G. T. Fraser

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...

The structure of NH3–acetylene

The microwave spectrum of the weakly bound complex NH3–acetylene has been measured by molecular beam electric resonance spectroscopy. A spectrum characteristic of a symmetric top is observed and the following spectroscopic constants are determined:  The observed spectrum is consistent with an axially symmetric complex in which the acetylene hydrogen bonds to the ammonia with a hydrogen bond length of 2.33 A.

Rotational spectrum and structure of the complex HCNCO2

Radiofrequency and microwave rotational spectra of the complexes HCN CO2 and DCN CO2 have been obtained using molecular beam electric resonance spectroscopy. The spectra are characteristic of prolate asymmetric rotors. The spectroscopic constants obtained are These are shown to be consistent with a C2v structure in which the nitrogen of the HCN bonds to the carbon of the CO2 with a weak bond length of 3.00 A. The HCN subunit subtends an average angle of 17.4 (2)° with the line joining the centers of mass of the two submolecules, and the average bending vibrational amplitude of the CO2 is 11(1)°. The difference between the average amplitudes of the in‐plane and out‐of‐plane bending vibrations of the HCN is less than 4°. The stretching force constant for the weak bond is 0.049 mdyne/A in HCNCO2, and the induced dipole moment is 0.361 D.

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...

Rotational spectrum and structure of CF3H–NH3

The rotational spectrum of CF3H–NH3 has been obtained using a pulsed nozzle Fourier transform microwave spectrometer. A symmetric top spectrum is observed that is consistent with free internal rotation of the NH3 subunit against the CF3H subunit. Rotational transitions have been measured for both the ground and first excited internal rotor state of the complex. The spectroscopic constants which have been obtained include: B0=1996.903(2) MHz, DJ =3.46(12) kHz, and eQqN =−3.186(8) MHz. From the quadrupole coupling constant of the nitrogen nucleus, eQqN, the bending amplitude of the NH3 unit is determined to be 22.57(10)°. The hydrogen bond length is 2.314(5) A and the weak bond stretching force constant is 0.066(2) mdyn/A. The bond length and stretching force constant for CF3H–NH3 are similar in value to those determined for HCCH–NH3 (2.33 A and 0.070 mdyn/A, respectively).

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...

The rotational spectrum, internal rotation, and structure of NH3–CO2

The radio frequency and microwave spectra of NH3–CO2 have been measured using the molecular beam electric resonance technique. The spectrum is characteristic of an asymmetric top in which the NH3 subunit exhibits effectively free internal rotation. The spectroscopic constants obtained for the ground internal rotor state are presented below: (B+C/2) (MHz)  3756.178(3),  (B−C/2) (MHz)  597.4(2), A−(B+C/2) (MHz)  8035.(8),  ΔJ (MHz)  0.0240(4), δK (MHz)  0.20(2),  ΔJK  (MHz)  0.23(5), δJ (MHz)  0.007(1),  eQqaa (MHz)  −3.175(4), eQqbb (MHz)  1.557(9),  eQqcc (MHz)  1.617(11),  μ(D)  1.7684(14). The N–CO2 framework of the complex has C2v symmetry with a N–C weak bond length of 2.9875(4) A. The average bending angle of the NH3 subunit is 22.71(5)° with a difference in amplitude of 1.0(4)° between the in plane and out of plane excursions. The weak bond stretching force constant is 0.070(1) mdyn/A and the induced dipole moment is 0.411(2) D. (B+C)/2 for the first excited internal rotor state (‖m‖=1) is 3753.008(...

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.