D. D. Nelson

Microwave and infrared characterization of several weakly bound NH3 complexesa)

We present the results of microwave and infrared spectroscopic studies of several van der Waals complexes of NH3. These results were obtained with a molecular beam electric resonance spectrometer. The microwave spectroscopy of the complexes (NH3)2 and Ar–NH3 show that both systems are nonrigid. The observed dipole moments for (NH3)2[0.74(2) D] and (ND3)2[0.57(1) D] are not compatible with the presently accepted theoretical structure. Ar–NH3, which has a complicated and currently unassigned microwave spectrum, exhibits Q branch inversion transitions near 19 GHz which indicate that the NH3 subunit is likely to be a near‐free rotor. Infrared studies of the complexes NH3–HCCH, NH3–CO2, (NH3)2, Ar–NH3, NH3–OCS, NH3–N2O, and NH3–HCN have been carried out with a line tunable CO2 laser. Only for NH3–HCN were no infrared resonances discovered. Photodissociative transitions are observed in all of the other systems. Band origins for the photodissociative infrared transitions involving the ν2 umbrella motion of NH3 w...

Ammonia dimer: A surprising structure

High resolution rotational spectra for (14NH3)2, (14ND3)2, 15NH3–14NH3, and 14NH3–15NH3 have been obtained using the molecular beam electric resonance technique. For 14NH3 dimer the spectra are simple and consist only of the J=0–1 and J=1–2 transitions with K=0 for two vibrational states (hereafter designated α and β) of the complex. The spectroscopic constants for (14NH3)2 are: 1/2 (B+C)α=5110.412(2), 1/2 (B+C)β=5110.564(2), DJ=0.0529(5), eqQ1aa =−0.639(13), eqQ2aa =0.904(8) (MHz);  μαa =0.7497(10),  μ βa =0.7376(9) (D). For (ND3)2 the vibrational states are unsplit 1/2 (B+C)=4190.300(50) MHz and  μa=0.567(4) D. These spectroscopic constants are inconsistent with the theoretically predicted structure for NH3 dimer. In the predicted structure one NH3 unit hydrogen bonds to the other with a nearly linear N–H‐ –N arrangement so that the C3 axis of the basic NH3 is collinear with the hydrogen bond. If θi is the angle between the C3 axis of the ith NH3 unit and the a‐inertial axis of the complex, θ1∼0° and θ2...

Ammonia dimer: Further structural studies

New experimental results on the structural and dynamical properties of NH3 dimer are reported in this work. J=1–0, K=0 transitions of 14NH3–15NH3, 15NH3–14NH3, ND3 dimer, and ND3–ND2H have been measured at high resolution and 14N electric quadrupole coupling constants are reported for each of these species. The NH3 subunits comprising the dimer are inequivalent. The quadrupole coupling constant associated with the first ammonia subunit eqQ1aa, is measured in 14NH3–15NH3 [−627(8)kHz], in ND3 dimer [−531(15) kHz], and in ND3–ND2H [−991(18) kHz]. For the other subunit, eqQ2aa is reported in 15NH3–14NH3 [892(8)kHz], in ND3 dimer [745(13) kHz], and in NH3–ND2H [1013(18) kHz]. These numbers can be used to estimate the vibrationally averaged polar angles of these isotopomers of NH3 dimer. The result is (including the primary isotopomer) θ1 for 14NH3–14NH3 is 48.6°, for 14NH3–15NH3 is 48.7°, for ND3 dimer is 49.6° and for ND3–ND2H is 45.3°; while θ2 for 14NH3–14NH3 is 64.5°, for 15NH3–14NH3 is 64.3°, for ND3 dime...

The rotational spectrum and structure of NH3–HCN

The microwave spectrum of H3N–HCN has been measured using the molecular beam electric resonance technique. A symmetric top spectrum is observed and the following spectroscopic constants were obtained::[RW2:B0(MHz):3016.756 1(24)]

Microwave and radiofrequency Stark spectrum of ArHCN: A highly nonrigid molecule

The rotational spectrum of the weakly bound complex ArHCN has been observed using molecular beam electric resonance spectroscopy. The spectrum is superficially characteristic of that of a linear molecule with both unusually large centrifugal distortion (requiring a J6 dependent distortion term to fit the data) and an unexpectedly large bending amplitude. The spectroscopic constants are The centrifugal distortion constant DJ is remarkably large and abnormally sensitive to isotopic substitution. Using the usual model, the stretching and bending force constants obtained from these data are an order of magnitude smaller than those similarly computed for the hydrogen halide complexes of argon. The calculated stretching and bending frequencies are 10 cm−1, predicting that excited vibrational levels should be populated in the beam. Three transitions have been observed which appear to correspond to an excited vibrational level of ArDCN, but poor signal‐to‐noise has prohibited their unambiguous assignment. W...

