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Pure rotational absorption of nitrosyl fluoride and nitrosyl chloride in the 80–250 μ region
Spectlochlmic8 Acta, 1963,Vol. 19,pp. 421to 119. PergamouPressLtd. Printedin NorthernIreland
Pure rotational absorptionof nitrosyl fluoride and nitrosyl chloride in the 80-250 p region J. R. DURIG* and R. C. LORD Spectroscopy
Laboratory, Massachusetts Institute Cambridge 39, Mass. (Received
of Technology,
3 August 1962)
Abs~~t-The pure
rotational infrared absorption spectra of nitrosyl fluoride (ONF) and nitrosyl chloride (OPUTCI)have been investigated with a small grating spectrometer in the spectral range 80-250 p (125-40 cm-l). For ONF twelve Q-branches were observed which occur at positions expected for a slightly asymmetricprolate topwithcentrlfugal distortion. The observed frequencies of these lines are in excellent agreement with thsfrequenclescalculated from the microwave value of 2.80261 cm-l for the rotational constant A - (B + C)/2 provided that a centrifugal distortion constant r>, with a value of 1.14 x lop4 cm-l is used. Eight sharp but rather werik Q-branches have been observed for ONCI. Frequencies are caIeulated which agree with the observed ones within experimental error by using a value of zi. - (B + C)/2) = 2.73 i O-02 cm-” and a D, value of I,0 x low4 cm-l. Combination of the former with the microwave values for B and C gives 9.60 & 0.07 x 10m40g cmz for the least moment of inertia. This leads to an inert& defect of 0.13 amu K and an asymmetry parameter b, -. - 2.21 X 10v3.
fluoride (ONF) is a nonlinear molecule with the nitrogen atom at the apex of a relatively flat triangle. The vibrational infrared spectrum has been previously investigated at prism resolution by JONES and WOLTZ [l] and by WOLTZ et al. [2] who reported the band positions and gave frequency assignments. Rotational analyses of bands due to y1 and v’3were carried out by ~MAGNUSON 133, who resolved these bands with a grating spectrometer. The structural parameters and dipoIe moment have been determined from the microwave spectrum 141. The ONE’ angle of 110” leads to a small value for I,, the least moment of inertia and thus Ib N I,. Therefore ONF is nearly a prolate symmetric-top rotator. The present results are consistent with the previously determined structure of the molecule. The first infrared absorption spectrum of OXCl gas was observed by BAILEY and CASSIE [5] who interpreted the spectrum in terms of three fundamentals. It was later shown by BEESON and YOST [6] that these fundamentals could not account
NITROSYL
* Present address: South Carolma. [l] [2] [3] [4] [5] [6]
E. P. D. D. C. C.
Department
of Chemistry,
University
of South
Carolina,
A. JONES and P. J. H. WOLTZ, J. Chem. Phys.18, 1516 (1950). J. H. WOLTZ, E. A. JONES and A. H. NIELSEN, J. Chem. Phys. 20,378 (1952). W. MAGNUSON, J. Chem. Phys. 20, 380 (1952). W. MAGNUSON, J. Chem. Phys. 19,1071 (1951). R. BAILEY and A. B. D. CASSIE, Proc. Roy. Sot. (London) A145, 336 (1934). M. BEESON and D. M. YOST, J. Chem. Phys. 7,44 (1939). 421
Columbia.
422
J. R. D~RIG and
R.C. LORD
for the observed entropy, and they reinterpreted the spectrum in terms of a lowlying bending frequency beyond the range of measurements made by BAILEY and CASSXE.The molecule was the object of four more recent infrared investigations [7-lo]. The lowest fundamental frequency was observed only in the work of EBERHARDT and BURKE [lo]. From the analysis of the rotational fine structure they obtained values for the least moment of inertia 1, in the ground and first excited vibrational states. Their ground-state value was lower than that calculated from electron-diffraction [ 1I] and microwave [ 121 data. Recently LANDAU and FLETCHER(131 confirmed the low fundamental frequency but did not report any rotational fine structure. There have been two microwave investigations in which the spectra of the two isotopic species 0NC135and ONCP7 were analyzed, the first by ROGERSet al. [12] and the second by MILLEEN and PAXNELL1141. The latter also studied a sample enriched in Ols. The ON%1angle of 114’ leads to a value of 1, so small that in both microwave investigations it was determined as the difference between 1, and I,, a relationship which holds only for instantaneous positions of the nuclei [IS]. In the present work rotational transitions were observed which directly depend upon the value of A - (B + C)/2 so that by using the microwave results for B and C, a value of A can be determined. From the measured value of A the inertial defect A has been calculated. EXPERIMENTAL DETAILSAXD RESULTS The far-infrared spectra of ONF and OXCl were observed with the small grating spectrometer of LORD and MCCUBBLN [t6]. Its wavenumber range was extended down to 35 cm-l by means of a Bausch and Lomb grating having 4 grooves per mm blazed at 45 cm-1 in the first order. Purification of the radiation was achieved with the help of replica gratings used as reflection filters. These filters were replicated by the Jarrell-Ash Company from a grating of 6.4 grooves per mm ruled at the University of Michigan and were of high quality. They were used as crossed polarizers to take advantage of the fact that the unwanted diffracted radiation is much more strongly polarized than the desired specularly reflected radiation. The normals to the surfaces of the gratings were not parallel but made an angle of about 120” to one another. The filters were located just before the Golay detector [fft]. Their apparent reflectivity above 70 cm-l was checked with a grating blazed at 90 cm-l in the spectrometer and with a mild filter of black polyethylene (10 mils thick) in the optical path. Under these conditions the integrated reflected radiation for all 171J. H. WISE and J. T. ELMER, f. Chem. Phys. 18, 1411 (1950). [9] W, G. BURNS and H. J. BERNSTEIN, J. Chem. Phye. 18, 1669(1950). [9] A. G. PULFORD and A. WALSR, Trans. Faraday Sot. 47, 347 (195X), [lo] 5V.I-I. EBERHARDT and T.G. BTRKE,J.C~~M.. P&s. 20,529 (1953). [ 1 l] J”. A. A. KETELAAR and K. J. PALMER, J. Am. Chem. Sot. 59, 2629 (1937). [12] J.D. ROGERS,~. J,PIETENPOL~~~D. WILLIAMS, Phys. Rew.83,431 (1951). [13] I,. LANDAU and W. H. FLETCHER, J. Mol. Spectrosc. 4, 276 (1960). [lb] D.J. MILLEN andJ.P~NN~~~,J.chern.Soc. 1322 (1961). [15] G. HERZBERG, Infrared and Rccman Spectra of Poly~~~e ~~alec~le~ p. 461,Van No&and, Sew York (1945). [lS] R.C. LORD and T K.M&UBBIR.
