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Journal of Research of the National Bureau of Standards. Section A, Physics and Chemistry logoLink to Journal of Research of the National Bureau of Standards. Section A, Physics and Chemistry
. 1964 Feb 1;68A(1):79–86. doi: 10.6028/jres.068A.006

Infrared Absorption Spectrum of Nitrous Oxide (N2O) From 1830 cm−1 to 2270 cm−1*

Earle K Plyler, Eugene D Tidwell, Arthur G Maki
PMCID: PMC5325172  PMID: 31834715

Abstract

The frequencies of the vibration-rotation spectrum of N2O have been measured from 1830 cm−1 to 2270 cm−1. A number of weak bands have been measured and assigned to “hot bands’’ and isotopic species in normal abundance. By using the Ritz principle and previously measured bands the bending frequency (v2) is calculated as 588.780 cm−1. Frequencies are given for lines arising from the three principal transitions found in this region.

1. Introduction

Recently there has been considerable interest in obtaining accurate values for the vibration-rotation potential constants for small molecules. Pliva [1]1 has measured the spectra of various isotopic species of nitrous oxide (N2O) in the hope of obtaining more data with which to check the anharmonic terms of a potential function which he has devised [2]. Tidwell, Plyler, and Benedict [3] have reported measurements on a large number of vibrational-energy levels for N2O and have derived a set of vibration-rotation constants.

Rank et al. [4] have reported the results of some very precise measurements on five absorption bands of N2O. McCubbin, Grosso, and Mangus [5] have made some further precise measurements on N2O which will be reported soon. While this work was in progress Fraley, Brim, and Rao [6] published results of measurements on the strongest absorption lines due to N2O in the 5-μ region. The latter measurements are in essential agreement with those reported here.

2. Experimental Procedure

The spectra were measured on the NBS high-resolution infrared spectrometer described elsewhere [7]. Most of the measurements were made using a 7,500 lines/in. grating although some measurements were obtained with a 1,860 lines/in. grating. A liquid-nitrogen cooled PbSe detector was used. Calibration was achieved by the combination of accurately measured rare-gas spectra and a Fabry-Perot interferometer fringe system in the manner described in reference 8.

Spectra were obtained with pathlengths of 1.2, 4, and 24 m and pressures ranging from 1 to 200 mm Hg. Representative spectra are shown in figures 1 and 2. The 1.2 m cell could be either cooled to 220 °K or warmed to 400 °K; representative spectra obtained at these temperature extremes are shown in figures 3 and 4. In these figures it is evident that many lines which are weak at low temperatures have intensified with increase in temperature. These lines must be attributed to transitions originating from excited vibrational states and accordingly have been assigned as “hot band” lines.

Figure 1. N2O absorption from 1840 to 1910 cm−1.

Figure 1

Pathlength 1.2 m; pressure 28 cm Hg.

Figure 2. N2O absorption from 2270 to 2160 cm−1.

Figure 2

Pathlength 4 m; pressure 1 mm Hg. Circle: indicates absorption lines due to atmospheric C13O216 in the optical path.

Figure 3. N2O absorption from 2184 to 2200 cm−1.

Figure 3

Upper curve is for a temperature of 400 °K and lower curve is with cell cooled to about 220 °K. Pathlength is 1.2 m; pressure 1 mm Hg.

Figure 4. N2O absorption from 2204 to 2220 cm−1.

Figure 4

Temperature of gas in upper eurve is 400 °K, temperature of gas in lower curve is 220 °K. Pathlength is 1.2 m; pressure is 2 mm Hg.

3. Analysis of Data

The microwave measurements of Burrus and Gordy [9] have given very precise values for the ground-state rotational constant, B0, and the l-doubling constant, q, of the molecule N14N14O16. Coles and Hughes [10] and Coles, Good, and Lide [11] have made further measurements from which one can obtain B0 for the isotopes N14N15O16, N15N14O16, and N14N14O18. Combining the results of these workers with the velocity of light (taken as 299,793 km/s) we have calculated these constants in wavenumbers as given in table 1. Since these molecular constants are more accurate than could be obtained from the measurements reported here, the values given in table 1 were used wherever applicable for the calculation of the other molecular constants. For this same purpose the value of D0 = 17.6×10−8 cm−1 given by Rank et al. [4] was used.