The microwave spectrum of the K=0 states of Ar–NH3

The microwave spectrum of Ar–NH3 has been obtained using molecular beam electric resonance spectroscopy and pulsed nozzle Fourier transform microwave spectroscopy. The spectrum is complicated by nonrigidity and most of the transitions are not yet assigned. A ΔJ=1, K=0 progression is assigned, however, and from it the following spectroscopic constants are obtained for Ar–14NH3: (B+C)/2=2876.849(2) MHz, DJ =0.0887(2) MHz, eqQaa =0.350(8) MHz, and μa =0.2803(3) D. For Ar–15NH3 we obtain (B+C)/2 =2768.701(1) MHz and DJ =0.0822(1) MHz. The distance between the Ar atom and the 14NH3 center of mass RCM is calculated in the free internal rotor limit and obtained as 3.8358 A. In the pseudodiatomic approximation, the weak bond stretching force constant is 0.0084 mdyn/A which corresponds to a weak bond stretching frequency of 35 cm−1. The NH3 orientation in the complex is discussed primarily on the basis of the measured dipole moment projection and the quadrupole coupling constant. It is concluded that the Ar–NH3 in...

Microwave and submillimeter spectroscopy of Ar-NH3 states correlating with Ar + NH3 (j=1,|k|=1)

Microwave and submillimeter transitions for Ar–NH3 have been observed and assigned for the ∑ and Π states correlating asymptotically with Ar+NH3 (j=1,‖k‖=1). The ∑ states are found to lie below the Π states and are separated by approximately the inversion splitting of free NH3. For the Π states the NH3 inversion tunneling is nearly quenched, being only weakly allowed through Coriolis interactions with the nearby ∑ states. The observed microwave and submillimeter spectra also allow the determination of 14N quadrupole coupling constants and relative submillimeter absorption intensities. All the above results are interpreted using a model internal–rotation inversion Hamiltonian, leading to detailed information about the anisotropy of the intermolecular potential.

Classification of the tunneling‐rotational energy levels of ammonia dimer in the molecular symmetry group

The symmetry properties of NH3 dimer are analyzed in the molecular symmetry group and the physical assumptions underlying the choice of this group are detailed. Several low lying tunneling states are predicted. Two of these states transform as four‐dimensional (G) representations of G36, the molecular symmetry group. These two states can have the same microwave selection rules as those found in the two experimentally observed states of NH3 dimer. That is, the microwave transitions can follow pure rotational selection rules (no tunneling splittings) even though interchange tunneling is included as a feasible motion in the molecular symmetry group. The observation of these selection rules is interpreted as internal rotation stabilizing the system against interchange tunneling. Hence, in these particular states the interchange tunneling is quenched. The group theory predicts this picture of ammonia dimer to be appropriate if certain internal rotation interactions are large compared to the interchange tunneli...

The microwave and radio frequency rotation–inversion spectrum of (SO2)2

The radio frequency and microwave spectrum of (SO2)2 has been measured using the molecular beam electric resonance technique. The spectrum is characteristic of an asymmetric top in which the two nonequivalent SO2 subunits interchange roles through a low frequency (70 kHz) tunneling motion. The spectroscopic constants obtained for SO2 dimer are: (B+C/2) (MHz)  926.160(2),    (B−C/2) (MHz)   22.3207(1),   A−(B+C/2) (MHz) 6032.3(6),      ΔJ(MHz)    0.00217(2),  ΔJK(MHz)    0.0995(1),   δinv(MHz)    0.070(1),    μa(D)    1.4052(13).  The average distance between the centers of mass of the two subunits, RCM, is 3.825(10) A. The magnitude of the weak bond stretching force constant ks is 0.0264(4) mdyn/A. The relative orientation of the subunits is not well determined, but is demonstrated to be unlike the orientation of the nearest neighbors in the sulfur dioxide crystal.

The rotational spectra of NH3–CO and NH3–N2

The rotational spectra of NH3–CO, ND3–CO, ND2H–CO, NDH2–CO, NH3–13CO, and NH3–N2 have been measured by molecular beam electric resonance. The K=0 ground vibrational state transitions for these species were fit to a linear molecule Hamiltonian and the following constants were obtained for NH3–CO; (B+C)/2 (MHz)=3485.757(2), DJ (kHz)=110.2(2), eQqNaa (MHz)=−1.890(7), μa (D)=1.2477(8). These constants were also determined forND3–CO [3078.440(7), 75.7(8), −2.028(15), 1.2845(9)], NHD2–CO [3202.303(4), 86.8(6), −1.972(11), 1.2686(8)], NH2D–CO [3338.235(4), 98.9(6), −1.916(12), 1.2546(8)], NH3–13CO [3451.684(5), 108.7(7), −1.870(15), 1.2452(8)]. For NH3–N2 (B+C)/2=3385.76(21), DJ =117.(10), and μa =1.069(14). For NH3–CO three ‖ΔJ‖=1, K=0 progressions were seen along with two ‖ΔJ‖=1, K=1 progressions, suggesting nonrigidity in the complex. The internal rotation of the NH3 subunit about its C3 axis is expected to be essentially free, but this motion, by itself, is not sufficient to explainthe observed spectra, thus...