jr..J. Opt. Sot. Am. 47, 689
(1957).
Pure rotational
423
absorption of nitrosyl fluoride
from 70 to 500 cm-l was less than 3 per cent of the radiation in one spectral slit width (about 1 cm-l) when the grating was scanned from 70 cm-l up. Nitrosyl fluoride gas contained in a Monel-metal cylinder was obtained from the Central Research Department, E. I. duPont deNemours and Company, and the purity was checked spectroscopically by the vibrational infrared spectrum. A prefluorinated Monel cell with silver-chloride windows attached with Kel-F wax was used to contain the sample for this check. It was found that NO, was present as an impurity in small amounts, but its far-infrared spectrum is known [17] and no interference was anticipated. For the far-infrared work, the ONF gas was contained at pressures slightly below one atmosphere in a 10 cm prefluorinated Monel cell with polyethylene windows. Preconditioning of the metal surfaces is very important, particularly in work with small quantities, to prevent contamination or decomposition of the pure gas [18]. When a pure sample of ONF was placed in a clean Monel absorption cell, no trace of the gas could be detected in the infrared spectrum. However, after conditioning of the cell overnight with ONF at one atmosphere, the spectrum of a fresh sample of gas showed only small amounts of nitrogen oxides. Pressures were measured with a special manometer having a liquid fluorocarbon cover for the mercury and a vacuum system constructed from polyethylene tubing and Monel valves. When the above precautions were taken, it was found that ONF contained in the Monel infrared gas cells showed no appreciable decomposition up to three months’ time Nitrosyl chloride was obtained from Matheson and Company, Inc., and initially used without further purification. A IO-cm stainless steel cell was used to contain the ONCl gas at pressures slightly below 1 atm, but the vibrational infrared spectra recorded before and then after the far-infrared study showed extensivedecomposition. Thereafter a Monel cell equipped with polyethylene windows was employed in all far-infrared work and the mercury manometer with a liquid fluorocarbon cover mentioned above was used to measure the pressure. No appreciable decomposition was observed when the above precautions were taken. The rather large permanent dipole (1.81 D) of ONF leads one to expect strong absorption comparable to that of NH, and H,O. Intense continuous absorption was found between 40 and 125 cm-l with a series of 12 rather strong Q-branches. The Q-branch frequencies are listed in Table 1 and recordings of some of them are shown in Figs. 1 and 2. The first five Q-branches were observed in the first order of the 4-grooves/mm grating. The next seven, along with the fourth and fifth from the previous region, were measured in the first order of the S-grooves/mm grating. The last t,wo Q-branches are heavily masked by strong H,O absorption at 96.15 There was indication of about three more, but strong H,O aband loo.55 cm-l. sorption also interfered with definite identification of these. The measured frequencies reported in Table 1 are expected to be correct to within 0.20 cm-l or better. Below 80 cm-l the wavenumber scale was calibrated with frequencies of HCN calculated from the values of B, and D, reported by ALLEN etal.[19].The region from 40 to 125 cm-l was calibrated with the absorpt,ion wavenumbers
1171 G.
R. BIRD, A. DANTI and*R C. LORD, Spectrochzm. Acta 12, 247 (1958)
[IS] S. ANDREADES, Private communication [19] H
(22 Dec., 1960). C’ ALLEN, Jr., E. D TIDWELL and E. K. PLYLER, J. Chew. Phys
25, 302 (1956)
J. R. DJRIG
424
Table 1. Rotational
K&
Observed frequency [cm-l (vat)]
(I&
and R. C. LORD
Q-Branches of ONI?
Calculated frequency = l-14 x 10-d)
Ohs-oalc.
2.80261
0
7 8 9 10 11 12 13 14 15 I6 17 18
Rigid molecule (talc.)
41.70 47.39 52.85 58.35 63.78 69.10 74.48 79 73 85.17 SO.55 95.65 (100.7)
41-85 47.36 52.86 58.33 63 77 69 17 74 55 79 88 85.18 SO.44 95.65 100-81
-0.15 + 0.03 --0.01 to.02 +0.01 -0.07 -- 0.07 -0*15 -0.01 i-o.11 + 0.00 - 0.11
42.04 47-64 53-25 58.85 64.46 70.07 75.67 81.28 86 88 92.49 98.09 103.70
Fig. 1. The lower curve is the mtrosyl fluoride and the upper one the background. The curves were recorded ( -0.5 cm-ljmin) under the following conditions: source, General Electric H-loo-_44 Hg arc; two grating reflection filters (6.4 grooves/ mm) ; lo-ml1 black polyethylene transmission filter; KBr chopper; sbt width 3.0 mm; lo-cm Monel-metal cell with 30-ml1 thick polyethylene windows; pressure, l/2 at,m of OSF.