Table 1.

Accurately known molecular constants used in the analysis of the N2O hands between 4.4 and 5.5 μ

B000 = 0.4190104 cm−1
D000 = 17.6×10−8 cm−1
q = 79.17×10−5 cm−1
B010 = 0.4195727 cm−1 (average of c and d levels)
B000(N14N15O16) =0.4189821 cm−1
B000(N15N14O16) = 0.4048564 cm−1
B000(N14N14O18) =0.395577 cm−1

Since the data for many of the bands reported here was rather fragmentary due to the high degree of overlapping, all of the absorption bands were analyzed by obtaining a least-squares fit to the polynomial

vobs=vo+(B+B)m+(BB)m22(D+D)m3(DD)m4

where the terms have their usual significance.

The data were also analyzed using the method of combinations and differences. The results of both methods were comparable, but in many cases the fact that more lines could be used in fitting to the polynomial given above resulted in a reduction in the uncertainty of the various unknown constants when this method was used, especially since the lower state constants were in most cases very accurately known.

For those bands which are split into resolved c and d components the two bands were analyzed simultaneously to obtain a least-squares fit to the same band center and other constants were made compatible. A more detailed description is given for the individual bands in the next section.

4. Results

4.1. The Region From 1830 to 1925 cm−1

In this region lines have been identified due to the three transitions 12°0–01lc0, 1220–0110, and 111c0– 000. For all these perpendicular bands the Q branches were observed, but the resolution was not good enough to measure any individual Q branch lines. The splitting of the Δ—II band was observed for all but a few low J lines. The analysis of this Δ—II band was carried out by analyzing the c and d components simultaneously. Since the data were quite fragmentary for these weak “hot bands,” the best available estimates of the values of Bc, Bd, D″, Dc, and Dd were used in order to obtain more accurate values of v0 and B′. For this purpose it was assumed that Bc=Bd. Since the c and d levels of the 1220 state undergo l-type resonance with different levels, it was necessary to use different values of D for the c and d levels. The values of the constants used are given in table 2. Only the c component of the v1 + v2 band was measured, therefore the value obtained for ΔB as given in table 2 is for the transition to the c level only.

Table 2. Vibration-rotation constants describing the absorption bands of N2O in the 2000 cm−1 Region.

The limits of error given are standard deviations

Isotopic species Assignment v0 ΔB×10+5cm−1 *D′ × 108 *D″ × 108 ΔD × 10+8







N14N14O16 1200–011c0 1873.206 ±0.009 −93**±3 23.8 17.6 …..
111c0–000 1880.271 ±.003 −154. 6±0.7 17.6 17.6 −0.2±0.3
122c0–011c0 }1886.033 ± 00.9 − 96 ± 3 {13.117.6 }17.6 …..
122d0–011d0
200–011c0 }1974.571 ± .003 − 397. 5 ± 1.0 16.1 17.6 − 2.3±0.6
200–011d0
2110–0220 1988.2±.3 — Q branch position
211d0–0200 1997.65 ± .15 —Q branrh position
022cl–022c0 }2195.406 ± .006 − 340. 3± 1.2 {11.617.6 11.6 …..
022d1–022d0 17.6 …..
020l–0200 2195.849±.025 −336±2 23.6 23.6 0±1.4
101–100 2195.93±.04 −352 ± 5 17.0 17.4 …..
O111–0110 2209.527±.004 −340. 3 ± 0.9 17.6 17.6 0.4±0.3
001–000 2223.764 ± .003 −345.6±0.3 17.4 17.6 −.26±0.05
N14N15O16 0111–0110 2164.13 ± .03 −315 ± 15 17.5 17.5 …..
N14N15O16 001–000 2177.659 −330 ± 3 17.5 17.5 …..
N15N14O16 001–000 2201.604±.015 −337 ±3 16.5 16.5 …..
N14N14O18 001–000 2219.678 ±.02 −411±10 16.5 16.5 …..
*

In all cases the values of D given were assumed in order to obtain the best possible values of v0, ΔB, and ΔD.

**

This ΔB value is from the average of the c and d levels of the lower state.

The calculated and observed frequencies of absorption lines due to the transition 111c0–000 are given in table 3. Figure 1 shows the appearance of the absorption in this region.

