181 1 ' Spartein . 2 THF kristallisieu in der monoklinen Raumgruppe P2,, u = 921.6(10), b =1766.7(10), r =1098(2)pm, fi =107.65(7)", V = 1703(4) A', Z = 2, Q~~~. =1.127 g~m -~ fur M = 577.8, p = 0.498 mm-'. An einem Kristall der GroBe 0.6 x 0.4 x 0.3 mm wurden auf einem EN- RAF-NONIUS-Vierkreisdiffraktometermit Graphit-monochromatisierter Cu,,-Strahlung bei 163 K 4606 Reflexe vermessen, von denen 4279 unabhingig (R,,, = 0.0533) und 3467 beobachtet ( F > 4u(F/) waren. Losung (PATSEE) [9] und Verfeinerung im SHELXTL PLUS (VMS)-Programmsystem [lo], R = 0.0837, wR = 0.0764, Nicht-Wasserstoffatome
Measurements of the inversion-rotation spectra of the deutero-ammonias have been made in the 7, 000 Mc/sec to 80, 000 Mc/sec region. Thirty-five lines of NH2D and NHD 2 have been measured and identified by means of the Stark effect. As expected, our results show that for these molecules only simultaneous rotational and inversion transitions can be observed. Thus, corresponding to each J-J ,transition two absorption T T lines can be observed, separated by either twice the rotational energy separation or twice the inversion splitting of the levels involved. From our observations the inversion splittings of the partially deuterated molecules can be calculated and are found to be in good agreement with the Dennison and Uhlenbeck inversion doubling theory. Thus the inversion splittings for the ammonia molecules are approximately as follows: 24, 000 Mc/sec for NH 3 ; 12, 000 Mc/sec for NH2D; 5, 000 Mc/sec for NHD 2 ; 2, 000 Mc/sec for ND 3. Our results are also in agreement with the structural parameters of ammonia as given by Herzberg. The sign of the nuclear quadrupole coupling constant was also measured and found to be negative, as expected. The centrifugal distortion for both rotation and inversion states has been experimentally determined.
I N an earlier report 1 we described a single-crystal detecting system for microwave spectroscopy which employs a low frequency cut-off filter to eliminate the contour of the oscillator mode and the spurious signals caused by reflections in the microwave line. We have now improved the sensitivity of our system by a modulation technique which allows the signal to be amplified at higher frequencies.Upon the slowly varying sawtooth voltage used to sweep the microwave oscillator over its mode we superimpose a radiofrequency voltage (^100 kc) of low amplitude which causes the oscillator to be frequency modulated over a small range of frequencies, at a rate determined by the superimposed radiofrequency. An absorption line of a gas then acts as a discriminator to produce intensity modulation of the output radiation at a frequency corresponding to that of the modulating voltage. A single-crystal detector is used with an amplifier tuned to the modulating frequency. After pre-amplification at this frequency a second detector is employed followed by the filter mentioned above and by an audio amplifier. The filter we find effective against reflections. There are sharp, strong signals caused by the abrupt beginning and ending of the mode which are not eliminated by the filter. These are easily identified and are useful as indicators for crystal tuning and as markers to define the limits of tube oscillation.Using the method described here with a 3.6-meter K-band wave-guide cell we easily detected the stronger lines of N 15 H 3 in its naturally occurring concentrations of 0.3 percent in ordinary NH 3 . A photograph of the 3,3 line of N 15 H 3 thus obtained is shown in Fig. 1. The photograph of the 3,3 line of N 14 H3 in Fig. 2 demonstrates that good FIG. 1. Cathode-ray scope display of the 3,3 line of N 15 Ha at natural concentrations of 0.3 percent in normal ammonia. Cell length, 3.6 meters. Pressure, 2 X10~3 mm of Hg. Modulation frequency 100 kc. FIG. 2. Cathode-ray scope display of the 3,3 line N 14 H3 showing satellite structure. Cell length, j3.6 meters. Pressure, 4X10~s. Modulation frequency 100 kc.resolution can also be obtained. The distortions apparent in these photographs are npt caused by the modulation but by the narrow band pass of the receiver used. Objectionable distortions can, of course, be produced by an incorrect modulation, but the adjustment of the modulation voltage is not critical.No greater sensitivity can be claimed for the system described above than for the Stark-effect modulation method of Hughes and Wilson. 2 In both systems the reception depends upon an intensity modulation produced by a high frequency motion of the signal in time (or components of the signal in the Stark-effect case). The present method has the advantage of being convenient to use with a cell of indefinite length. It is in general easier to modulate the oscillator than the molecules. The^ Hughes-Wilson method has the advantage of not requiring a filter to eliminate reflections.A more complete description of the system will be repo...
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