Pure rotational absorption of mtrosyl fluoride
425
spectrum of H,O, the frequencies used being those of YAROSLAVSKIIand STANEVICH [20]. These two methods of calibration, overlapping for t’he 50-80 cm-l region, agreed to within about 0.10 cm-l. The odd shape of the Q-branches is very similar to that predicted [21] and found [22] for ozone. This is not surprising since the asymmetry parameters of ozone and nitrosyl fluoride are about the same.
ONF
90
85
80
75
70 65 Wovenumber.
60
55
cm-’
Fig 2. The lower two curves show the mtrosyl fluoride absorption and the upper one the background. The curves were recorded ( -0.5 cm-l/mm) under the same condltlons as Frg. 1 except that two KRS-5 reflection filters replaced the gratmg filters and the pressure of ONF was slightly less than 1 atm. The traces of the mdrvidual Q-branches were recorded with higher amphfier gain and one TlBr reflection filter.
Strong absorption by ONCl comparable to that found for ONF was expected from the measured dipole moment of 1.83 D [23]. This expectation was not realized. Very weak generalized absorption was found between 40 amd 80 cm-l, on which was superimposed a series of sharp but rather weak Q-branches. Therefore a second lo-cm Monel cell was placed in the source compartment of the instrument [lS], which gave a total gas path of 20 cm atm. The observed Q-branches are listed in Table 2 and recordings of some of them are shown in Fig. 3. The first four Q-branches were observed in the first order of the 4-grooves/mm grating and the last four in the first order of the 8-grooves/mm grating. Additional Q-branches could not be detected because of the interference of strong water-vapor absorption in the background. In the initial investigation it was noted that the bands at 41.05 and 62.33 cm-i were both stronger and sharper than the others. Upon checking the spectrum from 80 to 110 cm-l, two additional bands were found at 83 and 104 cm-l, which frequencies indicated the presence of a trace amount (less than 1 per cent) of HCl in the sample. The amount was too small to be detected in the vibrational spectrum under the conditions of measurement. The sample was further purified by a number of [20] [21] [22] [23]
N. E. A. J.
G. Yaroslavsku and A. E. Stanevlch, Izveat.dkad. XuukS.S.S K. GORA, J. Mol. Spectros. 3, 78 (1959). DANTI and R C. LORD, J. Chem. Phys. 30, 1310 (1959). A. A. Ketelaar, Rec. Trav. Chum. 62, 289 (1943).
R , Ser. F?z 22,1145( 1958).
J. R. Durtrs and El. C.
446
Lown
Table 2. Rotation&l &-branches of ONCI
K=
Calculated frequency (DX = 1.0 x lWa)
Observed frequency
Ohs.-talc.
A - (I3 + G)/2 Value for this work 2.73
0
7 8
9 10 11 12 13 14
A - (B -+ C)/2 microwave value
40.78 46.16 51.53 56.87 62.18 67.47 72.72 77.95
41.05 46.25 51.47 (56.85) 62.33 67.39 72-55 77.83
2.66394
(talc. for rigld mol.) 40 95 39.96 46.41 45.29 51.87 50.61 57.33 55.94 62.79 61-27 68.25 66.60 71.92 73.71 79,17 77.25
$0~27
-t 0.09 -0.06 -- 0.02 $0.15 -0.08 -Lo*17 $0.12
k&* I
75
70
65
60
+p.----’
Wa~enumber~
I
55
1
50
’
45
40
cm-’
Fig. 3. The lower curve is the nitrosyl chloride and the upper one the background. The curves were recorded under the same conditions as Figs. 1 and 2 except that two lo-cm Monel-metal cells were employed with the pressure of ON’1 in each slightly less than 1 atm.
trap-to-trap distillations and contamination by HCI was no longer evident in the spectrum from 80 to 110 cm-l. The frequencies of the &-branches were then remeasured with the purified sample and are reported in Table 3. They are expected to be correct to within 0.20 cm-1 The line at 46 cm-l is partially overlapped by the 47.04 cm-1 wateror better. vapor line. By reducing the nitrogen flushing rate and thus increasing the watervapor absorption, two lines were clearly resolved. The band observed under strong flushing is thus expected to be due only to ONCI, and its frequency is not believed to be seriously affected by the overlap. Recent& the 30-p f~lndamental of ONCE was reinvestigated 1241 and the fine [24] Ohlo St&e Report,, Project Xo
2104-P.
Contract, So. DA-33-019-ORB-2722.
