the nitrogen lsttice [8]. In contrast, the far infrared spectrum of NH, isolated in an argon matrix [lo] consists of a number of lines, the analysis of which supports the assignments here proposed. [101 J. A. CUQLEY
8
and A. D. E. PULLIN,Chewa.Whys. L&t. 17, 406 (1972).
1672
J. A.
CUULEY
and A. D. E.
PULLIN
CONCLUSION
Ammonia rotates in an argon matrix but not in a nitrogen matrix, and the assignment proposed by Abouaf-Marguin et al. explains the temperature dependence of the observed lines. Acknowledgement-J. A. Cugley wishes to acknowledge the assistance of a Commonwealth postgraduate research award. The authors also thank Professor I. Mills, Dr. A. Hurley and Dr. M. F. O’Dwyer for informative discussions on statistical weights and line intensities.