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  • 1
    Electronic Resource
    Electronic Resource
    Symmetry breaking in HCl and DCl dimers: A direct near-infrared measurement of interconversion tunneling rates (1989)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 91 (1989), S. 4418-4419 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The interconversion tunneling frequencies for (HCl)2 and (DCl)2 are obtained from near-infrared absorption spectra of the H(D)Cl stretching transitions, to spectroscopic precision for the mixed 35Cl–37Cl dimers. A phenomenological model of the interconversion process explains several experimental observations, and provides good estimates of the splittings expected for the 35Cl–35Cl and 37Cl–37Cl species.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.456774
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  • 2
    Electronic Resource
    Electronic Resource
    Ammonia dimer: Further structural studies (1987)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 87 (1987), S. 6364-6372 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: 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 dimer is 62.6°, and for ND3–ND2H is 65.8°. The remarkable invariance of these values rules out the possibility of large vibrational averaging or tunneling averaging in this system and establishes that the angles θ1=49° and θ2=65° are near equilibrium. The isotope effect in thecomponent of the electric dipole moment along the a inertial axis μa, is shown to correlate well with the trend in polar angles given by the quadrupole coupling constants. The absence of interchange tunneling effects in the observed states of NH3 dimer implies that these states are asymmetrically excited internal rotor states of the complex. These experimental structural results are in disagreement with all previous theoretically determined structures for NH3 dimer except one. A recent electronic structure calculation which incorporates correlation through the coupled pair functional approach (while systematically varying geometry) obtains a compact, asymmetric structure for the dimer in close accord to observations.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.453466
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  • 3
    Electronic Resource
    Electronic Resource
    Classification of the tunneling-rotational energy levels of ammonia dimer in the molecular symmetry group (1987)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 87 (1987), S. 139-149 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: 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 tunneling matrix element. These conclusions are derived through correlation diagrams which are used to provide physical insight into the nature of the two states of G symmetry and approximate electric dipole selection rules for microwave transitions arising from these states. The selection rules for the infrared transitions of this dimer which correlate to the ν2 fundamental of NH3 are also given.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.453707
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  • 4
    Electronic Resource
    Electronic Resource
    Microwave and infrared characterization of several weakly bound NH3 complexesa) (1985)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 82 (1985), S. 2535-2546 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: 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 were determined for NH3–HCCH [984.4(9) cm−1], NH3–CO2 [987.1(2) cm−1]. NH3–OCS [981.5(15) cm−1], and NH3–N2O [980(2) cm−1]. The observation of an infrared transition for Ar–NH3 at 938.69 cm−1, which is 40 cm−1 lower than the band origins in the other NH3 complexes, lends support to the model of Ar–NH3 mentioned above. NH3–HCCH, NH3–CO2, (NH3)2, and Ar–NH3 were studied in microwave-infrared double resonance experiments in order to eliminate much of the inhomogeneous broadening present in their infrared spectra and to aid in the rotational assignment of the infrared spectra. Linewidths were determined for NH3–HCCH (0.15 GHz) and for NH3–CO2 [14(6) GHz]. An important result of this study is that the dissociation energies of all the complexes studied, except for NH3–HCN, are established to be less than 990 cm−1, i.e., 2.8 kcal/mol.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.448303
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  • 5
    Electronic Resource
    Electronic Resource
    The microwave and radio frequency rotation–inversion spectrum of (SO2)2 (1985)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 83 (1985), S. 945-949 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: 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(ring). The magnitude of the weak bond stretching force constant ks is 0.0264(4) mdyn/A(ring). 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.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.449421
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  • 6
    Electronic Resource
    Electronic Resource