Table 3.

Observed and calculated wavenumbers for the two strongest N2O absorption bands between 1830 and 2000 cm−1

J 111c 0–000
200–011c 0
200–011d 0
P
R
P
R
Q
Obs. Calc. Obs. Calc. Obs. Calc. Obs. Calc. Obs. Calc.











0 ….. ….. 1881.095 .106 ….. ….. ….. ….. ….. …..
1 ….. ….. 81.929 .937 ….. ….. b1976.222 .227 ….. …..
2 b1878.595 .591 82.751 .766 ….. ….. ….. ….. ….. …..
3 b77.766 .747 83.600 .592 ….. ….. b77.832 .853 ….. …..
4 76.894 .900 ….. ….. ….. ….. b78.643 .656 ….. …..
5 76.041 .050 ….. ….. ….. ….. ….. ….. ….. …..
6 75.185 .196 ….. ….. ….. ….. b80.253 .239 ….. …..
7 ….. ….. 86.865 .863 ….. ….. 81.016 .020 ….. …..
8 b73.472 .480 ….. ….. ….. ….. 81.790 .794 ….. …..
9 ….. ….. ….. ….. b1960.764 .769 82.559 .561 ….. …..
10 ….. ….. ….. ….. b65.880 .867 83.317 .320 ….. …..
11 ….. ….. ….. ….. 64.952 .957 84.081 .073 1973.995 .995
12 ….. ….. b90.861 .882 64.034 .040 84.821 .818 ….. …..
13 ….. ….. b91.684 .677 63.127 .117 85.553 .556 73.784 .777
14 ….. ….. b92.488 .468 62.181 .186 86.287 .287 73.636 .655
15 ….. ….. 93.253 .256 ….. ….. 87.030 .010 73.543 .524
16 ….. ….. 94.054 .041 60.302 .303 87.723 .727 73.400 .384
17 b65.590 .607 b94.850 .823 59.351 .351 ….. ….. 73.218 .236
18 b64.720 .717 b95.585 .601 58.399 .392 89.127 .139 73.088 .079
19 b63.844 .824 ….. ….. 57.422 .426 89.833 .834 72.920 .914
20 b62.958 .928 97.150 .149 56.448 .453 90.521 .522 72.754 .740
21 ….. ….. 97.910 .918 55.465 .473 ….. ….. 72.560 .557
22 61.130 .128 98.674 .684 54.494 .489 ….. ….. 72.381 .366
23 60.210 .223 1899.436 .447 53.490 .493 ….. ….. 72.160 .166
24 59.315 .315 b1900.190 .207 52.487 .4.92 93.222 .202 b71.986 .957
25 ….. ….. 00.964 .963 51.481 .484 93.859 .855 71.746 .740
26 57.496 .491 01.715 .716 50.469 .470 94.496 .500 71.518 .514
27 56.576 .574 b02.493 .466 b49.440 .449 95.138 .138 b71.258 .280
28 55.662 .654 03.221 .213 48.423 .420 95.777 .770 b71.032 .037
29 54.739 .732 03.960 .957 47.383 .386 96.389 .394 70.787 .786
30 53.808 .806 04.689 .697 ….. ….. ….. ….. 70.518 .526
31 52.884 .877 05.425 .435 ….. ….. b97.600 .621 b70.270 .258
32 51.950 .946 06.154 .169 44.234 .240 b1998.197 .224 69.987 .981
33 51.012 .012 06.883 .900 b43.171 .178 1969.700 .696
34 50.062 .074 b07.617 .627 ….. …..
35 49.125 .134 08.348 .352 41.034 .033
36 ….. ….. 09.070 .073 39.938 .951
37 b47.256 .245 ….. ….. 38.880 .862
38 b46.266 .296 10.517 .506 b37.783 .767
39 ….. ….. 11.220 .217 36.680 .665
40 ….. ….. 11.931 .926 35.542 .556
41 ….. ….. 12.626 .631 1934.443 .441
42 b42.475 .472 13.345 .333
43 41.518 .509 14.024 .032
44 40.544 .543 b14.720 .727
45 b39.591 .574 1915.421 .420
46 38.604 .602
47 ….. …..
48 36.662 .650
49 35.672 .670
50 34.696 .687
51 33.700 .701
52 32.712 .712
53 b1831.720 .721
b

Blended or weak lines.