Pure rotational absorption of n&rosy1fluoride
$27
structure reported by ~BER~~~T and B~XUCE[ 101 cuuld not be confirmed. The same band was reinvestigated by us with the gas contained at a pressure of 25 mm Hg in a IO-cm Monel cell with polyethylene windows. No fine structure was observed. In all the above work the spectral slit width was less than 1 cm-l. DISCUSSI~~N OF RESULTS
The rotational spectrum of ONP must be analyzed, strictly speaking, as that of an asy~~~~ri~ top+ The dipole moment [4] does not lie along any principal axis but may be resolved into two components ,H~and ph. The allowed transitions are therefore the sum of all those allowed for each component [25]. The component along the least axis (,u,) is by far the larger but as it contributes only to the generalized background absorption under the present moderate resolution, it will not be considered further. The observed &-branches arise from the component of the dipole moment component perpendicular to the least axis &). The energy levels of an asymmetric top are commonly characterized by their quantum numbers J, the total angular momentum apart from particle spins, K_,, the limiting value of_7i’ for a prolate symmetric top, and K,,, the limiting value of K for an oblate symmetric top. In terms of these quantum numbers, the observed transitions are a series of &-branches with the selection rules AJ = 0, AK_, = + 1, and AK+1 = - 1. Each &-branch is thus a collection of superimposed lines due to all transitions for which the levels JK_l,K+, (J > K_,) have appreciable populations at 300°K. Thus for example the Q-branch found at 58.35 cm-l is composed of lines due to the transitions
1%s - fO,,,: q,,,
- fO,*lr 111,,2- %,3, %,“l - lb.2
and so forth. Observation of these &-branches depends on the degree of asymmetry, Since the value of the Wang asymmetry parameter b, obtained from microwave studies C41is b, =
IC - f4 Z[A -
(B -L C)/2]
= -7-95
x IO-3
the asymmetry
correction for the K_, values observed here is smaller than the resolution and we may neglect the asymmetry and consider ONE’ initially as a rigid symmetric top. The energy levels and the series of &-branches are given by the limiting symmetric top formulae = J(J + 1) (R + C)$
WJX-A,, Y&
=
[A -
(I3 + C)Pl
+ [A -
(I.3 -I- C)/2jK_,2
f2-KL + 1)
(1)
(2)
where KL is the lower value of K_, for the transition. With the microwave value of R - (B + C)/2 = 2.80161 cm-l, the series of lines shown in column 5 of Table 1 is predicted. The observed spectrum deviates from these lines because of centrifugal f2Eiij C. Et. Towrms and A. T;. %‘HAWLOW,
York f1955).
Micrmave
Spertrmcopy,
Chap. 4. McGraw-Hill,
SW
428
J. R. DURIG
~stortion, but can be accounted (‘1) corrected for distortion:
~~(J,K-,) = w(J,K_l)r,g -
and R. C. LORD
for by using the limiting symmetric
DJJZ(J + 1)2 -
DJKJ(J
+ l)K_,2
-
top formula
D&L,4
.
(3)
For transitions with AJ = 0 there is no contribution from 11,, and D,, is assumed negligible compared to D,. The expression for the Q-branch frequencies then becomes yo = [A - (B + C)jz](zrr, + 1) - r)ll(iKL3 $- 6iu,’ t 4hT, + 1) (4) By using the microwave value for A - (23 + C)/2 and a value for DK of l-14 x IO-4 cm-l, the frequencies listed in coIumn 3 of Table 1 were calculated. The agreement with the observed frequencies is within experimental error. The value of DK may be compared to those found for other triatomic molecules (Table 3). The distortion Table 3. Molecular parameters (See Refs. [4, 14, 17, 22, 231) Dipole moment X-O Molecule
Apex angle
NO2 0, SO, ONF
134” 15’ 116” 49’ 119” 32’ 110”
ONCl
114”
distance (11)
1.197 1.278 1.432 l-13(N-0) 1.52(X-F) 1.139(X-O) 1.975(X-Cl)
(1)) --.
Asymmetry parameter -6, (10-3)
Centrifugal distortion constant DE ( 10P4 cm-“)
Pa
Pb
1.70
0.33 0.53 1.60 0.62
1.52 8.06 14.82 7.95
22.4 1.96 0.761 1.14
1.28
I.31
2.21
1.0
is smaller than that found in 0, but this is to be expected since the bond angle is somewhat smaller. The interpretation of the rotational spectrum of ONCI follows that for ONF. From the dipole moments [l 2, 231 and the relative values of rl, B and C, one would expect the Q-branches to be nearly four times as strong as those found for OXF. The asymmetry parameter is smaller (see Table 3) and therefore the Q-branches should be sharper. For this reason their peak absorption might be somewhat reduced by the spectral slit width of the instrument. That the Q-branches observed were not as strong as those of ONE” can be seen from a comparison of Figs. 1 and 3. Twice the path length of the gas had to be used to observe them distinctly as was used for ONF at the same pressure. The observed Q-branches can be interpreted in terms of the limiting symmotri~ top formulae (3) and (4). By using any two frequencies, values for d - (B + C)/2 and DK can be obtained. With a value of A - (B + C)/2 = 2.73 cm-l and a DK value of 1.0 x lo-* the frequencies listed in column 3 of Table 2 were calculated. The agreement with the measured frequencies is within experimental error for all except the band of lowest observed frequency. It can be seen from Table 2 that no values of the centrifugal distortion constant combined with the microwave value of A - (B -C Cf/2 will reproduce the observed
Pure rotational absorption of mtrosyl fluoride
429
bands. This failure appears to be due to neglect of the inertial defect. The microwave values for B and C combined with the present results give a value for A of 87,400 MC/S, as compared with 85,420 MC/S reported from the microwave work alone. Table 4 shows the various values of the smallest moment of inertia that have Table 4. Values reported for the least moment I, of ONCl
Ia k
cm2)
9.05 x 10-40 9.837 x 1O-4o 8 3 & 0.5 x 1O-4o 9.822 x 1O-4o 960 i_ 0.07 x 10-40
of inertia
Source Electron diffraction [ 1 l] Microwave [ 121 Vibrational infrared [lo] Microwave [ 141 This work
been reported. The result obtained from this work appears to be the most nearly correct of these. With the measured value of I,, the inertial defect A, as defined by A = I, - Ia - I,, is calculated to be 0.13 a.m.u. A2. The asymmetry parameter b, = -2.21 x lob3 is slightly smaller than that calculated from the microwave data. Acknoudedgements-We thank Dr. R. M. JOYCE and Dr. S. ANDREADES of the Central Research Department, E. I. duPont deNemours and Company, for the gift of the sample of nitrosyl fluoride and advice about handling it. This work was supported by the National Science Foundation under grants G-6230 and G-19,637.