    Ammonia dimer: A surprising structure (1985)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 83 (1985), S. 6201-6208 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: 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∼68° forthe theoretically predicted structure. In contrast, for 14NH3 dimer we measure the average values of θ1 and θ2 as 48.6(1)° and 115.5(1)°, respectively, using the quadrupole hyperfine structure. Furthermore, the component of the dipole moment along the a-inertial axis, μa, is found to be much smaller (0.75 D) than expected for the theoretical structure (∼2 D). The distance between the centers of mass of the NH3 subunits, RCM, is also determined and is found to be 3.3374(1) A(ring). No evidence exists for nonrigidity in the ground vibrational state of this complex, i.e., no high frequency tunneling splittings were observed in either the microwave or the infrared spectrum of the complex. The relationship between these structural results and the concept of hydrogen bonding in NH3 systems is discussed. The apparent inability of measured values of heats of formation for NH3 dimer to correctly predict dissociation energies of the complex is also commented upon. The dissociation energy of NH3 dimer has been previously determined as less than 2.8 kcal/mol.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.449566
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  • 7
    Electronic Resource
    Electronic Resource
    The rotational spectrum, barrier to internal rotation, and structure of NH3–N2O (1985)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 83 (1985), S. 5442-5449 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The rotational spectrum of NH3–N2O has been observed by the molecular beam electric resonance method. The spectrum is characteristic of a T-shaped complex in which the nitrogen of the NH3 subunit is directed toward the N2O subunit. The NH3 unit exhibits nearly free internal rotation about its C3 axis. The A internal rotor state transitions were fit to Watson's asymmetrical top Hamiltonian and the following spectroscopic constants were determined: A(MHz)=12 722.5(5), δK (MHz)=0.22(2), B(MHz)=4 083.5(2), δJ(MHz) =0.007(1), C(MHz)=3 070.8(2), ΔK(MHz) =−0.3(2), ΔJ(MHz) =0.016(9), ΔJK(MHz) =0.32(3), μa(D) =1.514(9), μb(D) =0.09(9). The E internal rotor state transitions were fit to the Hamiltonian of Kilb, Lin, and Wilson. For the E internal rotor state the rotational constants A [12 743(64) MHz], B[4077.1(12) MHz], and C[3069.9(9) MHz] were determined, as well as the height of the barrier to internal rotation, V3 [12.5(25) cm−1]. The distance between the nitrogen of the NH3 to the center of mass of N2O is 3.088 A(ring). The angle between the C3 axis of NH3 and the line joining the centers of mass of the NH3 and N2O subunits is 13°. The C3 axis of NH3 is pointed towards the nitrogen end of the N2O subunit.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.449714
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  • 8
    Electronic Resource
    Electronic Resource
    Electric dipole moments of HF–C2H2, HF–C2H4, and HF–C3H6 (1985)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 82 (1985), S. 4483-4485 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Electric dipole moments of HF–cyclopropane, HF–ethylene, and HF–acetylene have been determined using the molecular beam electric resonance technique. The dipole moments of HF–C3H6, HF–C2H4, and HF–C2H2 are 2.5084(28), 2.3839(45), and 2.3681(28) D, respectively. The induced dipole moments of these systems are discussed, along with that of HF–benzene which has been previously determined. The induced dipole moments do not scale simply with hydrocarbon polarizabilities. Moreover, they do not correlate well with the frequency shifts of the HF submolecule stretching vibration measured in matrix isolation studies.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.448752
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  • 9
    Electronic Resource
    Electronic Resource
    The microwave spectrum of the K=0 states of Ar–NH3 (1986)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 85 (1986), S. 5512-5518 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: 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(ring). In the pseudodiatomic approximation, the weak bond stretching force constant is 0.0084 mdyn/A(ring) 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 intermolecular potential is nearly isotropic and that the NH3 subunit undergoes practically free internal rotation in each of its angular degrees of freedom. Spectroscopic evidence is presented which indicates that the NH3 subunit also inverts within the complex. These conclusions concerning the internal dynamics in the Ar–NH3 complex support the model initially proposed in our previous study of the microwave and infrared spectra of this species.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.451562
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  • 10
    Electronic Resource
    Electronic Resource
    The rotational spectra of NH3–CO and NH3–N2 (1986)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 84 (1986), S. 2472-2480 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: 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, large amplitude dynamics are occurring in at least two degrees of freedom. The quadrupole coupling constants, eQqNaa indicate that in each of the isotopes of NH3–CO the NH3 subunit has its C3 axis relatively rigidly oriented at an angle of approximately 36° with respect to the line connecting the centers of mass of the two subunits. The structure is not hydrogen bonded; the N atom is closest to the CO subunit. The orientation of the CO subunit is not established. The distance between the N atom and the center of mass of the CO unit (RN–CO) is 3.54(3) A(ring). The spectroscopic constants suggest that the weak bond stretching force constant is quite small (0.01 mdyn/A(ring)) but compatible with the long bond length.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.450366
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