4.2. Absorption Lines in the Region 1925 to 2000 cm−1

The main band found in this region is a Σ–II band due to the transition 200–0110. In this case the Q branch was sufficiently well resolved so that measurements were obtained for both the c and d levels. Since microwave values for the lower state are quite good, these values were used in the analysis and transitions from both the c and d levels were analyzed simultaneously by a least-squares program in order to obtain the best values for v0 and B′.

Absorption in this region was very weak as might be expected from the transitions involved. The Q branches for the two “hot bands” 2110–0220 and 2110–0200 were also observed in this wavelength region, but they were not resolved. Nor were any lines of the P and R branches observed.

The calculated and observed frequencies of the lines for the 200–0110 transition are given in table 3.

4.3. N14N14O16 Absorption Between 2130 and 2270 cm−1

The fundamental v3 and associated “hot bands” are located in this region. v3 is a rather strong absorption band, consequently with the pathlength and resolution available it was possible to obtain measurements on four “hot bands” and four isotopic bands.

The splitting of the first “hot band” was observed for high-J levels but overlapping with various other bands was rather severe. For this reason it was felt that more accurate band constants could be obtained by averaging the frequencies of the c and d components. Even though this procedure did not permit the use of measurements where only one component was observed, the resultant constants are believed to be more reliable than those found by analyzing each band individually.

Lines due to the Δ—Δ transition 0221–0220 have been observed and the position of the Q, while overlapped, has been verified by observations at 220 °K and 400 °K. Figure 3 shows a few lines due to this transition and the manner in which the line intensities change with temperature. The Boltzman distribution predicts this transition will show an approximate six-fold increase in intensity in going from 220 °K to 400 °K.

Since the two Σ—Σ transitions 101–100 and 0201–0200 are predicted to lie quite close to each other, some difficulty was anticipated in assigning the two series of lines which must be due to these transitions. The assignments of these lines are, however, considered to be reliable due to the rather large differences in the lower-state B values. Δ2F″ plots for these two bands yield respective B″ values of 0.4175 and 0.4200 cm−1. The B″ values expected from the data of reference 3 are 0.41725 and 0.41991, respectively. Many of the low-J lines are badly overlapped because the two band centers lie so close together. As a consequence the band centers calculated from the data may be in error by several hundredths of a wavenumber. The statistical treatment of the data resulted in a standard deviation for the v0 values of 0.01 cm−1, but inspection of the data leads us to believe that this is not a realistic number. Therefore table 2 contains a more subjective evaluation of the accuracy of the band centers for these two bands.

Table 4 lists the frequencies of the v3 band as observed in this laboratory and as reported by Fraley, Brim, and Rao. Columns 3 and 6 compare the calculated frequencies with those observed.

Table 4.

Wavenumbers of absorption lines for the v1 band of N2O

J P Branch
R Branch
Observed wavenumber NBS Calc.NBS Observed wavenumber ref. 6 Observed wavenumber NBS Calc.NBS Observed wavenumber ref. 6