Pure rotational absorptionof nitrosyl fluoride and nitrosyl chloride in the 80-250 p region J. R. DURIG* and R. C. LORD Spectroscopy
Laboratory, Massachusetts Institute Cambridge 39, Mass. (Received
of Technology,
3 August 1962)
Abs~~t-The pure
rotational infrared absorption spectra of nitrosyl fluoride (ONF) and nitrosyl chloride (OPUTCI)have been investigated with a small grating spectrometer in the spectral range 80-250 p (125-40 cm-l). For ONF twelve Q-branches were observed which occur at positions expected for a slightly asymmetricprolate topwithcentrlfugal distortion. The observed frequencies of these lines are in excellent agreement with thsfrequenclescalculated from the microwave value of 2.80261 cm-l for the rotational constant A - (B + C)/2 provided that a centrifugal distortion constant r>, with a value of 1.14 x lop4 cm-l is used. Eight sharp but rather werik Q-branches have been observed for ONCI. Frequencies are caIeulated which agree with the observed ones within experimental error by using a value of zi. - (B + C)/2) = 2.73 i O-02 cm-” and a D, value of I,0 x low4 cm-l. Combination of the former with the microwave values for B and C gives 9.60 & 0.07 x 10m40g cmz for the least moment of inertia. This leads to an inert& defect of 0.13 amu K and an asymmetry parameter b, -. - 2.21 X 10v3.
fluoride (ONF) is a nonlinear molecule with the nitrogen atom at the apex of a relatively flat triangle. The vibrational infrared spectrum has been previously investigated at prism resolution by JONES and WOLTZ [l] and by WOLTZ et al. [2] who reported the band positions and gave frequency assignments. Rotational analyses of bands due to y1 and v’3were carried out by ~MAGNUSON 133, who resolved these bands with a grating spectrometer. The structural parameters and dipoIe moment have been determined from the microwave spectrum 141. The ONE’ angle of 110” leads to a small value for I,, the least moment of inertia and thus Ib N I,. Therefore ONF is nearly a prolate symmetric-top rotator. The present results are consistent with the previously determined structure of the molecule. The first infrared absorption spectrum of OXCl gas was observed by BAILEY and CASSIE [5] who interpreted the spectrum in terms of three fundamentals. It was later shown by BEESON and YOST [6] that these fundamentals could not account
NITROSYL
* Present address: South Carolma. [l] [2] [3] [4] [5] [6]
E. P. D. D. C. C.
Department
of Chemistry,
University
of South
Carolina,
A. JONES and P. J. H. WOLTZ, J. Chem. Phys.18, 1516 (1950). J. H. WOLTZ, E. A. JONES and A. H. NIELSEN, J. Chem. Phys. 20,378 (1952). W. MAGNUSON, J. Chem. Phys. 20, 380 (1952). W. MAGNUSON, J. Chem. Phys. 19,1071 (1951). R. BAILEY and A. B. D. CASSIE, Proc. Roy. Sot. (London) A145, 336 (1934). M. BEESON and D. M. YOST, J. Chem. Phys. 7,44 (1939). 421
Columbia.
422
J. R. D~RIG and
R.C. LORD
for the observed entropy, and they reinterpreted the spectrum in terms of a lowlying bending frequency beyond the range of measurements made by BAILEY and CASSXE.The molecule was the object of four more recent infrared investigations [7-lo]. The lowest fundamental frequency was observed only in the work of EBERHARDT and BURKE [lo]. From the analysis of the rotational fine structure they obtained values for the least moment of inertia 1, in the ground and first excited vibrational states. Their ground-state value was lower than that calculated from electron-diffraction [ 1I] and microwave [ 121 data. Recently LANDAU and FLETCHER(131 confirmed the low fundamental frequency but did not report any rotational fine structure. There have been two microwave investigations in which the spectra of the two isotopic species 0NC135and ONCP7 were analyzed, the first by ROGERSet al. [12] and the second by MILLEEN and PAXNELL1141. The latter also studied a sample enriched in Ols. The ON%1angle of 114’ leads to a value of 1, so small that in both microwave investigations it was determined as the difference between 1, and I,, a relationship which holds only for instantaneous positions of the nuclei [IS]. In the present work rotational transitions were observed which directly depend upon the value of A - (B + C)/2 so that by using the microwave results for B and C, a value of A can be determined. From the measured value of A the inertial defect A has been calculated. EXPERIMENTAL DETAILSAXD RESULTS The far-infrared spectra of ONF and OXCl were observed with the small grating spectrometer of LORD and MCCUBBLN [t6]. Its wavenumber range was extended down to 35 cm-l by means of a Bausch and Lomb grating having 4 grooves per mm blazed at 45 cm-1 in the first order. Purification of the radiation was achieved with the help of replica gratings used as reflection filters. These filters were replicated by the Jarrell-Ash Company from a grating of 6.4 grooves per mm ruled at the University of Michigan and were of high quality. They were used as crossed polarizers to take advantage of the fact that the unwanted diffracted radiation is much more strongly polarized than the desired specularly reflected radiation. The normals to the surfaces of the gratings were not parallel but made an angle of about 120” to one another. The filters were located just before the Golay detector [fft]. Their apparent reflectivity above 70 cm-l was checked with a grating blazed at 90 cm-l in the spectrometer and with a mild filter of black polyethylene (10 mils thick) in the optical path. Under these conditions the integrated reflected radiation for all 171J. H. WISE and J. T. ELMER, f. Chem. Phys. 18, 1411 (1950). [9] W, G. BURNS and H. J. BERNSTEIN, J. Chem. Phye. 18, 1669(1950). [9] A. G. PULFORD and A. WALSR, Trans. Faraday Sot. 47, 347 (195X), [lo] 5V.I-I. EBERHARDT and T.G. BTRKE,J.C~~M.. P&s. 20,529 (1953). [ 1 l] J”. A. A. KETELAAR and K. J. PALMER, J. Am. Chem. Sot. 59, 2629 (1937). [12] J.D. ROGERS,~. J,PIETENPOL~~~D. WILLIAMS, Phys. Rew.83,431 (1951). [13] I,. LANDAU and W. H. FLETCHER, J. Mol. Spectrosc. 4, 276 (1960). [lb] D.J. MILLEN andJ.P~NN~~~,J.chern.Soc. 1322 (1961). [15] G. HERZBERG, Infrared and Rccman Spectra of Poly~~~e ~~alec~le~ p. 461,Van No&and, Sew York (1945). [lS] R.C. LORD and T K.M&UBBIR.
jr..J. Opt. Sot. Am. 47, 689
(1957).
Pure rotational
423
absorption of nitrosyl fluoride
from 70 to 500 cm-l was less than 3 per cent of the radiation in one spectral slit width (about 1 cm-l) when the grating was scanned from 70 cm-l up. Nitrosyl fluoride gas contained in a Monel-metal cylinder was obtained from the Central Research Department, E. I. duPont deNemours and Company, and the purity was checked spectroscopically by the vibrational infrared spectrum. A prefluorinated Monel cell with silver-chloride windows attached with Kel-F wax was used to contain the sample for this check. It was found that NO, was present as an impurity in small amounts, but its far-infrared spectrum is known [17] and no interference was anticipated. For the far-infrared work, the ONF gas was contained at pressures slightly below one atmosphere in a 10 cm prefluorinated Monel cell with polyethylene windows. Preconditioning of the metal surfaces is very important, particularly in work with small quantities, to prevent contamination or decomposition of the pure gas [18]. When a pure sample of ONF was placed in a clean Monel absorption cell, no trace of the gas could be detected in the infrared spectrum. However, after conditioning of the cell overnight with ONF at one atmosphere, the spectrum of a fresh sample of gas showed only small amounts of nitrogen oxides. Pressures were measured with a special manometer having a liquid fluorocarbon cover for the mercury and a vacuum system constructed from polyethylene tubing and Monel valves. When the above precautions were taken, it was found that ONF contained in the Monel infrared gas cells showed no appreciable decomposition up to three months’ time Nitrosyl chloride was obtained from Matheson and Company, Inc., and initially used without further purification. A IO-cm stainless steel cell was used to contain the ONCl gas at pressures slightly below 1 atm, but the vibrational infrared spectra recorded before and then after the far-infrared study showed extensivedecomposition. Thereafter a Monel cell equipped with polyethylene windows was employed in all far-infrared work and the mercury manometer with a liquid fluorocarbon cover mentioned above was used to measure the pressure. No appreciable decomposition was observed when the above precautions were taken. The rather large permanent dipole (1.81 D) of ONF leads one to expect strong absorption comparable to that of NH, and H,O. Intense continuous absorption was found between 40 and 125 cm-l with a series of 12 rather strong Q-branches. The Q-branch frequencies are listed in Table 1 and recordings of some of them are shown in Figs. 1 and 2. The first five Q-branches were observed in the first order of the 4-grooves/mm grating. The next seven, along with the fourth and fifth from the previous region, were measured in the first order of the S-grooves/mm grating. The last t,wo Q-branches are heavily masked by strong H,O absorption at 96.15 There was indication of about three more, but strong H,O aband loo.55 cm-l. sorption also interfered with definite identification of these. The measured frequencies reported in Table 1 are expected to be correct to within 0.20 cm-l or better. Below 80 cm-l the wavenumber scale was calibrated with frequencies of HCN calculated from the values of B, and D, reported by ALLEN etal.[19].The region from 40 to 125 cm-l was calibrated with the absorpt,ion wavenumbers
1171 G.
R. BIRD, A. DANTI and*R C. LORD, Spectrochzm. Acta 12, 247 (1958)
[IS] S. ANDREADES, Private communication [19] H
(22 Dec., 1960). C’ ALLEN, Jr., E. D TIDWELL and E. K. PLYLER, J. Chew. Phys
25, 302 (1956)
J. R. DJRIG
424
Table 1. Rotational
K&
Observed frequency [cm-l (vat)]
(I&
and R. C. LORD
Q-Branches of ONI?
Calculated frequency = l-14 x 10-d)
Ohs-oalc.
2.80261
0
7 8 9 10 11 12 13 14 15 I6 17 18
Rigid molecule (talc.)
41.70 47.39 52.85 58.35 63.78 69.10 74.48 79 73 85.17 SO.55 95.65 (100.7)
41-85 47.36 52.86 58.33 63 77 69 17 74 55 79 88 85.18 SO.44 95.65 100-81
-0.15 + 0.03 --0.01 to.02 +0.01 -0.07 -- 0.07 -0*15 -0.01 i-o.11 + 0.00 - 0.11
42.04 47-64 53-25 58.85 64.46 70.07 75.67 81.28 86 88 92.49 98.09 103.70
Fig. 1. The lower curve is the mtrosyl fluoride and the upper one the background. The curves were recorded ( -0.5 cm-ljmin) under the following conditions: source, General Electric H-loo-_44 Hg arc; two grating reflection filters (6.4 grooves/ mm) ; lo-ml1 black polyethylene transmission filter; KBr chopper; sbt width 3.0 mm; lo-cm Monel-metal cell with 30-ml1 thick polyethylene windows; pressure, l/2 at,m of OSF.