0 ….. ….. ….. 2224.593 2224.595 2224.594
1 b2222.900 2222.926 2222.940 b25.41 25.419 25.430
2 b22.065 22.081 22.085 b26.237 26.236 26.243
3 b21.253 21.229 21.273 (b) 27.050
4 20.360 20.370 20.382 27.845 27.850 27.848
5 19.514 19.505 19.516 28.650 28.647 28.639
6 18.638 18.632 18.641 b29.459 29.436 29.450
7 17.745 17.753 17.756 b30.224 30.219 30.218
8 b16.833 16.867 16.840 b30.975 30.995 30.975
9 b15.975 15.973 15.983 31.784 31.763 31.741
10 b15.109 15.073 15.088 32.543 32.525 32.548
11 14.175 14.166 14.173 (b) 33.301
12 13.244 13.253 13.269 b34.024 34.028 34.023
13 12.304 12.332 12.330 34.784 34.769 34.806
14 11.404 11.405 11.420 35.526 35.503 35.509
15 b10.469 10.470 10.478 b36.253 36.229 36.246
16 (b) ….. 09.523 b36.942 36.950 36.945
17 b08.582 08.581 08.590 b37.670 37.663 37.663
18 09.625 07.626 07.622 38.364 38.369 38.368
19 06.669 06.665 06.668 b39.076 39.068 39.076
20 05.681 05.696 05.687 b39.728 39.760 39.761
21 04.719 04.721 04.717 40.454 40.445 40.444
22 b03.737 03.739 03.735 41.134 41.123 41.130
23 b02.736 02.750 02.741 41.780 41.794 41.805
24 b2201.747 2201.754 01.752 42.466 42.458 42.443
25 ….. ….. 2200.772 43.122 43.115 43.120
26 2199.720 2199.742 2199.735 43.759 43.765 43.773
27 98.726 98.726 98.735 44.424 44.409 44.416
28 97.707 97.704 97.696 45.043 45.045 45.049
29 96.677 96.674 96.667 b45.668 45.674 45.688
30 95.645 95.638 95.631 ….. ….. 46.289
31 94.600 94.594 94.602 46.895 46.911 46.918
32 b93.526 93.545 93.528 47.531 47.519 47.509
33 (b) ….. 92.428 48.114 48.120 48.119
34 b91.432 91.425 91.433 48.712 48.714 48.714
35 b90.345 90.355 90.349 49.304 49.301 49.328
36 89.264 89.278 89.280 49.887 49.881 49.889
37 88.184 88.194 88.189 50.460 50.454 50.449
38 87.134 87.104 87.106 51.033 51.020 51.047
39 86.002 86.007 86.004 (b) ….. …..
40 84.911 84.903 84.890 52.145 52.131 52.148
41 83.794 83.793 83.790 52.682 52.676 52.685
42 82.666 82.676 82.667 53.209 53.214 53.216
43 b81.560 81.552 81.545 (b) ….. …..
44 (b) ….. 80.380 54.236 54.268 54.253
45 b79.303 79.285 79.293 54.796 54.785 2254.799
46 (b) ….. 78.142 55.318 55.295 …..
47 76.982 76.991 76.983 55.796 55.797 …..
48 75.840 75.834 75.838 56.287 56.293
49 74.674 74.670 74.672 56.762 56.781 …..
50 73.497 73.500 73.488 b57.300 57.263 …..
51 ….. ….. 72.324 b57.707 57.737 …..
52 71.144 71.139 71.134 58.212 58.205 …..
53 b69.898 ….. 69.891 58.664 58.665 …..
54 68.751 68.752 68.759 59.131 59.119 …..
55 67.536 67.549 2167.579 (b) ….. …..
56 b66.318 66.339 ….. 60.001 60.004 …..
57 (b) ….. ….. 60.419 60.436 …..
58 63.926 63.900 ….. 60.882 60.862 …..
59 62.690 62.670 ….. b61.28 61.280 …..
60 61.418 61.434 ….. 61.675 61.691 …..
61 60.210 60.191 ….. 62.082 62.095 …..
62 58.979 ….. ….. 62.489 62.492 …..
63 57.693 57.686 ….. (b) ….. …..
64 56.422 56.424 ….. ….. ….. …..
65 55.168 55.155 ….. 63.636 63.640 …..
66 53.883 53.880 ….. 64.020 64.009 …..
67 ….. ….. ….. 64.362 64.371 …..
68 51.295 51.310 ….. 64.741 64.725 …..
69 b49.976 50.015 ….. (b) ….. …..
70 48.740 48.714 ….. 65.429 65.414 …..
71 47.369 47.406 ….. (b) …..
72 46.097 46.092 ….. b66.08 66.074 …..
73 44.736 44.771 ….. b66.40 66.393 …..
74 b43.46 43.444 ….. b66.713 66.705 …..
75 (b) ….. ….. (b) ….. …..
76 (b) ….. ….. (b) ….. …..
77 b39.438 39.424 ….. b2267.612 2267.600 …..
78 b2138.086 2138.072 ….. ….. ….. …..
b

Blended or weak lines.

4.4. Isotopic Absorption Bands From 2100 to 2240 cm−1

Within this region absorption bands due to N2O molecules containing N15 or O18 are expected. Lines due to four transitions in such isotopically substituted molecules have been identified in this study.