Pure rotational absorption of mtrosyl fluoride
425
spectrum of H,O, the frequencies used being those of YAROSLAVSKIIand STANEVICH [20]. These two methods of calibration, overlapping for t’he 50-80 cm-l region, agreed to within about 0.10 cm-l. The odd shape of the Q-branches is very similar to that predicted [21] and found [22] for ozone. This is not surprising since the asymmetry parameters of ozone and nitrosyl fluoride are about the same.
ONF
90
85
80
75
70 65 Wovenumber.
60
55
cm-’
Fig 2. The lower two curves show the mtrosyl fluoride absorption and the upper one the background. The curves were recorded ( -0.5 cm-l/mm) under the same condltlons as Frg. 1 except that two KRS-5 reflection filters replaced the gratmg filters and the pressure of ONF was slightly less than 1 atm. The traces of the mdrvidual Q-branches were recorded with higher amphfier gain and one TlBr reflection filter.
Strong absorption by ONCl comparable to that found for ONF was expected from the measured dipole moment of 1.83 D [23]. This expectation was not realized. Very weak generalized absorption was found between 40 amd 80 cm-l, on which was superimposed a series of sharp but rather weak Q-branches. Therefore a second lo-cm Monel cell was placed in the source compartment of the instrument [lS], which gave a total gas path of 20 cm atm. The observed Q-branches are listed in Table 2 and recordings of some of them are shown in Fig. 3. The first four Q-branches were observed in the first order of the 4-grooves/mm grating and the last four in the first order of the 8-grooves/mm grating. Additional Q-branches could not be detected because of the interference of strong water-vapor absorption in the background. In the initial investigation it was noted that the bands at 41.05 and 62.33 cm-i were both stronger and sharper than the others. Upon checking the spectrum from 80 to 110 cm-l, two additional bands were found at 83 and 104 cm-l, which frequencies indicated the presence of a trace amount (less than 1 per cent) of HCl in the sample. The amount was too small to be detected in the vibrational spectrum under the conditions of measurement. The sample was further purified by a number of [20] [21] [22] [23]
N. E. A. J.
G. Yaroslavsku and A. E. Stanevlch, Izveat.dkad. XuukS.S.S K. GORA, J. Mol. Spectros. 3, 78 (1959). DANTI and R C. LORD, J. Chem. Phys. 30, 1310 (1959). A. A. Ketelaar, Rec. Trav. Chum. 62, 289 (1943).
R , Ser. F?z 22,1145( 1958).
J. R. Durtrs and El. C.
446
Lown
Table 2. Rotation&l &-branches of ONCI
K=
Calculated frequency (DX = 1.0 x lWa)
Observed frequency
Ohs.-talc.
A - (I3 + G)/2 Value for this work 2.73
0
7 8
9 10 11 12 13 14
A - (B -+ C)/2 microwave value
40.78 46.16 51.53 56.87 62.18 67.47 72.72 77.95
41.05 46.25 51.47 (56.85) 62.33 67.39 72-55 77.83
2.66394
(talc. for rigld mol.) 40 95 39.96 46.41 45.29 51.87 50.61 57.33 55.94 62.79 61-27 68.25 66.60 71.92 73.71 79,17 77.25
$0~27
-t 0.09 -0.06 -- 0.02 $0.15 -0.08 -Lo*17 $0.12
k&* I
75
70
65
60
+p.----’
Wa~enumber~
I
55
1
50
’
45
40
cm-’
Fig. 3. The lower curve is the nitrosyl chloride and the upper one the background. The curves were recorded under the same conditions as Figs. 1 and 2 except that two lo-cm Monel-metal cells were employed with the pressure of ON’1 in each slightly less than 1 atm.
trap-to-trap distillations and contamination by HCI was no longer evident in the spectrum from 80 to 110 cm-l. The frequencies of the &-branches were then remeasured with the purified sample and are reported in Table 3. They are expected to be correct to within 0.20 cm-1 The line at 46 cm-l is partially overlapped by the 47.04 cm-1 wateror better. vapor line. By reducing the nitrogen flushing rate and thus increasing the watervapor absorption, two lines were clearly resolved. The band observed under strong flushing is thus expected to be due only to ONCI, and its frequency is not believed to be seriously affected by the overlap. Recent& the 30-p f~lndamental of ONCE was reinvestigated 1241 and the fine [24] Ohlo St&e Report,, Project Xo
2104-P.
Contract, So. DA-33-019-ORB-2722.
Pure rotational absorption of n&rosy1fluoride
$27
structure reported by ~BER~~~T and B~XUCE[ 101 cuuld not be confirmed. The same band was reinvestigated by us with the gas contained at a pressure of 25 mm Hg in a IO-cm Monel cell with polyethylene windows. No fine structure was observed. In all the above work the spectral slit width was less than 1 cm-l. DISCUSSI~~N OF RESULTS
The rotational spectrum of ONP must be analyzed, strictly speaking, as that of an asy~~~~ri~ top+ The dipole moment [4] does not lie along any principal axis but may be resolved into two components ,H~and ph. The allowed transitions are therefore the sum of all those allowed for each component [25]. The component along the least axis (,u,) is by far the larger but as it contributes only to the generalized background absorption under the present moderate resolution, it will not be considered further. The observed &-branches arise from the component of the dipole moment component perpendicular to the least axis &). The energy levels of an asymmetric top are commonly characterized by their quantum numbers J, the total angular momentum apart from particle spins, K_,, the limiting value of_7i’ for a prolate symmetric top, and K,,, the limiting value of K for an oblate symmetric top. In terms of these quantum numbers, the observed transitions are a series of &-branches with the selection rules AJ = 0, AK_, = + 1, and AK+1 = - 1. Each &-branch is thus a collection of superimposed lines due to all transitions for which the levels JK_l,K+, (J > K_,) have appreciable populations at 300°K. Thus for example the Q-branch found at 58.35 cm-l is composed of lines due to the transitions
1%s - fO,,,: q,,,
- fO,*lr 111,,2- %,3, %,“l - lb.2
and so forth. Observation of these &-branches depends on the degree of asymmetry, Since the value of the Wang asymmetry parameter b, obtained from microwave studies C41is b, =
IC - f4 Z[A -
(B -L C)/2]
= -7-95
x IO-3
the asymmetry
correction for the K_, values observed here is smaller than the resolution and we may neglect the asymmetry and consider ONE’ initially as a rigid symmetric top. The energy levels and the series of &-branches are given by the limiting symmetric top formulae = J(J + 1) (R + C)$
WJX-A,, Y&
=
[A -
(I3 + C)Pl
+ [A -
(I.3 -I- C)/2jK_,2
f2-KL + 1)
(1)
(2)
where KL is the lower value of K_, for the transition. With the microwave value of R - (B + C)/2 = 2.80161 cm-l, the series of lines shown in column 5 of Table 1 is predicted. The observed spectrum deviates from these lines because of centrifugal f2Eiij C. Et. Towrms and A. T;. %‘HAWLOW,
York f1955).
Micrmave
Spertrmcopy,
Chap. 4. McGraw-Hill,
SW
428
J. R. DURIG
~stortion, but can be accounted (‘1) corrected for distortion:
~~(J,K-,) = w(J,K_l)r,g -
and R. C. LORD
for by using the limiting symmetric
DJJZ(J + 1)2 -
DJKJ(J
+ l)K_,2
-
top formula
D&L,4
.
(3)
For transitions with AJ = 0 there is no contribution from 11,, and D,, is assumed negligible compared to D,. The expression for the Q-branch frequencies then becomes yo = [A - (B + C)jz](zrr, + 1) - r)ll(iKL3 $- 6iu,’ t 4hT, + 1) (4) By using the microwave value for A - (23 + C)/2 and a value for DK of l-14 x IO-4 cm-l, the frequencies listed in coIumn 3 of Table 1 were calculated. The agreement with the observed frequencies is within experimental error. The value of DK may be compared to those found for other triatomic molecules (Table 3). The distortion Table 3. Molecular parameters (See Refs. [4, 14, 17, 22, 231) Dipole moment X-O Molecule
Apex angle
NO2 0, SO, ONF
134” 15’ 116” 49’ 119” 32’ 110”
ONCl
114”
distance (11)
1.197 1.278 1.432 l-13(N-0) 1.52(X-F) 1.139(X-O) 1.975(X-Cl)
(1)) --.
Asymmetry parameter -6, (10-3)
Centrifugal distortion constant DE ( 10P4 cm-“)
Pa
Pb
1.70
0.33 0.53 1.60 0.62
1.52 8.06 14.82 7.95
22.4 1.96 0.761 1.14
1.28
I.31
2.21
1.0
is smaller than that found in 0, but this is to be expected since the bond angle is somewhat smaller. The interpretation of the rotational spectrum of ONCI follows that for ONF. From the dipole moments [l 2, 231 and the relative values of rl, B and C, one would expect the Q-branches to be nearly four times as strong as those found for OXF. The asymmetry parameter is smaller (see Table 3) and therefore the Q-branches should be sharper. For this reason their peak absorption might be somewhat reduced by the spectral slit width of the instrument. That the Q-branches observed were not as strong as those of ONE” can be seen from a comparison of Figs. 1 and 3. Twice the path length of the gas had to be used to observe them distinctly as was used for ONF at the same pressure. The observed Q-branches can be interpreted in terms of the limiting symmotri~ top formulae (3) and (4). By using any two frequencies, values for d - (B + C)/2 and DK can be obtained. With a value of A - (B + C)/2 = 2.73 cm-l and a DK value of 1.0 x lo-* the frequencies listed in column 3 of Table 2 were calculated. The agreement with the measured frequencies is within experimental error for all except the band of lowest observed frequency. It can be seen from Table 2 that no values of the centrifugal distortion constant combined with the microwave value of A - (B -C Cf/2 will reproduce the observed
Pure rotational absorption of mtrosyl fluoride
429
bands. This failure appears to be due to neglect of the inertial defect. The microwave values for B and C combined with the present results give a value for A of 87,400 MC/S, as compared with 85,420 MC/S reported from the microwave work alone. Table 4 shows the various values of the smallest moment of inertia that have Table 4. Values reported for the least moment I, of ONCl
Ia k
cm2)
9.05 x 10-40 9.837 x 1O-4o 8 3 & 0.5 x 1O-4o 9.822 x 1O-4o 960 i_ 0.07 x 10-40
of inertia
Source Electron diffraction [ 1 l] Microwave [ 121 Vibrational infrared [lo] Microwave [ 141 This work
been reported. The result obtained from this work appears to be the most nearly correct of these. With the measured value of I,, the inertial defect A, as defined by A = I, - Ia - I,, is calculated to be 0.13 a.m.u. A2. The asymmetry parameter b, = -2.21 x lob3 is slightly smaller than that calculated from the microwave data. Acknoudedgements-We thank Dr. R. M. JOYCE and Dr. S. ANDREADES of the Central Research Department, E. I. duPont deNemours and Company, for the gift of the sample of nitrosyl fluoride and advice about handling it. This work was supported by the National Science Foundation under grants G-6230 and G-19,637.