The band at 2201.60 cm−1 due to N15N14O16 has been previously measured by Pliva [1] but bands due to the molecules N14N15O16 and N14N14O18 found at 2164.13, 2177.66, and 2219.67 cm−1 have not been previously reported. The assignments for these latter three bands have been confirmed by determination of the values of B″ from the Δ2F″ plots. In the case of the II—II transition at 2164.13 cm−1 the sharp Q branch seems to be observable at 2164.128 cm−1, thus providing greater confidence in the position of the band center. Since many of the lines for these isotopic molecules were weaker than or of comparable intensity with the “hot bands” of the most abundant molecule, the identification of the lines was greatly aided by spectra obtained at 220 °K. At this temperature the intensity of the “hot band” lines is very greatly diminished. This leaves the isotopic lines as the most outstanding of the weak lines at low temperatures.

4.5. Discussion of Results

By means of the Ritz principle the position of some of the low-lying vibrational levels may be obtained. The bending vibration, v2, may be obtained in four different ways. Using the precise measurements of reference 4 and the recent measurements of reference 5, we find

v21=(1200000)(12000110)=2461.9981873.206=588.792cm1
v21=(200000)(2000110)=2563.3411974.571=588.770cm1
v21=(1110000)(11100110)=1880.2711291.496=588.775cm1

Taking the average of the values given for the 0111–000 transition in references 1 and 3, one obtains

v21=(0111000)(01110110)=2798.3082209.527=588.781cm1.

The average of these four indirect determinations, 588.780 cm−1, compares very favorably with the indirect measurement of Pliva [1] (588.767 cm−1), the four indirect measurements of Tidwell et al. [3] (588.773 cm−1), and the direct measurement of Lakshmi, Rao, and Nielsen [12] (588.78 cm−1).

The Ritz principle may also be applied to determine the values of 2v20, 2v22 and v1 as follows:

2v22=(0221000)(02210220)=3373.1842195.406=1177.778cm1
2v20=(0201000)(02010200)=3363.9972195.85=1168.15cm1
v1=(101000)(101100)=3480.8542195.93=1284.92cm1

where the values for the vibration levels given by Pliva [1] and Tidwell et al. [3] are used. By using similar combinations Tidwell et al., have previously determined 2v22=1177.78cm1. McCubbin et al. [5] have recently measured 2v20 and v1 at 1168.134 and 1284.907 cm−1, respectively. These were direct measurements and should be more accurate than the indirectly obtained values given above.

The band centers for the transitions 001–000 and 0111–0110 have now been measured quite carefully in three different laboratories. Some idea of the absolute accuracy of these measurements may be obtained by comparing the constants derived from the measurements. We may also compare the measurements of v3 for the N15N14O16 molecule with those reported by Pliva [1]. Table 5 shows how closely these independent measurements agree. Although it is seen that Pliva and Fraley et al., agree on a slightly larger value for B001 than has been reported in this work, nevertheless it is felt that the values given here are probably more accurate. The present measurements extend to considerably higher values of J than previous measurements and as a consequence a more accurate determination of ΔB and ΔD is expected. On the other hand Fraley, Brim, and Rao probably have a more accurate value for v0 since their resolution was slightly better so that blending of low-J lines would have less tendency to cause errors in the derived v0. Since Pliva worked with an isotopically enriched sample, it is to be expected that his constants for N15N14O16 are better than those reported here.

Table 5.

Comparison of the results of measurements on N2O in different laboratories

001–000
0111–0110
001–000
N15N14O16
v0 ΔB×105 v0 ΔB×105 V0 ΔB×105






NBS (this work) 2223.764 −345.6 2209.527 −340.3 2201.604 −337
Pliva (ref. 1) 2223.754 −344.7 2209.521 −341.2 2201.605 −336.0
Fraley, Brim, and Rao (ref. 6) 2223.759 −344.6 2209.535 ….. ….. …..

An attempt was made to compare the results of this work with the constants given by Tidwell et al., but, as noted by Pliva [1], the agreement is not entirely satisfactory. Perhaps the measurements given here will be of value in determining the accuracy of the revised constants which Pliva is calculating.

Footnotes

*

This work was supported in part by the Geophysic Research Directorate, Air Force Cambridge Research Laboratories.

1

Figures in brackets indicate the literature references at the end of this paper.

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